Chiral Solvation as a Means to Quantitatively Characterize Preferential Solvation of a Helical Polymer in Mixed Solvents

Abstract

Poly(n-hexyl isocyanate) adopts a helical structure in solution and the preference of one helical sense induced by a nonracemic chiral solvent can be detected by the circular dichroism (CD) of the backbone chromophore of the polymer. Addition of an achiral or racemic cosolvent leads to a reduction of the CD intensity, which can be interpreted quantitatively as reflecting the composition of the solvent mixture in contact with the helical backbone of the polymer. Equilibrium constants characterizing the relation between the solvent composition in contact with the polymer and the bulk of the system are given for mixtures of (S)-1-chloro-2-methylbutane and a number of achiral or racemic cosolvents. The relationship between the structure of the cosolvent and the extent of the preferential solvation was surprising in several instances. The polymer was preferentially solvated by a highly branched hydrocarbon although it was a precipitant and the preferential solvation by alcohols extended up to the point of polymer precipitation. This unusual method for characterizing preferential solvation of this rod-like polymer is compared with other methods used previously on flexible chain macromolecules.

Preferential Solvation of Organic Species in Binary Solvent Mixtures Probed by Intermolecular1H NOESY NMR Spectroscopy

“…PS has been investigated with a wide variety of experimental methods (electrochemical [10] and thermodynamic [11] measurements, solvatochromic effects on UV/Vis or fluorescence spectra, [12±15] circular dichroism, [16] IR, [11c, 17±19] and NMR [chemical shift [20] or line-width [21] measurements]) and with theoretical methods (semiempirical [22] or Monte Carlo [23] calculations on simple solvation models, and molecular dynamics simulations [24±26] ). The relationship between PS and the microscopic characteristics of the solvation shell has been described by means of various theoretical models, such as the stepwise solvent exchange, [27,28] competitive preferential solvation, [29] and quasi-lattice quasi-chemical [30] theories, and preferential solvation isotherms.…”

The Macromolecular Route to Chiral Amplification

“…Actual contact with the chiral solvent is necessary to cause a discrimination between the helical senses, and this is directly responsible for the magnitude of the circular dichroism signal. [25] Treatment of the circular dichroism data obtained on mixing various proportions of (S)-1-chloro-2-methylbutane with 13 achiral or racemic solvents allowed determination of a series of equilibrium constants K defining the preferential solvation (Table 2). [25] Why should the two isomeric alcohols 1-and 2-butanol and 1-and 2-octanol, which are nonsolvents for the polymer and act to precipitate the polymer on addition to a solution in (S)-1-chloro-2-methylbutane, preferentially solvate the polymer up to the point of precipitation?…”

A Molecular Orbital Approach to Conformational Study of the Polyisocyanates

“…[34] In addition, this molecular orbital approach to understanding the polymer and the effect of solvation may allow theoretical understanding of the unusual preferential solvation effects observed in the PIC's. [35] These directions are now in progress using the techniques described above.…”