Low molecular weight polydienes were obtained with Nd-Al bimetallic complex isolated from the NdCl3 3P350 (or TBP)-Al(i-Bu)3 system. IR and 13C-NMR data show that there is-CH2CH (CH3)2 group at the molecular end of the polydiene. Based on the results

it was proposed that the activce species composition of the Nd-Al bimetallic complex is Nd-CH2CH(CH3)2. The diene polymerization may be in accordance with the process that the monomer inserts into Nd-C bond. A possible diene polymerization mechanism was proposed.