SEMI OR FULLY CONTINUOUS PROCESS FOR PREPARATION OF POLYESTER BY TRANSESTERIFICATION POLYMERIZATION

Abstract: Linear aromatic polyesters of bisphenols and dicarbox­ ylic compounds are prepared by transesterification polymer­ ization in either a semi-continuous process or in a fully con­ tinuous process wherein a first stage is conducted batchwise or continuously and a second stage is conducted continu­ ously, wherein the second stage employs a wiped film reac­ tor, optionally followed by a vented extrusion reactor. The polymerisation is preferably conducted in the presence of a catalyst in liquid form, and further under conditions wherein the catalyst is introduced concurrently with the introduction of liquid phase reactants to avoid substantial heating of the reactants in contact with the catalyst prior to onset of the transesterification reaction. The products of the processes have an unexpectedly low polydispersity.

Description: "SEMI OR FULLY CONTINUOUS PROCESS FOR PREPARATION OF POLYESTER BY TRANSESTERIFICATION POLYMERIZATION" Background of the Invention and Relationship to the Prior Art Linear polyesters consisting essentially of residues of a bisphenol and dicarboxylic acids are important engineering thermoplastics especially noted for their excellent performance at high temperatures. The transesterification reaction of a bisphenol and a diaryl ester of a dicarboxylic acid, preferentially in the presence of a small effective amount of a catalyst to prepare a thermoplastic linear polyester has been disclosed for example by British Patent 924,697 assigned to Imperial Chemical Industries, Ltd. published April 24, 1963. The reaction, which is generally carried out in bulk, i.e. without solvent or diluent, with the reactants in molten condition generally employs temperatures in the range of above about 100" to about 4000 or higher, e.g. above about 1600 to about 350"C. for bisphenol A-isophthalate-terephthalate polyesters. Since the reaction is reversible, provision is made for removal of the monohydroxy aromatic compound, e.g. phenol, displaced from the diaryl ester reactant during transesterification when a diphenyl ester is employed as the diaryl ester reactant, to cause the reaction to proceed to completion. Monohydroxy aromatic compound removal is generally facilitated by gradually raising the temperature of the reaction mass from the minimum temperature required for onset of transesterification to about 3000-400"C. while gradually reducing the reaction pressure from about atmospheric pressure to about 0.1 nBn or lower of mercury. The viscosity of the polyester products is generally exceptionally high causing the viscosity of the reaction mass, as completion of the reaction is approached, to approach values above about 1 x 106 to above about 10 x 106 centipoises at reaction conditions, i.e. the range of viscosities of such polyesters as measured at shear rate of about 24 sec. -l and at about 320"C. Because of the above indicated wide variance in conditions, e.g. temperature and pressure, employed in the reaction and because of the necessity for wide variance in kind and degree of agitation, occasioned by the aforementioned viscosity increase during the reaction, the prior art has generally carried out the reaction in two stages, namely, a moderate temperature prepolymerization stage, to prepare a polyester oligomer prepolymer and a subsequent polymerization stage wherein said oligomer and the reactants are condensed to provide the polyester product. The prepolymerization stage is carried out at about 100" to about 350 C. under moderate agitation conditions to prepare a low molecular weight polyester, i.e. prepolymer or oligomer, of relatively low molecular weight, corresponding to an intrinsic viscosity of less than about 0.1 to about 0.3 dl/g. The polymerization stage is carried out at temperatures ranging to about 400"C. employing extreme agitation conditions. Because of difficulties in handling the aforementioned high viscosities of the molten reaction mass encountered in the transesterification reaction only batchwise mode or semi-continuous mode (wherein prepolymerization is batchwise and polymerization is continuous) have been suggested for preparation of these polyesters by transesterification. Batchwise transesterification is in general not cornercially attractive for production of large scale quantities of the polyester and generally provides an undesirably wide variance of product characteristics and properties from batch to batch of product. In batchwise transesterification processes it is generally impossible to vary substantially the product molecular weight distribution, as measured by the product's polydispersity (which is further discussed below) by change of reaction conditions. K. Eise et al., German Preliminary Application 2,232,877 published January 17, 1974, disclose a semi-continuous transesterification polymerization of the polyester wherein the prepolymerization stage to prepare the oligomer (referred to as precondensate in the patent) is carried out batchwise and the subsequent polymerization stage is carried out continuously employing a vented extruder reactor, i.e. an extruder having provision for removal of vapors of the phenolic side product of the redaction. Use of the extruder in accordance with the Eise et al. patent, generally has the disadvantage that the reaction mass in the extrusion reaction vessel is in the form of a thick mass, e.g. of greater than about 20 mm. thickness, principally adhering to the screw element of the extruder. The residence time of the reaction mass in an extruder is relatively long compared to a thin film reactor giving rise to an undesirably long thermal history to the polyester. Such a long thermal history leads to a product of greater coloration which in the case of extreme product discoloration makes the product unacceptable for many uses. Under normal operations, the rate of new face exposure is limited, thus limiting the rate at which phenol can be eliminated from the system. As a result, relatively costly expenditures of energy are necessary to razz the phenolic side product to maintain a satisfactory rate of transesterification. As an additional alternative to overcome the aforementioned difficulties in preparation of the polyester by transesterification especially the difficulties arising from the aforementioned high melt viscosity of the bisphenol-dicarboxylic acid polyester, the prior art (for example in US Patent 3,399,170, F. Blachke et al. assigned to Chemiche erke Witten A.G. issued August 27, 1968) has proposed modification of the structure of the polyester by introducing residues in minor proportion of alkylene and cycloalkylene glycols e.g. ethylene glycol, polybutylene glycol, into the polyester. Such structural modification of the polyester, while lowering the melt viscosity of the polyester thereby tending to overcome the aforementioned process difficulties, results in a modified polyester product which does not possess the high temperature performance characteristics of the unmodified bisphenoldicarboxylic acid polyesters of this invention. According to technology developed by Teijin Co., Ltd., with respect to solid state polymerization, molecular weight build-up does occur in this method, but the method is limited to materials that do crystallize. Summary of the Invention The improvement of the invention overcomes the aforementioned difficulties of prior art technology in the process of preparing a linear aromatic polyester by transesterification polymerization of monomer reactants consisting essentially of a bisphenol and a diaryl ester of a dicarboxylic acid in a polymerization reaction zone which comprises reacting said bisphenol and said diaryl ester in a first reaction stage of said zone to prepare a polyester oligomer, which is further polymerized in a second reaction stage of said zone to increase molecular weight thereby producing the polyester product. According to this iniproveirient the transesterification polymer- ization is carried out in the prepolymerization stage, batchwise or substantially continuously, and the transesterification polymerization is carried out in the polymerization stage, substantially continuously, with the proviso that when the polymerization in the prepolymerization stage is carried out batchwise, the polymerization reaction in the polymerization stage is carried out in a countercurrent-flow, mechanically agitated thin film reactor. In accordance with the invention substantially continuously means that the reactants are introduced into a particular reaction stage in substantially continuous fashion while reaction product of the reaction stage are withdrawn in substantially continuous fashion. The countercurrent-flow mechanically agitated thin film reactor is defined as a reaction vessel which is equipped with a motor driven rotary screw or a rotor equipped with suitable blades, and which is known in the art for continuous processing or reaction of viscous molten materials such as polymers. As more particularly described below, the viscous material (in the present invention, the transesterification reaction mass) is transported through the vessel by rotation of the screw in the form of a thin film which principally adheres to the inner walls of the vessel as the material moves through the vessel. In the present known type of thin film reactor the direction of movement of the material through the vessel is counter to direction of flow of gas or vapor evolved from the moving material. In the process of the invention, the vapor removed from the transesterification reaction mass is the aforementioned monohydroxy aromatic, i.e. phenolic, compound which is the side product of the transesterification reaction. In one aspect of the invention; there is contemplated a semicontinuous process for transesterification polymerization wherein the prepolymerization stage of the process is carried out bat wise and the subsequent polymerization stage is carried out substantially continuously in one or more of the aforementioned counter-current flow mechanically agitated thin film or wiped film reactors. Another aspect of the invention contemplates a fully continuour transesterification polymerization wherein, the prepolymerization is carried