Chemistry Reference
In-Depth Information
Another example of the neighboring group effect is the behavior of polyacrylamides in hydrolyses.
There are two distinct and successive rates [
18
]. After conversions of up to 40-50% are reached, the
reactions slow down. This is due to accumulations of negative electrostatic charges on the polymeric
backbones [
18
]. In alkaline media, the increasing negative charges along the chains exert electrostatic
repulsions toward the hydroxyl ions. This results in rate decreases.
9.1.3 Effect of Molecular Size
An example is the effect of DP on the rates of alkaline hydrolyses of poly(vinyl acetate)s. Rapid
increases in the rates can be seen [
19
] in large, but not in small molecules, as the reactions progress.
Solvents that are good for the products, like acetone-water mixtures, are used in these reactions.
These are, however, poor solvents for the staring materials with high DP. Low molecular weight
molecules are more soluble. This means that, at the start of the reaction, the large molecules are coiled
up and the reactive sites not readily available. As the reactions progress, the chains unravel and the
sites became more accessible with accompanying increases in the reaction rates. Because the small
molecules are more soluble, the reactive sites are accessible from the start of the reactions, and the
rates are constant.
There are many report in the literature on the
effects of chain conformation
[
19
-
25
]. One example
is radical bromination of poly(methyl styrene) [
20
] with
-bromosuccinimide-benzoyl peroxide or
Br
2
-K
2
CO
3
-light.
13
C NMR spectroscopy shows differences in reactivities of the methyl groups in the
3 and 4 positions on the benzene rings between isotactic and atactic polystyrenes [
20
].
The differences in reactivities in poly(vinyl alcohol)s between isotactic (
N
meso
) and syndiotactic
(
acetals [
26
-
28
] is another example.
In extending this to model compounds, reactions of stereo isomers of pentane-2,4-diol and heptane-
1,4,6-triol with formaldehyde take place much faster for the meso than for the
dl
-diol) portions of the polymers and between
cis
and
trans
-diol portions
[
26
-
28
]. Even more important are the steric effects imposed by restricted rotations. For instance,
quaternizations of chloromethylated polyether sulfones exhibit decreasing rates at high degrees of
substitution. This can be attributed to restricted rotations of the polymeric chains, because this
phenomenon is not observed with more flexible chloromethylated polystyrene under identical
conditions [
23
,
24
].
dl
9.1.4 Effects of Changes in Solubility
Changes in solubility can occur during the courses of various reactions. Such changes are observed,
for instance, during the chlorination of polyethylene in aromatic and chlorinated solvents [
29
].
There is an increase in the solubility until 30% conversion is reached. After that, solubility decreases
and reaches a minimum at 50-60% chlorine content. Following that it increases again. This, however,
is not typical of many reactions of polymers in solutions. More common is that the starting material is
soluble, but not the product or the opposite is true. Higher conversions are, usually, expected when the
polymers are solvated and the chains are fully extended. In such situations, the reagents have ready
access to the reactive sites [
29
]. If the products are insoluble in the reaction medium and tend to
precipitate as the reaction progresses, the sites become increasingly less accessible. This can result in
low conversions and premature terminations. If the opposite is true and the product is more soluble
