BiTEMPS methacrylate dynamic covalent cross-linker providing rapid reprocessability and extrudability of covalent adaptable networks: high-yield synthesis with strong selectivity for disulfide linkages

Abstract

The dialkylamino disulfide-based dynamic cross-linker bis(2,2,6,6-tetramethyl-4-piperidyl methacrylate) disulfide, also known as BiTEMPS methacrylate (BTMA), has been of recent interest in the preparation of various reprocessable cross-linked polymers, otherwise known as...

Reprocessable Covalent Adaptable Networks via Free-Radical Polymerization with an Aromatic Disulfide Cross-Linker

“…Comparison of our BPMA-containing CAN to previous, related CANs ,,,, made evident the different mechanisms behind aromatic disulfide and dialkylamino disulfide dynamic chemistries. Across the rubbery plateau, the BPMA-containing CAN exhibits a direct proportionality of elastic modulus with absolute T in accordance with Flory’s ideal rubber elasticity theory .…”

“…The relatively high reprocessing T of 200 °C is a consequence of the relatively high BDE of the aromatic disulfide bond, which is roughly a factor of 2 greater than the BDE of the dialkylamino disulfide bond in BiTEMPS methacrylate (BTMA), with BTMA yielding a reprocessing T of 130 °C in otherwise similar CANs. 39 Comparison of our BPMA-containing CAN to previous, related CANs 39,40,44,56,57 made evident the different mechanisms behind aromatic disulfide and dialkylamino disulfide dynamic chemistries. Across the rubbery plateau, the BPMAcontaining CAN exhibits a direct proportionality of elastic modulus with absolute T in accordance with Flory's ideal rubber elasticity theory.…”

“…As such, we first conducted TGA on both BPMA and the copolymer to determine a potential upper limit on processing temperature (represented by the degradation temperature T d , the point at which the sample loses 5% of its mass). As seen in Figure S2, the BPMA cross-linker reached this point slightly prior to the copolymer network, though its T d of 273 °C was still far in excess of the processing temperatures used in prior related CAN studies. ,− We also note that due to the rapidity of the temperature ramp (10 °C/min) and thus the low residence time at any given temperature, extended exposure of the samples to lower temperatures than their T d values may also result in degradation. With this in mind, we selected a milder range of temperatures (140, 160, 180, and 200 °C) to determine which would result in a robust and healed film after 1.0 h of compression molding.…”

“…One particularly interesting form of dynamic covalent chemistry employed in CANs is the disulfide linkage. − Inherently, though the disulfide bond is weaker than a corresponding carbon–carbon bond, its bond dissociation energy (BDE) is nevertheless substantial (often cited as 250–300 kJ/mol , ), meaning that catalysts are often employed to facilitate rearrangement. ,, However, the BDE of a disulfide bond can vary greatly depending on the adjacent atoms and functional groups, which can provide stabilization upon dissociation of the bond into sulfur radicals. ,,, For instance, some of the lowest BDEs reported for a disulfide-based dynamic bond are those of the dialkylamino disulfide chemistry (100–130 kJ/mol), − attributed to the stability imparted to the sulfur radical by the lone pair of electrons on the adjacent nitrogen atom. − As a result, studies have reported a variety of cross-linkers based on the disulfide linkage (with various adjacent substituents) which allow for catalyst-free dynamic behavior, though the exact nature of this behavior can vary depending on the particular system (aliphatic disulfide, dialkylamino disulfide, etc. ). ,− Studies have indicated that the disulfide dynamic chemistry often involves a [2 + 1] radical-mediated mechanism in which a disulfide bond dissociates into radicals which can then interact with still-intact disulfide bonds to result in exchange reactions; thus, this mechanism incorporates aspects of both dissociative and associative chemistries. ,− …”