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

ARTICLE Please do not adjust margins Please do not adjust margins Received 00th January 20xx, Accepted 00th January 20xx DOI: 10.1039/x0xx00000x One-pot terpolymerization of CHO, CO2 and L-lactide using chloride indium catalysts Marc Martínez de Sarasa Buchaca,a Felipe de la Cruz-Martínez,a Luis F. Sánchez-Barba,c Juan Tejeda,a Ana M. Rodríguez,a José A. Castro-Osma,b* Agustín Lara-Sáncheza* 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, polyesterpolycarbonate copolymers poly(lactide-co-cyclohexene carbonate) (PLA-co-PCHC) were obtained via 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 monomers feed, resulting in copolymers with different thermal properties. Introduction During the last few decades, the rapid depletion of fossil fuels and the stronger legislative requirements for a circular economy have driven the scientific and industrial community to search for greener and more sustainable catalytic processes to synthesize high valueadded chemicals and polymers.1–5 Finding new biorenewable-based monomers to produce biodegradable polymeric materials has been targeted by many research groups around the world. In this context, carbon dioxide has emerged as a highly desirable renewable C1 chemical feedstock for the synthesis of polycarbonate materials due to its low toxicity and abundancy.6–12 Among the different synthetic routes developed for their preparation, the ring-opening copolymerization (ROCOP) of epoxides and CO2 is the most extensively studied reaction.6,13–16 Similarly, a lot of attention has been devoted to the synthesis of biodegradable aliphatic polyesters through the ring-opening polymerization (ROP) of cyclic esters.17–20 Polylactide (PLA), synthesized from the ROP of lactide, represents the most studied biodegradable polyester due to its relatively facile production from agricultural renewable sources, its biodegradability and biocompatibility, and it has already found numerous applications ranging from food packaging to the biomedical and pharmaceutical industry (Scheme 1).18,21–25 Scheme 1 Synthesis of polycarbonate, polyester-polyether and polyesterpolycarbonate materials by different ROP and ROCOP processes. A wide range of different metal complexes has been reported for the development of biodegradable polycarbonates, polyesters and terpolymers.6,14,26,27 Amongst them, it is worth highlighting the bimetallic zinc/magnesium β-diiminate (BDI) complexes, firstly described by Coates and co-workers.28,29 They were found to be very efficient for the stereoselective ROP of lactide, affording the formation of isotactic, heterotactic and syndiotactic PLA from L-, racand meso-lactide respectively.30,31 The introduction of electrowithdrawing groups in the imine moieties of the BDI ligands increased the catalytic activity of these complexes for the ROCOP/terpolymerization of different epoxides and CO2, yielding the corresponding polycarbonate materials with high molecular weights and TOF values up to 5520 h-1.32 In addition, their catalytic activity was further tested towards the terpolymerization reaction of epoxides, CO2 and β-butyrolactone, affording the corresponding block polycarbonate-polyester copolymers with tuneable properties such as glass transition temperature or transparency.33,34 a. Universidad de Castilla-La Mancha, Departamento de Química Inorgánica, Orgánica y Bioquímica-Centro de Innovación en Química Avanzada (ORFEOCINQA), Facultad de Ciencias y Tecnologías Químicas and Instituto Regional de Investigación Científica Aplicada-IRICA, 13071-Ciudad Real, Spain. Email: [email protected] b. Universidad de Castilla-La Mancha, Dpto. de Química Inorgánica, Orgánica y Bioquímica, Facultad de Farmacia, 02071-Albacete, Spain. E-mail: [email protected] c. Departamento de Biología y Geología, Física y Química Inorgnica, Universidad Rey Juan Carlos, Mstoles, 28933 Madrid, Spain † Footnotes relating to the title and/or authors should appear here. Electronic Supplementary Information (ESI) available: experimental data, work-up for catalytic reactions, spectra of the 1 H and 13 C{ 1 H} NMR and IR for complexes 1 −  , X-ray crystallographic data for compound 1 and 1 H and 13 C NMR, IR and MALDI-ToF spectra, DSC and TGA of the corresponding polymers. See DOI: 10.1039/x0xx00000x Journal Name ARTICLE This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 2 Please do not adjust margins Please do not adjust margins Scheme 2. Indium catalysts reported for epoxide