F NMR Analyses of Terpolymers from Vinylidene ... - ACS Publications

May 22, 2015 - Department of Chemistry, University of Akron, Akron, Ohio ... Chemical Engineering and Materials Science, Soochow University, 199 Renai...
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Multidimensional 19F NMR Analyses of Terpolymers from Vinylidene Fluoride (VDF)−Hexafluoropropylene (HFP)−Tetrafluoroethylene (TFE) Eric B. Twum,*,†,‡ Elizabeth F. McCord,§ Donald F. Lyons,§ and Peter L. Rinaldi*,†,∥ †

Department of Chemistry, University of Akron, Akron, Ohio 44325-3601, United States Department of Chemistry, Indiana University, 800 E. Kirkwood Ave., Bloomington, Indiana 47405-7102, United States § Experimental Station, E. I. DuPont de Nemours and Co., Wilmington, Delaware 19880-0402, United States ∥ College of Chemistry, Chemical Engineering and Materials Science, Soochow University, 199 Renai Road, Suzhou Industrial Park, Suzhou , Jiangsu Province 21513, P. R. China ‡

S Supporting Information *

ABSTRACT: The use of multidimensional NMR methods for the characterization of polymer microstructure has been applied to terpolymers from vinylidene fluoride (VDF), hexafluoropropylene (HFP), and tetrafluoroethylene (TFE). By assembling the atomic connectivity information obtained from different multidimensional NMR experiments, selective 19 F−19F COSY (correlation spectroscopy), 19F−19F gradient double-quantum COSY, and 19F−13C gradient heteronuclear single-quantum coherence (gHSQC), among others, the detailed monomer sequence arrangements in the terpolymer were obtained. Obtaining the resonance assignments of the terpolymer was greatly aided by the extrapolation of known resonance assignments from PVDF homopolymer, poly(VDF-coHFP) copolymer, and poly(VDF-co-TFE) copolymer. A tabulated comparison of the microstructure assignment of resonances from PVDF homopolymer as well as poly(VDF-co-HFP) and poly(VDF-co-TFE) copolymers and the terpolymer is provided. Detailed comparisons of 19F spectra from 470 and 658.4 MHz spectrometers, revealing the AB patterns present in this terpolymer, are presented and discussed in this paper. The compositions of the comonomers in the terpolymers were calculated with different methods, all of which gave similar values. The percentages of VDF and HFP monomer inversions in the terpolymers were also calculated from the assigned NMR resonances.

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INTRODUCTION Terpolymers of vinylidene fluoride (VDF), hexafluoropropylene (HFP), and tetrafluoroethylene (TFE), poly(VDF-terHFP-ter-TFE), can be elastomers, thermoplastics, or thermoplastic elastomers depending on the content of the comonomers.1,2 The elastomeric terpolymers are more amenable to NMR studies since they are more soluble in organic solvents. The elastomeric terpolymer is marketed by DuPont under the trade name Viton B or Viton F fluoroelastomer depending on the amount of fluorine in the polymer. These fluoroelastomers, like poly(VDF-co-HFP) copolymers, are designed for use in applications requiring improved chemical resistance and in high-temperature environments. They have found applications in automotive, aerospace, chemical, and petroleum industries.1,3−5 Terpolymers with other compositions of comonomers are marketed by Solvay Solexis, 3M/Dyneon, and Daikin under the trade names Tecnoflon, THV (Dyneon terpolymers), and Dai-el fluoropolymers, respectively. These terpolymers are prepared commercially by free radical emulsion polymerization under high pressure6,7 using both batch and continuous processes. An emulsifier can be used to concentrate the dispersion from the emulsion polymerization. © XXXX American Chemical Society

However, the use of certain initiators such as ammonium persulfate produces ionic end groups which increase the colloidal stability of the latex formed during the polymerization process;8 this makes the use of emulsifiers unnecessary.9 The ratio of the comonomers in the terpolymer affects the melting point, chemical resistance, and flexibility of the polymer.10 NMR spectroscopy is the method of choice for quantifying the composition of the comonomers in the terpolymer and also for studying the statistical distribution of monomer units along their chains.6,11−16 Knowledge of the monomer sequence distribution is very important as many useful properties of the polymer depend on the polymer’s monomer sequence distribution. The use of 19F NMR to study fluoropolymers is appealing since 19F has a large chemical shift range (over 200 ppm) compared to 1H (10 ppm), and the 19F chemical shift is extremely sensitive to variations in chemical structure. The 19F NMR spectra also show a large range of J-couplings (couplings over 2−6 bonds are commonly observed which range from 0 to Received: January 29, 2015 Revised: May 13, 2015

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fluorinated terpolymer. However, this work did not provide detailed assignments for the many resonances in the terpolymer. In that paper, resonances of only nine structures were assigned, based on a few of the resolved correlations seen in a gCOSY spectrum. In this work, multidimensional NMR experimentsselective 19 F−19F COSY, 19F−19F gDQCOSY, and 19F−13C gHSQC, among othersare used to determine the monomer sequence distribution in a poly(VDF-ter-HFP-ter-TFE) terpolymer. The determination of resonance assignments was aided by the known resonance assignments of PVDF homopolymer and of other VDF-based copolymers described in earlier papers.22−27

