Butadiene was polymerized to a certain extent in the presence of a rare earth coordination catalyst, neodymium compound of neodymium chloride and i-propyl alcohol and triethyl aluminum (NdCl3.3i-PrOH-AlE3 in toluene and the allyl chloride was then added to the reactive solution in order to in situ cyclize the formed polybutadiene and cyclopolymerize the unreacted butadiene monomers. Effects of molar ratio of allylehloride to AlEt3 (Cl/Al), cyclization reaction time and temperature, butadiene and NdCl3.3i-PrOH concentrations on the cyclization reaction have been investigated. The cyclization reaction is very quick, only several minutes. The cyclization reaction temperature has few effects on the properties of the cyclized product. Cl/Al is a very important condition for this reaction system. Cyclized polybutadiene has a low value of intrinsic viscosity, free gelling and high yield at high Cl/Al. The microstructures and properties of the cyclized products have been characterized by Fourier transform infrared spectroscopy, nuclear magnetic resonance spectroscopy and gel permeation chromatography. The cyclization mechanism is put forward. (C) 2004 Elsevier B.V. All rights reserved.