Electronic structure, conformation and reactivity of active centres of anionic polymerization with an aluminium counterion

Abstract

The quantum-chemical CNDO/2 method was used in connection with the anionic polymerization of unsaturated compounds initiated by organoaluminium compounds. Comparison of the results for model compounds Al(XH n) 3 H 2AlXHP n (X = O, N, C) with the data for analogous lithium and magnesium derivatives showed that the polarity of the MtX bond (calculated taking account of the metal valency) and the order of this bond are reactivity indices. According to these indices, amido- and alkoxy-alkylaluminium derivatives should be reactive in initiation. Absent of activity for the alkoxy derivatives is probably due to the stability of their aggregates. The optimization of the geometry of the molecule CH 3CH(COOCH 3)AlH 2, modelling the propagation active centre, showed that, as a result of the competing interaction of the Al atom with the growing chain and the side radical, the AlC bond is greatly weakened. Its reactivity evaluated from the above mentioned indices exceeds that of alkylaluminium amide initiators. The geometry of the complex of the model compound, CH 3AlH 2, with the methyl acrylate molecule was favourable for the subsequent insertion of the monomer into the chain. Possible reasons for the discrepancy between the experimental and calculated data are considered for acrylonitrile.