300 Hz). In addition, for many fluoropolymers the presence of the three NMR active nuclei (1H, 13C, and 19F) provides many structure-dependent NMR parameters to measure. There have not been many NMR studies on this terpolymer. NMR spectra from poly(VDF-ter-HFP-ter-TFE) terpolymer are complicated by numerous permutations of monomer sequences and the many nJFF and nJHF couplings present. The CF of the HFP monomer (CF2CF(CF3)) creates a stereogenic center in the polymer. For instance, for the sequence HFP-VDF in a chain (−CF2CF(CF3)−CH2CF2−), the two fluorines from the HFP CF2 will be diastereotopic. The presence of these stereogenic centers in the polymer results in many AB patterns in the NMR spectra. These AB patterns from nonequivalent geminal fluorines may be interpreted incorrectly if thorough analysis is not made. The many resonances from this terpolymer, which result from the numerous permutations of monomer sequences, also make the assignment of the different resonances very difficult. This is especially true for the weak resonances from less probable sequences. The dearth of information on NMR characterization of this terpolymer is not surprising if one looks at the 19F 1D NMR spectrum of the polymer. Unlike PVDF homopolymer which has been the subject of extensive NMR studies, the terpolymer has many overlapping resonances which makes NMR characterization challenging. Another difficulty with the NMR studies of this terpolymer, which may be present in the NMR studies of most fluoropolymers, is the large chemical shift range of 19F. While the large chemical shift range may present an advantage (there are likely to be fewer overlaps of peaks in the 19F spectrum compared to 1H spectrum of the terpolymer), it also presents a challenge with regards to uniform excitation of the entire spectral width, especially in multidimensional experiments.17,18 This requires a probe with a short 19F 90° pulse width. In addition, the NMR probe for studying fluoropolymers should be made of minimal 19F containing materials. In spite of these difficulties, the availability of hardware for 1H/19F double resonance and 1H/19F/13C triple resonance experiments and the use of multidimensional pulse sequences specifically suited for fluoropolymers made it possible to perform a detailed structural characterization of the terpolymer. Murasheva et al.19 used an additivity scheme to interpret the resonances from 84.68 MHz 19F NMR spectra of poly(VDF-coTFE) copolymer and poly(VDF-ter-HFP-ter-TFE) terpolymer. They were only successful in assigning resolved resonances with relatively strong signal intensities. Because of the low resolution of their spectra, several resonances were not taken into account. High-speed magic-angle spinning (MAS) was used to study the poly(VDF-ter-HFP-ter-TFE) terpolymer by Dec et al.20 They obtained high-resolution spectra at 338.7 MHz spectrometer using MAS speeds greater than 18 kHz. Spectral assignments of five-carbon sequences were obtained by comparing their spectra with those obtained from solution state NMR studies.19 Pianca et al.6 determined the monomer sequence distribution of poly(VDF-co-HFP) copolymer and poly(VDF-ter-HFP-ter-TFE) terpolymer using NMR spectra from a 200 MHz spectrometer. By employing empirical relationships, they were able to assign the resolved 19F resonances from the copolymer. However, when similar relationships were applied to the spectra of the terpolymer, they were unsuccessful in assigning the many 19F resonances present. They suggested the use of model compounds to help decipher the many 19F resonances in the terpolymer. Recently, Ok21 used 2D NMR to study the microstructures of this

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EXPERIMENTAL SECTION

Materials. All the polymers described in this study were provided by DuPont. All materials were used as received. Acetone-d6 (99.9%), used as the solvent, was purchased from Cambridge Isotope Laboratories Inc. Trichlorofluoromethane (CFCl3, 99.5%), used as internal standard for 19F NMR, and TMS (99.9%), used as internal standard for 1H and 13C NMR, were purchased from Sigma-Aldrich. Sample Preparation for NMR Analyses. Samples for NMR analyses were prepared by dissolving ca. 70−80 mg of poly(VDF-terHFP-ter-TFE) terpolymer in about 600 μL of the acetone-d6 solvent contained in a 5 mm NMR tube. A drop of CFCl3, used as the internal chemical shift reference for the 19F NMR, was added. The NMR tubes were flame-sealed to prevent evaporation of the acetone. The samples were warmed to about 45−50 °C and agitated to dissolve all the terpolymer in solution before NMR analyses at 30 °C. The polymers were prepared by aqueous emulsion polymerization. The details of how the polymers were prepared, and the details of the NMR experimental parameters used can be found in the Supporting Information. Instrumentation. Except where indicated, all the NMR spectra were acquired on a Varian Direct-Drive 500 MHz spectrometer. Details of this instrument configuration are given in an earlier paper.22 Where decoupling was applied, the decoupler modulation used for 1H, 13 C, and 19F were WALTZ-16 (γHBH/2π = 2.6 kHz), WURST-40 (γCBC/2π = 10 kHz), and CHIRP (γFBF/2π = 16.8 kHz), respectively.

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RESULTS AND DISCUSSION Nomenclature and Structure. The nomenclature used to describe the structures in this paper consists of the numerals 0, 1, 2, and 3 representing CH2, CF, CF2, and CF3 groups, respectively; the numerals represent the number of 19F atoms attached to a carbon atom. The terpolymer under study is composed of VDF (CH2CF2) units which can be 02 (normal unit) or 20 (reverse unit), HFP (CF2CF(CF3)) units which can be 21 (normal unit) or 12 (reverse unit), and TFE (CF2CF2) units which are 22. The labeling used in this paper for the various 19F resonances is as follow: In figures, the backbone resonances are labeled with letters (e.g., A, B, C, etc.), and the 3-carbon sequence region to which they belong is indicated in the figure. However, in the text, the backbone resonances are described with letters, and the 3-carbon sequence regions to which they belong are indicated with subscripts (e.g., A020, B020, C020, etc.). Where the number of 19F resonances in a region is more than 26 (the total number of Latin alphabet), after the letter Z, the superscript 1 is used in addition to the letter, e.g., A1020, B1020, C1020, etc. The labeling of the resonances takes into account resonances from the PVDF homopolymer as well as poly(VDF-co-HFP) and poly(VDF-co-TFE) copolymers. Therefore, the labeling might skip the letters in their normal order as certain resonances in say PVDF homopolymer, would not be observed in this terpolymer; hence, the letters used for those resonances would not be used in the terpolymer. B

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Macromolecules In the terpolymerization of VDF, HFP, and TFE monomers via free radical polymerization, structures similar to those obtained from the homopolymerization of the VDF monomer and homopolymerization of TFE monomer would be present. Additionally, depending on the composition of the comonomers, the polymer is likely to be composed mainly of sequences containing randomly placed units of VDF, HFP, and TFE units. Under the conditions employed in the copolymerization process, HFP does not homopolymerize.1,6,28−30 The initiating radical as shown in Scheme 1 can attack any of

Scheme 2. Possible Odd-Numbered Carbon Sequences from Poly(VDF-ter-HFP-ter-TFE) Terpolymer

Scheme 1. Representative Equations Showing the Reaction Pathways for Free Radical Polymerization of VDF, HFP, and TFE Monomers

ter-TFE) terpolymer. We assume that the number of HFP-HFP sequences are insignificant and hence are not shown. Assignment of Resonances to Three-Carbon Sequences. Shown in Figures 1a−f are the peak containing regions from the 19F 1D NMR spectrum of poly(VDF-ter-HFP-terTFE) terpolymer (52.8:23.0:24.2 mol %). The CF3, CF2, and CF fluorine resonances fall in different regions of the 19F spectrum and hence are easily distinguished. Figure 1a shows the resonances of fluorines from CF3 groups (−69 to −76.5 ppm). (Note: it should be understood that in reference to the 3 structure element, the 3 unit is the trifluoromethyl branch attached to the backbone 1 unit.) Figures 1b−e show the resonances of fluorines from the CF2 region of the 19F spectrum. Figure 1f shows resonances of fluorines from CF groups (−178 to −186 ppm). Signals of sequences with monomer inversion will be relatively weak compared to signals of sequences from normal addition. The CFn centered 3-carbon sequence assignments for the various 19F resonances in the terpolymer were made using NMR spectral information from 19F−19F COSY (gDQCOSY and selective COSY) and 19F{13C} gHSQC and also by comparing the 19F spectra of the terpolymer with those of PVDF homopolymer and poly(VDF-co-TFE) and poly(VDFco-HFP) copolymers. Poly(VDF-ter-HFP-ter-TFE) terpolymer, like poly(VDF-coHFP) copolymer, has several CF2 fluorines and CH2 protons that are chemically nonequivalent due to the stereogenic centers created by the backbone CF groups in HFP. Extra caution should therefore be taken when interpreting the spectra, especially 19F spectra, as multiplets due to large couplings (e.g., 2JFF ≈ 280−300 Hz in AB patterns) might be incorrectly assigned as different resonances. Shown in Figure 2 are selected regions from the one-bond and two-bond gHSQC spectra of poly(VDF-ter-HFP-ter-TFE) terpolymer (52.8:23.0:24.2 mol %). Results from these data show that the CF fluorine resonances have two-bond correlations to CF3 carbon resonances (Figure 2e), and CF3 fluorine resonances show two-bond correlations to CF carbon resonances (Figure 2b). It is thus possible to pair the resonances of the CF3 groups with the corresponding resonances of directly attached CF groups in each of the many HFP-containing n-ad structures. The CF 19F resonances within −183.3 to −185.5 ppm region show two-bond correlations to both CH2 and CF2 carbon resonances (Figure 2d,e); the other CF 19F resonances show two-bond correlations only to CF2 carbon resonances but not to CH2 carbon

