Dynamic covalent hexahydrotriazine breakdown through nucleophilic attack by phosphine
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Research Letter
Dynamic covalent hexahydrotriazine breakdown through nucleophilic attack by phosphine Peter J. Boul, and Diana K. Rasner, Aramco Services Company, Houston, Texas 77061, USA Peter D. Jarowski, ChemAlive LLC, 1003, Lausanne, Switzerland Carl J. Thaemlitz, Aramco Services Company, Houston, Texas 77061, USA Address all correspondence to Peter J. Boul at [email protected] (Received 17 January 2019; accepted 5 March 2019)
Abstract In the current manuscript we discuss the response of dynamic metallogels that display reversion to the liquid state when exposed to phosphines. The metallogels are formed through the condensation of formaldehyde and poly(alkyloxide)amines in polar aprotic solvents. The gel formation can be catalyzed with trivalent metals (Al(III and Fe(III)) with concomitant enhanced dynamism (gelation/degelation). When various phosphines are introduced, the metallogel is irreversibly liquefied. This process adds a new vector for controlling the bulk properties of this class of materials. Here, we explore the mechanism in detail for the reaction of tris(carboxyethyl)phosphine with N,N,N-triethoxylethyl-1,3,5hexahydro-1,3,5-triazine (HEHT, 1) a stable derivative of the active hexahydrotriazine (HT) core in dimethylformamide in the presence or absence of Al(III). Additionally, density functional theory is used on the model N,N,N-trimethyl system (MHT, 2) to estimate reaction parameters and predict nuclear magnetic resonance spectra.
Introduction New reaction pathways are of special interest in constitutionally dynamic materials (CDMs)[1] particularly when they lead to greater possibilities and conditions where these material systems can be used for triggered release of contained chemical cargo[2,3] and reversible gelation.[4] CDMs are materials which are able to undergo dynamic rearrangements to their constitutions in an adaptive response to a stimulus or change in environment.[5] CDM systems have been programmed to show responses upon exposure to light,[6] electric fields,[7] pH,[8] and heat.[9] The nature of responses offered by CDMs is varied and have been shown to include mechanical modifications,[10] phase-change,[11] self-healing adaptability,[12] or selfreplication.[13] CDMs have shown great promise as materials in drug discovery[14] and materials science.[15] New chemical reactions which display utility in these dynamic materials are of substantial value in broadening the practical scope and application of CDMs. Reversible reactivity is an essential element in constitutional dynamics. The reversible covalent reactions which have shown utility in CDMs are Diels-Alder,[16,17] transamidation,[18–23] transesterification,[24,25] transetherification,[26] disulfide [27] boronate ester formation,[28] and hemiaminal/ exchange, aminal formation.[29] Recently, CDMs based on hemiaminal and hexahydrotriazine (HT) have been reported to display pH responsive phase change and network rearrangements of CDMs, which have given them promise as recyclable plastics, self-healing polymers, and stimulus-r
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