Article pubs.acs.org/Macromolecules
Controlled Synthesis of Fluorinated Copolymers via Cobalt-Mediated Radical Copolymerization of Perfluorohexylethylene and Vinyl Acetate Jérémy Demarteau,† Bruno Améduri,‡ Vincent Ladmiral,‡ Maarten A. Mees,§ Richard Hoogenboom,§ Antoine Debuigne,† and Christophe Detrembleur*,† †
Centre for Education and Research on Macromolecules (CERM), CESAM Research Unit, Department of Chemistry, University of Liege, Allée de la Chimie B6A, 4000 Liège, Belgium ‡ Ingénierie et Architectures Macromoléculaires, CNRS, UM, ENSCM, Institut Charles Gerhardt, UMR 5253, Place Eugène Bataillon, 34095 Montpellier, Cedex 5, France § Supramolecular Chemistry Group, Department of Organic and Macromolecular Chemistry, Ghent University, Krijgslaan 281 S4, 9000 Ghent, Belgium S Supporting Information *
ABSTRACT: Designing novel polyfluoropolymer architectures is attractive for the development of new applications, such as advanced coatings, purification membranes, or materials for energy. Nevertheless, controlling the radical polymerization of fluoroalkenes is very challenging due to the high reactivity of the propagating fluorinated macroradicals. This study aims at exploring the controlled copolymerization of perfluorohexylethylene (PFHE) and vinyl acetate (VAc) in order to prepare a range of well-defined statistical poly(PFHE-stat-VAc) copolymers with different compositions. Cobalt-mediated radical polymerization demonstrated to be active at 40 °C starting from an alkylcobalt(III) initiator, and copolymers with a fluorinated monomer content as high as ca. 80 wt % were successfully prepared. Reactivity ratios were determined to be rVAc = 0.18 and rPFHE = 0 at 40 °C and emphasized a clear tendency for alternation. Unprecedented PFHE/VAc containing block copolymers were also prepared via a single-step approach or through sequential copolymerizations. Finally, hydrolysis of the pendant ester groups of these copolymers led to the corresponding fluorinated copolymers bearing vinyl alcohol (VOH). A preliminary solution behavior study, carried out by dynamic light scattering and transmission electron microscopy on block copolymers composed of PFHE and VAc or VOH units, evidenced a marked amphiphilicity of the copolymer composed of an extremely hydrophobic PFHE block associated with a highly hydrophilic PVOH segment.
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ability of the fluorinated polymers, these fluoroalkenes have been copolymerized with various comonomers3,4,7 including ethylene (E) or vinyl acetate (VAc).11−14 TFE was also copolymerized with liquid α-olefin bearing a perfluorinated chain,15 i.e., perfluorohexylethylene (PFHE), for the preparation of membranes, films, or tapes.16 Similarly, copolymers having good flexibility, impact, and cold resistance have been produced via copolymerization of TFE and VDF.17 Interestingly PFHE, which does not homopolymerize under radical initiation, was also used as the sole source of fluorine in polymers via free radical copolymerization with non-fluorinated monomers such as VAc.18 The resulting copolymers showed valuable properties in various applications ranging from emulsifiers to protective colloids or components of adhesives.
INTRODUCTION Fluoropolymers have attracted great attention from the scientific community for decades because of their particular properties, such as chemical inertness (oxidation or acid/base treatment), high thermal stability, low flammability, optical properties (low refractive index), hydrophobicity, etc.1−6 The field of applications of such polymers ranges from coatings, membranes for fuel cells and water purification, high performance elastomers, aeronautics, and optics to wiring insulation or automobile industry.5,7−10 Poly(tetrafluoroethylene) (PTFE) and poly(vinylidene fluoride) (PVDF) are certainly two of the most famous fluorinated polymers, but they are highly crystalline fluoroplastics, which implies consequent costs for their processing and confers them low solubility in organic solvents. Moreover, their starting fluoroalkene monomers, TFE and VDF, are gaseous at room temperature and atmospheric pressure, and their handling requires specific equipment. In order to decrease the crystallinity and facilitate the process© XXXX American Chemical Society
Received: March 18, 2017 Revised: April 28, 2017
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DOI: 10.1021/acs.macromol.7b00578 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
amphiphilicity of the copolymer composed of an extremely hydrophobic PFHE block associated with a highly hydrophilic PVOH segment is regarded as a real challenge in the field of fluoropolymers.
