The Thermal Stability and Rheological Properties of Alkylated Carboxymethyl Starch

Several alkylated carboxymethyl starches (CMS) with different alkyl chain lengths were prepared. The influence of the number of alkyls on the thermal stability and rheological properties, such as thickening properties, salt-tolerance, temperature sensitivity and time-dependent rheological behavior, are discussed. The initial decomposition temperature (IDT) of alkylated CMS reached 263°C ～293°C which, as compared to the IDT for CMS itself (230°C), indicated that the thermal stability of CMS was improved after being alkylated. The solution viscosities of the alkylated carboxymethyl starch increased with the increasing of alkyl length. With the alkyl chain length increasing from C₂ to C₈, the viscosity increased from 400 mPa·s to 38000 mPa·s. The weak hydrophobic aggregation of the alkyl groups did not improve the shear-resistance and relative hysteresis area. But the temperature sensitivity of alkylated CMS was improved as the chain length of the alkyl groups increased, as the activation energy (E_a) value decreased from 2.082 kJ·mol^?1 to 0.077 kJ·mol^?1; Improving the rigidity of the molecular chains and reducing the network structure of the hydrophobic contribution to the viscosity of the solution are benefits for improving the salt-tolerance and shear-resistance of the aqueous solution.     

Mechanochemical Synthesis of Aryl Fluorides by Using Ball Milling and a Piezoelectric Material as the Redox Catalyst

Aryl fluorides are important structural motifs in many pharmaceuticals. Although the Balz–Schiemann reaction provides an entry to aryl fluorides from aryldiazonium tetrafluoroborates, it suffers from drawbacks such as long reaction time, high temperature, toxic solvent, toxic gas release, and low functional group tolerance. Here, we describe a general method for the synthesis of aryl fluorides from aryldiazonium tetrafluoroborates using a piezoelectric material as redox catalyst under ball milling conditions in the presence of Selectfluor. This approach effectively addresses the aforementioned limitations. Furthermore, the piezoelectric material can be recycled multiple times. Mechanistic investigations indicate that this fluorination reaction may proceed via a radical pathway, and Selectfluor plays a dual role as both a source of fluorine and a terminal reductant.