Abstract:
The invention relates to tetrahydrofuran derivatives of general formula (I), wherein X stands for *—(C═O)—O—, *—(CH2)n-O—, or *—(CH2)n-O—(C═O)—, wherein * represents the point of bonding to the tetrahydrofuran ring and n has the value 0, 1, or 2; and R1 and R2 are selected independently of each other from among C4-C5 alkyl and C5-C6 cycloalkyl, wherein the cycloalkyl groups are unsubstituted or can be substituted by at least one C1-C10 alkyl group, a plasticizer composition that contains said tetrahydrofuran derivatives, molding masses that contain a thermoplastic polymer or an elastomer and such a tetrahydrofuran derivative. The invention further relates to a method for producing said tetrahydrofuran derivatives, and to the use of said tetrahydrofuran derivatives

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to tetrahydrofuran derivatives, to a plasticizer composition which comprises said tetrahydrofuran derivatives, to molding compositions which comprise a thermoplastic polymer and a tetrahydrofuran derivative of this type, to a process for producing said tetrahydrofuran derivatives, and to use of these. 
       PRIOR ART 
       [0002]    Desired processing properties or desired performance characteristics are achieved in many plastics by adding what are known as plasticizers in order to render the plastics softer, more flexible and/or more extensible. Plasticizers generally serve to shift the thermoplastic region of plastics to lower temperatures, so as to obtain the desired elastic properties at lower processing temperatures and lower usage temperatures. 
         [0003]    Production quantities of polyvinyl chloride (PVC) are among the highest of any plastic. Because this material is versatile, it is nowadays found in a wide variety of products used in everyday life. PVC therefore has very great economic importance. PVC is intrinsically a plastic that is hard and brittle up to about 80° C., and is used in the form of rigid PVC (PVC-U) by adding heat stabilizers and other additives. Flexible PVC (PVC-P) is obtained only by adding suitable plasticizers, and can be used for many applications for which rigid PVC is unsuitable. 
         [0004]    Examples of other important thermoplastic polymers in which plasticizers are usually used are polyvinyl butyral (PVB), homo- and copolymers of styrene, polyacrylates, polysulfides, and thermoplastic polyurethanes (PUs). 
         [0005]    There are many different compounds marketed for plasticizing PVC and other plastics. Phthalic diesters with alcohols of different chemical structure have in the past often been used as plasticizers because they have good compatibility with PVC and advantageous performance characteristics, examples being diethylhexyl phthalate (DEHP), diisononyl phthalate (DINP) and diisodecyl phthalate (DIDP). Short-chain phthalates, e.g. dibutyl phthalate (DBP), diisobutyl phthalate (DIBP), benzyl butyl phthalate (BBP) or diisoheptyl phthalate (DIHP), are also used as gelling aids (“fast fuser”), for example in the production of what are known as plastisols. It is also possible to use dibenzoic esters, such as dipropylene glycol dibenzoates, for the same purpose alongside the short-chain phthalates. Phenyl esters of alkylsulfonic acids are another class of plasticizers with good gelling properties, and are marketed by way of example in the form of mixtures as Mesamoll® TP-LXS 51067. 
         [0006]    Plastisols initially are a suspension of finely pulverulant plastics in liquid plasticizers. The solvation rate of the polymer in the plasticizer here is very low at ambient temperature. The polymer is noticeably solvated in the plasticizer only on heating to relatively high temperatures. The individual isolated polymer aggregates here swell and fuse to give a three-dimensional high-viscosity gel. This procedure is termed gelling, and begins at a certain minimum temperature which is termed gel point or solvation temperature. The gelling step is not reversible. 
         [0007]    Since plastisols take the form of liquids, these are very often used for the coating of a very wide variety of materials, e.g. textiles, glass nonwovens, etc. This coating is very often composed of a plurality of sublayers. 
         [0008]    In a procedure often used in the industrial processing of plastisols, a layer of plastisol is therefore applied and then the plastic, in particular PVC, with the plasticizer is subjected to incipient gelling above the solvation temperature, thus producing a solid layer composed of a mixture of gelled, partially gelled, and ungelled polymer particles. 
         [0009]    The next sublayer is then applied to this incipiently gelled layer, and once the final layer has been applied the entire structure is processed in its entirety to give the fully gelled plastics product by heating to relatively high temperatures. 
         [0010]    Another possibility, alongside production of plastisols, is production of dry pulverulant mixtures of plasticizer and polymers. These dry blends, in particular based on PVC, can then be further processed at elevated temperatures for example by extrusion to give pellets, or processed through conventional shaping processes, such as injection molding, extrusion, or calendering, to give the fully gelled plastics product. 
         [0011]    In particular in the production and processing of PVC plastisols, for example for producing PVC coatings, it is inter alia desirable to have available, as gelling aid, a plasticizer with minimal gelling point and low viscosity. High storage stability of the plastisol is moreover also desirable, i.e. the ungelled plastisol is intended to exhibit no, or only a slight, viscosity rise over the course of time at ambient temperature. As far as possible, these properties are intended to be achieved by addition of a suitable plasticizer with rapid-gelling properties, with no need for the use of other viscosity-reducing additives and/or of solvents. 
         [0012]    Another known method for establishing the desired properties is to use mixtures of plasticizers, e.g. to use at least one plasticizer which provides good thermoplastic properties but has poor gelling effect, in combination with at least one gelling aid. 
         [0013]    There is a need to replace the phthalate plasticizers mentioned in the introduction, because these are not entirely free from toxicological concerns. This specifically applies to sensitive application sectors such as toys, food packaging, or medical items. 
         [0014]    Various alternate plasticizers for a variety of plastics, and specifically for PVC, are known in the prior art. 
         [0015]    A plasticizer class that is known from the prior art and that can be used as alternative to phthalates is based on the cyclohexanepolycarboxylic acids described in WO 99/32427. Unlike their unhydrogenated aromatic analogs, these compounds give rise to no toxicological concerns, and can be used even in sensitive application sectors. The corresponding lower alkyl esters generally have rapid-gelling properties. 
         [0016]    WO 00/78704 describes selected dialkylcyclohexane-1,3- and 1,4-dicarboxylic esters for the use as plasticizer in synthetic materials. 
         [0017]    U.S. Pat. No. 7,973,194 B1 teaches the use of dibenzyl cyclohexane-1,4-dicarboxylate, benzyl butyl cyclohexane-1,4-dicarboxylate, and dibutyl cyclohexane-1,4-dicarboxylate as rapid-gelling plasticizers for PVC. 
         [0018]    Some diether derivatives of 2,5-di(hydroxymethyl)tetrahydrofuran are already known materials. WO 2009/141166 describes a fuel composition composed of ring-hydrogenated alkylfurfuryl ethers of the general formula: R″-TF-CH 2 —O—R, in which IF is a 2,5-disubstituted tetrahydrofuran ring, R is a hydrocarbyl group having from 1 to 20 carbon atoms, R″ represents a methyl group, a hydroxymethyl group, or else the product of an aldol condensation, or represents an alkoxymethyl group of the general formula: —CH 2 —O—R′, in which R′ is a hydrocarbyl group having from 1 to 20 carbon atoms. Only methyl and ethyl are specifically used as moiety R and R′. Said document claims that these compounds are novel materials, and also describes a process for producing these, but teaches only use of these as fuel or fuel additives, rather than as plasticizer. 
         [0019]    The esters of 2,5-furandicarboxylic acid (FDCA) are another plasticizer class. 
         [0020]    WO 2012/113608 describes C 5 -dialkyl esters of 2,5-furandicarboxylic acid and use of these as plasticizers. These short-chain esters are specifically also suitable for producing plastisols. 
         [0021]    WO 2012/113609 describes C 7 -dialkyl esters of 2,5-furandicarboxylic acid and use of these as plasticizers. 
         [0022]    WO 2011/023490 describes C 9 -dialkyl esters of 2,5-furandicarboxylic acid and use of these as plasticizers. 
         [0023]    WO 2011/023491 describes C 10 -dialkyl esters of 2,5-furandicarboxylic acid and use of these as plasticizers. 
         [0024]    R. D. Sanderson et al. (J. Appl. Pol. Sci., 1994, vol. 53, 1785-1793) describe the synthesis of esters of 2,5-furandicarboxylic acid and use of these as plasticizers for plastics, in particular polyvinyl chloride (PVC), polyvinyl butyral (PVB), polylactic acid (PLA), polyhydroxybutyric acid (PHB) or polyalkyl methacrylate (PAMA). Specifically, the di(2-ethylhexyl), di(2-octyl), dihexyl, and dibutyl esters of 2,5-furandicarboxylic acid are described, and the plasticizing properties of these are characterized by way of dynamic mechanical thermal analyses. 
         [0025]    U.S. Pat. No. 3,259,636 describes a process for producing esters of cis-2,5-tetrahydrofurandicarboxylic acid, where hydrogen, 2,5-furandicarboxylic acid and an alcohol are reacted in the presence of a noble metal catalyst in a one-pot reaction. It is moreover disclosed that the esters of alcohols having 6 or more carbon atoms are suitable as plasticizers in resin compositions. 
         [0026]    It is an object of the present invention to provide novel compounds which can advantageously be used as, or in, plasticizers for thermoplastic polymers and elastomers. They are intended to be free from toxicological concerns and to be capable of production from readily obtainable starting materials which preferably at least to some extent derive from renewable raw materials. They are intended to have good gelling properties and/or to exhibit low viscosity in the ungelled state, and therefore to be particularly suitable for providing plastisols. The novel compounds are accordingly intended to be able to at least equally replace the standard petrochemically based plasticizers that are mainly used nowadays. 
         [0027]    Surprisingly, said object is achieved via tetrahydrofuran derivatives of the general formula (I) 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    in which
 
X is *—(C═O)—O—, *—(CH 2 ) n —O— or *—(CH 2 ) n —O—(C═O)—, where is the point of linkage to the tetrahydrofuran ring, and n has the value 0, 1, or 2;
 
and
 
R 1  and R 2  are selected mutually independently from C 4 -C 5 -alkyl and C 5 -C 6 -cycloalkyl, where the cycloalkyl moieties are unsubstituted or can have substitution by at least one C 1 -C 10 -alkyl moiety.
 
         [0028]    The invention further provides plasticizer compositions which comprise at least one compound of the general formula (I) as defined above and hereinafter, and at least one plasticizer different from the compounds of the formula (I). 
         [0029]    The invention further provides processes for producing compounds of the general formula (I). 
         [0030]    The invention further provides the use of compounds of the general formula (I) as, or in, plasticizers for thermoplastic polymers, in particular polyvinyl chloride (PVC). 
         [0031]    The invention further provides molding compositions which comprise at least one thermoplastic polymer and at least one compound of the general formula (I) as defined above and hereinafter. 
         [0032]    The invention further provides the use of said molding compositions for producing moldings and foils. 
       DESCRIPTION OF THE INVENTION 
       [0033]    The compounds (I) of the invention exhibit the following advantages:
       By virtue of their physical properties, the compounds (I) of the invention have very good suitability for applications as plasticizers or as component of a plasticizer composition for thermoplastic polymers, in particular for PVC.   By virtue of their low solvation temperatures in accordance with DIN 53408, the compounds (I) of the invention have very good suitability as gelling aids. They are therefore suitable for reducing the temperature required for gelling of a thermoplastic polymer and/or for increasing the gelling rate.   The compounds of the general formula (I) of the invention feature very good compatibility with a wide variety of different plasticizers. They are specifically suitable in combination with conventional plasticizers for improving gelling performance.   The compounds (I) of the invention are advantageously suitable for producing plastisols.   The compounds (I) of the invention are suitable for the use for producing moldings and foils for sensitive application sectors, for example medical products, food packaging, products for the interior sector, for example in dwellings and in vehicles, and for toys, child-care items, etc.   The compounds (I) of the invention can be produced by using readily obtainable starting materials. A particular economic and environmental advantage of the present invention derives from the possibility of using, in the production of the compounds (I) of the invention, not only petrochemical raw materials that are available in large quantities but also renewable raw materials. By way of example, therefore, it is possible to obtain the starting materials for the furan rings from naturally occurring carbohydrates, such as cellulose and starch, while the alcohols that can be used for introducing the side chains are available from large-scale industrial processes. It is thus possible on the one hand to comply with the “sustainable” materials requirement while on the other hand also permitting cost-effective production.   The processes for producing the compounds (I) of the invention are simple and efficient, and these can therefore be provided without difficulty on a large industrial scale.       
 
