DETAILED DESCRIPTION OF THE INVENTION

[0018] The present invention may be understood more readily by reference to the following detailed description of preferred embodiments of the invention and the examples included therein. In the following specification and the claims which follow, reference will be made to a number of terms which shall be defined to have the following meanings:

[0019] The singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise.

[0020] “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.

[0021] As used herein, the term “oligomeric polycarbonate” refers to a polycarbonate oligomer having a number average molecular weight of less than 5000 daltons and includes oligomeric polycarbonates comprising polycarbonate repeat units derived from one or more dihydroxy aromatic compounds.

[0022] As used herein, when describing an oligomeric polycarbonate, the expression “polycarbonate repeat units derived from at least one dihydroxy aromatic compound” means a repeat unit incorporated into an oligomeric polycarbonate by reaction of a dihydroxy aromatic compound with a source of carbonyl units, for example the reaction of bisphenol A with bis(methyl salicyl) carbonate.

[0023] As used herein, the term “high molecular weight polycarbonate” means polycarbonate having a number average molecular weight, M _n, of 8000 daltons or more.

[0024] As used herein, the term “solvent” can refer to a single solvent or a mixture of solvents.

[0025] As used herein, the term “solution comprising a solvent and an oligomeric polycarbonate” refers to a liquid comprising an oligomeric polycarbonate, said liquid comprising at least 10 percent by weight solvent.

[0026] As used herein, the term “melt polycarbonate” refers to a polycarbonate made by the transesterification of a diaryl carbonate with a dihydroxy aromatic compound.

[0027] “BPA” is herein defined as bisphenol A or 2,2-bis(4-hydroxyphenyl)propane.

[0028] As used herein the term “Fries product” is defined as a structural unit of the product polycarbonate which upon hydrolysis of the product polycarbonate affords a carboxy-substituted dihydroxy aromatic compound bearing a carboxy group adjacent to one or both of the hydroxy groups of said carboxy-substituted dihydroxy aromatic compound. For example, in bisphenol A polycarbonate prepared by a melt reaction method in which Fries reaction occurs, the Fries product includes those structural features of the polycarbonate which afford 2-carboxy bisphenol A upon complete hydrolysis of the product polycarbonate.

[0029] The terms “Fries product” and “Fries group” are used interchangeably herein.

[0030] The terms “Fries reaction” and “Fries rearrangement” are used interchangeably herein.

[0031] The terms “double screw extruder” and “twin screw extruder” are used interchangeably herein.

[0032] As used herein the term “monofunctional phenol” means a phenol comprising a single reactive hydroxy group.

[0033] The terms “vent port” and “vent” are used interchangeably herein.

[0034] The term “atmospheric vent” as used herein is meant to indicate a vent which is operated at or near atmospheric pressure. Thus, an atmospheric vent being operated under a slight vacuum, such as that commonly designated “house vacuum”, is meant to fall within the ambit of the term “atmospheric vent”.

[0035] As used herein the term “aliphatic radical” refers to a radical having a valence of at least one comprising a linear or branched array of atoms which is not cyclic. The array may include heteroatoms such as nitrogen, sulfur and oxygen or may be composed exclusively of carbon and hydrogen. Examples of aliphatic radicals include methyl, methylene, ethyl, ethylene, hexyl, hexamethylene and the like.

[0036] As used herein the term “aromatic radical” refers to a radical having a valence of at least one comprising at least one aromatic group. Examples of aromatic radicals include phenyl, pyridyl, furanyl, thienyl, naphthyl, phenylene, and biphenyl. The term includes groups containing both aromatic and aliphatic components, for example a benzyl group. Aromatic radicals are further illustrated by radicals which comprise both aromatic and cycloaliphatic structural components. For example, the radical 4-cyclohexylphenyl is ranked herein as an aromatic rather than a cycloaliphatic radical.

[0037] As used herein the term “cycloaliphatic radical” refers to a radical having a valence of at least one comprising an array of atoms which is cyclic but which is not aromatic. The array may include heteroatoms such as nitrogen, sulfur and oxygen or may be composed exclusively of carbon and hydrogen. Examples of cycloaliphatic radicals include cyclopropyl, cyclopentyl, cyclohexyl, tetrahydrofuranyl and the like.

[0038] The present invention provides a method of preparing polycarbonates whereby a solution comprising an oligomeric polycarbonate in a solvent, said oligomeric polycarbonate comprising ester substituted phenoxy endgroups having structure I, is extruded through an extruder adapted to remove solvent. The method of the invention effects both the conversion of the oligomeric polycarbonate to a product polycarbonate having higher molecular weight, and a separation of the solvent initially present in the solution of the oligomeric polycarbonate from the product polycarbonate. Additionally, the method provides for the removal of other volatile materials which may be present in the initial solution of oligomeric polycarbonate, or formed as by-products as the oligomeric polycarbonate is transformed in the extruder to the product polycarbonate.

