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.
| Identifer | oai:union.ndltd.org:tamu.edu/oai:repository.tamu.edu:1969.1/ETD-TAMU-1678 |
| Date | 15 May 2009 |
| Creators | Price, Craig Justin |
| Contributors | Miller, Stephen A. |
| Source Sets | Texas A and M University |
| Language | en_US |
| Detected Language | English |
| Type | Book, Thesis, Electronic Dissertation, text |
| Format | electronic, application/pdf, born digital |
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