Chain termination thietane

Substituted thietanes 102 and 132-134 are inhibitors of the cumene oxidation to cumyl hydroperoxide. These properties result from the termination of the radical oxidation chain process as well as from the catalysis of the hydroperoxide degradation <2001RJAC114>. 

An ingenious method of measuring kp was evolved by Goethals 17) for the polymerisation of thietans by Et30+BF4. These polymerisations stop at incomplete conversion, because the sulphur atom in the polymer is more basic than that in the monomer, and therefore the reaction of a growing chain (II) with polymer yields a stable, non-propagating tertiary sulphonium ion (III), so that this reaction is kinetically a termination (suicidal polymerisation) ... 

Higher conversions in thiirane polymerizations, however, proceed with chain scission transfer mechanism under the influence of BF3 (C2H5)20 [192]. This is indicated by a change in the molecular weight distribution, a bimodal character. When the reaction is complete there is a marked decrease in the average molecular weight of the polymer. When thietane polymerizes with triethyl-oxonium tetrafluoroborate initiation in methylene chloride, the reaction terminates after only limited conversion [193]. This results from reactions between the reactive chain ends (cyclic sulfonium salts) and the sulfur atoms on the polymer backbone. In propylene sulfide polymerization, however, terminations are mainly due to formations of 12-membered ring sulfonium salts from intramolecular reactions [193]. 

Cyclic Sulfides. The three-membered cyclic thiiranes can be polymerized cationically, anionically, or by a coordination mechanism. The four-membered cyclic thietanes can be polymerized by cationic and anionic mechanisms, but five-membered rings cannot be polymerized. Polymerization of propylenesulfide initiated with sodium naphthalene yields telechelics with naphthalene groups on both ends, if the living chain is terminated 1-chloromethylnaphthalene (321). 

The mechanism of the cationic polymerization of thietanes differs from the mechanism described for the thiiranes by the fact that the reaction between the active species and a sullin atom of the polymer chain appears to be irreversible so that it is a real termination reaction. As a consequence, the polymerizations of thietanes stop before all monomer is consumed and there is no formation of cyclic oligomers. The maximum conversions obtained depend on the amoimt of initiator used and on the ratio of the rate constant of propagation, fep, to the rate constant of the termination reaction, kt (Scheme 34). 

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