Chain extenders for polyesters. I. Addition‐type chain extenders reactive with carboxyl end groups of polyesters

Effective chain extenders for linear polyesters were investigated among some bis‐heterocycles, which were capable of coupling carboxyl terminals of the polyesters through addition reaction. Consequently, 2,2′‐bis(2‐oxazoline), 2,2′‐bis(5,6‐dihydro‐4H‐1,3‐oxazine) and N , N′ ‐hexamethylenebis(carbamoyl‐2‐ oxazoline) were found to be the most effective chain extenders. Starting from a poly(ethylene terephthalate) (PET) having intrinsic viscosity ([η]) of 0.66 and carboxyl content (CV) of 46 eq/10 6 g, one could obtain polyesters with [η] of above 1.0 and CV of below 5 eq/10 6 g in the presence of the chain extenders. Typical reaction condition for the coupling of PET was heating PET under atmospheric nitrogen above its melting temperature with 0.5 mol % of a chain extenders only for several minutes. Bis‐2‐thiazolines showed no effect under the condition investigated, while in case of bis‐2‐imidazolines definite degradation was observed. Bis‐ N ‐acylaziridines and bisiminocarbonates resulted in some gell formation, indicative of side reactions.