Electronic Effects of Asymmetric and Meta-alkoxy Substituent on the Polymerization Behavior of Bis-benzoxazines
Three isomers of benzoxazine monomers based on m-alkoxyphenol and 4,4'-methylenedianiline are synthesized and successfully isolated by column chromatography. The molecular structures of benzoxazine monomers are confirmed by proton nuclear magnetic resonance (1H NMR) and Fourier transform infrared (FT-IR) spectroscopy. The polymerization behavior evaluated by DSC shows that the asymmetric isomer, which has methoxy group both at 5-position and 7-positions (5,7’MO-ddm), has only one exotherm peak between temperatures of other two symmetric isomers. The 1H NMR spectrum of monomers evidences that the type and position of alkoxy groups can exert different effects on the electron density of oxazine ring, maybe resulting in a sensitive trend of ring-opening. The difference of electron densities is verified by calculation results of natural charges by Gaussian simulation. In this work, we firstly provide a fundamental molecular-level understanding of the polymerization mechanism of asymmetric bis-benzoxazines, which can open up possibilities for designing new benzoxazines in order to solve the potential disadvantage of benzoxazines/polybenzoxazines and/or enhance their advantages.