Synthesis and thermal properties of phenol- and amine-capped main-chain benzoxazine oligomers with multiple methyl substitutions

Abstract

A series of main-chain benzoxazine oligomers with different methyl substitutions are successfully synthesized. Chemical structures are analyzed by Fourier transform infrared spectroscopy, proton nuclear magnetic resonance spectroscopy, and gel permeation chromatography. Effects of methyl substitutions on chemical shifts of protons in oxazine ring and thermal properties, including glass transition temperature, thermal stability, and char yield, are discussed. The influences of methyl substitutions on different positions are demonstrated: (i) substitution on phenols induces obvious increase in curing temperature while substitution on amine does not show apparent impact; (ii) substitution at different positions results in T g variation, following the sequence of none-substitution > substitution at end-capping > substitution on diamines in main-chain > substitution on bisphenols in main-chain; and (iii) substitution at end-capping would cause apparent deterioration in thermal stability while substitution on diamines in main-chain would benefit thermal stability and char yield. Experimental results and related explanations are provided in detail.