Stereoselectivity and Chemoselectivity in Ziegler−Natta Polymerizations of Conjugated Dienes. 1. Monomers with Low-Energy s-Cis η⁴ Coordination

The mechanisms of stereoselectivity and chemoselectivity in the Ziegler−Natta polymerizations of conjugated dienes have been investigated by density functional methods, in the framework of the widely accepted polymerization scheme involving anti -allyl coordination of the growing chain. Most insertions would occur through intermediates with the diene monomer s- cis η 4 coordinated to the metal and with the monomer concavity oriented toward the terminal allyl group of the growing chain. Monomer coordinated intermediates presenting a backbiting of the anti -allyl coordinated growing chain (η 3 η 2 ) would essentially lead to 1,4- unlike insertions only, while monomer coordinated intermediates simply presenting an anti -allyl coordination of the growing chain would lead to 1,4- like and/or 1,2- unlike insertions. These models are able to rationalize several experimental data relative to stereoselectivity and chemoselectivity in Ziegler−Natta polymerizations of conjugated dienes.