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Synthesis and characterization of a new fluorinated polyether glycol prepared by radical grafting of hexafluoropropylene onto polytetramethylene glycol
EUROPEAN POLYMER JOURNAL
European Polymer Journal 42 (2006) 395–401
www.elsevier.com/locate/europolj
Synthesis and characterization of a new fluorinated polyether glycol prepared by radical grafting of hexafluoropropylene onto polytetramethylene glycol Zhen Ge, Xingyuan Zhang *, Jiabing Dai Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei 230026, PR China Received 21 January 2005; received in revised form 20 July 2005; accepted 8 August 2005 Available online 1 December 2005
Abstract A new fluorinated polyether glycol (PTMG-g-HFP) was prepared by radical grafting of hexafluoropropylene (HFP) onto polytetramethylene glycol (PTMG) in the presence of different initiators. The structure of PTMG-g-HFP was characterized by means of IR, 1H NMR and 13C NMR. The effects of nature and amount of initiator, reaction time and reaction temperature on grafting HFP onto PTMG were investigated. The results showed di-tert-butyl peroxide (DTBP) was the most efficient in the reaction and the optimal reaction conditions were: [DTBP]0/[PTMG]0, 0.12; reaction temperature, 140 °C; reaction time, 6 h. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Fluorinated polyether glycol; Polytetramethylene glycol; Fluoropolymers
1. Introduction It is well-known that fluoropolymers exhibit a series of unique properties, such as high thermal stability, chemical resistance (to acids, bases and solvents), resistance to aging, low dielectric constants and dissipation factors, low water absorptivity and attractive surface properties [1–8]. Therefore, they are widely used in numerous applications such as aerospace, water repellent textile finishing and engineering [9–13], and there is a continuous interest in the synthesis of novel fluoropolymers. Fluorinated *
Corresponding author. Tel./fax: +86 551 3607484. E-mail addresses: [email protected] (Z. Ge), zxym@ustc. edu.cn (X. Zhang).
polyether glycols are particularly valuable fluoropolymers. They can be used as chemical intermediates and react with some compounds containing a plurality of groups reactive towards hydroxyl groups to produce useful macromolecular materials such as elastomers, coatings, foams and thermoplastics [14–18]. Fluorinated polyether glycols are generally prepared by copolymerization of fluorinated vinyl ether alcohols with fluorinated diols [19] or by photoxidative polymerization of tetrafluoroethylene and the subsequent thermal and chemical treatment of resulting product [20]. However, these methods have some limitations. For example, the former reaction must be carried out very slowly under carefully controlled conditions to avoid generating cylic
0014-3057/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2005.08.008
Z. Ge et al. / European Polymer Journal 42 (2006) 395–401
dimer of fluorinated vinyl ether alcohols, and the later has a very complex process containing a fivestep reaction. Free radical additions of fluoro-olefin to various cyclic and acyclic alcohols, diols and their derivatives have been described previously [21–23], but little literature discussed the additions of fluoro-olefin to polyether glycols. In this paper, we report a new synthetic method of fluorinated polyether glycol PTMG-g-HFP by performing a radical way of grafting hexafluoropropylene (HFP) onto polytetramethylene glycol (PTMG). This reaction process is unsophisticated, easily controlled and reaction materials are inexpensive. It is hopeful that this reaction can be applied to commercial use. 2. Experimental 2.1. Materials Polytetramethylene glycol (Mn = 1000, Mitsubishi Chemical), Hexafluoropropylene (Zhejiang Juhua Co. Ltd), Di-tert-butyl peroxide (DTBP), 2,2-azobisisobutyronitrile (AIBN) and di-benzoyl peroxide (BPO) (China Medicine, Shanghai Chemical Reagent Corporation) were used as received. All the materials above mentioned were used without further purification unless otherwise specified. 2.2. Measurements 1
H NMR and 13C NMR spectra were respectively recorded on a Bruker AC 300 NMR spectrometer and on a Bruker AC 400 NMR spectrometer with tetramethylsilane (TMS) as internal standard and deuterated chloroform (CDCl3) as solvent unless otherwise stated. For 13C NMR experiment, the carbon atom in CDCl3 was taken as 77 ppm and all other peaks were assigned with respect to it. Infrared analysis was carried out on a Bruker Equinox 55 FTIR spectrometer and the spectra were recorded in the range of 4000–500 cm1. 2.3. General procedure for free radical grafting reaction A 500 ml Hastelloy lined autoclave equipped with a ‘‘flip-flop’’ stirrer, pressure gauge, bursting disc (maximum working pressure approx. 20 MPa) and inlet/outlet valve was charged with 140 g molten polytetramethylene glycol (Mn = 1000) and 0.0164 mol di-tert-butyl peroxide. The reaction
