Anionic Polymerization of Vinylcatechol Derivatives: Reversal of the

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Anionic Polymerization of Vinylcatechol Derivatives: Reversal of the Monomer Gradient Directed by the Position of the Catechol Moiety in the Copolymerization with Styrene Daniel Leibig,†,‡ Axel H. E. Müller,† and Holger Frey*,† †

Institute of Organic Chemistry, Johannes Gutenberg University Mainz, Duesbergweg 10-14, D-55128 Mainz, Germany Graduate School Materials Science in Mainz, Staudinger Weg 9, D-55128 Mainz, Germany

‡

S Supporting Information *

ABSTRACT: The catechol-containing vinyl monomers 4vinylcatechol acetonide (4-VCA) and 3-vinylcatechol acetonide (3-VCA) are introduced for carbanionic polymerization in THF, using sec-butyllithium as an initiator. Molecular weights (Mn) ranging from 3000 to 80 000 g mol−1 were obtained, with polydispersities (Mw/Mn) below 1.10 for 4VCA and 1.15 for 3-VCA homopolymerization. Furthermore, block copolymers and gradient copolymers with styrene have been prepared via living carbanionic copolymerization. The reactivity of the new monomers 4-VCA and 3-VCA in the copolymerization with styrene and the resulting monomer gradient in the copolymer chains were investigated via in situ 1 H NMR spectroscopic kinetic studies in toluene-d8. The results show lower reactivity of the 4-VCA monomer than styrene (rS = 4.0, r4‑VCA = 0.24) and a higher reactivity than styrene for 3-VCA (r3‑VCA = 2.4, rS = 0.48). Well-defined copolymers of styrene and 4-VCA exhibit a strong gradient structure within the polymer chains with the catechol functionalities preferentially incorporated near the chain terminus. However, in the case of 3-VCA, the gradient structure of the copolymers is reversed, and the catechol functionalities are preferentially incorporated in the vicinity of the initiator. The direction of the monomer gradient in the copolymers can be predicted from the difference of the chemical shift of the β-carbon signal of the respective vinyl monomers in 13 C NMR spectra, which has general implications for the copolymerization of vinyl monomers. All polymers were characterized by 1H NMR spectroscopy, size exclusion chromatography (SEC), MALDI-ToF mass spectrometry, and differential scanning calorimetry (DSC). Quantitative cleavage of the acetonide protecting group under mild acidic conditions rendered well-defined poly(vinyl catechol)s, which were used for pH-sensitive precipitation of iron(III) cations and for surface coating on a variety of materials, showing very stable and permanent catechol-promoted adhesion.

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INTRODUCTION Catechol-containing polymers are attracting immense attention at present. This bioinspired functional group, adapted from dopamine in the mussel foot protein, enables strong attachment to a large variety of surfaces, such as silicon, gold, and metal oxides, which renders catechol-bearing polymers interesting for surface coatings.1−6 The catechol group permits even the coating of nonconventional polymer surfaces with extremely low surface energies, such as PTFE or polycarbonates.7 Various synthetic strategies have been developed for catecholcontaining polymers. Most commonly the postpolymerization functionalization of polymers with dopamine8−13 or other catechol-containing reagents like dihydrocaffeic acid14−16 is employed. However, postmodification reactions are often not quantitative, resulting in low degrees of functionalization. Another synthetic option is the polymerization of monomers containing catechol moieties.17−19 However, the two hydroxyl groups impede direct polymerization because of protonation of the carbanions in the case of anionic polymerization and the © XXXX American Chemical Society

facile electron transfer and oxidation in radical polymerizations. Consequently, for the controlled synthesis of the respective polymers, protecting group chemistry is mandatory. Common protecting groups for the catechol moiety are trialkylsilyl groups,20−22 acetals,23−25 or ethers.26−28 Several polymer synthesis routes for catechol-containing polymers based on radical polymerization techniques have recently been reported.26,27,29 Carbanionic polymerization offers excellent control over the degree of polymerization and polydispersity. However, suitable protecting groups for the anionic polymerization must be stable under the harsh nucleophilic reaction conditions, as demonstrated by Hirao et al. for a variety of functional groups.30,31 Common protecting groups for the catechol functionality in this context are acetals and ethers. Ethers, as applied in the Received: April 20, 2016 Revised: June 19, 2016

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DOI: 10.1021/acs.macromol.6b00831 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

