|MadSci Network: Chemistry|
Dear Cynthia, The key to understanding polymerization is understanding INITIATION. To start any polymerization, the molecule with the double bond must add a RADICAL, so that it becomes a radical itself ( I think you know that a radical, in chemistry, is a molecule with an "unpaired" electron). The unpaired electron (symbolized by a period) then sits on that carbon of the ex-double bond, to which the radical has NOT added. R. + C=C -> R-C-C. The first radical, which starts the chain reaction, is generated by decomposition of a special molecule called an initiator. Initiator molecules are unstable towards heat or light, causing some SINGLE bond within themselves to break into two radicals. Those are very shortlived, but if a large excess of monomers surrounds them, each radical adds to one of them, and so becomes the beginning of a chain. Eventually, chains are terminated e.g. by adding another radical (eg. a partially grown chain: this is called RECOMBINATION). Since initiators are added in very small amounts, they are often not mentioned in popular discussions of polymerization. Under some conditions (e.g UV light or very high temperatures), ordinary molecules, solvents, impurities or even monomers can act as initiators, leading to "spontaneous" polymerization. Best regards Werner Sieber
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