Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
Chemoselective RAFT Polymerization of a Trivinyl Monomer Derived from Carbon Dioxide and 1,3-Butadiene: From Linear to Hyperbranched Lifeng Chen, Yao Li, Sicong Yue, Jun Ling, Xufeng Ni,* and Zhiquan Shen MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *
ABSTRACT: 3-Ethylidene-6-vinyltetrahydro-2H-pyran-2-one (EVL), a substituent δ-lactone synthesized through telomerization of CO2 with 1,3-butadiene, is highly functionalized containing a six-membered ring and two vinyl groups. It can hardly be polymerized directly through common methods due to its poor polymerizability. In this paper, we report the controlled polymerization of a highly reactive trivinyl monomer derived from EVL, i.e. methyl-2-ethylidene-5-hydroxyhept-6-enoate methacrylate (MEDMA), which is synthesized through ring cleavage of EVL and subsequent esterification with methacryloyl chloride. Chemoselective RAFT polymerizations of MEDMA mediated by 2-cyanoprop-2-yl-dithiobenzoate (CPDB) are achieved and well-defined polymers with linear and hyperbranched topologies are obtained under optimized polymerization conditions. RAFT polymerizations show good control in producing linear PMEDMAs with predetermined molecular weights (4000 g/mol) and moderate polydispersity indices (below 1.22). The resultant hyperbranched PMEDMAs are fully characterized by 1H NMR, 13C NMR, 1H−13C HMQC, DEPT, DLS, and TEM. The incorporations of vinyl group in allylic ester into polymer chains lead to the hyperbranched topology. Both PMEDMAs are ready for the introduction of amino and carboxy groups through thiol−ene click chemistry, and the products self-assemble to the micelles with different morphologies. This protocol develops the utilization of EVL in synthetic polymers and is significant to the carbon dioxide transformation.
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of CO2 with 1,3-butadiene for the first time in 1976.9 The resultant δ-lactone is highly functionalized with a sixmembered ring, an internal and a further terminal carbon− carbon double bonds, which leads to the conversions of the δ-lactone to various products of potential industrial relevance.10 Through this δ-lactone intermediate, CO2 can be efficiently converted to many organic substances.10,11 The telomerisation reaction is accompanied by several byproducts, and considerable efforts have been made to improve the conversion and selectivity to this six-membered δ-lactone.12−15 Behr and co-workers optimized the reaction to above 95% selectivity to the δ-lactone and an overall butadiene conversion of 45%.10,16 Reaction conditions were developed to miniplant scale.17 Recently Beller and co-workers reported the highest yield of 67% by use of a palladium/TOMPP-catalyst system.15 Although EVL contains two carbon−carbon double bonds and a six-membered ring, the reports of direct polyaddition or ring-opening polymerization still remain very few. Dinjus et al. investigated the radical, anionic, and cationic polymerizations of EVL, and only oligomers with low conversion were obtained.18 Its poor polymerizability stems from the structure similarities
INTRODUCTION Chemical transformations of carbon dioxide (CO2) are valuable but limited as a renewable C1 feedstock. Due to the stable chemical structures of CO2, its reactions often encounter high energy barrier; thus, coreagents with high energy are usually required to gain a thermodynamic driving force. Though the coupling reactions of epoxides with CO2 to produce cyclic carbonate or polycarbonates have been reported intensively,1−6 developing new methods for CO2 transformation to other chemical products is attractive and meaningful. More recently, Lu’s group developed new methods to synthesize polyesters from CO2 and 2-butyne via α-methylene-β-butyrolactone intermediate.7,8 Among the large abundance of chemicals, olefins, most commonly used ones today, are ideal coreagents with CO2 in the synthesis of novel chemical products. Inoue and colleagues reported the synthesis of 3-ethylidene-6-vinyltetrahydro-2H-pyran-2-one (EVL, Scheme 1) through telomerisation Scheme 1. Structures of EVL, Allylic Ester, and Tiglate Ester
