Cyclo-oligomerization of hydroxyl-containing mono-functional benzoxazines: a mechanism for oligomer formation

Abstract

The oligomerization of mono-functional benzoxazines is demonstrated with intermolecular cyclization via a Mannich bridge structure as the primary reaction pathway. The structures of oligomers of mono-functional benzoxazines were previously thought to be benzoxazine dimers stabilized by inter- and intramolecular hydrogen bonding. This study is based on four hydroxyl-containing mono-functional benzoxazines synthesized from phenol, o-allylphenol, o-cresol, p-cresol, diglycolamine, and paraformaldehyde. A reaction mechanism is proposed that ortho- and para-substituted-phenol-based mono-functional benzoxazines undergo thermally activated ring-opening cyclo-oligomerization (ROCO) and ring-opening oligomerization (ROO), whereas their phenol-based counterpart polymerizes into polybenzoxazine following ROCO and ROO, which is evidenced by the data of ¹H and ¹³C nuclear magnetic resonance spectra, Fourier transform infrared spectra, size exclusion chromatograms, and mass spectra of thermally treated samples.