Method of polycarbonate preparation

BACKGROUND OF THE INVENTION

0001This invention relates to the preparation of polycarbonates by the melt reaction of a bisphenol with an ester-substituted diaryl carbonate. More particularly, the instant invention relates to the formation under mild conditions of polycarbonates having extremely low levels of Fries rearrangement products and possessing a high level of endcapping.

0002Polycarbonates, such as bisphenol A polycarbonate, are typically prepared either by interfacial or melt polymerization methods. The reaction of a bisphenol such as bisphenol A (BPA) with phosgene in the presence of water, a solvent such as methylene chloride , an acid acceptor such as sodium hydroxide and a phase transfer catalyst such as triethylamine is typical of the interfacial methodology. The reaction of bisphenol A with a source of carbonate units such as diphenyl carbonate at high temperature in the presence of a catalyst such as sodium hydroxide is typical of currently employed melt polymerization methods. Each method is practiced on a large scale commercially and each presents significant drawbacks.

0003The interfacial method for making polycarbonate has several inherent disadvantages. First it is a disadvantage to operate a process which requires phosgene as a reactant due to obvious safety concerns. Second it is a disadvantage to operate a process which requires using large amounts of an organic solvent because expensive precautions must be taken to guard against any adverse environmental impact. Third, the interfacial method requires a relatively large amount of equipment and capital investment. Fourth, the polycarbonate produced by the interfacial process is prone to having inconsistent color, higher levels of particulates, and higher chloride content, which can cause corrosion.

0004The melt method, although obviating the need for phosgene or a solvent such as methylene chloride requires high temperatures and relatively long reaction times. As a result, by-products may be formed at high temperature, such as the products arising by Fries rearrangement of carbonate units along the growing polymer chains. Fries rearrangement gives rise to undesired and uncontrolled polymer branching which may negatively impact the polymer's flow properties and performance.

0005Some years ago, it was reported in U.S. Patent No. 4,323,668 that polycarbonate could be formed under relatively mild conditions by reacting a bisphenol such as BPA with the diaryl carbonate formed by reaction phosgene with methyl salicylate. The method used relatively high levels of transesterification catalysts such as lithium stearate in order to achieve high molecular weight polycarbonate. High catalyst loadings are particularly undesirable in melt polycarbonate reactions since the catalyst remains in the product polycarbonate following the reaction. The presence of a transesterification catalyst in a the polycarbonate may shorten the useful life span of articles made therefrom by promoting increased water absorption, polymer degradation at high temperatures and discoloration.

0006It would be desirable, therefore, to minimize the amount of catalyst required in the for the melt preparation of polycarbonate from bisphenols and ester substituted diaryl carbonates such as bis-methyl salicyl carbonate.

BRIEF SUMMARY OF THE INVENTION

0007The present invention provides a method for preparing polycarbonate comprising heating a mixture comprising a catalyst, at least one diaryl carbonate having structure I <chemistry id="chem0001" num="0001"><img file="EP1577333A2_D0001.tif" /></chemistry> wherein R¹ and R² are independently C₁ - C₂₀ alkyl radicals, C₄-C₂₀ cycloalkyl radicals or C₄-C₂₀ aromatic radicals, R³ and R⁴ are independently at each occurrence a halogen atom, cyano group, nitro group, C₁ - C₂₀ alkyl radical, C₄-C₂₀ cycloalkyl radical, C₄-C₂₀ aromatic radical, C₁- C₂₀ alkoxy radical, C₄-C₂₀ cycloalkoxy radical, C₄-C₂₀ aryloxy radical, C₁-C₂₀ alkylthio radical, C₄-C₂₀ cycloalkylthio radical, C₄-C₂₀ arylthio radical, C₁-C₂₀ alkylsulfinyl radical, C₄-C₂₀ cycloalkylsulfinyl radical, C₄-C₂₀ arylsulfinyl radical, C₁-C₂₀ alkylsulfonyl radical, C₄-C₂₀ cycloalkylsulfonyl radical, C₄-C₂₀ arylsulfonyl radical, C₁-C₂₀ alkoxycarbonyl radical, C₄-C₂₀ cycloalkoxycarbonyl radical, C₄-C₂₀ aryloxycarbonyl radical, C₂-C₆₀ alkylamino radical, C₆-C₆₀ cycloalkylamino radical, C₅-C₆₀ arylamino radical, C₁-C₄₀ alkylaminocarbonyl radical, C₄-C₄₀ cycloalkylaminocarbonyl radical, C₄-C₄₀ arylaminocarbonyl radical, and C₁-C₂₀ acylamino radical; and b and c are independently integers 0-4; and at least one dihydroxy aromatic compound, said catalyst comprising at least one source of alkaline earth ions or alkali metal ions, and at least one quaternary ammonium compound, quaternary phosphonium compound or a mixture thereof, said source of alkaline earth ions or alkali metal ions being present in an amount such that between about 10^-5 and about 10^-8 moles of alkaline earth metal ions or alkali metal ions are present in the mixture relative per mole of dihydroxy aromatic compound employed, said quaternary ammonium compound, quaternary phosphonium compound or mixture thereof being present in an amount between about 2.5x10^-3 and about 1x10^-6 moles per mole of dihydroxy aromatic compound employed.

0008The present invention further relates to a method for forming polycarbonates by reaction of an ester-substituted diaryl carbonate in which the level of Fries rearrangement product in the product polycarbonate is less than about 1000 parts per million (ppm) and the level of internal ester carbonate linkages in the product polycarbonate is less than about 1 percent of the total number of moles of dihydroxy aromatic compound employed and the level of terminal hydroxy ester groups in the product polycarbonate is less than about 1 percent of the total number of moles of dihydroxy aromatic compound employed.

DETAILED DESCRIPTION OF THE INVENTION

0009The present invention may be understood more readily by reference to the following detailed description of preferred embodiments of the invention and the examples included therein. In the following specification and the claims which follow, reference will be made to a number of terms which shall be defined to have the following meanings:

0010The singular forms "a", "an" and "the" include plural referents unless the context clearly dictates otherwise.

0011"Optional" or "optionally" means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where the event occurs and instances where it does not.

0012As used herein the term "polycarbonate" refers to polycarbonates incorporating structural units derived from one or more dihydroxy aromatic compounds and includes copolycarbonates and polyester carbonates.

0013As used herein, the term "melt polycarbonate" refers to a polycarbonate made by the transesterification of a diaryl carbonate with a dihydroxy aromatic compound.

