Light is a fascinating source of external stimulus for the regulation of polymerization processes. In this contribution, some salicylaldimine Zn(II) complexes bearing azobenzene moieties were prepared and characterized. Isomerization between the trans and cis forms of these complexes is achieved by exposure to light; both isomers are active in the ring-opening polymerizations of epsilon-caprolactone (CL), rac-lactide, trimethylene carbonate, beta-oxetanone, delta-valerolactone, cyclopentadecanolide, and S-methyl-S-propyl-1,3-dioxan-2-one but exhibit different kinetic profiles. A 6-fold reactivity difference was realized for the ring-opening polymerization of CL upon irradiation with light. The light-induced isomerization also led to reactivity discrimination between different monomers in copolymerization reactions. Reversible photoswitching between two catalytically active states provides a fascinating strategy for the control of polymerization processes, as well as the compositions of the polymers produced.