Poly(hexamethylene terephthalate-co-caprolactone) copolymers: Influence of cycle size on ring-opening polymerization

Poly(hexamethylene terephthalate) was cyclo-depolymerized in solution by heating to yield a fraction of cyclic oligomers of hexamethylene terephthalate (c(HT)_2-5) with a content around to 95% in dimer to pentamer. Ring-opening polymerization in the melt of c(HT)_2-5, either neat or in mixtures with ε-caprolactone (CL) covering a range of HT/CL ratios from 9/1 to 1/9 was carried out to produce polyesters with molecular weights above 30,000 in high yields. The copolyesters had a comonomer composition according to the feed and the microstructure evolved from random to blocky as the content in CL increases. The thermal and mechanical properties of the copolyesters were evaluated for a variety of compositions. Results obtained in this work were compared to those previously obtained by us in the ring-opening copolymerization of CL with a cyclic oligomeric fraction enriched in hexamer and heptamer (c(HT)_6-7). Although the polyesters resulting from the use of these two fractions were similar, significant differences were found in polymerization rate evidencing a lower reactivity of c(HT)_n with decreasing values of n.