Studies on the mesophase formation mechanism of polybenzoxazines: polymerisation induction

To study the mesophase formation mechanism of polybenzoxazine, a novel linear benzoxazine oligomer bearing cholesteryl side groups [poly(PC-AC)] was designed and synthesised through thermally activated ring-opening polymerisation of a monofunctional benzoxazine monomer (PC-AC). The PC-AC was obtained by Mannich condensation reaction using mesomorphic amine of cholesteryl 4-aminobenzoate, p-cresol and paraformaldehyde as starting materials. During the isothermal polymerisation of PC-AC monomer, the phase evolution from a crystal phase to an isotropic molten phase and then to a liquid crystal (LC) phase was observed. Since it is PC-AC oligomers that form the LC phase, ‘polymerisation-induced LC’ mechanism is put forward. We believe that the structure factors including the confined formation of intramolecular hydrogen bonding and the side chain position of mesogenic units also play an important role in the formation of the LC phase. Furthermore, poly(PC-AC) exhibits a smectic mesophase. This work provides new insight into the LC phase formation mechanism of polybenzoxazines. This is very helpful to guide the rational design and synthesis of polymers with high thermal conductivity and high-temperature resistance.