         [0041]    As previously mentioned, it has surprisingly been found that the compounds of the general formula (I), in particular the C 4 -C 5 -dialkyl esters of tetrahydrofurandicarboxylic acid, have very low solvation temperatures, and also excellent gelling properties in the production of plastisols, in particular of PVC plastisols: their solvation temperatures are markedly below the solvation temperatures of the corresponding dialkyl esters of 2,5-furandicarboxylic acid or phthalic acid, and have at least equivalent rapid-gelling properties. This was not to be expected, since by way of example ring-hydrogenated phthalates such as diisononyl cyclohexane-1,2-dicarboxylate generally have higher solvation temperatures than their unhydrogenated forms: by way of example, the solvation temperature of diisononyl 1,2-cyclohexanedicarboxylate is higher at 151° C. than that of diisononyl phthalate at 132° C., in accordance with DIN 53408. 
         [0042]    For the purposes of the present invention, the expression gelling aid means a plasticizer which has a solvation temperature below 120° C. in accordance with DIN 53408. Gelling aids of this type are in particular used for producing plastisols. 
         [0043]    The compounds of the general formula (I.1) of the invention can take the form either of pure cis-isomers or of pure trans-isomers, or of cis/trans-isomer mixtures. The pure isomers and the isomer mixtures of any desired composition are equally suitable as plasticizers. 
         [0044]    For the purposes of the present invention, the expression “C 1 -C 10 -alkyl” comprises straight-chain or branched C 1 -C 10 -alkyl groups. These preferably are straight-chain or branched C 1 -C 8 -alkyl groups. Among these are methyl, ethyl, propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, n-pentyl, 2-pentyl, 2-methylbutyl, 3-methylbutyl, 1,2-dimethylpropyl, 1,1-dimethylpropyl, 2,2-dimethylpropyl, 1-ethylpropyl, n-hexyl, 2-hexyl, 2-methylpentyl, 3-methylpentyl, 4-methylpentyl, 1,2-dimethylbutyl, 1,3-dimethylbutyl, 2,3-dimethylbutyl, 1,1-dimethylbutyl, 2,2-dimethylbutyl, 3,3-dimethylbutyl, 1,1,2-trimethylpropyl, 1,2,2-trimethylpropyl, 1-ethylbutyl, 2-ethylbutyl, 1-ethyl-2-methylpropyl, n-heptyl, 2-heptyl, 3-heptyl, 2-ethylpentyl, 1-propylbutyl, n-octyl, and the like. These particularly preferably are straight-chain or branched C 1 -C 5 -alkyl groups. 
         [0045]    The expression “C 4 -C 5 -alkyl” comprises straight-chain and branched C 4 -C 5 -alkyl groups. It is preferable that C 4 -C 5 -alkyl is selected from n-butyl, isobutyl, sec-butyl, tert-butyl, n-pentyl, 2-pentyl, 2-methylbutyl, 3-methylbutyl, 1,2-dimethylpropyl, 1,1-dimethylpropyl, 2,2-dimethylpropyl, and 1-ethylpropyl. It is particularly preferable that C 4 -C 5 -alkyl is n-butyl, isobutyl, or n-pentyl. 
         [0046]    The expression “C 5 -C 6 -cycloalkyl” comprises for the purposes of the present invention cyclic hydrocarbons having from 5 to 6, in particular having 6, carbon atoms. Among these are cyclopentyl and cyclohexyl. 
         [0047]    Substituted C 5 -C 6 -cycloalkyl groups can, as permitted by their ring size, have one or more (e.g. 1, 2, 3, 4, or 5) C 1 -C 10 -alkyl substituents. Examples of C 5 -C 6 -cycloalkyl groups are 2- and 3-methylcyclopentyl 2-, and 3-ethylcyclopentyl, 2-, 3-, and 4-methyl-cyclohexyl, 2-, 3-, and 4-ethylcyclohexyl, 2-, 3-, and 4-Propylcyclohexyl, 2-, 3-, and 4-isopropylcyclohexyl, 2-, 3-, and 4-butylcyclohexyl, 2-, 3-, and 4-sec-butylcyclohexyl, and 2-, 3-, and 4-tert-butylcyclohexyl. 
         [0048]    It is preferable that the definitions of the groups X in the compounds of the general formula (I) are identical. 
         [0049]    In a first preferred embodiment, both of the groups X in the compounds of the general formula (I) are *—(C═O)—O—. 
         [0050]    In another preferred embodiment, both of the groups X in the compounds of the general formula (I) are *—(CH 2 )—O—(C═O)—. 
         [0051]    In another preferred embodiment, both of the groups X in the compounds of the general formula (I) are *—(CH 2 ) n —O—, where n is 0, 1 or 2. It is particularly preferable that n is 2. 
         [0052]    It is preferable that the moieties R 1  and R 2  in the compounds of the general formula (I) are mutually independently an unbranched or branched C 4 -alkyl moiety. 
         [0053]    It is particularly preferable that the moieties R 1  and R 2  in the compounds of the general formula (I) are mutually independently n-butyl or isobutyl. 
         [0054]    In a preferred embodiment, the definitions of the moieties R 1  and R 2  in the compounds of the general formula (I) are identical. 
         [0055]    Preferred compounds of the general formula (I) are those selected from
   di(n-butyl) 2,5-tetrahydrofurandicarboxylate,   di-n-butyl ether of 2,5-di(hydroxymethyl)tetrahydrofuran,   2,5-di(hydroxymethyl)tetrahydrofuran di-n-butanoate,   di(isobutyl) 2,5-tetrahydrofurandicarboxylate,   diisobutyl ether of 2,5-di(hydroxymethyl)tetrahydrofuran,   2,5-di(hydroxymethyl)tetrahydrofuran diisobutanoate,
 
and also mixtures of 2 or more of the abovementioned compounds.
   
 
         [0062]    A particularly preferred compound of the general formula (I) is di(n-butyl) 2,5-tetrahydrofurandicarboxylate. 
         [0063]    Production of the Compounds of the General Formula (I) 
         [0064]    Production of the diesters of 2,5-tetrahydrofurandicarboxylic acid 
         [0065]    The invention further provides a process for producing compounds of the general formula (I.1), 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    in which
   R 1  and R 2  are selected mutually independently from C 4 -C 5 -alkyl and C 5 -C 6 -cycloalkyl, where the cycloalkyl moieties are unsubstituted or can have substitution by at least one C 1 -C 10 -alkyl moiety,
 
where
   a) optionally 2,5-furandicarboxylic acid or an anhydride or acyl halide thereof is reacted with a C 1 -C 3 -alkanol in the presence of a catalyst to give a di(C 1 -C 3 -alkyl) 2,5-furandicarboxylate,   b1) 2,5-furandicarboxylic acid or an anhydride or acyl halide thereof, or the di(C 1 -C 3 -alkyl) 2,5-furandicarboxylate obtained in step a), is reacted with at least one alcohol R 1 —OH and, if R 1  and R 2  are different, also with at least one alcohol R 2 —OH, in the presence of at least one catalyst to give a compound of the formula (I.1a),   
 
         [0000]    
       
                 
         
             
             
         
       
       
         c1) the compound (I.1a) obtained in step b1) is hydrogenated with hydrogen in the presence of at least one hydrogenation catalyst to give the compound of the general formula (I.1),
 
or
 
         b2) 2,5-furandicarboxylic acid or the di(C 1 -C 3 -alkyl) 2,5-furandicarboxylate obtained in step a) is hydrogenated with hydrogen in the presence of at least one hydrogenation catalyst to give a compound of the general formula (I.1 b), 
       
     
         [0000]    
       
                 
         
             
             
         
       
       
         the compound (I, 1b) obtained in step b2) is reacted with at least one alcohol R 1 —OH and, if R 1  and R 2  are different, also with at least one alcohol R 2 —OH, in the presence of a catalyst to give a compound of the formula (I.1). 
       
