Redox-triggered buoyancy and size modulation of a dynamic covalent gel

The development of stimuli-responsive materials capable of transducing external stimuli into mechanical and physical changes has always been an intriguing challenge and an inspiration for scientists. Several stimuli-responsive gels have been developed and applied to biomimetic actuators or artificial muscles. Redox active actuators in which the mechanical motion is driven chemically or electrochemically have attracted much interest and their actuation mechanism is based on the change in electrostatic repulsion and the loss or gain of counterions to balance newly formed charges. Actuation can also be promoted by changing the hydration state of the material leading to the release/adsorption of water molecules from the network, inducing a direct shrinking/swelling of the material respectively. A cationic crystalline dynamic covalent gel was obtained via the formation of imine bonds between 2,6-diformyl pyridine and triamino guanidinium chloride. The gel exhibits a reversible contraction/expansion behavior in response to base (oxidation, –H+, –e–) and acid (reduction +H+, +e–) respectively. The oxidation induces a color change and contraction of the gel with a concomitant increase in its strength. As synthesized, the cationic gel is denser than water and sinks when placed in water. Upon oxidation, the radical cationic gel expels water molecules rendering it less dense than water and the gel is propelled to the surface without any loss of its structural integrity. These results demonstrate that a careful choice of amine and aldehyde linkers can give rise to imine-linked materials capable of tolerating and resisting extreme acidic and basic conditions while performing work

Development of organocatalytic strategies for controlled ring-opening polymerisation for producing biodegradable polyesters and polycarbonates

This thesis investigates organocatalytic ring-opening polymerisation (ROP) approaches for the controlled synthesis of biodegradable polymers by exploring the post-polymerisation modifications of pendent groups and by photochemical regulations. Chapter 1 reviews the organocatalysts developed for ROP, including the mechanistic aspects involved in the polymerisation. In addition, an overview of photocatalysed ROPs and the photocatalysts that can be potentially applied for this strategy is presented. Chapter 2 describes the selective carbonate ROP of trimethylenepropane oxirane ether carbonate monomer by using 1,5,7-triazabicyclo[4.4.0]dec-5-ene as organocatalyst. Post-polymerisation modifications of the pendent epoxide group with a range of nucleophiles demonstrate to be especially effective for benzylamine, chlorine and aromatic thiols. Chapter 3 demonstrates the high catalytic activity of 1,1,3,3-tetramethylguanidine (TMG) towards the ROP of L-lactide, and of δ-valerolactone and ε-caprolactone when a thiourea cocatalyst is utilised. In addition, the photoinduced ROP is explored by protecting TMG with a photosensitive group and the system is shown to provide temporal control to the polymerisation. Chapter 4 details the synthesis a bicyclic monomer analogue to lactide that would potentially crosslink via ROP to provide spatial resolution to the photocatalyst system. The monomer is obtained from the synthesis of an allyl-functional lactide, and subsequent thiol-ene reaction with a dithiol to form a bis(cyclic diester). Chapter 5 describes the efforts in preparing a resin formulation composed of the bis(cyclic diester) prepared in Chapter 4 and the photocatalyst developed in Chapter 3 to achieve spatial control. Network materials with various mechanical properties could be obtained by using the crosslinker, poly(L-lactide) oligomers and TMG as a catalyst, although no curing was observed when utilising photocatalysts. Chapter 6 summarises the key findings of Chapters 2 to 5 and presents opportunities for future investigations in the area of research of this thesis. Chapter 7 provides the experimental protocols and characterisation data of the compounds and materials prepared in this thesis

Ring-opening polymerisation of ε-substituted-ε-caprolactones towards novel, semi-crystalline functional polyesters

This thesis investigates the synthesis of stereoenriched poly(ε-substituted-ε- caprolactones) based on organocatalytic methods. Moreover, the investigation into orthogonally controlled switchable catalysis for ring-opening polymerisation (ROP) is also explored. Chapter one reviews how substituent placement on a cyclic monomer effects the thermodynamics of the polymerisation. Moreover, an overview of organocatalysts for ROP, including those that display stereoselectivity during the polymerisation, is presented. Chapter two describes the enantioenriched synthesis and subsequent functionalisation of a ε-substituted-ε-caprolactone (εSεL) monomer. The effect of functionality on the polymerisation is then reviewed. Following this, the thermal properties of the resulting polymer was then analysed. Chapter three describes the screening of a range of chiral binaphthol-based phosphoric acids as stereoselective catalysts for the ROP of a variety of racemic εSεLs. The steric and electronic properties of the catalysts are investigated to understand how these influence the polymerisation. Moreover, the microstructure of the polymer is evaluated by 13C NMR spectroscopy, and the thermal properties of the resulting polymers are analysed. Chapter four summarises donor-acceptor Stenhouse adducts as switchable catalysts for the ROP of cyclic esters using light and heat to control the ON and OFF states of the polymerisation. Chapter five describes the key findings of chapters two to four and presents opportunities for future investigations in the area of research of this thesis. Chapter six provides the experimental protocols and characterisation data of the compounds and materials prepared in this thesis

