Star-like block copolymers

The form and the structure factor of the PEP-PEO star-like block-copolymer... 

Hedrick et al. reported on the synthesis of 6- and 12-arm star-like block copolymers (Fig. 49) of tBA and MMA using sequential ATRP reactions with a NiBr2(PPh3)2 catalyst [349]. The ferf-butyl esters were subsequently deprotected... 

Polymers with a star-like topology have attracted interest for many years. The rheological behavior in the melt and in solution of starpolymers differs from the behavior of linear polymers [172]. Polystyrene starpolymers with selectively deuterated core or corona chains were investigated by SANS and it was found that the chains are more stretched within the core (or close to the branching point), while the outer parts of the chains follow the single chain behavior of linear polymers [173]. This result confirmed theoretical predictions by Daoud and Cotton [174] and Birshtein et al. [175]. A similar behavior was found for the chain conformations in star-like block copolymer ionomer micelles, which were studied by SANS, too [176]. 

The star-polymer template approach allows preparation of spherical nanocrystals. In the initial step, a star-like block copolymer nanoreactor was prepared, with a polyacrylic acid (PAA) inner block and an outer block of polystyrene. Then, the inner PAA block was infiltrated with the precursor for preparation of the desired nanoparticles. Finally, the precursor is transformed into the nanoparticle inside the star-polymer template, resulting in formation of uniform well-defined nanocrystals. By using a star-like triblock copolymer template, core-shell and hollow nanocrystals can be prepared following similar strategies (Scheme 9). 

Li et al. utilized a solvent-assisted collocation strategy to prepare star-like block copolymers here an amphiphilic diblock copolymer was prepared by sequential monomer addition using a suitably designed RAFT initiator (11), which yielded diblock copolymers with both azide and alkyne groups at the same end of the polymer chain (Scheme 7.27). In this study, they have used either M-isopropylacrylamide (NIPAM) and dimethyl acrylamide (DMA) or styrene and NIPAM to first generate the diblock copolymer the PNIPAM segment in both of them gives the system a temperature-responsive amphiphilic character due the lower critical solution temperature (LCST) behavior of PNIPAM block. 

Star-shaped block copolymers can be made by using polyfunctional linking agents, like methyltrichlorosilane or silicon tetrachloride, to produce tribranch or tetrabranch polymers. 

The influence of the local chemical stracture of a macromolecule like its tactidty or the ds/trans distribution on its molecular dynamics has been known for a long time (see for instance References 150 and 222). In the following special chain stractures such as rings, stars, or block copolymers are discussed where both segmental and chain dynamics are considered. 

Fig. 2. Selected architectures of block copolymers (a) diblock (b) triblock (c) comb copolymer consisting of flexible chains (d) rod-coil diblock copolymer consisting of a rodlike block and a coil-like block (e) hairyrods, i.e., comb-block copolymers consisting of rodlike backbone and coil-like side chains and (f) LC coil with a side-chain liquid crystalline (LC) block and a flexible block. Many other variations have been introduced, such as multiblock copolymers, block copolymers consisting of several rodlike blocks, or star-shaped block copolymers. Comb-coil block copolymers with dense packing of side chains are also denoted as molecular bottle brushes, as is illustrated in (g) by a simulated structure of an isolated molecule dissolved in a solvent. (Courtesy of Mika Saariaho.)...

Scheme 5.2 Synthesis of 2G dendrimer-like star-branched block copolymer composed of PEO and PS segments reported by Gnanou et al.

Scheme 5.12 Synthesis of 4G dendrimer-like star-branched block copolymers having functionalized segment at the fourth generation.

More recent examples include end-functionalized multiarmed poly(vinyl ether) (44), MVE/styrene block copolymers (45), and star-shaped polymers (46—48). With this remarkable control over polymer architecture, the growth of future commercial appHcations seems entirely likely. 

Block (Star) Arrangement. The known star polymers, like their linear counterparts, exhibit microphase separation. In general, they exhibit higher viscosities in the melt than their analogous linear materials. Their rheological behavior is reminiscent of network materials rather than linear block copolymers (58). Although they have been used as compatibiUzers in polymer blends, they are not as effective at property enhancements as linear diblocks... 

Closely related to these but thermoplastic rather than rubber-like in character are the K-resins developed hy Phillips. These resins comprise star-shaped butadiene-styrene block copolymers containing about 75% styrene and, like SBS thermoplastic elastomers, are produced by sequential anionic polymerisation (see Chapter 2). 

Abstract This review highlights recent (2000-2004) advances and developments regarding the synthesis of block copolymers with both linear [AB diblocks, ABA and ABC triblocks, ABCD tetrablocks, (AB)n multiblocks etc.] and non-linear structures (star-block, graft, miktoarm star, H-shaped, dendrimer-like and cyclic copolymers). Attention is given only to those synthetic methodologies which lead to well-defined and well-characterized macromolecules. 

Hyperbranched polymers have also been prepared via living anionic polymerization. The reaction of poly(4-methylstyrene)-fo-polystyrene lithium with a small amount of divinylbenzene, afforded a star-block copolymer with 4-methylstyrene units in the periphery [200]. The methyl groups were subsequently metalated with s-butyllithium/tetramethylethylenediamine. The produced anions initiated the polymerization of a-methylstyrene (Scheme 109). From the radius of gyration to hydrodynamic radius ratio (0.96-1.1) it was concluded that the second generation polymers behaved like soft spheres. 

Many micellar catalytic applications using low molecular weight amphiphiles have already been discussed in reviews and books and will not be the subject of this chapter [1]. We will rather focus on the use of different polymeric amphiphiles, that form micelles or micellar analogous structures and will summarize recent advances and new trends of using such systems for the catalytic synthesis of low molecular weight compounds and polymers, particularly in aqueous solution. The polymeric amphiphiles discussed herein are block copolymers, star-like polymers with a hyperbranched core, and polysoaps (Fig. 6.3). 

Fig. 6.3 Different types of micelles and micelle analogous structures a) amphiphilic block copolymers, b) star-like polymers with a hyperbranched core, c) polysoaps.

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