Formation mechanism of anionic isotactic polystyrene initiated by Li carbanions in cyclohexane in the presence of Na-tosylate II: a DFT calculation study

Abstract

The stereochemistry of t-BuLi initiated styrene polymerization in cyclohexane yields isotactic-rich polystyrene in the presence of sodium 4-methylbenzenesulfonate (SMBS). Herein, we report the results of DFT calculations in cyclohexane, both in the presence and absence of SMBS, using the B3PW91 hybrid functional and 6-311++G(d,p) basis set with DFT-D3 correction, allowing for the detailed evaluation of the transition state structures confirmed by benchmark analysis. We focus on the addition of a single styrene to a 1-lithio-1,3(S)-diphenyl-pentane (LDPP) model dimer anion, producing the expected four stereoisomers of 1-lithio-1,3,5-triphenylheptane (LTPH). The addition proceeds through four styrene stereoisomeric complexes, consisting of two pseudo-enantiomeric sets with the corresponding transition states (TSs), which exhibit similar Gibbs free energy (≤0.1 kcal mol⁻¹), in agreement with previously reported results. The geometries of the complexes and transition states are consistent with the major rotational (rather than lateral) motions of the aryl and vinyl groups as the complexes evolve into transition states and trimer anions. In the presence of SMBS, similar but higher energy monomer complexes are formed that surprisingly have nearly the same free energy. However, the TS free energy barriers increase in the following order: m-pro-m <r-pro-m ≅ m-pro-r <r-pro-r. For instance, the TS energies for monomer addition differ by as much as 1.4 kcal mol⁻¹ for the r-pro-r compared to the m-pro-m transition states. However, due to the 1.9 kcal mol⁻¹ lower free energy of the pro-r compared to the pro-m dimer anions, the corresponding activation energies differ by as much as 3.3 kcal mol⁻¹. This would tend to favor the formation for mm triads over rr dyads by a factor of about 1.9 × 10², which is consistent with the prevailing isotactic stereochemistry.