Ethylene and styrene insertion into the metal-alkyl bond Of [Me2Si(C5Me4)(N'Bu)Ti (CH2CH2CH3)] + [mu-Me-Al(Me)(2)-(AlOMe)(6)Me](-)species has been investigated using a QM/M approach. Validation of the B3LYP//B3LYP/UFF theoretical model was performed by comparing some results with full QM calculations. Ion pairs which contain a bounded triniethylaluminium molecule give rise to active species, whereas direct coordination of the MAO cage to the catalyst leads to dormant species for polymerisation. Ion pair formation and dissociation for both ion-pair complexes have been performed. In addition, monomer insertions into the active ion-pair species have been studied. The monomer coordination step results to be endothermic in contrast to the values obtained for the 'naked' cation. The energetic insertion barriers starting from the pi-complexes are similar to those obtained for the 'naked' cationic species. The net effect of the cocatalyst is to increase the coordination barriers at a similar amount for two-monomer insertions. Some implications of the cocatalyst effect in ethylene/styrene copolymerisation are discussed. (c) 2005 Elsevier Ltd. All rights reserved.