formed on a Shimadzu Axima CFR MALDI-ToF mass spectrometer equipped with a nitrogen laser delivering 3 ns laser pulses at 337 nm. DSC measurements were performed using a PerkinElmer DSC 7 with an Elmer thermal analysis controller TAC7/DX in a temperature range from room temperature to 150 °C with heating rates of 10 K min−1 (second heating) under nitrogen, unless otherwise stated. 4-Methylcatechol Acetonide (1). 10.0 g (80.5 mmol) of 4-methyl catechol was dissolved in 100 mL of benzene, and 24.5 mL (20.8 g, 200 mmol, 2.5 equiv) of 2,2-dimethoxypropane and 100 mg of ptoluenesulfonic acid were added. The solution was refluxed with a Soxhlet extractor (CaCl2) for 4 h. Then the solvent was removed, and the residue was distilled in vacuum at 30 °C (1 × 10−3 mbar) to obtain 12.1 g (73.6 mmol) of a light yellow liquid (yield = 91%). 1H NMR (400 MHz, CDCl3): δ (ppm) = 6.63−6.56 (m, 3H, Ar), 2.27 (s, 3 H, CH3), 1.66 (s, 6H, C(CH3)2). All NMR spectra are shown in the Supporting Information. 3-Methylcetachol Acetonide (5). The reaction conditions were similar to the synthesis of 1. A light yellow liquid was obtained, bp = 36 °C (1 × 10−3 mbar). Yield = 92%. 1H NMR (400 MHz, CDCl3): δ (ppm) = 6.71−6.57 (m, 3H, Ar), 2.20 (s, 3H, CH3), 1.67 (s, 6H, C(CH3)2). 4-Bromomethylcatechol Acetonide (2). 10.0 g (60.9 mmol) of 1 was dissolved in 200 mL of dry and degassed cyclohexane. 10.8 g (60.6 mmol, 0.99 equiv) of N-bromosuccinimide and 400 mg of AIBN were added. The solution was refluxed for 4 h under an argon atmosphere. The solution was filtered to remove the white precipitate (succinimide), and the solvent was removed in vacuum to afford the crude product, which was used in the next reaction step without further purification (conversion: 80%). 1H NMR (400 MHz, CDCl3): δ (ppm) = 6.84 (dd, 1H, Ar−C5H, 7.8 Hz, 1.8 Hz), 6.81 (d, 1H, ArC3H, 1.8 Hz), 6.68 (d, 1H, ArC6H, 7.8 Hz), 4.48 (s, 2H, CH2Br), 1.70 (s, 6H, C(CH3)2). 3-Bromomethylcatechol Acetonide (6). The reaction was carried out as described for compound 2. Conversion = 67%. 1H NMR (400 MHz, CDCl3): δ (ppm) = 6.82−6.66 (3H, m, Ar−H), 4.46 (s, 2H, CH2Br), 1.71 (s, 6H, C(CH3)2). (4-Methylcatechol acetonide)triphenylphosphonium Bromide (3). The crude product of 2 was dissolved in 200 mL of dry acetone, and 15.0 g (57.1 mmol, 0.94 equiv) of triphenylphosphine was added. The solution was heated under reflux for 4 h and stirred at room temperature for an additional 10 h. The precipitate was filtered and washed with dry acetone to give 20.1 g (yield = 65%) of a colorless powder. 1H NMR (DMSO-d6): δ (ppm) = 7.96−7.62 (m, 15H, arom Ph−P), 6.70 (m, 1H Ar−C6−H), 6.45 (m, 1H Ar−C5−H), 6.35 (m, 1H Ar−C3−H), 5.09 (d, 2H, CH2−P, 15.0 Hz), 1.58 (s, 6H, C(CH3)2). 31P NMR (DMSO-d6): δ (ppm) = 22.8 (CH2−P+Ph3). (3-Methylcatechol acetonide)triphenylphosphonium Bromide (7). The same reaction conditions as described for compound 3 were applied. Colorless powder; yield = 58%. 1H NMR (400 MHz, methanol-d4): δ (ppm) = 7.96−7.68 (m, 15H, arom. Ph−P), 6.71 (m, 1H Ar−H), 6.65 (m, 1H Ar−H), 6.37 (m, 1H Ar−H), 4,85 (d, 2H, CH2−P, 14.6 Hz), 1.39 (s, 6H, C(CH3)2). 31P NMR (methanol-d4): δ (ppm) = 21.7 (CH2−P+Ph3). 4-Vinylcatechol Acetonide (4). 11.0 g of NaOH in 70 mL of H2O was added dropwise to a suspension of 20.0 g (39.5 mmol) of 3 in 200 mL of formaldehyde solution (37 wt %). The reaction mixture was stirred overnight. Subsequently, the solution was extracted three times with 25 mL of dichloromethane. The organic phase was dried and concentrated. The crude product was purified by column chromatography (silica, CH2Cl2, Rf = 1.0). After vacuum distillation at 41 °C (1 × 10−3 mbar) we received 6.35 g (yield, 90%) of a clear colorless liquid. 1H NMR (400 MHz, CDCl3): δ (ppm) = 6.94 (d, 1H, Ar− C3H, 1.7 Hz), 6.84 (dd, 1H, ArC5H, 8.0, 1.7 Hz), 6.72 (d, 1H, ArC6H, 8.0 Hz), 6.67 (dd, 1H, CHCH2, 17.5, 10.8 Hz), 5.61 (dd, 1H, cis-CHCH2, 17.5, 1.0 Hz), 5.15 (dd, 1H, trans-CHCH2, 10.9, 0.9 Hz), 1.72 (s, 6H,C(CH3)2). 13C NMR (100 MHz, CDCl3): δ (ppm) = 147.9 (C2), 147.4 (C1), 136.6 (CHCH2), 131.6 (C4), 120.6 (C5), 118.0 (C(CH3)2), 111.5 (CHCH2), 108.0 (C6), 105.2 (C3), 25.9 (C(CH3)2).

