Heteroarm polymer

Miktoarm star (or p-star) polymers are star polymers with branches of different polymers. These polymers are also called heteroarm or mixed-arm star polymers. Their synthesis is more difficult but success has been achieved by sequential coupling [Hadjichristidis, et al., 1999]. To obtain a 3-arm star with one polyisoprenyl (PI) and two polystyryl (PS) branches, polyisoprenyllithium is reacted with an excess of CH3SiCl3 to form the one-arm polymer, the unreacted CH3SiCl3 is removed, and then polystyryllithium is added ... 

This review covered recent developments in the synthesis of branched (star, comb, graft, and hyperbranched) polymers by cationic polymerization. It should be noted that although current examples in some areas may be limited, the general synthetic strategies presented could be extended to other monomers, initiating systems etc. Particularly promising areas to obtain materials formerly unavailable by conventional techniques are heteroarm star-block copolymers and hyperbranched polymers. Even without further examples the number and variety of well-defined branched polymers obtained by cationic polymerization should convince the reader that cationic polymerization has become one of the most important methods in branched polymer synthesis in terms of scope, versatility, and utility. 

For quite some time, there have been indications for a phase-separation in the shell of polyelectrolyte block copolymer micelles. Electrophoretic mobility measurements on PS-PMAc [50] indicated that a part of the shell exhibits a considerable higher ionic strength than the surrounding medium. This had been corroborated by fluorescence studies on PS-PMAc [51-53] and PS-P2VP-heteroarm star polymers [54]. According to the steady-state fluorescence and anisotropy decays of fluorophores attached to the ends of the PMAc-blocks, a certain fraction of the fluorophores (probably those on the blocks that were folded back to the core/shell interface) monitored a lower polarity of the environment. Their mobility was substantially restricted. It thus seemed as if the polyelectrolyte corona was phase separated into a dense interior part and a dilute outer part. Further experimental evidence for the existence of a dense interior corona domain has been found in an NMR/SANS-study on poly(methylmethacrylate-fr-acrylic acid) (PMMA-PAAc) micelles [55]. 

By the use of the polymer-linking method with 20a, a variety of starshaped poly(vinyl ethers) have been synthesized (Scheme 12) [208-212]. A focus of these syntheses is to introduce polar functional groups, such as hydroxyl and carboxyl, into the multiarmed architectures. These functionalized star polymers include star block (23a,23b) [209,210], heteroarm (24) [211], and core-functionalized (25) [212] star polymers. Scheme 12 also shows the route for the amphiphilic star block polymers (23b) where each arm consists of an AB-block copolymer of 1BVE and HOVE [209] or a vinyl ether with a pendant carboxyl group [210], Thus, this is an expanded version of triarmed and tetraarmed amphiphilic block copolymers obtained by the multifunctional initiation (Section VI.B.2) and the multifunctional termination (Section VI.B.3). Note that, as in the previously discussed cases, the hydrophilic arm segments may be placed either the inner or the outer layers of the arms. 

Similar host-guest interactions are found not only with the amphiphilic star block copolymers [210,220] but also with the corresponding heteroarm [211] and core-functionalized [212] versions. Overall, these starshaped polymers induce the interaction more efficiently than their linear counterparts [220],... 

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. 

From a practical point of view, the effective molecular weight distribution of an anionically prepared polymer can be broadened by reaction with less than a stoichiometric amount of a linking agent such as silicon tetrachloride [239]. This results in a product mixture composed of unlinked arm, coupled product, three-arm, and four-arm star-branched polymers. Heteroarm star-branched polymers can be formed by coupling of a mixture of polymeric organolithium chains that have different compositions and molecular weights. This mixture can be produced by the sequential addition of initiator as well as monomers [3, 259, 260]. 

Quirk RP, Yoo T, Lee BJ (1994) Anionic synthesis of heteroarm, star-branched polymers -scope and limitations. J Macromol Sci Pure Appl Chem A31 911-926... 

Miktoarm polymers are essentially heteroarm star polymers where two or more arms of the star are chemically unique. Therefore, the same general approaches for the synthesis of star polymers also apply to miktoarms, with some additional constraints. Many research groups have sequentially performed orthogonal polymerization techniques to access a variety ABC and ABCD miktoarm polymers, in a core-first approach. 

