Dynamic Exchange Reactions as Self-Blowing Agents for the Production of Reprocessable Foams

Thermosetting foams are crucial in the polymer industry due to their unique lightweight and durable properties. Recent developments in sustainable chemistry intend to align foam lifecycles toward circular economy principles and reduce their environmental impact. In this study, this issue is addressed thanks to an innovative and straightforward approach, enabling the manufacture of single-component self-blowing vitrimer foam. The self-blowing mechanism originates from the in situ formation and release of the blowing agent via the polycondensation of an alkylester. This alkylester, composed of a benzoxazine matrix terminated with a β-hydroxylamine, undergoes in a cascade mechanism ring opening polymerization (ROP) and irreversible transesterification exchange, which in turn generates an alcohol gas. The cascade polymerization and alcohol release were optimized and fine-tuned to develop various foams that have an open-cell morphology, volume expansion of 223%-853%, porosity of 67%-86%, and compression modulus in the range of 3-38 MPa. The polymerization process results in a dual crosslinked poly(benzoxazine-co-ester) network composed of β-hydroxylamine moieties, alkyl, and β-aminoester bonds, enabling reversible transesterification dynamic exchanges and mechanical reprocessability, achieving a foam-to-resin reprocessing.