vessel was closed, frozen in an acetone/liquid nitrogen mixture and placed under vacuum for several minutes. Then the required amount of HFP (300 g) was introduced. After being heated to 140 °C and stirred for 6 h in a thermostatically controlled furnace, the autoclave was cooled to room temperature. The product was collected and purified by distillation under reduced pressure. 3. Results and discussion 3.1. Effect of the initiator The reaction of grafting HFP onto PTMG was carried out with different initiators used separately at a temperature chosen at which their half life was close to 1 h: DTBP (140 °C); AIBN (80 °C); BPO (95 °C). Other reaction conditions were kept invariable. For each initiator, the relationship between grafting percentage (GP) and time was investigated. The results are shown in Fig. 1. The grafting percentage was calculated using the following equation: GPð%Þ ¼
W1W2 100 W2
where W1 and W2 are the weight of grafting product (PTMG-g-HFP) and monomer (PTMG), respectively. From Fig. 1, it is found that GP similarly increases with reaction time for each initiator. Whereas, the grafting efficiency is different. DTBP is better than AIBN and BPO for the reaction of
160
DTBP 140 120
AIBN 100
GP (%)
396
BPO
80 60 40 20 0
0
2
4
6
8
time (h)
Fig. 1. Dependence of grafting percentage on reaction time for several initiators. Initiator/PTMG = 0.12 (molar ratio).
Z. Ge et al. / European Polymer Journal 42 (2006) 395–401
grafting HFP onto PTMG. The curves in Fig. 1 give the series of radical reactivity in the order of DTBP (140 °C) > AIBN (80 °C) > BPO (95 °C). The phenomenon can be attributed to two facts. One can be that the induced decomposition of BPO is easily generated in the grafting reaction, which resulted in the decrease of initiator efficiency of BPO. The other can be that the temperature is the key factor although the reaction activation energy initiated separately by three initiators is nearly same. The reaction temperature initiated by DTBP is higher than the temperatures initiated by the other initiators. As a result, the reaction rate initiated by DTBP is the fastest. Therefore, DTBP is the most efficient in the grafting reaction. Then it was chosen as initiator for further reaction. 3.2. Effect of reaction time and temperature The change in the grafting percentage with reaction time at different temperatures for the grafting of HFP onto PTMG is shown in Fig. 2. Graft reaction was carried out at four different temperatures between 110 °C and 140 °C with other reaction conditions kept invariable. It can be clearly seen that GP increases rapidly during the first 6 h, and then increases slowly for all reaction temperatures. This indicates that more grafting sites are produced and more monomers are grafted during the first 6 h, and then grafting reaction is stagnated gradually. Fig. 2 also shows the effect of temperature on grafting percentage. It can be seen that GP shows a tendency to increase with increasing temperature, which conforms to the general law of radical reac-
397
tion. The result indicates that 140 °C is rather efficient reaction temperature, which is evidenced experimentally by a sharp decrease of the pressure when the reaction proceeds at the temperature. 3.3. Effect of the amount of initiator Fig. 3 shows the dependence of grafting percentage on the amount of initiator with other conditions maintained constant (140 °C, 6 h). In Fig. 3, R0 is defined as the molar ratio of DTBP/PTMG in the reaction. The grafting percentage increases with the increase of R0 initially and then decreases. When R0 approaches 0.12, GP reaches the maximum. This can be that the active points of PTMG, which produce radicals for initiating HFP, proliferate with the increase of the amount of DTBP at the primary stage of reaction. However, the excessive increase of DTBP engenders plenty of radicals, which could terminate the process of grafting reaction and decrease the grafting efficiency. 3.4. IR characterization The IR spectra of PTMG and PTMG-g-HFP are both shown in Fig. 4. Structural modification is evidenced by the spectrum of PTMG-g-HFP, in which there are new absorption bands dealing with the vibration absorption of C–F bond except all the characteristic absorption bands of PTMG. From the spectrum of PTMG-g-HFP, it is concluded that the stronger characteristic absorption bands locating at 1190 cm1 and 1287 cm1 can be attributed
140 160
140ºC
120
120
130ºC
100
120ºC
80
GP (%)
GP (%)
140
100
80
110ºC
60 60 40 20
40
0 0
2
4
6
8
time (h) Fig. 2. Change in grafting percentage with reaction time at different temperatures.
