Studies of the reactions of the anhydrosulphites of α-hydroxy-carboxylic acids. Part II. Polymerisation of glycollic and lactic acid anhydrosulphites

Abstract

The reactions of lactic and glycollic acid anhydrosulphites with simple alcohols to give the esters have been followed by gas-evolution techniques. The reaction is first-order in anhydrosulphite and hydroxy-group concentrations. A mechanism involving direct attack on the anhydrosulphite ring by the alcohol accounts for most of the observations. The kinetics of the formation of polylactic acid from the anhydrosulphite show that the polymerisation is resolvable into two processes; one is first-order in anhydrosulphite and zero-order in hydroxy-group, whereas the other is first-order in both anhydrosulphite and hydroxy-group. Polymerisation proceeds by thermal decomposition of the anhydrosulphite to the α-lactone which reacts rapidly with the terminal hydroxy-group and by direct attack of the terminal hydroxy-group on the hydrosulphite ring. The energies of activation and frequency factors support the proposed explanation. This system gives polymer with a Poisson molecular weight distribution even though the molecular weights are small.