Poly(lactide): from hyperbranched copolyesters to new block copolymers with functional methacrylates

The prologue of this thesis (Chapter 1.0) gives a general overview on lactone based poly(ester) chemistry with a focus on advanced synthetic strategies for ring-opening polymerization, including the emerging field of organo catalysis. This section is followed by a presentation of the state-of the art regarding the two central fields of the thesis: (i) polyfunctional and branched poly(ester)s in Chapter 1.1 as well as (ii) the development of new poly(ester) based block copolymers with functional methacrylates (Chapter 1.2). Chapter 2 deals with the synthesis of new, non-linear poly(ester) structures. In Chapter 2.1, the synthesis of poly(lactide)-based multiarm stars, prepared via a grafting-from method, is described. The hyperbranched poly(ether)-poly(ol) poly(glycerol) is employed as a hydrophilic core molecule. The resulting star block copolymers exhibit potential as phase transfer agents and can stabilize hydrophilic dyes in a hydrophobic environment. In Chapter 2.2, this approach is expanded to poly(glycolide) multiarm star polymers. The problem of the poor solubility of linear poly(glycolide)s in common organic solvents combined with an improvement of the thermal properties has been approached by the reduction of the total chain length. In Chapter 2.3, the first successful synthesis of hyperbranched poly(lactide)s is presented. The ring-opening, multibranching copolymerization of lactide with the “inimer” 5HDON (a hydroxyl-functional lactone monomer) was carefully examined. Besides a precise molecular characterization involving the determination of the degree of branching, we were able to put forward a reaction model for the formation of branching during polymerization. Several innovative approaches to amphiphilic poly(ester)/poly(methacrylate)-based block copolymers are presented in the third part of the thesis (Chapter 3). Block copolymer build-up especially relies on the combination of ring-opening and living radical polymerization. Atom transfer radical polymerization has been successfully combined with lactide ring-opening, using a “double headed” initiator. This strategy allowed for the realization of poly(lactide)-block-poly(2-hydroxyethyl methacrylate) copolymers, which represent promising materials for tissue engineering scaffolds with anti-fouling properties (Chapter 3.1). The two-step/one-pot approach forgoes the use of protecting groups for HEMA by a careful selection of the reaction conditions. A series of potentially biocompatible and partially biodegradable homo- and block copolymers is described in Chapter 3.2. In order to create a block copolymer with a comparably strong hydrophilic character, a new acetal-protected glycerol monomethacrylate monomer (cis-1,3- benzylidene glycerol methacrylate/BGMA) was designed. The hydrophobic poly(BGMA) could be readily transformed into the hydrophilic and water-soluble poly(iso-glycerol methacrylate) (PIGMA) by mild acidic hydrolysis. Block copolymers of PIGMA and poly(lactide) exhibited interesting spherical aggregates in aqueous environment which could be significantly influenced by variation of the poly(lactide)s stereo-structure. In Chapter 3.3, pH-sensitive poly(ethylene glycol)-b-PBGMA copolymers are described. At slightly acidic pH values (pH 4/37°C), they decompose due to a polarity change of the BGMA block caused by progressing acetal cleavage. This stimuli-responsive behavior renders the system highly attractive for the targeted delivery of anti-cancer drugs. In Chapter 3.4, which was realized in cooperation, the concept of biocompatible, amphiphilic poly(lactide) based polymer drug conjugates, was pursued. This was accomplished in the form of fluorescently labeled poly(HPMA)-b-poly(lactide) copolymers. Fluorescence correlation spectroscopy (FCS) of partially biodegradable block copolymer aggregates exhibited fast cellular uptake by human cervix adenocarcinoma cells without showing toxic effects in the examined concentration range (Chapter 4.1). The current state of further projects which will be pursued in future studies is addressed in Chapter 4. This covers the synthesis of biocompatible star block copolymers (Chapter 4.2) and the development of new methacrylate monomers for biomedical applications (Chapters 4.3 and 4.4). Finally, the further investigation of hydroxyl-functional lactones and carbonates which are promising candidates for the synthesis of new hydrophilic linear or hyperbranched biopolymers, is addressed in Chapter 4.5.

