Process for preparing high molecular weight polycarbonates
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a novel process for preparing high molecular weight polycarbonates. 
     2. Description of the Prior Art 
     There are known various processes for producing aromatic polycarbonates such as a process of reacting a dihydroxy diaryl alkane with phosgene in the presence of an acid acceptor such as an aqueous alkali, a tertiary amine and the like, and a solvent (hereinafter referred to as the &#34;phosgene process&#34;), a process of interesterification between a dihydroxy diaryl alkane and a di-ester of carbonic acid (hereinafter referred to as the &#34;interesterification process&#34;) and the like. 
     The phosgene process can produce polycarbonates of a desired degree of polymerization ranging from a low degree of polymerization to a high degree of polymerization, but in producing an aromatic polycarbonate this process gives an extremely high solution viscosity and therefore a lot of labor and a long time are necessay for purification such as washing, neutralization and the like, and the purification can not be conducted sufficiently. 
     In addition, when the solvent is removed by simply heating or reducing the pressure to produce a solid resin from the resulting resin solution, the concentrated resin solution becomes paste-like resulting in difficult handling, and therefore, a complicated procedure is required, such as, for example, adding a precipitating agent to precipitate the resin and then removing the solvent and the precipitating agent remaining in the resin. 
     When the interesterification process is employed for producing high molecular weight polycarbonates, such various complicated procedures after the reaction step as in the phosgene process are not required, but in general, heating and melting for a long period of time are necessary so that there occur various disadvantages such as coloring, deterioration and diminution of physical properties of the resulting resin. 
     In addition to the above mentioned two fundamental preparation processes, there have been proposed processes where a low molecular weight polymer (prepolymer) is firstly produced and then polymerized further as disclosed in Japanese Patent Publication Sho-36-13942, Sho-37-12550 and Sho-47-14742. 
     Japenese Patent Publication Sho-36-13942 discloses a process for producing high molecular weight polycarbonates comprising using a phenol as a chain terminater, adding a dihydroxy diaryl alkane to a polycarbonate prepolymer obtained by a phosgene process, and heating and polycondensing them under a reduced pressure. 
     Japanese Patent Publication Sho-37-12550 discloses a process for preparing a high molecular weight polycarbonate which comprises removing unreacted dihydroxy diaryl alkanes from a polycarbonate prepolymer interesterification, then adding an diphenylcarbonate and an interesterification catalyst and heating and melting under vacuum. 
     Japanese Patent Publication Sho-47-14742 discloses a process for producing a high molecular weight polycarbonate which comprises adding a quaternary ammonium compound as a polycondensation catalyst to a polycarbonate prepolymer obtained by an interesterification and heating and melting under vacuum. 
     However, the melting reaction in the latter half step of each of these three is the same as the polycondensation reaction in conventional interesterification processes and the reaction mechanism is just the same as that of the interesterifying process, and therefore, a long melting time is required to produce a high molecular weight polycarbonate according to these processes, just as is required in the conventional interesterifying processes. 
     For example, Japanese Patent Publication Sho-36-13942 reports that the melting reaction time is as long as 5.5 hours and thereby the product becomes yellowish brown or brown. 
     In Japanese Patent Publication Sho-37-12550 and Sho-47-14742, it takes 2-3 hours for the latter half polycondensation and 6-7 hours for the total melting reaction to produce a high molecular weight polycarbonate. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided a process for preparing a high molecular weight polycarbonate comprising the reaction of phosgene with a dihydroxy diaryl alkane in the presence of an acid acceptor and a solvent, which comprises adding a monohydric phenol and at leaast one member selected from the group consisting of the compounds having the following formulas: ##STR1## where R is a hydrocarbon residue having 1-10 carbon atoms selected from the group consisting of saturated aliphatic hydrocarbon residues, aliphatic hydrocarbon residues having an aromatic group substituent and alicyclic hydrocarbon residues; 
     X is chloro or bromo; 
     R&#39; and R&#34; are similar or dissimilar and are hydrocarbon residues having 1-9 carbon atoms selected from the group consisting of a saturated aliphatic hydrocarbon residues, aliphatic hydrocarbon residues having an aromatic group substituent and alicyclic hydrocarbon residues; 
     Y is hydroxy, chloro or bromo; 
     A +  is an alkali metal ion or an ammonium ion derived from a tertiary amine; 
     M and M&#39; are similar or dissimilar and are selected from the group consisting of alkyl of C 1  -C 5 , chloro and bromo; and n and n&#39; are similar or dissimilar and are 0, 1, 2 or 3; to produce a polycarbonate prepolymer having an intrinsic viscosity of 0.01-0.40, and heating and melting the prepolymer under a reduced pressure to cause the polycondensation. 
     An object of the present invention is to provide a process for preparing a high molecular weight polycarbonate free from the drawbacks of the prior art as mentioned above. 
