Article pubs.acs.org/Macromolecules
Novel Method to Assess the Molecular Weights of Fluoropolymers by Radical Copolymerization of Vinylidene Fluoride with Various Fluorinated Comonomers Initiated by a Persistent Radical Yogesh Patil,† Ali Alaaeddine,† Taizo Ono,‡ and Bruno Ameduri†,* †
Institut Charles Gerhardt, Ingénierie et Architectures Macromoléculaires, UMR CNRS 5253, Ecole Nationale Supérieure de Chimie de Montpellier, 8 Rue de l’Ecole Normale, 34296 Montpellier, France ‡ National Institute of Advanced Industrial Science and Technology, Research Institute of Instrumentation Frontier, 2266-98, Anagahora, Shimoshidami, Moriyama, Nagoya, Aichi, 463-8560, Japan S Supporting Information *
ABSTRACT: Batch radical homopolymerization of vinylidene fluoride (VDF) and its copolymerizations with seven fluorinated comonomers such as H2CCRACF3 (where RA stands for H, F, CO2R (R = H or tBu) or F2CCFRB (RB: H, CF3, OCF3) initiated by •CF3 radical generated from a perfluoro-3-ethyl-2,4-dimethyl-3-pentyl persistent radical (PPFR) are presented. That highly perfluorinated branched radical itself was unable to directly initiate the copolymerization in contrast to the generated trifluoromethyl radical that was successful. The microstructures of the resulting PVDFs, poly(VDF-co-H2CCRACF3) or poly(VDF-co-F2CCFRB) (co)polymers were assessed by 19F and 1H NMR spectroscopy. 19F NMR spectra displayed a quintet centered at −61 ppm assigned to a CF3 end-group which enabled one to determine both the molecular weights of the resulting (co)polymers and the amounts (numbers and molar percentages) of these above comonomers. In all cases, the regioselective radical addition of •CF3 onto the methylene site of VDF was noted as well as for 3,3,3trifluoropropene. In addition, the 19F NMR spectra showed the absence of signals attributed to PPFR or its derivatives. According to [PPFR]o/([VDF]o + [comonomers]o) initial molar ratios, this novel initiating system could produce high molecular weight-fluoropolymers (up to 200 000 g·mol−1). The lower that ratio, the higher their molecular weights as well as the higher their thermal stability (a 10% weight loss under air was noted at 425 °C). Although the kinetics of radical copolymerization were not studied, the presence of branched persistent radical did not disturb the reactivities of the comonomers and it was not involved in the primary recombination of radicals in the termination step of the polymerization while the macromolecular recombination was favored. For an initial PPFR concentration lower than 1 mol %, a feed ratio of ca. 20 mol % of 2-trifluoromethacrylic acid, MAF (or tert-butyl 2-trifluoromethacrylate, MAF−TBE) led to alternated poly(VDF-alt-MAF or MAF−TBE) copolymers. High yields (90%) of copolymers based on VDF and 2,3,3,3-tetrafluoroprop-1-ene, hexafluoropropylene (HFP), or perfluoromethyl vinyl ether (PMVE) comonomers were achieved while those from MAF, MAF−TBE, trifluoroethylene, and 3,3,3-trifluoropropene were fair to satisfactory. As expected, for a reasonable amount of HFP and PMVE inserted in the copolymers (>18 mol %), elastomers endowed with low Tg values (−35 and −42 °C, respectively) were produced. In addition, the higher the content of VDF, the lower the Tg, though the presence of tBu from MAF−TBE slightly increased the Tg values. Copolymers containing MAF or MAF−TBE exhibited poor thermostability (that arises from the decarboxylation and isobutylene elimination from MAF or MAF−TBE, respectively) in contrast to the other VDF-containing copolymers that showed decomposition from 250 °C under air.
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fluoropolymers is not easy and such characteristics, usually determined by size exclusion chromatography, SEC are not reliable. Thus, it was worth finding a model of the synthesis of fluoropolymers to predict their degree of polymerization, true molecular weights, and to provide new standards for SEC. Recently, for this topic we successfully used a perfluoro-3-ethyl2,4-dimethyl-3-pentyl persistent radical (PPFR) that, when
INTRODUCTION
Fluorinated polymers are attractive niche polymers with remarkable properties and can find High Tech applications in various areas: aerospace, electronics, medicine, nuclear, building and automotive industries. They are usually synthesized by radical (co)polymerization.1−3 However, fluorinated homopolymers are dealing with some drawbacks: they are highly crystalline, poorly soluble in common organic solvents (hence preventing from the complete characterization), and they are not easily cured or cross-linked.4−8 Because of solubility and standards issues, the assessment of molecular weights of © XXXX American Chemical Society
Received: February 11, 2013 Revised: March 14, 2013
A
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heated above 80 °C, straightforwardly generated •CF3 radicals which was able to initiate the radical homopolymerization of VDF9 and its copolymerization with tert-butyl 2-trifluoromethacrylate (MAF−TBE).10 These preliminary results led to CF3− poly(VDF)−CF3 homopolymers9 and CF3−poly(VDF-coMAF−TBE)−CF3 copolymers10 that bear CF3 end groups (this was evidenced by a characteristic quintet centered at −61.5 ppm noted in 19F NMR spectroscopy) which act as labels to assess the molecular weights of these (co)polymers. Following that relevant feature on VDF homopolymerization9 and its copolymerization with MAF−TBE,10 it was worth extending such a strategy toward other fluorinated comonomers. Actually, • CF3 is an interesting radical that can be generated from various precursors: CF3X (X = I,11−14 Br15 and SO2Y16 where Y = Na16a or Cl16b), bis(trifluoroacetyl) peroxide (CF3CO2)2,17 CF3− paracyclophane,18 alkyl (or aryl) trifluoromethanethiosulfonates (CF3SO2SR19). On the other hand, persistent radicals have been involved in controlled radical polymerization that is an attractive research area. Indeed, Fischer20 reported comprehensive studies on such radicals, their selective radical reactions, and their role in controlled radical polymerizations. However, the literature reveals that, up to now, persistent radicals have scarcely been involved in polymerization of fluoromonomers. Actually, Scherer and Ono et al.21 synthesized an original highly branched perfluoroalkyl persistent radical (PPFR) from hexafluoropropylene (HFP) trimer and also reported that its thermal degradation occurred via a β-scission to yield a trifluoromethyl radical and perfluoro-4-methyl-3-ethyl-2-pentene (E/Z forms in 8:3 ratio, Scheme 1). The use of PPFR in polymerization of fluorinated
are efficient partners of VDF in radical copolymerization. The molecular weights (Mns) of PVDF polymers can be assessed from the Mark−Houwink coefficients53−55 but those of VDFbased copolymers have not been reported yet as well as any suitable method and standards to assess the absolute Mn values (since size exclusion chromatography usually involves polystyrene and poly(methyl methacrylate) as the standards). Hence, the objectives of this present article are as follows: (i) to evidence the presence of the •CF3 radical, generated from a perfluorinated hyperbranched persistent radical, capable of initiating the radical copolymerization of VDF with seven fluorinated monomers (M) above, such as H2CCRACF3 where RA stands for H (TFP), F (1234yf), or CO2R (where R = H for MAF or tBu for MAF−TBE) and F2CCFRB where RB represents H (TrFE), CF3 (HFP), or OCF3 (PMVE), (ii) to use CF3 end-groups as labels to determine the molecular weights (Mns) of the resulting poly(VDF-co-M) copolymers by spectroscopic data (that also made it possible to assess the contents of comonomers in the copolymers and the defects of VDF−VDF dyads in the copolymeric chains), and (iii) to study the surface properties and thermal stabilities of these original VDF-containing (co)polymers.
