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

208 J. Roovers, B. Comanita
ume. Because of the branched nature of the substituent the true value is probably
much larger. The optical rotations of the second generation monomer and its
polymer are identical. This does not allow any conclusion about a possible secondary
structure, e.g., helix formation in the polymer [112].
In conclusion it has turned out to be difficult to polymerize all but the first
generation dendritic macromonomer. Higher generation dendrimers need to be
attached through a long series of bonds to the polymer backbone. This in turn
prevents any strong conformational influence on the backbone and no interesting
new properties have been detected so far.
4.2.2
Dendrimers on Stiff Backbones
If dendrimers are introduced on stiff backbone polymers cylindrical molecules
are created and an empty inner volume becomes available between the stiff
backbone and the outer dendrimer shield. Furthermore the peripheral groups of
the dendrimer can be manipulated to control the solubility characteristics of the
resulting polymers.
Two routes to dendrimer modified stiff backbone polymers have been explored.
The first involves the polycondensation of dendrimer substituted monomers.
Monomers of the type
in which the dendrimer is a Fréchet type poly(benzyl ether) have been crosscoupled
with phenylene diboronic acid in the presence of Pd[PPh 3] 4 under Suzuki
conditions. Good yields of polyphenylenes have been obtained in all cases
[113]. SEC provides polystyrene equivalent MWs in the 20–60 K range. Absolute
MW determination suggests that true MWs are about five times larger [113].
These results prove that the poly(paraphenylene) backbone can be covered with
one third-generation dendrimer on every second phenylene ring.
Synthesis of more densely covered poly(paraphenylene) has also been attempted
with two other monomers:
(6)
(7)