Chain-Transfer Agents in Photopolymer Networks

Chain-transfer agents can regulate radical photopolymer-network formation by terminating or redirecting growing chains, creating a new radical, delaying gelation, altering strand architecture or enabling reversible exchange. These effects can reduce stress or change mechanical response, but the term chain transfer covers chemically distinct mechanisms. Conventional thiol hydrogen transfer, irreversible addition–fragmentation chain transfer and reversible addition–fragmentation chain transfer cannot be assigned one common transfer efficiency or property ranking. This review separates the mechanisms and defines the evidence required to connect transfer chemistry with conversion history, gelation, stress evolution, network state and retained properties. Primary studies demonstrate stress relaxation through allyl-sulfide AFCT, loading-dependent effects in dimethacrylate networks, thiol and vinyl-sulfonate regulation of acrylate formulations, topology changes in photo-crosslinked poly(ε-caprolactone), and RAFT-mediated control in printed or film-cured systems. Their numerical outcomes remain specific to their chemistries and irradiation histories. Qualification therefore requires an explicit measurand for molecular weight, matched kinetic and network-state measurements, and independent tests of modulus, strength, elongation, toughness or durability.