Multifunctional polyether architectures : versatile materials for surface attachment and functionalization

Chapter 1 of this thesis comprises a review of polyether polyamines, i.e., combinations of polyether scaffolds with polymers bearing multiple amino moieties. Focus is laid on controlled or living polymerization methods. Furthermore, fields in which the combination of cationic, complexing, and pH-sensitive properties of the polyamines and biocompatibility and water-solubility of polyethers promise enormous potential are presented. Applications include stimuli-responsive polymers with a lower critical solution temperature (LCST) and/or the ability to gel, preparation of shell cross-linked (SCL) micelles, gene transfection, and surface functionalization.rnIn Chapter 2, multiaminofunctional polyethers relying on the class of glycidyl amine comonomers for anionic ring-opening polymerization (AROP) are presented. In Chapter 2.1, N,N-diethyl glycidyl amine (DEGA) is introduced for copolymerization with ethylene oxide (EO). Copolymer microstructure is assessed using online 1H NMR kinetics, 13C NMR triad sequence analysis, and differential scanning calorimetry (DSC). The concurrent copolymerization of EO and DEGA is found to result in macromolecules with a gradient structure. The LCSTs of the resulting copolymers can be tailored by adjusting DEGA fraction or pH value of the environment. Quaternization of the amino moieties by methylation results in polyelectrolytes. Block copolymers are used for PEGylated gold nanoparticle formation. Chapter 2.2 deals with a glycidyl amine monomer with a removable protecting group at the amino moiety, for liberation of primary amines at the polyether backbone, which is N,N-diallyl glycidyl amine (DAGA). Its allyl groups are able to withstand the harsh basic conditions of AROP, but can be cleaved homogeneously after polymerization. Gradient as well as block copolymers poly(ethylene glycol)-PDAGA (PEG-PDAGA) are obtained. They are analyzed regarding their microstructure, LCST behavior, and cleavage of the protecting groups. rnChapter 3 describes applications of multi(amino)functional polyethers for functionalization of inorganic surfaces. In Chapter 3.1, they are combined with an acetal-protected catechol initiator, leading to well-defined PEG and heteromultifunctional PEG analogues. After deprotection, multifunctional PEG ligands capable of attaching to a variety of metal oxide surfaces are obtained. In a cooperative project with the Department of Inorganic and Analytical Chemistry, JGU Mainz, their potential is demonstrated on MnO nanoparticles, which are promising candidates as T1 contrast agents in magnetic resonance imaging. The MnO nanoparticles are solubilized in aqueous solution upon ligand exchange. In Chapter 3.2, a concept for passivation and functionalization of glass surfaces towards gold nanorods is developed. Quaternized mPEG-b-PqDEGA diblock copolymers are attached to negatively charged glass surfaces via the cationic PqDEGA blocks. The PEG blocks are able to suppress gold nanorod adsorption on the glass in the flow cell, analyzed by dark field microscopy.rnChapter 4 highlights a straightforward approach to poly(ethylene glycol) macrocycles. Starting from commercially available bishydroxy-PEG, cyclic polymers are available by perallylation and ring-closing metathesis in presence of Grubbs’ catalyst. Purification of cyclic PEG is carried out using α-cyclodextrin. This cyclic sugar derivative forms inclusion complexes with remaining unreacted linear PEG in aqueous solution. Simple filtration leads to pure macrocycles, as evidenced by SEC and MALDI-ToF mass spectrometry. Cyclic polymers from biocompatible precursors are interesting materials regarding their increased blood circulation time compared to their linear counterparts.rnIn the Appendix, A.1, a study of the temperature-dependent water-solubility of polyether copolymers is presented. Macroscopic cloud points, determined by turbidimetry, are compared with microscopic aggregation phenomena, monitored by continuous wave electron paramagnetic resonance (CW EPR) spectroscopy in presence of the amphiphilic spin probe and model drug (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO). These thermoresponsive polymers are promising candidates for molecular transport applications. The same techniques are applied in Chapter A.2 to explore the pH-dependence of the cloud points of PEG-PDEGA copolymers in further detail. It is shown that the introduction of amino moieties at the PEG backbone allows for precise manipulation of complex phase transition modes. In Chapter A.3, multi-hydroxyfunctional polysilanes are presented. They are obtained via copolymerization of the acetal-protected dichloro(isopropylidene glyceryl propyl ether)methylsilane monomer. The hydroxyl groups are liberated through acidic work-up, yielding versatile access to new multifunctional polysilanes.

Expanding the scope of poly(ethylene glycol)s for bioconjugation to squaric acid-mediated PEGylation and pH-sensitivity

