Correlation and Prediction of the Refractive Indices of Polymers by QSPR

The refractive index n is a basic optical property of polymers that is directly related to other optical, electrical, and magnetic properties. The refractive index is also widely used in material science. The specific refractive index increment (dn/dc) is an important parameter in light scattering measurements of dilute polymer solutions, which can be used for the determination of molecular weight, size, and shape.1 Importantly, the refractive index can indicate the potential of a polymer for a specific purpose. A satisfactory quantitative structure-property relationship (QSPR) that would allow quantitative prediction of the refractive index of as yet unsynthesized polymers would clearly be of significant utility. In principle, combining the QSPR method with pattern recognition techniques should make possible the theoretical prediction of structures with desired property values. Theoretical methods for calculating the refractive indices of polymers generally utilize equations formulated by (i) Lorentz and Lorentz (eq 1) and (ii) Gladstone and Dale (eq 2). Both approaches require the availability (or theoretical estimation) of molar refraction and molecular volume data. A good summary of early attempts to estimate the molar refraction of polymers using group contributions was provided by Krevelen.1 In a recent review, Askadskii2 proposed several semiempirical equations for the calculation of various physical properties of polymers and copolymers (with accuracy usually within 3-5%). The calculation of refractive index is based on eq 1, where the molecular refraction (RLL) is calculated as a sum of corresponding atom and bond contributions, and the volume (V) is estimated as a van der Waals volume of the compound divided by the average coefficient of molecular packing.