methods may introduce additional processing requirements to isolate the product from a solvent. Photopolymerization Most photopolymerization reactions are chain-growth polymerizations which are initiated by the absorption of visible or ultraviolet light. The light may be absorbed either directly by the reactant monomer (direct photopolymerization), or else by a photosensitizer which absorbs the light and then transfers energy to the monomer. In general, only the initiation step differs from that of the ordinary thermal polymerization of the same monomer; subsequent propagation, termination, and chain-transfer steps are unchanged. In step-growth photopolymerization, absorption of light triggers an addition (or condensation) reaction between two comonomers that do not react without light. A propagation cycle is not initiated because each growth step requires the assistance of light. Photopolymerization can be used as a photographic or printing process because polymerization only occurs in regions which have been exposed to light. Unreacted monomer can be removed from unexposed regions, leaving a relief polymeric image. Several forms of 3D printing—including layer-by-layer stereolithography and two-photon absorption 3D photopolymerization—use photopolymerization. Multiphoton polymerization using single pulses have also been demonstrated for fabrication of complex structures using a digital micromirror device. See also Cross-link In situ polymerization Metallocene Plasma polymerization Polymer characterization Polymer physics Reversible addition−fragmentation chain-transfer polymerization Ring-opening polymerization Sequence-controlled polymers Sol-gel References