[0039] The oligomeric polycarbonate comprises polycarbonate repeat units and terminal phenoxy endgroups having structure I. Terminal phenoxy endgroups having structure I include ester substituted phenoxy endgroups generally. Ester substituted phenoxy engroups are exemplified by the 2-ethoxycarbonylphenoxy group, 2-propoxycarbonylphenoxy group, 4-chloro-2-methoxycarbonylphenoxy group, and the 4-cyano-2-methoxycarbonylphenoxy group. Among the various ester substituted phenoxy terminal groups, the 2-methoxycarbonylphenoxy group II is frequently preferred. 2

[0040] The oligomeric polycarbonate comprises repeat units derived from at least one dihydroxy aromatic compound. Dihydroxy aromatic compounds are illustrated by dihydroxy benzenes, for example hydroquinone (HQ), 2-methylhydroquinone, resorcinol, 5-methylresorcinol and the like; dihydroxy naphthalenes, for example 1,4-dihydroxynathalene, 2,6-dihydroxynaphthalene, and the like; and bisphenols, for example bisphenol A and 4,4′-sulfonyldiphenol. The oligomeric polycarbonate typically contains polycarbonate repeat units derived from at least one bisphenol, said polycarbonate repeat units having structure III 3

[0041] wherein R ³ -R ¹⁰ are independently a hydrogen atom, halogen atom, nitro group, cyano group, C ₁ -C ₂₀ alkyl group, C ₄ -C ₂₀ cycloalkyl group, or C ₆ -C ₂₀ aryl group; W is a bond, an oxygen atom, a sulfur atom, a SO ₂ group, a C ₁ -C ₂₀ aliphatic radical, a C ₆ -C ₂₀ aromatic radical, a C ₆ -C ₂₀ cycloaliphatic radical, or the group 4

[0042] wherein R ¹¹ and R ¹² are independently a hydrogen atom, C ₁ C ₂₀ alkyl group, C ₄ -C ₂₀ cycloalkyl group, or C ₄ -C ₂₀ aryl group; or R ¹¹ and R ¹² together form a C ₄ -C ₂₀ cycloaliphatic ring which is optionally substituted by one or more C ₁ -C ₂₀ alkyl, C ₆ -C ₂₀ aryl, C ₅ -C ₂ , aralkyl, C ₅ -C ₂₀ cycloalkyl groups, or a combination thereof.

[0043] Repeat units having structure III are illustrated by repeat units present in bisphenol A polycarbonate, bisphenol M polycarbonate, bisphenol C polycarbonate, and the like.

[0044] In one embodiment of the present invention the oligomeric polycarbonate comprises repeat units having structure IV 5

[0045] said repeat units IV being derived from bisphenol A. In an alternate embodiment of the present invention the oligomeric polycarbonate comprises repeat units having structures IV and V 6

[0046] said repeat units V being derived from 4,4′-sulfonyldiphenol.

[0047] The solution of the oligomeric polycarbonate used according to the method of the present invention comprises at least one solvent. The solvent may be a single solvent or a mixture of solvents. Typically the solvent present in the solution of the oligomeric polycarbonate comprises from about 10 percent by weight to about 99 percent by weight, preferably from about 10 percent by weight to about 70 percent by weight of the solution. For example a solution of oligomeric bisphenol A polycarbonate comprising phenoxy endgroups II dissolved in methyl salicylate, said solution being about 40 percent by weight of said oligomeric polycarbonate and about 60 percent by weight methyl salicylate. Alternatively the solution may comprise more than one solvent, for example a solution of oligomeric bisphenol A polycarbonate comprising phenoxy endgroups II dissolved in a mixture of ortho-dichlorobenzene (ODCB) and methyl salicylate, said solution being about 40 percent by weight of said oligomeric polycarbonate, 30 percent by weight ODCB, and about 30 percent by weight methyl salicylate.

[0048] In one embodiment of the present invention the solvent employed according to the method of the present invention comprises at least one ester substituted phenol having structure VI 7

[0049] wherein R ¹ , R ² , and b are defined as in structure I. Examples of ester-substituted phenols having structure VI include methyl salicylate, ethyl salicylate, butyl salicylate, 4-chloro methyl salicylate, and mixtures thereof. Solvent VI may be recovered and reused. For example, ester substituted phenols such as VI may be recovered, purified, and reacted with phosgene to make ester substituted diaryl carbonates which in turn can be used to prepare oligomeric polycarbonates comprising terminal phenoxy groups having structure I. Typically, purification of the recovered ester substituted phenol is efficiently carried out by distillation.

[0050] The solvent used according to the method of the present invention optionally comprises a halogenated aliphatic solvent, a halogenated aromatic solvent, a non-halogenated aromatic solvent, a non-halogenated aliphatic solvent, or a mixture thereof. Halogenated aromatic solvents are illustrated by ortho-dichlorobenzene (ODCB), chlorobenzene and the like. Non-halogenated aromatic solvents are illustrated by toluene, xylene, anisole, phenol; 2,6-dimethylphenol; and the like. Halogenated aliphatic solvents are illustrated by methylene chloride; chloroform; 1,2-dichloroethane; and the like. Non-halogenated aliphatic solvents are illustrated by ethanol, acetone, ethyl acetate, cyclohexanone, and the like.

[0051] In one embodiment of the present invention the solvent employed comprises a mixture of a halogenated aromatic solvent and an ester substituted phenol, for example a mixture of ortho-dichlorobenzene (ODCB) and methyl salicylate.

[0052] The transesterification catalyst used according to the present invention may be any catalyst effective in promoting chain growth of the oligomeric polycarbonate during the extrusion. The transesterification catalysts for use according to the method of the present invention may comprise onium catalysts such as a quaternary ammonium compound, a quaternary phosphonium compound, or a mixture thereof.

[0053] Quaternary ammonium compounds suitable for use as transesterifcation catalysts according to the method of the present invention include quaternary ammonium compounds having structure VII 8

[0054] wherein R ¹³ -R ¹⁶ are independently a C ₁ -C ₂₀ alkyl group, C ₄ -C ₂₀ cycloalkyl group, or a C ₄ -C ₂₀ aryl group; and X ⁻ is an organic or inorganic anion.