Poly(ethylene glycol) (PEG) is used in a broad range of applications due to its unique combination of properties and is approved use in formulations for body-care products, edibles and medicine. This thesis aims at the synthesis and characterization of novel heterofunctional PEG structures and the establishment of diethyl squarate as a suitable linker for the covalent attachment to proteins. Chapter 1 is an introduction on the properties and applications of PEG as well as the fascinating chemistry of squaric acid derivatives. In Chapter 1.1, the synthesis and properties of PEG are described, and the versatile applications of PEG derivatives in everyday products are emphasized with a focus on PEG-based pharmaceuticals and nonionic surfactants. This chapter is written in German, as it was published in the German Journal Chemie in unserer Zeit. Chapter 1.2 deals with PEGs major drawbacks, its non-biodegradability, which impedes parenteral administration of PEG conjugates with polyethers exceeding the renal excretion limit, although these would improve blood circulation times and passive tumor targeting. This section gives a comprehensive overview of the cleavable groups that have been implemented in the polyether backbone to tackle this issue as well as the synthetic strategies employed to accomplish this task. Chapter 1.3 briefly summarizes the chemical properties of alkyl squarates and the advantages in protein conjugation chemistry that can be taken from its use as a coupling agent. In Chapter 2, the application of diethyl squarate as a coupling agent in the PEGylation of proteins is illustrated. Chapter 2.1 describes the straightforward synthesis and characterization of squaric acid ethyl ester amido PEGs with terminal hydroxyl functions or methoxy groups. The reactivity and selectivity of theses activated PEGs are explored in kinetic studies on the reactions with different lysine and other amino acid derivatives, followed by 1H NMR spectroscopy. Further, the efficient attachment of the novel PEGs to a model protein, i.e., bovine serum albumin (BSA), demonstrates the usefulness of the new linker for the PEGylation with heterofunctional PEGs. In Chapter 2.3 initial studies on the biocompatibility of polyether/BSA conjugates synthesized by the squaric acid mediated PEGylation are presented. No cytotoxic effects on human umbilical vein endothelial cells exposed to various concentrations of the conjugates were observed in a WST-1 assay. A cell adhesion molecule - enzyme immunosorbent assay did not reveal the expression of E-selectin or ICAM-1, cell adhesion molecules involved in inflammation processes. The focus of Chapter 3 lies on the syntheses of novel heterofunctional PEG structures which are suitable candidates for the squaric acid mediated PEGylation and exhibit superior features compared to established PEGs applied in bioconjugation. Chapter 3.1 describes the synthetic route to well-defined, linear heterobifunctional PEGs carrying a single acid-sensitive moiety either at the initiation site or at a tunable position in the polyether backbone. A universal concept for the implementation of acetal moieties into initiators for the anionic ring-opening polymerization (AROP) of epoxides is presented and proven to grant access to the degradable PEG structures aimed at. The hydrolysis of the heterofunctional PEG with the acetal moiety at the initiating site is followed by 1H NMR spectroscopy in deuterium oxide at different pH. In an exploratory study, the same polymer is attached to BSA via the squarate acid coupling and subsequently cleaved from the conjugate under acidic conditions. Furthermore, the concept for the generation of acetal-modified AROP initiators is demonstrated to be suitable for cholesterol, and the respective amphiphilic cholesteryl-PEG is cleaved at lowered pH. In Chapter 3.2, the straightforward synthesis of α-amino ω2-dihydroxyl star-shaped three-arm PEGs is described. To assure a symmetric length of the hydroxyl-terminated PEG arms, a novel AROP initiator is presented, who’s primary and secondary hydroxyl groups are separated by an acetal moiety. Upon polymerization of ethylene oxide for these functionalities and subsequent cleavage of the acid-labile unit no difference in the degree of polymerization is seen for both polyether fragments.

Modellkomplexe oligonuklearer Metalloproteine auf Basis von Salen-Liganden und Spin-Crossover-Verbindungen als Chemosensoren

