Solution and Surface Properties of Helical Rod-like Polyguanidines

Abstract

Helical polymers are of increasing interest because of the wide variety of applications for them, which include uses in optical devices, chiral columns for HPLC separations, asymmetric supports for catalysis, components in organic/inorganic hybrid materials and biomimetic application.

Polymerization of achiral monomers (with optically inactive catalysts) always yields a racemic mixture of right and left helical senses since the enantiomers are of equal energy. Our research goal is to synthesize polymers with a biased helical sense and investigate their properties, such as chirality and liquid crystallinity. To this end, two kinds of approaches for the preparation of helical polyguanidines have been demonstrated: significant amplification of chirality was observed in polyguanidines containing chiral side chains and the optically active polyguanidines can also be produced from achiral monomers by helix-sense selective catalysts. The chirality of polymer arises from helical conformation of polymer chains with an excess of single screw sense.

Furthermore, we investigated a new method of preparing self-assembled monolayers (SAMs) of thiol terminated polyguanidines. These end-capped polymers were prepared through the living polymerization of carbodiimides. The features of living polymerization allow us to functionalize the polymer during the initiation step. Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), monitoring the infrared signature peak of polyguanidines at 1649 cm-1, ellipsometry, and contact-angle goniometry were used to detect the formation and properties of these polymeric SAMs on gold surfaces. The gold-polyguanidines nanocomposites have been also prepared by the surface initiated polymerization on gold nanoparticles. Because these polymers possess helical structures reminiscent of polypeptides, these work should allow us to probe chiral interactions and energies on surfaces that are biologically relevant.