J. Cent. South Univ. (2013) 20: 629−633 DOI: 10.1007/s11771­013­1528­5

Catalytic synthesis of perfluorolyethers

YANG Wei­jun(阳卫军) 1, FANG Chao(方超) 1, ZHOU Ji­cang(周济苍) 2, GUO Can­cheng(郭灿城) 1, WAN Wei(万伟) 2

1. College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China; 2. Hunan Non­ferrous Fluorine Chemistry Corporation, Changsha 410063, China © Central South University Press and Springer­Verlag Berlin Heidelberg 2013

Abstract: Perfluorolyether is characterized by highly chemical inertness, oxidative stability, anticorrosion as well as radiation resistance. It can be used as lubricant especially in harsh environmental conditions. In this work, hexafluoropylene oxide was catalytically polymerized at low temperature using the methods of anionic polymerization, and perfluorolyethers were obtained with number­average degree of polymerization more than 15. CsF and RbF were used as catalysts and their catalytic activities were investigated. Experimental results show that perfluorolyethers with number­average molar masses up to 3 000 g/mol could be obtained using the two kinds of catalysts, respectively. As compared to CsF, the number­average degree of polymerization is higher and the relative molecular mass distribution interval is narrower when RbF is used as catalyst. The effect of factors such as impurities’ content, reaction temperature and reaction time on the number­average degree of polymerization was also investigated. It is found that low impurities’ content and low temperature are beneficial to the generation of high number­average degree of perfluorolyethers. The optimization reaction time is 24 h, and further increase of reaction time does not significantly affect the average relative molecular mass. The product was characterized by IR, 19F NMR and GC­MS, and the catalytic mechanism was analyzed finally.

Key words: hexafluoropylene oxide; perfluorolyether; polymerization; purification

theoretical and practical significance. 1 Introduction At present, there are two main methods to synthesize perfluorolyethers: the anionic polymerization Perfluorolyethers are a kind of full fluorine polymer, of hexafluoropylene oxide and the photocatalytic which present colorless, odorless and transparent oil oxidation of perfluorolefine [6–7]. So far, research is on liquid under normal temperature, and their molecular the synthesis of perfluorolyether. DUAN et al [8] once structures only containing elements C, F and O [1]. used KF as catalyst，and glycol dimethyl as solvent. Because of the large bonding energy of C—F, the main Hexafluoropylene oxide derived into perfluorolyethers chain of the perfluorolyether molecular can be shielded under atmospheric pressure, but the conversion was only perfectly, which makes perfluorolyethers characterized 7.1%, the average relative molecular mass was only 1 by chemical inertness, oxidative stability, corrosion 160, and none of them was satisfactory. According to the resistance and radiation resistance. These unique research result of Ref. [9], when using CsF/glycol properties make such polymers very useful in wide dimethyl as catalysts, perfluorolyethers with application [2–4], for example, as lubricants in the number­average molar masses could reach from 2 500 to chemical industry, electronic industry, mechanical 3 500 g/mol, and the conversion could be 90%, but the industry, nuclear industry and aerospace engines [5]. In relative molecular mass distribution was too wide. addition to this, perfluorolyethers reaction with some After stabilizing treatment of chain ends, hydrocarbons are derived into fluorocarbon surfactants perfluorolyethers can be used as lubricants, which were and fluorocarbon coating paints, which are characterized characterized by thermostability, high chemical stability, by water repellent, oil repellent, stain resistance and anti­corrosion and anti­abrasion [10]. The main durable resistance. These high performances make manufacturer companies in the world are: DuPont in fluorocarbon surfactant as textile finishing agent and USA, Demnum in Japan and Montefluos in Italy. The fluorocarbon coating paint as special occasion paint. So, annual production capacity of perfluorolyethers was synthesis of high­degree polymerization has great about hundreds of thousands of kilograms, which was