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EXPERIMENTAL SECTION
Materials. All reagents were used as received unless it is stated. 2-Trifluoromethacrylic acid (MAF) and tert-butyl 2-trifluoromethacrylate (MAF−TBE) were kindly offered by Tosoh F-Tech Company (Shunan, Japan). 1,1-Difluoroethylene (vinylidene fluoride, VDF) and 1,1,1,3,3-pentafluorobutane (HFC-245 fa, Solkane 365mfc) were kindly supplied by Solvay S.A. (Tavaux, France and Brussels, Belgium), while perfluoromethyl vinyl ether (PMVE) was a gift from Dupont Performance Elastomers (Wilmington, DE). 3,3,3-Trifluoropropene (TFP) and hexafluoropropylene (HFP) were purchased from Fluorochem. Trifluoroethylene (TrFE) and 2,3,3,3-tetrafluoroprop-1ene (1234yf) were kindly supplied by Arkema (Colombes and PierreBernite, France). Acetonitrile was purchased from Fisher Scientific and distilled over calcium hydride. Deuterated acetone used for NMR spectroscopy was supplied by Euroiso-top (Grenoble, France) (purity >99.8%). Characterization. Nuclear Magnetic Resonance (NMR). The NMR spectra were recorded on Bruker AC 400 instruments, using deuterated chloroform or acetone as the solvent and tetramethylsilane (TMS) (or CFCl3) as the references for 1H (or 19F) nuclei. Coupling constants and chemical shifts are given in hertz (Hz) and parts per million (ppm), respectively. The experimental conditions for recording 1 H [or 19F] NMR spectra were as follows: flip angle 90° [or 30°], acquisition time 4.5 s [or 0.7 s], pulse delay 2 s [or 5 s], number of scans 36 [or 64], and a pulse width of 5 μs for 19F NMR. In the details of NMR characterization, s, d, t, q, and m stand for singlet, doublet, triplet, quintet, and multiplet, respectively. Size Exclusion Chromatography (SEC or Gel Permeation Chromatography, GPC). SEC was carried out in tetrahydrofuran at 30 °C, at a flow rate of 0.8 mL.min−1, by means of a Spectra Physics Winner Station, a Waters Associate R 401 differential refractometer, and a set of four columns connected in series: Styragel (Waters) HR4 5m, HR3 analyses 5m, PL Gel (Polymer Laboratories) 5m 100 Å. Monodispersed polystyrene standards were used for calibration. Aliquots were sampled from the reaction medium, diluted with tetrahydrofuran up to a known concentration (Cp,t) ca. 4 wt %, filtered through a 20 mm PTFE Chromafil membrane, and finally analyzed by SEC under the conditions described above. Thermogravimetric Analyses (TGA). TGA analyses were performed with a TGA 51 apparatus from TA Instruments, under air, at the heating rate of 10 °C·min−1 from room temperature up to a maximum of 600 °C. The sample size varied between 10 and 15 mg. Differential Scanning Calorimetry (DSC). DSC measurements were conducted using a Perkin-Elmer Pyris 1 apparatus. Scans were recorded
Scheme 1. Generating a Trifluoromethyl Radical by Heating Perfluoro-3-ethyl-2,4-dimethyl-3-pentyl (PPFR) Persistent Radical
alkenes has been preliminarily attempted by our group for the homopolymerization of VDF9 and its copolymerization with MAF−TBE.10 Notwithstanding, VDF is known to copolymerize with a series of different fluorinated comonomers despite the nonhomopolymerizability of some of them (such as MAF, MAF−TBE, HFP or perfluoroalkyl vinyl ethers).5,6 In fact, VDF copolymers are involved in valuable materials used in relevant applications including membranes, pyro-, ferroand piezo-electical devices, coatings, high performance elastomers, gaskets, O-rings.6 However, PVDF does not bear any functions and any strategy to insert a functional group onto PVDF is attractive for various interests: as precursor of PVDF-gpoly(M) graft copolymers (where M stands for a monomer; for example poly(M) such as poly(ethylene oxide) may induce some hydrophilicity in PVDF)6 or cross-linking (via cure site comonomers that contain epoxide, azido, cyanato or isocyanato groups).8 To our knowledge, one of the most suitable ways to insert any function onto PVDF is the radical copolymerization of VDF with a reactive functional comonomer. MAF,22,23 MAF− TBE,10 perfluoromethyl vinyl ether (PMVE),24−26 HFP,27−42 3,3,3-trifluoropropene (TFP), 4 3 , 4 4 trifluoroethylene (TrFE),45−50 and 2,3,3,3-tetrafluoroprop-1-ene (1234yf)51,52 B
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Table 1. Experimental Conditions and Results of the Radical Homopolymerization of VDF and Its Copolymerization with Various Fluorinated Monomers (M) Initiated by Perfluoro-3-ethyl-2,4-dimethyl-3-pentyl Persistent Radicala in feed (mol %)
in copolym (mol %)b
run
monomers M
VDF
M
VDF
M
[initiator]o/ [monomers]o (%)
Pmax (bar)
ΔP (bar)
yield (%)
1 2
− −
100 100
0 0
100 100
0 0
0 (TBPPi) 1.0
− 34
− 06
17 38
3 4 5 6
− − − MAF
100 100 100 0
0 0 0 100
100 100 100 0
0 0 0 0
5.0 10.0 20.0 2.0
− − − −
− − − −
7
MAF−TBE
0
100
0
0
2.0
−
8
MAF−TBE
80
20
74
26
10.0
9 10 11 12
MAF−TBE MAF−TBE MAF−TBE MAF
80 80 80 80
20 20 20 20
66 49 48 65
34 51 52 35
13
MAF
80
20
54
14
MAF
95
05
15
MAF
97
16
TFP
17
Mnd
PDI
Td,10%e (°C)
Tgf (°C)
− 9800
18 400 8200
1.4 1.8
454 439
−30 −19
72 58 41 0
7400 5300 4200 −
6100 3700 2800 −
2.0 2.3 1.8 −
422 402 381 −
−14 −15 −12 −
−
0
−
−
−
−
55
13
79
5500
3100
1.4
156
04
2.0 1.0 0.5 2.0
49 48 47 51
8 7 6 7
63 48 42 68
21 300 49 900 104 400 10 200
9800 11 200 16 800 −
1.6 1.6 1.7 −
169 177 187 235
05 07 10 19
46
1.0
46
7
54
23 100
−
−
263
22
82
18
1.0
49
11
56
40 100
−
−
304
16
03
93
07
1.0
56
18
57
62 000
−
−
339
10
70
30
31
69
1.0
33
11
67
4200
2200
1.6
270
−33
1234yf
90
10
83
17
1.0
35
20
94
62 000
49 200
1.3
402
−33
18
PMVE
62
38
82
18
1.0
29
10
75
11 200
4900
1.8
425
−42
19
HFP
71
29
86
14
1.0
28
12
80
16 400
4700
2.0
418
−35
20
TrFE
60
40
62
38
1.0
20
8
72
200 000
32 500
2.2
412
−
Mnc
−
ref 9 this work 9 9 9 this work this work this work 10 10 10 this work this work this work this work this work this work this work this work this work
Reaction conditions: solvent used, C4F5H5 + CH3CN = 60 mL; reaction temperature, 90 °C; reaction time, 4−14 h; ΔP, pressure drop observed in the autoclave during the reaction. Acronyms of comonomers are defined below. bCopolymer compositions were assessed by 19F NMR spectroscopy (the assignments of chemical shifts are listed in Table S1, Supporting Information) using the formula given in Appendix 1. cDetermined from 19F NMR spectroscopy using previous formula (see Appendix 2).31. dDetermined by SEC calibrated with PS standards. eDetermined by thermogravimetric analysis (TGA), under air with a temperature ramp of 10 °C/min. fDetermined by differential scanning calorimetry (DSC), from −60 to 150 °C with a temperature ramp of 5 °C/min. a
at a heating rate of 20 °C·min−1 from −80 to +150 °C, and the cooling rate was 20 °C·min−1. A second scan was required for the assessment of the Tg, defined as the inflection point in the heat capacity jump. The sample size was about 10−15 mg. Water Contact Angle (WCA). WCA measurements were carried out on a Contact Angle System from OCA-Data Physics. The water sessile drop method was used for the static contact angle (CA) measurements at ambient temperature. The probe liquid was water (θH2O) and the average CA value was determined on five different drops of 1.0 μL deposited on the same sample. Homo- and (Co)polymerizations Carried Out in Autoclave. The radical homopolymerization of VDF and its copolymerizations with various fluorinated monomers were performed in a 100 mL Hastelloy autoclave Parr system (HC 276) equipped with a manometer, a mechanical Hastelloy anchor, a rupture disk (3000 PSI), and inlet and outlet valves. An electronic device regulated and controlled both stirring and heating of the autoclave. Prior to reaction, the autoclave was pressurized with 30 bar of nitrogen for 1 h to check for leaks. The autoclave was then conditioned for the reaction with several nitrogen/ vacuum cycles (10−2 mbar) to remove any trace of oxygen. The liquid and dissolved solid phases were introduced via a funnel tightly connected to the autoclave, and then, the gases were transferred by
