Catalytic Synthesis and Characterization of Biodegradable ...

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Polymerization and Applications of Biodegradable Polyesters
small quantity of CO2 consumed in these processes is likely always to be a very small fraction
of the total CO2 produced from fossil fuel combustion and other sources. Although this
consumption would have a effect on global warming, its use is considered “green chemistry”
in the context of providing more environmentally benign routes to producing chemicals
otherwise made utilizing reagents detrimental to the environment.
Scheme 1.2.1
Both the monomeric and the polymeric products provided from the coupling of CO2 and
epoxides have important industrial applications. Polycarbonates possess outstanding
properties, which include strength, lightness, durability, high transparency, heat resistance,
and good electrical insulation; in addition, they are easily processed and colored. 2 Hence,
these materials have wide-scale uses in electronics, optical media, glazing and sheeting, the
automotive industry, the medical and healthcare industry, and many other consumable goods.
On the other hand, cyclic carbonates find numerous applicabilities, most importantly as high
boiling and flash point solvents with low odor/toxicity in degreasing, paint stripping, and
cleaning processes. 3 These monomeric compounds are also extensively employed as reactive
intermediates.
In 1969, Inoue and co-workers made the remarkable discovery that a mixture of ZnEt2 and
H2O was active for catalyzing the alternating copolymerization of propyleneoxide (PO) and
CO2, marking the advent of epoxide–CO2 coupling chemistry. 4, 5 An optimum 1:1 ratio of
ZnEt2/H2O gave the best yields of methanol-insoluble PPC with an activity of 0.12 h -1 (mol of
PO converted to polymer per mol Zn per h) at 80 o C and 20–50 atm CO2. On the basis of
elemental analysis, the copolymer contained 88% carbonate linkages. Notably, a 1:1 mixture
of ZnEt2 and MeOH did not generate an active catalytic species for polycarbonate synthesis.
Following this initial lead, Inoue investigated the use of dihydric sources, including
resorcinol, 6, 7 dicarboxylic acids, 8 and primary amines, 9 in mixtures with ZnEt2 for PO–CO2
copolymerization. These systems showed TOFs of 0.17, 0.43, and 0.06 h -1 , respectively.