Received: October 19, 2017 Revised: November 18, 2017
© XXXX American Chemical Society
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DOI: 10.1021/acs.macromol.7b02238 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules with tiglic acid ester and α-substituted allyl ester (Scheme 1), which are difficult to be polymerized.18,19 In 2014, Nozaki and co-workers reported the first successful polymerization of EVL through conventional radical polymerization, and the resultant polymers own molecular weights up to 8.5 × 104 g/mol. However, the process does not have good control over the structures, producing polymers with three different repeating units in uncontrollable ratios.19 To the best of our knowledge, no other polymerization of EVL has been published so far. The utilization of EVL in synthetic polymers is of great significance as a way of CO2 transformation. In addition, polymerizations of multivinyl monomers without cross-linking are challenging and require specific catalysts.20−22 Group transfer polymerization (GTP) is a robust tool to produce linear polymers in the polymerization of divinyl monomers, but a high purity of reactants and highly anhydrous conditions are required.23,24 In contrast, the polymerizations of divinyl monomers by controlled radical polymerizations like ATRP25 and RAFT26−28 only produce hyperbranched polymers. In some cases, copolymerization of divinyl monomers with other monovinyl monomer is required to avoid cross-linking. The polymerizations of the monomers containing more than two vinyl groups to produce soluble (linear) polymers remain even fewer because it is much easier to cross-link. Herein we propose two facile reactions of EVL to the synthesis of a novel trivinyl monomer methyl-2-ethylidene-5-hydroxyhept-6-enoate methacrylate (MEDMA; Scheme 2). Furthermore, chemoselective RAFT
polymerizations of MEDMA were realized, producing pure linear or hyperbranched polymers under optimized conditions. Abundant vinyl groups in PMEDMA are ready for postpolymerization functionalizations through thiol−ene click chemistry, and the products exhibit topology-dependent assembly behaviors.
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EXPERIMENTAL SECTION
Materials. Tricyclohexyl phosphine (PCy3; 98%+) was purchased from Admas-beta, palladium(II) acetylacetonate (Pd(acac)2; 99%) and 2-cyanoprop-2-yl-dithiobenzoate (CPDB; 97%) were purchased from Strem Chemicals and used as received. Methacryloyl chloride (97%) was purchased from J&K Scientific Co., Ltd., and distilled prior to use. Acetonitrile (CH3CN) and anisole were bought from Sinopharm Chemical Reagent Co., Ltd. and distilled prior to use. Tetrahydrofuran (THF) and triethylamine (TEA) were stirred over CaH2 and followed by distillation. AIBN (Sinopharm Chemical Reagent Co., Ltd.) was recrystallized from methanol twice. CO2 (>99.99%) was used as received. 1,3-Butadiene from Sinopec Shanghai Petrochemical Company, Ltd. was purified with columns filled with KOH and Al2O3. Measurements. Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker Advance DMX 400 spectrometer (1H: 400 MHz and 13C: 100 MHz). Distortionless enhancement by polarization transfer (DEPT) 135 technique was used to assist in the assignment of 13 C NMR spectra. Molecular weights (MWs) and polydispersity indices (PDIs) were determined by gel permeation chromatography (GPC) using a Waters-150C apparatus equipped with Waters Styragel HR3 and HR4 columns and a Water 2414 refractive index detector.