Photoschaltbare Polymerisationen

Die fortschreitende Entwicklung auf dem Gebiet der kontrollierten Polymerisationstechniken ermöglicht heutzutage die Synthese definierter Makromoleküle. Durch das Design der Primärstruktur dieser Makromoleküle kann ein starker Einfluss auf die sich bildende Sekundärstruktur ausgeübt werden. Um den Grad der Kontrolle über den Polymerisationsprozess zu erhöhen sollten in dieser Arbeit photoschaltbare Azobenzolfunktionalitäten in Guanidin- und Thioharnstoffmotive integriert werden, so dass durch die reversibel photoschaltbare Ausbildung von Wasserstoffbrücken die vorliegenden Polymerisationsprozesse beeinflusst werden können. In einem Ansatz wurden dazu neuartige azobenzolverknüpfte Guanidin- und Thioharnstoffkatalysatoren für die Ringöffnungspolymerisation (ROP) von Lactid (LA) synthetisiert. Im Fall der photoschaltbaren Guanidinkatalysatoren wurde eine Synthesemethode entwickelt, welche die zu Beginn langwierige Aufreinigung der Katalysatoren bedeutend vereinfacht und somit die Darstellung mehrerer Katalysatorgenerationen ermöglichte. Die erste Guanidinkatalysatorgeneration zeigte keine katalytische Aktivität. Durch die Synthese von verschiedenen Referenzguanidinen und deren Einsatz in der ROP von LA konnten die aromatischen Substituenten der Guanidinfunktionalität als Ursache der katalytischen Inaktivität identifiziert werden. Daraufhin wurde eine mit zwei Alkylsubstituenten versehene zweite Generation synthetisiert und erfolgreich in der ROP von Lactid eingesetzt. In einem anderen Ansatz wurde versucht ein azobenzolverknüpftes Guanidinium-Carboxylat-Zwitterion so zu gestalten, dass es als photoschaltbares Monomer zur Bildung supramolekularer Polymere verwendet werden kann. Hierzu wurden zwei Generationen photoschaltbarer Zwitterionen synthetisiert. Die Eigenschaften der zweiten Generation wurden mit verschiedenen spektroskopischen Methoden untersucht. Dabei wurden Hinweise auf die Bildung eines supramolekualren Polymers gefunden.Following the progressive development in the field of controlled polymerizations, it is now possible to synthesize well defined macromolecular structures. Controlling the primary structure in these macromolecules significantly influences the secondary structure, allowing the preparation of smart materials. In order to improve the achievable degree of control, this work aims to incorporate azobenzene functionalities into guanidine and thiourea moieties and, through the photo triggered reversible formation of hydrogen bridges, influence polymerization processes. Novel azobenzene substituted guanidine and thiourea catalysts for the ring opening polymerization (ROP) of lactide were synthesized. In the case of the photoswitchable guanidine catalysts, a new synthetic protocol was developed to overcome the difficult purification of the catalysts, allowing the facile preparation of multiple catalyst generations. The first generation of photoswitchable guanidines showed catalytic activity. Synthesis of reference guanidine catalysts demonstrated a negative effect between aromatic guanidine substituents and the catalytic performance. Following this observation, a second generation of alkyl substituted guanidine catalysts was synthesized and applied successfully in the ROP of lactide. In a concurrent approach, a guanidinium carboxylate zwitterion was rendered photoswitchable by the incorporation of an azobenzene functionality and used as a monomer in supramolecular polymerization processes. The first generation of photoswitchable zwitterions showed promising photochemical properties, but its poor solubility in apolar, aprotic solvents prevented a final proof of the concept. To achieve this, a second generation of photoswitchable zwitterions was synthesized, incorporating solubilizing functionalities into the molecular design. The properties of the second generation zwitterion were examined by various spectroscopical methods, indicating the formation of supramolecular polymers

Urea-based [2]Rotaxanes as Effective Phase-Transfer Organocatalysts: Hydrogen-Bonding Cooperative Activation Enabled by the Mechanical Bond

© 2024 The Authors. This manuscript version is made available under the CC-BY 4.0 license http://creativecommons.org/licenses/by/4.0/ This document is the Accepted version of a Published Work that appeared in final form in Journal of the American Chemical Society. To access the final edited and published work see https://doi.org/10.1021/jacs.4c0663

Peptide Nucleic Acids (PNA) as a versatile tool for modulation of biological systems with visible light

