Unity Makes Strength: Constructing Polymeric Catalyst for Selective Synthesis of CO ₂ /Epoxide Copolymer

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE17 May 2022Unity Makes Strength: Constructing Polymeric Catalyst for Selective Synthesis of CO2/Epoxide Copolymer Ruoyu Zhang, Qingxian Kuang, Han Cao, Shunjie Liu, Xuesi Chen, Xianhong Wang and Fosong Wang Ruoyu Zhang Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022 University of Science and Technology of China, Hefei 230026 , Qingxian Kuang Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022 University of Science and Technology of China, Hefei 230026 , Han Cao Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022 , Shunjie Liu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022 University of Science and Technology of China, Hefei 230026 , Xuesi Chen Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022 University of Science and Technology of China, Hefei 230026 , Xianhong Wang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022 University of Science and Technology of China, Hefei 230026 and Fosong Wang Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022 University of Science and Technology of China, Hefei 230026 https://doi.org/10.31635/ccschem.022.202201952 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Catalyst design strategies such as bi-functional and di-nuclear catalysts have been developed based on intramolecular interactions, achieving excellent catalytic performance. However, most of these catalysts work in a state of disunity. To make progress in this direction, we reckoned that enhancing the neglected intermolecular interactions of these catalysts might be a suitable approach. Herein, we report a strategy of constructing homogeneous polymeric catalysts based on the philosophy of "unity makes strength" to convert the intermolecular interactions into stronger intramolecular interactions. We united discrete active centers of aluminum (Al) porphyrin and tertiary amine (methyl methacrylate; MMA) via a random copolymerization process into one polymer chain with the subsequent metallization using low-toxic metal AlEt2Cl, to construct polymeric catalysts for selective copolymerization of CO2/epoxide. The spatial confinement enabled the multiple interactions among the active centers, which was distinct from the "point-to-point" interacting systems such as binary, bi-functional, or di-nuclear complexes. Through detailed tuning of the multiple synergistic effects between porphyrin/porphyrin (metal synergistic effect) and Al porphyrin/tertiary amine (Lewis pair effect), the optimized polymeric catalyst showed significantly boosted catalytic activity of 4300 h−1, much higher than their mono-nuclear (∼0 h−1) and homo-polymeric (750 h−1) counterparts. Our present approach for designing polymeric catalysts based on multiple synergistic effects provides a platform for developing highly active catalysts. Download figure Download PowerPoint Introduction The quest for an ideal catalyst with high reactivity, selectivity, and longevity has long provided momentum for breakthroughs in the synthesis of high-value-added chemical products. The catalytic performance of these catalysts could be tailored by tuning the catalytic structure. For example, regulating the electronic structure and the coordinative environment of the Ti3+ and Al3+ sites on the Ziegler–Natta catalyst could regulate the catalytic performance effectively, as well as the microstructures of the resulting polyolefins.1 In comparison with the insoluble attribute of the industrially favorable heterogeneous catalyst,2 the homogeneous counterparts with well-defined architectures are more suitable for unveiling structure-function relationships,3 which advance the prosperity and development of catalysis in coordination polymerizations,4 ring-opening polymerizations,5–7 and others. Currently, various bifunctional8–10 and dinuclear11–14 catalytic systems have been advanced to boost overall catalytic efficiencies. For example, in CO2/epoxide copolymerization,15 the bi-functional systems based on the direct attachment of metal complexes with nucleophilic moieties (steric hindrance organic base16–18 or quaternary ammonium19–23) significantly boosted the reaction rate, as well as the polymer selectivity.24–27 Similarly, the di-nuclear (homo-nuclear28,29 or hetero-nuclear30–33) catalysts achieved