Chemistry Reference
In-Depth Information
Figure 6.
Relative peak elution volume (or peak elution time) of linear PS standards (solid diamonds) and the four modes
observed in the PS star fractogram (open triangles) shown in Figure 5.
halide exchange reaction.
[8]
In the case of
the chemistry used to make this 3-arm star
sample, synthesis of species having more
than 3 arms is entirely possible.
Interpretation of the MTF fractogram
has revealed some interesting characteris-
tics of theMTF separation mode. First, long
chain branched molecules are retained
substantially longer than linear molecules
of the same molecular weight. Referring to
Figure 6, one can see that a 4-arm star is
retained nearly a factor of two longer than a
linear molecule of the same molecular
weight (the highest molecular weight linear
standard in the figure is approximately the
same molecular weight as the 4-arm star).
Additionally, the 3-arm stars are also
retained much longer than linear molecules
of the same molecular weight by about a
factor of 1.4. Secondly, the 3-arm star and
4-arm star co-eluted in SEC, but these
species are clearly separated in the MTF
mode. Thus, MTF would not only appear to
separate long chain branched molecules
from linear molecules, but appears to
separate long chain branched molecules
star would be expected to have a molecular
weight of
3,750 kg/mol based on the
molecular weight of the uniarm species.
Thus, the third mode appears to represent
the 3-arm star material. Finally, the fourth
mode includes molecular weights ranging
from
7,000 kg/mol. Near
the apex of the fourth mode, the M
W
is
4,500 kg/mol to
5,000 kg/mol. The expected Mw of a
4-arm star is 5,000 kg/mol. The fourth mode
would appear to be, predominantly, 4-arm
star material. The higher molecular weights
detected near the tail of the distribution
most likely stem from low signal to noise in
both the concentration detector and light
scattering detector traces due to the very
low concentration of material present.
Based on the reaction scheme used to
make the star sample, one would not
immediately expect the formation of a
4-arm star. However, in the synthesis of star
polymers, it has been observed that mole-
cules having more arms than the function-
ality of the coupling agent are often
produced. The chemistry responsible for
this observation is known as the lithium-
