Catalytic Synthesis and Characterization of Biodegradable ...

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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 ‐ 60 ‐
Synthesis and Electrochemistry of Schiff Base Cobalt(III) Complexes and Their Catalytic Activity for Copolymerization of Epoxide and Carbon Dioxide The Schiff base ligands were synthesized by the condensation of the corresponding diamine and 3,5-di-tert-butylsalicylaldehyde in a 1:2 molar ratio in methanol (Scheme 2.1). The ligands were further purified by recrystallization from ethanol. 2.2.2 Synthesis of the Complexes The L-Co III -dnp complex was prepared by the reaction of cobalt acetate tetrahydrate and the ligand, followed by subsequent oxidation in the presence of 1 equiv. of 2,4-dinitrophenol and an excess amount of oxygen. The formation of the Schiff base ligand and their complexes have been confirmed by spectroscopic methods and X-ray diffraction experiments (vide infra). All the Schiff base complexes were stable at room temperature, and were soluble in methylene chloride, chloroform and DMSO. Scheme 2.1 Preparation of the L-Co III -dnp complexes. 2.2.3 Complexes Structure Characterization The molecular structure of L 1 -Co III -dnp (Figure 2.1) revealed that the complex was monomeric with a six-coordinated central cobalt atom in the solid state. ‐ 61 ‐
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Catalytic Synthesis and Characteriz
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Preface Biodegradable polyesters ha
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Chapter 3 Polymerization of Lactic
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Chapter 1 Polymerization and Applic
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- 3 - Polymerization and Applicatio
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- 5 - Polymerization and Applicatio
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1.2.2 Aluminum Catalysts - 7 - Poly
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Page 88 and 89: Chapter 3 aluminum based complexes
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Page 94 and 95: Chapter 4 4.1 Introduction Stimulus
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‐ 109 ‐ Synthesis of Triblock C
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‐ 111 ‐ Synthesis of Triblock C
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Chapter 6 6.1 Introduction Since th
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Chapter 6 6.3 Synthesis of MPEG-b-P
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Chapter 6 used to characterize the
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Chapter 6 Figure 6.6 Cyclic voltamm
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Chapter 6 vivo cytotoxicity was als
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Chapter 6 NMR. Characterization of
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Chapter 6 CO2 for 24 h. RSC96 cells
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Chapter 7 Conclusion and Future Pro
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‐ 131 ‐ Conclusion and Future P
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‐ 133 ‐ Conclusion and Future P
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13. Xuan Pang, Xuesi Chen, Xiuli Zh
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Acknowledgments The present thesis
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