-rays, electrons of high energies) is carried out using
solid TFEO (18,31,32).Tetrafluoroethylene oxide polymerization.
The process of TFEO polymerization is of the greatest practical interest as it allows to obtain TFEO oligomer derivatives possessing high thermal and chemical stability together with high reactivity of the end fluoroanhydride group which by known chemical transformations can be easily transformed into other functional groupings depending on directions of further use of such perfluoropolyethers (2,3,11).
TFEO homopolymers are produced at exposure to ionizing radiation or fluoride-ion. When catalysts of anion polymerization are used, there may be functional groupings of the applied catalysts in the polymer structure. The polymers on TFEO base without functional groups are distinguished by higher thermal and chemical stability.
TFEO polymerization under exposure to radiation (Roentgen rays, -rays, electrons of high energies) is carried out using
solid TFEO (18,31,32).
The block TFEO polymerization at a temperature of liquid nitrogen and exposure to an electron flow with 2 MeV energy resulted in a solid TFEO polymer with a melting point of 40-42oC. The intrinsic viscosity of a 0.5% solution of the polymer produced in oil FC-75 is equal to 0.605. An analogous polymer with a melting point of 37oC and the intrinsic viscosity of 0.411 was produced at irradiation of TFEO cooled to -196oC by Roentgen rays (2.25 Mrad) during 1 hour (18). The investigation of TFEO polymerization under exposure to ionizing radiation (Co60) with a dose rate of 2.7 105 roentgen/hour was carried out in works (31,32). The TFEO polymerization was found to take place only in case of the solid TFEO. In this case a maximum polimerization rate and the formation of a polymer with a maximum molecular weight is observed at a temperature of ac. –122oC, close to the TFEO melting temperature. There is no data on the mechanism of solid phase polymerization of TFEO under irradiation in literature.
The process of TFEO polymerization in a solid phase can be resulted from such substances that form active particles at a temperature of polymerization, for example gaseous fluorine. So, at heating 1 g of TFEO cooled to –196oC, in the presence of gaseous fluorine there was produced 30mg of solid polytetrafluoroethylene oxide with a melting temperature of 38oC (18). The polyether produced by these methods is characterized by a high molecular weight (10 000 – 170 000), chemical and thermal stability. NMR and IRS studies of these polyethers have shown the absence of the end groups giving an additional thermal and chemical stability to the polymers. An important property of polyethers is their solubility in fluorocarbon solvents: perfluorodimethylcyclobutane, perfluorocyclohexane etc. That allows applying them in coatings resistant to chemical corrosion and providing good electrical insulation.
The process of homopolymerization of perfluorolefin oxides on activated carbon was described in details (33 –40). It is considered that the polymerization is also initiated by fluorine ions, which are formed somehow in the system of activated carbon/perfluorolefin oxide. Any activated carbon may be used for carrying out the process but American researchers mostly used activated carbon "Darco". Carbon activation is carried out by its heating up to 400oC under vacuum.
The absence of solvent is an advantage of the process because it makes easier separation of individual oligomers. In this case a change in pressure from 0.1 to 5000atm and a quantity of catalyst do not influence significantly the value of the molecular weight of the oligomers (34). In all the cases there is formed a mixture of the oligomers with a wide molecular weight distribution. The polymerization on carbon "Darco" (12,32) at a temperature of -80 - -45oC for 24 hours results in a mixture of the oligomers with the molecular mass from 400 to 4700:
The polymer with a molecular mass of 2550 obtained by this method sustained heating up to 500oC in dry nitrogen without noticeable decomposition. It was impossible to control the process proceeding that should be considered as an important disadvantage of the process of polymerization of perfluoroolefin oxides on activated carbon.
Just as conventional catalysts for polymerization of olefin oxides are unacceptable for
-oxides of
perfluoroolefins, so ,as anionic catalyst for the latter, there are used various ionic
fluorides either in individual form or in combination with other halides, also salts of
quaternary bases or their combinations with fluorides of alkali metals, with acetates and
cyanides, also tertiary amines, trialkylsulfonium fluorides etc.
