Rapid advancement of molecular weight in polybenzazole oligomer solutions

Abstract: Polybenzazole polymers can be advanced rapidly to high molecular weight at relatively high temperatures and high shear in a dehydrating solvent acid.

Description: This invention relates to the art of polybenzazole (PBZ) polymers and processes for synthesizing them. Polybenzazole polymers are a known class of polymers comprising polybenzoxazole (PBO), polybenzothiazole (PBT), polybenzimidazole (PBI) and copolymers of these polymers that are described in Wolfe, Liquid Crystalline Polymer Compositions, Process and Products , U S -A- 4,533,693 (August 6, 1985); and 11 Ency. Poly. Sci. & Eng., Polybenzothiazoles and Polybenzoxazoles , 601 (J. Wiley & Sons 1988). Polybenzazoles are generally classified as AB--polymers, containing a plurality of mer units depicted in formula 1(a), or AA/BB, containing a plurality of mer units which conform with formula 1(b) wherein: each Ar is an aromatic group, DM is a divalent organic moiety which is stable and inert under polymerization conditions, and each Z is independently an oxygen atom, a sulfur atom, or a nitrogen atom that is bonded to a hydrogen atom or an organic moiety which is stable and inert in acid under polymerization conditions. (For the purpose of this application, when the nitrogen atoms and Z moieties of a mer unit are depicted as bonded to an aromatic group without indicating their position, as in Formulae 1(a)-(b), it shall be understood that: (1) each nitrogen atom and Z group within a given azole ring are bonded to the aromatic group in ortho position with respect to each other; and (2) if the mer unit has two azole rings, one nitrogen atom and Z moiety may be in either cis position or trans position with respect to the other nitrogen atom and Z moiety, for example illustrated in 11 Ency. Poly. Sci. & Eng., Polybenzothiazoles and Polybenzoxazoles , at 602 (J. Wiley & Sons 1988). The same understandings apply with respect to amine groups and Z moieties in a BB-PBZ monomer.) Polybenzazoles are synthesized by (1) the reaction of at least one AA-PBZ monomer containing two electron-deficient carbon groups with at least one BB-PBZ monomer containing two o-amino-basic moieties and/or (2) the condensation of one or more AB-PBZ monomers containing an aromatic group, an o-amino-basic moiety and an electron-deficient carbon group. The reaction is illustrated generally in Formula 2: wherein each Q is an electron-deficient carbon group and all other moieties have the meaning and preferred embodiments previously given. The divalent organic moiety DM of the AA-monomer ordinarily comprises an aromatic group, which is most usually a p-phenylene group. The reaction is ordinarily carried out under non-oxidizing conditions in a non-oxidizing, dehydrating solvent acid, which is most frequently polyphosphoric acid. The known reaction conditions are unsuitable for synthesizing large quantities polybenzazole polymer. The most commonly used reaction techniques, as described in U S -A- 4,533,693, require days to synthesize a few hundred grams of high molecular weight polymer. What is needed is a process to quickly polymerize polybenzazole polymers of high molecular weight. In WO-A-90/03995 example 1 a process for making cis-polybenzoxazole from 4,6-diaminoresorcinol dihydrochloride and terephthaloyl chloride in polyphosphoric acid is described. The reaction mixture was rigorously stirred during reaction and in the final step of this process the reaction was continued for 24 hours at 190 °C. The object of the present invention is to provide a process yielding polybenzazole polymer of high molecular weight within shorter reaction times. The polymerization of PBZ monomers is a condensation reaction. It has surprisingly been found that the speed at which polybenzazole monomers and oligomers condense to form higher molecular weight polymer in a solvent acid solution is highly dependent upon the shear placed upon the solution during the polymerization. Under no shear polymerization occurs very slowly. As shear increases, the speed of reaction also increases. The present invention is a process for synthesizing a polybenzazole polymer wherein a liquid crystalline mixture containing solvent acid and functionally terminated polybenzazole oligomers having end groups selected from the class consisting of o-amino-basic moieties and electron-deficient carbon groups is reacted at a temperature of at least 150°C under conditions such that a polybenzazole polymer is formed, characterized in that the shear on the liquid crystalline mixture is at least 100 sec.