Scheme 2. Monomer Synthetic Scheme and RAFT Polymerization of MEDMA to Linear and Hyperbranched Polymers
Table 1. RAFT Polymerization of MEDMA Mediated by CPDBa entry
M:CPDB:AIBN
t (h)
conversionb (%)
Mn,calc (g/mol)
Mn,GPCd (g/mol)
PDId
1 2 3 4 5 6 7 8 9 10 11
30:1:0.2 30:1:0.2 30:1:0.2 60:1:0.2 60:1:0.2 60:1:0.2 100:1:0.2 100:1:0.2 100:1:0.2 200:1:0.2 200:1:0.2
6 8.5 14 4 9 13.5 3 9 16 2 8
40 52 69 28 51 67 11 32 60 8 29
3200 4100 5400 4400 7900 10 300 3100 8300 15 400 4200 14 800
3300 4000 4800 4200 7200 10 100 3900 7500 13 100 4500 13 400
1.21 1.22 1.42 1.25 1.36 1.88 1.21 1.40 2.19 1.23 1.94
Polymerizations were conducted with CPDB as CTA and AIBN as initiator in anisole at 70 °C. The monomer concentration was 1 mol/L. Conversion was determined by 1H NMR spectroscopy. cMn,cal = [M0] × MWmonomer × conversion/[CTA] + MWCTA. dAs determined by GPC in THF calibrated with PS standards. a b
B
DOI: 10.1021/acs.macromol.7b02238 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules THF was used as eluent with a flow rate of 1.0 mL/min at 40 °C, and narrow PDI polystyrene samples were used as calibration standard. Absolute molecular weights were determined by GPC/multiangle laser light scattering (GPC/MALLS) which consisted of a Waters 1515 isocratic high performance liquid chromatograph pump, a Wyatt DAWN DSP MALLS detector, a Wyatt Opitlab-DSP interferometric refractometer (RI), and a column of PLgel 5 μm MIXED-C. DMF containing 0.05 mol/L LiBr was used as the eluent with a flow rate of 1.0 mL/min at 60 °C. Matrix-assisted laser desorption ionization-timeof-flight (MALDI-ToF) mass spectra were collected on a Bruker UltraFLEX MALDI-ToF mass spectrometer in the linear mode, and dithranol was used as matrices. Intrinsic viscosities of polymer solutions in THF were measured by an Ubbelohde viscometer at 25 °C. The hydrodynamic diameters were measured by dynamic light scattering (DLS) using a particle size analyzer (Zetasizer Nano Series, Malvern Instruments) at 25 °C. Measurements were made at a fixed angle at 90° and a wavelength of 657 nm. Transmission electron microscope (TEM) images were recorded by a HITACHI HT7700 TEM instrument with the accelerating voltage of 100.0 kV. Samples were prepared by placing a drop of sample solution (0.3 mg/mL) onto a copper grid (water solution) or carbon film (organic solution) and stained with 2% phosphotungstic acid solution. Synthesis of 3-Ethylidene-6-vinyltetrahydro-2H-pyran-2one (EVL). EVL was synthesized according to the literature.19 Pd(acac)2 (0.101 g) and PCy3 (0.280 g) were dissolved to 100 mL of CH3CN in a 1 L stainless reactor. When the temperature of the reactor was reduced to −20 °C, 77 g of 1,3-butadiene was added slowly, and then the reactor was inflated with CO2 until the pressure reached 35 bar. The reactor was kept under 90 °C for 40 h, and the pressure decreased from 35 to 28 bar. After degassing the reactor, solvent was removed by distillation. Raw product was purified by column chromatography (with hexane/EtOAc = 3/1, Rf = 0.29). Distillation (110−120 °C, 140 Pa) was conducted to totally remove the catalyst. Yield: 19%. 