Monitoring Thiol–Ligand Exchange on Au Nanoparticle Surfaces

Surface functionalization of nanoparticles (NPs) plays a crucial role in particle solubility and reactivity. It is vital for particle nucleation and growth as well as for catalysis. This raises the quest for functionalization efficiency and new approaches to probe the degree of surface coverage. We present an (in situ) proton nuclear magnetic resonance (¹H NMR) study on the ligand exchange of oleylamine by 1-octadecanethiol as a function of the particle size and repeated functionalization on Au NPs. Ligand exchange is an equilibrium reaction associated with Nernst distribution, which often leads to incomplete surface functionalization following “standard” literature protocols. Here, we show that the surface coverage with the ligand depends on the (i) repeated exchange reactions with large ligand excess, (ii) size of NPs, that is, the surface curvature and reactivity, and (iii) molecular size of the ligand. As resonance shifts and extensive line broadening during and after the ligand exchange impede the evaluation of ¹H NMR spectra, one- and two-dimensional ¹⁹F NMR techniques (correlation spectroscopy and diffusion ordered spectroscopy) with 1<i>H</i>,1<i>H</i>,2<i>H</i>,2<i>H</i>-perfluorodecanthiol as the fluorinated thiol ligand were employed to study the reactions. The enhanced resolution associated with the spectral range of the ¹⁹F nucleus allowed carrying out a site-specific study of thiol chemisorption. The widths and shifts of the resonance signals of the different fluorinated carbon moieties were correlated with the distance to the thiol anchor group. In addition, the diffusion analysis revealed that moieties closer to the NP surface are characterized by a broader diffusion coefficient distribution as well as slower diffusion

Calcium Sulfate Nanoparticles with Unusual Dispersibility in Organic Solvents for Transparent Film Processing

Calcium sulfate is one of the most important construction materials. Today it is employed as high-performance compound in medical applications and cement mixtures. We report a synthesis for calcium sulfate nanoparticles with outstanding dispersibility properties in organic solvents without further functionalization. The nanoparticles (amorphous with small γ-anhydrite crystallites, 5–50 nm particle size) form long-term stable dispersions in acetone without any sign of precipitation. ¹H NMR spectroscopic techniques and Fourier-transform infrared spectroscopy (FTIR) reveal absorbed 2-propanol on the particle surfaces that induce the unusual dispersibility. Adding water to the nanoparticle dispersion leads to immediate precipitation. A phase transformation to gypsum via bassanite was monitored by an in situ kinetic FT-IR spectroscopic study and transmission electron microscopy (TEM). The dispersibility in a volatile organic solvent and the crystallization upon contact with water open a broad field of applications for the CaSO₄ nanoparticles, e.g., as nanogypsum for coatings or the fabrication of hybrid composites

Reactivity Studies of Alkoxy-Substituted [2.2]Paracyclophane-1,9-dienes and Specific Coordination of the Monomer Repeating Unit during ROMP

The polymerization of alkoxy-substituted [2.2]­paracyclophane-1,9-dienes via ring-opening metathesis polymerization (ROMP) to obtain soluble poly­(<i>p</i>-phenylenevinylene)­s is a versatile method due to its living nature which enables the possibility of block copolymerization and end group modification. However, detailed studies on the reactivity behavior and the polymerization process of alkoxy-substituted [2.2]­paracyclophane-1,9-dienes have not been reported so far. Herein we present a detailed study on the varying tendencies of the four isomers of dimethoxy-(2-ethyl­hexyloxy)-[2.2]­para­cyclophane-1,9-diene to undergo ROMP. Therefore, we carried out polymerization combining all individual isomers with five different metathesis catalysts and collected initiation and propagation kinetics for various combinations. Furthermore, we revealed a specific coordination of the monomer repeating unit to the catalyst during the polymerization process and succeeded to polymerize not only the pseudogeminal isomers but also one of the pseudo-<i>ortho</i> isomers

Supramolecular Linear‑<i>g</i>‑Hyperbranched Graft Polymers: Topology and Binding Strength of Hyperbranched Side Chains

Complex, reversible hyperbranched graft polymer topologies have been obtained by spontaneous self-assembly. Well-defined adamantyl- and β-cyclodextrin-functionalized polymers were employed to generate linear-<i>g</i>-(linear–hyperbranched) supramolecular graft terpolymers. For this purpose the synthesis of monoadamantyl-functionalized linear polyglycerols (Ada-<i>lin</i>PG) and hyperbranched polyglycerols (Ada-<i>hb</i>PG) as well as poly­(ethylene glycol)-<i>block</i>-linear polyglycerol (Ada-PEG-<i>b</i>-<i>lin</i>PG) and poly­(ethylene glycol)-<i>block</i>-hyperbranched poly­(glycerol) (Ada-PEG-<i>b</i>-<i>hb</i>PG) block copolymers was established. Isothermal titration calorimetry (ITC) with β-cyclodextrin revealed a shielding effect of hyperbranched polyglycerol for the adamantyl functionality, which was significantly less pronounced when using a linear spacer chain between the adamantyl residue and the hyperbranched polyglycerol block. Additionally, well-defined poly­(2-hydroxypropylamide) (PHPMA) with pendant β-cyclodextrin moieties was synthesized via RAFT polymerization and sequential postpolymerization modification. Upon mixing of the β-cyclodextrin-functionalized PHPMA with Ada-PEG-<i>b</i>-<i>hb</i>PG, a supramolecular linear-<i>g</i>-(linear–hyperbranched) graft terpolymer was formed. The self-assembly was proven by ITC, diffusion-ordered NMR spectroscopy (DOSY), and fluorescence correlation spectroscopy (FCS)