Structural consequences of complexing of polybutadienyl lithium and its analogues with different electron donors

Electronic excitation in anionic polymerization of butadiene: Nonempirical calculations of reaction complexes

“…In hydrocarbon solvents in the absence of free anions, polybutadiene with the predominant content of 1,4‐ cis ( trans ) units is formed (Table I). It is assumed that under these conditions chain propagation take place on associated forms OLC 5, 6, 8–16, 24–26, and the fixation of the 1,4‐structure in AC occurs as a result of BD insertion on the C α Met bond 5, 6, 8–21. The calculation of butadienyllithium (BDLi) by the AMI method shows that the Li cation ( q = +0.44) is located above the plane of the terminal unit at a distance of 2.01 Å from C α ( q = −0.54) and at a distance of 2.82 Å from C γ ( q = −0.38).…”

“…Actually, this state does not exhibit chemical activity in contrast to the electronically excited state (on the complex) in which acts of chemical bond breaking and formation take place 1. In extensive literature on BD polymerization 5–30, such an important characteristic as the content of 1,4‐ cis, 1,4‐ trans, and 1,2 units in its chain depending on polymerization conditions and the nature of the active center (AC) is discussed. According to the data of 1 H and 13 C nuclear magnetic resonance (NMR) spectroscopy 18–23, the structure of the AC terminal unit in both hydrocarbon (HCS) and electron‐donating (EDS) solvents is similar to the π‐allyl structure in which the negative charge is distributed between C α and C γ carbon atoms (Structure 1).…”

“…According to traditional concepts of the mechanism of anionic polymerization, the monomer molecule is inserted into the polymer chain on the CMet bond 5–30. Moreover, the content of 1,2‐ and 1,4‐( cis, trans ) units is determined by the probability that butadiene would attack the AC carbon atom bearing the maximum negative charge.…”

“…Under real polymerization conditions ([AC] < 10 −3 mol/L), polymer chain propagation in hydrocarbon solvents proceeds on monomer forms of OLC 7, 14, 17, 28 with the formation of considerable quantities of 1,4‐ cis units (about 90% 29). This is due to BD insertion into the C α Li bond with the fixation of the 1,4‐structure in the penultimate unit 5, 6, 8–29. At the same low AC concentration but in the presence of small amounts of strong electron donor solvents, 1,2‐units appear in the polybutadiene chain.…”

“…At the same low AC concentration but in the presence of small amounts of strong electron donor solvents, 1,2‐units appear in the polybutadiene chain. This reaction course results from butadiene insertion into the C γ Li bond 5, 6, 8–29.…”