The catalytic behaviour of tetravalent molybdenum catalyst system in the polymerization of dutadiene has been studied. The ca.talyst system is highly active when anions that combined with molybdenum are carboxylic acid group (BCOO-) or alkoxy (-OR) group. Obviously

the variety of substituent R in (i-Bu)2AlOR has great influence on the catalytic activity. The system is inactive

if the R group is carboxylic acid or alkoxy

while the activity of catalyst system increases if the R group is phenyl or tolyl. The content of 1

2-unit and the molecular weight of polymer decreases while the destri-bution index increases as polymerization temperature rises.