0014"BPA" is herein defined as bisphenol A or 2,2-bis(4-hydroxyphenyl)propane.

0015"Catalyst system" as used herein refers to the catalyst or catalysts that catalyze the transesterification of the bisphenol with the diaryl carbonate in the melt process.

0016The terms "bisphenol", "diphenol" and "dihydric phenol" as used herein are synonymous.

0017"Catalytically effective amount" refers to the amount of the catalyst at which catalytic performance is exhibited.

0018As used herein the term "Fries product" is defined as a structural unit of the product polycarbonate which upon hydrolysis of the product polycarbonate affords a carboxy-substituted dihydroxy aromatic compound bearing a carboxy group adjacent to one or both of the hydroxy groups of said carboxy-substituted dihydroxy aromatic compound. For example, in bisphenol A polycarbonate prepared by a melt reaction method in which Fries reaction occurs, among the Fries products within the product polycarbonate are those structural units, for example structure VIII below, which afford 2-carboxy bisphenol A upon complete hydrolysis of the product polycarbonate.

0019The terms "Fries product" and "Fries group" are used interchangeably herein.

0020The terms "Fries reaction" and "Fries rearrangement" are used interchangeably herein.

0021As used herein the term "aliphatic radical" refers to a radical having a valence of at least one comprising a linear or branched array of atoms which is not cyclic. The array may include heteroatoms such as nitrogen, sulfur and oxygen or may be composed exclusively of carbon and hydrogen. Examples of aliphatic radicals include methyl, methylene, ethyl, ethylene, hexyl, hexamethylene and the like.

0022As used herein the term "aromatic radical" refers to a radical having a valence of at least one comprising at least one aromatic group. Examples of aromatic radicals include phenyl, pyridyl, furanyl, thienyl, naphthyl, phenylene, and biphenyl. The term includes groups containing both aromatic and aliphatic components, for example a benzyl group.

0023As used herein the term "cycloaliphatic radical" refers to a radical having a valance of at least one comprising an array of atoms which is cyclic but which is not aromatic. The array may include heteroatoms such as nitrogen, sulfur and oxygen or may be composed exclusively of carbon and hydrogen. Examples of cycloaliphatic radicals include cyclcopropyl, cyclopentyl cyclohexyl, tetrahydrofuranyl and the like.

0024In the present invention it has been discovered that extremely low levels of catalyst may be employed to prepare polycarbonate using the melt reaction of an ester substituted diaryl carbonate with a bisphenol. The use of very low catalyst loadings is desirable from at least two perspectives. First, the use of low catalyst levels during melt polymerization tends to suppress the formation of undesired Fries rearrangement products. Second, because residual catalyst present in the polymer tends to decrease the useful life span of articles made from it by increasing water absorption, decreasing thermal stability and promoting discoloration, its minimization is desirable. The polycarbonate prepared by the method of the present invention is free of, or contains undetectable levels of Fries rearrangement products. Moreover, in the absence of an added exogenous monofunctional phenol the product polycarbonate is very highly endcapped with less than 50% of the endgroups being free hydroxyl groups. Where an exogenous monofunctional phenol is added to the polymerization mixture, high levels of incorporation of said phenol are observed. In this manner both the identity of the polymer endgroups and the polymer molecular weight may be controlled in the melt reaction.

0025In the process of the present invention an ester-substituted diaryl carbonate having structure I is reacted under melt reaction conditions with at least one dihydroxy aromatic compound in the presence of at least one source of alkaline earth ions or alkali metal ions, and an organic ammonium compound or an organic phosphonium compound or a combination thereof. Ester-substituted diaryl carbonates I are exemplified by bis-methyl salicyl carbonate (CAS Registry No. 82091-12-1), bis-ethyl salicyl carbonate, bis-propyl salicyl carbonate, bis-butyl salicyl carbonate, bis-benzyl salicyl carbonate, bis-methyl 4-chlorosalicyl carbonate and the like. Typically bis-methyl salicyl carbonate is preferred.

0026The dihydroxy aromatic compounds of the present invention are selected from the group consisting of bisphenols having structure II, <chemistry id="chem0002" num="0002"><img file="EP1577333A2_D0002.tif" /></chemistry> wherein R⁵-R¹² are independently a hydrogen atom, halogen atom, nitro group, cyano group, C₁-C₂₀ alkyl radical C_4-C₂₀ cycloalkyl radical, or C₆-C₂₀ aryl radical; W is a bond, an oxygen atom, a sulfur atom, a SO₂ group, a C₆-C₂₀ aromatic radical, a C₆-C₂₀ cycloaliphatic radical or the group <chemistry id="chem0003" num="0003"><img file="EP1577333A2_D0003.tif" /></chemistry> wherein R¹³ and R¹⁴ are independently a hydrogen atom, C₁-C₂₀ alkyl radical, C₄-C₂₀ cycloalkyl radical, or C_4-C₂₀ aryl radical; or R¹³ and R¹⁴ together form a C_4-C₂₀ cycloaliphatic ring which is optionally substituted by one or more C_1-C₂₀ alkyl, C_6-C₂₀ aryl, C_5-C₂₁ aralkyl, C₅-C₂₀ cycloalkyl groups or a combination thereof; dihydroxy benzenes having structure III <chemistry id="chem0004" num="0004"><img file="EP1577333A2_D0004.tif" /></chemistry> wherein R¹⁵ is independently at each occurrence a hydrogen atom, halogen atom, nitro group, cyano group, C_1-C₂₀ alkyl radical, C_4-C₂₀ cycloalkyl radical C_4-C₂₀ aryl radical, and d is an integer from 0 to 4; and dihydroxy naphthalenes having structures IV and V <chemistry id="chem0005" num="0005"><img file="EP1577333A2_D0005.tif" /></chemistry> wherein R¹⁶, R¹⁷, R¹⁸ and R¹⁹ are independently at each occurrence a hydrogen atom, halogen atom, nitro group, cyano group, C₁-C₂₀ alkyl radical, C_4-C₂₀ cycloalkyl radical C₄-C₂₀ aryl radical; e and f are integers of from 0 to 3, g is an integer from 0 to 4, and h is an integer from 0 to 2;

