ABSTRACT:The subject of this review is to present new synthetic methodologies recently developed by us, which are applicable to both regular and asymmetric star polymers with well-defined architectures. The first methodology involves the coupling reaction of a variety of living anionic polymers of styrene, α-methylstyrene, isoprene, tert-butyl methacrylate, and ethylene oxide with novel chain-end-and in-chain-functionalized polymers with a definite number of benzyl halide moieties intentionally designed as polymeric coupling agents. In the second methodology, we propose a new concept based on iterative approach, with which star polymers can be successively and, in principle, unlimitedly synthesized by repeating the iterative reaction sequence. Finally, a convenient synthesis of densely branched polymers with starlike structures is presented by the quantitative coupling reaction of living anionic polymers with reactive benzyl halide-functionalized backbone polymers based on a grafting-onto method.KEY WORDS Star Polymer / Asymmetric Star Polymer / Living Anionic Polymerization / Iterative Methodology / Polymeric Coupling Agent / 1,1-Diphenylethylene Derivative / Star polymers are branched polymers in which more than three linear polymer chains are linked together at one end of each chain by a central core or a single branch. They have physical properties in bulk, melt, and solution quite distinct from linear analogues. For this reason, star polymers have been widely studied from both synthetic and theoretical points of view. [1][2][3][4][5][6][7][8][9] It is of course essential to use star polymers with welldefined structures for evaluating fundamental understanding regarding the effect of chain branching on such physical properties.At the present time, the most successful methodologies for the synthesis well-defined regular star polymers have been developed mainly based on coupling reactions of living anionic polymers of styrene and 1,3-dinene monomers with multifunctional chlorosilane compounds as electrophilic coupling agents. A variety of star polymers with a definite number of three to eighteen arms have been synthesized. [10][11][12][13][14][15][16][17][18][19] Furthermore, the successful syntheses of the 32-, 64-, and even 128-arm star polymers by using specially designed carbosilane dendrimers have been reported so far. 20,21 Currently, great attention has been paid to more complex asymmetric star polymers whose arms differ in molecular weight and chemical composition, since star polymers of this type are expected to exhibit interesting and unique physical performance originated from their † To whom correspondence should be addressed.branched architectures as well as heterophase structures. [22][23][24][25][26][27][28][29][30][31][32][33] Synthesis of asymmetric star polymers is generally more difficult than that of regular star polymers. In the synthesis of regular star polymers, for example, their arms can be simultaneously introduced by one reaction. On the other hand, two or more highyielding reactions...

To synthesize star-branched polymers, we have developed a new methodology of using polymer anions (M w = ca. 10 kg/mol) consisting of the same or different two polymer chains in the coupling reaction of chain-end-multifunctionalized polystyrenes with 4, 8, and 16 benzyl bromide moieties. The coupling reactions efficiently proceeded to afford 9-, 17-, and 33-arm regular star-branched polystyrenes as well as A5B4, A9B8, and A17B16 asymmetric stars consisting of polystyrene (A) and poly(4-trimethylsilylstyrene) (B) segments in greater than 85% isolated yields. However, the yields of the coupling reactions were high but not quantitative when either chain-end-functionalized polystyrene with 32 benzyl bromide moieties or a high molecular weight polymer anion (M w = 20.0 kg/mol) was used.

Well‐defined five‐arm star polymers, having different arms in molecular weight or composition, were synthesized by linking reactions of end‐functionalized polystyrenes with two chloromethylphenyl (CMP) groups and polymer anions consisting of two identical or different polymer chains. The polymer anions were prepared by coupling living anionic polymers of styrene or isoprene with 1,1‐diphenylethylene (DPE)‐end‐functionalized polymers. They were then reacted in situ with the CMP‐end‐functionalized polystyrenes to afford heteroarm star polymers of the AA′2A″2, AA′2B2, and AB2C2 types where the A, B, and C segments were polystyrene, poly(a‐methylstyrene), and polyisoprene, respectively. 1H NMR spectroscopy, SEC, vapor pressure osmometry (VPO), and static light scattering measurements evaluated the well‐defined architecture of these polymers.

The synthesis of well-defined three-and five-arm star polymers, with varying arm lengths, was described by the method based on anionic living polymerization. A three-arm AB 2 star polymer was synthesized by the linking reaction of a specially designed two chloromethylphenyl-end-functionalized polystyrene prepolymer with the anionic living polymer of t-butyl methacrylate initiated with diphenylmethylpotassium. A five-arm AA' 2 B 2 star polymer was synthesized by the linking reaction of the same functionalized polystyrene having two chloromethylphenyl termini with the block copolymeric anion prepared from the coupling reaction of polystyryllithium with 1,1-diphenylethylene-end-functionalized poly(a-methylstyrene). Both star polymers were obtained in more than 95% yields and were found to possess well-defined structures in terms of molecular weight, molecular weight distribution, arm number, and composition.