     
         [0072]    In respect of suitable and preferred embodiments of the moieties R 1  and R 2 , reference is made to the entirety of the information provided above. 
         [0073]    The process of the invention permits the production of the 2,5-tetrahydrofurandicarboxylic esters of the general formula (I.1) by two different routes (hereinafter termed variant 1 and variant 2). 
         [0074]    Examples of C 1 -C 3 -alkanols suitable for use in step a) are methanol, ethanol, n-propanol, and mixtures thereof. 
         [0075]    In variant 1 of the process of the invention, the 2,5-furandicarboxylic acid or the di(C 1 -C 3 -alkyl) 2,5-furandicarboxylate obtained in step a) is subjected to esterification or transesterification with at least one alcohol R 1 —OH and, if R 1  and R 2  are different, also with at least one alcohol R 2 —OH, to give the compounds of the formula (I.1a), which are then hydrogenated to give compounds of the general formula (I.1) (step c1)). 
         [0076]    In variant 2, the 2,5-furandicarboxylic acid or the 2,5-di(C 1 -C 3 -alkyl) furandicarboxylate obtained in step a) is first hydrogenated to give 2,5-tetrahydrofurandicarboxylic acid or, respectively, a compound of the general formula (I.1 b) (step b2)), and the hydrogenation product is then reacted with at least one alcohol R 1 —OH and, if R 1  and R 2  are different, also with at least one alcohol R 2 —OH to give the compounds of the general formula (I.1) (step c2)). 
         [0077]    Esterification 
         [0078]    Conventional processes known to the person skilled in the art can be used to convert the 2,5-furandicarboxylic acid (FDCA) or the 2,5-tetrahydrofurandicarboxylic acid to the corresponding ester compounds of the general formulae (I.1), (I.1a), and (I.1b). Among these are the reaction of at least one alcohol component selected from C 1 -C 3 -alkanols or from the alcohols R 1 —OH and, respectively. R 2 —OH with FDCA or a suitable derivative thereof. Examples of suitable derivatives are the acyl halides and anhydrides. A preferred acyl halide is the acyl chloride. Esterification catalysts that can be used are the catalysts conventionally used for this purpose, e.g. mineral acids, such as sulfuric acid and phosphoric acid; organic sulfonic acids, such as methanesulfonic acid and p-toluenesulfonic acid; amphoteric catalysts, in particular titanium compounds, tin(IV) compounds, or zirconium compounds, e.g. tetraalkoxytitanium compounds, e.g. tetrabutoxytitanium, and tin(IV) oxide. The water produced during the reaction can be removed by conventional measures, e.g. by distillation. WO 02/038531 describes a process for producing esters where a) a mixture consisting essentially of the acid component or an anhydride thereof and of the alcohol component is heated to boiling point in the presence of an esterification catalyst in a reaction zone, b) the vapors comprising alcohol and water are fractionated to give an alcohol-rich fraction and a water-rich fraction, c) the alcohol-rich fraction is returned to the reaction zone, and the water-rich fraction is discharged from the process. Esterification catalysts used are the abovementioned catalysts. An effective amount of the esterification catalyst is used and is usually in the range from 0.05 to 10% by weight, preferably from 0.1 to 5% by weight, based on the entirety of acid component (or anhydride) and alcohol component. Other detailed descriptions of the conduct of esterification processes are found by way of example in U.S. Pat. No. 6,310,235, U.S. Pat. No. 5,324,853, DE-A 2612355 (Derwent Abstract No. DW 77-72638 Y) or DE-A 1945359 (Derwent Abstract No. DW 73-27151 U). The entirety of the documents mentioned is incorporated herein by way of reference. 
         [0079]    In one preferred embodiment, the esterification of FDCA or of the 2,5-tetrahydrofurandicarboxylic acid is carried out in the presence of the alcohol components described above by means of an organic acid or mineral acid, in particular concentrated sulfuric acid. The amount used of the alcohol component here is advantageously at least twice the stochiometric amount, based on the FDCA or the 2,5-tetrahydrofurandicarboxylic acid or a derivative. 
         [0080]    The esterification can generally take place at ambient pressure or at reduced or elevated pressure. It is preferable that the esterification is carried out at ambient pressure or reduced pressure. 
         [0081]    The esterification can be carried out in the absence of any added solvent or in the presence of an organic solvent. 
         [0082]    If the esterification is carried out in the presence of a solvent, it is preferably an organic solvent that is inert under the reaction conditions. Among these are by way of example aliphatic hydrocarbons, halogenated aliphatic hydrocarbons, and aromatic and substituted aromatic hydrocarbons and ethers. It is preferable that the solvent is one selected from pentane, hexane, heptane, ligroin, petrol ether, cyclohexane, dichloromethane, trichloromethane, tetrachloromethane, benzene, toluene, xylene, chlorobenzene, dichlorobenzenes, dibutyl ether, THF, dioxane, and mixtures thereof. 
         [0083]    The esterification is usually carried out in the temperature range from 50 to 250° C. 
         [0084]    If the esterification catalyst is selected from organic acids or mineral acids, the esterification is carried out typically in a temperature range from 50 to 160° C. 
         [0085]    If the esterification catalyst is selected from amphoteric catalysts, the esterification is carried out typically in a temperature range from 100 to 250° C. 
         [0086]    The esterification can take place in the absence of or in the presence of an inert gas. The expression inert gas generally means a gas which under the prevailing reaction conditions does not enter into any reactions with the starting materials, reagents, or solvents participating in the reaction, or with the resultant products. It is preferable that the esterification takes place without addition of any inert gas. 
         [0087]    Transesterification: 
         [0088]    Conventional processes known to the person skilled in the art can be used for the reaction, described in steps b1) and c2), of the di(C 1 -C 3 -alkyl) 2,5-furandicarboxylates and, respectively, the di(C 1 -C 3 -alkyl) 2,5-tetrahydrofurandicarboxylates to give the corresponding ester compounds I.1a and, respectively, I.1. Among these are the reaction of the di(C 1 -C 3 )-alkyl esters with at least one C 4 -C 5 -alkanol or C 5 -C 6 -cycloalkanol or a mixture thereof in the presence of a suitable transesterification catalyst. 
         [0089]    Transesterification catalysts that can be used are the conventional catalysts usually used for transesterification reactions, where these are mostly also used in esterification reactions. Among these are by way of example mineral acids, such as sulfuric acid and phosphoric acid; organic sulfonic acids, such as methanesulfonic acid and p-toluenesulfonic acid; and specific metal catalysts from the group of the tin(IV) catalysts, for example dialkyltin dicarboxylates, such as dibutyltin diacetate, trialkyltin alkoxides, monoalkyltin compounds, such as monobutyltin dioxide, tin salts, such as tin acetate, or tin oxides; from the group of the titanium catalysts: monomeric and polymeric titanates and titanium chelates, for example tetraethyl orthotitanate, tetrapropyl orthotitanate, tetrabutyl orthotitanate, triethanolamine titanate; from the group of the zirconium catalysts: zirconates and zirconium chelates, for example tetrapropyl zirconate, tetrabutyl zirconate, triethanolamine zirconate; and also lithium catalysts, such as lithium salts, lithium alkoxides; and aluminum(III) acetylacetonate, chromium(III) acetylacetonate, iron(III) acetylacetonate, cobalt(II) acetylacetonate, nickel(II) acetylacetonate, and zinc(II) acetylacetonate. 
         [0090]    The amount of transesterification catalyst used is from 0.001 to 10% by weight, preferably from 0.05 to 5% by weight. The reaction mixture is preferably heated to the boiling point of the reaction mixture, the reaction temperature therefore being from 20° C. to 200° C., depending on the reactants. 
         [0091]    The transesterification can take place at ambient pressure or at reduced or elevated pressure. It is preferable that the transesterification is carried out at a pressure of from 0.001 to 200 bar, particularly from 0.01 to 5 bar. The relatively low-boiling-point alcohol eliminated during the transesterification is preferably continuously removed by distillation in order to shift the equilibrium of the transesterification reaction. The distillation column necessary for this purpose generally has direct connection to the transesterification reactor, and it is preferable that said column is a direct attachment thereto. If a plurality of transesterification reactors are used in series, each of said reactors can have a distillation column, or the vaporized alcohol mixture can preferably be introduced into a distillation column from the final tanks of the transesterification reactor cascade by way of one or more collection lines. The relatively high-boiling-point alcohol reclaimed in said distillation is preferably returned to the transesterification. 
         [0092]    If an amphoteric catalyst is used, it is separated off generally by hydrolysis and subsequent removal of the metal oxide formed, by filtration, for example. Preferably, after reaction has taken place, the catalyst is hydrolyzed by washing with water and the precipitated metal oxide is removed by filtration. If desired, the filtrate may be subjected to further work-up for the isolation and/or purification of the product. The product is preferably isolated by distillation. 
         [0093]    In one preferred embodiment of steps 1b) and 2c), the transesterification of the di(C 1 -C 3 -alkyl) 2,5-furandicarboxylates and, respectively, di(C 1 -C 3 -alkyl) 2,5-tetrahydrofurandicarboxylates takes place in the presence of the alcohol component and in the presence of at least one titanium(IV) alcoholate. Preferred titanium(IV) alcoholates are tetrapropoxytitanium, tetrabutoxytitanium, and mixtures thereof. It is preferable that the amount used of the alcohol component is at least twice the stochiometric amount, based on the di(C 1 -C 3 -alkyl) ester used. 
         [0094]    The transesterification can be carried out in the absence of, or in the presence of, an added organic solvent. It is preferable that the transesterification is carried out in the presence of an inert organic solvent. Suitable organic solvents are those mentioned above for the esterification. Among these are specifically toluene and THF. 
         [0095]    The transesterification is preferably carried out in the temperature range from 50 to 200° C. 
         [0096]    The transesterification can take place in the absence of or in the presence of an inert gas. The expression inert gas generally means a gas which under the prevailing reaction conditions does not enter into any reactions with the starting materials, reagents, or solvents participating in the reaction, or with the resultant products. It is preferable that the transesterification takes place without addition of any inert gas. 
         [0097]    Hydrogenation 
         [0098]    Many processes and catalysts for the hydrogenation of the double bonds of the furan ring carried out in steps c1) and b2) of the invention are available to the person skilled in the art and these by way of example are also used in the hydrogenation of esters of aromatic polycarboxylic acids, examples being phthalates, isophthalates and terephthalates. By way of example, the ring-hydrogenation process described in WO 99/032427 is suitable. This comprises hydrogenation at from 50 to 250° C. and at a pressure of from 20 to 300 bar by means of catalysts which comprise at least one metal of transition group VIII of the Periodic Table of the Elements, for example platinum, rhodium, palladium, cobalt, nickel, or ruthenium, preferably ruthenium, either alone or together with at least one metal from transition group I or VII of the Periodic Table of the Elements, for example copper or ruthenium, deposited on a mesoporous aluminum oxide support material with bimodal pore distribution. The ring-hydrogenation process described in WO 02/100536 is moreover suitable. This comprises hydrogenation with use of a ruthenium catalyst on amorphous silicon dioxide as support. Other suitable processes are described in the following documents: EP-A 1266882—Use of a nickel/magnesium oxide on kieselguhr catalyst, WO 03/029181—Use of a nickel/zinc on silicon dioxide catalyst, WO 03/029168—Use of a palladium/ZnO on Al 2 O 3  catalyst and of a ruthenium/ZnO on α-Al 2 O 3  catalyst, or WO 04/09526—Use of a ruthenium on titanium dioxide catalyst. Other suitable catalysts are likewise Raney catalysts, preferably Raney nickel. Other suitable support materials alongside those already mentioned are by way of example zirconium dioxide (ZrO 2 ), sulfated zirconium dioxide, tungsten carbide (WC), titanium dioxide (TiO 2 ), sulfated carbon, activated charcoal, aluminum phosphate, aluminosilicates, or phosphated aluminum oxide, or else a combination thereof. 
         [0099]    The hydrogenation can take place by analogy with the known hydrogenation processes for hydrogenating organic compounds which have hydrogenatable groups. To this end, the organic compound in the form of liquid phase or gas phase, preferably in the form of liquid phase, is brought into contact with the catalyst in the presence of hydrogen. The liquid phase can by way of example be passed over a fluidized bed of catalyst (fluidized bed method) or can be passed over a fixed bed of catalyst (fixed bed method). 
         [0100]    In the process of the invention, is preferable that the hydrogenation takes place in a fixed-bed reactor. 
         [0101]    The hydrogenation can be designed to take place either continuously or else batchwise, preference being given here to the continuous design of the process. The batchwise hydrogenation can use a reaction apparatus conventionally used for this purpose, e.g. a stirred reactor. It is preferable that the hydrogenation of the invention is carried out continuously in fixed-bed reactors in the bottoms method or trickle-bed method. The hydrogen here can be passed over the catalyst cocurrently with the solution of the starting material to be hydrogenated, or else in countercurrent. 
         [0102]    Suitable apparatuses for conducting fluidized-bed-catalyst hydrogenation and fixed-bed-catalyst hydrogenation are known in the prior art, e.g. from Ullmanns Enzyklopädie der Technischen Chemie [Ullmann&#39;s Encyclopedia of Industrial Chemistry], 4 th  edition, volume 13, pp. 135 ff., and also from P. N. Rylander, “Hydrogenation and Dehydrogenation” in Ullmann&#39;s Encyclopedia of Industrial Chemistry, 5th edn. on CD-ROM. 
         [0103]    The hydrogenation generally takes place under elevated hydrogen pressure. Preference is given to hydrogen pressure in the range from 2 to 500 bar, particularly from 10 to 300 bar. 
         [0104]    It is preferable that the hydrogenation takes place in the presence of an organic solvent that is inert under the hydrogenation conditions. Suitable solvents are those previously defined for the esterification. Specifically, an ether is used, for example THF, or a dialkylene glycol, or a mono- or diether thereof, for example glyme. 
         [0105]    The hydrogenation is carried out at a temperature in the range from 20 to 350° C., particularly preferably from 50 to 300° C. 
         [0106]    The amount of hydrogen used for the hydrogenation is generally from 1 to 15 times the stochiometric amount of hydrogen theoretically needed for the complete hydrogenation of the furan ring. 
         [0107]    In one preferred embodiment of steps c1) and b2), the hydrogenation of the furan ring is carried out with platinum, rhodium, palladium, cobalt, nickel, or ruthenium, in particular platinum and palladium, deposited on aluminum oxide, on zirconium dioxide, on sulfated zirconium dioxide, on zinc oxide, or on silicon dioxide, in particular on zirconium dioxide, in the presence of an inert solvent, under hydrogen pressure of from 150 to 300 bar, at a temperature of from 150 to 250° C. 
         [0108]    The hydrogenation processes described can give preference to formation of the cis- or trans-isomer of the 2,5-tetrahydrofurandicarboxylic esters in accordance with the selected hydrogenation conditions, for example catalyst composition, or hydrogenation temperature: it is possible to produce cis- or trans-2,5-tetrahydrofurandicarboxylic esters that are in essence isomerically pure, or else a mixture with various proportions of cis- and trans-isomers. The expression “in essence isomerically pure” here means content of at least 95% by weight of a particular isomer, based on the total weight of the 2,5-tetrahydrofurandicarboxylic ester. 
         [0109]    The compounds of the general formula (I.1) of the invention can accordingly take the form of pure cis-isomers or take the form of pure trans-isomers, or take the form of cis/trans-isomer mixtures. The pure isomers and the isomer mixtures of any desired composition are equally suitable as plasticizers. 
         [0110]    In one particularly preferred embodiment of steps c1) and b2), FDCA and, respectively, the esters of the 2,5-furandicarboxylic acid from steps a) and b1) are dissolved in an inert solvent and fully hydrogenated in the presence of a heterogeneous Pd/Pt catalyst at a hydrogen pressure of from 50 to 300 bar and at from 100 to 250° C. The hydrogenation here preferably takes place continuously by the fixed-bed method, where the hydrogen is conducted in countercurrent over the catalyst. In this embodiment, it is preferable to use THF as solvent. In this embodiment it is moreover preferable to use a Pd/Pt catalyst on ZrO 2 . The preferred reaction temperature for this embodiment is in the range from 100 to 200° C. In this embodiment, the desired tetrahydrofuran derivatives are generally obtained with a proportion of cis-isomer of at least 90% by weight, based on the total amount of the cis/trans-isomers formed. 
         [0111]    One particularly preferred embodiment of the process of the invention comprises:
   a) reaction of 2,5-furandicarboxylic acid with methanol in the presence of concentrated sulfuric acid to give the dimethyl 2,5-furandicarboxylate,   2b) hydrogenation of the dimethyl 2,5-furandicarboxylate obtained in step a) with hydrogen in the presence of a Pd/Pt catalyst on ZrO 2  to give the dimethyl 2,5-tetrahydrofurandicarboxylate,   2c) reaction of the dimethyl 2,5-tetrahydrofurandicarboxylate obtained in step 2b) with at least one alcohol R 1 —OH in the presence of at least one titanium(IV) alcoholate to give the compounds of the general formula (I.1).   
 
