Communication: 2-Phenyl-1,3-butadiene and (E)-1-phenyl-1,3-butadiene were polymerized in the presence of homogeneous Ziegler-Natta catalytic systems. The products were analyzed by means of C-13 NMR spectroscopy. Monomer, reactivities and polymer structures were compared with those obtained by using the same catalytic systems in the polymerizations of isoprene, (E)-1,3-pentadiene and 1,3-butadiene. The results were tentatively rationalized by considering the stability of the allyl groups resulting after any insertion of the monomer into the growing chain.