Nanoconfinement in miniemulsion increases reaction rates of thiol–ene photopolymerization and yields high molecular weight polymers

Abstract

Thiol–ene polymerization is a powerful synthetic platform for the preparation of a variety of polymer materials but is often plagued by the formation of low molecular weight polymers. This is typical of step-growth polymerization, where high molecular weights are achieved only at nearly complete monomer conversions. However, experimental results suggest that it is possible to produce step-growth polymers with a high degree of polymerization by performing the reaction in a miniemulsion, where the dispersed droplets act as nanoreactors. Here, we investigate the effect of confinement arising from the reduction of the reaction loci from bulk to a nanoreactor and how it affects the thiol–ene reaction and the resulting polymers. The polymerization rates observed for the reaction in the miniemulsion were up to 35-fold higher than the rates observed in bulk. Different monomer pairs were evaluated using either a diallyl, divinyl, or diacrylate monomer as dienes. The reaction was followed by Raman spectroscopy to simultaneously quantify the conversion of thiols and enes in the system, which enabled the detection of side reactions, such as homopolymerization. Mixtures with a non-stoichiometric ratio of dithiol and diene monomer also benefited from the polymerization in nanoconfinement. In such cases, the polymerizations in bulk were limited to very low degrees of polymerization. However, when the polymerization was performed in the confinement of the miniemulsion droplets, high molecular weight polymers were produced.