The Polymerisation of Sulphur containing Heterocycles

Abstract

A number of α-thio-carboxylic acid anhydrocarboxylates and α-thio-carboxylic acid anhydrosulphites have been synthesised by the reaction of the parent α-thio acids with carbonyl chloride (COCL2) and thionyl chloride (SOCL2) respectively. The base-initiated decomposition of the anhydrocarboxylateshas been studied in some detail using both protic and aprotic bases. Kinetic studies have been carried out by means of gas evolution techniques and the reaction products (principally poly-a-thioesters) characterised. The characteristics of the reactions of the α-thioacidanhydrocarboxylates with protic and with aprotic bases are quite different. Mechanisms have been proposed that account for these  differences and for the observed effects of ring substituents on the polymerisability of monomers of this type. A third type of reaction has been studied, this is the thermal (non-initiated) decomposition of the anhydrocarboxylate of α-thioisobutyric acid. The presence of two methyl substituents makes this compound relatively stable to attack by bases but has the opposite effect on its rate of thermal decomposition. In this latter reaction a competition was observed between polymerisation and formation of tetramethyl thioglycollide. The overall kinetics of the reaction were studied by both gas evolution and spectroscopic techniques. The proposed mechanism involves the primary decomposition of the ring in a first-order rate-controlling process leading to the expulsion of carbon dioxide and the formation of an α-thiolactoneintermediate. This highly reactive species then takes part in a non rate-controlling chain addition process leading to polymer formation. The formation of tetramethyl  thioglycollidetakes place in competition with this latter reaction. The characterisation of the polyα-thioesters produced in these reactions has been carried out using a variety of techniques. The polymers are highly crystalline but of appreciably lower melting point and thermal stability than their oxygen-containing counterparts.