out in one or more stirred tank reactors, and preferably, the polymerization stage is carried out in one or more of the aforementioned wiped film reactors. in accordance with another embodiment of the invention, there is contemplated an improvement in the aforementioned process technology of transesterification preparation of the linear aromatic polyester wherein as the improvement, the polymerization is carried out in the first stage either batchwise or substantially continuously, and the polymerization is conducted in the second reaction stage continuously, with the provision that when the polymerization in the first reaction stage is carried out batchwise, the polymerization in the second stage comprises polymerization in the wiped film reaction zone followed by polymerization in a vented extrusion reaction zone, i.e. in a vented multi-screw extruder as reaction vessel. Preferably, in accordance with this embodiment, the continuous preparation of the polyester also employs this configuration of reaction vessels. This embodiment substantially eliminates from the polyester high molecular weight gel particles and charred degradation particulate products thereof of unsightly brown and black colors, respectively. The aforementioned gel and char particles are found to occur generally in trace amounts in the product when the latter is prepared continuously or seiiii-continuously employing the afore mentioned 'iiiped thin film reactor at high product molecular weight corresponding to a product intrinsic viscosity (measured as described in the present application) of above about 0.5 dl/g (with said particles increasing in amount in the product with increasing molecular weight of the product). These unsightly particles of polyester gel and polyester char are believed to result from entrainment of polyester on the shaft of the wiped thin film reactor rotor and the blade thereof. The thus entrained polyester is not propelled through the thin film reactor with the remainder of the polymerization mass. At the high temperatures reached at the final stage of the polymerization, the stationary entrained polyester undergoes polymerization to an excessively high molecular weight and even chars. Also, the entrained particles which build up in the wiped film reactor during semi- and fully continuous polymerization are irregularly discharged with the polymeriàtion mass and manifest themselves as gel particles. Also, the highly viscous polyester at the final stage of the polymerization may be subjected to excessive shear heating in the polymer-lubricated bottom bearing of the wiped film reactor. The local temperature increase in the polymer due to heating from shear mechanical forces can also result in occasional excessive polymer molecular weight build-up and char formation causing an occasional undesirable fluctuation in the nolecular weight of the product obtained in semi-continuous and fully continuous operation of the process, as well as brown and black particle contamination of the product. This fluctuation in product molecular weight, as well as brown and black particle contamination of the product, are overcome by the practice of this embodiment of the invention. The multi screw extrusion reaction vessel contemplated for use in this embodiment of the invention is a known apparatus employing a multiplicity (generally two) of rotating screws move a molten mass through zones or stages wherein relatively volatile organic by-products of the reaction (corresponding to the relatively volatile monohydric organic by-product of the present rcaction) can be removed. The apparatus ultimately provides for extrusion of the molten reaction mass through an appropriate orifice such as a die or nose plate. It is preferred to carry out the transesterification reaction in the presence of a ctfyst including acidic, neutral and especially basic catalyst for the transesterification. It is especially preferred to introduce basic catalyst in liquid form. It is also especially preferred to introduce the basic catalyst to the polymerization reaction mass concurrent with introduction of molten reactants to the pre-polymerization so as to avoid substantial heating of the reactants in contact with the catalyst prior to onset of the transesteri fi cation reaction. The present invention provides linear aromatic polyesters consisting essentially of residues of bisphenols and dicarboxylic acids of desirable low color and high melt viscosities, e.g. of viscosities ranging from below about 1 x 106 centipoises up to about 10 x 106 centipoises or higher (as measured at a shear rate of about 24 sec. at about 320"C.) employing either semi- or fully continuous modes of reaction. Brief Description of Drawing Figure 1 of the drawing illustrates the first stage or prepolymerization stage of the aforementioned semi-continuous transesterification process of the invention. Figure 2 of the drawing illustrates the second stage or polymerization stage of semi-continuous and fully continuous transesterification processes of the invention. Figure 3 of the drawing illustrates the first stage or polymerization stage of the fully continuous transesterification process of the invention. Figure 4 of the drawing illustrates the second stage or polymerization stage of semi-continuous and fully continuous transesterification processes of the invention wherein polymerization in the second stage comprises polymerization in a wiped film reaction zone followed by polymerization in a vented extrusion reaction zone. Detailed Description of the Drawings The semi-continuous process of the invention will be described with respect to Figures 1, 2 and 4 of the drawing, utilizing as typical reactants, diphenyl isophthalate, diphenyl terephthalate and bisphenol A. Referring first to Figure 1, these reactants are introduced through conduit 10 to melt tank 12 which is maintained at about 1800 C. to maintain the reactants in the molten state. Thorough mixing of the reactants can be obtained by circulating the mixture through conduits 14 and 18 and back to the melt tank '12. The mixture in the melt tank 12 is continuously purged with dry nitrogen to exclude oxygen. No significant reaction occurs at this stage. In accordance with the semi-continuous process, a batch of the monomer mixture is transferred through conduit 16 to the stirred reactor 20. Catalyst is added directly to the stirred reactor 20 or with the molten mixture of monomers through conduit 17 so that the catalyst does not contact the molten monomers for any substantial time prior to onset of the polymerization reaction in the stirred reactor 20. The reaction is initiated by elevating the temperature and diminishing the pressure in the stirred reactor 20. The prepolymerization reaction is conducted in stirred reactor 20 to the extent that the inherent viscosity of the resulting polymer is about 0.15 to about 0.25, preferably about 0.18, which is equivalent to an extent of reaction of slightly more than 90X. This prepolymer product is transferred to the surge tank 44 jn molten condition. Phenol vapor produced in the reaction process and some unreacted monomer passes overhead from the stirred reactor 20 through conduit 24 to distillation column 26 where the overhead mixture is contacted with recycle phenol entering distillation column 26 through conduit 31. The distillation column removes any monomer entrained in the phenol vapor and returns the monomer to stirred reactor 20 through conduit 29. Phenol vapor passing overhead from the distillation column 26 passes through conduit 28 to the condenser 30. A portion of the recycled phenol is returned to distillation column 26 through conduit 31 and the remainder is passed through conduit 32 to distillate tank 34. The distillate is recirculated from tank 34 through conduit 38 and back to distillate tank 34. The vapor from the distillate tank is exhausted through conduit 36 to a freeze trap (not shown). The liquid product from the distillate tank 34 is removed from the process through conduit 40. The prepolymer is maintained in molten condition in the surge tank 44 under a nitrogen blanket, and is continuously pumped through conduit 46 at the rate required in the subsequent polymerization stage which is operated in a continuous manner as described with reference to Figure 2. As shown in Figure 2, molten prepolymer is introduced from conduit 46 to the top portion of wiped film reactor 48 and passes downwardly through the reaction zone in response to rotating blades (not shown) in reactor 48 which is operated in a temperature range of about 2800 to 3100C. and at a pressure of about 1 to about 10 mm of mercury absolute. The polymer product from the bottom portion of reactor 48 passes through conduit 64 to the upper portion of wiped film reactor 66 and then dcrrriw?rdly through reactor 66 in response to the turning blades of the wiped film reactor 66. The polymer product is removed from the process through conduit 84 from the bottom portion of reactor 66. Reactor 48 is provided with a spray condenser 52 which receives overhead vapor through conduit 50 and discharges condensed phenol through conduit 54 to distillate tank 56. The liquid effluent from distillate tank 56 is divided, and a portion is discharged from the process through conduit 60, while the remainder is recycled through conduit 58 to the top of the spray condenser 52 for contact with additional vapor entering through conduit 50 from reactor 48. The vapors from the spray condenser pass overhead through conduit 62 to a freeze trap (not shown). In a similar manner, the vapor product of reactor 66 passes through conduit 68 to spray condenser 70, from whence the condensate passes through conduit 72 to distillate tank 74. The liquid effluent from distillate tank 74 is provided with a portion being recycled to distillate tank 56 and the remainder being recycled to conduit 78 to the top of spray condenser 70 for contact with the vapor effluent from reactor 66. The overhead vapor product from the spray condenser is discharged through conduit 80 to freeze trap (not shown). Ethylene glycol is introduced to the process through conduit 82 to the distillate tank 74 from which the ethylene glycol is recycled through conduit 78 to the top of the spray corldenser 70 where the ethylene