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Polymerization is a chemical process of adding units to produce a chain. Radical polymerization uses free radicals and initiators, such as organic peroxides or azo compounds, to increase the size of a molecule. The reaction continues until it is stopped, and side reactions like combination and disproportion can occur. The resulting polymer can have varying molecular weights.
The word polymer means “many units” and refers to a chemical process of adding one unit to another, to another – and so on – until a chain of the desired size has been produced. Some polymerization reactions employ positively charged carbon ions, or carbocations; while others employ negatively charged carbon atoms, or carbanions. However, a third mechanism does the job using free radicals and the process is called radical polymerization. Free radicals are atoms or molecular fragments with a reactive unpaired electron that can be used to increase the size of a smaller molecule. An “initiator” is needed to initiate the reaction in free radical polymerization.
Generally, the initiator is a molecule possessing a weak chemical bond that splits evenly into radicals, each fragment taking away a single electron, rather than a fragment receiving both electrons and the other, none. Chlorine gas (Cl2) when exposed to ultraviolet light splits into two radicals, each denoted as Cl∙, with a dot representing the lone electron. When this reactive radical combines with an electrically neutral organic molecule, the result is a larger radical that can react further, and so on. Additional common initiators include organic peroxides — with one –(O–O)– bond and azo compounds — compounds with one –(N=N)– bond.
Once the reaction begins, the propagation of the reaction continues, with the number of free radicals remaining essentially constant. An example of this can be seen in the free radical polymerization of ethylene gas using an organic peroxide initiator, ROOR. A small quantity of peroxide is split into its radical components (R–O∙) and fed into the ethylene gas (CH2=CH2). The two react, resulting in addition and the presence of a new species (R–O–CH2–CH2∙). This is itself reactive and attacks a second molecule of ethylene gas to produce yet another larger species (R–O–CH2–CH2CH2–CH2∙); eventually, the reaction is stopped.
Although the radical polymerization reaction is stopped at a convenient point, there are natural processes that reduce the number of free radical reaction participants available before that point is reached. One of these is called “combination,” the unintentional mingling of free radicals. In the example mentioned,
2 R–O–CH2–CH2∙ → R–O–CH2–CH2–CH2–CH2–O–R. Another side reaction is “disproportion”, in which one radical strips a hydrogen atom from another radical, as in R–O–CH2–CH2∙ + ∙CH2–CH2–O–R → R–O–CH2CH3 + CH2=CH– O. The resulting polymer, polyethylene, can be written –(CH2–CH2)n–. Since not all product chains in the radical polymerization process will be of equal length by the time the reaction ends, the molecular weight can be given as an average molecular weight or as some form of molecular weight distribution.