         [0115]    Production of the C 4 -C 6 -diether derivatives and, respectively, C 4 -C 6 -diester derivatives of the formulae (I.2) and, respectively, (I.3) 
         [0116]    The invention further provides a process for producing compounds of the general formula (I.2) or (I.3), 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    in which
 
R 1  and R 2  are selected mutually independently from C 4 -C 5 -alkyl and C 5 -C 6 -cycloalkyl, where the cycloalkyl moieties are unsubstituted or can have substitution by at least one C 1 -C 10 -alkyl moiety, and n has the value 1 or 2,
 
where
   for 2,5-di(hydroxymethyl)tetrahydrofuran (n=1) or for 2,5-di(hydroxyethyl)tetrahydrofuran (n=2), reaction is carried out with at least one alkylating reagent R 1 —Z and, if R 1  and R 2  are different, also with at least one alkylating reagent R 2 —Z, where Z is a leaving group, in the presence of a base to give compounds of the formula (I.2),
 
or
   for 2,5-di(hydroxymethyl)tetrahydrofuran (n=1) or for 2,5-di(hydroxyethyl)tetrahydrofuran (n=2), reaction is carried out with at least one acyl halide R 1 —(C═O)X and, if R 1  and R 2  are different, also with at least one acyl halide R 2 —(C═O)X, where X is Br or Cl, in the presence of at least one tertiary amine to give compounds of the formula (I.3).   
 
         [0119]    The alkylation is generally carried out in the presence of an organic solvent that is inert under the reaction conditions. Suitable solvents are those previously mentioned for the estification. Aromatic hydrocarbons, such as toluene, are preferred as solvent. 
         [0120]    The leaving group Z is preferably a moiety selected from Br, Cl, and the tosyl, mesyl, and triflyl group. 
         [0121]    It is particularly preferable that the leaving group Z is Br. 
         [0122]    The alkylation reagents R 1 —Z and R 2 —Z are commercially available or can be produced by way of suitable reactions or procedures familiar to the person skilled in the art, from the corresponding alcohols. By way of example, the alkyl bromides R 1 —Br and, respectively, R 2 —Br preferably used for this process can be produced in a known manner on a large industrial scale from the appropriate alcohols R 1 —OH and, respectively, R 2 —OH by using hydrogen bromide (HBr). 
         [0123]    Suitable bases that can be used in the process of the invention are mineral bases and/or strong organic bases. Among these are by way of example inorganic bases or base-formers, for example hydroxides, hydrides, amides, oxides, and carbonates of the alkali metals and of the alkaline earth metals. Among these are LiOH, NaOH, KOH, Mg(OH) 2 , Ca(OH) 2 , LiH, NaH, sodium amide (NaNH 2 ), diisopropylamide (LDA), Na 2 O, K 2 CO 3 , Na 2 CO 3  and Cs 2 CO 3 ; and also organometallic compounds, such as n-BuLi, or tert-BuLi. Preference is given to NaOH, KOH, K 2 CO 3 , and Na 2 CO 3 . 
         [0124]    The amount used here of the base is preferably at least a two-fold stoichiometric excess, based on the 2,5-di(hydroxymethyl)tetrahydrofuran and, respectively, 2,5-di(hydroxyethyl)tetrahydrofuran. It is particularly preferable to use an at least four-fold stoichiometric excess of base. 
         [0125]    The alkylation can be carried out in the absence of, or in the presence of, an organic solvent. The reaction is generally carried out in the presence of an inert organic solvent, such as pentane, hexane, heptane, ligroin, petroleum ether, cyclohexane, dichloromethane, trichloromethane, tetrachloromethane, benzene, toluene, xylene, chlorobenzene, dichlorobenzenes, dibutyl ether, THF, dioxane, or a mixture thereof. 
         [0126]    The alkylation can generally take place at ambient pressure, reduced pressure, or elevated pressure. It is preferable that the alkylation is carried out at ambient pressure. 
         [0127]    It is preferable that the alkylation is carried out in the temperature range from 30 to 200° C., preferably from 50 to 150° C. 
         [0128]    The alkylation can take place in the absence of, or in the presence of, an inert gas. It is preferable that the alkylation uses no inert gas. 
         [0129]    In one specific embodiment of the alkylation, 2,5-di(hydroxymethyl)tetrahydrofuran or 2,5-di(hydroxyethyl)tetrahydrofuran is converted to the diether compounds of the general formula (I.2) in the presence of an at least four-fold excess of base in an inert organic solvent and with at least one alkyl bromide R 1 —Br and, respectively, R 2 —Br. In relation to the moieties R 1  and R 2 , reference is made to the previous descriptions. As base, it is preferable to use an alkali metal hydroxide, in particular KOH. 
         [0130]    To produce the ester compounds of the general formula (I.3) of the invention, it is preferable to react 2,5-di(hydroxymethyl)tetrahydrofuran or 2,5-di(hydroxyethyl)-tetrahydrofuran with at least one acyl halide R 1 —(C═O)X and, if R 1  and R 2  are different, with at least one acyl halide R 2 —(C═O)X, where X is Br or Cl, in the presence of at least one tertiary amine, to give the compounds of the formula (I.3). 
         [0131]    There are also other familiar esterification methods, alongside this process, available to the person skilled in the art, as previously described in relation to the esterification of FDCA and, respectively, 2,5-tetrahydrofurandicarboxylic acid. 
         [0132]    The ester compounds of the general formula (I.3) can usually be produced by using any of the tertiary amines familiar to the person skilled in the art. Examples of suitable tertiary amines are:
       from the group of the trialkylamines: trimethylamine, triethylamine, tri-n-propylamine, diethylisopropylamine, diisopropylethylamine and the like;   from the group of the N-cycloalkyl-N,N-dialkylamines: dimethylcyclohexylamine and diethylcyclohexylamine;   from the group of the N,N-dialkylanilines: dimethylaniline and diethylaniline;   from the group of the pyridine and quinoline bases: pyridine, α-, β-, and γ-picoline, and 4-(dimethylamino)pyridine (DMAP).       
 
