Iodine Transfer Radical Polymerizations of Vinylidene Fluoride in

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Chapter 15

Iodine Transfer Radical Polymerizations of Vinylidene Fluoride in Supercritical Carbon Dioxide and Polymer Functionalization via Click Chemistry Muhammad Imran-ul-haq, Nadja Förster, Radovan Vukicevic, Kristin Herrmann, Rebekka Siegmann, Sabine Beuermann* Institute of Chemistry, University of Potsdam, Karl-Liebknecht-Str. 24-25, D-14476 Golm, Germany,

Vinylidene fluoride polymerizations were carried out in solution with supercritical carbon dioxide using mono and diiodo perfluorinated hexane as chain transfer agents. Characteristics of controlled radical polymerizations were observed leading to polymers with low polydispersities in the range from 1.2 to 1.4. Polymers with iodine end groups were subsequently functionalized with sodium azide and symmetrical alkynes to yield poly(vinylidene fluoride) with triazol end groups.

© 2009 American Chemical Society

233

In Controlled/Living Radical Polymerization: Progress in RAFT, DT, NMP & OMRP; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.


234 Controlling the molecular structure of polymers is a key issue for modern polymer synthesis, because more and more complex macromolecules are needed in the rapidly growing fields of nanotechnology or nanobiotechnology (i). To fulfill this demand versatile techniques of macromolecular engineering were developped, enabling work at the interface between polymer science and other synthetic fields such as biochemistry or inorganic chemistry (ii-v). The control over polymer functionalities (side groups or chain ends) is essential, since functional groups can be used for performing further modifications such as the reinitiation of polymerizations, creation of supramolecular linkages, conjugation of macromolecules or adsorption of polymers on surfaces. Sharpless and coworkers popularized the 1,3-dipolar cycloaddition of azides and terminal alkynes, catalyzed by copper(I), in organic synthesis (vi). Such reactions were proven to be very practical, because they can be performed in high yield, in multiple solvents (including water), and in the presence of numerous other functional groups (vii). Moreover, the formed 1,2,3-triazol is chemically very stable. Due to their efficiency and simplicity, these cycloadditions were classified as ‘‘click’’ reactions (viii). Combination of chainend functionality control via iodine transfer polymerization (ITP) and the efficiency of click chemistry is an interesting pathway for the synthesis of endfunctional polymers, because iodine chain ends originating from ITP are expected to be easily transformed into azides, and a plethora of functional alkynes is commercially available. Fluorinated polymers are of large interest for technical applications due to their unique properties, e.g., excellent chemical, thermal and mechanical stability as well as electroactivity ( ix ). Frequently, these polymers are synthesized in heterogeneous phase employing fluorinated stabilizers, which have a high potential for bioaccumulation. Supercritical carbon dioxide (scCO2) has emerged as an attractive alternate solvent ( x , xi ). To avoid the use of surfactants homogeneous phase polymerizations of vinylidene fluoride in scCO2 were studied. Homogeneity was established for polymerizations leading to polymers with number average molecular weights, Mn, below 10000 g·mol−1. In case of molecular weight control via initiation up to 10 wt.% of initiator had to be used (xii). To reduce the amount of initiator alternately molecular weights were controlled by chain transfer reactions ( xiii , xiv ). Degenerative transfer polymerizations are particularly interesting since they lead to controlled polymerization conditions giving access, e.g., to block copolymers. In this contribution the syntheses of poly(vinylidene fluoride), PVDF, with iodine end groups in solution with supercritical CO2 and subsequent functionalization of the polymer by click reactions is reported.

Experimental Materials All reagents were used without further purification: the monomer vinylidene fluoride (VDF, Solvay Solexis), carbon dioxide (CO2, grade 4.5,


235 Messer Griesheim), perfluorinated hexyl iodide (C6F13I, Dyneon), diiodoperfluorohexane (IC6F12I, Dyneon), di-tert butyl peroxide (DTBP, Akzo Nobel), N,N-dimethyl acetamide (DMAc, 99 % pure, Acros) and LiBr (Sigma-Aldrich, 99 %).


