Stress Relaxation via Addition-Fragmentation Chain Transfer in a Thiol-ene Photopolymerization

Allyl sulfide addition - fragmentation chain transfer was employed concurrently with the radical-mediated formation of a thiol-ene network to enable network adaptation and mitigation of polymerization-induced shrinkage stress. This result represents the first demonstration of simultaneous polymerization and network adaptation in covalently cross-linked networks with significant implications for the fabrication of low-stress polymer networks. For comparison, analogous networks incorporating propyl sulfide moieties, incapable of addition-fragmentation, were synthesized and evaluated in parallel. At the highest irradiation intensity, the allyl sulfide-containing material demonstrated a > 75% reduction in the final stress when compared with the propyl sulfide-containing material. Analysis of the conversion evolution revealed that allyl sulfide addition-fragmentation decreased the polymerization rate owing to thiyl radical sequestration. Slow consumption of the allyl sulfide functional group suggests that intramolecular hydrolytic substitution occurs by a stepwise, rather than concerted, mechanism. Simultaneous stress and conversion measurements demonstrated that the initial stress evolution was identical for both the allyl and propyl sulfide-containing materials but diverged after gelation. Whereas addition-fragmentation chain transfer was found to occur throughout the polymerization, its effect on the stress evolution was concentrated toward the end of polymerization when network rearrangement becomes the dominant mechanism for stress relaxation. Even after the polymerization reaction was completed, the polymerization-induced shrinkage stress in the allyl sulfide-containing material continued to decrease, exhibiting a maximum in the stress evolution and demonstrating the potential for continuing, longer-term stress relaxation.