Chemistry Reference
In-Depth Information
FIGURE 4.1
Comparison of achievable main chain length in dependence of generation for
the macromonomer (left) and attach-to route (right) to dendronized polymers. For the former
the main chain lengths decrease with increasing generation (dark gray cylinders) and as a result
there is no report on a fifth-generation polymer by this method. The light gray extensions
indicate that under supercritical carbon dioxide conditions longer chains may be achievable.
Accordingly, at least an oligomeric
PG5
could recently be obtained for the first time. For the
latter method the main chain length and its distribution may stay more or less constant if
appropriate synthetic measures are taken. This is the key advantage of the attach-to route.
Preliminary studies show that
PG6
may be within reach.
length is not the generation of the dendrons as such but rather their steric demand that
is reflected to some degree by molar mass [6]. This shows nicely when comparing
monomer
and should therefore
give the shorter chains at first glance. As expected for the macromonomer route, both
monomers gave relatively short chains with number average degrees of polymeri-
zation P
n
of 30-40 for
M1(G4)
that has a higher generation than
M2(G3)
P1(G4)
and 10-15 for
P2(G3)
. Thus, despite its higher
.
2
This can qualitatively be
understood in terms of the macromonomers' molar masses. Whereas
generation,
P1(G4)
has the longer chains than
P2(G3)
M1(G4)
has
7700 g/mol.
3
The latter monomer's steric demand is
therefore more pronounced, which results in shorter chains.
This disadvantage of the macromonomer route is known since the early days of
dendronized polymers and is of very similar nature to what had priory been observed
with linear macromonomers [7]. It has thus always been of some concern to develop
4900 g/mol,
M2(G3)
has
2
It is delicate to compare main chain lengths obtained with different monomers in different experimental
settings. Though the authors' laboratory tries to meet certain standards as to monomer quality (e.g., purity),
it cannot be excluded that both monomers have unknown impurities in unknown (low) molar ratios. Given
the substantial experience in macromonomer polymerizations, the authors nevertheless consider the
described difference relevant. Note that the above argumentation implies that the elution behavior of the
two different dendronized polymers on the GPC column is the same that is unproven.
3
The dendron of the lower generation
M2G3
has a higher branching multiplicity (3) than
M1G4
(2) and
also longer segments between the branch points.
