Internal Catalysis in Covalent Adaptable Networks: Phthalate Monoester Transesterification As a Versatile Dynamic Cross-Linking Chemistry

Abstract

Covalent adaptable networks (CANs) often make use of highly active external catalysts to provide swift exchange of the dynamic chemical bonds. Alternatively, milder species can act as internal catalysts when covalently attached to the matrix and in close proximity to the dynamic bonds. In this context, we introduce the dynamic exchange of phthalate monoesters as a novel chemistry platform for covalent adaptable networks. A low-molecular-weight (MW) model study shows that these monoesters undergo fast transesterification via a dissociative mechanism, caused by internal catalysis of the free carboxylic acid, which reversibly forms an activated phthalic anhydride intermediate. Using this dynamic chemistry, a wide series of CANs with a broad range of properties have been prepared by simply curing a mixture of diols and triols with bifunctional phthalic anhydrides. The dynamic nature of the networks was confirmed via recycling experiments for multiple cycles and via stress relaxation using rheology. The networks proved to be resistant to deformation but showed a marked temperature response in their rheological behavior, related to the swift exchange reactions that have a high activation energy (120 kJ/mol). While densely cross-linked and hydrolytically stable polyester networks with low soluble fractions can be obtained, we found that, by swelling the networks in a hot solvent, a gel-to-sol transition happened, which resulted in the full dissolution of the network.

Thiol–Anhydride Dynamic Reversible Networks

“…This effective reversibility was attributed to the planar structure of the phthalic anhydride, which after ring opening enforces close vicinity between the aromatic thioester anhydride and the neighboring carboxylic group. A similar ester–anhydride reversion was recently shown in polyester networks designed from multi‐functional alcohols and phthalic anhydride derivatives …”

Thiol–Anhydride Dynamic Reversible Networks

“…A similar ester-anhydride reversion was recently shown in polyester networks designed from multi-functional alcohols and phthalic anhydride derivatives. [11] To determine the reversibility of the thiol-anhydride adducts derived from aliphatic anhydrides, methyl succinic anhydride (MSA) was reacted in another model reaction with methyl 3-mercaptopropionate (MMP; Supporting Information, Figure S1). The combination of equimolar concentrations of MMP and MSA, this time with inclusion of 5 mol % Scheme 1.…”

Dynamic covalent chemistry in polymer networks: a mechanistic perspective

“…As discussed above, in principle, for some stoichiometries, networks with dissociative MNR pathways could behave as networks with exclusive associative pathways, as netdecross-linking does not occur. 85 Also, it should be stressed that in polymer matrices, reactive groups and catalysts interact with the macromolecular network and will exhibit their own dynamics, which may be different from that in 'free' solutions. Therefore, understanding how such interactions and motion dynamics in the network combine and influence the emergence of material properties is at this time an interesting and open research question.…”