Copper Catalysed Reactions for the Synthesis of Polymeric Architectures

Craig Bell (2011). Copper Catalysed Reactions for the Synthesis of Polymeric Architectures PhD Thesis, School of Chemistry & Molecular Bioscience, The University of Queensland.

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Author Craig Bell
Thesis Title Copper Catalysed Reactions for the Synthesis of Polymeric Architectures
School, Centre or Institute School of Chemistry & Molecular Bioscience
Institution The University of Queensland
Publication date 2011-08
Thesis type PhD Thesis
Supervisor Professor Michael Monteiro
A/Prof Lawrie Gahan
Total pages 256
Total colour pages 86
Total black and white pages 170
Language eng
Subjects 03 Chemical Sciences
Abstract/Summary Abstract The synthesis of well-defined polymeric architectures by copper mediated polymerisation and copper catalysed coupling is highly sought after amongst polymer chemists. Constructs of this design strategy have immediate applications in the biomedical and materials industry as both drug and gene carriers as well as viscosity modifiers. Therefore the main focus of this thesis was to understand the fundamental mechanisms of copper catalysed reactions in order to control the synthesis of well defined polymer chains and higher order complex polymeric architectures. A kinetic analysis of styrene polymerisations was initially conducted using the Cu(I)Br coordinated N3S3 catalyst, AMMe-N3S3sar. It was shown that using AMMe-N3S3sar with Cu(I)Br in toluene did not effectively control the polymerisation of styrene (STY) at 60 or 100 oC but only acted as an initiation source as deactivation by Cu(II)Br2/AMMe-N3S3sar was found to be negligible. This was reaffirmed with evidence of the order of reaction for the catalytic species, where the order for Cu(I)Br = L = 0.5 and, notably, Cu(II)Br2 having an order equal to 0. From inference and literature it was proposed that the Cu(I) species was encapsulated within the cage of the AMMe-N3S3sar and would most probably result in activating the initiator or polymeric end-group halides through an OSET mechanism. This system was then used to produce multiblocks from a difunctional starting block copolymer (Br-PSTY-Br). However, the rate of multiblock formation significantly increased in the presence of DMSO, suggesting that DMSO stabilises the radical-anion intermediate in an OSET process. Furthermore it was found that disproportionation of Cu(I)Br/AMMe-N3S3sar complex did not occur in DMSO, and as such use of this ligand could not produce polymer with well-defined and narrow MWDs. Since DMSO was thought to stabilise the radical-anion intermediate, a series of methyl acrylate polymerisations were run in DMSO at 25 oC with Cu(I)Br and AMMe-N3S3sar. Once again this catalyst combination did not control the polymerisation and only acted as an initiation source. The orders of reaction were analysed for this catalyst and showed similar results as for the styrene system with Cu(I)Br = 0.5 and Cu(II)Br2 = 0. The exception was the order for L = -2, suggesting a rate retardation. Analysis of the electrochemistry showed that the redox potential (E½ = -0.186 V) is comparative to a highly activating ligand, Me6TREN (E½ = -0.278 V) but electrochemical kinetic analysis with an alkyl halide, EBiB, as well as the molecular weight data suggest that Cu(I)Br/AMMe-N3S3sar is a slow initiating catalyst. Using this effect, as well as the unique ability for this redox couple to initiate at low temperatures, a series of RAFT mediated polymerisations were run under the same conditions with the result of controlled (linear) growth of molecular weight with conversion as well as low polydispersity (< 1.1). As the macrobicyclic ligand could not provide control in an Cu(I) mediated OSET polymerisation, the highly activating ligand Me6TREN was then considered. This ligand has been effectively used previously for the Cu(0) mediated polymerisation of methyl acrylate in DMSO because of its ability to stabilise the Cu(II) complex and promote disproportionation. The polymerisation kinetics of methyl acrylate in DMSO were assessed, using two initiation methods; addition of Cu(I)Br last, and addition of a degassed MBP solution last whereby Cu(I)Br/Me6TREN was allowed to stir in solution for 1 hour prior