Photopolymer Networks with Tunable Cross-Link Density via Chemically Gated Chain Transfer

Photopolymers derived from (meth)acrylate-based resins typically feature high cross-link densities and limited ability to tune their mechanical properties. Chain transfer agents (CTAs) based on thiols have been widely deployed to control molecular weights in free-radical polymerization of linear polymers but have shown limited use as additives for tuning properties of thermosetting polymers. In this work, we demonstrate that addition of a monofunctional thiol-based CTA (2-mercaptoethanol) significantly alters the cross-link density of photopolymer networks, with a corresponding decrease in the glass transition temperature and modulus. This thiol-based CTA can be incorporated as an additive to pre-existing photopolymer resins at low concentrations (under 10 wt %) to tune network properties and does not significantly compromise photopolymerization kinetics. By utilizing the inherent reactivity of the thiol and acrylate moieties in a gating reaction via a thiol-Michael addition, we demonstrate that the CTA concentration within the resin can be regulated on demand prior to photopolymerization. To demonstrate the ability to tune the photopolymer resin via the gating reaction, we produced networks with time-resolved modulus based on thiol-Michael addition conversion as well as networks with patterned solubility based on local cross-link density. This work establishes a chemically gated CTA strategy as a practical pathway for tunable photopolymer materials derived from a single polymer chemistry.