Attractive interactions play a major role in determining conformation and are easily identified in the structures as they result in a close proximity between the groups involved. Repulsive interactions are no less important. However, since repulsive interactions often result in some distance between the groups that repel each other, they are sometimes overlooked, and definitely less commonly used in design. Steric effects represent one very large class of repulsive interactions. As shown in the following examples, numerous families of oligomeric and polymeric backbones adopt well defined, generally helical, conformations as a result of steric repulsion between bulky peripheral groups. Among the polymer classes that fall into this category are polyisocyanides, isotactic polymethacrylates, poly-isocyanates, polyguanidines, and polyacetylenes. Since the behavior of these polymers is presented in great detail in Chapters 11 and 12, they will be only briefly discussed here.
Steric repulsions between the side chains in substituted polyisocyanides (Fig. 1.15a) are large and prevent them from adopting the planar all-trans structure. Restricted rotation about the single bonds of the polymer backbone results in right- or left-handed helical polymers with four monomer units per helix turn. The polymers can be resolved using chiral chromatography and the use of chiral isocyanides or chiral catalysts can influence the handedness of the helix [137, 138]. Polymethacrylates (Fig. 1.15b) and polymethacrylamides (Fig. 1.15c) are among vinyl polymers that adopt helical conformations. In order for these helices to be stable, bulky side groups are once again required, and the polymerization process (either anionic or radical) must yield isotactic polymers [139, 140]. The helical conformation of polyisocyanates results from a combination of electronic and steric effects. In essence, the -C(O)-N(R)- linkages are forced to be non-planar because the substituents are bulky . Polyisocyanates have a rich stereochemistry due to the stability of long helical segments and the ease of controlling handedness by incorporating chiral monomers . Polyguanidines (Fig. 1.15d), formed from carbodiimide monomers, have similar helical structures to those of polyisocyanates. The use of bulky (R) and chiral (R') substituents leads to stable one-handed helical polymers with high barriers for conformational racemization . In alternating poly-(dialkoxy-1,4-phenylene-a!t-2,5-furan)s (Figs. 1.15e and f), favorable interactions between the oxygen atoms of alkoxy side chains or furan units and the ortho hydrogen atoms connected to the aromatic rings lead to restricted rotation about the inter-aromatic bonds and well-defined conformations. By varying the substitution pattern of these conjugated polymers, rigid-rod-like (poly(dialkoxy-2,5-phenylene)-a!t-2,5-furan)s) or helical (poly-(dialkoxy-1,4-phenylene-a!t-2,5-furan)s) conformations can be obtained .
Several classes of oligomers have also been shown to adopt sterically-induced helical conformations. Their oligomeric nature (as opposed to the polymers shown above) simply reflects that these molecules were synthesized in a stepwise fashion. Similar conformations would be expected had polymerization techniques been used for their preparation. Thus, ortho-oligophenylenes (Fig. 1.16a) adopt a tight helical conformation with three aromatic rings per helix turn that was characterized in the solid state. The geometry and sterics of this hindered structure does not allow free rotation about the interaromatic single bonds . Similarly,
torsional effects play the major role in biasing a helical conformation of sexi-thiophenes in the solid state (Fig. 1.16b) . Finally, oligoimides of trans-1,2-diaminocyclohexane have been shown to fold into helical conformations (Fig. 1.16c). Although these oligomers have fixed (R,R) chirality, they can fold into either M or P helical conformers. The folding into helical conformations is once again driven by the strict steric constraints of the system which restricts rotation about the single bonds while the preference for M or P helicity has been attributed to intermolecular interactions .
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