         [0137]    Preferred tertiary amines are trialkylamines and pyridine bases, in particular triethylamine and 4-(dimethylamino)pyridine (DMAP), and also mixtures thereof. 
         [0138]    The esterification can take place at ambient pressure, or at reduced or elevated pressure. It is preferable to carry out the esterification at ambient pressure. 
         [0139]    The esterification can be carried out in the absence of, or in the presence of, an organic solvent. It is preferable to carry out the esterification in the presence of an inert organic solvent, as defined previously. 
         [0140]    The esterification is usually carried out in the temperature range from 50 to 200° C. 
         [0141]    The esterification can take place in the absence of, or in the presence of, an inert gas. 
         [0142]    In one preferred embodiment, 2,5-di(hydroxymethyl)tetrahydrofuran is reacted with an acyl chloride R 1 —(C═O)Cl in the presence of triethylamine and/or DAMP and of an inert organic solvent to give compounds of the formula (I.3). 
         [0143]    The preferred embodiments of the processes of the invention for producing compounds of the general formula (I) use C 4 -C 5 -alkanols, and also C 5 -C 6 -cycloalkanols, as starting materials for the transesterification, esterification, or alkylation. 
         [0144]    Preferred C 4 -C 5 -alkanols can be straight-chain or branched compounds, or can be composed of mixtures of straight-chain and branched butanols and, respectively, pentanols. Among these are 1-butanol 2-butanol, 2-methyl-1-propanol, 2-methyl-2-propanol, 1-pentanol, and 2-methylbutanol, and also mixtures thereof. Preference is given to 1-butanol, 2-methyl-1-propanol, and 1-pentanol. 
         [0145]    The C 5 -C 6 -cycloalkanols are those selected from cyclopentanol and cyclohexanol, and mixtures thereof. Preference is given to cyclohexanol. 
         [0146]    Substituted C 5 -C 6 -cycloalkanols can, as permitted by their ring size, have one or more (e.g. 1, 2, 3, 4, or 5) C 1 -C 10 -alkyl substituents. Examples of C 5 -C 6 -cycloalkanols are 2- and 3-methylcyclopentanol, 2-, and 3-ethylcyclopentanol, 2-, 3-, and 4-methyl-cyclohexanol, 2-, 3-, and 4-ethylcyclohexanol, 2-, 3-, and 4-Propylcyclohexanol, 2-, 3-, and 4-isopropylcyclohexanol, 2-, 3-, and 4-butylcyclohexanol, 2-, 3-, and 4-sec-butylcyclohexanol, and 2-, 3-, and 4-tert-butylcyclohexanol. 
         [0147]    The furan-2,5-dicarboxylic acid (FDCA, CAS No. 3238-40-2) needed as starting material for the preferred processes for producing compounds of the general formula (I) can either be purchased commercially or can be produced by synthesis routes known from the literature: possibilities for synthesis are found in the publication by Lewkowski et al. published on the Internet with the title “Synthesis, Chemistry and Application of 5-hydroxymethylfurfural and its derivatives” (Lewkowski et al., ARKIVOC 2001 (i), pp. 17-54, ISSN 1424-6376). A feature common to most of these syntheses is acid-catalyzed reaction of carbohydrates, particularly glucose and fructose, preferably fructose, to give 5-hydroxymethylfurfural (5-HMF), which can be separated from the reaction mixture by using technical processes such as a two-phase method. Appropriate results have been described by way of example by Leshkov et al in Science 2006, vol. 312, pp. 1933-1937, and by Zhang et al. in Angewandte Chemie 2008, vol. 120, pp, 9485-9488. 5-HMF can then be oxidized to FDCA in a further step, as cited by way of example by Christensen in ChemSusChem 2007, vol. 1, pp. 75-78. 
         [0148]    2,5-Bis(hydroxymethyl)tetrahydrofuran (CAS No. 104-80-3) can likewise either be purchased or can be synthesized. The syntheses described start from 5-HMF, which can be reduced in two steps by way of 2,5-bis(hydroxymethyl)furan (2,5-BHF) or directly to give 2,5-di(hydroxymethyl)tetrahydrofuran (Lewkowski et al., ARKIVOC 2001 (i), pp. 17-54, ISSN 1424-6376). 
         [0149]    2,5-Bis(hydroxyethyl)tetrahydrofuran can be obtained via reduction of methyl 2,5-furandiacetate. Methyl 2,5-furandiacetate can be synthesized by way of suitable reactions familiar to the person skilled in the art from 2,5-bis(hydroxymethyl)furan (2,5-BHF), for example by analogy with the process described by Rau et al. in Liebigs Ann. Chem., vol. 1984 (8, 1984), pp, 1504-1512. ISSN 0947-3440. Here, 2,5-bis(chloromethyl)furan is prepared from 2-5-BHF via reaction with thionyl chloride, and is reacted via exposure to KCN in benzene in the presence of [18]-crown-6 to give 2,5-bis(cyanomethyl)furan. 2,5-bis(cyanomethyl)furan can then be hydrolyzed to give 2,5-furandiacetic acid and esterified with methanol to give the dimethyl ester, or can be converted directly to methyl 2,5-furandiacetate via alcoholysis with methanol (pinner reaction). Methyl 2,5-furandiacetate can then either be first hydrogenated to dimethyl tetrahydro-2,5-furandiacetate (by analogy with steps b2) and, respectively, c1)) or can be reduced directly to 2,5-bis(hydroxyethyl)tetrahydrofuran. 
         [0150]    Methyl 2,5-furandiacetate can likewise be prepared by analogy with the process described by Kern et al. in Liebigs Ann. Chem., vol. 1985 (6. 1985), pp. 1168-1174, ISSN 0947-3440. 
         [0151]    Plasticizer Composition 
         [0152]    The compounds of the general formula (I) of the invention feature very good compatibility with a wide variety of plasticizers. They are specifically suitable in combination with other plasticizers which have gelling properties that still require improvement, in order to improve gelling performance: they permit reduction of the temperature required for the gelling of a thermoplastic polymer, and/or can increase the gelling rate of plasticizer compositions. 
         [0153]    If there are specific or complex requirements necessary for an application, for example high low-temperature resilience, high resistance to extraction or to migration, or very low plasticizer volatility, it can be advantageous to use plasticizer compositions for plasticizing thermoplastic polymers. This is true in particular for flexible-PVC applications. 
         [0154]    The invention therefore also provides plasticizer compositions which comprise at least one compound of the general formula (I) and at least one plasticizer different from the compounds (I). 
         [0155]    In relation to suitable and preferred compounds of the general formula (I) for producing plasticizer compositions, reference is made to the entirety of the suitable and preferred compounds of the general formula (I) described previously. It is preferable that the plasticizer compositions of the invention comprise at least one compound of the general formula (I) in which R 1  and R 2  are mutually independently unbranched or branched C 4 -alkyl, in particular n-butyl or isobutyl. A compound of the general formula (I) specifically suitable for producing plasticizer compositions is di(n-butyl) 2,5-tetrahydrofurandicarboxylate. 
         [0156]    It is preferable that the additional plasticizer different from the compounds of the general formula (I) is one selected from dialkyl phthalates, alkyl aralkyl phthalates, dialkyl terephthalates, trialkyl trimellitates, dialkyl adipates, alkyl benzoates, dibenzoic esters of glycols, hydroxybenzoic esters, esters of saturated mono- and dicarboxylic acids, esters of unsaturated dicarboxylic acids, amides and esters of aromatic sulfonic acids, alkylsulfonic esters, glycerol esters, isosorbide esters, phosphoric esters, citric triesters, alkylpyrrolidone derivatives, 2,5-furandicarboxylic esters, epoxidized vegetable oils based on triglycerides and saturated or unsaturated fatty acids, polyesters derived from aliphatic and aromatic polycarboxylic acids with polyhydric alcohols. 
         [0157]    Preferred dialkyl phthalates have mutually independently from 4 to 13 carbon atoms, preferably from 8 to 13 carbon atoms, in the alkyl chains. An example of a preferred alkyl aralkyl phthalate is benzyl butyl phthalate. It is preferable that the dialkyl terephthalates have mutually independently in each case from 4 to 13 carbon atoms, in particular from 7 to 11 carbon atoms, in the alkyl chains. Preferred dialkyl terephthalates are, for example, di(n-butyl)terephthalic acid dialkyl esters, di(2-ethylhexyl)terephthalic acid dialkyl esters, di(isononyl)terephthalic acid dialkyl esters, or di(2-propylheptyl)terephthalic acid dialkyl esters. It is preferable that the trialkyl trimellitates have mutually independently in each case from 4 to 13 carbon atoms, in particular from 7 to 11 carbon atoms, in the alkyl chains. Preferably the esters of saturated mono- and dicarboxylic acids are esters of acetic acid, butyric acid, valeric acid, succinic acid, adipic acid, sebacic acid, lactic acid, malic acid, or tartaric acid. It is preferable that the dialkyl adipates have mutually independently in each case from 4 to 13 carbon atoms, in particular from 6 to 10 carbon atoms, in the alkyl chains. Preferably the esters of unsaturated dicarboxylic acids are esters of maleic acid and of fumaric acid. It is preferable that the alkyl benzoates have mutually independently in each case from 7 to 13 carbon atoms, in particular from 9 to 13 carbon atoms, in the alkyl chains. Preferred benzoic acid alkyl esters are, for example, isononyl benzoate, isodecyl benzoate, or 2-propylheptyl benzoate. Preferred dibenzoic esters of glycols are diethylene glycol dibenzoate and dibutylene glycol dibenzoate. Preferred alkylsulfonic esters preferably have an alkyl moiety having from 8 to 22 carbon atoms. Among these are by way of example the phenyl and cresyl esters of pentadecylsulfonic acid. Preferred isosorbide esters are isosorbide diesters, preferably esterified mutually independently in each case with C 8 -C 13 -carboxylic acids. Preferred phosphoric esters are tri-2-ethylhexyl phosphate, trioctyl phosphate, triphenyl phosphate, isodecyl diphenyl phosphate, bis(2-ethylhexyl)phenyl phosphate, and 2-ethylhexyl diphenyl phosphate. The OH group in the citric triesters can be present in free or carboxylated form, preferably in acetylated form. It is preferable that the alkyl moieties of the citric triesters have mutually independently from 4 to 8 carbon atoms, in particular from 6 to 8 carbon atoms. Preference is given to alkylpyrrolidone derivatives having alkyl moieties of from 4 to 18 carbon atoms. Preferred dialkyl 2,5-furandicarboxylates have mutually independently in each case from 4 to 13 carbon atoms, preferably from 8 to 13 carbon atoms, in the alkyl chains. The epoxidized vegetable oils are preferably, for example, epoxidized fatty acids from epoxidized soybean oil, available under the trade name reFlex™ from PolyOne, USA, Preferably the polyesters derived from aliphatic and aromatic polycarboxylic acids are polyesters of adipic acid with polyhydric alcohols, in particular are dialkylene glycol polyadipates having from 2 to 6 carbon atoms in the alkylene moiety. 
         [0158]    In all of the abovementioned cases, the alkyl moieties can in each case be linear or branched and in each case identical or different. Reference is made to the general descriptions relating to suitable and preferred alkyl moieties in the introduction. 
         [0159]    In one particularly preferred embodiment, the plasticizer compositions of the invention comprise at least one plasticizer different from the compounds (I) and selected from dialkyl adipates having from 4 to 9 carbon atoms in the side chain. 
         [0160]    In another particularly preferred embodiment, the plasticizer compositions of the invention comprise at least one C 5 -C 11 -dialkyl ester of 2,5-furandicarboxylic acid. Particular preference is given to the C 7 -C 10 -dialkyl esters of 2,5-furandicarboxylic acid. 
         [0161]    Suitable and preferred dialkyl esters of 2,5-furandicarboxylic acid are described in WO 2012/113608 (C 5 -dialkyl esters), WO 2012/113609 (C 7 -dialkyl esters), WO 2011/023490 (C 9 -dialkyl esters), and WO 2011/023491 (C 10 -dialkyl esters). The dihexyl, di(2-ethylhexyl), and di(2-octyl) esters of 2,5-furandicarboxylic acid and their production are described by R. D. Sanderson et al. in J. Appl. Pol. Sci., 1994, vol. 53, 1785-1793. The entire disclosure of those documents is incorporated here by way of reference. 
         [0162]    Particularly preferred dialkyl esters of 2,5-furandicarboxylic acid are the isomeric nonyl esters of 2,5-furandicarboxylic acid disclosed in WO 2011/023490. The isomeric nonyl moieties here preferably derive from a mixture of isomeric nonanols as described in WO 2011/023490, page 6, line 32 to page 10, line 15. 
         [0163]    Molding Compositions 
         [0164]    The present invention further provides a molding composition comprising at least one thermoplastic polymer and at least one compound of the general formula (I). 
         [0165]    Thermoplastic polymers that can be used are any of the thermoplastically processable polymers. In particular, these thermoplastic polymers are those selected from:
       homo- and copolymers which comprise at least one copolymerized monomer selected from C 2 -C 10 -monoolefins, such as ethylene or propylene, 1,3-butadiene, 2-chloro-1,3-butadiene, vinyl alcohol and its C 2 -C 10 -alkyl esters, vinyl chloride, vinylidene chloride, vinylidene fluoride, tetrafluoroethylene, glycidyl acrylate, glycidyl methacrylate, acrylates and methacrylates with alcohol components of branched and unbranched C 1 -C 10 -alcohols, vinylaromatics, such as polystyrene, (meth)acrylonitrile, α,β-ethylenically unsaturated mono- and dicarboxylic acids, and maleic anhydride;   homo- and copolymers of vinyl acetals;   polyvinyl esters;   polycarbonates (PCs);   polyesters, such as polyalkylene terephthalates, polyhydroxyalkanoates (PHAs), polybutylene succinates (PBSs), polybutylene succinate adipates (PBSAs);   polyethers;   polyether ketones;   thermoplastic polyurethanes (TPUs);   polysulfides;   polysulfones;
 
and mixtures thereof.
       
