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Chapter 6 6.1 Introduction Since the discovery of nitroxyl radicals in 1956, there have been of continually profound interest in the synthesis and investigation of compounds containing nitroxyl free radicals. Applications such as spin-labeling/trapping, MR/ESR in vivo imaging, oxidation organocatalysts and mediating radical polymerization have emerged based on the unique physical, chemical and biological properties of the nitroxyl radicals. Polymers bearing stable nitroxyl radicals in the pendant, known as radical polymers, also have been investigated during past several decades initially for their potential reactivities, however, it is not until recently that the radical polymers was intensively investigated for organic electronic devices applications. A series of radical polymers have been synthesized and evaluated by our group as the electro-active materials for secondary batteries and organic memory devices. 1-4 Aside from the electronic applications, radical polymers are also receiving increasing interest for the application as biomaterials due to nitroxyl radical which plays critical roles in various metabolic processes. C. C. Chu and co-workers are pioneers in this field by conjugating 2,2,6,6-tetramethylpiperidinyloxy (TEMPO) radicals with biodegradable polymers. The resultant TEMPO-functionalized polymers could be degraded in the physiological condition to release the TEMPO nitroxyl radicals, which may found potential use as the scaffold or surface coating for the cardiovascular tissue engineering, the macromolecular drugs for anticancer therapy and the wound dressing with improved healing and antimicrobial capability. 5-7 On the other hand, the magnetic resonance relaxivity and electron paramagnetic resonance (EPR) property have endowed the nitroxyl radical-contained materials with the potential application as bioimaging probes. Much attentions have been paid on the development of the nitroxyl radical-based magnetic resonance imaging (MRI) probes since they have lower toxicity in comparison with the commercially gadolinium derivatives agents. 8-10 However, the MR relaxivities of the nitroxyl radical is relatively lower. In order to solve this issue, several strategies such as copolymerization or attachment of nitroxyl radical compound to the water soluble hyperbranched polymer have been investigated. 11-14 Also, the nitroxyl radical compounds, such as TEMPO, are now being investigated as EPR probes applied in vivo because of the sensitivity of the EPR is much higher than that of MRI. 15-19 However, the reducing environment in vivo, which could reduce or eliminate the EPR signal by reducing the nitroxyl radical, has brought a great obstacle to the application of these ‐ 114 ‐
Synthesis of Amphiphilic Triblock Copolymers of Acrylamide, Lactate, and Ethyleneoxide materials. It has been reported that the half-time of free TEMPO radical in the blood was about 15 s which was far from satisfactory for practical use. 20 Therefore, our purpose is to develop a kind of polymeric deliver system that can carry stable nitroxyl radicals and resist to the reduction environment, which may find potential use as the EPR probe for bioimaging in vivo. For this study, amphiphilic triblock copolymer, consisting of a hydrophilic PEG segment, a hydrophobic PLA segment and a TEMPO radical-contained hydrophobic PTAm segment, was synthesized. An amphiphilic diblock copolymer only contains PEG and PTAm segments were also synthesized. Both of them can self-assemble into micelles in aqueous solution with radicals in the core and generate stable EPR signal. The stability of the micelles against reducing environment was also evaluated. 6.2 Synthesis of MPEG-CPAD and MPEG-b-PLA-CPAD 4-Cyanopentnoic acid dithiobenzoate (CPAD) was prepared as described in the literature 26. 1 H NMR spectra with proton resonance absorption assignments was shown in Figure 6.1 A. Then, the macromolecular chain transfer agents, MEPG-CPAD and MPEG-b-PLA-CPAD, were synthesized by chemical coupling the CPAD to the chain-end of MPEG and MPEG-b-PLA in the presence of DCC and DMAP, respectively. The structures of the obtained polymers were verified by 1 H NMR characterization as shown in Figure 6.1 with the presence of the proton resonance absorption peaks from CPAD. Figure 6.1 1 H NMR characterizations of (A) CPAD; (B) MPEG-CPAD; (C) MPEG-b-PLA-CPAD ‐ 115 ‐
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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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1.2.2 Aluminum Catalysts - 7 - Poly
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1.3 Polylactide 1.3.1 Introduction
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1.3.4 Application of PLA Figure 1.3
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1.5.1.4 Polyaniline (PANi) - 21 - P
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1.6.1 Radical Polymers for Organic
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initialization of the device by app
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182. R. B. Weller, J. Invest. Derma
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Chapter 2 Synthesis and Electrochem
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Synthesis and Electrochemistry of S
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Page 84 and 85: Chapter 3 3.1 Introduction Poly(lac
Page 86 and 87: Chapter 3 polydispersity index (PDI
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Page 94 and 95: Chapter 4 4.1 Introduction Stimulus
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Page 104 and 105: Chapter 4 The 1 H NMR and 13 C NMR
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Page 149: Acknowledgments The present thesis
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