Miktoarm Star Polymer

The synthesis of A2B miktoarm star polymers has been discussed and exemplified using PIB as a component. The synthesis involves a quasi living cationic polymerization of isobutylene from a monofunctional cationic initiator. This initiator also contains a blocked hydroxyl group. Eventually, the blocked hydroxyl group of the initiator is deblocked, and functionalized with a branching agent. This activated reagent is then used for an atom transfer radical polymerization process of /erf-butyl acrylate (18). 

L.K. Breland and R.F. Storey, Polyisobutylene-based miktoarm star polymers via a combination of carbocationic and atom transfer radical polymerizations, Polymer, 49(5) 1154-1163, March 2008. 

The recent development of living cationic polymerization systems has opened the way to the preparation of rather well defined star homopolymers and miktoarm star polymers [19 and see the chapter in this volume]. Divinyl ether compounds were used as linking agents in a manner similar to the DVB method for anionic polymerization. Typically the method involves the reaction of living polymer chains with a small amount of the divinyl compound. A star polymer is formed carrying at the core active sites capable of initiating the polymerization of a new monomer. Consequently a miktoarm star copolymer of the type AnBn is produced. 

Much experimental work has appeared in the literature concerning the microphase separation of miktoarm star polymers. The issue of interest is the influence of the branched architectures on the microdomain morphology and on the static and dynamic characteristics of the order-disorder transition, the ultimate goal being the understanding of the structure-properties relation for these complex materials in order to design polymers for special applications. 

The term miktoarm (from the greek word fu/crog, meaning mixed), or heteroarm star polymers, refers to stars consisting of chemically different arms. In the past decade considerable effort has been made toward the synthesis of miktoarm stars, when it was realized that these structures exhibit very interesting properties.88-90 The synthesis of the miktoarm star polymers can be accomplished by methods similar to those reported for the synthesis of asymmetric stars. The chlorosilane, DVB, and DPE derivative methods have been successfully employed in this case. Furthermore, several other individual methods have appeared in the literature. The most common examples of miktoarm stars are the A2B, A3B, A2B2, kn >n (n > 2) and ABC types. Other less common structures, like the ABCD, AB5, and AB2C2 are now also available. 

Vora A, Singh K, Webste DC (2009) A new approach to 3-miktoarm star polymers using a combination of reversible addition-fragmentation chain transfer(RAFT) and ting opening polymerization (ROP) via/click chemistiy. Polymta 15(13) 2768-2774... 

In a similar convergent strategy, the Monteiro group have demonstrated the preparation of well-defined, three-miktoarm star polymers using a combination of ATRP and CuAAC [108]. As discussed previously, polymers prepared via ATRP can be easily transformed by a nucleophihc displacement of the terminal halides with sodium azide to yield cUck -functional materials. In this way, PS, poly(t-butyl acrylate) (PtBA), and poly(methyl acrylate) (PMA), all of which bore a terminal azide... 

The latter material was then added to a solution of the second azide-terminated polymer to produce three-miktoarm star polymers in excellent yields. 

Amphiphilic star polymers were also prepared using this approach via the post modification of miktoarm star polymers containing PtBA arms. The subsequent hydrolysis of PtBA with trifluoroacetic acid (TFA) afforded three-miktoarm stars that each included one hydrophobic arm and two hydrophilic poly(acryUc acid) arms (Scheme 30.13). 

Scheme 30.13 Synthesis of well-defined three-miktoarm star polymers using a combination of ATRP and CuAAC click" coupling. Amphiphilic star polymers were prepared that contain one hydrophobic arm and two hydrophilic poly(acrylic acid) arms. Reproduced with permission from Ref. [108] 2006, American Chemical Society.

The radical-to-anion transformation was further merged with click chemistry for the synthesis of ABC-type miktoarm star polymers. For this purpose, a trifimctional initiator, namely, propargyl 2-hydroxylmethyl-2-(a-... 

Fragouli, P. latrou, H. Hajichristidis, N. Sakurai, T. Matsunaga, Y. Hirao, A., Synthesis of Well-Defined Miktoarm Star Polymers of Poly(dimethylsiloxane) by the Combination of Chlorosilane and Benzyl Chloride Linking Chemistry. 

Analysis of miktoarm star polymers. PS eluted in SEC mode, PI eluted in adsorption mode. 

It is desired to synthesize (a) AA -type triarm asymmetric polystyrene (PSt) stars with asymmetry in the molar mass of their branches, (b) AB2-type miktoarm star polymer core-(PSt)(PtBA)2 (where PtBA = poly(ten-butyl acrylate)), and (c) amphiphilic core-(PSt)(PAA)2 (where PAA = poly(acrylic acid)). Suggest a methodology to synthesize these polymers entirely by ATRP processes. 

Khanna K, Varshney S, Kakkar A. Miktoarm star polymers advances in synthesis, self-assembly, and applications. Polym Chem. 2010 1 1171-85. 

SoUman GM, Sharma R, Choi AO, Varshney SK, Winnik FM, Kakkar AK, Maysinger D. Tailoring the efiBcacy of nimodipine drug delivery using nanocarriers based on A2B miktoarm star polymers. Biomaterials. 2010 31 8382-92. 

Zhu Y, Storey RF. Synthesis of polyisobutylene-based miktoarm star polymers from a dica-tionic monoradical dual initiator. Macromolecules. 2012 45 5347-57. 

Ito S, Goseki R, Ishizone T, Senda S, Hirao A. Successive synthesis of miktoarm star polymers having up to seven arms by a new iterative methodology based on living anionic polymerization using a trifunctional lithium reagent. Macromolecules. 2013 46 819-27. 

Burts AO, Gao AX, Johnson JA. Brush-first synthesis of core-photodegradable miktoarm star polymers via ROMP towards photoresponsive self-assemblies. Macromol Rapid Commun. 2014 35 168-73. 

Scheme 29.10 Synthetic strategy to synthesizing amphiphilic ferrocene-containing block copolymers (PVFc-BGE-PEO) and ABj miktoarm star polymers (PVFc-(PEO)2).

Gao H, Matyjaszewski K. Synthesis of miktoarm star polymers via ATRP using the in-out method determination of initiation efficiency of star macroinitiators. Macromolecules 2006 39 7216-7223. 

Keywords. Anionic polymerization. Living anionic polymerization, 1,1-Diphenylalkyl-lithiums. Functionalized polymers. Block copolymers. Macromonomers, Star-branched polymers. Dilithium initiators. Trilithium initiators. Multifunctional initiators. Living linking reactions. Heteroarm star polymers, Miktoarm star polymers... 

Ohishi et al. have reported homopolymer-arm, block-arm, and miktoarm star polymers consisting of poly(N-octylm-benzamide) and poly(N-H-m-benzamide) by means of a core cross-linking method [74]. The H-NMR spectra of the star polymers in DMSO revealed that the poly(N-octyl-m-benzamide) segments and arms of the block-arm and miktoarm star polymers, respectively. Star 2 and star 3 before deprotection showed rather similar solubility, and they were soluble in many kinds of organic solvents, except for alcohols. After deprotection, star 4 was only soluble in polar aprotic solvents, such as DMF and DMSO, whereas starS was soluble in dichloromethane (E>CM), CHCI3, and DMF and insoluble in DMSO. Hence, the character of the external segment of the arms dominates the solubility and aggregability of the block-arm star polymers. 

Recent Synthetic Developments in Miktoarm Star Polymers with More than Three Different Arms... 

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