the three monomers in solution. For VDF and HFP which are not symmetrical, the radical can attack either end of the monomer, although an attack on the CF carbon of the HFP and the CF2 carbon of VDF is less likely due to steric and electronic factors. For a detailed description of the production of certain end groups based on the normal/inverse addition to VDF or HFP monomer, see an earlier paper on the study of VDF/HFP copolymer.27 In the polymerization process, VDF monomers can add to a growing chain either through normal addition, resulting in the terminal free radical on the 2 or CF2 carbon, or through inverse addition where the terminal free radical will be on the 0 or CH2 carbon. Likewise, an HFP monomer can add to a growing chain through either normal or inverse addition. The propagation step in Scheme 1 shows only the situation where the free radical is on the CF2 carbon of the VDF monomer. For brevity, propagation and termination processes involving HFP and TFE monomer units are not shown. Scheme 1 also does not show branching structures which are known to form in VDFcontaining polymers.23 Termination of the chains occurs when any two polymer radicals couple with each other or by disproportionation. Only one example of a termination reaction involving two chains terminated by VDF units is shown in Scheme 1. Termination can also occur by hydrogen abstraction at a VDF unit by a radical at the end of a polymer chain (not shown). A detailed discussion of these processes can be found in earlier published work on the homopolymer of VDF and copolymers of VDF with TFE and HFP.23,25,27 Scheme 2 shows most of the theoretically possible oddnumbered 3- and 5-carbon sequences from poly(VDF-ter-HFPC

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Figure 1. Selected regions from the 470 MHz 19F{1H} 1D NMR spectrum of poly(VDF-ter-HFP-ter-TFE) terpolymer (52.8:23.0:24.2 mol %): (a) CF3 region, (b−e) CF2 regions, and (f) CF region. The insets in (b) and (d) are from poly(VDF-ter-HFP-ter-TFE) terpolymer (66.8:22.4:10.8 mol %).

resonances. The CF 19F resonances between −183.3 and −185.5 ppm are therefore from 210 3-carbon sequences (1); the other CF 19F resonances are from 212 3-carbon sequences (2). Because the CF 19F resonances between −183.3 and −185.5 ppm show two-bond HSQC correlations to the carbon resonances of the CF3 groups whose 19F resonances occur between −74.5 and −76 ppm, the CF3 19F resonances between −74.5 and −76 ppm are assigned to the 21(3)0 3-carbon sequences. The CF3 19F resonances between −69.5 and −71.0 ppm are attributed to 21(3)2 3-carbon sequences. The two most deshielded CF3 19F resonances (resonances A232 and B232 between −69.5 and −70.2 ppm) must come from 3-carbon sequences that involve TFE units. This is because these two

CF3 19F resonances are not present in poly(VDF-co-HFP) copolymer as seen in the Supporting Information Figure S6; also as evident in Figure S6, as the content of TFE increases, the intensity of those resonances also increases.

Using the information from Scheme 2 as a guide, the various F resonances from the terpolymer can be assigned to different 3-carbon sequences. Shown in Figure 3 is a selected region of 19

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Figure 2. Selected regions from the 470 MHz 19F{13C} gHSQC spectra of poly(VDF-ter-HFP-ter-TFE) terpolymer (52.8:23.0:24.2 mol %): (a) onebond 19F−13C correlation for 21(3)2 and 21(3)0 sequences; (b) two-bond 19F−13C correlation for 21(3)2 and 21(3)0 sequences; (c) one-bond 19 F−13C correlation for 212 and 210 sequences; (d, e) two-bond 19F−13C correlation for 212 and 210 sequences. The corresponding regions from the 1D NMR spectra are plotted along the sides and tops of each 2D NMR spectrum.

CF2 resonances from the two-bond 19F{13C} gHSQC spectrum. The 19F resonances from −90 to −96.5 ppm only show twobond correlations to CH2 carbon resonances; they are therefore assigned to 020 3-carbon sequences. Some of the expected correlations are not observed due to the weak signal intensities of those resonances; however, their 3-carbon sequences are confirmed from other experiments and also by comparing the 19 F spectra of the terpolymer with those of poly(VDF-co-HFP) and poly(VDF-co-TFE) copolymers (see Figures S1−S6). The 19F resonances from 022 3-carbon sequences show twobond correlations to both CH2 and CF2 carbon resonances as seen from Figure 3. The 19F resonances from 021 3-carbon sequences are expected to show two-bond correlations to both CH2 and CF carbon resonances. However, no two-bond correlations are seen for these groups in the two-bond 19F{13C} gHSQC spectrum (Figure 3). This might be due in part to their low signal intensities and in part due to their appearance as AM patterns. In such patterns the large 2JFF (280−300 Hz) couplings result in creation of out-of-phase magnetization components which diminish the intensities of cross-peaks due to JCF couplings.31 Of the 19F resonances from ca. −116 to −120 ppm that show two-bond correlations, only two-bond correlations to both CF2 and CF carbon resonances are observed. They do not show two-bond correlations to CH2

Figure 3. 470 MHz two-bond 19F{13C} gHSQC spectrum of poly(VDF-ter-HFP-ter-TFE) terpolymer (52.8:23.0:24.2 mol %) showing only the region containing CF2 fluorine resonances in the 19 F dimension. The 13C{1H,19F} 1D NMR spectrum is plotted along the side of the 2D NMR spectrum. Note: one-bond leakage is the occurrence of one-bond 19F−13C correlation in the two-bond 19F−13C correlation experiment.

carbon resonances. They are thus assigned to 221 3-carbon sequences. Lastly, the 19F resonances from 222 3-carbon E