[0055] Quaternary ammonium compounds VII are illustrated by tetamethylammonium hydroxide, tetrabutylammonium acetate, tetrabutylammonium hydroxide, and the like.

[0056] Quaternary phosphonium compounds suitable for use as transesterifcation catalysts according to the method of the present invention include quaternary phosphonium compounds having structure VIII 9

[0057] wherein R ¹⁷ -R ²⁰ are independently a C ₁ -C ₂₀ alkyl group, C ₄ -C ₂₀ cycloalkyl group, or a C ₄ -C ₂₀ aryl group; and X ⁻ is an organic or inorganic anion.

[0058] Quaternary phosphonium compounds VIII are illustrated by tetamethylphosphonium hydroxide, tetrabutylphosphonium acetate, tetrabutylphosphonium hydroxide, and the like.

[0059] In structures VII and VIII, the anion X ⁻ is typically an anion selected from the group consisting of hydroxide, halide, carboxylate, phenoxide, sulfonate, sulfate, carbonate, and bicarbonate. With respect to transesterifcation catalysts having structures VII and VIII, where X ⁻ is a polyvalent anion such as carbonate or sulfate it is understood that the positive and negative charges in structures VII and VIII are properly balanced. For example, in tetrabutylphosphonium carbonate where R ¹⁷ -R ²⁰ in structure VIII are each butyl groups and X ⁻ represents a carbonate anion, it is understood that X ⁻ represents ½ (CO ₃ ⁻² ).

[0060] In one embodiment, the transesterification catalyst used is a combination of a quaternary ammonium compound, a quaternary phosphonium compound, or a mixture thereof, with at least one alkali metal hydroxide, alkaline earth metal hydroxide, or a mixture thereof. For example, a mixture of tetrabutylphosphonium acetate and sodium hydroxide.

[0061] Other transesterification catalysts that may be used according to the method of the present invention include one or more alkali metal salts of carboxylic acids, one or more alkaline earth salts of a carboxylic acid, and mixtures thereof. Such transesterifcation catalysts are illustrated by simple salts of carboxylic acids such as sodium acetate, calcium stearate and the like. Additionally, alkali metal and alkaline earth metal salts of organic polyacids may serve as efficient transesterification catalysts according to the method of the present invention. Alkali metal and alkaline earth metal salts of organic polyacids, such as ethylene diamine tetracarboxylate, may be employed. Salts of organic polyacids are illustrated by disodium magnesium ethylenediamine tetracarboxylate (Na ₂ Mg EDTA).

[0062] In one embodiment of the present invention the transesterification catalyst comprises at least one salt of a non-volatile acid. By “non-volatile” it is meant that the acid from which the catalyst is made has no appreciable vapor pressure under melt polymerization conditions. Examples of non-volatile acids include phosphorous acid, phosphoric acid, sulfuric acid, and metal “oxo acids” such as the oxo acids of germanium, antimony, niobium and the like. Salts of non-volatile acids useful as melt polymerization catalysts according to the method of the present invention include alkali metal salts of phosphites; alkaline earth metal salts of phosphites; alkali metal salts of phosphates; alkaline earth metal salts of phosphates, alkali metal salts of sulfates, alkaline earth metal salts of sulfates, alkali metal salts of metal oxo acids, and alkaline earth metal salts of metal oxo acids. Specific examples of salts of non-volatile acids include NaH ₂ PO ₃ , NaH ₂ PO ₄ , Na ₂ HPO ₄ , KH ₂ PO ₄ , CsH ₂ PO ₄ , Cs ₂ HPO ₄ , NaKHPO ₄ , NaCsHPO ₄ , KCsHPO ₄ , Na ₂ SO ₄ , NaHSO ₄ , NaSbO ₃ , LiSbO ₃ , KSbO ₃ , Mg(SbO ₃ ) ₂ , Na ₂ GeO ₃ , K ₂ GeO ₃ , Li ₂ GeO ₃ , Mg GeO ₃ , Mg ₂ GeO ₄ , and mixtures thereof.

[0063] Typically the transesterification catalyst is employed in an amount corresponding to between about 1.0×10 ⁻⁸ and about 1×10 ⁻³ , preferably between about 1.0×10 ⁻⁶ and about 2.5×10 ⁻⁴ moles of transesterification catalyst per mole of polycarbonate repeat units derived from aromatic dihydroxy compound present in the oligomeric polycarbonate.

