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-(MeC=N-2,6-iPr2C6H3)2C5H)N]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 alpha-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.

How to Polymerize Ethylene in a Highly Controlled Fashion?

“…Reports on transition-metal-based catalyst systems can be divided into, reports on single-catalyst systems [17,18,19] and systematically accomplished studies, in which one CGS was investigated with regard to a variety of main-group alkyls [20] and vice versa. [21] Systems identified by the authors as very fast, reversible, PE chain-transfer catalysts are only taken for discussion.…”

“…[18] Systematic studies concerning the main group alkyl and CGS: The Fe + /Al-Zn catalyst system was systematically investigated with regard to the variation of the MGM (Table 6). [20] This excellent study shows how difficult it is to find the right combination of CGS and MGM. The CGS based on bisA C H T U N G T R E N N U N G (imino)pyridine iron complex 7 essentially transfers PE chains very fast and reversibly to zinc and gallium alkyls.…”

Fe(acac)n and Co(acac)n bearing different bis(imino)pyridine ligands for ethylene polymerization and oligomerization

“…Gibson et al 28 observed that the end groups of the low-molar-mass part of polyethylene obtained by bis(imino)pyridyl iron complexes were almost saturated. Britovsek and coworkers 29,30 added an effective chain-transfer agent to the bis(imino)pyridyl iron complexes. When ZnEt 2 was added, the low-molar-mass part of polyethylene was mainly prepared by chain transfer to ZnEt 2 .…”

Group 4 metal complexes for homogeneous olefin polymerisation: a short tutorial review

“…(M = Group 4 metal, L = supporting ligands), which themselves are not catalytically active, and result in chain transfer and can also act to deactivate the active species; they are also considered to be catalytic resting states [8,119,120]. Alkyl aluminium compounds can also have a negative effect on the polymerisation performance of Group 4 catalysts by acting as reducing agents; titanium(III) compounds are not electrophilic enough to achieve high polymerisation activities.…”