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

Abstract

ABSTRACT:A new mechanism of anionic polymerization of butadiene is proposed.In the elementary chemical act, the "living" polymer-monomer complex is excited into the low-lying triplet state. This state has the character of charge (electron) and cation (Li ϩ or Na ϩ ) transfer from the terminal unit of the active center to the monomer molecule. In the framework of this concept, the probability of chemical bond formation is determined by spin density on radical centers of reagent molecules. Semiempirical and ab initio 6-31G** quantum-chemical calculations showed stable interaction between components of the complex in the ground electronic state (9 -11 kcal/mol) and low energy levels of triplet excited states (Ͻ14 kcal/mol). This new approach is shown to be useful in the analysis of polymerization kinetics and the microstructure of polybutadiene depending on the cation type and the ion pair state. The mechanism of cis-trans isomerization in the terminal unit of the living polymer consists in concerted rotation about the C ␤ OC ␥ bond and the migration of Li between C ␣ and C ␥ atoms.

Structure of charge‐transfer reaction complexes in anionic polymerization of isoprene: Quantum chemical calculations

“…The present work continues the study of the mechanism of anionic butadiene polymerization 1, 2, taking as an example its close analogue, isoprene, which is used as a monomer in the synthesis of polyisoprene rubber. The theoretical basis is the previously developed concept 3 of thermal electronic excitation.…”

“…In the framework of concepts of electronic excitation, the mechanism of 1,2‐unit formation has already been discussed in Refs. 1, 2 taking as an example butadiene polymerization on lithium catalysts in the presence of TMEDA. It was shown that the TMEDA molecule is located above the Li atom like a hat, limiting its mobility and hindering the approach of a monomer molecule.…”

“…Its transition into the stable state of the product is possible only by the formation of a bond between closely situated C α atoms of the isoprenyl radical and the C 4 atom of isoprene radical‐anion with a simultaneous shift of the Li + cation to the C 3 atom (for further details, see Refs. 1, 2). In this case, the stable 4,3‐structure is formed directly in the end unit of living polyisoprene.…”

“…The mechanism of butadiene anionic polymerization was first considered from the standpoint of thermal electronic excitation in the elementary chemical process 1–3. The new view introduced physical clarity into the understanding of driving forces and relationships of polymerization process and showed that the principle of spin control of the reaction has a great predicting force in the analysis of polymerization kinetics and polybutadiene microstructure.…”