Multiarm star polymers

Table 1. Multiarm star polymers and copolymers with a microgel core ...

These are chemically homogeneous multiarm star polymers (usually homopolymers) with only excluded volume interactions [21,22], Due to their synthesis procedure (high vacuum anionic polymerization), they are stable and nearly monodisperse [23]. Their softness can be tuned at the synthesis level (number and size of arms) [23,24] and/or by varying the temperature in different solvents [25,26], Moreover, these systems can be functionalized in various ways [27]. What made these systems truly ideal soft colloids were the breakthroughs in both theoretical description and synthesis. The former refers to the ability to describe analytically their internal structure [28] and their softness in terms of an effective interaction potential [24,29]. 

Well-characterized systems. This depends on the appropriate chemistry and subsequent characterization (typical issues here are the polydispersity, control of grafting density, reproducibility of procedure to obtain identical particles). One frequent problem here is that the price one pays for such systems is tlie availability of small amounts (sometimes only fractions of 1 g) of material. For example, multiarm star polymers are in many ways unique, clean, soft colloids [ 19,23], but their nontrivial synthesis makes them not readily available. On the other hand, recent developments witli block copolymer micelles from anionically synthesized polymers [54-58] and arborescent graft copolymer synthesis [40] appear to have adequately addressed this issue for making available different alternative star-like systems. 

Figured displays simulated form factors for a multiarm star polymer of varying functionality and a hard sphere [41], The high-g asymptotic behavior, characteristic of the coil structure, is absent in the latter case. A handicap in the experimental determination of P(g) is often the narrow-g range accessible by the scattering techniques that can be overcome through the combination of low-g light scattering and high-g X-ray and/or neutron scattering (utilized on the same system). Size and shape also determine the translational diffusion Dq of the nanoparticles in dilute solution, and hence Dq can prove the consistency of the scattering results.

For a multiarm star polymer withf arms, the Daoud-Cotton model [28] (Fig. 2) implies that the radius of the star scales as R where v w 3/5 is the... 

Multiarm star polymers have recently emerged as ideal model polymer-colloids, with properties interpolating between those of polymers and hard spheres [62-64]. They are representatives of a large class of soft colloids encompassing grafted particles and block copolymer micelles. Star polymers consist of f polymer chains attached to a solid core, which plays the role of a topological constraint (Fig. Ic). When fire functionality f is large, stars are virtually spherical objects, and for f = oo the hard sphere limit is recovered. A considerable literature describes the synthesis, structure, and dynamics of star polymers both in melt and in solution (for a review see [2]). 

Essential Features and Properties of Stars and Dendrimer-Uke Polymers 821 Multiarm star polymers... 

Neutral and charged multiarm star polymers possessing homopolymeric arms functional groups can be introduced either along the arms (in-chain functionalized stars) or at the end of each arm (end-functionahzed star polymers). 

Figure 37 Relative zero-shear viscosity (normalized to the solvent tis) as a function of the effective volume fraction

Hb polyethers that have been widely applied for the preparation of star polymers are mainly hbPG and hb polyoxetanes. Frey and co-workers presented the synthesis of poly (propylene oxide) (PPO) multiarm star polymers via... 

Hb polyethers based on 3-alkyl-3-(hydroxymethyl)oxetane have also been widely applied as core molecules for the preparation of multiarm star polymers. Carlmark et al. synthesized PMA star polymers with a similar route as mentioned above by ATRP after modification of hb poly[3-ethyl-3-(hydroxymethyl) oxetane] (hbPEHO) with 2-bromoisobutyryl bromide. The star polymers were prepared with CuBr/Mce-TREN as the catalyst in ethyl acetate at room temjjerature. In order to prevent gelation, the polymerization was conducted at low concentra tions and stopped after low conversion or by deaeasing o t e amoimt of catalyst used. The authors reported that e ratio initiating sites/catalyst could not exceed 1 0.1, otherwise, ge a tion occurred or no control over polymerization was possi... 

Scheme 9 Synthesis of multiarm star polymers with cleavable arms.

Three-layered nanoparticles containing an hbPG core and cross-linked block copolymers based on N-isopropyl acrylate and N,N-dimethylaminoethyl acrylate as the respective arms were synthesized and proved to be thermoresponsive. ° Chu and co-workers" reported electrically conductive core-shell nanoparticles based on poly(n-butylacrylate-b-polystyrene) multiarm star polymers. The PS segments were converted to poly(p-styrenesulfonate) (PSS), thus generating amphiphilic tmimolecular micelles. Then the oxidative propagation of 3,4-ethylenedioxythiophene (EDOT) on the PSS chains was carried out by counterion-induced polymerization to produce a stable aqueous dispersion of the respective PEDOT complex. 

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... 

Another approach to core shell polymers, or multiarmed star polymers is the arm first approach, where a growing polymer formed by a CRP is copolymerized with a difunction monomer to form a crosslinked core with the attached first formed arms [131,376,377]. Other surfaces include organic resins and latexes [315]. 

In conclusion, the synthesis of multiarmed star polymers was not successful by either the repetition of the synthetic sequence or the coupling reaction of the star-branched polymer anion with a multifunctional core. However, it should be mentioned that the methodology is facile and of great value to be able to successfully obtain some synthetically difficult miktoarm star-branched polymers, such as 8-arm A4B4, 16-arm AgBg, 6-arm A2B2C2, and 12-arm A4B4C4 stars. 

Vlassopoulos, D., Fytas, G., Pakula, T., and Roovers, J. (2001) Multiarm star polymers dynamics. Journal of Physics-Condensed Matter, 13, R855-R876. 

Although these materials may find applications that are predicted for hyperbranched polymers, they are used rather as precursors allowing (either by using hydroxyl groups directly or after their transformation into required initiating groups) the synthesis of multiarm star polymers of different structures. Star polymers containing poly(EOx) core may... 

Well-established materials, such as hyperbranched polyether polyols, have been used to develop exciting hybrid stmctures, from multiarm star polymers to LHBCs. Chemical modification of the core as well as the multiple terminal and interior groups leads to multifunctional materials for... 

Knischka R, Lutz PJ, Sunder A, Mtilhaupt R, Frey H (2000) Functiraial poly(ethylene oxide) multiarm star polymers core-first synthesis using hyperbranched polyglycerol initiators. Macromolecules 33 315-320... 

Miros, A., Vlassopoulos, D., Ldchtman, A. E., Roovers, J. Linear rheology of multiarm star polymers diluted with short linear chains. /. Rheol. (2003) 47, pp. 163-176... 

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