double weighing (i.e., the difference of weight before and after filling the autoclave with the gases). Because VDF has the lowest boiling point (ca. −82 °C/atmospheric pressure), this gas was inserted just after the other gaseous comonomers. Synthesis. 1. Persistent Perfluoroalkyl Radical. Perfluoro-3-ethyl2,4-dimethyl-3-pentyl persistent radical (PPFR) was synthesized by fluorination of hexafluoropropylene (HFP) trimer according to a known procedure,21 using undiluted fluorine gas until all the starting material was consumed. The obtained PPFR solution was analyzed by gas chromatography (GC) (with a capillary column: 1.5 μm thick NB-1, and 0.25 mm diameter x 60 m) to be comprised of PPFR and perfluoro-3ethyl-2,4-dimethylpentane in a ca. 3:2 ratio. The PPFR solution was washed with 1 N aqueous Na2CO3 and distilled water and then distilled under reduced pressure. The fraction collected at 31−33 °C/25 mmHg (PPFR) was used for this investigation. 19F NMR data on the products (E- and Z- forms of perfluoro-3-ethyl-4-methyl-2-pentene) obtained by thermal decomposition of persistent perfluoroalkyl radical (PPFR-1; perfluoro-3-ethyl-2,4-dimethyl-3-pentyl) were as follows: E-form: −62.9 (m, 3F), −79.4 (d, 3JFF = 29.3 Hz, 6F), −80.5 (d, 3JFF = 20.3 Hz, 3F), −81.3 (overlapping, 1F), −103.3 (m, 2F), and −175.9 (m, 1F). Z-form: −62.7 (d, 3JFF = 48.5 Hz, 3F), −72.1 (s, 6F), −79.1 (overlapping, 4F), −103.5 (m, 2F), and −170.1 (q, 3JFF = 48.5 Hz, 1F). C
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The thermal decomposition rate of PPFR was examined at 90 °C by monitoring the amount of PPFR in the reaction mixture by GC. The peak area ratio of PPFR to the internal standard, perfluorotri-npropylamine (PTFA), was plotted versus the reaction time (h). The linear relationship showed that the thermal decomposition of PPFR followed a first order reaction (ln([PPFR]/[PTFA]) = −0.718t + 1.479, for which the correlation coefficient value of 0.999). The half-life of PPFR at 90 °C was assessed as 3.18 h. 2. Radical Homopolymerization of Vinylidene Fluoride (VDF) (Runs 1, 3−5 in Table 1). The radical polymerization of vinylidene fluoride (VDF) was performed in thick borosilicate Carrius tubes (length 130 mm, internal diameter 10 mm, thickness 2.5 mm, total volume 8 mL). In a typical polymerization, the different reactants including the initiator and solvent were added in the tube. The tubes were then degassed by at least 4 thaw-freeze cycles, and the required amount of VDF was introduced via a special manifold (picture in Figure S1 of the Supporting Information) from an intermediate cylinder from which the drop of pressure was beforehand calibrated with the amount of VDF. Then, the tubes were sealed under dynamic vacuum at the temperature of liquid nitrogen and inserted in a custom designed heated and shaken apparatus regulated at the desired temperature. The reaction was carried out for 12 h at constant temperature (90 °C). After cooling, the tubes were frozen and opened, and the polymers were isolated by evaporation of the solvent. The polymers were further dried under high vacuum at 50 °C until constant weight. Then, the obtained white powders were weighed to assess the yield. For characterizations, the polymers were dissolved in dimethyl sulfoxide and precipitated from a large excess of methanol. The polymers were filtrated off and then dried as above in a vacuum oven at 50 °C until constant weight. However, run 2 in Table 1 was carried out in the 100 mL Hastelloy autoclave which was filled under vacuum with PPFR persistent radical (4.020 g, 8.93 mmol) and a solvent mixture composed of 1,1,1,3,3pentafluorobutane (30 mL) and acetonitrile (30 mL). The vessel was cooled in an acetone/liquid nitrogen bath and 3 thaw freeze pump cycles were applied before the fluorinated VDF gas (27 g, 0.422 mol) was condensed into the autoclave under weight control. Then, the reactor was stirred and gradually heated to 90 °C and the evolutions of pressure and temperature were recorded. An initial increase of the pressure to 34 bar, and a decrease to 28 bar in 4 h were observed. The reaction was stopped after 6 h and the autoclave was cooled to room temperature and then placed in an ice bath. After purging the nonreacted VDF, the conversion of VDF was determined by double weighing (79%, i.e., the autoclave was weighed immediately after reaction and placed in ice for 30 min, and then the unreacted VDF was carefully released and the autoclave was weighed again). A white powder and trace of a colorless liquid were obtained after opening the autoclave. The total product mixture was precipitated from chilled pentane, filtered and then dried under vacuum (10−2 bar) at 50 °C for 12 h. The mass yield (i.e., the yield in weight percent) was 38 wt % (run 2, Table 1). CF3−PVDF−CF3 homopolymer, as a colorless fine powder, was characterized by 1H and 19 F NMR spectroscopy. 1 H NMR (acetone-d6) δ (ppm) (Figure S2 in the Supporting Information): 3.3 (CF3−CH2−CF2, regioselective addition of ·CF3 radical onto CH2 of VDF); 2.9 (CH2−CF2 of VDF, normal VDF− VDF dyad addition); 2.4 (CF2−CH2−CH2−CF2 reverse VDF−VDF dyad addition). 19 F NMR (acetone-d6) δ (ppm) (Figure S3 in the Supporting Information): −61 (CF3 chain end in CF3CH2CF2−); −91.5 (CF2 of VDF, normal addition or head-to-tail VDF−VDF dyad); −113 and −116 (CH2−CF2−CF2−CH2 reverse addition of VDF or head to head VDF−VDF dyad). 3. Radical Homopolymerizations of 2-Trifluoromethacrylic Acid (MAF) and tert-Butyl 2-Trifluoromethacrylate (MAF−TBE) (Runs 6 and 7, Table 1). In a general procedure, MAF and MAF−TBE polymerizations were carried out in a round-bottom flask (100 mL) equipped with a magnetic stirrer. The flask was filled with acetonitrile and 1,1,1,3,3-pentafluorobutane solvent mixture and further purged with nitrogen for 20 min. The monomers (MAF, 10.06 g or MAF−TBE, 10.09 g) were dissolved in the above-mentioned solvent mixture and finally the initiator (PPFR, 2.012 g) was added into the flask to start the
polymerization. The reaction medium was maintained at 90 °C by using a thermostated oil bath. The polymerization was stopped after 16 h by cooling the round-bottom flask to room temperature. A colorless liquid was obtained. After the solvent was completely removed by distillation, we attempted to precipitate the total product mixture in chilled pentane, diethyl ether, methanol, or water, but that did not lead to any polymer (runs 6 and 7, Table 1). Both 1H and 19F NMR spectra displayed the presences of signals assigned to unreacted MAF or MAF−TBE monomers. 4. Radical Copolymerizations of Vinylidene Fluoride (VDF) with CH2CR−CF3 (R = tert-Butyl Ester, COOH, H, and F) (Runs 8−17 in Table 1). 4.1. Radical Copolymerization of Vinylidene Fluoride (VDF) and tert-Butyl 2-Trifluoromethylacrylate (MAF−TBE) (Runs 8−11 in Table 1). After being pressurized by 30 bar nitrogen for 1 h and put under vacuum, the 100 mL Hastelloy autoclave was filled under vacuum with a mixture composed of the persistent radical (8.021 g, 17.9 mmol) and MAF−TBE (20.103 g, 0.10 mol) both dissolved in 1,1,1,3,3pentafluorobutane (30 mL) and acetonitrile (30 mL). The vessel was cooled in an acetone/liquid nitrogen bath and 3 thaw freeze pumpcycles were applied before the fluorinated VDF gas (27 g, 0.422 mol) was condensed into the autoclave under weight control. Then, the reactor was stirred and gradually heated up to 90 °C and the evolutions of pressure and temperature were recorded. An increase of the pressure to 62 bar followed by a decrease to 48 bar in 4 h were observed. The reaction was stopped after 6 h and the autoclave was cooled to room temperature and then placed in an ice bath. After purging the nonreacted monomer, the conversion of gaseous monomer was assesed as 84% by double weighing. A light yellow liquid was obtained after opening the autoclave. After the solvent was removed by distillation, the total product mixture was precipitated from chilled pentane, filtered off and then dried under vacuum (10−2 bar) at 50 °C for 12 h. The yield in weight percentage (or massic yield) was 79 wt % (run 8, Table 1). Poly(VDF-co-MAF−TBE) copolymer, as a colorless powder, was characterized by 1H and 19F NMR spectroscopy. 1 H NMR (acetone-d6) δ (ppm) (Figure S4 in Supporting Information): 1.5 (s, tert-butyl −C(CH3)3 of MAF−TBE in the copolymer); 2.4 (m, CF2−CH2−CH2−CF2 reverse tail-to-tail addition of VDF only in 10 and 2% of PPFR); 2.9 (m, CH2 of VDF and −CH2C(CF3)COOtBu of MAF−TBE overlap on each other). 