A study on the effect of phenoxy ligands of an donor or acceptor character in titanium half-sandwich catalytic systems of a general formula LTiCl2(OC6H4X-p)/MAO (where L is cyclopentadienyl, indenyl or pentamethylcyclopentadienyl; X=NO2, Cl, H, CH3, t-C4H9, OCH3; MAO: methylalumoxane) on their activity and selectivity in the reactions of polymerization of olefins and styrene and of their co-polymerization, showed that the effect of coordination of the polymerization centers by the phenyl rings substituted with donor groups is involved here. The effect resulted in the formation of polymers of a lower molecular weight and was crucial to the composition of the products obtained: polymer or oligomer, co-polymer or homopolymer mixture. (c) 2006 Elsevier B.V. All rights reserved.