monomer 3,4-dimethoxystyrene, have been demonstrated to lead to broad molecular weight distributions due to proton abstraction at the methyl groups or other side reactions.32,33 Ishizone et al. reported narrow distributed polymers with methylene acetal protecting groups.24 However, the cleavage of the respective protecting groups remains challenging.26,34,35 Consequently, in this work we introduce two novel catechol monomers with simple acetal protecting groups. Both 4vinylcatechol acetonide (4-VCA) and 3-vinylcatechol acetonide (3-VCA) (Scheme 1) have been prepared and studied with respect to anionic homo- and copolymerization. Scheme 1. Monomers 4-VCA (4) and 3-VCA (8) and the Resulting Polymer Structures after Anionic Polymerization and Acidic Deprotection Reaction

Statistical copolymerization of the new vinyl monomers 4VCA and 3-VCA with styrene was investigated with respect to the monomer sequence, i.e., formation of random or gradient copolymer structures. We rely on the in situ 1H NMR characterization established for epoxide copolymerizations36−38 and recently extended to living carbanionic copolymerizations.39,40 Styrene is a key monomer for a broad range of applications. The catechol functionality opens new potential in areas as diverse as surface coating and pH-responsive glues.41 The bioinspired materials unite high precision functional polymers with coatings and inorganic matter.

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EXPERIMENTAL PART

Reagents. Allyl glycidyl ether, AIBN, 4-methylcatechol, styrene (99.5%), p-toluenesulfonic acid, triphenylphosphine, and sec-butyllithium as a 1.3 M solution in cyclohexane/hexane (92/8) were purchased from Acros Organics. 3-Methylcatechol, N-bromosuccinimide, and triphenylphosphine were purchased from ABCR. 2,2-Dimethoxypropane was received from TCI Chemicals. Formaldehyde solution was purchased as a 37−41 wt % solution in water from Fisher Chemical. AIBN was recrystallized from methanol and dried in high vacuum prior to use. THF was dried using sodium/ benzophenone and cryo-transferred into the reaction vessel. Deuterated chloroform-d1, cyclohexane-d12, DMSO-d6, methanol-d4, and toluene-d8 were purchased from Deutero GmbH. Instrumentation. SEC measurements were carried out in THF on an instrument consisting of a Waters 717 plus autosampler, a TSP Spectra Series P 100 pump, a set of three PSS-SDV 5 mL columns (102/103/104 Å), and RI and UV detectors (254 nm). Polystyrene standards provided by Polymer Standard Service were used for calibration. 1H NMR spectra (300 and 400 MHz) and 13C NMR spectra (100 MHz) were recorded on a Bruker AC300 or an Avance II 400. All spectra are referenced internally to the residual proton signals of the deuterated solvent. Matrix-assisted laser desorption and ionization time-of-flight (MALDI-ToF) measurements were perB


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Macromolecules Scheme 2. Synthesis Route of 4-Vinylcatechol Acetonide (4) Based on 4-Methylcatechol

Figure 1. SEC traces of (a) P(4-VCA) and (b) P(3-VCA) in THF (UV signal, PS standards). 3-Vinylcatechol Acetonide (8). The reaction was carried out in analogy to 4. Yield = 95%. 1H NMR (400 MHz, CDCl3): δ (ppm) = 6.84 (dd, 1H, Ar−C4H, 8.0, 1.2 Hz), 6.75 (t, 1H, ArC5H, 7.8 Hz), 6.68 (dd, 1H, CHCH2, 15.4, 11.1 Hz), 6.65 (dd, 1H, ArC6H, 7.6, 1.2 Hz), 5.89 (dd, 1H, cis-CHCH2, 17.2, 1.5 Hz), 5.35 (dd, 1H, trans-CHCH2, 11.2, 1.4 Hz), 1.71 (s, 6H,C(CH3)2). 13C NMR (100 MHz, CDCl3): δ (ppm) = 147.3 (C1), 144.8 (C2), 131.1 (CHCH 2 ), 120.6 (C5), 119.8 (C3), 119.4 (C4), 117.6 (C(CH3)2), 116.1 (CHCH2), 107.2 (C6), 26.9 (C(CH3)2). Homopolymerization. Dry and degassed THF was distilled into the reaction flask, and the dry and degassed monomer was added into the flask via syringe in an argon atmosphere. The solution was cooled down to −78 °C, and the polymerization was initiated with secbutyllithium. The solution turned red immediately. The reaction mixture was allowed to stir for 10 min to 1 h, depending on the targeted molecular weight. Then the polymerization was terminated with degassed methanol. The solvent was removed in a vacuum, and the residue was dissolved in chloroform and precipitated in methanol to obtain a colorless powder. The average molecular weights were measured by SEC and 1H NMR spectroscopy end-group analysis. Deprotection: Poly(4-vinylcatechol). 500 mg of the poly(4) was dissolved in 50 mL of acetonitrile, and 5 mL of a 1 M HCl solution was added. The solution was stirred under reflux for 1 h. The solvent was removed in vacuo, resulting in a crystalline, light red solid. Poly(3-vinylcatechol). 200 mg of the poly(8) was dissolved in acetic acid. The solution was heated, and water was added dropwise. The solution was stirred under reflux overnight. The solvent was removed in vacuo to afford a crystalline, light red solid. Block Copolymerization. Dry and degassed THF was distilled into the reaction flask, and the dry and degassed first monomer was added into the flask via syringe in an argon atmosphere. The solution was cooled down to −78 °C, and the reaction was initiated with secbutyllithium. The reaction was allowed to stir for 30 min. Subsequently, the second monomer, also dry and degassed, was added, and the solution was allowed to stir for an additional 30 min. Afterward, the reaction was terminated with freshly distilled dry and degassed allyl glycidyl ether or methanol. The solvent was removed in vacuo, and the residue was dissolved in chloroform and precipitated in methanol to receive a colorless powder. The average molecular weight was measured by SEC and 1H NMR spectroscopy, relying on endgroup analysis.

