Cationic copolymerization of n-butyl glycidyl ether (BGE) and 3-isochromanone (ICM) was investigated using trifluoromethanesulfonic acid (TfOH) as an initiator at 100 degrees C. In the copolymerization, the reactive site of ICM with the propagating cation was completely different from that in its homopolymerization: in the former, the propagating cation reacted with the carbonyl oxygen of ICM, while in the latter, the propagating cation reacted with the aromatic ring of ICM. In spite of the potential of ICM to undergo the homopolymerization, in the present copolymerization, ICM was consumed smoothly only in the presence of epoxide. As a result, the copolymerization proceeded in a statistic manner to afford the corresponding copolymer bearing ICM-derived ester linkages distributed in the main chain. Cationic copolymerization of bisphenol A-diglycidyl ether and ICM was also performed to synthesize the corresponding networked polymer. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4213-4220