Nanostructured polyurethanes have attracted substantial interest for the generation of "smart" surfaces, but step-growth polymerization at the nanoscale has scarcely been reported. In this work, the synthesis of polyurethanes by step-growth polymerization in the nanocavities of anodized aluminum oxide (AAO) templates is studied, finding that the kinetics of the reaction was faster than in bulk, while the molecular weight and the dispersity were reduced. A mathematical model was developed to explain these differences, taking into account the chemical and physical interactions encountered in the reaction between the monomers (diisocyanate and dialcohol) for nanoconfined in AAO and bulk. In nanoconfinement, the polyaddition is catalyzed by hydroxyl groups on the pore wall, resulting in an increase in the rate of polymerization during the early stages of the reaction. At high conversions, the limited diffusion of the polymer chains results in a slower polymerization and a dispersity lower than that expected from bulk. (C) 2018 Elsevier Ltd. All rights reserved.