Cyclic polycarbonate oligomers: inhibition of polymerization to linear polycarbonates
This invention relates to cyclic polycarbonate oligomers and methods for their stabilization and polymerization. 
     The conversion of low molecular weight cyclic aromatic carbonate polymers to linear polycarbonates is known. Reference is made, for example, to the following U.S. patents: 
     
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       3,155,683                                                          
              3,386,954                                                   
       3,274,214                                                          
               3,422,119.                                                 
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     More recently, cyclic polycarbonate oligomer mixtures have been prepared and converted to linear polycarbonates, often of very high molecular weight, by contact with various polycarbonate formation catalysts. Reference is made to U.S. Pat. Nos. 4,605,731 and 4,644,053, the disclosures of which are incorporated by reference herein. The polycarbonate formation catalysts disclosed as useful in said application include various bases and Lewis acids. 
     The conversion of cyclic polycarbonate oligomer mixtures to linear polycarbonates has high potential for utilization in reactive processing methods, such as pultrusion, rotational molding and polymerization in a mold for direct production of molded articles, and in composite formation. One reason for this is the low melt viscosity of the oligomer mixtures, as a result of which handling thereof and wetting of filler materials are simple, convenient and efficient. 
     Certain problems are occasionally encountered in the handling of cyclic polycarbonate oligomer compositions under conditions of low melt viscosity. Such conditions typically involve temperatures of at least about 200° C., frequently in the range of about 250°-300° C. for oligomer mixtures. At such temperatures, polymerization is typically initiated by the majority of polycarbonate formation catalysts and the melt viscosity of the oligomer composition consequently rises, sometimes to a value higher than convenient for reactive processing. Moreover, it is sometimes found that slow autocatalyzed polymerization occurs in such compositions even in the absence of catalyst, as a result of the action of impurities therein. It is of interest, therefore to provide a method of stabilizing cyclic polycarbonate oligomer compositions to inhibit polymerization prior to the addition of polycarbonate formation catalyst, and also to modify the polymerization rate after catalyst has been introduced. 
     These objects are achieved by the present invention, which provides stabilized cyclic polycarbonate oligomer compositions, methods for stabilizing such compositions and for decreasing the polymerization rate of such compositions upon addition of a polycarbonate formation catalyst. 
     The invention is based on the discovery that substituted phenols of the formula ##STR1## wherein X 1  is an electron-withdrawing substituent and n is at least 1, and corresponding phenyl carbonates of the formula ##STR2## are effective to inhibit autocatalyzed polymerization of cyclic polycarbonate oligomer compositions. Moreover, such compounds may be employed in the presence of polycarbonate formation catalysts to decrease the polymerization rate of such oligomer compositions. 
     The compounds which are effective according to the present invention are, as previously stated, phenols of formula I and phenyl carbonates derived from said phenols. Said compounds are more particularly characterized by the presence of at least one electron-withdrawing substituent. These substituents are well known in the art and are frequently also characterized as meta-directing substituents with respect to electrophilic aromatic substitution. They include the nitro, cyano, carbalkoxy, acyl, imido, anhydrido and sulfone groups. Nitro groups are often preferred because of the availability of compounds containing them. The number of such substituents, represented by the n value, is at least 1 (i.e., from 1 to 5) and preferably 2. The most preferred compounds are 2,4-dinitrophenol and 2,4-dinitrophenyl carbonate. 
     The cyclic polycarbonate oligomers useful in the invention generally comprise structural units having the formula . ##STR3## wherein at least about 60% of the total number of R 1  values are aromatic organic radicals and the balance thereof are aliphatic, alicyclic or aromatic organic radicals. The aliphatic or alicyclic R 1  values generally contain up to about 8 carbon atoms. The R 1  values may contain substituents such as halo, nitro, alkoxy, lactone and the like. Most often, however, all R 1  radicals are hydrocarbon radicals. 