and CO2 copolymerization reactions Bimetallic and heterobimetallic complexes supported by salen-type ligands have also been widely used for the ROCOP of different epoxides and CO2 and their terpolymerization reactions with different lactones.35–50 In this context, a wide range of cobalt catalysts featuring chiral salen-type scaffolds were developed by Xiao Bing-Lu and co-workers’ for the enantioselective synthesis of different polymeric materials.36,37,43–46 Scorpionate metal complexes have been employed as catalysts for these processes.51–55 Recently, our research group reported a series of bimetallic heteroscorpionate zinc acetate complexes as catalysts for the ROCOP of CHO and CO2 in the absence of a co-catalyst and at one bar of CO2 pressure, and the terpolymerization reaction with phthalic anhydride, affording the corresponding polycarbonatepolyester material with tuneable PCHC content depending on the CO2 pressure applied.56 Indium complexes have proven to be highly active towards the ROP of lactide to afford PLA.57–62 Mehrkhodavandi et al. reported the first chiral indium catalyst for the living ROP of cyclic esters.58 It showed excellent catalytic activity for the living polymerization of rac-LA to produce isotactically enriched PLA with high molecular weights and narrow polydispersities. These catalysts were further investigated towards the polymerization of β-butyrolactone (BBL) and meso-LA, and their terpolymerization reaction to form diand triblock copolymers, as well as star-shaped copolymers.59,63,64 On the other hand, indium complexes remain almost unknown for the copolymerization of epoxides and CO2.65–67 In the last few years, the first salen-type indium catalysts for the ROCOP of epoxides and CO2 were reported (Scheme 2a).66 They showed great catalytic activity towards the synthesis of PCHC, with high selectivity and under one bar CO2 pressure, affording polymeric materials with narrow polydispersities and high molecular weights. New hemisalen H[PNNO] indium complexes have also been tested as catalysts for the copolymerization of CHO and CO2 along with different co-catalysts at 80 °C and 30 bar of CO2, affording PCHC with molecular weights up to 10000 g mol-1 and moderate to high selectivities (Scheme 2b).67 This, combined with that the use of indium complexes as catalysts for the terpolymerization reactions of epoxides, CO2 and cyclic esters to produce polyester-polycarbonate materials remains unknown, as far as we know, prompted us to consider the design of a new family of indium complexes which present activity in this terpolymerization reaction. Herein, we report novel scorpionate indium complexes which have showed to be very efficient catalysts for the ROCOP reaction of CHO and CO2 under mild reaction conditions and at one bar CO2 pressure. Among the synthesized complexes, complex 1 has proved to be the most active catalyst, and its activity has been further tested towards the terpolymerization reaction of CHO, CO2 and L-lactide to produce polyester-polycarbonate materials. To the best of our knowledge, this represents the first indium complex ever reported for terpolymerization reactions involving L-lactide as monomer (Scheme 2c). Results and discussion Synthesis and structural characterization of catalysts Bimetallic chloride indium complexes [InCl2{(  3-bpzbe)(μ-O)}]2 (1), [InCl2{(  3-bpzte)(μ-O)}]2 (2), [InCl2{(  3-bpzappe)(μ-O)}]2 (3) and [InCl2{(  3-bpzFerr)(μ-O)}]2 (4) were prepared in a two-step reaction by metathesis of the corresponding previously reported alcoholcontaining heteroscorpionate ligand precursors 1,1-bis(3,5 dimethyl 1H-pyrazol-1-yl)-3,3-dimethylbutan-2-ol (bpzbeH; L1), (2,2-bis(3,5dimethyl-1H-pyrazol-1-yl)-1-p-tolyl)ethan-1-ol (bpzteH; L2), (2,2 bis(3,5-dimethyl-1H-pyrazol-1-yl)-1-(4-(dimethylamino)phenyl)- 1phenylethan-1-ol (bpzappeH; L3) and the newly synthesized (2,2bis(3,5-dimethyl-1H-pyrazol-1-yl)-1-ferrocenyl)ethan-1-ol (bpzFerrH; L4), with one equivalent of indium trichloride in THF at room temperature (Scheme 3). Ligand L1-L4 were chosen in order to study the influence of an aromatic, a bulkier alkyl group, or a ferrocene moiety on the catalytic activity and selectivity of the indium complexes towards the synthesis of polycarbonates and/or terpolymer materials. Complexes 1-4 were isolated in high yields and showed to be stable in solution and under air conditions in solid state. The structural characterization of these compounds was performed by different spectroscopic techniques. The 