Thiol–Ene Photopolymerization: Scaling Law and Analytical Formulas for Conversion Based on Kinetic Rate and Thiol–Ene Molar Ratio

Kinetics and analytical formulas for radical-mediated thiol–ene photopolymerization were developed in this paper. The conversion efficacy of thiol–ene systems was studied for various propagation to chain transfer kinetic rate-ratio (R(K)), and thiol–ene concentration molar-ratio (R(C)). Numerical data were analyzed using analytical formulas and compared with the experimental data. We demonstrated that our model for a thiol–acrylate system with homopolymerization effects, and for a thiol–norbornene system with viscosity effects, fit much better with the measured data than a previous model excluding these effects. The general features for the roles of R(K) and R(C) on the conversion efficacy of thiol (C(T)) and ene (C(V)) are: (i) for R(K) = 1, C(V) and C(T) have the same temporal profiles, but have a reversed dependence on R(C); (ii) for R(K) >> 1, C(T) are almost independent of R(C); (iii) for R(K) << 1, C(V) and C(T) have the same profiles and both are decreasing functions of the homopolymerization effects defined by k(CV); (iv) viscosity does not affect the efficacy in the case of R(K) >> 1, but reduces the efficacy of C(V) for other values of R(K). For a fixed light dose, higher light intensity has a higher transient efficacy but a lower steady-state conversion, resulting from a bimolecular termination. In contrast, in type II unimolecular termination, the conversion is mainly governed by the light dose rather than its intensity. For optically thick polymers, the light intensity increases with time due to photoinitiator depletion, and thus the assumption of constant photoinitiator concentration (as in most previous models) suffers an error of 5% to 20% (underestimated) of the crosslink depth and the efficacy. Scaling law for the overall reaction order, defined by [A](m)[B](n) and governed by the types of ene and the rate ratio is discussed herein. The dual ratio (R(K) and R(C)) for various binary functional groups (thiol–vinyl, thiol–acrylate, and thiol–norbornene) may be tailored to minimize side effects for maximal monomer conversion or tunable degree of crosslinking.