The coordination step of 1-2 homopolymerization of 4-methyl-1,3-pentadiene in presence of Ziegler-Natta initiators is discussed on the basis of ab initio computations. The results show that, as for 1-4 homopolymerizations, the coordination of the incoming monomer requires a substantial rearrangement of the ending unit, whose coordination mode changes from the eta(3)-allylic type to the eta(1)-sigma one. This step, most probably the rate-determining one, appears to be easier for 4-methyl-1,3-pentadiene than for butadiene or isoprene homopolymerization, in agreement with the observed homopolymerization rates. (C) 1998 Elsevier Science B.V.