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
R0 Fig. 3. Change in grafting percentage with the amount of initiator.
398
Z. Ge et al. / European Polymer Journal 42 (2006) 395–401
PTMG-g-HFP
1287 1190
839
PTMG
680
1115
1106 4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1) Fig. 4. IR spectra of PTMG and PTMG-g-HFP.
to stretching vibrations of C–F [24] and the absorption bands at 839 cm1 and 680 cm1 are resulted from the stretching vibration of CF3 and deformation vibration of CF2, respectively [25]. The absorption band at 3467 cm1 is characteristic of O–H stretching. Four sharp peaks at 1368 cm1, 1430 cm1, 2857 cm1, 2940 cm1, are resulted from CH2 group. In addition, the absorption band at 1106 cm1 originated from C–O–C group of PTMG shifts to a high value of 1115 cm1 and becomes weaker in the spectrum of PTMG-g-HFP. This is due to the introduction of fluoroalkyl group, which is electron withdrawing. Therefore, it causes the hypsochromic shift of the C–O–C group. This shows that the grafting reaction is occurred at the carbon of the ether link. 3.5. 1H and
13
C NMR characterization
(–CH2OH), the peaks (1.6–1.8 ppm) corresponding to the –CH2CH2CH2– protons, the peaks (1.9– 2.2 ppm) assigned to –CH2CH2CH– protons and the peaks at 3.43 ppm dealing with –CH2O– protons are also observed in the 1H NMR spectrum of PTMG-g-HFP. The occurrence of grafting reaction is also evidenced by the corresponding 13C NMR spectrum of PTMG-g-HFP. Some peaks between 84 ppm and 88 ppm correspond to CFH–CF3. The peaks centered at 122 ppm is assigned to –CF2–CF3. The presence of the CF2 group adjacent to the methenyl group is evidenced by the peak at 119 ppm [26]. The peaks centered at 76 ppm is assigned to –CH–O–. Other peaks of Fig. 6(B) are assigned to the carbon atoms of methylene group of PTMG-g-HFP. All these evidence affirm that the radical reaction of grafting HFP onto PTMG has been performed successfully.
1
H NMR analysis results further support the chemical structure of PTMG-g-HFP. A comparing of the 1H NMR spectrum of PTMG-g-HFP with that of PTMG is shown in Fig. 5. As can be seen from Fig. 5(B), several peaks for the fluoromethenyl protons (–CF2CFHCF3) appear from 4.64 ppm to 5.15 ppm, and the peaks assigned to the methenyl protons (–CF2CHO–) appear at 3.65 ppm. This shows the new group (–CF2CFHCF3) was generated in the grafting reaction [22,23]. Besides, the peaks at 3.86 ppm assigned to the hydroxyl protons
3.6. Reaction mechanism IR, 1H NMR and 13C NMR spectra revealed that fluorinated PTMG had been prepared by radical grafting. The whole reaction process is depicted in Scheme 1. It is well-known that ethers readily form radicals like alcohols, especially at sites attached to oxygen [27]. Therefore PTMG initiated by initiator can generate radical 1. The developing radical 1 is strongly stabilized by adjacent oxygen
Z. Ge et al. / European Polymer Journal 42 (2006) 395–401
HOCH2CH2CH2CH2
OCH2CH2CH2CH2 a b c d
399
n-1OH
e
b +c
a +d
e A
8
7
6
5
HOCH2CH2CH2CH2 a
4
pp m
3
OCH2 CH2CH 2CH2 b x b c d
2
1
CF2CFHCF3 h OCHCH2CH 2CH 2 yOH a e f c g
c +d
b +g
a +e
f
h
B 8
7
6
5
4
3
2
1
ppm Fig. 5. 1H NMR spectra of PTMG (A) and PTMG-g-HFP (B).
atom. If the stabilizing interaction by oxygen atom is represented in valence-bond terms, the involvement of the electron pair on oxygen atom with the radical centre on the adjacent carbon atom enhances the nucleophilic character of the radical 1. Moreover, HFP is susceptible to nucleophilic attack. So radical 2 is formed by the reaction of radical 1 with HFP. The resultant radical 2 is electrophilic and therefore abstracts a hydrogen atom
from the relatively electron-rich C–H site (adjacent to oxygen in PTMG), giving product 3, which is fluorinated polyether glycol PTMG-g-HFP and regenerating radical 1. At this stage, the oxygen atom is strongly electron withdrawing and therefore reduces reactivity at other positions except the C–H site (adjacent to oxygen). The introduction of fluoroalkyl group also enhances the deactivation.
