Green Stereoregular Polymerization of Poly(methyl methacrylate)s Through Vesicular Catalysis

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 Apr 2022Green Stereoregular Polymerization of Poly(methyl methacrylate)s Through Vesicular Catalysis Beike Cai, Shaodong Zhang and Yongfeng Zhou Beike Cai School of Chemistry and Chemical Engineering, Frontiers Science Center for Transformative Molecules, State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240 , Shaodong Zhang School of Chemistry and Chemical Engineering, Frontiers Science Center for Transformative Molecules, State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240 and Yongfeng Zhou *Corresponding author: E-mail Address: [email protected] School of Chemistry and Chemical Engineering, Frontiers Science Center for Transformative Molecules, State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240 https://doi.org/10.31635/ccschem.021.202101011 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Stereoselective polymerization can yield polymers with specific tacticity and properties, and it is an essential topic throughout polymer synthesis history. Herein, we report for the first time on a vesicular catalysis method to realize the green stereoregular polymerization of isotactic-rich poly(methyl methacrylate) (PMMA) in water and at ambient temperature and pressure, namely by conducting the polymerization of MMA monomers in the confined 5–10 nm thick hydrophobic layers of hyperbranched polymer vesicles. The isotactic degree of the as-prepared PMMAs increases from 12% to 40% with the decrease of vesicle size from 840 to 85 nm in hydrodynamic diameter (Dh) due to the conformation confinement effect of MMA monomers inside the thin vesicle membranes through a curvature-dependent way. The present work has extended the scope of green chemistry, and it also represents a new application of polymer vesicles in stereoregular polymerization. Download figure Download PowerPoint Introduction Due to the sustainable and eco-friendly nature, the concept of green chemistry has attracted increasing attention in recent years.1–3 Therefore, to realize it, great efforts have been made to design and apply new catalysts and catalysis methods, sustainable solvents, and natural chemicals.4–6 For example, as pioneered by Lipshutz et al.,7–9 micelles can be used as nanoreactors to allow synthetic chemistry to proceed in water and under very mild conditions. Such a so-called "micellar catalysis" method has been successfully applied for green synthesis with the construction of C−C, C−H, and C−heteroatom bonds. Green polymerization is an important branch of green chemistry, but the related progress is still limited to date, especially in green stereoregular polymerization.10–12 Herein, inspired by the concept of "micellar catalysis," we report for the first time the use of confined space of polymer vesicle membranes as the nanoreactors to control the green stereoregular polymerization of poly(methyl methacrylate) (PMMA) in water. As we know, stereoselective polymerization can yield polymers with specific tacticity, which plays a significant role in determining polymer properties13,14 such as solubility,15 crystallinity,16 melting and glass transition temperatures,17 and mechanical strength.18 Taking PMMA, for instance, the most common type is an irregular structure with a randomly distributed syndiotactic (ca. 65%) and atactic (ca. 35%) configuration, thus, generally hard and brittle with poor toughness and heat resistance.19,20 On the other hand, isotactic PMMA exhibits a lower Tg (45 °C) and higher crystallinity than its atactic and syndiotactic analogues, leading to enhanced elastic properties,17 thermal conductivity, and thermal stability,21 which vastly expands the application of this polymer. Up to now, various strategies have been developed for the preparation of isotactic-rich PMMAs, including anionic polymerization,22 templates (matrix) polymerization,23,24 polymerization in the presence of Lewis acid,25 and polymerization within nanochannels of metal–organic frameworks (MOF).26,27 However, these methods generally involve operation in organic solvents, with heating or cooling processes, which do not comply with the principles of green chemistry. In addition, reports on the controlled range of the isotactic degree of PMMAs are limited. Vesicles are important supramolecular structures featured with bilayer membranes and aqueous lumens. Up to now, vesicles have been widely applied to biology, biomimetics, biomedicine, cosmetics, photocatalysis, and electrocatalysis.28–33 According to literature reports, vesicle membranes generally exhibit a uniform thickness of 5–20 nm.28 This means that vesicles can accommodate hydrophobic monomers within their membranes, which, in principle, can suppress free rotation and diffusion of the monomers and propagating species, and thus, effectively regulate the