Cycloadditions in mixed aqueous solvents: the role of the water concentration

Abstract

We examined the kinetics of a series of cycloaddition reactions in mixtures of water with methanol, acetonitrile and poly(ethylene glycol) (MW 1000). The reactions include the Diels-Alder (DA) reaction between cyclopentadiene and N-n-butylmaleimide or acridizinium bromide, the retro-Diels-Alder (RDA) reaction of 1,4,4a,9a-tetrahydro-4a-methyl-(1,4,4a,9a)-1,4-methaneanthracene-9,10-dione and the 1,3-dipolar cycloaddition of benzonitrile oxide with N-n-butylmaleimide. Plots of logk vs the molar concentration or volume fraction of water are approximately linear, but with a characteristic break around 40 M water. This break, absent for the RDA reaction, is ascribed to hydrophobic effects. Comparison with aqueous mixtures of the more hydrophobic 1-propanol shows that these mixtures induce qualitatively similar effects on the rate, but that preferential solvation effects cause the mixtures of 1-propanol to exhibit a more complex behavior of logk on composition. The results are analyzed using the Abraham-Kamlett-Taft model. The solvent effects in aqueous mixtures are not satisfactorily described by this model. For some cycloadditions, small maxima in rate are observed in highly aqueous mixtures of alcohols. The origin of these maxima and the aforementioned breaks is most likely the same.

Tuning the self-healing behavior of one-component intrinsic polymers

“…This is due to the decrease of the furan content and, therefore, an increase of the mobility of the chains appears [28]. Similar effects of the polarity on the activation energy (E A ) of the rDA of small molecules were reported by Wijnen et al [29], Rispens et al [30] and Kiseleva et al [31]. The authors indicated that polar solvents have a significant influence on the…”

Solvent Influence on the Diels‐Alder Reaction Rates of 9‐(Hydroxymethyl)anthracene and 9,10‐Bis(hydroxymethyl)anthracene with Two Maleimides

“…The acceleration effect of the DAR in water is unlikely to be attributed only to the formation of hydrogen bonds between water and reactants . The acceleration effects in the DARs with various dienophiles with different activating groups, capable of forming hydrogen bonds with water, are largely dependent on the molecular structure of the dienes . One should take into account that partial dehydration of the reaction centers on the way to the activated complex is accompanied by a large gain in energy due to the favorable water–water interaction (Eq.…”

Lowering the Healing Temperature of Photoswitchable Dynamic Covalent Polymer Networks

“…Hence, enhancing the flexibility of the linker units is one obvious possibility for lowering the T g and thus improving the flow of the material . Furthermore, it is well‐known for small molecular systems that polar surroundings lower the barrier for the retro Diels–Alder reaction . Therefore, the introduction of polar comonomers, for example methacrylates bearing (oligo)ethylene glycol side chains, could in principle also influence the healing temperature …”

“…[24,[26][27][28] Furthermore, it is well-known for small molecular systems that polar surroundings lower the barrier for the retro Diels-Alder reaction. [29][30][31] Therefore, the introduction of polar comonomers, for example methacrylates bearing (oligo)ethylene glycol side chains, could in principle also influence the healing temperature. [24,26,32,33] Herein, we describe a new set of bis-furyl-substituted diarylethene-type crosslinkers (X 2,2 and X 2,6 , Figure 1a,b) in combination with previously reported copolymer P(LMA-co-MIMA1)…”