Degradable Spiroacetal-Bridged Bioderived Polybenzoxazine Thermosets

Beyond the use of biofeedstocks, degradation of thermosets is crucial for effective waste management in the development of sustainable polymers. In this study, we report the synthesis of polybenzoxazine thermosets using spiroacetal-bridged diphenols, p-hydroxybenzaldehyde (PHace)/vanillin (Vace), combined with furfurylamine (fa)/hexylamine (ha), and formalin. The phenol and amine components in the monomers were chosen to affect the cross-linking density of the polybenzoxazine thermoset; the free-ortho position in the phenol component and the aromatic furan ring in the amine participate profoundly in cross-linking reactions. The developed polybenzoxazine addresses the challenge of degradation, which has been difficult otherwise, by leveraging the intrinsic aldehyde transformation in the phenol precursor into citric acid (CA)-cleavable spiroacetal linkages within the network. Poly(Vace-fa) and poly(PHace-fa) exhibit notable thermal properties and a typically high Tg value of 205 and >350 °C, respectively. Poly(Vace-fa), besides showing promising adhesive performance to adhere stainless steel coupons (lap shear strength of 19 kg/cm2), was also easily degraded by dilute CA in mild conditions, presenting 100% recovery of the metal substrate cleanly and with minimal interventions, increasing the overall sustainability aspect. This study opens opportunities for designing robust yet degradable thermosets by incorporating labile units at the molecular level, offering transformative potential in this emerging class of phenolic thermosets.