Use of Bis(2,2,6,6-tetramethylpiperidin-1-yl)trisulfide as a Dynamic Covalent Bond for Thermally Healable Cross-Linked Polymer Networks

Abstract

Dynamic covalent bonds (DCBs), which can undergo reversible cleavage and reformation upon exposure to readily useable stimuli, have attracted dramatic attention, but the library of such bonds still remains to be developed. Herein, we report molecular structures, dynamic behaviors, and healability of bis­(2,2,6,6-tetramethylpiperidin-1-yl)­trisulfide (BITEMPS-S3) to compare with its disulfide analogue (BITEMPS-S2) exchanged at moderate temperature. Unsymmetrical cleavage of trisulfide linkage induced relatively rapid disproportionation to di- and tetrasulfide derivatives. In the bulk, poly­(hexyl methacrylate) networks partially containing the BITEMPS-S3 moiety as a cross-linking point afforded nearly quantitative damage healability only by simple hot pressing at 110 °C under mild pressure. A slightly higher healability of BITEMPS-S2 compared to that of BITEMPS-S3 would be due to the differences in the chain-transfer reaction for the trisulfide linkage during free-radical polymerization rather than thermal exchangeability of the BITEMPS-S3 moiety. Therefore, not only BITEMPS-S2 but also BITEMPS-S3 should be regarded as one of DCBs triggered upon exposure to mild external stimuli.

Self-Strengthening of Cross-Linked Elastomers via the Use of Dynamic Covalent Macrocyclic Mechanophores

“…A rectangular specimen of each CPHMA polymer was held at 5% strain at 100 °C, and the stress transition was monitored over time. As has been previously reported, 41,42 BTM-CPHAM-1, which contains 1% of acyclic BiTEMPS at the cross-linked points, exhibited a fast release of the applied stress after stretching (Figure 4b). In contrast, BCM-CPHMA-1 showed stress-relaxation (11% after 60 min) that is slightly higher than that of HDM-CPHMA-1 (5% after 60 min), but substantially lower than that of BTM-CPHAM-1 (57% after 60 min).…”

Effect of bulky 2,6-bis(spirocyclohexyl)-substituted piperidine rings in bis(hindered amino)trisulfide on thermal healability of polymethacrylate networks

“…As has been previously reported, the trisulfide linkage in the BITEMPS-S3 moiety as a DCB disproportionated to the corresponding disulfide and tetrasulfide linkages because unsymmetrical and homolytic bond cleavage of the trisulfide linkage affords both thiyl and dithiyl radicals. 17 Therefore, to investigate the thermal disproportionation reaction of the BIBSCPS-S3 moiety, the phenyl urethanated derivative (BIBSCPS-S3-Ar) was prepared due to the poor solubility of BIBSCPS-S3-OH. As shown in Figure S3, the ESI mass spectrum of BIBSCPS-S3-Ar heated at 80 °C for 24 h in DMF under air atmosphere exhibits the corresponding disulfide (BIBSCPS-S2-Ar) and tetrasulfide (BIBSCPS-S4-Ar) analogues.…”

“…incorporated into poly(nHexMA) at a cross-linking point shows lower 0 ⁄ value than BITEMPS-S3, probably due to higher chain-transfer constant ( tr ) of the trisulfide linkage compared to the disulfide linkage. 17,29 Considering that the oxidative stability of the 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO)to ascorbic acid was increased by replacing the methyl groups in TEMPO with spirocyclohexyl groups due to effective steric shielding, 30 this considerable difference would indicate that chain-transfer reaction to S-S linkage in the BIBSCPS-S3 moiety during radical polymerization is highly suppressed by the steric hindrance of the bulky spirocyclohexyl groups.…”

“…[10][11][12][13][14][15][16] Afterward, we reported that the trisulfide analogue of BITEMPS-S2, bis(2,2,6,6-tetramethylpiperidin-1-yl)trisulfide (BITEMPS-S3), also behave as an DCB to produce air-stable thiyl and dithiyl radicals upon heating at mild temperature (as shown in Scheme 1b). 17 Additionally, poly(n-hexylmethacrylate) network containing BITEMPS-S3 units at cross-linking points afforded nearly quantitative damage healability only by simple hot pressing at 110 °C for 24 h, which were almost comparable to one containing BITEMPS-S2 units (100 °C for 24 h). Although these results indicate that not only bis(dialkylamino) disulfide but also bis(dialkylamino) trisulfide should be recognized as one of the intrinsic DCBs, it is necessary to develop DCBs triggered at more moderate temperature.…”

Synthetic Strategy for Mechanically Interlocked Cyclic Polymers via the Ring-Expansion Polymerization of Macrocycles with a Bis(hindered amino)disulfide Linker

“…Recently, exploiting the dynamic nature of bis­(2,2,6,6-tetramethylpiperidin-1-yl) disulfide (BiTEMPS), − we were able to selectively isolate some macrocyclic monomers (MMs) with one BiTEMPS unit (S-S linkage) in their cyclic structure . The BiTEMPS structure allows the exchange of disulfide bonds above 80 °C via relatively stable radicals (Scheme a).…”