Experimental and computational investigations on ring-opening polymerization mechanisms of amide-functional benzoxazines

We observed an unusual low polymerization temperature for the ortho-amide benzoxazine in comparison with its para-isomer. Density functional theory (DFT) calculations suggested that the intramolecular hydrogen bond between the oxazine ring and the adjacent amide softens the C–O bond, resulting in a reduced activation energy and thus a low ring-opening polymerization temperature. In addition, the polymerization kinetics of both para- and ortho-amide functional benzoxazines were investigated using the Starink method, which confirmed a relatively lower activation energy for the ortho-amide functional benzoxazine compared with its para-isomer. Our work suggests that softening chemical bonds by intramolecular hydrogen bonding may become a new strategy for the design of high-performance polybenzoxazine thermosets with low processing temperatures. Graphical abstract: [Figure not available: see fulltext.]