desirable enhancement in catalytic performance. These ingenious strategies achieved excellent catalytic performance and opened up new avenues for catalyst design. However, most of these catalysts work in a state of disunity. To make progress in this direction, we considered that enhancing the neglected intermolecular interactions in these systems might be a promising approach. Besides, the regulation of these conventional catalysts is mainly based on tuning a substitution group (small molecule level) of the organic ligand.34 Therefore, it is presumed interesting to construct an efficient yet simple catalyst design employing an intermolecular interactions strategy for a much higher catalytic performance. Herein, we report a strategy for the construction of homogeneous polymeric catalysts based on the philosophy of "unity makes strength" by converting intermolecular interactions into stronger intramolecular interactions. The polymeric catalysts were fabricated via a copolymerization process involving a reaction between a single-site Al porphyrin catalyst, tertiary amine co-catalyst and methyl methacrylate (MMA), with subsequent metallization using low-toxic metal AlEt2Cl (Figure 1), which were then utilized for the selective catalysis of epoxide copolymers (Figure 2a). By uniting the discrete active centers into one polymer chain, the spatial confinement effect enabled multiple intramolecular interactions among the active centers, distinct from the conventional "point-to-point" interacting systems such as binary, bi-functional, and di-nuclear complexes. The homogeneous characteristics of the polymeric catalysts prompted us to investigate structure-function relationships further. Precisely, the constructed polymeric catalyst was used for selective copolymerization of CO2/epoxide (Figure 2a). The key point in the design was the random distribution of the functional units on the side chain to facilitate intramolecular interactions (Figure 2b). Through this strategy, a combination of cooperative interactions between porphyrin/porphyrin (metal synergistic effect) and Al porphyrin/tertiary amine (Lewis pair effect) (Figure 2c) could be formed in the CO2/epoxide copolymerization. As most catalysts are proposed to enchain through the bi-metallic35–37 or bi-component38–40 mechanism, these synergistic effects could boost the catalytic activity without using an additional co-catalyst. Notably, the catalytic performance could be tailored by regulating the structural units (polymer level) of the catalyst, which was much simpler than tuning the substitution group (small molecule level) of the organic ligand. Due to the metal synergistic effect of the adjacent metal centers, the activity (turnover frequency; TOF) of homo-polymeric Al porphyrin catalyst could be enhanced from ∼0 h−1 (for mono-nuclear catalyst) to 750 h−1. This effect was further demonstrated through enlarging the spatial distance of the Al porphyrin centers, as the TOF decreased slightly from 750 to 700 h−1. The UV–vis and 1H NMR spectra indicated that the H-aggregation of the Al porphyrin units was beneficial for the metal synergistic effect and increased catalytic efficiency. Simultaneously, the Lewis pair interaction was optimized through the variation of porphyrin:amine, ranging from 1:0, 1:0.9, 1:1.6, to 1:2.8, resulting in the TOF changing from 700, 3200, 4300, and to 2000 h−1. Besides, a sterically hindered structure of the tertiary amine on the catalyst restricted the Lewis pair effect and reduced the activity from 3200 h−1 (with dimethylaminoethyl group) to 180 h−1 (with bulky dibutylaminoethyl group). Through structural engineering, the optimized catalyst exhibited a high activity of 4300 h−1 at diluted condition, much better than the control groups (0 h−1 for mono-nuclear and 750 h−1 for homo-polymeric catalyst). Our present study proposes a new catalyst constructing strategy based on a polymeric catalyst and provides a platform for designing highly efficient catalysts. Figure 1 | Schematic illustration of the conventional catalyst system (binary, di-nuclear, and bi-functional catalyst) and the proposed polymeric catalysts design strategy. Download figure Download PowerPoint Experimental Methods Polymeric aluminum porphyrin catalysts, Poly-TPPAl-N, were synthesized via reversible addition-fragmentation chain transfer (RAFT) copolymerization of methacrylic-based porphyrin (monomer 1), 2-(dimethylamino)-ethyl methacrylate (DMAEMA), and MMA, followed by metallization using low-toxic metal AlEt2Cl ( Supporting Infor>mation Scheme S1). The DMAEMA was introduced as an organic base to form Lewis pair interaction with the Al porphyrin, while MMA without an electron-donating group was utilized as a blank control unit to regulate the spatial distance of the porphyrin units. A mono-nuclear Al porphyrin (mono-TPPAl) and homo-polymeric Al porphyrin catalyst (CAT1; Figure 2b) were prepared as a control to study the interactions between the Al porphyrin units. To optimize the catalytic performance, the structures of polymeric porphyrin ligands were tailored by regulating the molar ratios of porphyrin:DMAEMA ranging from 1:0, 1:0.9, 1:1.6 to 1:2.8, while keeping a nearly constant porphyrin unit:(DMAEMA+MMA) as 1:2.8 (CAT2 to CAT5; Figure 2b). The number of Al centers on the catalyst was generally controlled by maintaining the molar ratio of porphyrin:RAFT agent at 10:1 ( Supporting Information Scheme S1). The chemical structures of polymeric porphyrin catalysts and the associated intermediates were confirmed by 1H NMR and gel permeation chromatography (GPC) ( Supporting Information Table S1 and Figures S1–S12). (Further details about these procedures can be found in the Supporting Information.) Figure 2 | (a) Synthetic route for CO2/epoxide copolymerization afforded by poly(carbonate-ether); (b) the general structure of monomeric (mono-TPPAl) and polymeric (poly-TPPAl-N) aluminum porphyrin catalysts. Note: in poly-TPPAl-N, the m:n:p represents the molar ratio of porphyrin unit:DMAEMA: MMA; (c) detailed schematic illustration of the different roles of metal synergistic effect and Lewis pair effect in CO2/PO copolymerization. Download figure Download PowerPoint Results and Discussion Metal synergistic effect All the as-prepared aluminum porphyrin complexes were applied as homogeneous catalysts, and the results of CO2/propylene oxide (PO) copolymerization catalyzed by these catalysts are shown in Table 1. The copolymerization products could be characterized as poly(carbonate-ether) with randomly distributed oligoether units along the backbone through 1H NMR, GPC, and matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) tests ( Supporting Information Figures S13–S15). For example, poly(carbonate-ether) with a molecular weight (Mn) of 68.2 kg/mol was hydrolyzed into oligoethers with Mn of 2.2 kg/mol ( Supporting Information Figure S16). First, a metal synergistic effect was observed on polymeric Al porphyrin catalysts (entries 1–3, Table 1). At the PO:[Al] molar ratio of 20,000:1, the mono-nuclear Al porphyrin catalyst showed negligible catalytic activity without detecting any polymer product at 80 °C and 4 MPa for 3 h (entry 1, Table 1). Once the mono-nuclear Al porphyrin polymerized, the resulting homo-polymeric catalyst CAT1 showed significantly enhanced activity with a TOF of 750 h−1 under similar conditions (entry 2, Table 1), indicating a positive metal synergistic effect had occurred due to the spatial proximity of the porphyrin unit. To further tune this metal synergistic effect, an MMA unit was introduced into the polymeric catalyst to regulate the spatial distance of the porphyrin unit. The resulting CAT2 with porphyrin:MMA of 1:2.8 showed a slight decrease in TOF of 700 h−1 (entry 3, Table 1). In addition, the difference between CAT1 and CAT2 was more evident at high catalyst loading (PO:[Al] = 5000:1) with the respective TOFs of 840 and 500 h−1 ( Supporting Information Table S2). These results suggested that enlarging the porphyrin distance restricted the metal synergistic effect, with a resultant decrease in the catalytic activity. Meanwhile, compared with the reported mono-nuclear Al porphyrin without co-catalyst,41,42 the polymeric catalyst displayed a higher polymer selectivity (97% vs 92%) and a slightly enhanced carbonate unit content (CU, ∼40% vs 20%). The high polymer selectivity was due to the stabilization of the growing polymer chain, which restricted the backbiting of the carbonate chain end to form cyclic carbonate by-products.43 Meanwhile, the low carbonate unit content resulted from an acceleration of the ring-opening step of epoxides rather than CO2 insertion by the metal synergistic effect.44 Table 1 | CO2/PO Copolymerization Catalyzed by Monomeric Catalyst (Mono-TPPAl) and Polymeric