     Preferably at least about 80% of the total number of R 1  values in the cyclic oligomer mixtures, and most desirably all of said R 1  values, are aromatic. The aromatic R 1  radicals preferably have the formula 
     
         --A.sup.1 --Y--A.sup.2 --                                  (III) 
    
     wherein each of A 1  and A 2  is a monocyclic divalent aromatic radical and Y is a bridging radical in which one or two carbon atoms separate A 1  from A 2 . The free valence bonds in formula II are usually in the meta or para positions of A 1  and A 2  in relation to Y. 
     In formula III, the A 1  and A 2  values may be unsubstituted phenylene or substituted derivatives thereof, illustrative substituents (one or more) being alkyl, alkenyl, halo (especially chloro and/or bromo), nitro, alkoxy and the like. Unsubstituted phenylene radicals are preferred. Both A 1  and A 2  are preferably p-phenylene, although both may be o- or m-phenylene or one o- or m-phenylene and the other p-phenylene. 
     The bridging radical, Y, is one in which one or two atoms, preferably one, separate A 1  from A 2 . It is most often a hydrocarbon radical and particularly a saturated radical such as methylene, cyclohexylmethylene, 2-[2.2.1]-bicycloheptylmethylene, ethylene, isopropylidene, neopentylidene, cyclohexylidene, cyclopentadecylidene, cyclododecylidene or adamantylidene, especially a gem-alkylene (alkylidene) radical. Also included, however, are unsaturated radicals and radicals which contain atoms other than carbon and hydrogen; for example, 2,2-dichloroethylidene, carbonyl, phthalidylidene, oxy, thio, sulfoxy and sulfone. 
     The R 1  values may be considered as being derived from dihydroxy compounds of the formula 
     
         HO--R.sup.1 --OH                                           (IV) 
    
     especially dihydroxyaromatic compounds and preferably bisphenols of the formula HO--A 1  --Y--A 2  --OH. The following dihydroxy compounds are illustrative: 
     Ethylene glycol 
     Propylene glycol 
     1,3-Propanediol 
     1,4-Butanediol 
     1,6-Hexanediol 
     1,12-Dodecanediol 
     2-Ethyl-1,10-decanediol 
     2-Butene-1,4-diol 
     1,3-Cyclopentanediol 
     1,3-Cyclohexanediol 
     1,4-Cyclohexanediol 
     1,4-Bis(hydroxymethyl)benzene (which is a vinylog of ethylene glycol and has similar properties) 
     Resorcinol 
     4-Bromoresorcinol 
     Hydroquinone 
     4,4&#39;-Dihydroxybiphenyl 
     1,6-Dihydroxynaphthalene 
     2,6-Dihydroxynaphthalene 
     Bis(4-hydroxyphenyl)methane 
     Bis(4-hydroxyphenyl)diphenylmethane 
     Bis(4-hydroxyphenyl)-1-naphthylmethane 
     1,1-Bis(4-hydroxyphenyl)ethane 
     1,2-Bis(4-hydroxyphenyl)ethane 
     1,1-Bis(4-hydroxyphenyl)-1-phenylethane 
     2,2-Bis(4-hydroxyphenyl)propane (&#34;bisphenol A&#34;) 
     2-(4-Hydroxyphenyl)-2-(3-hydroxyphenyl) propane 
     2,2-Bis(4-hydroxyphenyl)butane 
     1,1-Bis(4-hydroxyphenyl)isobutane 
     1,1-Bis(4-hydroxyphenyl)cyclohexane 
     1,1-Bis(4-hydroxyphenyl)cyclododecane 
     Trans-2,3-bis(4-hydroxyphenyl)-2-butene 
     2,2-Bis(4-hydroxyphenyl)adamantane 
     Bis(4-hydroxyphenyl)acetonitrile 
     2,2-Bis(3-methyl-4-hydroxyphenyl)propane 
