Document VJqaJBjKbYQ05G2eG4bGeRL1o
Kinetics of the Iodine Transfer Polymerization of Vinylidene Fluoride
CYRILLE BOYER, DAVID VALADE, PATRICK LACROIX-DESMAZES, BRUNO AMEDURI, BERNARD BOUTEVIN
Laboratoire de Chimie Macromoleculaire, Ecole Nationale Superieure de Chimie de Montpellier (UMR 5076-CNRS), 8 Rue de l'Ecole Normale, 34296 Montpellier Cedex 1, France
Received 16 May 2006; accepted 23 June 2006 DOI: 10.1002/pola.21654 Published online 29 August 2006 in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: The kinetics of the iodine transfer polymerization (ITP) of vinylidene fluoride (VDF) was achieved in the presence of three different chain-transfer agents (CTAs): 1-iodoperfluorohexane (C6F13I), 1-iodo-2H,2H-perfluorooctane (C6F13CH2CF2I), and 1,1,2,2-tetrafluoro-3-iodopropane (HCF2CF2CH2I). ITPs of VDF carried out in the presence of C6F13I and C6F13CH2CF2I showed the following: (1) a linear increase in DPn versus aVDF, which evidenced the controlled character of ITP, although the polydispersity indices were slightly high (ca 1.5), and (2) theoretical DPn values close to the targeted ones. In contrast, neither of these statements was observed for the ITP of VDF in the presence of HCF2CF2CH2I achieved under the same conditions, even if the synthesized oligomers could be reactivated. Although the CTr values of C6F13I and C6F13CH2CF2I were close (i.e., 7.7 at 75 8C), that of HCF2CF2CH2I was lower (0.3 at 75 8C). The percentages of CF2I and CH2I functionalities were also assessed, and in the course of the reaction, a reduction of CF2I end groups was noted. Then, the mechanism of the ITP of VDF was proposed. VC 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 5763-5777, 2006 Keywords: degree of polymerization (DP); fluoropolymers; living polymerization; molecular weight distribution/molar mass distribution; NMR; radical polymerization
INTRODUCTION
Though discovered in the late 1970s,1-3 controlled free-radical polymerization attracted much interest in the mid 1990s.4 This is based on adding to a polymerization medium a species able to react with active centers by reversible termination4-7 or transfer.1-3,8-13 Hence, equilibrium occurs between the living radicals (or active species) and dormant
This article includes Supplementary Material available from the authors upon request or via the Internet at www. interscience.wiley.com/jpages/0887-624X/suppmat.
Correspondence to: B. Ameduri (E-mail: bruno.ameduri@ enscm.fr)
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 44, 5763-5777 (2006) VC 2006 Wiley Periodicals, Inc.
(or nonpropagating) species. A radical polymerization can be called controlled when the three following conditions are fulfilled:4
1. The controlling species must be quickly consumed, and this consumption rate must be faster than that of the propagation step.
2. The polymeric chain must be created in a short time and not in the course of the polymerization, as in the traditional process.
3. The dormant (PX) and active (P) species must exchange quickly (Scheme 1).
In the last decade, nitroxide-mediated polymerization,5 atom transfer radical polymerization,6,7 and methods based on transfer [reversible addition-frag-
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5764 BOYER ET AL.
Scheme 1. General scheme for reversible activation in controlled radical polymerization.
mentation chain transfer (RAFT),12,13 macromolecular design by interchanges of xanthates,10,11 and iodine transfer polymerization (ITP)1,2,8,9,14] have been the three main emerging approaches.
Nowadays, ITP of fluorinated alkenes is a technique that enables various companies involved in fluorine chemistry to produce fluorinated thermoplastic elastomers (TPEs), which have been recently reviewed.14 These TPEs are diblock or triblock copolymers composed of hard and soft sequences prepared by the stepwise (or sequential) ITP of various fluoroalkenes. They have been marketed by Daikin,8,15 Dupont,16,17 and Ausimont18-20 (now Solvay Solexis) under the Daiel, Viton, and Tecnoflon trademarks, respectively. These triblock copolymers [based on a large variety of different monomeric units such as vinylidene fluoride (VDF), hexafluoropropylene, chlorotrifluoroethylene, tetrafluoroethylene, perfluoromethyl vinyl ether, ethylene, and propylene or a combination of two or three of these alkenes]3,18-21 can be synthesized because of the controlled behavior of that radical polymerization in the presence of a fluoroiodinated transfer agent. Besides, ITPs of other monomers have also confirmed their controlled character in the presence of alkyl iodides. These monomers are mainly hydrogenated, such as styrene,22 methyl acrylate,23 butyl acrylate,23-26 vinyl acetate,27 vinylidene chloride,28 and vinyl chloride.29-45 Although many industrial breakthroughs have been achieved, leading to various patents, ITP of fluorinated alkenes has been described in a few articles1-3,9 and in a chapter of a book.14 Nowadays, the evidence of the controlled character of that polymerization (especially the linear relationship between the molar masses of the resulting polymers and the monomer conversions) deserves to be proved, even if the kinetics of these gaseous fluoroalkenes seem more difficult to be determined than those of liquid, hydrogenated monomers.
In a recent article,46 the microstructure of oligo-VDFs is studied. This article considers the influence of defects of VDF chaining on the control of ITP, especially because the produced reversed
CF2CH2I end group can be reactivated with difficulty. This inversion was evidenced by the choice of three chain-transfer agents (CTAs) representing the possible end structures of growing chains: 1iodoperfluorohexane (C6F13I), 1-iodo-2H,2H-perfluoro-octane (C6F13CH2CF2I), and 1,1,2,2-tetrafluoro-3-iodopropane (HCF2CF2CH2I). In contrast to those observed from the last one, the first two lead to a good fit between the targeted and experimentally observed number-average degree of polymerization (DPn) values. Thus, it was worth investigating the kinetics of the ITP of VDF and assessing the values of the transfer constant (CTr) of these three CTAs at different temperatures. Then, it was also of interest to determine the ratio of the square of the propagation rate coefficient (kp) to the termination rate coefficient (kte) of VDF in the radical polymerization. These are the objectives of this article.
