Controlled polymerization of 3,4-dihydro-2H-1,3-benzoxazine and its properties tailored by Lewis acids

Polymerization reaction of a typical thermosetting benzoxazine monomer, BA-a, is tailored intentionally by particular Lewis acids including aluminium trichloride (AlCl3), phosphorus pentachloride (PCl5) and zinc chloride (ZnCl2), and polybenzoxazines with different crosslinking structures are obtained. Their rheological behaviors, polymerization mechanisms, chemical crosslinking structures and thermal properties are systematically studied. The results suggest that AlCl3 and PCl5 are much more efficient than ZnCl2 to affect the polymerization reaction of BA-a. PCl5 and AlCl3 can promote the generation of more carbocations, thus the polymerization of benzoxazine follows cationic chain polymerization mechanism. However, in the presence of less effective ZnCl2 or in the case of neat benzoxazine, the polymerization follows step polymerization mechanism. It is learned that the Lewis acids can promote crosslinking reaction of dangling arylamine benzene rings and form arylamine Mannich linkages that vary with the catalytic efficiency of the Lewis acids. As a result, the increase of impact strength by 36.4%, the increase of flexural strength by 243.4%, the increase of flexural modulus by 42% and the increase of char yield by 18.6% (absolute value) can be achieved by using different Lewis acids.