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Dinucleating Ligand Platforms Supporting Zinc and Indium

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Dinucleating Ligand Platforms Supporting Zinc and Indium DINUCLEATING LIGAND PLATFORMS SUPPORTING ZINC AND INDIUM CATALYSTS FOR STEREOSELECTIVE LACTIDE POLYMERIZATION by Alexandre Bertrand Kremer B.Sc., Ecole Supérieur de Chimie, Physique et Electronique de Lyon, 2012 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Chemistry) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) July, 2016 © Alexandre Bertrand Kremer, 2016 Abstract With the recent advances in cyclic ester polymerization to access biodegradable and bio-based plastic, an increasing number of metal-based initiators have been reported to mediate ring opening polymerization with high yield and stereoselectivity. However, the origin of the stereoselectivity and the factors affecting it are yet little understood. The role of the metal nuclearity for the polymerization of lactide has especially been subject to various speculations. In our group, we developed unique asymmetrically bridged indium initiators, explored their ring opening polymerization mechanism and found evidence for a type of tandem catalysis. In order to have a better understanding of the role of the two indium metal centres during polymerization, we aimed to synthesize dinucleating analogues. The syntheses of the first alkoxide bridged indium complex supported by a chiral dinucleating ligand platform, along with its zinc analogue, are reported. Both complexes were synthesized in a one pot reaction starting from a chiral dinucleating bis(diamino)phenolate ligand platform, NaOEt, and respective metal salts. The dinucleating indium species bearing an achiral ligand backbone previously synthesized in our group was also further investigated. Both indium complexes catalyze the ring opening polymerization of racemic lactide to afford highly heterotactic PLA (Pr > 0.85). The indium complex bearing the achiral ligand backbone affords atactic PLA (Pr = 0.46) from meso-LA. The role of the dinucleating ligand structure in catalyst synthesis and polymerization activity is discussed. ii Preface The first chapter of this thesis was written entirely by myself with minor corrections from my supervisor Dr. Parisa Mehrkhodavandi. The second chapter was also written by myself with major contributions from my supervisor. This work was published in collaboration with Dr. Kim Osten and Dr. Insun Yu who initiated this project and first isolated the indium complexes bearing achiral proligands. The synthesis, optimization and purification of the chiral proligands as well as the related complexes was developed entirely by myself. I completed all polymerization studies presented in this work. The single crystal structures presented in this work were obtained by Dr. Insun Yu, Dr. Kim Osten and myself. Dr. Insun Yu, Dr. Dinesh Aluthge and Tannaz Ebrahimi contributed to the solving of X-ray crystal structures. The second chapter of this thesis has been previously published in the American Chemical Society Journal Inorganic Chemistry (DOI: 10.1021/acs.inorgchem.6b00358). iii Table of Content Abstract .................................................................................................................................................... ii Preface .................................................................................................................................................... iii Table of Content ..................................................................................................................................... iv List of Tables .......................................................................................................................................... vi List of Figures ........................................................................................................................................ vii List of Abbreviations and Symbols.................................................................................................... xiii Acknowledgements................................................................................................................................ xv Chapter 1: General Introduction........................................................................................................... 1 1.1. Introduction to poly(lactic acid) ........................................................................................... 1 1.2. Metal mediated ring opening polymerization mechanism of lactide ................................... 5 1.3. Tacticity in PLA ................................................................................................................... 8 1.4. ‘PLA toward sustainability’ or ‘Towards sustainable PLA’? ............................................ 11 1.5. Defined bimetallic catalysts for the ROP of lactide .......................................................... 15 1.5.1. Dimeric initiators ............................................................................................................... .15 1.5.2. Tethered initiators ............................................................................................................... 42 1.5.3. Dinucleating initiators ......................................................................................................... 50 1.5.4. Heterobimetallic complexes ................................................................................................ 56 1.5.5. Conclusion .......................................................................................................................... 59 1.6. Scope of the thesis ............................................................................................................. 61 iv Chapter 2: Dinucleating ligand platforms supporting zinc and indium catalysts for stereoselective lactide polymerization ................................................................................................. 63 2.1. Introduction ........................................................................................................................ 63 2.2. Synthesis and characterization of chiral proligands ........................................................... 66 2.3. Synthesis of indium and zinc alkoxide complexes bearing chiral ligand backbones. ....... 68 2.4. Synthesis indium alkoxide complexes bearing achiral ligand backbone. .......................... 78 2.5. Polymerization of lactide. .................................................................................................. 81 2.6. Conclusions and perspectives ............................................................................................ 83 2.7. Experimental procedures. .................................................................................................. 85 References .............................................................................................................................................. 93 Appendices ........................................................................................................................................... 100 v List of Tables Table 2-1: Polymerization of lactide by initiators 2, 4, and 5. ............................................................... 82 Table A-1: Bernoullian equations of probability obtained for the different tetrad present in PLA. .... 100 Table E-1: Selected bond lengths and angles for complexes 7a, 7b, and 8. ........................................ 125 Table E-2: Selected crystallographic parameters for complexes 4 and 5. ........................................... 127 Table E-3: Selected bond lengths and angles for complexes 2, 3, and 4. ............................................ 128 Table E-4: Selected crystallographic parameters for complexes 1, 7, and 8. ...................................... 130 vi List of Figures Figure 1-1: Lifecycle of PLA from corn starch. ...................................................................................... 2 Figure 1-2: Homoleptic initiators commonly used in industrial ROP of PLA, respectively tin(II)octanoate and aluminumisopropoxide. ............................................................................................ 3 Figure 1-3: Coordination-insertion mechanism for the ROP of lactide. .................................................. 5 Figure 1-4: Activated monomer mechanism for ROP of lactide. ............................................................ 6 Figure 1-5: Living polymerization versus immortal polymerization. ...................................................... 7 Figure 1-6: Different microstructure of PLA arising from lactide polymerization. ................................ 9 Figure 1-7: PLA presenting an mrr or iss tetrad (A), 1H{1H} NMR representative of PLA arising from rac-LA (B), and meso-LA (C), tetrad associated to each tacticity value (D). ........................................ 10 Figure 1-8: Different classes of bimetallic catalysts .............................................................................. 15 Figure 1-9: Dinuclear systems based on monodentate ligand reported for ROP of lactide ................... 17 Figure 1-10: Dinuclear β-diiminate, β-diketiminate based complexes. ................................................. 19 Figure 1-11: β-enaminoketonate pyrazonolate zinc alkoxides, β-enaminoketonate zinc alkyls and β- diketonate aluminum alkoxides. ............................................................................................................. 20 Figure 1-12: Scorpionate and alkoxide
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