EXPERIMENTAL
Materials
VDF (or 1,1-difluoroethylene; bp 82 8C) and 1,1,1,3,3-pentafluorobutane were kindly offered by Solvay S.A. (Tavaux, France, and Brussels, Belgium). HCF2CF2CH2I (purity 99%) was purchased from Aldrich Chimie (Saint QuentinFallavier, France), and C6F13I or perfluorohexyliodide (purity 99%) were generously supplied by Atofina (now Arkema; Pierre-Benite, France). They were worked up with sodium thiosulfate and then distilled before use. tert-Butylperoxypivalate (TBPPI; purity 75%) and 2,5-dimethyl2,5-bis(tert-butylperoxide)hexane (DHBP; purity 90%) were gifts from Akzo Nobel (Chalons sur Marne, France). They were used as supplied. Acetonitrile, dimethylformamide (DMF), tetrahydrofuran, methanol, methyl ethyl ketone, and dimethylacetamide (DMAc) (analytical-grade) were purchased from Aldrich Chimie.
C6F13CH2CF2I was synthesized by the thermal telomerization of VDF with C6F13I at 200 8C for 12 h, and this monoadduct was purified by distillation, as previously reported.47 It was worked up with sodium thiosulfate and then distilled before use.
Analyses
The compositions and structures of the oligomers obtained by ITP were determined with 19F and 1H NMR spectroscopy. The NMR spectra were
Journal of Polymer Science: Part A: Polymer Chemistry DOI 10.1002/pola
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IODINE TRANSFER POLYMERIZATION 5765
recorded on Bruker AC 200, AC 250, and 400 (200-, 250-, and 400-MHz) instruments with deuterated acetone, dimethyl sulfoxide (DMSO), or DMF as the solvent and tetramethylsilane (or CFCl3) as the reference for 1H (or 19F) nuclei. Coupling constants and chemical shifts are given in hertz and parts per million, respectively. The experimental conditions for 1H (or 19F) NMR spectra were as follows: a flip angle of 908 (or 308), an acquisition time of 4.5 s (or 0.7 s), a pulse delay of 2 s (or 5 s), 16 (or 64) scans, and a pulse width of 5 ls (for 19F NMR).
Size exclusion chromatography (SEC) analyses were performed with a Spectra-Physics apparatus equipped with two PLgel 5-lm mixed-C columns from Polymer Laboratories and a Spectra Physics SP8430 refractive-index detector {the signals assigned to PVDF-I [where PVDF is poly(vinylidene fluoride)] gave negative values}. DMAc was chosen as the eluent at 70 8C with a flow rate of 0.6 mL min1. The standards were monodispersed polystyrene purchased from Polymer Laboratories.
Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) was performed with an Ultraflex Bruker (Universite de Montpellier II) in the reflectron mode (accelerating potential of 20 kV) on a Bruker apparatus equipped with a nitrogen laser (337 nm). 2,3,4,5,6-Pentafluorocinnamic acid and sodium iodide (NaI) (or silver trifluoroacetate AgTFA) were used as the matrix and the cationizing agent, respectively. The concentrations of the sample and matrix solutions were 10 g L1 in DMF, and the analyte/matrix ratio was 1/10 (v/v). The mixture (1 lL) was deposited on a stainless steel target, air-dried, and introduced into the spectrometer in vacuo.
Reaction in an Autoclave
ITPs of VDF were performed in the presence of C6F13I, C6F13CH2CF2I, or HCF2CF2CH2I as the CTA and initiated with TBPPI at 75 8C or DHBP at 135 8C according to a procedure described in the supplementary material.
After purification, the samples were characterized with 19F and 1H NMR spectroscopy. For example, for 19F NMR [deuterated acetone, d (ppm)] of C6F13CH2CF2-(VDF)n-I (DPn 13; experimental conditions for the ITP of VDF with C6F13CH2CF2I: [VDF]0/[C6F13CH2CF2I]0/[TBPPI]0 100.0:6.6:0.6 at 75 8C), we found.
19F NMR: 38.0 (t, CH2CF2I, 1.2F); 60.0 (m, CF2CF2I, 0.0F); 82.0 (m, CF3CF2,
Journal of Polymer Science: Part A: Polymer Chemistry DOI 10.1002/pola
3.0F); 91.0 (m, CH2CF2 CH2CF2, 23.1F); 95.7 (m, CH3CH2CF2, 0.0F); 108.0 (m, CH3CF2, CF2CF2 CH2I, CF2CF2 CH2CF2, 0.8F); 112.0 [m, (CH3)3 CCF2 CH2, 2.7F]; 113.4 (m, CH2CF2 CF2 CH2CH2, 2F); 114.8 (dm, CH2 CF2H, 0.0F); 115.7 (m, CH2 CF2CF2CH2 CH2, 0.0F). 1H NMR: 1.0 [s, (CH3)3CCH2CF2, 0.0H]; 1.2 (t, CH3CH2CF2, 0.0H); 1.3 [s, (CH3)3 CCF2CH2, 0.0H]; 1.8 (t, CH3CF2 CH2, 0.0H); 2.3 (m, CH2CF2CF2CH2C H2 CF2, 0.4H); 2.9 [m, CH2CF2CH2CF2 CH2CF2, CH3CH2CF2, (CH3)3CCH2 CF2, 21.5H]; 3.3 (q, 3JHF 15 Hz, C6F13CH2 CF2, 2.0H); 3.7 (q, 3JHF 14.0 Hz, CH2 CF2I, 1.1H); 3.9 (q, 3JHF 8.0 Hz, CF2CH2I, 0.9H); 6.5 (t, 2JHF 49.6 Hz, 3JHF 4.0 Hz, HCF2CF2, 0.0H).
