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Photo-induced polymerization is a chain-growth process in which absorption of light generates the initiating species that starts macromolecular propagation ^[1]. In photopolymerization the concept is bounded by photonic initiation: the light quantum is required to produce radicals, cations, or related active centers, after which chain growth may proceed by a conventional thermal mechanism. Essential components are a photoinitiator or photoactive monomer, a wavelength that matches the absorption of that species, and a monomer that can propagate from the photogenerated center. The definition excludes thermally initiated polymerizations that happen to be conducted under illumination and excludes photochemical small-molecule reactions that do not produce a macromolecule. Photo-induced initiation can be used to grow grafts from a surface, to lock a self-assembled chiral superstructure into a polymer matrix, or to trigger polymerization-induced self-assembly when a solvophilic block is chain-extended with a solvophobic monomer ^[1][2][3]. Because initiation is spatially and temporally confined to the illuminated region, the same concept underlies the formation of patterned or rapidly cured polymer networks. The process is distinguished from pulsed-laser polymerization used solely as a kinetic analysis method by its identity as a preparative polymerization rather than as a rate-coefficient experiment.