Nucleophilicity and Lewis basicity of N-heterocyclic carbenes (NHCs) affect reactions of NHCs, including their ability to act as organocatalysts. We are particularly interested in how structural features of NHCs influences their mechanochemical reactivity. In this study, different types of NHCs are paired with p-tolyl carbodiimide (CDI) and integrated into poly(methyl acrylate) (PMA) network as a mechanophore-rich material. As mechanophores are activated as a result of elongational force exerted on the polymer backbone, bonds between CDI and NHC are broken to release NHC. Under the hypothesis of that the ring structure and substituents of NHCs affect the equilibrium between bound and unbound states of NHC-CDI adduct (the mechanophore), we conducted a series of experiments aimed at isolating contributions from nucleophilicity and Lewis basicity. First, several NHC-CDI pairs were screened to see which pairs survive radical polymerization condition. NHCs of the surviving pairs then were subjected to nucleophilicity assessments by reaction with a number of electrophiles to obtain their nucleophilicity parameter N. Since Lewis basicity of NHCs is so high that it is hard to be determined experimentally, the basicity was calculated through DFT quantum calculation. Data from these experiments will be compared with the empirical activation of NHC-CDI mechanophores in polymer networks. From this study, we anticipate that structure-property relationship can be established for this mechanophore, and this relationship can be used for mechanophore design.

Influence of NHC structure on NHC-carbodiimide mechanophore reactivity in polymeric network

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