Radical-based Visible-light Initiator System with Extensive Dark Cure Potential

Free radical photopolymerization is very widely used in a variety of dental polymer applications. While radical-based photopolymerizations are favored for their convenience and versatility, polymerization rapidly ceases when the irradiation source is extinguished due to highly efficient radical-radical termination reactions. Objective: The development of a radical-based photo-initiator system that will allow (meth)acrylate monomers to continue polymerization to completion even when the visible-light exposure is ended at an early stage of conversion.

Methods: 2-Hydroxyethyl methacrylate (HEMA) was photo-activated with either a conventional camphorquinone (CQ; 0.5 M)/diazabicyclooctane (DABCO; 0.25 M) 2-component initiator or a 3-component initiator containing a zinc tetraphenylporphyrin (ZnTPP; 2.3x10-4 M), DABCO (0.25 M) and diphenyliodonium hexafluoroarsenate (DPI; 0.02 M). Activated monomer specimens (0.5 mm thick) were photopolymerized with a 100 W halogen lamp while conversion was simultaneously monitored using near-infrared spectroscopy.

Results: The 2-component CQ/DABCO photopolymerization of HEMA reached > 90% conversion with a continuous full exposure cycle while an abbreviated exposure, which was interrupted at 62.2% conversion, only marginally improved to achieve a final conversion of 64.0% after a 90 min post-cure in the dark. The 3-componment ZnTPP/DABCO/DPI initiator system produced a limiting HEMA conversion of 93.3% upon full exposure; however, in dramatic contrast, partial cure exposures ended at either 1.7%, 4.3% or 30.7% conversion, yielded final 90 min conversions of 81.5%, 84.7 or 93.2%, respectively. The conversion kinetic data demonstrated little change in reaction rate as the polymerization transitioned to and then continued in the dark state.

Conclusions: This preliminary study represents a truly novel display of a photo-induced, controlled radical polymerization where the common bimolecular termination process is effectively suppressed. Besides the extensive dark cure, this approach has significant potential to obtain photocuring into shadow regions and to yield enhanced depths of cure.

Support: NIH/NIDCR 2 R01DE14227