Molecule of any of a class of mostly organic compounds that can react with other molecules of the same or other compounds to form very large molecules (polymers). The essential feature of monomer molecules is the ability to form chemical bonds (see bonding) with at least two other monomer molecules (polyfunctionality). Those able to react with two others can form only chainlike polymers; those able to react with three or more can form cross-linked, network polymers. Examples of monomers (and their polymers) are styrene (polystyrene), ethylene (polyethylene), and amino acids (proteins).

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