The homogeneous catalyst CpTiCl(3)-MAO is able to produce a random copolymer of 4-methyl-1, 3-pentadiene with ethylene. C-13 NMR analysis of the copolymers shows that after insertion of ethylene units, the next 4-methyl-1,3-pentadiene unit can be inserted in either 1,2 or 1,4 arrangement. The high chemoselectivity observed in the 1,2-syndiotactic homopolymerization of 4-methyl-1,3-pentadiene with respect to the lower one observed in the copolymerization with ethylene is attributed to a back-biting coordination to the Ti of the active species of the penultimate monomer unit of the growing chain.