In Situ Supramolecular Polymerization via Organometallic-Catalyzed Macromolecular Metamorphosis

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE18 May 2022In Situ Supramolecular Polymerization via Organometallic-Catalyzed Macromolecular Metamorphosis Xiwen Yang, Qianqian Ji, Jiaxiong Liu and Yiliu Liu Xiwen Yang South China Advanced Institute for Soft Matter Science and Technology, School of Emergent Soft Matter, South China University of Technology, Guangzhou 510640 Google Scholar More articles by this author , Qianqian Ji South China Advanced Institute for Soft Matter Science and Technology, School of Emergent Soft Matter, South China University of Technology, Guangzhou 510640 Google Scholar More articles by this author , Jiaxiong Liu South China Advanced Institute for Soft Matter Science and Technology, School of Emergent Soft Matter, South China University of Technology, Guangzhou 510640 Google Scholar More articles by this author and Yiliu Liu *Corresponding author: E-mail Address: [email protected] South China Advanced Institute for Soft Matter Science and Technology, School of Emergent Soft Matter, South China University of Technology, Guangzhou 510640 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.022.202201936 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Integrating catalytic reactions with molecular assembly is a promising means of achieving controllable supramolecular polymerization. We report herein a novel and controllable method for in situ supramolecular polymerization via organometallic-catalyzed macromolecular metamorphosis. To this end, covalent polymers with polypentenamer backbones and pendant supramolecular motifs are designed and synthesized. By depolymerizing the polymers with Grubbs catalysts, the supramolecular motifs can be gradually released from the polymers to the solution. Supramolecular polymerization occurs when a critical concentration is reached. The supramolecular polymerization process was readily controlled by varying the rate of the depolymerization reaction. This work presents a novel approach that uses organometallic catalysis to transform covalent polymers into supramolecular polymers. It offers a new means of constructing complex molecular systems in a controllable manner. Download figure Download PowerPoint Introduction Supramolecular polymers, whose polymeric chains are formed by non-covalently connected monomers, hold great promise in developing functional materials with applications in optoelectronics, pharmaceutics, and recyclable plastics.1–6 The rational control of the supramolecular polymerization process is very important for the construction of supramolecular polymers with desired structures and functions. Therefore, the development of controllable supramolecular polymerization (CSP) methods is always a major focus of research in supramolecular polymer chemistry.7–11 Lately, a comprehensive understanding of molecular assembly processes, such as pathway selection and solvent effects, has greatly advanced the field of supramolecular polymerization.12–16 Several novel concepts, such as seeded living supramolecular polymerization,17–20 chain-growth supramolecular polymerization,21,22 supramolecular interfacial polymerization,23,24 and fuel-driven supramolecular polymerization,25–28 have been established. Nevertheless, compared with nature’s ability to construct biological analogues of supramolecular polymers (e.g., microtubules and actin filaments), enormous challenges remain in the preparation of supramolecular polymers under precise temporal and spatial control.11,29 One powerful tool that nature utilizes is catalysis, which offers kinetic control over the assembly process to achieve emergent structures and functions.30–32 Therefore, integrating catalytic reactions with molecular assembly is legitimately considered a promising means of achieving CSP. For example, enzymatic catalysis has been successfully introduced to instruct the formation of transient supramolecular polymers in a controllable manner.32 Organometallic-catalyzed reactions that cover a broad range of catalysts and substrates, however, have still rarely been explored to drive or