Sulfide crosslinks

Polyphenylene sulfide Melts at 270-315°C (578-599°F) crosslinked polymer stable to 450°C (842°F) in air adhesive and laminating applications. 

An even higher heat resistance of modified AN copolymers is attained as a result of cyclization by treating previously crosslinked fibres based on the copolymer of AN and 4-chloro-2-butenyl methacrylate44 with ammonium sulfide ... 

The synthesis of polysulfide elastomers involves the use of a small amount of trichloroalkane in addition to dichloroalkane and sodium sulfide in order to form a branched polymer. The prepolymer is treated with a mixture of sodium hydrosulfide and sodium sulfite followed by acidification to convert all end-groups to thiol groups. Further polymerization and crosslinking is achieved by oxidative coupling of the thiol end-groups by treatment with lead dioxide, p-quinone dioxime, or other oxidizing agent... 

The first reaction is p-elimination in cysteine, serine, phosphoserine, and threonine residues due to attack by hydroxide ion, leading to the formation of very reactive dehydroalanine (DHA). In a cystine residue, this results in rupturing of the disulfide bond and liberation of a sulfide ion and free sulfur (Figure 13.4). Nucleophilic additions of the s-amino group of the protein-bound lysine to the double bond of DHA residue causes crosslinking of the polypeptide chain. After hydrolysis, a mixture of L-lysino-L-alanine and L-lysino-D-alanine, with probably a small proportion of dl and dd isomers,... 

Note 2 A classic example of vulcanization is the crosslinking of c/s-polyisoprene through sulfide bridges in the thermal treatment of natural rubber with sulfur or a sulfur-containing compound. 

Vulcanization by heating with sulfur alone is a very inefficient process with approximately 40-50 sulfur atoms incorporated into the polymer per crosslink. Sulfur is wasted by the formation of long polysulfide crosslinks (i.e., high values of m in XHI), vicinal crosslinks (XIV), and intramolecular cyclic sulfide structures (XV). (Structures XIV and XV do not contribute significantly to the physical properties of the polymer.)... 

The action of zinc in increasing the efficiency and rate of crosslinking is thought to involve chelation of zinc with the accelerator as well as species XVIII and XIX. Zinc polysulfide compounds such as XX are also likely intermediates. Zinc chelated to sulfur or as zinc sulfide bonds probably facilitate cleavage of sulfur-sulfur bonds in the concerted reactions described by Eqs. 9-20 and 9-21. 

The dithioamines probably react with rubber molecules to give sulfidic intermediates (30 equation 10) which subsequently lead to crosslinking. The use of these vulcanizers produces cured materials with good resistance to aging. 

It has long been recognized that zinc plays an essential part in vulcanization, affecting both the rate and the extent of crosslinking (74MI11501, B-78MI11503). This behavior has been explained in terms of complex formation (36) between zinc ions and the various sulfidic intermediates involved in vulcanization (65MI11502). 

Among the most biologically reactive alkylating agents are the nitrogen and sulfur "mustards" such as bis-(2-chloroethyl)sulfide. These toxic bifunctional compounds cause lethal crosslinking of DNA chains... 

The new absorptions in the spectra of crosslinked rubber are assigned on the basis of 13C solution NMR chemical shifts for a variety of model compounds, such as pentenes and mono-, di- and tri-sulfidic compounds, by using the 13C chemical shift substituent effect. From the calculated values for particular structural units, the experimental spectra of a sulfur vulcanized natural rubber 194,195,106), natural rubber cured by accelerated sulfur vulcanization 197 y-irradiation crosslinked natural rubber198 and peroxide crosslinked natural rubber and cis-polybutadiene 193 1991 are assigned. 

By using this method, the chemical shifts of the resonances in the spectra of a sulfur vulcanized natural rubber (Fig. 32 expanded aliphatic region in shown in Fig. 33 [top]) are assigned to various units of the polymer network, which arise from structural modifications induced by the vulcanization 194,196 200). Different sulfidic structures are found for unaccelerated and accelerated sulfur vulcanizations, respectively. With increasing amount of accelerator (as compared to the sulfur), the network structure exhibits less crosslinking, fewer main chain structural modifications, and fewer cyclic sulfide structures 197). 

A networked supramolecular POM-based hybrid catalyst has been synthesized by Ikegami and coworkers via the self-assembly of [PWi2O40]3 and non-crosslinked copolymer based on N-isopropylacrylamide (NIPAM) and ammonium cations (Figure 6.7) [124, 125]. This heterogeneous NIPAM-POM hybrid catalyst showed very high catalytic activity for the epoxidation of allylic alcohols with H202 (e.g., up to 35 000 TON for phytol). In addition, the oxygenation of amines and sulfide could be... 

The experimental ratios of 2 ( 0.2) agree with the expected value of 2.0 converted ENB units for one chemical crosslink (ENB-Sn-ENB), indicating that crosslinks are formed predominantly. Several explanations are proposed for deviations from the theoretical value. Both pendent sulfur as in the crosslink precursor (ENB-Sn-X, X = accelerator residue) and sulfur loops (sulfur bridge between two ENB units in the same EPDM chain) will give rise to a ratio larger than 2.0. Formation of cyclic sulfide (addition of S to unsaturation) will give rise to a ratio lower than 2.0. The fact that within experimental error a ratio of 2.0 is found suggests that pendent sulfur, loops and cyclic sulfur hardly occur (less than 10% of the total amount of reacted sulfur). 

---