Cr(III)-salophen catalysts: efficient single-component and binary systems for sustainable CO₂/cyclohexene oxide copolymerization

Abstract

The rise in carbon dioxide (CO₂) emissions and the accumulation of human-made CO₂ in the atmosphere are recognized as the primary causes of the greenhouse effect. Therefore, one strategy to address this issue is capturing and utilizing CO₂ as a raw material for producing high-value chemical compounds. The most promising reactions involve the catalytic conversion of CO₂ with epoxides to produce polycarbonates (PC). For this purpose, Co(III) and Cr(III) complexes (Cat1–Cat9) with salophen-type ligands were synthesized and tested as catalysts for the copolymerization of cyclohexene oxide (CHO) with CO₂ to create poly(cyclohexene)carbonate (PCHC). Their catalytic activity was initially assessed in single-component systems at 100 °C and 8 bar of CO₂ for 24 hours, where Cat5 demonstrated the highest selectivity. Subsequent optimization studies explored the effect of solvents, CO₂ pressure, and co-ligand variation. The best performance was obtained with the Cat5:DMAP binary system, producing a semi-crystalline PCHC copolymer with a high degree of isotacticity.

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Introduction

Most of the world's energy is generated by burning fossil fuels such as coal, oil, and natural gas.¹ However, these will likely remain the predominant energy source for the next 30 years.^2–4 However, this dependence has significant environmental consequences, as it inevitably results in the emission of large amounts of CO₂, the principal greenhouse gas, into the atmosphere. These emissions have increased by more than 40% since the beginning of the Industrial Revolution, rising from a pre-industrial level of 280 ppm to 410 ppm in 2018.^5,6 According to projections by the Intergovernmental Panel on Climate Change (IPCC), the atmospheric concentration of CO₂ could reach 570 ppm in 2100. In June 2025, the monthly average CO₂ emissions reached a record high of 429.61 ppm, up from 415.2 ppm in April 2021.^7–9 To address this problem, a strategy based on the principles of the circular economy is required. In particular, carbon capture and utilization (CCU), using CO₂ as a feedstock to obtain value-added products.^10–12 CO₂ is a renewable source of carbon with multiple advantages, including its abundance, low cost, non-toxicity, and non-flammability.¹² However, it is a difficult molecule to activate, resulting in high thermodynamic stability (ΔG_f⁰ = −394.228 kJ mol⁻¹) and low reactivity.^13,14 Nevertheless, reacting CO₂ with high-energy substrates (hydrogen, amines, or epoxides) can overcome this low thermodynamic stability.^15–17 Among the various efforts to use CO₂ as a feedstock, the catalyzed copolymerization reaction between CO₂ and epoxides results in cyclic carbonates (CC) or in polycarbonates (PC) formation. The latter have become an attractive option at the industrial level due to their atom efficiency and their greater respect for environmental sustainability (Scheme 1).^17–19 CCs have been widely used as polar aprotic solvents,²⁰ electrolytes for lithium-ion batteries,²¹ and in the pharmaceutical industry.^2,7,22 On the other hand, PCs have been proposed as alternatives to petroleum-derived chemicals in industries such as automotive, medical, and electronics. They are also used as starting materials for manufacturing polyurethanes.^23–25 In recent years, this research has seen further development, with numerous studies focusing on generating catalysts that can synthesize CCs or PCs using CO₂.^5,21,26–29

Scheme 1 Copolymerization between epoxides and CO₂ using a catalyst to obtain cyclic carbonates (1) or polycarbonates (2).

The most studied catalytic systems are coordination complexes, as they can exhibit excellent activity and selectivity for the preparation of CCs or PCs.^30,31 The well-established epoxide activation mechanism governs the synthesis of CC and applies to a wide range of catalytic systems, both metallic and non-metallic.^32–35 The most commonly reported catalytic systems use Al(III) as the metal center. However, due to the instability of these complexes in air and humidity, other metals such as Zn(II), Co(III), Cr(III), and Fe(III) become interesting.^8,32 Catalytic complexes generally contain a Lewis acid (LA), which is the metal center responsible for activating the epoxide through coordination.^33,36–38 Additionally, the presence of a nucleophile is essential for opening the epoxide ring, and it is usually part of the co-catalyst.^39,40 These catalytic systems can be classified into three groups: binary, bifunctional, and single-component.^{3,32,38,41–44} Binary systems, or first-generation systems, are those in which the LA and the nucleophile exist in two separate compounds.¹⁸ In these systems, the LA is the metal in the complex, while the nucleophile co-catalysts are usually halides from onium salts, such as tetrabutylammonium (TBA) and bis(triphenylphosphine)iminium (PPN) cations, and phenyltrimethylammonium tribromide (PTAT). Additionally, Lewis bases such as dimethylaminopyridine (DMAP), N-methylimidazole (NMeIm), pyridine (Py), 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) are widely used to carry out this process. Generally, an ideal co-catalyst consists of a bulky, non-coordinating cation paired with a nucleophilic anion.^21,45