 
         [0176]    Mention may be made by way of example of polyacrylates having identical or different alcohol moieties from the group of the C 4 -C 6 -alcohols, particularly of butanol, hexanol, octanol, and 2-ethylhexanol, polymethyl methacrylate (PMMA), methyl methacrylate-butyl acrylate copolymers, acrylonitrile-butadiene-styrene copolymers (ABSs), ethylene-propylene copolymers, ethylene-propylene-diene copolymers (EPDMs), polystyrene (PS), styrene-acrylonitrile copolymers (SANs), acrylonitrile-styrene-acrylate (ASA), styrene-butadiene-methyl methacrylate copolymers (SBMMAs), styrene-maleic anhydride copolymers, styrene-methacrylic acid copolymers (SMAs), polyoxymethylene (POM), polyvinyl alcohol (PVAL), polyvinyl acetate (PVA), polyvinyl butyral (PVB), polycaprolactone (PCL), polyhydroxybutyric acid (PHB), polyhydroxyvaleric acid (PHV), polylactic acid (PLA), ethylcellulose (EC), cellulose acetate (CA), cellulose propionate (CP), and cellulose acetate/butyrate (CAB). 
         [0177]    Preferably the at least one thermoplastic polymer comprised in the molding composition of the invention is polyvinyl chloride (PVC), polyvinyl butyral (PVB), homo- and copolymers of vinyl acetate, homo- and copolymers of styrene, polyacrylates, thermoplastic polyurethanes (TPUs), or polysulfides. 
         [0178]    The present invention further provides molding compositions comprising at least one elastomer and at least one compound of the general formula (I). 
         [0179]    Preferably the elastomer comprised in the molding compositions of the invention is at least one natural rubber (NR), at least one rubber produced by a synthetic route, or a mixture thereof. Examples of preferred rubbers produced by a synthetic route are polyisoprene rubber (IR), styrene-butadiene rubber (SBR), butadiene rubber (BR), nitrile-butadiene rubber (NBR), and chloroprene rubber (CR). 
         [0180]    Preference is given to rubbers or rubber mixtures which can be vulcanized with sulfur. 
         [0181]    For the purposes of the invention, the content (% by weight) of elastomer in the molding compositions is from 20 to 99%, preferably from 45 to 95%, particularly preferably from 50 to 90%, and in particular from 55 to 85%. 
         [0182]    The molding composition of the invention can comprise, alongside at least one elastomer and at least one tetrahydrofuran derivative of the general formula (I), at least one plasticizer different from the compounds (I). 
         [0183]    Suitable plasticizers different from the compounds (I) are those of the type already defined above. 
         [0184]    For the purposes of the invention, the molding compositions which comprise at least one elastomer can comprise other suitable additives, in addition to the above constituents. By way of example, the materials may comprise reinforcing fillers, such as carbon black or silicon dioxide, other fillers, a methylene donor, such as hexamethylenetetraamine (HMT), a methylene acceptor, such as phenolic resins modified with Cardanol (from cashew nuts), a vulcanizing agent or crosslinking agent, a vulcanizing accelerator or crosslinking accelerator, activators, various types of oil, antioxidants, and other various additives which by way of example can be mixed into tire compositions and into other rubber compositions. 
         [0185]    Specifically, the at least one thermoplastic polymer comprised in the molding composition of the invention is polyvinyl chloride (PVC). 
         [0186]    Polyvinyl chloride is obtained via homopolymerization of vinyl chloride. The polyvinyl chloride (PVC) used in the invention can by way of example be produced via suspension polymerization, microsuspension polymerization, emulsion polymerization, or bulk polymerization. The production of PVC via polymerization of vinyl chloride, and also the production and composition of plasticized PVC, are described by way of example in “Becker/Braun, Kunststoff-Handbuch” [Plastics Handbook], vol. 2/1: Polyvinylchlorid [Polyvinyl chloride], 2nd edn., Carl Hanser Verlag, Munich. 
         [0187]    The K value, which characterizes the molar mass of the PVC, and is determined in accordance with DIN 53726, is mostly from 57 to 90 for the PVC plasticized in the invention, preferably from 61 to 85, in particular from 64 to 75. 
         [0188]    For the purposes of the invention, the content of PVC in the mixtures is from 20 to 99% by weight, preferably from 45 to 95% by weight, particularly preferably from 50 to 90% by weight, and in particular from 55 to 85% by weight. 
         [0189]    At least one plasticizer different from the compounds (I) can be comprised in the molding composition of the invention, alongside at least one thermoplastic polymer and at least one tetrahydrofuran derivative of the general formula (0. 
         [0190]    The proportion of the additional at least one plasticizer, different from the compounds (I), in the molding composition of the invention is from 10% to 90% by weight, preferably from 20% to 85% by weight, and particularly preferably from 50% to 80% by weight, based on the total amount of plasticizer present in the molding composition. 
         [0191]    Suitable plasticizers different from the compounds (I) are those of the type already defined above. 
         [0192]    It is particularly preferable that the at least one additional plasticizer comprised in the molding composition of the invention is selected from dialkyl adipates having from 4 to 9 carbon atoms in the side chain and 2,5-furandicarboxylic esters having from 4 to 10 carbon atoms in the side chain, where the ester groups can have either the same or a different number of carbon atoms. 
         [0193]    Amounts of plasticizer used differ in accordance with the choice of thermoplastic polymer or thermoplastic polymer mixture comprised in the molding composition. The total plasticizer content in the molding composition is generally from 0.5 to 300 phr (parts per 100 resins=parts by weight per 100 parts by weight of polymer), preferably from 0.5 to 130 phr, particularly preferably from 1 to 35 phr. 
         [0194]    Where the thermoplastic polymer in the molding compositions of the invention is polyvinyl chloride and where the plasticizer used comprises exclusively at least one of the (C 7 -C 12 )-dialkyl esters of tetrahydrofurandicarboxylic acid of the invention, the total plasticizer content in the molding composition is from 5 to 300 phr, preferably from 10 to 100 phr, and particularly preferably from 30 to 70 phr. 
         [0195]    If the thermoplastic polymer in the molding compositions of the invention is polyvinyl chloride, and if plasticizer mixtures are used that comprise at least one compound of the general formula (I) and at least one plasticizer different from the compounds (I), the total plasticizer content in the molding composition is from 1 to 400 phr, preferably from 5 to 130 phr, particularly preferably from 10 to 100 phr, and in particular from 15 to 85 phr. 
         [0196]    If the polymer in the molding compositions of the invention is rubbers, the total plasticizer content in the molding composition is from 1 to 60 phr, preferably from 1 to 40 phr, particularly preferably from 2 to 30 phr. 
         [0197]    Molding Composition Additives 
         [0198]    For the purposes of the invention, the molding compositions comprising at least one thermoplastic polymer can comprise other suitable additives. By way of example, the materials can comprise stabilizers, lubricants, fillers, pigments, flame retardants, light stabilizers, blowing agents, polymeric processing aids, impact modifiers, optical brighteners, antistatic agents, or biostabilizers. 
         [0199]    Some suitable additives are described in more detail below. However, the examples listed do not represent any restriction of the molding compositions of the invention, but instead serve merely for illustration. AU data relating to content are in % by weight, based on the entire molding composition. 
         [0200]    Stabilizers that can be used are any of the conventional PVC stabilizers in solid and liquid form, for example conventional Ca/Zn, Ba/Zn, Pb, or Sn stabilizers, and also acid-binding phyllosilicates, such as hydrotalcite. 
         [0201]    The molding compositions of the invention can have from 0.05 to 7% content of stabilizers, preferably from 0.1 to 5%, particularly preferably from 0.2 to 4%, and in particular from 0.5 to 3%. 
         [0202]    Lubricants are intended to be effective between the PVC pastilles, and to counteract frictional forces during mixing, plastification, and deformation. 
         [0203]    The molding compositions of the invention can comprise, as lubricants, any of the lubricants conventionally used for the processing of plastics. Examples of those that can be used are hydrocarbons, such as oils, paraffins, and PE waxes, fatty alcohols having from 6 to 20 carbon atoms, ketones, carboxylic acids, such as fatty acids and montanic acid, oxidized PE wax, metal salts of carboxylic acids, carboxamides, and also carboxylic esters, for example with the following alcohols: ethanol, fatty alcohols, glycerol, ethanediol, and pentaerythritol, and with long-chain carboxylic acids as acid component. 
         [0204]    The molding compositions of the invention can have from 0.01 to 10% lubricant content, preferably from 0.05 to 5%, particularly preferably from 0.1 to 3%, and in particular from 0.2 to 2%. 
         [0205]    Fillers have an advantageous effect primarily on the compressive strength, tensile strength, and flexural strength, and also the hardness and heat resistance, of plasticized PVC. 
         [0206]    For the purposes of the invention, the molding compositions can also comprise fillers such as carbon black and other organic fillers such as natural calcium carbonates, for example chalk, limestone, and marble, dolomite, silicates, silica, sand, diatomaceous earth, aluminum silicates, such as kaolin, mica, and feldspat, and synthetic calcium carbonates. It is preferable to use the following as fillers: calcium carbonates, chalk, dolomite, kaolin, silicates, talc powder, or carbon black. 
         [0207]    The molding compositions of the invention can have from 0.01 to 80% content of fillers, preferably from 0.1 to 60%, particularly preferably from 0.5 to 50%, and in particular from 1 to 40%. 
         [0208]    The molding compositions of the invention can also comprise pigments in order to adapt the resultant product to be appropriate to various possible uses. 
         [0209]    For the purposes of the present invention, it is possible to use either inorganic pigments or organic pigments. Examples of inorganic pigments that can be used are cadmium pigments, such as CdS, cobalt pigments, such as CoO/Al 2 O 3 , and chromium pigments, such as Cr 2 O 3 . Examples of organic pigments that can be used are monoazo pigments, condensed azo pigments, azomethine pigments, anthraquinone pigments, quinacridones, phthalocyanine pigments, dioxazine pigments, and aniline pigments. 
         [0210]    The molding compositions of the invention can have from 0.01 to 10% content of pigments, preferably from 0.05 to 5%, particularly preferably from 0.1 to 3%, and in particular from 0.5 to 2%. 
         [0211]    In order to reduce flammability and to reduce smoke generation during combustion, the molding compositions of the invention can also comprise flame retardants. 
         [0212]    Examples of flame retardants that can be used are antimony trioxide, phosphate esters, chloroparaffin, aluminum hydroxide, boron compounds, molybdenum trioxide, ferrocene, calcium carbonate, and magnesium carbonate. 
         [0213]    The molding compositions of the invention can have from 0.01 to 10% content of flame retardants, preferably from 0.1 to 8%, particularly preferably from 0.2 to 5%, and in particular from 0.5 to 2%. 
         [0214]    The molding compositions can also comprise light stabilizers in order to protect items produced from the molding compositions of the invention from surface damage due to the effect of light. 
         [0215]    For the purposes of the present invention it is possible by way of example to use hydroxybenzophenones or hydroxyphenylbenzotriazoles. 
         [0216]    The molding compositions of the invention can have from 0.01 to 7% content of light stabilizers, preferably from 0.1 to 5%, particularly preferably from 0.2 to 4%, and in particular from 0.5 to 3%. 
         [0217]    Plastisol Applications 
         [0218]    As described already, the good gelling properties of the compounds of the invention make them particularly suitable for producing plastisols. 
         [0219]    Plastisols can be produced from various plastics. In one preferred embodiment, the plastisols of the invention are a PVC plastisol. 
         [0220]    The plastisols of the invention may comprise not only at least one plastic and at least one tetrahydrofuran derivative of the general formula (I), but also, optionally, at least one plasticizer different from the compounds (I). 
         [0221]    The fraction of the additional at least one plasticizer, different from the compounds (I), in the plastisol is from 10% to 90% by weight, preferably from 20% to 85% by weight, and particularly preferably from 50% to 80% by weight, based on the total amount of plasticizer present in the plastisol. 
         [0222]    In the case of PVC plastisols comprising as plasticizers exclusively at least one of the (C 7 -C 12 )-dialkyl esters of tetrahydrofurandicarboxylic acid of the invention, the total plasticizer fraction is customarily from 5 to 300 phr, preferably from 10 to 100 phr. 
         [0223]    In the case of PVC plastisols which comprise as plasticizers at least one compound of the general formula (I) and at least one plasticizer different from the compounds (I), the total plasticizer fraction is customarily from 5 to 400 phr, preferably from 50 to 200 phr. 
         [0224]    Plastisols are usually converted to the form of the finished product at ambient temperature via various processes, such as spreading processes, casting processes, such as the slush molding process or rotomolding process, dip-coating process, spray process, and the like. Gelling then takes place via heating, whereupon cooling gives a homogeneous product with relatively high or relatively low flexibility. 
         [0225]    PVC plastisols are particularly suitable for producing PVC foils, for producing seamless hollow bodies, for producing gloves, and for use in the textile sector, e.g. for textile coatings. 
         [0226]    Molding Composition Applications 
         [0227]    The molding composition of the invention is preferably used for producing moldings and foils. Among these are in particular tooling; apparatuses; piping; cables; hoses, for example plastic hoses, water hoses, and irrigation hoses, industrial rubber hoses, or chemical hoses; wire sheathing; window profiles; vehicle-construction components, for example bodywork constituents, vibration dampers for engines; tires; furniture, for example chairs, tables, or shelving; cushion foam and mattress foam; tarpaulins, for example lorry tarpaulins or tent tarpaulins; gaskets; composite foils, such as foils for laminated safety glass, in particular for vehicle windows and for window panes; recording disks; synthetic leather; packaging containers; adhesive-tape foils, coatings, computer housings, and housings of electrical devices, for example kitchen machines. 
         [0228]    The molding composition of the invention is also suitable for producing moldings and foils which come directly into contact with people or with foods. These primarily are medical products, hygiene products, packaging for food or drink, products for the interior sector, toys and child-care items, sports and leisure products, apparel, and also fibers for textiles, and the like. 
         [0229]    The medical products which can be produced from the molding composition of the invention are by way of example tubes for enteral nutrition and hemodialysis, breathing tubes, infusion tubes, infusion bags, blood bags, catheters, tracheal tubes, gloves, breathing masks, or disposal syringes. 
         [0230]    The packaging that can be produced from the molding composition of the invention for food or drink is by way of example freshness-retention foils, food-or-drink hoses, drinking-water hoses, containers for storing or freezing food or drink, lid gaskets, closure caps, crown corks, or synthetic corks for wine. 
         [0231]    The products which can be produced from the molding composition of the invention for the interior sector are by way of example floorcoverings, which may have a uniform construction or a construction comprising a plurality of layers, consisting of at least one foamed layer, such as, for example, floorcoverings, sports floors, or luxury vinyl tiles (LVT), synthetic leathers, wallcoverings, or foamed or unfoamed wall papers in buildings, or are cladding or console covers in vehicles. 
         [0232]    The toys and child-care items which can be produced from the molding composition of the invention are by way of example dolls, inflatable toys, such as balls, toy figures, modeling clays, swimming aids, stroller covers, baby-changing mats, bed warmers, teething rings, or bottles. 
         [0233]    The sports and leisure products that can be produced from the molding composition of the invention are by way of example gymnastics balls, exercise mats, seat cushions, massage balls and massage rolls, shoes and shoe soles, balls, air mattresses, and drinking bottles. 
         [0234]    The apparel that can be produced from the molding compositions of the invention is by way of example latex clothing, protective apparel, rain jackets, or rubber boots. 
         [0235]    Non-PVC Applications: 
         [0236]    The present invention also includes the use of the compounds of the invention as and/or in auxiliaries selected from: calendering auxiliaries; rheology auxiliaries; surfactant compositions, such as flow aids and film-forming aids, defoamers, antifoams, wetting agents, coalescing agents, and emulsifiers; lubricants, such as lubricating oils, lubricating greases, and lubricating pastes; quenchers for chemical reactions; phlegmatizing agents; pharmaceutical products; plasticizers in adhesives; impact modifiers and antiflow additives. 
         [0237]    The figures described below and the examples provide further explanation of the invention. These figures and examples are not to be understood as restricting the invention. 
         [0238]    The following abbreviations are used in the examples and figures below:
   2,5-FDCA for 2,5-furandicarboxylic acid,   2,5-THFDCA for 2,5-tetrahydrofurandicarboxylic acid,   DMAP for 4-dimethylaminopyridine,   TBME for tert-butyl methyl ether,   THF for tetrahydrofuran,
 
