Polyguanidines (4) prepared by the polymerization of carbodiimides were previously considered to be an interesting class of dynamic helical polymers (Fig. 11.5A) (see Section 11.3.2). Novak et al. found that an optically active polyguani-dine (16) showed a specific rotation identical to its monomer ([a]D = +7.5°) just after the polymerization. However, the specific rotation significantly increased in solution and reached a plateau value ([a]D = —157.5°) upon annealing at elevated temperatures [33]. This unusual behavior was irreversible and ascribed to the conformational change from a kinetically controlled structure to a thermodynam-ically controlled helical one with the excess helical sense upon heating. An excess of one helical sense can be induced for an optically inactive polyguanidine (17) catalyzed by optically active camphorsulfonic acid [33]. Novak and coworkers further synthesized an optically active helical polyguanidine stable in solution by the

Fig. 11.5 (A) Synthetic scheme of polyguanidines. (B) Schematic illustration of the synthesis of a helical polyguanidine by helix-sense selective polymerization of achiral 18 using a chiral Ti catalyst and reversible switching of the pendant anthracene rings.

helix-sense selective polymerization of an achiral bulky carbodiimide (18) with a chiral titanium complex (Fig. 11.5B) [34]. This polymer exhibited a reversible, temperature and solvent-induced chiroptical switch due to a change in the orientation of the pendant anthracene rings as evidenced by vibrational CD measurements, while the main-chain helicity remained unchanged.

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