On the other hand, second-generation catalytic systems were developed, in which the co-catalyst is covalently bound to the main ligand, known as bifunctional systems. In these systems, both the LA and the nucleophile are contained within a single molecular entity, meaning the nucleophile is part of the functionalization of the ligand.^46,47 Generally, these systems consist of protic tetraalkylammonium halides, protic phosphonium halides, among others.^18,24 These changes have resulted in increased selectivity for PC formation over CC formation, as the ammonium arms prevent the cyclization of the anionic polymer chain through electrostatic interactions. In addition, these bifunctional catalysts enable reduced catalyst loadings while maintaining high selectivity even at elevated temperatures, thereby enhancing overall reactivity.^25,48,49 Finally, the latest generation of homogeneous catalysts, known as single-component catalysts, is characterized by the absence of a co-catalyst and the presence of a nucleophile directly attached to the metal, serving as its counterion. Several bimetallic catalysts have been synthesized in this category.^36,50 However, there are only a few reports in the literature based on monometallic catalysts of this class, the first being an Al(III) complex derived from a bis(amidinato) ligand.³³ The most widely used catalysts are highly efficient and selective homogeneous metal-based complexes. Representative examples include complexes with ligands such as porphyrins, aminophenolates, β-imidinates, amidinates, heteroscorpionates, salen, and macrocycles. These catalysts typically enable synthesis at room temperature (25–100 °C) and CO₂ pressures ranging from 1 to 100 bar.^{3,20,28,33,34,36,39,42,49–52} Although various metal-based catalytic systems have been explored for this transformation, a key challenge remains: the development of catalysts that are not only highly active and selective but also easy to synthesize, air-stable, and based on metals with relatively low toxicity. In this context, Cr(III) and Co(III) complexes have become highly effective and selective catalysts for forming CC and PCs, demonstrating excellent performance under mild conditions. These catalysts not only provide promising activity and selectivity but also offer improved air and moisture stability compared to traditional Al(III) systems. This enhanced robustness facilitates handling and storage, making them highly attractive candidates for CO₂-based polymerization. In this context, this work will focus on evaluating the catalytic performance of Cr(III) and Co(III) complexes derived from salophen ligands in the production of poly(cyclohexene carbonate) (PCHC) via the copolymerization of cyclohexene oxide (CHO) and CO₂. Through this study, we aim to contribute to the development of efficient catalysts for sustainable polymer synthesis.

Results and discussion

Salophen-type Co(III) and Cr(III) complexes Cat1–Cat9 (Scheme 2) were prepared in excellent yields, as previously reported,^53,54 and tested as catalysts for the copolymerization reaction between CHO and CO₂. It is known that the reaction for producing PCHC yields a mixture of cyclohexene carbonate (CHC-trans) and polyether (PE), the latter formed from consecutive epoxide insertions. These by-products arise from competing propagation and backbiting reactions, which reduce the selectivity toward PCHC.⁵⁵

Scheme 2 Synthesis of PCHC catalyzed by Co(III) and Cr(III) Cat1–Cat9 catalyst complexes.

Firstly, the single-component catalysts Cat1–Cat6 were tested for the obtention of PCHC using a 0.5 mol% catalyst loading for 24 h at room temperature and 1 bar of CO₂ pressure without a cocatalyst. Under these conditions, no conversion to PCHC was observed. Therefore, we decided to increase the CO₂ pressure and temperature to 8 bar and 100 °C, respectively, while maintaining all other conditions constant. Co(III) complexes Cat1, Cat2, and Cat3 achieved conversions of 37%, 31% and 49%, respectively; however, PE was the only product obtained. Subsequently, increasing the catalyst loading to 1 mol% (8 bar, 100 °C, 24 h) improved conversions to 60%, 57% and 41%, respectively, although PE remained the only product. These results suggest that, following nucleophilic opening of the epoxide, consecutive epoxide insertion was favored over CO₂ insertion. This behavior can be attributed to the Lewis acidity of Co(III), which stabilizes the alkoxide intermediate and reduces its nucleophilicity, thereby slowing down the CO2 insertion step and resulting in remarkable selectivity toward PE formation.^51,56