Foundation item: Project(53110704012) supported by the Fundamental Research Funds for the Central Universities, China Received date: 2012–01–08; Accepted date: 2012–06–12 Corresponding author: YANG Wei­jun, Professor; Tel: +86–731–88821449; E­mail: [email protected] 630 J. Cent. South Univ. (2013) 20: 629−633 mainly sold to Western Europe, Japan and other commercial hexafluoropropylene oxide through a developed countries [11–12]. Currently in China, China sequence of three scrubbers containing calcium oxide, Petrochemical Corp has a small amount of the lithium aluminum hydride and alumina. Content of water perfluorolyethers for sale, which cannot meet the needs was tested online by dew point transmitter. Content of of the market neither in quality nor in output. The the fluoride acid was tested using the method of alkali synthetic technique of perfluorolyethers is of high­degree absorption. Hexafluoroacetone and other organic business secret in related companies all along. At present, impurities were investigated by gas chromatography. the high performance perfluorolyethers still mainly rely on imports in China. So, it is necessary to study and 2.2.2 Polymerization procedure produce high quality perfluorolyethers to meet the needs In this part, CsF and RbF were used as catalysts, of Chinese market [13]. and glycol dimethyl as solvent. Catalysts were dried in In this work, a process for catalytic synthesis of 150 ºC over 1 h in order to remove internal water. hexafluoropropylene oxide to perfluorolyethers using the Molecular sieve was used to remove water from the methods of anionic polymerization was studied. Several solvent. In order to facilitate charging and control purification agents were used to purify polymerization process, catalyst was dissolved in solvent hexafluoropropylene, and the appropriate purification as solution and dropped continuously into the reactor, materials and purification parameters were obtained. which containing 0.5 g catalyst per milliliter solvent. The polymerization vessel consisted of a three­neck 2 Experiment flask which was equipped with a paddle stirrer, a gas inlet port and a constant pressure funnel. The purified 2.1 Materials and equipments monomer was prepared by passing hexafluoropropylene Hydrogen, oxygen and nitrogen were obtained from oxide monomer through a sequence of refining scrubbers Changsha Rizhen Gas Corporation; hexafluoropropylene at a rate of 100 mL/min then was filled into the cooled oxide was obtained from Shangdong Dongyue Shenzhou polymerization vessel. The liquid catalyst was dropped New Material Corporation; methanol, anhydrous, THF, continuously into the reactor with stirring. glycol dimethyl, CsF and RbF were of analytical grade Hexafluoropropylene oxide feed was stopped after 8 h and obtained commercially. and the reactor was allowed to stand for an additional GC9890A gas chromatograph was manufactured by 16 h to assure complete reaction with residual Nanjing Kejie Analysis Instrument Application Institute; hexafluoropropylene oxide. Number­average degree of LZB­3WB rotameter was manufactured by Changzhou polymerization and average relative molecular mass of Kede Thermo­technical Instrument Corporation; MDM the polymer could be calculated from the 19F ­NMR dew point transmitter was manufactured by Guangzhou spectra and gas chromatograph spectra. Oubaite Technology Corporation; BS­124S precision The polymerization reaction equation is electronic balance was manufactured by Beijing Sartorius Instrument Corporation; 101A­1B electro thermostatic blast oven was manufactured by Shanghai Experimental Instrument Corporation; DF­101Z constant temperature and heating magnetic blender was manufactured by Henan Yuhua Instrument Corporation; (1) INOVA­400 NMR was manufactured by Varian Corporation. 3 Results and discussion

2.2 Experimental procedure 3.1 Comparing with catalysis activity of catalysts 2.2.1 Purification of hexafluoropropylene oxide In this work, the polymerization reaction time were This work explored a process for catalytic set as 8, 16, 24 and 30 h, respectively, with KF and RbF synthesizing hexafluoropropylene oxide to the as the catalysts. The relationship of average relative perfluorolyethers. Typical supplies contain several trace molecular mass and reaction time is shown as Fig. 1. impurities including hexafluoroacetone, fluoride acids Further increase of reaction time up to 30 h did not and water. The presence of even very small quantities of significantly affect the average relative molecular mass. these impurities will reduce the maximum degree of The optimization reaction time is 24 h. polymerization of hexafluoropropylene markedly, so As shown in Fig. 2 and Fig. 3, when RbF was used hexafluoropropylene oxide must be purified deeply as catalyst, the number­average degree of polymerization before polymerization [7]. is higher, and the relative molecular mass distribution The purified monomer was prepared by passing interval is narrower. Because of the high hydroscopic J. Cent. South Univ. (2013) 20: 629−633 631 property of CsF, it is hard to control trace moisture in the reaction, and RbF is more dissolvable in glycol dimethyl ether, so the polymerization result is better when RbF was used as catalyst.

