polymer s).
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 {{link=amino acid">amino acid s ( protein s).