P-Hydroxybenzoic acid , use

Kaneda et al. synthesized [61] a series of high molecular weight extended chain copolyimides (XV) by the reaction of PMDA and 3,3, 4,4 -biphenyltetra-carboxylic dianhydride (PPDA) with 3,3 -dimethyl-4,4 -diaminobiphenyl. Solvents used for the one-step synthesis to the fully cyclized imide structure were phenol, p-chlorophenol, m-cresol, p-cresol and 2,4-dicholorophenol. The polycondensations were performed at 180°C for 2h with a monomer concentration of 6% by weight and p-hydroxybenzoic acid used as a catalytic accelerator. A maximum of 50 mol % of PMDA could be used before the copolymer precipitated from solution. Reconstituted copolymers as isotropic dopes (8-10% by weight) in p-chlorophenol were dry-jet wet spun between 80 and 100 °C [62]. 

We have reported a convenient method for the preparation of polyesters by direct polycondensation reaction of p-hydroxybenzoic acids using triphenyl phosphite or phenylphosphine dichloride in pyridine. However, the poly condensation of m-hydroxy-benzoic acid or of a combination of aromatic dicarboxylic acids and bisphenols gives polymers only in low yields34. ... 

The method of choice for determining carboxyl groups in lignin is based on potentiometric titration in the presence of an internal standard, p-hydroxybenzoic acid, using tetra-n-butylammonium hydroxide as a titrant (53). The carboxyl contents of different lignins are shown in Table 6. In general, the carboxyl content of lignin increases upon oxidation. 

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