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. 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.