Finally the morphologies of ternary linear block copolymers and heteroarm star terpolymers are compared. Here systems containing I or B are also compared, since these two polymers show rather similar interactions toward the other blocks (see Table 2 for solubility parameters). Figure 22 shows in the left column symmetrically composed SIM heteroarm star terpol5uners (134) and linear BSM and SBM triblock copolymers (76). In the right column corresponding SBV (135) and SIV (144) heteroarm star terpolymers are compared with their linear compositional... 

Hizal and coworkers [Durmaz, H., Karatas, E, Tunca, U., and Flizal, G., J. Polym. Sci., Part A Polym. Chem., 44, 499 (2006)] synthesized a compound, (XXI) (Fig. 12.22), having an anthracene functionality as also a NMP site and an ATRP site. Show how through combination of the DA click reaction, ATRP, and NMP, a heteroarm H-shaped terpolymer (PSt)(PrBA)-fc/7-(PEO)-fc/7-(PrBA)(PSt), containing PEG as a backbone and PSt and PtBA as side arms via a branch point (bp) at either end of PEO [where PSt is polystyrene, PtBA is poly(tert-butyl acrylate), and PEG is poly(ethylene oxide)] can be synthesized. 

It is interesting to compare monolayers from PS-PEO block copolymers formed by linear and heteroarm star polymers, where the polymer architecture plays a significant role in the morphology of the interfacial film. In particular, starshaped polymers tend to build well-defined circular domains after transfer to solid... 

Heteroarm or miktoarm star copolymers have attracted considerable attention in recent years due to the imique properties of these polymers, for example, they exhibit dramatic difference in morphology and solution properties. In comparison with the linear block and star-block copolymers, the s)mthesis of heteroarm or miktoarm star copolymers has been one of the more challenging projects available. A typical example is that the synthesis of the heteroarm H-shaped terpolymers, [(PLLA)(PS)]-PEO-[(PS)(PLLA)], in which PEO acts as a main chain and PS and PLLA as side arms (Fig. 4.11). The copolymers have been successfully prepared via combination of reversible addition-fragmentation transfer (RAFT) polymerization and ring-opening polymerization (ROP) by Han and Pan [166]. Another interesting example is that Pan et al. [167] successfully... 

Xu, J. and Zubarev, E.R. (2004) Synthesis of heteroarm star-shaped amphiphiles with 12 alternating arms ofpolybntadieneandpoly(ethylene oxide). ACSPo/yw. Prepr. (Div. Polym. Chem.), 45(1), 762-763. 

Peleshanko, S., Jeong, J., Gunawidjaja, R. and Tsukruk, V.V. (2004) Amphiphilic heteroarm PEO-b-PS, star polymers at the air-water interface aggregation and surface morphology. Macromolecules, 37,6511 522. 

Stepanek M, Matejicek P, Humpolickova J, Havrankova J, Podhajecka K, Spirkova M, Tuzar Z, Tsitsilianis C, Prochazka K (2005) New insights on the solution behavior and self-assembly of polystyrene/poly(2-vinylpyridine) hairy heteroarm star copolymers with highly asymmetric arms in polar organic and aqueous media. Polymer 46(23) 10493-10505. doi 10.1016/j.polymer.2005.08.031... 

The use of bis( 1,1-diphenylethylenes) and tris( 1,1-diphenylethylenes) as living linking agents to prepare heteroarm (miktoarm) star-branched polymers... 

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... 

Polymeric organolithium compounds react simply and quantitatively with 1,1-diphenylethylenes [3, 109]. These reactions have provided a new methodology for the synthesis of star-branched polymers, internally-functionalized polymer chains and stars, as well as heteroarm, star-branched polymers via living linking reactions as shown in Scheme 33 [3, 202, 203, 207]. 

In addition, 1,1-diphenylalkyllithium sites in the living linked polymer product (100) can initiate polymerization of a second monomer (M ) to generate a heteroarm (or mikto-arm), star-branched polymer, 102 [243, 244). 

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