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Addition polymerization is a polymer-forming process in which monomer molecules combine without the stoichiometric elimination of a small molecule. In its principal chain-growth form, an active radical, ionic, or coordination site adds successively to unsaturated monomers, transferring the active center to the newly incorporated repeat unit. The category also includes addition to heteroatom-containing multiple bonds, as demonstrated by polymerization across a phosphaalkene P=C bond ^[1]. Conjugate-addition polymerizations are mechanistically distinct polyaddition processes in which multifunctional nucleophiles and activated alkenes form successive bonds without a condensate ^[2]. N-heterocyclic-carbene-catalyzed conjugate-addition polymerization illustrates a chain process with identifiable initiation, propagation, and termination steps ^[3]. The absence of an eliminated byproduct distinguishes addition polymerization from condensation polymerization, whereas the manner in which molecular mass develops distinguishes chain-growth addition from step-growth polyaddition.