19 F NMR (acetone-d6) δ (ppm) (Figure S5 in Supporting Information): −61 (q, 3JFH = 4JFF = 10 Hz CF3 chain end); −68.7 (m, CF3 of MAF−TBE); −91.5 (m, CF2 of VDF, normal head-to-tail addition); −95 (m, alternating VDF−MAF−TBE dyad −CH2CF2CH2C(CF3CO2tBu)−); −113 and −116 (CH2−CF2− CF2−CH2 reverse head-to-head addition of VDF). 4.2. Radical Copolymerization of Vinylidene Fluoride (VDF) and 2Trifluoromethacrylic Acid (MAF) (Runs 12−15 in Table 1). Under vacuum, the 100 mL Hastelloy autoclave was filled with PPFR persistent radical (2.01 g, 4.30 mmol), MAF (13.71 g, 0.112 mol), 1,1,1,3,3pentafluorobutane (30 mL), and acetonitrile (30 mL). A similar procedure as that mentioned above enabled to transfer 27 g (0.422 mol) of VDF. In the course of the radical copolymerization, an increase of the pressure to 47 bar followed by a decrease to 35 bar in 4 h were observed. After the nonreacted VDF was purged, its conversion was 70%. After precipitation, filtration and drying, the massic yield was 68 wt %. Poly(VDF-co-MAF) copolymer, as a colorless powder, was characterized by 1H and 19F NMR spectroscopy. 1 H NMR (acetone-d6) δ (ppm) (Figure S6, Supporting Information): 2.4 (m, CF2−CH2−CH2−CF2 reverse VDF−VDF dyad, only from 2% of PPFR, otherwise absence of such a signal); 2.9 (m, CH2 of normal VDF−VDF dyad and −CH2C(CF3)COOH of MAF that overlapped on each other). 19 F NMR (acetone-d6) δ (ppm) (Figure S7, Supporting Information): −61 (q, 3JFH = 4JFF = 10 Hz, CF3−CH2CF2- chain end); −68.5 (m, CF3 of MAF); −91.5 (m, CF2 of VDF, normal head-to-tail addition of VDF−VDF dyad only from 2 mol % of PPFR otherwise absence of such a signal); −95.0 (alternating VDF−MAF dyad −CH2CF2CH2C(CF3CO2H)−); −113 and −116 (CH2−CF2−CF2−CH2 reverse VDF−VDF dyad addition, only when 2 mol % of persistent radical initiator was used). D
dx.doi.org/10.1021/ma400304u | Macromolecules XXXX, XXX, XXX−XXX
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Figure 1. 19F NMR spectrum of CF3−poly(VDF-co-TFP)−CF3 copolymer (run 16 in Table 1) recorded in acetone-d6 (VDF and TFP stand for vinylidene fluoride and 3,3,3-trifluoropropene, respectively). VDF:TFP feed molar ratio of 70:30; molar composition of copolymer of 31:69. 4.3. Radical Copolymerization of Vinylidene Fluoride (VDF) and 3,3,3-Trifluoropropene (TFP) (Run 16 in Table 1). As in experiments above, the 100 mL Hastelloy autoclave was filled, under vacuum, with persistent radical (2.04 g, 4.30 mmol), 1,1,1,3,3-pentafluorobutane (30 mL), and acetonitrile (30 mL). The reactor was cooled in an acetone/ liquid nitrogen bath and 3 thaw freeze pump cycles were applied before the fluorinated gases: first TFP (12 g, 0.12 mol) and then VDF (15 g, 0.234 mol) were condensed into the autoclave under weight control. The remaining procedure was the same as that above. An increase of pressure to 33 bar followed by a decrease to 21 bar were observed. After purging nonreacted TFP and VDF, the conversion of gaseous monomers was 71%. The massic yield was 67 wt %. The poly(VDFco-TFP) copolymer, as a light yellow elastomeric product was characterized by 1H and 19F NMR spectroscopy. 1 H NMR (acetone-d6) δ (ppm) (Figure S8 in the Supporting Information): 2.3 (m, CH2 of TFP); 2.8 (m, CH2 of normal head-to-tail VDF−VDF dyad); 4.4 (m, CH of TFP). 19 F NMR (acetone-d6) δ (ppm) (Figure 1): −61 (q, 3JHH = 4JFF = 10 Hz, CF3−CH2CF2− chain end); −64 (m, CF3CH2CH(CF3) chain end); −66 to −68.5 (CF3 of TFP); −70.5 (overlapping between CF3 of TFP in oligo(TFP) and CF2 of VDF); −91 to −96 (m, CF2 of VDF, normal addition, head-to-tail VDF−VDF dyad); −98 (m, CF2 of VDF in alternated addition of VDF−TFP); −113 and −116 (traces m, CF2 of VDF in reversed head-to-head VDF−VDF dyad). 4.4. Radical Copolymerization of Vinylidene Fluoride (VDF) with 2,3,3,3-Tetrafluoroprop-1-ene (1234yf) (Run 17 in Table 1). As above, the 100 mL Hastelloy autoclave was filled under vacuum with perfluorinated persistent radical (1.21 g, 2.61 mmol) dissolved in 1,1,1,3,3-pentafluorobutane (60 mL). After cooling and three thaw− freeze pump cycles in the autoclave, 2,3,3,3-tetrafluoroprop-1-ene (3 g, 0.026 mol) and then VDF (15 g, 0.234 mol) gases were condensed under weight control. Following the same procedure as above, an
increase of the pressure up to 35 bar, followed by a decrease to 15 bar in 2 h were observed. The reaction was stopped after 14 h and the autoclave was cooled to room temperature and then placed in an ice bath. After purging the nonreacted monomer, the conversion of gaseous monomer was determined by double weighing (96%). The solvent was completely removed by distillation and the product was precipitated from an excess of chilled pentane, filtered off and then dried under vacuum. The yield in weight percent of the obtained white powder was 94 wt %. The poly(VDF-co-2,3,3,3-tetrafluoropropene-1-ene) copolymer was characterized by 1H and 19F NMR spectroscopy. 1 H NMR (acetone-d6) δ (ppm) (Figure 2): 2.45 (CF2−CH2−CH2− CF2 reverse addition of VDF); 2.7 to 3.1 (CH2 of VDF and of 2,3,3,3tetrafluoroprop-1-ene overlapping each other) 19 F NMR (acetone-d6) δ (ppm) (Figure 3): −61 (q, 3JHH = 4JFF = 10 Hz; CF3−CH2CF2 chain end); −79 (m, −CH2−CF(CF3)); −92 (m, −CH2CF2−CH2CF2− normal VDF−VDF dyad addition); −96 (m, −CH2CF2−CH2CFCF3−); −113 and −116 (m, −CH2CF2−CF2CH2− CH2CF2− reverse tail to tail VDF−VDF dyad); −164 (m, −CH2− CF(CF3)). 5. Radical Copolymerization of Vinylidene Fluoride (VDF) with CF2CF-R′ (R′ = OCF3, CF3 and H) (Runs 18−20, Table 1). 5.1. Radical Copolymerization of Vinylidene Fluoride (VDF) with Perfluoromethyl Vinyl Ether (PMVE) (Run 18 in Table 1). In similar experiments as above, the 100 mL Hastelloy autoclave was filled under vacuum with persistent radical (2.01 g, 4.31 mmol), 1,1,1,3,3-pentafluorobutane (30 mL), and acetonitrile (30 mL). As above, the fluorinated gases, first PMVE (15 g, 0.089 mol) and then VDF (14 g, 0.218 mol) were condensed into the autoclave under weight control. Upon heating to 90 °C, an increase of the pressure to 29 bar followed by a decrease to 19 bar were observed in 4 h. After cooling and purging the nonreacted monomers, the conversion of gaseous monomers was 79%. After purification, the poly(VDF-co-PMVE) copolymer, as a brown elastomeric product E
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Article 1 H NMR (acetone-d6) δ (ppm) (Figure S9 in the Supporting Information): 2.4 (CF2−CH2−CH2−CF2 reverse VDF−VDF addition); 2.9 (CH2 of normal VDF−VDF addition). 19 F NMR (acetone-d6) δ (ppm) (Figure 4): −52 (m, CF2CF(OCF3); −61.2 (q, 3JHH = 4JFF = 10 Hz CF3CH2CF2- chain end); −91.5 (m, CF2 of VDF, head-to-tail normal VDF−VDF dyad addition); −110 (m, CF2 of VDF in alternated VDF−PMVE dyads); −113 and −116 (m, CH2− CF2−CF2−CH2 reverse addition of VDF−VDF dyad); −122 to −128 (m, CF2CF(OCF3); −146 (m, CF2CF(OCF3). 5.2. Radical Copolymerization of Vinylidene Fluoride (VDF) and Hexafluoropropylene (HFP) (Run 19 in Table 1). As in the experimental procedures above, the 100 mL Hastelloy autoclave was filled with persistent radical (2.02 g, 4.30 mmol), 1,1,1,3,3-pentafluorobutane (30 mL) and acetonitrile (30 mL), cooled, and followed by 3 thaw freeze pump cycles. Then, HFP (15 g, 0.100 mol) and VDF (15 g, 0.234 mol) were condensed into the autoclave under weight control. The remaining procedure was the same as above. An increase of the pressure to 28 bar followed by a decrease to 16 bar were observed. After venting the nonreacted monomers, the conversion of gaseous monomers was 83%. After purification of the poly(VDF-co-HFP) copolymer, a light brown elastomeric product was obtained in 80 wt % and was characterized by 1 H and 19F NMR spectroscopy. 1 H NMR (acetone-d6) δ (ppm) (Figure S10 in the Supporting Information): 2.5 (m, CF2CH2−CH2CF2 reverse VDF−VDF addition); 2.95 (q, 3JHF = 14 Hz, CH2 of normal VDF−VDF dyads) 19 F NMR (acetone-d6) δ (ppm) (Figure 5): −61 (q, 3JFH = 4JFF = 10 Hz, CF3CH2CF2− chain end); −71 and −75 ppm (m, CF3 of HFP);
Figure 2. 1H NMR spectrum of CF3−poly(VDF-co-1234yf)−CF3 copolymer (run 17, Table 1) recorded in acetone-d6 at room temperature (where 1234yf stands for 2,3,3,3-tetrafluoroprop-1-ene). VDF:1234yf feed molar ratio of 90:10; molar composition of copolymer of 83:17. obtained in 75 wt %, was characterized by 1H and spectroscopy.