For example, we determined 19F and 1H NMR [deuterated acetone, d (ppm)] for the oligomer obtained by the ITP of VDF with HC2F4CH2I (DPn 35; recorded in deuterated DMF, 400 MHz, 298 K; experimental conditions for the ITP of VDF with HC2F4CH2I: [VDF]0/[HC2F4CH2I]0/[ TBPPI]0 100.0:6.6:0.6 at 75 8C).
19F NMR: 38.0 (t, CH2CF2I, 0.0F); 91.0 (m, CH2CF2CH2CF2, 78.0F); 95.7 (m, CH3CH2CF2, 2.0F); 108.0 (m, CH3CF2, CF2CF2CH2I, CF2CF2CH2CF2, 1.9F); 112.0 [m, (CH3)3CCF2CH2, 2.7F]; 113.4 (m, CH2CF2CF2CH2CH2, 5.5F); 114.8 (dm, CH2CF2H, 0.0F); 115.7 (m, CH2 CF2CF2CH2CH2, 5.5F), 136.0 (d, CF2 CF2H, 2.0F). 1H NMR: 1.0 [s, (CH3)3CCH2 CF2, 0.1H]; 1.2 (t, CH3CH2CF2, 0.2H); 1.3 [s, (CH3)3CCF2CH2, 0.2H]; 1.8 (t, CH3CF2 CH2, 0.4H); 2.3 to 2.9 (m, CH2CF2CF2 CH2CH2CF2, m, CH2CF2CH2CF2 CH2CF2, CH3CH2CF2, (CH3)3CCH2 CF2, 82.0H); 3.3 (q, 3JHF 15 Hz, C6F13 CH2CF2, 0.0H); 3.7 (q, 3JHF 14.0 Hz, CH2 CF2I, 0.0H); 3.9 (q, 3JHF 8.0 Hz, CF2CH2I, 2.0H); 6.5 (t, 2JHF 49.6 Hz, 3JHF 4.0 Hz, HCF2CF2, 1.0H).
Kinetics of the ITP of VDF
Aliquots were periodically sampled from the autoclave (with a probe) in the course of the polymerization of VDF to monitor the following: (1) the conversions of both the monomer and the CTA and (2) the evolution of the molar masses. The collected samples were frozen into liquid nitrogen to stop the
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5766 BOYER ET AL.
Scheme 2. Chain-transfer reaction in ITP (RF represents C6F13, C6F13CH2CF2, or HCF2CF2CH2).
polymerization reaction and then diluted in DMAc and were characterized with SEC and NMR.
RESULTS AND DISCUSSION
Concept of ITP
ITP is a degenerative transfer polymerization requiring alkyl iodides:23 an initiating radical (A), generated by the thermal decomposition of a conventional radical initiator, reacts with a monomer (M) and leads to a propagating radical (Pn; direct initiation). This mechanism exhibits two transfer reactions:
1. The first one deals with the exchange of iodine from the transfer agent, RFI, to the propagating macroradical, Pn, resulting in the formation of the polymeric alkyl iodide, PnI, and in a new initiating radical, RF (first transfer reaction, Scheme 2). The ratio of the transfer rate coefficient (ktr) to kp is called CTr, which gives the intrinsic reactivity of the transfer agent. The reactivity of the CTA is defined by this value. Indeed, a high CTr value (i.e., >1) means that the CTA allows the molar masses of the resulting polymers to be controlled.
2. The second transfer reaction concerns the exchange reaction between Pn and PmI (Scheme 3). This reaction allows the growing chains to react with additional monomeric units in each activation-deactivation cycle. Ideally, in ITP, to lead to a polymer with a narrow molar mass distribution, the exchange rate coefficient has to be higher than the propagation rate coefficient [exchange constant (Cex) >1].23,48 Litvinenko and Muller49 showed that the value of the polydisper-
Scheme 3. Exchange reaction in ITP.
sity index (PDI) depends on Cex and can be approximated at a high monomer conversion as follows: PDI 1 [CTA]0/[M]0 (1/Cex) & 1 (1/Cex).
Thus, in the ITP mechanism, CTr indicates the activity of the CTA. Therefore, the amount of the CTA controls the number of chains and, hence, the molar masses of the oligomers (in the ideal case, e.g., when the chains formed by direct initiation from the initiator can be neglected).
In this work, the ITP of VDF was carried out in the presence of three fluorinated CTAs initiated with TBPPI at 75 8C (Scheme 4).
In each case, the reaction was monitored via sampling in the autoclave. The targeted DPn values were low enough (DPn 15-30) to allow the formation of soluble oligomers to be easily characterized with SEC in DMAc and with 19F NMR spectroscopy (deuterated DMF was used as the solvent). The former technique enabled us to determine the relative molar masses and their polydispersities, whereas the latter one allowed us to assess the conversions of both the monomer and CTA, DPn, and the nature of the end groups.
ITP of VDF in the Presence of C6F13I
The VDF conversion (aVDF) was determined by 19F NMR; we took into account the signals centered at 83.0, 91.0, 38.0, and 108.0 ppm, which were assigned to CF3 in the C6F13 end group, to CF2 of VDF units in PVDF-I, and to CH2CF2I and CF2CH2I end groups, respectively.46 As the initial amount of VDF (i.e., the quantity of VDF introduced into the autoclave) was known, aVDF could be assessed:
a R R RR R R VDF
CF238:0ppm CF2 91:0ppm CF2108:0ppm CF2 113ppm CF2 116ppm 2 CF3 83:0ppm 3
CTA0 VDF0
1
Journal of Polymer Science: Part A: Polymer Chemistry DOI 10.1002/pola
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IODINE TRANSFER POLYMERIZATION 5767
Scheme 4. Synthesis of PVDF-I oligomers by the ITP of VDF in the presence of different CTAs initiated with TBPPI.
where $CFxippm, [CTA]0, and [VDF]0 represent the integral of the signal assigned to the CFx group centered at i ppm and the concentrations of CTA and VDF at t 0, respectively.
