Termination by initiator radicals

Termination. Two radicals react to form one or two molecules having no unpaired spins. If the radicals are assumed randomly distributed in space, the termination rate is second order in radical concentration. Under conditions where diffusion is restricted, however, the twin chains initiated by radicals from the same initiator molecule may remain close together for a time, so that termination has a higher probability than in the case of random distribution of radicals. This case has been discussed by Allen and Patrick (I, 3). It results in a rate expression formally equivalent to a reaction first order in radical concentration, with half-life t. Termination by initiator radicals is also taken into account here. 

In contrast to ionic polymerizations, where the growing chain mostly continues to live, the monomacroradicals, P, in free radical polymerizations are destroyed by termination reactions. Frequent termination reactions are recombinations or disproportionations of two macroradicals or terminations by initiator radicals, shown schematically as... 

All the equations in this section, however, apply only to the case where no transfer reactions or terminations by initiator radicals occur. 

This is a chain reaction with many similarities to the chain reactions that occur in free-radical combustion processes in the previous chapter. For the chain reaction A B + C we represented the process as a kinetic chain involving the chain-propagating intermediate R, which was fed and terminated by initiation and termination reactions (Figure 1 1-2). In the preceding reaction sequence AMj is involved in a similar chain, but now each time the chain goes around the molecule is increased in size by one monomer unit We can represent this process in Figure 11-3. This reaction system is a series reaction in AMj,... 

In this paper the rate expressions have all been corrected for nitrogen evolution from the azo initiator, oxygen absorption by initiator radicals, and oxygen evolution in termination. It is assumed that the initiator which decomposes without starting oxidation chains does not react with oxygen (21). This correction involves the addition of (l-e)Ri/2e to the measured rate, where e is the efficiency of chain initiation, found to be 0.5 at 30 °C. and 0.6 at 56 °C. The rate constant for Reaction 7 has been written as 4ktCT in order that the three termination constants may be comparable (26, footnote 27).]... 

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). 

When the substituent R stabilizes radicals as in (A) and (C), chain scission is more likely than termination by coupling. Radicals (C) then propagate the depolymerization process with volatilization of polypropylene and polystyrene at a temperature at which these polymers would not give significant amounts of volatile products when heated alone. Moreover, unsaturated chain ends such as (B) would also initiate the volatilization process because of the thermal instability of carbon-carbon bonds in P position to a double bond (Equation 4.23). 

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... 

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. 

This equation is too complicated to be confronted with experimental results. It can be greatly simplified when reaction (46) is neglected. The assumption about the small importance of macroradical termination by phenyl radicals appears acceptable. First of all, the concentration of phenyl radicals is much smaller than that of benzoyloxyl radicals, kt pr should not substantially differ from kt pr, and finally the reactive phenyl radical should be immediately consumed by initiation. Equation (51) then assumes the form... 

Application of a moderate MF accelerates photopolymerization initiated by PI leading to triplet RPs. " The main effect is an increase in/and an increase in the rate of initiation. One should expect a second weak effect leading to deceleration of a chain termination by bimolecular radical reaction. A MFE on an F parr was observed for the first time in Refs 23,24. 

The qualitative features of paraffin pyrolysis, on the other hand, are reasonably well understood. The decomposition is, in general, initiated by rupture of C-C bonds, carried by chains of hydrogen atoms, methyl radicals, and to some extent ethyl radicals, and terminated by assorted radical recombinations. Product inhibition occurs through the reaction... 

As shown In Table IV, the total incorporation values are greater than 2 at both initiator concentrations. Since MA terminates predominately by combination (16), the presence of more than 2 Initiator fragments/polymer is suggestive of an additional mode of Incorporation. A possible mechanism Is H-abstractlon from polymer by Initiator radicals which provides radical sites for chain branching. However, further discussion of these findings must await confirmation of the molecular weight data by more direct means. 

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). 

Here we have assumed for simplicity that propagation and initiation arise solely from the parent hydrocarbon R H. If we can simplify further by assuming uniform termination by R radicals only, so that the steady-state concentration of R is given by ... 

Figure 20-7. End-group fraction in the polymerization of styrene at 60° C with AIBN as initiator for l%-3% conversion. Termination by mutual deactivation of two free radicals, t(pp), leads to one initiator free radical fragment (disproportionation) or two initiator free radical fragments (recombination) per chain with one monomer residue as end group in the case of disproportionation. With termination by initiator, /(/), each molecule has two initiator fragments, etc. tr(i), transfer to initiator tr(m), transfer to monomer tr(ah), transfer to the molecule. III, in Equation (20-32). (After Pryor and Coco.)...

According to Equation (A20.15), the number-average degree of polymerization is inversely proportional to p. According to Equation (A20.5), P is proportional to the monomer concentration. Consequently, since the degree of polymerization is proportional to the monomer concentration, it should decrease with increasing conversion. But it is often observed that the degree of polymerization in free radical polymerization is independent of the conversion. This behavior can be explained by the fact that termination by initiator free radicals was not considered in the above derivations. 

To date, matrix processes have only been partially reproduced in synthetic polymers. Monodisperse polyphenols with degrees of polymerization from 3 to 5, for example, can be esterified with acrylic acid, and the resulting compound can then be polymerized at a considerable dilution (preventing intermolecular polymerization) with an excess of radical-producing initiator (controlling the second end group by termination with initiator radicals) ... 

This steady state is reached within a relatively short time. Before the occurrence of the steady state, the production of polymer free radicals P (ignoring termination by initiator free radicals) is... 

According to these assumptions, the overall rate should be first order with respect to monomer and 0.5 order with respect to initiator. Thus, a plot of the overall rate versus the square root of the initiator concentration for constant monomer concentrations (low yields) should give a straight line according to equation (20-59). However, it is frequently observed that the overall rate increases more slowly than with the root of the initiator concentration for increasing initiator concentrations. This behavior is explained by additional termination by initiator free radicals ... 

For quantitative analysis, it is assumed that termination by initiator free radicals is negligibly small. In addition, termination should result solely from the recombination of two polymer free radicals. The number-average degree of polymerization is then equal to twice the kinetic chain length ... 

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