Authors: Yajun Zhao | Shuaishuai Zhu | Xiaojing Li | Xiaoyu Zhao | Jing Xu | Bijin Xiong | Yong Wang | Xingping Zhou | Xiaolin Xie
Source: CCS Chemistry (article, 2021-02-10)
Field: Chemistry — Organic Chemistry

Aluminum Porphyrin Complex Mediated Auto-Tandem Catalysis for One-Pot Synthesis of Block Copolymers Open AccessCCS ChemistryCOMMUNICATION1 Jan 2022Aluminum Porphyrin Complex Mediated Auto-Tandem Catalysis for One-Pot Synthesis of Block Copolymers Yajun Zhao, Shuaishuai Zhu, Xiaojing Li, Xiaoyu Zhao, Jing Xu, Bijin Xiong, Yong Wang, Xingping Zhou and Xiaolin Xie Yajun Zhao Key Lab for Material Chemistry of Energy Conversion and Storage, Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074 , Shuaishuai Zhu Key Lab for Material Chemistry of Energy Conversion and Storage, Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074 , Xiaojing Li Key Lab for Material Chemistry of Energy Conversion and Storage, Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074 , Xiaoyu Zhao Key Lab for Material Chemistry of Energy Conversion and Storage, Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074 , Jing Xu College of Chemistry and Material Science, Shandong Agricultural University, Taian 271018 , Bijin Xiong Key Lab for Material Chemistry of Energy Conversion and Storage, Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074 , Yong Wang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Lab for Material Chemistry of Energy Conversion and Storage, Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074 , Xingping Zhou *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Lab for Material Chemistry of Energy Conversion and Storage, Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074 and Xiaolin Xie Key Lab for Material Chemistry of Energy Conversion and Storage, Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074 https://doi.org/10.31635/ccschem.021.202000607 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Auto-tandem catalysis that uses a single catalyst to bridge and discriminate different catalytic cycles in a one-pot process is highly desirable for obtaining a high degree of structural complexity; however, it is a great challenge to develop auto-tandem catalytic systems in polymer chemistry. Herein, we report the auto-tandem catalysis by rationally designed aluminum porphyrin complexes, wherein well-controlled photoinduced electron/energy transfer–reversible addition-fragmentation chain transfer (PET-RAFT) polymerization of vinyl monomers and completely alternating ring-opening copolymerization (ROCOP) of epoxides/anhydrides can occur in a concurrent or sequential manner. With a carboxylic group incorporated trithiocarbonate compound bearing a carboxylic acid group (TTC-COOH) as the bifunctional chain transfer agent (CTA), the auto-tandem catalysis provides one-pot access to diblock copolymers with predictable molecular weights and narrow distributions. Notably, the efficient electron/energy transfer from 5,10,15,20-tetrakis(2-chlorophenyl)porphyrin aluminum(III) chloride [(TPP2-Cl)AlIII-Cl] to TTC-COOH and their axial group exchange reactions completely circumvent the formations of undesirable homopolymers. Download figure Download PowerPoint Introduction Tandem catalysis is an intriguing strategy evolved from biological systems, wherein mechanistically distinct catalytic cycles proceed via a precisely programed manner in a single reaction vessel without interfering with each other.1 As compared with traditional catalytic transformations, tandem catalysis involves no purification and isolation of intermediates, reduces both workup and waste, and may be thermodynamically favorable by coupling multiple catalytic cycles.2–5 By virtue of such merits, tandem catalysis is of special interest for