Star-shaped polymers miktoarm

The arm-first method based on a deactivation reaction was used to build a novel type of star-shaped polymers miktoarm star-shaped polymers and quaterarm star-shaped polymers. It has been shown that the reaction of the... 

Scheme 8.6 Graphical representation of a regular star-shaped polymer (left), and a miktoarm starshaped polymer (right).

Various methods have been used to synthesize miktoarm star-shaped polymers, ranging from combinations of ring-opening, ATRP, nitroxide-mediated, anionic polymerization, and the in-out ATRP method (Gao and Matyjaszewski, 2009). A combination of CRP and click chemistry facilitates the synthesis of these miktoarm star-shaped polymers. 

Chlorosilanes were used applying an identical reaction procedure to prepare either quaterarm pol)miers of type ABCD or 4- miktoarm polymers of type A2B2. A, B, C, D correspond to PS, PBd, PI, and poly(4-methylst5n ene), respectively [33]. That method was extended recently to well-defined poly(iso-prene)/poly(butadiene) A2B2 copolymers [34]. The presence on the same nodu-lus of chains exhibiting different chemical structures leads to original solution properties. Roovers et al. [35] have examined in detail the solution properties and compared the specific behavior of these miktoarm star-shaped polymers to linear diblock copolymers. That strategy was extended to the preparation of... 

In order to study the influence of an increase of functionality on the properties of these miktoarm star-shaped polymers, Avgeropoulos et al. have synthesized and characterized 16- miktoarm star copolymers (Figure 5) [37]. Further work is now in progress to investigate the different properties of these miktoarm star-shaped copol)miers [38]. 

Several excellent books and review articles have been published covering this particular area of polymer science [1-3]. Nevertheless, this review will highlight 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 will be given only to those synthetic methodologies which lead to well-defined and well-characterized macromolecules. 

Architectural polymers (Hirao etal, 2005 Hadjichristidis etai, 2006) refer, in this chapter, to polymers that have finite molecular weights and are architecturally more complex than linear chains. They can be cyclic, star-shaped, combed, balloon-shaped, and H-shaped. Illustrated in Figure 24.1 are the structures of cyclic, balloon-shaped, and H-shaped polymers, as well as of a miktoarm star ABC triblock copolymer. 

Figure 24.1 Structures for cyclic, balloon-shaped, and H-shaped polymers as well as a miktoarm star ABC triblock copolymer.

Similarly, Hadjichristidis [88] has s)mthesized miktoarm star-shaped terpolymers exhibiting PMMA branches. As mentioned earlier, the chlorosilane chemistry does not apply efficiently to the s)mthesis of star polymers containing PMMA branches (living PMMA does not react with chlorosilanes). For this reason, Hadjichristidis used the DPE method to s)mthesize such species. 

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... 

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... 

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