Structure-activity relationships in olefin polymerization catalysts



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The thermodynamic parameters associated with the copolymerization of ethylene and carbon dioxide were calculated using bond dissociation energies, the Benson additivity method and density functional theory calculations (DFT). In all cases, the formation of an alternation copolymer was found to be endergonic at any reasonable polymerization temperatures (the ceiling temperature is calculated to be -159 ?C). However, the polymerization was calculated to be exergonic at room temperature, as long as the incorporation of CO2 is less than 29.7 mol%. Experiments failed to provide evidence of any CO2 incorporation, despite previously published reports claiming up to 30 mol%. Octamethyloctahydrodibenzofluorenyl (Oct) has profound steric consequences when incorporated into metallocene olefin polymerization catalysts ? including increased catalytic activity and stereoselectivity. However, the electronic effect of the ligand?s four electron-donating tertiary alkyl groups is less understood. NMR and DFT calculations were used to study the electronic nature of the Oct moiety ? both as a part of ansa-metallocene pre-catalysts and as an independent molecule. The results show that Oct is more electron rich than other cyclopentadienyl analogues and that the electronics of the ligand are readily conveyed to the metal center. Upon activation, the steric bulk of the Oct moiety dominates the immediate environment around the metal center. Evidence is presented that supports previous theories about Oct?s ability to influence the counteranion distance, thereby increasing the catalytic activity. In addition, excess trimethyl aluminum (TMA) is known to be detrimental to catalytic activity and results uphold this belief ? although the magnitude of the effect varies depending on the metallocene being studied. However, UV-Vis data do not support the theory that TMA binds to the catalytically-active metal center, thereby decreasing the catalytic activity; but does not offer an alternate mechanism.