Star and Dendritic Polymers

As mentioned previously, PLAs and related copolymers are biodegradable and bio-assimilable, while their properties can be tuned by varying their structural parameters. AH of these aspects have given rise to a broad range of medical (surgical and pharmaceutical) and commodity applications (packaging, fibers) [3e-k]. 

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Benedicte Lepoittevin has been an Assistant Professor in Pol)nner Chemistry at the University of Paris-sud (France) since 2002. Prior to this she spent one year at the University of Bordeaux as postdoctoral fellow where she studied the synthesis of star and dendritic polymers using atom transfer radical polynierization. In 2000 she defended a Ph.D thesis at the University of Paris VI on the synthesis cyclic polymers by controlled radical and anionic polymerizations under the supervision of Prof. Patrick Hemery. Her scientific interests include PET surface modification, controlled radical polymerization and polymer synthesis using carbohydrates and essential oils derivatives in order to obtain polymer materials with antibacterial properties. 

Star polymers and dendrimers have been synthesized by Astruc using cyclopentadienyliron-mediated peralkylation, benzylation, and allylation reactions of cationic tri-, tetra-, and /iexa-methylbenzene cyclopentadienyliron com-plexes. " " These star and dendritic polymers contained cationic cyclopentadienyliron moieties at the core and/or the periphery. The cathodic reduction of nitrates and nitrites to ammonia has been achieved using a water-soluble dendrimer containing six cationic cyclopentadienyliron moieties as a redox catalyst. " The octametallic star (41) was prepared by deprotonation of permethylated iron complexes. ... 

The most common pH-sensitive polymer structures described in the literature are linear homopolymers or copolymers, amphiphilic block copolymers which form micelles, grafted copolymers, polymer brushes, star and dendritic polymers, NPs, vesicles or HGs (Fig. 3.4). Some of these examples are explained in this section. 

Jin, S., Jin, K.S., Yoon, J. etal. (2008) X-ray scattering studies on molecular structures of star and dendritic polymers. 

Star and miktostar polymers have been synthesized by living cationic polymerization using dienes and trienes as coupling agents and/or multifunctional initiators [Faust and Shaffer, 1997 Hadjichristidis, et al., 1999 Hadjikyriacou and Faust, 2000 Kennedy and Jacob, 1998 Puskas et al., 2001 Sawamoto, 1991]. Multifunctional halides such as hexaiodo-methylmelamines have also been used to obtain star and comb polymers [Ryu and Hirao, 2001 Zhang and Goethals, 2001], Hyperbranched and dendritic polymers have also been studied. 

To synthesise polymers with unusual properties from existing basic monomers one needs to place the monomer units in ordered arrays rather than at random. Thus polymer architecture control remains an important area of research. Possible structural elements include block, graft and comb copolymers as well as star and dendritic/hyperbranched topographies. Potential for such structures in the surface coatings and adjacent industries include use as... 

Ooya, T. Lee, J. Park, K. Effects of ethylene glycol-based 69. graft, star-shaped, and dendritic polymers on solubilization and controlled release of paclitaxel. Journal of Controlled Release 2003, 93, 121-127. 

This phenoltriallyl dendron has been functionalized at both the phenolic and allylic positions. For instance, the dendron can be bound, after suitable molecular engineering, to the branches of a phenolic-protected dendron (convergent construction), onto stars and dendritic cores (divergent construction), nonaparticles, surfaces, and polymers (Scheme 11.9). An example is... 

Hydrothermal synthesis, on the contrary, resulted in nanotube [153] and nano-particle [177] morphologies, as well as in hexagonal star- and dendrite-like 2-D nanostmctures [144] (Figure 19.11a and b). In this case, Mn(N03)2 and Mn(S04)2 salts were used as the most common sources of Mn, with reaction temperatures ranging between 100 and 250 °C. Kim ef al. [143] reported that the addition of a polyvinylpyrolidone (PVP) polymer to the reaction mixture, in combination with the post-treatment (calcination), resulted in a thin layer of a-Mn203 phase covering... 