1H NMR (400 MHz, CDCl3): δ ppm, 7.14 (m, 1H), 5.88 (ddd, J = 17.2, 10.6, 5.4 Hz, 1H), 5.47−5.15 (m, 2H), 4.77 (m, 1H), 2.72−2.52 (m, 1H), 2.50−2.36 (m, 1H), 2.05 (dtd, J = 13.9, 5.4, 2.8 Hz, 1H), 1.87−1.66 (m, 4H). 13C NMR (100 MHz, CDCl3): δ ppm, 166.32, 141.28, 135.80, 125.89, 116.97, 78.95, 27.63, 21.98, 14.14. ESI-MS: product + Na+, 175 Da; calculated 175 Da. NMR spectra are shown in Figure S1 and S2 in the Supporting Information. Synthesis of Methyl-2-ethylidene-5-hydroxyhept-6-enoate methacrylate (MEDMA). A total of 20 g (0.13 mol) of EVL was dissolved in 100 mL of methanol with 6.6 g (0.065 mol) of TEA, and the solution was refluxed under 70 °C for 24 h. Methanol and TEA were removed by rotary evaporation, and the crude alcoholized EVL was obtained. Conversion of EVL was 80.6% determined by 1H NMR. The product was directly used in next step without further purification. Then 10 g of alcoholized EVL was dissolved in 60 mL of dry THF with 5.5 g of TEA and kept under 0 °C. A solution of methacryloyl chloride (5.7 g, 20 mL THF) was added dropwise within 30 min. This solution was kept stirring overnight. After removing the salt and THF, the concentrated solution was dissolved in DCM and washed with saturated Na2CO3 aqueous solution three times and then dehydrated with sodium sulfate anhydrous. Column chromatography was conducted with hexane/EtOAc = 15:1. Pure MEDMA monomer was obtained as a colorless oil with Rf = 0.21. Total yield 76%. 1 H NMR (400 MHz, CDCl3): δ ppm, 6.87 (q, J = 7.2 Hz, 1H), 6.15 (t, J = 1.3 Hz, 1H), 5.83 (ddd, J = 17.0, 10.6, 6.0 Hz, 1H), 5.57 (p, J = 1.6 Hz, 1H), 5.35−5.12 (m, 3H), 3.72 (s, 3H), 2.37 (td, J = 7.1, 2.3 Hz, 2H), 1.96 (t, J = 1.2 Hz, 3H), 1.78 (t, J = 6.6 Hz, 5H). 13C NMR (100 MHz, CDCl3): δ ppm, 166.65, 138.19, 136.55, 136.19, 132.17, 125.39, 116.70, 77.22, 74.51, 51.66, 33.24, 22.12, 18.34, 14.16. ESI-MS: product + Na+, 275.2 Da; calculated 275.3 Da. Polymerization Procedures. RAFT polymerization of MEDMA was conducted in anisole with CPDB as chain transfer agent (CTA) and AIBN as initiator at 70 °C. Take entry 1 in Table 1 as an example for description. A total of 1097 μL (3.965 × 10−2 mmol) of CPDB (0.008 g/mL in anisole) and 651 μL (7.929 × 10−3 mmol) of AIBN (0.002 g/mL in anisole) were added to 0.300 g of monomer (1.189 mmol). After degassing the solution three times by freeze−vacuum−thaw cycles,
the mixture was immersed in an oil bath thermostated at 70 °C. Polymerization was quenched by rapid cooling to −20 °C after a certain time. A sample was taken for 1H NMR analysis to determine the conversion. The residual part was precipitated in hexane three times, collected, and dried under vacuum. Thiol−ene Click Reaction. A total of 0.030 g of PMEDMA (1.2 × 10−4 mol repeating unit), 1.2 × 10−4 mol of thiol compound, and 5 wt % of AIBN were dissolved in solvent (DMF for 2-aminoethane-1-thiol, THF for 3-mercaptopropionic acid). After degassing, the solution was stirred at 70 °C overnight. Products were isolated by precipitated in diethyl ether three times and dried under vacuum.