     Another object of the present invention is to provide a process for preparing a transparent and colorless polycarbonate of a high molecular weight within a short time. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     According to the present invention, the dihydroxy diaryl alkane and phosgene are usually reacted at a temperature of 0°-80° C, and preferably at 10°-40° C. 
     The dihydroxy diaryl alkane used in the present invention is a diaryl alkane having two phenolic hydroxy groups. 
     Representative dihydroxy diaryl alkanes used in the present invention are bis(hydroxyphenyl) alkanes and aryl alkyl hydroquinones, preferred with the bis(hydroxyphenyl) alkane having the formula ##STR2## where R 1  is an alkylene or alkylidene of not more than 5 carbon atoms, R 2  and R 3  are similar or dissimilar and selected from the group consisting of alkyl of not more than 4 carbon atoms, phenyl, chloro and bromo, and m and n are similar or dissimilar and selected from 0, 1 and 2. 
     Examples of the bis(hydroxyphenyl)alkanes are: 
     bis(2-hydroxyphenyl) methane, 
     bis(4-hydroxyphenyl) methane, 
     1,1-bis(4&#39;-hydroxyphenyl) ethane, 
     2,2-bis(4&#39;-hydroxyphenyl) propane, 
     2,3-bis(2&#39;-hydroxyphenyl) butane, 
     3,3-bis(4&#39;-hydroxyphenyl) pentane, 
     2,2-bis(2&#39;,6&#39;-dimethyl-4-hydroxyphenyl) propane, 
     2,3-bis(2&#39;-t-butyl-4&#39;-hydroxyphenyl) pentane, 
     2,2-bis(3&#39;,5&#39;-dichloro-4&#39;-hydroxyphenyl) propane, and 
     1,4-bis(2&#39;-phenyl-4&#39;-hydroxyphenyl) butane. 
     Among them, 2,2-bis(4&#39;-hydroxyphenyl) propane is preferable because it is easily available and inexpensive. 
     The dihydroxy diaryl alkanes may be used alone or in combination. 
     The monohydric phenols used upon producing the polycarbonate prepolymer are phenol and nuclear substituted phenols, and in other words, are monofunctional compounds having one phenolic hydroxy group. 
     Representative monohydric phenols are: 
     phenol, 
     o-cresol, 
     m-cresol, 
     p-cresol, 
     2,6-xylenol, 
     2,4,6-trimethyl phenol, 
     p-t-butylphenol, and 
     2,4,6-tribromophenol. 
     Representative monohydric alcohols of Formula (1) are methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, t-butanol, n-hexanol, 2-hexanol, 3-hexanol, 2-ethylhexanol, chlorobutanol, cyclopentanol, cyclohexanol, and benzyl alcohol. 
     Representative haloformates of Formula (2) are methyl chloroformate, ethyl chloroformate, isopropyl chloroformate, 2-ethylhexyl chloroformate and methyl bromoformate. 
     Representative monothiols of Formula (3) are methyl mercaptan, ethyl mercaptan, benzyl mercaptan, and cyclohexyl mercaptan. 
     Representative halothioformates of Formula (4) are methyl chlorothioformate, ethyl chlorothioformate, benzylchlorothioformate, and cyclohexyl chlorothioformate. 
     Representative monocarboxylic acids of Formula (5) where Y is hydroxy are acetic acid, propionic acid, n-butyric acid, isobutyric acid, n-octylic acid, iso-octylic acid, cyclohexane carboxylic acid, phenylacetic acid, and hydrocinnamic acid. 
     Representative monocarboxylic acids of Formula (8) where Y is hydroxy are benzoic acid, o-toluic acid, and p-toluic acid. 
     Representative monocarboxylic acid halides of Formula (5) where Y is chloro or bromo are acetyl chloride and propionyl chloride. 
     A representative monocarboxylic acid halide of Formula (8) where Y is chloro or bromo is benzoyl chloride. 
     Representative monocarboxylic acid salts of Formula (6) are sodium acetate and pyridine acetate. 
     Representative monocarboxylic acid salts of Formula (9) are sodium benzoate and pyridine benzoate. 
     Representative monocarboxylic acid anhydrides of Formula (7) are acetic anhydride, n-butyric anhydride, isobutyric anhydride and phenylacetic anhydride. 
     Representative monocarboxylic acid anhydrides of Formula (10) is benzoic anhydride. 
     The compounds of Formulas (1) and (2) are most preferable and inexpensive and the rate of reaction is very rapid. The compounds of Formula (3) and (4) give a rate of reaction similar to that of compounds of Formulas (1) and (2), but they have somewhat undesirable odors. The compounds of Formulas (5), (6) and (7) attach to the polymer ends more strongly than compounds (1) and (2) and therefore, the reactivity is lower than that of compounds (1) and (2). The compounds (8), (9) and (10) are less volatile than compounds (5), (6) and (7) and therefore, the rate of reaction thereof is lower than that of compounds (5), (6) and (7). 
     When the number of carbon atom in R, R&#39; and R&#34; is higher than the upper limit, the volatility of products derived from the polymer ends of the prepolymer upon melting polycondensation is so low that the rate of reaction is disadvantageously slow. 