one-pot or one-step synthesis of molecules with ever-increasing sophistication.6–9 Typically, tandem catalysis falls into three categories: orthogonal, assisted, and auto-tandem (Figure 1). Among them, orthogonal catalysis demands a different catalyst in each of the catalytic cycles. While auto-tandem catalysis and assisted tandem catalysis can make multiple use of a single catalyst, an intervention is required to trigger the switch of catalytic cycles for the assisted tandem catalysis strategy.10,11 Since 2005, significant achievements in orthogonal and assisted tandem catalysis have occurred.12–15 Nevertheless, the development of auto-tandem catalysis systems is a significant challenge and remains much less exploited. On the one hand, there are not many catalysts capable of mediating two distinct catalytic cycles. On the other hand, harsh requirements involving the compatibility between the reaction mechanisms, the tolerance among the reagents, and the match of reaction conditions should all be met for a one-pot catalytic process.16,17 Figure 1 | Flowchart of tandem catalysis in one-pot process. Download figure Download PowerPoint For fine-tuning the material properties at the molecular scale, a central focus in modern polymer chemistry is the rational design and precise synthesis of block copolymers.18–21 Due to their attractive phase separation behaviors, which promise a wide scope of applications, block copolymers incorporated with mechanistically incompatible monomers are of special research interest.22–27 It can be envisioned that tandem catalysis permits exceptional efficiency in the synthesis of block copolymers from monomer mixtures by integrating different polymerization cycles in a one-pot process. However, most of the research efforts in such attempts are dependent on orthogonal tandem catalysis.28–33 Recently, our group reported one-pot synthetic protocols based on the switchable catalysis of cobalt salcy [bis(salicylaldimine)] complexes involving ring-opening copolymerization (ROCOP) of epoxides, CO2, and anhydrides and organometallic-mediated radical polymerization (OMRP) using oxygen or carbon monoxide as external stimuli, which can be classified as assisted tandem catalysis.34,35 To the best of our knowledge, the only example utilizing auto-tandem catalysis to regulate block sequence was contributed by Grubbs et al.,36 wherein a multifunctional ruthenium complex could concurrently or sequentially mediate ring-opening metathesis polymerization (ROMP) and atom transfer radical polymerization (ATRP). Since the seminal work by Boyer's group,37–46 divalent metalloporphyrins such as Zinc (II) mesotetraphenylporphine [(TPP)ZnII] and chlorophyll (TPP)MgII have been widely used as photocatalysts for photoinduced electron/energy transfer–reversible addition-fragmentation chain transfer (PET-RAFT) polymerization, which allow for temporal and spatial control over the chain propagation processes. On the other hand, metalloporphyrins with trivalent metal centers (TPP)MIII-X (M = Co, Al, and Cr; X = halogen, carboxylate, etc.) display a well-controlled manner of ROCOP of epoxides, CO2, and anhydrides, wherein the ancillary X acts as an initiator and is essential for the copolymerization.6,44,47–52 Using 5,10,15,20-tetrakis(2-chlorophenyl)porphyrin aluminum(III) chloride [(TPP2-Cl)AlIII-Cl] as the catalyst and a trithiocarbonate compound bearing a carboxylic acid group (TTC-COOH) as the bifunctional chain transfer agent (CTA), our group successfully demonstrated the concurrent ROCOP of epoxides/CO2 and thermal induced RAFT polymerization of vinyl monomers for the one-step synthesis of well-defined CO2-based block copolymers. However, when azodiisobutyronitrile (AIBN) was employed as the exogenous radical resource, it