No abstract

Even late transition metal complexes function as active and selective catalysts for α-olefin polymerization. The discovery of a highly active family of catalysts 1 based on iron, a metal that had no previous track record in this field, has highlighted the possibilities for further new catalyst discoveries. As a result, an intense search has developed for new-generation catalysts, in both academic and industrial research laboratories. R =H, Me; R =Me, iPr; R =H, Me, iPr; R =H, Me; X=halide.

The synthesis, characterization, and ethylene polymerization behavior of a series of iron and cobalt halide complexes, LMX n (M ) Fe, X ) Cl, n ) 2, 3, X ) Br, n ) 2; M ) Co, X ) Cl, n ) 2), bearing chelating 2,6-bis(imino)pyridyl ligands L [L ) 2,6-(ArNCR 1 ) 2 C 5 H 3 N] is reported. X-ray diffraction studies show the geometry at the metal centers to be either distorted square pyramidal or distorted trigonal bipyramidal. Treatment of the complexes LMX n with methylaluminoxane (MAO) leads to highly active ethylene polymerization catalysts converting ethylene to highly linear polyethylene (PE). LFeX 2 precatalysts with ketimine ligands (R 1 ) Me) are approximately an order of magnitude more active than precatalysts with aldimine ligands (R 1 ) H). Catalyst productivities in the range 3750-20600 g/mmol‚h‚bar are observed for Fe-based ketimine catalysts, while Co ketimine systems display activities of 450-1740 g/mmol‚h‚bar. Molecular weights (M w ) of the polymers produced are in the range 14000-611000. Changing reaction conditions also affects productivity and molecular weight; in some systems, a bimodal molecular weight distribution is observed. On the basis of evidence gathered to date, the lower molecular weight fraction is a result of chain transfer to aluminum while the higher molecular weight fraction is produced by a combination of mainly -H transfer and some chain transfer to aluminum.

No abstract

No abstract

A new family of aluminum complexes bearing tetradentate bis(aminophenoxide) ligands is reported and shown to initiate the living ring-opening polymerization of rac-lactide. The microstructures of the polylactide products are found to be highly dependent upon the ancillary ligand substituents, ranging from highly isotactic (Pm = 0.79) to very highly heterotactic (Pr = 0.96).

No 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.