The ability to achieve maximum conversion throughout an optically thick photopolymer material requires extended irradiation of the exposed surface to obtain near-comparable levels of conversion at the light-attenuated, opposite surface. Due to the high efficiency of bi-radical termination processes in radical-based polymerizations, only limited post-cure occurs if access to the curing light and continued production of initiating radicals is interrupted. A strategy to uniformly maximize final conversion within photopolymers by extending radical initiation well beyond the temporal exposure of the curing light is advanced here through a photoinduced redox initiation. Redox photoinitiators (redox PI) release various reductants (amines) upon 365nm irradiation, which react over extended intervals with oxidants (peroxides) to provide initiating radicals. For a real-time FTIR study of dark curing, the redox PI system achieved an initial conversion of 15% during a brief irradiation period. However, the continuous generation of radicals from dark redox reactions achieved a final conversion of 80% over 30 min without reliance on thermal assistance in conventional comonomers. The initial photopolymerization was rapid due to a synergistic effect of direct radical production from photodegradation of the PIs while the released amines reacted with peroxides to provide latent initiating radicals via the redox mechanism. This dark cure result is juxtaposed against the analogous condition with a conventional photoinitiator such as 2,2-dimethoxy-2-phenylacetophenone, which when interrupted at 12% conversion plateaued with a final conversion of 14%. We further demonstrated that dark curing rates can be well-controlled based on the reduction potential difference between the redox pairs. The capacity to reach full conversion under non-ideal photocuring conditions or throughout light-attenuated and thick films, such as bulk fill dental composites, can be accomplished with the redox photoinitiation.

Designing free-radical redox photoinitiators for practical dark curing

255th National Meeting (2018)

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