is difficult to precisely manipulate the block compositions.53 For the first time, we report that (TPP)AlIII-X can serve as highly efficient and selective photoredox catalysts for PET-RAFT polymerization. Furthermore, the (TPP2-Cl)AlIII-Cl auto-tandem catalysis involving ROCOP of epoxides/anhydrides and PET-RAFT polymerization of vinyl monomers has been demonstrated, wherein the regulation of visible light permits on-demand sequence control over the block copolymer products (Scheme 1). Scheme 1 | One-pot synthesis of block copolymers via auto-tandem catalysis. Download figure Download PowerPoint Results and Discussion As compared with the Co analogues, (TPP)AlIII-X complexes typically show inferior selectivity and produce polymers with high polyether content in ROCOP.54–57 Yet, Al is not likely to impede the electron and energy transfer property of porphyrin ligands like Mg because of its lack of d-electron at the metal centers, while (TPP)CoIII-X complexes suffer from shortened lifetime of the triplet state and prove to be ineffective for PET-RAFT polymerization.39,40 In ROCOP catalyzed by (TPP)AlIII-X, it is well-accepted that the axial group, X, initiates the reaction and aluminum carboxylate (Al-O2CR) and aluminum alkoxide (Al-OR) intermediates serve as the active species. To develop an auto-tandem catalytic system that involves ROCOP and PET-RAFT polymerization, it is of primary importance to make sure that the ROCOP active species are efficient for the PET-RAFT process. In this regard, (TPP2-Cl)AlIII-Cl, (TPP2-Cl)AlIII-OAc, and (TPP2-Cl)AlIII-OMe were all synthesized and exploited as photocatalysts for PET-RAFT polymerization (Scheme 2). Despite the difference of the axial groups, all three aluminum porphyrin complexes shared a strong Soret band at 420 nm and red-shifted absorption peak at 560 nm on the UV–vis absorption spectra ( Supporting Information Figure S8). Boyer et al.58 and Qiao et al.59 demonstrated well-controlled radical polymerization via selectively activating the relatively weak n-π* electronic transition of TTC-COOH using visible light (∼460 nm). As TTC-COOH also exhibits weak signals in the visible region (λmax,n→π* = 400−550 nm, see Supporting Information Figure S9), polymerization of methyl methacrylate (MMA) via direct photolysis (blue light, λmax = 460 nm) of TTC-COOH under comparative reaction conditions were exploited: (1) [MMA]/[TTC-COOH] = 100/1 under a light intensity of 10 mW/cm2; (2) [MMA]/[TTC-COOH] = 200/1 under a light intensity of 10 mW/cm2; (3) [MMA]/[TTC-COOH] = 100/1 under a light intensity of 5 mW/cm2; and (4) [MMA]/[TTC-COOH] = 200/1 under a light intensity of 5 mW/cm2. After reaction for 8 h at room temperature, the conversions of MMA were 36%, 16.5%, 5.5%, 0%, respectively ( Supporting Information Figures S10–S13). Therefore, it was tentatively assumed that photolysis of TTC-COOH can be effectively suppressed under optimized reaction conditions. Based on these experimental results, the PET-RAFT polymerization of MMA was initially investigated with an [MMA]/[TTC-COOH]/[(TPP2-Cl)AlIII-X] ratio of 1000/5/0.1 at room temperature under blue-light irradiation (λmax = 460 nm, 5 mW/cm2). As revealed by 1H NMR spectra, (TPP2-Cl)AlIII-Cl, (TPP2-Cl)AlIII-OAc, and (TPP2-Cl)AlIII-OMe gave monomer conversions of 24%, 21%, and 26% within 4 h, respectively. The gel permeation chromatography (GPC) traces of resultant poly(methyl methacrylate) (PMMA) all displayed monomodal and narrow distributions ( Supporting Information Table S1, entries 1–3). As could be anticipated, no polymeric products were detected in comparative experiments without (TPP2-Cl)AlIII-X, TTC-COOH, or light source ( Supporting Information Table S1, entries 4–8). Moreover, the kinetic plots of PET-RAFT polymerization