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

A comprehensive theoretical investigation of the entire catalytic reaction course of the 1,4-polymerization of 1,3-butadiene assisted by the cationic [Ti III Cp(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 η 3 -π-butenyl− and η 1 -σ-butenyl−Ti III 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 η 3 -butenyl−Ti III bond, thus verifying the π-allyl-insertion mechanism as being operative. (2) The alternative insertion commencing from the s-trans configuration of butadiene is seen to be kinetically disfavored, with thermodynamic considerations found to be less decisive. (3) The allylic anti-syn isomerization is indicated to be much slower than the monomer insertion step. (4) The regeneration of the active catalyst species proceeds via a facile monomer uptake in the anti -η 3 -butenyl−Ti III insertion product, which occurs with preservation of the η 3 -π coordination mode of the terminal group of the growing polybutadienyl chain. (5) The formation of anti -η 3 -butenyl species as the result of the favorable cis -butadiene insertion and the slow allylic isomerization are the critical factors for the nearly exclusive generation of cis -1,4-polybutadiene, notwithstanding the comparable reactivity of anti -η 3 - and syn -η 3 -butenyl forms. (6) The formation of an isotactic cis -1,4-polymer is rationalized in terms of the higher reactivity of the cis -1,3-diene in its prone orientation together with the regulating influence of the axial Cp ligand.