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Macromolecules sequences are easily identified from Figure 3 as they show twobond 19F−13C correlations to only CF2 carbon resonances. The 222 3-carbon sequences can also be identified by comparing the 19 F spectrum of the terpolymer with that of poly(VDF-co-TFE) copolymer whose assignments were described in an earlier paper.24 Assignment of Resonances to Five-Carbon Sequences. The assigned resonances from 3-carbon sequences were used as a basis for the assignment of resonances from 5-carbon and higher level sequences. From Scheme 2 there are 32 possible 5-carbon sequences in the terpolymer; of these, some are less probable as they involve inversion of either VDF or HFP units. Resonances of some sequences would also overlap with others and might not be resolved. In the 020 3-carbon sequence region, resonances from the 020-centered 5-carbon sequence 20202 are identified as these 19 F resonances only show 4JFF COSY correlations to CF2 resonances as shown in Figure 4a. These 4JFF correlations are confirmed by using information from the two-bond 19F{13C} gHSQC spectra given in the Supporting Information (Figure S9); for instance, in Figure S9a, some resonances from the 020 and 022 3-carbon sequences region show 2-bond 19F−13C correlations to the same CH2 carbon resonances. Resonances from 10202 5-carbon sequences show 4JFF COSY correlations to CF resonances as seen in the selective COSY spectrum given in Figure S10a. Although 00202 5-carbon sequences are present, due to their lower signal intensities their correlations are not detected in the COSY spectrum. Previous work has shown that inversions of VDF units occur at a level of ca. 5 mol % in VDF homopolymer prepared under similar conditions.22 For copolymers of VDF with HFP prepared under similar conditions, the VDF inversion was found to be dependent on the amount of the VDF units in the copolymer; for copolymers consisting of 78.5 and 66 mol % VDF, the VDF inversions were ca. 2.9 and 2.2 mol %, respectively.26 The assignments of resonances from the 00202 5-carbon sequences were obtained by comparing the spectra of the terpolymer with those of PVDF homopolymer and copolymers of VDF with TFE and VDF with HFP (see Supporting Information). As outlined in Scheme 2, there are four possible 021centered 5-carbon sequences in the terpolymer. These 5-carbon sequences involve inversion of either VDF or HFP units in the chain; as such they are low probability sequences. Inversion of HFP units occur in less than 2 mol % in these poly(VDF-terHFP-ter-TFE) terpolymers. The 021-centered 5-carbon sequences (and therefore higher level sequences from the 021 3-carbon sequence region) overlap with each other. The 021centered 5-carbon sequences and higher levels sequences from this region are assigned using spectral information from poly(VDF-co-HFP) copolymer. Detailed resonance assignments of poly(VDF-co-HFP) copolymer can be found in refs 26 and 27. Shown in Figure 5 are the 19F spectra of poly(VDFco-HFP) copolymer and the terpolymers having different compositions. These spectra show that resonances of higher level sequences from the 021 region in the terpolymer and the copolymer have similar chemical shifts. It should be noted that with the introduction of TFE in the terpolymer newer signals that involve resonances of TFE containing structures are present in the spectra of the terpolymer. However, due to the overlap of the signals from higher level sequences in this region, these new signals are not resolved in the 1D NMR spectra. It should also be noted that poly(VDF-co-TFE) copolymer has no

Figure 4. Selected regions from the 470 MHz 19F−19F gDQCOSY spectrum of the CF2 region from poly(VDF-ter-HFP-ter-TFE) terpolymer (52.8:23.0:24.2 mol %): (a) correlations among resonances from the 020 and 022 regions of the 19F spectrum; (b) correlations among resonances from the 022 and 222 regions of the 19F spectrum.

peaks in the 021 3-carbon sequence region (Figures S1 and S3). By comparing the 19 F 1D NMR spectra of PVDF homopolymer, poly(VDF-co-HFP) copolymer, poly(VDF-coTFE) copolymer, and poly(VDF-ter-HFP-ter-TFE) terpolymer (Figure 6), the regions corresponding to the various 022centered 5-carbon sequences in the terpolymer (Scheme 2) can be identified. Some of these 5-carbons sequences involve inversion of a monomer unit; they therefore have very weak signals. The 5-carbon and higher level sequences of these weak resonances are assigned by extrapolation of the assignments from PVDF homopolymer, poly(VDF-co-HFP) and poly(VDFco-TFE) copolymers based on their chemical shift and also variations in signal intensities with changes in comonomer composition. The only 022-centered 5-carbon sequence which is not present in PVDF homopolymer, poly(VDF-co-HFP) copolymer, and poly(VDF-co-TFE) copolymer is the 10222 sequence (Figure 6). The central CF2 in this 5-carbon sequence F

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in the terpolymer is attributed to 19F resonances C022 and D022. Resonance C022 shows 4JFF COSY correlations to CF resonances (Figure S10b) and to CF2 resonances (Figure 4b) from 210 and 222 3-carbon sequence regions, respectively. The 4 JFF correlation between resonance C022 and the resonances from the 210 region is confirmed by the correlations observed in the gHSQC spectra given in Figure S9; resonance C022 and certain CF resonances show 2-bond 19F−13C correlations to the same CH2 carbon resonance. There are five possible 221-centered 5-carbon sequences in the terpolymer (Scheme 2). Some of these probable sequences are not present in poly(VDF-co-HFP) copolymer26 because they have TFE monomer(s) in the sequences. When the 19F spectra of the 221 3-carbon sequence region of poly(VDF-coHFP) copolymer is compared to that of the terpolymer (Figure 7), it is observed that only a few new signals appear in the

Figure 5. Comparison of the 021 3-carbon sequence regions from the 470 MHz 19F{1H} 1D NMR spectra of (a) poly(VDF-co-HFP) copolymer (78.5:21.5 mol %), (b) poly(VDF-ter-HFP-ter-TFE) terpolymer (66.8:22.4:10.8 mol %), and (c) poly(VDF-ter-HFP-terTFE) terpolymer (52.8:23.0:24.2 mol %).

