[GRAPHICS] Trialkylaluminum and sodium hydride form hydrocarbon soluble heterocomplexes which were used as initiating systems for the retarded anionic polymerization of styrene in hydrocarbon media at high temperature and in concentrated monomer, corresponding to conditions close to those used in industry for styrene radical polymerization. To be active towards styrene polymerization in hydrocarbons, these systems require the presence of a slight excess of metal hydride with respect to AIR(3) ([Al]/[Na] < 1), whereas for ratios [Al]/[Na] >= 1 only the initiation step takes place. However, in these last conditions the addition of a few equivalents of a polar additive, such as tetrahydrofuran, allows the polymerization to proceed. Polymerizations initiated by AIR(3)/NaH systems are characterized by initiation efficiency not far from one (> 0.7) with respect to sodium hydride and by the presence of an important induction period, corresponding to first styrene insertion into M-iota-H bonds. This behavior can be explained by the fact that most of the M-iota-H bonds have to convert into M-iota-styryl bonds before the propagation can start a particular situation which enables the control of the polymerization. The structure of the initiating and propagating sites, inside the bimetallic complexes, and the elementary reaction mechanisms are discussed to the light of experimental data. (c) 2007 Elsevier Ltd. All rights reserved.