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

## TL;DR

This study explores how structural changes in benzoxazine monomers affect their thermal and mechanical properties.

## Contribution

The paper reveals how meta-substitution and molecular weight influence benzoxazine polymerization and thermal behavior.

## Key findings

- Meta-substitution suppresses crystallinity and lowers polymerization onsets.
- Higher polymerization enthalpies and increased glass transition temperatures are observed with meta-substitution.
- A new polymerization pathway is identified that could enable benzoxazines with glass transition temperatures near 250 °C.

## Abstract

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.