1H and 13C{1H}-NMR spectra for complexes 1-4 exhibited two distinct sets of resonances for protons and carbons of both pyrazole rings, indicating they were not equivalent (Fig. 1). This fact was attributed to the presence of a stereogenic centre in the Ca carbon of the ligand L1-L4 precursors. These data indicate the possible existence of four stereoisomers (two diastereoisomers) for indium complexes (Fig. 1). Thus, the VT NMR analysis shows one diastereoisomer at –93 °C in which the two heteroscorpionate ligands are equivalent. NOESY-1D NMR experiments and 1H-13C heteronuclear correlation (g-HSQC) permitted the unequivocal assignment of most of the resonances. Journal Name ARTICLE This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 3 Please do not adjust margins Please do not adjust margins Based on the data obtained, an octahedral disposition for dinuclear indium complexes with the heteroscorpionate ligand in a  3-NNO-µO coordination mode with the ligand bridging the two indium centres can be proposed (Scheme 3). Scheme 3. Synthesis of chloride indium complexes 1-4. Fig. 1. 1H-NMR spectrum of [InCl2{(  3-bpzFerr)(μ-O)}]2 4 in CD3CN. The solid-state structure of these compounds was confirmed by single crystal X-ray diffraction analysis for complex 1. The corresponding ORTEP diagram is represented in Fig. 2. The crystallographic data and selected bond distances and angles are collected in Tables S1 and S2 respectively. The molecular structure revealed a dimeric compound in which each indium centre displays an octahedral geometry with the heteroscorpionate ligands coordinated in a  3-NNO fashion, two chloride atoms disposed in a cis-configuration and the last position of the coordination sphere occupied by the oxygen atom from the alkoxide group of the second heteroscorpionate ligand, which also acts as bridging atom between both indium centres. Bond distances between the indium and the chloride atoms adopt values of 2.407(2) Å and 2.415(1) Å, which are slightly shorter than the ones found for other indium chloride complexes.68,69 Also, bond distances between the indium and nitrogen atoms of 2.267(5) Å for the In(1)−N(1) bond and 2.371(4) Å for the In(1)−N(3) bond, are slightly longer than those found for other indium complexes with scorpionate ligands.68,69 Bond angles confirmed the proposed distorted octahedral structure, with the maximum distortion of 162.2(2)º observed for the O(1A)-In(1)-N(1) angle. ROCOP of Cyclohexene Oxide and CO 2 A first screening to evaluate the catalytic activity of complexes 1-4 was performed for the ROCOP reaction of CHO and CO2. The initial experiments were carried out using 0.5 mol% of catalyst, at 60 °C and 40 bar of CO2 without the use of a co-catalyst under solvent free conditions (Scheme 4), and the results are given in Table 1. Fig. 2. ORTEP diagram for complex [InCl2{(  3-bpzbe)(μ-O)}]2 (1). Hydrogen atoms are omitted. Thermal ellipsoids are shown at 30%. Scheme 4. Synthesis of poly(cyclohexene carbonate) (5) catalyzed by complexes 1-4.a Chloride indium complexes 1 and 3 with tert-butyl groups and dimethylamino substituted aromatic rings respectively, displayed high catalytic activity and excellent selectivity towards the synthesis of PCHC. On the other hand, complex 4 exhibited moderate activity, attributed to electronic and steric effects of the ferrocene moiety. Surprisingly, complex 2, featuring a toluene substituent in the alcohol group, showed no catalytic activity, which was ascribed to the insolubility of the catalyst in CHO. It is worth highlighting that no cocatalyst was used, indicating that chloride indium complexes catalyzed the reaction efficiently themselves. In addition, no formation of polyether or trans-cyclic carbonate was observed in any case, indicating the absence of homopolymerization or back-biting side-reactions. A control experiment using 1 mol% of InCl3 as catalyst under the same reaction conditions was carried out and no catalytic activity was observed in this entry (Table 1, entry 5). Among all the complexes tested, complex 1 was selected as the optimal catalyst to conduct the copolymerization reaction, achieving 77% conversion and a selectivity higher than 99% (Table 1, entry 1). Thus, it was further used for the optimization of the reaction temperature, CO2 pressure and reaction time (See Fig. 3-5 and SI). As it can be seen from Fig. 3, the temperature showed to have a great influence on the catalytic activity of complex 1, which remained almost constant in