An Environmentally Friendly Class of Fluoropolyether: Α,ω-dialkoxyfluoropolyethers

The α,ω-dialkoxyfluoropolyethers (DA-FPEs) characterized by the structure R H O(CF 2 CF 2 O) n (CF 2 O) m R H have been developed as a new class of environmentally friendly hydrofluoroethers (HFEs) suitable as solvents, long-term refrigerants, cleaning fluids, and heat transfer fluids. Synthetic methodologies for DA-FPEs described here consist of radical-initiated oxypolymerization of olefin, peroxy-elimination reaction in peroxidic perfluoropolyethers (P-PFPEs) and further chemical modification of α,ω-diacylfluoride PFPE. The physical properties of selected α,ω-dimethoxyfluoropolyethers (DM-FPEs) have been evaluated and compared with analogous hydrofluoropolyethers (HFPEs) having-OCF 2 H as end-groups. Atmospheric implications and global warming potentials (GWPs) of selected DA-FPEs are also considered.

The incorporation of two alkoxy-groups provides reactive sites, which limit their atmospheric lifetime [1][2][3]; therefore, when compared with perfluoropolyethers (PFPEs), they have a lower environmental impact in terms of global warming potential (GWP) [1].Furthermore, perfluorinated ether units from polymeric backbones also makes them well characterized by typical properties of PFPEs, like high thermal and chemical stability, no acute toxicity and excellent heat exchange properties [4].Lastly, due to the presence of the above-mentioned alkoxy-groups as ending groups, they show good solvent properties with several organic liquids, such as ketones and alcohols [5].All these appealing properties of DA-FPEs make them excellent candidates as CFC, perfluocarbons and halons substitutes in a number of applications, like foaming and fire extinguishing agents, cleaning agents for sophisticated electronic devices and heat transfer fluids [6][7][8].

Syntheses of α,ω-Dialkoxyfluoropolyethers
The synthesis of DA-FPEs described here involves a combination of a few innovative and conventional synthetic technologies.The whole process includes the following steps: (1) The oxidative polymerization of perfluoroolefinic monomers into peroxidic perfluoropolyethers (P-PFPEs) as a precursor; (2) the subsequent reduction of P-PFPEs to produce diacyl fluorides perfluoropolyether (DAF-PFPE); (3) and the further chemical modification of α,ω-diacyl functional groups of DAF-PFPE to yield the final products α,ω-dialkoxyfluoropolyethers.

Oxypolymerization of Perfluoroolefin
Oxidative polymerization of perfluoroolefinc monomers like tetrafluoroethylene (TFE) by molecular oxygen into P-PFPEs (A) described in Figure 1, is a well-known reaction [9].The forming peroxidic polymer has the following general structure: TO-(CF 2 CF 2 O) p -(CF 2 O) r -(O) q -T', comprising fluoroether repeating units (-CF 2 CF 2 O-and -CF 2 O-), interspersed peroxy units (-CF 2 CF 2 OO-and -CF 2 OO-) and perfluorinated alkyl groups, acyl fluoride or fluoroformate as chain end-groups (T, T').Practically, this oxypolymerization can be activated either by high energy UV light or by employing elemental fluorine or perfluoroalkyl hypofluorites, especially CF 3 OF, as chemical initiators [9].The peroxidic polymer is the result of a complex free radical oxidation reaction dominated by the chemistry of perfluoro oxyradicals [10].The composition and the reaction yields of forming PFPE polymers depend on irradiation condition, TFE concentration, fluid dynamics of the system and other classical physical parameters like temperature and pressure.Commonly, the average molecular weight of formed P-PFPEs (A) is more than 10 4 amu [11].Carbonyl fluoride and tetrafluoroethylene oxide are the main by-products.

Deperoxidation of P-PFPEs
α,ω-Diacyl PFPE can be obtained by thermal treatment or chemical reduction of peroxide bonds present in the above mentioned peroxidic precursor.Under the appropriate thermal treatment, homolysis of fragile peroxide bonds in P-PFPEs (A) produces PFPE alkoxy radicals; subsequently, the cleavage of the bond in ß-position to the oxygen-centered radicals leads to the formation of reactive carbon-centered PFPE radicals [12,13].The fast cage-combination reaction of these radicals takes place and leads to a PFPE material in which acyl fluoride and fluoroformyl moieties are nearly 50% of the end groups (Figure 2).Generally, above 150 °C those fluoroformate groups can also be converted into acyl fluorides.After the thermal treatment of the peroxidic PFPE, the reduced PFPE products can be distinguished into two fractions: The major one includes PFPE with monoacid end-groups, while the minor one comprises completely neutral (perfluoroalkyl end-groups) and diacyl fluoride PFPEs.3).The chemical reduction is performed with reducing agents, such as hydrogen iodide or with molecular hydrogen in the presence of a noble metal catalyst (e.g., Palladium) [14].

