Controlled RAFT Polymerization and Zinc Binding Performance of

Article pubs.acs.org/Macromolecules

Controlled RAFT Polymerization and Zinc Binding Performance of Catechol-Inspired Homopolymers Anna Isakova, Paul D. Topham,* and Andrew J. Sutherland Chemical Engineering and Applied Chemistry, Aston University, Birmingham B4 7ET, U.K. S Supporting Information *

ABSTRACT: Incorporation of catechols into polymers has long been of interest due to their ability to chelate heavy metals and their use in the design of adhesives, metal−polymer nanocomposites, antifouling coatings, and so on. This paper reports, for the first time, the reversible addition−fragmentation chain transfer (RAFT) polymerization of a protected catechol-inspired monomer, 3,4dimethoxystyrene (DMS), using commercially available trithiocarbonate, 2(dodecylthiocarbonothioylthio)-2-methylpropionic acid (DDMAT), as a chain transfer agent. Our identified RAFT system produces well-defined polymers across a range of molecular weights (5−50 kg/mol) with low molar mass dispersities (Đ, Mw/Mn < 1.3). Subsequent facile demethylation of poly(3,4-dimethoxystyrene) (PDMS) yields poly(3,4-dihydroxystyrene) (PDHS), a catechol-bearing polymer, in quantitative yields. Semiquantitative zinc binding capacity analysis of both polymers using SEM/EDXA has demonstrated that both PDMS and PDHS have considerable surface binding (65% and 87%, respectively), although the films deposited from PDMS are of a better quality and processability due to solubility and lower processing temperatures.

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INTRODUCTION The quest for materials which bind to metallic surfaces or particles has been ongoing for several years1−6 for the removal of heavy metals from water supplies,7−10 efficient charge transfer in organic electronics with long-term stability,11−13 antifouling coatings,14,15 and adhesives.15−28 Mussel foot proteins have been of particular interest in this area since they contain a specific protein-like substance that allows them to adhere to almost any type of surface. Waite and coworkers25,29−36 have demonstrated that the major contributors to its adhesive performance are L-3,4-dihydroxyphenylalanine (DOPA) residues.34,36 DOPA content in the protein usually ranges from 2 to 30%.16,25 Waite’s group has shown that DOPA units within the protein have two major roles. First, they serve as an anchor, linking the protein to different molecules through various intermolecular interactions, predominantly metal coordination37 and hydrogen bonding.29,38 Second, the catechol phenolic groups can be readily oxidized to produce quinones (e.g., with the enzyme, oxidase39), which can then react with different nucleophilic groups in the peptide and thus serve as strong cross-linking units. Finally, the outstanding ability of this material to adhere to wet surfaces has been demonstrated29 and undoubtedly results from the natural subaquatic environment of mussels.40 Inspired by nature, different groups have exploited the propensity of catechols to chelate heavy metals in the design and synthesis of metal oxide-containing nanoparticles (e.g., Fe2O3, ZnO, TiO2).11−13,41−49 The catechol linkers have been used by Werner et al.12,13,46 to anchor porphyrin complexes to zinc oxide nanoparticles in dye-sensitized solar cells. Chelation © 2014 American Chemical Society

was achieved by the simple soaking of nanoparticles in a solution of catechol-containing molecules, and the stability of the resultant catechol−nanoparticle complex was demonstrated by thermogravimetric analysis (TGA).13 The same group similarly reported several different complex architectures containing the active catechol motif and assessed their performance as stabilizers in zinc oxide electrodes.12,46 The ability of catechols to cross-link readily has been used in the design of new adhesive polymers, initially based on natural amino acid sequences. For example, in 1987, Yamamoto synthesized a polydecapeptide based on the repeating sequence Ala-Lys-Pro-Ser-Tyr-Hyp-Hyp-Thr-DOPA-Lys and tested the adhesive performance against iron and aluminum surfaces.50 Subsequently, Yamamoto went on to describe other polypeptide sequences, including those produced by biotechnological methods.51 In general, the decapeptides exhibited higher adhesion than the original natural mussel foot proteins (mfp1 and mfp-2).51 Various methods have been demonstrated for the incorporation of DOPA into peptides, including solid-phase peptide synthesis, which requires protection of the sensitive catechol group during synthesis.52−54 More recent approaches involve synthetic copolymers of DOPA or catechol-functionalized monomers with common monomers, such as methyl methacrylate (MMA),18,55 styrene,23,26 and sodium styrene-4sulfonate,56 or covalent attachment to polymers, predominantly poly(ethylene glycol) (PEG).57,58 Incorporating catechol bindReceived: February 13, 2014 Revised: March 27, 2014 Published: April 3, 2014 2561

