The Initiation Mechanism of Butadiene Polymerization in Aliphatic Hydrocarbons: A Full Mechanistic Approach

An <i>in situ</i> ¹H NMR study has been carried out to examine the anionic initiation mechanism of 1,3-butadiene and <i>tert</i>-butyllithium (<i>t</i>-BuLi) using <i>n</i>-heptane as solvent. Additionally, mixtures of model compounds have been investigated <i>ex situ</i> to simulate very early stages of polymerization. The analysis of the NMR spectra in combination with density functional theory (DFT) calculations proves the coexistence of cross-aggregates of <i>t</i>-BuLi and initiated chains and their crucial role for the initiation mechanism. From the low concentrations of these species showing a characteristic maximum at <i>t</i> ≈ 50 min and the increase of the overall initiation rate constant with ongoing initiation, we propose a double-stage autocatalytic mechanism for this process. We first assume a fairly small reactivity of butadiene and <i>t</i>-BuLi, which exists under these reaction conditions as a tetrameric aggregate. However, after the reaction of the first <i>t</i>-BuLi unit with a monomer molecule, the reactivity of the remaining three <i>t</i>-BuLi units in the aggregate is increased considerably. The crucial second step of the autocatalytic mechanism is based on the unimer exchange between partially or fully initiated <i>t</i>-BuLi aggregates and the residual unreacted <i>t</i>-BuLi tetramers. As a result, the initiation rate constantly increases and leads to a sigmoidal consumption of initiator molecules during the polymerization. In addition, the time-dependent cross-aggregate concentrations are used as a benchmark for a full mechanistic approach compiling all literature assumptions. Numerical modeling allows a semiquantitative description of the data