High-Molecular-Weight Bisalkoxy-Substituted Poly(para)phenylenes by Kumada Polymerization

Substituted poly­(para)­phenylenes (PPPs) are conjugated polymers with an attractive application potential in various fields of materials science. They are synthesized nearly exclusively using catalytic cross-coupling polymerization reactions based on Pd- or Ni-catalysts. Among these synthetic approaches to access alkoxy-substituted PPPs, Kumada catalyst transfer polymerization (KCTP or GRIM polymerization) would offer certain economic advantages over Suzuki-type polymerization as it relies on the utilization of a non-precious metal for catalysis. It also results in less total costs of the utilized reagents, avoiding additional preparative steps such as synthesis, isolation, and purification of boronic acid derivatives necessary for the Suzuki reaction. In fact, KCTP is nowadays the state-of-the-art method for the synthesis of polythiophenes. However, the application of KCTP for the synthesis of alkoxy-substituted PPPs leads to polymers with low molecular weights, limiting their practical applicability. Here, we developed a synthesis protocol that resulted in MEH-PPP with a molecular weight of Mn = 133 kg/mol and BHex-PPP with Mn = 153 kg/mol relative to polystyrene, outperforming the previous state of the art by a factor more than 5. Also, a tetra­(ethylene glycol)-substituted PPP has been prepared by this procedure, with a molecular weight exceeding the previously reported results for analogous structures. Such molecular weights can be obtained in a reasonable reaction time (5 days) using low concentrations of an N-heterocyclic carbene-coordinated Ni complex. The polymerization kinetics suggested a chain-growth mechanism with a chain transfer step. The latter is caused most likely by a bimolecular interaction of the Ni-species at the polymer chain ends

Conformationally Flexible Dimeric Salphen Complexes for Bifunctional Catalysis

Appropriate modification of the salphen ligand allows an easy modular design of flexibly linked dimeric salphen species and their complexes, which can act as bifunctional catalysts. A series of chromium salphen systems including monomeric compound and dimers with different spacer lengths were tested for their catalytic performance in β-butyrolactone polymerization and CO2/propylene oxide copolymerization toward biodegradable materials. The results clearly show an enhancement in activity upon dimerization, thus underlining the role of bifunctional catalysis in the studied processes and extending the possible strategies for improvement of catalysts in these reactions

Cobaltoporphyrin-Catalyzed CO₂/Epoxide Copolymerization: Selectivity Control by Molecular Design

A series of cobalt­(III) chloride porphyrin complexes of the general formula 5,10,15,20-tetra­(<i>p</i>-alkoxy)­phenylporphyrin cobalt chloride (<b>4b</b>–<b>e</b>) and the related 5,10,15,20-tetra­(<i>p</i>-nitro)­phenylporphyrin cobalt chloride (<b>4f</b>) are presented and their reactivity toward propylene oxide (PO)/CO₂ coupling/copolymerization is explored. While the nitro-substituted complex (<b>4f</b>), in conjunction with an onium salt, shows moderate activity toward cyclization, the <b>4b</b>–<b>e</b>/onium systems show superior copolymerization activity in comparison to tetraphenylporphyrin Co­(III) chloride (<b>4a</b>) with high selectivity and conversion to poly­(propylene carbonate) (PPC). A comprehensive copolymerization behavior study of the alkoxy-substituted porphyrin complexes <b>4b</b>–<b>e</b> in terms of reaction temperature and CO₂ pressure is presented. Complexes bearing longer alkoxy-substituents demonstrate the highest polymerization activity and molecular weights, however all substituted catalyst systems display a reduced tolerance to increased temperature with respect to PPC formation. Studies of the resulting polymer microstructures show excellent head-to-tail epoxide incorporation and near perfectly alternating poly­(carbonate) character at lower polymerization temperatures

2‑Methoxyethylamino-bis(phenolate)yttrium Catalysts for the Synthesis of Highly Isotactic Poly(2-vinylpyridine) by Rare-Earth Metal-Mediated Group Transfer Polymerization

Highly isotactic poly­(2-vinylpyridine) (P2VP) was synthesized by the group transfer polymerization of the prochiral 2-vinylpyridine (2VP) with 2-methoxyethylamino­bis­(phenolate)yttrium complexes. Isotacticities of up to <i>P</i>_<i>m</i> = 0.92, narrow molecular weight distributions, and high molecular weights were achieved by steric modifications of the variable bisphenolate ligand structure. The resulting polymer samples were characterized by thermoanalysis (DSC, TGA), GPC, and ¹³C NMR. The origin of the isotactic microstructure was attributed to an enantiomorphic site control mechanism based on ¹³C NMR mechanistic studies and allowed new insights into ¹³C pentad assignments

Ultrasensitive Picomolar Detection of Aqueous Acids in Microscale Fluorescent Droplets