the range of temperatures 60-100 °C but decreased notably to 25 % when the temperature was decreased to 50 °C. On the other hand, the selectivity of the process remained almost constant for all the range of temperatures tested, decreasing to 95% when the reaction temperature was increased to 100 °C, at which the formation of trans-cyclohexene carbonate by-product is observed. Cp’ Me3,3’ Cp H4,4’ CH Me5,5’ CaH Journal Name ARTICLE This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 4 Please do not adjust margins Please do not adjust margins Table 1. Synthesis of PCHC (5) catalyzed by complexes 1-4. Entry Catalyst Conv. (%)b Carbonate linkages (%)b Polycarbonate selectivity (%)b Mn,exp.c (g/mol) PDIc 1 1 77 >99 >99 5340 1.30 2 2 - - - - - 3 3 72 >99 >99 4930 1.21 4 4 26 >99 >99 1150 1.28 5 InCl3 - - - - - aCopolymerisation conditions: 25 µmol of catalyst; [CHO]:[catalyst] = 200:1, 16 hours, 60 ºC, 40 bar CO2. bDetermined by 1H-NMR spectroscopy of the crude reaction mixture through the comparison of the integrals of signals arising from the methylene protons in the 1H NMR spectra due to copolymer carbonate linkages against copolymer ether linkages and trans-cyclic carbonate. cDetermined by GPC using polystyrene standards in THF. Fig. 3. Effect of the reaction temperature on the catalytic activity of complex 1. The effect of the CO2 pressure was then investigated at the optimal reaction temperature of 60 °C, and the results are shown in Fig. 4. It is worth highlighting that complex 1 showed to be active for the copolymerization process even at one bar CO2 pressure, achieving 47% conversion and a selectivity higher than 99% towards the formation of PCHC (5). The selectivity of the process remained constant for all the range of pressures tested and the catalytic performance of complex 1 increased as the pressure increased, reaching 77% conversion at 40 bar CO2. Fig. 4. Effect of the reaction pressure on the catalytic activity of complex 1. In order to maximize the sustainability of the process and perform the copolymerization under the mildest reaction conditions, the reaction temperature and pressure were set at 60 °C and one bar CO2 pressure, respectively. Then, the effect of the reaction time on the copolymerization process was also studied (Fig. 5), obtaining TOF values ranging from 4 to 11 h−1 for reaction times between 1 and 24 hours (Table 2). As it can be seen, the conversion increased linearly until it reached 42% after 8 hours. Then, the conversion increased much slower reaching 52% after 24 hours, which can be attributed to mass transfer issues, due to the increase of the viscosity of the reaction mixture and the decreased concentration of CHO in the solution. Phosphasalen indium complexes previously described exhibited generally lower TOF values with longer reaction times.66 Similarly, hemisalen H[NNO] indium complexes recently reported also exhibited lower TOF values, using harsher reaction conditions of 30 bar CO2, 80 °C and 24 h.67 Fig. 5. Effect of the reaction time on the catalytic activity of complex 1 at 1 bar CO2. The molecular weights of the polycarbonate materials synthesized increased as the conversion of the copolymerization process increased, obtaining in all cases narrow polydispersity values ranging from 1.15-1.21 (Table 2). Generally, the molecular weights obtained for the polycarbonate materials proved to be approximately five times lower than the theoretical values calculated from the conversion obtained (Table 2), which was indicative of the presence of water or cyclohexenediol acting as chain-transfer agents, and, thus, reducing the molecular weight of the polycarbonates obtained.66,70,71 The polycarbonate materials synthesized were characterized by 1H-NMR (Fig. S9) and 13C-{1H}-NMR spectroscopy (Fig. S10), gel permeation chromatography (GPC) (see Supporting Information) 0 25 77 75 74 0 99 99 99 95 0 20 40 60 80 100 25 50 60 80 100 Conversion or selectivity (%) Temperature (°C) Conversion Selectivity 47 53 61 73 77 99 99 99 99 99 0 20 40 60 80 100 110 20 30 40 Conversion or Selectivity (%) Pressure (bar) Conversion Selectivity 2,5 915 20 32 42 47 52 99 99 99 99 99 99 99 99 0 20 40 60 80 100 1 2 3 4 6 8 16 24 Conversion or Selectivity (%) time (h) Conversion Selectivity Journal Name ARTICLE This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 5 Please do not adjust margins Please do not adjust margins Table 2. Effect of the reaction time on the synthesis of poly(cyclohexene carbonate) (5) catalyzed