Heteroarm

In miktoarm-star copolymers (or heteroarm star copolymers ), the unlike blocks are connected at one junction point, as shown in Fig. 34. As in linear-block copolymers, exactly two blocks are linked. Hence, it seems tempting to approximate those copolymers as a set of diblocks with the free ends of one block component joined together in a cluster [111]. 

Similar onion type micelles were obtained by the combination of PS-P2VP heteroarm-star copolymers with a P2VP-PEO diblock copolymer, as reported recently by Tsitsilianis et al. [269]. 

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 ... 

Bender JL, Corbin PS, Fraser CL, Metcalf DH, Richardson FS, Thomas IiL, Urbas AM. Site-isolated luminescent europium complexes with polyester macroUgands metal-centered heteroarm stars and nanoscale assemblies with labile block junctions. J Am Chem Soc 2002 124 8526-8527. 

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],... 

Fig. 19 Schematic representation of block-arm star copolymer of type (PS- -P2VP) (PS) and (PS) (P2VP) heteroarm star diblock copolymer complexed with DBSA. In the drawing n = 4, Reprinted with permission from [135,136]. 2005 and 2006 American Chemical Society...

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. 

Tsitsilianis et al. recently published [245] preliminary results on the micelliza-tion behavior of anionically synthesized amphiphilic heteroarm star copolymers with polystyrene and poly(ethylene oxide) branches in THF and water. The former solvent is not very selective for one of the segments whereas the latter is strongly selective for PEO. The apparent molecular weights found for the micelles in THF were two orders of magnitude larger than the ones measured for the unimers. By increasing concentration an increase in the depolarization ratio was observed supporting the conclusion that multimolecular micelles are formed by this kind of miktoarm star copolymer. 

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]. 

VOU Voulgaris, D. and Tsitsilianis, C., Aggregation behavior of polystyrene/poly(acrylic acid) heteroarm star copolymers in 1,4-dioxane and aqueous media, Macromol. Chem. Phys., 202, 3284, 2001. 

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... 

An interesting example of macromolecular co-assemblies derived from starshaped polyionic species was reported by Ge et al. [81]. The authors found that a star-shaped double hydrophilic poly(methacrylic acid)-poly(ethylene oxide) heteroarm copolymer [(PMAA)x-PDVB-(PEO)x, with PDVB being poly(divinylbenzene) and X denoting the number of PMAA and PEG arms] can interact in alkaline media with a double hydrophilic poly(ethylene oxide)-block-quaternized poly[2-(dimethylamino)ethyl methacrylate] (PEO- -PDMAEMAQ) diblock copolymer. At Z = [PDMAEMAQ]/[PMAA] = 1, well-defined water-soluble onion-like (core-shell-corona) macromolecular co-assemblies are formed, with a hydrophobic core consisting of a PDVB microgel. The interaction of the PMA arms of the hybrid coronas of such copolymer stars with the PDMAEMAQ+ blocks of the diblock copolymer generates an insoluble inner layer (shell) around a PDVB core. Meanwhile, PEG blocks from both PEG- -PDMAEMAQ and (PMAA)x-PDVB-(PEG)x build up a hydrophilic nonionic corona that stabilizes the whole complex in aqueous media. 

Miktoarm and heteroarm stars that contain chemically different polymeric... 

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. 

The topology of the hydrophobic block is critical in the double-hydrophilic ABC terpolymers since it determines the self-assemblies in aqueous media. For instance, P2VP-PMMA-PAA, prepared by sequential monomer addition, forms now micellar self-assemblies bearing oppositely charged corona chains (P2VP, PAA). These living self-assemblies, called heteroarm star-like micelles, are sensitive to pH changes and can further associate in a second level of hierarchy. 

Heteroarmed polymacromonomers including PS and PEO or PS and PB arms have been prepared by statistical ROM copolymerization of PS/PEO or PS/PB macromonomers... 

The so-called palm tree-like copolymers consist in an arrangement of nB blocks with one A block of much larger size in a heteroarmed star architecture [8]. These architectures were obtained by sequential copolymerization of norbornenyl-PS macromonomers with a molecular comonomer, namely cyclooctadiene (COD), Scheme 6. Dumbbell-shaped copolymers can be viewed as double stars that are linked through a linear block. They are obtained upon using palm tree-like copolymers to trigger the polymerization of a new amount of PS macromonomers. Scheme 6. 

For the synthesis of block copolymers the reader is refered to a previous article in this series (23) and some other works describing the application of various controlled polymerization techniques for their synthesis, such as anionic polymerization (24-26), cationic polsmierization (27,28), controlled radical polymerization (29,30), as well as combinations of various techniques (31-34). Synthetic procedures for star copolymers have been reviewed (35). The first synthesis of a heteroarm star terpolymer with three immiscible blocks has been described in Reference 36. 

The nomenclature of the block copolsrmers is as follows A ByCz is a block copolymer composed of the blocks A, B, and C, where subscripts denote the weight fraction (%) and M is the number-averaged overall molecular weight (kg/mol). Heteroarm star terpolymers are indicated by an asterisk (A ByCz ). The morphological schemes are presented in such a way that the typical colors found in tern images of correspondingly stained samples are used (Table 1). 

Fig. 17. Various cylindrical morphologies found in SIM heteroarm star terpolymers (OSO4, see Table 1).

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