The Cr(III) complexes Cat4, Cat5, and Cat6 displayed moderate to high activity at 0.5 mol% catalyst loading, where Cat5 exhibited the highest selectivity toward PCHC (Table 1). When the catalyst loading was increased to 1 mol%, Cat4–Cat6 presented higher conversions, and Cat5 again exhibited the highest selectivity toward PCHC. This is probably due to the presence of methoxy groups on the aromatic ring enhances CO₂ insertion and copolymerization by increasing the lability of the Cr(III)-alkoxy bond. On the other hand, Cat4 showed moderate catalytic activity due to the absence of electron donation. At the same time, Cat6, with electron-withdrawing groups, exhibited lower activity because the stronger Cr(III)-alkoxide interaction hinders CO₂ insertion and PCHC formation.^57,58 To investigate the effect of the coligand, Cr(III) catalysts Cat7 (Br⁻), Cat8 (I⁻), and Cat9 (OAc⁻) (Scheme 2) were designed as structural modifications of Cat5. The reaction time was then shortened to 9 hours while keeping the same conditions. Results showed that Cat5 remained the most efficient catalyst under these conditions, while Cat7–Cat9 exhibited significantly lower activity and were inactive in producing PCHC. Moreover, increasing Cat5 loading at short reaction time did not substantially enhance PCHC selectivity. These results indicate that the difference in selectivity toward PCHC between using 0.5 mol% and 1 mol% is not significant because increasing the metal content in the system does not necessarily improve epoxide conversion activation.⁵⁹

Table 1 Synthesis of PCHC catalyzed by complexes Cat1–Cat9^.a

Entry	Cat. (mol%)	Time (h)	% Conv.^b	% CHC^b	% Copolymer (% carbonate linkage)^b	TON^c	TOF^d (h^−1)
a Reactions were carried out at 100 °C and 8 bar CO2 pressure for 24 h.b Conversion, % of CHC-trans, % of PCHC, and % of carbonate linkages determined by ^1H NMR spectroscopy of the crude reaction mixture.c TON = moles of product/moles of catalyst.d TOF = TON/time (h); calculated from total epoxide conversion.
1	Cat1 (0.5)	24	37	0	37 (0)	74	3.07
2	Cat2 (0.5)	24	31	0	31 (0)	62	2.58
3	Cat3 (0.5)	24	49	0	49 (0)	98	4.07
4	Cat4 (0.5)	24	64	3	97 (47)	128	5.34
5	Cat5 (0.5)	24	68	2	98 (54)	136	5.65
6	Cat6 (0.5)	24	41	0	100 (16)	82	3.42
7	Cat1 (1)	24	60	0	60 (0)	60	2.49
8	Cat2 (1)	24	57	0	57 (0)	57	2.37
9	Cat3 (1)	24	41	0	41 (0)	41	1.70
10	Cat4 (1)	24	69	2	98 (48)	69	2.88
11	Cat5 (1)	24	79	2	98 (61)	79	3.29
12	Cat6 (1)	24	70	1	99 (8)	70	2.92
13	Cat5 (0.5)	9	42	3	97 (35)	84	9.31
14	Cat7 (0.5)	9	33	15	85 (5)	66	7.31
15	Cat8 (0.5)	9	27	0	27 (0)	54	5.98
16	Cat9 (0.5)	9	30	0	30 (0)	60	6.65
17	Cat5 (1)	9	50	3	97 (45)	50	5.54

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To enhance selectivity toward PCHC, a co-catalyst was added, as previous studies have confirmed that introducing a nucleophile source as a co-catalyst improves copolymerization selectivity (Table S1).^60–62 It is important to note that before this step, the co-catalysts were tested individually and showed no conversion to PCHC or CHC.

Using DMAP as a co-catalyst (0.5 : 0.5 mol% ratio, 8 bar CO₂, 100 °C, 24 h), Cat1–Cat3 exhibited low selectivity toward PCHC, and were therefore excluded from further evaluation.^63–65 In contrast, catalysts Cat4–Cat6 were tested with each of the co-catalysts (DMAP, PPNCl, TBAI, TBAC, TBAB), and Cat5:DMAP showed the highest selectivity toward PCHC.