by complex 1.a Entry t (h) Conv. (%)b Carbonate linkages (%)b TOF (h-1)c Mn,theo.d (g/mol) Mn,exp.e (g/mol) PDIe 1 1 3 >99 4.6 653 - - 2 2 9 >99 8.8 2556 - - 3 3 15 >99 10.0 4260 850 1.21 4 4 20 >99 10.1 5680 1150 1.20 5 6 32 >99 10.7 9088 1800 1.20 6 8 42 >99 10.5 11928 2400 1.18 7 16 47 >99 6.0 13348 2865 1.16 8 24 52 >99 4.3 14768 3100 1.15 9 48 63 >99 2.6 17892 3590 1.19 aCopolymerization conditions: 25 μmol of complex; [CHO]:[1]= 200:1, 16 hours, 60 ºC, one bar CO2. bDetermined by 1H-NMR from the reaction mixture; cTOF = (mol product) / (mol catalyst × time); dMn,theo. = (monomer/initiator) × (% conversion) × (Mw CHC). eDetermined by GPC using polystyrene standards in TH and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-ToF MS) (Fig. 6 and Fig. S11). 13C-{1H}- NMR study allowed to determine the tacticity of the synthesized polycarbonate materials. Thus, an atactic structure was proposed since both signals corresponding to the isotactic and syndiotactic dyads were detected. The MALDI-ToF spectrum for PCHC (5) showed two end-group series of peaks with a m/z interval of 142 mass units, indicating a controlled alternating microstructure (Fig. 5). The major series (blue diamond) is in good agreement with a polymeric chain with one hydroxyl and carbonic acid end-groups in the polycarbonate material. A second major series (green diamond) also containing two hydroxyl end groups corresponding to trans-cyclohexanediol was observed, indicating the existence of chain transfer agents during the copolymerization, as expected due to the low molecular weights obtained for the polycarbonate materials synthesized. Finally, the thermal properties of the polycarbonate material synthesized were determined by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). TGA showed that it is stable in the range of temperatures between 0-200 °C (Fig. S13), while differential scanning calorimetry (DSC) exhibited a Tg value at 65 °C (Fig. S14). This Tg value is lower than others found in the literature for atactic PCHC, attributed to the low molecular weights of the PCHC obtained.72 Terpolymerization of Cyclohexene Oxide, CO2 and L-lactide The versatility of complex 1 was further investigated towards the terpolymerization reaction of CHO, CO2 and L-LA to afford PLA-co-PCHC (6). To the best of our knowledge, only a few examples of zinc catalysts have ever been reported for this process, being complex 1 the first indium complex.73-76 However, some of these catalysts afforded the formation of block copolymers PLA-b-PCHC either by sequential addition methods or by tandem catalysis,75,76 and, in some cases, by different synthetic routes employing cyclohexene carbonate or L-lactide-Ocarboxyanhydride as monomers.74,75 The initial experiments were carried out under solvent-free conditions, at 60 °C and 40 bar CO2 pressure for 16 hours, using [1]:[L-lactide]:[CHO] of 0.5-1.0:100:700 molar ratio (Scheme 5) (the catalysts loading is respect to L-LA monomer), and the results are given in Table 3 (entries 1 and 2). As it can be seen, an increase in the catalyst loading resulted in a higher CO2 incorporation into the polymeric material, producing a higher PCHC content. On the other hand, the ROP of L-LA to afford poly(L-lactide) (PLA) was achieved quantitatively in both scenarios, which is indicative that the ROP of L-lactide is the first step in the terpolymerization process. In addition, no homopolymerization of CHO to produce poly(cyclohexene oxide) (PCHO) was observed in any case. Scheme 5. Synthesis of terpolymers derived from CHO, L-LA and CO2 catalyzed by complex 1. Fig. 6. MALDI-ToF spectrum for PCHC (5) obtained by complex 1 (Table 2, entry 7). Journal Name ARTICLE This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 6 Please do not adjust margins Please do not adjust margins Table 3. Influence of the pressure on the synthesis of terpolymer 6 catalyzed by complex 1.a Entry Pressure (bar) Conv.(%) CHOc Conv. (%) L-LAc PLA (%)c Ether linkages (%)c Carbonate linkages (%)c 1b 40 2.7 100 91 - 9 2 40 17 100 60 - 40 3 30 20 100 54 - 46 4 20 23 100 48 - 52 5 10 27 100 42 - 58 6 5 16 100 60 - 40 7 1 0 100 100 - - aCopolymerization conditions: [CHO]:[L-LA]:[1] = 700:100:1, 16 h, 60 °C; b[CHO]:[L-LA]:[1] = 700:100:0.5, 16 h, 60 °C; cDetermined by 1H-NMR from the reaction mixture. Table 4. Influence of the [CHO]/[L-LA] ratio and thermal properties of the different PLA-PCHC (6) copolymers synthesized.a Entry [CHO]:[L-LA] ratio Conv.