     According to the present invention, the amount of the monohydric phenol added upon preparing the polycarbonate prepolymer may be added in an optional amount, but when a large amount of the monohydric phenol is used, the rate of reaction at the latter half of the melting polycondensation reaction is lowered to a great extent so that a preferable amount of the monohydric phenol is 0.1-5 mole % and it is more preferred with 1-4 mole % based on the dihydroxy diaryl alkane. 
     The compounds represented by Formulas (1)-(10) added together with the monohydric phenol may be also used in an optional amount. Preferably the amount of said compound is 0.5-100 mole % based on the dihydroxy diaryl alkane, and more preferably said compound is in an amount of 0.5-40 mole % based on the dihydroxy diaryl alkane. 
     When the compound of Formulas (1)-(10) is used in an amount of more than 100 mole %, upon producing the polycarbonate prepolymer the compound of Formulas (1)-(10) reacts with phosgene to form a large amount of by-products such as low boiling dialkyl carbonates, dialkyl thiocarbonates and the like. Thus, an excess amount of phosgene is consumed and further upon the melting polycondensation reaction, a large amount of low boiling materials are contained in the prepolymer and therefore, the degree of vacuum is disadvantageously lowered. 
     On the contrary, when the amount of each of the compounds represented by Formula (1)-(10) is less than 0.5 mole % based on the dihydroxy diaryl alkane, the melting polycondensation reaction is not effectively accelerated since sufficient residues of these compounds available to the polymer ends of the polycarbonate prepolymer. 
     The molar ratio of a compound of Formulas (1)-(10) to the monohydric phenol ranges from 1000:1 to 0.1:1, preferably from 50:1 to 0.3:1, more preferably from 10:1 to 0.3:1. 
     As an acid acceptor, there may be used a 5-10% (by weight) aqueous alkali solution such as aqueous sodium hydroxide, potassium hydroxide and calcium hydroxide, or a tertiary amine such as pyridine. 
     As the solvent, there may be used an organic solvent, provided that it is insoluble or difficultly soluble in water and does not react with phosgene. It is preferable that the organic solvent can dissolve the dihydroxy diaryl alkane, monohydric phenol, compound of Formula (1)-(10) and the resulting polycarbonate prepolymer to some extent. Such preferable solvents are, for example, methylene chloride, chloroform, carbon tetrachloride, tetrachloroethane, chlorobenzene, and xylene. 
     Phosgene may be added to the reaction system of one of various ways such as blowing a gaseous phosgene into a mixture of dihydroxy diaryl alkane, an acid acceptor, chain terminator, and solvent; adding a phosgene dissolved in a solvent to the mixture; and the like. 
     The monohydric phenol and the compound of Formula (1)-(10) may be added to the reaction system at any time before, during and after the reaction with phosgene. 
     However, the monocarboxylic acid and the monocarboxylic acid salt in Formula (5)-(10) are preferably added before or during the reaction with phosgene. If they are added after the reaction is completed, the monocarboxylic acid residue can not be sufficiently attached to the polymer ends of polycarbonate prepolymer and therefore, the melting polycondensation reaction is not sufficiently accelerated in some cases. 
     When a tertiary amine is used as an acid acceptor, it is preferable to add the monohydric phenol and the compound of Formula (1)-(10) before the reaction or during the first half period of the reaction with phosgene so as to cause effectively the reaction of the added starting materials. 
     The degree of polymerization of the polycarbonate prepolymer may be adjusted by controlling types of the monohydric phenol and the compound of Formula (1)-(10), combination thereof, amounts thereof and the reaction condition appropriately. Preferably the intrinsic viscosity is 0.01-0.40, more preferably, 0.05-0.40. 
     When the intrinsic viscosity of polycarbonate prepolymer is higher than 0.40, it is difficult to conduct a purification of a solution of the polycarbonate prepolymer such as washing and neutralization and remove the solvent. On the contrary, when the intrinsic viscosity is lower than 0.01, the polycarbonate prepolymer vaporizes to a great extent in the melting polycondensation reaction and therefore, such range of the intrinsic viscosity is not preferable. 
     The resulting solution of the polycarbonate prepolymer may be purified by washing and neutralizing, and then concentrated or dried to give a polycarbonate prepolymer in a form of jelly, paste, block, crystal, powder or particle, and the resulting matter may be heated to effect the melting polycondensation under a reduced pressure and, if desired, 0.00005-0.1% by weight of a polycondensation catalyst. 
     As the polycondensation catalyst, there may be mentioned preferably a basic compound such as oxides, hydroxides, carboxylic acid salts, alcoholates, phenolates, and hydrides of an alkali metal and an alkaline earth metal. 
     The melting polycondensation reaction may be effected at 220°-320° C and under a reduced pressure, that is, preferably at not higher than 40 Torr. at the beginning of the reaction and at not higher than 10 Torr., and preferably not higher than 1 Torr., towards the end of the reaction. 