Statistical Copolymerization. 1.2 mL (1.1 g, 10 mmol, 23 equiv) of styrene was dried over CaH2 and degassed, and subsequently it was freshly distilled into the reaction flask. Then 30 mL of THF was also condensed into the reaction flask. 4-VCA was dried over CaH2, degassed, and distilled. 1 mL (approximately 1.0 g, 5.7 mmol, 13 equiv) of 4-VCA was added into the reaction flask via syringe in an argon atmosphere. The solution was cooled down to −78 °C, and then 0.35 mL of sec-butyllithium (1.3 M, 0.45 mmol, 1 equiv) was added under intensive stirring. The solution turned orange immediately and became red after about 15 min. The polymerization was terminated with degassed methanol after 1 h. Solvents were removed in a vacuum, and the residue was dissolved in chloroform and precipitated in methanol to receive a colorless powder. The average molecular weight was measured by SEC and 1H NMR spectroscopy end-group analysis.

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RESULTS AND DISCUSSION Monomer Synthesis. The synthesis of 4-VCA offers some challenges, since neither the attachment of the acetonide in 3,4dihydroxybenzaldehyde nor the Wittig reaction to 3,4dihdroxystyrene was successful. Consequently, we developed a four-step synthesis starting from 4-methylcatechol (Scheme 2). The attachment of the acetonide protecting group catalyzed with p-toluenesulfonic acid results in yields exceeding 90%. In a second step, the methyl group was transformed to the desired bromomethyl functionality. The benzyl bromide was prepared in a radical substitution in the benzyl position. This reaction was performed in various solvents with the best results obtained for cyclohexane. In the third step the phosphonium salt was formed using triphenylphosphine. The salt precipitates as a white, crystalline powder and can be used without further purification. Finally, the vinyl group is formed via Wittig reaction with an aqueous, alkaline formaldehyde solution. The product was extracted with dichloromethane and purified via column chromatography to remove all impurities. The resulting yield after purification is 90%, which leads to an overall yield of the four-step synthesis exceeding 50%. The synthesis of the second monomer 3-VCA was performed in an analogous manner, resulting in similar yields (Scheme S1).

C


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Macromolecules Homopolymer Synthesis. The homopolymerization was performed in THF at −78 °C. Polymers with number-average molecular weights between 2500 and 80 000 g mol−1 and PDIs below 1.10 for 4-VCA and polymers between 3000 and 26 000 g mol−1 with polydispersities of less than 1.15 for 3VCA were obtained (Table 1). Degrees of polymerization and polydispersity Mw/Mn were analyzed via size exclusion chromatography (SEC) in THF with PS standards (Figure 1). In all cases lower calculated theoretical molecular weights are tentatively ascribed to a variation of the concentration of the sec-butyllithium solution.

Samples P(3-VCA)19 and P(3-VCA)53 exhibit only a small amount of the high molecular weight byproduct. Both polymerizations were terminated after 10 min. P(3-VCA)26 and P(3-VCA)150 required longer polymerization times of 30 min and show a shoulder at higher molecular weights. Ishizone and co-workers demonstrated that the oxygen atoms in the neighborhood of the carbanion have a stabilizing character due to the interaction of the metal counterion with the carbanion and the oxygen atom.24 We propose that for our monomers due to steric effects the methyl groups of the acetal protecting group disturb this interaction, which leads to higher reactivity of the respective carbanions. This leads to an increasing extent of side reactions with reaction time, when the monomer is completely consumed. In summary, it is possible to prepare P(4-VCA) and P(3-VCA) via carbanionic polymerization in a controlled manner and with low polydispersity. In the case of 3-VCA the reaction time has to be kept as short as possible to prevent side reactions that occur already after a few minutes. MALDI-ToF mass spectra confirm homopolymerization without cleavage of the protecting groups or other termination reactions. The difference between peak molecular weights reflects the monomer repeating unit. Only one molecular weight distribution was detected. Consequently, side reactions can be excluded, and the living character and high stability of the acetonide protecting groups are confirmed (Figure 2 and Figure S49). The thermal properties of the homopolymers were investigated in bulk by DSC (Table 1, Figures S52 and S54). As expected, the glass transition temperatures increase with increasing chain length. Extrapolation to Mn → ∞ (Figures S53 and S55) resulted in Tg = 119 °C for P(4-VCA) and in a Tg = 108 °C for P(3-VCA). Thus, P(4-VCA) exhibits a higher Tg than both P(3-VCA) and polystyrene (Tg = 100 °C). Copolymer Synthesis. In a second set of polymerizations, both the statistical and the block copolymerization of 4-VCA and 3-VCA with styrene were investigated, aiming at catecholfunctionalized polystyrene. The statistical copolymerizations resulted in well-defined polymers within a molecular weight range of 5000−50 000 g mol−1 and polydispersities below 1.15 (Figures S44 and S45). The content of the VCA monomers in the copolymers was calculated from 1H NMR spectra by comparison of the aliphatic and aromatic proton signals (Figures S23 and S29). The VCA content for these copolymers is between 20 and 40 mol%. Incorporation of both monomers was verified by MALDI ToF mass spectrometry (Figures S48

