Star-shaped architecture groups

Problem 12.13 The combination of RAFT chemistry and the FIDA cydoadditicm provides a simple and synthetically nondemanding pathway to well-de ned macromolecular star-shaped architectures. In support of this contention describe the synthesis of polystyrene (PSt) star polymers with up to 4 arms using the aforesaid two different RAFT end groups, namely, diethoxy-phosphoryldithioformate and pyridin-2-yldithioformate, and HDA coupling reactions. 

In this study, lactide was polymerized by Sn-oct in the presence of polyfunctional alcohols such as glycerol or pentaerythritol. The resulting PLA had multi-armed chains to yield a star-shaped architecture. The microstructure, thermal properties, and degradation behaviour were studied to compare the effect of the different architecture, linear PLA with starshaped one. In addition, it was very beneficial to study the effect of various chain end groups on the thermal and hydrolytic stability of this multi-armed PLA. Therefore, OH terminal groups of PLA were converted into Cl, NH2 or COOH groups. 

Living polymerization processes pave the way to the macromolecular engineering, because the reactivity that persists at the chain ends allows (i) a variety of reactive groups to be attached at that position, thus (semi-)telechelic polymers to be synthesized, (ii) the polymerization of a second type of monomer to be resumed with formation of block copolymers, (iii) star-shaped (co)polymers to be prepared by addition of the living chains onto a multifunctional compound. A combination of these strategies with the use of multifunctional initiators andtor macromonomers can increase further the range of polymer architectures and properties. 

The spherical architectures of highly branched macromolecules, such as dendrimers, star-shaped polymers, and hyperbranched polymers, have attracted much attention from the viewpoint of nanotechnology, because their numerous terminal units can be converted into various functional groups leading to novel nanomaterials (Zeng and Zimmerman, 1997 Hirao etal., 2007 Satoh, 2009). Thus, various types of dendrimers, star polymers, and hyperbranched polymers have been synthesized and their properties were compared to the linear analogues (Stiriba et al., 2002 Hirao et al., 2005). 

On the other hand, when aiming at the synthesis of star-shaped polymers via thermally and photochemically induced radical thiol-ene click chemistry, serious limitations of side reactions and incomplete conversions have been clearly demonstrated by the groups of Du Prez and Bamer-Kowollik. From this study, it was concluded that this type of thiol-ene reactions cannot be applied for the well-defined synthesis of complex polymer architectures in which polymer-polymer conjugation is needed (Koo et al, 2010). 

The arm-first methods are efficient at synthesizing well-defined star-shaped macromolecules. Difficulty arises, however, in the functionalization of the outer chain ends, which is only possible through the use of functional initiators to generate the precursor chains [58]. Core-first methods were developed extensively in polar solvents to access star-shaped macromolecules exhibiting functional groups at the outer chain ends. Once such species are obtained, they can serve as valuable intermediates in the elaboration of a large scope of macro-molecular architectures. Attempts to prepare polyfunctional initiators have been described by Nagasawa and co-workers [62], who s)mthesized core-first star... 

Several others methods also exhibiting a living character are now being used for the construction of branched macromolecular architectures. One of them, group transfer polymerization, is briefly mentioned in the text. Special efforts are being made presently to apply living radical polymerization to the synthesis of star-shaped polymers. The performance of the different methods has been compared recently [94]. 

Multifunctional initiators based on, for example, cyclotriphosphazine [106], silesquioxane [107], porphyrin [108] and bipyridine metal complex [109, 110] cores were also successfully used for the living cationic ring-opening (co)polymerization of 2-oxazolines, resulting in star-shaped (co)polymers. The use of polymeric initiators also allowed the construction of well-defined complex macro molecular architectures, such as triblock copolymers with a non-poly(2-oxazo-line) middle block that is used to initiate the 2-oxazoHne polymerization after functionalization with tosylate end-groups [111-113]. In addition, poly(2-oxazoline) graft copolymers can be prepared by the inihation of the CROP from, for example, poly(chloromethylstyrene) [114, 115] or tosylated cellulose [116]. 

As discussed in Chapter 7, the absence of termination in living polymerization permits the synthesis of unusual and unique block polymers — star- and comb-shaped polymers. Living polymerization can also be employed to introduce a variety of desired functional groups at one or both ends of polymeric chains both in homo- and block polymers. In particular, living polymerization techniques provide the synthetic polymer chemist with a vital and versatile tool to control the architecture of a polymer complicated macromolecules can be synthesized to meet the rigid specification imposed by a scientific or technological demand. 

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