Termination Studies of Free-Radical Polymerizations

The first body of evidence has been obtained from the direct study of the average termination rate coefficient in polymerizing systems. The examples can be found in many papers dealing with kinetics of free-radical polymerizations and only a few examples will be given here. 

Evidence for kt being chain-length dependent came also from the initial results from Spatially Intermittent Polymerization (SIP) experiments [149]. O Driscoll and Mahabadi found that the termination rate coefficient was a strong function of chain length for both MMA [149] and S [150] up to chain lengths of approximately 1000 for MMA and 2000 for S. For chain lengths exceeding these values, the termination rate coefficient was found to be a 

More recent examples can be found in the work of e.g. Matsumoto and Mizuta [154]. They found a chain-length dependence of kt by studying the free-radical polymerization of a substituted methacrylate trans-A-tert-hu. y cyclohexyl methacrylate) by ESR. Their results for kt for various monomer, initiator, transfer agent and viscosity modifier concentrations, could be summarized in terms of equation 2.11 by = 0.28 (10 10 ). At larger chain 

Although tempting to conclude, constant C is not identical with k] , as equation 2.11 has been derived for the long chain limit [151]. 

Compared to the analytical approach, numerical methods have several advantages. 

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In the older literature one can encounter quite a number of papers suggesting that bimolecular termination is not chain-length dependent e.g. 36, 66, 68, 139-143] Experimental evidence, however, has disproved these suggestions and numerous papers have proven termination to be chain-length dependent. The evidence takes several forms and has been obtained employing different experimental approaches, which can roughly be divided in three classes (i) termination studies of free-radical polymerizations, (ii) termination studies of non-propagating species and (iii) diffusion-controlled mimic reactions. 

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