Multinuclear Zinc-Magnesium Hydroxide Carboxylates: A Predesigned Model System for Copolymerization of CO2 with Epoxides

Among numerous catalysts in the ring-opening copolymerization of epoxides with carbon dioxide (CO2), zinc dicarboxylate complexes are the most common type, and in the family of metal-based homogeneous catalysts, zinc and magnesium complexes have attracted widespread attention. We report on the synthesis and structural characterization of a zinc-magnesium benzoate framework templated by the central hydroxide anion with μ3-κ2:κ2:κ2 coordination mode, [ZnMg2(μ3-OH)(O2CPh)5]n (n = 1 or 2). The resulting heterometallic system forms stable Lewis acid-base adducts with tetrahydrofuran (THF) and cyclohexene oxide (CHO), which crystallize as the hexanuclear zinc-magnesium hydroxide carboxylate cluster [ZnMg2(μ3-OH)(O2CPh)5(L)2]2 (L = THF or CHO). Their X-ray crystal structure analysis revealed that the Zn center prefers 4-fold coordination and the Mg centers demonstrated the ability to accommodate higher coordination numbers, and as a result, the heterocyclic molecules are exclusively bonded to 6-fold Mg atoms. The heteronuclear carboxylate aggregates appeared active in the copolymerization reaction at elevated temperatures to produce an alternating poly(cyclohexene carbonate).

One-pot terpolymerization of CHO, CO2 and L-lactide using chloride indium catalysts

Ring-opening copolymerization reactions of epoxides, carbon dioxide and cyclic esters to produce copolymers is a promising strategy to prepare CO₂-based polymeric materials. In this contribution, bimetallic chloride indium complexes have been developed as catalysts for the copolymerization processes of cyclohexene oxide, carbon dioxide and L-lactide under mild reaction conditions. The catalysts displayed good catalytic activity and excellent selectivity towards the preparation of poly(cyclohexene carbonate) (PCHC) at one bar CO₂ pressure in the absence of a co-catalyst. Additionally, polyester-polycarbonate copolymers poly(lactide-co-cyclohexene carbonate) (PLA-co-PCHC) were obtained via an one-pot one-step route without the use of a co-catalyst. The degree of incorporation of carbon dioxide can be easily modulated by changing the CO₂ pressure and the monomer feed, resulting in copolymers with different thermal properties.

Perfluoroaryl Zinc Catalysts Active in Cyclohexene Oxide Homopolymerization and Alternating Copolymerization with Carbon Dioxide

The zinc complex Zn(C6F5)2(toluene) (1) behaves as a very active and selective catalyst in cyclohexene oxide (CHO) polymerization to produce poly(cyclohexene oxide) (PCHO) by the trans-ring-opening of CHO with remarkable TOF values at room temperature. The ring-opening copolymerization (ROCOP) of CO2 with CHO catalysed by 1 yields poly(cyclohexene carbonate) (PCHC) when using benzyl alcohol (BnOH) as an initiator at 120°C. The 1H NMR monitoring of the in situ reaction of 1 with BnOH highlighted the formation of the dinuclear species [(C6F5)2Zn2(BnO)2 (2) that was isolated and found an active catalyst in the ROCOP of CO2 with CHO in the absence of initiators. Interestingly, PCHCs by 2 in solventless conditions show polydispersity index (Mw/Mn) values close to 2, corresponding to those expected for a single-site catalyst; on the contrary, a broader polydispersity index of the polymer products was found in toluene solution, suggesting the formation of new zinc catalysts during the polymerization reaction.