400
Z. Ge et al. / European Polymer Journal 42 (2006) 395–401
HOCH2CH2CH2CH2 OCH2CH2CH2CH2 n-1OH e f f e a b c d
d+e
c+f
b
a
A
140
120
100
80
60
40
20
ppm h i j CF2CFHCF3 HOCH2CH2CH2CH2 OCHCH2CH2CH2 x OCH2CH2CH2CH2 OH g f f g y a b c d e f f g
c+f d+g
e
140
120
b
i
j h 100
a 80
B 60
40
20
ppm Fig. 6.
13
C NMR spectra of PTMG (A) and PTMG-g-HFP (B).
4. Conclusion It is possible to graft HFP onto PTMG in the presence of different initiators. The grafting position was at the carbon atom attached to oxygen of PTMG. DTBP gave better results than the other initiators. It was observed that factors such as temperature and initiator amount significantly affect
the graft yield. The reaction process is unsophisticated, easily controlled and reaction materials are inexpensive. Therefore, it is hopeful that the reaction can be applied to commercial use. Furthermore, as the hydroxyl group is still reserved, the reaction product can be precursors of novel fluorinated polycondensates, which are under investigation.
Z. Ge et al. / European Polymer Journal 42 (2006) 395–401
OCH2CH2CH2CH2
. ..
.O-CH-CH . . CH CH
initiator
2
n
2
401
+
_
O-CH-CH2CH2CH2
2 n
n
(1)
PTMG
.
CF2CFCF3 (1) + CF2=CFCF3
OCHCH2CH2CH2
n
(2)
HFP
CF2CFHCF3 (2) +
OCH2CH2CH2CH2
n
O CHCH2CH2CH2
n
+ (1)
(3)
Scheme 1. Synthesis of fluorinated polyether glycol PTMG-g-HFP.
Acknowledgement Financial supports from the National Natural Science Foundation of China (No. 50273035) and Hangzhou Zhijiang Silicone Chemical Industry Co., Ltd. are acknowledged. References [1] Wall LA. In: Wall LA, editor. Fluoropolymers. New York: Wiley; 1972. p. 381–418. [2] Pittman AG. In: Wall LA, editor. Fluoropolymers. New York: Wiley; 1972. p. 419–49. [3] Bur AJ. In: Wall LA, editor. Fluoropolymers. New York: Wiley; 1972. p. 475–505. [4] Scheirs J. In: Scheirs J, editor. Modern fluoropolymers. New York: Wiley; 1997. p. 446–52. [5] Turri S, Scicchitano M, Marchetti R, Sanguineti A, Radice S. In: Hougham G, Johns K, Cassidy PE, Davidson T, editors. Fluoropolymers, Vol. 2. New York: Kluwer; 1999. p. 145–69. [6] Turri S, Scicchitano M, Glanotti G, Tonelli C. Eur Polym J 1995;31(12):1227–33. [7] Ang C, Yu Z. Adv Mater 2004;16(2):979–82. [8] Khayet M. Appl Surf Sci 2004;238(1–4):269–72. [9] Banks BA. In: Scheirs J, editor. Modern fluoropolymers. New York: Wiley; 1999. p. 103–14. [10] Yamamoto S, Matsumoto M, Shimada T. European Patent EP256765, 1988.
