Controlling polymer architectures : high-throughput experimentation, tailor-made macromolecules and glycopolymers via "click" reactions

Abstract

In nature, complex three-dimensionally ordered macromolecular architectures, such as proteins and DNA, can be found which are dependent on a high level of structural control in order to perform their desired biological tasks. Such systems are up to now not accessible by synthetic methods; however, in the last decades tremendous progress was made in the development of advanced living and controlled polymerization techniques. Besides, several outstanding organic reactions have been discovered and perfectionated with their easy experimental conditions and resulting high yields, which are categorized as click reactions. These techniques allow researchers to prepare well–defined tailor–made macromolecules with before not accessible control. However, in particular living and controlled polymerization techniques require a delicate selection of the appropriate catalyst, initiator, and solvent at a certain polymerization temperature and period for each type of monomer. Therefore, high–throughput experimentation (HTE) tools and techniques are required to screen the effect of reaction parameters in relatively short times. These polymerization techniques and the application of HTE in polymer science have been reviewed in the first chapter. A major part of this thesis deal with the optimization of not only controlled radical polymerization techniques but also cationic ring opening polymerization (CROP) process. Nitroxide mediated radical polymerization (NMP) of several monomers have been performed in an automated parallel synthesizer to obtain the most optimum reaction conditions in means of polydispersity indices, number average molar masses, monomer conversions as well as block copolymerization. We have used for this purpose a unimolecular nitroxide initiator (s-phosphonylated alkoxyamine, Bloc Builder) which has a relatively low decomposition temperature and provides good control over the polymerization progress. Some of the obtained polymer libraries were examined for their thermal properties and lower critical solution temperature behavior. The results of these experiments are discussed in detail in the second chapter. In the third chapter, we have focused on the reversible addition fragmentation chain transfer (RAFT) polymerization technique to synthesize methacrylic acid containing thermo-responsive copolymer libraries. These polymers have been prepared using a synthesis robot and also parallel characterization techniques were employed. Furthermore, water uptake properties of the hydrophilic polymers as well as thermo-responsive polymers have been investigated. It was demonstrated that responsive polymers behave hydrophilic below their LCST and hydrophobic above their LCST, thus exhibiting a reversible water uptake–release profile. Atom transfer radical polymerization (ATRP) is one of the most important controlled/living polymerization techniques which has attracted significant attention in many fields of chemistry. We have contributed for the further development by introducing a new tetradentate nitrogen based ligand for the ATRP of methyl methacrylate and styrene. The optimization results revealed that this ligand is suitable to conduct ATRP of methyl methacrylate (MMA) in the presence of Cu(I) and Cu(II) metal ions. Besides, this ligand has been used for the ATRP of styrene initiated from functionalized surfaces. Grafting from the surface resulted in the formation of polymer brushes with controlled lengths depending on the reaction time. Transformation of the polymerization mechanisms by post polymerization modifications or by using functional initiating/terminating agents have been of great interest to combine different classes of monomers on the same backbone. Therefore, we have employed for the first time a heterofunctional initiator for the ATRP of styrene and the CROP of 2-ethyl-2-oxazoline (EtOx) to synthesize amphiphilic block copolymers. Furthermore, we determined the optimum polymerization temperature for EtOx using acetyl halide type of initiators. These reactions have been performed systematically in a microwave synthesizer and the results have been discussed in the fifth chapter. Click reactions have been employed in many fields of chemistry since 2001. These efficient reactions attracted also polymer chemists to introduce functional end groups or side groups to well–defined polymers. Several different techniques have been published in the last eight years and we discussed critically the ones which do not require a metal catalyst during the reactions in the last chapter. Besides, we have introduced a metal-free click reaction between thiol and pentafluorophenyl groups to synthesize glycopolymers. For this purpose, fluorinated polymers have been prepared by NMP and were further functionalized using this new click chemistry route. In conclusion, this thesis provides new insights into the most important controlled radical polymerization techniques by utilizing in the automated parallel synthesis platforms and by the systematical preparation of copolymer libraries. The detailed characterization of these libraries provided fundamental knowledge on the structure-property relationships. Moreover, a new ligand for the ATRP of MMA and styrene, a new type of heterofunctional initiator for the combination of ATRP and CROP, and a new type of click reaction for the synthesis of glycopolymers have been introduced during this thesis. These new compounds and routes will be employed further for the preparation of tailor-made macromolecules to be used in specific applications.