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Polyisocyanides

Polyisocyanides with a bulky side group adopt a stable 4/1 helical conformation even in solution, as first postulated by Millich [24] and confirmed by Nolte et al. through the direct resolution of poly(t-butyl isocyanide) into enantiomeric helices by chiral chromatography [25]. The resolved polymer with a positive rotation was postulated to have a left-handed helical conformation based on a CD spectral analysis [26]. The stable helical conformation of the polyisocyanides was further confirmed by the helix-sense selective polymerization of achiral bulky isocyanides by Nolte [27] and Novak [28]. Nolte and coworkers and other groups have further synthesized wide varieties of helical polyisocyanides with a controlled helicity [10, 29] and these results will be described in detail in Chapter 12.

Although the helical structure of the polyisocyanides has been postulated to be a 4/1 helical conformation on the basis of an X-ray analysis, the absolute configuration of the helical polyisocyanides remains obscure. Advanced microscopy techniques, in particular, atomic force microscopy (AFM) combined with circular di-chroism (CD) spectroscopy can reveal the structures of the helical polymers and their helix-senses. Diastereomeric right- and left-handed helical polyisocyanides were prepared from an unprecedented helix-sense controlled polymerization of enantiomerically pure phenyl isocyanides bearing an l- or d-alanine pendant with a long alkyl chain using a nickel catalyst in different solvents (13). Highresolution AFM revealed their helical conformations and enabled the determination of the helical sense (Fig. 11.4); poly(phenyl isocyanide)s showing a positive

Fig. 11.4 Schematic illustration of diastereomeric helical polyisocyanides produced by the helix-sense controlled polymerization of 13. The helix-sense can be controlled by the solvent polarity and temperature during the polymerization, resulting in the formation ofdiastereomeric helical polyisocyanides. The helix-senses of the diastereomeric 13s were determined by their AFM measurements. AFM phase images of self-assembled 13 on graphite (scale = 10 x 20 nm) with the left-handed (left) and right-handed helical 13 (right) together with their structures determined by X-ray are also shown. (Reproduced with permission from Ref. 30. Copyright 2006 American Chemical Society.)

Fig. 11.4 Schematic illustration of diastereomeric helical polyisocyanides produced by the helix-sense controlled polymerization of 13. The helix-sense can be controlled by the solvent polarity and temperature during the polymerization, resulting in the formation ofdiastereomeric helical polyisocyanides. The helix-senses of the diastereomeric 13s were determined by their AFM measurements. AFM phase images of self-assembled 13 on graphite (scale = 10 x 20 nm) with the left-handed (left) and right-handed helical 13 (right) together with their structures determined by X-ray are also shown. (Reproduced with permission from Ref. 30. Copyright 2006 American Chemical Society.)

first Cotton effect sign was assigned to have a right-handed helix [30]. This assignment agrees with that determined by the exciton-coupled CD method [31].

Poly(2,3-quinoxaline)s, which structurally resemble polyisocyanides by condensing two adjacent imine units in each heteroaromatic moiety, have been obtained by the polymerization of 1,2-diisocyanobenzene (14) using an organopalla-dium complex with an optically active imidazoline group (15) as the initiator produced a right-handed helical poly(2,3-quinoxaline) with an almost 100% helix-sense selectivity via a living and cyclopolymerization mechanism (Scheme 11.2) [16, 32]. The helical structure and handedness were postulated by X-ray crystallographic analysis of an active pentamer of a diastereomerically pure oligo(2,3-quinoxaline). In sharp contrast to other living polymerization systems, the active growing chains complexed with the palladium can be isolated and subsequent helix-sense selective block polymerization takes place [16].

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