ABSTRACT
All synthetic homopolymers are really mixtures of different molecular weights. Some polymers, such as polyethylene, poly(ethylene oxide), and dimethyl silicones, are commercially available in sizes ranging almost continuously from monomer or dimer up to molecular weights in the millions. Although there is no sharp dividing line, we can draw an imaginary one at a molecular weight of about 2000, because it is near the limit of convenient purification by distillation or extraction. For compounds below this molecular weight, the term oligomer (Section 1.1) is often used. Except for smaller members of each series, each of these products is a mixture of different-sized molecules having a distribution of molecular weights and an average molecular weight, which we can define and measure in several ways. Polymers with high molecular weights are responsible for many of the properties that make polymers valuable as a class of materials. For high-molecular-weight polymers, differences between members of a homologous series differing in steps of 100 or so (the molecular weight of a typical monomer) become so slight as to prevent clean separations. Distributions of molecular weights are determined routinely using chromatographic techniques.
