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 η⁴ 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 (η³η²) 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.