Xing-Hong Zhang

Perfectly Alternating and Regioselective Copolymerization of Carbonyl Sulfide and Epoxides by Metal‐Free Lewis Pairs

The preparation of perfectly alternating and regioslective copolymers derived from the copolymerization of carbonyl sulfide (COS) and epoxides by metal-free Lewis pair catalysts composed of a Lewis base (amidine, guanidine, or quaternary onium salts) and a Lewis acid (triethyl borane) is described. Colorless and highly transparent copolymers of poly(monothiocarbonate) were successfully obtained with over 99 % tail-to-head content and high molecular weight (up to 92.5 kg mol-1 ). In most instances, oxygen-sulfur exchange reactions (O/S ERs), which would generate random thiocarbonate and carbonate units, were effectively suppressed. The turnover frequencies (TOF) of these Lewis pair catalyzed processes were as high as 119 h-1 at ambient temperature.

Precise synthesis of sulfur-containing polymers via cooperative dual organocatalysts with high activity

Metal-free and controlled synthesis of sulfur-containing polymer is still a big challenge in polymer chemistry. Here, we report a metal-free, living copolymerization of carbonyl sulfide (COS) with epoxides via the cooperative catalysis of organic Lewis pairs including bases (e.g.: phosphazene, amidine, and guanidine) and thioureas as hydrogen-bond donors, afford well-defined poly(monothiocarbonate)s with 100% alternating degree, >99% tail-to-head content, controlled molecular weights (up to 98.4 kg/mol), and narrow molecular weight distributions (1.13–1.23). The effect of the types of Lewis pairs on the copolymerization of COS with several epoxides is investigated. The turnover frequencies (TOFs) of these Lewis pairs are as high as 112 h−1 at 25 °C. Kinetic and mechanistic results suggest that the supramolecular specific recognition of thiourea to epoxide and base to COS promote the copolymerization cooperatively. This strategy provides commercially available Lewis pairs for metal-free synthesis of sulfur-containing polymers with precise structure.Sulfur-containing polymers are useful commodities, but there is still a big challenge to produce such polymers in a controlled fashion. Here the authors show a metal-free living copolymerization between carbonyl sulfide and epoxides via cooperative catalysis.

Highly Efficient One-Pot Synthesis of COS-Based Block Copolymers by Using Organic Lewis Pairs

A one-pot synthesis of block copolymer with regioregular poly(monothiocarbonate) block is described via metal-free catalysis. Lewis bases such as guanidine, quaternary onium salts, and Lewis acid triethyl borane (TEB) were equivalently combined and used as the catalysts. By using polyethylene glycol (PEG) as the macromolecular chain transfer agent (CTA), narrow polydispersity block copolymers were obtained from the copolymerization of carbonyl sulfide (COS) and propylene oxide (PO). The block copolymers had a poly(monothiocarbonate) block with perfect alternating degree and regioregularity. Unexpectedly, the addition of CTA to COS/PO copolymerization system could dramatically improve the turnover frequency (TOF) of PO (up to 240 h−1), higher than that of the copolymerization without CTA. In addition, the conversion of CTA could be up to 100% in most cases, as revealed by 1H NMR spectra. Of consequence, the number-average molecular weights (Mns) of the resultant block copolymers could be regulated by varying the feed ratio of CTA to PO. Oxygen-sulfur exchange reaction (O/S ER), which can generate randomly distributed thiocarbonate and carbonate units, was effectively suppressed in all of the cases in the presence of CTA, even at 80 °C. This work presents a versatile method for synthesizing sulfur-containing block copolymers through a metal-free route, providing an array of new block copolymers.

Fluorescent linear CO2-derived poly(hydroxyurethane) for cool white LED

The fluorescent properties of a linear poly(hydroxyurethane) (P1) from carbon dioxide, siloxane (Si–O–Si)-containing bisepoxide and diamine are described. P1 showed strong photoluminescence with a quantum yield of up to 23.6%, high photostability, and broad absorption and emission spectra either in bulk or solution. The flexibility and hydrophobicity of the Si–O–Si linkage in P1 were utilized to drive the intense aggregation of hydroxyurethane chromophores which combined with the hydrogen bonding interactions lead to strong photoluminescence. P1 was used as a single phosphor film for fabricating a low voltage, cool white light-emitting diode device with competitive performances.

A fluorescein-centered polymer as a phosphor for fabricating pure white light-emitting diodes

White light-emitting diodes (LEDs) are very important energy-saving lighting devices. It is still a big challenge to fabricate pure white LEDs using low-cost, easy-to-process and environment-friendly phosphors. Herein, we report a pure white LED fabricated by combining rationally designed fluorescein-centered polymers with a blue LED chip. Pure white LEDs with Commission International de L’Eclairage (CIE) coordinates of (0.33, 0.33), a satisfactory color rendering index (CRI) of 82 and a correlated color temperature (CCT) of 5230 K were achieved. Covalently embedding fluorescein into the center of the polymer chain realized molecular dispersion of fluorescein. Of significance, by varying the types of the side groups of the polymer, the fluorescence properties of the embedded fluorescein unit can be regulated. The performance parameters (i.e., emission spectra, CIE coordinates, CRI and CCT) of the as-fabricated LEDs could also be tailored by varying the working voltages, and the types and amounts of fluorescein-centered polymers.

Metal-Free Access to Precise Synthesis of Poly(propylene oxide) and Its Blocks with High Activity

The fast and living ring-opening polymerization (ROP) of propylene oxide (PO) by metal-free catalysis is reported. By using triethyl borane (TEB) and organic Lewis bases (LBs, e.g.: phosphazene base, amidine and guanidine) as the catalysts, various alkyl alcohols can effectively initiate the ROP of PO, yielding tailor-made poly(propylene oxide)s (PPOs) with high regioregularity, predictable molecular weights, and narrow dispersity approaching Poisson distribution. The TEB/LB catalysts present unprecedentedly high activity (turnover frequency of up to 7500 h-1 ) and a truly living character for the polymerization, as evidenced by kinetic studies that showed fast initiation and growth, unobserved chain-transfer to PO, chain extension reactions, and the synthesis of various PPO-based block copolymers with narrow dispersities (Đ<1.1).

Alternating and regioregular copolymers with high refractive index from COS and biomass-derived epoxides

This study describes the catalytic formation of alternating and regioregular copolymers from carbonyl sulfide (COS) and epoxides along with eugenol-based glycidyl ether (EGE) and guaiacol-based glycidyl ether (GGE), derived from eugenol and guaiacol, respectively. The (salen)CrCl [salen = N,N′-bis(salicylidene) cyclohexanediimine] complex, accompanied with various organic bases, was highly active towards the EGE/COS, GGE/COS copolymerization and EGE/GGE/COS terpolymerization. The turnover of frequency (TOF) of the (salen)CrCl complex for the EGE/COS copolymerization was up to 12 000 h−1. The number-average molecular weight (Mn) of the resultant EGE/COS copolymer was up to 62.2 kg mol−1. In the presence of 0.5–1.5 mol% chlorohydrin, which was a by-product of the synthetic process of EGE, α-Cl, ω-OH EGE/COS copolymers were obtained. This result suggests that chlorohydrin could act as a very efficient chain transfer agent for the copolymerization. The EGE/COS, GGE/COS, and EGE/GGE/COS copolymers were soluble in most of the common solvents and exhibited a high refractive index of more than 1.58 with high Abbe numbers of up to 40.4. This study provides an unprecedented and sustainable synthetic route for making soluble sulfur-rich polymers with high optical properties.