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RESULTS AND DISCUSSION RAFT Polymerization of MEDMA. MEDMA was characterized by mass spectroscopy (see the Experimental Section), 1H NMR (Figure 1a), 13C NMR (Figure 1b), 1H−13C HMQC (Figure S3), and 1H−13C HMBC (Figure S4), and the results were discussed in the Supporting Information in detail. RAFT polymerizations of MEDMA mediated by CPDB with
Figure 1. NMR spectra of MEDMA and PMEDMA in CDCl3: (a) 1H NMR spectrum of MEDMA. (b) 13C NMR spectrum of MEDMA. (c) 1H NMR spectrum of linear PMEDMA. C
DOI: 10.1021/acs.macromol.7b02238 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules AIBN as initiator were carried out in anisole at 70 °C, and the results of the polymerization are summarized in Table 1. The polymerizations of MEDMA are investigated with various [M]/[CPDB]/[AIBN] molar ratios in feed. Under the condition of [M]/[CPDB]/[AIBN] = 30/1/0.2 (entries 1−3 in Table 1), the polymerization process shows good controllability with narrow to moderate PDIs, and the number-average molecular weights of PMEDMA determined from GPC measurements (Mn,GPC) are in good agreement with the molecular weights in theory (Mn,cal) which are calculated according to conversions from 1H NMR spectra. In the 1H NMR spectrum of PMEDMA (Figure 1c), the signals of HF, HJ, HK, and HD can be found and assigned to the remaining carbon−carbon double bonds and the CTA residue, respectively. When the polymerization condition changes to [M]/[CPDB]/ [AIBN] = 60/1/0.2, the resultant polymers show low PDIs with Mn,GPC close to Mn,cal up to 51% conversion, indicating the
polymerizations are under good control. Kinetic studies under the condition of [M]/[CPDB]/[AIBN] = 60/1/0.2 are investigated, and the results further prove the controllability of the polymerization (Figure 2). A linear plot of Ln([M0]/[Mt]) versus time (Figure 2a) appears up to monomer conversion of 57%, indicating the first order kinetics with respect to monomer concentration. The linear relationship between MW and conversion as well as the relatively low PDIs (≤1.56) further demonstrate the good control (Figure 2b) of polymerization. GPC curve of entry 2 in Table 1 (Figure 3a) shows unimodal and narrow distribution. The MALDI-ToF mass spectrum (Figure 3c) gives the absolute molecular weight of PMEDMA. Peak A is assigned to the chains following normal RAFT mechanism, while peak D represents those terminated during polymerization. Only linear polymers are generated without the observation of branched chains (see part II in the
Figure 2. (a) Kinetic plot of the polymerization of MEDMA under [M]/[CPDB]/[AIBN] = 60/1/0.2, the best fit line with R2 = 0.984. (b) Numberaverage molecular weights from GPC and PDI versus conversion, solid square: Mn; open square: PDI.
Figure 3. (a) GPC curve of the sample (entry 2, Table 1) in THF. (b) GPC curves of the samples in Table 1 in THF and (c) MALDI-ToF mass spectrum of PMEDMA (entry 2, Table 1). D
DOI: 10.1021/acs.macromol.7b02238 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Table 2. RAFT Polymerization of MEDMA Mediated by CPDBa entry
M:CPDB: AIBN
t (h)
conversionb (%)
Mwc (g/mol)
Mwd (g/mol)
PDIc
[η] (dL/g)e
12 13 14
60:1:0.5 100:1:0.5 200:1:0.5
26 24 12
92 90 76
146 000 108 100 149 700
312 900 221 700 252 900
6.32 4.95 5.06
0.19 0.14 0.17
Polymerizations were conducted in anisole with CPDB as CTA and AIBN as initiator in anisole at 70 °C. Monomer concentration was 1 mol/L. Conversion was determined by 1H NMR spectroscopy. cAs determined by GPC in THF with RI detector. dAs determined by GPC in DMF with laser-light scattering detector. eIntrinsic viscosity. a b
Figure 4. (a) 13C NMR spectrum and (b) DEPT135 spectrum of hyperbranched PMEDMA.
