MECHANISM OF EXCHANGE IN POLYESTERS - COMPOSITION AND MICROSTRUCTURE OF COPOLYMERS FORMED IN THE MELT MIXING PROCESS OF POLY(ETHYLENE-TEREPHTHALATE) AND POLY(ETHYLENE ADIPATE)

A study of the microstructure of the copolyesters formed in the exchange reactions which occur during the melt mixing of poly(ethylene adipate) (PEA) and poly(ethylene terephthalate) (PET) is reported. A PET sample stripped of the low molecular weight oligomers was melt-mixed with an equimolar amount of an PEA sample, and the exchange kinetics at 290-degrees-C was followed by analyzing the reaction mixture by H-1 NMR and FAB-MS. Mass spectra of partially aminolyzed melt-mixed samples were also examined. The copolyester initially formed in the exchange reaction proved to be a block copolymer containing an excess of EA units, as expected for a reaction proceeding by the attack of PEA on the PET chains. In the later stages of the exchange reaction, the copolyester microstructure was observed to approach a Bernoullian distribution and equimolar composition. The determination of the composition and microstructure of the copolyesters formed was attempted by NMR and mass spectra of the melt-mixed samples. This proved to be a difficult task by 200-MHz H-1 NMR spectroscopy, since only dyads were detected here because of the large repeating units in the copolymer. Instead, the FAB mass spectra showed series of oligomers up to the octamers, and the composition and microstructure of the copolyesters was monitored by applying some methods of statistical modeling of mass spectra of copolymers.