Figure 7. Comparison of 221 3-carbon sequence regions from the 470 MHz 19F{1H} 1D NMR spectra of (a) poly(VDF-co-HFP) copolymer (78.5:21.5 mol %), (b) poly(VDF-ter-HFP-ter-TFE) terpolymer (66.8:22.4:10.8 mol %), and (c) poly(VDF-ter-HFP-ter-TFE) terpolymer (52.8:23.0:24.2 mol %). The peaks labeled with an asterisk are the resolved peaks present in the terpolymer but not in the copolymer.

spectrum of the terpolymer. However, most of the peaks do change intensities from one polymer to another in the predicted way. The new signals in the 221 3-carbon sequence region of the terpolymer are indicated with an asterisk in Figure 7. The CF2 19F resonances in the 221 region also exhibit AB patterns (with 2JAB ≈ 280−300 Hz, and are further split by additional small unresolved long-range couplings) as they are diastereotopic fluorines (see Figures S16 and S17, for some AB patterns in the different regions of the 19F spectrum). For these reasons, weaker resonances from higher level sequences in this region might therefore overlap with the AB patterns from the strong resonances. The 221-centered 5-carbon sequences are therefore not all assigned to specific regions, but the sequences that give rise to strong resonances in the 19F spectrum are assigned to higher level sequences. There are six possible 222-centered 5-carbon sequences in the terpolymer, as given in Scheme 2. Of these six sequences, three involve only VDF and TFE units; these sequences can be

Figure 6. Comparison of the 022 3-carbon sequence regions from the 470 MHz 19F{1H} 1D NMR spectra of (a) PVDF homopolymer, (b) poly(VDF-co-HFP) copolymer (78.5:21.5 mol %), (c) poly(VDF-coTFE) copolymer (52:48 mol %), and (d) poly(VDF-ter-HFP-ter-TFE) terpolymer (52.8:23.0:24.2 mol %).

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Macromolecules assigned by comparing the 19F spectra of poly(VDF-co-TFE) copolymer24 and the terpolymer as shown in Figure 8. Of the

carbon sequence which involves inversion of a monomer unit will have low probability of occurrence. In addition, the broadness of the resonances from the 210 3-carbon sequence region suggests that the weaker resonances from 02102 sequences will overlap with the strong resonances from 22102 sequences. In poly(VDF-co-HFP) copolymer,26 the two 212-centered 5carbon sequences result from an inversion of a monomer unit; resonances from this region therefore have low intensities. In the terpolymer, there are three possible 212-centered 5-carbon sequences; a sequence containing both TFE and HFP units is possible. This new sequence can result from either the 022122 or 222122 monomer combinations. Neither of these involves monomer inversion; therefore, resonances from these structures would be expected to be strong. However, as discussed above, the addition of HFP to TFE and vice versa is unfavorable for steric and electronic reasons, and therefore resonances from these structures will have low probability of occurrence. The resonances overlap in the region containing signals of 212-centered 5-carbon sequences; they are therefore not easily assigned. Specific 19F resonances of higher order sequences from this region with relatively strong intensities are however assigned (vide inf ra). Assignment of Resonances to Seven-Carbon and Higher Order Sequences. The assigned resonances of 5carbon sequences were used as a foundation to assign 7-carbon and higher order sequences. Thus, the identification of 2D NMR correlations between pairs of resonances whose 5-carbon sequences are known is the basis for the higher level sequence assignments. Because every CF3 group has a CF group attached to it, the assignments of CF3 resonances are obtained from the assignments of their attached CF fluorine resonances. Weaker resonances whose higher order sequences cannot be assigned from correlations in the 2D NMR spectra are assigned by extrapolation of the resonance assignments for PVDF homopolymer and VDF-based copolymers, whose assignments can be found in earlier papers.22−27 For brevity, not all the 2D NMR correlations that are used to provide the resonance assignments in Table 1 will be discussed. A few of them, however, will be discussed to show how they are used; the NMR spectral information provided in this paper and in the Supporting Information contain data that can be used to assign many of the resonances not discussed. From the 5-carbon sequence assignments discussed above, 19 F resonances D022, A222, F222, and I222 have been assigned to 10222, 22222, 02222, and 02222 5-carbon sequences, respectively. From the gDQCOSY spectrum in Figure S11b, 19 F resonance F222 attributed to the 02222 5-carbon sequence shows correlations to 19F resonances A222 and I222 attributed to the 22222 and the 02222 5-carbon sequences, respectively (3). Resonance D022 (attributed to the 10222 5-carbon sequence) is correlated with resonances A222 and F222 in the gDQCOSY spectrum (Figure 4b). Resonance D022 also shows correlation to resonance E210 in the selective COSY spectrum given in Figure S10b. The 7-carbon and higher order sequences for the resonances involved in 3 are summarized in Table 1. Information from the gHSQC spectra can be used to confirm the different COSY correlations in 3. Table 1 summarizes the backbone 19F resonance assignments of poly(VDF-ter-HFP-terTFE) terpolymer on the basis of 2D NMR correlations and also from the extrapolation of known assignments from PVDF homopolymer and other VDF-based copolymers. The table also gives the literature assignments of resonances from PVDF

Figure 8. Comparison of 222 3-carbon sequence regions from the 470 MHz 19F{1H} 1D NMR spectra of (a) poly(VDF-co-TFE) copolymer (52:48 mol %) and (b) poly(VDF-ter-HFP-ter-TFE) terpolymer (52.8:23.0:24.2 mol %). Peaks in rectangular boxes are as result of HFP in the terpolymer; hence, they are not present in the copolymer. The peak(s) in the long dashed rectangular box consist of two overlapping resonances: one is present in the copolymer, and the other results from the presence of HFP in the terpolymer.

remaining three (3) 222-centered 5-carbon sequences, the 02221 sequence involves inversion of a monomer and its intensity is expected to be low. This sequence is not observed in our spectra. No 2D NMR correlations are seen in our spectra to assign the two remaining 222-centered 5-carbon sequences which involve HFP (12221 and 12222). For example, 4JFF COSY correlations between the central CF2 in a 222 3-carbon sequence and CF fluorine resonances are not observed in a selective COSY experiment between the CF2 resonances in the 222 region and the resonances in the CF region of the 19F spectrum. Although these sequences (12221 and 12222) do not involve inversion of monomers and therefore might be thought to be prevalent, the addition of TFE to HFP, and vice versa, is unfavorable for steric and electronic reasons, and hence resonances from structures having TFE and HFP adjacent to one another would have low occurrence. The resonances in the solid rectangular box in Figure 8b are unassigned; they are also not present in poly(VDF-co-TFE) copolymer. They can be from any of the three unassigned 222-centered 5-carbon sequences described above, although no 2D NMR correlations are observed to support this proposed assignment. The CF centered 3-carbon sequence (210) can occur in two CF centered 5-carbon sequences. Of the two possible 210centered 5-carbon sequences, only the 22102 sequence is identified from the correlations observed in the 2D NMR spectra. For instance, from Supporting Information Figure S10, the resonances from the 210 3-carbon sequence region show 4 JFF COSY correlations to CF2 resonances from the 020 3carbon sequence region (Figure S10a) and to the 022 3-carbon sequence region (Figure S10b), respectively. The 02102 5H

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Macromolecules Table 1. 19F Resonance Assignments of the Backbone Structures in Poly(VDF-ter-HFP-ter-TFE) Terpolymerb literature assignmentsa

assignment in this work CF3 fluorines peak label

δ19F vs CFCl3

A232 B232 C232 D232 E232 A230 B230 C230 D230

−69.78 −70.05 −70.38 −70.49 −70.59 −74.92 −75.05 −75.34 −75.44

poly(VDF-ter-HFP-ter-TFE)