glycol assists in condensing and dissolving the phenol vapors. Additional portions of the ethylene glycol are circulated through conduit 76 to distillate tank 56 and thence through recycle conduit 58 to the top of spray condenser 52 where a similar function is served. Alternatively, as shown in Figure 4, illustrating the embodiment of the invention employing an extruder in the polymerization stage of the invention, the molten prepolymer is introduced from conduit 46 to the top portion of wiped film reactor 86 and passes downwardly through the reaction zone in response to rotating blades (not shown) in reactor 86 which is operated in a temperature range of about 280O to 3100C. and at a pressure of about 1 to about 10 mn of mercury absolute. The polymer product from the bottom portion of reactor 86 passes through conduit 102 to the head of the vented extrusion reactor 104 and passes horizontally through said reactor in response to rotating screws (not shown) in reactor 104 which is operated at a temperature range of about 300 to about 3500C. and a pressure of above about 0.1 to less than about 1 mn of mercury absolute. The polymer product is removed from the process through conduit 106 from the end portion of reactor 104. Reactor 86 is provided with a spray condenser 90 which receives overhead vapor through conduit 88 and discharges condensed phenol through conduit 92 to distillate tank 94. The liquid effluent from distillate tank 94 is divided, and a portion is discharged from the process through conduit 98, while the re mainder is recycled through conduit 96 to the top of the spray condenser 90 for contact with additional vapor entering through conduit 88 fro reactor 86. The vapors from the spray condenser pass overhead through conduit 100 to a freeze trap (not shown). Ovcrhead vapor from reactor 104 is vented from reactor 104 through conduit 108 to a freeze trap (not shown). The continuous process of the invention will be described with respect to Figures 2, 3, and 4 of the drawings, utilizing as typical reactants, diphenyl isophthalate, diphenyl terephthalate and bisphenol A. Referring first to Figure 3, these reactants are introduced through conduit 10 to melt tank 12 which is sain- tained at about 1600 to 2100 C. to maintain the reactants in the molten state. Thorough mixing of the reactants can be obtained by circulating the mixture through conduits 14 and 18 and back to the melt tank 12. The nixture in the melt tank 12 is continuously purged with dry nitrogen to exclude oxygen. No significant reaction occurs at this stage. In accordance with the continuous process, a batch of the monomer mixture is transferred through conduit 16 to the stirred reactor 20. Catalyst is added directly to the stirred reactor 20 or with the molten mixture of monomers through conduit 17 so that the catalyst does not contact the molten monomers for any substantial time prior to onset of the polymerization reaction in the stirred reactor 20. The reaction is initiated by elevating the pressure and temperature in the stirred reactor 20. Stirred reactor 20 is operated at a temperature of about 200" to 300"C., and from 40 to 300 mm. Hg. with one hour residence time. About 60 percent of the total amount of phenol by-product is removed from stirred reactor 20. The viscosity of the reaction mixture is generally about 1000 cp. at 2400C. Phenol vapor produced in the reaction process and some unreacted monomer passes overhead from the stirred reactor 20 through conduit 24 to distillation column 26 where the overhead mixture is contacted with recycle phenol entering distillation column 26 through conduit 31. The distillation column removes any ilonomer untrained in the phenol vapor and returns the monomer to stirred reactor 20 through conduit 29. Phenol vapor passing overhead from the distillation column 26 passes through conduit 28 to the condenser 30. A portion of the recycled phenol is re turned to diitillation column 26 through conduit 31 and the re mainder is pissed through conduit 32 to distillate tank 34. The distillate li recirculated from tank 34 through conduit 38 and back to distillate tank 34. The vapor from the distillate tank is exhausted through conduit 36 to a freeze trap (not shown). The liquid product from the distillate tank 34 is removed from the process through conduits 40. The prepolymerization reaction is continued in stirred reactor 45, which is operated at a temperature in the range of about 250 to about 300 C., and a pressure of about 10 to 50 mm. Hg. absolute has a one hour residence time. Thirty percent of the total renlovable phenols is removed during this step of the process to produce a reaction mixture having a melt viscosity of about 10,000 cp at 280 C. The intrinsic viscosity of the reaction mixture at this stage of the process is about 0.18 dl./g. The reaction mixture or prepolymer is removed from re actor 45 though conduit 46. The phenol vapor produced in reactor 45 and some unreacted monomer is passed overhead in conduit 21 to distillation column 25 when the overhead mixture is contacted with recycle phenol entering distillation column 25 through conduit 35. The dis tillation column 25 removes any monomer entrained in the phenol vapor and returns the monomer to reactor 45 through conduit 23. Phenol vapor passing overhead from distillation 25 passes through conduit 27 to the condenser 33. A portion of the recycled phenol is returned to distillation column 25 through conduit 35 and the remainder is passed through conduit 37 and 32 to distillate tank 34 for processing as described hereinabove with respect of Figure 1. Referring again to Figure 2, the prepolymerization product is continuously passed through conduit 46 to wiped film reactor 48. In reactor 48, the molten prepolymer is introduced to the top portion of the reactor and passes downwardly through the reaction zone in response to slowly rotating blades in the wiped film reactor 48 which is operated in a temperature range of about 2800 to 3100C. and at a pressure of about 1 to about 10 mm of mercury absolute. The polymer product from the bottom portion of reactor 48 passes through conduit 64 to the upper portion of wiped film reactor 66, where the temperature is about 2900 to 3300C. and the pressure is 0.1 to 0.5 mm HG., and then downwardly through reactor 66 in response to the slowly turning blades of the wiped film reactor 66. The polymer product is removed from the process through conduit 84 from the bottom portion of reactor 66. Reactor 48 is provided with a spray condenser 52 which receives overhead vapor through conduit 50 and discharges condensed phenol through conduit 54 to distillate tank 56. The liquid effluent from distillate tank 56 is processed as described with respect to the semi-continuous process. In a similar manner, the vapor product of reactor 66 passes through conduit 68 to spray condenser 70, from whence the condensate passes through conduit 72 to distillate tank 74. The liquid effluent from distillate tank 74 is processed as described with respect to the semi-continuous process. Ethylene glycol is introduced to the process through conduit 82 to the distillate tank 74 from which the ethylene glycol is recycled through conduit 78 to the top of the spray condenser 70 where the ethylene glycol assists in condensing and dissolving the phenol vapors. Additional portions of the ethylene glycol are circulated through conduit 76 to distillate tank 56 and thence through recycle conduit 58 to the top of spray condenser 52 where a similar function is served. Alternatively, as shown in Figure 4, illustrating the embodiment of the invention employing an extruder in the polymerization stage of the invention, the molten prepolymer is introduced continuously from conduit 46 to the top portion of wiped film reactor 86 and passes downwardly through the reaction zone in response to rotating blades (not shown) in reactor 86 which is operated in a temperature range of about 2800 to 3100C. and at a pressure of about 1 to about 10 mm of mercury absolute. The polymer product from the bottom portion of reactor 86 passes through conduit 102 to the head of the vented extrusion reactor 104 and passes horizontally through said reactor in response to rotating screws (not shown) in reactor 104 which is operated at a temperature range of about 300 to about 3500C. and a pressure of above about 0.1 to less than about 1 mm of mercury absolute. The polymer product is removed from the process through conduit 106 from the end portion of reactor 104. Reactor 86 is provided with a spray condenser 90 which receives overhead vapor through conduit 88 and discharges condensed phenol through conduit 92 to distillate tank 94. The liquid effluent from distillate tank 94 is divided, and a portion is discharged from the process through conduit 98, while the remainder is recycled through conduit 96 to the top of the spray condenser 90 for contact with additional vapor entering through conduit 88 from reactor 86. The vapors from the spray condenser pass overhead through conduit 100 to a freeze trap (not shown). Overhead vapor from reactor 104 is vented from reactor 104 thrqugh conduit 108 to a freeze trap (not shown). Detailed Description of the Preferred Embodiments of the Invention A. Reactants The reaction of the process of the invention employes a diester of a dicarboxylic acid and a nionohydroxy aromatic compound of the benzene or naphthalene series of 6 to 20 carbon atoms. Examples of monohydroxy aromatic compounds are disclosed in EPC Applications 80301865.4 and 80301867.0 (Publications Nos. 0021652A and 0021654A). Preferred is a monohydroxy compound of the benzene series especially phenol. The dicarboxylic acids which are useful in the process are also well known and can be represented by the formula: EMI17.1 in which X is oxygen or sulfur, Z is alkylene, -Ar- or -Ar-Y-Arwhere Ar has the same definition as given with respect to the bisphenols, Y is alkylene of 1 to 10 carbons, haloalkylene, -0-, -S-, -SO-, -S02-, S03-, -CO-, EMI17.2 n is O or 1, and G is alkyl, haloalkyl, aryl, haloaryl, alkylaryl, haloalkylaryl, arylalkyl, haloarylalkyl, cycloalkyl, and halocycloalkyl. I1lustrative aromatic and aliphatic dicarboxylic acids are disclosed in US Patent 4,126,602, the disclosure of which is incorporated herein by