Rationalizing the regioselectivity of cationic ring-opening polymerization of benzoxazines

Polybenzoxazines have gained increasing interest both in industry and academia for the last few decades due to their unique structural features. The ring-opening polymerization mechanism of 1,3-benzoxazine monomers and the regioselectivity during polymerization, still need further clarification. In this study, ring-opening polymerization mechanisms of two methyl substituted benzoxazine derivatives 3,6-dimethyl-3,4-dihydro-2H-benzo[e][1,3] oxazine (pC-m) and 3,5,6,7-tetramethyl-3,4-dihydro-2Hbenzo[e][1,3] oxazine (345TMP-m) are investigated by quantum mechanical tools using density functional theory (DFT). Calculations have shown that in the presence of a nucleophile, pC-m can yield the phenolic polymer upon rearrangement of its intermediate phenoxy product. However, the polymerization of 345TMP-m results in a mixture of phenoxy and phenolic type polymers. The extra methyl groups on 345TMP-m have a dual role in preventing the π stacking interactions observed in pC-m, and in decreasing the barrier yielding phenolic polymers by electron donation.