The polymerization reaction is initiated by three classes of free-radical initiators:
- certain compounds that can be broken down in two radicals at temperatures just above room temperature. Such compounds include organic peroxides such as Benzoyl peroxide and certain azo compounds such as AIBN.
- RO-OR ----> 2 RO.
- R2.CN.CN = NC.CN.R2 -----> 2 R2C.CN + N2
- photosensitive molecules, which under the influence of light, get into an excited state or react with other molecules, forming radicals.
- a redox--system with transfer of one electron during the reaction. This often involves a metal-ion such as in the reaction of a ferrous ion with hydrogen peroxide to a ferric ion in which a hydroxyl radical is formed.
Emulsion polymerization is a special radical polymerization technique in which reactive sites are kept separated from each other by dispersing monomer in an aqueous medium.
Initiation is the creation of free radicals necessary for propagation. The radicals can be created from radical initiators, such as organic peroxide molecules, or other molecules containing an O-O single bond or by reacting oxygen with ethene. The products formed are unstable and easily break down into two radicals. In an ethene monomer, one electron pair is held securely between the two carbons in a sigma bond. The other is more loosely held in a pi bond. The free radical uses one electron from the pi bond to form a more stable bond with the carbon atom. The other electron returns to the second carbon atom, turning the whole molecule into another radical.
Propagation is the rapid reaction of this radicalised ethene molecule with another ethene monomer, and the subsequent repetition to create the repeating chain.
Termination occurs when a radical reacts in a way that prevents further propagation. The most common method of termination is by coupling where two radical species react with each other forming a single molecule. Another, less common method of termination is chain disproportionation where two radicals meet, but instead of coupling, they exchange a proton, which gives two terminated chains, one saturated and the other with a terminal double bond. Termination is suppressed in emulsion polymerization because the radical concentration is low. A chain transfer reaction is also a side-reaction in radical polymerization and serves to reduce the average chain length.
Free radical addition polymerization of ethylene must take place at high temperatures and pressures, approximately 300 °C and 2000 At. While most other free radical polymerizations do not require such extreme temperatures and pressures (for instance styrene will polymerise at 80 oC in benzene or toluene), they do tend to lack stereocontrol.
Another lack of control is a high degree of branching, this is due to the rearrangement of the free radical to cause branching, this is why free radical polymerised ethylene forms low density polyethylene (LDPE) which has very different properties to high density polyethylene (HDPE) which is made using a Ziegler-Natta catalyst.
As termination occurs randomly when two chains collide, it is impossible to control the length of individual chains.
Electron rich alkenes when used as monomers tend to form radicals which are more able to react with electron poor alkenes and vice versa, hence mixtures of electron poor and electron rich alkenes tend to copolymerise forming polymers where the two monomers alternate. Classic examples of these pairs include ethylene and tetrafluoroethylene, and maleic anhydride and styrene. These combinations of monomers form polymers which are of great industrial importance.