Based on typical 3-substituted benzoxazines

methyl

isopropyl and phenyl groups were introduced at the 2- position

respectively. Influences of 2-substituent on hydrogen bonds and properties of polybenzoxazine resins were studied by molecular dynamics simulation. Cross-linking structures of the four polybenzoxazines were established

and the free volume fraction

hydrogen bond

glass transition temperature

thermal expansion coefficient and density were calculated. The results show that the 2-substituent increases the free volume of polybenzoxazines and decreases the proportion of OH… N intramolecular chelating hydrogen bonds to a certain extent. The proportion of OH…O hydrogen bonds in methylsubstituted and isopropyl-substituted polybenzoxazines is increased

while phenyl substituted polybenzoxazine is difficult to form OH…O hydrogen bonds. The radial distribution function verifies the formation of hydrogen bonds. Steric hindrance

free volume and hydrogen bond affect the glass transition temperature and thermal expansion coefficient of polybenzoxazines. Polybenzoxazine substituted by the phenyl group at the 2-position has the highest glass transition temperature at 242.0℃

while the glass transition temperature of polybenzoxazine substituted by the isopropyl group is the lowest at 213.5℃

which is lower than that of unsubstituted polybenzoxazine.