reference. Most preferably, the dicarboxylic acid component comprises a mixture of about 60 to about 100 mol percent isophthalic acid and about 40 to about 0 mol percent terephthalic acid. Diesters of an aromatic dicarboxylic acid which, in general, are normally solids, i.e. solids at ambient temperature, are preferably employed. In carrying out the present process by the semi- or fully continuous process mode wherein liquid reaction mass is passes from one reaction vessel to another, it is much preferred to employ a dicarboxylic acid component comprising about 15 to about 85 mol percent isophthalic acid and about 85 to about 15 mol percent terephthalic acid. According to the invention, use of said proportion inhibits or prevents precipitation, i.e. crystallization, from the molten transesterification reaction mass of low molecular weight polyester, i.e. polyester oligomers, which precipitated oligorners can clog the conduits between the reaction vessels through which the molten mass flows. The bisphenols useful in the invention are disclosed in US Patent 4,126,602, the disclosure of which is incorporated herein by reference. Generally, the bisphenols which can be used in this process are known in the art and correspond to the general formula: EMI18.1 wherein Ar is aromatic, preferably containing 6-18 carbon atoms (including phenyl, biphenyl and napthyl); G is alkyl, haloalkyl, aryl, haloaryl, alkylaryl, haloalkylaryl, arylalkyl, haloarylalkyl, cycloalkyl, and halocycloalkyl; E is a divalent (or disubstituted) akylene, haloalkylene, cycloalkylene, halocycloalkylene, arylene, or haloarylene, -0-, -S-, -SO-, -SO2-, -SO3-, -CO-, EMI19.1 T and T' are independently selected from the group consisting of halogen such as chlorine or broinine, G and OG; m is an integer from 0 to the number of replaceable hydrogen atolls on E; b is an integer from 0 to the number of replaceable hydrogen atoms on Ar, and x is O or 1. When there is a plurality of G substituents in the bisphenols, such substituents may be the same or different. The T and Ti substituents may occur in the ortho, meta or parapositions with respect to the hydroxyl radical. The foregoing hydrocarbon radicals preferably have carbon atoms as follows: alkyl, haloalkyl, alkylene and haloalkylene or 1 to 14 carbons; aryl, haloaryl, arylene and haloarylene of 6 to 14 carbons; alkylaryl, haloalkylaryl, arylalkyl and haloarylalkyl of 7 to 14 carbons; and cycloalkyl, halocycloalkyl, cycloalkylene and halocycloalkylene of 4 to 14 carbons. Additionally, mixtures of the abovedescribed bisphenols nia; be er.p1Qved to obtain a polymer with especially desired properties. The bisphenols generally contain 12 to about 30 carbon atoms, and preferably 12 to about 25 carbon atoms. The bisphenols and biphenols are in general normally solids. The preferred bisphenol is commonly called bisphenol A. The conventional reaction practice for carrying out the transesterification preparation of the present polyesters is described in EPC Applications Nos. 80301865.4 and 80301867.0. There is generally employed a molar amount of dicarboxylic acid reactant, i.e. the diaryl ester, which is stoichiometrically equivalent to molar amount of the hydroxylic reactant, i.e. the bisphenol employed. If a carboxy,late ester group-teniliriated polyester product is desired, a molar excess of diaryl ester over the hydroxylic reactant, e.g. up to about 5 molar percent or more excess of the diaryl ester over the aforementioned stoichiometric equivalent proportion of diaryl ester is employed. Likewise, if b hydroxy group-tenilinated polyester is desired the bisphenol and any glycol modifier is charged in a similar molar excess over the aforementioned stoichiometrically equivalent proportion of the hydroxylic reactant is employed. If a specially end-capped polyerylate is desired, such as in aforesaid EPC applications Nos. 80301865.4 and 80301867.0, and also EPC application No. 80301866.2 (Publication No. 0021653A), the appropriate end-capping reagent is introduced at an appropriate time in the process. Since one or more of the aforementioned reactants is normally a solid it is generally necessary to melt the reactants prior to introduction of the reactants to the polymerization reaction which is carried out in molten state. Normally solid reactants can be individually melted and subsequently mixed together prior to their addition to the polymerization but it is preferred to mix the reactants together and subsequently melt the reactants since the mixture of solid reactants generally melts at a temperature lower than the melting point of many individual solid reactants. Use of the latter preferred embodiment generally conserves energy employed to melt the normally solid reactants. The invention is advantageously carried out employing above about a 0.05 to less than a 1.8 mol percent excess of the-diaryl ester reactant over the stoichiometric proportion thereof in relation to the hydroxy component used as is disclosed in the copending application of J. C. Rosenfeld et al., US Serial No. 198,980 filed October 21, 1980. B. Cala2ykt Also in accord with conventional transe.terification reaction practice, catalyst preferably present in the tralisesterification reaction of the invention is an acidic neutral or basic catalyst, such classifications are based on the reaction of a conventional acid-base indicator and the catalyst . cn the latter is dissolved in a polar ionizing solvent such as water. Preferably a basic catalyst is employed, such as disclosed in copending EPC application No. (Ref. N.33590), filed on even date herewith. Prior to its introduction into the reaction mass, the basic catalyst is preferably converted to liquid form, e.g. by melting or by dissolution in a liquid or normally solid, low melting solvent as described in this copending EPC application. Suitable basic catalysts include the alkali metals, such as lithium, sodium, potassium, rubidium, cesium and'francium; and the carbonates, hydroxides, hydrides, borohydrides, phenates, bisphenates, (i.e. salt of a bisphenol or bisphenol), carboxylates such as acetate or benzoate, oxides of the foregoing alkali metals. Group II and III elements can also be used in place of the alkali metals of the foregoing classes of compounds such as metals and compounds of calcium, magnesium and aluminum. Other bases include trialkyl or triaryl tin hydroxides, acetates, phenates, and the like. Examples of catalysts are lithium, sodium, potassium, rubidium, cesium and francium metals, potassium or rubidium carbonate, potassium hydroxide, lithium hydride, sodium borohydride, potassium borohydride, calcium acetate, magnesiuin acetate, aluminum tri-isopropoxide and triphenyl tin hydroxide. Phenol is the preferred solvent for the normally solid catalysts. Substituted phenols which can be used include thnim' having the formula EMI22.1 wherein K is alkyl of 1 to 10 carbon atoms, alkoxy of 1 to 10 carbon atoms, aryl of 6 to 10 carbon atoms, chloro, bromo or mixtures thereof, and wherein n is 1 or 2. Typical solvents include o-benzyl phenol, o-broino phenol, m-broluo phenol, m-chloro phenol1 p-chloro phenol, 2,4 dibroi:io phenol, 2,6 dichloro phenol, 3,5 dimethoxy phenol, o-ethoxy phenol, m-ethyl-phenol, p-ethyl-phenol, o-isopropyl phenol, m-methoxy phenol, m-propyl phenol, p-propyl phenol, and the like. Other solvents which are particularly useful are of the ether type, for example, tetrahydrofuran and the various glymes, for example, ethylene glycol dimethylether and the like; liquid phosphites such as triphenyl phosphite and tris(nonylphenyl)phosphite, and phosphates such as trimethyl or triethyl phosphate are also usefui as diluents. Combinations of catalysts and co,.lbinations of phenols or phenols and solvents may also be used. Especially preferred liquid basic catalysts are charged dissolved in molten normal solid-low melting organic solvent such as phenol. Especially preferred catalysts providing excellent results are the basic catalysts, rubidium phenoxide, potassium phenoxide, and potassium borophenoxide, each dissolved in molten phenol as described in the aforementioned copending EPC Application. Preferably also when a basic catalyst is employed, the catalyst is introduced concurrently with the aforementioned molten reactants to the polynerization to avoid heating the reactants in the presence of the catalyst prior to onset of the transesterification also as desribed in the copending EPC application. Less preferred catalysts include the metal oxides, nietal acetates, titanium and tin compounds. Suitable metal oxides include antimony trioxide, germanium oxide. arsenic trinyide, lead oxide, magnesium oxide, and zinc oxide. Suitable metal acetates include cobalt acetate, zinc acetate, cadmium acetate and manganese acetate. Suitable titanium compounds include the titanates such as tetrabutyl titanate and tetraisopropyl titanate. Suitable tin compounds include dibutyloxide, dibutyl tin methoxide and dibutyl tin dilaurate. In accord with conventional transesterification reaction practice a catalytically effective amount is employed for example about 0.005 to about 2 mol percent or more, preferably about 0.01 to 1 mole percent on the bisphenol in accordance with known techniques of transesterification. C. Prapolymerization¯Sttge for Semi-Continuous Transesterification In accordance with the aforementioned seri-continuous process embodiment of the invention, in the prepolymerization stage is carried out batchwise and polymerization stage is carried out continuously. The reaction in the prepolymerization stage to prepare a polyester oligomer corresponding to an Intrinsic Viscosity in the range of less than about 0.1 to about 0.3 dl/g and a melt viscosity up to about 100,000 centipoises can be carried out conveniently in one, two, three or more reaction tanks of conventional design which have conventional agitation means and which are connected in series. Preferably a sinyle agitated reaction tank is employed as reaction vessel for carrying out the prepolymerization reaction stage according to the semi-continuous process embodiment of the invention. The reaction tank or tanks are equipped with distillation means for removal overhead of the phenolic compound side product of the reaction which vaporizes at the temperature