These systems, Cat5:PPNCl, Cat5:TBAC, Cat5:DMAP, Cat4:DMAP, and Cat4:TBAC, were subsequently evaluated in THF and toluene to assess the effects of the solvent and cocatalyst, and the single-component catalysts Cat4 and Cat5 were also tested (Table 2). Initially, when tested in THF without a co-catalyst, Cat4 and Cat5 showed moderate conversion rates, but the PCHC selectivity was low in both cases, resulting in the exclusive formation of CHC-cis. This is attributed to the coordinating nature of THF, which promotes the early-stage formation of CHC-cis by competing with the growing polymer chain for coordination to the Cr(III) center. This mechanism facilitates chain release and cyclization through two SN₂ steps: epoxide ring opening followed by CO₂ insertion, and final ring closure to form the cyclic carbonate.^52,66,67 Cat5:DMAP showed the highest conversion among the catalytic systems in THF, due to the good solubility of DMAP in CHO.^24,68 However, PCHC was not obtained as a product in any of these assays, although CHC-cis and CHC-trans were obtained. Its formation is attributed to the depolymerization of a previously formed polymer-free anionic chain due to THF coordination. CHC-trans results from the cyclization of a terminal alkoxide, while CHC-cis forms via cyclization of a carbonate intermediate through two consecutive stereochemical inversions.^69,70 These findings are supported by solvent-free assays with Cat5, in which CHC-cis was not detected and PCHC was the predominant product. When toluene was used as a solvent (Table S2), both conversion and PCHC selectivity decreased, while CHC-trans selectivity increased due to ring-opening polymerization followed by decarboxylation, yielding PE and CO₂.^56,71 This suggests that toluene favors depolymerization pathways, enhancing CHC-trans and PE formation.⁷² Despite this, the highest conversion was achieved with Cat5:DMAP, followed by Cat4:DMAP. Overall, Cat5:DMAP showed the best performance among all binary systems tested. However, the presence of solvent did not improve the catalytic activity for PCHC production in these systems.

Table 2 Synthesis of PCHC catalyzed by complexes Cat4 and Cat5 using THF^a

Entry	Cat. (0.5 mol%)	Co-cat (0.5 mol%)	% Conv.^b	% CHC (cis/trans)^b	% Copolymer (% carbonate linkage)^b	TON^c	TOF^d (h^−1)
a Reactions were carried out at 100 °C and 8 bar CO2 pressure for 24 h.b Conversion, % of CHC-trans, % of CHC-cis, % of PCHC, and % of carbonate linkages determined by ^1H NMR spectroscopy of the crude reaction mixture.c TON = moles of product/moles of catalyst.d TOF = TON/time (h); calculated from total epoxide conversion.
1	Cat4	—	54	35/0	60 (4)	108	4.49
2	Cat4	DMAP	30	20/52	28 (0)	60	2.49
3	Cat4	TBAC	16	16/27	57 (0)	32	1.33
4	Cat5	—	71	30/0	66 (4)	142	5.90
5	Cat5	DMAP	64	9/32	59 (0)	128	5.32
6	Cat5	PPNCl	20	5/74	21 (0)	40	1.66
7	Cat5	TBAC	21	4/69	27 (0)	42	1.75

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Based on the results obtained, it can be observed that Cat5 exhibits the highest selectivity values towards PCHC, with and without solvent. These combinations, Cat5:PPNCl, Cat5:TBAC, and Cat5:DMAP, were tested at a molar ratio of 0.5 : 1 without solvent (Table 3). According to Table 3, the highest conversion and selectivity towards PCHC was achieved with the binary catalyst Cat5:DMAP. Therefore, the DMAP co-catalyst was chosen as the most active and was used in the other catalytic tests. Subsequently, Cat7–Cat9 with DMAP (0.5 : 1 mol%) were also tested. Although all three catalysts showed improved selectivity towards PCHC compared to the 0.5 : 0.5 mol% ratio (Table S3), they still showed lower selectivity than the Cat5:DMAP system. Even without a cocatalyst, the single-component Cat5 showed good selectivity toward PCHC (Table 1). In general, the trend of the influence of the co-ligand on catalyst activity was Cl⁻ > Br⁻ ≈ I⁻ > OAc⁻. This is probably because Cl⁻, which acts as a co-ligand, acts as an effective nucleophile and leaving group, favoring epoxide ring opening and promoting active propagation chains over cyclization reactions toward CHC. Furthermore, to verify whether the 0.5 : 1 mol% (Cat5:DMAP) ratio was the most catalytically active, additional tests were performed, keeping the catalyst loading at 1 mol% and varying the co-catalyst loading to 0.5, 1, and 2 mol% (Table S3). These tests revealed that none of these three ratios exceeded the selectivity achieved with 0.5 : 1 mol% (Cat5:DMAP), which is currently the highest (Table 3). This suggests that the ratio between the metal (catalyst) and the anion (co-catalyst) is more critical than the actual amount of metal (catalyst) present in the reaction mixture.

Table 3 Effect of co-catalyst on the synthesis of PCHC catalyzed by Cat5 (0.5 mol%)^a

Entry	Co-cat (1 mol%)	% Conv.^b	% CHC trans^b	% Copolymer (% carbonate linkage)^b	TON^c	TOF^d (h^−1)
a Reactions were carried out using 0.5 mol% Cat5 and 1 mol% co-cat at 8 bar CO2 pressure for 24 h at 100 °C.b Conversion, % of CHC-trans, % of PCHC, and % of carbonate linkages determined by ^1H NMR spectroscopy of the crude reaction mixture.c TON = moles of product/moles of catalyst.d TOF = TON/time (h); calculated from total epoxide conversion.
1	DMAP	95	14	86 (77)	189	7.89
2	PPNCl	86	13	87 (69)	172	7.15
3	TBAC	88	6	94 (55)	176	7.31

