Patent Publication Number: US-2003232957-A1

Title: Method for making an aromatic polycarbonate

Description:
BACKGROUND OF THE INVENTION  
       [0001] This invention relates to a method of preparing polycarbonate. More particularly the method relates to a method whereby a solution comprising a solvent and an oligomeric polycarbonate is introduced into a devolatilizing extruder wherein the oligomeric polycarbonate is converted into high molecular weight polycarbonate while simultaneously removing the solvent. More particularly, the instant invention relates to the formation under mild conditions of polycarbonates having extremely low levels of Fries rearrangement products, a high level of endcapping and low levels of residual solvent.  
       [0002] Polycarbonates, such as bisphenol A polycarbonate, are typically prepared either by interfacial or melt polymerization methods. The reaction of a bisphenol such as bisphenol A (BPA) with phosgene in the presence of water, a solvent such as methylene chloride, an acid acceptor such as sodium hydroxide and a phase transfer catalyst such as triethylamine is typical of the interfacial methodology. The reaction of bisphenol A with a source of carbonate units such as diphenyl carbonate at high temperature in the presence of a catalyst such as sodium hydroxide is typical of currently employed melt polymerization methods. Each method is practiced on a large scale commercially and each presents significant drawbacks.  
       [0003] The interfacial method for making polycarbonate has several inherent disadvantages. First it is a disadvantage to operate a process which requires phosgene as a reactant due to obvious safety concerns. Second it is a disadvantage to operate a process which requires using large amounts of an organic solvent because expensive precautions must be taken to guard against any adverse environmental impact. Third, the interfacial method requires a relatively large amount of equipment and capital investment. Fourth, the polycarbonate produced by the interfacial process is prone to having inconsistent color, higher levels of particulates, and higher chloride content, which can cause corrosion.  
       [0004] The melt method, although obviating the need for phosgene or a solvent such as methylene chloride requires high temperatures and relatively long reaction times. As a result, by-products may be formed at high temperature, such as the products arising by Fries rearrangement of carbonate units along the growing polymer chains. Fries rearrangement gives rise to undesired and uncontrolled polymer branching which may negatively impact the polymer&#39;s flow properties and performance. The melt method further requires the use of complex processing equipment capable of operation at high temperature and low pressure, and capable of efficient agitation of the highly viscous polymer melt during the relatively long reaction times required to achieve high molecular weight.  
       [0005] Some years ago, it was reported in U.S. Pat. No. 4,323,668 that polycarbonate could be formed under relatively mild conditions by reacting a bisphenol such as BPA with the diaryl carbonate formed by reaction phosgene with methyl salicylate. The method used relatively high levels of transesterification catalysts such as lithium stearate in order to achieve high molecular weight polycarbonate. High catalyst loadings are particularly undesirable in melt polycarbonate reactions since the catalyst remains in the product polycarbonate following the reaction. The presence of a transesterification catalyst in the polycarbonate may shorten the useful life span of articles made therefrom by promoting increased water absorption, polymer degradation at high temperatures and discoloration.  
       [0006] In copending U.S. application Ser. No. 09/911,439, extrusion of a mixture of an ester-substituted diaryl carbonate, such as bis-methyl salicyl carbonate, a dihydroxy aromatic compound, such as bisphenol A, and a transesterification catalyst, such as tetrabutylphosphonium acetate (TBPA), afforded high molecular weight polycarbonate. The extruder employed was equipped with one or more vacuum vents to remove by-product ester-substituted phenol. Similarly, a precursor polycarbonate having ester-substituted phenoxy endgroups, for example methyl salicyl endgroups, when subjected to extrusion afforded a polycarbonate having a significantly increased molecular weight relative to the precursor polycarbonate. The reaction to form a higher molecular weight polycarbonate may be catalyzed by residual transesterification catalyst present in the precursor polycarbonate, or by a combination of any residual catalyst and an additional catalyst such as TBPA introduced in the extrusion step. Fries rearrangement products were not observed in the product polycarbonates.  
       [0007] Although the methods described in copending U.S. application Ser. No. 09/911,439 represent significant enhancements in the preparation of polycarbonate relative to older methods, additional improvements are needed. For example, it would be highly desirable to increase the throughput rate of starting materials through the extruder in order to achieve greater efficiency. In addition, it would be highly desirable to avoid having to isolate a precursor polycarbonate having ester-substituted phenoxy endgroups prior to its extrusion to afford a higher molecular weight polycarbonate.  
       BRIEF SUMMARY OF THE INVENTION  
       [0008] The present invention provides a method for the preparation of polycarbonate, said method comprising extruding in the presence of a transesterification catalyst at one or more temperatures in a temperature range between about 100° C. and about 400° C. a solution comprising a solvent and an oligomeric polycarbonate, said extruding being carried out on an extruder equipped with at least one vent adapted for solvent removal, said oligomeric polycarbonate comprising polycarbonate repeat units derived from at least one dihydroxy aromatic compound, said oligomeric polycarbonate comprising ester substituted phenoxy terminal groups having structure I  
                 
 
       [0009] wherein R 1  is a C 1 -C 20  alkyl group, C 4 -C 20  cycloalkyl group, or C 4 -C 20  aryl group; R 2  is independently at each occurrence a halogen atom, cyano group, nitro group, C 1 -C 20  alkyl group, C 4 -C 20  cycloalkyl group, C 4 -C 20  aryl group, C 1 -C 20  alkoxy group, C 4 -C 20  cycloalkoxy group, C 4 -C 20  aryloxy group, C 1 -C 20  alkylthio group, C 4 -C 20  cycloalkylthio group, C 4 -C 20  arylthio group, C 1 -C 20  alkylsulfinyl group, C 4 -C 20  cycloalkylsulfinyl group, C 4 -C 20  arylsulfinyl group, C 1 -C 20  alkylsulfonyl group, C 4 -C 20  cycloalkylsulfonyl group, C 4 -C 20  arylsulfonyl group, C 1 -C 20  alkoxycarbonyl group, C 4 -C 20  cycloalkoxycarbonyl group, C 4 -C 20  aryloxycarbonyl group, C 2 -C 60  alkylamino group, C 6 -C 60  cycloalkylamino group, C 5 -C 60  arylamino group, C 1 -C 40  alkylaminocarbonyl group, C 4 -C 40  cycloalkylaminocarbonyl group, C 4 -C 40  arylaminocarbonyl group, or C 1 -C 20  acylamino group; and b is an integer 0-4.  
       [0010] The present invention further relates a method for preparing solutions comprising an ester substituted phenol solvent and an oligomeric polycarbonate, and the conversion of said oligomeric polycarbonate into high molecular weight polycarbonate with simultaneous removal said solvent, said method comprising:  
       [0011] Step (I) heating a mixture comprising at least one dihydroxy aromatic compound, an ester substituted diaryl carbonate and a transesterification catalyst at a temperature in a range between about 100° C. and about 300° C. to provide a solution of an oligomeric polycarbonate in an ester substituted phenol solvent; and  
       [0012] Step (II) extruding said solution of oligomeric polycarbonate in said ester substituted phenol at one or more temperatures in a range between about 100° C. and about 400° C., and at one or more screw speeds in a range between about 50 and about 1200 rpm, said extruding being carried out on an extruder comprising at least one vent adapted for solvent removal.  
       [0013] In another aspect the present invention relates to a polycarbonate prepared according to the method of the invention, said polycarbonate having a very high level of endcapping, a very low level of Fries product, and a very low level of residual solvent. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0014]FIG. 1 illustrates a devolatilizing extruder and feed system suitable for use according to the method of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0015] The present invention may be understood more readily by reference to the following detailed description of preferred embodiments of the invention and the examples included therein. In the following specification and the claims which follow, reference will be made to a number of terms which shall be defined to have the following meanings:  
     [0016] The singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise.  
     [0017] “Optional” or “optionally” means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where the event occurs and instances where it does not.  
     [0018] As used herein, the term “oligomeric polycarbonate” refers to a polycarbonate oligomer having a number average molecular weight of less than 5000 daltons and includes oligomeric polycarbonates comprising polycarbonate repeat units derived from one or more dihydroxy aromatic compounds.  
     [0019] As used herein, when describing an oligomeric polycarbonate, the expression “polycarbonate repeat units derived from at least one dihydroxy aromatic compound” means a repeat unit incorporated into an oligomeric polycarbonate by reaction of a dihydroxy aromatic compound with a source of carbonyl units, for example the reaction of bisphenol A with bis(methyl salicyl) carbonate.  
     [0020] As used herein, the term “high molecular weight polycarbonate” means polycarbonate having a number average molecular weight, M n , of 8000 daltons or more.  
     [0021] As used herein, the term “solvent” can refer to a single solvent or a mixture of solvents.  
     [0022] As used herein, the term “solution comprising a solvent and an oligomeric polycarbonate” refers to a liquid oligomeric polycarbonate comprising at least 10 percent by weight solvent.  
     [0023] As used herein, the term “melt polycarbonate” refers to a polycarbonate made by the transesterification of a diaryl carbonate with a dihydroxy aromatic compound.  
     [0024] “BPA” is herein defined as bisphenol A or 2,2-bis(4-hydroxyphenyl)propane.  
     [0025] As used herein the term “Fries product” is defined as a structural unit of the product polycarbonate which upon hydrolysis of the product polycarbonate affords a carboxy-substituted dihydroxy aromatic compound bearing a carboxy group adjacent to one or both of the hydroxy groups of said carboxy-substituted dihydroxy aromatic compound. For example, in bisphenol A polycarbonate prepared by a melt reaction method in which Fries reaction occurs, the Fries product includes those structural features of the polycarbonate which afford 2-carboxy bisphenol A upon complete hydrolysis of the product polycarbonate.  
     [0026] The terms “Fries product” and “Fries group” are used interchangeably herein.  
     [0027] The terms “Fries reaction” and “Fries rearrangement” are used interchangeably herein.  
     [0028] The terms “double screw extruder” and “twin screw extruder” are used interchangeably herein.  
     [0029] As used herein the term “monofunctional phenol” means a phenol comprising a single reactive hydroxy group.  
     [0030] The terms “vent port” and “vent” are used interchangeably herein.  
     [0031] As used herein the term “aliphatic radical” refers to a radical having a valence of at least one comprising a linear or branched array of atoms which is not cyclic. The array may include heteroatoms such as nitrogen, sulfur and oxygen or may be composed exclusively of carbon and hydrogen. Examples of aliphatic radicals include methyl, methylene, ethyl, ethylene, hexyl, hexamethylene and the like.  
     [0032] As used herein the term “aromatic radical” refers to a radical having a valence of at least one comprising at least one aromatic group. Examples of aromatic radicals include phenyl, pyridyl, furanyl, thienyl, naphthyl, phenylene, and biphenyl. The term includes groups containing both aromatic and aliphatic components, for example a benzyl group.  
     [0033] As used herein the term “cycloaliphatic radical” refers to a radical having a valence of at least one comprising an array of atoms which is cyclic but which is not aromatic. The array may include heteroatoms such as nitrogen, sulfur and oxygen or may be composed exclusively of carbon and hydrogen. Examples of cycloaliphatic radicals include cyclopropyl, cyclopentyl, cyclohexyl, tetrahydrofuranyl and the like.  
     [0034] The present invention provides a method of preparing polycarbonates whereby a solution comprising an oligomeric polycarbonate in a solvent, said oligomeric polycarbonate comprising ester substituted phenoxy endgroups having structure I, is extruded through an extruder adapted to remove solvent. The method of the invention effects both the conversion of the oligomeric polycarbonate to a product polycarbonate having higher molecular weight, and a separation of the solvent initially present in the solution of the oligomeric polycarbonate from the product polycarbonate. Additionally, the method provides for the removal of other volatile materials which may be present in the initial solution of oligomeric polycarbonate, or formed as by-products as the oligomeric polycarbonate is transformed in the extruder to the product polycarbonate.  
     [0035] The oligomeric polycarbonate comprises polycarbonate repeat units and terminal phenoxy endgroups having structure I. Terminal phenoxy endgroups having structure I include ester substituted phenoxy endgroups generally. Ester substituted phenoxy engroups are exemplified by the 2-ethoxycarbonylphenoxy group, 2-propoxycarbonylphenoxy group, 4-chloro-2-methoxycarbonylphenoxy group, and the 4-cyano-2-methoxycarbonylphenoxy group. Among the various ester substituted phenoxy terminal groups, the 2-methoxycarbonylphenoxy group II is frequently preferred.  
                 
