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

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184 J. Roovers, B. Comanita Scheme 2 out scheme starting from the core and proceeding to the periphery, Vögtle synthesized poly(alkylene imine)s by means of two alternating reactions [2]: (1) the Michael addition of a primary amino group to acrylonitrile, (2) the hydrogenation of the nitrile group to regenerate the amino group (Scheme 1). The primary amines are now available for a new cycle of Michael addition and hydrogenation. The overall yield was originally limited by the poor yield of the hydrogenation step. Improved hydrogenation methods have been found later independently by Wörner and Mülhaupt [4] and workers at DSM [5, 6]. This made the large scale synthesis of poly(propylene imine)s possible. The DSM dendrimers AS- TRAMOL are based on the 1,4-diaminobutane core and are available to generation 5 which contains 64 primary amine groups (see Scheme 1). Other commercially available dendrimers containing nitrogen branching points were introduced by Tomalia at Dow and Dendritech. They are based on the Michael addition of primary amines to methyl acrylate followed by aminolysis of the ester function with excess ethylene diamine [3] (see Scheme 2). The resulting dendrimers are poly(amidoamine)s (PAMAM) and have been prepared to the 10th generation. Details of the reaction conditions and limitations brought about by side reactions have been given [7]. Dendrimers with carbon branch points are more difficult to prepare. They have been synthesized and are known as “Arborols” [8]. Path B in Fig. 2 is the convergent method. It is the outside-inward method, proposed independently by Miller and Neenan [9] and by Hawker and Fréchet [10]. This method is well suited when the branch point is an aromatic ring. As an example of the convergent process we show in Scheme 3 the preparation of poly(benzyl ether) dendrimers. The phenol functionality of 2,5-dihydroxybenzyl alcohol is first protected by Williamson reaction with benzyl bromide to provide the first generation dendron [G-1]-OH. The benzyl alcohol in [G-1]-OH is then converted to the benzyl bromide form [G-1]-Br. This in turn reacts with
Dendrimers and Dendrimer-Polymer Hybrids 185 Scheme 3 2,5-dihydroxybenzyl alcohol to yield [G-2]-OH. Scheme 3 illustrates also the synthesis of the generation-3 dendrimer from a generation-3 dendron in a selfexplanatory pictorial manner. The advantages of the convergent method over the divergent method are that each generation requires limited (usually two) reactions per molecule. Furthermore, unreacted material is easily separable because it is substantially different in molecular weight from the product. As a consequence, organic reactions producing lower yields ( ‡90%) can be tolerated in convergent synthesis. In contrast, the divergent synthesis involves an increasing number of identical reactions per molecule and requires high yield (>99%) reactions in order to minimize imperfect products that are practically unseparable. The main disadvantage 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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Author Index Volumes 101-142 235 Og
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Author Index Volumes 101-142 237 Su
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Subject Index Addition-fragmentatio
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Subject Index 241 Microphases 11z,
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