     2,2-Bis(3-ethyl-4-hydroxyphenyl)propane 
     2,2-Bis(3-n-propyl-4-hydroxyphenyl)propane 
     2,2-Bis(3-isopropyl-4-hydroxyphenyl)propane 
     2,2-Bis(3-sec-butyl-4-hydroxyphenyl)propane 
     2,2-Bis(3-t-butyl-4-hydroxyphenyl)propane 
     2,2-Bis(3-cyclohexyl-4-hydroxyphenyl)propane 
     2,2-Bis(3-allyl-4-hydroxyphenyl)propane 
     2,2-Bis(3-methoxy-4-hydroxyphenyl)propane 
     2,2-Bis(3,5-dimethyl-4-hydroxyphenyl)propane 
     2,2-Bis(2,3,5,6-tetramethyl-4-hydroxyphenyl) 
     propane 
     2,2-Bis(3,5-dichloro-4-hydroxyphenyl)propane 
     2,2-Bis(3,5-dibromo-4-hydroxyphenyl)propane 
     2,2-Bis(2,6-dibromo-3,5-dimethyl-4-hydroxyphenyl)propane 
     α,α-Bis(4-hydroxyphenyl)toluene 
     α,α,α&#39;,α&#39;-Tetramethyl-α,α&#39;-bis(4-hydroxyphenyl)-p-xylene 
     2,2-Bis(4-hydroxyphenyl)hexafluoropropane 
     1,1-Dichloro-2,2-bis(4-hydroxyphenyl)ethylene 
     1,1-Dibromo-2,2-bis(4-hydroxyphenyl)ethylene 
     1,1-Dichloro-2,2-bis(5-phenoxy-4-hydroxyphenyl)ethylene 
     4,4&#39;-Dihydroxybenzophenone 
     3,3-Bis(4-hydroxyphenyl)-2-butanone 
     1,6-Bis(4-hydroxyphenyl)-1,6-hexanedione 
     Bis(4-hydroxyphenyl) ether 
     Bis(4-hydroxyphenyl) sulfide 
     Bis(4-hydroxyphenyl) sulfoxide 
     Bis(4-hydroxyphenyl) sulfone 
     Bis(3,5-dimethyl-4-hydroxyphenyl) sulfone 
     9,9-Bis(4-hydroxyphenyl)fluorene 2,7-Dihydroxypyrene 
     6,6&#39;-Dihydroxy-3,3,3&#39;,3&#39;-tetramethylspiro(bis)indane (&#34;spirobiindane bisphenol&#34;) 
     3,3-Bis(4-hydroxyphenyl)phthalide 
     2,6-Dihydroxydibenzo-p-dioxin 
     2,6-Dihydroxythianthrene 
     2,7-Dihydroxyphenoxathiin 
     2,7-Dihydroxy-9,10dimethylphenazine 
     3,6-Dihydroxydibenzofuran 
     3,6-Dihydroxydibenzothiophene 
     2,7-Dihydroxycarbazole. 
     Bisphenol A is often preferred for reasons of availability and particular suitability for the purposes of the invention. 
     The cyclic oligomers have degrees of polymerization from 2 to about 30. Cyclic oligomer mixtures are preferred, especially those in which the various molecular species have degrees of polymerization up to about 20, with a major proportion being up to about 12 and a still larger proportion up to about 15. Such mixtures have relatively low melting points as compared to single compounds such as the corresponding cyclic trimer. The cyclic oligomer mixtures are generally liquid at temperatures above 300° C., most often at temperatures above 225° C. and frequently above 200° C. 
     The cyclic oligomer mixtures contain very low proportions (if any) of linear oligomers. In general, no more than about 10% by weight, and most often no more than about 5%, of such linear oligomers are present. The mixtures also usually contain low percentages, if any (frequently less than 30% and preferably no higher than about 20%) of polymers (linear or cyclic) having a degree of polymerization greater than about 30. Such polymers are frequently identified hereinafter as &#34;high polymer&#34;. These properties, coupled with the relatively low melting points and viscosities of the cyclic oligomer mixtures, contribute to their utility as resin precursors, especially for high molecular weight resins, as described hereinafter. 