 
     [0036] The oligomeric polycarbonate comprises repeat units derived from at least one dihydroxy aromatic compound. Dihydroxy aromatic compounds are illustrated by dihydroxy benzenes, for example hydroquinone (HQ), 2-methylhydroquinone, resorcinol, 5-methylresorcinol and the like; dihydroxy naphthalenes, for example 1,4-dihydroxynathalene, 2,6-dihydroxynaphthalene, and the like; and bisphenols, for example bisphenol A and 4,4′-sulfonyldiphenol. The oligomeric polycarbonate typically contains polycarbonate repeat units derived from at least one bisphenol, said polycarbonate repeat units having structure III  
     [0037] wherein R 3 -R 10  are independently a hydrogen atom, halogen atom, nitro group, cyano  
                 
 
     [0038] group, C 1- C 20  alkyl group, C 4- C 20  cycloalkyl group, or C 6- C 20  aryl group; W is a bond, an oxygen atom, a sulfur atom, a SO 2  group, a C 1 -C 20  aliphatic radical, a C 6 -C 20  aromatic radical, a C 6 -C 20  cycloaliphatic radical, or the group  
                 
 
     [0039] wherein R 11  and R 12  are independently a hydrogen atom, C 1- C 20  alkyl group, C 4- C 20  cycloalkyl group, or C 4- C 20  aryl group; or R 11  and R 12  together form a C 4- C 20  cycloaliphatic ring which is optionally substituted by one or more C 1- C 20  alkyl, C 6- C 20  aryl, C 5 -C 21  aralkyl, C 5 -C 20  cycloalkyl groups, or a combination thereof.  
     [0040] Repeat units having structure III are illustrated by repeat units present in bisphenol A polycarbonate, bisphenol M polycarbonate, bisphenol C polycarbonate, and the like.  
     [0041] In one embodiment of the present invention the oligomeric polycarbonate comprises repeat units having structure IV  
                 
 
     [0042] said repeat units IV being derived from bisphenol A. In an alternate embodiment of the present invention the oligomeric polycarbonate comprises repeat units having structures IV and V  
                 
 
     [0043] said repeat units V being derived from 4,4′-sulfonyldiphenol.  
     [0044] The solution of the oligomeric polycarbonate used according to the method of the present invention comprises at least one solvent. The solvent may be a single solvent or a mixture of solvents. Typically the solvent present in the solution of the oligomeric polycarbonate comprises from about 10 percent by weight to about 99 percent by weight, preferably from about 30 percent by weight to about 70 percent by weight of the solution. For example a solution of oligomeric bisphenol A polycarbonate comprising phenoxy endgroups II dissolved in methyl salicylate, said solution being about 40 percent by weight of said oligomeric polycarbonate and about 60 percent by weight methyl salicylate. Alternatively the solution may comprise more than one solvent, for example a solution of oligomeric bisphenol A polycarbonate comprising phenoxy endgroups II dissolved in a mixture of ortho-dichlorobenzene (ODCB) and methyl salicylate, said solution being about 40 percent by weight of said oligomeric polycarbonate, 30 percent by weight ODCB, and about 10 percent by weight methyl salicylate.  
     [0045] In one embodiment of the present invention the solvent employed according to the method of the present invention comprises at least one ester substituted phenol having structure VI  
                 
 
     [0046] wherein R 1 , R 2 , and b are defined as in structure I. Examples of ester substituted phenols having structure VI include methyl salicylate, ethyl salicylate, butyl salicylate, 4-chloro methyl salicylate, and mixtures thereof. Solvent VI may be recovered and reused. For example, ester substituted phenols such as VI may be recovered, purified, and reacted with phosgene to make ester substituted diaryl carbonates which in turn can be used to prepare oligomeric polycarbonates comprising terminal phenoxy groups having structure I. Typically, purification of the recovered ester substituted phenol is efficiently carried out by distillation.  
     [0047] The solvent used according to the method of the present invention optionally comprises a halogenated aliphatic solvent, a halogenated aromatic solvent, a non-halogenated aromatic solvent, a non-halogenated aliphatic solvent, or a mixture thereof. Halogenated aromatic solvents are illustrated by ortho-dichlorobenzene (ODCB), chlorobenzene and the like. Non-halogenated aromatic solvents are illustrated by toluene, xylene, anisole, phenol; 2,6-dimethylphenol; and the like. Halogenated aliphatic solvents are illustrated by methylene chloride; chloroform; 1,2-dichloroethane; and the like. Non-halogenated aliphatic solvents are illustrated by ethanol, acetone, ethyl acetate, cyclohexanone, and the like.  
     [0048] In one embodiment of the present invention the solvent employed comprises a mixture of a halogenated aromatic solvent and an ester substituted phenol, for example a mixture of ortho-dichlorobenzene (ODCB) and methyl salicylate.  
     [0049] The transesterification catalyst used according to the present invention may be any catalyst effective in promoting chain growth of the oligomeric polycarbonate during the extrusion. The transesterification catalysts for use according to the method of the present invention may comprise onium catalysts such as a quaternary ammonium compound, a quaternary phosphonium compound, or a mixture thereof.  
     [0050] Quaternary ammonium compounds suitable for use as transesterifcation catalysts according to the method of the present invention include quaternary ammonium compounds having structure VII  
                 
 
     [0051] wherein R 13 -R 16  are independently a C 1- C 20  alkyl group, C 4- C 20  cycloalkyl group, or a C 4- C 20  aryl group; and X −  is an organic or inorganic anion.  
     [0052] Quaternary ammonium compounds VII are illustrated by tetamethylammonium hydroxide, tetrabutylammonium acetate, tetrabutylammonium hydroxide, and the like.  
     [0053] Quaternary phosphonium compounds suitable for use as transesterifcation catalysts according to the method of the present invention include quaternary phosphonium compounds having structure VIII  
                 
 
     [0054] wherein R 17 -R 20  are independently a C 1- C 20  alkyl group, C 4- C 20  cycloalkyl group, or a C 4- C 20  aryl group; and X −  is an organic or inorganic anion.  
     [0055] Quaternary phosphonium compounds VIII are illustrated by tetamethylphosphonium hydroxide, tetrabutylphosphonium acetate, tetrabutylphosphonium hydroxide, and the like.  
     [0056] In structures VII and VIII, the anion X −  is typically an anion selected from the group consisting of hydroxide, halide, carboxylate, phenoxide, sulfonate, sulfate, carbonate, and bicarbonate. With respect to transesterifcation catalysts having structures VII and VIII, where X −  is a polyvalent anion such as carbonate or sulfate it is understood that the positive and negative charges in structures VII and VIII are properly balanced. For example, in tetrabutylphosphonium carbonate where R 17 -R 20  in structure VIII are each butyl groups and X −  represents a carbonate anion, it is understood that X −  represents ½ (CO 3   −2 ).  
     [0057] In one embodiment, the transesterification catalyst used is a combination of a quaternary ammonium compound, a quaternary phosphonium compound, or a mixture thereof, with at least one alkali metal hydroxide, alkaline earth metal hydroxide, or a mixture thereof. For example, a mixture of tetrabutylphosphonium acetate and sodium hydroxide.  
     [0058] Other transesterification catalysts that may be used according to the method of the present invention include one or more alkali metal salts of carboxylic acids, one or more alkaline earth salts of a carboxylic acid, and mixtures thereof. Such transesterifcation catalysts are illustrated by simple salts of carboxylic acids such as sodium acetate, calcium stearate and the like. Additionally, alkali metal and alkaline earth metal salts of organic polyacids may serve as efficient transesterification catalysts according to the method of the present invention. Alkali metal and alkaline earth metal salts of organic polyacids, such as ethylene diamine tetracarboxylate, may be employed. Salts of organic polyacids are illustrated by disodium magnesium ethylenediamine tetracarboxylate (Na 2 Mg EDTA).  
     [0059] In one embodiment of the present invention the transesterification catalyst comprises at least one salt of a non-volatile acid. By “non-volatile” it is meant that the acid from which the catalyst is made has no appreciable vapor pressure under melt polymerization conditions. Examples of non-volatile acids include phosphorous acid, phosphoric acid, sulfuric acid, and metal “oxo acids” such as the oxo acids of germanium, antimony, niobium and the like. Salts of non-volatile acids useful as melt polymerization catalysts according to the method of the present invention include alkali metal salts of phosphites; alkaline earth metal salts of phosphites; alkali metal salts of phosphates; alkaline earth metal salts of phosphates, alkali metal salts of sulfates, alkaline earth metal salts of sulfates, alkali metal salts of metal oxo acids, and alkaline earth metal salts of metal oxo acids. Specific examples of salts of non-volatile acids include NaH 2 PO 3 , NaH 2 PO 4 , Na 2 HPO 4 , KH 2 PO 4 , CsH 2 PO 4 , Cs 2 HPO 4 , NaKHPO 4 , NaCsHPO 4 , KCsHPO 4 , Na 2 SO 4 , NaHSO 4 , NaSbO 3 , LiSbO 3 , KSbO 3 , Mg(SbO 3 ) 2 , Na 2 GeO 3 , K 2 GeO 3 , Li 2 GeO 3 , Mg GeO 3 , Mg 2 GeO 4 , and mixtures thereof.  
     [0060] Typically the transesterification catalyst is employed in an amount corresponding to between about 1.0×10 −8  and about 1×10 −3 , preferably between about 1.0×10 −6  and about 2.5×10 −4  moles of transesterification catalyst per mole of polycarbonate repeat units derived from aromatic dihydroxy compound present in the oligomeric polycarbonate.  
     [0061] Typically, the oligomeric polycarbonate employed is prepared in a step involving heating a dihydroxy aromatic compound with an ester substituted diaryl carbonate in the presence of a transesterification catalyst. Thus, the reactants are combined in a vessel in a ratio between about 0.95 and 1.3 moles, preferably between about 1.0 and about 1.05 moles of ester substituted diaryl carbonate per mole of dihydroxy aromatic compound. The amount of transesterification catalyst employed is between about 1.0×10 −8  and about 1×10 −3 , preferably between about 1.0×10 −6  and about 2.5×10 −4  moles of transesterification catalyst per mole of dihydroxy aromatic compound employed. Upon heating the mixture at one or more temperatures in a range from about 100° C. to about 400° C., preferably from about 100° C. to about 300° C., and still more preferably from about 100° C. to about 250° C., reaction occurs to produce a solution comprising an equilibrium mixture of product oligomeric polycarbonate, by-product ester substituted phenol (solvent), transesterification catalyst, and low levels of the starting materials, dihydroxy aromatic compound and ester substituted diaryl carbonate. This is referred to as “equilibrating” the reactants. Typically the equilibrium strongly favors the formation of product oligomeric polycarbonate and by-product ester substituted phenol and only traces of the starting materials are observed. The “equilibrated” product mixture may then be introduced into a devolatilizing extruder to effect removal of the by-product ester substituted phenol solvent while converting the oligomeric polycarbonate into a higher molecular weight product polycarbonate. Because, the transesterification catalyst is typically neither consumed in the equilibration step nor removed prior to extrusion, there is typically no need to add additional catalyst during extrusion. Where no additional catalyst is added, the amount of catalyst present during extrusion step (expressed in terms of moles catalyst per mole of polycarbonate repeat units in the oligomeric polycarbonate) will closely approximate the amount of catalyst used in the equilibration step, expressed in moles catalyst per mole dihydroxy aromatic compound.  
     [0062] Typically the ester substituted diaryl carbonate will have structure IX  
                 