Table 1. Analytical Data of Poly(4-vinylcatechol acetonide) and Poly(3-vinylcatechol acetonide) samples

Mnth (g mol−1)

Mn (g mol−1) (1H NMR)a

Mn (g mol−1) (SEC)b

Mw/Mn (SEC)b

Tg (°C)

P(4-VCA)14 P(4-VCA)34 P(4-VCA)40 P(4-VCA)43 P(4-VCA)64 P(4-VCA)480 P(3-VCA)19 P(3-VCA)26 P(3-VCA)53 P(3-VCA)150

2000 4200 4500 5700 7300 60000 2500 3200 6700 10700

2470 6000 7100 7610 11300 c 3340 4600 9460 c

2250 5950 6880 8230 12200 79200 2620 4930 9230 26100

1.07 1.07 1.07 1.06 1.09 1.10 1.09 1.12 1.09 1.15

82 105 106 109 112 119 85 93 102 105

a1

H NMR measurements with 400 MHz in deuterated chloroform. Molecular weights were calculated by end-group analysis. bSEC measurements in THF with PS standards. cMolecular weight too high for end-group analysis.

Mn for polymers with less than 20 000 g mol−1 was also calculated via 1H NMR spectroscopy based end-group analysis. Typical spectra are shown in the Supporting Information (Figures S19 and S25). The signal of the six methyl protons of the initiator was used as a reference, and the molecular weight was determined from the integral of the protons in the aromatic region. The NMR and SEC Mn values agree well, indicating that the polystyrene calibration can be applied for both polymers. All homopolymers of 4-vinylcatechol acetonide showed monomodal molecular weight distributions. For the 3-vinylcatechol acetonide we observed a small shoulder at higher molecular weight. The SEC results show clearly that the shoulder increases with longer reaction time (Figure 1b).

Figure 2. (a) MALDI-ToF spectrum of P(4-VCA)43. (b) Zoom with assignment of the single peak signals and the illustration of the peak distance of exactly 176 g mol−1. D


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(δ = 5.61 ppm, dd, J = 17.6 Hz, δ = 5.09 ppm, dd, J = 10.9 Hz) and 4-VCA (δ = 5.45 ppm, dd, J = 17.5 Hz and δ = 4.99 ppm, dd, J = 10.8 Hz). Successful initiation is visible by the appearance of the typical orange color of the active carbanionic chain end. Figure 3 shows the relevant signals in the 1H NMR spectra obtained in the kinetic measurements. The time between the measurements was 42 s. With increasing time, the monomer signals of the vinyl protons disappear. Simultaneously, the typical broad polymer resonances occur in the spectra (Figure S57). The zoom-in shows the conversion of the monomers, manifested by the decreasing intensity of the vinyl doublets. The styrene signals (highlighted in blue) decrease faster than the 4-VCA double bond signals (red), reflecting the higher reactivity of styrene and confirming our previous observations regarding the block copolymerization.

and S50). The mass peaks of the polymer show the molecular weight distance of styrene as well as that of 4-VCA or 3-VCA. The exact molecular microstructure of the copolymers was analyzed by in situ 1H NMR kinetic studies, as shown in the following section. Table 2. Analytical Data of the Copolymers of 4-VCA and 3VCA with Styrene sample

a

P(S26-co-(4-VCA)16) P(S95-co-(4-VCA)31) P(S199-co-(4-VCA)59) P(S38-co-(3-VCA)14) PS31-b-P(4-VCA)12 P(4-VCA)10-b-PS26 PS47-b-P(3-VCA)6

VCA content (mol %)

Mn (g mol−1) (1H NMR)b

Mn (g mol−1) (SEC)c

Mw/Mn (SEC)c

39 25 23 27 29 27 12

5490 15400 31000 6370 5480 4600 5930

5190 18000 48700 6520 6120 5290 6120

1.10 1.09 1.15 1.11 1.09 1.20 1.10

a

Monomer repeating units were calculated by 1H NMR spectroscopy. H NMR measurements with 400 MHz in chloroform-d. cSEC measurements in THF with PS standards. b1