regulate supramolecular polymerization. Herein, we report a new method for in situ supramolecular polymerization via organometallic-catalyzed macromolecular metamorphosis. The basic concept utilizes organometallic-catalyzed depolymerization reactions to controllably transform covalent polymers into supramolecular polymers (Scheme 1). Generally, supramolecular motifs intended for supramolecular polymerization are modified on degradable covalent polymers as pendant groups. In this state, the supramolecular motifs remain unaggregated or ill-aggregated because of the structural constraints imposed by the polymer chains. Upon depolymerization, the supramolecular motifs are gradually released from the covalent polymers to the solution, where they form supramolecular polymers when a critical concentration is reached. The macromolecular metamorphosis from covalent polymers to supramolecular polymers can be readily tuned by varying the rate of depolymerization, thereby providing a means of kinetically controlling the supramolecular polymerization. Scheme 1 | Organometallic-catalyzed macromolecular metamorphosis: in situ transformation from covalent polymers to supramolecular polymers. Download figure Download PowerPoint Experimental Methods All reagents were purchased from commercial suppliers and used as received without further purification. 1H, 13C, and 19F NMR spectra were recorded with a JNM-ECZ500R 500 MHz NMR (JEOL, Tokyo, Japan) or Bruker Avance 400 MHz NMR spectrometer or Bruker Avance 500 MHz NMR spectrometer (Bruker, Switzerland. The high-resolution mass spectra were recorded on an Agilent1290/maXis impact high-resolution liquid chromatography mass spectrometer (LC-MS). Molecular weights and distributions were determined by gel permeation chromatography (GPC) with a Waters ACQUITY UPLC sample manager and a Waters ACQUITY differential refractive index detector; polystyrene was used as the standard. UV–vis absorption spectra in solution were recorded using a UV-3600Plus spectrophotometer (Shimadzu, Nakagyo-ku, Kyoto, Japan). The fluorescence spectra were recorded on an RF-6000 spectrofluorophotometer (Shimadzu, Nakagyo-ku, Kyoto, Japan). Atomic force microscopy (AFM) measurements were performed under ambient conditions using a Cypher VRS system operating in tapping mode in air (Oxford Instruments Asylum Research, Santa Barbara, California, United States). Silica cantilevers (AC200TS-R3, Oxford Instruments, Taiwan, China) with a resonance frequency of ∼150 kHz and a spring constant of ∼9 N m−1 were used. More experimental details are available in Supporting Information. Results and Discussion Polymer design and synthesis In our design, the covalent polymers (PCPs) was composed of a polypentenamer-based backbone functionalized with 3,6-di(thiophen-2-yl)-4,5-dihydropyrrolo[3,4-c]pyrrol-1(2H)-one (DPP) and polyoctahedral silsesquioxane (POSS) derivatives as pendant groups (Scheme 2). The polypentenamer-based backbone was chosen for its unique catalytic degradability by the ring-closing metathesis reaction.33–35 In particular, Kennemur and coworkers34 demonstrated that the polypentenamers can be transformed into other architectures through post-depolymerization reactions. The DPP derivative, which contains a planar π-conjugated core and hydrogen-bond-forming amide moieties, was chosen as the supramolecular motif.36–38 POSS-based pendant groups were introduced to endow the polymer with good solubility in low-polarity solvents.39,40 The steric structure of the POSS pendant groups was also thought to provide structural constraints that prevent the aggregation of the DPP moieties on the polymer chains. Scheme 2 | Polymer design: a polypentenamer backbone with DPP- and POSS-based pendant groups. Download figure Download PowerPoint The protocol of the organometallic-catalyzed macromolecular metamorphosis is described in Scheme 3. The PCP polymers are depolymerized continuously by ring-closing metathesis reactions catalyzed by the Grubbs catalysts. Thus, the cyclopentene-based monomers with different pendant groups, namely POSS-M and DPP-M, are gradually released from the polymeric chains into the solution. Without the structural