isotactic polymers were not restricted to the crystalline regions where the helical
state could be directly observed by the diffraction data.
From the intense interest in polymers at Brooklyn Polytechnic and needless to
say the presence of Herbert Morawetz, Eli Pearce, and Fred Eirich in addition to
Herman Mark, one could not avoid an attraction to the field, whose signature
characteristic, one early learns, is cooperativity. The results from Pisa [3, 5], in
fact, were a perfect example of the cooperativity arising from the helical conformation. However, the helical state of vinyl-derived polymers does not exist in a
deep energy well and therefore the helix is easily interrupted by defects in this
conformational state. The ease of these interruptions and the inherent flexibility of
the bonds along the backbone of the polymer lead to a limited persistence length.
The polymer can be described as a random coil on a small length scale.
At DuPont corporation in the 1950s, as it became clear that control of nylon 6,6
production and sale would inevitably be out of the corporation’s control, a polymer
was synthesized, nylon 1, which was designed to allow DuPont to control another
nylon-forming fiber that was of possible commercial importance. Anionic polymerization of alkyl isocyanates yields a polymer À(R)NCOÀ with stiff fiberforming properties. However, hopes were dashed when the ceiling temperature of
the polyisocyanate (nylon 1) was discovered to be near the boiling point of water.
There was, however, a theoretical interest in these polymers because of their
very high viscosity at moderate molecular weights, and diffraction data that showed
a helical conformation. The viscosity properties demonstrated a resistance of the
polymer chain to distort from a single conformation, consistent with an unusually
high measured persistence length, and this was further confirmed by the observation
for poly(n-hexyl isocyanate) of lyotropic liquid crystal formation. Walter
Stockmayer and other polymer physicists took an interest in the question of the
source of the limit to the persistence length of such a helical polymer. This interest
turned out, in a surprising manner, to be connected to work on this polymer at the
Polymer Research Institute. Murray Goodman, in the late 1960s, showed optical
activity properties for the polyisocyanates when the alkyl pendant groups on each
nitrogen atom of the chain were chiral. The experimental results were seen as
consistent with polymer dissymmetry, which was interpreted as a “preferred conformation of the polymer backbone” [6].
One possibility for the limit to the persistence length in the polyisocyanates was
seen as the presence of helical reversals along the chain backbone. These defects in
the conformational regularity could arise as a consequence of the stereochemical
necessity that the left- and right-handed helices are enantiomerically related and
therefore of equal probability. Helical reversals are especially interesting considering that such states are blocked (with rare exceptions) in biological helical polymers.
If one blocked or reduced the number of helical reversals in this synthetic polymer,
by favoring one helical sense, would the persistence length increase? However, how
is one to accomplish favoring one helical sense without decreasing the torsional
motions along the chain backbone, which is another source of the polymer flexibility
and therefore also a limit to persistence length? Chiral pendant groups, as used by
Pino on the vinyl polymers, and Goodman on the polyisocyanates, are inherently
266
M.M. Green
state could be directly observed by the diffraction data.
From the intense interest in polymers at Brooklyn Polytechnic and needless to
say the presence of Herbert Morawetz, Eli Pearce, and Fred Eirich in addition to
Herman Mark, one could not avoid an attraction to the field, whose signature
characteristic, one early learns, is cooperativity. The results from Pisa [3, 5], in
fact, were a perfect example of the cooperativity arising from the helical conformation. However, the helical state of vinyl-derived polymers does not exist in a
deep energy well and therefore the helix is easily interrupted by defects in this
conformational state. The ease of these interruptions and the inherent flexibility of
the bonds along the backbone of the polymer lead to a limited persistence length.
The polymer can be described as a random coil on a small length scale.
At DuPont corporation in the 1950s, as it became clear that control of nylon 6,6
production and sale would inevitably be out of the corporation’s control, a polymer
was synthesized, nylon 1, which was designed to allow DuPont to control another
nylon-forming fiber that was of possible commercial importance. Anionic polymerization of alkyl isocyanates yields a polymer À(R)NCOÀ with stiff fiberforming properties. However, hopes were dashed when the ceiling temperature of
the polyisocyanate (nylon 1) was discovered to be near the boiling point of water.
There was, however, a theoretical interest in these polymers because of their
very high viscosity at moderate molecular weights, and diffraction data that showed
a helical conformation. The viscosity properties demonstrated a resistance of the
polymer chain to distort from a single conformation, consistent with an unusually
high measured persistence length, and this was further confirmed by the observation
for poly(n-hexyl isocyanate) of lyotropic liquid crystal formation. Walter
Stockmayer and other polymer physicists took an interest in the question of the
source of the limit to the persistence length of such a helical polymer. This interest
turned out, in a surprising manner, to be connected to work on this polymer at the
Polymer Research Institute. Murray Goodman, in the late 1960s, showed optical
activity properties for the polyisocyanates when the alkyl pendant groups on each
nitrogen atom of the chain were chiral. The experimental results were seen as
consistent with polymer dissymmetry, which was interpreted as a “preferred conformation of the polymer backbone” [6].
One possibility for the limit to the persistence length in the polyisocyanates was
seen as the presence of helical reversals along the chain backbone. These defects in
the conformational regularity could arise as a consequence of the stereochemical
necessity that the left- and right-handed helices are enantiomerically related and
therefore of equal probability. Helical reversals are especially interesting considering that such states are blocked (with rare exceptions) in biological helical polymers.
If one blocked or reduced the number of helical reversals in this synthetic polymer,
by favoring one helical sense, would the persistence length increase? However, how
is one to accomplish favoring one helical sense without decreasing the torsional
motions along the chain backbone, which is another source of the polymer flexibility
and therefore also a limit to persistence length? Chiral pendant groups, as used by
Pino on the vinyl polymers, and Goodman on the polyisocyanates, are inherently
266
M.M. Green