Conformation

Millich et al. found that the polymerization of the optically active a-phenylethyl isocyanide in the presence of acid treated glass yields polymers with a high optical rotation per repeat unit [33]. On the basis of this observation, combined with Debye-Scherrer X-ray patterns and space-filling molecular models [34], they proposed that in a polyisocyanide chain the transition dipoles of the absorbing imine chromophores are coupled, leading to a highly organized helical polymer backbone containing 4 repeat units per turn and a pitch of 4.1-4.2 A [35].

The presumed helical conformation of polyisocyanides was confirmed by Nolte et al. when poly( t-butyl isocyanide), which has no chiral centers, was resolved into (+)- and (—)-rotating fractions on the basis of CD spectroscopy [10, 36]. Theoretical studies on the conformation of t-butyl isocyanide oligomers using consistent force field conformational calculations indicated that a helical conformation was favored with an increasing number of monomer units. The average dihedral angle N=C-C=N in the hexamer was found to be +78.6°, corresponding to 3.75 monomer units per helical turn [37, 38]. The same calculations for a hexade-camer of the t-butyl isocyanide resulted in a dihedral angle of 84.3°, corresponding to 3.60 units per helical turn. Substitution of the t-butyl group by a methyl-, ethyl-, or isopropyl group was calculated to give a smaller dihedral angle and more units per helical turn. In the case of poly(methylisocyanide), calculations revealed that the methyl group was too small to lead to a fixed dihedral angle and hence no atropisomerism was proposed to be possible.

In the late 1970s, Kollmar and Hoffmann carried out molecular orbital calculations using an extended Huckel approach on a series of polyisocyanides [39], namely RNC, where R = H, CH3, C(CH3)3. They concluded that N lone-pair repulsion between the nitrogens that are second nearest neighbor in the polymer chain (Fig. 12.2) plays a dominant role in the structural conformation and as a result the polyisocyanide backbone must adopt a conformation that is not planar.

In the case of isocyanides with bulky R-substituents, electronic repulsion is of minor importance and the non-planar conformation is mainly dictated by the steric interactions between the side groups [39]. According to calculations, the helical angle that is adopted by the polyisocyanide backbone varies from a fairly broad range of helical conformations for the R = H polymer, to a narrow range of configurations around the 4-fold helix as the steric bulk of the substituent in-

Steric N-N repulsion repulsion

Steric N-N repulsion repulsion

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