to initiation. The polymerisation kinetics showed differences in apparent propagation rates and conversions when either Cu(I)Br or MBP was added last. This led us to infer, based on literature reports, that disproportionation plays a critical role in polymerisations conducted in DMSO, with both an elevated kact for Cu(I)Br and the formation of Cu(0) elevating the rate of activation for alkyl halide initiators and polymer chain ends. Lower concentrations of Cu(I)Br (0.1 – 0.2 equivalents to initiator) were shown to control the polymerisation of MA to form well defined polymers with control of the molecular weight distribution and low polydispersity. UV-Vis analysis showed a saturation concentration of Cu(II)Br2/Me6TREN in these polymerisations. In DMSO, the saturation concentration is still high, however control is still lost at higher Cu(I)Br concentrations due to the high activation rate. Despite this loss of control, this system shows potential in controlling “click” type radical coupling reactions for polymeric architecture construction. Copper catalysed azide-alkyne coupling (CuAAC) chemistry has been previously explored as a technique for higher order polymeric architecture synthesis. However there have been virtually no kinetic analyses conducted for polymeric coupling rates. Using linear PSTY with azide and alkyne functionalities as a model system, it was shown that the ligand, solvent and copper combination had a significant effect on the rate of CuAAC reaction. A rapid rate of CuAAC was found using Cu(I)Br/PMDETA in toluene, reaching near full conversion after 15 min at 25 oC. For the same catalyst system, DMF also showed fast rates of ‘click’ with 95% conversion in 25 min. Copper wire was also shown to mediate CuAAC in these two solvents, and in toluene gave a conversion of 98% after 600 min which was much faster than that observed in DMF. When the PSTY chain had an incorporated (or chemically bound) triazole ring close to the site of reaction, the rate of CuAAC in toluene increased significantly, reaching 97 % in 180 min at 25 oC. These results showed that the catalytic activity can be modulated by a choice of solvent, ligand and copper source combinations for CuAAC reactions and that the choice of these reagents can be crucial for achieving high yielding reactions. Finally, polymeric dendrimers were then constructed in one-pot via a divergent, convergent or parallel sequence, modulated by the copper catalyst activity. This approach reduced the number of purification and chemical protection steps, allowing the production of 3rd generation dendrimers in one-pot at 25 oC. Using the model PSTY building block system, 3rd generation dendrimers could be formed divergently, convergently or in parallel by modulating the Cu(I) activity for CuAAC and nitroxide radical coupling (NRC) reactions. The parallel approach was found to be the fastest generating a G3 dendrimer (< 30 min), followed by the divergent pathway (30 min), and finally the very slow convergent pathway (24 h). This G3 dendrimer was cleaved at the alkoxyamine sites back to linear polymers through heating at 120 oC in the presence of an excess hydroxyl-nitroxide. The utility of this new approach allowed us to produce 3rd generational layered dendrimers, consisting of a wide range of chemically different polymer building blocks, in one-pot at 25 oC in less than 30 min using solvent and ligand conditions to facilitate the parallel process. The range of dendrimers was formed with high efficiencies through coupling linear telechelic polymer building blocks, consisting of PSTY, PtBA, PEG and PNIPAM, and subsequent purification by preparative SEC to fractionate starting reactants and intermediate polymer species.
Keyword Atom Transfer Radical Polymerisation
Single Electron Transfer-Living Radical Polymerisation
CuAAC “Click” Chemistry
Nitroxide Radical Coupling
Additional Notes Colour pages (of the PDF): 34,36,39-41,45-46,51,54,57,78-80,82,88,95,112,115,127-129,135,138,148-149,151-153,155,157,159-160,172-173,180-183,185,188-192,198-203,205-209,214,216-226,228,231,234,237,239-245,247-254 Landscape pages (of the PDF): 158

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Created: Sun, 04 Dec 2011, 18:35:53 EST by Mr Craig Bell on behalf of Library - Information Access Service