Polyfunctional mercaptans

Polysulfide Impression Materials. In 1953 the first nonaqueous, elastic dental impression material based on the room-temperature conversion of a Hquid polymer, a polyfunctional mercaptan (polysulfide), to a strong, tough, dimensionally accurate elastomer, was introduced. The conversion of the Hquid polymer to an elastic soHd has been achieved in most products by lead peroxide [1309-60-0]. Significant improvements in strength, toughness, and especiaHy dimensional stabiHty of the set polysulfide elastomers over the aqueous elastic impression materials made these materials popular. 

The polysulfide base material contains 50—80% of the polyfunctional mercaptan, which is a clear, amber, sympy Hquid polymer with a viscosity at 25°C of 35, 000 Pa-s(= cP), an average mol wt of 4000, a pH range of 6—8, and a ntild, characteristic mercaptan odor. Fillers are added to extend, reinforce, harden, and color the base. They may iaclude siUca, calcium sulfate, ziac oxide, ziac sulfide [1314-98-3] alumina, titanium dioxide [13463-67-7] and calcium carbonate. The high shear strength of the Hquid polymer makes the compositions difficult to mix. The addition of limited amounts of diluents improves the mix without reduciag the set-mbber characteristics unduly, eg, dibutyl phthalate [84-74-2], tricresyl phosphate [1330-78-5], and tributyl citrate [77-94-1]. 

Polyfunctional mercaptans. Polyfunctional mercaptans are some of the most powerful odorants in nature, and they must be quantified at extremely low levels. The methods developed for their determination make use either of a selective separation using -hydroxy-mercurybenzoate (Tominaga et al. 1998 Tominaga and Dubourdieu 2006 Ferreira et al. 2007), of covalent chromatography (Schneider et al. 2003) or of the derivatization with pentaflurobenzyl bromide (Mateo-Vivaracho et al. 2006, 2007). A recent report using non-selective headspace isolation with SPME has also been published, but the limits of quantification are more than one order of magnitude above the odor thresholds (Fedrizzi et al. 2007). 

Mateo-Vivaracho, L., Cacho, J., and Ferreira, V. (2007). Quantitative determination of wine polyfunctional mercaptans at nanogram per liter level by gas chromatography-negative ion mass spectrometric analysis of their pentafluorobenzyl derivatives. J. Chromatogr. A., 1146, 242-250. 

Polyfunctional mercaptans can be used to impart some control to free-radical polymerisation. For methacrylate monomers, it has been demonstrated that polymers with polydispersities as low as 1.25 can be made. Unlike conventional radical polymerisations, this proceeds with an increase in molecular weight with time. The reason for this is that the thiol groups react sequentially as polymerisation proceeds. The polymerisation is statistically more controlled. 

The polyaddition reaction is the most commonly used type of reaction for the cure of epoxy resins. The curing agents used in this type of reaction have an active hydrogen compound, and they include amines, amides, and mercaptans. With this reaction mechanism, the most important curing agents for adhesives are primary and secondary amines containing at least three active hydrogen atoms and various di- or polyfunctional carboxylic acids and their anhydrides. 

---