of the reaction mixture. Since some of the reactants may be entrained with the phenolic side product vapor stream, the removal system for the phenolic side product desirably includes means for fractional distillation and condensation of the phenolic overhead vapor stream so as to separate, condense and return entrained monomer to the reaction mixture of the prepolymerization stage. In semi-continuous operation, the phenolic overhead vapors of the prepolymerization stage and the phenolic overhead vapors from the polymerization stage are condensed in the presence of ethylene glycol or equivalent high boiling point solvent for phenol as described below in connection with the operation of the polymerization stage of the process. The phenolic overhead streams can be combined before the condensate step. The prepolymerization stage of semi-continuous mode of carrying out the process of the invention generally includes a holding or surge tank connected in series with the aforementioned batch tank reactor or reactors. This tank, which, if desired, can be equipped with agitation means, serves to collect and hold batches of molten oligomer including unreacted molten bisphenol and diaryl ester monomer for feed on a continuous basis to the polymerization stage. Holding tank is maintained at substantially the temperature of the immediately preceding tank to maintain the aforementioned reactants in molten state. In the semi-continuous mode of carrying out the present invention, the prepolymerization is carried out as a batch reaction at a reaction temperature of above about 1000 to about 3500C. preferably at about 1600 to 3300 especially at about 2000 to about 3000C. under diminished pressure in the range of from about atmospheric down to about 10 mn of mercury absolute, especially down to about 1.0 sgn of mercury or lower, to facili- tate removal of the phenolic side product with the temperature being raised and pressure being lowered as the reaction progresses. Conveniently, catalyst is added with ramping reaction temperature and staged diminution of the reaction pressure in the batch reactor or reactors of the prepolymerization stage in accordance with the semi-continuous process embodiment of the invention. D. Pre-polymerization Stage for Continuous Transesterification In fully continuous operation of the prepolymerization stage the agitated reaction tanks such as were used in the batchwise mode of operation are employed for carrying out the reaction on a substantially continuous basis. Preferably two agitated tank reaction vessels in series are employed for continuous operation of the prepolymerization stage. If desired, one or all of the conventional agitated tank reactors can be replaced by a mechanically agitated thin film reactor of the type more particularly described below which operates on co-current flow basis. In fully continuous operation of the process, the prepolymerization stage preferably employs a distillative means or system for removal of the phenolic side product of the transesterification and separation of entrained monomer which is separate from that employed for the polymerization stage. In the phenolic side product distillative removal system employed for continuous prepolymerization the stream of phenolic compound vapor (containing traces of entrained monomer) from each reaction vessel of the stage passes overhead to a fractionating column which separates and returns liquid entrained monomer to the reaction vessel, while phenolic vapor flows from the fractionation column, and passes through a condenser for collection as a liquid condensat , which then flows two a freeze trap wherein it is solidified. The surge or holding tank employed at the end of the batchwise prepolymerization stage is omitted so that the oligomer and unreacted bis phenol and diaryl ester is flowed directly to the polymerization stage described hereinbelow. Substantially similar reaction conditions to those described for the aforementioned batchwise operation of the prepolymerization are used except that ramping of the reaction temperature and pressure is desirably omitted. E. The Polymerization Stage One, two, three or more counter-current mechanically agitated thin film reactors in series are desirable for use in the polymerization stage of the present process which is carried out in substantially continuous fashion. As mentioned above, thin film reactors known for carrying out, on a continuous basis, reactions of viscous materials employ a rotary screw for mechanical agitation and transport of the material through the reactor as a thin film adherring to the walls of the reactor. The thin film reactors employed in invention generally provide for transport of a thin agitated film of less than about 0.1 mm to about 10 mm thickness preferably less than about 6 mm. Mechanically agitated thin film reactors are available of appropriate construction and motor horse power capacity for reaction of materials of varying viscosities. Mechanically agitated thin film reactors of light construction and of relatively low horsepower suitable for reaction of viscous materials of relatively low viscosities up to about 0.1 x 106 centipoises are available as Viscon Polymer Machines manufactured by Luwa Corp. under the designation "MK". Mechanically agitated thin film reactors of appropriate heavier construction and higher horsepower are available for reaction of more viscous materials, i.e. of viscosities above 0.1 x 106 centipoises are available as Filmtruder Polymer Machines manufactured and sold by Luwa Corp. under the designation "HM" (designed for use with moderately viscous material) and "HS", (designed for use with especially viscous materials.) Mechanically agitated thin film reactors of the types described above are available in designs for both counter-current and cocurrent removal of vapor from material which is treated in the thin film reactors. In the. polymerization stage of the present process counter-current flow mechanically agitated thin film reactors are employed so as to effect removal of phenolic side product countercurrent to flow of the reaction mass through the thin film reactor. Thin film reactors, especially mechanically agitated thin film reactors (both co-current and counter-current flow types) are described and illustrated in detail in the bulletin "Luwa Polymer Technology--Luwa Polymer Machines - HS-19", Luwa Corp. and F. Widmer, "The Treatment of Viscous Substances in Thin Film Apparatus", Luwa Corp. 1970. If desired a multi screw extrusion reaction vessel may be substituted for the final wiped thin film reaction vessel of the present process according to the embodiment of the invention described above to eliminate substantially from the product polyester gel and char particles and provide a product of consistent, even molecular weight. Such extruder apparatus typically includes an elongated, generally horizontal chamber, or barrel, the head or inlet end of which communicates with a feed means. An extrusion section such as an extrusion die (or nose plate) terminates the other extremity of the chamber. Intermediate the two extremities are means for applying vacuum and controlling the temperatilre. Ports or vents are provided for withdrawal of vapors. Within the chamber arid providing for the forward movement of the composition are mechanical means such-as one-or more screws appropriately mounted to impel forwardly the molten polyester reaction mass from the inlet end towards the nose plate and through the extruder. These screws force the molten polyester reaction mass forwardly even when it is highly viscous. During the volatilization of by-product, these screws work and knead residual polyester reaction mass in the chamber facilitating its handling and forward movement through the extruder die so as to avoid sub stantial entrainment of polyester within the extruder chamber. In general, in accordance with the invention, a twin screw extruder is employed wherein the screws rotate co-current to each other. More than one multi-stage extruder can be employed in series but it is preferred to employ a single extruder. Also, more than one wiped thin film reactor can preceed the extruder but the combination of one wiped film reactor followed by the present extrusion reactor is preferred for the polymerization stage of the present process. Extruders of the type employed in the practice of this embodiment of the invention are described generally in US Patent 3,335,111 of B. O. Pray et al., the disclosure of which is incorporated herein by reference. A preferred vented twin-screw extruder contemplated by the invention is similar to that described in aforementioned German Preliminary application 22 32 877.4 of K. Eise et al. The disclosure of the latter application is incorporated herein by reference. The' preferred extruder of the invention contains five vacuum zones although corresponding extruders containing 3 to 6 such zones may be used. Alternatively, but less preferably, in carrying out a fully continuous transesterification polymerization in accordance with the invention, other reactors equipped with heavy duty agitation means known in the art as suitable for treatment of highly viscous masses can be employed, such as: a "Diskpack" pdlymer processor reactor as described by Z. Tadmor et al., Plastics Engineering, November 1979, pages 20-25. In the polymerization stage of the present semi or fully continuous transesterification process, it is preferred to employ two counter-current flow mechanically agitated thin film reactors connected in series. Preferably both of the thin film reactors are of the aforementioned high shear type. If convenient, the first thin film reactor of this stage can be of the aforementioned type suitable for handling moderately viscous materials while the second thin film reactor is of the aforementioned high shear type. In accordance with the invention, the polymerization stage of the process is in general carried out at a temperature somewhat greater than that employed in the prepolymerisation in the range of about 150 to about 400 C. or higher, preferably at 2250 to about 3500C. and especially at about 1500 to about 3300C. with a reaction pressure being in the range of about 100 to about 0.1 mm of mercury absolute; the reaction