Unusual product distribution in ethylene oligomerization promoted by in situ ansa-chloroneodymocene-dialkylmagnesium systems

Ethylene polymerization using in situ combinations between a chloroneodymocene precursor and a dialkylmagnesium reagent has been investigated to prepare tailor-made oligomers. Combinations of [Cp*2NdCl2Li(OEt2)(2)] (1) with 40 equiv. of n-butylethylmagnesium (BEM) or di(n-hexyl)magnesium (DHM) gave oligoethylenes with M up to 2500 and narrow molecular weight distributions (M-w/M-n < 1.10) in moderate activity (A(1h) = 79 kg/(mol of Nd h atm) at 80degreesC, 1 atm). Under these conditions, ethylene polymerization proceeded in a controlled fashion, with a linear growth of M vs monomer conversion, ascribed to an effective chain transfer between the Nd and Mg centers. Combinations of [rac-{Me2Si(eta(5)-2-SiMe3-4-t-Bu-C-5 H-2)(2)}Nd(mu-Cl)(2)Li(THF)(2)] (2) with either BEM or DHM (20-40equiv.) showed decreased activity, suggesting possibly a different rate-determining-step for ethylene polymerization than for that of higher (alpha-olefins. The oligoethylenes obtained from combinations based on 2 have narrow molecular weight distributions (M-W/M-n < 1.2) but higher contents of vinyl terminations. Monitoring of the reactions showed also a non-linear growth of M vs monomer conversion, especially marked when DHM was used as co-reagent. The 2/DHM combination behaves as a "self-correcting" catalyst system that deviates from the calculated M values for a controlled-living polymerization in the early stage of the reaction and re-approach them progressively in the second stage. (C) 2002 Elsevier Science B.V All rights reserved.