5-Vinyl mercaptals

Whether a monomer can be polymerized free-radically also depends in certain cases on the initiator. Azobisisobutyronitrile, for example, polymerizes 5-vinyl mercaptals CH2=CH—S—CH2—S—R to products of high molecular weight, whereas no polymer is formed at all under the same conditions with benzoyl peroxide. Here, in fact, the result is an induced decomposition of the benzoylperoxide by the ——CH2—S— group to form an (unstable) ester CH2=CH—S—CH(OCOC6H5)—S—R and benzoic acid. 

Under certain conditions, no free radical polymerization at all occurs with certain free radical initiators. For example, azobisisobutyronitrile polymerizes vinyl mercaptals, CH2=CH—S—CH2—S—R, to high-molar-mass compounds. But no polymer at all is produced under the same conditions by dibenzoyl peroxide the mercaptal groups induce dibenzoyl peroxide decomposition, producing benzoic acid and an unstable ester, CH2=CH—S— CH(00CC6H5)—S—R. Thus, the initiator is completely consumed by this side reaction. 

An alternate approach to the synthesis of polymercaptan precursor was considered by H. Ringsdorf et al. 28-31). They prepared vinyl mercaptals by either dehydration of the analc es S- -hydroxyethyl... 

Treatment of acetals with thiols in excess produces an equilibrium mixture from which the corresponding dithioacetal may be obtained a favourable case from this point of view is the conversion of formamide acetals R2NCH(OMe)2 into corresponding mercaptals. Vinyl sulphides CH2=CHSR give acetaldehyde dithioacetals CHs—CH(SR)2, in 40% yield, and acetylene, on being heated at 170—180 °C in the presence of KOH, through synchronous intramolecular transfer of hydride and thiolate. 

Methyl glyoxylate diethyl mercaptal added at 4° to a soln. of NaH in dimethoxy-ethane, stirred 40 min., then methyl vinyl ketone added product. Y 93%. F. e. s. J. L. Herrmann, R. H. Schlessinger et al., Tetrah. Let. 1973, 2595 with cyclic mercaptals cf. ibid. 1973, 2599. ... 

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