Pyrogallol-based benzoxazines with latent catalytic characteristics: The temperature-dependent effect of hydrogen bonds on ring-opening polymerization

Two kinds of pyrogallol-based di-benzoxazines (PG-FA and PG-A) with a free phenolic hydroxyl between two oxazine rings attaching to the same benzene were synthesized. The chemical structures of PG-FA and PG-A were identified by H-1 NMR, C-13 NMR and FTIR tests. The ring-opening polymerization (ROP) behavior of PG-FA and PG-A was monitored by DSC, TGA, FTIR and in situ FTIR measurements. The pyrogallol-containing benzoxazines showed latent catalytic activity as they reached melting condition. The inter- and intramolecular hydrogen bonds from the phenolic hydroxyl and oxygen or nitrogen were converted into weak OH re intramolecular hydrogen bonding, which easily released the free phenolic hydroxyl. Then, the free hydroxyl could accelerate the breakage of oxazine ring. The para-position of free phenolic hydroxyl in pygarollol-based benzoxazines was preferentially polymerized with the carbocations. The temperature-dependent evolution of the chemical structure of PG-FA and PG-A on ROP was consistent with the change of hydrogen bonds.