Generally, the relative molecular mass of low molecular weight is less than one thousand, and the relative molecular mass of high molecular weight is more than 5,000. Therefore, the relative molecular mass is a characteristic of a polymer compound, and the most fundamental difference between a polymer and a low molecule. It is also a basic reason why high molecular substances have various unique properties, such as small specific gravity, high strength, and high elasticity and plasticity. As for whether a substance with a relative molecular mass between one thousand and five thousand is a low molecule or a polymer, it is determined by their physical and mechanical properties. In general, polymer compounds have good strength and elasticity. Low-molecular compounds are not, that is, their relative molecular mass must be significantly different from low-molecular compounds in terms of their physical and mechanical properties.
Although the relative molecular mass of a polymer is large, its chemical composition is generally relatively simple, and many polymer chains are often repeatedly bonded by covalent bonds to form a polymer chain. For example, polyvinyl chloride is polymerized from many vinyl chloride molecules:
A low molecular compound which is polymerized into a polymer compound like vinyl chloride is called a monomer. The repeating structural unit (such as -CH2-CHCI-) constituting the polymer chain is called a chain link. The number n of links is called the degree of polymerization. Therefore, the relative molecular mass of the polymer = degree of polymerization × chain amount.
It should be pointed out that the current technology of synthesizing polymers is not as strict and precise as the synthesis of proteins in living organisms - with certain order, structure and relative molecular mass. Therefore, the degree of polymerization of synthetic polymer chains is always different. That is to say, the relative molecular mass of each molecule in the same synthetic polymer compound is always different (except, of course, synthetic proteins such as insulin). Therefore, the synthetic polymer compound is actually a homologous mixture having a different molecular mass. The relative molecular mass of the polymer compound we are talking about refers to the average relative molecular mass, and the degree of polymerization is also the average degree of polymerization. The phenomenon in which the relative molecular masses in the polymer compound are not equal is called the polydispersity (that is, the heterogeneity) of the polymer. This phenomenon does not exist in low molecules, but the performance of polymer compounds has a great influence. Generally, the greater the dispersibility, the worse the performance. The relative molecular mass and dispersion problems are all issues that must be controlled when synthesizing polymers.
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