In the past decades, huge efforts have been devoted to control the radical (co)polymerization of fluoroalkenes in order to build more complex fluorinated architectures and finely tune their properties. Examples of controlled radical (co)polymerization (CRP or RDRP) of fluoroalkenes are relatively scarce due to the very particular reactivity of the growing fluorinated macroradical. Tatemoto pioneered the controlled radical polymerization of VDF through iodide transfer polymerization (ITP),19−21 notably leading to well-defined VDF containing copolymers7 including thermoplastic elastomers marketed by the Daikin company. The photochemical activation of alkyl iodides was also developed by Asandei et al. to initiate the controlled polymerization of VDF and synthesize block copolymers.22−26 Organoborane reagents have also been used for the controlled synthesis of fluoropolymers.27−30 Recently, the controlled copolymerization of VDF and VAc was achieved by macromolecular design through interchange of xanthate (MADIX).31−33 Sequential MADIX of VDF and VAc followed by hydrolysis of the pendant esters notably led to block copolymers containing PVDF and poly(vinyl alcohol) (PVOH) sequences and to the formation of fluorinated colloids in water.34 On the other hand, the controlled radical copolymerization of PFHE with non-fluorinated monomers has only been briefly reported by Sen et al.35,36 Among other monomers, VAc was copolymerized by ITP with a series of 1-alkenes including PFHE by using azobis(isobutyronitrile) (AIBN) and ethyl iodoacetate as radical initiator and as chain transfer agent (CTA), respectively. One poly(PFHE-stat-VAc) copolymer with low molar mass and moderate dispersity (Đ ∼ 1.6) was prepared. The current work aims at exploiting cobalt-mediated radical polymerization (CMRP) for the precision design of welldefined PFHE/VAc copolymers over a broad comonomer composition. CMRP, a controlled radical polymerization method based on the reversible deactivation of the growing radical chains by a cobalt complex, appears as the technique of choice given its ability to control the polymerization of numerous “less activated monomers” (LAMs), characterized by the high reactivity of their propagating radicals due to the lack of resonance stabilization, like VAc.37 The latter monomer has also been copolymerized in a controlled fashion by CMRP with other LAMs such as α-olefins like octene38,39 or ethylene.38,40,41 Nevertheless, implementing CMRP with novel α-olefins is always a new challenge due to the risk of forming too stable cobalt−carbon bonds by combination of the metal and nonstabilized radicals, and so is the case for the PFHE having peculiar electronic properties. In this article, the CMRP conditions are explored to properly control the copolymerization of PFHE and VAc and to prepare a range of well-defined statistical poly(PFHE-stat-VAc) copolymers with different compositions. Reactivity ratios were determined in order to have a precise idea of the distribution of the comonomers along the chains and to exploit them for preparing unprecedented PFHE/VAc containing block copolymers in a facile single step. This type of block copolymers was also produced via a sequential polymerization approach. Finally, hydrolysis of the pendant ester groups of these copolymers led to the corresponding fluorinated vinyl alcohol (VOH)containing copolymers. A preliminary solution behavior study was carried out by dynamic light scattering (DLS) and transmission electronic microscopy (TEM) on block copolymers composed of PFHE and VAc or VOH units. The marked
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EXPERIMENTAL PART
Materials. All manipulations were performed by classical Schlenk techniques under argon. Vinyl acetate (VAc, >99%, Aldrich) was dried over calcium hydride, degassed by several freeze−thawing cycles before distillation under reduced pressure, and stored under argon. Perfluorohexylethylene, also called 1H,1H,2H,2H-perfluorooct-1-ene (CH2CHC6F13 or PFHE, >98%), synthesized from the ethylene end-capping of 1-iodoperfluorohexane followed by a dehydroiodination (reported in ref 42), was dried over molecular sieves and degassed by purging Ar during several minutes. The organocobalt(III) adduct initiator [R0-(CH(OCOCH3)CH2)99%, VWR), ethanol (EtOH, >99%, VWR), and tetrahydrofuran (THF, >99%, VWR) were used as received. Characterizations. The number-average molar masses (Mn) and dispersity (Đ) of poly(PFHE-stat-VAc) for the kinetics having f PFHE ≤ 0.24 were determined by size exclusion chromatography (SEC) in tetrahydrofuran (THF) at 45 °C at a flow rate of 1 mL/min. The apparatus is a Viscotek 305 TDA liquid chromatograph