It is sometimes possible, especially in the field of organic chemistry, to join small molecules together to form long chains. The term for the long chains is polymer and the process is called polymerization. Poly- means many, whereas -mer means unit. Many units are combined to form a new, single unit. There are two primary methods by which small chains can polymerize into larger chains -- addition and condensation polymerization.
Condensation polymerization refers to the joining together of smaller molecules through the loss of a small molecule, such as water, to form a larger molecule. One of the simplest examples is the reaction of glycine, or aminoacetic acid, HOOC-CH2-NH2, to form the dimer HOOC-CH2-NH-CO-CH2-NH2. Polymerization requires at least one double or two single reaction sites.
Styrene, or C6H5-CH=CH2, can form even lengthy chains, via free radical polymerization. This involves the breakage of the double bond that allows the addition of another molecule of styrene. Repetition allows the addition of another, and yet another, styrene molecule. The process can be controlled to limit the number of additions.
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Another addition polymerization involves carbocations. Double- or triple-bonded compounds interact with acids to form positively charged carbocations. These can combine with additional molecules to form lengthier carbocations capable of further repeating the process.