CpTiCl3-MAO (methylaluminoxane) catalytic system promotes the butadiene polymerization leading to 85% of 4,1-cis polymer. The generally well accepted homopolymerization mechanism involves a diene insertion on a pi-allylic terminal growing chain. In alpha-olefin-diene copolymerizations, diene monomer can also insert on a sigma carbon-metal bond. In order to simulate an insertion of a conjugated diene on a sigma chain, experimental and theoretical studies, relative to the first insertion step, are reported. Unexpectedly 56% of 1,2-primary and 44% of 4,1-trans first inserted units were obtained, by using as catalyst CpTiCl3/MAO/ Al((CH3)-C-13)(3). The experimental results were rationalized by DFT (Density Functional Theory) calculations, by including a solvent molecule coordinated to the catalytic site during the first insertion step. (c) 2007 Elsevier Ltd. All rights reserved.