Molecular Weight and Molar Mass

In this book, we shall retain the term molecular weight because of its widespread use in the polymer literature. However, when using in numerical calculations we shall substitute the molecular weight by its numerically equivalent molar mass (g/mol) to facilitate dimensional balancing. 

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The term molecular weight was traditionally used instead of molar mass. The terms molecular weight and molar mass mean exactly the same thing. Because the term molar mass more accurately describes the concept, it will be used in this text. 

Note that when using metric units, atomic weight, molecular weight, and molar mass are expressed in grams per mole (g/mol) and then the mass of the species comes out in grams (g). When looking at the units, it makes sense ... 

The molecular weight or molar mass of proteins and nucleic acids (DNA, RNA) is identical for each specific species, e.g., the molecular weights of all casein molecules from a specific source are identical. These polymers are members of a homologous series and are said to be monodisperse or molecularly homogeneous. 

Table 15.3 Molecular weights and molar volumes of active mass constituents [4].

The correct term now for molecular weight is molar mass. The old name molecular weight has, however, been retained because of its familiarity and in order to avoid misunderstanding.)... 

The polymerisation reactions are statistically driven processes. Therefore, unlike some natural polymers such as DNA, synthetic polymers always show, due to the reaction mechanisms involved in the production processes, a certain distribution of molar mass and not a distinct molecular weight. The molar mass of S5mthetic polymers can range from some thousand g/mol up to some million g/mol. As an example. Figure 1.4 shows the normalised molar mass distribution (MMD) curves of two different polyethylene samples. 

Nitric oxide is a low-activity free radical and can be used as a counter of radicals in gas and liquid phases. The reactions of alkyl radicals with NO lead to the formation of nitroso compounds, which are spin traps. Thus, the initiation of free-radical reactions in solid polymers in the presence of nitric oxide provides further information on their mechanism. It is well established that at room temperature NO is not able to remove allylic and tertiary hydrogen atoms and add to isolated double bonds [24-26]. There are discrepant opinions on the capability of NO to react with low molecular weight (low molar mass) dienes and polyenes. Some authors believe that NO is able to add to dienes and polyenes, for example, to substituted o-quinonedimethane, phorone and P-carotene, with the formation of free radicals [27-29]. Another way of looking at these reactions lies in the fact that they can be initiated by NO2 impurities [25, 26]. 

The molecular weight of a substance is, for a large number of calculations, far more convenient than unit mass. This depends on the fact that a large number of properties are independent of the nature of the substance, and depend only on the number of molecular weights present ( molar, or colligative, properties) (cf. Chap. XI. on Solutions ). 

A mole of a substance is the number of elementary particles (atoms, molecules) found in the mass (in grams) of that substance that corresponds to its atomic or molecular weight. In molar volumes, which amount to 22.4 L for gases and are different from solid to solid, there are always the same number of atoms or molecules. This "magic number", Avogadro s number, is 6.022xl023 mol-1. 

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