     Suitable cyclic oligomer mixtures may be prepared by a condensation reaction involving at least one compound selected from the group consisting of bishaloformates and thiol analogs thereof, said compounds having the formula 
     
         R.sup.1 (OCOX.sup.2).sub.2                                 (V) 
    
     or a mixture thereof with at least one dihydroxy compound having formula IV, wherein R 1  is as defined hereinabove and X 2  is chlorine or bromine. (The compound of formula V or mixture thereof with that of formula IV is frequently referred to hereinafter as &#34;bishaloformate composition&#34; or &#34;bischloroformate composition&#34;.) The condensation reaction typically takes place interfacially when a solution of said compound in a substantially non-polar organic liquid is contacted with a tertiary amine from a specific class and an aqueous alkali metal hydroxide solution. 
     In addition to compounds of formula V and, optionally, formula IV, the bishaloformate composition may also contain other compounds, including oligomers of the formula ##STR4## wherein R 1  and X 2  are as previously defined and m is a small number, typically about 1-4. 
     While the X 2  values in formula V may be chlorine or bromine, the bischloroformates, in which X 2  is chlorine, are most readily available and their use is therefore preferred. (Frequent reference to bischloroformates will be made hereinafter, but it should be understood that other bishaloformates may be substituted therefor as appropriate.) Suitable dihydroxy compounds of formula IV include those having divalent radicals of formula II which are different from the corresponding divalent radicals in the compound of formula V. When such dihydroxy compounds are present, they generally comprise up to about 50%, most often up to about 20% and preferably up to about 10%, of the bischloroformate mixture. Most preferably, however, said mixture consists essentially of bischloroformates. 
     The tertiary amines useful in the oligomer formation reaction (&#34;tertiary&#34; in this context denoting the absence of N--H bonds) generally comprise those which are oleophilic; i.e., which are soluble in and highly active in organic media, especially those used in the oligomer preparation method, and in particular those which are useful for the formation of polycarbonates. Reference is made, for example, to the tertiary amines disclosed in U.S. Pat. Nos. 4,217,438 and 4,368,315, the disclosures of which are incorporated by reference herein. They include aliphatic amines such as triethylamine, tri-n-propylamine, diethyl-n-propylamine and tri-n-butylamine and highly nucleophilic heterocyclic amines such as 4-dimethylaminopyridine (which, for the purposes of this invention, contains only one active amine group). The preferred amines are those which dissolve preferentially in the organic phase of the reaction system; that is, for which the organic-aqueous partition coefficient is greater than 1. This is true because intimate contact between the amine and bischloroformate composition is essential for the formation of the cyclic oligomer mixture. For the most part, such amines contain at least about 6 and preferably about 6-14 carbon atoms. 
     The most useful amines are trialkylamines containing no branching on the carbon atoms in the 1- and 2- positions. Especially preferred are tri-n-alkylamines in which the alkyl groups contain up to about 4 carbon atoms. Triethylamine is most preferred by reason of its particular availability, low cost, and effectiveness in the preparation of products containing low percentages of linear oligomers and high polymers. 
     Also employed in the oligomer formation reaction is an aqueous alkali metal hydroxide solution. It is most often lithium, sodium or potassium hydroxide, with sodium hydroxide being preferred because of its availability and relatively low cost. The concentration of said solution is about 0.2-10M and preferably no higher than about 3 M. 
     The fourth essential component in the cyclic oligomer preparation method is a substantially non-polar organic liquid which forms a two-phase system with water. The identity of the liquid is not critical, provided it possesses the stated properties. Illustrative liquids are aromatic hydrocarbons such as toluene and xylene; substituted aromatic hydrocarbons such as chlorobenzene, o-dichlorobenzene and nitrobenzene; chlorinated aliphatic hydrocarbons such as chloroform and methylene chloride; and mixtures of the foregoing with ethers such as tetrahydrofuran. 
     To prepare the cyclic oligomer mixture according to the above-described method, in the first step the reagents and components are placed in contact under conditions wherein the bischloroformate composition is present in high dilution, or equivalent conditions. Actual high dilution conditions, requiring a large proportion of organic liquid, may be employed but are usually not preferred for cost and convenience reasons. Instead, simulated high dilution conditions known to those skilled in the art may be employed. For example, in one embodiment of the method the bischloroformate composition or a mixture thereof with the amine is added gradually to a mixture of the other materials. It is within the scope of this embodiment to incorporate the amine in the mixture to which the bischloroformate is added, or to add it gradually, either in admixture therewith or separately. Continuous or incremental addition of amine is frequently preferred, whereupon the cyclic oligomer mixture is obtained in relatively pure form and in high yield. 