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The effect of increasing CO₂ pressure from 8 bar to 16 bar was examined using the single-component Cat5 catalyst and the Cat5:DMAP catalytic system at 100 °C for 24 hours. Initially, with Cat5 loadings of 0.5 mol% and 1 mol%, the variation in selectivity towards PCHC was found to be minimal (Table S4). Similarly, using Cat5:DMAP at a 0.5 : 1 mol% catalyst to cocatalyst ratio, it was observed that this was the most catalytically active molar ratio at both 8 and 16 bar CO₂, with 8 bar showing the highest selectivity towards PCHC (Table 3). Along with the tests mentioned above, an experiment was carried out at 100 °C, 50 bar of CO₂ with a catalyst loading of 1 mol% Cat5, which resulted in good conversion and selectivity towards PCHC (Table S4). However, when comparing these results with those obtained with Cat5 (Table 1), lower CO₂ pressures favor the formation of PCHC, while higher pressures increase the selectivity of trans-CHC, probably due to the depolymerization of PCHC caused by high CO₂ pressure. Furthermore, the observed decrease in selectivity may be due to dilution of the catalyst/epoxide mixture⁷³ or precipitation caused by increased CO₂ solubility, ultimately reducing catalytic activity.^74–76 From this, it can be concluded that the optimal pressure for catalytic tests is 8 bar of CO₂, since doubling the pressure to 16 bar did not significantly change the selectivity toward PCHC. However, when the pressure is raised to 50 bar of CO₂, the PCCH production rate starts to decline.

Characterization of PCHC obtained with Cat5:DMAP

The polymerization reaction performed with the binary catalyst Cat5:DMAP (Table 3, entry 5) reached 95% conversion, as determined by ¹H NMR, with 77% selectivity toward PCHC (Fig. S2). Subsequent purification effectively removed PE from the crude mixture; however, complete elimination of CHC-trans remained challenging, as indicated by residual signals in the ¹H NMR spectrum (Fig. 1). As a result, the selectivity increased from 77% to 95%, suggesting that the initially obtained material was likely a physical mixture of PCHC and PE, rather than a true copolymer containing both ether and carbonate linkages.^77–80 In the PCHC repeating unit, the methine protons were designated as H_c and H_c′, while the protons corresponding to the polymer chain end were assigned as H_a and H_b. The terminal hydroxyl group was identified based on literature reports, as the chemical shift of H_a remains consistent across all PCHC samples, regardless of the halide initiator used.^81,82 This consistency further suggests that halogen-terminated copolymers, if present, are only present in trace amounts.^80,83

Fig. 1 ¹{H} NMR spectrum of PCHC obtained using Cat5:DMAP in CDCl₃. Signals H_a and H_b are attributed to the terminal methine protons of the copolymer, while H_c/H_c′ correspond to the methine protons within the copolymer backbone. Additionally, the signal at 4.0 ppm corresponds to 14% of the CHC-trans.

In the ¹³C{¹H} NMR spectrum of PCHC, signals corresponding to the methine carbon (a/a') were identified at 76.29 ppm and to the methylene carbons at 27.18 (b/b') and 19.58 (c/c') ppm (Fig. S3). Additionally, signals indicating the stereoselectivity and thus the tacticity of PCHC were observed in the carbonyl region of the ¹³C NMR spectrum. The assignment of these signals was based on previous reports describing PCHC synthesized from copolymerization between CHO and CO₂.⁸⁴ The stereoselectivity of a PCHC is determined by the relative stereochemistry of the carbons of the cyclohexane units into which the main chain enters. Literature consistently reports a trans orientation of the entry and exit bonds in these rings.^85,86

According to convention, [m] and [r] refer to meso and racemic dyads, respectively, depending on whether the incorporated monomer units have identical or opposite configurations.^84,87 Tetrads arise from combinations of such dyads and are classified as isotactic ([m]) or syndiotactic ([r]), as illustrated in Fig. 2.⁸⁸ Isotactic sequences (RR–RR or SS–SS) yield highly ordered PCHC, while alternating RR–SS configurations form syndiotactic chains.^89,90 In the carbonyl region (152–156 ppm), two signals were detected: a low-intensity peak at 155.61 ppm (15%) assigned to syndiotactic tetrads, and a more intense signal at 155.91 ppm (85%) attributed to isotactic tetrads (Fig. 2). These results indicate a predominantly ditactic microstructure, favoring crystallization.^85,89

Fig. 2 Carbonyl region of the ¹³C{¹H} NMR spectrum in CDCl₃ of PCHC obtained using Cat5:DMAP.