Dinuclear Nickel Complexes Using Hexadentate Benzothiazole-Based Diamine-Bisphenolate Ligands: Highly Active Catalysts for Copolymerization of Carbon Dioxide with Epoxides

We reported for the first time the utilization of hexadentate benzothiazole-based diamine-bisphenolate ligands to synthesize structurally well-characterized dinickel dicarboxylate complexes and studied their catalysis for copolymerization of carbon dioxide with epoxides. Dinickel carboxylate complexes having a 1,3-diamine-bridged backbone were demonstrated to be high-performance catalysts for alternating copolymerization of CO₂ and cyclohexene oxide (CHO) with high product selectivity. Particularly, acetate-supported nickel complex 2 enabled us to promote such CO₂-copolymerization of this kind with a maximum turnover frequency of up to 2600 h^-1 and gave good molecular weight controllability under high-pressure conditions. It is worth noting that bimetallic Ni catalyst 2 was also capable of mediating the catalytic CO₂-polymerization of alicyclic epoxides at atmospheric pressure. Kinetic investigations of CO₂/CHO copolymerization by 2 allowed us to determine the rate equation of -d[CHO]/dt = k_p[2]¹[CHO]¹, and such catalysis exhibited a first-order dependence on both dinickel complex and CHO concentrations.

Ring-Opening Copolymerization of Cyclohexene Oxide and Cyclic Anhydrides Catalyzed by Bimetallic Scorpionate Zinc Catalysts

The catalytic activity and high selectivity reported by bimetallic heteroscorpionate acetate zinc complexes in ring-opening copolymerization (ROCOP) reactions involving CO₂ as substrate encouraged us to expand their use as catalysts for ROCOP of cyclohexene oxide (CHO) and cyclic anhydrides. Among the catalysts tested for the ROCOP of CHO and phthalic anhydride at different reaction conditions, the most active catalytic system was the combination of complex 3 with bis(triphenylphosphine)iminium as cocatalyst in toluene at 80 °C. Once the optimal catalytic system was determined, the scope in terms of other cyclic anhydrides was broadened. The catalytic system was capable of copolymerizing selectively and efficiently CHO with phthalic, maleic, succinic and naphthalic anhydrides to afford the corresponding polyester materials. The polyesters obtained were characterized by spectroscopic, spectrometric, and calorimetric techniques. Finally, the reaction mechanism of the catalytic system was proposed based on stoichiometric reactions.

Recent Developments in Ring-Opening Copolymerization of Epoxides With CO2 and Cyclic Anhydrides for Biomedical Applications

The biomedical applications of polyesters and polycarbonates are of interest due to their potential biocompatibility and biodegradability. Confined by the narrow scope of monomers and the lack of controlled polymerization routes, the biomedical-related applications of polyesters and polycarbonates remain challenging. To address this challenge, ring-opening copolymerization (ROCOP) has been exploited to prepare new alternating polyesters and polycarbonates, which would be hard to synthesize using other controlled polymerization methods. This review highlights recent advances in catalyst development, including the emerging dinuclear organometallic complexes and metal-free Lewis pair systems. The post-polymerization modification methods involved in tailoring the biomedical functions of resultant polyesters and polycarbonates are summarized. Pioneering attempts for the biomedical applications of ROCOP polyesters and polycarbonates are presented, and the future opportunities and challenges are also highlighted.

Efficient Production of Poly(Cyclohexene Carbonate) via ROCOP of Cyclohexene Oxide and CO2 Mediated by NNO-Scorpionate Zinc Complexes

New mono- and dinuclear chiral alkoxide/thioalkoxide NNO-scorpinate zinc complexes were easily synthesized in very high yields, and characterized by spectroscopic methods. X-ray diffraction analysis unambiguously confirmed the different nuclearity of the new complexes as well as the variety of coordination modes of the scorpionate ligands. Scorpionate zinc complexes 2, 4 and 6 were assessed as catalysts for polycarbonate production from epoxide and carbon dioxide with no need for a co-catalyst or activator under mild conditions. Interestingly, at 70 °C, 10 bar of CO₂ pressure and 1 mol % of loading, the dinuclear thioaryloxide [Zn(bpzaepe)₂{Zn(SAr)₂}] (4) behaves as an efficient and selective one-component initiator for the synthesis of poly(cyclohexene carbonate) via ring-opening copolymerization of cyclohexene oxide (CHO) and CO₂, affording polycarbonate materials with narrow dispersity values.

