Thermally switchable polymers: From thermo-reversibly self-healing hybrid polymers to irreversibly crosslinked flame-retardant networks

Diels-Alder (DA) reactions can be easily triggered by mere heating, sparing catalysts or stimulus, that are widely used in Covalent Adaptable Networks (CANs). Such CANs are used for making recyclable crosslinked materials due to their dynamic covalent networks. Herein we report the preparation and characterization of novel thermally switchable polymers capable of transitioning from thermo-reversibly self-healing hybrid polymers (re-CP-co-BMI) into irreversibly highly crosslinked networks with higher flame-retardancy (i-CP-co-BMI). Re-CP-co-BMI is characterized by excellent self-healing properties and reprocess-ability originating from DA/rDA reaction between maleimide of 4,4′-bismaleimidodiphenylmethane (BMI) and furan of cyclotriphosphazene bearing three allyl and three furan groups (CP-3AF). The DA reaction proceeds in the melt, avoiding the need for additives, such as solvents and catalysts. At higher temperatures, irreversible alkene addition reactions are triggered by reactions between maleimide groups of BMI and allyl groups of CP-3AF, forming i-CP-co-BMI. This polymer is characterized by outstanding flame retardancy with 30.4% limiting oxygen index (LOI) and V-0 grade in the UL-94 test. Such a convenient synthesis and flexible structure design of phosphazene will open new windows for CANs with tunable properties.