[0064] Typically, the oligomeric polycarbonate employed is prepared in a step involving heating a dihydroxy aromatic compound with an ester substituted diaryl carbonate in the presence of a transesterification catalyst. Thus, the reactants are combined in a vessel in a ratio between about 0.95 and 1.3 moles, preferably between about 1.0 and about 1.05 moles of ester substituted diaryl carbonate per mole of dihydroxy aromatic compound. The amount of transesterification catalyst employed is between about 1.0×10 ⁻⁸ and about 1×10 ⁻³ preferably between about 1.0×10 ⁻⁶ and about 2.5×10 ⁻⁴ moles of transesterification catalyst per mole of dihydroxy aromatic compound employed. Upon heating the mixture at one or more temperatures in a range from about 100° C. to about 400° C., preferably from about 100° C. to about 300° C., and still more preferably from about 100° C. to about 250° C., reaction occurs to produce a solution comprising an equilibrium mixture of product oligomeric polycarbonate, by-product ester substituted phenol (solvent), transesterification catalyst, and low levels of the starting materials, dihydroxy aromatic compound and ester substituted diaryl carbonate. This is referred to as “equilibrating” the reactants. Typically the equilibrium strongly favors the formation of product oligomeric polycarbonate and by-product ester substituted phenol and only traces of the starting materials are observed. The “equilibrated” product mixture may then be introduced into a devolatilizing extruder to effect removal of the by-product ester substituted phenol solvent while converting the oligomeric polycarbonate into a higher molecular weight product polycarbonate. Because, the transesterification catalyst is typically neither consumed in the equilibration step nor removed prior to extrusion, there is typically no need to add additional catalyst during extrusion. Where no additional catalyst is added, the amount of catalyst present during extrusion step (expressed in terms of moles catalyst per mole of polycarbonate repeat units in the oligomeric polycarbonate) will closely approximate the amount of catalyst used in the equilibration step, expressed in moles catalyst per mole dihydroxy aromatic compound.

[0065] Typically the ester substituted diaryl carbonate will have structure IX 10

[0066] wherein R ¹ , R ² , and b are defined as in structure I. In addition, the dihydroxy aromatic compound is typically, though not always, at least one bisphenol having structure X 11

[0067] wherein R ³ -R ¹⁰ and W are defined as in structure III.

[0068] In one embodiment, the method of the present invention comprises:

[0069] Step (I) heating a mixture comprising at least one dihydroxy aromatic compound, an ester substituted diaryl carbonate and a transesterification catalyst at a temperature in a range between about 100° C. and about 300° C. to provide a solution of an oligomeric polycarbonate in an ester substituted phenol solvent; and

[0070] Step (II) extruding said solution of oligomeric polycarbonate in said ester substituted phenol at one or more temperatures in a range between about 100° C. and about 400° C., and at one or more screw speeds in a range between about 50 and about 1200 rpm, said extruding being carried out on an extruder comprising at least one vent adapted for solvent removal.

[0071] In some instances it may be desirable to remove a portion of the ester-substituted phenol formed during the equilibration of the monomers. This may be effected conveniently by heating the mixture of monomers and the transesterification catalyst under vacuum, typically from about 0.01 atmospheres to about 0.9 atmospheres, and distilling off a portion of the ester substituted phenol. As ester substituted phenol is distilled from the mixture undergoing the equilibration reaction, the molecular weight of the oligomeric polycarbonate will tend to increase. If sufficient ester substituted phenol by-product is removed, the number average molecular weight (M _n ) of the polycarbonate product may be in excess of 5000 daltons and in some instances in excess of 8000 daltons. Thus, in one aspect of the present invention a mixture comprising at least dihydroxy aromatic compound is reacted with at least one ester substituted diaryl carbonate in the presence of a transesterification catalyst at a temperature between about 100° C. and about 300° C. and a portion of the by-product ester substituted phenol is removed by distillation. The equilibration product may be a mixture comprising an ester substituted phenol solvent and a polycarbonate comprising terminal phenoxy groups having structure I and having a number average molecular weight in excess of 5000 daltons. This equilibration product is then fed to a devolatilizing extruder wherein the polycarbonate is converted to still higher molecular weight product polycarbonate, said product polycarbonate having a high level of endcapping, a low level of Fries product, and a low level of residual solvent. In one embodiment of the present invention, a portion of the ester substituted phenol formed during equilibration is distilled from the mixture undergoing equilibration and a like amount of ODCB is added to provide a solution comprising a polycarbonate having a number average molecular weight in excess of 5000 daltons, ester substituted phenol and ODCB. This solution is then fed to a devolatilizing extruder wherein the polycarbonate is converted to a product polycarbonate having a higher molecular weight, said product polycarbonate having a Fries content of under 10 ppm, an endcapping level of at least 97%, and less than 1 percent by weight solvent. Typically, in instances in which the polycarbonate formed in the equilibration reaction has a number average molecular weight in excess of 5000 daltons, it will have a M _n value in a range between 5000 daltons and about 15000 daltons.

[0072] Oligomeric polycarbonates comprising ester substituted terminal phenoxy groups I may be prepared by a variety of other methods in addition to the equilibration method described. For example, oligomeric bischloroformates of bisphenols may be prepared by reaction of one or more bisphenols with phosgene under interfacial conditions in a methylene chloride water mixture at low pH. Such bischloroformates may then be further reacted under interfacial conditions with an ester substituted phenol, for example methyl salicylate, to afford an oligomeric polycarbonate comprising ester substituted terminal phenoxy groups in methylene chloride solution. The product oligomeric polycarbonate in solution may then be subjected to the method of the present invention. Catalysts employed during the interfacial reaction are typically removed from the solution of the oligomeric polycarbonate in a series of washing steps in which the methylene chloride solution of the oligomeric polycarbonate is washed repeatedly with water to remove sodium chloride. Under such circumstances, additional catalyst may be required and may be added during or just prior to the extrusion step.

[0073] In one embodiment, a monofunctional phenol chainstopper is added to a solution of an oligomeric polycarbonate comprising ester substituted phenoxy terminal groups, said oligomeric polycarbon ate being prepared using the equilibration technique described herein. The solution is then subjected to extrusion devolatilization to afford a product polycarbonate incorporating terminal phenoxy groups derived from said chainstopper. Suitable monofunctional phenol chainstoppers include p-cumylphenol and cardanol.