phr for parts by weight per 100 parts by weight of polymer.
   
 
     
    
     
       DESCRIPTION OF FIGURES 
         [0244]      FIG. 1  shows, in the form of a bar chart, the Shore A hardness of flexible PVC test specimens which comprise different amounts of the plasticizer 2,5-THFDCA dibutyl ester (white hatched) and, as comparison, the commercially available plasticizer Mesamoll® TP-LXS 51067 (black). The Shore A hardness has been plotted against the plasticizer content of the flexible PVC test specimens (stated in phr). The values measured were always determined after a time of 15 seconds. 
           [0245]      FIG. 2  shows, in the form of a bar chart, the Shore D hardness of flexible PVC test specimens which comprise 50 and, respectively, 70 phr of the plasticizer 2,5-THFDCA dibutyl ester of the invention (white hatched) and, as comparison, the commercially available plasticizer Mesamoll® TP-LXS 51067 (black). The Shore D hardness has been plotted against the plasticizer content of the flexible PVC test specimens (stated in phr). The values measured were always determined after a time of 15 seconds. 
           [0246]      FIG. 3  shows, in the form of a bar chart, the 100% modulus of flexible PVC test specimens which comprise 50 and, respectively, 70 phr of the plasticizer 2,5-THFDCA dibutyl ester of the invention (white hatched) and, as comparison, the commercially available plasticizer Mesamoll® TP-LXS 51067 (black). The 100% modulus has been plotted against the plasticizer content of the flexible PVC test specimens (stated in phr). 
           [0247]      FIG. 4  shows, in the form of a bar chart, the cold crack temperature of flexible PVC foils which comprise the plasticizer 2,5-THFDCA dibutyl ester of the invention and, as comparison, the commercially available plasticizer Mesamoll® TP-LXS 51067. The chart shows the cold crack temperature in ° C. for flexible PVC foils with plasticizer content of 50 and 70 phr. 
           [0248]      FIG. 5  shows, in the form of a bar chart, the glass transition temperature (T g ) of flexible PVC foils which comprise the plasticizer 2,5-THFDCA dibutyl ester of the invention and, as comparison, the commercially available plasticizer Mesamoll® TP-LXS 51067. The chart shows the glass transition temperature (T g ) ° C. for flexible PVC foils with plasticizer content of 50 and 70 phr. 
       
    
    
     EXAMPLES 
     I) Production Examples 
     Example 1 
       [0249]    Synthesis of di(n-butyl) 2,5-tetrahydrofurandicarboxylate from dimethyl 2,5-furandicarboxylate via transesterification and hydrogenation 
       Example 1.1 
     Production of dimethyl 2,5-furandicarboxylate (=Step a) 
       [0250]    3.30 kg of methanol were used as initial charge together with 0.10 kg of concentrated sulfuric acid in a 10 L glass reactor equipped with heating jacket, reflux condenser, and mechanical stirrer. 1.6 kg of 2,5-furandicarboxylic acid (2,5-FDCA) were slowly added to this mixture, with vigorous stirring. The dense white suspension that forms was then heated to 70° C. (reflux). The course of the reaction was monitored by means of HPLC analysis, whereupon after about 20 h a clear solution was obtained, with complete conversion of the 2,5-FDCA. The reaction mixture was then cooled to 65° C., and neutralized with saturated NaHCO 3  solution and solid NaHCO 3  (pH 7). During the neutralization, a dense white suspension again formed, and was cooled to 10° C., stirred for a further 0.5 h, and then filtered by way of a P2 sintered glass frit. The filtercake was washed three times with 1 L of cold water, whereupon about 2 kg of wet solid was obtained. For purification and recrystallization, the wet solid was added to 6.00 kg of 2-butanone in a 10 L glass reactor equipped with heating jacket, reflux condenser, and mechanical stirrer. The suspension was heated to 70° C., whereupon a clear solution was obtained. 1.00 kg of water was then added, and this led to formation of a brownish orange aqueous phase. It was sometimes necessary to add 900 mL of saturated sodium chloride solution in order to achieve phase separation. The aqueous phase was removed, and the organic phase was cooled to 20° C., without stirring, whereupon the crystallization of the product began (usually at about 35° C.). The crystalline suspension was then cooled to 0° C. and stirred overnight. The suspension was then filtered by way of a P2 sintered glass frit, and the filtercake was washed with 1 L of cold methanol. The solid residue was dried at room temperature in vacuo. The desired dimethyl 2,5-furandicarboxylate was obtained in a yield of from 50 to 60% and in a purity of &gt;99%. The identity and purity of the final product was determined by means of NMR and HPLC (HPLC column: Varian Polaris 3μ C18-A, 150×4.6 mm). 
       Example 1.2 
     Catalytic Hydrogenation (=Step b2) 
       [0251]    A 20% by weight solution of dimethyl 2,5-furandicarboxylate in THF was charged to a nitrogen-filled 2.5 L Hastelloy C autoclave from Parr Instrument, equipped with a mechanical stirrer with magnetic coupling, thermocouple, sampling tube, and baffles. 120 g of a heterogeneous Pd/Pt catalyst (0.4% by weight of Pd/0.4% by weight of Pt on ZrO 2 , produced by analogy with DE4429014, example 6) were then added, and the nitrogen atmosphere was replaced by a hydrogen atmosphere by filling and ventilating the autoclave with hydrogen three times. The final pressure of hydrogen was increased to 200 bar, and the autoclave was heated to 180° C. The progress of the reaction was monitored by means of GC analysis. After complete conversion (usually after from 40 to 60 hours), the autoclave was cooled and ventilated, and the contents were filtered in order to remove the solid catalyst. The solvent in the filtrate was then removed by distillation under reduced pressure, and the retained crude product was diluted in 300 mL of tert-butyl methyl ether and transferred to a separating funnel. The organic phase was washed twice with saturated NaHCO 3  solution and once with saturated sodium chloride solution. The solvent and other volatile constituents were then removed by distillation under reduced pressure. The crude product was purified by fractional distillation, whereupon dimethyl 2,5-tetrahydrofurandicarboxylate was obtained in the form of colorless to brownish, viscous liquid. The desired dimethyl 2,5-tetrahydrofurandicarboxylate was obtained here in a yield of 57% and in a purity of 98.2%. The identity and purity of the final product were determined by means of NMR and GC-MS analysis (GC column: Agilent J&amp;W DB-5, 30 m×0.32 mm×1.0 μm). 
       Example 1.3 
     Transesterification of dimethyl 2,5-tetrahydrofurandicarboxylate (=Step c2) 
       [0252]    204 g (1.08 mol, 1.0 equivalent) of dimethyl 2,5-tetrahydrofurandicarboxylate were dissolved in 200 g of n-heptane in a 2 L round-necked flask equipped with a dropping funnel with pressure equalization, and 325 g (4.38 mol, 4.0 equivalents) of n-butanol, and also a mixed titanium(IV) propoxide/butoxide complex (3 mol % of titanium) were added. The mixture was heated to reflux (from 100 to 126° C.) for 22 hours, with stirring. The course of the reaction was monitored by means of GC analysis. After complete conversion, the reaction mixture was cooled to room temperature and filtered, and the titanium(IV) alkoxide was hydrolyzed via addition of 100 mL of water. The two-phase mixture was transferred to a separating funnel, the aqueous phase was removed, and the organic phase was washed once with saturated sodium chloride solution. The solvent and other volatile constituents were then removed by distillation under reduced pressure. The crude product was purified by means of fractional distillation, whereupon di(n-butyl) 2,5-tetrahydrofurandicarboxylate was obtained in the form of clear colorless liquid in a yield of 72% and in a purity of 98.3%. The identity and purity of the final product was determined by means of NMR and GC-MS analysis (GC column: Agilent J&amp;W DB-5, 30 m×0.32 mm×1.0 μm). 
       Example 2 
     Synthesis of di(n-butyl) 2,5-tetrahydrofurandicarboxylate Via Direct Esterification and Hydrogenation 
     Example 2.1 
     Production of di(n-butyl) 2,5-furandicarboxylate (=Step b1) 
       [0253]    445 g (6.00 mol, 4.0 equivalents) of n-butanol were used as initial charge in 500 g of toluene in a 2 L round-necked flask equipped with a Dean-Stark water separator and a dropping funnel with pressure equalization. The mixture was heated to reflux, with stirring, and 234 g (1.50 mol, 1.0 equivalent) of 2,5-furandicarboxylic acid were added, followed by 11.5 g (0.12 mol, 8 mol %) of 99.9% sulfuric acid in from 3 to 4 portions whenever the reaction slowed. The course of the reaction was monitored on the basis of the amount of water separated in the Dean-Stark apparatus. After complete conversion, a specimen was taken from the reaction mixture and analyzed by GC. The reaction mixture was cooled to room temperature, transferred to a separating funnel, and washed twice with saturated NaHCO 3  solution. The organic phase was washed with saturated sodium chloride solution and dried with anhydrous Na 2 SO 4 , and the solvent was removed under reduced pressure. The crude product was purified by means of fractional distillation. The desired di(n-butyl) 2,5-furandicarboxylate was obtained here in a yield of 80% and in a purity of 98.9%. The identity and purity of the final product was determined by means of NMR and GC-MS analysis (GC column: Agilent J&amp;W DB-5, 30 m×0.32 mm×1.0 or Ohio Valley OV-1701 60 m×0.32 mm×0.25 μm). 
         [0254]    Catalytic Hydrogenation (=Step c1): 
         [0255]    A 20% by weight solution of di(n-butyl) 2,5-furandicarboxylate in THF was charged to a nitrogen-filled 2.5 L Hastelloy C autoclave from Parr Instrument, equipped with a mechanical stirrer with magnetic coupling, thermocouple, sampling tube, and baffles. 120 g of a heterogeneous Pd/Pt catalyst (0.4% by weight of Pd/0.4% by weight of Pt on ZrO 2 , produced by analogy with DE4429014, example 6) were then added, and the nitrogen atmosphere was replaced three times with hydrogen at superatmospheric pressure. The final pressure of hydrogen was increased to 200 bar, and the autoclave was heated to 180° C. The progress of the reaction was monitored by means of GC analysis. After complete conversion (usually after from 40 to 60 hours), the autoclave was ventilated, and the contents were filtered in order to remove the solid catalyst. The solvent in the filtrate was then removed by distillation under reduced pressure, and the retained crude product was diluted in 300 mL of TBME and transferred to a separating funnel. The organic phase was washed twice with saturated NaHCO 3  solution and once with saturated sodium chloride solution. The solvent and other volatile constituents were then removed by distillation under reduced pressure. The crude product was purified by fractional distillation, whereupon di(n-butyl) 2,5-tetrahydrofurandicarboxylate was obtained in the form of colorless to brownish, viscous liquid in a yield of 30% and in a purity of 97.9%. The identity and purity of the final product were determined by means of NMR and GC-MS analysis (GC column: Agilent J&amp;W DB-5, 30 m×0.32 mm×1.0 μm). 
       Example 3 
     Synthesis of the di-n-butyl ether of 2,5-di(hydroxymethyl)tetrahydrofuran 
       [0256]    10.6 g of 2,5-di(hydroxymethyl)tetrahydrofuran (80 mmol, 1.0 equivalent) were dissolved in 140 ml of toluene in a 500 mL four-necked flask equipped with a mechanical stirrer, dropping funnel, thermometer, and reflux condenser. 22.4 g (400 mmol, 5.0 equivalents) of powdered KOH were added in portions to this mixture at room temperature over a period of 30 minutes and with continuous stirring. The mixture was then stirred at reflux for from 3 to 4 hours. 60.0 g of molecular sieve (3 Å) were then added, and the mixture was stirred at reflux for a further hour, whereupon a cream-colored suspension was obtained. The mixture was cooled to 90° C., and 28.5 g (208 mmol, 2.6 equivalents) of 1-bromobutane dissolved in 40 mL of toluene were added dropwise over 1.5 hours. The dropping funnel was washed with 20 mL of toluene, and the wash solution was combined with the reaction mixture. The course of the reaction was monitored by means of GC analysis. After the end of the reaction, (usually from 40 to 80 hours) the mixture was cooled to room temperature. The glass containers were washed with TBME, combined with the reaction mixture, and the resultant white suspension was filtered. The salt residues removed by filtration were washed with TBME. The combined organic phases were in each case washed in succession once with saturated sodium chloride solution, with saturated ammonium chloride solution, and again with saturated sodium chloride solution, and finally dried over Na 2 SO 4 . The solvent and other volatile constituents were then removed by distillation under reduced pressure, and the residue was dried under high vacuum. The crude product was purified by means of fractional distillation, whereupon the di-n-butyl ether of 2,5-di(hydroxymethyl)tetrahydrofuran was obtained in the form of clear colorless liquid in a yield of 55% and in a purity of 98.7%. The identity and purity of the final product were determined by means of NMR and GC-MS analysis (GC column: Agilent J&amp;W DB-5, 30 m×0.32 mm×1.0 μm). 
       II) Production of Plasticized PVC Foils on a Roll Mill and of PVC Test Specimens 
       [0257]    II.a) Production of PVC Foils on a Roll Mill: 
         [0258]    To assess the plasticizing properties of the plasticizers of the invention and of the comparative compounds during thermoplastic processing, flexible PVC foils of thickness 0.5 mm were produced. These foils were produced via rolling and pressing of plasticized PVC. 
         [0259]    In order to eliminate effects due to different additives, the formulation below was used in each case for producing the plasticized PVC: 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Additive 
                 phr 
               
