Search

The electron-rich nature of vinyl ethers (CH2═CHOR) enables their versatile reactivity patterns with transition metal catalysts. Furthermore, they are highly attractive monomers for the synthesis o...

Precisely controlling the stereochemistry of a copolymerization by single catalyst, and achieving copolymers with predicted sequences, is a challenging project in organometallic and macromolecular sciences. This article reports that the copolymerization of isoprene and butadiene with the 3,4- and cis-1,4 different regioselectivities has been accessed through employing a pyridylmethylene functionalized fluorenyl ligated yttrium cationic species. The thus unprecedented regularity combination is attributed mainly to the backbiting interaction of the penultimate unit of the polymer chain to the active central metal ion according to the DFT calculation. Moreover, the compositions can be adjusted by regulating the monomer feed ratio according to the copolymerization kinetics study.

This paper reports a DFT study on trans-1,4-specific polymerization of isoprene catalyzed by the cationic heterobimetallic half-sandwich complex [(C5Me5)La(AlMe4)]+. The possible structures of the active species, viz., [(C5Me5)La(μ2-Me)3AlMe]+ (A), [(C5Me5)La(μ2-Me)2AlMe2]+ (B), and [(C5Me5)La(Me)(μ2-Me)AlMe2]+ (C), have been investigated. On the basis of the chain initiation and the structure transformations among these three species, C has been proposed to be the true active species smoothly producing trans-1,4-polyisoprene observed experimentally. Both La/Al bimetal-cooperating monomer insertion and La-center-based insertion pathways have been calculated, and the latter is found to be more favorable, where the AlMe3 moiety serves as a ligand coordinating to the La center via a methyl group. In contrast to this, in the Y analogous system, the AlMe3 ligand is proposed to leave away from the Y center during the chain propagation and the cis-1,4-selectivity is preferred, showing a consistence with experime...