19
F NMR
Figure 3. 19F NMR spectrum of CF3−poly(VDF-co-1234yf)−CF3 copolymer (run 17, Table 1) recorded in acetone-d6 at room temperature (where 1234yf stands for 2,3,3,3-tetrafluoroprop-1-ene). VDF:1234yf feed molar ratio of 90:10; molar composition of copolymer of 83:17. F
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Figure 4. 19F NMR spectrum of CF3−poly(VDF-co-PMVE)−CF3 copolymer (run 18 in Table 1) recorded in acetone-d6 at room temperature (where PMVE stands for perfluoromethyl vinyl ether). VDF:PMVE feed molar ratio of 62.4:37.6; molar composition of copolymer of 82:18. −91.5 (CF2 of VDF, normal head-to-tail VDF−VDF addition); −110 (CF2 of VDF in alternated VDF−HFP dyad); −113 and −116 (m, CH2CF2CF2CH2 reverse VDF−VDF addition); −119 (m, CF2CFCF3); −186 (m, CF2CFCF3). 5.3. Copolymerization of Vinylidene Fluoride (VDF) with Trifluoroethylene (TrFE) (Run 20, Table 1). The procedure was similar as that above, using perfluorinated persistent radical (0.998 g, 2.13 mmol), 60 mL of 1,1,1,3,3-pentafluorobutane, 1,1,2-trifluoroethylene (TrFE) (7 g, 0.085 mol) and VDF (8 g, 0.125 mol). The copolymerization was carried out at 90 °C and, as above, an increase of the pressure to 25 bar followed by a decrease to 9 bar in 2 h were observed. A similar work-up procedure was carried out leading to the poly(VDF-co-TrFE) copolymer as a white powder in 64 wt % yield. Then, it was characterized by 1H and 19F NMR spectroscopy. 1 H NMR (acetone-d 6 ) δ (ppm) (Figure S11, Supporting Information): 2.5 (m, CF2CH2CH2CF2 reverse VDF−VDF addition); 2.9 (m, CH2 of normal VDF−VDF addition); 5.5 (m, −CHF). 19 F NMR (acetone-d6) δ (ppm) (Figure 6): −61 (q, 3JFH = 4JFF = 10 Hz, CF3−CH2CF2− chain end); −92 to −94 (m, CH2CF2−CH2CF2 and CH2CF2−CH2CF2−CF2CH2−); −104 and −105 to −111 (m, overlapping of 2 AB systems assigned to −CH2CF2−CFHCF2 and CH2CF2−CF2CFH−); −113 and −116 (m, −CH2CF2−CF2CH2−); −121 and −129 (AB system, −CF2CFH−); −199 (m, CFH).
(that corresponds to ca. 3 h half-life; see experimental section) for ca. 4−14 h to be sure that all PPFR decomposed into •CF3 radical. The results are summarized in Table 1. 1,1,1,3,3-Pentafluorobutane and acetonitrile were used as the reaction medium due to their high solubility toward the starting fluorinated monomers, and because they do not induce any chain transfer reactions as reported in previous studies.9,10,14,19,22−26,42−44 An effectiveness of the persistent radical has been first attempted for the radical homopolymerization of VDF and its copolymerization with a series of different fluorinated monomers that are depicted below. 1. Radical Homopolymerization of VDF (Runs 1−5, Table 1). The efficiency of such persistent radical was already tested for the homopolymerization of VDF9 and reported that (i) PPFR persistent radical did not disturb the homopolymerization and (ii) the molecular weights and thermostability of the resulting PVDF homopolymers increased for lower initiator concentrations (runs 1, 3−5, Table 1). Interestingly, the melting and glass transition temperatures were not affected much by the [PPFR]o/[VDF]o initial molar ratios. In addition, it was also reported that •CF3 radical’s attack was selective onto the methylene site of VDF and confirms Tedder and Walton’s survey12 and another study of our group,14 and is also in good agreement with ·CCl3’s selective radical addition onto CH2 of VDF.56 The characterizations of the resulting PVDF showed several features: (i) CF3−PVDF−CF3 structures of these fluoropolymers were obtained by macromolecular radical recombination, (ii) the released branched persistent derivative (Scheme 1) (absence of any signals in the 19F NMR spectra) was unabled to initiate the polymerization, and (iii) the generated
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RESULTS AND DISCUSSION The radical homopolymerization of vinylidene fluoride (VDF) and its copolymerization with seven comonomers such as H2C CRACF3 (where RA stands for H, F, CO2R (R = H or tBu) or F2CCFRB (RB: H, CF3, OCF3) were initiated by •CF3 radical generated from a branched perfluoro-3-ethyl-2,4-dimethyl-3pentyl persistent radical (PPFR) (Scheme 2) under different initiator concentrations. The reactions were carried out at 90 °C G
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Figure 5. 19F NMR spectrum of CF3−poly(VDF-co-HFP)−CF3 copolymer (Run 19 in Table 1) recorded in acetone-d6 at room temperature (where HFP stands for hexafluoropropylene). VDF:HFP feed molar ratio of 70.6:29.4; molar composition of copolymer of 86:14.
hindered perfluoroalkene (Scheme 1) was not involved in any termination step or in any copolymerization with VDF.9 Moreover, the effect of the initiator concentrations has been further studied in this present article by performing an additional experiment at 1% of persistent radical concentration (run 2, Table 1) under exactly the same conditions as reported previously.9 Molecular weights (compared run 2 with runs 3− 5, Table 1) and thermostabilty (Figure 7) of the resulting CF3− PVDF−CF3 increased when the initiator concentrations decreased. Furthermore, 1H and 19F NMR spectra, in agreement with those previously disclosed,9 showed the characteristic signals centered at 2.95 ppm (assigned to methylene group in head to tail addition of VDF−VDF dyads), 2.5 ppm (chain defect of VDF−VDF tail to tail reverse addition, CF2CH2CH2CF2), and a signal of small intensity at 3.3 ppm that arises from the regioselective addition of •CF3 radical onto the CH2 of VDF (Figure S2, Supporting Information). Besides, 19F NMR spectra exhibit the characteristic signals of PVDF signals (−92, −113, and −116 ppm, attributed to normal and reverse VDF−VDF addition, respectively), and an expected quintet centered at −61.5 ppm assigned to the CF3 chain end. This quintet stems from the same 3JFH and 4JFF coupling constant values that worth 10 Hz and is consistent with CF3−CH2−CF2− (Figure S3, Supporting Information), hence confirming VDF telomers with CF3I.14
Then, it was of interest to extend this new and efficient initiating system to the radical copolymerization of VDF with other fluoroolefins. 2. Radical Homopolymerization of MAF−TBE and Its Copolymerization with VDF (Runs 6−11, Table 1). In the literature, the radical copolymerizations of VDF with 2trifluoromethacrylic acid (MAF) and tert-butyl 2-trifluoromethacrylate (MAF−TBE) have been reported in satisfactory yields with different initiating systems and it was found that MAF22,23 and MAF−TBE10,57,58 are suitable comonomers for VDF copolymerization although they do not homopolymerize under radical conditions.59 In the present study, we also confirmed that these monomers (MAF and MAF−TBE) did not homopolymerize from •CF3 radical of PPFR (runs 6 and 7, Table 1) but copolymerized with VDF very well and led to alternated copolymers inspite of their lower concentrations in the feed.10 However, our team recently reported the radical copolymerization of VDF with MAF−TBE initiated by PPFR and observed analogous behavior as that of VDF homopolymerization.10 Concisely, when [PPFR]o/([VDF]o+[MAF−TBE]o) initial molar ratios decreased, both the molecular weights and the thermostability of these poly(VDF-co-MAF−TBE) copolymers increased (runs 9−11, Table 1). In addition, the surface properties of these copolymers were assessed by means of water contact angle measurements and showed hydrophobic surfaces. This •CF3 radical initiated the copolymerization by a regioselective addition onto the methylene group of VDF H
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Figure 6. 19F NMR spectrum of CF3−poly(VDF-co-TrFE)−CF3 copolymer (run 20, Table 1) recorded in acetone-d6 at room temperature (where TrFE stands for trifluoroethylene). VDF:TrFE feed molar ratio of 59.5:40.5; molar composition of copolymer of 62:38.
