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

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200 J. Roovers, B. Comanita tions. However, dendrimers with COOH end groups have a 35% larger hydrodynamic radius in neutral or basic than in acidic solution. Dendrimers terminated with CH 2 NH 2 groups have values of R h 35% larger in neutral or acidic conditions than in a basic medium. These documented expansions are clearly due to electrostatic repulsion. Because measurements have been reported only at one concentration (c£10 –3 mol/l) it is not clearly established that ionizable dendrimers act as polyelectrolytes [72]. The observation that ionic strength has little effect on dimension suggests that considerable screening occurs at the concentration of the measurements. Dubin et al. measured [h] and D o of seven generation of carboxylated PAMAM dendrimers in NaNO 3. NaH 2 PO 4 aqueous buffer at pH 5.5 and 0.38 mol/l in order to minimize electrostatic interaction [85]. The values of R h of the 2.5 and 3.5 generation dendrimer agree closely with those of the corresponding methyl ester dendrimer in MeOH. Rather surprisingly, the quoted values of [h] do not go through a maximum expected for the higher generation dendrimers. 4 Dendrimer-Polymer Hybrids 4.1 Polymeric Dendrimers The dendrimers discussed in the previous sections have short spacers containing usually between two and eight bonds between neighboring branch points. It is, however, also possible to work with polymeric chains rather than short spacers albeit with some some sacrifices to regularity and symmetry. Arborescent [86] and combburst [87] polymers contain linear polymer chains at the core and in each generation. These polymers have several trifunctional branch points along each polymer chain, rather than the end-standing branch points of the classical dendrimer. The branching functionality, considered per polymer chain is usually high (7–15) compared to 2 to 4 in a dendrimer. As a consequence the molecular weight and number of branch points increases very rapidly with each generation (see Scheme 8a and Table 1). The arborescent and combburst polymers are expected to have a high segment density and near spherical conformation. At this point there is no experimental evidence on the segment density distribution with a possible shell like structure or extensive backfolding. Polymeric dendrimers have been synthesized by anionic, cationic and free radical polymerization. Gauthier and Möller and their collaborators used two alternating reactions of chloromethylation of polystyrene and grafting with polystyryllithium to create arborescent polystyrenes [86] (see Scheme 9a). Tomalia and co-workers used hydrolysis of poly(ethyloxazoline) to create secondary amines in poly(ethylene imine) onto which poly(ethyloxazoline) chains are grafted by means of the living oxazolinium end group [87] (see Scheme 9b). In the original work [86, 87] the individual polymer chains are kept small (DP=20
Dendrimers and Dendrimer-Polymer Hybrids 201 Scheme 8 or 50) and up to one third of the monomer units are converted into branch points. The spacers between two neighboring branch points are therefore small and stiff and do not have flexible polymer characteristics. Both laboratories noted that the grafting efficiency decreased with increasing generation. This is ascribed to steric congestion [86, 87]. It is also possible that there is strong thermodynamic repulsion between the highly branched polymer and linear polymer preventing a fast and complete grafting reaction. In later work the MW of
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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 157 and 158: Poly(macromonomers), Homo- and Copo
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Page 249 and 250: Subject Index 241 Microphases 11z,
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142 Advances in Polymer Science Editorial Board: A. Abe. A.-C ...

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