The radical polymerization behavior of 1-cyano-o-quinodimethane generated by thermal isomerization of 1-cyanobenzocyclobutene in the presence of 2,2,6,6-tetramethylpiperidine-N-oxide (TEMPO) and the block copolymerization of the obtained polymer with styrene are described. The radical polymerization of 1-cyanobenzocyclobutene was carried out in a sealed tube at temperatures ranging from 100 to 150 degrees C for 24 h in the presence of di-tert-butyl peroxide (DTBP) as a radical initiator and two equivalents of TEMPO as a trapping agent of the propagation end radical to obtain hexane-insoluble polymer above 130 degrees C. Polymerization at 150 degrees C with 5 mol % of DTBP in the presence of TEMPO resulted in the polymer having a number-average molecular weight (M-n) of 2900 in 63% yield. The structure of the obtained polymer was confirmed as the ring-opened polymer having a TEMPO unit at the terminal end by H-1 NMR, C-13 NMR, and IR analyses. Then, block copolymerization of the obtained polymer with styrene was carried out at 140 degrees C for 72 h to give the corresponding block copolymer in 82% yield, in which the unimodal GPC curve was shifted to a higher molecular weight region.