[0074] The extruder used according to the method of the present invention is of the devolatilizing extruder type. That is, it is an extruder adapted for separating substantial amounts of solvent from a polymer-solvent mixture. The extruder, therefore must possess at least one and preferably a greater number of vents adapted for solvent removal. FIG. 1 , FIG. 2 and FIG. 3 illustrate devolatilizing extruders and feed systems suitable for use according to the method of the present invention. In one embodiment of the invention (illustrated here with reference to FIG. 1 ) reactants, ester substituted diaryl carbonate, dihydroxy aromatic compound and a transesterification catalyst are combined in a reaction vessel 10 and heated at a temperature in a range between about 100° C. and about 300° C., preferably between about 150° C. and about 250° C., at a pressure between about I atmosphere and about 10 atmospheres, preferably between about 1 and about 2 atmospheres, to provide a solution of an oligomeric polycarbonate in an ester substituted phenol. The solution is transferred by means of a gear pump 14 via piping 17 which is directly plumbed into a fourteen barrel, vented, twin screw extruder 20 , said extruder possessing screw design 30 . The extruder is operated at a temperature between about 100° C. and about 400° C., preferably between about 200° C. and about 350° C., at a screw speed between about 50 and about 1200 rpm. The solution is introduced into the upstream edge of barrel one 22 . The segmentations along the extruder indicate the transitions from one extruder barrel to the next. Barrel two is labeled 24 . (The remaining barrels 3 - 14 are not labeled.) The extruder screw design 30 consists of conveying screw elements illustrated by 32 and mixing sections which include an initial mixing section 34 and four zones of intense mixing 36 . The extruder is equipped with four atmospheric vents 40 , said vents being connected to a manifold 42 for removal of ester substituted phenol solvent and other volatile by-products formed as the oligomeric polycarbonate is converted into product polycarbonate within the extruder. Solvent vapors and other volatile by-products are condensed in a shell and tube condenser 44 which is attached to a source of house vacuum 46 . The extruder is further equipped with two vacuum vents 50 . Vacuum vents 50 are connected via a cold trap 52 to a vacuum pump 54 . As mentioned, the extruder comprises four mixing sections which provide for intense mixing of the contents of the extruder. These are indicated in the screw design 30 as the mixing sections labeled 36 . Mixing sections labeled 36 in the screw design correspond to reaction zones 26 of the extruder. Said reaction zones are believed to provide for enhanced rates of polycarbonate chain growth relative to other domains within the extruder.

[0075] In an alternate embodiment of the invention (illustrated here with reference to FIG. 2 ) reactants, ester substituted diaryl carbonate, dihydroxy aromatic compound and a transesterification catalyst are combined in a reaction vessel 10 and heated at a temperature in a range between about 100° C. and about 300° C., preferably between about 150° C. and about 250° C., at a pressure between about 0.0001 and about 10 atmospheres, preferably between about 0.001 and about 2 atmospheres, to provide a solution of an oligomeric polycarbonate in an ester substituted phenol. The reaction vessel 10 is adapted for operation at either subambient, ambient or elevated pressure, and is equipped with a mixer 11 . The reaction vessel 10 is connected via a solvent recovery manifold 12 to a condenser 44 . The solution is transferred by means of a gear pump 14 through a heat exchanger 15 wherein the solution is superheated. Heat exchanger 15 is linked to pressure control valve 16 which is directly plumbed into a fourteen barrel, vented, twin screw extruder 20 , said extruder possessing screw design 30 . The extruder is operated at a temperature between about 100° C. and about 400° C., preferably between about 200° C. and about 350° C., at a screw speed between about 50 and about 1200 rpm. The solution is introduced into the upstream edge of barrel one 22 . The segmentations along the extruder indicate the transitions from one extruder barrel to the next. Barrel two is labeled 24 . (The remaining barrels 3 - 14 are not labeled.) The extruder screw design 30 consists of conveying screw elements illustrated by 32 and mixing sections which include an initial mixing section 34 and four zones of intense mixing 36 . The extruder is equipped with four atmospheric vents 40 , said vents being connected to a manifold 42 for removal of ester substituted phenol solvent and other volatile by-products formed as the oligomeric polycarbonate is converted into product polycarbonate within the extruder. Solvent vapors and other volatile by-products are condensed in a shell and tube condenser 44 which is attached to a source of house vacuum 46 . The extruder is further equipped with two vacuum vents 50 . Vacuum vents 50 are connected via a cold trap 52 to a vacuum pump 54 . As mentioned, the extruder comprises four mixing sections which provide for intense mixing of the contents of the extruder. These are indicated in the screw design 30 as the mixing sections labeled 36 . Mixing sections labeled 36 in the screw design correspond to reaction zones 26 of the extruder. Said reaction zones are believed to provide for enhanced rates of polycarbonate chain growth relative to other domains within the extruder.