We report on a fluorescent-droplet-based acid-sensing scheme that allows limits of detection below 100 pM for weak acids. The concept is based on a strong partitioning of acid from an aqueous phase into octanol droplets. Using salicylic acid as a demonstration, we show that at a high concentration, the acid partitions into the organic phase by a factor of 260, which is approximately consistent with literature values. However, at lower concentrations, we obtain a partition coefficient as high as 106, which is partly responsible for the excellent sensing performance. The enhanced equilibrium partitioning is likely due to the interaction of the dissociated acid phase with the sensor dye employed for this work. The effect of droplet size was determined, after which we derived a simple model to predict the time dependence of the color change as a function of droplet size. This work shows that color-change fluorescent-droplet-based detection is a promising avenue that can lead to exceptional sensing performance from an aqueous analyte

Kinetic and Mechanistic Investigation of Mononuclear and Flexibly Linked Dinuclear Complexes for Copolymerization of CO₂ and Epoxides

Mono- and dinuclear salphen-type complexes were developed and investigated in CO2/epoxide copolymerization reactions. Kinetic investigations indicate that the reaction occurs predominately in a bimetallic fashion in the absence of cocatalysts for both mono- and dinuclear complexes. The dinuclear system, therefore, maintains its activity even under highly diluted conditions of [PO]/[M] = 20000 at which the mononuclear system loses its efficiency. The effect of the nature and amount of added cocatalyst on catalytic performance was investigated as well, indicating a binary propagation mechanism both in mononuclear and dinuclear systems in the presence of cocatalysts

Versatile 2‑Methoxyethylaminobis(phenolate)yttrium Catalysts: Catalytic Precision Polymerization of Polar Monomers via Rare Earth Metal-Mediated Group Transfer Polymerization

The present study is one of the first examples for rare earth metal-mediated group transfer polymerization (REM-GTP) with non-metallocene catalyst systems. 2-Methoxyethylaminobis­(phenolate)yttrium trimethylsilylmethyl complexes were synthesized and showed moderate to high activities in the rare earth metal-mediated group transfer polymerizations of 2-vinylpyridine, 2-isopropenyl-2-oxazoline, diethyl vinylphosphonate, diisopropyl vinylphosphonate, and <i><i>N,N</i></i>-dimethylacrylamide as well as in the ring-opening polymerization of β-butyrolactone. Reaction orders in catalyst and monomer were determined for the REM-GTP of 2-vinylpyridine. The mechanistic studies revealed that the catalyst systems follow a living monometallic group transfer polymerization mechanism allowing a precise molecular-weight control of the homopolymers and the block copolymers with very narrow molecular weight distributions. Temperature-dependent reaction kinetics were conducted and allowed conclusions about the influence of the bulky substituents around the metal center on the polymerization activity. Additional polymerization experiments concerning the combination of REM-GTP and ROP to obtain block copolymers were performed

DataSheet1_A Fluorescent Alcohol Biosensor Using a Simple microPAD Based Detection Scheme.pdf

A paper-based microfluidic detection device for the detection of ethanol is demonstrated in this work. The method is based on a fluorophore consisting of short-chain conjugated molecular unit susceptible to the protonation of its terminal pyridine groups, along with a carboxyl-functionalized sidechain that acts as a binder and renders it water-soluble. The resulting fluorescent paper device yields large fluorescence changes when exposed to reactions that yield H2O2 in aqueous solutions. Using an enzyme-catalyzed rection that produces H2O2 from ethanol, we developed a two-zone, cut-out paper device containing a reaction zone in which the ethanol-containing analyte is placed, and an adjacent sensor zone where we observe a fluorescence color shift proportional to the ethanol concentration. The limit of detection of the fluidic ethanol biosensor was 0.05 v/v% and the dynamic range was 0.05–2 v/v%. This method was employed to detect the alcohol concentration of consumer vodkas using only a paper sensor and a smartphone camera.</p

A Nanometric Probe of the Local Proton Concentration in Microtubule-Based Biophysical Systems

We show a double-functional fluorescence sensing paradigm that can retrieve nanometric pH information on biological structures. We use this method to measure the extent of protonic condensation around microtubules, which are protein polymers that play many roles crucial to cell function. While microtubules are believed to have a profound impact on the local cytoplasmic pH, this has been hard to show experimentally due to the limitations of conventional sensing techniques. We show that subtle changes in the local electrochemical surroundings cause a double-functional sensor to transform its spectrum, thus allowing a direct measurement of the protonic concentration at the microtubule surface. Microtubules concentrate protons by as much as one unit on the pH scale, indicating a charge storage role within the cell via the localized ionic condensation. These results confirm the bioelectrical significance of microtubules and reveal a sensing concept that can deliver localized biochemical information on intracellular structures