Electrochemical Synthesis of Poly(trisulfides)

Synthesis of Polycyclic Olefinic Monomers from Norbornadiene for Inverse Vulcanization: Structural and Mechanistic Consequences

The preparation of high-sulfur content organosulfur polymers has generated considerable interest as an emerging area in polymer science that has been driven by advances in the inverse vulcanization polymerization of elemental sulfur with organic comonomers. While numerous new inverse vulcanized polysulfides have been made over the past decade, insights into the mechanism of inverse vulcanization and structural characterization of the high-sulfur-content copolymers remain limited in scope. Furthermore, the exploration of new molecular architectures for organic comonomer synthesis remains an important frontier to enhance the properties of these new polymeric materials. In the current report, the first detailed study on the synthesis and inverse vulcanization of polycyclic rigid comonomers derived from norbornadiene was conducted, affording a quantitative assessment of polymer microstructure for these organopolysulfides and insights into the inverse vulcanization polymerization mechanism for this class of monomers. In particular, a stereoselective synthesis of the endo-exo norbornadiene cyclopentadiene adduct (Stillene) was achieved, which enabled direct comparison with the known exo-exo norbornadiene dimer (NBD2) previously used for inverse vulcanization. Reductive degradation of these sulfur copolymers and detailed structural analysis of the recovered sulfurated organic fragments revealed that remarkable exo-stereospecificity was achieved in the inverse vulcanization of elemental sulfur with both these polycyclic dienyl comonomers, which correlated to the robust thermomechanical properties associated with organopolysulfides made from NBD2 previously. Melt processing and molding of these sulfur copolymers were conducted to fabricate free-standing plastic lenses for long-wave infrared thermal imaging.

Sustainable Electrosynthesis of N,N-Dimethylformamide via Relay Catalysis on Synergistic Active Sites

Electrified synthesis of high-value organonitrogen chemicals from low-cost carbon- and nitrogen-based feedstocks offers an economically and environmentally appealing alternative to traditional thermocatalytic methods. However, the intricate electrochemical reactions at electrode surfaces pose significant challenges in controlling selectivity and activity, especially for producing complex substances such as N,N-dimethylformamide (DMF). Herein, we tackle this challenge by developing relay catalysis for efficient DMF production using a composite WO2-NiOOH/Ni catalyst with two distinctive active sites. Specifically, WO2 selectively promotes dimethylamine (DMA) electrooxidation to produce strongly surface-bound (CH3)2N*, while nearby NiOOH facilitates methanol electrooxidation to yield more weakly bound *CHO. The disparity in binding energetics of the key C- and N-intermediates expedites C-N coupling at the WO2-NiOOH interface. In situ infrared spectroscopy with isotope-labeling experiments, quasi-in situ electron paramagnetic resonance trapping experiments, and electrochemical operating experiments revealed the C-N coupling mechanism and enhanced DMF-synthesis selectivity and activity. In situ X-ray absorption spectroscopy (XAS) and postreaction transmission electron microscopy (TEM) studies verified the stability of WO2-NiOOH/Ni during extended electrochemical operation. A Faradaic efficiency of ∼50% and a production rate of 438 μmol cm-2 h-1 were achieved at an industrially relevant current density of 100 mA cm-2 over an 80 h DMF production period. This study introduces a new paradigm for developing electrothermo relay catalysis for the sustainable and efficient synthesis of valuable organic chemicals with industrial potential.

Modification of Polysulfide Surfaces with Low-Power Lasers

The modification of polymer surfaces using laser light is important for many applications in the nano-, bio- and chemical sciences. Such capabilities have supported advances in biomedical devices, electronics, information storage, microfluidics, and other applications. In most cases, these modifications require high power lasers that are expensive and require specialized equipment and facilities to minimize risk of hazardous radiation. Additionally, polymer systems that can be easily modified by lasers are often complex and costly to prepare. In this report, these challenges are addressed with the discovery of low-cost sulfur copolymers that can be rapidly modified with lasers emitting low-power infrared and visible light. The featured copolymers are made from elemental sulfur and either cyclopentadiene or dicyclopentadiene. Using a suite of lasers with discreet wavelengths (532, 638 and 786 nm) and powers, a variety of surface modifications could be made on the polymers such as controlled swelling or etching via ablation. The facile synthesis and laser modification of these polymer systems were exploited in applications such as direct laser lithography and erasable information storage.

