Mechanism and Modeling of a Thiol−Ene Photopolymerization

Thiol−ene photopolymerizations proceed via a sequential radical propagation/chain transfer mechanism that leads to polymer and network formation much like a step growth polymerization. Here, the chain transfer step in this sequential reaction series is shown to be the rate-limiting step. A model has been developed that accurately predicts the observed polymerization kinetic behavior under a variety of circumstances. Chain transfer is modeled as a rate-limiting step with the rate parameter (kp) for the propagation reaction being a factor of 10 greater than that for the chain transfer process (kCT) (kp/kCT = 10). The polymerization rate is first-order overall with first-order dependence on thiol functional group concentration and independent of the ene functional group concentration; Rp ∝ [SH]1[CC]0. Polymerization rate behavior vs functional group concentration change is shown to be only a function of the ratio of propagation to chain transfer kinetic parameters.