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

Dendrimers and Dendrimer-Polymer Hybrids 211
sidered unlikely to be successful. The reverse Williamson substitution, using a
iodo leaving group on the polymer, is more successful yielding 100, 100, and
70% substitution for [G-1]-OH, [G-2]-OH, and [G-3]-OH respectively (see
Scheme 13b).
The most successful substitution reaction is via the isocyanate route. The poly
(paraphenylene) backbone polymer chosen for this work
has one extra carbon-carbon bond between the phenyl backbone and reactive
hydroxyl group. Reaction with isocyanate focal group of a third generation Fréchet
type poly(benzyl ether) dendrimer gave a 91–92% substitution yield, the
highest attained with a third generation dendrimer [113, 117]. The status of this
type of research at this point indicates that only one dendrimer, up to generation
three, can be incorporated in every second phenyl ring along the polymer backbone.
Moreover, the chemical route followed is of importance. The isocyanate
route with a reaction center slightly further removed from the backbone gives
the highest rate of substitution.
4.3
Linear Polymers on Dendrimers
4.3.1
Single Polymer-Dendrimer Hybrids
Hybrids of linear polymers and dendrimers are expected to be unusual block copolymers
because they combine, in one molecule, a long flexible chain with a
random coil conformation with a dense globular dendrimer. Gitsov et al. prepared
the first polymer-dendrimer hybrids by reacting monofunctional narrow
MWD PEO with their [G-3] and [G-4] poly(benzyl ether) dendrimers [118–121]
(see Scheme 14a). This linear polymer-dendrimer hybrid is comparable with an
AB type block copolymer. The ABA hybrids are similarly obtained from difunctional
poly(ethylene glycols) (PEG) (see Scheme 14b). The Williamson reactions
are performed in dry THF at room temperature with a small excess of dendrimer.
Yields are in excess of 90%. A four-arm star PEO has similarly been modified
with [G-2],[G-3], and [G-4] dendrimers by means of the focal benzyl bromide
group in yields of 90, 85, and 80%, respectively [122]. The same hybrids can also
be obtained by transesterification in the melt catalyzed by tin or cobalt salts (see
Scheme 14c). Yields range between 50 and 80%. There is one report in which the
linear polymer and dendrimer are joined by means of a peripheral group rather
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