Das Hauptziel dieser Arbeit war die Kombination neuartiger photoschaltbarer Moleküle, die möglicherweise im Bereich des sichtbaren Lichts operieren, mit PNA (Peptidonukleinsäuren) - künstlichen Oligonukleotidanaloga mit hoher Hybridisierungsaffinität und Sequenzselektivität gegenüber komplementären DNA- und RNA-Strängen. Die Bindung ist so stark, dass unter optimalen Bedingungen Genexpressionen (Antigene) und Übersetzungen in Proteine (Antisense) in vivo verhindert werden können. Aufgrund einer hochspezifischen Hybridisierung mit einem Oligomer von >10 Nukleobasen können kurze PNA-Oligomere selektiv auf bestimmte Ziele innerhalb lebender Zellen oder Organismen abzielen, ohne die übrige biosynthetische Maschiner ie der Zelle zu stören. Das langfristige Ziel dieser Arbeit war die Implementierung photoschaltbarer Einheiten, wie das Azobenzol oder ein Spiropyran, um ein reversibles Schalten des Antigens oder des Antisense- Effekts mit sichtbarem Licht zu erreichen. In einem Nachfolgeprojekt wurde die PNA für Untersuchungen gegen das HIV (humanes Immunschwächevirus) eingesetzt. HIV verursacht AIDS, eine der derzeit anspruchsvollsten Krankheiten. Das Virus selbst greift das Immunsystem an und vernichtet es systematisch. Da das Virus unsere DNA benutzt, um sich zu reproduzieren, wird es nicht als "Eindringling" erkannt. Es breitet sich leicht aus, und eine kommerziell verfügbare Heilung ist noch nicht bekannt, obwohl kürzlich eingeführte antiretrovirale Therapien die Virusmenge vorübergehend unter die Nachweisgrenze senken können. Die derzeitige Hypothese ist, dass das Virus irgendwo im Organismus konserviert wird, wo es von den üblichen Medikamenten nicht erreicht werden kann. Da es Fluoreszenzfarbstoffe gibt, die eine Erhöhung der Fluoreszenzintensität (einen Aufhellungseffekt) zeigen, während sie innerhalb eines Doppel-Oligonukleotidstra ngs (RNA/RNA; RNA/DNA; DNA/DNA; PNA/DNA; PNA/RNA) hybridisiert sind, haben wir versucht, ein analytisches Werkzeug zum Nachweis von HIV-RNA zu konstruieren. Die fluoreszierende Lokalisierung der HI-Viren würde wiederum dazu beitragen, Medikamente zu entwickeln, die die letzten Reservoire des Virus erreichen und es effizient aus dem Organismus ausrotten können. Die Region der HIV-RNA, auf die wir mit PNA-Sequenzen abzielen wollen, ist bekanntlich die wichtigste Region für die Vermehrung. Eine Blockierung dieser Region mit einer komplementären PNA würde verhindern, dass sie die richtige Faltung annimmt. Dies würde zu einem Knockdown führen, was die weitere Ausbreitung des Virus behindern würde. Eine andere Strategie bestand darin, das Ende der PNA-Sequenzen mit einer chemischen Ribonuklease zu dekorieren. Ein solches PNA-Oligomer würde die komplementäre RNA sequenzspezifisch ausschneiden und so die RNA des Virus und damit das Virus selbst zerstören

The development of switchable initiators for rac-Lactide ring-opening polymerisation

This thesis describes various systems to achieve switchability for the ring-opening polymerisation of lactide by means of catalyst design. Heterobimetallic cooperativity is reported whereby a combination of hard/soft metals have been shown to enhance the catalytic activity compared to each independent species. A traditional hard titanium salen bis(isopropoxide) complex in combination with a soft, non-oxophilic silver salt induces catalytic activity at room temperature, where the presence of cation- interactions between the silver cation and the phenolate ring of the ligand have been supported by experimental work and theoretical studies. The nature of the silver salt is important, in particular the anion, where unprecedented salt metathesis reactions occurred for coordinating anions, leading to the isolation and characterisation of a new family of titanium salen complexes. Dinuclear and mononuclear aluminium salen complexes featuring a disulfide or thiomethyl motif respectively were isolated and fully characterised by multinuclear NMR spectroscopy, elemental analysis, and in some cases X-ray diffraction. All complexes could successfully initiate the polymerisation with a high activity and good control, and they all proved to be active under immortal conditions. Homolytic cleavage of the disulfide bond was not achieved, but little differences were observed between the disulfide and thiomethyl complexes, with respect to their catalytic capability. Investigations into azo-functionalised ligands have commenced, as an alternative way to achieve photoswitching between the cis and trans isomers. Novel homoleptic zinc complexes featuring a Schiff base ligand scaffold functionalised with ferrocene were isolated and fully characterised using multinuclear NMR, high-resolution mass spectrometry and elemental analysis. Cyclic voltammetry, spectroelectrochemistry and NMR analysis evidenced oxidation of the ferrocene could occur using stoichiometric amounts of chemical oxidant (AgOTf). The catalyst exhibited good activity towards the ring-opening polymerisation of lactide, with very narrow dispersities and predictable molecular weights. Investigations into the formation of lactide/cyclohexene oxide (LA/CHO) block copolymers using established titanium salen bis(isopropoxide) catalysts were not successful, with the preferred formation of oligomers of CHO (Mn < 600 g mol-1) despite good overall activity towards the ring-opening polymerisation of lactide.Open Acces