⁻¹ during at least part of the reaction. Advancement of molecular weight according to the present invention may be completed in only a few minutes. The polymers can be formed into fibers and films that are useful structural materials. Definitions The following terms are used repeatedly throughout this application, and have the meaning and preferred embodiments defined herein unless otherwise specified. AA-PBZ Monomer - A monomer suitable for synthesizing polybenzazole polymers. comprising two electron-deficient carbon groups linked by a divalent organic moiety (DM) which is stable so that it does not interfere with the synthesis, fabrication and use of the polybenzazole polymer. The electron-deficient carbon groups have the definition and preferred embodiments given herein. The divalent organic moiety is preferably alkyl or an aromatic group, as herein defined, is more preferably an aromatic group, and is most preferably a six-membered aromatic group. Examples of suitable AA-PBZ monomers and references to their synthesis are provided in U S -A 4,533,693 at Col. 25-32, Tables 4-6. Preferred examples of AA-PBZ monomers include terephthalic acid, isophthalic acid, bis-(4-benzoic) acid and oxy-bis-(4-benzoic acid) and acid halides thereof. AB-PBZ Monomer - A monomer suitable for synthesizing polybenzazole polymers, comprising an aromatic group, an o-amino-basic moiety bonded to the aromatic group, and an electron-deficient carbon group linked to the aromatic group. The aromatic group, the electron-deficient carbon group and the o-amino-basic moiety have the definitions and preferred embodiments given herein. Examples of suitable AB-monomers and processes for their synthesis are provided in U S -A-4,533,693 at Col. 33-35, Tables 7-8. Preferred examples of AB-monomers include 3-amino-4-hydroxybenzoic acid, 3-hydroxy-4-aminobenzoic acid and the acid halides thereof. AB-monomers are frequently stored as salts of hydrogen chloride or phosphoric acid, because the free-base of the monomer is susceptible to air oxidation. o-Amino-basic moiety - a moiety, which is bonded to an aromatic group, consisting of. (1) a primary amine group bonded to the aromatic group and (2) a hydroxy, thiol or primary or secondary amine group bonded to the aromatic group ortho to said primary amine group. It preferably comprises a hydroxy, thiol or primary amine moiety, more preferably comprises a hydroxy or thiol moiety, and most preferably comprises a hydroxy moiety. Secondary amine groups comprise an aromatic or an aliphatic group and preferably an alkyl group. The secondary amine group preferably comprises no more than about 6 carbon atoms, more preferably no more than about 4 carbon atoms and most preferably no more than about 1 carbon atom. Aromatic group (Ar) - any aromatic ring or ring system. Size is not critical as long as the aromatic group is not so big that it prevents further reactions of the moiety in which it is incorporated. Each aromatic group independently preferably comprises no more than about 18 carbon atoms, more preferably no more than about 12 carbon atoms and most preferably no more than about 6 carbon atoms. Each may be heterocyclic but is preferably carbocyclic and more preferably hydrocarbyl. If the aromatic group is heterocyclic, the heteroatom is preferably nitrogen. Unless otherwise specified, each aromatic group may comprise a single aromatic ring, a fused ring system or an unfused ring system containing two or more aromatic moieties joined by bonds or by divalent moieties (DL) which are inert with respect to PBZ polymerizing reagents under polymerization conditions. Suitable divalent moieties comprise, for example, a carbonyl group, a sulfonyl group, an oxygen atom, a sulfur atom, an alkyl group and/or and or a perfluorinated alkyl group. Each aromatic group is preferably a single six-membered ring. Each aromatic group may contain substituents which are stable in solvent acid and do not interfere with further reactions of the moiety which the aromatic group is part of. Examples of preferred substituents include halogens, alkoxy moieties, aryloxy moieties or alkyl groups. More preferred substituents are either an alkyl group having no more than about 6 carbon atoms or a halogen. Most preferably, each aromatic group contains only those substituents specifically called for hereinafter. Azole ring - an oxazole, thiazole or imidazole ring. The carbon atom bonded to both the nitrogen atom and the oxygen, sulfur or second nitrogen atom is the 2-carbon, as depicted in Formula 3 wherein Z is -O-, -S- or -NR-; and R is hydrogen, an aromatic group or an aliphatic group, preferably hydrogen or an alkyl group, and most preferably hydrogen. R preferably comprises no more than about 6 carbon atoms, more preferably no more than about 4 and most preferably no more than about 1. Each azole ring is independently preferably oxazole or thiazole and more preferably oxazole. In PBZ polymers, the 4- and 5-carbon of each azole ring is ordinarily fused with an aromatic group. Azole-forming moiety - an "o-amino-basic moiety" or "electron-deficient carbon group," as those terms are defined herein. BB-PBZ Monomer - A monomer suitable for synthesizing polybenzazole polymers, comprising an aromatic group and two o-amino-basic moieties which are bonded to the aromatic group. The aromatic group and the o-amino-basic moieties have the definitions and preferred embodiments given herein. Examples of suitable BB-PBZ monomers and processes for synthesis are provided in U S -A- 4,533,693 at Col. 19-24, Tables 1-3. Examples of preferred BB-PBZ monomers include 4,6-diaminoresorcinol, 2,5-diaminohydroquinone and 1,4-dithio-2,5-diaminobenzene. BB-PBZ monomers are frequently stored as salts of hydrogen chloride or phosphoric acid, because the free base of the monomer is susceptible to air oxidation. Electron-deficient carbon group (Q) - any group containing a carbon atom which can react in the solvent acid with an o-amino-basic moiety to form an azole ring, such as the groups listed in Col. 24, lines 59-66 of the 4,533,693 patent. Preferred electron-deficient carbon groups are carboxylic acids, acid halides, metal carboxylate salts, cyano groups and trihalomethyl groups. Halogens in electron-deficient carbon groups are preferably chlorine, bromine or iodine and more preferably chlorine. Polybenzazole (PBZ) polymer - A polymer from the group of polybenzoxazoles and polybenzobisoxazoles (PBO), polybenzothiazoles and polybenzobisthiazoles (PBT) and polybenzimidazoles or polybenzobisimidazoles (PBI). For the purposes of this application, the term "polybenzoxazole (PBO)" refers broadly to polymers in which each mer unit contains an oxazole ring bonded to an aromatic group, which need not necessarily be a benzene ring. The term "polybenzoxazole (PBO)" also refers broadly to poly(phenylene-benzo-bis-oxazole)s and other polymers wherein each mer unit comprises a plurality of oxazole rings fused to an aromatic group. The same understandings shall apply to the terms polybenzothiazole (PBT) and polybenzimidazole (PBI). Rigid Rod PBZ polymer - An "intrinsic" or "articulated" rigid rod PBZ polymer as the terms "intrinsic" and "articulated" are defined in the Hwang, "Processing, Structure and Properties of Liquid Crystalline PBT Polymer", Kansai Committee of the Society of Fiber Science and Technology, Japan, Post Symposium on Formation, Structure and Properties of High Modulus and High Tenacity Fibers 23-26 (August 26, 1985); Evers et al., "Articulated All-Para Polymers with 2,6-Benzobisoxazole, 2,6-Benzobisthiazole, and 2,6-Benzobisimidazole Units in the Backbone," 14 Macromolecules 925 (1981); Evers, "Thermooxadatively Stable Articulated Benzobisoxazole and Benzobisthiazole Polymers," 24 J. Poly. Sci. Part A 1863 (1986) and Evers et al., Articulated Para-Ordered Aromatic Heterocyclic Polymers Containing Diphenoxybenzene Structures , U S -A-4,229,566 (October 21, 1980). Intrinsic rigid rod polymers are essentially rectilinear and consist essentially of mer units having an angle of catenation of at least about 150°. Articulated rigid rod polymers comprise a plurality of essentially rectilinear moieties joined by a relatively small number of non-linear moieties. Rigid rod PBZ polymers used in the present invention are preferably intrinsic rigid rod polymers. If articulated, they preferably comprise on average no more than about 1 non-linear mer unit for each 9 essentially rectilinear mer units. Solvent acid - any non-oxidizing liquid acid capable of dissolving PBZ polymers, such as sulfuric acid, methanesulfonic acid, trifluoromethylsulfonic acid, polyphosphoric acid and mixtures thereof. It must be sufficiently non-oxidizing that it does not substantially oxidize AB- and BB-PBZ monomers which are dissolved therein. Solvent acids are preferably dehydrating acids, such as polyphosphoric acid or a mixture of methanesulfonic acid and phosphorus pentoxide. Preferred concentrations of P₂O₅ in the methanesulfonic acid are described in U S -A- 4,847,350 and 4,722,678. Concentrations of P₂O₅ in the polyphosphoric acids are described in U S -A- 4,533,693 and 4,722,678. Polybenzazole polymers, their properties and their synthesis are discussed in detail in the following references: Sybert et al., Liquid Crystalline Polymer Compositions, Process and Products. U S -A-4,772,678 (September 20, 1988); Wolfe et al., Liquid Crystalline Polymer Compositions, Process and Products , U S -A- 4,703,103 (October 27, 1987); Wolfe et al., Liquid Crystalline Polymer Compositions, Process and Products , U S -A- 4,533,692 (August 6, 1985); Wolfe et al., Liquid Crystalline Poly(2.6-Benzothiazole) Compositions, Process and Products , U S -A- 4,533,724 (August 6, 1985); Wolfe, Liquid Crystalline Polymer Compositions, Process and Products , U S -A- 4,533,693 (August 6, 1985); Imai et al., "Polybenzoxazoles and Polybenzothiazoles," 83 Makromol. Chem. 167 (1965), Evers, Thermooxadatively Stable Articulated p-Benzobisoxazole and p-Benzobisthiazole Polymers , U S -A- 4,359,567 (November 16, 1982); Tsai et al., Method for Making Heterocyclic Block Copolymer , U S -A- 4,578,432 (March 25, 1986) and 11 Ency. Poly. Sci. & Eng., Polybenzothiazoles and Polybenzoxazoles , 601 (J. Wiley & Sons 1988). It is convenient to think of the synthesis of polybenzazole polymers as occurring in four stages. In the first (mixing) stage, at least the AB-PBZ and/or BB-PBZ monomers are mixed with a solvent. In the second (devolatilization) stage, volatile protecting acids are drawn off from the reaction mixture, if necessary. In the third (oligomerization) stage, the monomers react to form oligomers. In the fourth (advancement) stage, the oligomers react to form higher molecular weight polymer. The processes of the present invention preferably correspond roughly to the advancement stage. The first three stages are preferably used to make a reaction mixture suitable for use in the processes of the present invention. There is no sharp dividing line for these stages. Devolatilization may commence as the monomers are mixed with the solvent. The devolatilization and