prominent members of the Italian school, Piero Pino and Mario Farina, chemists
who had been present at the genesis of the discoveries of stereoregular polymers in
Milan under the guidance of Giulio Natta. I discovered from this reading how
important was the year 1953, when the Nobel Prize was awarded to Hermann
Staudinger, who fought, finally successfully, to convince the world of science
that the kinds of long chains arising from styrene, vinyl acetate, vinyl chloride,
and ethylene, among others, were what he imagined them to be and what his
experimental work demonstrated – long chains linked by the kinds of covalent
bonds well accepted in small molecules. Staudinger expressed this view in the
beginning of his Nobel lecture [1]: “The only difference between macromolecules
and the small molecules of low molecular substances is one of structural size.”
Nevertheless, he fully realized that new phenomena would arise as a consequence
of the considerable size differences, as he expressed a few sentences later in the
same lecture: “It is desired to lay down a boundary between macromolecular and
low molecular compounds – there are of course transitions linking the two groups –
the substances with a molecular weight greater than 10,000, i.e. the molecules of
which consist of 1,000 or more atoms, may be classified as macromolecular.
Beyond roughly this size, characteristic macromolecular properties occur.” As
discussed below, stereochemical considerations of macromolecules and especially
issues of chirality are excellent examples of Staudinger’s prescient views about
“characteristic macromolecular properties.” The story begins in Milan when polypropylenes of various tacticities were produced.
There is something very interesting about the stereochemistry of atactic polypropylene. From inspection of the structure of a single chain, even stretched out in a
planar zig-zag conformation so as to avoid dissymmetry arising from chiral shapes,
it is apparent that the irregular configurations of the pendant methyl groups would
mean that mirror images do not superimpose, even for a portion of the chain. This is
the foundation of chirality and must mean that atactic polypropylene is chiral.
However, although it follows that there is a potential for the observation of optical
activity, this observation was never made for atactic polypropylene or for other
atactic polymers such as polystyrene. Why not?
In small molecules, the absence of optical activity at some wavelength for chiral
molecules is almost always ascribed to the fact that the ensemble of molecules
contains equal number of both enantiomers – a racemic mixture. However, in a
chain longer than a hundred or so units, statistical considerations demonstrate that
the presence of mirror image isomers of enantiomeric chains, and therefore for
racemic states, is virtually impossible, which leaves the absence of optical activity
in atactic vinyl polymers an open question. The answer turned out to be one that
never arises in small molecule stereochemistry: an ensemble of polymer chains of
an atactic polymer is a mixture of diastereomeric chains, each one chiral but
without the enantiomeric chain present in the ensemble. If a single chain could be
studied by a method that could reveal chiral optical properties, optical activity
should be observed. However, each chain in the ensemble (a very large number of
chains) would exhibit a different optical activity, even of differing sign. The optical
activity properties of a sample of an atactic polymer would arise as the sum
264
M.M. Green
who had been present at the genesis of the discoveries of stereoregular polymers in
Milan under the guidance of Giulio Natta. I discovered from this reading how
important was the year 1953, when the Nobel Prize was awarded to Hermann
Staudinger, who fought, finally successfully, to convince the world of science
that the kinds of long chains arising from styrene, vinyl acetate, vinyl chloride,
and ethylene, among others, were what he imagined them to be and what his
experimental work demonstrated – long chains linked by the kinds of covalent
bonds well accepted in small molecules. Staudinger expressed this view in the
beginning of his Nobel lecture [1]: “The only difference between macromolecules
and the small molecules of low molecular substances is one of structural size.”
Nevertheless, he fully realized that new phenomena would arise as a consequence
of the considerable size differences, as he expressed a few sentences later in the
same lecture: “It is desired to lay down a boundary between macromolecular and
low molecular compounds – there are of course transitions linking the two groups –
the substances with a molecular weight greater than 10,000, i.e. the molecules of
which consist of 1,000 or more atoms, may be classified as macromolecular.
Beyond roughly this size, characteristic macromolecular properties occur.” As
discussed below, stereochemical considerations of macromolecules and especially
issues of chirality are excellent examples of Staudinger’s prescient views about
“characteristic macromolecular properties.” The story begins in Milan when polypropylenes of various tacticities were produced.
There is something very interesting about the stereochemistry of atactic polypropylene. From inspection of the structure of a single chain, even stretched out in a
planar zig-zag conformation so as to avoid dissymmetry arising from chiral shapes,
it is apparent that the irregular configurations of the pendant methyl groups would
mean that mirror images do not superimpose, even for a portion of the chain. This is
the foundation of chirality and must mean that atactic polypropylene is chiral.
However, although it follows that there is a potential for the observation of optical
activity, this observation was never made for atactic polypropylene or for other
atactic polymers such as polystyrene. Why not?
In small molecules, the absence of optical activity at some wavelength for chiral
molecules is almost always ascribed to the fact that the ensemble of molecules
contains equal number of both enantiomers – a racemic mixture. However, in a
chain longer than a hundred or so units, statistical considerations demonstrate that
the presence of mirror image isomers of enantiomeric chains, and therefore for
racemic states, is virtually impossible, which leaves the absence of optical activity
in atactic vinyl polymers an open question. The answer turned out to be one that
never arises in small molecule stereochemistry: an ensemble of polymer chains of
an atactic polymer is a mixture of diastereomeric chains, each one chiral but
without the enantiomeric chain present in the ensemble. If a single chain could be
studied by a method that could reveal chiral optical properties, optical activity
should be observed. However, each chain in the ensemble (a very large number of
chains) would exhibit a different optical activity, even of differing sign. The optical
activity properties of a sample of an atactic polymer would arise as the sum
264
M.M. Green