     If necessary, the reaction vessel may be flushed with nitrogen gas and the reaction may be carried out in a nitrogen atmosphere and the reaction mixture may be stirred. 
     When the polycondensation proceeds up to a certain degree of polymerization, a neutralizer may be added to deactivate the polycondensation catalyst, and a stabilizer, a dye, a pigment, a filler or an ultraviolet ray absorber may be incorporated therein. 
     After the completion of the melting polycondensation, the resulting molten resin is usually extruded to a strand or plate form and then cut to produce pellets. 
     Hereinafter there is shown an excellent effect of the present invention, i.e. acceleration of a melting polycondensation. The polycarbonate prepolymer having residues of the chain terminater according to the present invention at the polymer ends can be converted to a high polymer in a very short time in the polycondensation reaction. 
     Table 1 below shows a result of comparison experiments from which the excellent acceleration of the melting polycondensation reaction is clear according to the present invention. 
     As a polycondensation reaction catalyst, 0.0010% by weight of potassium benzoate was added to each of the polycarbonate prepolymers No. 1-No. 8 below heated and melted in a glass tube at 1.0 Torr. at 300° C for 60 minutes and the difference between the intrinsic viscosity before the melting polycondensation and that after the melting polycondensation, which corresponds to the rate of reaction is compared with each other. 
     No. 1: Bisphenol A polycarbonate prepolymer where 7.5 mole %, based on bisphenol A, of phenol is used as a chain terminator 
     No. 2: Bisphenol A polycarbonate prepolymer where 7.5 mole %, based on bisphenol A, of p-t-butylphenol is used. 
     No. 3: Bisphenol A polycarbonate prepolymer where 3 mole %, based on bisphenol A, of phenol and 3 mole %, based on bisphenol A, of methyl chloroformate are used. 
     No. 4: Bisphenol A polycarbonate prepolymer where 3 mole %, based on bisphenol A, of p-t-butylphenol and 3 mole %, based on bisphenol A, of methanol are used. 
     No. 5: Bisphenol A polycarbonate prepolymer where 3 mole %, based on bisphenol A, of p-t-butylphenol and 3 mole %, based on bisphenol A, of isopropyl chloroformate are used. 
     No. 6: Bisphenol A polycarbonate prepolymer where 3 mole %, based on bisphenol A, of p-t-butylphenol and 3 mole %, based on bisphenol A, of methylmercaptan are used. 
     Np. 7: Bisphenol A polycarbonate prepolymer where 3 mole %, based on bisphenol A, of p-t-butylphenol and 3 mole %, based on bisphenol A, propionyl chloride are used. 
     No. 8: Bisphenol A polycarbonate prepolymer where 3 mole %, based on bisphenol A, of p-t-butylphenol, 1 mole %, based on bisphenol A, of methanol, 1 mole %, based on bisphenol A, of methyl mercaptan and 1 mole %, based on bisphenol A, of acetyl chloride are used. 
     
                                           Table I                                 
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          Chain terminator    Result of                                   
          used for preparation                                            
                              melting polycondensation                    
          of prepolymer       Intrinsic                                   
                       Amount viscosity                                   
                                     Rate                                 
                       (Mol %  η!                                     
                                     of                                   
                       based on                                           
                              (a)*   reaction                             
                                            Example                       
       No.                                                                
          Kind         bisphenol A)                                       
                              (b)**  Δ η!/hr.                   
                                            No.                           
__________________________________________________________________________
Comparison                                                                
       1  Phenol       7.5    (a) 0.278                                   
                                     0.030  Comparison                    
example                       (b) 0.308     example 1                     
       2  p-t-butylphenol                                                 
                       7.5    (a) 0.305                                   
                                     0.017  Comparison                    
                              (b) 0.322     example 2                     
       3  Phenol       3      (a) 0.371                                   
                                     0.635  Example 1                     
          Methylchloroformate                                             
                       3      (b) 1.006                                   
The    4  p-t-butylphenol                                                 
                       3      (a) 0.382                                   
                                     0.585  Example 8                     
present   Methanol     3      (b) 0.967                                   
invention                                                                 
       5  p-t-butylphenol                                                 
                       3      (a) 0.353                                   
                                     0.295  Example 11                    
          Isopropylchloroformate                                          
                       3      (b) 0.648                                   
       6  p-t-butylphenol                                                 
                       3      (a) 0.362                                   
                                     0.317  Example 12                    
          Methylmercaptan                                                 
                       3      (b) 0.679                                   
       7  p-t-butylphenol                                                 
                       3      (a) 0.383                                   
                                     0.152  Example 16                    
          Propionylchloride                                               
                       3      (b) 0.535                                   
          p-t-butylphenol                                                 
                       3                                                  
       8  Methanol     1      (a) 0.344                                   
                                     0.386  Example 19                    
          Methylmercaptan                                                 
                       1      (b) 0.730                                   
          Acetylchloride                                                  
                       1                                                  
__________________________________________________________________________
 *(a) : Before the reaction (Polycarbonate prepolymer)                    
 **(b) : After the reaction (Product obtained by the melting              
 polycondensation reaction)                                               
 
    
     It is well known that aromatic polycarbonate resins have excellent heat resistance and impact strength when the intrinsic viscosity is not lower than 0.45 (the average molecular weight being 19,500) while the physical properties are lowered to a great extent when the intrinsic viscosity is lower than 0.45. 
