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

216 J. Roovers, B. Comanita
phase boundaries is dependent on the polymer concentration. The boundary
between the monomolecular and micellar phases depends on whether the experimental
concentration is above or below the critical micelle concentration.
The boundary between micellar solution and gel is defined by the equilibrium
between loops and bridges formed by the soluble PEO blocks. Bridges are entropically
favored. A similar, less complete, phase diagram is also obtained for AB
type polymer-dendrimer block copolymers. However, in that case, no gel phase
is produced for lack of bridging.
The structures formed by polystyrene-poly(propylene imine) dendrimers
have also been analyzed. Block copolymers with 8, 16, and 32 end-standing
amines are soluble in water. They have a critical micelle concentration of the order
of 10 –7 mol/l. At 3·10 –4 mol/l they form different types of micelles. The dendrimer
with eight amine groups (80% PS) form bilayers. The dendrimer with 16
amine groups (65% PS) forms cylinders and the dendrimer with 32 amine
groups (50% PS) forms spherical micelles [38, 130, 131]. These are the classical
lamellar, cylindrical, and spherical phases of block copolymers. However, the
boundary between the phases occurs at very different volume fractions, due to
the very different packing requirements of the linear polymer and spherical
dendrimer at the interphase.
4.3.2
Multiple Polymer-Dendrimer Hybrids
Dendrimers with their multiple end-standing functional groups are ideally suited
for the construction of star-shaped polymers. Indeed, the end-standing functional
groups can be used as initiators for polymerization (“grafting from”
method) or as functional groups for “grafting onto”. They can also be used as redistribution
centers in equilibrium polymerization.
“Grafting from” has not been a successful method in anionic polymerization
because the required low molecular weight multifunctional organometallic initiators
are almost always insoluble and this is also expected to be the case when
dendrimers are modified. However, in cationic polymerizations the dormant
species is less polar and more soluble. For example, the hexabenzyl bromide
(10)
has been used in the polymerization of 2-methyl-2-oxazoline [133] and it can be
envisioned that larger dendritic initiators based on phosphorus can also be used
[134].
The “grafting from” method is also successful in other ring opening polymerizations.
For example, e-caprolactone polymerization is initiated from poly(propylene
imine) dendrimers with n=2–16 end-standing amine groups [135]. To be