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styrene, and styrenic-based monomers,75−79 disulfide-based dimethacrylate,70,73,80,81 methyl acrylate,82,83 tert-butyl acrylate,84 2-(dimethylamino)ethyl methacrylate,85 glycerol monomethacrylate,86,87 various acrylamides,88−94 and butadienes.95,96 In our report, we demonstrate a RAFT system capable of producing PDMS across a broad molecular weight range (from 4 to 50 kg/mol, where the attained degrees of polymerization closely match the targeted values), with low molar mass dispersities (Đ, Mw/Mn < 1.3) and monomer conversions exceeding 80%. Subsequently, a facile demethylation step allowed us to obtain well-defined PDHS polymers, which have been compared with PDMS and polystyrene (PS) for their surface zinc binding capacity.

ing sites into a macromolecule in this manner brings about several advantages. Most notably, a polymer scaffold can be manipulated to confer a wide range of different properties, such as solubility, wettability, thermal properties, flexibility, durability, and hydrophilicity/hydrophobicity, among many others. Accordingly, the judicious combination of macromolecular constructs with the heavy metal chelating ability of catechols has led to the production of porous resins. For example, the catechol unit has been grafted onto the Amberlite XAD-4 resin through a reduced7,9 and nonreduced59 imine linker using 3,4dihydroxybenzaldehyde as the catechol-containing unit. This grafting approach has also been applied in the fabrication of polymer−metal hybrids,60 where a catechol-containing unit was grafted onto poly(vinyl alcohol) (PVA), with the resultant material possessing metal scavenging properties. In an alternative strategy employing catechol-containing monomers, Bernard et al.8 produced poly(vinyl catechol-co-styrene) resins, using divinylbenzene (DVB) as a cross-linker and 3,4dimethoxystyrene (DMS) as a parental compound for the catechol comonomer. It is important to note that the protected monomer form (DMS) of the catechol is needed owing to the ability of the unprotected catechol unit to readily scavenge radicals and thus inhibit radical polymerization. The use of a catechol-containing monomer in a polymer construct confers the distinct advantage that each unit in the polymer bears an adhesive unit. Accordingly, poly(3,4dimethoxystyrene) (PDMS) has been synthesized by lowtemperature free radical polymerization using tri-n-butylborane as an initiator; however, low monomer conversions were typically obtained.61 Well-defined polymers of PDMS have also been prepared by controlled polymerization techniques. For example, PDMS was synthesized by both nitroxide-mediated polymerization (NMP), using bespoke bifunctional initiators62 or 2,2,6,6-tetramethyl-piperidin-1-yl)oxyl (TEMPO),63 and anionic polymerization.23 The former example demonstrated the sequential employment of uncontrolled and controlled freeradical polymerizations in an elegant approach to produce a wide range of PDMS-containing copolymers for use as polymer supports for liquid-phase organic synthesis.62 In the latter example, PDHS was subsequently synthesized; Wilker’s group reported the homo- and copolymerization of DMS [with styrene (S)] and subsequent demethylation using BBr3, to produce poly(3,4-dihydroxy) (PDHS) and P(S-co-DHS), respectively, for use as adhesives to a wide range of typically metal-containing substrates.23 Furthermore, Rooney reported the first cationic polymerization of DMS using trityl hexachloroantimonate to produce low molecular weight PDMS (ca. 1 kg/mol),64 before Daly et al.65−68 subsequently reported both cationic and anionic polymerization of DMS. In this later work, it was shown that cationic polymerization was capable of generating moderately high molecular weight polymers (41.7 kg/mol) with 46% yield at low temperatures (−50 °C), whereas anionic polymerization only yielded low molecular weight polymers in low yields. Interestingly, the cleavage of one of the methoxy groups was observed in anionic polymerization, even when other anion radicals (such as sodium naphthalide) were used.68 Herein, we report the reversible addition−fragmentation chain transfer (RAFT) polymerization of DMS for the first time. RAFT polymerization is a versatile (controlled) freeradical polymerization in the presence of a thioester-based chain transfer agent (CTA), shown to be suitable for a wide range of monomers, including methyl methacrylate,69−74