oligomerization stages may be carried out simultaneously in the same piece of equipment. There is not a generally recognized point at which a polymer is at too high a molecular weight for the oligomerization stage or too low a molecular weight for the advancement stage. Oligomerization and advancement may be carried out in a single reactor as a single continuous process, and even under similar conditions (although this is not preferred). Certain reaction conditions are equally applicable in all stages. The reaction should take place in a solvent acid under dehydrating conditions. The conditions should be non-oxidizing, such as nitrogen or noble gas atmosphere or vacuum. The pressure may be atmospheric, subatmospheric or superatmospheric, but should permit the removal of volatile compounds that interfere with polymerization. In the first stage of the process, a selected amount of AB-PBZ monomer and/or BB-PBZ monomer is mixed with a reaction solvent, which is preferably a solvent acid. The monomer is preferably a BB-PBZ monomer. The monomers are more preferably chosen to provide a rigid rod polybenzoxazole or polybenzothiazole, and are most preferably chosen to provide rigid rod cis-polybenzoxazole. The monomers are defined and further described in the definitions. The amount of monomer is preferably sufficient to provide a liquid crystalline solution after the polymerization is complete. The concentration of monomer is preferably sufficient to provide a finished dope containing at least 7 percent polymer, more preferably at least about 12 percent polymer and most preferably at least about 14 percent polymer, by weight. The maximum concentration of monomer is typically limited by practical considerations such as solubility and viscosity. Ordinarily the concentration of monomers is no more than would be necessary to provide a solution containing about 30 percent polymer, more typically no more than about 21 percent polymer, and most often no more than about 18 percent polymer. Optimum concentrations may vary depending upon the desired utility of the resulting polymer and the desired molecular weight of the resulting polymer. AA-PBZ monomers may be added simultaneously with the BB-PBZ monomers, or the monomers may be added in any order sequentially. Preferably, AA-PBZ monomers are added about simultaneously with the BB-PBZ monomers. More preferably, the ratio of AA-PBZ monomers to BB-PBZ monomers added is about that described hereinafter for the oligomerization step. In a preferred embodiment the oligomers are the reaction product of at least one AA-PBZ monomer which is terephthalic acid, bis-(4-benzoic acid) or an acid halide thereof, and at least one monomer BB-PBZ monomer which is 4,6-diaminoresorcinol, 2,5-diaminohydroquinone or 2,5-diamino-4,6-dithiobenzene or an acid salt thereof. The solvent is preferably a solvent acid, as previously defined, and is most preferably a polyphosphoric acid. The solvent may be a low P₂O₅ polyphosphoric acid (such as about 77 weight percent P₂O₅) or a commercial polyphosphoric acid (such as about 83 weight percent P₂O₅) at the time that mixing is commenced, but it preferably contains higher levels of P₂O₅ at the commencement of the oligomerization step, as described hereinafter. This is preferably accomplished by adding extra P₂O₅ to the solvent acid during the mixing step. The extra P₂O₅ is more preferably added to the solvent and mixed in at about the same time that the monomers are added and mixed in. A reducing agent, such as tin (II) chloride, may also be added to control oxidation or thermal degradation of the monomer. The temperature of mixing is preferably at least about 25°C, more preferably at least about 45°C and most preferably at least about 55°C. It is preferably low enough that the monomer does not substantially degrade. It is preferably no more than about 150°C, more preferably no more than about 100°C and most preferably no more than about 70°C. Certain monomers, such as terephthalic acid, may be substantially insoluble in the solvent acid. Mixing need not be carried out to dissolve all monomers, but it is preferably carried out for a time sufficient to at least wet all monomer. The monomers and P₂O₅ are preferably micronized. Then the second (devolatilization) stage of the reaction is preferably commenced immediately. The second (devolatilization) stage may be unnecessary with proper choice of monomers. Certain monomers, such as BB-PBZ monomer phosphate salts and terephthalic acids, do not release volatile compounds and do not require devolatilization. Reactions carried out without devolatilization are described in Harris et al., U.S. Patent Application Ser. No. 341,502, the priority of which is claimed in EP-A-0 393 559. If the monomers do release volatile components, such as hydrogen chloride from BB-PBZ monomer hydrohalide salts and terephthaloyl halides, then the reaction mixture should be devolatilized. The temperature of the devolatilization stage should be high enough to drive out volatile protecting acids in a rapid fashion, and low enough to avoid substantial decomposition of the unpolymerized monomers. The temperature is preferably no more than about 190°C, more preferably no more than about 150°C, and most