Catalytic Synthesis and Characterization of Biodegradable ...
Catalytic Synthesis and Characterization of Biodegradable ...
Catalytic Synthesis and Characterization of Biodegradable ...
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Polymerization <strong>and</strong> Applications <strong>of</strong> <strong>Biodegradable</strong> Polyesters<br />
in the presence <strong>of</strong> a quaternary organic salt or triphenylphosphine. At ambient temperature<br />
<strong>and</strong> a relatively high catalyst loading, reaction rates were slow, generally requiring 12-23 days<br />
<strong>and</strong> thereby affording rather low molecular weight polymers. Nevertheless, copolymers from<br />
ethylene oxide, propylene oxide, or cyclohexene oxide <strong>and</strong> carbon dioxide with very narrow<br />
molecular weight distributions (1.06-1.14) were isolated. Approximately a decade later,<br />
Kruper <strong>and</strong> Dellar investigated the use <strong>of</strong> (tpp)CrX complexes in the presence <strong>of</strong> 4-10 equiv<br />
<strong>of</strong> nitrogen donors, such as N-methylimidazole (N-MeIm) or (4-dimethylamino)pyridine<br />
(DMAP), for the coupling <strong>of</strong> a wide variety <strong>of</strong> epoxides <strong>and</strong> carbon dioxide to provide cyclic<br />
carbonates. 15 For example, propylene oxide readily afforded propylene carbonate at 50 bar <strong>of</strong><br />
CO2 pressures with a TOF <strong>of</strong> 158 h -1 at 80 . Conversely, cyclohexene oxide <strong>and</strong> CO2<br />
yielded predominantly copolymer under similar reaction conditions. In more recent studies,<br />
Holmes <strong>and</strong> co-workers have reported the copolymerization <strong>of</strong> cyclohexene oxide <strong>and</strong> CO2 in<br />
the presence <strong>of</strong> a fluorinated (tpp)CrCl catalyst (Figure 1.2.2) along with a cocatalyst such as<br />
DMAP. 16 TOFs greater than 150 h -1 were observed for reactions performed at 110 in scCO2<br />
(225 bar) where the catalyst solubility is greatly enhanced over its (tpp)CrCl analogue, with<br />
the resulting copolymers having a high percentage <strong>of</strong> carbonate linkages (>97%) <strong>and</strong> narrow<br />
polydispersities (PDIs). The rapid advances currently being experienced in the<br />
copolymerization <strong>of</strong> cyclohexene oxide <strong>and</strong> carbon dioxide were fueled by Darensbourg’s<br />
development <strong>of</strong> a series <strong>of</strong> discrete zinc phenoxide derivatives as catalysts in 1995. 17 A<br />
typical bis(phenoxide)Zn(THF)2 complex (where THF = tetrahydr<strong>of</strong>uran) used in these<br />
studies is illustrated in Figure 1.2.3, where the steric bulk <strong>of</strong> the phenoxide lig<strong>and</strong>s limits<br />
aggregate formation.<br />
Figure 1.2.2 Fluorinated porphyrin derivative <strong>of</strong> chromium(III) chloride.<br />
These catalysts were extremely effective for homopolymerizing epoxides to polyether;