b S Supporting Information
' INTRODUCTIONStar-like polymers have attracted considerable attention over the past decade due to their unique solution and solid-state properties. The main feature of star-shaped polymers, differing from the linear analogues of identical molar mass, is their compact structure (i.e., smaller hydrodynamic volume and radius of gyration) and high concentration of functional terminal groups, which enable them with higher solubility in common solvents, lower solution and melt viscosities, and modified thermal properties. 1À5 As such, star-like polymers provide most of the properties of high molecular weight materials without the solution viscosity penalty of linear materials of similar molecular weight for potential applications in coatings, additives, drug and gene delivery, and supramolecular science. 6À10 Living polymerization, atom transfer radical polymerization (ATRP) and reversible additionÀfragmentation chain-transfer (RAFT) polymerization in particular, 4,11,12 has been extensively used for synthesis of star-like polymers through either a core-first method or an arm-first approach. 13À19 In the arm-first method, a living linear monofunctional macromolecule is initially synthesized. The star is then prepared either through the cross-linking by a difunctional comonomer during propagation 20 or by connecting a number of arms with a multifunctional terminating agent. 21 However, the number of arms in these star-like polymers cannot be precisely controlled. The resulting structures are not well-defined, and certain stringent experimental conditions may be required. 20,21 In stark contrast, in the core-first method, starlike polymers are produced with a multifunctional initiator to induce the growth of arms. It has been successfully implemented to achieve well-defined stars with a discrete number of arms. For example, several styrenic and (meth)acrylic star-like polymers have been prepared by living radical polymerization using the multifunctional core of initiators, such as cyclotriphosphazenes, cyclosiloxanes, and organic polyols. 22 The synthesis of star-like polystyrene by RAFT polymerization 23 and (tetramethylpiperidine-1-oxyl) (TEMPO)-mediated living radical polymerization have also been demonstrated. 24 However, it is noteworthy that these multiarm polymers were only star-like homopolymers, and the number of arms were rarely greater than four. 20 More importantly, limited work was reported on the preparation of star-like block copolymers 17 because it is difficult to purify star-like macroinitiators and grow the second block at the end of star-like first block.Amphiphilic linear block copolymers are well-known to selfassemble into micelles composed of hydrophobic core and hydrophilic shell in aqueous solutions. 25 These micelles known as "polymeric micelles" are expected to play an important role in the drug delivery, analytical chemistry, etc. 26,27 Self-assembled ABSTRACT: A series of novel amphiphilic multiarm, star-like block copolymers, poly(acrylic acid)-b-polystyrene (PA...