Catalyst (Poly-TPPAl-N)a Entry Catalyst P(CO2) (MPa) T (°C) PO/[Al]b t (h) Conv. of PO (%) TOFc (h−1) Polymer Selectivityd (%) CUe (%) Mnf (kg mol−1) PDIf 1 Mono-TPPAl 4 80 20,000 3 — — — — — — 2 CAT1 4 80 20,000 3 11.3 750 97 28 10.8 1.23 3 CAT2 4 80 20,000 3 10.4 700 98 32.3 7.5 2.14 4 CAT3 4 80 20,000 3 47.3 3200 >99 41.6 54.5 1.15 5 CAT4 4 80 20,000 3 64.2 4300 >99 38.2 68.2 1.13 6 CAT5 4 80 20,000 3 30.0 2000 99 29.7 37.0 1.33 7 CAT4 2 80 20,000 3 27.4 1800 >99 29.6 61.4 1.17 8 CAT4 3 80 20,000 3 45.6 3000 >99 38.9 83.7 1.14 9 CAT4 5 80 20,000 3 63.4 4200 >99 40.4 48.1 1.32 10 CAT4 4 120 100,000 2 10.1 5000 >99 36.3 28.8 1.30 11 CAT4 6 60 50,000 36 88.5 1200 >99 32.7 232.3 1.31 aThe copolymerization reaction was carried out in neat PO. For entries 1–9, 4 mL of PO was used, while for entries 10 and 11, 20 mL of PO was used. bMolar ratio of PO to Al center of catalyst. cDetermined by 1H NMR spectroscopy. The TOF represents the conversion of PO to products including both polymer and cyclic carbonate based on [Al] centers, TOF = ([PO] − [PO]0)/([Al]*t) = ([PO]0*conv. of PO)/([Al]*t) = ([PO]0/[Al])*(A5.0–4.7 ppm+A4.3–3.9 ppm+A4.5 ppm+A3.7–3.3 ppm)/[(A5.0–4.7 ppm+A4.3–3.9 ppm+A4.5 ppm+A3.7–3.3 ppm+A2.9–2.2 ppm)*t]. dSelectivity for polymer over cPC, polymer selectivity = [102*(A5.0–4.7 ppm+A4.3–3.9 ppm−2*A4.5 ppm)+58*A3.7–3.3 ppm]/[102*(A5.0–4.7 ppm+A4.3–3.9 ppm+A4.5 ppm)+58*A3.7–3.3 ppm]. eThe content of carbonate unit, CU = (A5.0–4.7 ppm+A4.3–3.9 ppm−2*A4.5 ppm)/(A5.0–4.7 ppm+A4.3–3.9 ppm+A3.7–3.3 ppm−2*A4.5 ppm). fThe polydispersity index (PDI) was determined by gel permeation chromatography in CH2Cl2 at 35 °C calibrated against polystyrene standards. To demonstrate the influence of polymeric catalyst structure on the metal synergistic effect, we determined the 1H NMR and UV–vis properties of the polymeric porphyrin ligands. In 1H NMR spectra, the chemical shift of hydrogens in the ring-shaped porphyrin could partly reflect the electron density of the Al center. As shown in Figure 3a, the proton peaks up-shifted from −2.8 to −3.2 and then to −3.6 ppm, corresponding to mono-nuclear porphyrin, porphyrin-MMA copolymer, and porphyrin homo-polymer, respectively. These results indicated an increase in electron density of ring hydrogen with spatial proximity of porphyrin unit, which could affect the Lewis acidity of the subsequent Al complex ( Supporting Information Figure S17). The UV–vis experiments were carried out to examine the intermolecular interactions of the porphyrin units. As displayed in Figure 3b, the polymeric porphyrin showed a blue-shifted Soret band (406 nm) than that of the mono-nuclear counterpart (419 nm). Similar to the 1H NMR results, the Soret band gradually widened towards 406 nm with an increased porphyrin content ( Supporting Information Figure S18). This blue shift in the Soret band indicated an H-aggregation45 of porphyrin units owing to their strong intermolecular π–π interaction46,47 ( Supporting Information Figure S19). Collectively, the polymerization process ensured a valid way to enhance the synergistic effect of the porphyrin unit through spatial confinement by decreasing the Lewis acidity of the Al center because of the improved electron-donating ability of polymeric porphyrin units. It is for this reason that the growing chain dissociated readily from the Al active center and induced a nucleophilic attack on another activated epoxide,48 giving polymers with high ether content (∼60%) and polymer selectivity (>97%). Despite the introduction of MMA units restricting the formation of H-aggregate, the activity of CAT2 (TOF of 700 h−1) was still higher than that of mono-TPPAl (TOF of ∼0 h−1). These data indicated that besides the tight H-aggregation, the loose spatial closeness of the porphyrin units was also beneficial for activity enhancement (Figure 3c and Supporting Information Figure S20). Figure 3 | (a) 1H NMR spectra of the ring hydrogen on monomeric porphyrin (4BrTPP) and polymeric porphyrin ligands with varied ratios of porphyrin:MMA, the proton peaks up-shifted from −2.8 to −3.2 and then to −3.6 ppm, which correspond to mono-nuclear porphyrin, porphyrin-MMA copolymer, and porphyrin homo-polymer, respectively; (b) The UV–vis spectra of porphyrin polymer ligands in chloroform, the polymeric porphyrin showed a blue-shifted Soret band (406 nm) than that of the mono-nuclear counterpart (419 