AbstractMethods are presented by which the limiting viscosity number ηθ and the limiting sedimentation coefficientS0of a monodisperse linear polymer in its theta solvent as functions of the molecular weightMmay be deduced from data taken with a series of polydisperse samples of the polymer. The necessary data are the limiting viscosity numbers and the distribution functions ofS0of the chosen samples in the theta solvent, plus their number‐average molecular weights. The methods are applied to unfractionated and fractionated samples of a styrene–butadiene copolymer rubber (SBR) having 24 wt.‐ bound styrene in a theta solvent, methyln‐propyl ketone (MNPK), at 21.0°C. The following relations are deduced for monodisperse unbranched SBR in this theta solvent: ηθ = 1.73× 10−3M1/2andS0= 0.83 × 10−15M1/2, where ηθ is expressed in deciliters/gram andS0in seconds. Besides these, the viscosity–molecular weight relations for this cold rubber in toluene and in cyclohexane, both at 30°C., are established. The new relation for the toluene system does not accord with the French‐Ewart relation for the hot rubber in the same solvent. The integral distribution of molecular weight in an unfractionated SBR is calculated from its distribution function ofS0in MNPK at 21.0°C. by using the derivedS0versusMrelationship, and is found to coincide well with the mass distribution obtained from fractionation data if the new viscosity–molecular weight relation is used for the molec
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