Neighbouring group effect on the kinetics of acetalisation of poly(vinyl alcohol)

A kinetic model for acetalisation of poly(vinyl alcohol) (PVOH) involving a neighbouring group effect has been developed using a statistical approach. Assuming that the acetalisation proceeds irreversibly, an effectiveness factor (defined as the ratio of the concentration of reactable hydroxyl groups to the total concentration of hydroxyl groups of the polymer) has been obtained as a function of conversion. When the effect of neighbouring groups on the reactivity of OH groups is neglected, the model predicts a maximum attainable conversion of 87% which compares well with that postulated by Flory for irreversible acetalisation. Possibilities of conversions > 87%, as reported by some investigators, are discussed. Flory accounted for such higher conversions by postulating reversibility of the acetalisation. Experimental studies reported in the literature, however, indicate the acetalisation to be essentially irreversible. It is shown here that the present model, with the inclusion of the neighbouring group effect, can account for such higher conversions even for irreversible acetalisation of PVOH.