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This work dealt with the preparation and the study of supramolecular block copolymers based on hydrogen-bonding between homocomplementary or heterocomplementary stickers. The synthetic strategy was based on the combination of RAFT-mediated controlled radical polymerization and supramolecular chemistry. In the Chapter 2, we developed a strategy relying on the design of RAFT agents bearing thymine/diaminopyridine (DAP) recognition pairs and capable to grow well-defined miktoarm star supramolecular copolymers. To further extend the scope of H-bonding RAFT agents, in the Chapter 3, we also investigated the preparation of RAFT agents functionalized with motifs exhibiting very high binding constants. The Hamilton/barbiturate couple (log(K)≈4-5) was selected to generate more stable supramolecular block copolymers. Aiming at elaborating original associating macromolecules and at simplifying the strategy of synthesis, we finally explored the preparation ABC triblock supramolecular copolymers based on PA11 oligomers (OPA11) in Chapter 4. Ligation of a relevant dithiobenzoate group on the oligomers afforded oligomeric RAFT agents that allow for the preparation of ABC triblock supramolecular copolymers, where A is semi-crystalline, B in rubbery state and C in glassy state. Studies on the incorporation of such copolymers in epoxy networks are under progress.
In this thesis, Bernhard Schmidt describes his research into two fields in the chemical sciences: supramolecular and macromolecular chemistry. Schmidt first investigates cyclodextrins (CDs), which are well known for the formation of supramolecular host/guest complexes with hydrophobic molecules in aqueous solution. Schmidt then also examines reversible addition-fragmentation chain transfer (RAFT) polymerization as a well-suited toll for the synthesis of water-soluble end-functionalized polymers. The author skillfully combines both concepts as a powerful tool to access reversibly forming macromolecular architectures. The novel methods and architectures presented in this work are highly interesting from both a fundamental point of view as well as a basis for the design of efficient drug release systems. The work in this thesis has led to a number of publications in top peer-reviewed journals.
Biocompatible and biodegradable poly(lactide)-2-methacryloyloxyethyl phosphorylcholine (PLA-PMPC) amphiphilic block copolymers were synthesized by a combination of Ring Opening Polymerization (ROP) and Reversible Addition-Fragmentation Chain Transfer (RAFT) polymerization techniques. The PLA-macroRAFT agent was synthesized by the derivatization of PLA-OH with RAFT agent 4-cyano-4-(phenylcarbonothioylthio)pentanoic acid (CPADB) achieving high levels of functionalization and narrow weight distributions (PDI range of 1.02-1.17). PLA-PMPC with varied MPC block lengths were synthesized yielding polymers with a narrow polydispersity PDI = 1.16-1.21. Triblock copolymers PMPC-PLA-PMPC with varying hydrophilic weight ratios were synthesized following an analogous method, the polymerizations were shown to be controlled with PDI's of 1.24 and 1.36. PLA-PMPC block copolymers with varied compositions were self-assembled using several techniques to target different morphologies. Nanostructures were characterised by DLS and TEM. Block copolymers with a larger PLA block length were shown to generate smaller aggregates i.e. micelles. The morphologies observed for the various block copolymers were consistent amongst different preparative techniques. Vesicle structures were reproducible by the self-assembly of PMPC50-PLA51-PMPC50, however, by preparing nanoparticles by direct dissolution micelles formed. The block copolymers were shown to encapsulate a hydrophobic dye in aqueous media thereby demonstrating its potential drug delivery applications.