temperature being raised and the reaction pressure being lowered in the course of the reaction in this stage of the process. In accord with especially preferred operation of the process of the invention employing two thin film reactors in the polymerization stage, the first thin film reactor operates at a temperature of about 2950 to about 305 C. especially at about 3000C.; at a reaction pressure of about 5 to about 15 mm of mercury absolute, especially at about 10 mlii of mercury absolute, at a rate of flow of molten reaction mixture through the reactor of about 4 to 16 kilograms per hour per square meter of thin film reactor-surface in the polymerization stage, especially about 8 to 12 kilograms per hour per squire meter and at the screw speed of about 10 to 200 rpm, especially a screw speed that yields rotor tip speeds of about 0.5 to 1 meter per second. In accord with foregoing especially preferred operation of the process of the invention, the second thin film reactor of the polymerization stage operates at a temperature of about 2800 to about 3250C., more preferably of about 3000 to 3150C. and especially of about 3050; â reaction pressure in the range of less than about 0.5 mm to about 5 mm. of mercury (absolute), preferably at less than about 0.5 to about 1.0 mm of mercury (absolute), at a rate of flow of reaction mixture substantially the same as that described for the preceeding thin film reactor of the stage, and at screw tip speed as described above. The final thin film reactor of the polymerization stage is desirably equipped with a heated die through which the molten polyester product is removed. Preferably the die is maintained at a temperature of about 100C. higher than that of the thin film reactor to facilitate removal of the product from the reactor. In accordance with the embodiment of the invention wherein a multi-screw vented extruder or extruders is used as the final reaction vessel of the polymerization, the extruder is generally operated under diminished pressure. The diminished pressure may follow a pressure profile or be substantially uniform. If a diminished pressure profile is employed in operating the extruder, the diminished pressure is highest at the feed end of the extruder and decreases along the extruder so that the lowest diminished pressure is at the outlet end of the extruder. Advantageously, the dinlinished pressure profile ranges from less than about 1.0 m;n of mercury absolute, at the feed erid of the extruder, to about 0.1 mm of mercury, absolute, at the outlet end of the extruder and preferably ranges from about 0.8 mm of mercury, absolute, to about 0.2 mm of mercury, absolute. Preferably, the diminished pressure is maintained substantially uniform in the extruder, advantageously in the range of from less than about 1.0 mm of mercury, absolute, to about 0.1 mm of mercury, absolute, preferably from about 0.8 mm of mercury, absolute, to about 0.2 mm of mercury, absolute. The extruder is operated at temperature conditions to provide a substantially uniform temperature to the molten reaction mass or melt in the extruder or alternatively, to provide a temperature profile for the melt in the extruder. The substantially uniform melt temperature in all zones of the extruder.is advantageously, in the range of at least about 2500C to about 390"C., preferably from about 300QC. to about 350"C. and especially from about 3200C. to about 3400C. Preferably, according to the invention, in operating the extruder, a melt temperature profile is employed wherein the lowest melt temperature is at the feed end of the extruder, and the melt temperature increases gradually or in stages along the extruder so that the highest melt temperature is at the outlet end of the extruder. According to this profile, the melt temperature, advantageously, ranges from about 2500C. at the feed end of the extruder to about 390"C. at the outlet end of the extruder, preferably from about 300"C. to about 350"C. and especially from about 320"C. to about 3400C The screw speed of the present extruder is generally about 50 to 200 rpm and preferably is about 100 to 150 rpm. If desired, additional monomer reactants can be introduced to the process in the polymerization stage especially by introduction in the first reaction vessel of said stage, preferably, however, unreacted monomers present ir the polyinerizotion stage are those which have not reacted in the prepolymerization, 'generally amount to less than about 10X by weight of the total monomer employed by weight. In coimnencing the continuous polymerization in the polymerization stage of the present process, to avoid overhead loss of oligomer, it is generally desirable to diminish the pressure in the thin film reactor or reactors gradually to reach the aforementioned reaction pressures employed in the reaction. In general in commencing the reaction in the polymerization stage it is convenient to diminish the pressure from atmospheric pressure to about 40 mm of mercury absolute at a rate of about 144 ril of mercury (absolute) per min. end subsequently to about 10 mm of mercury (absolute) at a rate of about 15 nini of mercury (absolute) per hour while heating the filled reactors to the temperature employed in the polymerization stage. When the process of the invention is carried out as fully continuous process, the fractional distillation means or system which is employed to remove phenolic side product vapor from the thin film reactor or reactors of this stage is desirably distinctive and separate from the corresponding fractional distillation means or system of the prepolymerisation stage. It is preferred to pass the phenolic compound vapor stream which is removed overhead frora each of the thin film reactor of this stage to.a spray condenser operating on a circulation loop for a solvent for phenol and vapor pressure depressant such as ethylene glycol or equiva lent solvent and vapor pressure depressant for the phenolic vapor. Other solvents include glycols such as ethylene glycol and higher glycols. Preferably, the glycol-circulating loops of each spray condenser operate independently of each other. F. Products of the Invention The processes of the invention provide products of improved, diminished color. The CO.Ot of the product of the semi-continuous process of the invention is unexpected in view of the aforementioned detrimental product color results which are obtained in carrying out semi-continuous polymerization employing an extruder in the polymerization stage. According to the aforementioned preferred embodiment of the invention which contemplates introducing a basic catalyst in liquid form to the polymerization concurrently with the liquid reactants, there is obtained unexpectedly a polyester of low polydispersity, D. D is defined by the following equation D = Mw, wherein Mw is the weight average molecular weight of Mn the polyester product and Mn is the number average molecular weight of the polyester product. Polydispersity, also known as polydispersity index, heterogeneity ratio, non-uniformity coefficient or dispersion ratio is discussed at page 78 of F.W. Billmeyer Jr., "Textbook of Polymer Science", Second Edition, Wiley Interscience, 1971; and at page 332 of E.M. McCaffery "Laboratory Preparation for Macromolecular Chemists", McGraw Hill Book Co., 1970. The polydispersity of a linear condensation polymer such as a polyester of a bisphenol and a dicarboxylic acid, is a known measure of the spread of the product molecules' molecular wciyht distribution. A tow value in the range of above about 2.0 to about 2.3 for D indicates a narrow molecular weight distribution and is known to be characteristic of linear products of batch condensation polymerization as indicated by K.G. Denbiytl, Trans. Faraday Soc. 43 648 (1947). A high value for D, e.g. ranging frora about 2.5 to about 6 or higher1 is known to be characteristic of linear products of continuous or semi-continuous condensation polymerization. Unexpectedly, the polyester product of the foregoing preferred embodiment of. the invention, although prepared employing a semi-continuous polyesterification reaction technique has a low polydispersity, i.e. a polydispersity indicative of a narrow molecular weight distribution, characteristic of a product of batch polymerization. The polyester product of low polydispersity obtained by the aforementioned preferred embodiment of the invention using a semi-continuous mode of reaction with addition of a liquid basic catalyst concurrent with addition of molten reactants is generally additionally characterized by enhanced impact strength, lower gel concentrations and lower free monomer concentrations, properties which heretofor could be obtained only with the corresponding polyester of low polydispersity prepared by the batch mode of transesterification polymerization. However, as is well known, batch mode phase preparation is commercially undesirable for large scale production of polymers such as the polyesters of bisphenols and dicarboxylic acid in comparison with the semi- or fully continuous polymer preparatory process. Products of outstanding properties are also produced by the continuous transesterification polymerization process. Moreover, the continuous mode of the present process permits the product polydispersity to be varied, i.e. increased or broadened, by change of the reaction conditions in contrast to the corresponding batch process. By varying reaction conditions which affect residence time distribution of the reacting mass, the product polydispersity can be varied. High molecular weight polyester, i.e. polyester of a molecular weight corresponding to a polyester intrinsic viscosity above about 0.5 dl/g, which is produced by the above-descri3rd etbodi- ment of the invention wherein a vented multi-screw extruder reactor is substituted for a wiped thin film reactor as final reaction vessel in the polymerization is characterized by the foregoing advantageous properties and, in addition, by substantial absence of small unsightly particles of brown gel and black char. High molecular weight polyester is generally produced at a substantially consistent molecular weight throughout semi- and fully continuous processes of the invention employing this embodiment. As used herein the polymer viscosities are as defined at pages 717-719 of "Encyclopedia of Polymer Science and Technology" edited by Mark and Gaylord, Volume 14, Interscience Publishers (Wiley and Sons) 1964. The following examples further illustrate the invention but are not intended to limit it. Various modifications can be made in the invention without departing from the scope and spirit thereof. Where not otherwise specified in this specification and claims, temperatures are given in degrees centigrade, and all parts, percentages and proportions are by weight. Illustrative Examples 'EXAMPLE 1 A. Catalyst Preparation Approximately 800 cc. molten phenol is charged to a oneliter 3-necked flask, put under a blanket of dry nitrogen with mechanical agitation and heated to reflux. The phenol is dis- tilled through al5 cm vigreaux column at atmospheric pressure and a center cut of about 500 cc. phenol is taken. 