Scheme 2. Radical Copolymerizations of Vinylidene Fluoride (VDF) with H2CCRACF3 (Where RA Stands for H, F, CO2R (R = H or tBu) or with F2CCFRB (RB: H, CF3, OCF3) Initiated by •CF3 Radical Generated from a Perfluoro-3-ethyl-2,4-dimethyl-3pentyl-Branched Persistent Radical (PPFR) at 90 °C
evidenced by the chain defect of VDF in the 1H NMR spectra by the signal centered at 2.4 ppm (reversed addition of VDF, Figure S4, Supporting Information) and in 19F NMR spectra by the signals centered at −113 and −116 ppm assigned to the reversed addition of VDF−VDF (head to head, CH2CF2CF2CH2, Figure S5, Supporting Information). In addition, as observed in a previous study,10 the decreases of both the molecular weights (Figure 8) and the thermostability (Figure S12, Supporting Information) were noted. This was attributed to lower molecular weights (compare run 8 with runs 9−11 in Table 1).
selectively (probably because of the excess of VDF in the feed). Such a CF3 end group of these copolymers acted as an original label to assess VDF and MAF−TBE units, leading to original standards in SEC of CF3−poly(VDF-co-MAF−TBE)−CF3 copolymers.10 Interestingly, this study showed that a higher initiator concentration (2% of PPFR) enabled to produce chain defect of VDF−VDF in the system.10 To illustrate this effect, a supplementary experiment was carried out at higher initiator concentration (10% of PPFR, run 8, Table 1) under similar reaction conditions. The same observation was confirmed as I
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Figure 9. TGA thermograms of CF3−poly(VDF-co-MAF)−CF3 copolymers (heated at 10 °C min−1 under air) at 1.0% PPFR and VDF(80):MAF(20) in feed, 2.0% PPFR and VDF(80):MAF(20) in feed, 1.0% PPFR and 95(VDF):05(MAF) mole% in feed and 1.0% PPFR and 97(VDF):03(MAF) mol % in feed (runs 12−15 in Table 1).
Figure 7. TGA thermograms of CF3−poly(VDF)−CF3 homopolymers at 0, 1, 5, 10 and 20% of PPFR (runs 1−5, Table 1) under air atmosphere at a temperature ramp of 10 °C/min (for 0% PPFR, tert butyl peroxypivalate was used in 0.8 mol % as the radical initiator).
above and the molecular weights were calculated by 19F NMR spectroscopy and not by SEC due to the presence of COOH group in the copolymers which can be trapped in the SEC columns. On the contrary, the yields increased while glass transition temperatures (Tg) and surface properties (hydrophobic materials were obtained) (Figure S13, Supporting Information) were not affected much due to lower initiator concentrations (compare runs 12−13 with 9−10, Table 1). Besides, 1H NMR spectra exhibit the characteristic signals centered at 2.9 and 2.4 ppm as mentioned for normal and reverse additions of VDF−VDF (Figure S6, Supporting Information). 19 F NMR spectra were also in good agreement and showed signals centered at −61.5 (CF3 chain end), −68.5 (CF3 of MAF in copolymer), −91.5 (CF2 of VDF), −95 (alternated VDF− MAF dyads) and −113 and −116 ppm (chain defects, VDF− VDF reverse addition, CH2CF2CF2CH2, only at higher initiator concentration) (Figure S7, Supporting Information). Beside the initiator concentration study, the effect of comonomer concentrations on the resulting copolymers was also examined. Three different VDF:MAF feed molar ratios of 80:20, 95:05 and 97:03 have been used (runs 13−15, respectively in Table 1) at the same [PPFR]o initiator concentration. The increase of molar feed ratios of VDF caused a marked increase of the pressure in autoclave, yield, mole amount of VDF in the copolymer, molecular weights and thermostability of the resulting copolymers (runs 12−15, Table 1). Hence, glass transition temperatures (Tgs) decreased because of the higher amount of VDF in the copolymer. Furthermore, the fluorinated chains containing more VDF units in the copolymer exhibited better surface properties and, as expected, hydrophobicity increased as the VDF content increased, taking into account that the water contact angle of PVDF is 120° (Figure S13, Supporting Information),60 whereas more carboxylic acid functions brought by MAF units induced hydrophilic moieties. Additionally, the comparison of the copolymerization of VDF with MAF or MAF−TBE shows that the copolymers based on MAF produced lower molecular weights (compared runs 12−15 with 7−11, Table 1). This can be explained by the steric effect (i.e., the bulkier tert-butyl group) that could efficiently suppress any transfer reactions, thus increasing molecular weights but it also induced a reverse effect on the yields. However, MAF−based copolymers exhibit better thermostability61 than those containing MAF−TBE. These latters heated above 160 °C led to the decomposition of the tert-butyl ester group that released
Figure 8. Size exclusion chromatograms of CF3−poly(VDF-co-MAF− TBE)−CF3 copolymers at 0.5 (full line), 1.0 (dotted line), 2.0 (dashed line), and 10% (long dashed line) (runs 8−11 in Table 1) with polystyrene standards (MAF−TBE stands for tert-butyl 2-trifluoromethyl acrylate).
After the successful study on VDF homopolymerizations9 and its copolymerizations with MAF−TBE,10 it has been worth checking the usefulness of this novel and efficient initiating system for the radical copolymerizations of VDF with other fluorinated olefins. This present article aims at reporting the radical copolymerizations of VDF with 2-trifluoromethacrylic acid (MAF), 3,3,3-trifluoropropene (TFP), 2,3,3,3-tetrafluoroprop-1-ene (1234yf), perfluoromethyl vinyl ether (PMVE), hexafluoropropylene (HFP), and trifluoroethylene (TrFE). The detailed study and results are described below. 3. Radical Copolymerization of VDF with 2-Trifluoromethacrylic Acid (MAF, Runs 12−15, Table 1). As mentioned above, MAF and MAF−TBE are suitable comonomers with VDF copolymerization. The latter comonomer has been successfully involved in radical copolymerization with VDF10 and hence, it can be anticipated that MAF may behave similarly as MAF−TBE does. A couple of experiments aimed at comparing this copolymerization with that obtained with MAF− TBE under similar conditions. Runs 12 and 13 in Table 1 (2 and 1% of PPFR, respectively) confirmed that, when copolymerized with VDF, MAF has an analogous tendency in reactivity as that of MAF−TBE (runs 9−10, Table 1) and unexpectedly a high exotherm up to 120 °C was noted. The results show that, when the initiator concentration decreased, the molecular weights and thermostability (Figure 9) of the resulting poly(VDF-co-MAF) copolymers increased. These are comparable observations as J
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isobutene and a carboxylic acid group (Scheme 3). This statement confirms various studies62−67 which explain the first Scheme 3. Mechanism of Decomposition of tert-Butyl 2Trifluoromethacrylate (MAF−TBE) Units in Poly(VDF-coMAF−TBE) Copolymers into Isobutene and a Carboxylic Acid Function That Further Undergoes a Decarboxylation
Figure 10. TGA thermograms of CF3−poly(VDF)−CF3 homopolymers and its copolymers with perfluoromethyl vinyl ether (PMVE), hexafluoropropylene (HFP), trifluoroethylene (TrFE), 2,3,3,3-tetrafluoroprop-1-ene (1234yf), 3,3,3-trifluoropropene (TFP), 2-trifluoromethacrylic acid (MAF), and tert-butyl 2-trifluoromethacrylate (MAF−TBE) at 1.0% PPFR, heated at 10 °C min−1 under air.