PVDF

2221(3)220 0221(3)220 02021(3)202 02021(3)2022 22021(3)2022 20221(3)0202 0221(3)022X 20221(3)0221 20221(3)0222 assignment in this work

poly(VDF-co-HFP)

NA NA NA NA NA NA NA NA NA

poly(VDF-co-TFE)

NA NA 02021(3)202 021(3)2022 NA 0221(3)020 0221(3)0220 0221(3)0221 NA literature assignmentsa

NA NA NA NA NA NA NA NA NA

CF2 fluorines peak label

δ F vs CFCl3

poly(VDF-ter-HFP-ter-TFE)

PVDF

poly(VDF-co-HFP)

poly(VDF-co-TFE)

A020 B020 E020 L020 N020 O020 R020 S020 T020 U020 V020 W020 X020 Y020 Z020

−91.18 −91.47 −91.60 −91.77 −92.09 −92.26 −92.67 −92.83 −92.95 −93.14 −93.29 −93.70 −93.85 −95.06 −95.37

A1020 B1020 A021 B021 C021 A022 B022 C022 D022 E022 F022 G022 H022 J022 K022 L022 M022 O022 P022 Q022 R022 S022 T022 U022 V022 X022 Y022 B1022 A221

−95.96 −96.13 −101.40 −102.67 −103.25 −108.53 −109.44 −109.55 −109.97 −110.16 −110.31 −110.49 −110.49 −110.91 −111.01 −111.26 −111.26 −111.68 −111.98 −112.14 −112.25 −113.07 −113.29 −113.48 −113.52 −115.20 −115.29 −115.86 −117.67

0202022(0) 202020202 222020202 202020221 221020202 221020202 222020221 222020221 222020222 221020222 221020221 221020222 221020221 220020202 120020202 220020220 220020222 220020221 2102120 0212022 0212020 221022102 02220222022 221022202 221022222 202022102 21022202220 202220222 202220222 222022222 102220202 102022202 202022202 222220202 2002210 X21022021 X21022021 220022202 220220X 202022002 102022002 21022021X 21022021X 02022002X 220221022

0202022 202020202 NA NA NA NA NA NA NA NA NA NA NA 2002020 NA 2002022 NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA 202022002 NA NA NA 020220020 NA

0202022 202020202 NA 0202022(1) 2102020 NA NA NA NA NA 2102022(1) NA NA 2002020 NA 2002022 NA 2002022(1) 2102120 0212022 0212020 221022102 NA NA NA 202022102 NA NA NA NA NA NA NA NA 2002210 X21022021 X21022021 NA NA 202022002 102022002 21022021X 21022021X 02022002X NA

0202022 0202020 222020202 NA NA NA NA NA 2202022 NA NA NA NA 2002020 NA 2002022 2002022 NA NA NA NA NA NA NA NA NA NA 202220222 202220222 222022222 NA NA 202022202 222220202 NA NA NA 220022202 2002220 202022002 NA NA NA 2022002 NA

19

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Macromolecules Table 1. continued literature assignmentsa

assignment in this work CF2 fluorines peak label

δ19F vs CFCl3

B221 C221

−118.30 −118.38

A222 B222 C222 D222 E222 F222 G222 H222 I222 J222 K222 L222 M222 N222 O222

−120.69 −120.80 −121.01 −121.42 −122.30 −122.30 −122.57 −122.97 −123.24 −124.24 −124.63 −124.85 −125.25 −125.51 −125.83

poly(VDF-ter-HFP-ter-TFE)

PVDF

020221020 210221020 020221022 102222202 202222202 222222202 022222220 222222022 210222220 220222220 020222222 020222220 210222022 220222022 210222020 220222020 200222020 020222020 assignment in this work

poly(VDF-co-HFP)

NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA

poly(VDF-co-TFE)

020221020 210221020 020221022 NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA literature assignmentsa

NA NA NA NA 202222202 222222202 022222220 222222022 NA 220222220 020222222 020222220 NA 220222022 NA 220222020 200222020 020222020

CF fluorines peak label

δ19F vs CFCl3

poly(VDF-ter-HFP-ter-TFE)

PVDF

poly(VDF-co-HFP)

poly(VDF-co-TFE)

B212 C212 D212 K212 L212 A210 B210 C210 D210 E210 F210 G210 H210

−179.09 −179.17 −179.48 −181.29 −181.45 −183.93 −184.04 −184.13 −184.26 −184.38 −184.55 −184.87 −185.15

2221222(1) 2221222(2) 2221220 021202 021202 0221020 0221020 0221022 0221022 0221022 2221022 2021022 2021021

NA NA NA NA NA NA NA NA NA NA NA NA NA

NA NA NA 021202 021202 202210202 NA 0221022 202210221 NA NA 2021022 2021021

NA NA NA NA NA NA NA NA NA NA NA NA NA

Literature assignments. Spectra were also collected in acetone-d6. bAll spectra were collected in acetone-d6 at 30 °C. The chemical shift of the assignments from literature may differ slightly from that of the terpolymer. NA = not applicable.

a

expected to arise from 3JFF couplings. Resonance K022 is further correlated to 19F resonance S020 attributed to the 20202 5carbon sequence. Resonance C022 also shows a correlation to a CF 19F resonance with 22102 5-carbon sequence. Putting these pieces of information together produces the sequence given in 4.

homopolymer, poly(VDF-co-HFP) copolymer, and poly(VDFco-TFE) copolymer for comparison.

Fluorine resonance L222 (attributed to the 02220 5-carbon sequence) shows a correlation in the COSY spectrum (Figure 4b) to 19F resonances C022 (attributed to the 10222 5-carbon sequence) and K022 (attributed to the 20222 5-carbon sequence). From the 5-carbon sequence of resonance L222, the correlations between it and the other 19F resonances are

Fluorine resonance D222 (attributed to the 22222 5-carbon sequence) shows COSY correlations to 19F resonances H222, E222, and C222 in the gDQCOSY spectrum given in Figure J

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Macromolecules S11b. Because of the low intensities of these signals, 2-bond 19 F−13C HSQC correlations between the resonances of these fragments were not observed. The different COSY correlations were distinguished based on their intensities. The COSY correlations between resonances D222 and E222 and those between resonances D222 and H222 are assigned as 4JFF correlations since they are the strongest correlations among the above resonances. The other correlations as seen in 5 are assigned to those arising from 3JFF couplings.

