Studies of a click reaction route to antimicrobial polymer latexes

The objective of this project was to prepare alkyne-functionalized polymer latexes using surfactant-free emulsion polymerization, and then functionalize these polymer latexes with three quaternary ammonium azides via Cu(I)-catalyzed azide/alkyne cycloaddition (CuAAC) in order to produce antimicrobial polymer latexes.Three quaternary ammonium azides with different linear alkyl chain lengths (C4, C8 and C12) were successfully synthesised in high yield (>70%) using established procedures, and their purity determined by elemental analysis, Fourier transform infrared (FTIR) spectroscopy and nuclear magnetic resonance (NMR) spectroscopy. Alkyne-functionalized polymer latexes were prepared via surfactant-free emulsion polymerization using 2,2?-azobis(2-methylpropionamidine)dihydrochloride (AIBA) as initiator, [2-(methaccryloyloxy)ethyl]trimethylammonium chloride (MATMAC) as cationic comonomer, propargyl methacrylate (PMA) to provide the alkyne groups, and for some latexes, ethylene glycol dimethacrylate (EGDMA) as crosslinking comonomer. The effects of temperature and the concentrations of AIBA, MATMAC, PMA and EGDMA on monomer conversion, the rate of polymerization, particle diameter and colloidal stability have been investigated. The studies showed that the very high rates of polymerization were due to high values of the number of radicals per particle (in the range 3-2300). The observations also determined that the reaction conditions required to produce small particles (diameter of 150-350 nm) of narrow size distribution were: 75 oC reaction temperature, AIBA at 0.2 wt% to the total mass of monomer, MATMAC level of ? 12 mol% to total monomer (including MATMAC), and EGDMA level of < 2.0 mol% to total monomer (excluding EGDMA). Three series of alkyne-functionalized polymer latexes have been synthesised using these conditions: non-crosslinked (NCL), crosslinked (CL) and core-shell (CS). All the latex particles were functionalized with the three quaternary ammonium azides by CuAAC. Zeta potential analysis, FTIR and Raman spectroscopy analysis confirmed the success of the click reactions. The quantitative analysis of FTIR and Raman spectra showed similar values of conversion of click reaction for both NCL and CL particles, indicating NCL and CL particles have similar swellability. The data also showed that significantly higher click reaction conversions were achieved for CS particles (around 60%) than for NCL/CL particles (less than 40%), which indicates that the click reaction only occurred at the surface of particles and that a higher proportion of alkyne groups are located on the surface of CS particles than on NCL/CL particles. The antimicrobial properties of all QAAs, MATMAC, NCL, CL and CS polymer latexes against E. coli bacteria (ATCC 25922) have been investigated using a modified liquid microdilution method in M9 medium, which was shown not to affect latex colloidal stability. It was found that all the polymer latexes showed much higher antimicrobial activities (MIC 6.5-75 �g ml-1) than many antimicrobial polymers reported recently in the literature (MIC 100-2000 �g ml-1); (Ganewatta, M.S. and C.B. Tang, Controlling macromolecular structures towards effective antimicrobial polymers. Polymer, 2015. 63: p. A1-A29). Polymer latexes with clicked-on QAAs showed significantly higher antimicrobial activities than the original latexes. The magnification of the increase in antimicrobial properties of CS particles after click reaction (~3.5 times) was greater than for NCL/CL particles (~2.5 times), showing that a larger amount of QAAs have been clicked onto the surface of CS particles than NCL/CL particles and that the clicked-on QAAs enhance the antimicrobial activity significantly.