In the present thesis, the self-assembly of multi thermoresponsive block copolymers in dilute aqueous solution was investigated by a combination of turbidimetry, dynamic light scattering, TEM measurements, NMR as well as fluorescence spectroscopy. The successive conversion of such block copolymers from a hydrophilic into a hydrophobic state includes intermediate amphiphilic states with a variable hydrophilic-to-lipophilic balance. As a result, the self-organization is not following an all-or-none principle but a multistep aggregation in dilute solution was observed. The synthesis of double thermoresponsive diblock copolymers as well as triple thermoresponsive triblock copolymers was realized using twofold-TMS labeled RAFT agents which provide direct information about the average molar mass as well as residual end group functionality from a routine proton NMR spectrum. First a set of double thermosensitive diblock copolymers poly(N-n-propylacrylamide)-b-poly(N-ethylacrylamide) was synthesized which differed only in the relative size of the two blocks. Depending on the relative block lengths, different aggregation pathways were found. Furthermore, the complementary TMS-labeled end groups served as NMR-probes for the self-assembly of these diblock copolymers in dilute solution. Reversible, temperature sensitive peak splitting of the TMS-signals in NMR spectroscopy was indicative for the formation of mixed star-/flower-like micelles in some cases. Moreover, triple thermoresponsive triblock copolymers from poly(N-n-propylacrylamide) (A), poly(methoxydiethylene glycol acrylate) (B) and poly(N-ethylacrylamide) (C) were obtained from sequential RAFT polymerization in all possible block sequences (ABC, BAC, ACB). Their self-organization behavior in dilute aqueous solution was found to be rather complex and dependent on the positioning of the different blocks within the terpolymers. Especially the localization of the low-LCST block (A) had a large influence on the aggregation behavior. Above the first cloud point, aggregates were only observed when the A block was located at one terminus. Once placed in the middle, unimolecular micelles were observed which showed aggregation only above the second phase transition temperature of the B block. Carrier abilities of such triple thermosensitive triblock copolymers tested in fluorescence spectroscopy, using the solvatochromic dye Nile Red, suggested that the hydrophobic probe is less efficiently incorporated by the polymer with the BAC sequence as compared to ABC or ACB polymers above the first phase transition temperature. In addition, due to the problem of increasing loss of end group functionality during the subsequent polymerization steps, a novel concept for the one-step synthesis of multi thermoresponsive block copolymers was developed. This allowed to synthesize double thermoresponsive di- and triblock copolymers in a single polymerization step. The copolymerization of different N-substituted maleimides with a thermosensitive styrene derivative (4-vinylbenzyl methoxytetrakis(oxyethylene) ether) led to alternating copolymers with variable LCST. Consequently, an excess of this styrene-based monomer allowed the synthesis of double thermoresponsive tapered block copolymers in a single polymerization step.
Understanding the reactivity of monomers is crucial in creating copolymers and determining the outcome of copolymerization. Covering the fundamental aspects of polymerization, Synthesis and Applications of Copolymers explores the reactivity of monomers and reaction conditions that ensure that the newly formed polymeric materials exhibit desired properties. Referencing a wide-range of disciplines, the book provides researchers, students, and scientists with the preparation of a diverse variety of copolymers and their recent developments, with a particular focus on copolymerization, crystallization, and techniques like nanoimprinting and micropatterning.
A novel methodology has been developed for the formation of block copolymers that combines ring-opening metathesis polymerization (ROMP) with functional chain-transfer agents (CTAs), functional chain-terminators (CTs) and self-assembly. Telechelic homopolymers of cyclooctene derivatives that are end-functionalized with hydrogen-bonding or metal-coordination sites are formed via the combination of ROMP with a corresponding functional CTA. These telechelic homopolymers are fashioned with a high control over molecular weight and without the need for post-polymerization procedures. The homopolymers undergo fast and efficient self-assembly with their complement homopolymer or small molecule analogues to form block copolymer architectures. The block copolymers have similar association constants to small molecule analogues described in the literature, regardless of size or the nature of the complementary unit or the polymer side-chain. The ROMP of side-chain functionalized norbornene polymers is coupled with functional CTs to produce block copolymer with main- and side-chain self-assembly sites. Combinations of these norbornene polymers with their complement polymer via self-assembly produce non-covalent AB type block copolymers fast and efficiently. ABA type block copolymers are realized by combining the difunctional homopolymer formed via the CTA pathway with the CT synthesized mono-functional polymer. These polymers show similar association constants regardless of the sequence of polymer formation.
