The C5Me4R-ligated samarocene(II) complexes, (C5Me4R)(2)Sm(THF)(x) (1, R = Me; 2, R = Et; 3, R = Pr-i; 4, R = Bu-n, 5; R = TMS; x = 1 or 2), in combination with modified methylaluminoxane (MMAO) were examined in cyclohexane with respect to their catalytic performances for the polymerization of butadiene. Complexes 2-5 showed considerable promotion in both catalytic activity and regularity of the resulting polymer as compared to parent 1: activity, 3 much greater than 5 similar to 4 > 2 > 1; 1,4-cis selectivity at 50 degreesC, 3 similar to 5 (98.6%) > 4 (97.5%) > 2 (97.1%) > 1 (96.2%). The system 3/MMAO showed an excellent catalytic activity even at very low catalyst concentration ([butadiene](0)/[Sm](0) = 150 000), where the turnover number (TON) reached about 115 600 butadiene-mol/(Sm-mol h) at 50 degreesC (78% polymer yield in 1 h). At room temperature, the polymerization reaction in cyclohexane using 3/MMAO catalyst gave polybutadiene with extremely high 1,4-cis selectivity of 99.1% in reasonable rate (polymer yield = 87%, in 10 min) while the performance by 1/MMAO was 97.8% in 1,4-cis selectivity and 12% in polymer yield under the same reaction conditions.