The effects of a series of eompooinds having varied structure and polarity on the polymerization of butadiene with the rare-earth catalytic system were studied. It was found that for most of these compounds the catalytic activity is lowered and the molecular weight of the polymers is increased

with the exception of allyl derivatives both the catalytic activity and the molecular weight are lowered. In view of its relative long lifetime and the insensitiveness towards modifiers usually used in Ziegler-Natta cataly-tic systems

the rare-earth catalytic system is assumed to have its special nature

lead-ing to somewhat different polymerization mechanism.