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

200 J. Roovers, B. Comanita
tions. However, dendrimers with COOH end groups have a 35% larger hydrodynamic
radius in neutral or basic than in acidic solution. Dendrimers terminated
with CH 2 NH 2 groups have values of R h 35% larger in neutral or acidic conditions
than in a basic medium. These documented expansions are clearly due to electrostatic
repulsion. Because measurements have been reported only at one concentration
(c£10 –3 mol/l) it is not clearly established that ionizable dendrimers
act as polyelectrolytes [72]. The observation that ionic strength has little effect
on dimension suggests that considerable screening occurs at the concentration
of the measurements.
Dubin et al. measured [h] and D o of seven generation of carboxylated
PAMAM dendrimers in NaNO 3. NaH 2 PO 4 aqueous buffer at pH 5.5 and
0.38 mol/l in order to minimize electrostatic interaction [85]. The values of R h of
the 2.5 and 3.5 generation dendrimer agree closely with those of the corresponding
methyl ester dendrimer in MeOH. Rather surprisingly, the quoted values of
[h] do not go through a maximum expected for the higher generation dendrimers.
4
Dendrimer-Polymer Hybrids
4.1
Polymeric Dendrimers
The dendrimers discussed in the previous sections have short spacers containing
usually between two and eight bonds between neighboring branch points. It
is, however, also possible to work with polymeric chains rather than short spacers
albeit with some some sacrifices to regularity and symmetry. Arborescent
[86] and combburst [87] polymers contain linear polymer chains at the core and
in each generation. These polymers have several trifunctional branch points
along each polymer chain, rather than the end-standing branch points of the
classical dendrimer. The branching functionality, considered per polymer chain
is usually high (7–15) compared to 2 to 4 in a dendrimer. As a consequence the
molecular weight and number of branch points increases very rapidly with each
generation (see Scheme 8a and Table 1). The arborescent and combburst polymers
are expected to have a high segment density and near spherical conformation.
At this point there is no experimental evidence on the segment density distribution
with a possible shell like structure or extensive backfolding.
Polymeric dendrimers have been synthesized by anionic, cationic and free
radical polymerization. Gauthier and Möller and their collaborators used two
alternating reactions of chloromethylation of polystyrene and grafting with polystyryllithium
to create arborescent polystyrenes [86] (see Scheme 9a). Tomalia
and co-workers used hydrolysis of poly(ethyloxazoline) to create secondary
amines in poly(ethylene imine) onto which poly(ethyloxazoline) chains are
grafted by means of the living oxazolinium end group [87] (see Scheme 9b). In
the original work [86, 87] the individual polymer chains are kept small (DP=20