The polymerization of 1,3-dienes has been well-studied using an array of metal-containing complexes. Although a variety of (pi-allyl) Ni(II) catalysts have been shown to be active for diene polymerization, details concerning the mechanism of chain growth are largely speculative and supported primarily by DFT calculations. The observation by low-temperature NMR spectroscopy of the key intermediates and the catalyst resting states in the chain growth are described here and provide a more complete and detailed mechanism of butadiene polymerization.