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EXPERIMENTAL SECTION

Materials. 3,4-Dimethoxystyrene (DMS, technical grade, 99%) and styrene (S, ReagentPlus, ≥99%) were purchased from Sigma-Aldrich and were extracted with 10% aqueous NaOH solution to remove the inhibitor. Anhydrous tetrahydrofuran (THF, ≥99.9%, inhibitor free, Sigma-Aldrich) was purged with nitrogen before use. 2,2′-Azobis(isobutyronitrile) (AIBN, Fisher Scientific) was recrystallized from methanol and dried in vacuo before use. 2-(Dodecylthiocarbonothioylthio)-2-methylpropionic acid (DDMAT) was synthesized according to Lai et al.71 and recrystallized from hexane before use. Boron tribromide (≥99.99%, Sigma-Aldrich), dichloromethane (DCM), hexane, and methanol (Fisher Scientific, Laboratory grade), zinc acetate (99.99% trace metal basis, Sigma-Aldrich), dithranol (matrix substance for MALDI-MS, ≥98.0%, Sigma-Aldrich), and triethylamine (≥99%, Sigma-Aldrich) were all used as received. Synthesis. Synthesis of Poly(3,4-dimethoxystyrene), PDMS, via RAFT Polymerization. The following example describes the polymerization of DMS in THF at 60 °C with [AIBN]0/[DDMAT]0/[DMS]0 = 1/1/30 (i.e., a target degree of polymerization, Dp of 30); this is representative of all DMS polymerizations undertaken in this work. A 25 mL polymerization tube equipped with a magnetic follower was charged with a mixture of DMS (3 g, 18.27 mmol), AIBN (101 mg, 0.61 mmol), DDMAT (222 mg, 0.61 mmol), and THF (6 mL). The flask was sealed with a rubber septum, and the solution was stirred and purged with nitrogen for 15 min, following which the flask was placed in an oil bath at 60 °C. Aliquots were taken periodically, and the polymerization was monitored to high conversion. Upon completion, the polymerization was cooled rapidly to 0 °C to allow immediate termination. 10 mL of THF was added, and the resulting solution was precipitated in 250 mL of hexane. The precipitate was collected by filtration, washed with hexane several times, and dried in vacuo to obtain a pale yellow powder. For the comparative studies, polystyrene was also synthesized via RAFT polymerization using the procedure outlined above except at the ratio of [AIBN]0/[DDMAT]0/[S]0 = 0.1/ 1/250. Synthesis of Poly(3,4-dihydroxystyrene) (PDHS). PDMS (3 mmol) was dissolved in 30 mL of DCM in a 100 mL round-bottomed flask equipped with a magnetic follower. The flask was sealed with a rubber septum, and the solution was stirred and purged with nitrogen for 15 min. After purging, the solution was cooled to −20 °C, stirred for 10 min, and then boron tribromide (2.26 g, 870 μL, 9 mmol) was added slowly via the rubber septum. After 30 min, the mixture was allowed to warm to room temperature and then stirred for a further 18 h. The dark purple solution was subsequently cooled to −20 °C, and 30 mL of deionized water was added. Following evolution of fumes, the mixture was slowly warmed to room temperature. It should be noted that PDHS is insoluble in both DCM and water and thus precipitates from the reaction mixture, facilitating its purification. The precipitate was collected by filtration and sequentially washed with water and DCM, before the resultant purple powder was dried in vacuo. Zinc Binding Experiments. A standard SEM aluminum substrate was washed sequentially with water, acetone, and propan-2-ol and then sonicated in propan-2-ol for 20 min, before being dried in vacuo for 10 min at 100 °C. Polymers were deposited by spin-coating (at 700 rpm 2562

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Scheme 1. Synthetic Route for the Preparation of Poly(3,4-dihydroxystyrene), PDHS

