Toward Rigid Foams by Aza-Michael Chemistry: Tuning Foam Morphology through Controlled Prepolymerization without Altering Polymer Structure

Increasing regulatory restrictions on toxic isocyanates are expected to significantly affect the production and application of polyurethane (PU) foams in the near future. Although nonisocyanate PU foams are being actively investigated, their development remains largely at the laboratory scale. Beyond conventional urethane chemistry, alternative approaches, particularly those based on aza-Michael addition, have been scarcely explored for foam generation. In this study, we employed the aza-Michael reaction to produce foams, whose properties provide a promising foundation for achieving performance comparable to conventional PU foams through further optimization. A modified reaction pathway incorporating a prepolymerization step prior to foaming was introduced, enabling precise control over foam growth and morphology. With a single set of multifunctional monomers, tunable reaction kinetics was achieved, allowing tuning of foam expansion without measurably altering the final polymer structure. This strategy provides a promising platform for the future development of high-performance, isocyanate-free rigid foams for thermal insulation applications.