0027Suitable bisphenols II are illustrated by 2,2-bis(4-hydroxyphenyl)propane (bisphenol A); 2,2-bis(3-chloro-4-hydroxyphenyl)propane; 2,2-bis(3-bromo-4-hydroxyphenyl)propane; 2,2-bis(4-hydroxy-3-methylphenyl)propane; 2,2-bis(4-hydroxy-3-isopropylphenyl)propane; 2,2-bis(3-t-butyl-4-hydroxyphenyl)propane; 2,2-bis(3-phenyl-4-hydroxyphenyl)propane; 2,2-bis(3,5-dichloro-4-hydroxyphenyl)propane; 2,2-bis(3,5-dibromo-4-hydroxyphenyl)propane; 2,2-bis(3,5-dimethyl-4-hydroxyphenyl)propane; 2,2-bis(3-chloro-4-hydroxy-5-methylphenyl)propane; 2,2-bis(3-bromo-4-hydroxy-5-methylphenyl)propane; 2,2-bis(3-chloro-4-hydroxy-5-isopropylphenyl)propane; 2,2-bis(3-bromo-4-hydroxy-5-isopropylphenyl)propane; 2,2-bis(3-t-butyl-5-chloro-4-hydroxyphenyl)propane; 2,2-bis(3-bromo-5-t-butyl-4-hydroxyphenyl)propane; 2,2-bis(3-chloro-5-phenyl-4-hydroxyphenyl)propane; 2,2-bis(3-bromo-5-phenyl-4-hydroxyphenyl)propane; 2,2-bis(3,5-disopropyl-4-hydroxyphenyl)propane; 2,2-bis(3,5-di-t-butyl-4-hydroxyphenyl)propane; 2,2-bis(3,5-diphenyl-4-hydroxyphenyl)propane; 2,2-bis(4-hydroxy-2,3,5,6-tetrachlorophenyl)propane; 2,2-bis(4-hydroxy-2,3,5,6-tetrabromophenyl)propane; 2,2-bis(4-hydroxy-2,3,5,6-tetramethylphenyl)propane; 2,2-bis(2,6-dichloro-3,5-dimethyl-4-hydroxyphenyl)propane; 2,2-bis(2,6-dibromo-3,5-dimethyl-4-hydroxyphenyl)propane; 1,1-bis(4-hydroxyphenyl)cyclohexane; 1,1-bis(3-chloro-4-hydroxyphenyl)cyclohexane; 1,1-bis(3-bromo-4-hydroxyphenyl)cyclohexane; 1,1-bis(4-hydroxy-3-methylphenyl)cyclohexane; 1,1-bis(4-hydroxy-3-isopropylphenyl)cyclohexane; 1,1-bis(3-t-butyl-4-hydroxyphenyl)cyclohexane; 1,1-bis(3-phenyl-4-hydroxyphenyl)cyclohexane; 1,1-bis(3,5-dichloro-4-hydroxyphenyl)cyclohexane; 1,1-bis(3,5-dibromo-4-hydroxyphenyl)cyclohexane; 1,1-bis(3,5-dimethyl-4-hydroxyphenyl)cyclohexane; 1,1-bis(3-chloro-4-hydroxy-5-methylphenyl)cyclohexane; 1,1-bis(3-bromo-4-hydroxy-5-methylphenyl)cyclohexane; 1,1-bis(3-chloro-4-hydroxy-5-isopropylphenyl)cyclohexane; 1,1-bis(3-bromo-4-hydroxy-5-isopropylphenyl)cyclohexane; 1,1-bis(3-t-butyl-5-chloro-4-hydroxyphenyl)cyclohexane; 1,1-bis(3-bromo-5-t-butyl-4-hydroxyphenyl)cyclohexane; 1,1-bis(3-chloro-5-phenyl-4-hydroxyphenyl)cyclohexane; 1,1-bis(3-bromo-5-phenyl-4-hydroxyphenyl)cyclohexane; 1,1-bis(3,5-disopropyl-4-hydroxyphenyl)cyclohexane; 1,1-bis(3,5-di-t-butyl-4-hydroxyphenyl)cyclohexane; 1,1-bis(3,5-diphenyl-4-hydroxyphenyl)cyclohexane; 1,1-bis(4-hydroxy-2,3,5,6-tetrachlorophenyl)cyclohexane; 1,1-bis(4-hydroxy-2,3,5,6-tetrabromophenyl)cyclohexane; 1,1-bis(4-hydroxy-2,3,5,6-tetramethylphenyl)cyclohexane; 1,1-bis(2,6-dichloro-3,5-dimethyl-4-hydroxyphenyl)cyclohexane; 1,1-bis(2,6-dibromo-3,5-dimethyl-4-hydroxyphenyl)cyclohexane; 1,1-bis(4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3-chloro-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3-bromo-4-hydroxyphenyl)-3,3,5-trimethyleyclohexane; 1,1-bis(4-hydroxy-3-methylphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(4-hydroxy-3-isopropylphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3-t-butyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3-phenyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3,5-dichloro-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3,5-dibromo-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3,5-dimethyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3-chloro-4-hydroxy-5-methylphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3-bromo-4-hydroxy-5-methylphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3-chloro-4-hydroxy-5-isopropylphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3-bromo-4-hydroxy-5-isopropylphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3-t-butyl-5-chloro-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3-bromo-5-t-butyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; bis(3-chloro-5-phenyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3-bromo-5-phenyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3,5-disopropyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3,5-di-t-butyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3,5-diphenyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(4-hydroxy-2,3,5,6-tetrachlorophenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(4-hydroxy-2,3,5,6-tetrabromophenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(4-hydroxy-2,3,5,6-tetramethylphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(2,6-dichloro-3,5-dimethyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(2,6-dibromo-3,5-dimethyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 4,4'dihydroxy-1,1-biphenyl; 4,4'-dihydroxy-3,3'-dimethyl-1,1-biphenyl; 4,4'-dihydroxy-3,3'-dioctyl-1,1-biphenyl; 4,4'-dihydroxydiphenylether; 4,4'-dihydroxydiphenylthioether; 1,3-bis(2-(4-hydroxyphenyl)-2-propyl)benzene; 1,3-bis(2-(4-hydroxy-3-methylphenyl)-2-propyl)benzene; 1,4-bis(2-(4-hydroxyphenyl)-2-propyl)benzene and 1,4-bis(2-(4-hydroxy-3-methylphenyl)-2-propyl)benzene.

0028Suitable dihydroxy benzenes III are illustrated by hydroquinone, resorcinol, methylhydroquinone, phenylhydroquinone, 4-phenylresorcinol and 4-methylresorcinol.