preferably no more than about 130°C. It is preferably at least about 40°C, more preferably at least about 60°C, and most preferably at least about 100°C. The devolatilization is preferably carried out with vigorous mixing. Examples of suitable mixing equipment includes all of the equipment listed in U S -A- 4,772,678 in Col. 44-45. Preferred examples include single or multiple screw extruders, piston-agitated reactors, Sigma blade mixers, helical mixers (such as dual helical ribbon mixer) and multiple impeller mixers. When the solvent is polyphosphoric acid, it may contain as little as 76 percent P₂O₅ or less at the beginning of the devolatilization step, but it preferably contains at least about 80 weight percent P₂O₅, more preferably at least about 85 weight percent, and most preferably at least about 88 weight percent. The concentration of P₂O₅ in the polyphosphoric acid must be low enough to allow adequate mixing. It is preferably less than about 92 weight percent and more preferably no more than about 90 weight percent. Under preferred conditions, devolatilization of 4.54 Kg (10 lbs.) or more of diaminoresorcinol bis(hydrogen chloride) salt can be essentially completed in a few hours. The rapid evolution of volatiles in viscous reaction mixtures may cause foaming, as previously described in the literature. Foaming may be minimized by several techniques. For instance, the devolatilization reactor may have a high surface-to-volume ratio as described in U S -A-4,772,678. The BB-monomer salt may have a large particle size, such as at least about 40 mesh or greater. The larger particle is dissolved more slowly into the polyphosphoric acid, thus spreading out the evolution of the volatile acid. The monomers may be chosen to minimize the level of volatile components in the system, such as polymerizing terephthalic acid (rather than terephthaloyl chloride) with the BB-PBZ monomer bis(hydrogen chloride) salt. The mixing apparatus may be chosen to help break foam. The pressure in the reactor may be controlled to minimize foaming. The devolatilization reactor should have a reasonable amount of head space to permit some level of foaming. If the reaction mixture contains both AA-monomers and BB-PBZ monomers during mixing and/or devolatilization, then the oligomers ordinarily begin forming during the devolatilization stage. Otherwise, the AA-PBZ monomer is preferably added and oligomerization is preferably commenced almost immediately after devolatilization is completed. If storage of the devolatilized solution is contemplated, the devolatilized reaction mixture is preferably stored after the oligomerization stage, rather than before it. The oligomerization stage is preferably commenced as quickly as possible. The oligomerization step is carried out under known conditions. The solvent acid should be dehydrating, as described in U S -A- 4,533,693 and 4,847,350. The solvent acid is preferably a polyphosphoric acid containing at least about 84 weight percent P₂O₅. It more preferably contains at least about 86 weight percent P₂O₅ and most preferably contains at least about 88 weight percent P₂O₅ at the commencement of the oligomerization stage. The temperature is preferably at least about 60°C, more preferably at least about 100°C and most preferably at least about 120°C. It is preferably no more than about 190°C, more preferably no more than about 170°C, and most preferably no more than about 150°C. The oligomerization is carried out with vigorous agitation as previously described. Staged reactions of monomers to form stable oligomers that may be stored and reacted later are described in Gregory et al., U.S. Patent Application Serial Number 600,551 which is filed simultaneously herewith, the priority of which is claimed in EP-A-0 481 402. The stoichiometry in such staged reactions may be slightly unbalanced or may be essentially equimolar. Preferably, neither monomer is in an excess of more than about 25 mole percent, more preferably no more than about 4 mole percent, during the oligomerization stage. It is critical that the oligomers and/or low molecular weight polymers resulting from the oligomerization step have active end groups selected from the class consisting of o-amino-basic moieties and electron-deficient carbon groups. If the oligomers are capped, for instance by monofunctional reagents, the condensation reaction can not continue during the advancement stage and no further advancement is possible. Therefore, monofunctional capping agents are preferably not added during the oligomerization step. The active end groups are preferably predominantly o-amino-basic moieties. No hard and fast rule governs the maximum molecular weight that can be reached in the oligomerization stage. When the oligomerization stage is carried out in a batch reactor, the stage should be terminated while the dope is still pumpable and stirrable. The oligomerization is preferably carried out until the polybenzazole reaches an average molecular weight at which the polybenzazole is substantially stable. "Substantially stable" means that the reaction mixture can be stored for at least 24 hours at pumpable temperatures and still be advanced to high molecular weight. The dope is preferably storable for at