88.1 Grams of the distilled phenol is transferred to a 250 cc., 3-necked flask with a nitrogen inlet, mechanical stirrer and a y-tube with thermometer and a drying tube (filled with Drierite). The system is put under e blanket of dry nitrogen and heated to 70 C. 1.00 Gram KBH4 is added. After about 20 minutes, most of the solid is in solution, the temperature is 550C., and 3.23 grams more of KBH4 is added. After two hours more, the reaction temperature is at 71 C. and the solution is clear and water-white and is allowed to ccol. B. Prepolymer Preparation 4.61 Moles (1052.5 grams) bisphenol-A, 3.46 moles (1100.6 gn) diphenyl isophthalate, and 1.15 moles (366.9 gm) diphenyl terephthalate are dried overnight in a vacuum oven at about 750C. and several mm. Hg. and charged to a 5-liter resin kettle equipped with a nitrogen inlet, mechanical stirred thermometer and combination vigreax column-distillation head with a 3-liter flask cooled in ice-water as the receiver. The reaction mixture is heated to 1860C. with a heating mantel so that all reactants are in the liquid state. Then 2.5 cc (0.002305 mole catalyst) of the above-molten KSH4-phenol solution was added with a warmed syringe under a blanket of dry nitrogen. Phenol is removed steadily over about 3 hours at 1900 to 2550C. and under vacuum applied gradually down to about 1 r. Hg. The clear light yellow prepolymer is then poured into an aluminium foil-lined glass tray. Intrinsic viscosity is 0.19 dl/g (sym-tetrachloroethane, 30 C). The prepolymer is very light in color, slightly haze in appearance and brittle. 798.1 Grains of phenol is collected in the receiver. C. Polymerization The above prepolyter is crushcd and 1500 gn charged to a ?gallon, oil-heated, stirred reactor. The reactor is put under a blanket of nitrogen and left to sit overnight. Vacuum is applied to the reactor. The reaction mixture is heated to 2900C. over 2.1 hours (stirring at 5 rpm began at 1.0 hour and 1390C) at which point the vacuum is 0.6 mm ig. The polymerisation is continued under these conditions (temperature about 295 C., 0.6 mm, 5 rpm) for 4.3 hours. Then the reactor is opened and the viscous, light yellow, clear polymer removed. When cool, the polymer is clear and very tough and has an intrinsic viscosity of 0.61 (sym-tetrachloroethane, 300C). Examples 2 to 5 The process of Example 1 is repeated using different catalysts and conditions which are set forth in Table 1, together with the intrinsic viscosities of the resulting polyester products. TABLE 1 Catalyst Preparation Prepolymer Preparation Polymer Preparation Catalyst Time to Conc. Equiv. per Nominal Catalyst Intrinsic Time(hrs.) Intrinsic Example Ingredient Solvent Dissolve Temp. C, cc. Solution Catalyst Level(2) Viscosity at 290-300 C. Viscosity(3) 1 KSH4 Phenol 2.5 hr. 39-71 0.00093 Potassium 0.05 0.19 4.3 0.61 Borophenoxyde 2 Lithium Phenol 2.0 hr. Warmed 0.00051 Lithium 0.05 0.15 7.2 0.62 Tetrahydro- Phenoxyde furan(1) 3 NaoH Phenol 35 min. Room Temp. 0.00050 Sodium 0.05 0.17 6.0 0.57 to 130 C. 4 K2CO3 Phenol 50 min. Room Temp. 0.00050 Potassium 0.05 0.17 5.0 0.63 to 132 C. Phenoxide 5 Rb2CO3 Phenol 17 min. Room Temp. 0.00050 Rubidium 0.05 0.17 3.5 0.65 to 100 C. Phenoxide (1) 1521.2 ml THF and 284 g. phenol (2) mole percent based on moles bisphenol A (3) dl./g. measured in sym. tetrachlorethane at 30 C. Examples 6 to 10 compare the results of adding the catalyst before melting the reactants (Examples 6 and 7) with the inventive step of concurrent addition of catalyst and liquid reactants. Example 6 0.0500 mole (5.21 gm) neopentyl glycol, 0.0513 mole (11.60 gm) bisphenol-A and 0.1000 mole (31.80 gm) diphenyl terephthalate are charged to a small glass tube shaped reactor (with oilheated jacket) with a gas inlet, a mechanically driven glass stirrer with a flat, circular tip and a curved tube leading to a trap. The system is put under nitrogen blanket. 0.3 cc. (0.00015 mole) of a solution of lithium metal in phenol-tetrahydrofuran solvent is added to the reaction mixture and the mixture heated to 230"C. with stirring. Vacuum is applied gradually over about one hour at 2300C. to 1.15 mm. Hg. then heated for an additional 30 minutes at about 1.13 mm. The product is then removed from the reactor. It is light in color with a slight haze, but has some dark particles in it. Example 7 The reaction is set up and then heated to 230"C. as above and checked for appearance. Black particles are observed floating in the melted monomers and the run is terminated. Example 8 The monomers are charged as above but not the catalyst. The reaction mixture was heated to 210"C. and then the catalyst is added to the molten reactants and the polymerization begun. Phenol is removed over 1.25 hours with increasing vacuum at 230"C. to 2400C. and the product is clear (slight haze), light yellow. No black specks are observed. Intrinsic viscosity of the polymer is 0.33 dl/g (sym-tetrachloroethane, 30"C). A thin film polymerization is run by distributing 0.7 gm. of the above prepolymer (0.33 dl/g) around the bottom of a 250 cc. round-bottom flask and heating in an oil bath at 3100C. for 10 minutes,at,about 0.1 III lig. The resulting polymer is Scraped out of the flask while still soft. It is clear, light yellow and tough, with intrinsic viscosity of 0.59 dl/g (sym-tetrachloroethylene &commat; 300C). Example 9 Using 0.10 mole (22.81 gm) bisphenol A, 0.050 mole (15.92 g) diphenyl isophthalate and 0.50 mole (15.92 gm) diphenyl terephthalate are charged to the reactor as above and melted at 2100C. and then 0.3 cc. catalyst solution is added and the polymerization carried out'as above. No black specks are observed. The product had the following properties: Prepolymer; clear, light yellow, 0.27 dl/g Poylmer; clear, light yellow, 0.93 dl/g after 10 min. 310 C. clear, light yellow, 1.23 dl/g after 30 min. 310 C. Example 10 The process is repeated as in Example 9 using 0.1 cc catalyst solution. The following results are obtained: Prepolymer; light, cloudy, 0.29 dl/g Polymer; light, slightly hazy 0.58 dl/g after 30 min 310 C. light, slightly hazy 0.74 dl/g after 30 min. 3100C. Example 11 Following describes the process of the invention using the semi-continuous mode described in Figures 1 and 2 of the drawing: In an agitated 10 U.S gallon (38 dim3) melt tank, equipped with a turbine agitator and inert gas supply, 60.65 g-moles diphenyl isqalate, 20.25 g-moles of diphenyl terephthalaté and 81.25 g-moles of bisphenol-A are melted. This and all subsequent vessels/reactors are heated by circulating hot oil (Therminol 66) in their jacket in a controlled manner. Internal temperature of this melt tank is maintained at 1800C.+ 50C,A slow stream of dry nitrogen was purged through the melt tank throughout the run. The molten monomers are then transferred to a 10 U.S. gallon (38 dm3) stainless steel stirred tank reactor. The transfer is done by pressuring the melt tank to 2-3 atm (2-3 x 101 kPa) gauge and opening a bottom valve. A liquid catalyst for the process was prepared by forming a solution by reacting KBH4 with purified phenol. The resulting catalyst solution was employed at the level of 0.05 mol percent by KBH4 based on the moles of bisphenol-A. The catalyst is added to the stirred tank reactor in a liquid form. The internal temperature of the reactor is increased to 2800C. over a period of 2 hours and 15 minutes, while at the same time the internal pressure is reduced to 70 mm Hg. During this time the by-product phenol is continuously distilled off. At the end of this reaction time, a sample is collected and the intrinsic viscosity determined to be 0.175 + 0.005 dl/g. The material is then transferred to a stirred surge tank by pressurizing the reactor and opening the bottom valve. The holding tank is maintained at atmospheric pressure and 260 + 50C. and under constant nitrogen purge. The prepolymer is then continuously fed through a metering gear pump to the top feed port of a filmtruder, type HM, with a 5.4 square feet (0.5 mê) of heating surface, from Luwa Corp. The internal temperature, pressure and rotor speed maintained in this first filmtruder is 2950C., 4 mm Hg. and 140 rpm, respectively. The material is continuously pumped out of the bottom and fed to a second filmtruder of type HS of the same size. The internal temperature, pressure and rotor speed in the second filmtruder is maintained at 305 C., i mm Hg and 100 rpm, respectively. Under this conditions and at a capacity of 20lbs. (9kg)per hour a polyester with a weight average molecular weight of 27,800 and polydispersity of 2.2 is produced. The properties of the polyester product are as follows: Tensile Strength, psi 10158 (7.142xl06kg/m2) Tensile Modulus, psi 3.44 x 105 (24x106 kg/mê) Elongation at yield, percent 7.3 Heat Distortion Temperature O (unannealed) C 155.6 Izod Impact Strength ft/lb/inch (0.3m/0.45kg/2.54cm) of notch 4.8 Intrinsic Viscosity dl/g in symtetrachloroethane &commat; 30 C 0.51 Yellowness Index ca 27 Example 12 Following describes the process of the invention using the continuous mode as described in Figures 2 and 3 of the drawing. An essentially stoichiometric mix of reactants is continuously charged to the melt tank, and oxygen is excluded by suitable purging with nitrogen. A catalyst solution is prepared as in Example 11 and introduced to the first stirred reactor together with the liquid monomers. The reaction is conducted in the four reactors listed in