Chromium-catalyzed ethylene oligomerization

The selective trimerization and tetramerization of ethylene to form 1-hexene and 1-octene has become an important process to generate monomers for the synthesis of LLDPE. 1-Hexene was detected as a byproduct in the polymerization of ethylene catalyzed by homogeneous chromium complexes. Chromium complexes have now been identified that catalyze this oligomerization to form 1-hexene with remarkably high selectivity. Phillips patented the combination of 2,5-dimethylpyrrole, triethylalu-minum, and diethylaluminum chloride with a chromium(lll) salt, and researchers at Union Carbide patented a catalyst generated from chromium(lll) 2-ethylhexanoate and hydrolyzed triisobutylaluminum. - Researchers at BP have described a particularly active and selective catalyst based on a chromium(lll) precursor, a bis(diphosphino)amine ligand, (o-MeO-C H ),PN(CH,)P(o-MeOC H ) and methylaluminoxane (MAO). " ... 

Klemps C, Payet E, Magna L, et al PCNCP ligands in the chromium-catalyzed oligomerization of ethylene tri- versus tetrametization, Chem EurJ 15(33) 8259-8268, 2009. 

It is important to note the following points. First, conversion of 7.16 to 7.17 is a formal oxidative addition where the oxidation state of chromium increases by two. Second, unlike Ni-catalyzed di- or oligomerization, here the chain growth takes place by insertion of ethylene into the five- and seven-membered metallacycles. Finally, in the metallacycfic mechanism there is no hydride intermediate. 

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