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Surface modification with polymers using living radical polymerisation and click chemistry

By Gaojian Chen


Thin organic and polymer layers on solid substrates play a key role in many processes\ud aimed at modifying surface properties. Both "grafting to" and "grafting from"\ud methods have been used in this project to modify a variety of surfaces including\ud cellulose, resins and carbon nanotubes (CNT) with functional polymers. Living\ud radical polymerisation and Huisgen [2+3] cycloaddition (often termed "click"\ud reaction) were used to carry out these modifications.\ud Living radical polymerisation was first used to synthesize different α-functional\ud polymers and used for surface modification. For example, Living radical\ud polymerisations of methyl methacrylate and a fluorescent comonomer with 2-bromo-\ud 2-methyl-propionic acid 3-azido-propyl ester and 2-bromo-2-methyl-hept-6-yn-3-one\ud as initiators have been successfully employed for the synthesis of fluorescently tagged\ud azide and alkyne terminated PMMA with molecular weight (Mn) close to that\ud predicted and polydispersity index (PDi) less than 1.20 and good first order kinetics\ud that would be expected for living radical polymerisation. Cotton and organic resin\ud surfaces have been functionalised with alkyne groups using simple condensation with\ud 4-chlorocarbonyl-butyric acid prop-2-ynyl ester. The surfaces have been further\ud modifies using a Huisgen [2+3] cycloaddition (click) reaction of both polymeric and\ud small molecule azides. Different functional azides, namely mono azido-PEG and a\ud new fluorescent hostasol derivative have also been prepared and tested as model\ud substrates for cotton surface modification. This approach is shown to be very general\ud allowing soft and hard surfaces with different geometries to be modified. In particular\ud it is an excellent method to alter the nature of organic resins allowing the\ud incorporation of many different functionalities.\ud The covalent immobilization of a range of carbohydrate derivatives onto resin beads\ud was then carried out. Copper-catalysed Huisgen [2+3] cycloaddition was used to graft\ud mannose-containing azides to complementarily functionalised alkyne surfaces,\ud namely: a) Wang resin or b) "Rasta" particles consisting of a "clickable" alkyne\ud polymer loose outer shell and a Wang resin inner core. For the second approach,\ud Wang resin beads were first converted into immobilized ATRP initiators, and then\ud polymerisation of trimethylsilanyl-protected propargyl methacrylate followed by\ud deprotection with TBAF·3H2O afforded the desired polyalkyne clickable scaffold. An\ud appropriated α-mannopyranoside azide was then clicked onto it, to give a mannose\ud functionalized "Rasta" resin. The binding abilities of these D-mannose-modified\ud particles were then tested using fluorescein labelled Concanavalin A (Con A), a lectin\ud known for its ability of binding certain mannose-containing molecules. Our\ud preliminary results indicated that the novel glycohybrid materials presented in this\ud work are able to efficiently recognize mannose-binding model lectins such as Con A,\ud opening the way for their potential application in affinity chromatography, sensors\ud and other protein recognition/separation fields.\ud Other functional polymers with antibiotic or chiral properties were also grafted from\ud surfaces. Living radical polymerisation of poly(ethylene glycol) methyl ether\ud methacrylate (PEGMA) and a metronidazole monomer (MTD-MA) has been\ud successfully employed for the synthesis of antibiotic metronidazole containing\ud polymers with Mn close to that predicted, narrow polydispersity and good first order\ud kinetics that would be expected for living radical polymerisation. Using the\ud monomers PEGMA and MTD-MA, with preformed immobilized initiator on cotton,\ud surface initiated LRP was carried out to give cotton bearing antibiotic polymers.\ud Surface initiated living radical polymerisation of GMA was then successfully carried\ud out for the synthesis of PGMA containing bead base on Aquagel resin. The hydroxyl\ud groups of the PGMA moiety were then reacted with a single enantiomer (R)-(+)-1-\ud phenylethyl isocyanate (EtPhNCO). This demonstrates a convenient way of\ud immobilise enantiomer moiety onto resin surface and the resulting solid support may\ud be used as chiral stationary phases (CSP) for HPLC chromatography.\ud To modify CNTs with functional polymers not only increase the dispersability of the\ud CNTs, it has also enlarged the application areas of CNT’s due to the polymers' own\ud functional properties. MWCNTs were first converted to a solid support LRP initiator\ud by an esterification reaction and styrene was grafted from MWCNTs through surfaceinitiated\ud LRP, the PSt modified CNTs were then used to form isoporous membranes.\ud Similarly, Poly(amidoamine) (PAMAM) dendrons were covalently attached to\ud MWCNTs and dendron-MWCNT-Ag(0) hybrid materials were made afterwards\ud which occurred via Ag(I) coordination to the PAMAM dendron nitrogen donors,\ud followed by reduction with formaldehyde. Finally, noncovalent method was used to\ud make a thermo-sensitive water soluble CNTs. The homopolymerisations and\ud copolymerisation of poly(ethylene glycol) methyl ether methacrylate (PEGMA) and\ud di(ethylene glycol) methyl ether methacrylate (DEGMA) using a pyrene-containing\ud initiator and a Cu(0)/Me6-Tren catalyst system was investigated. The pyrenefunctionalised\ud polymers synthesised were then used to modify CNTs and thus thermosensitive\ud water-dispersible CNTs were made

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