Polymeric Frustrated Lewis Pairs
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This thesis has been submitted in fulfilment of the requirements for a postgraduate degree (e.g. PhD, MPhil, DClinPsychol) at the University of Edinburgh. Please note the following terms and conditions of use: This work is protected by copyright and other intellectual property rights, which are retained by the thesis author, unless otherwise stated. A copy can be downloaded for personal non-commercial research or study, without prior permission or charge. This thesis cannot be reproduced or quoted extensively from without first obtaining permission in writing from the author. The content must not be changed in any way or sold commercially in any format or medium without the formal permission of the author. When referring to this work, full bibliographic details including the author, title, awarding institution and date of the thesis must be given. Polymeric Frustrated Lewis Pairs Meng Wang A thesis submitted to the University of Edinburgh for the Degree of Doctor of Philosophy 2019 Abstract Frustrated Lewis Pair (FLP) chemistry is a significant and growing field since it offers a novel non-metal catalyst for hydrogenation and small molecule activation. Once it was discovered, different FLPs with varying reactivity towards small molecules have been extensively investigated. Its research has mainly focused on small molecule- based FLPs, however, especially in the aspect of hydrogenation reactions. In the field of polymer chemistry, several examples of conventional Lewis pair adduct containing polymers have been reported but there has yet been no exploration of FLPs incorporated into polymers up to the date of this project. Dynamic crosslinked polymeric networks have attracted more attention in recent years as their shape can be post-modified after polymerisation due to their exchangeable crosslinks. This dynamic crosslinking also makes the material stimuli-responsive and provides self-healing properties. This thesis introduces the synthesis of a polymeric network with combined features of frustrated Lewis pairs and dynamic crosslinking. New monomers containing Lewis acid or Lewis base centres were designed and synthesised successfully. For the pair 4- styryl-diphenylborane and 4-styryl-diphenylphosphine, the two monomers were found to be able to bind together at high concentration in toluene so as to form a weak conventional Lewis pair (CLP) adduct. An FLP can be obtained when the phosphine monomer was replaced to its more hindered analogue, 4-styryl-dimesitylphosphine, which is reactive enough to form a complex with diethyl azodicarboxylate (DEAD), where the DEAD bridges the boron and phosphorous centres. The monomers obtained were copolymerised with styrene by RAFT polymerisations. It was also found to be possible to control both the molecular weight and the dispersity. The FLP polymers i synthesised in this way were characterised by NMR spectroscopy and gel permission chromatography. The Lewis acidity of both the monomer and resultant polymer were tested using the Gutmann-Beckett Method, and a decrease in Lewis acidity was observed when the boron monomer was polymerised. The network was synthesised by addition of DEAD into the solution containing both Lewis acid and Lewis base polymers. A gel was quickly generated (in 10 seconds). The mechanical properties of the network formed were determined by rheology. The gel was responsive to heat, in that it would break and return to a polymer solution at high temperatures. The gel formed also shows the ability to self-heal with the assistance of a solvent after physical cracking. The synthesis of the next generation of polymeric FLPs was also examined. A much more Lewis acidic boron monomer, (2,3,5,6-tetrafluorostyryl)- bis(pentafluorophenyl)borane was synthesised. This boron monomer was paired with 4-styryl-dimesitylphosphine to form a reactive FLP that was able to activate small molecules, including dihydrogen molecules and carbon dioxide. The catalysis reactivity of the hydrogenation reactions of this FLP was also explored. The copolymers made from these reactions readily formed a supramolecular gel upon mixing, which also proved temperature responsive. These early-stage results proved that this new boron-monomer is capable of generating a novel stimuli-responsive smart polymer for carbon capture and hydrogenation catalysis. Except for the polymeric FLP, some early-stage research about polymeric CLP and novel synthetic methods for boron-monomers were also introduced and discussed. ii Lay Summary Dynamic crosslinked polymer materials have received a lot of attention because they can be reshaped, reprocessed and self-healable. The reshaping and healing of this class of material is normally achieved by stimuli-triggered crosslinking exchanges. There are many dynamic bonds, either covalent or supramolecular, used as crosslinks in polymer networks, including carbon-carbon bonds based on reversible Diels-Alder or cycloaddition reactions, boronic ester or boroxine bonding, siloxane bonds, disulfide/thiol bonds, hydrogen bonds, ionic bonds, π-π stacking, and Lewis pair complexation. However, use of frustrated Lewis pairs (FLPs) as the dynamic crosslinking of a polymer gel was unprecedented. This thesis investigates the synthesis of macromolecular FLPs as a macro-gelators for generating novel dynamic crosslinked polymer networks. 