of MMA catalyzed by (TPP2-Cl)AlIII-Cl exhibited no induction period and a first-order relationship between ln([M]0/[M]t) and reaction time, strongly indicative of a constant radical concentration (Figure 2a). Regardless of the change in monomer conversions, the number average molecular weight (MW; Mn) of the products was in agreement with the theoretical values (Mn,th), and the dispersities (Р= Mw/Mn) stayed low (Figures 2b and 2c). Furthermore, the temporal control of the system was demonstrated using an "ON/OFF" experiment (Figure 2d). It is notable that the polymeric product (Mn = 1.7 kDa, Р= 1.07) exhibited only two series of signals corresponding to [TTC-COOH + (MMA)n + Na]+ and [TTC-COOH + (MMA)n + K]+, respectively, indicative of the formation of homogenous α-TTC-ω-COOH PMMA ( Supporting Information Figure S17, PMMA-COOH). Scheme 2 | Catalysis, co-catalyst, and chain transfer reagent. Download figure Download PowerPoint To match the reaction conditions of PET-RAFT polymerization and ROCOP, (TPP2-Cl)AlIII-X complexes were further investigated as catalysts for ROCOP of propylene oxide (PO) and phthalic anhydride (PA) with PMMA-COOH as the macro-CTA (Mn = 3.5 kDa, Р= 1.10). The polymerizations were carried out at room temperature under a [PO]/[PA]/[PMMA-COOH]/[(TPP2-Cl)AlIII-X]/[PPNCl] (PPNCl = Bis(triphenylphosphine)iminium chloride)ratio of 5000/500/ 5/1/0.8. Typically, the nucleophilic axial group X of LnM-X (Ln = salcy, porphyrin) complexes serve as the initiator for ROCOP, giving access to α-X-ω-OH polymers despite the existence of the CTAs.60–62 In this regard, (TPP2-Cl)AlIII-OAc was expected to produce PMMA-b-poly(propylene phthalate) (PMMA-b-PPE) as well as α-OH-ω-OAc PPE, which could be revealed by the bimodal distribution on the GPC trace of the polymeric product (Schemes 3a and 3b). To our delight, the polymeric product obtained by (TPP2-Cl)AlIII-Cl displayed monomodal and narrow distribution on the GPC curve and the Mn was in good agreement with the theoretical value, indicating that the axial Cl of (TPP2-Cl)AlIII-Cl did not initiate the reaction. This interesting phenomenon could be explained by the fast and quantitative axial group exchange reaction between the carboxylate group and (TPP2-Cl)AlIII-Cl, which leads to the rapid and quantitative formation of (TPP2-Cl)AlIII-O2C-PMMA (Scheme 3c).53 Therefore, the ring-opening of PO occurs via nucleophilic attack of [PMMA-CO2]− to generate the Al-alkoxide intermediates that initiate the chain propagation process and, with the chain transfer effect of TTC-COOH, results in the formation of homogeneous PPE-b-PMMA. The 1H NMR spectroscopic investigation of mixture of (TPP2-Cl)AlIII-OMe and acetic acid (AcOH) in dimethyl sulfoxide (DMSO)-d6 at room temperature revealed that an axial group exchange reaction could also occur between the carboxylate group and (TPP2-Cl)AlIII-OMe ( Supporting Information Figure S18). However, (TPP2-Cl)AlIII-OMe produced α-OH-ω-OMe PPE as the side product because the MeOH, formed in situ via axial group exchange reaction, could still serve as the CTA (Schemes 3d and 3e). Furthermore, several comparative experiments were further conducted to exploit the effect of light on the ROCOP of PO and PA catalyzed by (TPP2-Cl)AlIII-Cl. It is clear from Figure 3a that the polymerization rates under blue light were lower than that in the dark with or without TTC-COOH, suggesting that the (TPP2-Cl)AlIII-Cl is much less active in the excited state than in the ground state for ROCOP. However, high product selectivity and low MW distribution was maintained under irradiation. Based on these experimental results, it could be tentatively envisioned that (TPP2-Cl)AlIII-Cl would make an ideal auto-tandem catalyst for ROCOP and PET-RAFT