The substituent effect on the radical polymerization of o-quinodimethanes, generated by thermal isomerization of benzocyclobutenes, was investigated. Polymerizations of three benzocyclobutenes bearing electron-withdrawing groups were studied, namely 1-cyanobenzocyclobutene (1), 1-chlorobenzocyclobutene (2), and 1-bromobenzocyclobutene (3). While radical polymerizations of 2 and 3 did not afford any polymer, radical polymerization of 1 afforded n-hexane-insoluble polymer(M-n = 5000) in moderate yields at temperatures above 120 degrees C. The structure of the obtained polymer was confirmed to be a ring-opened polymer(4) by IR, H-1-, and C-13-NMR. The yield of the polymer increased with an increase in the initiator concentration. The polymer yield reported in this paper is higher than those of benzocyclobutenes bearing electron-donating groups, reported previously by the authors. The semi-empirical molecular orbital calculation supported the contribution of ring-opening polymerization of spiro-compounds, rejecting the possibility of 1,4-polymerization. Lastly, radical copolymerizations of 1 with various comonomers were also performed to obtain the corresponding copolymers.