Combination, termination

The inlet monomer concentration was varied sinusoidally to determine the effect of these changes on Dp, the time-averaged polydispersity, when compared with the steady-state case. For the unsteady state CSTR, the pseudo steady-state assumption for active centres was used to simplify computations. In both of the mechanisms considered, D increases with respect to the steady-state value (for constant conversion and number average chain length y ) as the frequency of the oscillation in the monomer feed concentration is decreased. The maximum deviation in D thus occurs as lo 0. However, it was predicted that the value of D could only be increased by 10-325S with respect to the steady state depending on reaction mechanism and the amplitude of the oscillating feed. Laurence and Vasudevan (12) considered a reaction with combination termination and no chain transfer. 

Termination (Step 4) can normally occur by either combination or disproportionation reactions, as shown in Fig. 2.20. For a combination termination, two chains with active centers combine to form a new chain without an active center. The molecular weight of the chain produced is the sum of both radicals. If these are two... 

Popcorn polymers are also formed by some pure monovinyl compounds—for example, methyl acrylate, ethyl acrylate, and n-butyl methacrylate. In these cases, chemical crosslinking by combined polymer chain transfer and combination termination also seems to take place. 

In a very interesting paper, Ray (42) has applied the methods of optimal design, for instance, minimizing the breadth of the molecular weight distribution within other constraints. He used generating functions to solve for the moments of the distribution and peak-seeking methods for reaching the optimum. One and two CSTR s were considered with combination termination as well as transfer to monomer. 

The compartmentalization of radicals may produce another important effect when large-sized branched polymer molecules are formed by chain transfer to polymer plus combination termination. As clarified in Sect. 4.1, when the n value is small, the frequency of bimolecular termination reactions between large polymer radicals drops significantly compared to models that do not account for compartmentalization of radicals. From this fact, it is easy to see that the size of branched polymer molecule is smaller than that calculated without considering compartmentalization effects [281]. 

The instantaneous MWD of the primary chains formed during Interval II in a zero-one system (assuming combination termination) is given by the most probable distribution, whose number fraction distribution is given by ... 

The aryloxyl radicals formed in the initial antioxidant reaction of phenols (equation 1) may undergo several different kinds of secondary reactions, including Type (1), rapid combination (termination) with the initiating oxygen-centered radicals (equation 11) Type (2), self-reactions Type (3), initiation of new oxidation chains by H-atom abstraction from the substrate, the so-called prooxidant effect and Type (4), reduction or regeneration by other H-atom donors resulting in synergistic inhibition. The relative importance of these secondary reactions will be considered briefly here, since they may affect the overall efficiency of the antioxidant, which includes the antioxidant activity, as measured by the rate constant, (equation 10), and the number of radicals trapped, n. 

Random chain scission initiation combination termination... 

Structure (i) is formed by combination termination or tail-to-tail addition. 

Crosslinked Copolymers. Crosslinking one linear polymer with a second polymer can be achieved by generating free-radical sites on a preformed polymer in the presence of a monomer. Grafting occurs on this site and combination termination results in crosslinking. C. H. Bamford and G. C. Eastmond have prepared such crosslinked copolymers of polycarbonate and polychloro-prene using molybdenum and manganese carbonyls as initiators. 

It should be noted that Reaction A is an H-abstraction, and thus it is usually reversible. In contrast. Reaction B is not readily reversible after the RO and 0H radicals escape from the solvent cage. The net result of both Reactions A and B is the formation of secondary products and the generation of additional radicals. Figure 2 outlines the progress of a hypothetical autoxidation of a lipid. The initiation phase is followed by rapid accumulation of radicals that promote both the formation and destruction of hydroperoxides. Finally, radical combination (termination) leads to nonradical secondary products. As discussed later, both secondary products and radical reactions per se are involved in food deterioration. 

If the pressure in the reaction vessel is now decreased, the rate at which chains are broken will also decrease because the rates of the disproportionation and combination (termination) reactions will diminish. Eventually, the pressure will decrease to a point at which the rates of the termination processes will become equal to the rate at which the radical concentration is increasing because of... 

Initiator decomposition Initiation step Propagation step Termination by combination Termination by disproportionation Transfer to monomer Transfer to the solvent Transfer to a transfer agent... 

The degree of polymerization x is equal to twice for combination termination and equal to for disproportionation terminator. Deviations that occur are due to chain transfer reactions in which the following occurs ... 

Termination by combination Termination by disproportion Transfer to monomer... 

P ratio of combination termination rate to propagation rate as defined by eqn [57]... 

The instantcineous polydispersities are between 1.5, if dominated by combination termination, and 2, if dominated by disproportionation termination and chain transfer reactions. Commercial polymer produas are cumulative polymers and have polydispersity values higher than 2. 

Figwe 22 (a) MWD of free-radical polymerization in three different reactors and (b) change of polydispersity with conversion (solid lines, combination termination dotted lines, disproportionation termination). 

The two growing chains become two polymer chains, one with a saturated group at the end and the other with an unsaturated group at its end. Also the chains have initiator fragments at only one end whereas polymer molecules formed by combination termination reactions have initiator fragments at both ends. 

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