Interestingly, the 19F NMR spectrum shows
only traces of both multiplets centered at 113.0
and 116.0 ppm, which were assigned to CH2 CF2CF2CH2CH2CF2 reversed head-to-head addition.46
The C6F13I conversion (aC6F13I) could also be calculated from 19F NMR by the monitoring of
the decrease in the integral of the signal cen-
tered at 60.0 ppm (characteristic of the CF2I end group in C6F13I) and that of the signal located at 83.0 ppm, which was attributed to the CF3 end group:
R
0 R
0
CF3 83:0ppm 3 CF2 I60:0ppm 2
aC6F13I
R
0
CF3 83:0ppm 3
2
After 10 min at 75 8C (aVDF was ca. 25%) in the presence of TBPPI, C6F13I was totally converted.
Figure 1 presents the evolution of DPn versus aVDF, showing that the higher aVDF was, the higher DPn was. The experimental number-average degree of polymerization (DPn,19F NMR) was assessed with 19F NMR spectroscopy:
DPn;19 F NMR aVDF VDF0=aCTA CTA0 3
where aCTA is the CTA conversion. Interestingly, the experimental DPn linearly increased with the monomer conversion, and this evidenced the controlled character of that reaction [Fig. 1(a)].4 This figure clearly demonstrates the controlled character of the ITP of VDF, and
Journal of Polymer Science: Part A: Polymer Chemistry DOI 10.1002/pola
this is the first time that this behavior has been reported in the literature. In addition, the experimental values of DPn were close to the theoretical number-average degree of polymerization (DPn,theoretical) values, which were determined as follows:
DPn;theoretical aVDF VDF0=CTA0 4
where [VDF]0 and [CTA]0 represent the initial concentrations of VDF and the CTA (C6F13I in this case), respectively.
The controlled character of the ITP of VDF with C6F13I was also confirmed with SEC chromatograms of sampled PVDF-I. As expected, the SEC signals assigned to PVDF-I were negative. The SEC chromatograms were shifted to lower elution volumes, that is, to higher molar masses [Fig. 2(a)]. This behavior is typical of a controlled character and also corresponds to a high CTr value.49,50
Because it is usually difficult to get absolute values of the molar masses of fluoropolymers (a lack of standards), the structures of the polymers obtained by the ITP of VDF in the presence of C6F13I were further studied with MALDITOF (Fig. 3). Figure 3 exhibits the spectrum of C6F13-(VDF)n-I oligomers and shows a series of peaks separated by 64 Da (corresponding to the molecular weight of one VDF unit). This population was centered at 1193 Da (n 10).
The MALDI-TOF spectrum of the expected C6F13-(VDF)n-I/Ag structure exhibits signals centered at 1001.88(n 7), 1065.90(n 8), 1129.94 (n 9) and others containing an increment of 64 Da. Interestingly, there is an absence of peaks assigned to oligomers such as (CH3)3C-(VDF)nm- C(CH3)3/Ag,(CH3)3C-(VDF)n-I/Ag,CH3-(VDF)n- I/Ag,(CH3)3C-(VDF)nm-CH3/Ag, and CH3- (VDF)nm-CH3/Ag, which could be obtained by direct initiation from a radical arising from the decom-
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5768 BOYER ET AL.
Figure 1. Variations of DPn versus the monomer conversion for the ITP of VDF performed with different CTAs and at two temperatures (targeted DPn 15): (a) at 75 8C with (^) C6F13I and () C6F13CH2CF2I and (b) with HCF2CF2CH2I at (n) 75 and (~) 135 8C. The straight lines represents the theoretical DPn values.
position of TBPPI [CH3 or C(CH3)3].51 These results confirm those found by 19F and 1H NMR characterizations (see Figs. 4 and 5 in the supplementary material).
ITP of VDF in the Presence of C6F13CH2CF2I
This CTA was chosen for checking its reactivity in the ITP of VDF because its CH2CF2I end group mimics a PVDF-I model. It also completes this investigation when it is compared with HCF2CF2CH2I, which also exhibits an isomeric VDF-I end group (reversed unit). It was worth comparing the reactivity of the iodine atom in both these models because the chemical environments are different.
C6F13CH2CF2I was synthesized by the thermal telomerization of VDF with C6F13I at 200 8C for 12 h, and this monoadduct was puri-
fied by distillation, as previously reported.47 As expected, the 19F NMR spectrum shows, besides
the unchanged chemical shift of the fluorinated groups in the CF3(CF2)4 group, the absence of a signal centered at 60.0 ppm characteristic of C5F11CF2I and the presence of that signal centered at 38.0 ppm assigned to the CH2CF2I end group.47,52
As previously stated, in the ITP of VDF in the
presence of HCF2CF2CH2I, aCTA was monitored with 19F NMR on sampled aliquots. Indeed, the chemical shift of CH2CF2I underwent a slight low-field shift from 39.8 to 38.6 ppm [Fig. 4(a)]. Interestingly, the 19F NMR spectrum exhibits the chemical shifts of the CH2CF2I end groups of the monoadduct, diadduct, triadduct, and higher adducts centered at 39.8, 39.1, 38.7 and 38.6 ppm, respectively. Incidentally, the quintet observed for the peak assigned to CH2CF2I in
Journal of Polymer Science: Part A: Polymer Chemistry DOI 10.1002/pola
Figure 2. SEC chromatograms for various ITPs of VDF with different CTAs (the percentages are the aVDF values): (a) PVDF-I obtained from the polymerization of VDF with C6F13I initiated with TBPPI at 75 8C (experimental condition for the ITP of VDF: [VDF]0/[C6F13I]0/[TBPPI]0 100.00:6.60:0.66), (b) PVDF-I obtained from the polymerization of VDF with HCF2CF2CH2I initiated with TBPPI at 75 8C (experimental condition for the ITP of VDF: [VDF]0/ [HCF2CF2CH2I]0/[TBPPI]0 100.00:6.60:0.66), and (c) PVDF-I obtained from the polymerization of VDF with HCF2CF2CH2I initiated with DHPB at 135 8C (experimental condition for the ITP of VDF: [VDF]0/ [HCF2CF2CH2I]0/[DHPB]0 100.00:6.60:0.66).
