(eta(3)-C3H5)(2)CeCl5Mg2(tmed)(2) combined with HAl(i-Bu)(2) or Al(i-B
u)(3) can initiate the polymerization of isoprene with about 50% of th
e cis-1, 4 microstructure contained in the polymer. The insertion reac
tion of isoprene occurring between Ce3+ and eta(3)-allyl was identifie
d by NMR techniques.