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 ( k p ) for the propagation reaction being a factor of 10 greater than that for the chain transfer process ( k CT ) ( k p / k CT = 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; R p ∝ [SH] 1 [C C] 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.