We investigate different couplings of TiCln (dimerisation to TiCln-1CH3 and
complexation by magnesium chloride) and their consequences for the reactiv
ity towards ethene. The dimerisation results from the donation of an electr
on pair from a chlorine ligand to the metal and leads to bridging structure
s. Alkyl groups also bridge two metal centres of the dinuclear complexes. T
his is much more favourable than the connection as terminal ligands that is
the only possibility in the case of mononuclear complexes. This change in
the adsorption mode favours the first step of the propagation reaction of t
he Ziegler-Natta reaction (CH3-->C3H7) by a stabilisation of the end produc
t. (C) 2000 Elsevier Science B.V. All rights reserved.