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

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206 J. Roovers, B. Comanita 4.2 Dendrimers on Polymers 4.2.1 Dendrimers on Flexible Polymers In general, the polymerization of macromonomers leads to low MW materials and the DP of the backbone is often similar to the DP of the macromonomer. Consequently, these polymers are nearly spherical and resemble star polymers in dilute solution. However, it has been shown recently that high MW poly(macromonomers) have a rodlike conformation in dilute solution [102, 103]. In the particular case studied, polystyrene macromonomers linked by an ethylene oxide spacer to an end-standing methacrylate group have been polymerized to DP»1000. It is concluded that the central methacrylic backbone adopts a stretched conformation due to the constraining long and bulky polystyrene side chains. The polymer resembles a “bottle brush” [104]. X-ray scattering experiments in semi-dilute solution further revealed a lyotropic phase consistent with the high aspect ratio of the polymer [105]. The second impetus to consider monomers with dendritic side groups comes from the work of Percec et al. on monomers with tapered side chains, e.g., polymerization of 3,4,5-tris (4'-dodecyloxybenzyloxy) benzoic acid ethylene glycol (n=1,2,3,4) methacrylates yields polymers which self-assemble into a tubular supramolecular architecture with radii between 27 and 36 Å [106]. Dendrimers on polymers may therefore be expected to influence the backbone conformation and be a possible source of new polymeric materials. The first macromonomers carrying dendrimers have been prepared by Fréchet and Hawker who attached their poly(benzyl ether) dendrimer [G-3], [G-4], and [G-5] (see Scheme 3) to a styrenic functional group [107] thereby placing seven bonds between the backbone carbon and the focal point. Copolymerization with styrene produced high MW material. The copolymer composition is equal to the monomer composition, implying that the dendritic substituent does not affect the reactivity of the styrenic double bond. Styrenic monomers with poly(benzyl ether) dendrimer of general structure (1) (2)
Dendrimers and Dendrimer-Polymer Hybrids 207 have been homopolymerized radically in concentrated solution in toluene or in the bulk [108]. The higher generation monomer requires higher polymerization temperatures and a higher radical initiator concentration. Yields are over 50%. Analysis by SEC (polystyrene equivalent) indicates that the polymers with larger dendrons appear to have a lower degree of polymerization. This apparent result can be due in part to the bulkiness of the substituents. More troublesome are the observed wide MWDs (in bulk MWD>10) which suggest that extensive chain transfer may occur in the bulk polymerization. Absolute MW determinations of two fractions based on the [G-2 ] dendrimer indicate MWs of 2·10 5 and 2·10 6 with very small radii of gyrations and hydrodynamic radii. A second generation dendrimer has also been linked to a styrenic moiety by a urethane bond [109] and polymerized to an apparent (SEC) DP=51. After hydrolysis of the peripheral tetrahydropyran groups a water soluble polymer is expected [109]. Schlüter and coworkers have made systematic studies of the polymerizability of methacrylic monomers modified by Fréchet type dendrons of generations 1–3[109]. They observe that clean polymerization is limited to methacrylate monomer with generation 1 dendron when the dendron is directly linked to the monomer, i.e., when the focal branch point of the dendron is separated by four bonds from the nearest backbone carbon. Insertion of a p-benzyloxy group causes a nine bond separation and affords homopolymerization of [G-1] and [G-2] substituted monomers. These observations on dendrimer carrying methacrylate monomers confirm earlier results [110, 111]. These observations are also consistent with results of Drahem and Ritter on the polymerization of methacrylamide monomers carrying four and eight peripheral ester functions dendritically bound to an L-aspartic acid chiral center [112]. The number average DP of the first generation dendrimer monomer is estimated by SEC to be only 23, based on polystyrene equivalent SEC elution vol- (3) (4) (5)
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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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Poly(macromonomers): Homo- and Copo
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Poly(macromonomers), Homo- and Copo
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