4-styryl-dimesitylphoshine and 4-styryl-diphenylborane were used to make copolymers with styrene. The resultant copolymers cannot form polymer gel upon mixing due to the steric hindrance around the boron and phosphorus. Gelation was only triggered when a small molecule diethyl azodicarboxylate was added, which linked up the Lewis acids and Lewis bases. Rheology study showed that the resultant poly(FLP) gel is supramolecularly crosslinked, temperature responsive and self- healable. Finally, a fully fluorinated boron containing monomer was synthesised. This monomer showed dramatically increased Lewis acidity, and was able to activate dihydrogen and carbon dioxide when paring with the phosphine monomer. This makes it ideal to be used in the next generation macromolecular FLPs for polymer network synthesis and catalysis. iii Declaration I declare that this thesis was composed by myself, and all the work described here is of my own, unless I have acknowledged help from a named person or referenced a published work. This thesis has not been submitted, in whole or in part, for any other degree or professional qualification. Meng Wang May 2019 iv Acknowledgments The research work involved in this thesis would not have been possible without the kind support of many people, which I deeply appreciate. First of all, I would like to express my sincere gratitude to my supervisor, Prof. Michael P. Shaver, not only for providing me with an opportunity to join the Green Materials Laboratory, but also for his patient supervision of the project and enthusiasm for chemistry. His expertise ensured the completion of my project, and his valuable advice has also helped me to become a more competent chemist. I would also like to thank the School of Chemistry at the University of Edinburgh for awarding me a Tercentenary Scholarship. I would like to thank all the past and present group members, for their huge support for my research and for providing lots of valuable advice in the lab. Special thanks to: Stefan, who makes the research life of all the group members so colourful. Ben, for his knowledge that all of us benefit from. Jarret, Emily, Fern, Genny for teaching me and for their familiarity with the lab work. Panos and Rebecca, for their hard work in my mentoring. Utku, who also joined the same project, made outstanding contributions with his rheology skills. Mitch, for his patient help with this thesis. Yuechao, for his conscientiousness and sense of responsibility to the group. Thanks to all the members in the McKeown group, who are nice neighbours to work nearby. Also thanks to all the people from the Garden group for their kind help. I would also like to show my appreciation to all the people in the College of Science and Engineering who helped me with my research. Dr Lorna Murray and Mr Juraj Bella for NMR spectroscopy. Dr Logan Mackay for training and assistance with the mass spectrometer. Mr Stuart Johnstone for accepting my unlimited amount of glassware orders. Special thanks to Dr Fabio Nudelman, for his support on TEM and v SEM in my published work. Dr John Royer from the School of Physics, for his kind help with rheology measurements. Finally, I wish to thank my friends and family for their support throughout my life. Yi Li and Tairan Wang, for their 15-year-friendship, which for sure will continue and lead to more stories among us. My parents, thank you both for understanding and for your support for all the life decisions I made since from I left home. You have brought so much to me, but what I have done for you is certainly far from enough. vi Table of Contents Abstract ......................................................................................................................... i Lay Summary .............................................................................................................. iii Declaration .................................................................................................................. iv Acknowledgments ........................................................................................................ v Table of Contents ......................................................................................................