Termination, by coupling and disproportionation

Not being aware of the earlier work, the present author first noticed the phenomenon in 1981. Geiger and Huber10 had photolyzed dimethylnitrosamine in the gas phase at 1 Torr and under 100 Torr N2 buffer. This compound fragments from the first excited singlet state into dimethylaminyl radicals and nitrous oxide NO with unity quantum yield, but neither photoproducts nor a decrease of the initial compound pressure were observed. Even after 20 h photolysis the back-reaction was complete to more than 99.9% (Scheme 6). This seemed quite puzzling because sterically unhindered aminyl radicals are transient and readily self-terminate by coupling and disproportionation. 

Let X fraction of chain radicals terminate by coupling and (1 — x) fraction by disproportionation. Therefore 1 mol of chain radicals give rise to 0.5a5 + (1 — x) or (1 — 0.5x) mol polymer molecules. Hence... 

Problem 6.21 A vinyl monomer of molecular weight 132 is polymerized by a free-radical initiator in the presence of dodecyl mercaptan (C12H25SH). The rate of polymerization is not depressed by the mercaptan. The purified polymer has a sulfur content of 0.02% (w/w) and its DP is 450. If 80% of the kinetic chains are terminated by coupling and 20% by disproportionation, what should be the extent of terminal unsaturation of the chains ... 

During a nonliving polymerization, such as conventional radical processes, four distinct mechanistic steps occur. As shown in Scheme 2.19 for monosubstituted vinyl monomers, these include initiation, propagation, irreversible termination by coupling or disproportionation, and chain transfer [76]. 

If half the kinetic chains are terminated by coupling and half by disproportionation, what is the expected value of C ... 

The main reason that the decreases as the polymerization temperature increases is the increase in the initiation and termination reactions, which leads to a decrease in the kinetic chain length (Fig. 17). At low temperature, the main termination mechanism is polystyryl radical coupling, but as the temperature increases, radical disproportionation becomes increasingly important. Termination by coupling results in higher PS than any of the other termination modes. 

The mode of termination varies with monomer and reaction conditions. While styrene macroradicals typically terminate by coupling, methyl methacrylate macroradicals terminate by coupling at temperatures below 60°C, but by disproportionation at higher temperatures. 

Termination can also occur by a combination of coupling and disproportionation. The two different modes of termination can be represented in general terms by... 

The first term in the denominator denotes termination by a combination of coupling and disproportionation, and the other terms denote chain transfer by monomer, chain-transfer agent, and initiator, respectively. 

In the case of styrene the polymer contains at both ends a carboxyl group (chain termination by coupling), while polymers characterized by a termination reaction by disproportionation (polyvinyl acetate, polymethyl methacrylate) contain only one carboxyl end group. The carboxyl groups in these polymers were transformed to acid chlorides and coupled with a diol, e.g. 1,6-hexanediol. The comparison of the molecular weight of the polymers before and after condensation permits to elucidate the... 

Since the nitroxide and the carbon-centered radical diffuse away from each other, termination by combination or disproportionation of two carbon-centered radicals cannot be excluded. This will lead to the formation of dead polymer chains and an excess of free nitroxide. The build-up of free nitroxide is referred to as the Persistent Radical Effect [207] and slows down the polymerization, since it will favor trapping (radical-radical coupling) over propagation. Besides termination, other side reactions play an important role in nitroxide-mediated CRP. One of the important side reactions is the decomposition of dormant chains [208], yielding polymer chains with an unsaturated end-group and a hydroxyamine, TH (Scheme 3, reaction 6). Another side reaction is thermal self-initiation [209], which is observed in styrene polymerizations at high temperatures. Here two styrene monomers can form a dimer, which, after reaction with another styrene monomer, results in the formation of two radicals (Scheme 3, reaction 7). This additional radical flux can compensate for the loss of radicals due to irreversible termination and allows the poly-... 

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