Rheology of Living Bifunctional Polybutadienyl Dilithium Chains in Benzene:Viscoelastic Evaluation of Aggregate Lifetime

Mechanisms for anionic butadiene polymerization with alkyl lithium species

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.

Anin situstudy of the t-butyllithium initiated polymerization of butadiene in d-heptane via small angle neutron scattering and H1-NMR

We present a combined 1H-NMR and small angle neutron scattering in situ study of the anionic polymerization of butadiene using t-butyllithium as the initiator. Both initiation and propagation phases were explored. This combined approach allows the structural and kinetic characteristics to be accessed and cross compared. The use of the D22 instrument (ILL Grenoble) permits the attainment of Q approximately equal to 2 x 10(-3) A. This, in turn, led to the identification of coexisting large-scale and smaller aggregates during all stages of the polymerization. The smaller aggregates contain most of the reacted monomers. Their structure changes from high functionality wormlike chains at early stages of the reaction to starlike aggregates where the crossover occurs at a degree of polymerization of approximately equal to 40. The initiation event involved these small, high functionality (approximately equal to 120) aggregates that apparently consisted of cross-associated t-butyllithium with the newly formed allylic-lithium head groups. As the initiation event progressed the initiation rate increased while the functionality of these small aggregates decreased and their size increased. Propagation, in the absence of initiation, was found to have a rate constant that was molecular weight dependent. At approximately 11 kg/mol the measured polymerization rate was found to increase while no further structural changes were seen.