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

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216 J. Roovers, B. Comanita phase boundaries is dependent on the polymer concentration. The boundary between the monomolecular and micellar phases depends on whether the experimental concentration is above or below the critical micelle concentration. The boundary between micellar solution and gel is defined by the equilibrium between loops and bridges formed by the soluble PEO blocks. Bridges are entropically favored. A similar, less complete, phase diagram is also obtained for AB type polymer-dendrimer block copolymers. However, in that case, no gel phase is produced for lack of bridging. The structures formed by polystyrene-poly(propylene imine) dendrimers have also been analyzed. Block copolymers with 8, 16, and 32 end-standing amines are soluble in water. They have a critical micelle concentration of the order of 10 –7 mol/l. At 3·10 –4 mol/l they form different types of micelles. The dendrimer with eight amine groups (80% PS) form bilayers. The dendrimer with 16 amine groups (65% PS) forms cylinders and the dendrimer with 32 amine groups (50% PS) forms spherical micelles [38, 130, 131]. These are the classical lamellar, cylindrical, and spherical phases of block copolymers. However, the boundary between the phases occurs at very different volume fractions, due to the very different packing requirements of the linear polymer and spherical dendrimer at the interphase. 4.3.2 Multiple Polymer-Dendrimer Hybrids 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. “Grafting from” has not been a successful method in anionic polymerization because the required low molecular weight multifunctional organometallic initiators are almost always insoluble and this is also expected to be the case when dendrimers are modified. However, in cationic polymerizations the dormant species is less polar and more soluble. For example, the hexabenzyl bromide (10) has been used in the polymerization of 2-methyl-2-oxazoline [133] and it can be envisioned that larger dendritic initiators based on phosphorus can also be used [134]. The “grafting from” method is also successful in other ring opening polymerizations. For example, e-caprolactone polymerization is initiated from poly(propylene imine) dendrimers with n=2–16 end-standing amine groups [135]. To be
Dendrimers and Dendrimer-Polymer Hybrids 217 Scheme 17 successful this method requires stable dendrimers and preferably little or no reversibility leading to depolymerization and ring formation (see Scheme 17a). The polymerization in bulk at 230 ˚C of e-caprolactam with poly(ethylene imine) dendrimers is also described [136] (Scheme 17b). The properties of the resulting semicrystalline six-arm star polymers are in all aspects similar to those of the linear nylon-6 except for a 40% lower melt viscosity [137] which is of significant industrial importance. Recently, PEO has been grafted onto PAMAM dendrimers by means of N-succinimidyl propionic acid spacers [138] (see Scheme 17c). When n=16 and 32, experimental and expected functionalization agree satisfactorily. However, for n= 64, 128, and 256, progressively lower functionalization is observed. It is not clear whether this result is due to imperfections of the PAMAM dendrimer used or to steric limitations on the extent of the substitution reaction. At present this reaction scheme has been tested only on PEO chains with MW=5000. Anionic polymerization is uniquely suited for the preparation of star polymers via grafting onto dendrimers. At least for a few monomers under well defined conditions narrow MWD chains having stable but reactive end groups are available for reaction with multifunctional dendrimers. It is required, however, that the dendrimers are essentially of a hydrocarbon nature except for the electophilic functional groups. In the original disclosure of the formation of 18-arm star polyisoprenes the required octadecachlorocarbosilane (see Fig. 3a for related structure) coupling agent was prepared by an embryonic recursive method which foreshadowed divergent dendrimer synthesis [139]. This octadecachlorocarbosilane compound is equivalent to a generation 1.5 dendrimer. The grafting-onto process consist of the nucleophilic displacement of Cl with the carbanionic end groups of living polymer chains. The coupling reaction between 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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