Catalytic Synthesis and Characterization of Biodegradable ...

Chapter 2
2.1 Introduction
Polycarbonate is one of the important thermoplastic with a good heat resistance, excellent
optical and electrical properties and the unique biodegradability. 1-2 The mechanical properties
are predominated by the stereostructure of the regioselectivity of the linear carbonate linkage
in the polycarbonate. The enantioselectivity of the polycarbonate, head-to-tail connectivity,
and carbonate linkage are important factors to determine the thermal stability and the
mechanical properties. The reactivity and enantioselectivity are mainly related to the type and
chemical stereostructure of the catalyst system. Several efficient catalyst systems, such as zinc,
aluminum, cadmium, manganese, cobalt, chromium and rare-earth-metal complexes, in
combination with other reagents for the copolymerization of CO2 and alicyclic epoxides such
as propylene oxide have been developed. 3-7 Recently, salen-type catalysts have become the
focus of many groups. The chemical structures of the complexes as catalysts are related to the
overall catalytic copolymerization performance of CO2 with an epoxide. 8 Transition metal
Schiff-base complexes with N,N,O,O-tetradentate ligands have been extensively studied, 9-16
because of their various applications 17-19 and importance in the area of coordination
chemistry. 20 Cobalt(III) Schiff-base complexes are characterized by the asymmetric structure
containing axial ligands, which are frequently labile, and exhibit high activities and yet a
significant stability. 21 Especially, binary catalytic system of many forms of the
(salen)Co(III)/Lewis base as catalysts is widely used in the fixation of CO2 with epoxide to
form polycarbonates. 22-26 Recent studies have focused on how to improve the efficiency of the
copolymerization and to enhance the catalytic activity of these complexes. 22, 24, 25, 27-30 In this
chapter, we describe the electrochemical properties of several structurally related cobalt
complexes and the X-ray crystal structure, which provided an important insight into the
copolymerization mechanism of CO2 and propylene oxide catalyzed by these complexes.
To elucidate factors that dominate the cobalt-catalyzed polymerization, a series of cobalt
complexes as catalyst containing N,N,O,O-tetradentate Schiff bases were synthesized. These
complexes were different in bridging lengths between the two nitrogen atoms in the Schiff
base ligands. The modification of the electronic and steric environments by altering the
substituents on the diimine-bridge of the salen ligand around the metal center significantly
affected the catalytic activity for the poly(propylene carbonate) formation, which was
successfully ascribed to the stability of the six-coordinate cobalt complex and the lability of
the propagating polymer chain as the axial ligand.
2.2 Synthesis and Characterization of L-CoIII-dnp Complexes
2.2.1 Synthesis of the Ligands
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