Arborescent graft copolymers: Highly branched macromolecules with a core-shell morphology

Macromolecules incorporating a highly branched polystyrene core and a poly(ethylene oxide) shell were synthesized. A comb-branched (generation G = 0) polystyrene was prepared by initiating the polymerization of styrene with sec-butyllithium, capping with 1,1-diphenylethylene, and titrating the living anions with a solution of chloromethylated linear polystyrene. A twice-grafted (G = 1) core with protected hydroxyl end groups was obtained using (6-lithiohexyl)acetaldehyde acetal to initiate the polymerization of styrene, followed by capping and grafting on the chloromethylated comb polymer. The acetal functionalities were hydrolyzed, and the core was titrated in solution with potassium naphthalide, before adding ethylene oxide. To maintain a narrow apparent molecular weight distribution, it was necessary to eliminate residual chloromethyl sites by a metal-halogen exchange reaction, prior to shell growth. Core-shell polymers based on a G = 1 core with (M) over bar(w) = 7 x 10(5) g . mol(-1) containing 19% and 66% poly(ethylene oxide) by weight were prepared, with apparent polydispersities (M) over bar(w)/(M) over bar n approximate to 1.1-1.2. Another sample incorporating a G = 4 core with (M) over bar w of similar to 10(8) g . mol(-1) containing 36% poly(ethylene oxide) by weight was also synthesized. The hydrodynamic radii of the core and core-shell polymers were determined by dynamic light scattering. Based on the (M) over bar w estimated for the poly(ethylene oxide) chains, the hydrophilic chains exist in a randomly coiled conformation. The solubility behavior of the macromolecules is consistent with a core-shell morphology: the amphiphilic copolymers are easily desolvated from tetrahydrofuran solutions, giving transparent dispersions in water or methanol.