Der erste Teil der hier vorgestellten Arbeit verfolgt die Synthese potentieller Modellverbindungen oligonuklearer Metalloproteine auf Basis von Salen-Liganden. Dazu wurden zwei Ligandensysteme mit unterschiedlich raumerfüllenden Alkyl-Substituenten modifiziert und auf ihre koordinativen Eigenschaften hin untersucht. Für das Ligandensystem auf Basis des Bis-(salicylidenamino)-propan-2-ols konnten fünf Derivate (H3L1, H3L2A,H3L2B, H3L3, H3L4), für das zweite verwendete Ligandensystem auf Basis des 1H-3,5-Bis-(salicylidenaminomethyl)-pyrazols konnten zwei weitere Derivate (H3L5A, H3L5B) dargestellt und zu Koordinationsverbindungen umgesetzt werden.rnFür den hier verwendeten Bis-(salicylidenamino)-propan-2-ol Liganden H3L1, welcher die geringsten sterischen Anforderungen stellt, konnten mono-, tri- und tetranukleare Koordinationsverbindungen synthetisiert werden. Dabei gelingt es dem Liganden, sich sowohl in planarer als auch in unterschiedlich stark gewinkelter Konformation um ein oder mehrere Metallzentren anzuordnen, wobei der Ligand ein N2O2- seines N2O3-Donorsets zur Koordination nutzt. Die Verbindung {[Ni7(HL1)2(L1)2(OBz)4(OMe)(H2O)]}n zeigt, dass eine Verkettung der so gestalteten dreikernigen Einheiten über das freie Propanol-Sauerstoffatomdes Ligandenrückgrats möglich ist. Mit zunehmendem sterischen Anspruch der angefügten Alkylsubstituenten nimmt die geometrische Flexibilität und somit das Potential des Liganden zur Ausbildung höhernuklearer Strukturen ab. So ist für Liganden mit mittlerem sterischen Anspruch neben mononuklearen Komplexen noch die Gestaltung dinuklearer Systeme möglich. Erhöht man den sterischen Anspruch des Liganden weiter, findet nur noch eine Reaktion zu mononuklearen Verbindungen statt.rnMit den Pyrazol-basierten Ligandensystemen H3L5A und H3L5B konnten dinukleare Kupfer- und Nickelverbindungen synthetisiert werden.rnDer zweite Teil dieser Arbeit befasst sich mit der Gestaltung von Spin-Crossover Systemen (SCO). Dazu soll ein Spinübergang innerhalb des gestalteten schaltbaren Systems an die Anwesenheit eines Signalstoffs gekoppelt werden, so dass diese SCO-Verbindung als Sensor für den Signalstoff eingesetzt werden kann. Dazu wurden zwei unterschiedliche Ansätze entwickelt und untersucht.rnDie erste Methode beruht auf der Kombination eines zum Spin-Crossover befähigten Metallzentrums, eines Capping-Liganden, eines zur Signalstofferkennung funktionalisierten Co-Liganden sowie eines entsprechenden Signalstoffs. Als Capping-Liganden wurden tetra- und pentadentateLigandensysteme eingesetzt und mit unterschiedlich Picolyl-substituierten Monoaza-[12]-krone-4-Derivaten umgesetzt, wobei die Monoazakrone zur Komplexierung des Signalstoffs,hier in Form eines Alkalimetallions, zur Verfügung steht. Nach dieser ersten Methode konnten im Zeitraum dieser Arbeit noch keine zufriedenstellenden Ergebnisse erzielt werden.rnEine vielversprechende zweite Möglichkeit beruht auf der Verwendung eines mehrzähnigen, etablierten Spin-Crossover Liganden,welcher in seiner Peripherie mit einer Bindungstasche zur Aufnahme des Signalstoffmodifiziert wird.Mit Hilfe des so gestalteten Liganden 4'-(4'''-Benzo-[15]-krone-5)-methyloxy-2,2':6',2''-terpyridin ([b15c5]-tpy) gelang die Umsetzung zu entsprechenden Eisen(II)- und Kobalt(II)komplexen der Zusammensetzung [M([b15c5]-tpy)2]2+. Alle synthetisierten Eisen(II)-Komplexe liegen aufgrund der hohen Ligandenfeldstärke des Terpyridins über einen Temperaturbereich von 300 – 400 K in ihrer diamagnetischen Low Spin Form vor. Die entsprechenden Kobalt(II)-Komplexe zeigen über einen Temperaturbereich von 2 – 350 K ein kontinuierliches, aber unvollständiges Spin-Crossover Verhalten.rnDer Einfluss von Signalstoffen auf das Spin-Crossover Verhalten der Kobalt(II)-Systeme wurde in einem ersten Versuch unter der Verwendung von Natriumionen als Signalstoff untersucht. Dabei stellte sich heraus, dass Natriumionen für dieses System zwar nicht als Auslöser eines SCO verwendet werden können, sie aber dennoch eine starke Auswirkung auf den Verlauf des Spin-Crossovers haben.

Sulfur-containing polymers for surface attachment

Different concepts for the synthesis of sulfur-containing polymers as well as their adsorption onto gold surfaces were studied. The present work is divided into three parts. The main part focuses on the synthesis of poly(1,2-alkylene sulfides) (“polysulfides”) with complex architectures on the basis of polyether-based macroinitiators by the anionic ring-opening polymerization of ethylene sulfide and propylene sulfide. This synthetic tool kit allowed the synthesis of star-shaped, brush-like, comb-like and pom-pom-like polysulfides, the latter two with an additional poly(ethylene glycol) chain. Additionally, the number of polysulfide arms as well as the monomer composition could be varied over a wide range to obtain copolymers with multiple thioether functionalities.rnThe second section deals with the synthesis of a novel lipoic acid-based initiator for ring-opening polymerizations for lactones and epoxides. A straightforward approach was selected to accomplish the ability to obtain tailored polymers with a common used disulfide-anchoring group, without the drawbacks of post-polymerization functionalization. rnIn the third part, a new class of block-copolymers consisting of polysulfides and polyesters were investigated. For the first time this approach enabled the use of hydroxyl-terminated poly(propylene sulfide) as macroinitiator for the synthesis of a second block.rnThe adsorption efficiency of those different polymer classes onto gold nanoparticles as well as gold rnsupports was studied via different methods.rn