tion limit. In this respect, the performance of the a-methyl substituted catalysts approaches that of heterogeneous Ziegler catalysts, in which these regio-irregularities are not observed .The molecular weights of the polymers obtained with the catalysts 1-4 show a surprising correlation with the respective stereo-and regio-regularities of the catalysts: Complex 3 produces by far the longest and most isotactic, complex 1 the shortest and least isotactic polymer chains. This unexpected observation can be explained based on structural models for the reactive zirconocene alkyl olefin ~a t i o n [ "~: The P-agostic binding required for chain termination by P-H transfer places the polymer chain (Pl) of a stereoregular unit into that ligand sector (Fig. la) which is occupied by the ring substituent R'. If R' is sufficiently large, chain termination will therefore be largely suppressed except in those cases where the positions of the polymer and CH, substituents at the P-carbon are inverted, i.e. in a stereo-irregular unit, or where the P-carbon carries two H atoms, as in a regio-irregular unit.