Photopolymerization is a technique where light acts as a contactless stimulus, creating free radicals that most frequently convert multifunctional (meth)acrylate monomers into crosslinked, thermosetting polymers. The materials generated by photopolymerization are inherently highly crosslinked and suffer from significant shrinkage stress, are often brittle, and have a limited range of material properties. Various classes of chain transfer agents (CTAs) have been investigated and developed to reduce the crosslinking density of photopolymers. Although CTAs are successful in manipulating the mechanical properties of photopolymers, they are required in high loadings (up to 20 wt% of total formulation) and frequently contain sulfur which is malodorous and can create unstable formulations. Presented here is a catalytic, sulfur-free CTA that can be added in ppm quantities to existing commercial monomer feedstocks to create photopolymers similar to those prepared using traditional CTAs, but at 10,000x lower loadings. These catalysts were benchmarked against other commonly utilized CTAs and the chain transfer constants were determined by digestion of the crosslinks of the photopolymer and analysis of the resultant linear polymer by size exclusion chromatography. The impact of this catalytic CTA in commercial resins to narrow and reduce the glass transition temperature, reduce storage modulus, and other mechanical properties will be presented. Ultimately, we believe that catalytic CTAs, as presented here, will eventually supplant existing, consumptive CTAs in commercial photopolymer products.

Development of catalytic, sulfur-free chain transfer agents for application in crosslinked photopolymers

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