               
                   
                   
               
             
             
               
                   
                 Solvin 271 SP 1)   
                 100 
               
               
                   
                 Plasticizer 
                 50 and, respectively, 70 
               
               
                   
                 SLX 781 2)  reagent 
                  2 
               
               
                   
                   
               
               
                   
                   1) commercially obtainable PVC from Solvin GmbH &amp; Co. KG, produced via suspension polymerization (K value in accordance with ISO 1628-2: 71) 
               
               
                   
                   2) liquid Ba—Zn stabilizer from Reagens Deutschland GmbH 
               
             
          
         
       
     
         [0260]    The ingredients were mixed at room temperature with a manual mixer. The mixture was then plastified in a steam-heated laboratory mixing unit from Collin (150) and processed to give a milled sheet. The rotation rates were 15 rotations/minute (front roll) and 12 rotations/minute (rear roll), and the roll-milling time was 5 minutes. This gave a milled sheet of thickness 0.55 mm. The cooled milled sheet was then pressed in a 400 P Collin press within a period of 400 seconds under a pressure of 220 bar to give a flexible PVC foil of thickness 0.50 mm. 
         [0261]    The respective conditions for the roll mill and press can be found in the table below: 
         [0000]    
       
         
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                 Plasticizer 
                 Roll- 
                   
               
               
                   
                   
                 content 
                 milling 
                 Pressing 
               
               
                 Ex. No. 
                 Product 
                 [phr] 
                 [° C.] 
                 [° C.] 
               
               
                   
               
             
             
               
                 1 
                 2,5-THFDCA di(n-butyl ester) 
                 50/70 
                 160/160 
                 160/160 
               
               
                 comp 1 
                 Mesamoll ® TP-LXS 51067 3)   
                 50/70 
                 175/165 
                 185/175 
               
               
                   
               
               
                   3) Mixture of phenyl alkylsulfonates from Lanxess Deutschland GmbH (CAS No. 91082-17-6) 
               
             
          
         
       
     
         [0262]    The test specimens needed for the tests were produced from the resultant roll-milled and pressed foils. 
         [0263]    II.b) Production of Test Specimens: 
         [0264]    The test specimens with dimensions 49 mm×49 mm×10 mm (length×width×thickness) were produced via pressing from roll-milled foils at a temperature which was 10° C. above the roll-milling temperature. For the performance tests, the test specimens were aged for 7 days at 23° C. and 50% relative humidity. 
       III) Performance Tests 
       [0265]    III.a) Determination of Solvation Temperature in Accordance with DIN 53408: 
         [0266]    To characterize the gelling performance of the plasticizers of the invention in PVC, solvation temperature was determined in accordance with DIN 53408. In accordance with DIN 53408, a droplet of a slurry of 1 g of PVC in 19 g of plasticizer is observed in transmitted light under a microscope equipped with a heatable stage. The temperature here is increased linearly by 2° C. per minute, starting at 60° C. The solvation temperature is the temperature at which the PVC particles become invisible, i.e. it is no longer possible to discern their outlines and contrasts. The lower the solvation temperature, the better the gelling performance of the relevant substance for PVC. 
         [0267]    The table below lists the solvation temperatures of the di(n-butyl) 2,5-tetrahydrofurandicarboxylate plasticizer of the invention and, as comparison, of Mesamoll® TP-LXS 5106, and also of dibutyl phthalate. 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
               
               
                   
                   
                 Solvation temperature 
               
               
                   
                   
                 in accordance with 
               
               
                   
                   
                 DIN 53408 
               
               
                 Ex. No. 
                 Substance 
                 [° C.] 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 Di(n-butyl) 2,5- 
                 71 
               
               
                   
                 tetrahydrofurandicarboxylate 
               
               
                 comp 1 
                 Mesamoll ® TP-LXS 51067 3)   
                 114 
               
               
                 comp 2 
                 Dibutyl phthalate 4)   
                 100 
               
               
                   
               
               
                   3) Mixture of phenyl alkylsulfonates from Lanxess Deutschland GmbH (CAS No. 91082-17-6) 
               
               
                   4) Di(n-butyl) benzene-1,2-dicarboxylate (CAS No. 84-74-2) 
               
             
          
         
       
     
         [0268]    As can be seen from the table, the plasticizer of the invention exhibits the lowest salvation temperature. 
         [0269]    III.b) Physical Properties: 
         [0270]    The table below lists the most significant physical properties of di(n-butyl) 2,5-tetrahydrofurandicarboxylate (example 1) in comparison with the Mesamoll® TP-LXS 51067 plasticizer used in the market (comparative example comp 1). 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 Di(n-butyl) 2,5- 
                   
               
               
                   
                   
                 tetrahydrofuran- 
                 Mesamoll ® TP- 
               
               
                   
                 Plasticizer: 
                 dicarboxylate 
                 LXS 51067 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Density (20° C.) 
                 1.048 
                 1.071 
               
               
                   
                 [g/cm 3 ] 
               
               
                   
                 Viscosity (20° C.) 
                 10 
                 90 
               
               
                   
                 [mPa · s] 
               
               
                   
                   
               
             
          
         
       
     
         [0271]    Relevant physical properties for the plasticizer application alongside the salvation temperature in accordance with DIN 53408 are specifically density and viscosity. In comparison with the plasticizer Mesamoll® TP-LXS 51067, which is commercially available and regarded as having advantageous properties, 2,5-THFDCA dibutyl ester exhibits markedly lower, and therefore more advantageous, viscosity with comparable density. 
         [0272]    III.c) Shore Hardness Determination: 
         [0273]    Shore A and D hardness were determined in accordance with DIN EN ISO 868 with a DD-3 digital durometer from Hildebrand. The test specimens were produced as in example II.c). The values shown in  FIG. 1  and  FIG. 2  are in each case the average value from 20 measurements per test specimen (10 measurements on the front side and 10 measurements on the reverse side). The value measured was always determined after a time of 15 seconds. 
         [0274]    As can be seen from the charts of  FIG. 1  and  FIG. 2 , 2,5-THFDCA dibutyl ester of the invention exhibits markedly better plasticizing effect than the commercially available plasticizer Mesamoll® TP-LXS 51067. 
         [0275]    III.d) Determination of 100% Modulus: 
         [0276]    100% modulus is another property, alongside Shore hardness, that characterizes the plasticizing effect of plasticizers, i.e. plasticizer efficiency. 
         [0277]    100% modulus was determined in accordance with DIN EN ISO 527 part 1 and 3 with a TMZ 2.5/TH1S tester from Zwick. The test specimens of dimensions 150 mm×10 mm×0.5 mm (length×width×thickness) correspond to type 2 in accordance with DIN EN ISO 527 part 3, and are punched out from the rolled/pressed foils by means of a hole punch. The test specimens are conditioned for 7 days before the test. The conditioning and the tensile tests take place at 23° C.+/−1.0° C. and 50%+/−5 relative humidity in accordance with DIN EN ISO 291. The values plotted in  FIG. 3  are in each case average values from the testing of 10 individual test specimens. 
         [0278]    As can be seen from the chart of  FIG. 3 , 2,5-THFDCA dibutyl ester of the invention exhibits markedly better plasticizing effect than the commercially available plasticizer Mesamoll® TP-LXS 51067. 
         [0279]    III.e) Determination of Low-Temperature Flexibility: 
         [0280]    To determine low-temperature flexibility, PVC foils were used which comprised different concentrations of the respective plasticizer to be tested. Two methods were used. Firstly, cold crack temperature was determined by a method based on the standard DIN 53372, which is no longer current, and secondly the glass transition temperature T g  of the foils was determined by means of DMA (dynamic mechanical analysis) in accordance with ISO 6721-7 from the maximum of the loss modulus “G”.  FIGS. 4 and 5  show the results from the two test methods. 
         [0281]    As is apparent from the charts of  FIGS. 4 and 5 , the PVC foils which comprise 2,5-THFDCA dibutyl ester of the invention exhibit a lower, and thus more advantageous, cold crack temperature in comparison with PVC foils using Mesamoll® TP-LXS 51067. The same applies to the glass transition temperature. The excellent value for the glass transition temperature of 2,5-THFDCA dibutyl ester of the invention at 70 phr plasticizer content is surprising.