Mechanisms for anionic butadiene polymerization with alkyl lithium species1

Anionic butadiene polymerization by means of [Li-polybutadienyl]x species (x = 1-6, 8) without polar agents was investigated by means of density functional theory (DFT) under conditions relevant to industrial application, namely in a low-dielectric hydrocarbon solvent and at room temperature. The calculations indicate that the dimeric and tetrameric catalyst species together account for the bulk of Li-polybutadienyl species in the polymerization mixture under typical conditions. It is likely that each type of oligomer produces its own fingerprint signature polymer microstructure, as there is a systematic variation in the amounts of 1,2- and 1,4-insertions as well as in the preference of cis- and trans-butadiene. According to the calculations, higher aggregated Li species tend to produce more 1,2-insertions and prefer trans- over cis-butadiene insertion, while the dimer prefers trans-butadiene and 1,4-insertions. The dimer closely reproduces the experimentally observed polybutadiene microstructure (5%-10% 1,2-insertion, approximately equal ratios of cis and trans units with a slight predominance of trans). The monomeric catalyst species shows a clear preference for insertion of cis-butadiene over trans-butadiene. Thus, the monomer species is predicted to be present in the polymerization mixture in very small concentrations under normal conditions and the overall polymerization is predicted to be mainly carried out by the dimeric catalyst species.