least about 48 hours and more preferably for at least about one week. The average degree of polymerization at the end of the oligomerization stage is preferably at least 5, more preferably at least 10, and most preferably at about 15. It is preferably at most 75, more preferably at most 50, and most preferably no more than 40. The degree of polymerization may conveniently be tracked by measuring the viscosity of the reaction mixture, for instance by measuring the level of power needed for agitation. Such measurements are necessarily dependent upon the equipment used and the specific contents of the reaction mixture. They can easily be optimized by persons of ordinary skill in the art. In one preferred embodiment, the mixing, devolatilization and oligomerization stages are carried out in a batch reactor. The solvent acid, AA- and BB-PBZ monomers and extra P₂O₅ (if any) are added simultaneously to the reactor with agitation at a temperature suitable for mixing. As mixing is accomplished, the temperature and agitation are adjusted for devolatilization and oligomerization. Following the oligomerization stage, the dope may be stored as previously described or may be pumped directly over to the advancement stage. Storage is preferably at a temperature at which the reaction mixture remains pumpable, as previously described. The temperature for storage is preferably no more than about 160°C and more preferably no more than about 120°C. The storage should be under non-oxidizing conditions, and preferably under essentially no shear. It is more preferably in a separate storage tank to free up the reaction vessels for mixing, devolatilization and oligomerization of a new batch. The advancement stage is carried out under high temperature and, at least in part, under high shear. The temperature is preferably at least about 175°C and more preferably at least about 190°C. It is preferably at most about 300°C, more preferably at most about 230°C, and most preferably no more than about 220°C. The shear is at least 100 sec.⁻¹, more preferably at least 300 sec.⁻¹ and most preferably at least 500 sec.⁻¹. The shear is limited primarily by practical considerations, such as viscosity. It is preferably no more than 1000 sec.⁻¹, and more preferably no more than 700 sec.-¹. High shear. as previously described, may be used in the mixing, devolatilization and oligomerization stages. In reactions of the present invention at least some advancement is carried out under high shear. Several different conditions may be used to measure when high shear is desirable in reactions of the present invention High shear is preferably maintained with polymers having an average degree of polymerization beyond that spelled out for the oligomerization step, as previously described. The reaction is preferably continued under high shear for at least part of the polymerization after the conversion of monomer is at least about 90 percent, and more preferably until the conversion reaches at least about 100 percent. Most preferably, high shear is maintained throughout the entire advancement stage. The dope under high shear is liquid crystalline. High shear is useful for advancement of dopes having a viscosity at 15.9 sec.⁻¹ shear and 150°C of at least 1000 poise, more preferably at least about 5000 poise and most preferably at least about 6500 poise. The viscosity is typically governed by several factors, including oligomer molecular weight and oligomer concentration. The advancement stage is preferably carried out under subatmospheric pressure. The pressure is preferably no more than about 0.2·10⁵ Pa (150 mmHg). The minimum pressure is dictated primarily by pratical concerns. It is conveniently at least about 133.3 Pa (1mmHg). The solvent in the advancement stage should be dehydrating. For instance, polyphosphoric acid preferably contains at least about 78 weight percent P₂O₅, more preferably at least about 80 weight percent and most preferably at least about 83 weight percent, at the beginning of advancement. The stoichiometry in the advancement stage must allow the oligomers to condense to form polymers of a relatively higher average molecular weight. If the oligomerization stage was run using a stoichiometric excess of one monomer (particularly a BB-PBZ monomer), the monomer level in the reaction mixture may be adjusted to provide a polymer having the desired molecular weight, such as by adding about a stoichiometric amount of AA-PBZ monomer as a chain extender. A chain terminator may be added if it is desirable to hold the molecular weight down during advancement. Examples of suitable chain terminators are described in Wolfe, U S -A- 4,772,678 at Col. 22-27 (September 20, 1988). Suitable chain terminators are typically monofunctional carboxylic acids or derivatives, or o-aminophenols or o-aminothiols or o-diamines. Examples of suitable monofunctional carboxylic acid type terminators include benzoic acid, phenylbenzoic acid, cyclohexane carboxylic acid. picolinic acid, naphthoic acid, acetic acid, propionic acid, and acid halide. ester or nitrile derivatives thereof. Examples of other suitable terminators include o-aminophenol, phenyl-2--aminophenol, o-aminonaphthol and variations wherein the hydroxy group is replaced