Block copolymerization of styrene and 4-VCA showed interesting reactivity differences. The crossover from the living polystyrene block to the P(4-VCA) block is possible. Welldefined polymers with PDI < 1.10 were obtained. Fast addition of the 4-VCA monomer to the chain end was observed by a pronounced and fast color change of the reaction mixture from orange to red. In contrast, when starting with a P(4-VCA) block and subsequently adding styrene, block copolymers with a rather broad PDI of 1.20 were formed. In addition, a slow color change of the reaction medium from red to orange indicated slow initiation of styrene by P(4-VCA). These experiments demonstrate that 4-VCA is less reactive than styrene, and block copolymerization leading to well-defined materials is only possible when starting with the PS block. The SEC traces of the block copolymers show monomodal distributions; thus, we assume a stable character of the propagating 4-VCA carbanions (Figure S44). Both polymers were terminated with allyl glycidyl ether to enable the synthesis of complex polymer architectures, e.g., by ring-opening polymerization (not discussed in this work). The block copolymerization of 3-VCA, started with living PS as a macroinitiator yielded block copolymers with low PDIs of 1.10. MALDI-ToF spectra confirmed successful copolymerization; i.e., both repeating units are detected in the respective spectra (Figures S49 and S51). Online 1H NMR Kinetic Studies. The results of block copolymerization already indicated different reactivities of the comonomers, which should lead to gradient copolymers in the statistical copolymerization. This motivated us to further investigate the copolymerization kinetics of the new monomers with styrene by in situ 1H NMR kinetic studies. To this end, we conducted the anionic copolymerization directly in an NMR tube. We used toluene-d8 as an apolar solvent in all kinetic studies and performed the kinetic experiments at 23 °C. The orange color of the propagating carbanion remained after the kinetic studies, indicating the stability of the propagating chain end. For the copolymerization of 4-VCA (40 mol%) with styrene the comonomer composition as a function of time was determined by comparison of the vinyl integrals of styrene

Figure 3. Online 1H NMR copolymerization kinetics of styrene and 4VCA in toluene-d8. Overlay of a selection of spectra of the online 1 H NMR kinetics study. The zoom-in show the decrease of the monomer signals of styrene (vinyl signals highlighted in blue) and 4VCA (vinyl signals highlighted in red).

The integrals of the vinyl signals enable calculation of the monomer conversion, which is visualized in Figure 4a. Both monomer concentrations are normalized to the starting concentration of 4-VCA. The normalized concentrations are plotted versus the total monomer concentration. The data show that the polymerization proceeds much faster for styrene than for 4-VCA. The resulting gradient structure of the copolymer chain is illustrated in Figure 8a. The first half of the chain nearly only consists of styrene (blue), whereas the catechol-containing comonomer becomes the predominant unit toward the chain end. Thus, the kinetic studies show a pronounced gradient structure for the P(S-co-(4VCA)) copolymers, with styrene as the more reactive monomer. The reactivity ratios for the system were determined by the Kelen−Tüdös formalism42 as rS = 4.0 ± 0.1 and r4‑VCA = 0.24 ± 0.01 (Figure S61). The polymer obtained from the kinetics experiment in the NMR tube was monomodal with a molecular weight of Mn = 9600 g mol−1 (PDI = 1.34) (Figure S46). The copolymerization kinetics of 3-VCA and styrene was also studied in toluene-d8. The sample was prepared in analogy to the styrene/4-VCA system. The starting content of the comonomer was 41 mol%. NMR spectra were recorded at different time intervals varying from 20 s to 1 min due to a noticeable deceleration of the polymerization reaction. The vinyl signals of styrene at δ = 5.61 and 5.09 ppm and the signals of 3-VCA at δ = 5.97 and 5.29 ppm were used for integration. The NMR spectra are shown in Figures S58 and S59. Surprisingly, the 3-VCA signals decreased much faster than the signals of styrene. The single monomer consumption is plotted versus the total monomer conversion in Figure 5a, E


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Figure 5. (a) Residual monomer concentrations normalized to the starting concentration of 3-VCA vs the total monomer conversion. (b) First-order time−conversion plot.

Figure 4. (a) Residual monomer concentrations normalized by the starting concentration of 4-VCA vs the total monomer conversion. (b) First-order time−conversion plot.

the comonomers (Table 3). Consequently, 3-VCA is more reactive than styrene and styrene is more reactive than 4-VCA.

illustrating the monomer incorporation in the polymer chain. 3VCA shows higher reactivity, which in this case leads to a copolymer structure with the catechol functionalities preferentially located in proximity of the initiator and the styrene units increasingly incorporated at the chain end (Figure 8b). The polymerization was also much faster than for the 4-VCA/ styrene system because of the higher reactivity of the monomer system 3-VCA and styrene (Figure 5b). The corresponding reactivity ratios of this monomer combination are r3‑VCA = 2.4 ± 0.1 and rS = 0.48 ± 0.01 (Figure S62). The polymer isolated from the NMR experiment had an average molecular weight of 4300 g mol−1 and a PDI of 1.28 (Figure S46). The in situ NMR kinetic studies of the two copolymers permit detailed insight into the microstructures of styrene/ vinylcatechol acetonide copolymers. It is interesting to note that the position of the catechol functionality leads to a reversal of the monomer gradient: 4-VCA results in P(S-co-(4-VCA)) copolymers with functional units close to the chain end, whereas 3-VCA is positioned near the initiator in the copolymerization with styrene. Hirao and co-workers showed that the effectivity of the crossover reaction in block copolymer synthesis depends on the difference of the 13 C NMR β-carbon shifts of the comonomers.43,44 This shift reflects the charge distribution in the vinyl group. Higher values of the chemical shift correspond to a more positive charge at the β-carbon and thus to a higher reactivity of the monomer toward reaction with a carbanion. For statistical copolymerizations the crossover reaction is an essential process, determining the gradient structure. Thus, our results can also be correlated to the β-carbon shift difference of