 
     [0063] wherein R 1 , R 2 , and b are defined as in structure I. In addition, the dihydroxy aromatic compound is typically, though not always, at least one bisphenol having structure X  
                 
 
     [0064] wherein R 3 -R 10  and W are defined as in structure III.  
     [0065] In one embodiment, the method of the present invention comprises:  
     [0066] Step (I) heating a mixture comprising at least one dihydroxy aromatic compound, an ester substituted diaryl carbonate and a transesterification catalyst at a temperature in a range between about 100° C. and about 300° C. to provide a solution of an oligomeric polycarbonate in an ester substituted phenol solvent; and  
     [0067] Step (II) extruding said solution of oligomeric polycarbonate in said ester substituted phenol at one or more temperatures in a range between about 100° C. and about 400° C., and at one or more screw speeds in a range between about 50 and about 1200 rpm, said extruding being carried out on an extruder comprising at least one vent adapted for solvent removal.  
     [0068] In some instances it may be desirable to remove a portion of the ester substituted phenol formed during the equilibration of the monomers. This may be effected conveniently by heating the mixture of monomers and the transesterification catalyst under vacuum, typically from about 0.01 atmospheres to about 0.9 atmospheres, and distilling off a portion of the ester substituted phenol. As ester substituted phenol is distilled from the mixture undergoing the equilibration reaction, the molecular weight of the oligomeric polycarbonate will tend to increase. If sufficient ester substituted phenol by-product is removed, the number average molecular weight (M n ) of the polycarbonate product may be in excess of 5000 daltons and in some instances in excess of 8000 daltons. Thus, in one aspect of the present invention a mixture comprising at least dihydroxy aromatic compound is reacted with at least one ester substituted diaryl carbonate in the presence of a transesterification catalyst at a temperature between about 100° C. and about 300° C. and a portion of the by-product ester substituted phenol is removed by distillation. The equilibration product may be a mixture comprising an ester substituted phenol solvent and a polycarbonate comprising terminal phenoxy groups having structure I and having a number average molecular weight in excess of 5000 daltons. This equilibration product is then fed to a devolatilizing extruder wherein the polycarbonate is converted to still higher molecular weight product polycarbonate, said product polycarbonate having a high level of endcapping, a low level of Fries product, and a low level of residual solvent. In one embodiment of the present invention, a portion of the ester substituted phenol formed during equilibration is distilled from the mixture undergoing equilibration and a like amount of ODCB is added to provide a solution comprising a polycarbonate having a number average molecular weight in excess of 5000 daltons, ester substituted phenol and ODCB. This solution is then fed to a devolatilizing extruder wherein the polycarbonate is converted to a product polycarbonate having a higher molecular weight, said product polycarbonate having a Fries content of under 10 ppm, an endcapping level of at least 97%, and less than 1 percent by weight solvent. Typically, in instances in which the polycarbonate formed in the equilibration reaction has a number average molecular weight in excess of 5000 daltons, it will have a M n  value in a range between 5000 daltons and about 15000 daltons.  
     [0069] Oligomeric polycarbonates comprising ester substituted terminal phenoxy groups I may be prepared by a variety of other methods in addition to the equilibration method described. For example, oligomeric bischloroformates of bisphenols may be prepared by reaction of one or more bisphenols with phosgene under interfacial conditions in a methylene chloride water mixture at low pH. Such bischloroformates may then be further reacted under interfacial conditions with an ester substituted phenol, for example methyl salicylate, to afford an oligomeric polycarbonate comprising ester substituted terminal phenoxy groups in methylene chloride solution. The product oligomeric polycarbonate in solution may then be subjected to the method of the present invention. Catalysts employed during the interfacial reaction are typically removed from the solution of the oligomeric polycarbonate in a series of washing steps in which the methylene chloride solution of the oligomeric polycarbonate is washed repeatedly with water to remove sodium chloride. Under such circumstances, additional catalyst may be required and may be added during or just prior to the extrusion step.  
     [0070] In one embodiment, a monofunctional phenol chainstopper is added to a solution of an oligomeric polycarbonate comprising ester substituted phenoxy terminal groups, said oligomeric polycarbonate being prepared using the equilibration technique described herein. The solution is then subjected to extrusion devolatilization to afford a product polycarbonate incorporating terminal phenoxy groups derived from said chainstopper. Suitable monofunctional phenol chainstoppers include p-cumylphenol and cardanol.  
     [0071] The extruder used according to the method of the present invention is of the devolatilizing extruder type. That is, it is an extruder adapted for separating substantial amounts of solvent from a polymer-solvent mixture. The extruder, therefore must possess at least one and preferably a greater number of vents adapted for solvent removal. FIG. 1 illustrates a devolatilizing extruder and feed system suitable for use according to the method of the present invention. In one embodiment of the invention reactants, ester substituted diaryl carbonate, dihydroxy aromatic compound and a transesterification catalyst are combined in a reaction vessel  10  and heated at a temperature in a range between about 100° C. and about 300° C., preferably between about 150° C. and about 250° C., at a pressure between about 1 atmosphere and about 10 atmospheres, preferably between about 1 and about 2 atmospheres, to provide a solution of an oligomeric polycarbonate in an ester substituted phenol. The solution is transferred by means of a gear pump  12  via piping  14  which is directly plumbed into a fourteen barrel, vented, twin screw extruder  20 , said extruder possessing screw design  30 . The extruder is operated at a temperature between about 100° C. and about 400° C., preferably between about 200° C. and about 350° C., at a screw speed between about 50 and about 1200 rpm. The solution is introduced into the upstream edge of barrel one  22 . The segmentations along the extruder indicate the transitions from one extruder barrel to the next. Barrel two is labeled  24 . (The remaining barrels  3 - 14  are not labeled.) The extruder screw design  30  consists of conveying screw elements illustrated by  32  and mixing sections which include an initial mixing section  34  and four zones of intense mixing  36 . The extruder is equipped with four atmospheric vents  40 , said vents being connected to a manifold  42  for removal of ester substituted phenol solvent and other volatile by-products formed as the oligomeric polycarbonate is converted into product polycarbonate within the extruder. Solvent vapors and other volatile by-products are condensed in a shell and tube condenser  44  which is attached to a source of house vacuum  46 . The extruder is further equipped with two vacuum vents  50 . Vacuum vents  50  are connected via a cold trap  52  to a vacuum pump  54 . As mentioned, the extruder comprises four mixing sections which provide for intense mixing of the contents of the extruder. These are indicated in the screw design  30  as the mixing sections labeled  36 . Mixing sections labeled  36  in the screw design correspond to reaction zones  26  of the extruder. Said reaction zones are believed to provide for enhanced rates of polycarbonate chain growth relative to other domains within the extruder.  
     [0072] The extruder used according to the method of the present invention, which may be a single screw or multiple screw extruder is typically operated at one or more temperatures in a range between about 100° C. and about 400° C. and at one or more screw speeds in a screw speed range, said range being between about 50 revolutions per minute (rpm) and about 1200 rpm, preferably between about 50 rpm and about 500 rpm.  
     [0073] Extruders suitable for use according to the method of the present invention include co-rotating intermeshing double screw extruders, counter-rotating non-intermeshing double screw extruders, single screw reciprocating extruders, and single screw non-reciprocating extruders.  
     [0074] It is a general principle of extruder operation that as the feed rate is increased a corresponding increase in the screw speed must be made in order to accommodate the additional material being fed. Moreover, the screw speed determines the residence time of the material being fed to the extruder, here the solution of the oligomeric polycarbonate and transesterification catalyst. Thus the screw speed and feed rate are typically interdependent. It is useful to characterize this relationship between feed rate and screw speed as a ratio. Typically the extruder is operated such that the ratio of starting material introduced into the extruder in pounds per hour to the screw speed expressed in rpm falls within a range of from about 0.01 to about 100, preferably from about 0.05 to about 1. For example, the ratio of feed rate to screw speed where the solution of comprising an oligomeric polycarbonate and transesterification catalyst are being introduced at 1000 pounds per hour into an extruder being operated at 400 rpm is 2.5. The maximum and minimum feed rates and extruder screw speeds are determined by, among other factors, the size of the extruder, the general rule being the larger the extruder the higher the maximum and minimum feed rates.  
     [0075] In one embodiment of the present invention, a mixture of an oligomeric polycarbonate comprising endgroups having structure I and a solvent is heated under pressure to produce a “superheated” solution, meaning that the temperature of said superheated solution is greater than the boiling point of the solvent at atmospheric pressure. Typically, the temperature of the superheated oligomeric polycarbonate will be between about 2° C. and about 200° C. higher than the boiling point of the solvent at atmospheric pressure. In instances where there are multiple solvents present, the solution of oligomeric polycarbonate is “superheated” with respect to at least one of the solvent components. Where the solution of oligomeric polycarbonate contains significant amounts of both high and low boiling solvents, it may be advantageous to superheat the solution of oligomeric polycarbonate with respect to all solvents present (i.e. above the boiling point at atmospheric pressure of the highest boiling solvent). Superheating of the solution of the oligomeric polycarbonate may be achieved by heating the mixture under pressure, typically at a pressure less than about 10 atmospheres but greater than one atmosphere. Superheated solutions of oligomeric polycarbonates are conveniently prepared in pressurized heated feed tanks, pressurized heat exchangers, extruders, pressurized reaction vessels and the like. The superheated solution is then introduced into a devolatilizing extruder through a pressure control valve, the pressure control valve having a cracking pressure higher than atmospheric pressure. The back pressure generated by the pressure control valve prevents evaporation of the solvent prior to introducing the solution into the extruder. Typically, the pressure control valve is attached (plumbed) directly to the extruder and serves as the principal feed inlet of the extruder. The extruder is equipped with at least one side feeder.  
     [0076] The extruder in combination with the side feeder is equipped with one or more atmospheric vents in close proximity to the principal feed inlet comprising the pressure control valve. The side feeder is typically positioned in close proximity to the pressure control valve through which the superheated oligomeric polycarbonate is introduced into the extruder. The side feeder comprises at least one atmospheric vent. Alternatively, the pressure control valve through which the superheated oligomeric polycarbonate is introduced may be attached to the side feeder itself in which instance the pressure control valve is attached to the side feeder at a position between the point of attachment of the side feeder to the extruder and the atmospheric vent located on the side feeder. In yet another alternative embodiment, the superheated solution of oligomeric polycarbonate may be introduced through multiple pressure control valves which may be attached to the side feeder, the extruder, or to both extruder and side feeder. The heated zones of the extruder are typically operated at one or more temperatures between about 100° C. and about 400° C. The expression “wherein the extruder is operated at a temperature between about 100° C. and about 400° C.” refers to the heated zones of the extruder, it being understood that the extruder may comprise both heated and unheated zones.  
     [0077] The superheated solution of oligomeric polycarbonate passes through the pressure control valve into the feed zone of the extruder which due to the presence of the aforementioned atmospheric vents is at atmospheric pressure. The solvent present in the superheated solution of oligomeric polycarbonate undergoes sudden and rapid evaporation thereby effecting at least partial separation of the oligomeric polycarbonate and the solvent. The solvent vapors emerge through the atmospheric vents. The atmospheric vents are attached to a solvent vapor manifold and condenser in order to recover solvent and prevent its adventitious release. Additionally, the extruder is equipped with at least one vent operated at subatmospheric pressure which serves to remove solvent not removed through the atmospheric vents. Vents operated at subatmospheric pressure are referred to herein as “vacuum vents” and are maintained at from about 1 to about 30, preferably from about 10 to about 29 inches of mercury as measured by a vacuum gauge measuring vacuum (as opposed to a pressure gauge measuring pressure). Typically, at least two vacuum vents are preferred.  
     [0078] Extruders suitable for use in embodiments of the present invention wherein a superheated oligomeric polycarbonate solution is being fed include co-rating intermeshing double screw extruders, counter-rotating non-intermeshing double screw extruders, single screw reciprocating extruders, and single screw non-reciprocating extruders.  
     [0079] In some instances, it may be found that the product polycarbonate prepared according to the method of the present invention is of insufficient molecular weight or retains too much of the solvent originally present in the solution of the oligomeric polycarbonate. In such instances, simply subjecting the product polycarbonate to a second extrusion on the same or a different devolatilizing extruder typically results in a product polycarbonate having an increased molecular weight and a reduced level of residual solvent. Thus, in one embodiment of the present invention, a solution of an oligomeric polycarbonate comprising terminal groups having structure I and a solvent is subjected to devolatilization extrusion at a temperature between about 100° C. and about 400° C. on an extruder equipped with at least one vent adapted for solvent removal to provide an initial product polycarbonate. The initial product polycarbonate is then introduced into a second extruder, said second extruder being equipped with at least one vacuum vent. The second extruder is operated at a temperature in a range between about 100° C. and about 400° C., and at a screw speed in a range between about 50 and about 1200 rpm.  
     [0080] The method of the present invention may be carried out in a batch or continuous mode. In one embodiment, the method of the present invention is carried out as a batch process wherein monomers and transesterification catalyst are equilibrated in a batch reactor to form a solution of the oligomeric polycarbonate. This solution is then fed to a devolatilizing extruder and the product polycarbonate is isolated until the solution is consumed. Alternatively, the method of the present invention may be carried out as a continuous process wherein the monomers and catalyst are continuously fed to, and the solution of oligomeric polycarbonate is continuously removed from a continuous reactor. Thus a mixture of BMSC, BPA and transesterification catalyst may be fed to one end of a tube reactor heated to a temperature between about 160° C. and about 250° C. A solution of an oligomeric polycarbonate comprising phenoxy endgroups II emerges at the opposite end of the tube reactor and is fed to a devolatilizing extruder from which emerges the product polycarbonate.  
     [0081] It is understood, especially for melt reactions of the type presented in the instant invention, that purity of the monomers employed may strongly affect the properties of the product polycarbonate. Thus, it is frequently desirable that the monomers employed be free of, or contain only very limited amounts of, contaminants such as metal ions, halide ions, acidic contaminants and other organic species. This may be especially true in applications such as optical disks, (e.g. compact disks) where contaminants present in the polycarbonate can affect disk performance. Typically the concentration of metal ions, for example iron, nickel, cobalt, sodium, and postassium, present in the monomer should be less than about 10 ppm, preferably less than about 1 ppm and still more preferably less than about 100 parts per billion (ppb). The amount of halide ion present in the polycarbonate, for example fluoride, chloride and bromide ions, should be minimized in order to inhibit the absorption of water by the product polycarbonate as well as to avoid the corrosive effects of halide ion on equipment used in the preparation of the polycarbonate. Certain applications, for example optical disks, may require very low levels of halide ion contaminants. Preferably, the level of halide ion present in each monomer employed should be less than about 1 ppm. The presence of acidic impurities, for example organic sulfonic acids which may be present in bisphenols such as BPA, should be minimized since only minute amounts of basic catalysts are employed in the oligomerization and subsequent polymerization steps. Even a small amount of an acidic impurity may have a large effect on the rate of oligomerization and polymerization since it may neutralize a substantial portion of the basic catalyst employed. Lastly, the tendency of polycarbonates to degrade at high temperature, for example during molding, with concomitant loss of molecular weight and discoloration correlates strongly with the presence of contaminating species within the polycarbonate. In general, the level of purity of a product polycarbonate prepared using a melt reaction method such as the instant invention will closely mirror the level of purity of the starting monomers.  
     [0082] Product polycarbonates prepared by the method of the present invention frequently contain only very low levels of Fries products. In many cases no Fries product is detectable when the polycarbonate is subjected to a Fries product analysis. The Fries product analysis is carried out by completely hydrolyzing the polycarbonate and analyzing the hydrolysis product by HPLC. For bisphenol A polycarbonate produced by the method of the present invention, the level of Fries product is a value expressed as parts 2-carboxy bisphenol A per million parts of the product bisphenol A polycarbonate which was subjected to hydrolysis. For bisphenol A polycarbonates prepared using the method of the present invention this value is frequently zero or very close to it.  
     [0083] The product polycarbonates prepared according to the method of the present invention are found to have very high levels, frequently 97 percent or higher, of endcapping. Typically product polycarbonates will be from about 97 to about 99 percent endcapped. Free hydroxyl groups at the polycarbonate chain ends are typically comprise less than about 100 ppm of the total polymer weight. Two types of free hydroxyl chain ends are typically observed for polycarbonates prepared according to the method of the present invention from BPA and BMSC: hydroxyl groups attached to a BPA residue (“BPA OH”), and hydroxyl groups attached to a salicyl ester residue (“salicyl OH”). Typically, the concentration of “BPA OH” endgroups is less than about 100 ppm based on the total weight of the product polymer. Likewise, the concentration of “salicyl OH” is typically less than about 100 ppm. Endgroups bearing “salicyl OH” groups have the structure indicated by structure XI  
                 
