A C-13 NMR-STUDY OF TRANSESTERIFICATION IN MIXTURES OF POLY(ETHYLENE-TEREPHTHALATE) AND POLY(BUTYLENE TEREPHTHALATE)

Poly(ethylene terephthalate) and poly(butylene terephthalate) homopolymers and mixtures of these two polymers have been heated in the absence of oxygen at 573 K for 30 min and 476 K for 6 h. The resultant polymers have been analysed using solution viscometry, d.s.c., density determination and C-13 n.m.r. Degradation has been observed when the polyesters are heated at 573 K and this is particularly evident for the poly(butylene terephthalate). Assignment of resonances due to homogeneous and heterogeneous dyads and degradation products in the n.m.r spectra have been made using model compounds and a random copolyester which have been synthesized by us. Using these assignments we have calculated sequence length distributions of ethylene and butylene sequences in the materials obtained. On heating at 573 K transesterification of the polyester present in the minority is complete and a single random copolymer is obtained. From the melting point data, it is clear that two copolyesters result when the mixtures are heated at 476 K for 6 h. The values for the number average sequence length suggest that block copolyesters are produced.