constraints imposed by the polymer chains, the DPP-Ms are expected to undergo supramolecular polymerization in solution. Scheme 3 | Proposed protocol of the organometallic-catalyzed macromolecular metamorphosis. Download figure Download PowerPoint A post-polymerization modification strategy was used to synthesize the designed PCP polymers.41–43 This involves first synthesizing a precursor polypentenamer (PCP-0) that contains pentafluorophenyl ester moieties. Direct polymerization of the pentafluorophenyl ester containing cyclopentene monomer was infeasible; therefore, an indirect multistep synthetic route was employed instead. Polymerization of methyl cyclopent-3-ene-1-carboxylate, followed by hydrolysis of the methyl ester and esterification of the obtained carboxylic acid moieties with pentafluorophenyl trifluoroacetate, readily gave the desired PCP-0 (see Supporting Information Figure S4 for detailed synthetic procedure). Due to the high reactivity between pentafluorophenyl esters and amines, PCP-0 reacted efficiently with the amino-group containing DPP and POSS derivatives, straightforwardly producing the desired polymers PCP-1, PCP-2, and PCP-3 after dialysis (Figure 1). PCP-1 contained only POSS pendant groups, whereas PCP-2 and PCP-3 contained 5% and 20% DPP pendant groups, respectively. The GPC results of the PCP polymers are included in Table 1, and reveal that the molar mass dispersity values of all PCP polymers remained narrow after the post-polymerization modification of PCP-0. Because a low-polarity solvent was necessary for the proposed macromolecular metamorphosis, the solubility of the obtained PCP polymers in methyl cyclohexane (MCH) was determined first. To our delight, although these polymers contain many amide moieties alongside the polymer backbone, they solubilize well in MCH, most likely because of the solubilizing POSS pendant groups. Figure 1 | Synthesis of the polypentenamer-based covalent polymers (PCP-1, PCP-2, and PCP-3) via post-polymerization modification. Download figure Download PowerPoint Table 1 | Composition and GPC Characterization of the PCPs Polymer xa ya Mn,GPC (kDa)b Đb PCP-0 — — 37.4 1.07 PCP-1 1.00 — 42.0 1.04 PCP-2 0.95 0.05 39.2 1.05 PCP-3 0.80 0.20 36.4 1.07 aDetermined by 1H NMR analysis (in CDCl3). bMeasured by gel permeation chromatography in tetrahydrofuran. Organometallic-catalyzed depolymerization of the polypentenamer backbone Polypentenamers are known to undergo depolymerization by olefin metathesis catalyzed by the Grubbs catalysts.33 However, the depolymerization reaction is heavily dependent on several influencing factors, including the pendant groups, the catalysts, and the solvents.35 The depolymerization of PCP-1 was investigated first to determine the proper conditions for the subsequent macromolecular metamorphosis experiments (Figure 2a). Considering that the depolymerization must be performed in a low-polarity solvent, the 2nd generation Grubbs catalyst (G2) was chosen due to its decent solubility in MCH. In a test reaction, 0.20 equiv G2 (relative to the total olefin moieties) was added to a PCP-1 solution in MCH, and the reaction mixture was kept at 27 °C for 24 h. The reaction mixture was monitored by 1H NMR, GPC, and electrospray ionization mass spectroscopy (ESI-MS). As shown in Figure 2b, the GPC trace of PCP-1 in the high-molecular-weight region disappears after the reaction, suggesting that the polymers depolymerized into small molecules. 1H NMR analysis revealed that the broadened olefin proton signal of PCP-1 became sharp and moved from 5.62 to 5.51 ppm after the reaction (Figure 2c). This indicates the quantitative conversion of the PCP-1 into POSS-M. ESI-MS revealed an m/z signal at 990.3473 corresponding to [(POSS-M)+Na]+, further confirming that the depolymerization had proceeded smoothly ( Supporting Information Figure S16). In summary, these experiments confirmed that the PCP polymer can be readily depolymerized by using the 2nd generation Grubbs catalyst at 27 °C in MCH solution. Figure 2 | Depolymerization of PCP-1 by G2 in MCH solution: (a) scheme of the depolymerization reaction; (b) GPC traces of PCP-1 (red) and PCP-1 after