The formation of PCHC was further confirmed by comparing the FT-IR spectra of CHO and PCHC (Fig. S4).⁹¹ Two strong absorption bands were observed at 1755 and 1261 cm⁻¹, corresponding to the C=O and C–O stretching vibrations of the linear carbonate bond, respectively. Additional bands at 2931 and 2862 cm⁻¹ were assigned to the asymmetric and symmetric stretching modes of the CH₂ groups.^92,93 The thermal properties of PCHC were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). PCHC exhibited thermal behavior consistent with a semicrystalline polymer. TGA analysis showed thermal stability up to 120 °C, followed by 95% mass loss and a decomposition onset at 220 °C (Fig. S5). DSC analysis revealed a glass transition (T_g) at ∼50 °C (Fig. S6), indicative of relatively high chain mobility.^89–91 A crystallization exotherm at 60.7 °C (ΔH_c = 2.47 J g⁻¹) and a melting endotherm at 171.0 °C (ΔH_f = 4.04 J g⁻¹) confirmed the semicrystalline nature of the polymer and its suitability for thermoplastic processing.^94,95 Overall, despite its semicrystalline character, the moderate crystallinity and relatively low T_g suggest that PCHC retains adequate chain mobility for conventional thermoplastic processing. Thus, the material should balance mechanical integrity with good processability under standard thermoplastic conditions. Finally, ESI-MS analysis of the crude product revealed two series of PCHC chains (● and ■), attributed to distinct initiation pathways involving the Cat5:DMAP binary system (Fig. S7). Both series showed regular m/z spacing of 142, corresponding to the carbonate–cyclohexane repeat unit.^93,96,97 Series ● was assigned to chains with hydroxyl termini, resulting from chain transfer from chloride to water.^20,98 Series ■ contained DMAP as the initiator and a hydroxyl end group, consistent with epoxide ring-opening initiated by DMAP (Fig. S8). In both cases, hydroxyl termination is attributed to HCl-induced hydrolysis. The mass distribution confirms the alternating structure of the copolymer.

Finally, the molecular weight (M_nexp) and polydispersity (PDI) of the obtained PCHC catalyzed by Cat5:DMAP were determined by gel permeation chromatography (GPC) calibrated with PMMA standards in chloroform (Table S5 and Fig. S9). It is relevant to comment that the PDI (M_w/M_n) observed was 1.06, indicating a narrow molecular weight distribution. This low PDI is characteristic of a well-controlled polymerization process, suggesting that the catalyst system effectively mediates the ring-opening copolymerization (ROCOP) of CO₂ and CHO with minimal side reactions or chain-transfer events. The M_n value observed (18 797 Da) reflects a high molecular weight, suitable for applications where mechanical robustness and thermal stability are desired.^99,100 These results highlight the potential of Cat5:DMAP as a highly selective catalyst system in producing uniform PCHC chains, which is particularly relevant for tuning polymer properties in advanced material applications.

Proposed mechanism for Cat5:DMAP catalyzed CHO/CO₂ copolymerization

According to the results obtained when using the binary catalyst Cat5:DMAP, the possible formation of a hexacoordinated complex was proposed, commonly observed when using a pentacoordinate (salophen)Cr(III)X complex (A) in the presence of a co-catalyst nucleophile (Nu) (Scheme 3).^101–103 An equilibrium between a mono-DMAP species (B) and a hexacoordinated cationic complex (C) with two DMAP molecules was suggested.

Scheme 3 Initiation stage and formation of the hexacoordinate complex.

In the presence of excess epoxide, these two intermediates underwent competitive binding with CHO to form species (D) and (E), activating the epoxide. CHO activation can proceed via two possible pathways: either through an intramolecular nucleophilic attack on the coordinated CHO (F), or via ring-opening promoted by the external co-catalyst nucleophile (G). In both cases (Nu = Cl⁻, DMAP), the nucleophile initiates polymerization and becomes incorporated into the growing chain. Coordination of CHO to Cr(III) weakens the adjacent metal–Nu₁ bond, enabling rapid epoxide ring-opening (H). When DMAP binds trans to the propagating chain, it facilitates CO₂ insertion by weakening the metal–alkoxide bond, leading to the initial carbonate complex (I) (Scheme 4).^24,105 Intermediate (I) then undergoes CHO insertion, likely via nucleophilic attack by a second metal-bound nucleophile rather than by the growing polymer chain, resulting in intermediate (J). Subsequent CO₂ insertion yields intermediate (K), from which alternating CHO and CO₂ insertions occur at the active chain ends. This enables the simultaneous growth of two polymer chains from the metal center, producing the intermediate (L). This alternation is due to the planar nature of the salophen ligand, a structural feature transferred to the complex (Scheme 4).^24,104 Alternatively, a complementary hypothesis might explain the higher conversion and selectivity observed with the binary Cat5:DMAP catalyst compared to the single-component Cat5. This involves an alternative initiation mechanism, in which CO₂ incorporation occurs before epoxide activation, a pathway often seen when N-heterocyclic amines are used as co-catalysts.^24,102 However, further mechanistic studies are needed to confirm this. It is proposed that DMAP activates CO₂ in two steps: first, they interact with CO₂ in the presence of (salophen) Cr(III)X (Cat5), forming a weakly stabilized carbamate zwitterionic intermediate via a bidentate complex (Scheme S1). This intermediate then reacts with CHO, producing a stabilized zwitterionic intermediate. This mechanism aligns with those proposed in the literature for related systems.¹⁰⁴

