AbstractPolymerization of 1.3‐dimethylcyclopentadiene was carried out with FRIEDEL‐CRAFTSand ZIEGLER‐type catalysts. All polymers obtained were soluble. The polymers had 45 to 55% 1.4‐structure and 55−45% 3.4‐structure. The polymerization appears to go through a tertiary carbonium ion, in contrast to that of monosubstituted cyclopentadienes. In copolymerization of 1.3‐dimethylcyclopentadiene (M1) with cyclopentadiene (M2), the former showed higher reactivity: r1= 6.85 ± 1.10, r2= 0.30 ± 0.10, at −78°C.Comparison of the structures of polymers obtained by cationic polymerization of substituted cyclopentadienes revealed a general mechanism which explains the mode of th