nm); (c) Schematic illustration of the metal synergistic effect including tight H-aggregate and loose spatial closeness between porphyrin units. Download figure Download PowerPoint Lewis pair effect By introducing the electron-donating unit, DMAEMA, into the backbone of the polymeric catalyst, a Lewis pair effect was formed. To optimize this effect, we varied the molar ratio of porphyrin:DMAEMA, as follows: 1:0.9, 1:1.6, and 1:2.8. As displayed in entries 3–6, Table 1, the DMAEMA unit could significantly boost the catalytic activity from 700 to 4300 h−1, indicating a positive effect of the naturally formed Lewis pairs between Al centers (Lewis acid) and amines (Lewis base), owing to the steric constraint of the catalyst backbone. By varying the porphyrin:DMAEMA (1:0, 1:0.9, 1:1.6, and 1:2.8), changes in the TOF were noted (700, 3200, 4300, and 2000 h−1), respectively (Figure 4a), which could be related mainly to a combination of metal synergistic effect and Lewis pair effect. The best catalytic activity (TOF of 4300 h−1) was achieved by a porphyrin:DMAEMA of 1:1.6 (CAT4), which could be related to the Lewis pairs interaction of Al center with two molecules of DMAEMA on both sides of the porphyrin plane.49 Besides, the polymeric organic base, PDMAEMA ( Supporting Information Figures S21 and S22), was utilized as an external co-catalyst to study the intermolecular Lewis pair effect between Al and the organic base on different polymer chains. Supporting Information Table S3 showed that no enhancement of the activity of CAT2 was observed (from 700 to 600 h−1) with the molar ratio of the DMAEMA unit:[Al] = 10:1. This result proved that the Lewis pair formed on the same chain of the poly-TPPAl-N structure was a crucial factor in achieving high activity. By introducing an organic base, the Lewis pair effect on the polymeric catalyst could further increase the catalytic activity in combination with the metal synergistic effect. Figure 4 | (a) Plots of TOF of different catalysts, mono-TPPAl shows negligible catalytic activity while polymeric catalysts (from CAT2 to CAT5) boost the reaction efficiency. The best catalytic activity was attained at a TOF of 4300 h−1, and originated from porphyrin:DMAEMA of 1:1.6 (CAT4); (b) plots of TOF versus CO2 pressure using CAT4, which has the organic base on the side chain of catalyst (red) and CAT2 without organic base (blue); (c) three-dimensional stack plot of the IR spectra collected every 1 min during the reaction of CO2 and PO with CAT4; (d) plots of initial rate versus CO2 pressure. Reaction conditions: PO:[Al] = 25000:1 (molar ratio), 80 °C. Download figure Download PowerPoint To investigate the effect of the introduced organic base (dimethylaminoethyl group), (Ph3P=N=PPh3)+Cl− (PPNCl) was added as a co-catalyst to CAT2-CAT5 (the molar ratio of porphyrin:DMAEMA ranging from 1:0 to 1:2.8). The PPNCl:[Al] ratio was increased from 0:1 to 0.5:1, then to 1:1, as excess PPNCl might cause undesirable side reactions (cyclic product formation). The results showed that PPNCl could boost the activity of CAT2 (without the dimethylaminoethyl group) from 940 to 3800 then to 3900 h−1 with an increment of the molar ratio of PPNCl:[Al] (entries 1–3, Supporting Information Table S4). However, the variations of TOF value in CAT3 ([Al]:base = 1:0.9), CAT4 ([Al]:base = 1:1.6), and CAT5 ([Al]:base = 1:2.8) under different molar ratios of PPNCl:[Al] (from 0:1 to 0.5:1, then to 1:1) were 1700, 2800, 3400 h−1; 3800, 4000, 4200 h−1; 3200, 3100, 3200 h−1; respectively (entries 4–12, Supporting Information Table S4 and Figure S23), indicating that the effect of PPNCl on the enhancement of activity decreased with an increased dimethylaminoethyl group in the polymeric catalyst. The results preliminarily demonstrated that the dimethylaminoethyl group might act as a nucleophile for the ring-opening of PO. To further determine whether the dimethylaminoethyl group could nucleophilic attack the epoxide and initiate the copolymerization, a monomeric DMAEMA molecule (10 equiv) was added as a co-catalyst for CAT2 fabrication. The data in entry 13, Supporting Information Table S4, demonstrated that the activity rose from 940 to 2700 h−1 with the DMAEMA/[Al] ratio of 10/1, but the effect was weaker than that of anchored amine, as demonstrated by CAT4 (TOF increased to 3800 h−1, entry 7, Supporting Information Table S4). Similar to PPNCl, the