Table 3. Chemical Shift of the β-Carbon Signal of the Monomers in 13C NMR Spectra monomer

β-carbon shift [ppm]

Δδ [ppm]

3-VCA (Figure S15) styrene 4-VCA (Figure S6)

116.1 113.7 111.5

−2.4 +2.2

Overall, both copolymerizations lead to a strong gradient structure. To sum up, the differences of the β-carbon shift permit to predict the direction and nature of the monomer gradient in the statistical vinyl monomer copolymerization. Deprotection of Catechol Moieties. A major advantage of the monomers 4-VCA and 3-VCA lies in the facile cleavage of the acetonide protecting groups in the polymer under mild conditions. P(4-VCA) was dissolved in acetonitrile, which is a good solvent for the protected polymer as well as for the deprotected poly(4-vinylcatechol). Cleavage of the protecting groups was achieved by the addition of 10 vol% of 1 M HClaq. The mixtures were refluxed for 30 min up to 4 h, depending on the molecular weight of the polymer. Full cleavage of the acetonide groups is evidenced by 13C NMR spectroscopy (Figure 6) as well as in 1H NMR spectra (Figures S19 and S21). Both methods demonstrate quantitative disappearance of the acetonide signals. The proton signals at 1.60 ppm disappear as well as the respective carbon signals at 117.1 and 25.9 ppm in 13 C NMR spectra. Poly(4-vinylcatechol) (P-4VC) also shows a characteristic change in the solubility properties from nonpolar F


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Figure 6. 13C NMR spectrum (100 MHz) of P(4-VCA) (bottom) in CDCl3 and P(4-VC) (top) in MeOD-d4. Verification of the removal of the acetonide carbon signals at 117.1 and 25.9 ppm (highlighted in green and red).

functionalities undergo irreversible oxidiation reactions. This oxidation does not occur in strong basic solution under an argon atmosphere. Surface Coating. A key aspect of catechol-containing polymers is their strong attachment to a variety of surfaces. The homopolymer P(4-VC)39 was studied with respect to surface deposition on silicon, copper, and stainless steel, applying a dipcoat procedure (Table 4). Details are given in the Supporting

to polar, becoming insoluble in cyclohexane, chloroform, or toluene. After acidic treatment the polymer shows good solubility in methanol, acetone, and alkaline water solutions, which is another evidence of the cleavage of the protection groups. The benefit of the mild cleavage protocol is the stability of other functional groups, e.g., ether linkages. Consequently, block copolymers with epoxides, such as P(4-VC)-blockpoly(ethylene oxide) or P(4-VC)-block-poly(propylene oxide), are accessible. Cleavage of the protecting group for the P(3-VCA) was not possible in acetontrile because of the low solubility in this solvent. The acetonide group was removed in acetic acid under reflux by addition of small amounts of water. After the addition of water the polymer precipitated at first, until the water was consumed by the hydrolysis of the catechol functionality. The end of the reaction can be detected, when the polymer does not precipitate any more upon the addition of water. The formation of free catechol moieties is indicated by the change in the solubility and the appearance of a blue color after the addition of alkaline solutions, which is a consequence of the chinone formation in the case of oxygen exposure. Successful deprotection of the P(3-VCA) was verified via 13C NMR spectroscopy. The signals of the acetonide protecting group at 25.8 and 116.2 ppm vanish completely (Figure S28). Effect of Bases on P(4-VC). Previous works demonstrate that an increase of the catechol complexation activity with iron(III) cations depends on the pH value.9 UV−vis measurements in different acidic and alkaline methanol solutions enable to define the pH at which the catechol moieties become deprotonated. This leads to multiple complexation of metal cations, which is shown in the following section of in this work. The catechol group absorbs at λ = 282 nm in neutral methanol solution. We prepared various NaOH and HCl solutions in methanol with concentrations ranging from 0.5 mM to 1.5 M. In acidic methanol solutions no variation of the absorption wavelength was observed (Figure S67). Alkaline methanol solutions show a shift to λ = 312 nm for c ≥ 0.5 M (Figures S64 and S65), which is a consequence of the deprotonation of the hydroxyl groups of the catechol moieties. In addition, a broad absorption in the visible light area with a band at 600 nm appears (Figure S64). The solution turns blue, which is a typical color for quinoid systems resulting from oxidation reaction of deprotonated catechol units. Figure S66 shows that the onset of the acid−base reaction occurs at a ratio of 1−2 hydroxyl ions per catechol moiety. The inflection point is at λ ≈ 300 nm, with a ratio of around 20−30 hydroxyl ions to catechol units. Full conversion is reached at λ ≈ 312 nm and an excess of more than 100 hydroxyl ions per catechol unit. In general, poly(4-vinylcatechol) is stable in neutral and acidic solutions. In basic solutions and in the presence of oxygen the catechol

Table 4. Contact Angles of the Blank Surfaces and of the Coated Surfaces after the Dip-Coating Procedure Subsequent Treatment in Ultrasound Bath material

CA (deg) (blank)