(%) CHOb Conv. (%) L-LAb PLA (%)b Ether linkages (%)b Carbonate linkages (%)b Mn, exp.b (kg mol-1) PDIb Tg (°C)c Td,5% (°C)d 1 1:1 0 75 100 - - 5.1 1.7 47 197 2 2:1 18 100 81 - 19 5.8 1.8 55 201 3 3:1 20 100 71 - 29 6.4 1.7 58 200 4 4:1 21 100 64 - 36 7.5 1.7 59 205 5 5:1 24 100 55 - 45 8.3 1.8 63 222 6 7:1 27 100 40 - 60 9.1 1.6 74 244 aCopolymerization conditions: [L-LA]:[1] = 100:1, 16 h, 60 °C, 10 bar; bDetermined by 1H-NMR from the reaction mixture. bDetermined by GPC using polystyrene standards in THF. cDetermined by Different Scanning Calorimetry. dDetermined by Thermal Gravimetric Analysis (TGA). Reported as temperature at 5% mass loss. The effect of the reaction pressure on the terpolymerization process was then investigated using a catalyst loading of both 0.5 (Table S5) and 1.0 mol% (Table 3 and Table S6). The CO2 pressure showed to have a significant effect on this process making it possible to modulate the polycarbonate content in the resulting terpolymer. The same trend was observed in both cases, with the highest PCHC content obtained at 10 bar CO2. The increase of the reaction pressure resulted in a lower PCHC content in the resulting terpolymer, in agreement with previously reported results.77 This can be ascribed to a dilution effect resulting from the increased amount of CO2 in the reactor. Once the [1] and CO2 pressure were optimized to 1 mol% respect to the L-LA monomer and 10 bar respectively, the optimal [CHO]:[L-LA] ratio for the terpolymerization process was investigated and the results are presented in Table 4. When the [CHO]:[L-LA] ratio employed was 1:1, only 75% conversion of L-LA toward the formation of PLA was achieved (Table 4, entry 1) and no formation of polycarbonate was observed, giving hints of a mechanism in which the ROP of L-LA was proposed as the first step of the terpolymerization. The increase in the [CHO]:[L-LA] ratio resulted in the complete consumption of L-LA towards the formation of PLA and the increase of the polycarbonate content in the resulting terpolymer, with the highest PCHC content of 60% when the [CHO]:[L-LA] ratio employed was 7:1 (Table 4, entry 6). The 1H-NMR spectrum (Fig. 7 and S17) of the terpolymer materials synthesized exhibited two different sets of signals with high intensity corresponding to the PLA and the PCHC moieties and low intensity signals corresponding to chain end and junction groups. PLA exhibited a doublet at 1.57 ppm corresponding to the methyl group CHa and a quartet at 5.17 ppm corresponding to the methine proton CHb. The latter is increasingly shifted as the amount of CHO in the reaction mixture increases (Fig. S18), which can be indicative of terpolymers with a statistical morphology. On the other, PCHC moiety exhibited five signals with high intensity corresponding to the methylene and methine groups of the cyclohexyl ring.74-76 13C-{1H}-NMR spectrum (Fig. S19) suggested the formation of a copolymer since different signals in the C=O region were observed, corresponding to both polycarbonate (154.3, 153.8 ppm) and polyester (169.6 ppm) moieties, in agreement with previously reported results.74-76 2D-DOSY NMR spectroscopy confirmed the formation of only one polyester-polycarbonate (PLA-co-PCHC) copolymer, since only one diffusion coefficient was observed for all the signals (Fig. S20). The reactions showed progressively increasing molar masses with the PCHC content in the resulting terpolymer, achieving its maximum value at 9100 g mol-1 (Table 4, Entry 6), corresponding with the highest CO2 incorporation. PDI values remained almost constant in the range of 1.6-1.8 for all the ratios employed (Fig. S21-S23). However, low mass values can be ascribed due to the presence of transesterification reactions. The experimental data support that complex 1 first does a ROP of Llactide followed by polycarbonate formation, although the presence of traces of water or diols in the reaction mixture can make both the initial PLA polymer and the subsequent PCHC chains scramble over time generating copolymers with different structures with low molecular weights. TGA analysis revealed that all terpolymers were stable in the range between 0-200 ºC. ARTICLE Please do not adjust margins Please do not adjust margins Terpolymer with the highest [CHO]:[L-LA] ratio (Table 4, entry 6) showed to be stable up to 244 °C, due to the higher PCHC content. Glass transition temperatures for the different PLA-co-PCHC copolymers exhibit a single value between the one for pure PLA (ca 50 °C, depending on chain length) and pure PCHC (120°C), which also sustains the premise of the formation of statistical copolymers. The Tg also increased as the [CHO]:[L-LA] ratio increased, and thus, with the polycarbonate content in the terpolymer, achieving its maximum value at 74 °C when a 7:1 molar ratio was applied (Fig. 8). Fig. 7. 