(499c) Extensive Dark Cure from Controlled Polymerization Based on a Method Using Visible-Light Activated Initiator System

Free-radical photopolymerization has been widely used for polymeric biomaterials because of rapid reaction, excellent mechanical properties and the versatility available with a broad array of monomers. In contrast, cationic polymerization shows greater sensitivity to trace impurities and involves a much more restrictive selection of monomers; however, cationic polymerization allows significant dark curing because of long active center lifetimes. The dark curing process is one of the distinct differences between cationic and free-radical photopolymerization. In this contribution, we report the development of a photo-initiator system that allows extensive dark curing from visible light-initiated controlled radical polymerization. The photo-initiator is a radical-based, three-component initiator system that includes a light-absorbing photosensitizer (PS), an electron donor and an electron acceptor, which is usually an onium salt. In this system, 5,10,15,20-tetraphenyl-21H,23H-porphyrin zinc (Zn-tpp) is used as a PS, diazabicyclooctane (DABCO) is used as an electron donor and diphenyliodonium hexafluoroarsenate (DPI) is used as an electron acceptor. The three-component initiator system, Zn-tpp/DABCO/DPI produced a limiting conversion of 93.3 % conversion in 2-hydroxyethyl methacrylate (HEMA) upon full exposure time over 90 min. Real time conversion was monitored using near-infrared spectroscopy while photopolymerizations were conducted with a 100 W halogen lamp at an irradiance of 0.15 mW/cm2. Remarkably, three-component initiator system did not exhibit rapid termination of HEMA polymerization when the irradiation source was turned off. For example, only 30, 60 or 300 sec partial illumination coincided with conversion of 1.7 %, 4.3 % and 30.7 % respectively, but the final conversions reached 81.5 %, 84.7 % and 93.3 % respectively, at 90 min. These conversion kinetic results illustrate a unique controlled radical dark polymerization where the radical active centers are not terminated even when the light source is extinguished. Among many applications, this approach has practical significance for polymeric biomaterials including dental composite materials and bone cements where depth of cure and shadow cure are important considerations.