the monoadduct arose from the same coupling constants (3JHF 4JFF 10.0 Hz), in contrast to the triplets (3JHF 15.0 Hz) of triplets (4JFF
Journal of Polymer Science: Part A: Polymer Chemistry DOI 10.1002/pola
IODINE TRANSFER POLYMERIZATION 5769
13.0 Hz)47,52 of the same signal in the diadduct and the triadduct. This difference can be explained by the electronegativity effect of the CTA (i.e., C6F13I) on the end group (CF2I). Figure 4(b) also presents the 19F NMR spectrum in these PVDF-I polymers after the irradiation of the fluorine atoms in the CF2I group, and the series of triplets (3JHF 15.0 Hz) confirms these assignments.
Thus, it was possible to monitor the CTA consumption versus time by the shift of the peak corresponding to signals of CH2CF2I end groups centered at 40 ppm (C6F13CH2CF2I) to 38 ppm [C6F13(CH2CF2)nI]. The CTA was quickly consumed.
As discussed previously, the experimental DPn,19F NMR values linearly increased with aVDF [Fig. 1(a)] and were close to the theoretical ones. Actually, the evolutions of DPn versus aVDF in the ITPs of VDF achieved in the presence of both C6F13I and C6F13CH2CF2I are similar [Fig. 1(a)]. This shows that the reactivities of these two CTAs are close. Hence, in the course of transfer reactions, chains terminated by a CH2CF2I end group are involved in a controlled radical polymerization.
As in the case of C6F13I, the SEC chromatograms gave a shift of the oligomeric distribution toward the lower elution times (i.e., toward an increase in DPn or in the molar mass) versus the monomer conversions, in agreement with the results of 19F NMR. This statement confirms the controlled character of the ITP reaction performed in the presence of C6F13CH2CF2I. The PDI values were close to 1.2 even after 50 min (aVDF & 95%, aCTA 1) at 75 8C.
Figure 3. MALDI-TOF mass spectrum of low-molecular-weight C6F13-(VDF)n-I (DPn,19F NMR 12; experimental condition for the ITP of VDF: [VDF]0/ [C6F13I]0/[TBPPI]0 100.00:6.60:0.66). The analysis was performed in the reflectron mode.
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5770 BOYER ET AL.
Figure 4. Expansion of the 38 to 40 ppm zone in the 19F NMR spectra of the oligomer obtained by the ITP of VDF with the CTA C6F13CH2CF2I at 75 8C (400 MHz, 298 K, in deuterated DMSO): (a) after the irradiation of the proton in CH2CF2I and (b) without any irradiation of the proton.
Furthermore, MALDI-TOF analysis was performed on a sample obtained by the ITP of VDF with this CTA. The result was similar to that involving C6F13I as the CTA. We have noted the absence of the peaks assigned to oligomers obtained by direct initiation.
ITP of VDF in the Presence of HCF2CF2CH2I
Two ITPs of VDF were achieved at 75 and 135 8C, initiated with TBPPI and DHBP, respectively. The temperatures of these ITPs were chosen because these radical initiators exhibit halflives of 1 h. The conversion of HCF2CF2CH2I was monitored by 19F NMR, and we took into account the integral of the signal centered at 136.0 ppm and attributed to the HCF2 a group of the polymer, which underwent a slight high-field shift to 138.0 ppm of the CTA (see Fig. 7 in the supplementary material).
In contrast to both cases previously described, the HCF2CF2CH2I consumption versus aVDF (Fig. 6 in the supplementary material) was much slower and may have evidenced a lower value of CTr than those of both CTAs bearing an
x-CF2I group. However, an increase in the temperature slightly increased aCTA. This statement was confirmed by the plotting of the evolution of DPn,19F NMR values versus aVDF at 75 and 135 8C. The experimental DPn values of both these experiments were far from the theoretical ones and could indicate that CTr of HCF2CF2CH2I was lower than 1 [Fig. 1(b)]. Moreover, at high monomer conversions, the experimental DPn values obtained at 75 and 135 8C were close.
Figure 2(b,c) presents the SEC chromatograms
of oligomers produced from the ITPs of VDF car-
ried out in the presence of HCF2CF2CH2I at 75 and 135 8C, respectively. The oligomeric distributions did not shift toward higher molar masses
versus the monomer conversion, in contrast to
reactions carried out in the presence of the other
CTAs (C6F13I and C6F13CH2CF2I). This result confirmed the poor control of ITP with HCF2 CF2CH2I and could be correlated to Litvinenko and Mueller's theory.49 Indeed, the PDI values
were close to 2.0 (with polystyrene calibration)
and confirmed the poor control of this radical po-
lymerization with this CTA.
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IODINE TRANSFER POLYMERIZATION 5771
This result was confirmed by MALDI-TOF. In contrast to the previous spectra, a Gaussian distribution was not observed (Fig. 5), and the spectrum of HCF2CF2CH2-(VDF)n-I oligomers (Fig. 5) shows a series of peaks separated by 64 Da, which corresponds to the molecular weight of the VDF monomer unit. The expansion of the zone ranging from 1200 to 1264 Da exhibits three different series (A-C) that can be distinguished (Fig. 5).
