Polymerization killing

Chain reactions do not go on forever. The fog may clear and the improved visibility ends the succession of accidents. Neutron-scavenging control rods may be inserted to shut down a nuclear reactor. The chemical reactions which terminate polymer chain reactions are also an important part of the polymerization mechanism. Killing off the reactive intermediate that keeps the chain going is the essence of these termination reactions. Some unusual polymers can be formed without this termination these are called living polymers. 

Conditions that are important to all chemical reactions such as stoichiometry and reactant purity become critical in polymer synthesis. In step growth polymerization, a 2% measuring and/or impurity error cuts the degree of polymerization or the molecular weight in half. In chain growth polymerization, the presence of a small amount of impurity that can react with the growing chain can kill the polymerization. 

Inhibitors can be injected into the system in order to kill active species present, for example, by neutralizing the catalyst or by capturing free radicals in a polymerization. For example, the Lewis acid, BF3-complex can be killed using gaseous NH3 since the inactive compound BF3 NH3 is formed, and the reaction stops for lack of active centers. An antioxidant such as hydroquinone can be used to capture peroxide radicals to control reactions involving vinyl-type monomeric substances. 

Different antimalarials selectively kill the parasite s different developmental forms. The mechanism of action is known for some of them pyrimethamine and dapsone inhibit dihydrofolate reductase (p. 273), as does chlorguanide (proguanil) via its active metabolite. The sulfonamide sulfadoxine inhibits synthesis of dihydrofolic acid (p. 272). Chlo-roquine and quinine accumulate within the acidic vacuoles of blood schizonts and inhibit polymerization of heme, the latter substance being toxic for the schizonts. 

Monomers devoid of polar groups generally undergo anionic polymerization in a predictable manner. With polar monomers sometimes side reactions occur during the process transfer reactions in the case of acrylonitrile, or propylene oxide, and even more so with alkylacrylates deactivations (or "killing") reactions in the case of halogen substituted styrene or dienes. 

McCormick (11) and Worsfold and Bywater (72) initiated the polymerization of a-methylstyrene by using sodium naphthalene, brought the system to equilibrium at the desired temperature, and then killed the living polymers. They showed that at 0° C the concentration of the monomer attains its equilibrium value in less than 16 hours, and they observed no further polymerization over periods lasting for as long as 124 hours. Furthermore, they showed that the monomer equilibrium concentration was independent of the concentration of living ends, i.e. of the amount of catalyst applied. 

Sometimes experimenters are tempted to determine the number of chains formed during polyerization and assume each site makes one chain, but a site terminates and reinitiates chains continuously, making this approach invalid except at very short reaction times. Quick-kill experiments in this laboratory (69) tend to confirm Hogan s number (77), but to actually see the first chain growing with time, the polymerization must be artificially slowed by using noncommercial conditions, and the results are not very reproducible. 

Zakharov et al. have used a radio tagging technique to measure the active site density in which polymerization is killed with labeled methanol (72, 73). They found only about 1 % or less of the chromium to be active, or about one tenth of Hogan s number. But because they calcined Cr/silica at only 400-500°C, their catalyst was probably only one-tenth as active. So the two studies are not necessarily in conflict. As expected, the active site density found by tagging increases with time during a polymerization run. 

Fig. 22. Flouride greatly improves polymerization activity in a series or Cr/alumina catalysts calcined at 600°C, but too much fluoride sinters the catalyst and kills the activity.

The active site concentration on the organochromium catalysts may be higher than that of the oxide catalysts. The activity usually assumes a more linear increase with chromium loading than on the oxide catalysts, at least up to 2% Cr. Yermakov and Zakharov, studying allyl-Cr(III)/silica catalysts, stopped the polymerization with radioactive methanol, and found that the kill mechanism is different from that on the oxide catalysts (59). The proton of the methanol, and not the alkoxide, became attached to the polymer. This suggests a polarity opposite to that of the oxide catalysts, with the site being more positive than the chain. 

The conditional killing agents may be monomers in their own right. For example, ethylene oxide added to living polystyrene terminates the polymerization of styrene. It is, therefore, a terminator in respect to styrene and, since it will not initiate polymerization of ethylene oxide at 0° C, or at lower temperatures, it is a terminator for polymerization under these conditions. However, if more ethylene oxide is added and the... 

Fig. 5 Concept of contact-killing membrane-active biocides surface-coupled via a polymeric...

Polymers with specific end groups can be prepared by deliberately introducing particular reagents that kill living polymers. Thus in the anionic polymerization of butadiene with bifunclional initiators, carboxyl end groups are produced by termination with CO ... 

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