with a thiol or amino group. The quantity of chain terminator added is preferably equal to no more than about 5 mole percent of the monomers added, more preferably no more than about 2 mole percent and most preferably no more than about 1 mole percent. Preferably, the stoichiometry of the reaction is adjusted such that the number of reactive sites provided by AA-monomers is essentially equal to the number of sites provided by the BB-PBZ monomer and aminophenolic terminator. The advancement may be carried out in a batch reactor, such as a piston-agitated reactor. It may be carried out in the same reactor used for the oligomerization and/or other stages. In a preferred embodiment of the present invention, the advancement step is carried out in a reactor that reacts relatively small quantities of the reaction mixture in a continuous fashion for a relatively short residence time under conditions that will cause the reaction to complete in that time frame. Examples of suitable reactors include single and multiple screw extruders. The build up in molecular weight during the advancement step can be measured for material leaving the reactor. Then the level of AA-PBZ monomer and/or chain terminator added before or during advancement step can be adjusted until the polymer leaving the reactor has the desired molecular weight. The residence time of dope in the advancement stage is preferably no more than about one hour and more preferably no more than about 30 minutes. Under preferred conditions, dope can be advanced to final molecular weight in no more than about 15 minutes. The minimum time is preferably at least about 1 minute and more preferably at least about 5 minutes The optimum residence time for advancement varies depending upon several factors. including the contents of the dope entering the reactor and the reactor conditions. Persons of ordinary skill in the art readily determine the optimum conditions for their own equipment without undue experimentation. If the reaction is not completed to the desired level of advancement, the reaction mixture may even be recirculated through the advancement reactor with a chain extender, such as AA-PBZ monomer. The reaction is preferably completed in one pass. The dope resulting from advancement contains solvent and polybenzazole polymer. The average molecular weight of the polymer should be higher than the average molecular weight of oligomers after the oligomerization stage. The polymer preferably has an average degree of polymerization of at least 20, more preferably at least 30 and most preferably at least 40. The optimal degree of polymerization may vary depending upon the application. For some applications the best degree of polymerization may be at least about 50, 100 or even 150. When the polymer is a rigid rod polybenzoxazole or polybenzothiazole, it preferably has an single point intrinsic viscosity (in methanesulfonic acid at 25°C) of at least 5 dL/g, more preferably at least 10 dL/g and most preferably at least 15 dL/g. The single point intrinsic viscosity may go as high as 60 dL/g or more, but it is preferably no more than 50 dL/g and more preferably no more than 40 dL/g. The optimal single point intrinsic viscosity may vary depending upon the application. For some applications the best single point intrinsic viscosity may be at least about 20, 25 or even 30 dL/g. The dope can be spun to form fibers or extruded to form films according to known processes, such as the processes described in The Materials Science and Engineering of Rigid Rod Polymers , 245-297 (Materials Research Society 1989). The fibers are useful in fiber-based composites and the films are useful laminates. Advancement to final molecular weight under the conditions of the present invention proceeds more rapidly and, if desired, to higher molecular weight than was previously possible. The residence time in a continuous reactor may be as low as 10 to 20 minutes. Combined with an efficient procedure for mixing, devolatilization and oligomerization, the process of the present invention may yield dopes containing high concentrations of high molecular weight polybenzazole polymer only a few hours after the reaction is commenced. The following examples are given to illustrate the invention and should not be interpreted as limiting it in any way. Unless stated otherwise, all parts and percentages are given by weight. Example 1 A standard procedure for mixing, dehydrohalogenation and oligomerization of polybenzazole dopes is established. A 45,4 Kg (100 lb) quantity of polyphosphoric acid containing 83.7 weight percent P₂O₅ is added to a 113,75 l (25 gallon) reactor with agitation by a helical dual ribbon impeller. The following dry materials are added at about 60°C under nitrogen atmosphere: 8,63 Kg (19.01 lbs) of diaminoresorcinol bis(hydrogen chloride); 6,73 Kg (14.822 lbs)of terephthalic acid; and 9.98 Kg (21.988 lbs) of P₂O₅. Mixing is continued for 1 hour at 60°C and for about 6 hours at about 120°C. Hydrogen halide gas evolved is drawn off and recovered. The reaction temperature is raised to 140°C, until a viscosity of about 300 poise is reached. The resulting solution is pumped into a tank and stored under nitrogen atmosphere at about 120°C. The reaction mixture is pumped through a twin screw extruder having about a 35:1 length to diameter ratio and an internal diameter of about 30 mm. The shear within the extruder is about 500 sec.