Preparation of DA-FPEs by DAF-PFPE Alkylation
Finally, hydrofluoropolyethers DA-FPEs are prepared through the reaction of diacyl fluoride PFPEs (DAF-PFPEs) with electrophiles in the presence of a source of fluorine ions, generally, metal fluorides.This process actually comprises two steps (Figure 4): Firstly, in aprotic polar solvent, DAF-PFPEs react with fluoride ions released from metal fluorides to form metal perfluoroalkoxides M +− OCF 2 RFCF 2 O − M + as intermediates; secondly, metal perfluoroalkoxide is alkylated by a suitable alkylating agent X-R H leading to the introduction of alkoxy groups at the end of the perfluoropolyether chain.Metal perfluoroalkoxides are known as compounds characterized by low nucleophilicity [15] and they decompose into acylfluorides and metal fluorides, generally around room temperature.In order to increase the stability and the nucleophilic strength of these species, polar aprotic solvents (e.g., diglyme, tetraglyme) are generally required.The stability of these species depends on the nature of metal fluoride [16], temperature, and the solvent [17].Metal perfluoroalkoxides are also important industrial intermediates in the synthesis of hypofluorites like CF 3 CF 2 OF [18] and fluorinated vinylethers [19,20].Completely anhydrous reaction conditions are required, because in presence of water the hydrolysis of acyl fluoride groups generates hydrofluoric acid, HF, which can be absorbed by the solid metal fluoride giving MHF 2 [16].Because of the low nucleophilicity of perfluoroalkoxides, strong alkylating agents with high electrophilicity are needed to effectively react with metal perfluoroalkoxides and form DA-FPEs.

Alkylation of PFPE Dialkoxides
In the past years, there have been many disclosures reported on finding effective alkylating agents suitable for perfluoroalkoxides.Dialkylsulfate was one of the first specific reagents used [21], because of its strong alkylating ability.In fact, it has been used to alkylate almost every nucleophile [22,23], but the alkylating temperature of dialkylsulfate needs to be lower than 20 °C and undesired viscose gels can be formed at this temperature and remarkably reduce the alkylation rate and yield.Moreover, dialkyl sulfates are highly toxic and their carcinogenic activity makes them inapplicable for large-scale industrial production.Dialkyl sulfites and the alkyl ester of polysulphonic PFPE (Figure 5) were also proposed and applied as alkylating agents in reactions with acylfluorides to synthesize hydrofluoroethers (HFE) [6,24].Compared with dialkyl sulfate, dialkyl sulphite and the alkyl ester of polysulphonic PFPE are considered non-carcinogenic, and their alkylating temperature over 100 °C avoids the formation of undesired gels.However, the molar yield of the product resulting from the use of these two alkylating agents is low (below 20%), in addition, the preparation of alkyl ester polysulphonic PFPE is particularly difficult.
Fluorovinyl alkyl ethers, (CF 3 ) 2 C=CFO-R H , were also considered as one of the most effient but expensive class of alkylating agent for metal perfluoroalkoxides [25].A high molar yield of HFEs was achieved by using heptafluoro-iso-butenyl methyl ether [26] in the reaction with metal perfluoroalkoxides, shown in Figure 6(a).A nonvolatile conjugated base (CF 3 )C=CO − FK + was formed as by-product after the reaction that can be hydrolyzed into its carboxylic acid (CF 3 ) 2 CHC(O)OH.Lastly, the carboxylic acid was decarboxylated with an aqueous base to form hexafluoropropane as another valuable product; this reaction is shown in Figure 6(b).Fluorovinyl alkyl ethers are commercially available products and they can also be prepared following the synthetic route described in Figure 6(c): The reaction between perfluoro-iso-butene (1) and a proper alcohol produces an octafluoro-iso-butyl alkyl ether (2) and by its subsequent dehydrofluorination the alkylating agent (3) and HF are synthesized.Newly developed alkylating agents which have strong a alkylating ability and less toxicity have been emerging in the past years, but these kinds of alkylating methods have an unavoidable common drawback: A stoichiometric amount of metal fluoride is usually needed to provide metal perfluoroalkoxides, and large amounts of partially reacted solid metal fluoride is left over after the reaction.

