Search

The development of effective and low-energy-consumption catalysts for CO2 conversion into high-value-added products by constructing versatile active sites on the surface of heterogeneous compounds is an urgent and challenging task. In this study, a stable and well-defined heterogeneous cobalt hexacyanocobaltate (Co3[Co(CN)6]2), typical cobalt Prussian blue analogue (CoCo-PBA) modified with tetrabutylammonium bromide (TBAB), is proven to be the superior catalyst for CO2 and epoxide coupling to produce cyclic carbonates with > 99% yield under mild reaction conditions (1.0 MPa, 65 °C). Based on a series of characterizations, it is revealed that the CoCo-PBA structure can maintain relatively high thermal and chemical stability. Recycling experiments exhibited that the CoCo-PBA system could retain 98% of the original activity after six reaction rounds. The CoCo-PBA/TBAB catalytic system was also highly active for coupling CO2 with other industrial-grade epoxides. These results show the CoCo-PBA catalytic system potential flexibility and the generality of the catalyst preparation strategy.

Novel constrained Schiff-base ligands (inden) were developed based on the well-known salen ligands. Chromium complexes supported by the constrained inden ligands were successfully synthesized and used as catalysts for the synthesis of cyclic carbonates from epoxides and carbon dioxide (CO2). The catalyst having tert-butyl (tBu) groups as substituents in combination with tetrabutylammonium bromide (TBAB) as a cocatalyst exhibited very high catalytic activity with a turnover frequency of up to 14800 h-1 for the conversion of CO2 and propylene oxide into propylene carbonate exclusively at 100 °C and 300 psi of CO2 under solvent-free conditions. The catalyst was found to be highly active for various epoxide substrates to produce terminal cyclic carbonates in 100% selectivity.

This work discloses a zwitterionic approach for selective copolymerization of carbon dioxide (CO₂) and propylene oxide (PO), producing poly(propylene carbonate) (PPC), a biodegradable polymer with broad applications. Small-molecule catalysts composed of triethylamine (TEA) and trialkyl boranes are effective for CO₂/PO copolymerization with an alternating degree of >99% and a productivity of 171 g PPC/g catalyst. A diamine N,N,N′,N′-tetraethyl ethylenediamine (TEED) paired with trialkyl borane exhibited improved activity and productivity (up to 216 g PPC/g catalyst). By adjusting the Lewis acid–base pair, the PPC selectivity can be regulated to 99%. In addition, PPCs have medium regioregularity with a head-to-tail diad content of 80–82% and number-average molecular weights of up to 56.0 kg/mol with narrow polydispersity (below 1.2). The overall catalytic performance of these readily available simple molecules is better than that of previously reported organic catalysts for CO₂/PO copolymerization. Successive insertion of PO and CO₂ into the Lewis pair leads to the formation of an end-to-end zwitterion featuring a TEB-masked anion and an onium cation, which is highly selective to the alternating copolymerization, as demonstrated by quantum mechanical calculations.

The copolymerization of carbonyl sulfide (COS) with epoxides has been developed to be a facile method to synthesize well-defined sulfur-containing polymers. This work describes the steric effect of tertiary amines on the copolymerization of COS with propylene oxide (PO) via a zwitterionic approach, using triethyl borane (TEB) accompanied by various tertiary amines. N,N-Dimethylcyclohexylamine accompanied by TEB (1/1) showed an exceedingly high turnover frequency of up to 69 800 h−1 and a copolymer selectivity of up to >99% for organocatalytic COS/PO copolymerization at 60 °C under solvent-free conditions. The obtained poly(propylene monothiocarbonate)s (PPMTC)s have fully alternating sequences, regioregularity of >99% tail-to-head (T–H) content, and high-number average molecular weights of up to 221.8 kg mol−1 and narrow dispersities (1.1–1.3) within 1 min. We disclosed that tertiary amines (pKa values range from 8.19 to 10.80) with less steric hindrance exhibited higher catalytic activity for COS/PO copolymerization.

