Bis(cyclic carbonate)s were quantitatively prepared with high efficiency via the coupling reaction of carbon dioxide (CO2) with diglycidyl ethers by a [Fe(BPMCDAC)]/TBAB catalytic system, where glycol diglycidyl ether (1a) could be completely converted to the corresponding bis(cyclic carbonate) (2a) with a turnover number of 1000 at 100 °C and 3 MPa in 4 h. The obtained bis(cyclic carbonate) (2a) could be used to prepare hydroxyl-functional polyurethanes via reaction with diamines, which may be one alternative for obtaining conventional polyurethanes without the use of toxic phosgene or isocyanates. The number-average molecular weights of the obtained non-isocyanate polyurethanes (NIPUs) were up to 25.4–30.2 kg mol−1, and the polydispersity indexes (PDIs) were relatively narrow between 1.18 and 1.22. A typical NIPU showed a glass transition temperature of 9 °C and an initial degradation temperature (Td 5%) of 206 °C.

Bifunctional aluminum porphyrin complexes were designed to synthesize poly(propylene carbonate) (PPC) by copolymerization of propylene oxide and carbon dioxide. The catalytic performance is adjustable via delicate control of the electronic environment of the central Al by the number of methoxy groups in the ligand framework as well as the length of the alkyl chain in the quaternary ammonium cation. The optimal catalyst having six methoxy groups in the ligand framework, two trihexylammonium cations linked to benzene via a six-methylene spacer, and NO3− as the axial ligand and quaternary ammonium anions exhibited a TOF of 1320 h−1 at 80 °C and 3 MPa, and a PPC selectivity of 93%, and the TOF even reached 2824 h−1 at 90 °C and 3 MPa, while the PPC selectivity remained at 89%, the highest recorded in aluminum porphyrin complexes to date. In another concern, even though the bifunctional aluminum porphyrin complex has a soil-compostable feature and can be left in PPC without separation, the depolymerization was very rapid even at 25 °C under an ambient atmosphere, and over a 50% decrease in number average molecular weight was observed in 8 days, which could be stabilized by treatment with aqueous HCl solution.

A one-pot synthesis of oligo(carbonate-ether) triol was realized by the copolymerization of CO2 and propylene oxide (PO) using a zinc-cobalt double metal cyanide (Zn-Co-DMC) catalyst in the presence of 1,3,5-benzenetricarboxylic (trimesic) acid (TMA). The catalytic activity ranged from 0.3 to 1.0 kg g−1 DMC under various copolymerization conditions. The structure of the oligo(carbonate-ether) triol was clearly confirmed, providing sound evidence for the special role played by the TMA, i.e., that it acted as an initiation-transfer agent. In the first stage the TMA initiated PO homo-polymerization to afford oligo-ether triol via a core-first approach in the presence of Zn-Co-DMC. After all of the TMA was consumed, the in situ formed oligo-ether triol acted as new chain transfer agent to participate in the copolymerization, forming carbonate-ether segments and therefore the oligo(carbonate-ether) triol. Since every molecule of TMA participated in initiation and propagation steps, the molecular weight of the triol depended on the amount of TMA used rather than the amount of Zn-Co-DMC. Consequently, the number average molecular weight (Mn) of the oligo(carbonate-ether) triol could be well controlled from 1400 to 3800 g mol−1 with a relatively narrow polydispersity index (PDI) (1.15–1.45), and its carbonate unit content (CU) could be adjusted between 20% and 54%.

