Thermally induced polymerizations of a series of 1,3-benzoxazines with a variety of substituents on the nitrogen atom were investigated in detail, particularly in the following three aspects of the polymerization: (1) N-alkyl-1,3-benzoxazines are much more reactive than N-phenyl-1,3-benzoxazine. (2) The polymerization rate depended on the bulkiness of the N-substituent. The bulkier the substituent was, the slower the polymerization was. (3) The polymerizations accompanied weight loss due to the elimination of the corresponding imine (R-N = CH2), and its extent became larger when R was more bulky. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2777-2782, 2010