Oligomeric Cp2Zr+-allyl species were detected in reaction mixtures of either [Cp2ZrMe][MeB(C6F5)(3)] or [Cp2ZrMe][B(C6F5)(4)] with propylene by a combination of H-1 NMR spectroscopy and electrospray ionization (tandem) mass spectrometry techniques. Conjointly, the data imply that formation of Cp2Zr+-allyl species occurs via (re)coordination of alkene (propylene and vinylidene end groups of unsaturated polymer chains, respectively) to Cp2Zr+R (R = CH3, H, polymeryl), followed by intramolecular proton transfer from C gamma to R and release of RH. Analysis of the olefinic region of the H-1 NMR spectra of the reaction mixture obtained from [Cp2ZrMe][MeB(C6F5)(3)] and propylene reveals the presence of a triplet resonance at delta similar to 5.2, which was attributed to the unprecedented 3-propenyl end group. A plausible mechanism for the origin of the 3-propenyl end groups is discussed. Additionally, a mechanism for incorporation of stereodefects into stereoregular polymers is also discussed. The cationic Cp2Zr+-allyl intermediates formed during polymerization may contribute to catalyst deactivation. (C) 2014 Elsevier Inc. All rights reserved.