Poly-␣-methystyrene (P-␣MS), while not as extensively studied as polystyrene, has been the subject of numerous investigations. Different methods of polymerization have been examined, for example, anionic 1,2 and free radical 3 polymerization, and the system has been a model for study of polymerization thermodynamics because of its relatively low ceiling temperature. The physical properties of P-␣MS have also been the subject of considerable study; for example, intrinsic viscosities, 1 sedimentation coefficients, 4 and second virial parameters 5 have all been evaluated.
A number of previous studies have included determinations of the refractive index increment (dn/dc) for P-␣MS in a variety of solvents. Most of the determinations have been made as a necessary adjunct to the assessments of molecular weight averages using light scattering techniques. Values obtained previously are presented in Table I. It is apparent that there is a variation in the value of dn/dc, depending on the solvent used and the wavelength of measurement (it should also be noted that some of the previous studies indicate a variation in dn/dc values with temperature 5 ).
At the onset of our study of copolymerization reactions involving ␣MS, it was noted that the dn/dc value was not available in tetrahydrofuran (THF) for P-␣MS at 633 nm. For molecular weight average measurements, our laboratories utilize room temperature (25°C) gel permeation chromatography (GPC) that uses THF as the solvent. The detectors within the GPC system are a Dawn multi-angle laser light scattering (MALLS) detector with a He-Ne laser operating at 633 nm in series with a Waters DRI detector. The Astra software package for molecular weight analysis (Wyatt Technology) uses dn/dc values as a key parameter to calculate molecular weights. The variation in dn/dc