Novel polyolefins derived from common feedstocks can be obtained by using Ziegler-Natta catalytic systems. In particular, rac-(CH2-(3-tert-butyl-1-indenyl)2)ZrCl2/ MAO catalyzes the copolymerization of ethene and 1,3-butadiene leading to a polyethene presenting prevailingly cyclopropane, cyclopentane, or 1,1- and 1,3-units, depending on the reaction conditions. Copolymerizations conducted in the presence of 13C-labeled 1,3-butadiene was able to clarify the polymerization mechanism leading to 1,1- and 1,3-units and to confirm the hypothesis, previously reported, relative to the mechanism leading to cyclopropane and cyclopentane units. In particular, all kinds of inserted butadiene units arise from an η2 primary butadiene coordination and insertion. Evidently, the sterical hindrance of the catalytic system induces the η2 primary butadiene coordination and insertion, instead of the more typical η4-cis or η2-trans coordination followed by formally secondary insertion.