Table 2. Polymer Compositions (mol %) Determined by Different Methods terpolymer A target compositiona FTIR NMR (lit. method)b NMR (this work)c

terpolymer B

VDF

HFP

TFE

VDF

HFP

TFE

67.7 66.4 70.0 66.8

21.9 20.9 21.9 22.4

10.4 12.7 8.1 10.8

53.8 53.7 50.2 52.8

21.7 22.4 23.3 23.0

24.5 23.9 26.5 24.2

a

Based on monomer feed. bBased on ref 34. cSee Supporting Information for NMR spectra used in calculating the compositions.

can sometimes lead to underestimation of the number of TFE units. The method of calculating the composition described herein is based on the use of an internal standard (in this case, 1,4-dichloro-2-(trifluoromethyl)benzene (DCTFMB)). This method also takes into account both the 1H and the 19F 1D NMR spectra (see eqs 1−3). The letters A−I correspond to the integral areas of the 21(3)2, 21(3)0, 020, 021, 022, 221, 222, 212, and 210 regions, respectively. The letter K is a proportionality constant relating the number of VDF, HFP, and TFE units with the integral areas.

The assignments of resonances from end groups and branching chains are not discussed in detail in this paper. Most of the resonances from end groups and branching structures in the terpolymer are very weak; all these resonances are also found in PVDF homopolymer and the copolymers poly(VDF-co-HFP) and poly(VDF-co-TFE) whose end group and chain branching resonance assignments are described in detail elsewhere.23,25,27,32,33 The Supporting Information shows some spectra comparing the resonances from the end groups in PVDF homopolymer, poly(VDF-co-HFP) copolymer, poly(VDF-co-TFE) copolymer, and poly(VDF-ter-HFP-ter-TFE) terpolymer. The assignments of the end group resonances in the terpolymer can be extrapolated from those of the homopolymer and the copolymers. At first, one might expect to see new end groups resulting from the presence of all three monomers in the terpolymer. However, if the chain end resonances are sensitive to the structures of the last two monomer units in the chain, the same pairwise combinations in the terpolymers are also present in the VDF homopolymer and its copolymers with HFP or TFE. Note that terminations involving VDF as the ultimate monomer in the chain seem to be more prevalent than those involving TFE or HFP. New structures are only predicted in the terpolymers if combinations of all three monomers are considered, which would require consideration of the antepenultimate monomer. This unit has very little influence on the chemical shift of the last atoms of the polymer chain and may be responsible for the broadness of some of the end group resonances in the spectra of the terpolymers compared to those of the homo- and copolymers. Composition of Polymer from NMR Measurements. The composition of each comonomer in the terpolymer was determined with both 19F and 1H 1D NMR spectra using an internal standard. The table below (Table 2) compares target compositions based on monomer feed, with those obtained from FTIR and NMR measurements. Isbester et al.34 calculated the comonomer composition of poly(VDF-ter-HFP-ter-TFE) terpolymer by NMR spectroscopy; their calculation was made on the assumption that the majority of TFE centered resonances will be found in the 221 and 222 3-carbon sequence regions of the 19F 1D NMR spectrum. While this is a reasonable approximation, since there can be considerable number of TFE centered resonances in the 022 3-carbon sequence region of the spectrum, their method

NHFP = K (A + B)/3

(1)

NVDF = K(proton integral)/2

(2)

NTFE = K (C + D + E + F + G − 2NHFP − 2NVDF)/4 (3)

The integrals of the CF3 group and three protons from the internal standard are set to the same number (1.00) in both 1H and 19F spectra. Spectra given in Figures S21−S24 were used to calculate the composition of the terpolymer in this work. The Supporting Information also gives details of the calculation using eqs 1−3. Figures S25 and S26 also show spectra with integral areas for calculating the comonomer compositions using the method described in ref 34. The relatively high HFP content is not surprising as other studies have reported high HFP content terpolymers.34,35 The percentage of HFP units involved in inverse enchainment can also be calculated from the resonance assignments of the signals at −102.67 and −103.25 ppm which are from inverse HFP monomer units. It should be noted that the signal at −103.25 ppm is composed of two overlapping signals; one is an HFP centered resonance, and the other is from VDF centered resonance. It is assumed in this work that the two overlapping signals are of similar intensity. Therefore, the mol % of HFP monomers units involved in inverse enchainment in the entire terpolymer can be calculated from integral ratio of HFP CF2 resonances involved in inverse addition to the total integral area of all CF2 resonances in the terpolymer as given in eq 4. The factor of 1/2 in the equation is explained above. The integral areas given in Figures S27 and S28 were used. From Table 3, the amount of HFP monomer units involved in inverse addition in the two terpolymers are almost equal, taking into consideration errors from the measurements; this is not surprising as the two terpolymers have similar amounts of HFP monomer units. In theory, there are several other monomer combinations that would also result in inversion of HFP units; examples include 021222, 021220, 221202, 221220, 201202, and 201222 combinations. However, some of these sequences are statistically improbable, and for those that are K

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Macromolecules

There are an insufficient number of resolved integrals to totally characterize the quantities of all the triads in the polymers. However, based on the relative integral areas of groups of resonances from the different monomers, it is possible to quantify some of the dominant monomer sequences in the polymer. The relative integral areas of the 222 centered 5-carbon sequences for terpolymer A show that only about 11.4% of the TFE units are involved in sequences with at least two consecutive TFE monomers (Figures S36). This 11.4% corresponds to the total integral areas of resonances A222−I222 (−120.5 to −123.5 ppm) which have either 22222 or 02222 5carbon sequence. The integral area of only the 22222 centered sequences (resonances A222−D222) show that only about 3.9% of the TFE monomers are in sequences with at least two consecutive TFE units, and the integral area of only the 02222 centered sequences (resonances E222−I222, from the ends of TFE runs) show that about 7.5% of the TFE monomers are in sequences with two consecutive TFE units. The percentage of sequences having three consecutive TFE units in terpolymer A will be less than 1%. The actual value is impossible to estimate as some of the sequences with three consecutive TFE units (resonances C222 and D222) are not resolved or are below the quantitation limit in the spectrum of terpolymer A. Of the other resonances with three consecutive TFE units, resonance E222 even if present in terpolymer A overlaps resonance F222. The only resonance with three consecutive TFE units that is measurable in terpolymer A is H222; its intensity is below the quantitation limit in the spectrum of terpolymer A. For terpolymer B, the integral areas (Figure S37) show that about 41% of the TFE units are involved in sequences that have at least two TFE monomers. There is an appreciable number of sequences in terpolymer B with three or more consecutive TFE units, however, because these resonances overlap with other resonances their estimate is not possible. In both terpolymers most of the TFE units are adjoining VDF units. There are also few TFE units adjoining HFP units. The high amount of TFE−TFE units in terpolymer B may be explained by the fact that HFP does not want to add to TFE, whereas VDF is more prone to add to HFP.36 From ref 36, the rate constant for HFP adding to TFE radical is extremely low, and the rate constant of VDF adding in a normal way to HFP radical is about 12 times faster than that of TFE adding to HFP radical. Thus, in terpolymer B, most of the VDF units will be tied up in HFP-VDF sequences, and also in VDF-VDF sequences. With so much of the VDF tied up in HFP-VDF and VDF-VDF diads, there is more opportunity to form long runs of TFE in the terpolymer. Integral areas of the CF3 fluorine resonances (Figure S32) from the HFP show that for terpolymer A about 92% of the HFP units are followed by VDF units that add to terminal HFP in a normal way, while for terpolymer B (Figure S33) about 91% of the HFP units are followed by VDF units that add to terminal HFP in normal way. The remaining HFP units are followed by either TFE units or reverse VDF units (Figures S32 and S33). No detectable signals were found for HFP-HFP sequences. This is far from that expected based on the relative amounts of the comonomers in the polymer. The integral areas of the CF2 fluorine resonances from VDF monomer (Figure S33) also show that in terpolymer A about 57% of the VDF units are in sequences comprising of three or more VDF monomers, while in terpolymer B only about 28% of the VDF units are in sequences with at least three VDF monomers.