Based on the mechanistic features of metal salen catalysis systems, titanium(IV) complexes from salen (salen-H2 = N,N-bis(3,5-di-tert-butylsalicylidene)-1,2-benzenediamine) and its half saturated form salalen have been prepared, which were used as catalysts in conjugation with bis(triphenylphosphino)iminium chloride ([PPN]Cl) for the coupling reaction of CO2 and cyclohexene oxide (CHO). The salen titanium complex (salen)Ti(IV)Cl2 showed moderate activity, producing a unique cis-isomer of cyclic carbonate with high conversion up to 100% in 8 h, however, it could not catalyze the copolymerization reaction. Meanwhile, the salalen titanium complex (salalen)Ti(IV)Cl was effective for the copolymerization of CO2 and CHO, where only one chain grew on Ti during the chain propagation reaction, yielding completely alternating copolymers with –OH and –Cl as terminal groups. Moreover, the nearly complete conversion of CHO indicated that (salalen)Ti(IV)Cl might be used to synthesize multiblock poly(cyclohexene carbonate)s with controllable sequences.

Due to the deep concern over residual, toxic cobalt or chromium from catalysts in biodegradable poly(propylene carbonate) (PPC), bifunctional aluminum porphyrin complexes with quaternary ammonium salts anchored on the ligand framework were prepared, and a delicate design of the porphyrin ligand was obtained. An optimized catalyst was complex 6b, which had two para-bromine benzenes and two quaternary ammonium cations linked to benzene via a six-methylene spacer in the meso-position of the porphyrin framework and NO3− as axial ligand and quaternary ammonium anion, showed TOF of 560 h−1 at 80 °C and 3 MPa to yield PPC with 94% carbonate linkage and number average molecular weight of 96 kg mol−1. The PPC selectivity reached 93%, which was the highest record in this copolymerization for aluminum porphyrin complexes. The soil tolerant bifunctional aluminum porphyrin complexes are becoming increasingly competitive catalysts, since they can be left with the plastics without any extra separation.

The novel iron complexes [N,N′-bis-2-pyridinylmethylene-cyclohexane-1,2- diamine]iron(II) chloride (1) and [N,N′-bis-2-pyridinylmethyl-cyclohexane-1,2-diamine]iron(II) chloride (2) were designed, and they showed excellent activity for the coupling reaction epoxides and CO2 to generate the corresponding cyclic carbonates. When complex 2 was used alone as a catalyst for the cycloaddition of propylene oxide (PO) and CO2, the PO conversion was 95% at 130 °C and 4 MPa CO2 pressure in 4 h. Once a co-catalyst like tetrabutylammonium bromide (TBAB) was added, the conversion could reach 100% with nearly 100% selectivity for propylene carbonate (PC), with a turnover number (TON) of 1000 at 100 °C and 4 MPa CO2 pressure in 6 h, i.e. the quantitative synthesis of propylene carbonate can be realized. Moreover, in combination with TBAB, the iron complex can also catalyze the cycloaddition of cyclohexene oxide (CHO) and epichlorohydrin (ECH) with CO2 to produce the corresponding cyclic carbonates, and the cyclohexene carbonate obtained was mainly the cis-isomer.Graphical abstractNovel iron complexes in combination with TBAB showed excellent activity for the coupling reaction of epoxides and CO2 to generate the corresponding cyclic carbonates.

Based on the mechanistic features of metal salen catalysis systems, titanium(IV) complexes from salen (salen-H2 = N,N-bis(3,5-di-tert-butylsalicylidene)-1,2-benzenediamine) and its half saturated form salalen have been prepared, which were used as catalysts in conjugation with bis(triphenylphosphino)iminium chloride ([PPN]Cl) for the coupling reaction of CO2 and cyclohexene oxide (CHO). The salen titanium complex (salen)Ti(IV)Cl2 showed moderate activity, producing a unique cis-isomer of cyclic carbonate with high conversion up to 100% in 8 h, however, it could not catalyze the copolymerization reaction. Meanwhile, the salalen titanium complex (salalen)Ti(IV)Cl was effective for the copolymerization of CO2 and CHO, where only one chain grew on Ti during the chain propagation reaction, yielding completely alternating copolymers with –OH and –Cl as terminal groups. Moreover, the nearly complete conversion of CHO indicated that (salalen)Ti(IV)Cl might be used to synthesize multiblock poly(cyclohexene carbonate)s with controllable sequences.