[11] Skutnik K. Australia Patent AU569145, 1988. [12] Yamamoto S, Matsumoto, M. Japanese Patent JP63430104, 1988. [13] Kobayashi T, Munekuta S, Unoki E. European Patent EP239935, 1987. [14] Mpore EP, Milian AS, Eleuterio HS. American Patent US3250808, 1966. [15] Miller WT. American Patent US3342218, 1966. [16] Tonelli C, Trombetta T, Scicchitano M, Simeone C, Ajroldi C. J Appl Polym Sci 1996;59:311–27. [17] Brady RF, Aronson CL. Biofouling 2003;19:59–62. [18] Ullett J. Prog Org Coat 2005;52(3):196–209. [19] Hung M, Fadam WB, Feiring AE, Rozent S. J Am Chem Soc 1993;115:8954–9. [20] Mitsch RA, Zollinger JL. American Patent US4094911, 1978. [21] Lazerte JD, Koshar RJ. J Am Chem Soc 1955;77:910–4. [22] Chambers RD, Diter P, Dunn SN, Farren C, Sandford G, Batsanov AS, et al. J Chem Soc Perkin Trans 1 2000:1639–49. [23] Chambers RD, Farren C, Sandford G, Yufit DS, Howard JAK. Z Anorg Allg Chem 2002;628:1857–60. [24] Romano RM, Czarnowski J, Ve´dova COD. Inorg Chem 2001;40:3039–47. [25] Ryan ME, Fonseca JLC, Tasker S, Badyal JPS. J Phys Chem 1995;99:7060–4. [26] Paleta O, Cı´rkva V, Kvı´cˇala J. J Fluorine Chem 1996;80:125–34. [27] Mu GZ. Radical reaction. Bei Jing: High Education Press; 1985.
European Polymer Journal 42 (2006) 395–401
www.elsevier.com/locate/europolj
Synthesis and characterization of a new fluorinated polyether glycol prepared by radical grafting of hexafluoropropylene onto polytetramethylene glycol Zhen Ge, Xingyuan Zhang *, Jiabing Dai Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei 230026, PR China Received 21 January 2005; received in revised form 20 July 2005; accepted 8 August 2005 Available online 1 December 2005
Abstract A new fluorinated polyether glycol (PTMG-g-HFP) was prepared by radical grafting of hexafluoropropylene (HFP) onto polytetramethylene glycol (PTMG) in the presence of different initiators. The structure of PTMG-g-HFP was characterized by means of IR, 1H NMR and 13C NMR. The effects of nature and amount of initiator, reaction time and reaction temperature on grafting HFP onto PTMG were investigated. The results showed di-tert-butyl peroxide (DTBP) was the most efficient in the reaction and the optimal reaction conditions were: [DTBP]0/[PTMG]0, 0.12; reaction temperature, 140 °C; reaction time, 6 h. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Fluorinated polyether glycol; Polytetramethylene glycol; Fluoropolymers
1. Introduction It is well-known that fluoropolymers exhibit a series of unique properties, such as high thermal stability, chemical resistance (to acids, bases and solvents), resistance to aging, low dielectric constants and dissipation factors, low water absorptivity and attractive surface properties [1–8]. Therefore, they are widely used in numerous applications such as aerospace, water repellent textile finishing and engineering [9–13], and there is a continuous interest in the synthesis of novel fluoropolymers. Fluorinated *
Corresponding author. Tel./fax: +86 551 3607484. E-mail addresses: [email protected] (Z. Ge), zxym@ustc. edu.cn (X. Zhang).
polyether glycols are particularly valuable fluoropolymers. They can be used as chemical intermediates and react with some compounds containing a plurality of groups reactive towards hydroxyl groups to produce useful macromolecular materials such as elastomers, coatings, foams and thermoplastics [14–18]. Fluorinated polyether glycols are generally prepared by copolymerization of fluorinated vinyl ether alcohols with fluorinated diols [19] or by photoxidative polymerization of tetrafluoroethylene and the subsequent thermal and chemical treatment of resulting product [20]. However, these methods have some limitations. For example, the former reaction must be carried out very slowly under carefully controlled conditions to avoid generating cylic
0014-3057/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2005.08.008
Z. Ge et al. / European Polymer Journal 42 (2006) 395–401
dimer of fluorinated vinyl ether alcohols, and the later has a very complex process containing a fivestep reaction. Free radical additions of fluoro-olefin to various cyclic and acyclic alcohols, diols and their derivatives have been described previously [21–23], but little literature discussed the additions of fluoro-olefin to polyether glycols. In this paper, we report a new synthetic method of fluorinated polyether glycol PTMG-g-HFP by performing a radical way of grafting hexafluoropropylene (HFP) onto polytetramethylene glycol (PTMG). This reaction process is unsophisticated, easily controlled and reaction materials are inexpensive. It is hopeful that this reaction can be applied to commercial use. 2. Experimental 2.1. Materials Polytetramethylene glycol (Mn = 1000, Mitsubishi Chemical), Hexafluoropropylene (Zhejiang Juhua Co. Ltd), Di-tert-butyl peroxide (DTBP), 2,2-azobisisobutyronitrile (AIBN) and di-benzoyl peroxide (BPO) (China Medicine, Shanghai Chemical Reagent Corporation) were used as received. All the materials above mentioned were used without further purification unless otherwise specified. 2.2. Measurements 1