Using the grafting-from approach, three novel well-defined bottlebrush block copolymers were synthesized using various controlled polymerization techniques. Resulting macromolecules were characterized by GPC and 1H NMR and then self-assembled into nanostructured materials for further characterization. Asymmetric polystyrene-polylactide (PS-PLA) bottlebrush block copolymers have been shown to self-assemble into a cylindrical morphology with large domain spacings. PLA cylinders can be selectively etched out of the shear-aligned polymer monoliths to generate nanoporous materials with an average cylindrical pore diameter of 55 nm. The remaining bottlebrush backbone provides a functional, hydrophilic coating inside the nanopores. This methodology significantly expands the range of pore sizes attainable in block copolymer based nanoporous materials.^A polystyrene-poly(methyl methacrylate) (PS-PMMA) bottlebrush block copolymer with asymmetric branches was synthesized by grafting from a symmetrical backbone containing a novel dual vinyl initiation system and characterized by atomic force microscopy (AFM) and small-angle x-ray scattering (SAXS). The block copolymer backbone was prepared by sequential reversible addition-fragmentation chain transfer (RAFT) copolymerization of solketal methacrylate and 2-(bromoisobutyryl)ethyl methacrylate (BIEM). From the poly(BIEM) segment, PMMA branches were grafted by atom transfer radical polymerization (ATRP). Solketal groups were then subjected to hydrolysis and functionalized with a RAFT agent. Subsequently, from RAFT sites of the poly(SM) segment, polystyrene branches were grafted to yield the final bottlebrush block copolymer.^The resulting polymer was found to have a vertically oriented cylindrical morphology by AFM with an average cylinder diameter of 45 nm; morphology was also confirmed by SAXS analysis. A novel polylactide-poly(methyl methacrylate)-polystyrene (PLA-PMMA-PS) triblock bottlebrush block copolymer was synthesized from a triblock backbone employing ATRP, RAFT and ROP techniques. Novel 5-TMS-4-pentyn-1-ol methacrylate (TPYM) with protected alkyne functionality was developed and characterized as a homopolymer. Click chemistry was used to introduce RAFT groups along the homopolymer backbone. Subsequently, PS branches were grafted from RAFT initiators along the backbone. A well-defined poly(SM-b-BIEM-b-TPYM) backbone was synthesized followed by grafting of PMMA brushes from the BIEM segment by ATRP.^Post PMMA grafting, the ATRP end groups were removed and the TMS group was deprotected to allow functionalization of the alkyne with an azido terminated RAFT agent by way of Huisgen 1,3-dipolar cycloaddition. From the RAFT sites, PS was grafted followed by deprotection of the hydroxyl functionality of the SM block. Resulting liberated hydroxyls were used to initiate PLA grafts, yielding the final well-defined triblock bottlebrush block copolymer (PLA-PMMA-PS).
Chapter 1 is the main introduction of this work and it features the two main concepts of this study. First living polymerisation techniques are introduced with a special focus into RAFT and ROP. Secondly solution self-assembly is briefly discussed. In Chapter 2 we describe the synthesis of an amphiphilic block copolymer where the two blocks are connected through a reversible bond. A Diels-Alder (DA) adduct consisted of a maleimide-furan pair was chosen as the reversible linker. The solution self-assembly of this polymer was studied by TEM and DLS giving rise to the unexpected formation of cylindrical micelles. In Chapter 3 the main objective was to synthesise new amphiphilic block copolymers without the DA motif in order to investigate their self-assembly behaviour compared to those for DA containing polymers obtained in Chapter 2. To further understand this self-assembly behaviour our method has been extended to the synthesis of other hydrophilic blocks and end group modified polymers. In addition, some key properties of the polymers synthesised have been investigated. In Chapter 4 our main goal is to understand the origins of the cylindrical micelle formation seen in Chapter 2. We investigated the aggregation behaviour under the aqueous thermal conditions in which the PTHPA block hydrolysis is performed. Studies at different concentrations and solvent mixtures provide valuable information regarding the self-assembly mechanism. In addition, the polymers with modified end groups and the triblock copolymers synthesised in Chapter 3 are studied and all the results compared. In Chapter 5 we explore the living crystallisation driven self-assembly of PLA-b- PAA block copolymers in aqueous media towards the formation of cylindrical micelles of controlled length. Interestingly, in many of the unstained TEM images presented in this work the particles demonstrate a non-uniform contrast along their width. This unexpected result is fully investigated in Chapter 6.