Bio-based, main-chain type polybenzoxazine precursor derived from sustainable furfurylamine and salicylaldehyde: Synthesis, characterization and properties

Main-chain type polybenzoxazine precursors (MCPPs) are a current focus in the polybenzoxazine field for their improved char yield, thermal stability and filmforming ability compared to monomeric benzoxazines resins. To improve sustainability, there is an urgent need to utilize renewable feedstocks instead of fossil-based chemicals in the preparation of MCPPs. Here, renewable furfurylamine and salicylaldehyde were used as staring materials to synthesize high biomass content MCPPs through a three-steps polymerization route. The number-average molecular weight of a bio-based MCPP is 6100 g/mol, and it has a polydispersity index of 1.7. The furan-containing MCPP shows two polymerization stages during heating; the main exothermic stage at approximately 250 degrees C is attributed to the ring-opening polymerization of benzoxazine units, and the secondary stage at higher temperature is due to a crosslinking reaction between furan rings, forming furfurylamine Mannich bridges, and an oxidization-induced coupling reaction. The average activation energy of an MCPP is approximately 173.8 kJ/mol. The 240 degrees C-cured film is transparent with a glass transition temperature of over 300 degrees C. The crosslinked film shows a high thermal stability and a char yield of greater than 66% in a nitrogen atmosphere. Scanning electron microscopy (SEM) images indicated that a defect-free film can be made by controlling the curing parameters.