Copolyesters
Mixed dicarboxylic acids are usually used to form copolyesters. For instance, terephthalic and isophthalic acids are reacted together with 1,4-dimethylol cyclohexane to form copolyester. The product is amor- phous and transparent. Another copolyester is manufactured from terephthalic, isophthalic, and an aliphatic dicarboxylic acid like adipic with either 1,4 butanediol or 1,6-hexanediol. The aliphatic dicarboxylic acid is used in minor quantities. Many such copolyesters are used as high-strength adhesives. In addition, there are thermoplastic polyester elastomers. These are produced by equilibrium melt transesterification of dimethyl terephthalate, 1,4-butanediol, and a poly(tetramethylene ether) glycol (molecular weight about 1,000). Because equilibrium conditions exist in the melt, the products are random copolymers:
Wholly aromatic polyesters are produced for high temperature applications. The materials must also have good abrasion resistance. One such commercial polyester is prepared from p hydroxybenzoic acid:
Another one is a copolyester prepared from p-hydroxybenzoic acid, p,p0-biphenol, and terephthalic or isophthalic acids:
The above shown polyester does not melt and decomposes at 550C. Turner et al. [38] reported preparation of triptysene containing polyesters. These polyesters were illustrated as follows:
where x ¼ 2,4, or 6. The polyester was prepared by melt-phase synthesis by a transesterification reaction. Higher thermal stability was claimed for the polyesters containing triptysene than the analogous polyesters without it. The properties of various polyesters were summarized by Wilfong [15]. Table 7.1 presents Tm and Tg values of some polyesters based on information from Wilfong and other sources in the literature. Among new methods of forming polyesters is a preparation of completely aromatic polyesters by direct condensation of hydroxyaromatic acids (like hydroxybenzoic) with the aid of triphenylpho sphorous compounds or dichlorophenylphosphine [38]:
Hexachlorotriphosphatriazine can also be used to attain direct polycondensations of hydroxybenzoic acid [43].
