Stereoselectivity and chemoselectivity in Ziegler-Natta polymerization of conjugated dienes. 2. Mechanism for 1,2 syndiotactic polymerization of diene monomers with high energy s-cis η⁴ coordination

Possible mechanisms of Ziegler-Natta polymerizations of conjugated dienes have been investigated by density functional methods, by considering η ² or η⁴ (both cis and trans) coordination of the monomer and η¹ or η³ (syn or anti) allyl coordination of the growing chain. In agreement with previous studies, for diene monomers presenting a low s-cis-η⁴ coordination energy (like butadiene), the favored insertion reaction would involve a s-cis-η⁴ monomer coordination and an anti η³ allyl coordination of the growing chain (mechanism I). On the other hand, for diene monomers presenting high s-cis-η⁴ coordination energy (like (Z)-pentadiene and 4-methyl-pentadiene), the favored insertion reaction would generally involve a 5-trans η² monomer coordination and a back-biting syn allyl (η³-η²) coordinated growing chain (mechanism II). However, these monomers would present an s-cis-η ⁴ coordination whenever the formation of the back-biting of the penultimate unit of the growing chain would be unfeasible, as for initiation steps as well as for insertion steps following an ethylene insertion. A switch from mechanism II toward mechanism I is able to rationalize the correspondingly observed loss of chemoselectivity. Mechanism II is also able to account for the high stereoselectivity in favor of 1,2 syndiotactic polymerization which has been observed for these dienes. The chain end stereocontrol would be dictated by the chirality of coordination of the syn allyl terminal of the back-biting growing chain.