 
     [0084] and are quantified by nuclear magnetic resonance spectroscopy (NMR). It should be noted that the concentrations of hydroxyl endgroups and percent endcapping described above refers to product polycarbonate and not the oligomeric polycarbonate. Additionally, in instances in which the product polycarbonate has been prepared by first equilibrating a mixture of an ester substituted diaryl carbonate with one or more dihydroxy aromatic compounds to afford a solution comprising an oligomeric polycarbonate and subsequently subjecting said solution to extrusion on a devolatilizing extruder, the concentrations of hydroxyl endgroups and percent endcapping in the product polycarbonate will reflect the molar ratio of ester substituted diaryl carbonate to total dihydroxy aromatic compound. Typically, this ratio should be in a range between about 1.01 and about 1.1. Typically, the product polycarbonate prepared by the method of the present invention will contain only very small amounts of residual starting dihydroxy aromatic compound (generally less than about 20 ppm) and ester substituted diaryl carbonate (generally less than about 350 ppm).  
     [0085] The product polycarbonates prepared by the method of the present invention may optionally be blended with any conventional additives used in thermoplastics applications, such as preparing molded articles. These additives include UV stabilizers, antioxidants, heat stabilizers, mold release agents, coloring agents, antistatic agents, slip agents, antiblocking agents, lubricants, anticlouding agents, coloring agents, natural oils, synthetic oils, waxes, organic fillers, inorganic fillers, and mixtures thereof. Typically, it is preferable to form a blend of the polycarbonate and additives which aid in processing the blend to form the desired molded article, such as an optical article. The blend may optionally comprise from 0.0001 to 10% by weight of the desired additives, more preferably from 0.0001 to 1.0% by weight of the desired additives.  
     [0086] Examples of UV absorbers include, but are not limited to, salicylic acid UV absorbers, benzophenone UV absorbers, benzotriazole UV absorbers, cyanoacrylate UV absorbers and mixtures thereof.  
     [0087] Examples of the aforementioned heat-resistant stabilizers, include, but are not limited to, phenol stabilizers, organic thioether stabilizers, organic phosphite stabilizers, hindered amine stabilizers, epoxy stabilizers and mixtures thereof. The heat-resistant stabilizer may be added in the form of a solid or liquid.  
     [0088] Examples of the mold-release agents include, but are not limited to natural and synthetic paraffins, polyethylene waxes, fluorocarbons, and other hydrocarbon mold-release agents; stearic acid, hydroxystearic acid, and other higher fatty acids, hydroxyfatty acids, and other fatty acid mold-release agents; stearic acid amide, ethylenebisstearamide, and other fatty acid amides, alkylenebisfatty acid amides, and other fatty acid amide mold-release agents; stearyl alcohol, cetyl alcohol, and other aliphatic alcohols, polyhydric alcohols, polyglycols, polyglycerols and other alcoholic mold release agents; butyl stearate, pentaerythritol tetrastearate, and other lower alcohol esters of fatty acids, polyhydric alcohol esters of fatty acids, polyglycol esters of fatty acids, and other fatty acid ester mold release agents; silicone oil and other silicone mold release agents, and mixtures of any of the aforementioned.  
     [0089] The coloring agent may be either pigments or dyes. Inorganic coloring agents and organic coloring agents may be used separately or in combination in the invention.  
     [0090] The polycarbonates prepared by the method of the present invention may be random copolymers, block copolymers, branched or linear When the product polycarbonate is branched a suitable branching agent, such as THPE, 9-carboxyoctadecandioic acid, or 1,3,5-trihydroxybenzne is employed. For example, the inclusion of about 0.02 moles of THPE per mole of BPA in the equilibration reaction of 1 mole of BPA with 1.03 moles of BMSC to form an a solution comprising an oligomeric polycarbonate in methyl salicylate, and subsequent extrusion of the solution on a devolatilizing extruder according to the method of the present invention will provide a branched bisphenol A polycarbonate.  
     [0091] Molded articles, such as a molded optical article, comprising the polycarbonates prepared by the method of the present invention, may be obtained by conventional molding techniques, for example injection molding and compression molding. Additionally molded articles may be prepared from a blend of the product polycarbonate with one or more additional polymers. Such blends, typically prepared using extrusion methods, may be molded using conventional techniques. Injection molding is the more preferred method of forming the molded article.  
     [0092] Because the polycarbonates prepared by the method of the present invention possess advantageous properties such as high impact strength, high clarity, low water absorption, good processability and low birefringence, they can be advantageously utilized to produce optical articles. End-use applications for the optical article of the invention include, but are not limited to, a digital audio disk, a digital versatile disk, an optical memory disk, a compact disk, an ASMO device and the like; optical lenses, such as contact lenses, lenses for glasses, lenses for telescopes, and prisms; optical fibers; magneto optical disks; information recording media; information transferring media; disks for video cameras, disks for still cameras and the like.  
     [0093] The polycarbonates prepared by the method of the present invention may function as the medium for data storage, i.e. the data may be fixed onto or into the polycarbonate. The polycarbonate may also function as the substrate onto which a data storage medium is applied. Further, some combination of both functions may be employed in a single device, as for instance when the polycarbonate is imprinted with tracking to aid in reading a data storage medium which is applied to the polycarbonate.  
     EXAMPLES  
     [0094] The following examples are set forth to provide those of ordinary skill in the art with a detailed description of how the methods claimed herein are evaluated, and are not intended to limit the scope of what the inventors regard as their invention. Unless indicated otherwise, parts are by weight, temperature is in ° C.  
     [0095] Molecular weights are reported as number average (M n ) or weight average (M w ) molecular weight and were determined by gel permeation chromatography (GPC) analysis, using a polycarbonate molecular weight standard to construct a broad standard calibration curve against which polymer molecular weights were determined. The temperature of the gel permeation columns was about 25° C. and the mobile phase was chloroform.  
     [0096] Fries content is measured by the KOH methanolysis of resin and is reported as parts per million (ppm). The Fries content is determined as follows. First, 0.50 grams of polycarbonate is dissolved in 4.0 ml of THF (containing p-terphenyl as internal standard). Next, 3.0 mL of 18% KOH in methanol is added to this solution. The resulting mixture is stirred for two hours at room temperature. Next, 1.0 mL of acetic acid is added, and the mixture is stirred for 5 minutes. Potassium acetate by-product is allowed to crystallize over 1 hour. The solid is filtered off and the resulting filtrate is analyzed by high performance liquid chromatography (HPLC) using p-terphenyl as the internal standard.  
     [0097] Concentrations of “BPA-OH” and “salicyl-OH” endgroups were measured by  31 P-NMR. Terminal hydroxy groups were first derivatized with 2-chloro-1,3,2-dioxaphospholane (Aldrich).  
     Examples 1-5  
     [0098] Solutions of oligomeric polycarbonate in methyl salicylate were prepared by equilibrating a mixture of bis(methyl salicyl) carbonate (BMSC), bisphenol A (BPA) and transesterification catalyst, tetrabutylphosphonium acetate (TBPA), at a temperature in a range between about 160° C. and about 220° C. in a batch melt reactor under a nitrogen atmosphere. The reaction mixture was stirred and heated until equilibrium was reached Equilibrium was reached in about 80 minutes at about 165° C. and in about 10 minutes at about 220° C. At equilibrium, the solution of oligomeric polycarbonate prepared from mixtures of BMSC (1.03 moles BMSC per mole BPA), BPA and TBPA (2.5×10 −4  moles per mole BPA) was about 45 percent by weight polycarbonate oligomer and about 54 to about 55 percent by weight methyl salicylate.  
               TABLE 1                          SOLUTIONS OF OLIGOMERIC POLYCARBONATE       IN METHYL SALYCILATE                                                         Oligo-               Mole TBPA       Time to   meric           [BMSC]/   Catalyst per   Equilibration   Equi-   Polycar-       Example   [BPA]   Mole BPA   Temperature   librium   bonate Mn               1   1.03   2.5 × 10 −4     220° C.   10   1385                       minutes       2   1.03   2.5 × 10 −4     210° C.   18   1474                       minutes       3   1.03   2.5 × 10 −4     195° C.   30   1670                       minutes       4   1.03   2.5 × 10 −4     180° C.   52   1849                       minutes       5   1.03   2.5 × 10 −4     160° C.   90   2090                       minutes                  
 
