Free-radical propagation rate coefficients

The pulsed laser polymerization-size-exclusion chromatography (PLP-SEC) technique has become the International Union of Pure and Applied Chemistry (IUPAC)-recommended method for the accurate determination of propagation rate coefficients, k_p, for radical polymerization. The fundamental insight provided by Heuts, Gilbert, and Radom into the relevance of transition state dynamics allows for an adequate understanding of the dependence of k_p on radical chain length, solvent environment, and monomer conversion. The associated entropic effects are particularly pronounced in aqueous solution, but are also found in less polar media and with non-polar monomers. Moreover, these arguments are applicable towards the interpretation of copolymerization reactivity ratios. The addition of sodium hydroxide to aqueous solutions of (meth)acrylic acid yields partially and even fully ionized systems with clearly reduced k_p, which indicates the action of repulsive forces between the equally charged monomer and radical species. Increasing the concentration of monomer as well as the addition of salts may reverse this effect and increase k_p due to counterion activity.     

Biased Symmetry Breaking in the Formation of Intercalated Layered Double Hydroxides: toward Control of Homochiral Supramolecular Assembly

Homochiral supramolecular assembly (HSA) based on achiral molecules has provided important clues to understand the origin of biological homochirality from the aspect of symmetry breaking. However, planar achiral molecules still face the challenge of forming HSA due to the lack of driving force for twisted stacking, which is a prerequisite for homochirality. Here, with the benefit of the formation of 2D intercalated layered double hydroxide (LDH, host–guest nanomaterials) in vortex motion, planar achiral guest molecules can form the chiral units with spatially asymmetrical structure in the confinement space of LDH. Once the LDH is removed, these chiral units are in a thermodynamic non-equilibrium state, which can be amplified to HSA by self-replicating. Especially, the homochiral bias can be predicted in advance by controlling the vortex direction. Therefore, this study breaks the bottleneck of complicated molecular design and provides a new technology to achieve HSA made of planar achiral molecules with definite handedness.