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