H NMR and 13C NMR spectra were respectively recorded on a Bruker AC 300 NMR spectrometer and on a Bruker AC 400 NMR spectrometer with tetramethylsilane (TMS) as internal standard and deuterated chloroform (CDCl3) as solvent unless otherwise stated. For 13C NMR experiment, the carbon atom in CDCl3 was taken as 77 ppm and all other peaks were assigned with respect to it. Infrared analysis was carried out on a Bruker Equinox 55 FTIR spectrometer and the spectra were recorded in the range of 4000–500 cm1. 2.3. General procedure for free radical grafting reaction A 500 ml Hastelloy lined autoclave equipped with a ‘‘flip-flop’’ stirrer, pressure gauge, bursting disc (maximum working pressure approx. 20 MPa) and inlet/outlet valve was charged with 140 g molten polytetramethylene glycol (Mn = 1000) and 0.0164 mol di-tert-butyl peroxide. The reaction
vessel was closed, frozen in an acetone/liquid nitrogen mixture and placed under vacuum for several minutes. Then the required amount of HFP (300 g) was introduced. After being heated to 140 °C and stirred for 6 h in a thermostatically controlled furnace, the autoclave was cooled to room temperature. The product was collected and purified by distillation under reduced pressure. 3. Results and discussion 3.1. Effect of the initiator The reaction of grafting HFP onto PTMG was carried out with different initiators used separately at a temperature chosen at which their half life was close to 1 h: DTBP (140 °C); AIBN (80 °C); BPO (95 °C). Other reaction conditions were kept invariable. For each initiator, the relationship between grafting percentage (GP) and time was investigated. The results are shown in Fig. 1. The grafting percentage was calculated using the following equation: GPð%Þ ¼
W1W2 100 W2
where W1 and W2 are the weight of grafting product (PTMG-g-HFP) and monomer (PTMG), respectively. From Fig. 1, it is found that GP similarly increases with reaction time for each initiator. Whereas, the grafting efficiency is different. DTBP is better than AIBN and BPO for the reaction of
160
DTBP 140 120
AIBN 100
GP (%)
396
BPO
80 60 40 20 0
0
2
4
6
8
time (h)
Fig. 1. Dependence of grafting percentage on reaction time for several initiators. Initiator/PTMG = 0.12 (molar ratio).
Z. Ge et al. / European Polymer Journal 42 (2006) 395–401
grafting HFP onto PTMG. The curves in Fig. 1 give the series of radical reactivity in the order of DTBP (140 °C) > AIBN (80 °C) > BPO (95 °C). The phenomenon can be attributed to two facts. One can be that the induced decomposition of BPO is easily generated in the grafting reaction, which resulted in the decrease of initiator efficiency of BPO. The other can be that the temperature is the key factor although the reaction activation energy initiated separately by three initiators is nearly same. The reaction temperature initiated by DTBP is higher than the temperatures initiated by the other initiators. As a result, the reaction rate initiated by DTBP is the fastest. Therefore, DTBP is the most efficient in the grafting reaction. Then it was chosen as initiator for further reaction. 3.2. Effect of reaction time and temperature The change in the grafting percentage with reaction time at different temperatures for the grafting of HFP onto PTMG is shown in Fig. 2. Graft reaction was carried out at four different temperatures between 110 °C and 140 °C with other reaction conditions kept invariable. It can be clearly seen that GP increases rapidly during the first 6 h, and then increases slowly for all reaction temperatures. This indicates that more grafting sites are produced and more monomers are grafted during the first 6 h, and then grafting reaction is stagnated gradually. Fig. 2 also shows the effect of temperature on grafting percentage. It can be seen that GP shows a tendency to increase with increasing temperature, which conforms to the general law of radical reac-
397
tion. The result indicates that 140 °C is rather efficient reaction temperature, which is evidenced experimentally by a sharp decrease of the pressure when the reaction proceeds at the temperature. 3.3. Effect of the amount of initiator Fig. 3 shows the dependence of grafting percentage on the amount of initiator with other conditions maintained constant (140 °C, 6 h). In Fig. 3, R0 is defined as the molar ratio of DTBP/PTMG in the reaction. The grafting percentage increases with the increase of R0 initially and then decreases. When R0 approaches 0.12, GP reaches the maximum. This can be that the active points of PTMG, which produce radicals for initiating HFP, proliferate with the increase of the amount of DTBP at the primary stage of reaction. However, the excessive increase of DTBP engenders plenty of radicals, which could terminate the process of grafting reaction and decrease the grafting efficiency. 3.4. IR characterization The IR spectra of PTMG and PTMG-g-HFP are both shown in Fig. 4. Structural modification is evidenced by the spectrum of PTMG-g-HFP, in which there are new absorption bands dealing with the vibration absorption of C–F bond except all the characteristic absorption bands of PTMG. From the spectrum of PTMG-g-HFP, it is concluded that the stronger characteristic absorption bands locating at 1190 cm1 and 1287 cm1 can be attributed
140 160
140ºC
120
120
130ºC
100
120ºC
80
GP (%)
GP (%)
140
100
80
110ºC
60 60 40 20
40
0 0
2
4
6
8
time (h) Fig. 2. Change in grafting percentage with reaction time at different temperatures.