CA (deg) (coated)

silicon copper steel

27 42 30

54 61 55

Information. Contact angles of the freshly coated surfaces varied from 42° for stainless steel to 59° for copper. The coated silicon surface showed a contact angle of 45° (Figure S63). To ensure that the polymer is not merely physically absorbed, the samples were treated with ultrasound in methanol for 30 min. After drying under high vacuum, the contact angles were 54° (Si), 55° (stainless steel), and 61° (Cu). A slight increase of the contact angles is observed after this procedure, which can arise from the reorganization of the polymer chains on the surface. In conclusion, the surface coating shows no major alteration after ultrasonic treatment, and strong attachment of the polymer chains to the surface is evidenced by a significant increase of the contact angle. Complexation Behavior of P(4-VC). A highly interesting property of the catechol moiety is the pH-dependent complexation behavior, e.g., with Fe(III). The number of catechol ligands for each iron(III) cation increases from one to three with increasing pH.9,41,45 Polymers with multiple catechol moieties lead to cross-linked structure for pH > 7. A Fe(III) chloride solution was prepared in methanol (Figure 7). After addition of poly(4-vinylcatechol), no change of the solution can be observed. However, the addition of 2 M NaOHMeOH leads to the precipitation of black, insoluble polymer particles. These particles are cross-linked polymer structures, in which two or three catechol functionalities coordinate to one iron(III) cation. This leads to formation of an insoluble polymer network. The network can be destroyed again by the addition of 1 M HClMeOH, reducing the number of coordinating catechols per iron(III) cation. Consequently, the cross-links are disrupted and the black precipitate dissolves. This process was repeated 10 times, evidencing its fully reversible nature. G


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work we demonstrate that the β-carbon shift differences can be used to obtain a monomer reactivity order for vinyl monomers, emphasizing the predictive nature of this parameter for the direction of the monomer gradient in carbanionic copolymerization. Importantly, the copolymers’ microstructure and the position of the catechol functionalities in the polymer chains can be reversed by the selection of the more or less reactive vinylcatechol acetonide monomer. The acteonide protecting group is stable under the very strong basic polymerization conditions of the carbanionic polymerization but can be completely cleaved in a mild acidic environment. Consequently, well-defined poly(vinylcatechol) homo- and copolymers in a broad molecular weight range are accessible. The strong binding properties of the catechol moieties were demonstrated with dip-coating experiments on various substrates; e.g., contact angles for freshly oxidized silicon surfaces show a clear shift from 27° to 54° when coated with P(4-Vc). UV−vis measurements revealed that transparent solutions in methanol form blue quinoid structures under alkaline conditions. In summary, the new vinyl monomers with protected catechol moiety offer intriguing potential for a variety of precise polymer architectures for tailored surfaces and interfaces.

Figure 7. (a) Reversible precipitation of iron(III) cations with poly(vinylcatechol). (b) Complexation at low pH with the coordination of one catechol moiety and network formation by increasing pH and the following complexation of three catechol groups per iron cation.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b00831. Additional experimental part, NMR spectra, IR spectra, SEC data, MALDI-ToF mass spectrometry data, DSC data, NMR kinetic data, Kelen−Tüdös plots, contact angle measurements, UV−vis spectra (PDF)

Figure 8. (a) Corresponding visualization of the polymer chain for styrene (blue) and 4-VCA (red). (b) Corresponding visualization of the polymer chain for styrene (blue) and 3-VCA (orange).

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CONCLUSION Two new acetonide-protected, catechol-containing vinyl monomers 3-VCA and 4-VCA have been introduced, which can be polymerized by living carbanionic polymerization in THF and toluene. Homopolymers as well as statistical and block copolymers with styrene were prepared in THF. Narrow molecular weight distributions and a range of molecular weights from 3000 to 80 000 g mol−1 are accessible. Besides homopolymerization, we elucidated the copolymerization characteristics of both protected vinylcatechol monomers with styrene. In both cases, pronounced gradient copolymer microstructures were obtained upon copolymerization with styrene in apolar media. Polymerization kinetics studies in toluene-d8 offer information on the monomer reactivity and the corresponding copolymer microstructure, demonstrating a gradient reversal upon going from 4-VCA to 3-VCA as a comonomer (Figure 8). 4-VCA is less reactive than styrene (rS = 4.0 and r4‑VCA = 0.24), which leads to the gradient copolymer with the catechol functionalities close to the terminus, whereas 3-VCA gives the opposite structure with the catechol functions close to the initiator (r3‑VCA = 2.4 and rS = 0.48). Although reactivity ratios in anionic polymerization depend on the solvent, we believe that the results in toluene can be qualitatively applied to THF as well. This is corroborated by the observations for block copolymerization. The monomer reactivity is related to the charge distribution in the vinyl group of the monomers, which translates to the β-carbon shift in 13C NMR spectra. In this

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (H.F.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS D.L. acknowledges a fellowship through the Excellence Initiative (DFG/GSC 266) in the context of MAINZ “Materials Science in Mainz”. The authors thank Steffen Hildebrand and Christian Jochum for technical assistance, Monika Schmelzer for SEC, Maria Müller for DSC, and Elena Berger-Nicoletti for MALDI-ToF measurements.

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REFERENCES

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Anionic Polymerization of Vinylcatechol Derivatives: Reversal of the

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