0029Suitable dihydroxy naphthalenes IV are illustrated by 2,6-dihydroxy naphthalene; 2,6-dihydroxy-3-methyl naphthalene; and 2,6-dihydroxy-3-phenyl naphthalene.

0030Suitable dihydroxy naphthalenes V are illustrated by 1,4-dihydroxy naphthalene; 1,4-dihydroxy-2-methyl naphthalene; 1,4-dihydroxy-2-phenyl naphthalene and 1,3-dihydroxy naphthalene.

0031The catalyst used in the method of the present invention comprises at least one source of alkaline earth ions or alkali metal ions, and at least one quaternary ammonium compound, quaternary phosphonium compound or a mixture thereof, said source of alkaline earth ions or alkali metal ions being used in an amount such that the amount of alkaline earth or alkali metal ions present in the reaction mixture is in a range between about 10^-5 and about 10^-8 moles alkaline earth or alkali metal ion per mole of dihydroxy aromatic compound employed.

0032The quaternary ammonium compound is selected from the group of organic ammonium compounds having structure VI <chemistry id="chem0006" num="0006"><img file="EP1577333A2_D0006.tif" /></chemistry> wherein R²⁰ -R²³ are independently a C_1-C₂₀ alkyl radical, C₄-C₂₀ cycloalkyl radical or a C₄-C₂₀ aryl radical and X^- is an organic or inorganic anion. In one embodiment of the present invention anion X^- is selected from the group consisting of hydroxide, halide, carboxylate, sulfonate, sulfate, formate, carbonate, and bicarbonate.

0033Suitable organic ammonium compounds comprising structure VI are illustrated by tetramethyl ammonium hydroxide, tetrabutyl ammonium hydroxide, tetramethyl ammonium acetate, tetramethyl ammonium formate and tetrabutyl ammonium acetate.

0034The quaternary phosphonium compound is selected from the group of organic phosphonium compounds having structure VII <chemistry id="chem0007" num="0007"><img file="EP1577333A2_D0007.tif" /></chemistry> wherein R²⁴ -R²⁷ are independently a C_1-C₂₀ alkyl radical, C_4-C₂₀ cycloalkyl radical or a C_4-C₂₀ aryl radical and X- is an organic or inorganic anion. In one embodiment of the present invention anion X^- is an anion selected from the group consisting of hydroxide, halide, carboxylate, sulfonate, sulfate, formate, carbonate, and bicarbonate.

0035Suitable organic phosphonium compounds comprising structure VII are illustrated by tetramethyl phosphonium hydroxide, tetramethyl phosphonium acetate, tetramethyl phosphonium formate, tetrabutyl phosphonium hydroxide, and tetrabutyl phosphonium acetate.

0036Where X- is a polyvalent anion such as carbonate or sulfate it is understood that the positive and negative charges in structures VI and VII are properly balanced. For example, where R²⁰ -R²³ in structure VI are each methyl groups and X-is carbonate, it is understood that X^- represents ½ (CO₃^-2).

0037Suitable sources of alkaline earth ions include alkaline earth hydroxides such as magnesium hydroxide and calcium hydroxide. Suitable sources of alkali metal ions include the alkali metal hydroxides illustrated by lithium hydroxide, sodium hydroxide and potassium hydroxide. Other sources of alkaline earth and alkali metal ions include salts of carboxylic acids, such as sodium acetate and derivatives of ethylene diamine tetraacetic acid (EDTA) such as EDTA tetrasodium salt, and EDTA magnesium disodium salt.

0038In the method of the present invention an ester-substituted diaryl carbonate I, at least one dihydroxy aromatic compound and a catalyst are contacted in a reactor suitable for conducting melt polymerization. The relative amounts of ester-substituted diaryl carbonate and dihydroxy aromatic compound are such that the molar ratio of carbonate I to dihydroxy aromatic compound is in a range between about 1.20 and about 0.8, preferably between about 1.10 and about 0.9 and still more preferably between about 1.05 and about 1.01.

0039The amount of catalyst employed is such that the amount of alkaline earth metal ion or alkali metal ions present in the reaction mixture is in a range between about 1 x 10^-5 and about 1 x 10^-8, preferably between about 5 x 10^-5 and about 1 x 10^-7, and still more preferably between about 5 x 10^-5 and about 5 x 10^-7 moles of alkaline earth metal ion or alkali metal ion per mole dihydroxy aromatic compound employed. The quaternary ammonium compound, quaternary phosphonium compound or a mixture thereof is used in an amount corresponding to about 2.5 x 10^-3 and 1 x 10^-6 moles per mole dihydroxy aromatic compound employed.

0040Typically the ester-substituted diaryl carbonate, at least one dihydroxy aromatic compound and the catalyst are combined in a reactor which has been treated to remove adventitious contaminants capable of catalyzing both the transesterification and Fries reactions observed in uncontrolled melt polymerizations of diaryl carbonates with dihydroxy aromatic compounds. Contaminants such as sodium ion adhering to the walls of a glass lined reactor are typical and may be removed by soaking the reactor in mild acid, for example 3 normal hydrochloric acid, followed by removal of the acid and soaking the reactor in high purity water, such as deionized water.

0041In one embodiment of the present invention an ester-substituted diaryl carbonate, such as bis-methyl salicyl carbonate, at least one dihydroxy aromatic compound, such as BPA, and a catalyst comprising alkali metal ions, such as sodium hydroxide, and a quaternary ammonium compound, such as tetramethyl ammonium hydroxide, or a quaternary phosphonium compound, such as tetrabutyl phosphonium acetate, are charged to a reactor and the reactor is purged with an inert gas such as nitrogen or helium. The reactor is then heated to a temperature in a range between about 100°C and about 340°C, preferably between about 100°C and about 280°C, and still more preferably between about 140°C and about 240°C for a period of from about 0.25 to about 5 hours, preferably from about 0.25 to about 2 hours, and still more preferably from about 0.25 hours to about 1.25 hours. While the reaction mixture is heated the pressure over the reaction mixture is gradually reduced from ambient pressure to a final pressure in a range between about 0.001 mmHg and about 400 mmHg, preferably 0.01 mmHg and about 100 mmHg, and still more preferably about 0.1 mmHg and about 10 mmHg.