[0076] In yet another embodiment of the invention (illustrated here with reference to FIG. 3 ) monomers; ester substituted diaryl carbonate, and dihydroxy aromatic compound, are combined in a vessel 1 and heated at a temperature in a range between about 100° C. and about 300° C., preferably between about 150° C. and about 250° C., to provide a molten mixture of monomers which is transferred forward into the system the system by means of gear pump 14 through. Vessel 1 is optimally equipped with a means for stirring the molten mixture. Flow meter 2 provides for accurate metering of the molten mixture of monomers into the downstream sections of the system. A transesterification catalyst may be added to the molten mixture of monomers at point 3 or directly into vessel 1 . The molten mixture of monomers optionally comprising a transesterification catalyst is introduced via static mixer 4 into heat exchanger 15 and from thence into reaction vessel 10 . Reaction vessel 10 comprises at least one stirrer 11 and is adapted for removal of ester substituted phenol solvent formed as the monomers equilibrate to form a solution comprising an oligomeric polycarbonate, said oligomeric polycarbonate comprising ester-substituted phenoxy endgroups. Ester-substituted phenol solvent is removed via solvent removal manifold 12 which is attached to condenser 44 , cold trap 52 and vacuum pump 54 . Reaction vessel 10 is further adapted for introduction of additional monomers, catalyst or solvents at one or more of points 17 and 18 . Typically the solution of oligomeric polycarbonate is heated to a temperature between about 150° C. and 300° C. The solution of oligomeric polycarbonate is then transferred via gear pump 14 via pressure control valve 16 which is directly plumbed into the feed inlet zone of a fourteen barrel, vented, twin screw, devolatilizing extruder 20 , said extruder possessing screw design 30 . The extruder is operated at a temperature between about 100° C. and about 400° C., preferably between about 200° C. and about 350° C., at a screw speed between about 50 and about 1200 rpm. The solution is introduced into the upstream edge of barrel 3 . The segmentations along the extruder indicate the transitions from one extruder barrel to the next. Barrel two is labeled 24 . (The remaining barrels 3 - 14 are not labeled.) The extruder screw design 30 consists of conveying screw elements illustrated by 32 and mixing sections which include an initial mixing section 34 and four zones of intense mixing 36 . The extruder is equipped with an atmospheric vent 40 connected to manifold 42 for removal of ester substituted phenol solvent and other volatiles. Solvent vapors and other volatile by-products are condensed in a shell and tube condenser 44 which may be attached to a source of house vacuum or operated at atmospheric pressure. Atmospheric vent 40 is positioned so that at least one kneading block 34 is interposed between the feed inlet zone (the zone directly under pressure control valve 16 ) and the atmospheric vent. This interposition of the kneading block between the feed inlet zone and the atmospheric vent serves to prevent entrainment of solids out through the atmospheric vent by escaping solvent vapors. The extruder 20 is further equipped with four vacuum vents 50 . Here again, a kneading block 34 is interposed between the feed inlet zone the vacuum vents in order to prevent the entrainment of solids by the rapidly escaping solvent vapors. Vacuum vents are located over conveying elements 32 to minimize the movement of polymer into the vents themselves. Vacuum vents 50 are connected via solvent manifolds 42 , condensers 44 , and cold traps 52 , to vacuum pumps 54 . As mentioned, the extruder comprises four mixing sections which provide for intense mixing of the contents of the extruder. These are indicated in the screw design 30 as the mixing sections labeled 36 . Mixing sections labeled 36 in the screw design correspond to reaction zones 26 of the extruder. The reaction zones 26 are believed to provide for enhanced rates of polycarbonate chain growth relative to other domains within the extruder. Additionally, the system illustrated in FIG. 3 comprises optional water injection inlets 60 which provide for enhanced removal of volatile components from the polycarbonate being prepared. Finally, the extruder is equipped with one or more sensors 70 which may be used to monitor melt temperature, die pressure, torque or other system parameters which in turn may be used as when operating the system according to a closed loop control strategy. For example, because melt temperature, die pressure and torque are strongly dependent upon the molecular weight of the product polycarbonate, sensors monitoring melt temperature and torque might indicate that additional monomer or catalyst should be added upstream (e.g. at inlet 17 ) in order to reduce or increase the molecular weight of the product polycarbonate emerging from the extruder die face.

[0077] The extruder used according to the method of the present invention, which may be a single screw or multiple screw extruder is typically operated at one or more temperatures in a range between about 100° C. and about 400° C. and at one or more screw speeds in a screw speed range, said range being between about 50 revolutions per minute (rpm) and about 1200 rpm, preferably between about 50 rpm and about 500 rpm.

[0078] Extruders suitable for use according to the method of the present invention include co-rotating intermeshing double screw extruders, counter-rotating non-intermeshing double screw extruders, single screw reciprocating extruders, and single screw non-reciprocating extruders.

[0079] It is a general principle of extruder operation that as the feed rate is increased a corresponding increase in the screw speed must be made in order to accommodate the additional material being fed. Moreover, the screw speed determines the residence time of the material being fed to the extruder, here the solution of the oligomeric polycarbonate and transesterification catalyst. Thus the screw speed and feed rate are typically interdependent. It is useful to characterize this relationship between feed rate and screw speed as a ratio. Typically the extruder is operated such that the ratio of starting material introduced into the extruder in pounds per hour to the screw speed expressed in rpm falls within a range of from about 0.01 to about 100, preferably from about 0.05 to about 5. For example, the ratio of feed rate to screw speed where the solution of comprising an oligomeric polycarbonate and transesterification catalyst are being introduced at 1000 pounds per hour into an extruder being operated at 400 rpm is 2.5. The maximum and minimum feed rates and extruder screw speeds are determined by, among other factors, the size of the extruder, the general rule being the larger the extruder the higher the maximum and minimum feed rates.

