Photoinduced cross-linking polymerization in the presence of aromatic thiols

The aim of this work was to find the effect of two aromatic thiols (one heteroaromatic and one aromatic), efficient co-initiators in two-component photoinitiating systems, on the termination mechanism of a photoinduced polymerization occurring in stationary and nonstationary conditions. This task has been approached by modeling using two termination models: the bimolecular termination model and the mixed termination model. For the first time the influence of thiols was considered separately on the monomolecular and bimolecular termination. The effect of the thiols consisting in the increasing contribution of the second-order termination was assumed to reside in higher values of the bimolecular termination rate coefficient (k(t)(b)) in comparing to polymerization in the absence of thiol. On the other hand, the decreasing contribution of the first-order termination (considered as a significant decrease in the mobility and/or reactivity of radical sites) was assumed to be involved in lower values of the monomolecular termination rate coefficient (k(t)(m)). It was found that the presence of the thiol increases the contribution of the bimolecular termination and decreases the contribution of the monomolecular termination at the early polymerization stages. Monomolecular termination becomes less important as the contribution of the reaction diffusion in bimolecular termination increases.