Scheme 4 Epoxide ring-opening through pathway 1 and pathway 2, followed by CO₂ insertion and chain propagation step, after consecutive CO₂ and CHO insertions.

Additionally, increasing the DMAP loading from 0.5 to 2 mol% (Table S3, entries 4–6) led to lower PCHC selectivity, suggesting that the mono-DMAP intermediate (B) is more catalytically active than the bis-DMAP cationic complex (C). While intermediate (B) is expected at Cat5:DMAP ratios of 0.5 : 0.5 (Table S1, entry 5) or 1 : 1 (Table S3, entry 5), high selectivity was not observed under these conditions. This implies that a slight excess of DMAP (0.5 : 1 ratio) (Table 3, entry 1) is necessary to favor an optimal equilibrium between intermediates (B) and (C), thereby promoting the formation of active species (D) and (E). In contrast, an excess of DMAP (1 : 2 ratio) (Table S3, entry 6) likely shifts the equilibrium toward (C), which reduces selectivity by displacing the growing polymer chain and favoring depolymerization via backbiting. Mass spectrometric (HRMS) evidence supports the coexistence of both species at the 0.5 : 1 mol% (Table 3, entry 1) Cat5:DMAP ratio. At lower DMAP loadings, the major species detected corresponded to intermediate (B), observed as a sodium adduct with the molecular formula [C₃₅H₂₈ClCrN₄O₈Na] and a m/z = 742.0818, in agreement with the theoretical value of 742.0898 m/z (Fig. 3a). Upon addition of an extra equivalent of DMAP, a new peak appeared with the molecular formula [C₄₂H₃₈CrN₆O₈]⁺ and a m/z = 806.2131, matching the theoretical value for intermediate (C) of 806.2156 m/z (Fig. 3b). The simultaneous presence of both signals indicates an equilibrium between these complexes, consistent with their proposed role in CHO activation. Conductivity measurements further supported the HRMS results. At a 0.5 : 0.5 Cat5:DMAP ratio, the solution exhibited a molar conductivity of 57.6 Ω⁻¹ cm² mol⁻¹, consistent with the formation of a neutral species, attributed to intermediate (B).¹⁰⁵ Upon increasing the DMAP ratio to 0.5 : 1, the conductivity increased to 63 Ω⁻¹ cm² mol⁻¹, approaching the value expected for a 1 : 1 electrolyte (65 Ω⁻¹ cm² mol⁻¹). This shift aligns with the formation of cationic intermediate (C), reinforcing the coexistence of both species in solution.

Fig. 3 (a). Intermediate [(B) + Na], and (b). Intermediate (C), identified by HRMS (ESI-MS).

Experimental

Methods and general procedures

All chemicals were purchased from commercial suppliers (Merck and Aldrich). 7-Hydroxycoumarin, trifluoroacetic acid, acetic anhydride, sodium bicarbonate, o-phenylenediamine, 4,5-dichloro-o-phenylenediamine, hexamine, catechol, methyl iodide, Pd/C 10 wt%, hydrazine monohydrate. Commercial CO₂ was obtained from Gas-Lab and used without further purification. The commercially available cyclohexene oxide (CHO) (99.8%) was purchased from Sigma-Aldrich. Fourier transform infrared (FTIR) spectra were obtained on a Shimadzu IR Tracer-100 spectrometer using a single-reflection ATR accessory. High-resolution Mass Spectrometry (HRMS) was performed using a Micromass Quattro Q-TOF LC/MS system equipped with electrospray ionization (ESI+) ionization. The NMR spectra were recorded on a Bruker AV-400 spectrometer and referenced to the residual NMR solvent signals. Molecular weight estimations of the synthesized polymers were carried out using a gel permeation chromatograph (GPC, Jasco, Japan) equipped with a refractive index detector (RI-4030, Jasco) and a divinylbenzene-based column (DVB column, Jordi Labs) enclosed in a column oven at 40 °C (CO-4060, Jasco). A 10 mg sample of the polymer was dissolved in 1.0 mL of chloroform and stirred overnight until the total dissolution of the polymer into the solvent occurred. GPC measurements of the samples were carried out with chloroform as mobile phase at 1.0 mL min⁻¹. Molecular weight calculations (M_w, M_n, and polydispersity index) were done using ChromNAV-GPC software (Jasco), employing a molecular weight calibration curve prepared using different narrow polymethylmethacrylate (PMMA) standards (ReadyCal kit, Polymer Standard Service GmbH).

