The neodymium iso-propoxide [Nd(Oi-Pr)(3)] catalyst activated by modified methylaluminoxane (MMAO) is homogeneous and effective in isoprene polymerization in heptane to provide polymers with high molecular weight (M-n similar to 10(5)), narrow molecular weight distribution (M-w/M-n = 1.1-2.0) and mainly cis-1,4 structure (82-93%). The polymer yield increased with increasing [Al]/[Nd] ratio (50-300 mole ratio) and polymerization temperature (0-60 degreesC), while the molecular weight and cis-1,4 content decreased. On the other hand, the same catalyst resulted in relatively low polymer yield and low molecular weight in toluene. The cyclized polyisoprene was formed in dichloromethane, which is attributable to the cationic active species derived from MMAO alone. When chlorine sources (Et2AlCl, t-BuCl, Me3SiCl) were added, the cis-1,4 stereoregularity of polymer improved up to 95% even at a high temperature of 60 degreesC, though the polymer yield decreased. (C) 2003 Elsevier Science Ltd. All rights reserved.