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
R0 Fig. 3. Change in grafting percentage with the amount of initiator.
398
Z. Ge et al. / European Polymer Journal 42 (2006) 395–401
PTMG-g-HFP
1287 1190
839
PTMG
680
1115
1106 4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1) Fig. 4. IR spectra of PTMG and PTMG-g-HFP.
to stretching vibrations of C–F [24] and the absorption bands at 839 cm1 and 680 cm1 are resulted from the stretching vibration of CF3 and deformation vibration of CF2, respectively [25]. The absorption band at 3467 cm1 is characteristic of O–H stretching. Four sharp peaks at 1368 cm1, 1430 cm1, 2857 cm1, 2940 cm1, are resulted from CH2 group. In addition, the absorption band at 1106 cm1 originated from C–O–C group of PTMG shifts to a high value of 1115 cm1 and becomes weaker in the spectrum of PTMG-g-HFP. This is due to the introduction of fluoroalkyl group, which is electron withdrawing. Therefore, it causes the hypsochromic shift of the C–O–C group. This shows that the grafting reaction is occurred at the carbon of the ether link. 3.5. 1H and
13
C NMR characterization
(–CH2OH), the peaks (1.6–1.8 ppm) corresponding to the –CH2CH2CH2– protons, the peaks (1.9– 2.2 ppm) assigned to –CH2CH2CH– protons and the peaks at 3.43 ppm dealing with –CH2O– protons are also observed in the 1H NMR spectrum of PTMG-g-HFP. The occurrence of grafting reaction is also evidenced by the corresponding 13C NMR spectrum of PTMG-g-HFP. Some peaks between 84 ppm and 88 ppm correspond to CFH–CF3. The peaks centered at 122 ppm is assigned to –CF2–CF3. The presence of the CF2 group adjacent to the methenyl group is evidenced by the peak at 119 ppm [26]. The peaks centered at 76 ppm is assigned to –CH–O–. Other peaks of Fig. 6(B) are assigned to the carbon atoms of methylene group of PTMG-g-HFP. All these evidence affirm that the radical reaction of grafting HFP onto PTMG has been performed successfully.
1
H NMR analysis results further support the chemical structure of PTMG-g-HFP. A comparing of the 1H NMR spectrum of PTMG-g-HFP with that of PTMG is shown in Fig. 5. As can be seen from Fig. 5(B), several peaks for the fluoromethenyl protons (–CF2CFHCF3) appear from 4.64 ppm to 5.15 ppm, and the peaks assigned to the methenyl protons (–CF2CHO–) appear at 3.65 ppm. This shows the new group (–CF2CFHCF3) was generated in the grafting reaction [22,23]. Besides, the peaks at 3.86 ppm assigned to the hydroxyl protons
3.6. Reaction mechanism IR, 1H NMR and 13C NMR spectra revealed that fluorinated PTMG had been prepared by radical grafting. The whole reaction process is depicted in Scheme 1. It is well-known that ethers readily form radicals like alcohols, especially at sites attached to oxygen [27]. Therefore PTMG initiated by initiator can generate radical 1. The developing radical 1 is strongly stabilized by adjacent oxygen
Z. Ge et al. / European Polymer Journal 42 (2006) 395–401
HOCH2CH2CH2CH2
OCH2CH2CH2CH2 a b c d
399
n-1OH
e
b +c
a +d
e A
8
7
6
5
HOCH2CH2CH2CH2 a
4
pp m
3
OCH2 CH2CH 2CH2 b x b c d
2
1
CF2CFHCF3 h OCHCH2CH 2CH 2 yOH a e f c g
c +d
b +g
a +e
f
h
B 8
7
6
5
4
3
2
1
ppm Fig. 5. 1H NMR spectra of PTMG (A) and PTMG-g-HFP (B).
