Star-shaped polymers core-functionalized

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. 

Scheme 20 Preparation of star-shaped polymer conjugates with cholic acid (CA) core and polyether arms functionalized with pendant COOH or NH2 groups [150] (reprinted with permission from American Chemical Society)...

Star-shaped polymers have gained increasing interest because of their compact structure and high segment density, and because very efficient synthetic methods have made possible the functionalization of the outer branch ends. Until recently, anionic polymerization was one of the best methods to obtain well-defined star-shaped polymers of predetermined branch molar mass. This technique provided the long lifetime for the active sites necessary to allow the formation of star-shaped macromolecules. Anionic polymerization also limited the polymolecularity of the samples. Given the appropriate reaction conditions, the functionality of the core can be controlled in advance. 

In the second method, the core-first method, a polyfunctional core is used to initiate the polymerization of the branches of the star. This method allows for easy access to chain end functionalization by simple deactivation of the active sites. The extension of the core-first method to the preparation of functional star-shaped polymers in nonpolar solvents will be discussed. 

Burchard [65] was the pioneer for the preparation of functional starshaped pol)miers in nonpolar solvents by an anionic core-first method. To build the cores pure / -DVB was reacted with /i-butyllithium in dilute cyclohexane solution. Suspensions of small crosslinked poly(DVB) noduli were obtained that contained numerous lithium organic sites. In a second step, styrene (or isoprene) was added to the living cores and polymerized. The polymeric species obtained exhibit huge molar mass distribution and rather large polydispersity indices. Even if these star-shaped polymers could exhibit active sites at the outer end of the branches, the efficiency of initiation of a second generation of monomers or of hmctionalization was never given by the authors. 

It is not possible to cite here all the other attempts to synthesize star polymers however, the most important methods, yielding well-characterized species, have been quoted. The use of fullerenes as the central core provides an original and easy access to well-defined star-shaped polymers where the central fullerene entity is far better defined in size and functionality, as for example, in the DVB nodulus. As in DVB, arm-first , core-first and in-out have been developed. One must remain aware that the intrinsic properties of the fullerene core may be affected by the presence of the arms [91]. 

The so-called core-first method has been extensively used for the synthesis of various kind of star-shaped polymers, water-soluble or not. The functionalizable outer end of the branches offers an original access to a large scope of macromolecular architecture star-shaped polymers with copolymeric branches, functional star-shaped polymer networks, etc. The in-out method combines the advantage of the arm-first method and the core-first method allowing good control of the structure and the presence of functionalizable outer end of the branches. Different unsaturated compounds have been used to generate the core, such as DVB and DPE, the latter compound giving access to star-shaped terpolymers. 

Further advances along that line include the development of anionic polymerization methods with the aim of attaining better control of the functionality in core-first methods and developing original strategies to access so-called heteroarm star-shaped polymers. 

C. Core-Functionalized Amphiphilic Star-Shaped Polymers of Vinyl Ethers with Hydroxyl Groups... 

Such amphiphilic graft polymers may have almost uniform number of branches, provided that living polymerization of 23 is available. In addition, unlike the various types of star-shaped polymers previously reported [7,12,13], graft polymer 24 has no hydrophobic microgel core that may possess some dimensions. Therefore, the amphiphilic graft polymers are expected to possess properties and functions differing from those of the corresponding star block amphiphiles. 

The breakthrough in the synthesis of multiarm star polymers was made by Roovers, who reported the high-vacuum anionic synthesis of 1,4-polybutadiene stars via two distinct routes (1) using chlorosilane chemistry, central dendritic cores of spherical shape and different generations were synthesized, to which the desired number of polymeric arms were grafted. With this approach regular stars with typical nominal functionality / in the range 18-128 and nominal... 

PEO is well known for its good water solubility but the use of PEO was limited either to the linear soluble polymers or to hydrogels. The core-first method enables easy preparation of functional star-shaped polyethylene oxide. This easy access to water-soluble multifunctional PEO molecules has widely extended the application of these polymers [80]. 

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]. 

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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