Different half-sandwich titanium trifluorides and constrained geometry catalysts of titanium and vanadium were used to polymerize butadiene, isoprene, and 1,3-pentadiene. The polybutadienes produced with these catalysts have nearly identical microstructures of about 80% 1,4-cis, 1% 1,4-trans and 19% 1,2-linked units. An optimum activity is obtained by an aluminium/titanium ratio of about 700. There is no dependence of the aluminium/titanium ratio on the microstructure. Vanadium-based complexes are more active for 1,3-pentadiene than titanium complexes.