Introducing Materials-Chirality

Mark M. Green

Herman F. Mark Polymer Research Institute, Polytechnic University, Six Metrotech Center, Brooklyn, New York 11201 USA (mgreen@duke.poly.edu)

Since the Italian School, centered in Milan and Pisa, introduced rational study of optically active polymers after the development of the Ziegler-Natta catalyst, it has been clear that cooperativity plays an important role in stereochemical work in polymers. This cooperativity, following on more recent work in stiff helical polymers, can be described theoretically by adaptations of one-dimensional statistical physical theories. A variety of experiments become possible yielding new ways to measure chiral forces so minute as to be beyond interpretation [1] but that can be adapted to the study of phenomena involving larger energies such as preferential solvation [2]. Concepts such as the sergeants and soldiers experiment and majority rule are manifestations of the amplification of chirality [3] pointing to the possibility that chiral materials might be developed in which enantiomeric excess and enantiomeric content may be nonlinearly related to uses such as chiral chromatography and asymmetric catalysis. New kinds of sensors and switches based on chirality, involving circularly polarized light or temperature become possible. The behavior of liquid crystal forming optically active helical polymers offers insight into the forces exerted on the accessible conformational states [4].

There is another face of cooperativity, the disorderly face. Here chirality can also play a role and here polymers are the perfect materials. Chirality, in the form of atropisomeric molecules, may be introduced into polyelectrolytes in water solution. This allows new understanding of the glass-like restrictions to motions associated with the disorderly cooperative hydrophobic capsules formed in various pH conditions and also opens the possibility of developing new kinds of chiral based switches [5]. In addition these polyelectrolytes can be adapted with photoinitiating groups allowing the formation of graft copolymers of from water soluble initiators. These formation of these grafts depend on pH since the polyelectrolyte can act as a pH dependent cage about the radicals formed on irradiation. In the situation of polyacrylamide grafted onto polymethacrylic acid, strong hydrogels are formed.

In another direction, amorphous chiral states may be developed around the properties of polymeric melt and glass states allowing new methods to study polymer conformation and new kinds of experiments with fiber optic-wave guides. This research as well yields insight into the length scale of entanglements in these disorderly cooperative arrays and shows the transition to the glassy state and certain characteristics of the glass by the crossover of segmental with atropisomeric motions [6]. Study of atropisomeric racemization motions in highly viscous polymer melts yields the first deviations from transition state theory for a high barrier process with a relationship between rate and viscosity causing difficulty in interpretation for existing theories [7]. Finally the chiral optical changes associated with this work offer new possible materials for archival holographic information storage.

1. A Helical Polymer with a Cooperative Response to Chiral Information, M. M. Green, N. C. Peterson, T. Sato, A. Teramoto, R. Cook, S. Lifson, Science, 268, 1860(1995); The Chiral Optical Properties of a Helical Polymer Synthesized from Nearly Racemic Chiral Monomers Highly Diluted with Achiral Monomers, S. K. Jha, K. S. Cheon, M. M. Green, J. V. Selinger, J. Am. Chem. Soc., 121, 1665(1999).

2. Use of Circular Dichroism for the Quantitative Characterization of Preferential Solvation of a Polymer in Mixed Solvents, C. K. Khatri, Y. Pavlova, M. M. Green, H. Morawetz, J. Am. Chem. Soc., 119, 6991(1997).

3. The Macromolecular Route to Chiral Amplification. M. M. Green, J.-W. Park, T. Sato, A. Teramoto, S. Lifson, R. Selinger and J. V. Selinger, Angewandte Chemie, 38, 3138(1999).

4. The Mechanism of the Transformation of a Stiff Polymer Lyotropic Nematic Liquid Crystal to the Cholesteric State by Dopant Mediated Chiral Information Transfer. M. M. Green, S. Zanella, H. Gu, T. Sato, G. Gottarelli, S. K. Jha, G. P. Spada, A.M. Schoevaars, B. Feringa, A. Teramoto, J. Am. Chem. Soc., 120, 9810(1998).

5. An On/Off Circular Dichroism Signal Reveals a pH Dependent Competition Between a Cyclodextrin and a Polyelectrolyte for an Atropisomeric Aromatic Guest, J.Am.Chem.Soc., 119, 12404(1997); The Clustering of Poly(methacrylic acid) around Appended Binaphthyl Labels as Reflected by the Disruption of g -cyclodextrin Complexation and Racemization Kinetics, S. Y. Yang, G. Schultz, M. M. Green, H. Morawetz, Macromolecules, 32, 2577(1999).

6. Polymeric Glasses with Chiral Optical Properties, J. W. Park, M. D. Ediger, M. M.Green, Polymer Preprints (ACS), 2000, San Francisco ACS meeting March 2000.

           7. Creating Chiral Tools to Explore Polymeric Melt and Glass States, J. W. Park, M. D. Ediger, M. M. Green, submitted.