The expected HCF2CF2CH2-(VDF)15-I/Na structure (series B, Fig. 5) appears at 1225.28. This structure was confirmed by 19F NMR analysis. Similar structures stem from the following equation: 1225.28 m 64 Da (with m ranging from 1 to 7). Indeed, in the ITP reaction, the polymerization of VDF can be initiated by the radicals obtained from the decomposition of TBPPI [CH3 or C(CH3)3].51 The distribution at 1205.27 y 64 Da (with y ranging from 0 to 7; series A, Fig. 5) could be ascribed to the coupling of CH3-(VDF)n with another CH3- (VDF)m radical to give CH3-(VDF)nm-CH3/Na, with n m 18. This hypothesis was confirmed by 1H NMR spectra (see Figs. 7 and 8 in the supplementary material), in which the characteristic signals centered at 1.0, 1.2, 1.3, and 1.8 ppm were assigned to (CH3)3CCH2, (CH3)3CCF2, CH3CH2, and CH3CF2 groups, respectively.51 The (CH3)3C-(VDF)nm- C(CH3)3/Na structures gave signals at 1225.28 and 1289.29 Da. These distributions overlapped that of the HCF2CF2CH2-(VDF)n-I/Na. The distribution at 1245.28 Da (series C, Fig. 5) could be attributed to the (CH3)3C-(VDF)nm- CH3/Nastructure, which was generated by the coupling of CH3-(VDF)n with CH3-(VDF)m radicals. Hence, there is a competition between the transfer reaction and the direct initiation. Thus, MALDI-TOF spectroscopy confirmed the poor efficiency of HCF2CF2CH2-I as the CTA in the ITP of VDF.
Assessment of the CTr Values of Iodofluorinated CTAs and Cex Values of PVDF-I
The literature reports several methods for assessing CTr values, such as those developed by Mayo,53 O'Brien and Gornick,54 David and Gosselain,55 Bauduin et al.,56 and Maeder and Gilbert,57 but none of them took the living character into account because they were applied only for telomerization.58 For controlled radical polymerizations such as ITP or RAFT, this becomes more complex
Journal of Polymer Science: Part A: Polymer Chemistry DOI 10.1002/pola
Figure 5. MALDI-TOF mass spectrum of HCF2CF2CH2-(VDF)n-I (DPn 32, as assessed by 19F NMR; experimental condition for the ITP of VDF at 75 8C: [VDF]0/[HCF2CF2CH2I]0/[TBPPI]0 100.00:6.60:0.66; targeted DPn 15). The analysis was performed in the reflectron mode. The A series corresponds to the CH3-(VDF)nm-CH3/Na structure. The B series corresponds to the HCF2CF2CH2-(VDF)15-I/Na and (CH3)3C-(VDF)nm-C(CH3)3/Na structures. The C series corresponds to the (CH3)3C-(VDF)nm-CH3/ Nastructure.
because the dormant chains also act as transfer agents (Scheme 3). In the ITP mechanism, two different transfer reactions can occur, and we must well differentiate these two reactions:
1. The first one is based on the CTA itself and is defined by CTr (CTr ktr/kp). CTr is actually ascribed to the activity of the iodinated transfer agent, and thus it determines the evolution of DPn versus the monomer conversion (see Fig. 10 in the supplementary material). There are two cases: if CTr is very low (CTr < 1), the CTA consumption is slow, and the experimental molar masses will start high (and with very different values from the targeted ones) and decrease versus the monomer conversion. On the contrary, if CTr is high (CTr > 1), the experimental molar masses will increase almost linearly versus the monomer conversion and will be close to the targeted values.
2. The second one occurs between two polymeric chains (degenerative transfer) and is defined by Cex (Cex kex/kp). Unlike CTr, Cex is characteristic of the degenerative transfer, that is, the evolution of the molecular weight distribution versus the monomer conversion. It is noteworthy that an increase in Cex gives a lower PDI of the polymeric chains (see Fig. 11 in the supplementary material).
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5772 BOYER ET AL.
Determination of CTr
In this work, O'Brien and Gornick's method54 was used to determine the CTr constant. This method was previously used by Chong et al.12 to determine the high CTr value of a dithiobenzoate used in the RAFT polymerization of styrene and methyl methacrylate (MMA). Actually, this kind of controlled radical polymerization requires a transfer step as in the case of ITP. These authors monitored both the CTA and monomer consumptions, and plotting ln [CTA] versus ln [VDF] led to a straight line, the slope of which gave CTr (eqs 5 and 6, the demonstration being given in the supplementary material):
Rp=VDF kp 1
5
Rtr=CTA ktr CTr
where [VDF] and [CTA] represent the monomer and transfer agent concentrations, respectively, and Rp and Rtr stand for the propagation and transfer rates, respectively.
After integration, eq 5 gives
lnCTA0=CTA CTr lnVDF0=VDF 6
This method was successfully used for this
investigation, and monitoring the ITP of VDF
with three CTAs (Fig. 6) led to different values
of CTr (Table 1). The CTr values of C6F13I and of C6F13CH2CF2I were rather close (7.9 and 7.4 at 75 8C, respectively), whereas that of HCF2 CF2CH2I was much lower (0.3 and 0.4 at 75 and 135 8C, respectively, Table 1). Thus, the CTr value of a CTA bearing a CF2I end group (CTr * 7.7 at 75 8C) was about 20 times higher than that of a CTA terminated by CF2CH2I (CTr * 0.3 and 0.4 at 75 and 135 8C, respectively).
An alternative method for assessing CTr is based on the evolution of DPn versus the monomer conversion (eq 7), as suggested by Mueller and coworkers.49,59,60 This method arises from the Bauduin et al.'s law56 established in the
1980s for the telomerization process. Litvinenko and Mueller49 demonstrated that eq 7 can be
used to assess the CTr value in the case of degenerative transfer polymerizations, whatever
the polymerizations (i.e., anionic, cationic, group
transfer, and radical polymerization):
VDF0 CTA
aVDF
DPn %
0
7
1
1
P CTA
1 a CTr VDF
0
where [P8] represents the concentration of macroradicals at a given time.
