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low-molecular-weight substances composed of molecules that interact with one another (polymerization or polycondensation) or with the molecules of other substances to form a polymer. Most of the monomers that take part in polymerization processes belong to one of two classes: (1) compounds that undergo polymerization because of the scission of multiple bonds—for example, C=C, O≡C, C=O, and C≡N (olefins, diene and acetylene hydrocarbons, aldehydes, and nitriles); and (2) compounds that polymerize because of scission of ring groups (for example, olefinic oxides, lactames, and lactones).
The monomers that take part in polycondensation reactions may be any compounds that contain no fewer than two reactive (functional) groups per molecule—for example, diamines, dicarboxylic acids, amino acids, and glycols. During polycondensation, bifunctional compounds form linear polymers, whereas compounds with more than two functions produce branched and reticular polymers.