stereoregularity of the resulting polymers. However, to our surprise, almost no work so far has been done on regulating the tacticity of polymers using vesicle membranes as nanoreactors. Our group has been focusing on the biomimetics and biomedical applications of vesicles self-assembled from hyperbranched polymers (HBPs).34–36 The HBP vesicles generally have a thin membrane of around 5–10 nm.35 In addition, they have either excellent membrane stability comparable to block copolymer vesicles or a good membrane fluidity like liposomes, which is a trade-off property difficult to be achieved together in conventional vesicles.36 Therefore, we assumed that HBP vesicles could be an ideal nanoreactor for stereoregular polymerization. Thus, as proof of concept, we have successfully realized the preparation of isotactic-rich PMMAs confined within HBP vesicle membranes in an aqueous solution. The entire synthetic procedure conforms to green chemistry, as the reaction is conducted in water and at ambient temperature and pressure. We also managed to control the isotactic degree of PMMAs from 12% to 40% by simply adjusting the vesicles' curvature. Experimental Methods Materials Methyl methacrylate (MMA) and fluorescein (95%) were purchased from Tansoole (Shanghai, China), and 4-cyanopentanoic acid dithiobenzoate (CPADB, >97%) was purchased from Macklin (Shanghai, China). MMA was purified by vacuum distillation before use. Boron trifluoride etherate (BF3·OEt2) and dichloromethane (CH2Cl2) were from Greagent (Shanghai, China), and ethylene oxide (EO) was from Sinopec Shanghai Petrochemical Co., Ltd. Synthesis of HBPO-star-PEO A series of hyperbranched poly(3-ethyl-3-oxetanemethanol) (HBPO)-star-polyethylene oxide (PEO) samples were synthesized by "one-pot" but two-step cationic ring-opening polymerization (CROP) based on our previous reports ( Supporting Information Figure S1). Before feeding, repeatedly vacuuming to maintain the N2 atmosphere and baked the flask with open air to remove all the moisture. In a dried three-neck bottle equipped with a constant pressure drop funnel filled with 0.1 mol (11.6 mL) 3-ethyl-3-oxetanemethanol (EHO) monomer, 100 mL dry CH2Cl2 with 0.05 mmol (6.4 mL) BF3·OEt2 were injected inside first, and then the EHO monomer was quickly added. The reaction lasted for 24 h under vigorous stirring at room temperature. The CROP of EHO monomers generated the HBPO. In the second step, HBPO was further used to initiate the CROP of EOs to obtain hyperbranched multiarm copolymer HBPO-star-PEOs. Further, the reaction temperature was reduced to about −20 °C, and EO of varying equivalents (0.25, 0.6, 1.0, 2.0, and 3.0 mol; 10, 20, 30, 50, and 70 mL) added to the constant pressure drop funnel, then dropped EO monomer to the reaction system extremely slowly. The reaction took another 24 h to obtain HBPO-star-PEOs with different degrees of polymerization of PEO arms (DParm). The hydrophilic fraction (fEO), DParm, molar mass and dispersity of five kinds of HBPO-star-PEOs (HSP2–HSP14) are listed in Supporting Information Table S1. Aqueous self-assembly of HSP vesicles According to our previously reported protocols,35,37 vesicles with a wall thickness of ca. 5–10 nm were prepared by self-assembly of HBPO-star-PEOs (HSPs). Precisely, 20 mL of distilled water was added to 0.2 g HSP polymers in a 50 mL quartz flask under room temperature. Water is a good solvent for the PEO arms and a nonsolvent for the HBPO cores. The polymer slowly dissolved under stirring, and finally, a colorless but slightly turbid solution was obtained, indicating the successful self-assembly of HSPs. Encapsulation of MMA, CPADB, and fluorescein into vesicle membranes The reactants, including the monomer MMA (10 mmol, 1.06 mL), reversible addition–fragmentation chain transfer (RAFT) reagent CPADB (50 μmol, 14.0 mg), and the catalyst of fluorescein (5 μmol, 1.7 mg), were encapsulated into the vesicle membranes. For the experiments, CPADB and fluorescein were solubilized in MMAs first and then dropped together into the aqueous vesicle solution in a 50 mL quartz flask. A small amount of THF (0.05 V‰) was used to solubilize reactants better to facilitate their penetration into vesicle membranes. Then the flask was wrapped with aluminum foil and stirred at 25 °C in the dark; 100 μL sample was taken from the vesicle solutions every 1 h for UV, dynamic light scattering (DLS), transmission electron microscopy (TEM), and optical microscopy tests. PET-RAFT polymerization of PMMA within the vesicle membranes After encapsulation for 24 h, the irradiation source (4.8 W blue LED light, λmax = 465 nm) was turned on to conduct the photo-induced electron transfer-RAFT (PET-RAFT) polymerization of MMA within the vesicle membranes at ambient temperature and pressure for 24 h. The distance between the light and the flask was 4 cm. The polymerization was terminated by removing the light source, and