Nitroxide-Mediated Polymerization-Induced Self-Assembly of Poly(poly(ethylene oxide) methyl ether methacrylate-co-styrene)-b-poly(n-butyl methacrylate-co-styrene) Amphiphilic Block Copolymers

Well-defined water-soluble brush-type homopolymers and copolymers entirely composed of poly(ethylene)oxide methacrylate (PEOMA) units with PEO side groups of various chain lengths (Mn = 300 and 950 g mol–1) were synthesized by nitroxide-mediated polymerization (NMP) using a low molar mass unimolecular alkoxyamine initiator (so-called BlocBuilder) and N-tert-butyl-N-(1-diethyl phosphono-2,2-dimethyl propyl) nitroxide (SG1) in the presence of a small amount of styrene. The resulting SG1-capped macroalkoxyamines possessed the predicted molar masses based on the monomer/initiator molar ratio and narrow molar mass distributions. They were then employed to initiate the emulsion polymerization of n-butyl methacrylate with a low amount of styrene under mild conditions (85 °C) leading to amphiphilic block copolymers that in situ self-assembled into sterically stabilized particles. Kinetics study confirmed the living character of the polymerization, while SEC analysis indicated the presence of a small fraction of dead chains formed by irreversible homotermination reactions. Spherical latex particles with diameters comprised between 60 and 300 nm were obtained at pH 4.2 when decreasing the macroinitiator concentration from 7.2 to 1.2 mM in agreement with the in situ formation of block copolymers. As expected, only the macroinitiators with long PEO side groups, and whose cloud points were above the reaction temperature, led to stable latex suspensions. A drastic change in particle morphology with the formation of vesicles and anisotropic objects was observed when increasing the suspension pH from 4.2 to 6.7. As the macroalkoxyamine initiator contained only one terminal methacrylic acid unit, such a strong effect of pH on particles morphology was not expected. The observed results were interpreted in terms of a salting out effect induced by the concomitant increase of ionic strength upon neutralization of the alkoxyamine initiator. Hence, particle morphologies could be tuned from spherical to elongated micelles and vesicles by the addition of increasing amounts of sodium chloride at a fixed pH of 4.2, which confirmed the above hypothesis.