Supporting Information for details) though peaks B and C remain unknown. The GPC curves of entries 4 (Figure 3b), 7, and 10 (Figure S5a,b) in Table 1 show unimodal and narrow distributions. On the contrary, samples of entries 6, 9, and 11 with high monomer conversions over 67% exhibit high molecular weights and broad PDIs up to 2.19. Polymerizations with high conversions were conducted, and the corresponding results are summarized in Table 2. Polymers with higher molecular weights and broader PDIs are obtained, and in addition, their absolute molecular weights are much larger than the relative ones. The intrinsic viscosities of polymers characterized by Ubbelohde viscometer are relatively small and arise with the increase of molecular weights. A stable signal in DLS measurement indicates that unimolecular nanoparticles of PMEDMA (entry 12) with the diameter of 26.9 nm exist in the solution of good solvent THF (0.5 mg/mL; Figure S6b). Furthermore, TEM image shows nanoparticles with the diameters between 10 and 30 nm (Figure S6c). In comparison, there is no stable signals for linear PMEDMA (entry 2 in Table 1) in DLS (Figure S6a). Therefore, a hyperbranched topology of PMEDMA samples is proved.29 To compare the polymerization reactivities of vinyl groups of allylic ester and tiglate ester, we carried out a model reaction by copolymerizing cis-3-hexenyl tiglate and allyl phenyl ether separately with MMA. The vinyl group of allyl phenyl ether incorporated into PMMA backbones, while cis-3-hexenyl tiglate
Figure 5. Polymerization for kinetic study was conducted under [M]/[CPDB]/[AIBN] = 100/1/0.5. (a) Conversion of MEDMA versus time. (b) Molecular weights (GPC), PDI versus conversion, solid square: number-average molecular weights; open square: weightaverage molecular weights; solid triangle: PDI. (c) GPC curves of samples collected at different time during the polymerization.
did not (Figures S7 and S8 and detailed discussion in the Supporting Information). Therefore, we conclude that the incorporation of the ally group into backbones during MEDMA polymerizations results in the formation of hyperbranched E
DOI: 10.1021/acs.macromol.7b02238 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Scheme 3. Thiol−ene Click Reactions of PMEDMAs with Different Thiol Compounds
pendent vinyl groups are valuable since they can be easily modified by thiol−ene click chemistry to fluorescent materials,35 functional biomaterials,36 zwitterionic polymers,37 brush polymers,38 etc. Here we demonstrate the functionalization of both linear and hyperbranched PMEDMAs with different thiol compounds (Scheme 3). Double bonds conversions in Table S1 are determined through the ratio of the integrations of Hg to Hh and Hd shown in Figures S11, S12, and S13. PMEDMAs bearing amino groups exhibit pH sensitive behavior after modification. As shown in Figure 6, both
topology. Hyperbranched PMEDMA has been further characterized by 13C NMR, 1H−13C HMQC, and DEPT135 spectra. In the 13C NMR spectrum (Figure 4a), the signals at 138.05, 135.19, 132.21, and 118.91 ppm are assigned to the carbons in the pendent vinyl groups Ck, Cf, Cj, and Cg, respectively. Co at the branching point locates at 22.60 ppm, while no such signal is observed in linear PMEDMA (Figure S9). In addition, the 1H−13C HMQC spectrum (Figure S10) confirms the correlation of Co with Ho that overlaps with Hb, Hh, and He in 1 H NMR spectrum. The DEPT135 spectrum distinguishes primary and tertiary carbons with positive signals from secondary carbons.30 A positive signal is found at 22.60 ppm in Figure 4b, further supporting the assignment of Co at the branching point. No evidence is observed for the participation of the vinyl groups of tiglate ester. Therefore, the incorporation of ally groups leads to the hyperbranched PMEDMA while tiglate ester remains intact. Kinetic studies were conducted to better understand the polymerization process. Reacting samples were taken out of the polymerization mixture and quenched by rapid cooling to −20 °C. 1H NMR and GPC analyses were conducted without precipitation to determine the conversions and molecular weight distributions, respectively (Figure 5a−c). PMEDMA with relatively low PDI of 1.70 at 49% conversion was obtained within 2 h, and its GPC curve remained monomodal distribution. When the conversion reached 68% after 4 h, a high molecular weight shoulder appeared and the corresponding PDI increased to 2.32. With the increase of monomer conversion, the molecular weight distributions went broader. The bimodal distribution of GPC curves became significant. Similar phenomena were observed in the syntheses of hyperbranched polymers via self-condensing vinyl polymerization (SCVP).31,32 The monomer conversion reached 90% after 14 h, and the PDI of the product was observed as 4.83. In the polymerization kinetics of MEDMA, Mw of the products rapidly expands while conversion hardly increases at the last period of polymerization, which shows a similarity with the polycondensation nature of SCVP.31,33,34 Postpolymerization Functionalization of PMEDMA through Thiol−ene Click Chemistry. Polymers with
Figure 6. Transmittance versus pH of L-PMEDMA-NH2 aqueous solution (0.3 mg/mL) at λ = 500 nm.