In this case independently of the anion in the initial salt of a quaternary base, the polymerization process is initiated by fluorides forming as a result of the primary reaction of the initial salt with TFEO, for example:
Also, further fluorides are a source of fluoride ions converting oxide molecules to alkoxyanions. To initiate the reaction of TFEO polymerization besides fluorides of alkali metals there are used fluorides of different metals such as silver, thallium and mercury but they have no advantages in comparison with fluorides of alkali metals. There are no data in literature on the mechanism of solid-phase polymerization of TFEO. As regards the polymerization of TFEO under exposure to anionic catalysts, authors (3) proposed the following mechanism:
Initiation stage
Chain growth
Termination of the chain growth
where Rf = F, CF3;
M = K+, Na+, Rb+, Cs+, RuN+, RuP+, RuAs+ etc.
R = alkyl
As it is seen from the given scheme, the active centers remain in the system and due to their presence the process recommences with a repeated addition of TFEO. Solvents play a great role in the process of anionic polymerization. Their choice is determined by a quantity of fluoride ions soluble in them and by a degree of interaction of the solvent with the starting and intermediate compounds.
The most effective solvents are glymes, i.e. dimethyl ethers of ethylene glycol and polyethylene glycols. This is accounted for by the high ability of glymes to coordinate with cations resulting in high increase in nucleophilic activity of fluorine anion. In this case cesium fluoride is the most active catalyst because it has high ionic nature and minimum energy of crystal lattice among the metals of group I (3,32,33). Data on thermal stability of perfluoroalkoxydes confirm this.
It is very important to carefully dry the solvents and ionic fluorides. Due to the small size and high charge density, fluorine ion forms the strongest hydrogen bond among all the mononuclear ions (123 kcal/mol) and thus is a very weak nucleophile in solutions containing water (41).
Complexation between sodium and lithium cations and glymes has been described in work (42). It has been established, the coordination power of glymes of general formula CH3O(CH2CH2O)mCH3 increases with increasing m. Further investigation of solvating power of glymes with respect to cesium fluoride and cesium perfluoroalkoxides was carried in works (43-45). By means of cryoscopy and electrodialysis methods there were obtained data on the nature of active center in interaction of HFPO with cesium fluoride and cesium perfluoroalkoxides (of C3F7O[CF(CF3)CF2O]nCs formula ,where n =0,1,2) in the presence of glymes. According to data (46), the HFPO polymerization takes place according to the following scheme where the formation of solvates of CF3CF2CF2O-,Cs+,L type is of great importance (L=glime):
The stronger the glime coordination ability, the more equilibrium is shifted to the right by cesium coordination by glyme:
The coordination power of glymes for cesium ions increases with increasing m (43) that confirms data (42). Molecular-weight distribution of oligomers of perfluorinated oxides is determined by the experimental conditions. Polymerization temperature (from –80 to 200oC) is of great importance (34,45).
Investigation of stability of perfluoroalkoxides of alkali metals has shown that the equilibrium of the reaction
RfCF2OM = > RfC(O)F + MF
where M=alkali metal
is shifted to the right with temperature. Thus a temperature increase above 20oC results in products of lower molecular weight. The growth of molecular mass depends on solubility of the forming polymer and the viscosity of the reaction medium also. The polymerization process can proceed until the polymer molecular mass exceeds the limits of solubility (11).
According to data of (44,45), the main cause of failure to obtain polyethers with higher molecular weight in the anionic polymerization of perfluoroolefins oxides is the reaction of inherent chain transfer. Kinetic investigations have shown that the chain growth takes place on ionic pairs and the chain transfer occurs on free ions.
In ionic pair the attack to electrophilic center of epoxyde is performed by oxygen atom accompanied with polymer chain growth (45):
Tetraglyme (TG) added to a solution of cesium alkoxyde in tetrahydrofurane (up to a ratio of 1:1) converts contact ion pair of cesium alkoxyde into solvent-separated ion pair which activity is by an order of magnitude higher due to bond loosening:
RfCF2 O-,Cs+ TG = > RfCF2 O-,TG, Cs+
This does not change the rate constant of chain transfer but significantly increases the constant of rate growth. But at a ratio of tetraglyme/alkoxyde above 2, electroconductivity of the solution jumps, which may be resulted from the formation of complex, dissociated to ions:
An increase in the concentration of free alkoxy anions leads to an increase in the reaction rate of inherent chain transfer. In this case nucleophilic attack is performed by more electronegative fluorine ion (45):
Ponomarenko et al.(47,48) have established, that the oligomerization rate of hexafluoropropylene oxide at small conversions is in proportion to the first degree of the oxide concentration or to the concentration of CsF that is close to the first order (0.8).