0042Control of the pressure over the reaction mixture allows the orderly removal of the phenolic by-product formed when the dihydroxy aromatic compound undergoes a transesterification reaction with a species capable of releasing a phenolic by-product, for example bis-methyl salicyl carbonate or a growing polymer chain endcapped by a methyl salicyl group. As noted above the reaction may be conducted at subambient pressure. In an alternate embodiment of the present invention the reaction may be conducted at slightly elevated pressure, for example a pressure in a range between about 1 and about 2 atmospheres.

0043As noted, the use of excessive amounts of catalyst may affect negatively the structure and properties of a polycarbonate prepared under melt polymerization conditions. The present invention provides a method of melt polymerization employing a highly effective catalyst system comprising at least one source of alkaline earth or alkali metal ions, and a quaternary ammonium compound or quaternary phosphonium compound or mixture thereof which provides useful reaction rates at very low catalyst concentrations, thereby minimizing the amount of residual catalyst remaining in the product polycarbonate. Limiting the amount of catalyst employed according to the method of the present invention provides a new and useful means of controlling the structural integrity of the product polycarbonate as well. Thus, the method of the present invention provides a product polycarbonate having a weight average molecular weight, as determined by gel permeation chromatography, in a range between about 10,000 and about 100,000 Daltons, preferably between about 15,000 and about 60,000 Daltons, and still more preferably between about 15,000 and about 50,000 Daltons said product polycarbonate having less than about 1000, preferably less than about 500, and still more preferably less than about 100 parts per million (ppm) Fries product. Structure VIII below illustrates <chemistry id="chem0008" num="0008"><img file="EP1577333A2_D0008.tif" /></chemistry> the Fries product structure present in a polycarbonate prepared from bisphenol A. As indicated, the Fries product may serve as a site for polymer branching, the wavy lines indicating polymer chain structure.

0044In addition to providing a product polycarbonate containing only very low levels of Fries products, the method of the present invention provides polycarbonates containing very low levels of other undesirable structural features which arise from side reactions taking place during melt the polymerization reaction between ester-substituted diaryl carbonates I and dihydroxy aromatic compounds. One such undesirable structural feature has structure IX and is termed an internal ester-carbonate linkage. Structure IX is thought to arise by reaction of an ester-substituted phenol by-product, for example methyl salicylate, at its ester carbonyl group with a dihydroxy aromatic compound or a hydroxy group of a growing polymer chain. Further reaction of the ester-substituted phenolic hydroxy group leads to formation of a carbonate linkage. Thus, the ester-substituted phenol by-product of <chemistry id="chem0009" num="0009"><img file="EP1577333A2_D0009.tif" /></chemistry> reaction of an ester-substituted diaryl carbonate with a dihydroxy aromatic compound, may be incorporated into the main chain of a linear polycarbonate. The presence of uncontrolled amounts of ester carbonate linkages in the polycarbonate polymer chain is undesirable.

0045Another undesirable structural feature present in melt polymerization reactions between ester-substituted diaryl carbonates and dihydroxy aromatic compounds is the ester-linked terminal group having structure X which possesses a <chemistry id="chem0010" num="0010"><img file="EP1577333A2_D0010.tif" /></chemistry> free hydroxyl group. Structure X is thought to arise in the same manner as structure IX but without further reaction of the ester-substituted phenolic hydroxy group. The presence of uncontrolled amounts of hydroxy terminated groups such as X is undesirable. In structures VIII, IX and X the wavy line shown as represents the product polycarbonate polymer chain structure.

0046The present invention, in sharp contrast to known methods of effecting the melt polymerization of an ester-substituted diaryl carbonate and a dihydroxy aromatic compound, provides a means of limiting the formation of internal ester-carbonate linkages having structure IX as well as ester-linked terminal groups having structure X, during melt polymerization. Thus in a product polycarbonate prepared using the method of the present invention structures, IX and X, when present, represents less than 1mole percent of the total amount of all structural units present in the product polymer derived from dihydroxy aromatic compounds employed as starting materials for the polymer synthesis.

0047An additional advantage of the method of the present invention over earlier methods of melt polymerization of ester-substituted diaryl carbonates and dihydroxy aromatic compounds, derives from the fact that the product polymer is endcapped with ester-substituted phenoxy endgroups and contains very low levels, less than about 50 percent, preferably less than about 10 percent, and still more preferably less than about 1 percent, of polymer chain ends bearing free hydroxy groups. The ester substituted terminal groups are sufficiently reactive to allow their displacement by other phenols such as p-cumylphenol. Thus, following the melt polymerization the product polycarbonate may be treated with one or more exogenous phenols to afford a polycarbonate incorporating endgroups derived from the exogenous phenol. The reaction of the ester substituted terminal groups with the exogenous phenol may be carried out in a first formed polymer melt or in a separate step.

0048In one embodiment of the present invention an exogenous monofunctional phenol, for example p-cumylphenol, is added at the outset of the reaction between the ester substituted diaryl carbonate and the dihydroxy aromatic compound. The product polycarbonate then contains endgroups derived from the exogenous monofunctional phenol. The exogenous monofunctional phenol serves both to control the molecular weight of the product polycarbonate and to determine the identity of the polymer endgroups. The exogenous monofunctional phenol may be added in amounts ranging from about 0.1 to about 10 mole percent, preferably from about 0.5 to about 8 mole percent and still more preferably from about 1 to about 6 mole percent based on the total number of moles of dihydroxyaromatic compound employed in the polymerization. Additional catalyst is not required apart from the catalytically effective amount added to effect the polymerization reaction. Suitable exogenous monofunctional phenols are exemplified by p-cumylphenol; 2,6-xylenol; 4-t-butylphenol; p-cresol; 1-naphthol; 2-naphthol; cardanol; 3,5-di-t-butylphenol, p-nonylphenol; p-octadecylphenol; and phenol. In alternative embodiments of the present invention the exogenous monofunctional phenol may be added at an intermediate stage of the polymerization or after its completion. In such alternative embodiments the exogenous phenol may exert a controlling effect upon the molecular weight of the product polycarbonate and will control the identity of the polymer terminal groups.

0049The present invention may be used to prepare polycarbonate products having very low levels (less than 1 ppm) of trace contaminants such as iron, chloride ion, and sodium ion. Where such extremely low levels of trace contaminants is desired it is sufficient to practice the invention using starting materials, ester-substituted diary carbonate and dihydroxy aromatic compound having correspondingly low levels of the trace contaminants in question. For example, the preparation bisphenol A polycarbonate containing less than 1 ppm each of iron, chloride ion and sodium ion may be made by the method of the present invention using starting materials bis-methyl salicyl carbonate and bisphenol A containing less than 1 ppm iron, chloride ion and sodium ion.

