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

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