Thioacetone polymerization

Thioacetone polymerizes adventitiously even at very low temperature, in contrast to the inability of acetone to polymerize. 

As has been mentioned earlier, thioacetone polymerizes adventitiously even at very low temperatures. The product is a white powder of very high crystallinity. Bailey and Chu (34) state that pure thioacetone gives only polymer, but others always have obtained hexamethyl-s-trithiane in addition to polythioacetone. The... 

Though thioacetone polymerizes spontaneously, by free-radical initiation, and by base initiation, it does not form copolymers in any of these systems. Efforts to obtain copolymers with dienes, vinyl compounds, acrylates, aldehydes, and epoxides have failed. 

One of the great surprises of fluorothiocarbonyl chemistry is the ease with which these compounds undergo free-radical polymerization. This behavior is unique among thiocarbonyl compounds. Though thioacetone polymerizes in free-radical systems, it does not do so with anything like the avidity of fluorothiocarbonyl compounds. Thioacetone does not copolymerize with compounds containing carbon-carbon unsaturation, which is a most important property of fluorothiocarbonyl compounds. 

Compounds with active hydrogen add to the carbonyl group of acetone, often followed by the condensation of another molecule of the addend or loss of water. Hydrogen sulfide forms hexamethyl-l,3,5-trithiane probably through the transitory intermediate thioacetone which readily trimerizes. Hydrogen cyanide forms acetone cyanohydrin [75-86-5] (CH2)2C(OH)CN, which is further processed to methacrylates. Ammonia and hydrogen cyanide give (CH2)2C(NH2)CN [19355-69-2] ix.orn. 6<55i the widely used polymerization initiator, azobisisobutyronitrile [78-67-1] is made (4). 

Treatment of 2-methylthiirane with t-butyl hydroperoxide at 150 °C in a sealed vessel gave very low yields of allyl disulfide, 2-propenethiol and thioacetone. The allyl derivatives may be derived from abstraction of a hydrogen atom from the methyl group followed by ring opening to the allylthio radical. Percarbonate derivatives of 2-hydroxymethylthiirane decompose via a free radical pathway to tar. Acrylate esters of 2-hydroxymethylthiirane undergo free radical polymerization through the double bond. 

After thioformaldehyde, the thiocarbonyl compound that has received most attention is thioacetone. Unlike thioformaldehyde, it can be kept in monomeric form, but only if kept below - 50° C (30). It is a red oil that freezes at about — 55° C and boils at about 70° C. If pure it polymerizes to a white solid in a few hours at — 78° C or quite rapidly at room temperature. It also exists as a thioenol tautomer, which is stable at temperatures below — 50° C but which tautomerizes at higher temperatures to give an equilibrium mixture of thioketo and thioenol forms. 

Polymerization of thioacetone is also initiated by visible light. In fact, it has been shown that exposure of frozen crystals of thioacetone monomer to light leads to rapid solid phase polymerization (33). 

Fluorine-substituted thiocarbonyl compounds have been studied even more intensively than thioformaldehyde and thioacetone. These compounds have a very rich chemistry of which polymerization is only a part. The simplest member of this class is fluorothiocarbonyl fluoride, CF2==S, which also forms the most interesting polymers. Other members that have been investigated include a variety of fluorothioacyl halides and a number of fluorothioketones. Because... 

Figure 16-1. Dependence of the monomer concentration at polymerization equilibrium on the temperature for a-methyl styrene in cyclohexane (from data from F. S. Dainton and K. J. Ivin) and thioacetone, TA (from data from V. C. E. Burnop and K. G. Latham). The temperature at which the molar concentration, [A/]b, of the pure monomer is reached is the thermodynamic transition temperature Tc...

The equilibrium polymerization of a monomer melt to a polymer insoluble in the melt is quite rare. The ceiling temperature in this case corresponds to a phase transition temperature, below which the monomer is completely converted into polymer. Examples are the polymerization of chloral (TV = 58° C), sulfur trioxide (Tc = 30.4° C), and thioacetone (Tc = 95°C). 

---