Disulfide
In chemistry, a disulfide (or disulphide in British English) is a compound containing a R−S−S−R′ functional group or the S2−
2 anion. The linkage is also called an SS-bond or sometimes a disulfide bridge and usually derived from two thiol groups.
In inorganic chemistry, the anion appears in a few rare minerals, but the functional group has tremendous importance in biochemistry. Disulfide bridges formed between thiol groups in two cysteine residues are an important component of the secondary and tertiary structure of proteins.
Compounds of the form R−S−S−H are usually called persulfides instead.
The disulfide anion is S2−
2, or −S−S−. In disulfide, sulfur exists in the reduced state with oxidation number −1. Its electron configuration then resembles that of a chlorine atom. It thus tends to form a covalent bond with another S− center to form S2−
2 group, similar to elemental chlorine existing as the diatomic Cl2. Oxygen may also behave similarly, e.g. in peroxides such as H2O2. Examples:
Applications[edit]
Rubber manufacturing[edit]
The vulcanization of rubber results in crosslinking groups which consist of disulfide (and polysulfide) bonds; in analogy to the role of disulfides in proteins, the S−S linkages in rubber strongly affect the stability and rheology of the material.[19] Although the exact mechanism underlying the vulcanization process is not entirely understood (as multiple reaction pathways are present but the predominant one is unknown), it has been extensively shown that the extent to which the process is allowed to proceed determines the physical properties of the resulting rubber- namely, a greater degree of crosslinking corresponds to a stronger and more rigid material.[19][20] The current conventional methods of rubber manufacturing are typically irreversible, as the unregulated reaction mechanisms can result in complex networks of sulfide linkages; as such, rubber is considered to be a thermoset material.[19][21]
Covalent adaptable networks[edit]
Due to their relatively weak bond dissociation energy (in comparison to C−C bonds and the like), disulfides have been employed in covalent adaptable network (CAN) systems in order to allow for dynamic breakage and reformation of crosslinks.[22] By incorporating disulfide functional groups as crosslinks between polymer chains, materials can be produced which are stable at room temperature while also allowing for reversible crosslink dissociation upon application of elevated temperature.[20] The mechanism behind this reaction can be attributed to the cleavage of disulfide linkages (RS−SR) into thiyl radicals (2 RS•) which can subsequently reassociate into new bonds, resulting in reprocessability and self-healing characteristics for the bulk material.[22] However, since the bond dissociation energy of the disulfide bond is still fairly high, it is typically necessary to augment the bond with adjacent chemistry that can stabilize the unpaired electron of the intermediate state.[21][22] As such, studies usually employ aromatic disulfides or disulfidediamine (RNS−SNR) functional groups to encourage the dynamic dissociation of the S−S bond; these chemistries can result in the bond dissociation energy being reduced to half (or even less) of its prior magnitude.[20][21][22]
In practical terms, disulfide-containing CANs can be used to impart recyclability to polymeric materials while still exhibiting physical properties similar to that of thermosets.[21][22] Typically, recyclability is restricted to thermoplastic materials, as said materials consist of polymer chains which are not bonded to each other at the molecular level; as a result, they can be melted down and reformed (as the addition of thermal energy allows the chains to untangle, move past each other, and adopt new configurations), but this comes at the expense of their physical robustness.[22] Meanwhile, conventional thermosets contain permanent crosslinks which bolster their strength, toughness, creep resistance, and the like (as the bonding between chains provides resistance to deformation at the macroscopic level), but due to the permanence of said crosslinks, these materials cannot be reprocessed akin to thermoplastics.[21][22] However, due to the dynamic nature of the crosslinks in disulfide CANs, they can be designed to exhibit the best attributes of both of the aforementioned material types. Studies have shown that disulfide CANs can be reprocessed multiple times with negligible degradation in performance while also exhibiting creep resistance, glass transition, and dynamic modulus values comparable to those observed in similar conventional thermoset systems.[20][21]
