The present invention relates to monomers that are useful to make polybenzazole polymers, processes to make those monomers and methods to use those monomers.
Polybenzazole polymers are synthesized by the reaction of AA-PBZ monomers with BB-PBZ monomers. The AA-PBZ monomer (which is sometimes also referred to as a dicarboxylic acid monomer) contains two electron-deficient carbon groups linked by a divalent organic moiety. It is frequently illustrated by Formula 1: EQU Q-DM-Q (1)
wherein each Q is an electron-deficient carbon group and DM is a divalent organic moiety.
The BB-PBZ monomer (which is sometimes also referred to as a diamino-dihydroxy monomer or a diamino-dithio monomer) contains an aromatic group having two o-amino-basic moieties bonded to it. Each o-amino-basic moiety contains a first primary amine group bonded to the aromatic group and a hydroxyl, thiol or primary or secondary amine group bonded to the aromatic group in the ortho position with respect to the first primary amine group. The BB-PBZ monomer is ordinarily represented by Formula 2: ##STR1## wherein Ar.sup.1 is the aromatic group, each Z is --O--, --S--, or --NR--, and R is a hydrogen atom, an alkyl group or an aromatic group. The representation of bonds to an aromatic group without indicating position, as in Formula 2, indicates that the amine groups and --ZH groups may be in either cis- or trans- position with respect to each other. Each amine group must be ortho to one of the --ZH groups.
The BB-PBZ monomer is highly susceptible to oxidation when it is in a free-base state. It is ordinarily stored and handled as a hydrogen chloride salt, which is more stable. The hydrogen chloride is removed in a devolatilization step after the BB-PBZ monomer is dissolved in a polymerization solvent but prior to polymerization. The devolatilization step is time-consuming, but necessary because the hydrogen chloride interferes with polymerization of the monomer. It would be desirable to find new monomers that eliminate the need for the devolatilization step.
The monomers are usually polymerized in a dehydrating and non-oxidizing acid mixture. Proper stoichiometry is very important in the polymerization, since the reaction is a condensation polymerization. A small excess of BB-PBZ monomer can prevent the reaction from producing high-molecular weight polybenzazole polymer. It would be desirable to find new monomers that are simple to load in stoichiometric quantities.