Photopolymerization of difunctional cyclopolymerizable monomers with low shrinkage behavior

Cyclopolymerizable monomers (CPM) have been the focus of investigations for over 70 years due to favorably low shrinkage upon polymerization, yet little research dealt with difunctional CPMs, especially in the field of radical photopolymerization. Herein, we synthesized novel difunctional 1,6-diene CPMs based on the isomeric mixture of (2,2,4)/(2,4,4)-trimethylhexane-1,6-diamine as spacer unit, which undergoes cyclopolymerization forming five- or six-membered ring structures in the polymer backbone upon photopolymerization. Different photopolymerizable moieties (allyl-, methacryloyl-, and ester-activated allyl-moieties) were chosen for modification of the spacer unit to investigate their influence on reactivity and shrinkage behavior. The (thermo)mechanical properties of the cured difunctional 1,6-diene CPMs further reveal the effect of reactivity-enhancing electron-withdrawing groups (e.g., ester and carbonyl groups) on the final polymer network. For comparison, similar difunctional monoene compounds were are also synthesized and characterized to illustrate the low shrinkage behavior of the novel difunctional 1,6-diene CPMs.