     As is clear from Table 1 above, the intrinsic viscosity of the polycarbonate prepolymer, having residues of the chain terminator at the polymer ends according to the present invention, increases abruptly by heat melting under a reduced pressure and thereby the intrinsic viscosity of each No. 3-No. 8 far exceeds the above mentioned critical value, 0.45, in one hour. On the contrary, in cases of No. 1 and No. 2 (Comparison Examples) the rate of reaction of the polycarbonate prepolymer is so low that a further long heating and melting is necessary to attain the critical value, 0.45. This indicates clearly the remarkable acceleration of the melting polycondensation reaction caused by the residue of the chain terminator attached to the polymer end of polycarbonate prepolymer according to the present invention. 
     The process of the present invention uses a polycarbonate prepolymer as an intermediate so that the purification procedure such as washing and neutralizing is very easy, and furthermore, since the solid matter containing only a little amount of a remaining solvent can be obtained only by evaporating off the solvent and further, the resulting solid matter can be easily ground. Therefore, the solid matter can be simply produced and the productivity increases and the manufacturing apparatus can be rationalized. 
     Furthermore, the melting time can be shortened to a great extent as compared with conventional interesterification processes and the above mentioned improved interesterification processes, and thereby coloring and deterioration of the resulting resin can be prevented. 
     According to the process of the present invention, there are easily obtained high molecular weight polycarbonates of excellent color and of the same physical properties as high molecular weight polycarbonates produced by the conventional phosgene process and interesterification process, within a short time. Therefore, the process of the present invention is very useful for a commercial and continuous production of high molecular weight polycarbonates having an intrinsic viscosity of not lower than 0.45. 
     The intrinsic viscosity of polycarbonates used in this specification is calculated by the following formula: 
     
         intrinsic viscosity  η! = .sub.c→o lim η.sub.sp /c 
    
     where η sp  is a specific viscosity of a solution of a polycarbonate in methylene chloride determined at 25° C by using an Ostwald&#39;s viscosimeter and &#34;c&#34; is the amount (g.) of the polymer in 100 ml. of the solution. 
     The following examples are given for illustrating the present invention in more detail. 
    
    
     EXAMPLE 1 
     To a solution of 43.2 g. (1.08 m.) in 530 ml. of distilled water were subsequently added 250 ml. of methylene chloride, 1.13 g. (0.012 m.) of phenol, and 91.2 g. (0.4 m.) of bisphenol A, i.e. 2,2-bis(4&#39;-hydroxyphenyl)propane; the resulting mixture was kept at 20° C; and 39.0 g. (0.394 m.) of phosgene was bubbled through the mixture with stirring over 55 minutes. Immediately after the completion of bubbling the phosgene, 1.13 g. (0.012 m.) of methyl chloroformate dissolved in 25 ml. of methylene chloride was added to the mixture at one time and then 4.0 g. (0.04 m.) of phosgene was bubbled through it over 5 minutes. After the completion of bubbling phosgene, 0.1 g. of triethylamine was added followed by vigorous stirring for 90 minutes. 
     The methylene chloride layer was separated, and phosphoric acid was used for the neutralization and then water washing was repeated. 
     After distilling off the methylene chloride, the remaining matter was dried at 70° C under vacuum for one night to obtain a polycarbonate prepolymer having a phenol residue and a methyl chloroformate residue. Its intrinsic viscosity was 0.371 and it could be easily ground. 
     The resulting powders of the polycarbonate prepolymer were placed in a glass test tube and melted at 1.0 Torr. at 300° C for one hour to give a colorless and transparent product having an intrinsic viscosity of 0.468. 
     To the above mentioned powders of the polycarbonate prepolymer was added 0.0010% by weight of potassium benzoate as a polycondensation reaction catalyst and the resulting mixture was melted under the same reduced pressure for 30 minutes and one hour, respectively. 
     Each of the resulting resins was a strong and transparent resin, and the intrinsic viscosities thereof were 0.722 (average molecular weight = 34,500) and 1.006 (average molecular weight = 51,800), respectively. Their APHA were 20 and 25, respectively. 
     COMPARISON EXAMPLE 1 
     Repeating the procedure of Example 1 except that 2.82 g. (0.03 moles) of phenol was used and methyl chloroformate was not added, there was obtained a polycarbonate prepolymer having an intrinsic viscosity of 0.278. 
     The resulting powders of the polycarbonate prepolymer alone were melted at 1.0 Torr. at 300° C for one hour to obtain an intrinsic viscosity of 0.285. When the powders of the polycarbonate prepolymer were melted together with 0.0010% by weight of potassium benzoate for one hour to result in an intrinsic viscosity of 0.308. 