plateau in TGA thermograms. Chains bearing carboxylic acid functions also underwent decarboxylation and, as the temperature increased, a second weight loss was observed from 335 °C. On the other hand, Tg values of poly(VDF-co-MAF−TBE) copolymers were lower than those of poly(VDF-co-MAF) copolymers and this may be explained by the presence of MAF−TBE that acts as a plasticizer whereas MAF might bring hardness to the system by intra or inter hydrogen bonding between carbonyl and hydroxyl functions. Finally, it can also be concluded that a careful control of the initiator concentrations not only produced high molecular weights, better thermostability, and surface properties but also led to alternating structure (without any defect of (VDF)n chaining) of the resulting copolymers. 4. Radical Copolymerization of VDF with 3,3,3Trifluoropropene (TFP) (Run 16, Table 1). The above persistent radical was also successfully involved in the radical copolymerization of VDF with 3,3,3-trifluoropropene (TFP). To the best of our knowledge only a few surveys6,43,44,68 reported that this olefin can be (co)polymerized under radical conditions. In a first attempt, a copolymer was prepared from an initial VDF:TFP feed of 66:34 molar ratio to obtain a model compound compared to copolymers above in terms of molecular weight, and thermal and surface properties. That reaction led to a high amount of poly(VDF-alt-TFP) alternated structure and satisfactory yield (67%) (run 16, Table 1) which also states that such a branched persistent radical does not disturb both TFP and VDF’s reactivities. This confirms the higher reactivity of TFP.43,44 All signals assigned to VDF and TFP can be noted in 19 F and 1H NMR spectrum (Figures 1 and S8 (Supporting Information), respectively), that also display the characteristic signals of CF3 chain end and the absence of chain defects in VDF−VDF dyads (absences of signals centered at 2.4 and at −113 and −116 ppm in 1H and 19F NMR spectra, respectively), and alternated structure and microblocks of oligo(TFP) in the resulting copolymer (even starting from 30 mol % of TFP in the feed). In contrast to the copolymerizations of VDF with other comonomers, the resulting poly(VDF-co-TFP) copolymer’s microstructure displays CF3 end groups adjacent to both VDF (characteristic quintet centered at −61 ppm) and TFP (multiplet centered at −64 ppm assigned to CF3CH2CH(CF3) group). The thermal analysis of that copolymer revealed a fast degradation as compared to other fluorinated copolymers (except those containing MAF−TBE) under oxygen atmosphere (Figure 10). This may be due to the low molecular weight oligomer (1% of PPFR led to 4200 g·mol−1 by size exclusion chromatogram (SEC), Figure S14, Supporting Information) that
evaporate and do not decompose. Consequently, that reaction led to low Tg (−33 °C, run 16, Table 1). 5. Radical Copolymerization of VDF with 2,3,3,3Trifluoroprop-1-ene (1234yf) (Run 17, Table 1). Such a copolymerization was disclosed in a 3 M patent in 195552 that claims that the persulfate-initiated copolymerization starting from 50:50 mol % of VDF:1234yf led to a copolymer that contained 80 mol % of VDF showing a higher reactivity of such a fluoroalkene. In the present study, the same initiating system (•CF3 radical from 1 mol % of PPFR) as that of above copolymerizations also enabled the preparation of a statistical CF3−poly(VDF-co1234yf)−CF3 copolymer based on 83:17 mol % (in copolymer) VDF:1234yf starting from 90:10 mol % (in feed of run 17, Table 1). This copolymer was obtained in high yield (94%) and molecular weight (62 000 g·mol−1), low Tg (−33 °C) while Tm = 155 °C, TC = 116 °C, and satisfactory thermal degradation (Td10% = 402 °C under air). Hence, in contrast to the 3 M patent, this first attempt shows that 1234yf is likely more reactive than VDF. As in the above copolymers, 1H NMR spectrum (Figure 2) of poly(VDF-co-1234yf) copolymer displays only two signals centered at 2.7 ppm (as a broad quintet) and at 2.45 ppm assigned to normal (head to tail) and reversed (tail to tail) VDF− VDF chaining, respectively. The signal of changing to enchainment/methylene group in 1234yf comonomer overlaps with that of the former one. Interestingly, no triplet of triplets centered at 6.3 ppm was noted and justifies the absence of −CH2CF2−H end-group arising from any transfer to monomer, solvent or copolymer. The 19F NMR spectrum (Figure 3) of such a copolymer exhibits the methylenes of normal head to tail and reversed head to head VDF−VDF dyads by characteristic signals centered at −91 to −93 ppm, −113 and −116 ppm, while that in central position in −CH2CF2−CH2CF(CF3)− dyad led to a signal at −95 ppm. In addition, tertiary fluorine and trifluoromethyl groups in 1234yf units are assigned to signals centered at −164 to −168 ppm and −78 to −80 ppm, respectively. Figure 3 displays the expected quintet (3JFH = 4JFF = 10 Hz) assigned to trifluoromethyl end-group in CF3− CH2CF2− as noted in previous work9,10 while that of CF3 in CF3−CH2−CF(CF3)− was not observed. 6. Radical Copolymerization of VDF with Perfluoromethyl Vinyl Ether (PMVE) (Run 18, Table 1). One of the most important areas for the development of fluoropolymers based on VDF is their uses as elastomers that are endowed with a K
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°C) and characterized the comonomer-sequence distribution (or normalized monomer−dyad and −tryad fractions) versus the polymer composition in the resulting random copolymers. Other authors46−49 followed that strategy and also investigated the terpolymerization of VDF, TrFE and chlorotrifluoroethylene. The above persistent radical was successfully involved in radical copolymerization of VDF (90 mol % in feed) with TrFE (10 mol % in feed) initiated by 1 mol % PPFR (run 20, Table 1) that led to 74% yield and also confirmed the regioselectivity of ·CF3 addition onto the methylene site of VDF. This CF3−poly(VDF-co-TrFE)−CF3 copolymer was obtained in high molecular weight (200.000 g·mol−1) and thermal degradation (Td10% = 412 °C under air). The composition of this CF3−poly(VDF-coTrFE)−CF3 copolymer was assessed by 19F NMR spectroscopy, which indicated that the VDF:TrFE molar content was 62:38. The 1H NMR spectrum (Figure S11, Supporting Information) displays the expected broad signals assigned to VDF (as depicted in above examples) as well as those of TrFE unit (CFH group) in the 5.2 to 5.9 ppm range. Three small quintets and a major one centered between −60.5 to −62.0 ppm assigned to CF3− CH2CF2−X− (where X stands for CH2, CF2 and CFH), confirm Tedder and Walton’s study12 on the high selectivity of the ·CF3 radical’s attack onto CH2 site of VDF much higher than that onto CFH of TrFE. The 19F NMR spectrum (Figure 6, Table S1, Supporting Information) displays all expected multiplets as reported by the literature including the AB system at −122 and −129 ppm assigned to the difluoromethyl between two CHF groups.45−49 The absence of signal at ca. − 82 ppm confirms the absence of CF3−CF2CH2− or CF3−CF2CFH− since highly electrophilic ·CF3 radical cannot react onto electrophilic CF2 site of VDF or TrFE. As for C4F9−CH2CF2−CFHCF2−I and C4F9− CH2CF2−CF2CHFI isomers50 achieved by radical telomerization of TrFE with C4F9−CH2CF2−I, AB systems were attributed to anisochronous fluorine atoms in CF2 adjacent to CHF group that was evidenced in −CFHCF2−CFHCF2− dyads. In conclusion, the results gathered in Table 1 deserve the following additional comments: (a) The yields achieved from all copolymerizations are higher than those obtained for the preparation of PVDF, showing that these chosen comonomers are suitable partners of VDF. (b) For the respective reactivity of VDF about comonomers, all kinetics of copolymerizations did not change much and except for HFP, TrFE and PMVE comonomers, VDF seems less reactive (even at high mol % in the feed) than the other comonomers. (c) Regarding the efficiency of PPFR, the lower the PPFR concentration, the higher both the molecular weights (Figure 11) and thermal stabilities of the resulting copolymers. (d) The decreasing ranking of comonomers to achieve higher molecular weights is as follows: TrFE > MAF−TBE > 1234yf > PMVE > MAF > HFP > TFP, while that of the decreasing order for the yield was: 1234yf > HFP > PMVE > TrFE > TFP > MAF = MAF−TBE (Figure S17, Supporting Information). (e) Molecular weights assessed from SEC (PS standard) are always lower than those determined by 19F NMR. This shows that there is a need to get fluorinated standards for SEC and hence such VDF (co)polymers are potential ones.
good stability at low and high temperatures (from −25 °C to high temperatures).3,5,6 However, PVDF or PTFE cannot be potential rubbers because of the presence of highly crystalline domains. One way to decrease their crystallinity is the insertion of a comonomer, such as HFP (discussed below in subsection 7) and PMVE, yielding interesting fluoroelastomers since the CF3 or OCF3 groups induce some kinks to reduce the crystallinity.1−9,69 Boyer et al.25 reported the synthesis of poly(VDF-co-PMVE) copolymers and commented that their various Tgs and degradation temperatures depended on their molecular weights and on the contents of both comonomers. As in the cases above, starting from a molar VDF:PMVE feed ratio of 62.4:37.6, the persistent radical at low concentration produced desirable statistical CF3−poly(VDF-co-PMVE)−CF3 fluoroelastomers (run 18, Table 1) in satisfactory yield (75%), lower Tg (−42 °C) and higher degradation temperature (Td10% = 425 °C under air) (Figure 10). This result also showed that this new initiating system was efficient in the copolymerization of VDF with PMVE as for other ones. The 1H NMR spectrum (Figure S9, Supporting Information) exhibits signals centered at 3.3 and 2.5 ppm attributed to −CH2CF2- in normal VDF−VDF dyads and reversed tail-to-tail addition of VDF, respectively. The 19F NMR spectrum (Figure 4) is in agreement with those of previous work25 and shows key signals centered at −52, −61, −92, −110, −113 to −116, −123, −127, −147 ppm assigned to −OCF3 of PMVE, CF3 end group, CF2 group in normal VDF−VDF dyads, CF2 of VDF unit adjacent to a PMVE in VDF−PMVE dyad, reverse head to head addition of VDF−VDF (−CH2CF2CF2CH2−), CF2 of PMVE and −CF(OCF3) of PMVE, respectively. As stated above, a higher amount of VDF in the copolymer leads to satisfactory Mn and polydispersity index (Đ) (run 18, Table 1) (Figure S15, Supporting Information) as compared to that of alternating structure. From VDF:PMVE mol feed of 70:30, the 19F NMR spectrum also enabled us to assess the microstructure (i.e., a respective 74:26 mol. amounts in the copolymer) in agreement with conventional reactivities of both comonomers.7,25,69 7. Radical Copolymerization of VDF with Hexafluoropropylene (HFP) (Run 19, Table 1). From the same initiating system as that of the above, a fluoroelastomer based on VDF and HFP units (run 19, Table 1) was prepared in good yield (80%) endowed with a low Tg (−35 °C) and high thermal degradation (Td10% = 418 °C under air). Besides this efficient initiating system, the kinetics of this reaction was not modified. In addition to the characteristic signals assigned to VDF including that of VDF−HFP dyad at −110 ppm and the chain end, the 19F NMR spectrum (Figure 5) of CF3−poly(VDF-co-HFP)−CF3 copolymer exhibits the signals attributed to HFP at −71 and −75 (characteristic signals of CF3), −119 (CF2CFCF3) and −186 ppm (CF2CFCF3). Equation in footnotes of Table 1 leads to the determination of molecular weights (>16400 g. mol−1). Both CF 3 −poly(VDF-co-PMVE)−CF 3 and CF 3 −poly(VDF-coHFP)−CF3 fluoroelastomers (runs 18 and 19, Table 1) show similar properties concerning copolymer compositions, yields, degradation temperatures, and molecular weights (Figures S15 and S16 in Supporting Information, respectively) with a small increase of Đ for CF3−poly(VDF-co-HFP)−CF3 copolymer to 2.0. 8. Radical Copolymerization of VDF with trifluoroethylene (TrFE) (run 20, Table 1). The radical copolymerization of VDF with TrFE was first reported by Yagi and Tatemoto45 who extensively investigated its kinetics to assess their respective reactivity ratios (rVDF = 0.7 and rTrFE = 0.5 at 22 L
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mol% of VDF in copolymer = 100
∫ CF2at −92 to −95 + ∫ CF2at −113 to −116)/2)] /[(∫ CF2 at − 92 to − 95 + ∫ CF2 at − 113 to − 113)/2) + (∫ CF3at − 68.5/3)] × [(
mol % of MAF and MAF−TBE in copolymer Figure 11. Molecular weights of poly(VDF-co-MAF−TBE) copolymers versus PPFR initiator concentrations (runs 9−11, Table 1).
