Miktoarm stars of poly(ethylene oxide) and poly(dimethylaminoethyl methacrylate): manipulation of micellization by temperature and light

Abstract

A novel method for preparation of miktoarm stars is presented, first employing Williamson ether synthesis with protected dipentaerythritol and preformed poly(ethylene oxide) (PEO) as reactants. This arm-first reaction gave, after modification, PEO macroinitiators with 4 or 6 initiation sites, which are located in the center of the main chain. The initiators were used for atom transfer radical polymerization of N,N-dimethylaminoethyl methacrylate (DMAEMA; core-first method). Heteroarm stars were obtained with two hydrophilic PEO chains and 4 or 3 stimuli responsive PDMAEMA chains respectively. Both polymers had almost the same molecular weights. The star-shaped polymers were analyzed by NMR, size exclusion chromatography SEC, osmometry and mass spectrometry. The micellization of the polymers was investigated by light scattering, fluorescence spectroscopy and cryogenic transmission electron microscopy. At the conditions used (0.1 g/L in pH 8 buffer), PDMAEMA turns hydrophobic around 55 °C, forming micelles at higher temperatures. At low temperature, trivalent counterions like hexacyanocobaltate(III) allow additional micellization of the weak polyelectrolyte PDMAEMA, with PEO as the solubilizing agent. For this unique behavior the notion “confused micellization” is introduced, which is in analogy to schizophrenic micelles. The morphology of the aggregates depends strongly on the macromolecular architecture, giving spherical micelles for the star with 4 shorter PDMAEMA arms and vesicles for the star with 3 longer arms. The diameter of the vesicles, varying between 200 nm and 4000 nm at 10 °C, can be tuned by the cooling rate. This ionically induced micellization can then be reversed by UV-illumination, leading to disaggregation upon a photoinduced valency change of the counterion.