Controlled radical polymerization (CRP) techniques have allowed great opportunities to synthesize materials with controlled molecular weights, compositions, and topologies. The polymeric materials discussed in this dissertation were prepared by capitalizing on the structural homogeneity afforded by CRP and the highly orthogonal and efficient nature of recently developed "click" chemistry reactions. By describing the synthesis and characterization of a variety of functional polymers with advanced macromolecular architectures, these results demonstrate the promise of combining these two general fields to yield previously inaccessible materials. A variety of materials, including macromonomers, hyperbranches, and hydrogels, were prepared by various combinations of copper-catalyzed azide-alkyne cycloaddition (CuAAC), atom transfer radical polymerization (ATRP), and reversible addition-fragmentation chain transfer (RAFT) polymerization.
Initial research involved the combination of ATRP and CuAAC to prepare well-defined ω-(meth)acryloyl macromonomers in a highly efficient manner. Poly(n-butyl acrylate) (PBA), polystyrene (PS), and PS- b-PBA were prepared by ATRP and subsequently derivatized to contain azido end-groups. The reaction of the azidoterminated polymers with alkyne-containing acrylate and methacrylate monomers resulted in near-quantitative chain end functionalization. Macromonomers of various molecular weights and compositions were prepared by this method. The end-group transformations required to incorporate the polymerizable functionality were accomplished either as a step-wise series of discrete reactions or as an in situ process, wherein azidation was immediately followed by azide-alkyne coupling in situ. In both cases, the degree of end-group functionalization generally exceeded 90%. In order to demonstrate polymerizability, examples of ω-methacryloyloxy-PBA and ω-acryloyloxy-PS macromonomers were homopolymerized by conventional radical polymerization. This versatile method of incorporating polymerizable end-groups from commercially available reagents should be applicable to a variety of (co)polymers accessible by ATRP.
To further explore click chemistry in conjunction with a newer and arguably more versatile CRP technique, two novel azido-functionalized chain transfer agents (CTAs) were prepared and employed to mediate the RAFT polymerizations of styrene and N,N-dimethylacrylamide (DMA). Control was obtained as evidenced by both RAFT polymerizations exhibiting pseudo-first order kinetics and a linear Mn dependence with conversion. The resulting homopolymers were demonstrated to have retained ω-end group functionality as evidenced by the successful formation of block copolymers. The α-azido terminal polymers and the azido functionalized CTAs were coupled by CuAAC to various alkynes in the presence of a Cu(I) catalyst, demonstrating the ability to prepare a range of functional telechelics and CTAs. One small molecule CTA prepared by this methodology contained both a trithiocarbonate and an acryloyl vinyl group. RAFT polymerization of styrene and N -isopropylacrylamide (NIPAM) in the presence of this compound that was capable of both reversible chain transfer and propagation led to highly branched copolymers by a method analogous to self-condensing vinyl copolymerization. Appropriate selection of the reaction stoichiometry allowed precise control over the distribution and length of branches in the resulting polymers. The degree of branching increased with CTA concentration, as proven by NMR spectroscopy, size exclusion chromatography, and viscometry. Retention of the thiocarbonylthio compound during the polymerization was evidenced by successful chain extension of a branched poly(N-isopropylacrylamide) (PNIPAM) macroCTA by RAFT polymerization of DMA. The branched polymers led to reduced lower critical solution temperatures as compared to linear PNIPAM, an effect attributed to an increased contribution of hydrophobic end groups. End group cleavage by radical-induced reduction resulted in an increased transition temperature more similar to that expected for linear PNIPAM. In a separate study, ABA triblock copolymers prepared by RAFT polymerization were capable of forming hydrogels that were both temperature- and redox-responsive. Temperature-responsive PNIPAM outer blocks allowed gelation at elevated temperatures, and the amine-labile trithiocarbonate in the center of the PDMA middle block allowed nucleophile and redox responsiveness.
|Advisor:||Sumerlin, Brent S.|
|Commitee:||Iovu-Stefan, Mihaela C., Lattman, Michael, Son, David Y., Wisian-Neilson, Patty J.|
|School:||Southern Methodist University|
|School Location:||United States -- Texas|
|Source:||DAI-B 70/08, Dissertation Abstracts International|
|Keywords:||Branched, Click chemistry, Controlled radical polymerization, Hydrogels, Macromolecules, Polymers, Raft|
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