While Ti(III) alkyl species are the proposed active sites in Ziegler–Natta ethylene polymerization catalysts, the corresponding well-defined homogeneous catalysts are not known. We report that well-defined neutral β-diiminato Ti(III) alkyl species, namely [Ti(nacnac)(CH2tBu)2] and its alumina-grafted derivative [(AlsO)Ti(nacnac)(CH2tBu)], are active towards ethylene polymerization at moderate pressures and temperatures and possess an electron configuration well-adapted to insertion of ethylene. Advanced EPR spectroscopy showed that ethylene insertion into a Ti(III)–C bond takes place during polymerization from Ti(nacnac)(CH2tBu)2. A combination of pulsed EPR spectroscopy and DFT calculations, based on a crystal structure of [Ti(nacnac)(CH2tBu)2], enabled us to reveal details about the structure and electronic configurations of both molecular and surface-grafted species. For both compounds, the α-agostic C–H interaction, which involves the singly occupied molecular orbital, indicates a π character of the metal–carbon bond; this π character is enhanced upon ethylene coordination, leading to a nearly barrier-less C2H4 insertion into Ti(III)–C bonds after this first step. During coordination, back donation from the SOMO to the π*(C2H4) occurs, leading to stabilization of π-ethylene complexes and to a significant lowering of the overall energy of the C2H4 insertion transition state. In d1 alkyl complexes, ethylene insertion follows an original “augmented” Cossee–Arlman mechanism that involves the delocalization of unpaired electrons between the SOMO, π*(C2H4) and σ*(Ti–C) in the transition state, which further favors ethylene insertion. All these factors facilitate ethylene polymerization on Ti(III) neutral alkyl species and make d1 alkyl complexes potentially more effective polymerization catalysts than their d0 analogues.