Polymer Characterization Size-exclusion chromatography (SEC) of the polymers was carried out with DMAc containing 0.1 % LiBr as eluent and a column temperature of 45 °C. The samples were analyzed on a SEC set-up consisting of an Agilent 1200 isocratic pump, an Agilent 1200 refractive index detector, and two GRAM columns (10µm, 8 x 300mm, pore sizes 100 and 1000) from Polymer Standards Services. The SEC set-up was calibrated using low polydispersity polystyrene standards (PSS), since PVDF standards were not available. Polymer end groups were determined via NMR spectroscopy (Bruker, 300 MHz), Fourier Transform Infrared (FTIR, Vertex 70, Bruker) spectroscopy and electrospray ionization mass spectrometry (ESI-MS, Micromass Manchester, UK). NMR spectra were measured at room temperature using DMSO-d6 or acetone-d6 as solvent and TMS as an internal reference. Iodine Transfer Polymerizations of VDF using C6F13I and IC6F12I VDF was polymerized in solution with around 73 wt.% CO2 at 120 °C and 1500 bar with 0.06 mol·L−1 DTBP in the presence of variant amounts of C6F13I or IC6F12I. Monomer concentrations were around 3.7 mol·L−1 in all cases. The reactions were carried out in optical high-pressure cells allowing for in-line measurement of monomer conversion via Fourier Transform Near Infrared (FTNIR) spectroscopy (Vertex 70, Bruker). Details of the experimental set-up and preparation of the reactions mixtures are given in refs. xii and xiv. General Procedure for the Functionalization of PVDF with Iodine End Groups PVDF synthesized using perfluorinated hexyl iodide as chain transfer (Mn = 2040 g·mol–1, 200 mg, 0.10 mmol), sodium azide (200 mg, 3.07 mmol), symmetrically alkyl substituted alkyne (2-butyne, 0.5 ml, 6.4 mmol), and 15 ml of DMF were added in a flask. The flask was connected with a reflux condenser and the reaction mixture was stirred for 72 hours at 90 °C. As the reaction proceeds the colour of the solution changes from transparent to brown. Functionalized poly(vinylidene fluoride) was precipitated in water, filtered and washed with diethyl ether to remove the unreacted organic reactants and side products. The final product was dried under vacuum. The polymers were analyzed by 1H-NMR and FTIR spectroscopy.


236


Results and Discussion Based on previous VDF polymerizations, in which molecular weights were controlled by initiation, polymerizations in the presence of C6F13I or IC6F12I were carried out at 120 °C and 1500 bar in solution with around 70 wt.% of CO2. FT-NIR spectra recorded during the polymerization are depicted in Figure 1. The peak at 6303 cm−1 is assigned to the CH-stretching vibration at the double bond of the monomer. As conversion increases with reaction time the intensity of the peak decreases until the peak disappears completely. The two small peaks at 6214 and 6332 cm−1 referring to the absorbances of CO2 remain unchanged. The spectra in Figure 1 indicate that the reaction mixture stayed homogeneous until complete monomer conversion was reached.

Figure 1: FT-NIR spectra series recorded during a polymerization at 120 °C and 1500 bar in the presence of 0.21 mol·L−1 C6F13I as degenerative chain transfer agent. For end group analyses of the polymers 1H-NMR and ESI-MS spectra were recorded. As detailed in ref. xiii predominantly chains are seen that were initiated by a C6F13-group and terminated by transfer of an iodine atom. The finding is in accordance with a controlled radical polymerization (xv): C6F13-(VDF)n. + C6F13-(VDF)m–I

C6F13-(VDF)n–I + C6F13-(VDF)m .

Molecular weight analyses of polymers obtained from polymerizations with 0.2 mol·L−1 C6F13I resulted in a number average molecular weight of Mn = 1800 g·mol−1 and a polydispersity of 1.2. According to eq (1) an Mn of 1600 g·mol−1 is calculated, which is in good agreement with the experimental value. Mn = [VDF]/[C6F13I]·x·M(VDF) + M(C6F13I),

(1)



237 with VDF concentration, [VDF], C6F13I concentration, [C6F13I], monomer conversion, x, and the molar masses of VDF, M(VDF), and C6F13I, M(C6F13I). To test for livingness VDF polymerizations with a fixed amount of C6F13I were stopped at different conversions. Since the VDF polymerizations are rather fast and the chain transfer agent contributes to the initiation (xiv), a comparably low C6F13I concentration of 0.081 mol·L−1 leading to polydispersities of 1.4 had to be chosen. At higher concentrations of C6F13I and associated lower polydispersities stopping the reaction at rather low conversions was not possible. The molecular weight distributions (MWDs) are shown in Figure 3. As expected for living radical polymerizations the MWDs are shifted in direction of higher molecular weights with increasing conversion. The MWDs given in Figure 2 refer to 23, 54 and 86 % of VDF conversion. Considering that the SEC was calibrated against polystyrene standards, the Mn values of 1800, 2900, and 4700 g·mol−1 are in reasonable agreement with the theoretical values of 1100, 2000, and 3000 g·mol−1, respectively. It is remarkable to note that controlled radical polymerization conditions are not restricted to low or intermediate conversions.

