Haishen Yang

Organocatalyzed atom transfer radical polymerization driven by visible light

Precise control from a metal-free catalyst Polymerization can be a rather dangerous free for all, with molecules joining randomly in chains at a chaotic pace. One of modern chemistrys great accomplishments has been the development of methods to assemble polymers in steady, orderly steps. However, order comes at a price, and often its the need for metal catalysts that are hard to remove from the plastic product. Theriot et al. used theory to guide the design of a metal-free light-activated catalyst that offers precise control in atom transfer radical polymerization, alleviating concerns about residual metal contamination (see the Perspective by Shanmugam and Boyer). Science, this issue p. 1082; see also p. 1053 A metal-free catalyst offers comparable control to more commonly used metals without the drawback of product contamination. Atom transfer radical polymerization (ATRP) has become one of the most implemented methods for polymer synthesis, owing to impressive control over polymer composition and associated properties. However, contamination of the polymer by the metal catalyst remains a major limitation. Organic ATRP photoredox catalysts have been sought to address this difficult challenge but have not achieved the precision performance of metal catalysts. Here, we introduce diaryl dihydrophenazines, identified through computationally directed discovery, as a class of strongly reducing photoredox catalysts. These catalysts achieve high initiator efficiencies through activation by visible light to synthesize polymers with tunable molecular weights and low dispersities.

Ionic Covalent Organic Frameworks with Spiroborate Linkage

A novel type of ionic covalent organic framework (ICOF), which contains sp(3)  hybridized boron anionic centers and tunable countercations, was constructed by formation of spiroborate linkages. These ICOFs exhibit high BET surface areas up to 1259 m(2)  g(-1) and adsorb a significant amount of H2 (up to 3.11 wt %, 77 K, 1 bar) and CH4 (up to 4.62 wt %, 273 K, 1 bar). Importantly, the materials show good thermal stabilities and excellent resistance to hydrolysis, remaining nearly intact when immersed in water or basic solution for two days. The presence of permanently immobilized ion centers in ICOFs enables the transportation of lithium ions with room-temperature lithium-ion conductivity of 3.05×10(-5)  S cm(-1) and an average Li(+) transference number value of 0.80±0.02. Our approach thus provides a convenient route to highly stable COFs with ionic linkages, which can potentially serve as absorbents for alternative energy sources such as H2, CH4, and also as solid lithium electrolytes/separators for the next-generation lithium batteries.

Synthesis of a conjugated porous Co(II) porphyrinylene–ethynylene framework through alkyne metathesis and its catalytic activity study

The development of efficient catalysts for the oxygen reduction reaction (ORR) is crucial for a number of emerging technologies, to counter energy and environment crises. Herein, we report an alkyne metathesis polymerization protocol to synthesize a conjugated microporous metalloporphyrin-based framework containing interconnected ORR catalytic centers. A simple composite of the framework and carbon black shows excellent ORR electrocatalytic activity and specificity through a four-electron reduction mechanism under both acidic and alkaline conditions. The pyrolysis of the catalyst, which is commonly involved in the preparation of ORR catalytic systems, is not necessary. Compared to monomeric metalloporphyrins, the framework shows enhanced ORR catalytic activity, presumably due to the porous and conjugated nature of the framework structure, which allows better exposure of the catalytically active sites, and efficient electron/mass transport. More importantly, the composite electrocatalyst exhibits superior durability and methanol tolerance over commercial Pt/C and metalloporphyrin monomers. Given the highly structural tunability of conjugated microporous polymers, it is conceivable that such a non-pyrolytic approach could enable the systematic exploration of the structure–activity relationship of organic framework-based ORR catalysts and eventually lead to the development of cost-effective replacements for Pt/C.

Application of alkyne metathesis in polymer synthesis

Alkyne metathesis has attracted growing attention in the past two decades as a versatile synthetic method for formation of the CC bond. It has been applied to the synthesis of various small organic molecules, linear polymers, as well as polymer networks. Herein, we review the recent progress in the development of alkyne metathesis polymerization, which includes acyclic diyne metathesis polymerization (ADIMET) and ring-opening alkyne metathesis polymerization (ROAMP).

Highly Active Multidentate Ligand‐Based Alkyne Metathesis Catalysts

Alkyne metathesis catalysts composed of molybdenum(VI) propylidyne and multidentate tris(2-hydroxylbenzyl)methane ligands have been developed, which exhibit excellent stability (remains active in solution for months at room temperature), high activity, and broad functional-group tolerance. The homodimerization and cyclooligomerization of monopropynyl or dipropynyl substrates, including challenging heterocycle substrates (e.g., pyridine), proceed efficiently at 40-55 °C in a closed system. The ligand structure and catalytic activity relationship has been investigated, which shows that the ortho groups of the multidentate phenol ligands are critical to the stability and activity of such a catalyst system.

