Homopolymerizations of butadiene (BD), isoprene (IP), and 2,3-dimethyl-butadiene (DMBD) were carried out by a Gd(OCOCCl3)(3)-based catalyst, to investigate the effects of the energy levels of the monomers or the sterical factor of the methyl substituents on the polymerizability and the cis-selectivity of the monomers. The order of the polymerizability at 50 degrees C was as follows : ED (4.5 kg of polymer/(mol of Gd h)) similar to IP (4.8) > DMBD (0.6). On the other hand, the cis-selectivity of the polymers was as follows : ED (98%) > IP (94%) much greater than DMBD (35%). These results suggest that the terminal ED and IP units are controlled by the cis configuration by the coordination between the penultimate cis-vinylene unit and the catalyst metal, whereas the penultimate DMBD unit unfavorably controls the terminal DMBD unit to the cis-1,4 configuration through the back-biting coordination with difficulty by two methyl substituents compared with the penultimate ED and TP units. The validity of the back-biting coordination was examined by MO calculation with sigma-allylnickel complexes. According to the formation energy with respect to the BD-BD diad, the cis-cis form is somewhat preferable to the trans-cis form through the coordination of the penultimate ED unit by Delta E = 0.028 au (ca. 17.6 kcal/mol).