DRIFT spectroscopy was successfully used to investigate interactions of allyl lanthanide complexes with aluminium organic cocatalysts and for in situ studies of the insertion/polymerisation of gaseous 1,3-butadiene on the 'adducts' of both catalyst components as well as on silica-supported allyl complexes/cocatalysts. Shifts of characteristic bands revealed electronic interactions between the allyl species and the Lewis-acidic methylaluminoxane (MAO) as well as the trialkyl aluminium. compounds. It was found, that only trialkyl aluminium compounds with steric-demanding alkyl groups enable a polymerisation of 1,4-polybutadiene corresponding to a pi-allyl insertion mechanism. Start and progress of the heterogeneous catalytic polymerisation could be semi-quantitatively followed by means of in situ-DRIFTS, recording the appearance and the changes of characteristic vibration bands of the allyl species and the formed polymer. (C) 2002 Elsevier Science B.V. All rights reserved.