polymerization. Scheme 3 | (a–e) Mechanistic aspects for ROCOP of PO/PA in the presence of PMMA-COOH catalyzed by (TPP2-Cl)AlIII-X complexes at room temperature in the dark. [PO]/[PA]/[PMMA-COOH]/[(TPP2-Cl)AlIII-X]/[PPNCl] = 5000/500/5/1/0.8. Download figure Download PowerPoint In this regard, one-pot terpolymerization of epoxides, anhydrides, and vinyl monomers was conducted using PO, PA, and MMA as modal substrates and PPNCl as a co-catalyst at room temperature. The feed ratio of [PO]/[MMA]/[PA]/[TTC-COOH]/[(TPP2-Cl)AlIII-Cl]/[PPNCl] was set as 5000/2500/1250/5/1/0.8 to balance the catalytic activity and chain transfer efficiency for both PET-RAFT and ROCOP. As revealed by the 1H NMR spectrum (Figure 3b), both PPE with alternating structure and PMMA formed in the reaction, while no signals corresponding to junction units could be observed. As determined by 1H NMR spectroscopy, the conversions of both MMA and PA steadily increased with reaction time. Yet, all the polymeric products exhibited monomodal and narrow distributions on GPC traces and the Mn agreed well with the theoretical values (Table 1, entries 3–6) despite the difference in monomer conversions. The diffusion ordered spectroscopy (DOSY) NMR spectrum of the resultant polymeric product shared a single diffusion coefficient for all the characteristic signals, whereas two different diffusion coefficients could be observed for that of the constituted PMMA/PPE mixture (Figure 3d). This not only verified the block structure of the product, but also revealed that no homopolymers formed during the polymerization. We have previously reported that Cl− of PPNCl coordinates to Al, facilitates the dissociation of Al–O bonds, and therefore, accelerates ROCOP of CO2 and epoxides.53,62 As anticipated, the rate of PO/PA ROCOP decreased significantly without PPNCl (Table 1, entries 1 and 2). Yet, PPNCl showed no obvious effect on PET-RAFT polymerization of MMA. Higher monomer conversion was also achieved using dichloromethane as the solvent (Table 1, entry 7). Differential scanning calorimetry (DSC) of the resulting PPE-b-PMMA (Mn = 78.8 kDa, Р= 1.19) exhibited amorphous structures with two glass transition temperatures (Tg1 = 33.6 °C, Tg2 = 90.5 °C; Supporting Information Figure S42), indicative of the immiscibility between PPE and PMMA. However, no clear scattering peaks were observed in the small-angle X-ray scattering (SAXS) curve of its thermally annealed film, indicating a disordered microstructure ( Supporting Information Figure S44). Table 1 | Auto-Tandem Catalysis for One-Step Synthesis of Diblock Copolymersa Entry Monomer Time (h) Conv. %b (Anhydride) Conv. %b (Vinyl Monomer) Mn,NMRc (kg·mol−1) Mn,GPCd (kg·mol−1) Ðd 1 [PO]/[PA]/[MMA] 2 13 12 13.1 12.7 1.07 2e [PO]/[PA]/[MMA] 4 3 16 10.0 8.9 1.08 3 [PO]/[PA]/[MMA] 4 25 17 21.8 20.4 1.09 4 [PO]/[PA]/[MMA] 6 35 21 29.0 27.7 1.12 5 [PO]/[PA]/[MMA] 8 51 27 40.2 38.8 1.12 6 [PO]/[PA]/[MMA] 9 60 29 45.8 43.9 1.10 7f [PO]/[PA]/[MMA] 15 100 62 82.5 78.8 1.19 8 [EO]/[HHPA]/[St] 3 16 10 13.5 12.9 1.07 9 [PO]/[THPA]/[MMA] 4 21 16 19.4 18.6 1.07 10 [BO]/[PA]/[MMA] 8 45 14 32.2 30.4 1.10 11 [PO]/[NBA]/[St] 6 32 25 28.3 26.7 1.10 aThe reactions were carried out at room temperature under blue-light irradiation (λmax = 460 nm, 5 mW/cm2). bDetermined by 1H NMR spectroscopy. cCalculated by the equations: Mn,NMR = Conv.(anhydride) × [MW(epoxide) + MW(anhydride)] × [anhydride]0/[TTC-COOH]0 + Conv.