Solution processable polydiacetylenes (PDAs) through acyclic enediyne metathesis polymerization

Novel polydiacetylenes (PDAs) bearing alkyl and phenyl substituents have been synthesized, for the first time, by solution polymerization using acyclic enediyne metathesis. The resulting polymers are soluble in common organic solvents and show distinct physical and photophysical properties both in solution and as thin films, caused by different steric and electronic effects from the side-groups. Bulk heterojunction solar cells employing these PDAs have been fabricated and evaluated.

A titanium-based porous coordination polymer as a catalyst for chemical fixation of CO2

A novel type of porous coordination polymer with titanium alkoxide linkages (Ti-PCP) was successfully constructed through reaction of a 1,3-diol-substituted shape-persistent arylene–ethynylene macrocycle with Ti(OiPr)4. The polymer exhibits a high Brunauer–Emmett–Teller (BET) surface area (1029 m2 g−1) and good adsorption selectivity for CO2 over N2. With n-Bu4NBr as a co-catalyst, Ti-PCP can serve as a highly efficient catalyst to convert CO2 into cyclocarbonates through the cycloaddition reaction with epoxides under mild conditions. This framework-based heterogeneous catalyst not only has excellent catalytic activity, but also can be recycled and reused many times with easy product separation, thus showing its great potential for industrial applications as a green catalyst. With high uptake of CO2 and catalytic ability to convert CO2 to useful chemical compounds, this type of porous polymer with embedded catalytic centers could have important applications to solve environmental problems regarding post-combustion CO2 capture and conversion.

Solvent effects on the intramolecular charge transfer character of N,N-diaryl dihydrophenazine catalysts for organocatalyzed atom transfer radical polymerization

The nature of intramolecular charge transfer of N,N-diaryl dihydrophenazine photocatalysts (PCs) in different solvents is explored in context of their performance in organocatalyzed atom transfer radical polymerization (O-ATRP). PCs having a computationally predicted lowest energy excited state exhibiting charge transfer (CT) character can operate a highly controlled O-ATRP in a wide range of solvent polarities, from non-polar hexanes to highly polar N,N-dimethylacetamide. For PCs having a computationally predicted lowest energy excited state not possessing CT character, their ability to operate a controlled O-ATRP is decreased. This study confirms the importance of CT character in the excited state for N,N-diaryl dihydrophenazine PCs, and a deeper understanding of the activity of CT PCs has enabled the synthesis of polymers of low dispersity ( < 1.10) in a controlled fashion.

Strongly Reducing, Visible‐Light Organic Photoredox Catalysts as Sustainable Alternatives to Precious Metals

Photoredox catalysis is a versatile approach for the construction of challenging covalent bonds under mild reaction conditions, commonly using photoredox catalysts (PCs) derived from precious metals. As such, there is need to develop organic analogues as sustainable replacements. Although several organic PCs have been introduced, there remains a lack of strongly reducing, visible-light organic PCs. Herein, we establish the critical photophysical and electrochemical characteristics of both a dihydrophenazine and a phenoxazine system that enables their success as strongly reducing, visible-light PCs for trifluoromethylation reactions and dual photoredox/nickel-catalyzed C-N and C-S cross-coupling reactions, both of which have been historically exclusive to precious metal PCs.

Aromatic-rich hydrocarbon porous networks through alkyne metathesis

Purely hydrocarbon-based porous polymers have generally been prepared through various irreversible transition metal-catalyzed cross-coupling reactions forming C–C bonds. Herein, we report an alternative synthetic approach, namely reversible alkyne metathesis, for the preparation of ethynylene-linked porous polymers. Planar and tetrahedral-shaped monomers were explored to construct poly(aryleneethynylene) (PAE) networks. We systematically varied the size of the monomers and studied the structure–property relationships. The resulting polymers exhibit high Brunauer–Emmett–Teller (BET) surface areas in the range of 736 m2 g−1 to 2294 m2 g−1. The advantages of such aromatic-rich PAE networks are their lightweight, high thermal/chemical stabilities, and superior hydrophobicity, which are beneficial for their application in adsorption/separation of toxic organic pollutants from water. We found that PAEs can adsorb a significant amount of common aromatic solvents, e.g. up to 723 wt% of nitrobenzene. Our study thus demonstrates an encouraging novel approach to prepare purely hydrocarbon-based porous materials.