1H-NMR spectrum for PLA-co-PCHC (6) terpolymer in CDCl3. Fig. 8. DSC traces for terpolymers using conditions described in Table 4. In terms of the reaction mechanism, it is worth highlighting from the results obtained that the terpolymerization process proceeds in a different pathway compared to the previously reported mechanism for β-diiminate or salen-type catalysts (Scheme 6). Different studies showed that the catalytic activity of the metal complex could be switched towards the polymerisation of one monomer or another depending on the CO2 pressure.33,40,78,79 In the presence of high CO2 pressures, only PCHC (5) was generated, which was assigned due to the rapid formation of the metalcarbonate group, which is not able to perform the ROP of the corresponding cyclic ester. However, in this work and as previously commented, the ROP of the L-lactide monomer occurs in the first place regardless the presence and pressure of CO2, and once all the monomer is converted to PLA, the ROCOP of CHO and CO2 takes place. This is indicative of the high affinity of 1 to ringopen the cyclic ester an generate alkoxide species, even in the presence of CO2. In any case, this would lead to the formation of well-defined block copolymers, which does not correlate well with the presumably formation of statistical polymeric materials according to the data from the 1H NMR and DSC spectra. As aforementioned, it could be assumed that, given the transesterification reactions occurring, the initial block structure can be scrambled over time to ultimately form a statistical copolymer. Therefore, although we are yet unsure on the precise mechanism operating with these catalysts, we have already gathered some relevant information that set the basis of our future mechanistic studies. Conclusions New dinuclear chloride indium complexes supported by heteroscorpionate ligands have been synthesized and characterized by different spectroscopic techniques. The solid structure of complex 1 has been confirmed by X-Ray diffraction analysis, showing an octahedral geometry for both indium atoms, with the heteroscorpionate ligand coordinated in a  3-NNO mode, and with the oxygen atom from the alkoxide group acting also as bridging group between both indium centres. Indium complexes 1-4 were tested as catalysts for the copolymerization reaction between CHO and CO2 to afford poly(cyclohexene carbonate) 5 in the absence of a co-catalyst at 60 °C and 40 bar CO2 pressure. Amongst them, complex 1 showed to be the most active, and its catalytic activity was further evaluated at one bar CO2 pressure, affording PCHC in 52% conversion after 16 hours, with 99 % selectivity and TOF values up to 10.7 h-1. Tg = 47 °C Tg = 74 °C Tg = 59 °C [CHO]/[L-LA] 1:1 [CHO]/[L-LA] 4:1 [CHO]/[L-LA] 7:1 e a c d g f b Journal Name ARTICLE This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 8 Please do not adjust margins Please do not adjust margins Scheme 6. Mechanisms for terpolymerisation of epoxides, CO2 and lactones using: a) β-diiminate and salen-type catalysts, b) heteroscorpionate indium 1 complex Compound 1 was also found to be an efficient catalyst for the terpolymerization reaction of CHO, CO2 and L-lactide, obtaining the corresponding polyester-polycarbonate material. Optimization of the reaction conditions was also carried out by performing experiments at different CO2 pressures and CHO:Llactide ratios. Results showed different CO2 incorporation, making it easy to modulate the polycarbonate content in the resulting terpolymer depending on the CO2 pressure applied. Thus, complex 1 represents the first indium complex ever reported for terpolymerization processes, which we hope will broaden the scope of this element within this topic and serve as inspiration for future works. Experimental section Experimental details and spectroscopic and crystallographic data for chloride indium complexes 1-4, poly(cyclohexene carbonate) 5 and terpolymer 6 are provided in the Supporting Information. Representative procedure for the synthesis of [InCl2{(  3bpzbe)(μ-O)}]2 1 is as follows: [InCl2{(κ3-bpzbe)(μ-O)}]2 (1): In a 100 cm3 Schlenk tube, bpzbeH (1.00 g, 3.45 mmol) was dissolved in 30 mL of dry THF and cooled down to -78 °C. Then, a solution of nBuLi (1.6M in hexanes, 2.30 mL, 3.62 mmol) was added dropwise and the mixture was maintained at -78 °C for one hour. After that time, the lithiated adduct was transferred via cannula to a pre-cooled slurry of InCl3 (0.80 g, 3.62 mmol) in THF. The resulting mixture was warmed to room temperature and left stirring overnight. The white solid precipitated was filtered and dried in vacuo for two hours to afford complex 1 in 80% yield. Suitable crystals for X-Ray analysis were obtained from a CH2Cl2 solution at room temperature. 