Free-radical polymerization using a photobase/redox initiating system to provide light-activated dark cure
Jeff Stansbury, University of Colorado
Abstract: The ability to produce maximum conversion throughout an optically thick photopolymer material, such as with dimensionally thick or formulations involving filled or pigmented resins, requires extended irradiation of the exposed surface to obtain near-comparable levels of limiting conversion at the light-attenuated opposite surface. Similar situations exist when attempting to photocure irregular surfaces or using heterogeneous light outputs, which lead to poor control over the local irradiant intensity. Due to the high efficiency of bi-radical termination processes in free radical-based polymerizations, only limited post-cure occurs if access to the curing light and continued production of initiating radicals is interrupted prior to complete through-cure. A strategy to uniformly maximize final conversion within photopolymers by extending radical initiation beyond the temporal exposure of the curing light is advanced here through a photoinduced redox initiation reaction. New photobase generators have been designed to release various reductants (amines) upon UV irradiation to then react over extended intervals in the dark with oxidants (peroxides) to provide initiating free radicals. As a demonstration of this approach using real-time FTIR analysis, the redox photoinitiator system in a glass-forming triethylene glycol dimethacrylate/acrylonitrile resin developed an initial conversion of ~15% during a brief, low irradiance exposure. However, based on the continuous generation of radicals from the redox reaction in the dark, a final conversion of 80% was achieved over 30 min without reliance on any exothermic temperature rise. This vitrification-limited final conversion is equivalent to that produced by continuous direct irradiation. The highly efficient photo-redox photopolymerization involves synergistic direct radical production from photodegradation of the photobase while the released amine reacts with peroxide to provide latent initiating radicals via the redox mechanism. The photo-activated dark cure result contrasts markedly with the analogous condition using a conventional photoinitiator, 2,2-dimethoxy-2-phenylacetophenone (DMPA), which when interrupted at 12% conversion rapidly plateaued to yield a final resin conversion of only 14%. We further demonstrated that the rate of dark curing can be well-controlled based on the reduction potential difference between the photo-released amine and the oxidant in the redox pair. The capacity to reach full conversion under non-ideal photocuring conditions or throughout highly light-attenuating thick films, such as dental composites, can be reliably accomplished with the photobase/redox photoinitiation approach. This offers a means to extend the use of free-radical photopolymerization well beyond its current scope and limitations.