Mercapto acids do not react readily with dry natural rubber (NR), NR in solution, or NR latex, but some mercapto esters (mainly those derived from thioglycollic acid) have been added efficiently to latex and somewhat less efficiently to dry NR. The reaction between NR latex and ethyl thioglycollate has been examined in detail, and it has been found that addition proceeds efficiently at pH ∼ 1 in the presence of hydroperoxide catalysts at 0°. The NR-ethyl thioglycollates so obtained are rubberlike materials which can be readily hydrolyzed and crosslinked. Calcium hydroxide in the presence of water or glycols is an effective crosslinking agent and the vulcanizates so obtained have physical properties typical of salt crosslinked polymers; i.e., their high tensile strengths and modulus properties are markedly dependant upon testing conditions.
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Cunneen et al. (1960) studied this question.