1HNMR (500MHz, CD3CN, 298 K): δ = 6.18 (d, JHH = 2.4 Hz, 1H, CH), 5.98 (s, 1H, H4,4’), 5.92 (s, 1H, H4,4’), 3.78 (d, JHH = 2.3 Hz, 1H, CaH), 2.45, 2.41 (s, 6H, Me3,3’), 2.32 (brs, 6H, Me5,5’), 0.68 (s, 9H, tBu). 13C-{1H}-NMR (125MHz, CD3CN, 298 K): 151.3, 150.5, 141.4, 139.4 (C3,3’, C5,5’), 107.8, 106.6 (C4,4’), 86.0 (Ca), 64.4 (CH), 36.3 (C-tBu), 26.1 (tBu), 13.6 (Me3,3’), 11.7, 11.1 (Me5,5’). Elemental analysis calcd. (%) for C32H50Cl2In2N8O2: C, 43.7; H, 5.7; N, 12.7; found: C, 43.9; H, 5.8; N, 12.5. General procedure for ROCOP of CHO-CO2 at high CO2 pressure Cyclohexene oxide (0.50 g, 5.09 mmol) and catalysts 1-4 (25.18 µmol) were placed into a stainless-steel reactor equipped with a magnetic stirrer bar in the glovebox. The autoclave was sealed, pressurized to 5 bar with CO2, heated to the desired temperature and then pressurized to 10-40 bar with CO2. The reaction mixture was subsequently stirred at 25–100 °C for 16 h. The conversion of cyclohexene oxide into poly(cyclohexene carbonate) 5 was determined by analysis of the crude reaction mixture by 1H-NMR spectroscopy. Polymers were isolated by precipitation using MeOH to yield white powders. The solid was filtered and dried to constant weight. General procedure for ROCOP of CHO-CO2 at 1 bar CO2 pressure In the glovebox, chloride indium complex 1 (25.18 µmol) was dissolved in cyclohexene oxide (0.50 g, 5.09 mmol) under N2 atmosphere in a Schlenk flask equipped with a stirring bar. The Schlenk flask was sealed, brought outside the glovebox and connected into a vacuum-CO2 line. The reaction mixture was then purged with three cycles of vacuum-CO2 and heated at 60 °C under one bar of CO2 pressure, for the desired time (2-24h). The conversion of cyclohexene oxide into poly(cyclohexene carbonate) 5 was determined by analysis of the crude reaction mixture by 1H-NMR spectroscopy. Polymers were isolated by precipitation using MeOH to yield white powders. The solid was filtered and dried to constant weight. General procedure for the ROCOP of CHO, L-LA and CO2 Journal Name ARTICLE This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 9 Please do not adjust margins Please do not adjust margins In the glovebox, complex 1 (25.18-50.09 μmol), cyclohexene oxide (5.09-35.63 mmol) and L-lactide (5.09 mmol) were placed into a stainless-steel reactor with a magnetic stirrer bar. The autoclave was sealed, pressurized to 5 bar with CO2, heated to the desired temperature and then pressurized to 5-40 bar with CO2. The reaction mixture was subsequently stirred at 60 °C for 16 h. Then, the conversion and the selectivity were determined by NMR. The viscous mixture was then dissolved in the minimum amount of dichloromethane and precipitated with an excess of methanol. The polymer was filtered and dried to afford 6 as white powder. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Conflicts of interest There are no conflicts to declare. Acknowledgements We gratefully acknowledge the financial support; grant PID2020-117788RB-I00 funded by MCIN/AEI/ 10.13039/501100011033, grant RED2018-102387-T funded by MCIN/AEI/ 10.13039/501100011033, grant SBPLY/21/180501/000132 funded by Junta de Comunidades de Castilla-La Mancha and by the EU through “Fondo Europeo de Desarollo Regional” (FEDER), and grant 2021GRIN-31240 funded by Universidad de Castilla-La Mancha. Marc Martínez de Sarasa Buchaca acknowledges the Universidad de Castilla-La Mancha (UCLM) for the PhD Fellowship. Felipe de la Cruz-Martínez acknowledges the Ministerio de Educación, Cultura y Deporte (MECD) for the FPU Fellowship. Notes and references 1 Z. Sun, B. Fridrich, A. de Santi, S. Elangovan and K. Barta, Chem. Rev., 2018, 118 , 614–678. 2 C. Zhang, T. F. Garrison, S. A. Madbouly and M. R. Kessler, Prog. Polym. Sci., 2017, 71 , 91–143. 3 V. Froidevaux, C. Negrell, S. Caillol, J. P. Pascault and B. Boutevin, Chem. Rev., 2016, 116 , 14181–14224. 4 T. 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