     Although addition of bischloroformate composition neat (i.e., without solvents) is within the scope of this embodiment, it is frequently inconvenient because many bischloroformates are solids. Therefore, it is preferably added as a solution in a portion of the organic liquid, especially when it consists essentially of bischloroformate. The proportion of organic liquid used for this purpose is not critical; about 25-75% by weight, and especially about 40-60%, is preferred. 
     The reaction temperature is generally in the range of about 0°-50° C. It is most often about 0°-40° C. and preferably 20°-40° C. 
     For maximization of the yield and purity of cyclic oligomers as opposed to polymer and insoluble and/or intractable by-products, it is preferred to use not more than about 0.7 mole of bischloroformate composition (calculated as bisphenol bischloroformate) per liter of organic liquid present in the reaction system, including any liquid used to dissolve said composition. Preferably, about 0.003-0.6 mole thereof is used when it consists entirely of bischloroformate, and no more than about 0.5 mole is used when it is a mixture of compounds of formulas IV and V. It should be noted that this is not a molar concentration in the organic liquid when the bischloroformate composition is added gradually since said composition is consumed as it is added to the reaction system. 
     The molar proportions of the reagents constitute another important feature for yield and purity maximization. The preferred molar ratio of amine to bischloroformate composition (calculated as bisphenol bischloroformate) is about 0.1-1.0:1 and most often about 0.2-6:1. The preferred molar ratio of alkali metal hydroxide to said composition is about 1.5-3:1 and most often about 2-3:1. 
     In a second step, the oligomer mixture may be separated from at least a portion of the polymer and insoluble material present. When other reagents are added to the alkali metal hydroxide solution and the preferred conditions and material proportions are otherwise employed, the cyclic oligomer mixture (obtained as a solution in the organic liquid) typically contains less than 30% by weight and frequently less than about 20% of polymer and insoluble material. When all of the preferred conditions are employed, the product may contain 10% or even less of such material. Depending on the intended use of the cyclic oligomer mixture, the separation step may then be unnecessary. 
     Therefore, a highly preferred method for preparing the cyclic oligomer mixture comprises the single step of conducting the reaction using at least one aliphatic or heterocyclic tertiary amine which, under the reaction conditions, dissolves preferentially in the organic phase of the reaction system, and gradually adding all the reagents simultaneously to a substantially non-polar organic liquid or a mixture of said liquid with water, said liquid or mixture being maintained at a temperature in the range of about 0°-50° C.; the amount of bischloroformate composition used being up to about 0.7 mole for each liter of said organic liquid present in the reaction system, and the molar proportions of amine and alkali metal hydroxide to bischloroformate composition being approximately 0.2-1.0:1 and 2-3:1, respectively; and recovering the cyclic oligomers thus formed. 
     As in the embodiment previously described, another portion of said liquid may serve as a solvent for the bischloroformate composition. Addition of each reagent is preferably continuous, but may be incremental for any or all of said reagents. 
     Among the principal advantages of this preferred embodiment are the non-criticality of the degree of dilution of the reagents and the ability to complete the addition and reaction in a relatively short time, regardless of reaction scale. It ordinarily takes only about 25-30 minutes to complete cyclic oligomer preparation by this method, and the cyclic oligomer yield may be 85.90% or more. The crude product usually also contains only minor amounts of high molecular weight linear polycarbonates as by-products. By contrast, use of a less preferred embodiment may, depending on reaction scale, require an addition period as long as 8-10 hours and the crude product may contain substantial proportions of linear by-products with molecular weights of about 4,000-10,000, which, if not removed, may interfere with subsequent polymerization of the cyclic oligomers by acting as chain transfer agents. 