Figure 1. (a) Plot of molecular weight, Mn, and dispersity (Mw/Mn, Đ) against RAFT polymerization time. (b) Plot of Mn versus monomer conversion. (c) Semilogarithmic plot for the RAFT polymerization of PDMS with a target degree of polymerization of 30. (d) GPC traces of PDMS27, PDMS154, and PDMS258. for 15 s and then 2000 rpm for 30 s) from the appropriate 20 mg/mL solution (toluene for PS and PDMS, methanol for PDHS). The films were dried in vacuo without heating to remove the residual solvent. A saturated aqueous zinc acetate solution was prepared with sonication to ensure dissolution before being spin-coated onto the polymer films at 2000 rpm for 60 s. The films were annealed for 20 min in the air at 65, 100, and 120 °C for PDMS, PDHS, and PS, respectively. After annealing, the films were thoroughly washed with water and sonicated in water for 10 min to remove any weakly bound zinc material. Finally, the films were annealed again at the appropriate temperature for 10 min and dried in vacuo for 10 min to remove all residual water. Each polymer was prepared in this way in triplicate prior to assessment. Characterization. 1H and 13C NMR spectra were recorded using a Bruker NMR spectrometer (300 MHz). Water peaks in MeOD were suppressed manually using a Watergate sequence, incorporated within the TopSpin Bruker software. All chemical shifts are reported in ppm

(δ) and referenced to the chemical shifts of the residual solvent resonances. Fourier transform infrared (FTIR) spectra were obtained from KBr disks on a PerkinElmer Spectrum One spectrometer over the range 4000−500 cm−1 for 16 scans with a resolution of 4 cm−1. Number-average molecular weight (Mn) and dispersity (Mw/Mn, Đ) were measured using gel permeation chromatography (GPC) (flow rate 1 mL/min) through three PL gel 5 mm 300 × 7.5 mm mixed-C columns using a degassed THF eluent system containing 2% (v/v) TEA. The system, operating at 40 °C, was calibrated with narrow polystyrene standards (Mp range = 162−6 035 000 g/mol). All data were analyzed using PL Cirrus software (version 2.0) supplied by Agilent Technologies (previously Polymer Laboratories). Mn was also evaluated using 1H NMR spectroscopy (via end-group analysis) and matrix-assisted laser desorption/ionization time-of-flight/mass-spectrometry (MALDI-ToF-MS). MALDI-ToF-MS was conducted using a Bruker Daltonics Ultraflex II MALDI-ToF mass spectrometer, 2563

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Table 1. Summary of Monomer Conversion and Molecular Weight Data for the Homopolymerization of DMS using AIBN and DDMAT in THF at 60 °C target Dp

Mnth (kg/mol)

time (h)

conv (%)

Mna (kg/mol)

Mnb (kg/mol)

Dpb

Mw/Mn (Đ)

30 150 300

5.3 25.0 49.6

24 48 48

87 84 84

3.4 10.2 25.0

4.8 25.7 42.7

27 154 258

1.14 1.29 1.28

Calculated by GPC against polystyrene standards in THF at 40 °C. bCalculated using 1H NMR spectroscopy, comparing the integrals for the methanediyl peak of the CTA fragment to those of the aromatic peaks of the polymer (see Supporting Information). a

equipped with a nitrogen laser delivering 2 ns laser pulses at 337 nm with positive ion ToF detection performed using an accelerating voltage of 25 kV. A saturated solution of dithranol in THF as a matrix (0.5 μL), sodium iodide as cationization agent (1.0 mg/mL), and sample (1.0 mg/mL) were mixed, and 0.7 μL of the mixture was applied to the target plate. Spectra were recorded in linear mode using calibration with PEG-Me 1100 kDa. Ultraviolet and visible (UV−vis) absorbance spectra were obtained using a PerkinElmer Lambda 35 system in the wavelength range 300− 800 nm, using chloroform or methanol as solvent at 0.01 mg/mL. The glass transition temperatures, Tg, of the polymers were measured using differential scanning calorimetry (DSC, DSC 1 STARe, Mettler Toledo) from two heating cycles over a temperature range of 25−250 °C at a heating rate of 10 °C/min. Surface morphology and zinc binding performance were studied using a scanning electron microscopy with energy dispersive X-ray analysis (SEM/EDXA) on a Link system AN10000 and Cambridge scanning electron microscope Stereoscan S90, 22× resolution surface in bulk and an acceleration voltage of 25 kV. The polymer was deposited (via spin-coating) directly onto the aluminum SEM holders before the zinc acetate solution was spin-coated on top and the sample annealed at the appropriate temperature (65 °C for PDMS, 100 °C for PDHS and 120 °C for PS; vide inf ra).

