On the mechanism of formation of isotactic and syndiotactic polydiolefins

The mode of formation of isotactic and syndiotactic polymers from 1,3- dienes is examined in the light of the most recent results. An interpretation is given for the formation of trans-1,4 isotactic polymers from CH2=CH-CH=CHR (R = Me, Et, Pr, etc.) type monomers with eterogeneous VCl3-based systems. Evidence is reported showing that stereoregular 1,2 or cis-1,4 polymers derive from a growing polymer chain, anti-h3-bonded to the transition metal and a cis-eta4 coordinated monomer. The influence on stereoselectivity of the substituents at the central carbon atoms of the monomer is discussed. The peculiar behavior of (Z)-1,3-pentadiene and 4-methyl-1,3-pentadiene, which give 1,2 polymers with catalysts that give 1,4 polymers from other monomers, is attributable to the fact that they can coordinate trans-eta2, in addition to cis-eta4.