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For the past two years work has been in progress at the National Bureau of Standards to determine accurately the molecular weights of macromolecules through the measurement of dilute solution properties. This may at first seem like an easy task, although those acquainted with the details of the necessary measurements and their interpretation know that it is not (1).2 Difficulties are caused by the problems that usually arise when measurements are made on an absolute basis rather than on a relative basis, and from complexities of macromolecular interactions, even in very dilute solutions. Molecular weight determinations, however, are not the only physicochemical measurements on high polymeric materials that give difficulties; density measurements also demand considerably more care and patience than do similar measurements on low molecular weight compounds. Precise measurements of densities have revealed anomalies, the resolution of which has led to a much better understanding of the molecular properties of crystallinity and viscoelasticity of high polymeric materials. Likewise, improved precision in measurements of solution properties has given us much additional information on properties such as size and shape of the molecules, and distribution of molecular weights.
Chemist-Project Leader, National Bureau of Standards, Washington, D. C.