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The classical light-scattering tl~cory for polyrner solutions is extended to solutions of copolylners which may be polydisperse in chain composition as nrell as in lnolccular weight. I t is shol\ln that m~~c h too high molecular weights \\rill r c s~~l t from light-scattering data for copolymers owing to f l~r c t~~a t i o n s in chair1 composition; the magnitude of this efcct increases rapidly as tlie absolute value of tlie refractive index increrncnt approaches zero. The methods for deterlnining the usual n-eight-average ~nolecular \\?eight and polydispersity of composition are described.The theory was testecl \\-it11 experimental results 011: (1) a mixt~tre of polystyrene and poIy(methyln~et-hacrylate), considered as a copolymer ~vith the highest possible polydispersity of composition; (2) a high coriversion, statistical copoly~ner of styrcnc and mcthylmethacrylate (copolymer I ) ; ancl (3) a block copolyrner of the same monomers (copolymer 11). T h e apparent molecular weight fcr copolpn~er I (ilf,,, = l.83X1O5) varied markcdly with refracti1.e inclex increnie~it in much the same way as the total rnolec~llar weight for the mixture.The n~olecular weight of copolylner I1 (ill,, = 1.20X1OC) remained essentially constant in the same series of sol\~ents. 'The polydispersities of composition obtained Irere 0.72 and 0.05 for copolymer I and I1 respectively compared with the m a x i n l~~m possible value ccl~~al to 1.0. INTRODUCTIONSince the demonstration by Debye (1) that the amount of light scattered by solutions of high polymers is related to the mass of the solute molecules, light scattering has rapidly become one of the standard methods for the deterrninatioil of molecular weights of macromolecules. The application of this technique to copolymers has not been equally successful. On one occasion (2) the molecular weight obtained for a sample of GR-S (butadiene-styrene copolymer) was found to vary with refractive index of the solvent and it was suggested that this inight be due to heterogeneities in the composition of the particular sample. Similar anomalous molecular weights were obtained in this laboratory for other types of copolymers (3). Accordingl~., it seemed desirable to study the problem of light scattering by copolymers and the use of this techniclue for the determination of molecular parameters both from theoretical and experimental viewpoints.The theoretical treatment for the molecular weights of copolymers given in this paper is similar to that indicated by Stockmayer et al. (4) except that the final equations are arranged in somewhat inore useful form. A preliminary report of the present study has already been published (5).T o test the theory, experimental data are presented for solutions of mixtures of polystyrene (PS) and poly(met11ylmethacrylate) (PhIIhII) and two types of styrenemethylmethacrylate copolymers. These polymers were selected primarily because of the large difference between the refractive indices of the two homopolymers, polystyrene and poly (methylmethacrylate).

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