Figure 2: Molecular weight distributions of PVDF obtained from polymerizations at 120 °C and 1500 bar in around 70 wt.% CO2 with 0.081 mol·L−1 C6F13I stopped at different reaction times. For VDF polymerizations with 0.2 mol·L−1 C6F13I up to complete monomer conversion polydispersities as low as 1.2 were obtained. This is remarkable, because for various systems employing degenerative transfer agents, e. g., using dithiobenzoates, control of molecular weight decreases with conversion, as indicated by higher polydispersities (xvi). One reason for the livingness up to very high conversion may be seen in the high pressure used. Due to high pressure the diffusion controlled rate coefficient of the termination reaction is significantly reduced, which is reflected by typical activation volumes ΔV# of 20 cm3mol−1 for termination rate coefficients ( xvii ). Thus, unfavorable bimolecular termination reactions associated with a loss of control are suppressed. Previously it was shown that high pressure may significantly increase the conversion range in which reversible addition fragmentation transfer polymerizations led to low polydispersity (~1.1) polymer material (xvi).


238


Just recently, the favorable influence of high pressure on AGET (activators generated by electron transfer) atom transfer radical polymerizations was reported (xviii). In addition to C6F13I the corresponding diiodo compound, IC6F12I, was used for molecular weight control. Polymerizations were also carried out at 120 °C, 1500 bar and with 70 wt.% CO2. The experimental details and results are listed in Table 1. Table 1: Experimental details and results for VDF polymerizations at 120 °C and 1500 bar with 0.06 mol·L−1 DTBP and the indicated amounts of IC6F12I as chain transfer agent. exp.

c(IC6F12I)/ mmol·L−1

c(VDF)/ mol·L−1

c(CO2)/ wt.%

t/ min

x/ %

Mn / g·mol−1

PDI

1

14

3.7

76

54

47

13200

1.4

2

29

3.6

76

18

48

12900

1.2

3

29

3.6

76

42

87

29500

1.3

4

63

3.6

74

41

82

7600

1.2

5

64

3.6

74

185

96

9100

1.4

6

80

3.6

74

60

99

6100

1.2

7

79

3.6

74

109

82

11200

1.4

8

235

3.5

68

114

97

2650

1.1

Table 1 shows that again polymers with very narrow MWDs were obtained. As for C6F13I as chain transfer agent at high conversions low PDI values were obtained. For example, polymerizations up to conversions of more than 90 % lead to polymer material with PDI values below 1.2. It is interesting to note that rather high molecular weights of Mn > 10000 g·mol−1 were accessible. In case of experiment 3 the reaction mixture showed two phases at the end of the reaction. Still a low polydispersity of 1.3 was obtained.



239

Figure 3: Increase of Mn with conversion for VDF polymerizations at 120 °C, 1500 bar and IC6F12I concentrations as indicated. PVDF with C6F13I-derived end groups, Mn = 2040 g·mol−1 and Mw/Mn = 1.3 was used for functionalization. Due to the rather labile C−I bond it was expected that these polymers can be transformed into various triazol functional end groups with alkyl substituents (methyl, ethyl and propyl) by a one-pot reaction: substitution of the terminal iodine atom by an azide function as intermediate and subsequent 1,3-dipolar cycloaddition of the terminal azide and functional alkynes (methyl, ethyl and propyl). The scheme for the functionalization with 2butyne is shown in Scheme 1.

Scheme 1: Synthesis of end functionalized PVDF. The so-called ‘‘click’’ cycloaddition ( xix ) was performed without any catalyst because of the symmetric nature of the alkynes employed. The polymer end groups were determined by 1H-NMR, FT-IR spectroscopy and by ESI-MS analyses. For functionalization of PVDF with 2-butyne, 3-hexyne, and 4-octyne polymers with number average molecular weights ranging from 1500 to 2700 g·mol−1 were used. The reaction temperature was always 90 °C. After carrying out the first experiments with 2-butyne and 3-hexyne for 72 hours, for reactions with 1-octyne it was tested whether the reaction time may be lowered. It turned out that already after 24 hours the majority of iodine end groups is transformed


240 to triazol end groups. Further, it was found that variations in Mn do not affect the functionalization.


