Ring opening reactions of some dialkyl substituted 1, 3, 2-dioxathiolan-4-one-2-oxides

Abstract

The synthesis and polymerisation under various conditions of a series of 1,3,2-dioxathiolan-4-one-2-oxides (α-hydroxy carboxylic acid anhydrosulphites) derived from disubstituted α-hydroxy alkanoic acids has been studied. In thermal decomposition reactions the length of the nalkyl group in methyl, n-alkyl substituted compounds of this type was found to primarily affect the rate rather than the mechanism of the reaction. The first-order rate determining ring scission process, leading to the formation of a reactive -lactone intermediate which then undergoes spontaneous polymerisation, is the predominant mechanism even with substituents up to Cg: As the length of the n-alkyl substituent is increased the rate of decomposition and thus polymerisation increases until a plateau value is reached at around C3. Further increase in substituent length decreases the rate of decomposition, an effect which is attributed to steric hindrance of the leaving group by long (Ce a Cg) substituents. The 5,5-dibutyl derivative of the closely related anhydrocarboxylate ring system (1,3-dioxolan-2,4-diones) has been synthesised and its decomposition briefly studied. In the temperature range required for a reasonable rate of decomposition a competitive non-polymer forming reaction is found to be significant. Of the various initiating systems studied, lithium t-butoxide was found to be particularly successful in inducing the decomposition of anhydrosulphites. A kinetic analysis indicated that, in its early stages, the reaction is first order with respect to monomer and initiator. In the latter stages the reaction becomes slow, possibly due to the onset of heterogeneity and a diffusion controlled situation. Molecular weight studies indicate that the polymer produced by this route has a lower molecular weight and broader molecular weight distribution than that obtained by thermal methods. Various possible mechanisms for initiated anhydrosulphite decomposition are discussed, and a plausible reaction scheme is suggested.