[0080] As noted, in one embodiment of the present invention, a mixture of an oligomeric polycarbonate comprising endgroups having structure I and a solvent is heated under pressure to produce a “superheated” solution, meaning that the temperature of said superheated solution is greater than the boiling point of the solvent at atmospheric pressure. Typically, the temperature of the superheated oligomeric polycarbonate will be between about 2° C. and about 200° C. higher than the boiling point of the solvent at atmospheric pressure. In instances where there are multiple solvents present, the solution of oligomeric polycarbonate is “superheated” with respect to at least one of the solvent components. Where the solution of oligomeric polycarbonate contains significant amounts of both high and low boiling solvents, it may be advantageous to superheat the solution of oligomeric polycarbonate with respect to all solvents present (i.e. above the boiling point at atmospheric pressure of the highest boiling solvent). Superheating of the solution of the oligomeric polycarbonate may be achieved by heating the mixture under pressure, typically at a pressure less than about 10 atmospheres but greater than one atmosphere. Superheated solutions of oligomeric polycarbonates are conveniently prepared in pressurized heated feed tanks, pressurized heat exchangers, extruders, pressurized reaction vessels and the like. The superheated solution is then introduced into a devolatilizing extruder through a pressure control valve, the pressure control valve having a cracking pressure higher than atmospheric pressure. The backpressure generated by the pressure control valve prevents evaporation of the solvent prior to introducing the solution into the extruder. Typically, the pressure control valve is attached (plumbed) directly to the extruder and serves as the principal feed inlet of the extruder. In one embodiment of the present invention in which the oligomeric polycarbonate comprising ester-substituted phenoxy endgroups is introduced into a devolatilizing extruder as a superheated solution, the extruder is equipped with at least one side feeder.

[0081] In one embodiment, the extruder in combination with the side feeder is equipped with one or more atmospheric vents in close proximity to the principal feed inlet comprising the pressure control valve. The side feeder is typically positioned in close proximity to the pressure control valve through which the superheated oligomeric polycarbonate is introduced into the extruder. The side feeder comprises at least one atmospheric vent. Alternatively, the pressure control valve through which the superheated oligomeric polycarbonate is introduced may be attached to the side feeder itself in which instance the pressure control valve is attached to the side feeder at a position between the point of attachment of the side feeder to the extruder and the atmospheric vent located on the side feeder. In yet another alternative embodiment, the superheated solution of oligomeric polycarbonate may be introduced through multiple pressure control valves which may be attached to the side feeder, the extruder, or to both extruder and side feeder. The heated zones of the extruder are typically operated at one or more temperatures between about 100° C. and about 400° C. The expression “wherein the extruder is operated at a temperature between about 100° C. and about 400° C.” refers to the heated zones of the extruder, it being understood that the extruder may comprise both heated and unheated zones.

[0082] The superheated solution of oligomeric polycarbonate passes through the pressure control valve into the feed zone of the extruder which due to the presence of the aforementioned atmospheric vents is at atmospheric pressure. The solvent present in the superheated solution of oligomeric polycarbonate undergoes sudden and rapid evaporation thereby effecting at least partial separation of the oligomeric polycarbonate and the solvent. The solvent vapors emerge through the atmospheric vents. The atmospheric vents are attached to a solvent vapor manifold and condenser in order to recover solvent and prevent its adventitious release. Additionally, the extruder is equipped with at least one vent operated at subatmospheric pressure which serves to remove solvent not removed through the atmospheric vents. Vents operated at subatmospheric pressure are referred to herein as “vacuum vents” and are maintained at from about 1 to about 30, preferably from about 10 to about 29 inches of mercury as measured by a vacuum gauge measuring vacuum (as opposed to a pressure gauge measuring pressure). Typically, at least two vacuum vents are preferred.

[0083] Extruders suitable for use in embodiments of the present invention wherein a superheated oligomeric polycarbonate solution is being fed include co-rotating intermeshing double screw extruders, counter-rotating non-intermeshing double screw extruders, single screw reciprocating extruders, and single screw non-reciprocating extruders.

[0084] In some instances, it may be found that the product polycarbonate prepared according to the method of the present invention is of insufficient molecular weight or retains too much of the solvent originally present in the solution of the oligomeric polycarbonate. In such instances, simply subjecting the product polycarbonate to a second extrusion on the same or a different devolatilizing extruder typically results in a product polycarbonate having an increased molecular weight and a reduced level of residual solvent. Thus, in one embodiment of the present invention, a solution of an oligomeric polycarbonate comprising terminal groups having structure I and a solvent is subjected to devolatilization extrusion at a temperature between about 100° C. and about 400° C. on an extruder equipped with at least one vent adapted for solvent removal to provide an initial product polycarbonate. The initial product polycarbonate is then introduced into a second extruder, said second extruder being equipped with at least one vacuum vent. The second extruder is operated at a temperature in a range between about 100° C. and about 400° C., and at a screw speed in a range between about 50 and about 1200 rpm.

[0085] The method of the present invention may be carried out in a batch or continuous mode. In one embodiment, the method of the present invention is carried out as a batch process wherein monomers and transesterification catalyst are equilibrated in a batch reactor to form a solution of the oligomeric polycarbonate. This solution is then fed to a devolatilizing extruder and the product polycarbonate is isolated until the solution is consumed. Alternatively, the method of the present invention may be carried out as a continuous process wherein the monomers and catalyst are continuously fed to, and the solution of oligomeric polycarbonate is continuously removed from a continuous reactor. Thus a mixture of BMSC, BPA and transesterification catalyst may be fed to one end of a tube reactor heated to a temperature between about 160° C. and about 250° C. A solution of an oligomeric polycarbonate comprising phenoxy endgroups II emerges at the opposite end of the tube reactor and is fed to a devolatilizing extruder from which emerges the product polycarbonate.

