Chemical devulcanization, applied since the 1960s, is nowadays among the most diffused. It employs various organic and inorganic chemical compounds that selectively break carbon- sulfur and/or sulfur-sulfur bonds. Generally, a supply of thermal and mechanical energy accelerates the treatment. Most chemical devulcanization methods are batch processes, in which ground waste rubber is mixed with chemical agents, at a controlled temperature and pressure. Many types of chemical compounds can be used, such as sulfides, mercaptans, amine-based compounds, inorganic solvents like propane thiol/piperidine, triphenylphosphine (PPh3), trialkyl phosphites, lithium aluminum hydride, and methyl iodide, organic solvents like alcohols and ketones, and ionic liquids (ILs). The drawback relates to toxicity of chemicals. This problem could be partially mitigated by employing less toxic, more environmentally friendly, non-sulfured agents.

Sulfides and mercaptans: The most widely used chemicals in rubber devulcanization are sulfides and mercaptans such as disulfides (like DD, thiophenols, and their zinc salts), thiol- amine reagents, hydroxide or chlorinated hydrocarbons, added typically in concentrations of 0.5 to 10 wt%.

Amine-based devulcanization: Amine-based devulcanization was patented in 2003 by Van Duin et al. Their work showed that amines might help high temperature devulcanization, which is mainly radical. They also evidenced that the treatment is preferably done with 0.1 to 15 wt% of amine compounds and works only with amines of at least an α-H atom, to reduce the crosslink density mainly by scission. Sulfides and mercaptans, also amine-based chemicals, are often used in combination with other devulcanization treatments. Dijkhuis et al. employed hexadecylamine (HDA) as an agent for EPDM thermal devulcanization and reported a reduction of crosslink density of 50% in the temperature range 225 to 275 C, even though di-and poly-sulfide bonds were cleaved. Devulcanizates were then blended with virgin EPDM and the revulcanized showed good mechanical properties compared to virgin vulcanizate. Hexadecyl- and other amines such as tetraethylenepentamine (TEPA) have also been combined with ultrasonic or mechanical devulcanization, allowing treatment at lower temperatures.

Other organic and inorganic compounds: Other catalysts, inorganic, and sulfur-free organic were developed in the past decades. Some are propane thiol/ piperidine, Grubbs catalysts, PPh3, trialkyl phosphites, lithium aluminum hydride, methyl iodide, 1,8-diazobicyclo [5.4.0]undec-7-ene (DBU), and benzoyl peroxide (BPO). Another organic solvent, 2-butanol, is employed between 200 and 350 C. To decrease the process temperature and increase the eco-sustainability of the devulcanization, 2-butanol can be partly substituted with turpentine liquids and particularly α-terpineol, derivable from renewable resources. Deep eutectic solvents (DES), such as mixtures of choline chloride (ChCl) or ZnCl 2 with urea, p- toluene sulfonic acid, or glycerol, were also proposed as devulcanization agents, especially in combination with ultrasonic processes. These solvents are widely tunable, non-flammable, and have low volatility and toxicity.

Ionic liquids: Ionic liquids (ILs), such as phosphonium, imidazolium, and pyrrolidinium salts, are also added to physical and chemical devulcanization processes due to high conductivity, high thermal stability, low flammability, and low volatility. Not only are ILs safer and less toxic than other solvents, they can solubilize a large variety of compounds and their properties are tunable to the selected cations and anions. Ionic liquids have been used alone or combined with other treatments. Further ILs proved effective as devulcanizing agents on NR like trihexyl(tetradecyl)phosphonium chloride or N,N-dioctylimidaolium bromide combined with Grubbs catalyst, in producing telechelic oligomers such as acetoxy telechelic polyisoprene with high (95% and 99%) yield..