     When the powders of the polycarbonate prepolymer were melted together with 0.010% by weight of potassium benzoate for 30 minutes or one hour. In both cases the resulting product was light yellow. The product after melting for 30 minutes had an intrinsic viscosity of 0.442. The product after melting for one hour contained a lot of matter insoluble in methylene chloride and the intrinsic viscosity could not be measured. 
     EXAMPLES 2-7 
     Repeating the procedure of Example 1 except that the type of chloroformate and/or its amount is different from those in Example 1, there were obtained powders of a polycarbonate prepolymer, which were then melted together with 0.0010% by weight of potassium benzoate in a glass test tube at 1.0 Torr. at 300° C for 1 hour. The results are shown in Table 2 below. 
     
                                           Table 2                                 
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                        Intrinsic viscosity (η)                       
                                After the                                 
Chloroformate                   reaction                                  
                Amount  Before the                                        
                                (Product obtained                         
                (Mole % reaction                                          
                                by the melting                            
                based on                                                  
                        (Polycarbonate                                    
                                polycondensation                          
Example                                                                   
     Type       bisphenol A)                                              
                        prepolymer)                                       
                                reaction)                                 
__________________________________________________________________________
2    Methyl chloroformate                                                 
                5.0     0.291   0.928                                     
3    Methyl chloroformate                                                 
                8.0     0.247   0.792                                     
4    Methyl chloroformate                                                 
                15.0    0.156   0.536                                     
5    Ethyl chloroformate                                                  
                3.0     0.344   0.751                                     
6    Isopropyl  3.0     0.322   0.657                                     
     chloroformate                                                        
7    Cyclohexyl 3.0     0.372   0.614                                     
     chloroformate                                                        
__________________________________________________________________________
 
    
     EXAMPLE 8 
     To a four-necked separable flask fitted with a thermometer, a stirrer, and a phosgene inlet, were added 114 g. (0.5 m.) of bisphenol A, 2.25 g. (0.015 m.) of p-t-butylphenol, 0.48 g. (0.015 m.) of methanol; 125 g. (1.58 m.) of dry pyridine and 500 ml. of dry methylene chloride, and 50 g. (0.505 m.) of phosgene was bubbled through the resulting mixture at 23° C over 80 minutes. 
     300 ml. of dry methylene chloride was further added and then 4 g. (0.04 m.) of phosgene dissolved in 40 ml. of dry methylene chloride was added dropwise over 10 minutes. 
     The resulting mixture was stirred for a further 90 minutes and then the resulting yellow mixture in the flask was poured into 700 ml. of a 10% aqueous HCl and stirred vigorously. 
     The methylene chloride layer was separated, washed with 750 ml. of a distilled water three times, neutralized with 500 ml. of a 0.5% aqueous sodium hydroxide with an ice cooling for 5 minutes, washed with water twice, and then 5 ml. of a concentrated phosphoric acid was added thereto followed by a vigorous stirring for 30 minutes. The resulting solution was repeatedly washed with water and then methylene chloride was distilled off to obtain a polycarbonate prepolymer having a p-t-butylphenol residue and a methanol residue at the polymer ends. 
     Intrinsic viscosity of the polycarbonate thus obtained was 0.382. The polycarbonate was melted at 300° C at 1 Torr. for one hour to give an intrinsic viscosity of 0.461. 
     To the powders of the polycarbonate prepolymer was added 0.0010% by weight of potassium benzoate and melted for 30 minutes and one hour, respectively. Intrinsic viscoscities of the resulting products were 0.698 and 0.967, respectively, and their APHA were 15 and 20, respectively. 
     When the polycarbonate prepolymer was mixed with 0.0050% by weight of disodium salt of bisphenol A and melted for 15 minutes, there was obtained a slightly yellow transparent resin, which had an intrinsic viscosity of 0.743. 
     COMPARISON EXAMPLE 2 
     Repeating the procedure of Example 8 except that 5.62 g. (0.0375 m.) of p-t-butylphenol was used and methanol was not added, there was obtained a polycarbonate prepolymer having an intrinsic viscosity of 0.305. 
     To the powders of the aromatic polycarbonate was added 0.0010% by weight of potassium benzoate and melted for 30 minutes and one hour as in Example 8. The resulting intrinsic viscosities were 0.316 and 0.323, respectively. 
     EXAMPLES 9-18 
     Repeating the procedure of Example 8 except that, in place of methanol, there was used 3 mole %, based on bisphenol A, of cyclohexanol, benzyl alcohol, isopropyl chloroformate, methyl mercaptan, benzyl chlorothioformate, acetic acid, pyridine acetate, propionyl chloride, benzoyl chloride or acetic anhydride, there was obtained each polycarbonate prepolymer in the a form of powder, to which 0.0010% by weight of potassium benzoate was added. The resulting mixture was placed in a glass test tube, and melted at 1.00 Torr. at 300° C for 1 hour. The results are shown in Table 3. 