equipped with two PSS SDV analytical linear M 8 mm columns protected by a PL gel 5 μm guard column and calibrated with polystyrene standards. For the kinetic determination of poly(PFHE-stat-VAc) with f PFHE = 0.5, SEC was performed using an Agilent HPLC equipped with a 1260 refractive index detector (RID) using 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) containing 20 mM sodium trifluoroacetate as eluent, which was dried with molecular sieves (3 Å). The flow rate during the measurements was 0.53 mL/min, and poly(methyl methacrylate) (PMMA) standards were used to calculate the molar mass values. Measurements were performed with a column set consisting of two PSS PFG 300*8 Å gel 5 μm MIXED-D columns and a similar guard column (PSS) in series with a molar mass range from 100 to 1 000 000 g/mol in a column oven set at 35 °C. Chromatograms were analyzed with Agilent Chemstation software using the GPC add-on. All NMR experiments were carried out at 298 K. 1H NMR spectra of reaction mixtures for the determination of the conversions were recorded in CDCl3 with a 400 MHz Bruker spectrometer. For the kinetic determination of poly(PFHE-stat-VAc) with f PFHE = 0.5, 10− 60 μL of TFT was used in order to increase the copolymer solubility in the tube for reaction mixtures. The chemical shifts (δ) are reported in ppm. 19F NMR experiments were also carried out at 298 K in CDCl3 with a 400 MHz Bruker spectrometer. The determination of monomer composition in the copolymer was evaluated by integrating the methine (−CH−) peaks of PVAc at 4.7−5.3 ppm, the methyl (−OCH3) peaks of the end-chain at 3.16−3.19 ppm and the methine (−CH−) peaks of PFHE at 2.3−2.6 ppm (eqs 1−4). Concerning the determination of DPPVAc, four repetitive protons have been subtracted from the VAc total integral of the methine (−CH−) peak because of the organocobalt(III) adduct initiator: 2.6
DPPFHE =
∫2.3 CH(PFHE) 3.19
∫3.16 OCH3(end‐chain)/3 5.3
DPPVAc =
(1)
3.19
∫4.7 CH(PVAc) − (∫3.16 OCH3(end‐chain)/3) × 4 3.19
∫3.16 OCH3(end‐chain)/3 (2)
B
DOI: 10.1021/acs.macromol.7b00578 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules FPFHE =
DPPFHE DPPFHE + DPPVAc
FPVAc = 1 − FPFHE
final copolymer composition are shown in Table S2 and were used for the determination of reactivity ratios by Fineman−Ross (FR),44 Kelen−Tüdos (KT),45 and the nonlinear (NL)46,47 least-squares fitting curve methods (Figure 3). The nonlinear least-squares method uses the Mayo−Lewis equation (eq 5 and Figure 3a). The Fineman−Ross linearization method generates a straight line, the slope of which and the intercept with the ordinate (Y-axis) correspond to r1 and r2, respectively (eq 6 and Figure 3b). The Kelen−Tüdos method involves η and ζ parameters, mathematical functions of the mole ratios in the monomer feed ( f) and in the copolymer (F), and of a parameter α calculated on the basis of the lowest and highest values of ( f 2/F). The determination of r1 and r2 was made possible by the extrapolation and the intercept at ξ = 1 and ξ = 0, giving r1 and −r2/α, respectively (eq 7 and Figure 3c).
(3) (4)
Differential scanning calorimetry (DSC) was performed on a TA Instruments Q1000 DSC, using hermetic aluminum pans, indium standard for calibration, nitrogen as the purge gas, a sample weight of ∼5 mg, from 0 to 140 °C at 10 °C/min heating rate, followed by an isotherm at −90 °C for 5 min and heating up to 180 °C at 10 °C/min heating rate. TGA analyses were carried out with a Hi-Res TGA Q500 from TA Instruments at a heating rate of 20 °C min−1 under nitrogen with a sample weight of ∼10 mg. Fourier transform infrared (FT-IR) spectra were measured by using a Nicolet IS5 spectrometer (Thermo Fisher Scientific) equipped with a transmission or with a diamond attenuated transmission reflectance (ATR) device. Spectra were obtained in transmission or ATR mode as a result of 32 spectra in the range of 4000−500 cm−1 with a nominal resolution of 4 cm−1. Spectra were analyzed with an ONIUMTM (Thermo Fisher Scientific) software. TEM pictures were recorded with a Gatan 673 CCD camera and transferred to a computer equipped with Kontron KS 100 software. Samples were prepared by the deposition of a droplet of the aqueous/ methanolic copolymer solution (0.7 g L−1) onto a Formvar-coated copper TEM grid, which was allowed to evaporate overnight. DLS experiments were performed on a Malvern CGS-3 apparatus equipped with a He−Ne laser with a wavelength of 633 nm. The measurements were performed in water or in methanol at 90° angle at a concentration of 0.7 g L−1 at 25 °C. Typical Procedure for the Controlled Synthesis of Poly(PFHE-stat-VAc) by CMRP. A solution of organocobalt(III) adduct initiator [R0-(CH2CH(OCOCH3))