3.2 Effect of impurities on polymerization In order to investigate the effect of the impurities on the polymerization in hexafluoropropylene oxide, two groups of experiments were run: one group using commercial hexafluoropropylene oxide as reactant without refining, another using purified hexafluoropropylene oxide as reactant by passing commercially available hexafluoropropylene oxide through a sequence of three scrubbers containing calcium oxide, lithium aluminum hydride and alumina (reaction Fig. 1 Relationship of number average molar masses t and condition: temperature: –30 ºC; solvent: glycol dimethyl reaction time (Reaction conditions: Temperature: –30 ºC; ether; catalyst: RbF). The number­average degree of Solvent: glycol dimethyl ether; Amount of catalyst: 1.0 g/150 g polymerization is listed in Table 1. hexafluoropropylene oxide) As given in Table 1, compared to the untreated hexafluoropropylene oxide, the numberaverage degree of polymerization is higher and the polymer yield is up to 98% when hexafluoropropylene oxide is refined. At the same time, the yield is 10%–20% when hexafluo­ ropropylene oxide is untreated. Typical supplies contain several trace impurities including hexafluoroacetone, hexafluoropropylene, carbon dioxide, acid fluoride and water. Even very small quantities of these impurities could reduce remarkably the maximum degree of polymerization of hexafluoropropylene. So, the polymerization reaction should be operated under vacuum or inertness atmosphere. Related reagents and glass apparatus should be clean, and the key impurities, such as hexafluoroacetone, acid fluoride and water, should be Fig. 2 Relative molecular mass distribution in presence of RbF removed before polymerization. as catalyst (n=19.12; reaction condition: temperature: –30 ºC; solvent: glycol, dimethyl ether; catalyst: RbF) 3.3 Effect of temperature on polymerization As shown in Fig. 4, an increase in the polymerization temperature from –30 to –10 ºC leads to a decrease in average molar masses. The lower the temperature is, the higher the average molar masses are.

3.4 Synthesis and characterization of polymers When the polymerization reaction continued for 24 h at –30 ºC with CsF as catalyst, the polymer formed and appeared as whitish viscous liquid. IR spectrum is shown in Fig. 5 and 19F ­NMR spectrum is shown in Fig. 6. From IR spectrum of polymers, it can be seen that the band of the stretching of —C—O—C— groups is at 985 cm– 1, the peaks at 1 240 cm –1, 1 190 cm– 1, 1 132 cm –1 and 1 306 cm– 1 are from the band of the stretching of C—F groups, and the band of the stretching of —COF Fig. 3 Relative molecular mass distribution in presence of CsF groups is at 1 884 cm– 1. as catalyst (n=15.625; reaction condition: temperature: –30ºC; The formed polymers can be confirmed as 19 solvent: glycol dimethyl ether; catalyst: CsF) hexafluorolyethers by comparing the F­NMR spectrum 632 J. Cent. South Univ. (2013) 20: 629−633

Table 1 Effect of impurities to polymerization in hexafluoropropylene oxide Type Untreated HFPO Purified HFPO –6 –6 Impurity H2O 1 071×10 0.5×10 Impurities HF/% 0.23 0.014 Hexafluoropropylene/% 1.5 0.1 Number­average degree of polymerization 4.296 19.12 Average relative molecular mass 713.0 3 137.92

HFPO— Hexafluoropropylene oxide.

Fig. 4 Effect of different temperatures on polymerization Fig. 6 19F­ NMR spectrum of polymers (reaction condition: Catalyst, RbF; Solvent, glycol dimethyl ether) as the ratio of (1+2) to 2 integrals. The average molar

mass (Mm) and average degree of polymerization are calculated as

I1 + I 2 1.17 + 0.08 Dp = = = 15.625 I 2 0.08

M m = Dp × M w = 15.625×166 = 2 593.75

where Mw is the relative molecular mass. The polymers were systematically characterized by GC­MS spectroscopy. The relative molecular mass distributions of the polymers are shown in Fig. 5. With RbF as catalyst, other conditions remaining unchanged, the formed polymers were systematically characterized by GC and 19F ­NMR spectroscopy, respectively. The average degree of polymerization is Fig. 5 IR spectrum of polymer 19. 12, the average molar mass is 3 137.92 g/mol (Fig. 4). of polymers (Fig. 6) to the data from Refs. [9, 14], where It is evident that the catalytic properties of RbF is better weight­average molar masses and average degree of than CsF. polymerization of the polymers can be figured out according to the concentrations of I1 and I2. 4 Polymerization mechanism analysis In the 19 F­NMR spectrum, signals centered at –6 –6 –142.5×10 (I1) and –126×10 (I2) are ascribed to Taking RbF catalyst for example, the catalytic —CF—groups of main­chain fluorine atoms, and the reaction mechanism of hexafluoropylene oxide –6 –6 resonances at –125.5×10 (I3) and –80.5 ×10 (I4) are polymerization can be deduced as follows according to assigned to the —CF2— and —CF3, respectively. The the experimental results above and related references, average degree of polymerization (DP) can be calculated DME representing glycol dimethyl ether. J. Cent. South Univ. (2013) 20: 629−633 633 (2) References

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