the reaction (blue); (c) stacked 1H NMR spectra (benzene-d6) of PCP-1 (red), PCP-1 after the reaction (blue) and POSS-M (black). Download figure Download PowerPoint DPP-based supramolecular motif According to our protocol, DPP-based supramolecular motifs were released from the polymers in the form of DPP-M upon depolymerization. Therefore, the intrinsic supramolecular polymerization property of DPP-M was carefully investigated prior to the macromolecular metamorphosis experiments. The detailed procedure for the synthesis of DPP-M is included in the Supporting Information Figure S1. It was thought DPP-M forms supramolecular polymers via the joint force of π–π stacking and intramolecular hydrogen bonds (Figure 3a). First, solutions of DPP-M in dichloromethane (DCM) or MCH were studied by UV–vis and fluorescence spectroscopy. As shown in Figure 3b, the solution of DPP-M in DCM was orange and produced an absorption peak at approximately 547 nm, whereas the solution of DPP-M in MCH was purple and produced an absorption maximum peak at 586 nm. We rationalized this profound difference in absorption stems from the different aggregation states of DPP-M. The DPP-M is molecularly dissolved in DCM, thereby showing the characteristic absorption of the DPP chromophore. In the low-polarity solvent MCH, DPP-M underwent supramolecular polymerization in a J-aggregated manner, which led to the observed bathochromic shift in absorption. This was further confirmed by the fluorescence spectroscopy results. As shown in Figure 3b, DPP-M fluoresced strongly in DCM, whereas it negligibly fluoresced in MCH. This indicates that DPP-M was strongly aggregated in MCH, which led to severe fluorescence quenching. The morphology of the DPP-M based supramolecular polymers was studied by AFM. Multi-micrometer-long fibers with an approximate height of 3.5 nm, which were presumably single-stranded supramolecular polymers formed by stacked DPP-M molecules, were found (Figure 3c). Figure 3 | Supramolecular polymerization behavior of DPP-M: (a) molecular structure of DPP-M; (b) UV–vis spectra (solid lines) and fluorescence spectra (dashed lines) of DPP-M in DCM (red) or MCH (blue); (c) AFM height image of a sample prepared from an MCH solution of DPP-M. Download figure Download PowerPoint Temperature-dependent UV–vis spectroscopy was further employed to elucidate the mechanism by which the supramolecular polymerization of DPP-M occurs. The solution of DPP-M in MCH was first heated to 75 °C, then slowly cooled to 10 °C at a cooling rate of 0.5 °C/min. As shown in Figure 4a, the absorption maximum of DPP-M at high temperatures peaked at 547 nm, then it underwent a bathochromic shift to 586 nm upon cooling to certain temperatures. A plot of the absorption at 586 nm against temperature reveals curves with non-sigmoidal transition, which is indicative of a cooperative nucleation–elongation of supramolecular polymerization mechanism (Figure 4b).44–48 Fitting the non-sigmoidal transition by applying the cooperative nucleation–elongation model introduced by Meijer and co-workers gives a critical elongation temperature (Te) of 291.8 K and an elongation enthalpy (ΔHe) of −171.9 kJ mol−1 at the DPP-M concentration of 20.0 μM ( Supporting Information Figure S12). Moreover, the Te is positively correlated with the concentration of the DPP-M solution; DPP-M concentrations of 20.0, 40.0, and 60.0 μM correspond to Te values of 291.8, 302.1, and 306.0 K, respectively. This also suggests that, to guarantee supramolecular polymerization at room temperature, the total concentration of the DPP moiety ([DPP]tot.) needs to be >40.0 μM. Figure 4 | (a) Temperature-dependent UV–vis spectra of a 40.0 μM solution of DPP-M in MCH; (b) plots of the absorption at 586 nm against temperature of MCH solutions of DPP-M of various concentrations (black, 20.0 μM; red, 40.0 μM; blue, 60.0 μM). Download figure Download PowerPoint In situ supramolecular polymerization via macromolecular metamorphosis With the aforementioned information in hand, the catalytic macromolecular metamorphosis of PCP-2 was performed. A MCH solution of PCP-2 with [DPP]tot. of 60.0 μM was used for all experiments. Notably, the