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Abstract1,4‐Cyclohexadiene underwent monomer‐isomerization polymerization to yield poly(1,3‐cyclohexadiene) with a Ziegler‐Natta catalyst comprising TiCl4–Al(C2H5)3 catalyst with Al/Ti molar ratios of 0.5–3.0 at 60°C for 96 hr. Good yields of polymer were obtained (49.5% yield at Al/Ti = 3.0; [η] = 0.04 dl/g). The infrared and NMR spectra of the polymer were identical to those of poly‐(1,3‐cyclohexadiene), confirming that 1,4‐cyclohexadiene first isomerizes to 1,3‐cyclohexadiene and then homopolymerizes to give poly‐1,3‐cyclohexadiene. 1,3‐Cyclohexadiene polymerized without isomerization easily in the presence of TiCl3–Al(C2H5)3 catalyst at Al/Ti molar ratios of 0.5–3.0 at 60°C for 3 hr (76.3% yield at Al/Ti = 3.0; [η] = 0.06 dl/g).