Poly(butylene terephthalate) copolyesters derived frorn L-arabinitol and xylitol

The synthesis, characterization, and some properties of new copolyesters of poly(butylene terephthalate) based on L-arabinitol and xylitol are described. These copolyesters were obtained by polycondensation reaction in the melt Of mixtures of 1,4-butanediol and 2,3,4-tri-O-methyl-L-arabinitol or 2,3,4-tri-O-methylxylitol with dimethyl terephthalate. Their weight-average molecular weights ranged between 20 000 and 40 000, with polydispersities oscillating from 1.5 to 2.2. All them had a statistical microstructure and were thermally stable well above 300 degrees C. Copolyesters containing up to 30% of alditol-derived units were found to be crystalline and to adopt the same crystal structure as the parent homopolyester poly(butylene terephthalate). The melting temperature and crystallinity were observed to decrease, and the glass transition temperature to increase, with increasing amounts of alditol incorporated in the copolyester. Crystallizability was depressed by copolymerization, whereas the hydrolytic degradability was significantly enhanced by the presence of alditol units. No relevant differences in either structure or properties were observed between the L-arabinitol and xylitol copolyester series.