Synthesis and characterization of star-shaped Poly(N,N-dimethylaminoethyl methacrylate) and its quaternized ammonium salts

We report on the synthesis and characterization of star-shaped strong polyelectrolytes and their precursor stars with up to 24 arms. To achieve this we polymerized 2-(N,N-dimethylamino)ethyl methacrylate (DMAEMA) by atom transfer radical polymerization employing a core-first attempt. Sugar-based scaffolds as well as silsesquioxane nanoparticles were used as oligofunctional initiators. Subsequent quaternization of the obtained poly(DMAEMA) stars yielded star-shaped poly{[2-(methacryloyloxy)ethyl] trimethylammonium iodide} (PMETAI). The initiation site efficiency was determined both by molecular weight measurements of the cleaved arms and by a statistical method after partial destruction of the inorganic core. The rather low efficiency of the initiation sites (30-75%) leads to a moderate arm number distribution of the prepared polyelectrolyte stars. As expected, the hydrodynamic radii of these polyelectrolyte stars decrease with increasing ionic strength. However, if the ionic strength was adjusted with NaI instead of NaCl, pronounced ion-specific effects were observed; the star polyelectrolyte first strongly shrinks with increasing salt concentration and becomes insoluble at about 0.5 M NaI ("salting out"). Still higher concentrations of NaI lead to a redissolution and a reswelling of the star polyelectrolyte ("salting in"). The measured osmotic coefficients are low and decrease with increasing arm number from phi similar to 0.12 for a 3-arm star down to phi similar to 0.04 for an 18-arm star, confirming the expected strong counterion confinement within these objects with high charge density.