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N-Heterocyclic Carbene Mediated Zwitterionic Polymerization for Cyclic Polymers

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N-Heterocyclic Carbene Mediated Zwitterionic Polymerization for Cyclic Polymers N-HETEROCYCLIC CARBENE MEDIATED ZWITTERIONIC POLYMERIZATION FOR CYCLIC POLYMERS A DISSERTATION SUBMITTED TO THE DEPARTMENT OF CHEMISTRY AND THE COMMITTEE ON GRADUATE STUDIES OF STANFORD UNIVERSITY IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY Eun Ji Shin July 2011 © 2011 by Eun Ji Shin. All Rights Reserved. Re-distributed by Stanford University under license with the author. This work is licensed under a Creative Commons Attribution- Noncommercial 3.0 United States License. http://creativecommons.org/licenses/by-nc/3.0/us/ This dissertation is online at: http://purl.stanford.edu/tj619sw3147 ii I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Robert Waymouth, Primary Adviser I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Wray Huestis I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Barry Trost Approved for the Stanford University Committee on Graduate Studies. Patricia J. Gumport, Vice Provost Graduate Education This signature page was generated electronically upon submission of this dissertation in electronic format. An original signed hard copy of the signature page is on file in University Archives. iii ABSTRACT Zwitterionic polymerization involves a propagating species with both positively and negatively charged groups. Previous investigations on zwitterionic polymerization concern alternating copolymerization of nucleophilic and electrophilic monomers and polymerization of isolated stable zwitterionic monomers. More recently, the ring-opening polymerization of cyclic monomers using nucleophilic initiators have been studied. Specifically, the zwitterionic polymerization of cyclic esters using N-heterocyclic carbenes (NHCs) is the focus of this thesis. The N- heterocyclic carbene mediated zwitterionic polymerization of cyclic monomers provides an expedient route to polymers of various architectures, such as cyclic polymers, cyclic gradient copolymers and linear telechelic polymers. The ring-opening polymerization of lactide initiated by NHCs generates cyclic poly(lactide)s of defined molecular weight and molecular weight distribution. Kinetic studies implicate a mechanism that involves a slow initiation step and a propagation step that is much faster than depropagation and chain termination by cyclization. Stochastic simulations and chain extension experiments showed that only a fraction of the NHC forms the active zwitterion in solution, leading to both chain extension of the zwitterions and re-initiation of the NHC upon addition of the second batch of monomer. These results prompted investigation of a more efficient way to prepare cyclic block copolymers. The difference in reactivity of NHCs towards different monomers was exploited to synthesize cyclic block copolymers of -valerolactone (VL) and -caprolactone (CL). The faster ring-opening of VL relative to CL resulted in a gradient cyclic copolymer comprised of VL-rich sequences that transition to CL- rich sequences in a cyclic macromolecule, instead of a cyclic diblock copolymer. This work not only provides a simple batch copolymerization protocol to produce cyclic gradient copolymers, but also demonstrates the marked difference in reactivity of the NHCs compared to metal catalysts, which produce random copolymers. Stereocomplexation behavior has been observed in blends of linear poly(L- lactide) and linear poly(D-lactide). The influence of topology on the formation of v stereocomplex was investigated using blends of linear and cyclic poly(lactide)s prepared by NHC mediated zwitterionic polymerization. The linear/cyclic and cyclic/cyclic blends all form stereocomplexes when annealed. Analyses of data from various characterization techniques indicate that the cyclic topology does not impede the formation of stereocomplexes. The purity of the cyclic polymers is always a concern in the synthesis and physical property studies. Attempts to identify and quantify the linear contamination in cyclic poly(-caprolactone) samples are described. Esterification reactions targeting the hydroxyl endgroups of linear contaminants were not successful, but the macroinitiator approach where the linear contaminant in a cyclic polymer sample is used as the macroinitiator to grow polymers to identify and remove the linear contamination shows promise. A cyclic polymer more robust to post-polymerization chemistry may be needed for more thorough purity studies. vi PREFACE Chapter 1 is a review of zwitterionic polymerization with a particular emphasis on the synthesis of cyclic polymers. It is intended to give the reader a background on the brief history of zwitterionic polymerization and systems which use N-heterocyclic