often adopt well-defined secondary structures by virtue of specific intra-chain interactions that help overcome the entropic penalty associated with such structure
generation. Control of secondary structures of synthetic macromolecules in solution could be one approach to creating well-defined nanometer-size entities that
could further organize to generate hierarchically ordered macroscopic systems [23].
Although this approach, via the utilization of conformationally controlled macromolecular entities for the generation of nanostructured polymeric materials, is in
its infancy, precisely directed folding of synthetic macromolecules, in itself, has
generated a lot of interest among chemists, physicists and theoreticians alike, because of the elegant parallel in biology [24, 25]. Hence, I have chosen to include a
short discussion on this topic with the conviction that, in the near future, the
polymer nanostructuring repertoire will be more heavily reliant on these pre-folded
macromolecular entities.
At the two extreme ends of the flexibility spectrum are polymer chains that are
very rigid rod-like macromolecules with limited conformational freedom, and
those that are very flexible chains and can adopt a wide range of randomly coiled
conformations. The majority of polymers have conformational properties that lie in
between these extremes. Conformational restrictions can be introduced into macromolecules either by way of steric or bond angle constraints or through specific
intra-chain inter-segment interactions that stabilize certain conformations over the
others. A few examples of macromolecules that adopt preferred conformations by
virtue of some of these constraints are depicted in Figure 15.1. In the case of steric
constraint, the bulkiness of the side group imposes a helical motif to the backbone,
which provides the maximum possible volume for the accommodation of the bulky
substituent. Examples of such a system are poly(tritylmethacrylate), polychloral,
etc. [25]. Bond angle constraint comes into operation most effectively in cases
where the backbone is relatively rigid with very limited conformational freedom,
such as in the case of poly(meta-phenylene ethynylene) [24] and cis-poly(phenylacetylene) [26]. Additional coercions, such as solvophobic effects and salt-formation,
have been utilized to effect folding into helical motifs. Other examples of conformationally rigid polymers that adopt helical conformations include, poly(isocyanide)s, poly(isocyanate)s etc. [25].
Imposition of conformational restrictions through specific inter-segment intrachain interactions, often aided by restricted rotational freedom, is the approach that
nature has chosen to fold biological macromolecules. Like in proteins, efforts that
use H-bonding to effect folding of synthetic macromolecules have led to several
interesting systems that are now often clubbed under a newly coined term: foldamers [27]. The general problem of folding of conformationally flexible macromolecules may be simplified by a schematic depiction of the macromolecule as
containing well-defined and equally-spaced segments along the polymer backbone
that interact strongly with each other (Figure 15.2).
A straightforward consequence of such strong interactions (in the absence of
inter-chain interactions, such as in very dilute solutions) would be aggregation (or
clustering) of these segments, a situation that can be termed self-association. Such a
15.3 Polymer Conformational Control 481
generation. Control of secondary structures of synthetic macromolecules in solution could be one approach to creating well-defined nanometer-size entities that
could further organize to generate hierarchically ordered macroscopic systems [23].
Although this approach, via the utilization of conformationally controlled macromolecular entities for the generation of nanostructured polymeric materials, is in
its infancy, precisely directed folding of synthetic macromolecules, in itself, has
generated a lot of interest among chemists, physicists and theoreticians alike, because of the elegant parallel in biology [24, 25]. Hence, I have chosen to include a
short discussion on this topic with the conviction that, in the near future, the
polymer nanostructuring repertoire will be more heavily reliant on these pre-folded
macromolecular entities.
At the two extreme ends of the flexibility spectrum are polymer chains that are
very rigid rod-like macromolecules with limited conformational freedom, and
those that are very flexible chains and can adopt a wide range of randomly coiled
conformations. The majority of polymers have conformational properties that lie in
between these extremes. Conformational restrictions can be introduced into macromolecules either by way of steric or bond angle constraints or through specific
intra-chain inter-segment interactions that stabilize certain conformations over the
others. A few examples of macromolecules that adopt preferred conformations by
virtue of some of these constraints are depicted in Figure 15.1. In the case of steric
constraint, the bulkiness of the side group imposes a helical motif to the backbone,
which provides the maximum possible volume for the accommodation of the bulky
substituent. Examples of such a system are poly(tritylmethacrylate), polychloral,
etc. [25]. Bond angle constraint comes into operation most effectively in cases
where the backbone is relatively rigid with very limited conformational freedom,
such as in the case of poly(meta-phenylene ethynylene) [24] and cis-poly(phenylacetylene) [26]. Additional coercions, such as solvophobic effects and salt-formation,
have been utilized to effect folding into helical motifs. Other examples of conformationally rigid polymers that adopt helical conformations include, poly(isocyanide)s, poly(isocyanate)s etc. [25].
Imposition of conformational restrictions through specific inter-segment intrachain interactions, often aided by restricted rotational freedom, is the approach that
nature has chosen to fold biological macromolecules. Like in proteins, efforts that
use H-bonding to effect folding of synthetic macromolecules have led to several
interesting systems that are now often clubbed under a newly coined term: foldamers [27]. The general problem of folding of conformationally flexible macromolecules may be simplified by a schematic depiction of the macromolecule as
containing well-defined and equally-spaced segments along the polymer backbone
that interact strongly with each other (Figure 15.2).
A straightforward consequence of such strong interactions (in the absence of
inter-chain interactions, such as in very dilute solutions) would be aggregation (or
clustering) of these segments, a situation that can be termed self-association. Such a
15.3 Polymer Conformational Control 481