then the reaction mixture was precipitated in ethanol under stirring. A white precipitate was collected, re-dissolved in a minimal amount of dichloromethane, followed by three times precipitation in ethanol. Ethanol was selected because it is a good solvent for HSPs but a poor solvent for PMMA. Subsequently, the pure PMMA polymers were dried under vacuum at 50 °C for 24 h prior to further characterizations. Results and Discussion As shown in Figure 1, each HSP has a hydrophobic core of HBPO and many hydrophilic PEO arms. In this work, five HSP polymers were prepared, and the DParm were varied at 2, 4, 6, 10, and 14. The synthesis and characterization of the HSP polymers were described in detail in the Supporting Information Figures S1–S3 and Table S1. To differentiate one sample from the other, we denoted the polymers as HSPn, where n means DParm. For example, the HSP with DParm = 2 was named HSP2. The self-assembly of HSP vesicles was performed by a direct hydration process of putting water into the polymers. We previously showed that the average size of the HSP vesicles could be tuned by varying the DParm, and the larger the DParm, the smaller the vesicle size.35,37 Herein, as shown in Table 1, the average Dh of the used HSP vesicles measured by DLS was decreased from 840 to 85 nm with increasing DParm from 2 to 14 ( Supporting Information Figure S4). The atomic force microscopy (AFM) measurement results ( Supporting Information Figure S5) indicated that the thickness of the HSP2, HSP6, and HSP14 vesicle membranes was 7.21 ± 1.30 nm, 7.43 ± 1.61 nm, and 7.08 ± 1.48 nm, respectively, according to the statistical analyses of 50 vesicle samples for each. In other words, the HSP2–HSP14 vesicles have a similar vesicle wall thickness, despite the apparent difference in their size and curvature. Figure 1 | Schematic illustration of vesicle membrane-confined PET-RAFT polymerization of isotactic PMMA. The HBPO cores are in magenta, and the PEO arms are in cyan. Download figure Download PowerPoint Table 1 | Characterizations of Stereoregular PMMA Through Vesicular Catalysis Reaction medium DParm Dh (nm) Tacticity(mm∶mr∶rr)a Mnb Mw/Mnc Tg/°Cd DMSO – – 2∶33∶65 2.0 × 104 1.17 127 Water/Tween – – 3∶34∶63 2.5 × 104 1.25 127 HSP2 2 840 12∶28∶60 7.1 × 104 1.47 114 HSP4 4 627 19∶25∶56 5.6 × 104 1.66 105 HSP6 6 308 28∶29∶43 4.1 × 104 1.20 90 HSP10 10 195 32∶26∶42 3.5 × 104 1.35 86 HSP14 14 85 40∶23∶37 2.6 × 104 1.27 72 Reaction conditions: 25 °C, [MMA]∶[CPADB]∶[fluorescein] = 200∶1∶0.1. aThe ratios of isotacticity, atacticity, and syndiotacticity determined by 1H NMR analysis, mm = isotactic triads; mr = atactic triads; rr = syndiotactic triads. bNumber average molar mass determined by gel permeation chromatography (GPC). cMolar-mass dispersity determined by GPC. dGlass transition temperature of PMMA determined by DSC. The encapsulation process of reactants into vesicle membranes For the stereoregular polymerization process, the reactants of hydrophobic monomer MMA, RAFT agent CPADB, and the photocatalyst fluorescein were mixed together and then added into the aqueous HSP vesicle solution. With stirring, the reactants were encapsulated gradually within the HBPO layers of the vesicles spontaneously, which was subsequently submitted to photopolymerization (Figure 1). We first monitored the encapsulation process of the reactants within the vesicle membrane. Taking HSP14 vesicles as an example, 20 mL HSP14 vesicle solution (10 mg/mL) was first charged with MMA (0.5 M), CPADB, and fluorescein at a molar ratio of 200∶1∶0.1. Under stirring, the erstwhile turbid solution—due to the immiscibility of the reactants in water, formed an emulsion and became more transparent rapidly with time ( Supporting Information Figure S6a). Such a process was followed by the UV–vis spectroscopy at a wavelength of 510 nm, which showed an increasing transmittance with time until a plateau was established (Figure 2a). In contrast, without vesicles, the hydrophobic reactants quickly precipitated at the bottom of the vessel ( Supporting Information Figure S6b). Thus, we presumed that these hydrophobic reagents were encapsulated gradually within the vesicle membranes, and thus, the solution became transparent and stable. Figure 2 | Characterizations of the HSP14 vesicle solution in encapsulation with reactants of MMA, CPADB, and fluorescein. (a) Time evolution of UV transmittance with MMA concentrations ranging from 0.5 to 1.0 M. All samples were diluted 60 times before measurements. (b) The particle size distributions of pure vesicles (in green) and the reaction solution for different encapsulation periods (in other colors). (c) TEM image and (d) Fluorescent micrograph of the vesicles after encapsulation for 12 h. Download figure Download PowerPoint With the encapsulation, the HSP14 vesicle size was kept within 8 h and then expanded with time through a fusion process, followed by DLS (Figure 2b) and optical microscope ( Supporting Information Figure S7) measurements. As a control experiment, the pure HSP14 vesicle solution was stirred under the same reaction condition for 36 h; the size of vesicles was almost unchanged ( Supporting Information Figure S8), indicating that the increase of vesicle size, as shown in Figure 2b, is due to the solubilization of the reagents in the membranes. In addition, both the optical microscopy ( Supporting Information Figure S7) and TEM (Figure 2c and Supporting Information Figure S9) measurements indicated that the vesicle morphology was intact after the encapsulation. Furthermore, the vesicles showed green fluorescent rims after encapsulation under fluorescent microscopy (Figure 2d), which provided direct evidence to support the successful confinement of fluorescein into vesicle membranes. After encapsulation, such green fluorescent vesicles were much clearer for larger HSP2 vesicles ( Supporting Information Figure S10). Besides, the qualitative analysis of sulfur elements within vesicles from the field-emission TEM (FE-TEM) image also proved the successful encapsulation of CPADB into vesicles ( Supporting Information Figure S11). The encapsulation of hydrophobic reactants, including MMA monomers, CPADB initiators, and fluorescein catalysts into hydrophobic layers of vesicle membranes driven by hydrophobic interactions, was well expected. The saturated adsorption capacity can be calculated from the transmittance experiments. As shown in Figure 2a, with the variation of reactant concentration, represented by MMA from 0.5 to 1.0 M, the maximum transmittance at the plateau, the so-called saturated transmittance, was nearly constant from 0.5 to 0.8 M, then decreased with further increase in reactant concentration. The result indicated that almost all of the MMA were encapsulated into vesicles when the concentration was lower than 0.8 M. As a result, the vesicle solution reached a similar saturated transmittance. However, when the concentration was higher than 0.8 M, some residual reactants were kept in the solution, which led to a lower saturated transmittance. Herein, the saturated adsorption concentration is 0.8 M. Thus, in the following experiment, an MMA concentration of 0.5 M was selected, assuming that all monomers would be encapsulated into vesicle membranes at this concentration. After encapsulation equilibrium, the vesicles were relatively stable, and no decantation was observed when it was left standing or even centrifuged at a rate of 4000 rpm. PET-RAFT polymerization within the membrane of vesicles The PET-RAFT polymerization of MMA38,39 within the vesicle membrane was conducted after encapsulation of MMA, CPADB, and fluorescein into HSP2-HSP14 vesicles with different sizes. The mechanism for PET-RAFT polymerization mediated by fluorescein is shown in Supporting Information Figure S12. The PMMAs obtained have a number average molar mass (Mn) of around 20–70 kDa and molar mass dispersity of around 1.2–1.7 (Table 1). In the 1H NMR spectrum of obtained PMMAs, α-methyl protons showed three peaks at 1.21, 1.02, and 0.86 ppm (Figure 3), which represented the isotactic (mm), atactic (mr), and syndiotactic (rr) PMMA triads, respectively.40–43 It has been widely accepted that percentage could be used among these three stereo-triads (mm:mr:rr) of PMMAs to determine tacticity, where mm represents isotacticity, mr represents atacticity, and rr represents syndiotacticity.40,41 As shown here, we found that the isotacticity of the obtained PMMAs increased from 12% to 40% with decreased HSP vesicle size (Figure 3; Table 1). In addition, the 40% PMMA isotacticity obtained from HSP14 vesicles was much higher than that of PMMAs prepared in the presence of Lewis acid25 and the confined nanochannels of MOF.26 Figure 3 | 1H NMR spectra of PMMA produced by PET-RAFT polymerization within the vesicle membrane (CDCl3, 500 MHz, 298 k). The numbers show the ratios of isotacticity, atacticity, and syndiotacticity of the resulting polymers. Download figure Download PowerPoint We believed the polymerization was performed inside HSP vesicle membranes. First, as proved above, the reactants of hydrophobic MMA, CPADB, and fluorescein were encapsulated inside the vesicle membranes before polymerization. Second, as control experiments, the RAFT polymerization of these reactants in water with Tween surfactants ( Supporting Information Figure S13) or in DMSO was performed, and almost no PMMA with isotactic configuration was produced; however, PMMA with syndiotacticity and atacticity, accounting for 65% and 35%, respectively, were obtained (Figure 3). Collectively, these results indicated that the isotactic-rich PMMAs obtained in this work were attributed to the confined polymerization inside the vesicle membranes. The glass transition temperature (Tg) of the isotactic-rich PMMAs obtained was evaluated by differential