5 Nanostructures of Helical Polymer–Metal Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
6 Potential Uses of HPMC Nanospheres: Encapsulation Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
7 Helical Sense and Backbone Elongation by Polar and Donor Solvent Effects . . . . . . . . . . . 137
8 Conclusions . . . . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . 139
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
1 Introduction
Polyacetylenes (for reviews on polyacetylenes and poly(phenylacetylene)s see
[1, 2]) are a versatile family of helical polymers that have attracted the attention
of a number of research groups in recent years, mostly due to their capacity to adopt
helical structures and, correspondingly, to display axial chirality and due to the
properties associated with this structural feature.
Poly(phenylacetylene)s [1, 2] (PPAs) are a class of polyacetylenes in which the
monomer repeating units (mru) are derivatives of ethynylbenzene, usually bearing
substituents at the para position of the phenyl ring. They belong to the so-called
dynamic helical polymers, characterised by a low barrier to the inversion between
the two helical senses (left-handed and right-handed; M and P, respectively).
In order to adopt a helical structure, it is necessary for the conjugated double
bonds at the polymeric backbone to present a cis-cisoid or cis-transoid configuration; otherwise, the helix is not formed (trans-cisoid or trans-transoid configurations) [3]. Nowadays, a number of efficient catalysts, e.g. [Rh(nbd)Cl] 2 (where nbd
is norbornadiene), are available that provide conjugated double bonds with a high
cis content in good yields. The cis configuration can be easily detected in the
polymers by spectroscopic techniques such as NMR or Raman (measuring characteristic vinylic hydrogen chemical shifts and vibrational frequencies from the
phenylacetylene backbone, respectively) [3].
Since the pioneering work of researchers such as Percec [4, 5] and others [1, 2],
the possibility of achieving helix-sense bias (i.e. helix inversion) on PPAs has
stimulated the development of a number of tools for their manipulation; for
instance, through specific non-covalent interactions of achiral pendants with chiral
molecules [1, 2].
The presence of stereogenic centres in the pendants of PPAs usually leads to the
predominance of one helical sense at the helical backbone and thus to an optically
active polymer, with a clear CD spectra. These helical polymers can present inversion
of their helicity in response to external stimuli such as solvent polarity. This is the case
with PPAs containing L- or D-alanine pendants with long alkyl chains, the inversion
being ascribed to modifications on the intramolecular hydrogen bonds [6].
Those findings raised the prospect of a rational selection of the pendants, whose
conformation could be manipulated by an external stimuli and that change transferred from the pendants to the helical backbone of the PPA. This strategy could be
based either on de novo pendant design or by resorting to structures with wellestablished conformational behaviour under certain stimuli.
124
F. Freire et al.
6 Potential Uses of HPMC Nanospheres: Encapsulation Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
7 Helical Sense and Backbone Elongation by Polar and Donor Solvent Effects . . . . . . . . . . . 137
8 Conclusions . . . . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . 139
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
1 Introduction
Polyacetylenes (for reviews on polyacetylenes and poly(phenylacetylene)s see
[1, 2]) are a versatile family of helical polymers that have attracted the attention
of a number of research groups in recent years, mostly due to their capacity to adopt
helical structures and, correspondingly, to display axial chirality and due to the
properties associated with this structural feature.
Poly(phenylacetylene)s [1, 2] (PPAs) are a class of polyacetylenes in which the
monomer repeating units (mru) are derivatives of ethynylbenzene, usually bearing
substituents at the para position of the phenyl ring. They belong to the so-called
dynamic helical polymers, characterised by a low barrier to the inversion between
the two helical senses (left-handed and right-handed; M and P, respectively).
In order to adopt a helical structure, it is necessary for the conjugated double
bonds at the polymeric backbone to present a cis-cisoid or cis-transoid configuration; otherwise, the helix is not formed (trans-cisoid or trans-transoid configurations) [3]. Nowadays, a number of efficient catalysts, e.g. [Rh(nbd)Cl] 2 (where nbd
is norbornadiene), are available that provide conjugated double bonds with a high
cis content in good yields. The cis configuration can be easily detected in the
polymers by spectroscopic techniques such as NMR or Raman (measuring characteristic vinylic hydrogen chemical shifts and vibrational frequencies from the
phenylacetylene backbone, respectively) [3].
Since the pioneering work of researchers such as Percec [4, 5] and others [1, 2],
the possibility of achieving helix-sense bias (i.e. helix inversion) on PPAs has
stimulated the development of a number of tools for their manipulation; for
instance, through specific non-covalent interactions of achiral pendants with chiral
molecules [1, 2].
The presence of stereogenic centres in the pendants of PPAs usually leads to the
predominance of one helical sense at the helical backbone and thus to an optically
active polymer, with a clear CD spectra. These helical polymers can present inversion
of their helicity in response to external stimuli such as solvent polarity. This is the case
with PPAs containing L- or D-alanine pendants with long alkyl chains, the inversion
being ascribed to modifications on the intramolecular hydrogen bonds [6].
Those findings raised the prospect of a rational selection of the pendants, whose
conformation could be manipulated by an external stimuli and that change transferred from the pendants to the helical backbone of the PPA. This strategy could be
based either on de novo pendant design or by resorting to structures with wellestablished conformational behaviour under certain stimuli.
124
F. Freire et al.