UV–vis spectra of the PCP-2 solution had a similar absorption profile to that of the DCM solution of DPP-M, without noticeable absorption at approximately 586 nm. This suggests that the DPP motifs on the polymer chain were not J-aggregated, most likely due to the structural constrains imposed by the POSS-based polymer chains. Immediately after the addition of G2, the PCP-2 solution was continuously monitored by UV–vis spectroscopy. The time-dependent UV–vis spectra obtained from the sample with 0.20 equiv G2 is shown in Figure 5a. Interestingly, the changes in the absorption profile can be divided into two distinct stages. In the first stage, before approximately 120 min, there was no profound change in the absorption profile, only a small hypochromic shift of the absorption maximum from 550 nm to 547 nm was found (Figure 5c). Starting at 120 min after the addition of G2, the intensity of the absorption peak at 547 nm underwent a significant reduction. An emergent shoulder absorption peak at 586 nm grew rapidly and reached a plateau after approximately 6 h (Figure 5d). The two different stages are also reflected in the appearance of the solution (Figure 5b). The solution gradually turned from reddish to orange, with fluorescence detectable by the naked eye in the first stage, and then turned purple in the second stage. A plot of the changes in absorption at 586 nm against reaction time is shown in Figure 6, together with a schematic rationale of the results. During the first stage, the depolymerization of PCP-2 catalyzed by G2 proceeds, and the DPP-M is gradually released from the polymer chains to the solution phase. Upon reaching a critical point, the DPP-M starts to nucleate, then undergoes elongation to form J-aggregated supramolecular polymers. Figure 5 | Time-dependent UV–vis spectra of the PCP-2 solution with 0.2 equiv G2, (a) the full spectra; (c) stage I; (d) stage II; (b) color of the reaction mixture at various stages. Download figure Download PowerPoint Because the process of supramolecular polymerization directly correlates with the concentration of DPP-M released from PCP-2, we supposed that kinetic control over the supramolecular polymerization can be realized by varying the rate of the depolymerization reaction. To this end, experiments on the depolymerization of PCP-2 using various amounts of G2 were performed. The plots of absorption at 586 nm against time for all the experiments are shown in Figure 7a. All the obtained curves have similar profiles that comprise two stages, but with different transition times. For example, the transition starts at 70 min in the sample with 1.0 equiv G2, which is much sooner than in the sample with 0.20 equal G2. As the amount of catalyst decreases, the transition starts later: 170 min for the sample with 0.10 equiv G2 and over 300 min for the sample with 0.05 equiv G2. Notably, although the transition times are different, the maximum of absorption at 586 nm reaches a plateau with similar values after 24 h in all cases. Figure 6 | The macromolecular metamorphosis of PCP-2. Download figure Download PowerPoint Figure 7 | Plots of the absorption at 586 nm against reaction time: (a) PCP-2 solutions ([DPP]tot. = 60.0 μM) with various amounts of the G2 (brown, 0.05 equiv; green, 0.10 equiv; red, 0.20 equiv; blue, 1.0 equiv); (b) PCP-3 solutions at various concentration ([DPP]tot.: 60.0 μM, black; 80.0 μM, red; 120.0 μM, blue) with the same amount of G2 (120.0 μM). Download figure Download PowerPoint The depolymerization of PCP-3, which contains a higher content of DPP-based pendant groups, was performed as well. In the depolymerization experiments of PCP-3, the G2 catalysts were set at 120.0 μM but the concentration of PCP-3 varied. The plots of the absorption at 586 nm against reaction time are shown in Figure 7b. With identical [DPP]tot. of 60.0 μM and G2 catalyst of 120.0 μM, the transition in the PCP-3 reaction mixture started at 140 min, sooner than in PCP-2, which started at 170 min. In addition, the time of transition decreased with the increase of the concentration of PCP-3. These results indicate that, besides varying the amount of G2 catalyst, the