Table 3. Calculated Compositions of the Two Poly(VDF-terHFP-ter-TFE) Terpolymers mol % monomer unit

terpolymer A

terpolymer B

normal VDF (V) reverse VDF (B) normal HFP (H) reverse HFP (R) TFE (T) total

64.9 1.9 20.7 1.7 10.8 100

52.2 0.6 21.2 1.8 24.2 100

statistically probable, their resonances are not observed in our NMR spectra. mol % of inverse HFP 1 A −102.67 + 2 (A −103.25) = 100% × integral of all CF2 resonances

(4)

The amount of VDF units involved in inverse enchainment were calculated from the 1H 1D NMR spectrum. Integral areas from spectra in Supporting Information Figures S29 and S30 were used to calculate the amount of VDF monomer units involved in inverse addition. The proton resonances from inverse VDF units (200) and those from normal VDF units 202 and 102 are distinguishable in the 1H NMR spectrum. Equation 5 was used to calculate the mol % of inverse VDF units in the terpolymers, where A200, A102, and A202 represent the integrated peak areas of the 200, 102, and 202 proton resonances, respectively. The factor of 1/2 in the equation results from the fact that in the 200 sequence the two 00 methylene groups are produced from each monomer inversion. The percentages of VDF units involved in inverse addition in the polymers based on the 1H{19F} 1D NMR spectra were calculated to be ca. 1.9 and 0.6 mol % for terpolymers A and B, respectively. There may be some error introduced from the fact that there could be VDF units involved in inverse addition in the 202 sequence. Thus, not all 202 sequences are from normal VDF addition. The 202 sequence could be from the monomer combinations 0202, 2022, and 2021, and the chemical shifts of the central CH2’s (indicated in bold) overlap. Although the 0202 combination would be predominant, the 2022 and 2021 would not be zero, especially in polymers with higher amounts of HFP and TFE monomer units. Again, 2022 and 2021 combinations do not necessarily indicate an inversion of VDF, as these are identical to 2202 and 1202, respectively, in NMR. Estimation of the amount of VDF units involved in inverse enchainment using the 19F NMR spectrum gave 1.8 mol % for terpolymer A. An attempt to use the 19F spectrum for terpolymer B to estimate the number of VDF inversions in the polymer did not give reasonable results as the resonances arising from inverse VDF units in terpolymer B are too weak to obtain accurate integration. From the assigned resonances in Table 1, resonances T022, X022, Y022, and B1022 are due to sequences with inverted VDF units. The sum of the integral areas of these CF2 fluorines divided by the total integral area of all the CF2 fluorine resonances gives the amount of VDF units involved in inverse enchainment in the polymer. mol % inverse VDF = 100% ×

A 202

A 200 /2 + A 200 + A102

(5) L

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CONCLUSIONS

Several multidimensional NMR methods have been used as complementary techniques to provide resonance assignments of the backbone structures in poly(VDF-ter-HFP-ter-TFE) terpolymer to a level which has not been previously reported. Compared to the few NMR studies of this terpolymer found in the literature, this work provides a comprehensive assignment of almost all the detectable resonances from the 19F spectra of the terpolymer. This work also provides a tabulated comparison of the assignment of microstructures from PVDF homopolymer, poly(VDF-co-HFP) copolymer, poly(VDF-co-TFE) copolymer, and poly(VDF-ter-HFP-ter-TFE) terpolymer. The improved NMR hardware and software and the ability to simultaneously decouple two nuclei and detect a third nucleus provided enhanced resolution of the NMR spectra (both 1D and 2D) of poly(VDF-ter-HFP-ter-TFE) terpolymer. Weak resonances which would ordinarily be lost due to poorer resolution in fully coupled spectra are identified and assigned. The resonance assignments are greatly aided by the extrapolation of known resonance assignments of PVDF homopolymer, poly(VDF-co-HFP) copolymer, and poly(VDF-co-TFE) copolymer. The NMR methods used for studying this terpolymer can be applied to a broad range of polymers, including hydrocarbon polymers, fluoropolymers, and other polymers containing three NMR active nuclei. The significant advancement in the development of NMR hardware and software have made it possible to study polymers which hitherto were very difficult to characterize. The information obtained from polymer characterization provides insight into the microstructures in polymers which ultimately help to probe structure−property correlations, reveal polymerization mechanism, and also help in the development of polymers with improved performance. Signals from a few of the very weak peaks in the 19F 1D NMR spectra of poly(VDF-ter-TFE-ter-HFP) terpolymer and poly(VDF-co-HFP) copolymer samples did not produce detectable correlations in the 19F{13C} HSQC in this work or in the 19F DOSY in prior work.27 This may be in part due to the lower sensitivity of the 2D NMR experiments and partly due to loss of signal intensity from the presence of large homonuclear couplings between fluorines. The latter situation can potentially be a large problem when there are many large couplings between CF2 groups which produce wide multiplets or when large geminal couplings produce AM or AX splitting patterns with 2JFF = 200−300 Hz. Detection and assignment of these weak signals await the developments of improved versions of HSQC and DOSY pulse sequences for fluoropolymers, which are in progress.

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ACKNOWLEDGMENTS

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REFERENCES

We acknowledge the support of The Ohio Board of Regents and The National Science Foundation (CHE-0341701 and DMR-0414599) for funds used to purchase the NMR instrument used for this work. We thank the NSF (DMR0905120) and E. I. du Pont de Nemours and Co. for their support for this work. We also thank the staff of the Magnetic Resonance Center at the University of Akron for their help in maintaining the instruments used for this work. The authors are also grateful to Chun Gao at the University of Akron for running some spectra for us.

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S Supporting Information *

Figures S1−S37. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.macromol.5b00200.

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*E-mail [email protected] (E.B.T.). *E-mail [email protected] (P.L.R.). Notes

The authors declare no competing financial interest. M

DOI: 10.1021/acs.macromol.5b00200 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.5b00200 Macromolecules XXXX, XXX, XXX−XXX