Table 2 which are operated in series. Table 2 summarizes conditions used in each reactor at steady state. TABLE 2 REACTOR SUl11 & RY MEAN % PHENOL MELT VIS- TEMP. PRESSURE RESIDENCE TYPE REMOVAL COSITY, CP C mmHgA TIME, HR Melt Tank 0 50 200 758 1.5 Stirred Reactor 20 60.0 1,000 230 200 Stirred Reactor 45 31.2 10,000 280 20 1 Wiped Film 48 4.4 200,000 302 2 Wiped Film 66 2.0 4,000,000 320 0.4 Also included in Table 2 are the sequential degrees of phenol removal which are effected in each reactor. The process produces about 20 pounds (9kg) per hour of polyester having a final product I.V. of about 0.60 to 0.65 dl/g (sym-tetrachloroethane at 300C). EXAMPLES 13, 14 and 15 Mixtures of a 75/25 diphenyl isphthalate/diphenyl terephthalate mixture and bisphenol-A were vacuum oven dried for about 3 hours at 2 mm Hg in large flat trays then weighed into polyethylene-lined 5-U.S. gallon (19 dm ) pails and sealed until used. The amounts used in these Examples are presented in Table III below. The diphenyl isophthalate, diphenyl terephthalate, and bisphenol-A are charged to a stainless steel melt tank and melted at about 180 C under a blanket of dry rlitrOgell. The molten monomer mixture is transferred via heated lines to a continuously stirred oil heated reactor. Catalyst (potassium phenoxide, 0.041 molar solution in phenol, 0.00"1 moles, i.e. 0.05 mole percent based on bisphenol-A) is added. Vacuum is applied and the temperature (which is initially 220 ) is raised gradually. After about 2.5 hours, the pressure is about 70 mn and the temperature is 2804C. Intrinsic viscosity of the polyester prepolymer at this point is about 0.17 dl/g. The Prepolymer is transferred via heated lines to a second stirred reactor and another prepolymer is prepared similarly in the first stirred reactor. The prepolymer from the second stirred reactor is fed continuously at about 20-25 ibs (9-llkg)/bour to a vertical wiped thin film reactor which is maintainea at an internal temperatu?e of 300 C. and a pressure of 1.50 mu Hg (abs.). The reaction mass fed to the top of the wiped thin film reactor flows down through the reactor propelled by gravity and by the downward thrust of the pitched rotor blades. These blades also agitate and renew the polymer on the heated reaction surface of the thin film reactor. The material leaving the thin film reactor has an intrinsic viscosity of about 0.4 dl/g. The polymer is pumped out from the bottom of the wiped thin film reactor and fed to a 5-stage twin screw horizontal extruder having five vacuum vents (one from each stage). The extruder is operated at about 0.8 mm Hg (abs.) pressure, and a screw speed of 125 rpm. The pressure is maintained uniformly throughout the barrel, i.e. through the extruder. The temperature of the melt in the different stages of the extruder is maintained between, about 320"C and 340"C, the fonner temperature being the melt. temperature at the feed end of the extruder and the latter temperature being the melt temperature at the outlet end of the extruder. The aforementioned melt temperature profile within the extruder is controlled by fl!u'iiii.dininrj the following three temperature zones on the outside of the extruder barrel: zone 1 (near the feed end of the extruder): 3050; zone 2 (near the mid-section of the extruder): 3100C; zone 3 (near the out- let end of the extruder): 3150C. Under these operating conditions, light yellow, clear, tough polymer is produced at a rate of 20 libs (9 kg)/hour. The product is essent,ially free of black char and brown polyester gel particles in accordance with the teach 'ing of the present invention. The foregoing semi-continuous polyester preparatory process is initially run with a 0.5 mole percent excess of the diaryl ester reactant over the stoichiometric proportion corresponding to the molar amount of the bisphenol used. After a steady reaction is reached, five samples of polyester product are removed at two hour intervals and tested for polyester intrinsic viscosity and Yellowness Index color. The statistical deviation in results for the intrinsic viscosity and yellowness index color of the samples is also determined. This data is reported as Example 13 in Table III below. The proportion of the diaryl ester reactant charged to the process is then adjusted to provide a mole percent excess of 0.75 of the diaryl ester. After the reaction has reached a steady state, the foregoing product sampling and testing procedures are repeated substantially as described. The resultant intrinsic viscosity and yellowness index color data are reported as Example 14 in Table III below. The proportion of the diaryl ester reactant charged to the process is then adjusted-to provide a 1.0 mole percent,excesCj of the diaryl ester monomer. After the semi-continuous reaction has again reached a steady operating state, the foregoing product sampling and testing procedure is repeated substantially as described. The resultant data is reported in Table III below as Example 15. The low standard elevation obtained for the average Intrinsic Viscosity results in Examples 13, 14 and 15 is illustrative of the consistency of the product molecular weight as achieved in carrying out semi-continuous production of the polyester employ -;ng the extrusion reaction vessel as the last reaction vessel of the polymerization according to the invention. The invention has been described in the above specification and illustrated by reference to specific embodiments in the illustrative examples. However, it is to be understood that these embodiments are not intended to limit the invention since, changes and modifications in the specific details disclosed hereinabove can be made without departing from the scope and spirit of the invention. TABLE III PART A Amounts of Reactants Used (moles, weight (KG)) Example 13 Example 14 Example 15 Bisphenol-A 16.25 (3.710) 16.25 (3.710) 16.25 (3.710) Mixture of 75% Diphenyl Isophthalte/ and 25, Diphenyl Terephthalate charged 16.33 (5.199) 16.37 (5.211) 16.41 (5.224) Mole-% Excess Diphenyl Isophthalate and Diphenyl Terephthalate Employed 0.50 0.75 1.0 PART B: Intrinsic Viscosity and Yellowness Index Color of Product Samples Example 13 Example 14 Example 15 I.V. (dl/g)Ú Color (Y.I.)ê I.V. (dl/g) Color (Y.I.) I.V.(dl/g) Color (Y.I) Sample 1 0.56 37 0.49 26 0.45 19 2 0.56 38 0.50 28 0.50 22 3 0.58 35 0.54 34 0.50 22 4 0.58 38 0.52 35 0.50 30 5 0.58 37 0.53 27 0.54 30 6 -- -- -- -- 0.49 28 Average 0.57 37 0.52 30 0.50 25 Standard Deviation of Average Resultant #0.01 #1 #0.02 #4 #0.03 #5 1 Intrinsic viscosity determined at 30 C in 1,1,2,2, tetrachlorethane o at a concentration of 0.5 gm of polymer per 100 cc of solution of polymer. 2 Yellowness index of a compression molded 1/8" (0.3 cm) plate of polymer (Hunterlab Spectrophotometer). Yellowness index color is defined in ASTM D-124-70.

Claims: CLAIMS 1.- A process for preparing a linear aromatic polyester by transesterification polymerization of monomer reactants consisting essentially of a bisphenol and a diaryl ester of a dicarboxylic acid in a polymerization reaction zone which comprises reacting said bisphenol and said diaryl ester in a first reaction stage of said zone to prepare a polyester oligomer, and thereafter continuing the reaction in a second reaction stage of said zone to produce a polyester product, characterised in that the reaction is carried out in the first reaction stage either batchwise or substantially continuously and is continued in the second reaction stage substantially continuously, with the proviso that when the reaction in the first reaction stage is carried out batchwise, the reaction in the second stage is carried out in a wiped film reaction zone. 2. A process for preparing a linear aromatic polyester by transesterification polymerization of monomer reactants consisting essentially of a bisphenol and a diaryl ester of a dicarboxylic acid in a polymerization zone, said zone comprising a series of at least two reaction stages1 characterised in that the polymerization reaction in te final stage of the series is carried out continuously in a wiped film reaction zone. 3. A process according to claim 1 or 2 wherein polymerization is a wiped film reaction zone is followed by polymerization in a vented extrusion reaction zone. 4. A process according to claim 1, 2 or 3 wherein the wiped film reaction zone provides for countercurrent flow of reaction mixture and vapor by-product of the reaction. 5, A process for preparing a linear aromatic polyester by transesterification polymerization of monomer reactants consisting essentially of a bisphenol and a diaryl ester of a dicarboxylic acid in a polymerization reaction zone, characterised by introducing said reactants substantially continuously to said polymerization reaction zone, and withdrawing a polyester product substantially continuously from the polymerization zone. 6. A process according to any one of the preceding claims wherein the polymerization is conducted in the presence of a basic catalyst that is normally solid at 300C but which is introduced to the polymerization process in liquid form. 7. A process according to any one of the preceding claims wherein the polymerization is conduced in the presence of a basic catalyst that is introduced into the polymerization process substantially concurrently with the introduction of liquid state reactants. 8. "A A process according to claim 6 or 7 wherein the catalyst is introduced in molten form or in solution in a solvent. 9. A process according to any one of the preceding claims wherein the bisphenol is bisphenol A and the diaryl ester is diphenyl isophthalate and/or diphenyl terephthalate. 10. A process according to any one of the preceding claims wherein the diaryl ester is a diaryl ester of a mixture of 15 to 85 mole percent isophthalic acid and 85 to 15 mole percent terephthalic acid. 11. A process according to any one of the preceding claims wherein there is used as polymerization catalyst an alkali metal phenoxide selected from potassium phenoxide, rubidium phenoxide, lithium phenoxide and sodium phenoxide, or a reaction product of potassium borohydride and phenol.

Inventors: KOSANOVICH, GEORGE M., SALEE, GIDEON
Applicants: OCCIDENTAL CHEMICAL CORPORATION
Application year: 1981
Main classification: C08G63
IPC codes: C08G63