Figure 6. Plots of ln [CTA] versus ln [VDF] for the ITP of VDF at 75 8C initiated with TBPPI in the presence of three different CTAs: (a) C6F13I, (b) C6F13CH2CF2I, and (c) HCF2CF2CH2I (experimental condition for the ITP of VDF: [VDF]0/[CTA]0/[TBPPI]0 100.00:6.60:0.66). The CTr values were evaluated with the method of O'Brien and Gornick.54
In the case of radical polymerization, eq 7 can
be simplified to obtain Bauduin's equation (eq 8,
the demonstration being given in the supple-
mentary material). Indeed, in radical polymerization, the concentration of the radicals, [P8], is negligible (typically < 106 mol L1), and thus the value of 1 ([P8]/[CTA]0) is close to 1:
VDF0 CTA
aVDF
DPn %
0
C
8
1 1 aVDF Tr
The CTr values of the CTAs were obtained by the fitting of the experimental curves with eq 8. A comparison of the theoretical and experimental curves is given in the supplementary material. A good agreement was obtained between
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IODINE TRANSFER POLYMERIZATION 5773
Table 1. Assessments of the CTr Values of CTAs for the ITP of VDFa
Temperature
CTA
(8C)
CTr
Cex
C6F13I
75
7.9b (7.5c)
--
C6F13CH2CF2I
75
7.4b,c
7.4 (PVDF-CH2CF2I)
HCF2CF2CH2I
75
0.3b,c
0.3 (PVDF-CF2CH2I)
HCF2CF2CH2I
135
0.4b,c
--
a Experimental condition: [VDF]0/[CTA]0/[TBPPI]0 100.00:6.60:0.66. b According to O'Brien and Gornick's method.54 c According to Litvinenko and Mueller's method.49
the CTr values determined by O'Brien and Gornick's method54 and the Litvinenko and Mueller's method49 (Table 1).
Determination of Cex of PVDF-I
The Cex value can be determined by the polymerization being started from a macrotransfer agent with the same structure as that of the dormant polymeric chains because, in this case, CTr is equal to Cex. Fukuda and coworkers48,61-63 used O'Brien and Gornick's method54 to monitor the consumption of the macrotransfer agent, that is, polystyrene oligomers with an iodine end atom (PS-I), by SEC chromatography versus monomer conversion, and they assessed the Cex value. For instance, this method was used to determine Cex of PS-I (Cex 3.4 in the polymerization of styrene48). Boutevin and coworkers64,65 used a poly (methyl acrylate)-I macrotransfer agent in the polymerization of methyl acrylate and applied Mayo's method to assess Cex 2.2 for poly(methyl acrylate)-I at 70 8C.64 The same method was used to assess Cex 2.6 for poly(methyl methacrylate)-I at 80 8C.65
In this case, because C6F13CH2CF2I mimics the PVDF-I dormant chains bearing a normal terminal VDF unit, the value of CTr for this CTA allows us to approximate Cex of PVDF- CH2CF2-I (normal dormant species). Thus, Cex for normal PVDF-I is approximately 7.4 at 75 8C.
Moreover, because HCF2CF2CH2I mimics the PVDF-I dormant chain bearing a reversed terminal VDF unit, the CTr value of this CTA allows us to assess Cex of PVDF-CF2CH2I (reversed dormant chains): thus, Cex for reversed PVDF-I is approximately 0.3 at 75 8C. To the best of our knowledge, this is the first time that Cex values have been determined for the ITP of VDF.
Journal of Polymer Science: Part A: Polymer Chemistry DOI 10.1002/pola
This study also enables us to understand why oligomers of low DPn values contain only a CH2I end group. Indeed, in the course of the ITP of VDF, two macromolecular transfer agents have been produced (bearing either CF2I or CH2I end groups), and they do not exhibit the same reactivity. Hence, when the reaction medium contains macromolecular chains terminated by CF2I, the polymerization almost exclusively occurs on these end groups rather than on the poorly reactive CH2I end groups.
Mechanism of ITP of VDF
Among the growing PVDF-I chains in the reaction medium, the presence of two types of end groups (CF2I and CH2I) can be observed. To confirm and check the different reactivities of these structures and also to investigate the competition between them, the ITP of VDF was carried out in the presence of stoichiometric amounts of C6F13I and HC2F4CH2I as the CTAs. Methyl ethyl ketone was chosen as the solvent (which allowed us to preserve the solubility of high-molar-mass PVDF, thus leading to a homogeneous mixture), and it was initiated with TBPPI. As previously stated, both aCTA and aVDF were monitored by 19F NMR. Figure 7 presents the conversion versus time for several species present in the reaction medium, allowing us to understand better the mechanism of the ITP of VDF when both CTAs and oligomers possess CF2I and CH2I end groups.
Figure 7 shows that the ITP of VDF occurs according to three steps:
1. At the beginning of the reaction, C6F13I was quickly consumed, whereas the concentration of HC2F4CH2I remained constant. This behavior confirms the high reactivity of C6F13I (CTr 7.9 at 75 8C) compared with that of HC2F4CH2I (CTr 0.3 at 75 8C).
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5774 BOYER ET AL.
Figure 7. Evolution of the normalized concentrations of CTAs C6F13I and HC2F4CH2I and oligomers bearing CH2I and CF2I end groups versus time for the ITP of VDF initiated with TBPPI at 75 8C. The normalized concentration represents the ratio of the concentrations at different times to the concentration at the initial time.
2. In a second stage, after the consumption of C6F13I, the transfer reaction took place on the oligomers bearing a CH2CF2I end group, whereas HC2F4CH2I still did not react significantly. Because of the lower reactivity of the reversed dormant chains (i.e., PVDF- CF2CH2I), the number of CH2CF2I end groups decreased versus time, whereas the concentration of PVDF-CF2CH2I increased. This second step lasted until the DPn value was about 40, at which the concentration of the CH2CF2I end groups vanished, whereas that of the CF2CH2I end groups reached its maximum.
3. In a last stage, because of the low CTr value of HCF2CF2CH2I, only a small number of new polymeric chains were created (small effect on DPn). However, the low Cex value of the PVDF-CF2CH2I dormant chains now led to a slow exchange between active and dormant chains, and therefore the molecular weight distribution became poorly controlled (increase in PDI) (Figure 12 of the supplementary material).