     [0099] Examples 1-5 in Table 1 illustrate both the characteristics of the equilibrated solution at different temperatures and document the truly oligomeric nature of the material being fed to the extruder. The column heading “[BMSC]/[BPA]” indicates the molar ratio of BMSC and BPA employed in the equilibration reaction. The heading “Mn” indicates the number average molecular weight as determined by gel permeation chromatography measured using a polycarbonate molecular weight standard. Values of M n  are given in daltons. The data in Table 1 illustrate the speed at which equilibration of the reactants can be achieved. Example 1 indicates that solid reactants can be converted into a solution of an oligomeric polycarbonate and transesterification catalyst in methyl salicylate solvent in as little as ten minutes. Because residence times in the extruder are short (from about 0.5 to about 2 minutes on the equipment being used in the Examples which follow), the overall process of converting starting monomers into product polycarbonate can be achieved in under 15 minutes.  
     Examples 6-101  
     [0100] Solutions of oligomeric bisphenol A polycarbonate in methyl salicylate were prepared as in Examples 1-5 at an equilibration temperature of about 160° C. using as a catalyst either TBPA alone (as in Examples 1-5) or a combination of TBPA with sodium hydroxide. The amount of catalyst employed was 2.5×10 −4  moles TBPA per mole BPA and (when present) 2×10 −6  moles sodium hydroxide per mole BPA. Following the equilibration reaction, the solution was transferred by means of nitrogen pressure (about 80 psi) to a gear pump which pumped the solution via an insulated pipe connected directly (hard plumbed) to the upstream edge of barrel one of a 25 millimeter diameter, 14 barrel, co-rotating intermeshing twin screw extruder having a length to diameter datio of 56. The extruder comprised 6 vents V1-V6 located at the upstream edges of barrel four (V1, vacuum or atmospheric vent), barrel five (V2, optionally closed but at times operated as atmospheric or vacuum vent), barrel seven (V3, vacuum vent), barrel nine (V4, vacuum vent), barrel eleven (V5, vacuum vent) and barrel thirteen (V6, vacuum vent). Vent V1 was operated at atmospheric pressure or alternatively under a slight vacuum (5-10 in. Hg, as measured by a vacuum gauge). Vacuum vents V3 and V4 were operated at moderate vacuum (10-28 in. Hg). Vacuum vents V5 and V6 were operated at moderate to high (&gt;29 in Hg) vacuum. When operated at slight to moderate vacuum (5-28 in Hg), vacuum was supplied to vents V2-V6 with “house” vacuum. In a number of instances, vacuum vent V6 or vacuum vents V5 together with V6 were operated under high (i.e. “full”) vacuum (˜29 in. Hg, as measured by a vacuum gauge). The vacuum vents were connected through solvent recovery manifold and condenser systems to their respective sources of vacuum. In instances in which either V6 alone or V5 and V6 were operated at “full” vacuum, the vent or vents being operated at “full” vacuum were connected via a cold trap to a vacuum pump. Vents V1 and V3-V6 were equipped with type “C” vent port inserts. Vent port inserts are available from the Werner &amp; Pfleiderer Company. Vent port inserts differ in the cross sectional area available for the solvent vapors to escape the extruder: Type “A” inserts are the most restrictive (smallest cross section) and Type “C” are the least restrictive (largest cross section). As noted, V2 was kept closed in some instances and remained open in others. The screw design comprised conveying elements under the feed inlet and all vents. The screw design further comprised kneading blocks in four “reaction zones” (zones comprising screw elements providing intense mixing) located between vents. The four reaction zones were located between V2 and V3, between V3 and V4, between V4 and V5, and between V5 and V6 respectively. The data in Tables 2-5 below demonstrate the effects of changes in reaction conditions on the properties of the product polycarbonate.  
               TABLE 2                       EXTRUSION OF SOLUTIONOF OLIGOMERIC       POLYCARBONATE IN METHYL SALICYLATE,       BMSC/BPA = 1.017, Catalyst = TBPA + NaOH                                                                Vacuum level at   Mass Flow       Melt   Screw   Die           Vent (in. Hg)   Rate   Torque   Temperature   speed   Pressure                                                         Example   V1   V3   V4   V5   V6   (rpm)/(lb/hr)   (%)   (C.)   (rpm)   (psi)                6   atm   10   22   22   22   40/17.6   29   288   104   163        7   Atm.   18   26   25   25   40/17.6   35   290   104   242        8   Atm.   15   26   25   25   46/20.2   42   293   144   329        9   Atm.   15   26   25   25   50/22.0   50   302   200   347       10   Atm.   15   26   25   25   55/24.2   51   308   248   342       11   Atm.   15   26   25   25   55/24.2   50   306   248   316                                         Actual Barrel Temperatures   Molecular Weight   Residual MS       Example   (C.)   Mw/Mn/PDI   (ppm)                6   256/277/277/280/280/280/281/281/280   22779/10042/2.268   9792        7   254/278/277/280/280/280/281/280/280   25295/11167/2.265   6115        8   245/279/278/280/280/280/281/280/280   26930/11938/2.256   4419        9   241/279/279/280/280/280/284/281/280   27063/11921/2.270   4544       10   238/279/278/280/280/280/283/281/280   26683/11629/2.295   4989       11   236/280/279/280/280/280/280/280/280   26474/11622/2.278   4296                  
 
     [0101] In Examples 6-11 (Table 2) the ratio of BMSC to BPA employed in the equilibration reaction was 1.017. The catalyst employed for the equilibration reaction was a mixture of tetrabutylphosphonium acetate and sodium hydroxide. The oligomeric polycarbonate in methyl salicylate (MS) solution was found to have a weight average molecular weight, Mw, of 6865 daltons and a number average molecular weight, Mn, of 2980 daltons. In Examples 6-11 the polycarbonate emerging from the extruder was observed to be clear and free of color. Following pelletization the level of residual methyl salicylate present in the product polycarbonate was determined by gas chromatography. The data provided in Examples 6-11 illustrate that polycarbonate may be prepared using the method of the present invention and that the product so obtained contains less than about 1 percent by weight methyl salicylate (MS).  
               TABLE 3                       EXTRUSION OF SOLUTIONOF OLIGOMERIC POLYCARBONATE IN       METHYL SALICYLATE, BMSC/BPA = 1.02, Catalyst = TBPA + NaOH                                                                Vacuum level at Vent   Mass Flow       Melt   Screw   Die           (in. Hg)   Rate   Torque   Temperature   speed   Pressure                                                         Example   V1   V3   V4   V5   V6   (rpm)/(lb/hr)   (%)   (C.)   (rpm)   (psi)               12   Atm.   26   27   25   29   40/17.6   62   307   154   520       13    5   27   27   25   29   40/17.6   74   312   182   558       14   15   28   28   28   29   40/17.6   74   313   182   576       15   12   28   28   28   29   40/17.6   54   299   118   562                                                         ppm       Residual           Actual Barrel Temperatures   Molecular Weight   OH   %   MS       Example   (C.)   Mw/Mn/PDI   total   Endcap   (ppm)               12   249/303/282/280/281/280/280/282/281   31545/14191/2.223   115   95.2   1046.8       13   249/316/282/281/281/280/282/280/280   33106/14788/2.239   103   95.5   752.1       14   249/283/276/280/280/280/281/280/280   35070/15568/2.253    98   95.5   643.9       15   250/273/273/279/279/280/276/279/280   31437/14026/2.241   186   92.3   1403.2                  
 
     [0102] In Examples 12-15 the solution of the oligomeric polycarbonate in methyl salicylate was prepared from a mixture of BMSC and BPA. The molar ratio of BMSC to BPA was 1.02. The catalyst employed in the equilibration step was a mixture of TBPA and sodium hydroxide. The data in Table 3 illustrate that use of higher levels of vacuum provides higher molecular weight polycarbonate containing still lower levels of residual methyl salicylate than is observed in Examples 6-11.  
               TABLE 4                       EXTRUSION OF SOLUTIONOF OLIGOMERIC POLYCARBONATE IN       METHYL SALICYLATE, BMSC/BPA = 1.025, Catalyst = TBPA + NaOH                                                                Vacuum level at Vent   Mass Flow       Melt   Screw   Die           (in. Hg)   Rate   Torque   Temperature   speed   Pressure                                                         Example   V1   V3   V4   V5   V6   (rpm)/(lb/hr)   (%)   (C.)   (rpm)   (psi)               16   12   28   28   29   29   40/17.6   55   295   106   486       17   10   28   28   29   29   40/17.6   62   293   106   508       18   10   28   28   29   29   46/20.2   60   302   133   592       19    5   28   28   29   29   46/20.2   60   302   150   599                                         Actual Barrel Temperatures   Molecular Weight   Residual MS       Example   (C.)   Mw/Mn/PDI   (ppm)               16   252/278/278/280/280/280/282/281/270   30670/13858/2.213   922.8       17   251/291/292/281/281/280/280/280/265   31436/14215/2.211   770.2       18   248/289/290/280/280/280/281/280/265   32470/14647/2.217   654.3       19   248/290/290/280/280/280/280/280/265   31944/14369/2.223   704.4                  
 
