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Helical Polymers with Low Helix Inversion Barriers (Dynamic Helical Polymers)

Green and coworkers have thoroughly investigated the structures and chiroptical properties of polyisocyanates, a typical stiff, rigid rod-like polymer with a long persistent length (q), and the substantial nature of the dynamic macromolecular helicity of polyisocyanates has been experimentally and theoretically revealed [8, 35]. The most important feature of dynamic helical polymers, such as polyisocya-nates, polysilanes and polyacetylenes, is their high sensitivity to a chiral environment, and therefore, a small chiral bias can be transformed into a main-chain conformational change with a large amplification through covalent or noncova-lent bonding with high cooperativity, resulting in the formation of an excess of the preferred helical sense. Such systems provide the basis for the construction of novel chirality-sensing materials. The underlying principle observed in dynamic helical polyisocyanates may be universal and applicable to other polymeric and supramolecular systems [13, 14, 36].

Dynamic Helical Polymers Assisted by Covalent Bonding 11.3.1.1 Polyisocyanates

Polyisocyanates are characterized by an N-substituted amide repeat unit (Nylon-1) and possess a helical conformation (8/3 helix) rather than a restricted coplanar conformation. The conjugated partial double-bond characteristic of the backbone amide bonds is responsible for their stiffness. Even the optically inactive poly(n-hexyl isocyanate) (19, q = 20-40 nm) and poly(2-butylhexyl isocyanate) (20), which have no stereogenic centers, consist of an equal mixture of interconvertible right-and left-handed helical conformations separated by the rarely occurring helical reversals (Fig. 11.6A). Therefore, helical polyisocyanates in dynamic equilibrium are chiral (or dynamically racemic) macromolecules. However, the helix inversion

Fig. 11.6 Schematic illustration of dynamic helical conformation of polyisocyanates (A), ''sergeants and soldiers'' effect (B), majority rule (C), and energy diagram of dynamic helical polymers (D).

barriers are very small, so that optically active polyisocyanates with an excess single-handed helix can be obtained through the copolymerization of achiral monomers using a small amount of optically active monomers (less than 1 mol%) [37, 38] or polymerization of achiral isocyanates with optically active initiators [39]. This can be considered as a typical example of chiral amplification in a polymer. This highly cooperative phenomenon is called the ''sergeants and soldiers effect'' (Fig. 11.6B). The underlying principle for this unique chiral amplification phenomenon was theoretically and quantitatively solved using a statistical theory, where each monomer unit in the helical polymer chains can take either a right-handed helical state, left-handed helical state or helix reversal state [8, 35, 40]. According to Lifson, Green, Teramoto and coworkers, the helix-sense excess of the preferred helical state in helical homopolymers, such as a deuterium-substituted helical polyisocyanate (21, Fig. 11.7), can be calculated as a function of the thermodynamic stability parameters, the free energy difference between the right- and left-handed helical states (2AGh), the excess free energy of the helical reversal state (AGr) (per monomer unit), the degree of polymerization (N), and the absolute temperature (Fig. 11.6D) [41]. The key energy parameters (2AGh and AGr) arising from 21 were estimated to be 0.74 cal mol-1 and 3.9 kcal mol-1 on a monomer unit basis in hexane at 25 °C, respectively. Importantly, the former value is about three orders of magnitude smaller than the latter. The 2AGh value indicates that 21 favors the right-handed helix over the left-handed helix only by 0.12%, whereas for a longer polymer chain of 21 (N = 2000), this minute excess is remarkably amplified by the cooperative mechanism to 67:33, which results in the appearance of intense Cotton effects in the polymer backbone region (Fig. 11.7) as well as a large optical rotation [42]. In addition, the helix reversal costs 3.9 kcal mol-1 and appears only once in every 762 monomer

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