These results confirm the difference in the reactivities of the different CTAs. Thus, it is possible to classify the reactivities of the end groups in the following decreasing order: CF2CF2I > CH2CF2I >> CF2CH2I.
In conclusion, the ITP of VDF occurs according to two stages. In the first stage, when CF2I end-group species are still present in the reaction medium, the polymerization is well controlled. This behavior corresponds to a conventional ITP mechanism. On the contrary, in the second stage, when CF2I end-group species are almost totally consumed, the CH2I endgroup species can still transfer, but the exchange between active and dormant chains becomes slower than propagation (Cex < 1). Thus, the polymerization is no longer controlled, and this second behavior is closer to that of a telomerization mechanism. However, in the second stage of the polymerization, because CF2I end groups can be created again by a new inversion, the controlled character of the polymerization is preserved to some extent.
Assessment of kp/Hkte for the VDF for ITP
The evolution of ln([VDF]0/[VDF]) versus time
(Fig. 8) is linear and shows that the macromolec-
ular radical concentration, [P8], is constant. The
slope of the straight line led to the assessment of kp [P8] 5 104 s1 at 75 8C (Fig. 8).
In radical polymerization, kp/Hkte usually rep-
resents the reactivity of the monomer. The over-
all rate of polymerization depends on the kinetic
constant: k kp/Hkte. Such a value can be assessed with Tobolsky's equation:66,67
sffiffiffiffiffiqffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ln M0 2 pkffiffipffiffiffiffi f TBPPI 1 expkd t=2
M
kte kd
0
9
where f, kd, and t represent the efficiency, the decomposition rate coefficient of the initiator,
Figure 8. Plots of ln([M]0/[M]) versus time for the ITP of VDF initiated with TBPPI in the presence of HCF2 CF2CH2I at 75 8C (for the assessment of the apparent propagation rate constant, kpapp kp[P8] 5.104 s1; experimental condition for the ITP of VDF: [VDF]0/ [HCF2CF2CH2I]0/[TBPPI]0 100.00:6.60:0.66).
Journal of Polymer Science: Part A: Polymer Chemistry DOI 10.1002/pola
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IODINE TRANSFER POLYMERIZATION 5775
and time, respectively (the other symbols being defined previously).
Thus, plotting ln([M]0/[M]) versus 1 exp[(kdt)/2] enabled us to assess the kp/Hkte value from the slope of the obtained straight line. Taking into account f 0.9, kd 2.79 104 s1,68 and [TBPPI]0 2.6 102 mol L1, we find a value of kp2/kte 1.9 102 L mol1 s1 at 75 8C. To the best of our knowledge, this is the first value for the radical polymerization of VDF supplied in the literature.
Taking into account k2p/kte values, we can consider VDF a reactive monomer in comparison with other usual monomers, such as styrene (k2p/ kte 2.23 103 L mol1 s1 at 70 8C)69-71 or MMA (k2p/kte 3.1 102 L mol1 s1 at 80 8C).72
CONCLUSIONS
This work highlights the existence of two different mechanisms for the ITP of VDF. Hence, the kinetics of this reaction were determined in the presence of three different transfer agents. The reactivities of C6F13I and C6F13CH2CF2I were rather close, and these CTAs were quickly and totally consumed at 75 8C. In addition, the evolutions of DPn versus the monomer conversion were similar and linear. Thus, the controlled character of the radical polymerization of VDF in the presence of these transfer agents was proved. However, reverse additions of the macroradical onto VDF were also observed. This reaction was evidenced by the presence of CH2-I end groups in PVDF-I, leading to an unfavorable ITP process. Indeed, in the case of the ITP of VDF in the presence of the HCF2CF2CH2I transfer agent, the DPn evolution versus aVDF was similar to that of a telomerization. This statement was also confirmed by a low CTr value of HCF2CF2CH2I (<1), which allowed nevertheless the polymeric chains to preserve a controlled character in the course of the polymerization. Furthermore, with O'Brien and Gornick's method, the CTr values of these three CTAs were assessed. C6F13I and C6F13CH2CF2I had similar CTr values (ca. 7.7 at 75 8C) whereas that of HCF2CF2CH2I was much lower (0.3 or 0.4 at 75 or 135 8C, respectively). Cex of PVDF-CH2CF2I dormant chains was estimated to be Cex & CTr (C6F13CH2CF2I) 7.4 at 75 8C, whereas Cex of PVDF-CF2CH2I dormant chains was approximated to be Cex & CTr (HCF2CF2CH2I) 0.3 at 75 8C. Having carried out the ITP of VDF in the
Journal of Polymer Science: Part A: Polymer Chemistry DOI 10.1002/pola
presence of CTAs C6F13I and HCF2CF2CH2I, we can propose a general mechanism for the ITP of VDF in two stages. The first stage corresponds to a conventional ITP mechanism in which C6F13I is first mainly consumed, and this is followed by the produced oligomers bearing CF2I end groups. The second stage occurs once the concentration of CF2I species becomes negligible, and the polymerization then mainly proceeds as a telomerization, in which species bearing CH2I end groups behave as transfer agents. Finally, k2p/kte of VDF was determined to be 1.9 102 L mol1 s1 at 75 8C. Further investigations of the synthesis of block copolymers from these PVDF-Is prepared by ITP are in progress.
The authors thank Solvay S.A. Co. (Brussels, Belgium, and Tavaux, France) for free samples of VDF and 1,1,1,3,3-pentafluorobutane, Akzo Nobel (Cha^lons sur Marne, France) for tert-butylperoxypivalate, Gilles Valette (University of Montpellier II) for matrix-assisted laser desorption/ionization time-of-flight analyses, and Alain Fruchier (the head of NMR for Ecole Nationale Superieure de Chimie de Montpellier) for high-resolution NMR characterizations.
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