142 Advances in Polymer Science Editorial Board: A. Abe. A.-C ...

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214 J. Roovers, B. Comanita Scheme 16 Poly(benzyl ether) [G-2]-TEMPO, [G-3]-TEMPO, and [G-4]-TEMPO compounds have been synthesized and used as additives in the benzoyl peroxide initiated polymerization of styrene [127] (see Scheme 15c). After an induction period, chain growth is observed. However, the MWD is larger than in a dendrimer-free TEMPO modulated system (Mw/Mn»2). The expectation that the dendrimer would isolate the growing chain end and prevent side reactions is not borne out. Polymerizations of methylmethacrylate, vinylacetate, and n-butylacrylate with the same initiator/TEMPO recipe are disappointing. A special type of ABA block copolymer is formed when dendrimers are used as stoppers on rotaxanes. Rotaxanes are linear polymers onto which one or several large macrocycles are threaded. In order to prevent unthreading, sufficiently bulky groups must be attached to the two ends of the rotaxane. The [G-3] poly(benzyl ether) dendrimer proved to be highly suited for trapping [128] (see Scheme 16). The linear polymer is made of stiff bipyridinium groups that spontaneously assemble with diparaphenylene-34-crown-10 to form the rotaxane. The dendrimers are attached via quaternation of pyridine with benzyl bromide
Dendrimers and Dendrimer-Polymer Hybrids 215 Fig. 6. Phase diagram of ABA poly(benzyl ether) dendrimer PEO in MeOH:H 2 O (1:1) at the focal point. The nonpolar dendrimer exerts a large solubilizing effect on the highly polar rotaxane which allows its study in a wide range of solvents. Dendrimers that are prepared by the divergent method lack the focal group to attach them to a polymer chain. Such dendrimers must be grown from a preformed polymer having an end-standing functional group. The first synthesis by this method used glycine modified PEO and poly(a,e-lysine) to grow a poly(a,e- L-lysine) dendrimer [129] (See Fig. 3b for structure). Up to four generations of lysine have been added. The PEO-dendrimer is purified at each generation by precipitation in diethylether similar to the Merrifield procedure. At the fourth generation the MW of the poly(a,e-L-lysine) dendrimer and PEO are comparable. These amphiphilic block copolymers decrease the surface tension of water and form micelles in solution. Poly(propylene imine) dendrimers (see Scheme 1 for structure) have been constructed step by step onto an amine functionalized polystyrene [38, 130, 131]. The challenge in this synthesis is finding conditions for the poly(propylene imine) synthesis under which the low MW polystyrene (MW=3200) is soluble [38]. Similar poly(imine) dendrimers with carboxylic acid end groups have also been prepared [130]. The poly(propylene imine) dendrimer has also been synthesized on an amino-terminated poly(2-methyl-2-oxazoline) [132]. Because of the commercial interest in amphiphilic block copolymers the properties of the amphiphilic linear polymer-dendrimer have been most extensively studied. An illustration of the various phases that can be observed is shown in Fig. 6 for the case of an ABA type block copolymer, where A is the hydrophobic dendrimer and B the hydrophilic PEO [119]. The exact location of the
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142 Advances in Polymer Science Edi
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Branched Polymers I Volume Editor:
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Volume Editor Dr. Jacques Roovers I
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Preface While books have been writt
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Contents Synthesis of Branched Poly
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Synthesis of Branched Polymers by C
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72 N. Hadjichristidis, S. Pispas, M
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100 N. Hadjichristidis, S. Pispas,
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Poly(macromonomers): Homo- and Copo
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Poly(macromonomers), Homo- and Copo
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Page 171 and 172: Poly(macromonomers), Homo- and Copo
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