0050The method of the present invention can be conducted as a batch or a continuous process. Any desired apparatus can be used for the reaction. The material and the structure of the reactor used in the present invention is not particularly limited as long as the reactor has an ordinary capability of stirring and the presence of adventitious catalysts can be controlled. It is preferable that the reactor is capable of stirring in high viscosity conditions as the viscosity of the reaction system is increased in later stages of the reaction.

0051Polycarbonates prepared using the method of the present invention may be blended with conventional additives such as heat stabilizers, mold release agents and UV stabilizers and molded into various molded articles such as optical disks, optical lenses, automobile lamp components and the like. Further, the polycarbonates prepared using the method of the present invention may be blended with other polymeric materials, for example, other polycarbonates, polyestercarbonates, polyesters and olefin polymers such as ABS.

EXAMPLES

0052The following examples are set forth to provide those of ordinary skill in the art with a detailed description of how the methods claimed herein are evaluated, and are not intended to limit the scope of what the inventors regard as their invention. Unless indicated otherwise, parts are by weight, temperature is in °C.

0053Molecular weights are reported as number average (M_n) or weight average (M_w) molecular weight and were determined by gel permeation chromatography (GPC) analysis, using a polycarbonate molecular weight standard to construct a broad standard calibration curve against which polymer molecular weights were determined. The temperature of the gel permeation columns was about 25°C and the mobile phase was chloroform.

0054Fries content was measured by the KOH methanolysis of resin and is reported as parts per million (ppm). The Fries content for each of the melt polycarbonates listed in Table 1 was determined as follows. First, 0.50 grams of polycarbonate was dissolved in 4.0 ml of THF (containingp-terphenyl as internal standard). Next, 3.0 ml of 18% KOH in methanol was added to this solution. The resulting mixture was stirred for two hours at this temperature. Next, 1.0 ml of acetic acid was added, and the mixture was stirred for 5 minutes. Potassium acetate by-product was allowed to crystallize over 1 hour. The solid was filtered off and the resulting filtrate was analyzed by liquid chromoatograph usingp-terphenyl as the internal standard.

0055Internal ester-carbonate and terminal hydroxy-ester groups were measured by ¹³C- and ³¹P-NMR respectively. Terminal hydroxy ester groups were first derivatized with 2-chloro-1, 3, 2-dioxaphopholane (Aldrich).

0056Melt polymerization reactions were run in a 100 mL glass reactor adapted for distillation under vacuum equipped with a solid nickel helical agitator. The reactor was configured such that by-product phenol or methyl salicylate could be distilled out of the reaction vessel and condensed in a chilled receiving vessel. Prior to its use, the reactor was soaked in 3N HCl for a period of 12 hours and was then soaked for an additional 12 hours in deionized water (18-Mohm) and placed in a drying oven overnight. Reaction temperature was controlled by immersion of the reactor into a fluidized sand bath equipped with a PID temperature controller. The temperature of the sand bath was monitored at the reactor sand bath interface. The pressure of the reaction vessel was controlled by means of a vacuum pump coupled to a nitrogen bleed. The pressure within the reactor was measured with an MKS pirani gauge. Sodium hydroxide (J.T. Baker, 1 x 10^-6 mole per mole bisphenol A) and tetramethyl ammonium hydroxide (Sachem, 2.5 x 10^-4 mole per mole bisphenol A) or tetrabutyl phosphonium acetate (Sachem, 2.5 x 10^-4 mole per mole bisphenol A) were added as solutions in deionized (18 Mohm) water. Where the catalyst level was varied, the concentration of the catalyst solution was adjusted such that the volume of water introduced in the catalyst introduction step was held constant.

Examples 1-3 and Comparative Examples 1-4

0057The reactor was charged at ambient temperature and pressure with solid bisphenol A (General Electric Plastics Japan Ltd., 0.08761 mol) and solid bis-methyl salicyl carbonate (0.0880-0.0916 mol) or solid diphenyl carbonate (General Electric Plastics Japan Ltd., 0.0946 mol). In some instances a monofunctional phenol such as p-cumylphenol (0.0027- 0.0088 mol) was added in order to limit the molecular weight of the polymer and control chain endgroup identity. The catalyst was then injected into the bisphenol A layer and the reactor was assembled. The reactor was then evacuated briefly and nitrogen was reintroduced. This step was repeated three times. The reactor was then lowered into the sand bath maintained at 180°C. After a five minute period stirring at 250 rpm was initiated. After a total of 10 minutes the reaction mixture had fully melted. The temperature of the bath was raised to 210°C over a five minute period. The pressure in the reactor was then reduced to 180 mmHg at which point the phenolic by-product began to distill from the reaction vessel into the receiving vessel. The reaction mixture was held at 210°C and 180 mmHg for 20 minutes. The pressure was then lowered to 100 mmHg stirred for an additional 20 minutes after which time the temperature was raised to 240 °C over a five minute period. The pressure was then lowered to 15 mmHg and the reaction mixture was stirred at 240°C at 15 mmHg for 20 minutes. The temperature was then raised to 270°C over a five minute period and the pressure was then lowered to 2 mmHg. The reaction mixture was stirred at 270°C at 2 mmHg for 10 minutes. The temperature was then raised to 310°C over a ten minute period and the pressure was lowered to 1.1 mmHg. The reaction mixture was stirred at 310°C at 1.1 mmHg for 30 minutes after which the reaction vessel was raised from the sand bath and the molten product polymer was scooped from the reaction vessel into a liquid nitrogen bath in order to quench the reaction.

