Free-Radical Polymerization

FREE-RADICAL POLYMERIZATION

The first major use of free radicals was in olefin polymerization reactions. Polymerization reactions are amenable to free-radical initiation for several reasons. First the olefin is the only reagent present so as to minimize competing reactions. Second the initiator radical is produced by heat or light or catalysis in the pres-ence of a huge excess of the olefin. Under these conditions free-radical addition to the double bond is virtually the only process that occurs. Moreover, the new radical species resulting from olefin addition is also produced in the presence of a huge excess of olefin so that it adds to another olefin molecule to give a larger free radical. The process continues. By controlling the purity of the starting olefin and the reaction conditions so that terminations are rare, chain lengths in the tens of thousands can be achieved. This leads to the formation of thousands of carbon–carbon bonds per polymer molecule and extremely long polymer chains.

As a consequence of the fact that free-radical reactions are chain processes, they are very well suited for the preparation of polymers rather than single products. That is, products are obtained whose size is determined by the number of propagation cycles that occur before a termination event stops the growing chain.

If the number of propagation cycles is between 200 and 300, then the product mixture will contain molecules which contain between 200 and 300 monomers. It is more reasonable to describe the product mixture in terms of the “average molecular weight” rather than a single product with a discrete molecular weight. The physical properties reported for a polymer are those of a mixture of polymeric molecules rather than of a single polymeric compound.

Free-radical polymerization was a mainstay of the plastics industry for many years. While new and better methods have been developed for the polymerization of many substrates, free-radical polymerization is still used for the preparation of many plastics and composites. The success of these methods is based on an under-standing of the process. Huge amounts of effort have been expended in finding initiation reactions that produce free radicals controllably and reproducibly. The reaction environment has been studied intensively so that propagation reactions are maximized and termination events minimized. Finally the rational control of termination reactions, which are necessary to control the chain length and thus the average size of the polymer molecules produced, has been successfully developed. It is important to emphasize that the properties of the product mixture were the gauge by which the understanding and control were measured.

Thus it is not necessary to produce one product molecule with a defined molecular weight. It is only necessary to obtain a product mixture whose average molecular weight and physical properties fall within a defined range. Now it is true that obtaining a more narrow range of molecular weights in a polymer leads to much more consistent physical properties. This explains why so much effort was made to control free-radical polymerization. Those efforts played a large role in the creation of the “plastic society” in which we live.