There has been a great deal of interest in the design of dendrimers using arene complexes of transition metals (246-252). Astruc has developed an efficient route to core molecules suitable for the synthesis of star and dendritic materials via peralkylation or allylation of methyl-substituted arene complexes of cyclopen-tadienyliron. The resulting branched polymers contained cationic cyclopentadi-enyliron moieties at the core and/or the periphery. Complexes containing aryl ethers coordinated to six CpFe+ moieties (110) were synthesized via SNAr reactions (248). 

Branched macromolecules fall into three main classes star-branched polymers, characterized by multiple chains linked at one central point (Roovers, 1985), comb-branched polymers, having one linear backbone and side chains randomly distributed along it (Rempp et al., 1988), and dendritic polymers, with a multilevel branched architecture (Tomalia and Frechet, 2001). The cascade-branched structure of dendritic polymers is typically derived from polyfunctional monomers under more or less strictly controlled polymerization conditions. This class of macromolecules has a unique combination of features and, as a result, a broad spectrum of applications is being developed for these materials in areas including microencapsulation, drag delivery, nanotechnology, polymer processing additives, and catalysis. 

Polyphosphazenes form amphiphilic block copolymers with organic polymers, such as poly (ethylene oxide), polystyrene, or poly(dimethylsiloxane). Comb, star, and dendritic architectures were also obtained. There are also... 

Astruc and coworkers have reported the synthesis of highly branched polymers coordinated to cyclopentadienyliron and pentamethylcyclopentadienyl-ruthe-niiun cations." The catalytic and sensing ability of star polymers and dendrimers has also been reviewed. Multifunctional core molecules suitable for the synthesis of star and dendritic materials were synthesized by peralkylation or aUylation of methyl-substituted arene complexes of cyclopentadienyliron." The benzylic protons on these complexed arenes are acidic, which permits their facile alkylation. These branched polymers contained cationic cyclopentadienyliron moieties at the core and/or the periphery. The synthesis of a water-soluble metallodendrimers... 

Synthesis of complex polymeric molecules such as random and block copolymers, star and graft polymers, hyperbranched and dendritic structures by NMP and other CRP techniques has been reviewed several times. Good overviews of telechelic polymers or the coupling of NMP with other polymerization techniques are available. Synthesis of bioconjugates through CRDRP, including NMP, is treated by Nicolas et... 

Star and hyperbranched (dendritic) polymers have attracted increasing attention in organic, supermolecular, and polymer chemistry as well as coordination chemistry because of their specific structures and characteristics [1-5]. For the preparation of these highly branched polymers, two kinds of synthetic methods have been developed a one-step pol)nnerization and a stepwise method. By the one-step method, various star and hyperbranched polymers with many structural and functional group variations have been prepared [6-10]. On the other hand, the stepwise method is quite useful, especially for the synthesis of dendritic polymers. Both divergent and convergent s)mthetic approaches have been employed [2,11]. 

This cutting-edge reference supplies the very latest advances in research on star, hyperbranched, and dendritic polymers—providing design strategies needed for a wide variety of industrial applications. 

The use of dendritic cores in star polymer synthesis by NMP, ATRP and RAFT polymerization was mentioned in Section 9.9.1, In this section wc describe the synthesis of multi-generation dendritic polymers by an iterative approach. 

The synthesis of well-defined LCB polymers have progressed considerably beyond the original star polymers prepared by anionic polymerization between 1970 and 1980. Characterization of these new polymers has often been limited to NMR and SEC analysis. The physical properties of these polymers in dilute solution and in the bulk merit attention, especially in the case of completely new architectures such as the dendritic polymers. Many other branched polymers have been prepared, e.g. rigid polymers like nylon [123], polyimide [124] poly(aspartite) [125] and branched poly(thiophene) [126], There seems to be ample room for further development via the use of dendrimers and hyperbran-... 

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