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

Dendrimers and Dendrimer-Polymer Hybrids 203
butadienes have also been prepared. A branch MW=10,000 is equivalent to
about 200 monomers, 6% of which are converted to branch points [91].
The physical properties of these polymeric dendrimers have been studied to
some extent. Intrinsic viscosity measurements combined with MW afford values
of R h according to Eq. (5). Alternatively, the translational diffusion coefficient
leads to R h according to Eq. (6). These equations may well be applicable, since it
is observed that R h and R h scale with the 1/3 power of MW in support of the
equal density hard-sphere assumption [88].
Comparison of R h or R h of two consecutive generations yields an apparent
shell thickness. It is significant that the shell thickness of each generation increases
with increasing generation at constant branch MW (see Table 1). The apparent
thickness of the outer shell is always larger that the unperturbed end-toend
distance of the polymer chain. In some cases the value approaches the dimensions
of the fully stretched chain. This appears to be a good indication that
the addition of a new generation also enlarges the radius of the interior parent
dendritic polymer. A full proof would require measurements of R g on an inner/outer
labeled polymeric dendrimer. Because the branching process is random,
it is unlikely that the polymeric dendrimers have strongly segregated generational
shells. Increasing crowding in each generation and the resulting tendency
of all chains to stretch should, however, introduce some radial segregation
of the material in consecutive shells.
The intrinsic viscosities of the dendritic polymers are extremely small compared
with those of linear polymer of the same MW [86, 91]. Furthermore, the
dendritic polymers expand very little in going from a q solvent to a good solvent
[92]. This is to be expected. When steric congestion forces the polymer chains to
expand in a q solvent, further expansion in a good solvent is limited. In this regard
it is important to note that the q condition must be carefully specified. It is
known that branched polymers have different q conditions to the linear counterpart
[93].
Gauthier and coworkers have expanded the synthesis of dendritic polymers
to dendritic graft copolymers in which the inner generations are polystyrene
and the outer generation consists of polyisoprene [94] (Scheme 8b). They have
also prepared amphiphilic graft copolymers in which the inner generations are
polystyrene and the polymer chains in the peripheral shell are extended by poly(ethylene
oxide) (PEO) [95] (Scheme 8c). In order to accomplish this, the last
PS generation has been initiated with a lithium compound carrying a protected
hydroxyl group. After deprotection, the hydroxyl group was activated with the
potassium counterion for the polymerization of ethylene oxide. Comparison of
the hydrodynamic radii before and after the extension with PEO indicated a
rather small expansion due to the PEO chains, in spite of possible internal phase
separation of the PS and PEO units in the polymer.
Monolayers of arborescent polystyrenes have been investigated by scanning
force microscopy [90]. Dense polymers with a small branch spacing (M b~500 D)
are nearly spherical and become more spherical on annealing, thereby causing
the break-up of the film. On the other hand less densely branched polymers