Reaction Mechanism in the Stereospecific 1,4-Polymerization of Butadiene with Ziegler−Natta Type Catalysts of Early Transition Metals: Comprehensive Density Functional Investigation for the Cationic [TiIIICp(polybutadienyl)(butadiene)]+ Active Catalyst

Abstract

A comprehensive theoretical investigation of the entire catalytic reaction course of the 1,4-polymerization of 1,3-butadiene assisted by the cationic [TiIIICp(polybutadienyl)(butadiene)]+ active catalyst complex, employing a gradient-corrected DFT method, is presented. Critical elementary processes for a tentative catalytic cycle have been scrutinized, taking into account monomer insertion, allylic anti-syn isomerization, and the monomer uptake. The present investigation examines, in terms of the located key structures and the energy profile of individual elementary steps, the role played by η3-π-butenyl− and η1-σ-butenyl−TiIII species as possible intermediates in the catalytic reaction course. The mechanistic insights are improved by clarifying the following aspects. (1) The chain propagation preferably proceeds via cis-butadiene insertion into the η3-butenyl−TiIII bond, thus verifying the π-allyl-insertion mechanism as being operative. (2) The alternative insertion commencing from the s-trans configurat...