There is proposed a wide range of substances of different classes as solvents for the polimerization in the presence of ionic fluorides besides glymes such as dioxane, acetonitrile, propionitrile, benzonitrile, tetrahydrofurane, dimethylsulfoxide, acetone and so on (3).
The data mentioned above were received at the investigation of polimerization conditions of hexafluoropropylene oxide, and it was considered impossible to use these catalytic systems and solvents for TFEO polymerization because they reacted vigorously with monomer (15-17). The authors believe that this statement is mistaken because TFEO is polymerized in the mentioned systems in good yields.
USA patent (22) propose alkane halides as solvents of the following general formula: XCpF2pCH2Cl
where X = H or Cl, p =1-12, such as methylene chloride, chloroform, 1,2-dichloroethane, 1,1,2-trichloroethane.
These compounds dissolve the oxide and are inert with respect to it. Just as fluorides of alkali metals are practically insoluble in solvents inert to TFEO, so to initiate TFEO polymerization there are mostly used salts of quaternary ammonium, phosphonium and arsonium bases (22,36,37).
The reaction temperature is usually kept within a range of -80-50oC, preferably in a range of -25-30oC. Thus, TFEO polymerization in a medium of 1-chlor-2,2,3,3-tetrafluoropropane in the presence of tetraethylammonium cyanide at a temperature of -25oC results in an oligomer mixture of the general formula CF3CF2O-(CF2CF2O)nCF2C(O)F with a degree of polymerization n =0-50. The TFEO conversion is 100% (36). Individual oligomers with n= 0-10 were obtained by rectification.
A similar method is used for producing TFEO copolymers with fluorinated ketones and fluoroanhydrides of mono- and dibasic carbonic acids (22, 24,26).
TFEO polymerization in the presence of aliphatic and cyclic tertiary amines at a temperature of -110 to -30oC results in polytetrafluoroethylene oxide with an intrinsic viscosity of 0.05-0.155 (49). Catalysts, trimethylamine, triethylamine, tributylamine, pyridine, N-methylmorpholine, N-ethylpyridine etc., are taken in amount of 0.05-5% mol with respect to TFEO. To increase the molecular weight of polymer there are used additions of copper salts, such as carbonate, fluoride, in amount of 0.0005-0.001 equiv per mole of epoxyde (50).
TFEO addition to perfluoroketones and fluoroanhydrides of perfluorocarbonic acids may be performed in a system of glyme/fluoride of alkali metal also (27-29). In this case products of TFEO reaction with solvent described by Warnell (15,16) do not appear.
Obviously, a reason of this is more high reactivity of fluoroanhydrides and perfluoroketones in the reaction with ionic fluorides in comparison with perfluorinated oxides (43). The reaction starts with alkoxide formation:
RfC(O)F + Cs+F- = > RfCF2O-Cs+
which reacts in its turn with TFEO:
Therewith the rate of the TFEO reaction with alkoxide is higher than the rate of the TFEO reaction with diglyme. Thus, the system diglyme/cesium fluoride can be used successfully for the synthesis of TFEO oligomers if the oligomerization is carried out in the presence of fluoroanhydrides. At these conditions TFEO does not react with diglyme but some amount of TFEO homopolymers are observed to form.
Conclusion.
The TFEO polymerization is of the main interest among the mentioned TFEO reactions because polyethers produced have found a wide application due to their excellent performance (high thermal and chemical stability, low glass transition temperature, radiation and oxidizing resistance etc.). The polyethers base on TFEO in contrast to perfluoropolyethers, produced by copolymerization of TFE and oxygen, have a more regular structure and hence higher qualitative indices.
Much data available in literature are of probabilistic and hypothetical nature due to
the absence of research on elemental acts of the polymerization process of perfluorinated -oxides, especially TFEO
because of their exceptional complexity. But a detailed analysis of influence of different
factors (temperature, pressure, solvents, catalysts etc.) on the TFEO polymerization
performed in the study of Ponomarenko V.A., Kroukovsky S.P., Alybin A.Yu. " Fluorine
containing heterochain polymers" makes it possible to clear the process nature and to
use it for development of advanced materials on TFEO base.