L- and HB-PMEDMA-NH2 are able to be dissolved directly in water at pH 1.7 to form a transparent aqueous solution. Transmittance of the solution decreases rapidly with the increasing of pH value, and a turbid solution is finally formed at pH 5.6. Figure 7a,b shows the TEM images of L-PMEDMA-NH2 and HB-PMEDMA-NH2 from aqueous solutions. Nanoparticles are observed with the average diameters of 18 nm for L-PMEDMANH2 and 26 nm for HB-PMEDMA-NH2. After slow addition of water to the DMF solutions (0.3 mg/mL) and dialysis against deionized water, HB-PMEDMA-NH2 and F
DOI: 10.1021/acs.macromol.7b02238 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
Figure 7. TEM images of modified PMEDMAs in aqueous solution (0.3 mg/mL; stained with 2% phosphotungstic acid solution), (a) L-PMEDMA-NH2 at pH 2.1 and (b) HB-PMEDMA-NH2 at pH 2.6. DLS results for assemblies of modified PMEDMAs in aqueous solution with the concentration of 0.3 mg/mL at 25 °C and corresponding TEM images (stained with 2% phosphotungstic acid solution), (c) and (e) for HB-PMEDMA-NH2 and (d) and (f) for L-PMEDMA-NH2.
by AIBN with CPDB as CTA under the condition of [CPDB]/ [AIBN] = 1/0.2, producing linear PMEDMA with predetermined molecular weights and low PDIs. Hyperbranched PMEDMAs are obtained at the late period of polymerization with high monomer conversions according to kinetic studies. Model reactions and the characterizations of 13C NMR, 1H−13C HMQC, and DEPT confirm that the branching points result exclusively from the incorporation of the allylic carbon−carbon double bonds rather than tiglate ester. Both linear and hyperbranched PMEDMAs are ready for thiol−ene click chemistry to introduce functional groups like amino and carboxy groups. The NH2-modified PMEDMAs show topology-dependent self-assembly behaviors. Thus, we provide a novel strategy to transform EVL to clickable polymers with well-defined structures and different topologies.
L-PMEDMA-NH2 assemble to micelles with different morphologies shown in Figure 7. DLS analysis suggests the diameters of 178.3 nm (PDI 0.199) for HB-PMEDMA-NH2 (Figure 7c) and 229.2 nm (PDI 0.248) for L-PMEDMA-NH2 (Figure 7d). The TEM image confirms the diameter of approximate 160 nm for HB-PMEDMA-NH2 micelles with thin hydrophilic shells (Figure 7e). On the contrary, L-PMEDMANH2 assembles to multicomponent micelles according to TEM (Figures 7f and S14) presenting obvious small hydrophobic domains in thick hydrophilic shells.39−41
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CONCLUSION EVL with poor polymerizability is successfully transformed to a trivinyl monomer (MEDMA) by two facile ring cleavage and esterification steps. Well controlled RAFT polymerizations enable chemoselective polyaddition of MEDMA initiated G
DOI: 10.1021/acs.macromol.7b02238 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02238. 1 H NMR, 13C NMR, 1H−13C HMQC, and 1H−13C HMBC spectra, more discussion about MALDI-ToF mass spectrum, and TEM images. (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Jun Ling: 0000-0002-0365-1381 Xufeng Ni: 0000-0001-5386-0950 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful for the financial support from the National Natural Science Foundation of China (No. 21674089). REFERENCES
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DOI: 10.1021/acs.macromol.7b02238 Macromolecules XXXX, XXX, XXX−XXX
