POLYMERIZATION OF UNSATURATED MONOMERS BY FREE RADICAL MECHANISMS

Polymerization of Unsaturated Monomers by Free Radical Mechanisms 

The processes by which unsaturated monomers are converted to polymers of high molecular weight exhibit the characteristics of typical chain reactions. They are readily susceptible to catalysis, photoactivation, and inhibition. The quantum yield in a photoactivated polymerization in the liquid phase may be of the order of 10 or more, expressed as the number of monomer molecules polymerized per quantum absorbed. The efficiency of certain inhibitors is of a similar magnitude, thousands of monomer molecules being prevented from polymerizing by a single molecule of the inhibitor.  

Not only are intermediates of this nature generally undetectable, but in the case of styrene the known dimer and trimer are themselves quite inert toward further polymerization.  

The following free radical chain mechanism, first suggested by Taylor and Bates to explain the polymerization of ethylene induced by free radicals in the gas phase and independently proposed by Staud-inger for liquid phase polymerizations, offers an explanation for the above general characteristics of vinyl polymerizations. 

An impressive number of substances capable of generating free radicals have been shown to be potent accelerators for the polymerization of typical vinyl monomers such as styrene, methyl methacrylate, butadiene, and vinyl acetate. The most commonly employed initiators (often referred to inaccurately as catalysts) are organic peroxides, such as benzoyl peroxide. These are known to decompose slowly at temperatures of 50° to 100°C with release of free radicals as follows 

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