Iron Catalyzed Polyethylene Chain Growth on Zinc: A Study of the Factors Delineating Chain TransferversusCatalyzed Chain Growth in Zinc and Related Metal Alkyl Systems

Abstract

The bis(imino)pyridine iron complex, [{2,6-(MeCN-2,6-ⁱPr₂C₆H₃)₂C₅H₃N}FeCl₂] (1), in combination with MAO and ZnEt₂ (> 500 equiv.), is shown to catalyze polyethylene chain growth on zinc. The catalyzed chain growth process is characterized by an exceptionally fast and reversible exchange of the growing polymer chains between the iron and zinc centers. Upon hydrolysis of the resultant ZnR₂ product, a Poisson distribution of linear alkanes is obtained; linear α-olefins with a Poisson distribution can be generated via a nickel-catalyzed displacement reaction. Other dialkylzinc reagents such as ZnMe₂ and ZnⁱPr₂ also show catalyzed chain growth; in the case of ZnMe₂ a slight broadening of the product distribution is observed. The products obtained from Zn(CH₂Ph)₂ show evidence for chain transfer but not catalyzed chain growth, whereas ZnPh₂ shows no evidence for chain transfer. The Group 13 metal alkyl reagents AlR₃ (R = Me, Et, octyl, ^IBu) and GaR₃ (R = Et, ⁿBu) act as highly efficient chain transfer agents, whereas GaMe₃ exhibits behavior close to catalyzed chain growth. LiⁿBu, MgⁿBu₂ and BEt₃ result in very low activity catalyst systems. SnMe₄ and PbEt₄ give active catalysts, but with very little chain transfer to Sn or Pb. The remarkably efficient iron catalyzed chain growth reaction for ZnEt₂ compared to other metal alkyls can be rationalized on the basis of: (1) relatively low steric hindrance around the zinc center, (2) their monomeric nature in solution, (3) the relatively weak Zn−C bond, and (4) a reasonably close match in Zn−C and Fe−C bond strengths.