Polymerization of NCAs: Physical Aspects

Abstract

The great interest numerous research groups had, and still have, in the molecular weights and molecular weight distributions (MWDs) of polypeptides prepared by polymerization of NCAs has two sources. First, both absolute molecular weights and MWDs give useful information on the polymerization mechanism. Second, many of the properties of dissolved or solid polypeptides vary considerably with molecular weight. A broad variety of analytical methods have been applied to determine the molecular weight or MWD of polypeptides. These methods can be subdivided into four groups: I Viscosity measurements. Such measurements do not give absolute molecular weights, and thus, do not require detailed discussion. II Quantitative endgroup analyses, e.g., by means of titration, reaction with chromophores, or NMR spectroscopy. III Physical measurements, such as vapor pressure osmometry (VPO), membrane osmosis, cryoscopy, light scattering, or ultracentrifuge. IV Chromatographic methods, e.g., gel permeation chromatography (GPC). With the exception of viscosity measurements, the success of most methods is reduced by the low solubility of most homopolypeptides in organic solvents. In particular, information on the MWD of polypeptides is quite rare, because GPC is not applicable to most polypeptides and fractionated precipitation combined with methods suitable for the determination of absolute molecular weights is time consuming.