Evaluation of a Potential Ionic Contribution to the Polymerization of Highly Reactive (Meth)acrylate Monomers

Monovinyl methacrylic and acrylic monomers that exhibit enhanced photopolymerization reactivities were studied to discern a possible secondary polymerization mechanism that involves an anionic contribution to the polymerization. To test for this possible anionic contribution, a strong acid, methanesulfonic acid, was added in small quantities to each of the (meth)acrylic monomers, and the steady-state photopolymerization rate was monitored. Initial control studies with conventional free-radical polymerizing monomers showed that the polymerization rate of these conventional (meth)acrylic monomers are not affected by small amounts of acid. Acid studies of secondary functionalized (meth)acrylate monomers, which typically exhibit rapid polymerization kinetics, demonstrated that the polymerization rate is decreased in these monomers by the addition of the strong acid. More specifically, the acid addition had a greater inhibitory effect on the monomers with faster bulk polymerization rates. The acid addition had a much more profound effect on the polymerization rate of the acrylate monomers compared to analogous methacrylate monomers. More specifically, addition of 250 ppm of the strong acid to the phenyl carbamate acrylate monomer showed that both the steady-state and unsteady-state polymerization profiles were similar to that of a traditional free-radical acrylate monomer. All experiments indicate that an anionic contribution to the polymerization is probable and has a profound impact on the polymerization rate.