Quantum-Chemical Calculations of the Structure of the Triplet Reaction Complexes in Anionic Polymerization of Butadiene

Abstract

A new mechanism of anionic polymerization of butadiene, radically changing the existing concepts on the structure and properties of the reaction complex, is suggested. It is stated that an elementary chemical process involves excitation into the low-lying triplet state of the “living” polymer–monomer complex, characterized as a transfer of a charge (electron) and cation (Li+ or Na+) from the terminal unit to the monomer molecule. In the framework of this concept, the probability of chemical bonding is determined by the spin density on the radical centers of reagent molecules. The semiempirical and ab initio 6-31G** calculations have revealed a strong interaction in the ground electronic state of the complex (5-10 kcal/mole) and low energies of the excited triplet levels (< 10 kcal/mole). A mechanism of cis-trans-isomerization in the terminal unit of the “living” polymer is suggested. The mechanism involves concerted rotation around the Cβ - Cβ bond and migration of Li between the Cα and Cγ atoms. The new approach is shown to be fruitful for microstructure analysis of polybutadiene in relation to the type of cation and the state of the terminal unit of the “living” polymer.