This work aims to disclose the reason that prohibited the preparation of highly crosslinked polybenzoxazines. Based on experimental study and computer simulations, we found that the dominant –OH⋯N hydrogen bond (Type I –OH⋯N hydrogen bond) of polybenzoxazines blocked high-degree polymerization and resulted in a low crosslink density by decreasing the charge densities of corresponding hydroxyl groups on phenols. As a solution, by introducing additional hydrogen-bond acceptors, the formation of Type I –OH⋯N hydrogen bonds could be suppressed, which enables a high-degree of polymerization of benzoxazines. This novel insight about benzoxazine polymerization is anticipated to help researchers explore more types of polybenzoxazines with enhanced properties.