(MMA) × MW(vinyl monomer) × [vinyl monomer]0/[TTC-COOH]0 + MW(TTC-COOH). dDetermined by using GPC in THF and calibrated with polystyrene standards. eIn absence of PPNCl. fConducted in dichloromethane, [PA]0 = 1.8 M. Figure 2 | Kinetic study of PET-RAFT polymerization of MMA by (TPP2-Cl)AlIII-Cl at room temperature under blue-light irradiation (λmax = 460 nm, 5 mW/cm2), [MMA]/[TTC-COOH]/[(TPP2-Cl)AlIII-Cl] = 1000/5/0.1. (a) Plots of ln([M]0/[M]t) vs time. (b) Evolution of Mn and Ð with the MMA conversion; the straight line represents the theoretical mass weight evolution. (c) Evolution of GPC traces. (d) Investigation of the "ON/OFF" switching by light. Download figure Download PowerPoint Figure 3 | The effects of irradiation on the polymerizations (λmax = 460 nm, 5 mW/cm2). (a) PA conversion vs time of ROCOP of PO/PA under different conditions. (b) Representative 1H NMR spectrum of PPE-b-PMMA (Table 1, entry 3). (c) Monomer conversion vs time of auto-tandem catalysis of PET-RAFT polymerization of MMA and ROCOP of PO/PA in the "ON/OFF" experiment (blue dash lines represent plots of PA conversion vs time in dark). (d) DOSY NMR spectrum of PPE-b-PMMA (Table 1, entry 3) and the mixture of PPE/PMMA. Download figure Download PowerPoint Moreover, the auto-tandem catalysis was conducted under a periodic light on–off process, wherein at regular intervals samples were withdrawn from the reaction mixture for 1H NMR analysis. Figure 3c reveals that light could give precise "ON/OFF" control over the chain propagation of PET-RAFT polymerization. During the whole process, ROCOP proceeded smoothly, except that the polymerization rates in the dark were slightly higher than that under blue light. Also, the GPC curves displayed monomodal and narrow distributions regardless of the steadily increasing MWs, suggesting that the auto-tandem catalysis of (TPP2-Cl)AlIII-Cl allows both concurrent and sequential chain propagation of ROCOP and PET-RAFT polymerization via regulation of visible light. Furthermore, we evaluated the generality of our one-pot synthetic protocol using various epoxides, anhydrides, and vinyl monomers (Table 1, entries 8–11). In all cases, we successfully obtained the well-defined diblock copolymers with low MW distributions as confirmed by 1H and 13C NMR and GPC analysis ( Supporting Information Figures S47–S58). Interestingly, the alkene functional groups in anhydrides, like cis-5-norbornene-endo-2,3-dicarboxylic anhydride (NBA) and cis-1,2,3,6-tetrahydrophthalic anhydride (THPA), could be substituted with either hydrophilic or hydrophobic side-chains using a thiol–ene reaction, which provides future potential in coating materials.63,64 It is notable that tetrablock copolymers could be obtained via a second auto-tandem catalytic cycle with PMMA-b-PPE (Mn = 12.7 kDa, Р= 1.07, Table 1, entry 1) as the bifunctional CTA in the monomer mixture of styrene (St), ethylene oxide (EO), and cis-1,2-cyclohexanedicarboxylic anhydride (HHPA), further confirming the homogeneous α-TTC-ω-OH telechelic structure of the resulting diblock copolymers ( Supporting Information Figures S59 and S60). Conclusion The first example of an auto-tandem catalytic system that involves ROCOP of epoxides/anhydrides and PET-RAFT polymerization of vinyl monomers has been demonstrated using aluminum porphyrin complexes as the common catalysts. With TTC-COOH as a bifunctional CTA, sequential and concurrent ROCOP and PET-RAFT polymerization were achieved, enabling the one-pot synthesis of well-controlled diblock copolymers from monomer mixtures. The double chain transfer effect allows for independent and precise control over the MW of the two blocks and ensures low polydispersity of the resultant block copolymers (Đ < 1.15). Considering the excellent end-group fidelity of the resultant block copolymer, the auto-tandem catalysis can be readily applied for one-pot synthesis of even-block copolymers (tetra-, hexa-, octo-, etc.) with advanced properties and special phase separation behaviors by combing the sequential monomer addition strategy. 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