= 100 − mol% of VDF
2. For copolymers of VDF with perfluoromethyl vinyl ether (PMVE):
■
CONCLUSIONS A novel initiating system based on perfluorinated hyperbranched persistent radical, perfluoro-3-ethyl-2,4-dimethyl-3-pentyl (PPFR), was able to generate a valuable source of •CF3 radical. It successfully initiated the radical homopolymerization of vinylidene fluoride (VDF) and its copolymerizations with tertbutyl 2-trifluoromethacrylate (MAF−TBE), 2-trifluoromethacrylic acid (MAF), 3,3,3-trifluoropropene (TFP), 2,3,3,3tetrafluoroprop-1-ene (1234yf), perfluoromethyl vinyl ether (PMVE), hexafluoropropylene (HFP) and trifluoroethylene (TrFE) in fair to good yields. 19F NMR spectra showed that the released hindered perfluoroalkene was not involved in the radical copolymerization or in the termination of the polymerization. Interestingly, •CF3 radical initiated the copolymerization by a regioselective addition onto the methylene group of VDF, and in the case of TFP (that is more reactive than VDF), a CF3− CH2CH(CF3)- end-group was also noted. Such a CF3 end group acted as an original label to assess both the numbers of VDF and M comonomer units and the molecular weights of the resulting poly(VDF-co-M) copolymers leading to original standards in SEC of CF3−poly(VDF-co-M)−CF3 copolymers. In addition, the PPFR concentration directed their molar masses and thus the degradation temperatures. From 1% of PPFR, an increasing yield series was suggested from various above comonomers involved in the radical copolymerizations with VDF: 1234yf < HFP < PMVE < TrFE < TFP < MAF < MAF−TBE; while an increasing ranking in higher molecular weights (Mns) was proposed: TrFE < 1234yf < MAF−TBE < PMVE < HFP < TFP < MAF. For certain copolymers, high thermal stabilities (up to 450 °C under air) were achieved depending on their Mn values. Additionally, a wide range of thermoplastics and fluoroelastomers can be prepared by this new and efficient initiating system. Deeper work regarding the defects of VDF−VDF chaining from homopropagation of VDF or recombination of macroradicals, the viscosities of these copolymers, and their melting points is under investigation. These original (co)polymers appear as efficient SEC standards used for forthcoming studies. In addition, challenging work also concerns the use of PPFR in controlled radical (co)polymerization as a counter-radical, under progress.
■
mol% of VDF in copolymer = 100 × [(
∫ CF2at −92 to −95 + ∫ CF2at −110
∫ CF2at −113 to −116)/2)] /[(∫ CF2 at − 92 to − 95 + ∫ CF2 at − 110 + ∫ CF2 at − 113 to − 116)/2) + (∫ CF3at − 52 /3)] +
mol % of PMVE in copolymer = 100 − mol % of VDF
3. For copolymers of VDF with hexafluoropropylene (HFP): mol % of VDF in copolymer = 100 × [(
∫ CF2at −92 to −95 + ∫ CF2at −110
∫ CF2at −113 to −116)/2)] /[(∫ CF2 at − 92 to − 95 + ∫ CF2 at − 110 + ∫ CF2 at − 113 to − 116)/2) + (∫ CF3at − 71 to − 75)/3)] +
mol % of HFP in copolymer = 100 − mol % of VDF
4. For copolymers of VDF with 3,3,3-trifluoropropene (TFP): mol % of VDF in copolymer = 100
∫ CF2at −91 to −110/2]/[(∫ CF2at −91 to −100/2) + (∫ CF3at − 66 to − 72 /3)] ×[
APPENDIX 1
The equations from footnote b in Table 1 are given here.
mol % of TFP in copolymer = 100 − mol % of VDF
1. For copolymers of VDF with 2-trifluoromethacrylic acid (MAF) and tert-butyl 2-trifluoromethacrylate (MAF− TBE):
5. For copolymers of VDF with 2,3,3,3-tetrafluoroprop-1ene (1234yf): M
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∫ CFat −111 to −112 + ∫ CFat −121 to −130) /2]/(∫ CF3at − 61/6)
mol % of VDF in copolymer = 100
DPn of TrFE = [(
∫ CF2at −92 to −95 + ∫ CF2at −113 to −116)/2)] /[(∫ CF2 at − 92 to − 95 + ∫ CF2 at − 113 to − 116)/2 + (∫ CF3at − 79 to − 81/3)] × [(
(where ∫ CFiat −I to −j stands for the integral of the signal assigned to CFi group that ranges from −I to −j ppm in the 19F NMR spectra).
■
mol % of 1234yf = 100 − mol % of VDF
S Supporting Information *
6. For copolymers of VDF with trifluoroethylene (TrFE):
Picture of manifold used to prepare the sealed Carius tubes, 1H and 19F NMR spectra, thermogravimetric analyses (TGA), water contact angles (WCAs) and size exclusion chromatograms (SECs) of various poly(VDF-co-M) copolymers, plot of yield and molecular weight versus monomer, and a table of 19F NMR signal assignments. This material is available free of charge via the Internet at http://pubs.acs.org.
mol % of VDF in copolymer = 100 × [(
ASSOCIATED CONTENT
∫ CF2at −92 to −95 + ∫ CF2at −104 to −110
∫ CF2at −113 to −116)/2] /[(∫ CF2 at − 92 to − 95 + ∫ CF2 at − 104 to − 110 + ∫ CF2 at − 113 to − 116)/2) + (∫ CFat − 111 to − 112 + ∫ CF2 at − 121 to − 130)/2] +
■
AUTHOR INFORMATION
Corresponding Author
*(B.A.) Telephone: +33-467-144-368. Fax: +33-467-147-220. Email:
[email protected]. Notes
The authors declare no competing financial interest.
mol % of TrFE = 100 − mol % of VDF
■
■
ACKNOWLEDGMENTS The authors are grateful to the Institut de Chimie of the CNRS and the French National Research Agency (l’Agence Nationale de la Recherche, ANR PREMHYS project) for financial support. Arkema (Colombes and Pierre-Benite, France), Solvay S.A. (Tavaux, France and Brussels, Belgium), Tosoh F-Tech Company (Shunan, Japan), and Dupont Performance Elastomers (Wilmington, DE) are acknowledged for providing TrFE and 1234yf, VDF, MAF and MAF−TBE, and PMVE comonomers, respectively. B.A. also thanks the French Fluorine Network (GIS).
APPENDIX 2 The equations from footnote c in Table 1 are given here. M n = 2 × MCF3 + MVDF × (DPn of VDF) + MMo × (DPn of Mo)
where Mo designates the comonomer and MCF3 = 69 g·mol−1, MVDF = 64 g·mol−1, and MMo = 196 for MAF−TBE, 140 for MAF, 166 for PMVE, 150 for HFP, 96 for TFP, 114 for 1234yf, and 82 g·mol−1 for TrFE.
■
∫ CF2at −92 to −95 + ∫ CF2at −113 to −116) /2]/(∫ CF3at − 61/6)
DPn of VDF = [(
DPn of MAF − TBE = (
DPn of MAF = (
DPn of HFP = (
DPn of TFP = (
∫ CF3at −68.5/3)/(∫ CF3at −61/6)
∫ CF3at −68.5/3)/(∫ CF3at −61/6)
DPn of PMVE = (
∫ CF3at −52 /3)/(∫ CF3at −61/6)
∫ CF3at −71 to −75/3)/(∫ CF3at −61/6)
∫ CF3at −66 to −72 /3)/(∫ CF3at −61/6)]
DPn of 1234yf = (
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