     [0103] In Examples 16-19 the solution of the oligomeric polycarbonate in methyl salicylate was prepared from a mixture of BMSC and BPA. The molar ratio of BMSC to BPA was 1.025. The catalyst employed in the equilibration step was a mixture of TBPA and sodium hydroxide. During the extrusion step vents V5 and V6 were connected to a vacuum pump via a cold trap. Vents V1-V4 were connected to the “house vacuum” via a solvent recovery manifold and condenser. Vent V2 was employed and was connected to the same solvent recovery system and house vacuum as were vents V1, V3 and V4. The vacuum level at which V2 was operated was not determined, however.  
               TABLE 5                       EXTRUSION OF SOLUTIONOF OLIGOMERIC POLYCARBONATE IN       METHYL SALICYLATE, BMSC/BPA = 1.03, Catalyst = TBPA + NaOH                                                                Vacuum level at Vent   Mass Flow       Melt   Screw   Die           (in. Hg)   Rate   Torque   Temperature   speed   Pressure                                                         Example   V1   V3   V4   V5   V6   (rpm)/(lb/hr)   (%)   (C.)   (rpm)   (psi)               20   13   28   28   29   29   40/17.6   37   284   114   250       21   13   28   28   29   29   40/17.6   37   285   114   235       22   12   28   28   29   29   40/17.6   50   301   230   181       23   10   28   28   29   29   40/17.6   56   313   351   152       24   10   28   28   29   29   40/17.6   58   325   464   123                                                                 Residual           Actual Barrel Temperatures   Molecular Wt.   ppm OH   %   MS       Example   (C.)   Mw/Mn/PDI   total   Endcap   (ppm)               20   246/281/280/280/280/280/281/279/270   24456/11218/2.180   45   98.5   720.8       21   252/281/280/280/280/280/279/280/270   24237/11095/2.184   68   97.8   1003.9       22   253/280/280/280/281/280/284/281/271   25247/11527/2.190   48   98.4   559.4       23   252/280/280/280/281/280/285/280/270   26102/11891/2.195   23   99.2   307.1       24   251/281/281/281/281/280/283/281/270   26863/12030/2.233   18   99.4   183.4                  
 
     [0104] Examples 20-24 illustrate the application of the method of the present invention to a solution of an oligomeric polycarbonate prepared from a mixture of BMSC and BPA having an initial molar ratio of 1.03 moles BMSC to BPA. The catalyst employed in the equilibration reaction was a mixture of tetrabutylphosphonium acetate and sodium hydroxide. Vacuum vent V2 was employed as in Examples 16-19 but the precise pressure at which it was operated was not determined. As in previous Examples, the product polycarbonates of Examples 20-24 were clear and free of color when inspected visually. The column heading “ppm OH total” refers to the concentration of “BPA-OH” and “salicyl-OH” present in the product polycarbonate as determined by  31 P-NMR following derivatization with 2-chloro-1,3,2-dioxaphospholane. The column heading “% Endcap” refers to the percentage of the product polycarbonate chain ends which do not terminate in either “BPA-OH” or “salicyl-OH” groups. The data in Table 5 provide evidence that a very high level of endcapping is achieved using the method of the present invention and that the concentration of terminal OH groups in the product polycarbonate is very low.  
               TABLE 6                       EXTRUSION OF SOLUTIONOF OLIGOMERIC POLYCARBONATE IN       METHYL SALICYLATE, BMSC/BPA = 1.03, Catalyst = TBPA ONLY                                                                Vacuum level at Vent   Mass Flow       Melt   Screw   Die           (in. Hg)   Rate   Torque   Temperature   speed   Pressure                                                         Example   V1   V3   V4   V5   V6   (rpm)/(lb/hr)   (%)   (C.)   (rpm)   (psi)               25   14   28   28   29   29   40/17.6   35   282   113   286       26   14   28   28   29   29   40/17.6   35   283   113   280       27   12   28   28   29   29   40/17.6   46   298   229   238       28   14   28   28   29   29   40/17.6   54   313   355   183       29   14   28   28   29   29   40/17.6   56   325   464   149       30   14   28   28   29   29   70/30.7   48   323   464   209                                         Actual Barrel Temperatures   Molecular Weight   Residual MS       Example   (C.)   Mw/Mn/PDI   (ppm)               25   257/280/280/280/280/280/278/280/265   24549/11261/2.180   1134.4       26   258/280/280/280/280/280/279/280/265   24795/11367/2.181   959.5       27   257/280/280/280/280/280/283/280/265   25767/11797/2.184   402.2       28   256/280/280/280/281/280/284/281/265   26544/11656/2.277   216.8       29   255/280/281/281/281/280/283/281/265   26967/12255/2.200   148.9       30   251/276/277/278/278/280/280/280/265   26070/11877/2.195   435.9                  
 
     [0105] Examples 25-30 illustrate the application of the method of the invention to a solution of an oligomeric polycarbonate prepared using only tetrabutyl phosphonium acetate and no sodium hydroxide. Here again, reasonably high molecular weight polycarbonate is obtained upon extrusion of the solution. Vacuum vent V2 was employed as in Examples 16-19 but the precise pressure at which it was operated was not determined. As in previous Examples the product polycarbonates of Examples 25-30 were clear and free of color when inspected visually.  
     Examples 31-37 Preparation of Copolymers  
     [0106] In Examples 31-37 copolymers were obtained by first equilibrating a mixture of bis(methyl salicyl) carbonate (BMSC), bisphenol A (BPA), and hydroquinone (HQ). The ratio of bis(methyl salicyl) carbonate to the total number of moles of the dihydroxy aromatic compounds bisphenol A and hydroquinone was 1.017. The equilibration was carried out as in Examples 1-5 and the resultant solution in methyl salicylate was extruded on an extruder configured as in Examples 6-11. The catalyst was a mixture of TBPA and sodium hydroxide. The temperature of the solution of the oligomeric copolycarbonate fed to the extruder was about 160° C. and was introduced into the extruder using a positive displacement pump. Examples 31-35 employed 20 mole percent hydroquinone (based on the total number of moles of BPA and HQ) in the equilibration step. Examples 36 and 37 employed 40 mole percent hydroquinone (based on the total number of moles of BPA and HQ) in the equilibration step. Data for the preparation of the copolycarbonates are provided in Table 7.  
               TABLE 7                       EXTRUSION OF SOLUTIONOF OLIGOMERIC POLYCARBONATE IN       METHYL SALICYLATE, BMSC/(BPA and HQ) = 1.017, Catalyst = TBPA + NaOH                                                                Vacuum level at   Mass Flow       Melt   Screw   Die           Vent (in. Hg)   Rate   Torque   Temperature   speed   Pressure                                                         Example   V1   V3   V4   V5   V6   (rpm)/(lb/hr)   (%)   (C.)   (rpm)   (psi)               31   Atm   21   21   21   25   ˜12   57   315   138   347       32   Atm   21   21   21   25   ˜12   58   306   138   637       33   Atm   21   21   21   23   30   16   301   138   251       34   Atm   21   21   21   24   20   31   288   138   250       35   Atm   21   21   21   25   36   14   285   138   181       36   Atm   22   22   22   25   ˜15   43   315   140   275       37   Atm   21   21   21   24   15   17   308   118    31                                     Actual Barrel Temperatures   Molecular Weight       Example   (C.)   Mw/Mn/PDI               31   245/271/271/301/321/320/332/331/270   30600/13000/2.4       32   239/271/270/300/311/310/311/311/270   34700/14800/2.3       33   233/261/261/296/308/310/308/309/270   22500/9800/2.3       34   235/271/267/289/289/290/288/288/270   22400/9700/2.3       35   233/266/262/287/289/289/289/289/270   19400/8900/2.2       36   237/272/267/291/311/310/331/330/270   29400/13400/2.2       37   239/273/267/313/311/311/331/330/271   18900/8800/2.1                  
 
     [0107] Examples 31-37 illustrate the use of the present invention for the preparation of copolycarbonates.  
     [0108] Examples 38-50 were run in order to demonstrate the consistency of the method of the present invention. A solution of an oligomeric polycarbonate was prepared as in Example 5. The molar ratio of BMSC to BPA was 1.03. The transesterification catalyst was tetrabutylphosphonium acetate (TBPA, 2.5×10 −4  moles TBPA per mole BPA). The extruder was configured as in Examples 16-19. The data presented in Table 8 illustrate consistent molecular weight build in the conversion of the oligomeric polycarbonate into the product polycarbonate.  
               TABLE 8                       EXTRUSION OF SOLUTIONOF OLIGOMERIC POLYCARBONATE IN       METHYL SALICYLATE, BMSC/(BPA) = 1.03, Catalyst = TBPA ONLY                                                                Vacuum @ Vents   Mass Flow       Melt   Screw   Die           (in. Hg.)   Rate   Torque   Temperature   speed   Pressure                                                         Example   V1   V3   V4   V5   V6   (rpm)/(lb/hr)   (%)   (C.)   (rpm)   (psi)               38   12   28   28   29   29   40/17.6   38   289   201   155       39   12   28   28   29   29   40/17.6   38   289   201   159       40   12   28   28   29   29   40/17.6   39   289   201   166       41   12   28   28   29   29   40/17.6   39   289   201   173       42   12   28   28   29   29   40/17.6   39   290   201   177       43   12   28   28   29   29   40/17.6   40   290   201   183       44   12   25   28   29   29   40/17.6   39   291   201   192       45   12   25   28   29   29   40/17.6   39   292   201   181       46   14   28   28   29   29   40/17.6   41   292   201   188       47   14   28   28   29   29   40/17.6   41   292   201   209       48   14   28   28   29   29   40/17.6   42   292   201   214       49   14   28   28   29   29   40/17.6   42   293   201   214       50   14   28   28   29   29   40/17.6   42   293   201   200                                         Actual Barrel Temperatures   Molecular Weight   Residual MS       Example   (C.)   Mw/Mn/PDI   (ppm)               38   258/281/281/280/281/280/282/281/265   23194/10642/2.333   647.9       39   257/280/280/280/280/280/281/280/265   23551/10693/2.202   648       40   257/280/280/280/280/280/281/280/265   23978/10978/2.184   594.1       41   257/280/280/280/280/280/280/280/265   23926/11117/2.152   614.9       42   257/280/280/280/280/280/280/280/265   23861/10709/2.228   722       43   257/280/280/280/280/280/280/280/265   23961/10915/2.195   590.6       44   257/280/280/280/280/280/280/280/265   24318/11194/2.172   667       45   257/280/280/280/280/280/280/280/266   24192/11339/2.134   655.2       46   257/280/280/280/280/280/280/280/265   24348/11007/2.212   613.4       47   257/280/280/280/280/280/280/280/265   24717/11317/2.184   644.5       48   257/280/280/280/280/280/280/280/265   24826/11317/2.194   743.3       49   257/280/280/280/280/280/280/280/265   24987/11412/2.190   662.6       50   257/280/280/280/280/280/280/280/265   24878/11375/2.187   698                  
 
