Reducing volumetric shrinkage of photopolymerizable materials using reversible disulfide-bond reactions

We introduce a new strategy for reduction in volumetric shrinkage of free radical photopolymerization. Our strategy is based on the reversible reaction of disulfide bonds under UV irradiation. Here, we synthesized 2,2′-dithiodiethanol diacrylate (DSDA), an acrylate monomer with disulfide bonds. The homolytic photocleavage of DSDA under UV irradiation generates thiyl radicals that can initiate polymerization. Volumetric shrinkage can decrease to 0.1% through a repeated "contraction–expansion–contraction" volume-adjustable process. We identified the mechanism that underlies volumetric shrinkage reduction. The photocleavage rate of DSDA under UV irradiation is slower than that of the added photoinitiator. Moreover, in the presence of the photoinitiator, most of the generated thiyl radicals undergo restoration and exchange reactions instead of polymerization initiation or chain termination. The free volume and structure of the polymer network are effectively tuned by the dynamic and reversible processes of gradual disulfide-bond homolysis and recombination during fast photopolymerization.