P-star 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 ... 

Hadjichristidis N (1999) Synthesis of miktoarm star (p-star) polymers. J Polym Sci A Polym Chem 37 857-871... 

Stenzel, M.H. Davis, T.P. Star polymer synthesis using trithiocarbonate functional P-cyclodextrin cores (reversible addition-fragmentation chain-transfer polymerization). J. Polym. Sci. A 2002,40 (24), 4498-4512. 

In general, it is far more difficult to synthesize miktoarm star polymers than regular stars having identical arms because there are strict requirements in terms of multistep quantitative reactions that correspond to introducing different arms. In addition, the isolation of intermediate polymers is often required to obtain pure products. Although several successful examples of well-defined p-star polymers have been reported, most of them are composed of less than three different arms. Only a few examples of miktoarm star-branched polymers with four different arms have been synthesized to date. For this difficult synthetic problem, general and versatile methodologies for the synthesis of multiarmed and multicomponent p-star polymers have been vitally desired. 

For this reason, the focus of this chapter will be on the recent developments (since 2000) in p-star polymers synthesized by the above living anionic polymerization systems, with emphasis on the control of synthetic factors necessary to achieve well-defined structures of p-star polymers, that is, molecular weight, molecular-weight distribution, arm number, and composition. In the last 20 years, rapid progress in living/controlled radical polymerization systems as well as the application of click makes possible the synthesis of several new p-star polymers. Therefore, representative examples will also be described. The syntheses of p-star polymers before 2000 are beyond the scope of this chapter, although they will be briefly described in Section 4.2, since such subjects have been covered elsewhere by several excellent reviews (Hadjichristidis, 1999 Hadjichristidis et al, 2001). 

As mentioned in the introduction, p-star polymers are much more difficult to synthesize than corresponding regular stars. The methodologies currently developed can cover the synthesis of two-component A By-type and several 3-arm three-component ABC-type p-star polymers. 

Most of the p-star polymers synthesized by the three methodologies possess well-defined strucmres with precisely controlled arm segments and narrow molecular-weight distributions MJMn < 1.1 or smaller) as introduced above. 

The compositions observed by H NMR were consistent with those calculated from the feed ratios. Accordingly, these results clearly indicate that well-defined 3-arm ABC, 4-arm ABCD, and 5-arm ABCDE p-star polymers are successfully synthesized by repeating the processes with five different tiving polymers. Thus, the first proposed iterative methodology with the regeneration of DPE reaction sites works very well. 

The 5-arm ABCDE star synthesized by this method is the first successful p-star polymer with five different arm segments. Furthermore, the E segment of the resulting star was readily and quantitatively converted to a poly(4-vinylphenol) segment by treatment with BU4NF, resulting in a new functional 5-arm ABCDE star with an acidic and ionic segment. Since the final 5-arm star still possesses a DPE function at the core, this procedure enables further synthetic sequence of stars with more arms and components. 

Since all p-star polymers still possess several DPE functions, the synthetic sequence may possibly further continue to synthesize higher armed and compositional star-branched polymers. It should be mentioned that styrene derivatives were generally used herein both for facile synthesis of highly reactive living anionic polymers and for easy characterization of the resulting stars by H NMR, but living anionic polymers of 1,3-butadiene, isoprene, and certain functional styrene and 1,3-dienes can also be employed in the iterative methodology. 

Hirao et al. successfully synthesized a 3-arm ABC, followed by a 4-arm ABCD p-star polymer using the DPE functionalized agent, 7 (Higashihara and Hirao, 2004). As illustrated in Scheme 4.15, a chain-end-functionalized PI with both benzyl chloride and DPE moieties was prepared in 62% yield by the slow addition of PILi, after end capping with DPE, with a 8.7-fold excess of 7. After the isolation of the chain-end-functionalized PI by fractional precipitation, DPE-end-capped PMOSLi was added to react selectively with the benzyl chloride functionality, followed by reacting PSLi with the residual DPE functionality, resulting in a 3-arm ABC star-branched polymer having an anion at the core (Af = 31 800 g/mol, M /Mn = 1 03). 

The ABC star-branched polymer anion thus prepared was then reacted with a chain-end-butyl bromide-functionalized PSiS, prepared from 1,4-dibromobutane and PSiSLi end capped with DPE (yield = 95%). The linking reaction was observed to proceed nearly quantitatively to afford the objective 4-arm ABCD p-star polymer (M = 43 000 g/mol, MJM = 1.04). This is the first successful synthesis of a 4-arm ABCD p-star polymer by the methodology using the dual functionality of the DPE derivative. 

Wang et al. also synthesized a 4-arm ABCD p-star polymer by ingeniously utilizing twice the dual functionality of l,4-bis(l-phenylethenyl)benzene (10) in the living anionic polymerization (Wangef fl/., 2007). As illustrated in Scheme 4.16,10 undergoes a 1 1 addition reaction... 

The first example was an ABC p-star polymer composed of poly(tertahydrofuran) (PTHF), poly(l,3-dioxepane) (PDOP), and PS segments and was synthesized by Pan and coworkers in 2002 (Feng and Pan, 2002). The objective star-branched polymer was synthesized by the... 

In addition to several examples of ABC stars synthesized by the Uving/controUed polymerization systems mentioned above, three different ABCD p-star polymers were synthesized almost at the same time in 2008 using very similar methodologies (combining living/controlled polymerization systems with click chemistry) (Wang et al., 2008 Altintas et al., 2008), and (Yang et al., 2008). 

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