Structure–Property Relationships of Aryl Ether Diamine-Based Benzoxazines: Role of Aromatic Substitution and Molecular Weight Between Crosslinks

A systematic evaluation of meta-substitution and backbone molecular weight in diamine-based benzoxazines was conducted to investigate the impact on melt processability, network development, and the structure-property relationships in polybenzoxazines. Six benzoxazine monomers derived from aryl ether diamines were synthesized, with controlled levels of meta-substitution and varying numbers of ether-bridged phenyl rings in the monomer backbone. Meta-substitution was found to suppress crystallinity in high-purity benzoxazine monomers and lower onsets of polymerization were observed due to meta-positioning of the terminal diamine rings. Terminal diamine meta-substitution also led to higher polymerization enthalpies, attributed to the emergence of an additional polymerization mechanism that increased the glass transition temperature up to 60 °C and delayed the onset of mass loss degradation. Benzoxazines with glass transition temperatures approaching 200 °C are susceptible to Mannich bridge degradation during polymerization, and this additional polymerization pathway both illustrates the nuanced complexities of benzoxazine structure-property relationships as well as provides a potential design strategy for benzoxazines with high glass transition temperatures approaching 250 °C.