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

Dendrimers and Dendrimer-Polymer Hybrids 213
Scheme 15
esterification. These results are very different from those obtained with potassium
tert-butoxide or the initiator derived from [G-1]-OH. It is therefore proposed
that the large dendrimer provides temporary protection for the growing
chain against transesterification and backbiting reactions.
The focal functional group of the dendrimer has also been used to initiate living
free radical polymerization. For the TEMPO mediated free radical polymerization
of styrene the focal point is converted to a TEMPO derivative [126] (see
Scheme 15b). This species initiates the bulk living polymerization of styrene at
123 ˚C. Alternatively, the benzyl bromide focal group of poly(benzyl ether) dendrimers
can be used in ATRP. In the presence of Cu I Cl/4,4' (di-n-heptyl)-2,2'
dipyridyl complex styrene is polymerized at 130 ˚C [126]. The formation of
block copolymers of polystyrene and dendrimer is confirmed by the absence of
free dendrimer in SEC analysis of the crude product. Furthermore, the polystyrene
peak in SEC has a superposable UV-absorption at 283 nm which is the characteristic
wavelength of absorption of the dendrimer. In both systems the MWs
vary between 10,000 and 100,000 D. Calculated and experimental MWs agree at
low MW but the experimental MW tends progressively to lower values above
30,000 D with a concomitant widening of MWD from 1.19 to 1.5. Radical transfer
reactions to the benzyl ether groups on the dendrimer seem not to be important.
The TEMPO modified dendrimers [G-3] and [G-4] have been used to polymerize
4-acetoxystyrene. The resulting copolymer can be hydrolyzed to an
amphilic block copolymer consisting of a hydrophilic poly(vinylphenol) and hydrophobic
poly(benzyl ether) dendrimer.