supramolecular polymerization process can also be controlled by tuning the composition and the concentration of the precursor PCP polymers. As with PCP-1, the GPC and 1H NMR measurements confirmed the quantitative depolymerization of PCP-2 and PCP-3 in all cases (see Supporting Information). The solutions of the reaction mixture were spin-coated onto silicon wafers and investigated by AFM (Figure 8). Micrometer-long fibers were found in the samples from the depolymerized PCP-3 with [DPP]tot. of 60.0 and 80.0 μM (Figure 8a and Supporting Information Figures S34–S35). The height of the obtained fibers is in the range of 3.0–4.0 nm, which is consistent with the supramolecular polymers formed directly by DPP-M. This confirms the successful in situ transformation of the covalent PCP polymers into DPP-M based supramolecular polymers. Furthermore, in the samples from the depolymerized PCP-2 ([DPP]tot. = 60.0 μM, Figure 8b, Supporting Information Figures S23–S26) or the depolymerized PCP-3 with higher concentration ([DPP]tot. = 120.0 μM, Supporting Information Figure S36), fibers and some large-size aggregates co-existed. These large-size aggregates were identified as assemblies formed by POSS-M as “side products” by comparing with the samples from the depolymerized PCP-1 ( Supporting Information Figure S15). This is attributed to the relatively high concentration of the formed POSS-M in these samples that exceeds its critical aggregation concentration (CAC), thus aggregating. It suggests that, to prepare neat supramolecular polymers in situ, the solubilizing pendant groups should have a high CAC value or be kept at a relative low concentration. Figure 8 | AFM height image of a sample prepared from (a) the PCP-3 solution ([DPP]tot. = 60.0 μM) after depolymerization; (b) the PCP-2 solution ([DPP]tot. = 60.0 μM) after depolymerization. Download figure Download PowerPoint Conclusions Here, we describe a new approach to controlling supramolecular polymerization through organometallic-catalyzed macromolecular metamorphosis. Polypentenamers functionalized with DPP-based supramolecular motifs and POSS pendant groups were synthesized. The POSS pendant groups endow the polymers with greater solubility in the low-polarity solvent MCH and provide structural constraints to prevent the aggregation of DPP-based supramolecular motifs. The polypentenamers can be readily depolymerized at room temperature using the second-generation Grubbs catalyst in MCH. Upon depolymerization, the DPP-based supramolecular motifs are gradually released from the covalent polymers and transformed into supramolecular polymers. This unique macromolecular metamorphosis approach takes advantage of an organometallic-catalyzed reaction to provide kinetic control of the supramolecular polymerization. The technique, which catalytically transforms covalent polymers into supramolecular polymers, is potentially useful as a new effective method for fabricating complex supramolecular architectures. In our method, the precursor polymers act as a reservoir to provide monomers for supramolecular polymerization in situ, which provides a unique alternative to the popular “combination of good solvent and poor solvent” approach. Our current efforts are directed toward the application of this method in living supramolecular polymerization to fabricate complex supramolecular structures, for example multi-block supramolecular polymers.49–51 Supporting Information Supporting Information is available and includes experimental details, synthesis of chemical compounds, and other supplementary results. Conflicts of Interest The authors declare no competing interests. Acknowledgments We gratefully acknowledge the financial support from National Key R&D Program of China (grant no. 2021YFA1501600), National Natural Science Foundation of China (grant no. 21901077), Natural Science Foundation of Guangdong Province (grant no. 2016ZT06C322), Open Project of State Key Laboratory for Supramolecular Structure and Materials (grant no. SKLSSM2021012) and the Research Fund Program of Guangdong Provincial Key Laboratory of Functional and Intelligent Hybrid Materials and Devices (grant no. 2019B121203003).