Understanding Relative Reactivity-Governed Kinetics and Network Formation in Thiol-Yne and Other Competitive Consecutive Polymerizations

Thiol-yne polymerizations couple two sequential additions of thiol: first, to an alkyne and subsequently, to a vinyl sulfide, making the polymerization and the resulting polymer structure dependent on the relative reactivity. Herein, kinetic, analytical, and random-graph frameworks are developed that predict the reaction kinetics and network formation of thiol-yne polymerizations. The kinetic model predicts a chemically driven form of rate acceleration that arises from the interplay between vinyl sulfide generation and consumption and results in the maximum rate being up to 33% faster than the initial rate. Time-independent solutions are found for the conversion trajectories that are then used to derive expressions for gel-point conversion, sol fraction, and elastically active network chain concentrations. The results generalize to other competitive consecutive reactions, such as epoxy-amine reactions. The propagation ratio (γ=kp,enekp,yne) and the stoichiometric ratio of thiol to yne functional groups (χ=[SH0][Y0])are found to govern the relative conversions and network formation, while chain-transfer determines the time scale. In the presence of conversion limitations, the final network develops a kinetic path dependence, transforming the propagation ratio from a transient kinetic property to a governing parameter of the final network structure.