Step-growth Polymerization of Aziridines with Elemental Sulfur: Easy Access to Linear Polysulfides and Their Use as Recyclable Adhesives

The bulk radical polymerization of bis(aziridine) with molten elemental sulfur resulted in brittle, cross-linked polymers. However, when the bis(aziridine) was treated with elemental sulfur in the presence of an organobase, the ring-opening reaction of aziridine with oligosulfide anions occurred, leading to the formation of linear polymers by step-growth polymerization. These newly synthesized polymers possess repeating units containing a sulfonamide or amide functional moiety and oligosulfide bonds with an average sulfur segment of about two. A small molecular model reaction confirmed the nucleophilic addition reaction of elemental sulfur to aziridine. It was verified that S-S dynamic bond exchange takes place in the presence of an organic base within the linear chains. The mixture of the synthesized polysulfides with pyridine exhibits exceptional adhesive properties when applied to steel, and aluminum substrates. Notably, these prepared adhesives displayed good reusability due to the dynamic S-S exchange and complete recyclability due to their solution processability. This elemental sulfur-involved polymerization approach represents an innovative method for the synthesis of advanced sulfur-containing polymers, demonstrating the potential for various applications in adhesives and beyond.

Synthesis of Seven Indolizine-Derived Pentathiepines: Strong Electronic Structure Response to Nitro Substitution in Position C-9

Seven new 1,2,3,4,5-pentathiepino[6,7-a]indolizines were synthesized in which the pentathiepine moieties bear an indolizine backbone that is derivatized from C-H to F-, Cl-, Br-, I-, NO2-, and CH3-substitutions, respectively, in a meta position relative to the aza group on the pyridine moiety. Their preparation took place via two common steps: (i) a Sonogashira coupling between (4-substituted) 2-bromo- or 2-chloropyridines and propynyl 3,3-diethylacetal, and (ii) a ring closing reaction mediated by a molybdenum oxo-bistetrasulfido complex and elemental sulfur. The latter simultaneously facilitates the 1,2,3,4,5-pentathiepino chain/ring- and indolizine ring-formations. The fluoro derivative was addressed with 2-bromo-5-aminopyridine as the starting material via a Sandmeyer reaction. The iodo derivative was obtained from 5-bromo-2-alkynylpiridine using a metal-assisted variation of the Finkelstein reaction. The requirement to explore different reaction conditions and the varied respective yields of the final products are discussed. The influence of the distinct substitutions on the pyridine moieties, their electronic structures, and respective chemical properties was investigated through a set of spectroscopic/analytical characterizations. Intriguingly, in all cases, the nitro-substituted derivative exhibited a distinct behavior compared to the six other investigated derivatives, which was also addressed computationally. All seven new pentathiepines were crystallized, and their respective molecular structures were determined using single crystal X-ray diffraction. These structures are compared and discussed as are their respective packing patterns.

Base-Assisted Polymerizations of Elemental Sulfur and Alkynones for Temperature-Controlled Synthesis of Polythiophenes or Poly(1,4-dithiin)s

With the increasing demand for functional polythiophenes in extensive applications such as organic solar cells, electronic skins, thermoelectric materials, and field effect transistors, efficient and economic synthetic approaches for polythiophenes are urgently required. In this work, KOH-assisted polymerizations of elemental sulfur and alkynones were developed to directly afford polythiophenes with various backbones, regioselective structures, and high molecular weights (Mns up to 20700 g/mol) in high yields (up to 97%) at 80 °C in 30 min. When the same polymerization was conducted at room temperature, stable and unique poly(1,4-dithiin)s (Mns up to 21800 g/mol) could be rapidly obtained in high yields (up to 87%) in 10 min. The temperature-controlled KOH-assisted polymerizations of sulfur and alkynones possessed high efficiency, mild conditions, and simple operation, which had provided an economic, efficient, and convenient approach for the direct conversion from elemental sulfur to functional polythiophenes and poly(1,4-dithiin)s with the in situ constructed aromatic or nonaromatic heterocycles embedded in the polymer backbones, demonstrating great synthetic simplicity, high efficiency, good selectivity, and robustness. It is anticipated to accelerate the development of semiconducting polymer materials and their applications.

Controlled and Regioselective Ring-Opening Polymerization for Poly(disulfide)s by Anion-Binding Catalysis

Poly(disulfide)s are an emerging class of sulfur-containing polymers with applications in medicine, energy, and functional materials. However, the constituent dynamic covalent S-S bond is highly reactive in the presence of the sulfide (RS-) anion, imposing a persistent challenge to control the polymerization. Here, we report an anion-binding approach to arrest the high reactivity of the RS- chain end to control the synthesis of linear poly(disulfide)s, realizing a rapid, living ring-opening polymerization of 1,2-dithiolanes with narrow dispersity and high regioselectivity (Mw/Mn ∼ 1.1, Ps ∼ 0.85). Mechanistic studies support the formation of a thiourea-base-sulfide ternary complex as the catalytically active species during the chain propagation. Theoretical analyses reveal a synergistic catalytic model where the catalyst preorganizes the protonated base and anionic chain end to establish spatial confinement over the bound monomer, effecting the observed regioselectivity. The catalytic system is amenable to monomers with various functional groups, and semicrystalline polymers are also obtained from lipoic acid derivatives by enhancing the regioselectivity.