General procedure for catalyst screening

The catalyst complexes Cat1–Cat9 were synthesized previously following previous reports in our research group.^53,54 These catalysts are stable in air and humidity, which allows for easier handling when performing catalytic tests. CHO (0.107 g, 1.09 mmol), catalysts Cat1–Cat9 5.45 × 10⁻³ mmol (0.5 mol%) and 0.0109 mmol (1 mol%), and co-catalyst TBAI, TBAB, TBAC, PPNCl, or DMAP (0.5 mol%/1 mol%) were placed in a steel reactor with a magnetic stirrer bar. The autoclave was sealed, pressurized to 8–50 bar with CO_2, and heated to the desired temperature, 50 °C/100 °C. The reaction mixture was heated at 50–100 °C for 24 h. Finally, the reaction was stopped, and the reactor was carefully cooled in a liquid nitrogen bath. The conversion of the CHO to PCHC was determined by analyzing the crude reaction product using ¹H NMR spectroscopy. PCHC is a known compound, and the spectroscopic data of samples prepared using catalysts Cat1–Cat9 were consistent with those reported in the literature.

Purification of PCHC

The crude product was dissolved in 20 mL of MeOH and extracted with 5 mL portions of hexane. The combined hexane layers were collected in a round-bottom flask and evaporated under reduced pressure. The residue was treated with 5 mL of 1 M HCl, affording a white precipitate corresponding to PCCH. The PCHC obtained was analyzed by various techniques, ¹H and ¹³C{¹H} NMR, FTIR, TGA/DSC, ESI-MS, and GPC.

Conclusions

Nine air-stable Co(III) and Cr(III) complexes (Cat1–Cat9) with coumarin-derived salophen-type ligands were evaluated as single-component and binary catalysts in the presence of co-catalysts such as TBAB, TBAC, TBAI, PPNCl, and DMAP in the copolymerization of CO₂ and CHO to produce PCHC. Among them, Cat5 stood out and was the most selective for obtaining PCHC with a selectivity of 54% (0.5 mol%) and 61% (1 mol%) at 100 °C and 8 bar of CO₂ for 24 hours. Similarly, under the same conditions, Cat5:DMAP in a 0.5 : 1 mol% (cat: co-cat) ratio was the binary system that allowed up to 77% selectivity towards PCHC. However, the use of solvents such as THF and toluene in the reaction was ruled out because no increase in selectivity towards PCHC was observed, but in some cases, the selectivity towards CHC-trans and CHC-cis increased. Likewise, Cat7 (Br⁻), Cat8 (I⁻), and Cat9 (OAc⁻) were tested as a structural modification of Cat5, where the trend of the influence of the co-ligand on the activity of the catalyst was Cl⁻ > Br⁻ ≈ I⁻ > OAc⁻. By varying the pressure (8/16/50 bar CO₂), it was concluded that the best results were obtained at a pressure of 8 bar CO₂, since using higher CO₂ pressures caused the depolymerization of PCHC. Finally, the PCHC obtained with this Cat5:DMAP binary catalyst can be described as ditactic, with a repeating unit of 142 m/z, composed mainly of syndiotactic (15%) and isotactic (85%) tetrads. Furthermore, thanks to its high percentage of isotacticity, it is classified as a semi-crystalline copolymer, which was confirmed by DSC analysis, showing a T_c of 60.7 °C and a T_g of ∼50 °C. This synthesis of PCHC from epoxides and CO₂ is a good start towards the development of a more environmentally friendly route, offering materials with excellent properties for various applications.

Author contributions

The manuscript was written through the contributions of all authors. All authors have approved the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data underlying this article are available in the supplementary information (SI). Supplementary information: the optimization tables and the characterization of poly(cyclohexene carbonate) (PCHC). See DOI: https://doi.org/10.1039/d5ra08569a.

Acknowledgements

J. J. H., D. F. L., and D. E. S. acknowledge the Facultad de Ciencias and Chemistry Department at the Universidad de los Andes for providing funding (Project INV-2025-213-3347). J. M. is grateful for FONDECYT Iniciación Fellowship 11230124. R. R. acknowledges project FONDECYT 1230537.

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Footnote

† Current address: David Ezenarro-Salcedo, Organometallic CO₂ Chemistry, Molecular Catalysis, Max-Planck-Institut für Kohlenforschung (MPI CEC), Stiftstraβe 34–36, 45470 Mülheim an der Ruhr, Germany.

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