of random numbers and such a large number of random numbers would yield a sum
of zero. The absence of optical activity in such a sample of polymers does not
therefore arise from the absence of a chiral structure, nor does it arise from the
chiral structures with equal numbers of enantiomers (a racemic mixture), but rather
arises from what is never seen in small molecules, a large number of chiral
diastereomers each one in an enantiomerically pure state. Remarkable! [2].
Is there something equally remarkable about isotactic polymers? In any isotactic
polymer, the segment at the initiating end and that at the terminus of the chain
must differ and, therefore, if one considers the entire chain the conclusion is that
this is a chiral structure. In his review in Topics in Stereochemistry in 1987 [3],
Mario Farina noted that the overwhelming number of units along the chain
are oblivious to the end groups of the polymer chain. Considering a limited number
of these internal units, one sees a plane of symmetry and therefore an achiral
structure. Alternatively, one could assign the absence of observed optical activity
in isotactic polypropylene to cryptochirality, a term coined by Mislow [4] for
molecules in which optical activity is too small to be detected at observable
wavelengths. If these factors were not reason enough for the absence of any chiral
optical measure, the nature of the polymerization suffices in producing a racemic
mixture of chains. The enantiotopic faces of the incoming propylene monomers
approach the Ziegler–Natta catalyst with equal probability.
There is, however, another aspect of the chirality of isotactic polymers, which is
associated with the helical form of isotactic polypropylene first observed in the
X-ray diffraction experiments conducted in Milan by the Natta group. In this
manner, Natta discovered the reason for the crystalline properties of this polymer
synthesized from the catalyst developed by Karl Ziegler. But, in a sample of
isotactic polypropylene, or other isotactic polymers arising from vinyl monomers,
both left- and right-handed helical conformations are present so that no chiral
optical property is observed. Each crystalline state in a sample of isotactic polypropylene is a racemic mixture of helical forms.
There is more to the story of isotactic polypropylene arising from Piero Pino’s
interest in answering the following question: Does the helical property of isotactic
polypropylene in the crystalline regions of the sample extend into the amorphous
regions, as in the melted state? Pino decided to address this question by observing
the possibility of chiral optical properties in structural variations of polypropylene.
Pino synthesized variations of isotactic polypropylene in which the methyl group
was substituted for by a chiral alkyl group [5]. When these groups, which replaced
the pendant methyl groups along the chain, were enantiomerically distinct or
enriched, or even placed randomly among the methyl groups along the chain, the
optical activity properties of the polymer could be shown to arise from a helical
conformation and not simply from the chiral optical properties of the chiral pendant
group. Pino was able to further demonstrate that placing such chiral groups among
phenyl pendant groups, as in a random copolymer, gave rise to chiral optical
properties for the chromophore of the phenyl groups on the chain. These experiments, carried out in Pisa, left no doubt that the helical conformations of the
Stereochemical Studies at the Herman F. Mark Polymer Research Institute
265
of zero. The absence of optical activity in such a sample of polymers does not
therefore arise from the absence of a chiral structure, nor does it arise from the
chiral structures with equal numbers of enantiomers (a racemic mixture), but rather
arises from what is never seen in small molecules, a large number of chiral
diastereomers each one in an enantiomerically pure state. Remarkable! [2].
Is there something equally remarkable about isotactic polymers? In any isotactic
polymer, the segment at the initiating end and that at the terminus of the chain
must differ and, therefore, if one considers the entire chain the conclusion is that
this is a chiral structure. In his review in Topics in Stereochemistry in 1987 [3],
Mario Farina noted that the overwhelming number of units along the chain
are oblivious to the end groups of the polymer chain. Considering a limited number
of these internal units, one sees a plane of symmetry and therefore an achiral
structure. Alternatively, one could assign the absence of observed optical activity
in isotactic polypropylene to cryptochirality, a term coined by Mislow [4] for
molecules in which optical activity is too small to be detected at observable
wavelengths. If these factors were not reason enough for the absence of any chiral
optical measure, the nature of the polymerization suffices in producing a racemic
mixture of chains. The enantiotopic faces of the incoming propylene monomers
approach the Ziegler–Natta catalyst with equal probability.
There is, however, another aspect of the chirality of isotactic polymers, which is
associated with the helical form of isotactic polypropylene first observed in the
X-ray diffraction experiments conducted in Milan by the Natta group. In this
manner, Natta discovered the reason for the crystalline properties of this polymer
synthesized from the catalyst developed by Karl Ziegler. But, in a sample of
isotactic polypropylene, or other isotactic polymers arising from vinyl monomers,
both left- and right-handed helical conformations are present so that no chiral
optical property is observed. Each crystalline state in a sample of isotactic polypropylene is a racemic mixture of helical forms.
There is more to the story of isotactic polypropylene arising from Piero Pino’s
interest in answering the following question: Does the helical property of isotactic
polypropylene in the crystalline regions of the sample extend into the amorphous
regions, as in the melted state? Pino decided to address this question by observing
the possibility of chiral optical properties in structural variations of polypropylene.
Pino synthesized variations of isotactic polypropylene in which the methyl group
was substituted for by a chiral alkyl group [5]. When these groups, which replaced
the pendant methyl groups along the chain, were enantiomerically distinct or
enriched, or even placed randomly among the methyl groups along the chain, the
optical activity properties of the polymer could be shown to arise from a helical
conformation and not simply from the chiral optical properties of the chiral pendant
group. Pino was able to further demonstrate that placing such chiral groups among
phenyl pendant groups, as in a random copolymer, gave rise to chiral optical
properties for the chromophore of the phenyl groups on the chain. These experiments, carried out in Pisa, left no doubt that the helical conformations of the
Stereochemical Studies at the Herman F. Mark Polymer Research Institute
265