concentration. The latter produced higher monomer conversion, although at lower rates, in agreement with other reports for both more-activated (e.g., p-acetoxystyrene104,105) and less-activated (e.g., vinyl chloride106) monomers. Concomitantly, our system gives better control over the molecular weight distribution at the higher dilution tested (1:2 DMS:THF) than the other systems mentioned above. In short, the optimal RAFT system was found to have a 1:1 ratio of initiator to CTA ([AIBN]/[DDMAT] = 1) at 1:2 DMS:THF. This system afforded good control over the final polymer molecular weight and displayed first-order kinetics with respect to monomer concentration with an apparent rate constant for propagation, kapp, of 0.085 h−1 (see Figure 1). Finally, the monomer:CTA ratio was varied to target three different molecular weights: 5, 25, and 50 kg/mol (target degree of polymerization, Dp = [DMS]/[DDMAT] = 30, 150, and 300, respectively), as shown in Table 1. 1H NMR spectroscopy revealed that our actual molecular weights were close to those targeted for all three PDMS sample: 4.8, 25.7, and 42.7 kg/mol, corresponding to PDMS27, PDMS154, and PDMS258, respectively. The degree of polymerization was obtained from the 1H NMR spectra based on the ratio of the aromatic polymer protons to those of the methanediyl group of the CTA end group (see Supporting Information). It is important to note that the number-average molar mass, Mn, measured by GPC, is much lower than the corresponding Mn value obtained via 1H NMR spectroscopy (Table 1). This is attributed to the use of polystyrene standards in GPC, providing a relative Mn value only. The MALDI spectrum for low molecular weight PDMS (4.8 kg/mol) was used to verify the accuracy of the NMR results. As expected, the MALDI data are more consistent with the calculated Mn value obtained by NMR, with the most intense peak at 4377 m/z corresponding to Dp = 24 (see Supporting Information, Figure S10). Demethylation. Demethylation of PDMS was carried out in a simple procedure, previously reported by other groups.8,68 PDMS was treated with boron tribromide, a very strong Lewis acid, at low temperatures. Previous studies have reported that temperatures as low as −70 °C were employed for quantitative demethylation.68 Herein, however, a temperature of −20 °C was shown to be sufficient. The success of demethylation has been demonstrated by 1H NMR, FTIR, and UV−vis spectroscopies. The 1H NMR spectrum of PDMS (Figure 2a) shows two broad peaks associated with the methoxy groups at 4.0−3.5 ppm, which are no longer present in that of PDHS (Figure 2b), indicating complete cleavage of the methoxy groups. Concurrently, the characteristic methanediyl peak of the CTA fragment (at approximately 3.25 ppm) is not detectable in this solvent (MeOD) due to the overlapping of peaks. Finally, the peak splitting in the aromatic region (ca. 6−7 ppm) changes following demethylation, demonstrating that the substituents

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RESULTS AND DISCUSSION Polymer Synthesis. Our synthetic strategy comprises a straightforward two-step approach (Scheme 1), where the first step was to obtain a well-defined parental polymer (PDMS) with a predetermined molar mass. To this end, RAFT polymerization was employed owing to its good control over the polymer chain length and architecture. Moreover, the end group of the CTA provides an important opportunity for further material design through block copolymerization (using the polymer as a macroCTA in a “graf ting f rom” approach71,94 or as a building block in a “graf ting to” approach97,98), derivatization of the polymer with a wide variety of single molecules99 or particles,100,101 or anchoring to a solid substrate.102 The second step of the synthetic route was a facile and rapid cleavage of the methoxy groups with a strong Lewis acid, boron tribromide (BBr3), which resulted in quantitative demethylation of PDMS. To identify an appropriate RAFT system to produce welldefined polymers of controllable molecular weight, the initiator:CTA ratio was systematically varied ([AIBN]0/ [DDMAT]0 = 0.1, 0.5, 0.75, 1.0, and 2.0). It was shown that [AIBN]0/[CTA]0 = 1.0 was the most effective ratio, as it produced PDMS with low Đ (84%), displayed first-order polymerization kinetics with respect to monomer, and afforded low molar mass dispersities (Đ < 1.3). Quantitative demethylation of PDMS produced poly(dihydroxystyrene), PDHS, a polymer with catechol-bearing functionality. All polymers were assessed for their surface zinc binding capacity by SEM/EDXA. The average zinc surface coverage of PDHS was shown to be 85%, whereas that of PDMS was approximately 67%. The polymer 2566

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