0058Data for Examples 1-3 and Comparative Examples 1-5 are gathered in Table 1 and illustrate the utility of the method of the present invention. In the Comparative Examples 1-5 and Examples 1-3 no p-cumylphenol was added as a chainstopper. The column heading "DAC" indicates which diaryl carbonate was employed. "DPC" is diphenyl carbonate used in Comparative-Example 1 (CE-1). "BMSC" is bis-methyl salicyl carbonate. The ratio "DAC/BPA" represents the initial molar ratio of diaryl carbonate to bisphenol A employed in the reaction. "M_n" represents number average molecular weight of the product polymer. "Fries level" indicates the concentration of Fries rearrangement product present in the product polymer. Fries levels were determined by complete solvolysis (KOH catalyzed methanolysis) of the product polymer and quantitative measurement of the amount of Fries product, 2-carboxybisphenol A (CAS No. 101949-49-9), present by HPLC. "EC (%)" represents the percentage of polymer chain ends not terminating in a hydroxyl group. Hydroxyl endgroup concentrations were determined by quantitative infrared spectroscopy. Phenol and salicyl endgroups were determined by HPLC analysis after product solvolysis. <tables id="tabl0001" num="0001"><table frame="all"><title>TABLE 1</title><tgroup cols="6" colsep="1" rowsep="1"><colspec colnum="1" colname="col1" colwidth="26.25mm" /><colspec colnum="2" colname="col2" colwidth="26.25mm" /><colspec colnum="3" colname="col3" colwidth="26.25mm" /><colspec colnum="4" colname="col4" colwidth="26.25mm" /><colspec colnum="5" colname="col5" colwidth="26.25mm" /><colspec colnum="6" colname="col6" colwidth="26.25mm" /><thead valign="top"><row><entry namest="col1" nameend="col6" align="left">REACTION OF BPA WITH DIARYLCARBONATE (DAC)</entry></row><row><entry namest="col1" nameend="col1" align="center">Example</entry><entry namest="col2" nameend="col2" align="center">DAC</entry><entry namest="col3" nameend="col3" align="center">DAC/BPA</entry><entry namest="col4" nameend="col4" align="center">M_n</entry><entry namest="col5" nameend="col5" align="center">Fries level</entry><entry namest="col6" nameend="col6" align="center">EC(%)</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="center">CE-1</entry><entry namest="col2" nameend="col2" align="center">DPC</entry><entry namest="col3" nameend="col3" align="char" char=".">1.08</entry><entry namest="col4" nameend="col4" align="center">6250</entry><entry namest="col5" nameend="col5" align="center">946</entry><entry namest="col6" nameend="col6" align="center">41%</entry></row><row><entry namest="col1" nameend="col1" align="center">Example 1</entry><entry namest="col2" nameend="col2" align="center">BMSC</entry><entry namest="col3" nameend="col3" align="char" char=".">1.08</entry><entry namest="col4" nameend="col4" align="center">4293</entry><entry namest="col5" nameend="col5" align="center">not detected</entry><entry namest="col6" nameend="col6" align="center">>99%</entry></row><row><entry namest="col1" nameend="col1" align="center">Example 2</entry><entry namest="col2" nameend="col2" align="center">BMSC</entry><entry namest="col3" nameend="col3" align="char" char=".">1.04</entry><entry namest="col4" nameend="col4" align="center">8499</entry><entry namest="col5" nameend="col5" align="center">not detected</entry><entry namest="col6" nameend="col6" align="center">>99%</entry></row><row><entry namest="col1" nameend="col1" align="center">Example 3</entry><entry namest="col2" nameend="col2" align="center">BMSC</entry><entry namest="col3" nameend="col3" align="char" char=".">1.02</entry><entry namest="col4" nameend="col4" align="center">13762</entry><entry namest="col5" nameend="col5" align="center">not detected</entry><entry namest="col6" nameend="col6" align="center">98%</entry></row><row><entry namest="col1" nameend="col1" align="center">CE-2¹</entry><entry namest="col2" nameend="col2" align="center">BMSC</entry><entry namest="col3" nameend="col3" align="char" char=".">1.04</entry><entry namest="col4" nameend="col4" align="center">7683</entry><entry namest="col5" nameend="col5" align="center">43</entry><entry namest="col6" nameend="col6" /></row><row><entry namest="col1" nameend="col1" align="center">CE-3²</entry><entry namest="col2" nameend="col2" align="center">BMSC</entry><entry namest="col3" nameend="col3" align="char" char=".">1.04</entry><entry namest="col4" nameend="col4" align="center">7892</entry><entry namest="col5" nameend="col5" align="center">282</entry><entry namest="col6" nameend="col6" /></row><row><entry namest="col1" nameend="col1" align="center">CE-4³</entry><entry namest="col2" nameend="col2" align="center">BMSC</entry><entry namest="col3" nameend="col3" align="char" char=".">1.04</entry><entry namest="col4" nameend="col4" align="center">gel</entry><entry namest="col5" nameend="col5" align="center">8502</entry><entry namest="col6" nameend="col6" /></row><row rowsep="1"><entry namest="col1" nameend="col1" align="center">CE-5³</entry><entry namest="col2" nameend="col2" align="center">BMSC</entry><entry namest="col3" nameend="col3" align="char" char=".">1.0</entry><entry namest="col4" nameend="col4" align="center">5910</entry><entry namest="col5" nameend="col5" align="center">98</entry><entry namest="col6" nameend="col6" align="center">47%</entry></row></tbody></tgroup><tgroup cols="6" colsep="0" rowsep="0"><colspec colnum="1" colname="col1" colwidth="26.25mm" /><colspec colnum="2" colname="col2" colwidth="26.25mm" /><colspec colnum="3" colname="col3" colwidth="26.25mm" /><colspec colnum="4" colname="col4" colwidth="26.25mm" /><colspec colnum="5" colname="col5" colwidth="26.25mm" /><colspec colnum="6" colname="col6" colwidth="26.25mm" /><tbody valign="top"><row><entry namest="col1" nameend="col6" align="justify">¹ Catalyst was NaOH at 1 x 10^-6 mole per mole BPA ² Catalyst was tetrabutylphosphonium acetate at 2.5 x 10^-4 mole per mole BPA ³ Catalyst was lithium stearate at 1 x 10^-3 moles per mole BPA</entry></row></tbody></tgroup></table></tables>

0059Comparative Example 1 highlights differences between melt polymerization behavior of DPC and BMSC in reactions with bisphenol A. Melt polymerization using BMSC (Example 1) affords an undetectable level of Fries product and a very high level of endcapping, whereas melt polymerization using DPC under the same conditions using the same catalyst as that used in Example 1 leads to a high level (946 ppm) of Fries product and a far lower endcapping level. Examples 1-3 illustrate the melt polymerization of BMSC with BPA according to the method of the present invention which affords polycarbonate containing undetectable levels of Fries product and very high endcapping levels.

