Chemistry Reference
In-Depth Information
Conformational energy calculations
17,
44
on this polymer indicate that the
attractions should be particularly strong in the case of a pair of phenyl
groups in adjacent repeat units. Therefore, the chains should have a ten-
dency to adopt conformations in which two phenyl groups are apposed on
the same side of the chain.
17
For the syndiotactic polymer this effect favors
gauche
states, which favor relatively high spatial extension, but suppress
of such states for the isotactic polymer. As a result, the characteristic ratio
is predicted to be very small for the isotactic polymer and to increase lin-
early with increases in the number of syndiotactic placements in the
chain. These results are quite different from those calculated for mono-
substituted [-CHRCH
2
-]
x
vinyl or disubstituted [-CRR'CH
2
-]
x
vinylidene
chains, including the structurally analogous poly(α-methylstyrene)
[-C(CH
3
)(C
6
H
5
)-CH
2
-]
x
.
17
A characteristic ratio of 8.8 was reported for several samples of
poly(methylphenylsiloxane),
17,
45
at least some of which were known to be
essentially atactic. This experimental result, however, can be reproduced
from the model only by assuming a large fraction of syndiotactic place-
ments; the temperature coefficient predicted for this degree of syndiotac-
ticity is then also in good agreement with experiment. The assumption
of significant syndiotacticity is in disagreement with NMR results and
with the results of cyclization studies, both of which suggest that
poly(methylphenylsiloxane) is essentially atactic. The cyclization results,
however, yield a prediction for the characteristic ratio that is significantly
larger than the experimental value of 8.8. The two tentative conclusions
regarding the stereochemical structure might be brought into closer
agreement by improving the calculation of the interaction energy of two
apposed phenyl groups to take into account the fact that they would be
less exposed to favorable interactions with the solvent in such conforma-
tions. This effect is apparently quite important in polystyrene [-CH(C
6
H
5
)
-CH
2
-]
x
but may be less so in poly(methylphenylsiloxane) because of the
larger distance of separation between side groups in the siloxane poly-
mers. Such revision could increase the number of isotactic placements
without decreasing its predicted value of the characteristic ratio to below
its known experimental value. In any case, resolution of this point really
requires reliable experimental values of the characteristic ratio, deter-
mined on samples of known stereochemical structure.
Cyclization measurements have also been conducted on other stereo-
chemically variable polysiloxanes [-Si(CH
3
) R-O-]
x
, where R is H, CH
2
CH
3
,
CH
2
CH
2
CH
3
, and CH
2
CH
2
CF
3
. The conclusion from these investigations
was that such polymers are generally atactic and that an increase in the
length or size of the side chains increases the characteristic ratio.
37,
38,
42