atom. If the stabilizing interaction by oxygen atom is represented in valence-bond terms, the involvement of the electron pair on oxygen atom with the radical centre on the adjacent carbon atom enhances the nucleophilic character of the radical 1. Moreover, HFP is susceptible to nucleophilic attack. So radical 2 is formed by the reaction of radical 1 with HFP. The resultant radical 2 is electrophilic and therefore abstracts a hydrogen atom
from the relatively electron-rich C–H site (adjacent to oxygen in PTMG), giving product 3, which is fluorinated polyether glycol PTMG-g-HFP and regenerating radical 1. At this stage, the oxygen atom is strongly electron withdrawing and therefore reduces reactivity at other positions except the C–H site (adjacent to oxygen). The introduction of fluoroalkyl group also enhances the deactivation.
400
Z. Ge et al. / European Polymer Journal 42 (2006) 395–401
HOCH2CH2CH2CH2 OCH2CH2CH2CH2 n-1OH e f f e a b c d
d+e
c+f
b
a
A
140
120
100
80
60
40
20
ppm h i j CF2CFHCF3 HOCH2CH2CH2CH2 OCHCH2CH2CH2 x OCH2CH2CH2CH2 OH g f f g y a b c d e f f g
c+f d+g
e
140
120
b
i
j h 100
a 80
B 60
40
20
ppm Fig. 6.
13
C NMR spectra of PTMG (A) and PTMG-g-HFP (B).
4. Conclusion It is possible to graft HFP onto PTMG in the presence of different initiators. The grafting position was at the carbon atom attached to oxygen of PTMG. DTBP gave better results than the other initiators. It was observed that factors such as temperature and initiator amount significantly affect
the graft yield. The reaction process is unsophisticated, easily controlled and reaction materials are inexpensive. Therefore, it is hopeful that the reaction can be applied to commercial use. Furthermore, as the hydroxyl group is still reserved, the reaction product can be precursors of novel fluorinated polycondensates, which are under investigation.
Z. Ge et al. / European Polymer Journal 42 (2006) 395–401
OCH2CH2CH2CH2
. ..
.O-CH-CH . . CH CH
initiator
2
n
2
401
+
_
O-CH-CH2CH2CH2
2 n
n
(1)
PTMG
.
CF2CFCF3 (1) + CF2=CFCF3
OCHCH2CH2CH2
n
(2)
HFP
CF2CFHCF3 (2) +
OCH2CH2CH2CH2
n
O CHCH2CH2CH2
n
+ (1)
(3)
Scheme 1. Synthesis of fluorinated polyether glycol PTMG-g-HFP.
Acknowledgement Financial supports from the National Natural Science Foundation of China (No. 50273035) and Hangzhou Zhijiang Silicone Chemical Industry Co., Ltd. are acknowledged. References [1] Wall LA. In: Wall LA, editor. Fluoropolymers. New York: Wiley; 1972. p. 381–418. [2] Pittman AG. In: Wall LA, editor. Fluoropolymers. New York: Wiley; 1972. p. 419–49. [3] Bur AJ. In: Wall LA, editor. Fluoropolymers. New York: Wiley; 1972. p. 475–505. [4] Scheirs J. In: Scheirs J, editor. Modern fluoropolymers. New York: Wiley; 1997. p. 446–52. [5] Turri S, Scicchitano M, Marchetti R, Sanguineti A, Radice S. In: Hougham G, Johns K, Cassidy PE, Davidson T, editors. Fluoropolymers, Vol. 2. New York: Kluwer; 1999. p. 145–69. [6] Turri S, Scicchitano M, Glanotti G, Tonelli C. Eur Polym J 1995;31(12):1227–33. [7] Ang C, Yu Z. Adv Mater 2004;16(2):979–82. [8] Khayet M. Appl Surf Sci 2004;238(1–4):269–72. [9] Banks BA. In: Scheirs J, editor. Modern fluoropolymers. New York: Wiley; 1999. p. 103–14. [10] Yamamoto S, Matsumoto M, Shimada T. European Patent EP256765, 1988.
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