End Group Analyses A poly(vinylidene fluoride) sample with iodine end groups from polymerization in scCO2 was purified and analyzed by 1H-NMR. The spectrum (A) is given in lower part of Figure 4. The two signals centered at 3.62 and 3.87 ppm (a, b) are due to methylene protons of the terminal VDF unit carrying an iodine atom as end group either attached to CF2 or to CH2, respectively. (xv). The signals from 2.6 to 3.2 ppm are due to the methylene protons present in the polymer backbone (d). The peak at 3.25 ppm corresponds to protons of the methylene group directly attached to CF3(CF2)5 (c). For a detailed description of the peak assignments and the influence of head to tail, head to head, and tail to tail addition on the polymer microstructure the reader is referred to the original work by Boyer et al. (xv). The 1H-NMR spectra do not show any indication of initiator-derived end groups, which was confirmed by ESI-MS analysis (xiii). To perform a 1,3-dipolar azide/alkyne cycloaddition at the iodide-chain end, the iodide functional PVDF was transformed into an azide functional polymer by nucleophilic substitution. The obtained azide functional PVDF was subsequently involved in ‘‘click’’ reactions with three symmetric alkynes to prepare PVDF with triazol end groups. Typically, in the absence of an appropriate catalyst, the reaction between azides and terminal alkynes is quite slow, because these alkynes are poor 1,3-dipole acceptors. The upper spectrum in Figure 4 was recorded for PVDF after reaction between the azide group and a methyl substituted alkyne. After the click reaction, in all cases the signals assigned to the methylene protons in the terminal VDF unit were shifted to slightly higher field at 3.90 and 4.08 ppm (f, g). When 2-butyne is used for the cycloaddition the methyl group directly attached to the triazol ring should give rise to a peak between 2.5 and 3 ppm (h). Using 3-hexyne as alkyne the methyl protons occur at 1.14 ppm and in case of 4-octyne at 0.96 ppm. A:

C6F13

c CH2 CF2

a d CH2 CF2 n CH2 CF2

I

b CF2 CH2 I

h

f, g B:

CH2 CF2 n CH2 CF2

CH3

N N

N

CH3


241


h

B

acetone d6

f/g

b

a

c

d

A δ / ppm 1

Figure 4: H-NMR spectrum of purified poly(vinylidene fluoride) with iodine end groups (A) and a triazol end group (B) obtained after functionalization with NaN3 and 2-butyne. Spectra were recorded at room temperature in acetone-d6. The existence of triazol end groups was confirmed by electrosprayionization mass spectrometry. Characteristic peaks referring to a VDF chain with a C6F13 end group originating from the chain transfer agent and the second end group being a triazol group with two methyl substituents were found at e.g. m/z of 608.1, 672.1 and 736.1. Contributions from species with iodine end groups were negligible. Further, FTIR spectra of PVDF with iodine end groups show a characteristic peak at 613 cm−1. After transformation to an azide the peak disappears and an absorbance at 2112 cm−1 assigned to the azide is observed. After formation of the triazol a peak at 1654 cm−1 occurs, which may be assigned to C=C and C=N of the triazol ring (xx). Molecular weight analyses Molecular weight distributions of the original PVDF material with iodine end groups and of the final material after performing the “click” reactions with three different symmetrical alkynes were measured by SEC. The results listed in


242 Table 2 indicate that Mn is slightly enhanced with increasing size of the end groups. Further, Table 2 shows that polydispersities, Mw/Mn, of the material after performing the click reaction are slightly lower than the original material. The lowering in Mw/Mn should be due to additional purification steps.


Table 2: SEC results of the original PVDF sample (PVDF-I) and after performing “click” reactions with the alkynes indicated. alkynes

Mn / g·mol−1

Mw/Mn

PVDF-I

2040

1.4

2-butyne

2200

1.2

3-hexyne

2640

1.2

4-octyne

2800

1.3

Conclusions PVDF obtained from homogeneous phase iodine transfer polymerization in supercritical CO2 with iodide end groups allows for efficient functionalization of the polymer. After substitution of the iodine end group by an azide goup 1,3dipolar cycloadditions with alkynes yield polymers with 1,2,3-triazol end groups. Using symmetrical alkynes the reactions may be carried out in the absence of any catalyst. In future, the work will be extended to functionalization of the end groups with non-symmetric alkynes. With respect to applications, e.g., as liquid crystalline materials, the introduction of mesogenic end groups into the polymer appears to be attractive.

Acknowledgements The authors thank Dyneon GmbH for financial support of this research. Additional support by the Deutsche Forschungsgemeinschaft within the European Graduate School on „Microstructural Contral in Free-Radical Polymerization“ is acknowledged.

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Iodine Transfer Radical Polymerizations of Vinylidene Fluoride in

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