     It is believed that the advantageous results obtained by employing the preferred embodiment are a result of the relatively low pH of the reaction mixture, typically about 9-10. When bischloroformate composition (and optionally amine) is added to alkali metal hydroxide, on the other hand, the initial pH is on the order of 14. 
     When the polymer separation step is necessary, the unwanted impurities may be removed in the necessary amounts by conventional operations such as combining the solution with a non-solvent for said impurities. Illustrative nonsolvents include ketones such as acetone and methyl isobutyl ketone and esters such as methyl acetate and ethyl acetate. Acetone is a particularly preferred non-solvent. 
     Recovery of the cyclic oligomers normally means merely separating the same from diluent (by known methods such as vacuum evaporation) and, optionally, from high polymer and other impurities. As previously suggested, the degree of sophistication of recovery will depend on such variables as the intended end use of the product. 
    
    
     The preparation of cyclic oligomer mixtures useful in this invention is illustrated by the following examples. All parts and percentages in the examples herein are by weight unless otherwise indicated. Temperatures are in degrees Celsius. Molecular weights, whenever referred to herein, are weight average unless otherwise indicated and were determined by gel permeation chromatography relative to polystyrene. 
     EXAMPLE 1 
     The crude bischloroformate composition used was a bisphenol A bischloroformate composition corresponding roughly to the dimer. 
     A 300-ml. Morton flask was charged with 128 ml. of methylene chloride, 10 ml. of water, 2 ml. of 4.9M aqueous sodium hydroxide, 1.16 ml. of triethylamine and 5 ml. of 0.66M aqueous disodium salt of bisphenol A. The mixture was heated under reflux, with stirring, as 40 ml. of a 1.06M solution of the bischloroformate in methylene chloride was added over 37 minutes. There were concurrently added an additional 35 ml. of the bisphenol A disodium salt solution over 32 minutes, 10 ml. of sodium hydroxide solution over 30 minutes, and 0.36 ml. of triethylamine in 10 equal increments 31/2 minutes apart. Stirring was continued for several minutes, after which the aqueous and organic phases were separated and the aqueous layer was washed with methylene chloride. The combined organic phases were washed once with dilute aqueous sodium hydroxide, twice with aqueous hydrochloric acid, once again with sodium hydroxide and twice with water, and dried over magnesium sulfate. Upon filtration, vacuum stripping and drying in an oven, there was obtained a white solid comprising the desired cyclic oligomer mixture, containing about 89% cyclic oligomers. 
     EXAMPLE 2 
     A solution of 1.4 mmol. of bisphenol A bischloroformate and 0.6 mmol. of 1,4-benzenedimethanol bischloroformate in 10 ml. of a tetrahydrofuran-methylene chloride solution comprising 10% by volume tetrahydrofuran was added over 30 minutes at 30° C., with stirring, to a mixture of 10 ml. of methylene chloride, 2 ml. of 2.5 M aqueous sodium hydroxide and 1 mmol. of triethylamine. After addition was complete, the mixture was washed three times with dilute aqueous hydrochloric acid and the organic layer was separated, dried by filtration through phase separation paper and evaporated under vacuum. The product was the desired mixed cyclic polycarbonate oligomer of bisphenol A and benzenel,4-dimethanol. 
     EXAMPLES 3-12 
     Following the procedure of Example 2, products containing at least about 80% mixed cyclic polycarbonate oligomers were prepared from mixtures of bisphenol A bischloroformate and the dihydroxy compounds listed in Table I. In each case, a total of 2 mmol. of bischloroformate composition was used. The proportion of the listed dihydroxy compound therein was 10 mole percent unless otherwise indicated. 
