Star polymers heteroarm

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 ... 

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. 

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]. 

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. 

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... 

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. 

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... 

Star polymers consist of several linear polymer chains connected at one point. Prior to the development of CRP, star molecules prepared by anionic polymerization had heen examined. However, due to the scope of ionic polymerization, the composition and functionality of the materials were limited. The compact structure and globular shape of stars provide them with low solution viscosity and the core-shell architecture facilitates entry into several applications spanning a range from thermoplastic elastomers (TPEs) to dmg carriers. Based on the chemical compositions of the arm species, star polymers can be classified into two categories homoarm star polymers and miktoarm (or heteroarm) star copolymers... 

Following the excellent examples, the syntheses of amphiphilic heteroarm and core-functionalized star polymers were achieved using similar but modified arm-first methods. In the s)mthesis of heteroarm star polymers, polymer-linking... 

The DPE scheme can also be used to generate a controlled version of heteroarm stars produced by using DVB. A2B2 star polymers of I or Bd and MMA or nBMA were obtained by sequential sBuLi-initiated polymerisation of I in hexane at RT followed by solvent exchange to THE, chain-end modification by a compound of the DPE-X-DPE type and polymerisation of MMA at —78°C in the presence of LiCl. (38 < Mn/(kg/mol) < 70 1.01 < M / Mn < 1.06) [131,132]. 

The core functionalization in 21 may lead to the higher accumulation of polar hydroxyl groups in a core region that should be smaller in size than the arm moiety of the star-shaped polymers with functionalized arms. Another important feature of 21 is that an outer hydrophilic shell of 21 can effectively surround the hydrophilic microgel core with many hydroxyl groups. Therefore, the core-functionalized star polymers are amphiphilic but are expected to possess properties that differ from those of the star block and the heteroarm star amphiphiles. 

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. 

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],... 

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... 

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... 

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