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.

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The condensation of formaldehyde with p-alkylphenols under alkaline conditions, a reaction dating back to the days of Baeyer and Zinke, affords initially linear polyphenols, which, once a certain temperature is achieved, yield cyclic oligomeric phenolic compounds (Scheme 1). One-step, multigram synthetic procedures have now been developed, which by varying either the temperature or amount of base used in the preparation, readily afford the tetrameric, hexameric, and octameric phenolic ring systems, bridged by methylene (-CH2-) spacers. Ring systems with up to 20 phenolic residues are now known, although methods of preparation for these higher members, and for those with an odd number of phenolic residues, are usually low yielding. The smaller members of the family adopt a cone-shaped structure, hence the name calix[n]arene coined by Gutsche, from the Greek “calix” meaning vase, where n denotes the number of phenolic residues. Such a bowl shape results in the formation of a hydrophobic, electron-rich cavity, which is well-suited to the formation of inclusion complexes, particularly cations.5 The synthetic methodology has also been adapted to allow for the incorporation of bridging groups other than methylene, such as dimethyleneoxa (-CH2OCH2-), thia (-S-), and aza [-CH2N(R)CH2-] bridged calixarenes,6 and these additional donors can provide extra binding sites

Economic pressure, environmental issues and ever increasing demand are driving the shift from oil-based polymers to those available from renewable resources. Ring opening polymerisation of cyclic esters is currently a topical field with metal complex-induced coordination/insertion type polymerisation leading the way. Such a polymerisation method offers a wide range of advantages from control over the polymer structure to kinetic enhancement; however industrially available catalysts based on tin suffer from inherent toxicity and as a result a policy change from the Food and Drug Administration (FDA) is not unrealistic. This review underlines the efforts made in the past five years or so, and shows how low toxicity metals are attracting increasing attention in the field of e-caprolactone polymerisation