     [0109] In Examples 38-50 a single batch of oligomeric polycarbonate in methyl salicylate solution was extruded and the polycarbonate which emerged from the extruder was sampled at six minute intervals over the course of two hours.  
     [0110] In Examples 51-74 a solution of an oligomeric polycarbonate prepared as in Example 5 was extruded on the same devolatilizing extruder used in Examples 38-50. The ratio of BMSC to BPA was 1.02. Examples 51-71 demonstrate the level of consistency achieved. As in Examples 38-50, a single solution of oligomeric polycarbonate was fed to the extruder for a period of about 2 hours. Examples 51-74 represent samples of the product polycarbonate collected about every six minutes as the product polycarbonate emerged from the extruder. The molecular weight and level of residual methyl salicylate were determined for each sample. Examples 72-74 demonstrate that the process may be operated at higher feed rates than the 17.6 pounds of solution per hour used in Examples 51-71. Examples 72 and 73 demonstrate that feed rates as high as 35.1 and 41.6 pounds of solution per hour may be used without sacrificing the molecular weight of the product polycarbonate. Moreover, low levels of residual methyl salicylate may be maintained. Example 74 highlights the effect of screw speed on product polycarbonate molecular weight and the level of residual methyl salicylate contained in the product polycarbonate.  
               TABLE 9                       EXTRUSION OF SOLUTIONOF OLIGOMERIC POLYCARBONATE IN       METHYL SALICYLATE, BMSC/(BPA) = 1.02, Catalyst = TBPA ONLY                                                                Vacuum @ Vents   Mass Flow       Melt   Screw   Die           (in. Hg.)   Rate   Torque   Temperature   speed   Pressure                                                         Example   V1   V3   V4   V5   V6   (rpm)/(lb/hr)   (%)   (C.)   (rpm)   (psi)               51   15   28   28   29   29   40/17.6   65   302   200   477       52   15   28   28   29   29   40/17.6   65   304   199   486       53   15   28   28   29   29   40/17.6   66   304   199   506       54   15   28   28   29   29   40/17.6   67   304   199   488       55   15   28   28   29   29   40/17.6   67   305   199   496       56   15   28   28   29   29   40/17.6   66   305   199   504       57   15   28   28   29   29   40/17.6   67   305   199   507       58   15   28   28   29   29   40/17.6   67   306   199   521       59   15   28   28   29   29   40/17.6   67   306   199   512       60   15   28   28   29   29   40/17.6   67   306   199   519       61   15   28   28   29   29   40/17.6   67   307   199   523       62   15   28   28   29   29   40/17.6   67   307   199   550       63   15   28   28   29   29   40/17.6   67   308   199   500       64   15   28   28   29   29   40/17.6   67   308   199   524       65   15   28   28   29   29   40/17.6   67   308   199   496       66   15   28   28   29   29   40/17.6   67   308   199   550       67   15   28   28   29   29   40/17.6   67   308   199   535       68   15   28   28   29   29   40/17.6   67   308   199   523       69   15   28   28   29   29   40/17.6   67   309   199   548       70   15   28   28   29   29   40/17.6   67   308   199   562       71   15   28   28   29   29   40/17.6   67   309   199   596       72   15   28   28   29   29   80/35.1   62   372   655       73   15   28   28   29   29   95/41.6   53   368   655       74   15   28   28   29   29   40/17.6   69   369   650                                                         Residual                   Actual Barrel Temperatures   Molecular Weight   MS   %   Total OH/       Example   (C.)   Mw/Mn/PDI   (ppm)   Endcap   “MS” OH               51   257/280x5/282/280/265   31417/14266/2.202   428.3   100.0       52   256/280x7/265   31470/13923/2.260       100.0       53   255/280x7/265   31597/13989/2.259   481.7   100.0       54   255/280x7/265   31678/14010/2.261   476.3   100.0       55   255/280x7/265   31766/14063/2.259   486.7   100.0       56   255/280x7/265   32002/14136/2.264   532.6   100.0       57   256/280x2/281/280x4/265   32008/14354/2.230   477.5   100.0       58   256/280x7/265   32109/14409/2.228   434.5   100.0       59   256/280x7/265   32631/14369/2.271   569.6   100.0       60   256/280x7/265   32569/14352/2.269   470.9   100.0       61   256/280x7/265   32314/14225/2.272   499.2   100.0       62   257/280x7/265   32964/14524/2.270   477.5   100.0       63   256/280x6/281/266   32417/14293/2.268   604.4   100.0       64   256/280x6/281/265   32884/14464/2.274   509.5   100.0       65   256/280x7/265   32725/14394/2.274   513.8   100.0       66   256/280x7/265   32930/14499/2.271   493.2   100.0       67   256/280x7/265   33647/14571/2.309   493.1   100.0       68   256/280x7/265   33752/16821/2.007   535.8   100.0       69   256/280x7/265   32809/14406/2.277   527.2   100.0       70   256/280x7/265   32794/14363/2.283   505.9    97.9   49/16       71   256/280x7/265   33202/14488/2.292   529.6   100.0       72   256/280x7/265   33604/14619/2.299   309.9    98.5   36/10       73   256/280x7/265   33143/15448/2.145   416.9    97.7   51/13       74   256/280x7/265   35339/14654/2.412    62.6    99.2   19/0                   
 
     [0111] In Table 9 the column heading “Total OH/“MS” OH” provides the total concentration of OH endgroups (expressed in ppm) present in the product polycarbonate (numerator) and the measured concentration of “salicyl-OH” groups (“S” OH) expressed in parts per million (denominator). The data demonstrate very high levels of product polycarbonate endcapping. The product polycarbonate of example 70 was analyzed for the presence of residual monomer. Less than 350 ppm residual BMSC, and less than 20 ppm residual BPA were found in the product polycarbonate.  
     [0112] Examples 75-80 illustrate the method of the invention in which a chain stopper, p-cumylphenol was included in the equilibration step. Thus BMSC, BPA and p-cumylphenol (0.03 mole per mole BPA) were equilibrated as in Example 5 to provide a solution of an oligomeric polycarbonate in methyl salicylate. The molar ratio of BMSC to BPA was 1.03. As in Examples 38-50, a single solution of the oligomeric polycarbonate was fed to the extruder configured as in Examples 16-19. Examples 75-80 represent samples of the product polycarbonate which were collected at regular intervals over a period of about 1.5 hours. The molecular weight and level of residual methyl salicylate determined for each sample. Data for Examples 75-80 are gathered in Table 10 and demonstrate the successful use of a chain stopper to control molecular weight within the context of the present invention.  
               TABLE 10                       EXTRUSION OF SOLUTION OF PCP-CHAINSTOPPED       OLIGOMERIC POLYCARBONATE IN METHYL SALICYLATE, PCP LEVEL =       0.03 MOLE PER MOLE BPA, BMSC/(BPA) = 1.03, Catalyst = TBPA ONLY                                                                Vacuum @ Vents   Mass Flow       Melt   Screw   Die           (in. Hg.)   Rate   Torque   Temperature   speed   Pressure                                                         Example   V1   V3   V4   V5   V6   (rpm)/(lb/hr)   (%)   (C.)   (rpm)   (psi)               75   15   25   25   29   29   40/17.6   20   282   125   65       76   22   28   27   29   29   40/17.6   30   288   200   60       77   22   28   28   29   29   40/17.6   35   296   299   40       78   22   28   28   29   29   40/17.6   39   304   402   35       79   22   28   28   29   29   40/17.6   41   311   503   32       80   25   28   28   29   29   30/13.2   34   288   201   65                                         Actual Barrel Temperatures   Molecular Weight   Residual MS       Example   (C.)   Mw/Mn/PDI   (ppm)               75   264/278/280x5/282/265   19572/8847/2.212   1442.8       76   260/280x2/281x2/280/281x2/265   20298/9185/2.210   1158.7       77   257/280/281x3/280/283/282/265   20546/9315/2.206   934.5       78   254/280/281x3/280/284/281/265   20922/9486/2.206   723.7       79   253/280/281x2/282/280/285/282/265   21007/9516/2.208   595.5       80   257/280/279x2/278/280/274/277/264   21718/9839/2.207   619.9                  
 
     [0113] Examples 81-86 illustrate the use of the method of the invention to obtain a polycarbonate comprising methyl salicyl endgroups and a very low level of residual solvent in a single extrusion step. Thus BMSC and BPA were equilibrated as in Example 5 to provide a solution of an oligomeric polycarbonate in methyl salicylate. The molar ratio of BMSC to BPA was 1.035. As in Examples 38-50, a single solution of the oligomeric polycarbonate was fed to the extruder configured as in Examples 16-19. Examples 81-86 represent samples of the product polycarbonate which were collected at regular intervals over a period of about 1.5 hours. The molecular weight and level of residual methyl salicylate were determined for each sample. Data for Examples 81-86 are gathered in Table 11 and demonstrate that still lower levels of residual methyl salicylate can be achieved by increasing the melt temperature and screw speed. The product polycarbonate was very clear and colorless throughout the experiment giving rise to the Examples in Table 11.  
               TABLE 11                       EFFECT OF SCREW SPEED AND MELT TEMPERATURE ON       PRODUCT POLYCARBONATE. EXTRUSION OF OLIGOMERIC       POLYCARBONATE IN METHYL SALICYLATE, BMSC/(BPA) = 1.035,       Catalyst = TBPA ONLY                                                                Vacuum @ Vents   Mass Flow       Melt   Screw   Die           (in. Hg.)   Rate   Torque   Temperature   speed   Pressure                                                         Example   V1   V3   V4   V5   V6   (rpm)/(lb/hr)   (%)   (C.)   (rpm)   (psi)               81   15   28   28   29   29   40/17.6   35   294   251   31       82   15   28   28   29   29   40/17.6   26   281   125   56       83   15   28   28   29   29   40/17.6   42   304   378   17       84   15   28   28   29   29   40/17.6   43   315   500   12       85   25   28   28   29   29   40/17.6   47   324   630    8       86   15   28   28   29   29   99/43.4   37   324   630   66                                         Actual Barrel Temperatures   Molecular Weight   Residual MS       Example   (C.)   Mw/Mn/PDI   (ppm)               81   255/279/272/280/281/280/285/282/280   21711/9728/2.232   298.9       82   257/280/272/279/279/280/277/278/279   21815/9760/2.235   583.7       83   255/281/275/281x4/283/281   22417/10012/2.239   165.4       84   254/281/279/282x2/280/285/284/281   22589/9487/2.381   102.5       85   252/282/283x3/280/288/283/281   22874/10196/2.243   66.2       86   248/273/274/276/279/280/286/283/281   21689/9692/2.238   40.9                  
 
     [0114] Examples 87-90 further illustrate the use of the method of the invention to obtain a polycarbonate comprising methyl salicyl endgroups and a very low level of residual solvent in a single extrusion step in the context of a still higher initial molar ratio of BMSC to BPA. BMSC and BPA were equilibrated as in Example 5 to provide a solution of an oligomeric polycarbonate in methyl salicylate. The molar ratio of BMSC to BPA was 1.0375. As in Examples 38-50, a single solution of the oligomeric polycarbonate was fed to the extruder configured as in Examples 16-19. Examples 87-90 represent samples of the product polycarbonate which were collected at regular intervals over a period of about 1.5 hours. The molecular weight and level of residual methyl salicylate determined for each sample. Data for Examples 87-90 are gathered in Table 12 and demonstrate that very low levels of residual methyl salicylate can be achieved by increasing the melt temperature and screw speed. The lower molecular weight of the product polycarbonate reflects the higher level of BMSC employed. The product polycarbonate was very clear and colorless throughout the experiment giving rise to the Examples in Table 12.  
               TABLE 12                       EFFECT OF SCREW SPEED AND MELT TEMPERATURE ON       PRODUCT POLYCARBONATE. EXTRUSION OF OLIGOMERIC       POLYCARBONATE IN METHYL SALICYLATE, BMSC/(BPA) = 1.0375,       Catalyst = TBPA ONLY                                                                Vacuum @ Vents   Mass Flow       Melt   Screw   Die           (in. Hg.)   Rate   Torque   Temperature   speed   Pressure                                                         Example   V1   V3   V4   V5   V6   (rpm)/(lb/hr)   (%)   (C.)   (rpm)   (psi)               87   15   28   28   29   29   40/17.6   15   276   125   TLTM*       88   17   28   28   29   29   40/17.6   25   282   250   TLTM        89   20   28   28   29   29   40/17.6   30   290   374   TLTM        90   21   28   28   29   29   40/17.6   33   297   501   TLTM                                          Actual Barrel Temperatures   Molecular Weight   Residual MS       Example   (C.)   Mw/Mn/PDI   (ppm)               87   259/269/266/279/280x2/282/280/266   16233/7385/2.198   588       88   253/281/277/280x5/265   16500/7706/2.141   262       89   250/280/281x3/280/282/281/265   17008/7182/2.368   93.1       90   249/280/281x3/280/283/281/265   17149/7773/2.206   59.1                          
 