A series of tin(II) complexes supported by N₂O₂ bis(phenol)‐amine ligands were prepared from the reactions of the corresponding ligands with Sn[N(SiMe₃)₂]₂ in benzene at room temperature. The ligands were designed to have different substituted group at the ortho‐position on the aryl rings (R = ᵗBu, CH₃) and N‐containing side arm (E = CH₂NEt₂ and pyridine) giving a variation of tin(II) complexes (R = ᵗBu, E = CH₂NEt₂, 2a; R = ᵗBu, E = py, 2b; R = CH₃, E = CH₂NEt₂, 2c; R = CH₃, E = py, 2d). All complexes were characterized by NMR spectroscopy and single‐crystal X‐ray analysis. The single‐crystal X‐ray crystallography revealed that all complexes have a monomeric four‐coordinate tin center with a distorted seesaw structure. All complexes are active for solvent‐free polymerization of l‐lactide at 120 °C giving poly(l‐lactide) with narrow to moderate dispersity (Ð = 1.12–1.56). In the presence of benzyl alcohol during the polymerization, the resulting polymer was found to be linear having benzyl alcohol as the end group while, in the absence of benzyl alcohol, the polymer was cyclic. The large ᵗBu group at the ortho‐position was found to decrease polymerization activity while the more basic CH₂NEt₂ group was found to increase the polymerization activity. The polymerization of rac‐lactide under a similar condition gave PLA having a slight heterotactic bias for all catalysts. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 2104–2112

Metal-free synthesis of fully alternating polycarbonates from carbon dioxide (CO2) and epoxides is highly desired but a huge challenge. Here, we disclose the combination of tertiary amines with trialkyl boranes for completely alternating copolymerization of CO2 with epoxides. Triethylamine (TEA) pairing with triethyl borane (TEB), the simplest catalyst for the copolymerization of CO2 and propylene oxide (PO), afforded fully alternating poly(propylene carbonate) (PPC) with a turnover frequency (TOF) of 54 h-1 at 60 oC. Remarkably, diamine such as, N,N,N',N'-tetraethylethylenediamine (TEED) and triamine such as N,N,N',N'',N''-pentamethyldiethylene triamine (PMDETA) pairing with TEB exhibited improved the copolymer selectivity of up to 99%. Supplementally, the effect of trialkylborane structure on the copolymerization were also investigated. Moreover, PPCs prepared from these tertiary amines/ trialkyl boranes Lewis pairs showed the head-to-tail diad content of around 80%. The number-average molecular weight of PPC was up to 56.0 kg/mol with narrow distribution (Đ =1.16) and could be easily adjusted by simply varying the feeding ratios. The 1H NMR spectra and MALDI-TOF-MS spectra suggest that both tertiary amine-activating CO2 and TEB-masked end anions cooperatively promote the fully alternating CO2/epoxide copolymerization. This work provides an organocatalytic platform for well-defined CO2/epoxide copolymers using simple small molecules with high atomic utilization.

The synthesis, structural characterization and reactivity of aluminum complexes supported by a novel tetradentate aminophenolate ligand containing furfuryl groups (LH), LAlMe2 (1), LAlMeCl (2) and LAlMeOtBu (3), are described. The molecular structures of ligand LH and complexes 1–3 are determined by X-ray structural analysis. Complexes 1–3 contain a four-coordinated mononuclear aluminum center. Activation of complex 1 with either B(C6F5)3 or [Ph3C][B(C6F5)4] afforded the corresponding cationic complex, [LAlMe][MeB(C6F5)3] or [LAlMe][B(C6F5)4], respectively. All cationic complexes were stable at room temperature in the absence of an external Lewis base over a week. The cationic complex [LAlMe][MeB(C6F5)3] decomposed upon heating at 70 °C, giving a neutral LAlMe(C6F5) complex. Complexes 1–3 were inactive for the ring-opening polymerization (ROP) of e-caprolactone (CL) at room temperature. However, only the cationic aluminum complex [LAlMe][MeB(C6F5)3] in the presence of benzyl alcohol was found to be active in the ROP of CL at room temperature in a well-behaved manner, giving a first-order reaction with respect to [CL].

The rapid one-pot catalytic synthesis of double-capped oligoesters was carried out by the ring-opening polymerization of various cyclic ester monomers (i.e. e -caprolactone ( e -CL), δ -valerolactone (DVL), γ -butyrolactone (GBL), and L -lactide ( L -LA)) followed by the transesterification reaction with a variety of alkyl esters as an end-capping agent. Simple metal amides were found to be an effective catalyst under mild condition. Among them, LiN(SiMe 3 ) 2 was shown to be the most active catalyst giving 94% conversion of double-capped oligo( e -caprolactone) within 5 min at room temperature. The average chain length of the oligoesters can be controlled by the adjustment of the ratio of an alkyl ester to monomer, the reaction time and reaction temperature.