Copolymerization of CHO/CO2 catalyzed by a series of aluminum amino-phenolate complexes and insights into structure-activity relationships

Two series of monometallic aluminum complexes were prepared and characterized by elemental analyses, ¹H and ¹³C{¹H} NMR spectroscopy, and X-ray crystallography: Al[L]X, where [L] = dimethylaminoethylamino-N,N-bis(2-methylene-4,6-tert-butylphenolate) and X = Cl, OEt, and Al[L]₂Cl, where [L] = 6-{[(2R,6R)-2,6-dimethyl-4-morpholino]methylene}-2,4-bis(tert-butyl)phenolate or 6-(piperidinomethylene)-2-(tert-butyl)-4-(methyl)phenolate. All the complexes, including the previously reported morpholinyl complex Al[L]Cl, where [L] = 4-(2-aminoethyl)morpholinylamino-N,N-bis(2-methylene-4,6-tert-butylphenolate), were tested as catalysts for copolymerization of cyclohexene oxide and CO₂ in the presence and absence of PPNCl. When coupled with 1 equiv. PPNCl, the complexes exhibit similar activities and the best selectivity for poly(cyclohexenecarbonate) vs. the cyclic product, cyclohexene carbonate, was obtained with the morpholinyl complex (ca. 90%) whereas significantly lower selectivities (<1-63%) were obtained with the other complexes. Preliminary DFT calculations investigating this difference in selectivity were carried out by analyzing the aluminum partial atomic charges in the Al-carbonate intermediates.

Bimetallic Zinc Catalysts for Ring-Opening Copolymerization Processes

Novel bimetallic zinc acetate complexes supported by heteroscorpionate ligands have been developed for the ring-opening copolymerization of cyclohexene oxide and CO₂ and the terpolymerization of cyclohexene oxide, phthalic anhydride, and CO₂. Heteroscorpionate ligands precursors L₁-L₃ were reacted with two equivalents of zinc acetate to afford the dinuclear zinc complexes [{Zn(κ³-bpzappe)}(μ-O₂CCH₃)₃-{Zn(HO₂CCH₃)}] (1), [{Zn(κ³-bpzbdmape)}(μ-O₂CCH₃)₃-{Zn(HO₂CCH₃)}] (2), and [{Zn(κ³-bpzbdeape)}(μ-O₂CCH₃)₃{Zn(HO₂CCH₃)}] (3) in excellent yields. The molecular structure of these compounds was determined spectroscopically and confirmed by X-ray diffraction analysis. Zinc acetate complexes 1-3 were screened as catalysts for the copolymerization of cyclohexene oxide and CO₂ to produce poly(cyclohexene)carbonate, and complex 3 was found to be the most active catalyst for this process in the absence of a cocatalyst. Furthermore, the terpolymerization of cyclohexene oxide, phthalic anhydride, and CO₂ was studied using the combination of complex 3 and 4-dimethylaminopyridine as catalyst system yielding the corresponding polyester-polycarbonate materials.

Update and Challenges in Carbon Dioxide-Based Polycarbonate Synthesis

The utilization of carbon dioxide as a comonomer to produce polycarbonates has attracted a great deal of attention from both industrial and academic communities because it promises to replace petroleum-derived plastics and supports a sustainable environment. Significant progress in the copolymerization of cyclic ethers (e.g., epoxide, oxetane) and carbon dioxide has been made in recent decades, owing to the rapid development of catalysts. In this Review, the focus is to summarize and discuss recent advances in the development of homogeneous catalysts, including metal- and organo-based complexes, as well as the preparation of carbon dioxide-based block copolymer and functional polycarbonates.

Efficient CO2 fixation into cyclic carbonates catalyzed by NNO-scorpionate zinc complexes

Chiral bifunctional and bicomponent NNO-scorpionate zinc-based catalysts have been developed for the fixation of CO₂ into cyclic carbonates with broad substrate scope and functional group tolerance under mild and solvent-free conditions.

Mono- and binuclear chiral N,N,O-scorpionate zinc alkyls as efficient initiators for the ROP of rac-lactide

Activity enhancement in the ROP ofrac-LA through the cooperative effect of two remote zinc metals in N,N,O-scorpionate binuclear trisalkyls.