⁻¹ The temperature is between 190°C and 210°C. The residence time is about 6 minutes. The flow through the extruder is about 4.54 Kg (10 lb) of dope per hour. The extruder has syringe pump that can introduce a measured flow of liquid into the extruder. A solution of polyphosphoric acid and o-aminophenol is introduced as shown in Table 1. The dope leaving the extruder passes through a series of four static mixing elements. The effect of the chain terminator on the dope is monitored by monitoring the pressure drop across the mixers. The resulting polymer is recovered by precipitation in water. Its single point intrinsic viscosity is measured in methanesulfonic acid at 30°C and a concentration of 0.05 g/dL. The single point intrinsic viscosity is shown in Table 1. It relates in a linear manner to the pressure drop across the mixers. Sample terminator concentration (%) terminator solution (cc/hour) inherent viscosity (dL/g) 1 1 52.5 33.4 2 1 105 31.5 3 10 10.5 32.9 4 10 21 29.9 5 10 31.5 26.7 6 - 0 42.3 Example 2 The process of Example 1 is repeated using an excess of 4.6-diaminoresorcinol and no terminator. The resulting polymer has an intrinsic viscosity of less than 25 dL/g. The syringe pump is used to add a mixture of 10 weight percent terephthalic acid and polyphosphoric acid at the rate indicated in Table 2. The pressure drop across the static mixers is measured, and an estimated single point intrinsic viscosity is calculated using the previously calculated relationship. Sample chain extender solution feed rate (g/hr) Pressure Drop (psi)10⁵Pa estimated inherent viscosity (dL/g) 7 0 6.27 (91) <25 8 0.59 8.96 (130) 28 9 0.89 11.03 (160) 31 10 1.19 13.86 (201) 34 11 1.48 14.13 (205) 35 12 1.78 12.62 (183) 33 13 2.37 11.38 (165) 31 14 2.97 10.55 (153) 30 15 3.56 10.55 (133) 28 Example 3 A mixture containing 200 g of 4,6-diaminoresorcinol bis(hydrogen chloride), 116.85 g of micronized terephthalic acid, 947 g of 82.5 percent polyphosphoric acid, 6.3 g of tin (II) chloride dihydrate and 140 g of P₂O₅ is agitated under 133.3 Pa (1 mmHg) pressure using a helical dual ribbon impeller at 120°C for 23 hours and at 150°C for 3 hours, using a nitrogen purge with the outflow leading to a hydrochloric acid scrubbing device. Agitation is continued at 150°C for 62 hours under 1 atmosphere nitrogen pressure. A 197 g quantity of P₂O₅ and 38.95 g of terephthalic acid are added at 120°C. The mixture is heated under nitrogen atmosphere at 210°C in a piston-agitated reactor until it is completed. The shear within the reactor commences at about 300 sec.⁻¹, and declines to about 10 sec.⁻¹ as the viscosity of the dope increases with continued advancement. The resulting cis-PBO polymer has an single point intrinsic viscosity of about 31 dL/g in methanesulfonic acid at 25°C and a concentration of 0.05 g/dL.

Claims: A process for synthesizing a polybenzazole polymer wherein a liquid crystalline mixture containing solvent acid and functionally terminated polybenzazole oligomers having end groups selected from the class consisting of o-amino-basic moieties and electron-deficient carbon groups is reacted at a temperature of at least 150°C under conditions such that a polybenzazole polymer is formed, characterized in that the shear on the liquid crystalline mixture is at least 100 sec. ⁻¹ during at least part of the reaction. The process of claim 1 wherein the liquid crystalline mixture has a viscosity of at least 1000 poise at a shear of 15.9 sec.⁻¹ and a temperature of 150°C. The process of any of the preceding claims wherein the oligomers are polybenzoxazole and/or polybenzothiazole oligomers. The process of any one of the preceding claims wherein the shear is at least 300 sec.⁻¹ during at least part of the reaction. The process of any one of the preceding claims wherein the liquid crystalline mixture contains at least 7 weight percent oligomers. The process of any one of the preceding claims wherein the oligomers are the reaction product of at least one AA-PBZ monomer which is terephthalic acid, bis--(4-benzoic acid) or an acid halide thereof, and at least one BB-PBZ monomer which is 4,6-diaminoresorcinol, 2,5-diaminohydroquinone or 2,5-diamino--4,6-dithiobenzene or an acid salt thereof. The process of any one of the preceding claims wherein the average degree of polymerization of the polybenzazole oligomers is between 5 and 40, and the average degree of polymerization of the polybenzazole polymer is greater than 40. The process of claim 7 wherein the average degree of polymerization of the polybenzazole oligomers is at least 10. The process of any one of the preceding claims wherein the single point intrinsic viscosity of the polybenzazole polymer in methanesulfonic acid at 25°C is between 15 dL/g and 60 dL/g.

Inventors: GREGORY THOMAS, HURTIG CARL W, LEDBETTER HARVEY D, QUACKENBUSH KENNETH J, ROSENBERG STEVEN, GREGORY, THOMAS, HURTIG, CARL W., LEDBETTER, HARVEY D., QUACKENBUSH, KENNETH J., ROSENBERG, STEVEN
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1991
Main classification: C08G73
IPC codes: C08G75, C08G73