Star-shaped polymers functionalized

More recent examples include end-functionalized multiarmed poly(vinyl ether) (44), MVE/styrene block copolymers (45), and star-shaped polymers (46—48). With this remarkable control over polymer architecture, the growth of future commercial appHcations seems entirely likely. 

The purpose of this review is to show how anionic polymerization techniques have successfully contributed to the synthesis of a great variety of tailor-made polymer species Homopolymers of controlled molecular weight, co-functional polymers including macromonomers, cyclic macromolecules, star-shaped polymers and model networks, block copolymers and graft copolymers. 

Fig. 47a, b. Structure and dynamics of star-shaped polymers with different functionalities, a Kratky plot of the static structure factor (S(Q, 0) Q2 vs. Q Rg. b Q(Q)/Q3 vs. Q Rg, as derived from Eqs (94) and (123), assuming Rouse dynamics... 

Dendrimers with their multiple end-standing functional groups are ideally suited for the construction of star-shaped polymers. Indeed, the end-standing functional groups can be used as initiators for polymerization ( grafting from method) or as functional groups for grafting onto . They can also be used as redistribution centers in equilibrium polymerization. 

In 2009, Crecelius et al. [42] reported the characterization of homopolymers, copolymers, and star-shaped polymers by tandem MS. The main aim of the work was the analysis of end-group functionalization, which can be determined faster by this technique. 

Various forms of polymeric fullerenes have been prepared in the past decade side chain polymers, main chain polymers, dendritic fullerenes, star-shaped polymers, fuUer-ene endcapped polymers, etc. [71a-d]. With the invention of the carbon nanotubes [71e,f] and the development of methods to functionalize them [71g-i], their applications in the area of polymers range from opto-electronic devices to biosensors [71j-m]. 

In these two decades remarkable progress has been made in the development of excellent catalysts for living and stereospecific acetylene polymerizations (10,26-28). The r-conjugated polymers prepared by the sequential polsrmerization are strictly limited to polyacetylenes, except for only a few examples. Thus, synthesis of tailor-made conjugated macromolecules such as end-functionalized polymers, block copolymers, star-shaped polymers is possible only in the case of substituted acetylenes. 

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). 

Another approach to synthesize multiarm star block copolymers is based on a combination of CuAAC and the arm-first method (Durmaz et al, 2010). Protected alkyne PS polymers were prepared via ATRP and subsequently crosslinked by a divinyl containing compound. The formed 27-arm star-shaped polymers containing a protected alkyne periphery, was deprotected and subsequently coupled with azide-end-functionalized PEG and PtBuA to form star block or mixed block copolymers. The CuAAC reactions occurred at room temperature for 24 h, surprisingly leading to a full click efficiency. The quantitative character of the latter click reaction at ambient temperatures for such dense polymer structures is in contrast to those obtained by other research groups, as mentioned in previous paragraphs. 

Figure 8.12 The SEC analysis of the PiBA star-shaped polymer, P(EtOx)2o and the coupling product before (bottom) and after preparative SEC (top) (Lammens et al, 2009). (Reprinted from M. Lammens, D. Fournier, M.W.M. Fijten et aL, Star-shaped poly acrylates Highly functionalized architectures via CuAAC click conjugation, Macromolecular Rapid Communications, 30, 23, 2049-2055 (Table 2, Figure 3), 2009, with the permission of John Wiley Sons, Inc.)...

In addition to the broad application of the CuAAC reaction, other types of highly efficient click reactions were used for the synthesis of star-shaped polymers. Tunca and coworkers used the Diels-Alder click reaction for the preparation of 3-arm star-shaped polymers using furan protected maleimide end-fiinctionalized PEO, PMMA or PtBuA and trianthracene functional linking agents (Dag et al., 2008). However, the resulting yields are comprised between 82 and 93% depending on the coupled polymer, the reaction conditions are more drastic than those used in CuAAC. 

This result proves that well-defined structures with low degree of heterogeneity of the multiarm star-shaped polymers can be synthesized. Moreover, the method reported herein can also provide a synthetic pathway for the introduction of block copolymers synthesized via different polymerization routes (RAFT, ROP, etc.) onto the anthracene-end-functionalized multiarm star-shaped polymers. Although the Diels-Alder cycloaddition between anthracene and maleimide derivatives has proven to provide good results in the formation of complex architectures, the major drawback of this method remains the requirement of high temperature and relatively long reaction times. 

On the other hand, as soon as many stars are in the matrix, the entanglement points of the matrix are increased due to additional permanent crosslink points (i.e., centers of star polymers). Thus the average mesh size of the matrix decreases. In consequence, the cooperative diffusion becomes faster. But in the case of 12-arm star polystyrene as the test chain, the effect of star centers was not so obvious. According to the model of star shaped polymers by Daoud and Cotton,there is a region with size around the center of a star, inside which the chains of other polymers do not penetrate. The distance x is a function of the number of arms, i.e., / where f is the arm number of the star. So the unpenetrable distance of 12-arm star is larger than that of a 4-arm star, leading to an effective decrease in the number of entangled points. 

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)...

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