     
                                           Table 3                                 
__________________________________________________________________________
                       Intrinsic viscosity (η)                        
                                After the                                 
                                reaction                                  
                                (Product                                  
                                obtained                                  
                       Before the                                         
                                by the                                    
Chain terminator       reaction melting                                   
                  Amount                                                  
                       (Polycarbonate                                     
                                polycondensation                          
Example                                                                   
     Type         (g.) prepolymer)                                        
                                reaction)                                 
__________________________________________________________________________
 9   Cyclohexanol 1.50 0.336    0.724                                     
10   Benzyl alcohol                                                       
                  1.63 0.372    0.697                                     
11   Isopropyl chloroformate                                              
                  1.84 0.353    0.648                                     
12   Methyl mercaptan                                                     
                  0.72 0.362    0.679                                     
13   Benzyl chlorothioformate                                             
                  2.59 0.405    0.653                                     
14   Acetic acid  0.90 0.387    0.641                                     
15   Pyridine acetate                                                     
                  2.08 0.398    0.723                                     
16   Propionyl chloride                                                   
                  1.38 0.383    0.535                                     
17   Benzoyl chloride                                                     
                  2.11 0.354    0.476                                     
18   Acetic anhydride                                                     
                  1.53 0.370    0.602                                     
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     EXAMPLE 19 
     Repeating the procedure of Example 8 except that a mixture of 0.16 g. (0.005 m.) of methanol and 0.24 g. (0.005 m.) of methyl mercaptan was used in place of 0.48 g. (0.015 m.) of methanol; there was bubbled 15 g. (0.15 m.) of phosgene; and then 0.39 g. (0.005 m.) of acetyl chloride dissolved in 20 ml. of dry methylene chloride was added to produce polycarbonate prepolymers having methanol residues, methyl mercaptan residues and acetic acid residues in addition to p-t-butylphenol residues at the polymer ends. 
     Intrinsic viscosity of the resulting product was 0.344. When 0.0010% by weight of potassium benzoate was added and melting was effected for one hour at a reduced pressure as in Example 8, the intrinsic viscosity value increased up to 0.730. 
     EXAMPLE 20 
     To 55 g. (1.37 m.) of sodium hydroxide dissolved in 660 ml. of a distilled water were subsequently added 300 ml. of methylene chloride, 2.25 g. (0.015 m.) of p-t-butylphenol, and 114 g. (0.5 m.) of bisphenol A, and kept at 20° C, and 52 g. (0.525 m.) of phosgene was bubbled through the resulting solution over 50 minutes. 
     Immediately after the completion of the bubbling phosgene, 0.48 g. (0.015 m.) of methanol diluted with 20 ml. of methylene chloride was added and stirred for 90 minutes. 
     The methylene chloride layer was separated, neutralized with phosphoric acid, and repeatedly washed with water; and methylene chloride was distilled off; and the residue was dried over-night at 70° C under vacuum. Intrinsic viscosity of the resulting polycarbonate prepolymer was 0.351. After adding 0.0010% by weight of potassium benzoate, the resulting mixture was melted at 300° C at 1.0 Torr. for 30 minutes and for one hour, and the intrinsic viscosities increased to 0.627 and 0.841, respectively. 
     EXAMPLE 21 
     Repeating the procedure of Example 20, except that 3.2 g. (0.1 m.) of methanol was added before bubbling the phosgene, in place of adding 0.48 g. (0.015 m.) of methanol after bubbling the phosgene, the resulting polycarbonate prepolymer was subjected to the reaction procedure as in Example 20. The result is shown in Table 4. 
     EXAMPLE 22 
     Repeating the procedure of Example 20, except that 0.72 g. (0.015 m.) of methyl mercaptan was added 15 minutes after the beginning of bubbling the phosgene, in place of adding 0.48 g. (0.015 m.) of methanol after the beginning of bubbling the phosgene, the result is shown in Table 4. 
     EXAMPLE 23 
     Repeating the procedure of Example 20, except that 0.70 g. (0.005 m.) of benzoyl chloride was added in place of adding 0.48 g. (0.015 m.) of methanol, the result is shown in Table 4. 