addition of DMAEMA caused a slight enhancement of activity for CAT3 (from 1700 to 1800 h−1) and CAT4 (from 3800 to 4300 h−1) and even resulted in a decreased activity for CAT5 (from 3200 to 1500 h−1) with an anchored organic base (entries 4, 7, 10, and 14–16, Supporting Information Table S4). These data demonstrated that the anchored dimethylaminoethyl group could form Lewis pair with an Al center more efficiently than the free DMAEMA molecule. Notably, further increase in DMAEMA (DMAEMA/[Al] = 1000/1) to negligible even 3 h (entry Supporting Information Table S4). the reaction to 36 h resulted in the conversion of PO by (entry Supporting Information Table S4). the 1H NMR showed a chemical for compared with DMAEMA ( Supporting Information Figure indicating that the amine group in the copolymerization. Besides, DMAEMA without the Al porphyrin catalyst, could initiate the copolymerization process but cyclic carbonate and (entry Supporting Information Table S4 and Figure the from these data was that the dimethylaminoethyl group as a nucleophile for the ring-opening of PO and the we carried out a control using the sterically hindered dibutylaminoethyl group to investigate the coordination between the Al center and the dimethylaminoethyl group ( Supporting Information Figures and As shown in Supporting Information Table in comparison with with sterically hindered amine group showed significantly decreased catalytic activity from 3200 to 180 h−1, that the dibutylaminoethyl group with steric hindrance could with the Al center. This result demonstrated the of coordination between the dimethylaminoethyl group and the Al center to enhance the polymerization to demonstrate the CO2 ability of the dimethylaminoethyl we utilized the to study the of CO2 at with PO the of DMAEMA in PO displayed negligible variation in the IR ( Supporting Information Figure These data suggested that the of CO2 could be enhanced by introducing an organic Collectively, the proposed catalytic of the of polymeric catalyst and subsequent selective catalysis of CO2/epoxide copolymerization is displayed in Scheme Through copolymerization of Al porphyrin and organic base into polymeric catalysts, a combination of metal synergistic effect and Lewis pair effect was In the the metal synergistic effect could boost the initial rate of CO2/epoxide copolymerization through the two proximity metal In this one Al center could epoxide for the nucleophilic attack by the group on another Al center to initiate copolymerization. For the Lewis pair effect, the anchored organic base boosted the step by a direct nucleophilic attack of the activated epoxide or with the Al center to as the the H-aggregation of Al porphyrin (metal synergistic effect) and the coordination of the Al center with the organic base (Lewis pair effect) both reduced the Lewis acidity of the Al center to the nucleophilic attack of the chain Therefore, through the polymeric catalyst construction strategy, the metal synergistic effect and Lewis pair effect could efficiently the catalytic performance. Scheme 1 | The proposed catalytic of catalysis by a polymeric catalyst during CO2/epoxide copolymerization. Download figure Download PowerPoint Notably, an interesting of was observed in that the activity of CAT4 was boosted with increased CO2 pressure. to the results of the on CO2 indicated that high CO2 pressure was to activity. In this CAT4 showed a distinct in that the TOF increased from 1800 to 4200 h−1 with increased CO2 pressure from 2 to 5 MPa (entries in Table 1). To further that this originated from the introduction of tertiary a control using CAT2 without an organic base anchored on the side chain was As shown in Figure the TOF of CAT2 decreased from to h−1 with increased pressure from 2 to 5 MPa (entries 1–3, Supporting Information Table These results indicated that the organic were the key for the activity at high pressure. To study the effect of was into the reaction and the pressure was as 4 The TOF increased from to h−1 with an increase of CO2 pressure from 1 to 3 MPa (entries 4 and Supporting Information Table which proved the of CO2 in the CO2/epoxide copolymerization For CO2/PO copolymerization under varied pressure was using in IR (Figure the structure of the resulting the initial copolymerization rate could be from the variation of the IR at

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