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