     
                                           Table 4                                 
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          Example         21       22        23                           
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Monohydric                                                                
          Type            p-t-butylphenol                                 
                                   p-t-butylphenol                        
                                             p-t-butylphenol              
phenols   Mole % based on bisphenol A                                     
                          3        3         3                            
          Type            Methanol Methyl mercaptan                       
                                             Benzoyl                      
                                             chloride                     
          Time of addition                                                
                          Before bubbling                                 
                                   At 15 minutes                          
                                             After bubbling               
Chain terminator          phosgene after the phosgene                     
                                   beginning of                           
                                   bubbling                               
                                   phosgene                               
          Mole % based on bisphenol A                                     
                          20       3         1                            
          Before the reaction                                             
          (Polycarbonate prepolymer)                                      
                          0.098    0.382     0.415                        
Intrinsic After the                                                       
                  After melting                                           
viscosity reaction                                                        
                  for 30 minutes                                          
                          0.264    0.549     0.522                        
(η)   (Product of                                                     
          the melting                                                     
          polyconden-                                                     
                  After melting                                           
                          0.450    0.716     0.593                        
          sation  for one hour                                            
          reaction)                                                       
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     EXAMPLE 24 
     To 55 g. (1.37 m.) of sodium hydroxide dissolved in 660 ml. of distilled water were added 300 ml. of methylene chloride, 2.25 g. (0.015 m.) of p-t-butylphenol, 114 g. (0.5 m.) of bisphenol A; the resulting mixture was kept at 20° C; and phosgene was bubbled through with stirring. When 15 g. (0.151 m.) of phosgene had been bubbled through, there was added 1.39 g. (0.015 m.) of propionyl chloride dissolved in 20 ml. of methylene chloride. 
     The total amount of bubbled phosgene was 52 g. (0.525 m.). After completion of the phosgene bubbling, 0.48 g. (0.015 m.) of methanol diluted with 20 ml. of methylene chloride was added and then stirred for further 90 minutes. Thus, there were obtained polycarbonates having p-t-butylphenol residues, methanol residues and propionyl residues at the polymer ends and having an intrinsic viscosity of 0.220. 
     To the powders of the polycarbonate prepolymer was added 0.0005% by weight of sodium acetate and melted at 0.1 Torr. at 300° C for 15 minutes, 30 minutes and one hour, respectively. The resulting products are colorless and transparent and their intrinsic viscosities were 0.337, 0.402 and 0.648, respectively. 
     EXAMPLE 25 
     Repeating the procedure of Example 8, except that 0.80 g. (0.025 m.) of methanol was used in place of 0.48 g. of methanol, there was produced polycarbonate prepolymers having an intrinsic viscosity of 0.286. 
     The resulting polycarbonate powder (90 g.) was mixed with 1.2 mg. (0.0013% by weight based on the polycarbonate) of potassium benzoate, heated and melted in a 300 ml. stainless steel reaction vessel fitted with a stirrer, vacuum outlet, and a heater, at 290° C. at 1.0-1.5 Torr. for 30 minutes and one hour respectively. 
     The resulting reaction products were transparent and strong, and almost colorless. 
     The product was heat-rolled at 270° C to a film of 0.2 mm. in thickness, and sample pieces were taken therefrom to measure the tensile strength and elongation. 
     Further, the product was pressed to a plate of 3 mm. in thickness, and some samples were measured to determine the heat distortion. 
     It was not possible to make the polycarbonate prepolymer into a film or a press-shaped plate. 
     The results of measuring the physical properties are compared with those obtained by a conventional phosgene process in Table 5 below. 
     
                                           Table 5                                 
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                                         Heat distortion                  
                 Intrinsic    Stress at  temperature                      
                 viscosity                                                
                       Elongation                                         
                              brake                                       
                                   Yield stress                           
                                         (Load:264 psi.)                  
        Sample   (η)                                                  
                       (%)    (Kg/cm.sup.2)                               
                                   (Kg/cm.sup.2)                          
                                         (° C)                     
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The present                                                               
        Melted for                                                        
                 0.458 106    574  593   133.5                            
invention                                                                 
        30 min.                                                           
        Melted for                                                        
                 0.574 116    637  591   135.2                            
        one hour                                                          
Comparison                                                                
        Polycarbonate                                                     
Example produced by                                                       
        conventional                                                      
                 0.506 118    659  593   134.5                            
        phosgene                                                          
        process                                                           
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     EXAMPLE 26 
     Repeating the procedure of Example 15, except that 3.0 g. (3 mole %) of pyridine benzoate was used in place of pyridine acetate, the intrinsic viscosities before the reaction and after the reaction were 0.403 and 0.636, respectively. 
     EXAMPLE 27 
     Repeating the procedure of Example 18 except that 3.39 g. of benzoic anhydride was used in place of acetic anhydride, the intrinsic viscosity before the reaction and that after the reaction were 0.384 and 0.551, respectively. 
     EXAMPLE 28 
     To a solution of 43.2 g. (1.08 m.) in 530 ml. of a distilled water were subsequently added 250 ml. of methylene chloride, 1.13 g. (0.012 m.) of phenol, 1.23 g. (0.012 m.) of sodium acetate, and 91.2 g. (0.4 m.) of bisphenol A, i.e., 2,2-bis(4&#39;-hydroxyphenyl)propane; the resulting mixture was kept at 20° C; and 39.0 g. (0.394 m.) of phosgene was bubbled through the mixture with stirring over 55 minutes. 
     After the completion of bubbling phosgene, 0.1 g. of triethylamine was added followed by vigorous stirring for 90 minutes. 
     After the above procedure, the same procedure as in Example 1 was repeated and a similar result was obtained.