Stoichiometric engineering of aza-Michael chemistry for controlled foam generation

With rising concerns over the toxicity and environmental impact of isocyanates, it is high time to explore viable isocyanate-free alternatives for polymer foams. While polyurethane foams have long dominated the market due to decades of refinement, achieving comparable properties with alternative chemistries remains a challenge. The aza-Michael addition reaction, which proceeds under mild conditions, offers a promising route to synthesize crosslinked polymer foams without isocyanates. In this study, we demonstrate the preparation of rigid polymer foams via aza-Michael addition between multifunctional monomers using physical blowing agents. A key innovation lies in introducing an intermediate pre-reaction step, where varying the reactant stoichiometry allowed us to finely control the final polymerization kinetics and exothermicity. This tunability enabled modulation of foam rise, density and morphology, without altering the base formulation. The resulting foams were partially open-cell with densities as low as ~71 kg·m −3 , average cell radius of ~500 μm, and thermal conductivity of ~48 mW·m −1 ·K −1 . The foams were mechanically robust, with excellent recovery after 80% compression. With further optimization of the foaming process and the use of tailored surfactants and blowing agents, aza-Michael-based foams could emerge as strong candidates to replace rigid polyurethane foams in commercial applications. • Rigid foams via aza-Michael chemistry as an isocyanate-free alternative route. • Used tunable pre-reaction to control polymerization kinetics and exothermicity. • Pre-reaction stoichiometry modulated foam rise and properties at fixed formulation. • Obtained low density foams with low thermal conductivity and high shape recovery.