Solvent-free preparation of imine vitrimers: leveraging benzoxazine crosslinking for melt processability and tunable mechanical performance

Abstract

Covalent adaptable networks, particularly imine vitrimers, have received considerable attention for the design of recyclable and reprocessable thermoset-like materials. However, the preparation of imine vitrimers often relies on solvent casting approaches that lack compatibility with existing polymer processing techniques and limit their practical application as thermoset replacements. Therefore, alternative vitrimer precursors with inherent melt processability are necessitated to realize the full potential of these recyclable materials. In this work, dynamic imine vitrimers were prepared in a solvent-free approach by employing orthogonal benzoxazine polymerizations. A series of imine-containing benzoxazine monomers were prepared to yield vitrimer precursors with tailorable rheological profiles and melt-state processability. Upon further heating, the benzoxazine functionalities undergo a condensate-free homopolymerization to yield networks with varied glass transition temperatures (34–160 °C) and impressive tensile strengths (up to 80 MPa). The incorporation of imine bonds into the backbone of the benzoxazine networks imparts efficient stress relaxations through metathesis exchange reactions, enabling malleability and repeated reprocessability at elevated temperatures. Dynamic exchange kinetics are found to be strongly correlated with crosslink density, with activation energies ranging from 70–177 kJ mol⁻¹. Additionally, we demonstrate that exposure to acidic conditions results in hydrolysis of the dynamic imine linkages, leading to full dissolution of the benzoxazine networks. This work demonstrates a solvent-free approach for preparing imine vitrimers by employing an orthogonal polymerization, and provides insight for the molecular design of processable recyclable thermosets.