0060According to the method of the present invention the catalyst comprises at least one source of alkaline earth ions or alkali metal ions, and at least one quaternary ammonium compound, quaternary phosphonium compound or a mixture thereof. Comparative Examples 2 and 3 illustrate this requirement. In Comparative Example 2, a source of alkali metal ions, NaOH, was present but no quaternary ammonium compound or quaternary phosphonium compound was present. In Comparative Example, 3 a quaternary phosphonium compound, tetra butyl phosphonium acetate, was present but no source of alkali metal ions was present. In both instances detectable levels of Fries product were observed.

0061The data in Table 1 further illustrate the superiority of the method of the present invention over earlier polymerization methods using BMSC. Comparative Examples 4 and 5 show the effect of using lithium stearate at a level (1 x 10^-3 mole per mole BPA) taught by US Patent 4, 323, 668. Comparative Example 4 was run using the protocol used in Examples 1-3 and the product polycarbonate showed such a high level of Fries product that its molecular weight could not be determined due to the very high level of branching which occurred. The product polycarbonate was recovered as a gel which could not be dissolved for gel permeation chromatography. Comparative Example 5 was run under a milder temperature regime, the maximum temperature was 260°C (See below), than that used in Examples 1-3 and Comparative Examples 1-4, yet nonetheless the product polymer contained a significant amount of Fries product, 98 ppm.

Comparative Example No. 5 (CE-5, Table 1)

0062The reactor, equipped and passivated as described above, was charged at ambient temperature and pressure with solid bisphenol A (General Electric Plastics Japan Ltd., 0.100 mol) and solid bis-methyl salicyl carbonate (0.100). Lithium stearate catalyst (Kodak, 1 x 10^-3 mole per mole bisphenol A) was added as a solid and the reactor was assembled. The reactor was then evacuated briefly and nitrogen was reintroduced. The degassing step was repeated three times. The reactor was then lowered into the sand bath maintained at 150°C. After a five minute period stirring at 250 rpm was initiated. These conditions were maintained for 60 minutes. The temperature of the bath was then raised to 260°C. The pressure in the reactor was then reduced to 10 mmHg at which point the methyl salicylate by-product began to distill from the reaction vessel into the receiving vessel. The reaction mixture was held at 260°C and 10 mmHg for 10 minutes after which the reaction vessel was raised from the sand bath and the molten product polymer was scooped from the reaction vessel into a liquid nitrogen bath in order to quench the reaction. The product polycarbonate was characterized by gel permeation chromatography and found to have M_w = 14353 M_n = 5910. The level of Fries product was deterinined to be 98 ppm.

0063As noted, it has been found that the inclusion of an exogenous phenol such as p-cumylphenol in the melt reaction of a bisphenol with an ester-substituted diaryl carbonate according to the method of the present invention affords polycarbonate containing p-cumylphenol endgroups. Data are gathered in Table 2 which demonstrate the surprising efficiency of this transformation relative to the analogous reaction using diphenyl carbonate (DPC). Comparative Example 7 illustrates the low levels of PCP incorporation in the product polycarbonate encountered when a mixture of a bisphenol, DPC and an endcapping agent, p-cumylphenol (PCP) are reacted in the melt. Conversely, Example 5 conducted utilizing the method of the present invention reveals a high level of PCP incorporation. In the polycarbonate product formed in Comparative Example 7 roughly half of the endgroups were found by NMR to be derived from PCP and half derived from phenol. <tables id="tabl0002" num="0002"><table frame="all"><title>TABLE 2</title><tgroup cols="7" colsep="1" rowsep="1"><colspec colnum="1" colname="col1" colwidth="22.50mm" /><colspec colnum="2" colname="col2" colwidth="22.50mm" /><colspec colnum="3" colname="col3" colwidth="22.50mm" /><colspec colnum="4" colname="col4" colwidth="22.50mm" /><colspec colnum="5" colname="col5" colwidth="22.50mm" /><colspec colnum="6" colname="col6" colwidth="22.50mm" /><colspec colnum="7" colname="col7" colwidth="22.50mm" /><thead valign="top"><row><entry namest="col1" nameend="col7" align="left">REACTION OF BPA WITH DAC IN THE PRESENCE OF PCP</entry></row><row><entry namest="col1" nameend="col1" align="center">Example</entry><entry namest="col2" nameend="col2" align="center">DAC</entry><entry namest="col3" nameend="col3" align="center">DAC/BPA</entry><entry namest="col4" nameend="col4" align="center">PCP</entry><entry namest="col5" nameend="col5" align="center">M_n</entry><entry namest="col6" nameend="col6" align="center">EC(%)</entry><entry namest="col7" nameend="col7" align="center">%PCP</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="center">CE-6</entry><entry namest="col2" nameend="col2" align="center">DPC</entry><entry namest="col3" nameend="col3" align="char" char=".">1.08</entry><entry namest="col4" nameend="col4" align="center">---</entry><entry namest="col5" nameend="col5" align="center">6250</entry><entry namest="col6" nameend="col6" align="center">41%</entry><entry namest="col7" nameend="col7" align="center">---</entry></row><row><entry namest="col1" nameend="col1" align="center">CE-7</entry><entry namest="col2" nameend="col2" align="center">DPC</entry><entry namest="col3" nameend="col3" align="char" char=".">1.08</entry><entry namest="col4" nameend="col4" align="center">3.05</entry><entry namest="col5" nameend="col5" align="center">5674</entry><entry namest="col6" nameend="col6" align="center">60%</entry><entry namest="col7" nameend="col7" align="center">25%</entry></row><row><entry namest="col1" nameend="col1" align="center">Example 4</entry><entry namest="col2" nameend="col2" align="center">BMSC</entry><entry namest="col3" nameend="col3" align="char" char=".">1.04</entry><entry namest="col4" nameend="col4" align="center">---</entry><entry namest="col5" nameend="col5" align="center">8499</entry><entry namest="col6" nameend="col6" align="center">100%</entry><entry namest="col7" nameend="col7" align="center">---</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="center">Example 5</entry><entry namest="col2" nameend="col2" align="center">BMSC</entry><entry namest="col3" nameend="col3" align="char" char=".">1.03</entry><entry namest="col4" nameend="col4" align="center">5.07</entry><entry namest="col5" nameend="col5" align="center">9744</entry><entry namest="col6" nameend="col6" align="center">99%</entry><entry namest="col7" nameend="col7" align="center">97%</entry></row></tbody></tgroup></table></tables>

0064The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood by those skilled in the art that variations and modifications can be effected within the spirit and scope of the invention.