[0086] It is understood, especially for melt reactions of the type presented in the instant invention, that purity of the monomers employed may strongly affect the properties of the product polycarbonate. Thus, it is frequently desirable that the monomers employed be free of, or contain only very limited amounts of, contaminants such as metal ions, halide ions, acidic contaminants and other organic species. This may be especially true in applications such as optical disks, (e.g. compact disks) where contaminants present in the polycarbonate can affect disk performance. Typically the concentration of metal ions, for example iron, nickel, cobalt, sodium, and postassium, present in the monomer should be less than about 10 ppm, preferably less than about 1 ppm and still more preferably less than about 100 parts per billion (ppb). The amount of halide ion present in the polycarbonate, for example fluoride, chloride and bromide ions, should be minimized in order to inhibit the absorption of water by the product polycarbonate as well as to avoid the corrosive effects of halide ion on equipment used in the preparation of the polycarbonate. Certain applications, for example optical disks, may require very low levels of halide ion contaminants. Preferably, the level of halide ion present in each monomer employed should be less than about 1 ppm. The presence of acidic impurities, for example organic sulfonic acids which may be present in bisphenols such as BPA, should be minimized since only minute amounts of basic catalysts are employed in the oligomerization and subsequent polymerization steps. Even a small amount of an acidic impurity may have a large effect on the rate of oligomerization and polymerization since it may neutralize a substantial portion of the basic catalyst employed. Lastly, the tendency of polycarbonates to degrade at high temperature, for example during molding, with concomitant loss of molecular weight and discoloration correlates strongly with the presence of contaminating species within the polycarbonate. In general, the level of purity of a product polycarbonate prepared using a melt reaction method such as the instant invention will closely mirror the level of purity of the starting monomers.

[0087] Product polycarbonates prepared by the method of the present invention frequently contain only very low levels of Fries products. In many cases no Fries product is detectable when the polycarbonate is subjected to a Fries product analysis. The Fries product analysis is carried out by completely hydrolyzing the polycarbonate and analyzing the hydrolysis product by HPLC. For bisphenol A polycarbonate produced by the method of the present invention, the level of Fries product is a value expressed as parts 2-carboxy bisphenol A per million parts of the product bisphenol A polycarbonate which was subjected to hydrolysis. For bisphenol A polycarbonates prepared using the method of the present invention this value is frequently zero or very close to it.

[0088] The product polycarbonates prepared according to the method of the present invention are found to have very high levels, frequently 97 percent or higher, of endcapping. Typically product polycarbonates will be from about 97 to about 99 percent endcapped. Free hydroxyl groups at the polycarbonate chain ends are typically comprise less than about 100 ppm of the total polymer weight. Two types of free hydroxyl chain ends are typically observed for polycarbonates prepared according to the method of the present invention from BPA and BMSC: hydroxyl groups attached to a BPA residue (“BPA OH”), and hydroxyl groups attached to a salicyl ester residue (“salicyl OH”). Typically, the concentration of “BPA OH” endgroups is less than about 100 ppm based on the total weight of the product polymer. Likewise, the concentration of “salicyl OH” is typically less than about 100 ppm. Endgroups bearing “salicyl OH” groups have the structure indicated by structure XI 12

[0089] and are quantified by nuclear magnetic resonance spectroscopy (NMR). It should be noted that the concentrations of hydroxyl endgroups and percent endcapping described above refers to product polycarbonate and not the oligomeric polycarbonate. Additionally, in instances in which the product polycarbonate has been prepared by first equilibrating a mixture of an ester substituted diaryl carbonate with one or more dihydroxy aromatic compounds to afford a solution comprising an oligomeric polycarbonate and subsequently subjecting said solution to extrusion on a devolatilizing extruder, the concentrations of hydroxyl endgroups and percent endcapping in the product polycarbonate will reflect the molar ratio of ester substituted diaryl carbonate to total dihydroxy aromatic compound. Typically, this ratio should be in a range between about 1.01 and about 1.1. Typically, the product polycarbonate prepared by the method of the present invention will contain only very small amounts of residual starting dihydroxy aromatic compound (generally less than about 20 ppm) and ester substituted diaryl carbonate (generally less than about 350 ppm).

[0090] The product polycarbonates prepared by the method of the present invention may optionally be blended with any conventional additives used in thermoplastics applications, such as preparing molded articles. These additives include UV stabilizers, antioxidants, heat stabilizers, mold release agents, coloring agents, antistatic agents, slip agents, antiblocking agents, lubricants, anticlouding agents, coloring agents, natural oils, synthetic oils, waxes, organic fillers, inorganic fillers, and mixtures thereof. Typically, it is preferable to form a blend of the polycarbonate and additives which aid in processing the blend to form the desired molded article, such as an optical article. The blend may optionally comprise from 0.0001 to 10% by weight of the desired additives, more preferably from 0.0001 to 1.0% by weight of the desired additives.

[0091] Examples of UV absorbers include, but are not limited to, salicylic acid UV absorbers, benzophenone UV absorbers, benzotriazole UV absorbers, cyanoacrylate UV absorbers and mixtures thereof.

[0092] Examples of the aforementioned heat-resistant stabilizers, include, but are not limited to, phenol stabilizers, organic thioether stabilizers, organic phosphite stabilizers, hindered amine stabilizers, epoxy stabilizers and mixtures thereof. The heat-resistant stabilizer may be added in the form of a solid or liquid