carbenes to give interesting architectures, such as cyclic polymers. Chapter 2 describes polymerization of lactide with the N-heterocyclic carbene, 1,3- dimesitylimidazol-2-ylidene (IMes) by sequential addition of monomer. The products obtained are analyzed in the light of the mechanism and kinetics of this system. Also, a short section on the effect of different solvents on the polymerization of lactide with IMes is included. The introduction part and the stochastic simulations and experiments relating to them have been published in the Journal of the American Chemical Society 2009, 131, 4884-4891. The experiments in section 2.3 were performed together with Dr. Wonhee Jeong. Chapter 3 shows the stereocomplexation in various blends of linear and cyclic poly(lactide)s. The effect of the cyclic topology on the crystallization and stereocomplexation was explored. The linear poly(lactide)s were synthesized by a CPIMA SURE program undergraduate student Alexandra Jones. The wide-angle X-ray scattering experiments were performed by Dr. Jihoon Kang at Seoul National University, Korea. This work will be submitted to Macromolecules for publication. Chapter 4 outlines the synthesis and characterization of cyclic gradient copolymers using N-heterocyclic carbenes. This work was published in Angewandte Chemie, International Edition 2011, 50, 6388-6391. The reactivity ratios of - valerolactone and -caprolactone were determined by Silvia Gonzalez and Dr. Wonhee Jeong and 1,3-diethyl-4,5-dimethylimidazol-2-ylidene was synthesized by Hayley Brown. Chapter 5 details the synthesis of cyclic poly(-caprolactone) and the efforts to identify, quantify and remove linear contamination in these cyclic polymers. Parts of section 5.2 has been published in Macromolecules 2011, 44, 2773-2779. Synthesis of 1,3,4,5-tetramethylimidazol-2-ylidene via the deprotonation of the imidazolium salt was performed by Hayley Brown. All of the work in this thesis is done by me, except where noted. vii viii ACKNOWLEDGEMENTS I would like to express my deepest gratitude to my advisor Professor Waymouth. He is the ideal research advisor and has become a role model for me. Without his ideas and support, my Ph.D would have never been possible. Thank you so much. Dr. Hedrick has always been a great co-advisor and I thank him for always supportive and enthusiastic about my work. I would like to thank my committee members, Professor Trost and Professor Huestis for giving me a lot of feedback and advice on my research and research proposals. Also, I thank Professor Frank for being a substitute member on one of my research proposals. Professors Kanan and Sattely were so kind to find the time to be the non-readers for my thesis defense. I am so glad that I had the privilege to discuss research with such great minds. I thank Professor Do Yoon for the continuing support he gave me even after I finished my degree with him. Wonhee Jeong was the person who led me into the field of polymer synthesis. Ever since I met him he has been so kind and patient in teaching me everything I know and giving advice on not only research but also ways of life. I am grateful for everything. Another person that made my Ph.D possible was Hayley Brown. She is so smart and such a great person to work with. I owe her a lot and cannot thank her enough. The Waymouth group has been such a fun place to work in. I have met so many good friends. I thank Kyung-sun Son for always being there. She is the nicest person I’ve ever met. I could talk to her about everything and I hope this relationship lasts. I loved and miss the jokes of Matt Kiesewetter. His presence made my trips to lab 1 so eventful and enjoyable. Not only that, he gave so much advice when I got stuck with my research. I really appreciate his help. Liz Kiesewetter was the model ‘lab mother’ and I thank her for always being so nice to me. I thank David Pearson for all his help he gave me whenever I had a synthetic problem. Justin Edward makes me proud that I was his mentor, if only for a short time. I wish him all the best in the ix future. Dr. Sangjin Jeon has been and still is a great person to go for advice on anything. I thank him for being such a great senior. I would like to thank all other past and present members for making my time in the Waymouth lab so productive and pleasant. I should also thank all the collaborators who helped and contributed to my work; Dr. John Pople, Dr. Jihoon Kang, Allie Jones. Without them my work would not have been complete. Yeonju Kwak and Jean Chung joined the Ph.D program with me in 2006. Despite their young age, they have been really good friends and I am very lucky to have met them. I am very sorry that I won’t be able to be there for their thesis defense but I wish them all the best. I wouldn’t have been able to get my degree without the support from my friends.
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