     
                       TABLE I                                                     
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Example   Dihydroxy compound                                              
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3         1,1-Bis(4-hydroxyphenyl)cyclohexane                             
4         1,1-Bis(4-hydroxyphenyl)cyclododecane                           
5         2,2-Bis(3,5-dimethyl-4-hydroxyphenyl)propane                    
6         2,2-Bis(3,5-dibromo-4-hydroxyphenyl)propane                     
7         2,2-Bis(4-hydroxyphenyl)-1,1-dichloroethylene                   
8         Hydroquinone                                                    
9         Hydroquinone (15 mole percent)                                  
10        Bis(4-hydroxyphenyl) sulfide                                    
11        Bis(4-hydroxyphenyl) sulfone                                    
12        Bis(3,5-dimethyl-4-hydroxyphenyl) sulfone                       
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     One aspect of the invention is a method for stabilizing a composition consisting essentially of cyclic polycarbonate oligomers as defined hereinabove. Stabilization is achieved by incorporating therein about 0.1-0.5 mole percent, based on structural units in said oligomer composition, of said substituted phenol or phenyl carbonate. It is believed that the role of said compound is to inhibit polymerization initiated by impurities present in the oligomer composition. 
     Another aspect of the invention is a method for decreasing the polymerization rate of such oligomer compositions by contact under polymerization conditions with a basic polycarbonate formation catalyst. Such decrease is also achieved by incorporating the substituted phenol or phenyl carbonate in said composition in the amount of at least about 0.5 mole and generally no more than about 5 moles per mole of said catalyst. 
     The polycarbonate formation catalysts which can be used in this aspect of the invention include various bases. It is known that basic catalysts may be used to prepare polycarbonates by the interfacial method, as well as by transesterification and from cyclic oligomers. Reference is made to the aforementioned U.S. Pat. Nos. 3,155,683, 3,274,214, 4,271,438 and 4,368,315. Such catalysts may also be used to polymerize the cyclic oligomer mixtures. Examples thereof are organometallic compounds such as n-butyllithium; basic alkali metal salts such as lithium phenoxide, sodium phenoxide, lithium 2,2,2-trifluoroethoxide, sodium benzoate and lithium stearate; and quaternary ammonium hydroxides such as tetramethylammonium hydroxide. 
     A particularly useful class of Lewis bases is disclosed in the aforementioned U.S. Pat. No. 4,605,731. It comprises alkali metal, quaternary ammonium and quaternary phosphonium tetraarylborates, including lithium tetraphenylborate, sodium tetraphenylborate, sodium bis(2,2&#39;-biphenylene)borate, potassium tetraphenylborate, tetramethylammonium tetraphenylborate, tetra-n-butylammonium tetraphenylborate, tetramethylphosphonium tetraphenylborate, tetra-n-butylphosphonium tetraphenylborate and tetraphenylphosphonium tetraphenylborate. 
     Polymerization of the cyclic oligomer composition is typically effected by simply contacting said composition with the catalyst at temperatures up to 350° C., preferably about 200°-300° C., until polymerization has proceeded to the extent desired. Although the use of a solvent is within the scope of the invention, it is not required and is frequently not preferred. In general, the amount of catalyst used is about 0.001-1.0 mole percent based on structural units in the oligomer mixture. 
     The invention is illustrated by the following examples. All polymer molecular weights were determined by gel permeation chromatography. 
     EXAMPLE 13 
     Three-gram samples (12 mmol. based on structural units) of a cyclic bisphenol A polycarbonate oligomer mixture similar to that of Example 1 but of relatively low purity and 0.0058 mmol. of 2,4-dinitrophenol and 2,4-dinitrophenyl carbonate, respectively, were dissolved in methylene chloride. The solvent was removed by rotary evaporation and the solids dried for 3 hours at 110° C. to produce intimate mixtures of cyclics and inhibitor. 
     Samples comprising one gram of the resulting mixtures were heated in an oven at 250° C. for 21/2 hours. Also heated at that temperature was a control comprising cyclics from the same batch with no inhibitor incorporated therein. After the heating period, the molecular weights of the samples were determined and percentage of polymerization determined therefrom. The percentage values in each sample and control is listed in Table II. 
     
                       TABLE II                                                    
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Lewis acid         % polymerization                                       
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2,4-Dinitrophenol  36                                                     
2,4-Dinitrophenyl carbonate                                               
                   20                                                     
Control            100                                                    
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     These results clearly show the inhibitory effect of substituted phenols and derived phenyl carbonates on the polymerization rate of the cyclic oligomer composition.