A new generation of anionic initiators (butyl-lithium free), based on trialkylaluminum and a readily available alkali metal hydride, has been developed for the control of styrene polymerization at high temperature. Triisobutylaluminum and sodium hydride form heterocomplexes that are efficient for the initiation of styrene polymerization at 100 degrees C in toluene or in bulk. To be active under such conditions, these systems require the presence of an excess of metal hydride with respect to AlR3 ([Al]/[Na] < 1). PS chains are specifically initiated by the hydrides coming from NaH, and molar masses are controlled in the range 0.8 < [Al]/[Na] < 1. Fast exchange between dormant 1:1 and active 1:2 complexes (Al:Na), and ligand rearrangements within the 1:2 complex, can explain the observed results.