Abstract The title complexes (CH 2CHCH 2CH 2C 5H 4) 2LnCl·2THF (Ln=Sm, Y, Dy, Er) were synthesized as air and moisture sensitive free-flowing oils that have been fully characterized by MS, EA and IR. The EA and IR results indicate the presence of coordinated THF molecules. No evidence of intra-molecular coordination of the butenyl side-chain was observed. In association with Al( i-Bu) 3, the complexes have shown high activity for the bulk polymerization of MMA. At a MMA/catalyst (MMA/cat) molar ratio of 2000, temperature of 80 °C and polymerization time of 30 h, the catalyst system produced at a 70% conversion a syndiorich polymer (rr=62%), M η =1.65×10 6. Effects of temperature, time and molar ratios of MMA/catalyst and catalyst/cocatalyst on the polymerization have been studied. The result showed that higher temperature, longer time and Al/catalyst molar ratio=10 are conditions suitable for polymerization.