Kinetic modeling of thiol-ene reactions with both step and chain growth aspects

A kinetic model is presented for thiol ene cross linking photopolymerizations including the allowance for chain growth reaction of the ene, i.e., homopolymerization. The kinetic model is based on a description of the average chain lengths derived from differential equations of the type of Smoluehowski coagulation equations. The method of moments was applied to obtain average properties of thiol-ene reaction systems. The model predicts the molecualr weight distribution of active and inactive species in the pre-gel regime of the thiol enes as well as the gel points depending on the synthesis parameters. It is shown that, when no homopolymerizations is allowed the average molecular weights and the gel point conversion are given by the typical equations valid for the step-growth polymerization. Increasing the extent of homopolymerization also increases the average molecular weights and shifts the gel point toward lower conversions and shorter reaction times. It is also shown that the ratio of thiyl radical propagation to the chain transfer linetic parameter (k(vl)/k(tr)) affects the gelation time, t(cr). Gelation occurs earlier as the k(pl)/k(tr) ratio is increased due to the predominant attack of thiyl radicals on the vinyl groups and formation of more stable carbon radicals. The gel point in thiol-ene reactions is also found to be very sensitive to the extent of cyclization particularly, if the monomer functionalities are low.