Catalytic Synthesis and Characterization of Biodegradable ...

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