Hydroxynaphthoic acid unit

In some cases it is possible to directly polycondense acid and phenols evolving water at about 300 C [41]. This process works well when catalysed with compoimds of group IV or V metals tin salts are preferred, especially dialkyl tin dialkanoates or oxides. The process is not really suitable if p-hydroxybenzoic acid is involved because this can imdergo decarboxylation at >200 "C. The polymerisation is not affected because the phenol formed volatilises at the high temperatures involved, but the resultant polyester will contain less than the anticipated mole ratio of hydroxybenzoate-derived units. Hydroxynaphthoic acid does not suffer from this problem, and direct esterification processes may be used if non-benzoate copolymers are required [42]. 

Figure 6.17 Repeating unit structures of (a) poly(p-phenylene terephthalamide) (Kevlar) (b) copolyester of p-hydroxybenzoic acid and ethylene terephthalate (c) copolyester of p-hydroxybenzoic acid and 2,6-hydroxynaphthoic acid (Vectra).

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