Termination by primary radicals

It has been deduced from the order of reaction with respect to monomer that the rate of initiation is not independent of monomer (122). One could assume that initiator and monomer may form a complex from which the actual initiator is derived, or else that the AIBN and its radical products are in equilibrium within a cage of solvent. Although kinetic data are consistent with such assumptions, it appears that the results can be explained reasonably on the grounds of termination by primary radicals (21). The order of reaction with respect to AIBN initiator in nearly all of these studies is the expected 0.50 or possibly slightly higher (0.55 to 0.59). 

Termination by primary radicals in vinyl polymerization. Trans. 

For ideal radical polymerization to occur, three prerequisites must be fulfilled for both macro- and primary radicals, a stationary state must exist primary radicals have to be for initiation only and termination of macroradicals only occur by their mutual combination or disproportionation. The rate equation for an ideal polymerization is simple (see Chap. 8, Sect. 1.2) it reflects the simple course of this chain reaction. When the primary radicals are deactivated either mutually or with macroradicals, kinetic complications arise. Deviations from ideality are logically expected to be larger the higher the concentration of initiator and the lower the concentration of monomer. Today termination by primary radicals is an exclusively kinetic problem. Almost nothing has been published on the mechanism of radical liberation from the aggregation of other initiator fragments and from the cage of the... 

Sometimes termination by primary radicals is negligible [reaction (19)]. Polymerizations where termination occurs by primary radicals and/or by degradative transfer are called non-ideal . A number of other causes may lead to deviation from ideality . 

When the assumptions under which relations (23), (28), (31) and (35) have been derived are not fulfilled, these relations are no longer valid. This is manifested by a change in the reaction orders with respect to initiator and to monomer with changing concentration of these components. One of the most general causes of deviations from ideality is termination by primary radicals. 

Otsu, T. Yoshida, M. Makromol. Chem. Rapid Commun. 1982, 3, 127. Otsu, T. Yoshida, M. Makromol. Chem. Rapid Commun. 1982, 3, 133. Otsu, T. Matsunaga, T. Kuriyama, A. Yoshioka, M. Eur. Polym. J. 1989, 25, 643, and references therein. In a lucid highlight, Prof. Otsu has recently reviewed his work and the iniferter technique. An iniferter is a compound the fragments of which Initiate and preferably cross-terminate by primary radical termination and which is subject to chain transfer from the propagating species, although for many of Otsu s examples chain transfer is unlikely. Otsu, T. J. Polym. Sci, Part A Polym. Chem. 2000, 38, 2121. 

Eq.(5) by Tobolsky (9 ) does not take into account the monomer consumption in the initiation step and the termination by primary radicals. This oversimplification may explain the deviations from this equation found for telechelics (10). 

The template in this case was rather stiff, and excess of an initiator leads to termination by primary radicals. In this case the degree of polymerization of the daughter polymer was exactly the same as the degree of polymerization of the template (not taking end groups into account). 

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