The invention relates to the use of nitrogenous aluminium organyl complexes of general formula (I) as co-catalysts in heterogeneous polymerisation reactions of propene. In said formula: R, R′, R1 and R1′ independently of one another represent branched or unbranched C1-C7 alkyl, cycloalkyl, alkenyl, cycloalkenyl, aryl or alkynyl; R2 represents unsubstituted, monoalkylated or polyalkylated and/or monofluorinated or polyfluorinated aromatic hydrocarbons from group (II); R3 and R4 independently of one another represent CH2, CF2 oder C(R1)2; m stands for 0, 1 or 2; n stands for 0, 1 or 2; o stands for 0 or 1, all independently of one another. Said systems exhibit improved characteristics in terms of activity and stereoselectivity in comparison to conventional co-catalysts such as AlEt3 and can act simultaneously as co-catalysts and stereoselectivity promoters