Investigation of Copper Wire-Mediated Controlled/Living Radical Polymerization
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Copper wire mediated controlled living radical polymerization (CLRP) is a powerful tool that provides numerous opportunities for the development of new materials. It requires low catalyst loading and mild reaction conditions, but does involve use of organic solvents. There is a need for greener solvent systems to replace traditional volatile organic solvents without detrimentally affecting polymerization. The effects of greener solvents on control over polymerization and polymer characteristics were investigated with polyethylene glycol (PEG) and polypropylene glycol (PPG) for copper wire mediated CLRP. Polymerization of low molecular weight poly(methyl acrylate) (PMA) with bromine chain-end functionality was initiated by ethyl 2-bromoisobutyrate (EBiB). Two effective green solvent systems were established, 75-25 PEG-Ethanol and 75-25 PPG-Ethanol. Both provide good control over polymerization, livingness, narrow dispersity (~1.1), high retention of chain-end functionality (>90%), and can be recycled, which should be considered for an industrial scale implementation. Furthermore, utilization of a bromine chain-end functionality of synthetic homopolymer (PMA) synthesized via copper wire mediated CLRP with EBiB was evaluated for grafting to cellulose substrates, microcrystalline cellulose and cellulose acetate. Cellulose was functionalized with thiol groups using ring opening reaction with ethylene sulfide. The grafting via thio-bromo click reaction was investigated, where grafting of EBiB was performed first to evaluate the reaction and then repeated with PMA. One-pot and two-pot reactions were performed. In one-pot reaction, functionalization of cellulose and grafting were performed in the same reaction solution to avoid exposure of thiols to oxygen. In a two-pot reaction, functionalized cellulose was purified before grafting. Nuclear Magnetic Resonance analysis showed that grafting of EBiB had low degree of substitution (DS) of 0.026 (1 unit per 39 AGU units), whereas grafting of polymer chains to cellulose substrate was not successful.