Butadiene and isoprene can be polymerized by potassium hydride in aromatic hydrocarbons at various temperatures and in short times. A first additive, triethylaluminum, is needed to favour the homogeneity of the initiating system in the solvent of reaction. The importance of a second additive, a potassium alkoxide, is discussed in terms of activation of the polymerization. Experimental parameters like temperature, initiator concentration, activator/initiator ratio and monomer concentration are also investigated with the aim to reach a high content of 1,4 units. Transfer reactions occur for isoprene even though a good control of the polymerization is obtained for butadiene. (c) 2006 Elsevier Ltd. All rights reserved.