Chemistry Reference
In-Depth Information
with charged reagents. Sometimes, chain flexibility or folding can cause functional groups to come
together and interact, though they may be located well apart on the polymer backbone. Polymer
solutions of this type are comparable to dispersions of individual droplets of concentrated solutions.
Some statements above may require additional clarification. An elaboration, therefore, follows.
9.1.1 Diffusion-Controlled Reactions
Reactions that are bimolecular can be affected by the viscosity of the medium [
9
]. The translational
motions of flexible polymeric chains are accompanied by concomitant segmental rearrangements.
Whether this applies to a particular reaction, however, is hard to tell. For instance, two dynamic
processes affect reactions, like termination rates, in chain-growth polymerizations. If the termination
processes are controlled by translational motion, the rates of the reactions might be expected to vary
with the translational diffusion coefficients of the polymers. Termination reactions, however, are not
controlled by diffusions of entire molecules, but only by segmental diffusions within the coiled chains
[
10
]. The reactive ends assume positions where they are exposed to mutual interaction and are not
affected by the viscosity of the medium.
9.1.2 Paired Group and Neighboring Group Effects
When
reactions occur on polymeric backbones with the
functional groups adjacent to each other, they can be expected to react. There is, however, an upper
limit to conversions. This upper limit is due to statistical probability that some functional groups are
bound to become isolated. The limit for conversion was calculated to be 86.5% [
11
].
Theoretically, quantitative conversions should be possible with
random
,
irreversible
, and
intramolecular
reactions of paired
functional groups on macromolecular backbones. The ability, however, of isolated reactive groups
to find each other and then pair off depends either upon particularly high driving forces, or upon the
time required to accomplish complete conversions [
12
]. For reactions initiated randomly, at different
sites, the probability is high that two groups on the terminal units will eventually meet and react.
Since the reactions are reversible, at least in theory, very high conversions are possible.
Neighboring group participation can usually be deduced from three types of evidence:
1. If the reactions occur more rapidly during the rate-determining step than can be expected from
other considerations.
2. If the stereochemistry of the reactions suggests neighboring group involvement.
3. If molecular rearrangements occur and the groups remain bonded to reaction centers, but break
away from the atoms to which they were originally attached on the substrates.
reversible
There are many examples in the literature that describe neighboring group effects in reactions of
polymers. One example is hydrolysis of poly(
-nitrophenyl methacrylate-co-acrylic acid). The high
reaction rate at a neutral pH is due to attacks by the carboxylic moieties upon the neighboring
carbonyl carbons [
13
-
15
]. Decomposition rates of
p
-butyl acrylate-styrene copolymers [
16
] can serve
as an another example. Experimental data show pronounced acceleration for all samples. This is
interpreted in terms of both intra and intermolecular interactions of the esters and the carboxylic
groups. It follows a suggestion of Cherkezyan and Litmanovich [
16
,
17
] that the instantaneous
reactivity of any group depends on its microenvironment. That includes (for reactions of polymer
in molten condition) two nearest units on the same chain (internal neighbors) and two units belonging
to two different chains (external neighbors).
t
