Mechanism of rare-earth catalysis in coordination polymerization

The microstructure and stereospecificity of a variety of 1.3-diene polymers prepared with lanthanide coordination catalysts were examined. In contrast to the conventional d-orbital transition-metal catalysts, the lanthanide catalysts would polymerize 1,3-diene monomers prevailingly in a 1,4-addition manner. The cis and trans 1,4-stereoregular polymerizations depend on the steric and electronic structures of the dienes. Under the same lanthanide catalysts the 1,4-dimethylbutadiene (e.g., trans, trans-2,4-hexadiene) leads exclusively to trans-1,4 polymers, the 2,3-dimethylbutadiene gives mainly cis-1,4 polymers, and the 1,3-dimethylbutadiene (e.g., trans-2-methyl-1,3-pentadiene) produces mixed trans/cis (60/40) 1,4-polymers. The ligands in the lanthanide catalyst and the polymerization conditions have relatively little effect on the stereoregulation.