     [0115] Examples 91-94 illustrate the preparation of copolycarbonates using the method of the present invention. The copolycarbonates are characterized as having a high level of methyl salicyl endgroups, very low Fries group concentrations, and a low level of residual solvent. The solution of oligomeric copolycarbonate employed in Examples 91-93 was prepared as follows. A mixture of hydroquinone (0.2 moles HQ per 0.8 mole BPA), BPA and BMSC (1.02 moles BMSC per 0.8 mole of BPA) was equilibrated as in Example 5. TBPA (2.5×10 −4  moles TBPA per 0.8 mole BPA) was used as the catalyst to provide a solution of an oligomeric copolycarbonate in methyl salicylate. The molar ratio of BMSC to BPA+HQ was 1.02. The solution of oligomeric polycarbonate used in Example 94 was prepared as follows. A mixture of hydroquinone (0.35 moles HQ per 0.65 mole BPA), BPA and BMSC was equilibrated as in Example 5. TBPA (2.5×10 −4  moles TBPA per 0.65 mole BPA) was used as the catalyst to provide a solution of an oligomeric copolycarbonate in methyl salicylate. The molar ratio of BMSC to BPA+HQ was 1.015. The two solutions were fed sequentially to an extruder configured as in Examples 16-19. Examples 91-94 represent samples of the product copolycarbonate which were collected at regular intervals over a period of about 1.5 hours. The molecular weight and level of residual methyl salicylate were determined for each sample. Data for Examples 91-94 are gathered in Table 13 and demonstrate the formation of copolycarbonates using the method of the present invention. The product copolycarbonates were clear but had a slightly yellow color. 1   
               TABLE 13                       COPOLYCARBONATES OF BPA &amp; HQ BMSC/(BPA + HQ) = 1.02       (EXAMPLES 91-93), AND 1.015 (EXAMPLE 94), Catalyst = TBPA ONLY                                                                Vacuum @ Vents   Mass Flow       Melt   Screw   Die           (in. Hg.)   Rate   Torque   Temperature   speed   Pressure                                                         Example   V1   V3   V4   V5   V6   (rpm)/(lb/hr)   (%)   (C.)   (rpm)   (psi)               91   15   28   28   29   29   40/17.6   52   296   127   447       92   15   28   28   29   29   50/22.0   53   315   250   356       93   15   28   28   29   29   50/22.0   51   329   375   254       94   21   28   28   29   29   40/17.6   57   316   298   325                                         Actual Barrel Temperatures   Molecular Weight   Residual MS       Example   (C.)   Mw/Mn/PDI   (ppm)               91   229/282/272/280/279/280/278/278/280   30355/13295/2.283   1241.5       92   234/280/272/280x3/281/282/281   30242/13320/2.270   1076       93   244/280/273/281x3/283x2/281   30429/13338/2.281   1048.6       94   250/280/276/280x3/281/278/279   30156/13012/2.318   761.1                  
 
     [0116] Examples 95-97 illustrate the use of the method of the present invention for the preparation of copolycarbonates containing about 30 mole percent polycarbonate repeat units are derived from biphenol (BP=4,4′-dihydroxybiphenyl) and about 70 mole percent are derived from bisphenol A (BPA). The solution of oligomeric copolycarbonate employed in Examples 95-97 was prepared as follows. A mixture of biphenol (0.3 moles BP per 0.7 mole BPA), BPA and BMSC (1.015 moles BMSC per 0.7 mole of BPA) was equilibrated as in Example 5. TBPA (2.5×10 −4  moles TBPA per 0.7 mole BPA) was used as the catalyst to provide a solution of an oligomeric copolycarbonate in methyl salicylate. The molar ratio of BMSC to the total member of moles of BPA+HQ was 1.015. The solution was fed to a devolatilizing extruder configured as in Examples 16-19. Examples 95-97 represent samples of the product copolycarbonate which were collected at regular intervals over a period of about 1.5 hours. The molecular weight and level of residual methyl salicylate were determined for each sample. Data for Examples 95-97 are gathered in Table 14 and are consistent with the formation of copolycarbonates comprising both BP and BPA residues. The product copolycarbonate samples were clear and had no visible yellowness. The molecular weights observed for the copolycarbonate samples were lower than anticipated, prompting a post extrusion examination of the reaction vessel in which the initial equilibration of monomers was conducted. It was observed that some of the biphenol, a relatively insoluble dihydroxy aromatic compound, did not dissolve during the equilibration reaction. This effectively gave a molar ratio of BMSC to the combined number of moles of BP and BPA which was higher than 1.015. The molecular weights observed for the product polycarbonates are more consistent with a molar ratio of BMSC to BP+BPA of about 1.037. (See for example, the data provided in Table 12 for the preparation of bisphenol A polycarbonate in which the molar ratio of BMSC to BPA was 1.0375.)  
               TABLE 14                       COPOLYCARBONATES: 70% BPA &amp; 30% BIPHENOL (BP)       BMSC/(BPA + BP) = 1.015, Catalyst = TBPA ONLY                                                                Vacuum @ Vents   Mass Flow       Melt   Screw   Die           (in. Hg.)   Rate   Torque   Temperature   speed   Pressure                                                         Example   V1   V3   V4   V5   V6   (rpm)/(lb/hr)   (%)   (C.)   (rpm)   (psi)               95   24   28   28   29   29   40/17.6   45   302   350   106       96   24   28   28   29   29   40/17.6   44   302   350   94       97   26   28   28   29   29   40/17.6   42   300   401   40                                         Actual Barrel Temperatures   Molecular Weight   Residual MS       Example   (C.)   Mw/Mn/PDI   (ppm)               95   256/305/284/282/281/280/283/281/265   19691/9264/2.125   1835       96   261/302/290/280x3/281/280/265   19415/9031/2.150   2004       97   263/300/290/280x5/265   17867/8486/2.105   1942                  
 
     [0117] Examples 98-101 illustrate the use of the method of the present invention for the preparation of copolycarbonates containing polycarbonate repeat units derived from 4,4′-sulfonyldiphenol (BPS) and bisphenol A (BPA). The solution of oligomeric copolycarbonate employed in Examples 98-99 was prepared as follows. A mixture of 4,4′-sulfonyldiphenol (0.2 moles BPS per 0.8 mole BPA), BPA and BMSC (1.02 moles BMSC per 0.8 mole of BPA) was equilibrated as in Example 5. TBPA (2.5×10 −4  moles TBPA per 0.8 mole BPA) was used as the catalyst to provide a solution of an oligomeric copolycarbonate in methyl salicylate. The molar ratio of BMSC to BPA+BPS was 1.02. The solution of oligomeric polycarbonate used in Examples 100-101 was prepared as follows. A mixture of BPS (0.40 moles BPS per 0.60 mole BPA), BPA and BMSC (1.022 moles BMSC per 0.6 mole of BPA) was equilibrated as in Example 5. TBPA (2.5×10 −4  moles TBPA per 0.60 mole BPA) was used as the catalyst to provide a solution of an oligomeric copolycarbonate in methyl salicylate. The molar ratio of BMSC to BPA+BPS was 1.022. The two solutions were fed sequentially to an extruder configured as in Examples 16-19. Examples 98-101 represent samples of the product copolycarbonate which were collected at regular intervals over a total time period of about 3.0 hours. The molecular weight and level of residual methyl salicylate were determined for each sample. Data for Examples 98-101 are gathered in Table 15 and are consistent with the formation of copolycarbonates comprising both BPA and BPS derived repeat units using the method of the present invention.  
               TABLE 15                       COPOLYCARBONATES: 20% and 40% 4,4′-SULFONYLDIPHENOL       (BPS) BMSC/(BPA + BPS) = 1.02-1.022, Catalyst = TBPA ONLY                                                                Vacuum @ Vents   Mass Flow       Melt   Screw   Die           (in. Hg.)   Rate   Torque   Temperature   speed   Pressure                                                         Example   V1   V3   V4   V5   V6   (rpm)/(lb/hr)   (%)   (C.)   (rpm)   (psi)               98   15   28   28   29   29   40/17.6   80   324   200   744       99   15   28   28   29   29   40/17.6   79   323   200   690       100   16   28   28   29   29   35/15.4   76   334   225   615       101   17   28   28   29   29   35/15.4   80   330   201   694                                         Actual Barrel Temperatures   Molecular Weight   Residual MS       Example   (C.)   Mw/Mn/PDI   (ppm)               98   246/280/279/280x3/273/280x2   37069/15446/2.40   556       99   251/280x5/281/280/280   37031/15227/2.43   522       100   249/281/280/281x2/280/286/281/281   39014/13708/2.85   578       101   252/280x8   38194/13843/2.76   563                  
 
     Example 102  
     [0118] A solution of oligomeric polycarbonate is prepared as in Example 5 and is heated to a temperature of about 160° C. in a feed tank under a nitrogen atmosphere (50-60 psig N 2 ). Nitrogen is used to provide enough pressure to feed the pump head of a gear pump in communication with the feed tank by means of heated transfer lines. Additionally, the polymer-solvent mixture further comprises the commercial stabilizers IRGAFOS 168 (about 0.12 percent by weight based on the weight of the oligomeric polycarbonate) and IRGANOX 1010 (about 0.10 percent by weight based on the weight of the oligomeric polycarbonate). The solution is transferred from the heated feed tank by means of the gear pump at a rate of about 30 pounds of solution per hour to a heat exchanger maintained at about 290° C. The solution emerges from the heat exchanger at a temperature of about 265° C. and is then fed through a pressure control valve plumbed into the upstream edge of barrel 3 of a 10-barrel, 25 mm diameter, co-rotating, intermeshing twin screw extruder having a length to diameter ratio (L/D) of about 40. The cracking pressure of the pressure release valve is electronically controlled such that a steady stream of the superheated solution of the oligomeric polycarbonate is introduced into the extruder, the heated zones of which are maintained at a temperatures in a range between about 260° C. and about 290° C. The feed rate to the extruder is about 80 pounds per hour. The transfer lines between the heat exchanger and the pressure control valve are heated such that the temperature of the solution as it is introduced into the extruder through the pressure control valve is about 40° C. higher than the boiling point of methyl salicylate (boiling point 221° C.). The extruder is operated at a screw speed of about 460 rpm. The extruder is further equipped at barrel two with a side feeder positioned orthogonal to the barrel of the extruder. The side feeder is not heated, has an L/D of about 10, and comprises two screws consisting of forward conveying elements only. At the end most distant from the extruder barrel, the side feeder is equipped with a single atmospheric vent (V1). The conveying elements of the screws of the side feeder are configured to convey toward the extruder and away from the side feeder vent. The extruder is further equipped with two additional atmospheric vents at barrel 1 (V2) and barrel 4 (V3) and vacuum vents (vents operated at subatmospheric pressure) at barrel 6 (V4) and barrel 8 (V5). The three atmospheric vents, two on the extruder and one on the side feeder, are each connected to a solvent removal and recovery manifold comprising solvent vapor removal lines, a condenser and liquid solvent receiving vessel. The vacuum vents are similarly adapted for solvent recovery. Recovered ester substituted phenol may be purified by distillation or other means and recycled to prepare additional ester substituted diaryl carbonate. The extruder screw elements consist of both conveying elements and kneading elements. All of the conveying elements in both the extruder and the side feeder are forward flighted conveying elements. Kneading elements include neutral, forward flighted and rearward flighted kneading elements depending on function. In barrels 2 and 3 of the extruder, kneading blocks consisting of forward and neutral flighted kneading elements are employed. The extruder screws are equipped with melt seals consisting of kneading blocks made up of rearward flighted kneading elements. The melt seals are located at barrels 5, and 7. The vacuum vents are located downstream of the melt seals on barrel 6 and barrel 8 and are operated at vacuum levels of about 28 inches of mercury (a vacuum gauge indicating full vacuum, or zero absolute pressure, would read about 30 inches of mercury). The product polycarbonate which emerges from the die face (melt temperature about 325° C.) of the extruder is stranded and pelletized. The pelletized product polycarbonate is found to have a weight average molecular weight, M w , in excess of about 20000 daltons (GPC analysis) and to contain less than about 1 percent by weight residual ester substituted phenol. The product polycarbonate has a high level of endcapping (&gt;95%) and contains less than 10 ppm Fries product.  
     [0119] The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood by those skilled in the art that variations and modifications can be effected within the spirit and scope of the invention.