scanning calorimetry (DSC). As a disubstituted vinyl polymer, PMMA has a higher Tg when it was syntactic or atactic—owing to the good symmetry and small entropy of the repeating units, thus having poor flexibility.44 As a control experiment, the PMMAs synthesized in solution (DMSO) exhibited a Tg of 127 °C (Figure 4), similar to that of commercial PMMAs ( Supporting Information Figure S14). On the other hand, the Tg values of the fabricated isotactic-rich polymers produced from the HSP vesicles dropped from 114 °C to 72 °C with an increased degree of isotacticity (Figure 4; Table 1), becoming closer to that of fully isotactic PMMAs at 45 °C.45 Figure 4 | The DSC traces of PMMAs with different stereoregularity, synthesized within the HSP vesicles with DParm increasing from 2 to 14 (solid color lines) and DMSO (dashed gray line). The subscripts in brackets indicate different isotacticities of the resulting PMMA polymers. Download figure Download PowerPoint The mechanism of vesicle membrane-confined polymerization Similar to MOFs26 and silica nanopores,43 the vesicle membranes might suppress the molecular motions of MMA monomers (rotation and vibration) due to the confined space, leading to the high isotacticity of the resulting PMMAs. In addition, as a smaller diameter of a vesicle corresponds to a higher curvature, it provides a more pronounced restriction of molecular motions, thereby leading to a higher ratio of isotacticity of the polymers. To prove these hypotheses and analyze the mechanism of vesicle membrane-confined polymerization, we employed in spectroscopy to the of the at of MMA monomers the encapsulation by vesicles (Figure For all HSP polymer vesicles, were with increased encapsulation time from 1 to 10 h; the higher the DParm, the the Figure | In of vesicle solution after encapsulated with MMA monomers for different (a) spectra of HSP vesicle solutions HSP6, and (b) Schematic of to the and of MMA after (c) of ratio of at filled with lines) and with lines) of HSP vesicle solution with encapsulated Download figure Download PowerPoint we the into with an maximum at and (Figure Supporting Information Figures The at was attributed to the the at was attributed to the and related to the the and of due to the confined effect of vesicle the of these we can the ratio of MMA monomers in free and confined Taking HSP2 as an example, decreased from to with the of from to after 10 encapsulation (Figure Supporting Information Table results indicated that the MMA monomers were confined within the vesicle membranes, with a in an isotactic The other HSP vesicles showed a similar monomer conformation confinement effect with however, such an effect was related to vesicle As shown in Figure with a decreased vesicle size, the of after encapsulated for 1 h increased from for HSP2 to for Furthermore, the of was more pronounced and when smaller vesicles were For all vesicle the of MMA monomers would after encapsulation for a specific The was about and the time was 4 h for and h for HSP6, and and h for HSP2, (Figure Supporting Information Table The increase in the and the decrease of time of for HSP vesicles with smaller indicated that the MMA monomers were more in the encapsulation the smaller vesicles have a larger curvature, we believed that the of isotactic of the MMA monomers from an increased membrane as in Figure This is in with the increasing ratio of isotactic PMMAs from 12% for HSP2 to 40% for HSP14 vesicles (Table 1). In to further prove the we the dynamic of stereoregularity of PMMAs the confined polymerization inside HSP6 vesicle membranes. As shown in Supporting Information Table with increased polymerization time from 4 to 24 h, the of PMMA the tacticity was kept almost the of the 1H NMR measurement results further that the isotacticity of PMMA was from the confined conformation of the MMA monomers inside the vesicle membranes, as described in the mechanism in Figure We have developed a green polymerization for the preparation of isotactic-rich PMMA in aqueous solution under mild using vesicle membranes as the confined reaction The vesicle membranes the of the monomers in an isotactic which isotactic PMMAs Furthermore, decreased vesicle size increased isotacticity of the resulting polymers to due to the To the of our this the first of such confinement effect of vesicles for our a in the application of vesicles in stereoselective green polymerization. We that such a vesicular process also can be a green to stereoselective Supporting Information The Supporting Information is and and Experimental Figures and of are no to This work was by the Science of and and the for of Shanghai Science and for a Green Chemistry from and and of Green Chemistry in and M. and of Green the of of for the of in Catalysis and Lipshutz and of in Zhou Lipshutz the and Aqueous 10, Lipshutz and by and in Water at Synthesis of Polymerization in a Cai

Topics & Concepts