Arborescent Graft Copolymers:  Highly Branched Macromolecules with a Core-Shell Morphology

Abstract

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 M̄ w = 7 × 105 g·mol-1 containing 19% and 66% poly(ethylene oxide) by weight were prepared, with apparent polydispersities M̄ w/M̄ n ≈ 1.1−1.2. Another sample incorporating a G = 4 core with M̄ w of ∼108 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 M̄ 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.

Phase-Segregated Dendrigraft Copolymer Architectures

“…The first example of an arborescent copolymer structure with a clearly heterogeneous morphology, published in 1996, was for polymers incorporating an arborescent polystyrene core grafted with poly(ethylene oxide) (PEO) segments at the chain termini [20]. The synthesis of micelle-like core-shell macromolecules with different core sizes (generations) and varying PEO contents or shell thicknesses was demonstrated.…”

Arborescent polymers and other dendrigraft polymers: A journey into structural diversity

“…2(b)]. The synthesis of arborescent copolymers containing either short or long corona side chains was demonstrated initially for arborescent polystyrene substrates terminally grafted with poly(ethylene oxide) (PEO) segments, 13 and for copolymers obtained by grafting polyisoprene 14 and poly(2-vinylpyridine) 15 side chains randomly onto chloromethylated arborescent polystyrene substrates. Coupling reactions of reactive macroanions with chloromethylated polystyrene substrates are clean and proceed in high yield, but the chloromethylation procedure suffers from serious limitations.…”

Interfacial properties of amphiphilic dendritic polymers

“…The 'grafting from' technique also served in the synthesis of arborescent polystyrene-graft-poly(ethylene oxide) copolymers, whereby poly(ethylene oxide) chains were grown from alcoholate groups attached at the chain termini of the arborescent polystyrene substrates. 50 A similar technique was employed by Taton et al 51 to synthesize a dendritic graft copolymer by anionic growth of 12 outer poly(ethylene oxide) chains from a hexaarm polystyrene star core. Inverse dendrigraft micelles have also been derived 52 from triand tetraarm star-branched poly(ethylene oxide) substrates.…”