Catalytic Alkylation of Perfluoro(mono)acyl Fluorides and DAF-PFPE
A classical example of catalytic alkylation is the alkylation of perfluoroacyl fluorides performed by employing SbF 5 as acid catalyst in anhydrous medium that reacts with alkyl fluoride to produce HFE (Figure 7).The first step in this pathway is the reaction of SbF 5 with alkyl fluoride forming high valence metal SbF 6 -.The extra F ion from SbF 6 -attacks the carbonyl group of acyl fluorides forming alkoxides.Subsequently, the nucleophilic addition to in situ generated alkoxides produces the PFPE with alkoxy-group endings.In this reaction, work-up with a large amount of partially reacted metal fluoride is avoided; however, the drawback of this method is that the active acid catalyst may promote undesired isomerization or decomposition reactions and consequently deliver low or erratic yields [27].This fluoride-catalyzed fluoroformates alkylating reaction gives DA-PFPEs in good yield and CO 2 is the only by-product.Due to the big differences of boiling points among alkyl fluoroformates, DAF-PFPE and DA-PFPEs, un-reacted alkylating reagents can be easily restored through a standard distillation.Alkyl fluoroformate can be methyl, ethyl as well as allyl fluoroformates.The great variety of fluoroformate alkylating reagents improves the utility of this straightforward alkylating reaction.Examples of several successfully employed fluoroformates, which reacted with high boiling point linear DAF-PFPE with good selectivity and yield are reported in Table 1 [28,29].This catalytic alkoxides alkylation followed two supposed pathways (Figure 9): (a) A nucleophilic addition of the perfluoroalkoxide to alkyl fluoroformate directly forming the DA-FPE and CO 2 ; or (b) through the carbonate intermediate, R F CFOC(O)OR H , followed by a second nucleophilic substitution to generate the corresponding products [28].According to both pathways, the rearrangement of the fluoroformate group can release fluorides and replenish the perfluoro-alkyloxy-anions formation by reacting with a new acylfluoride.Therefore, alkyl fluoroformates can be applied not only as alkylating agents but also as an alternative fluorine ion source for the alkoxide formation.

Boiling Point
The boiling points (b.p.) of DM-FPEs are reported in Figure 10 as a function of the number of backbone chain atoms (N), i.e., carbon plus oxygen atoms.They increase regularly with N. In Figure 10, DM-FPEs boiling points are also compared with those of α,ω-dihydrofluoroethers (HFEs) [31] and perfluoropolyethers (PFPEs) [32].The highest boiling points are those of DM-FPEs, the intermediates are HFEs and the lowest ones are PFPEs.The above-mentioned result reflects the typical low intermolecular interactions of perfluorinated compounds.Partially fluorinated ethers, like HFEs and DM-FPEs, display higher boiling temperatures and this is indicative of greater intermolecular polar interactions [11].
Considering three similar fluoromethylethers, CF 3 OCF 3 (b.p. 214 K; μ = 0.49 D), CF 2 HOCF 3 (b.p. 238 K, μ = 1.18 D) and CH 3 OCF 3 (b.p. 250 K, μ = 2.56 D), the boiling points and the dipole moments have analogous tendencies [33].The progressive substitution of fluorine atoms with hydrogen in the methoxy end caps leads to a stronger dipole and consequently, to an increase of the boiling point, as shown in Figure 10.The boiling point differences between PFPEs and HFEs and between PFPEs and DM-FPEs are not constant but decrease continuously with the chain length.Since the attractive forces are due to polar terminal groups, their relative intensity decreases as N increases and, as a result, the boiling points of HFEs, DM-FPEs and PFPE tend to an asymptote at higher N values.

Density
A comparison of the specific volume (V sp ) at 298 K of DM-FPEs, HFEs and PFPEs series is shown in Figure 11.The DM-FPEs sequence shows the highest specific volume and the PFPEs the lowest at identical number of chain atoms, N. The V sp of PFPEs is almost constant and independent of N. The specific volumes of DM-FPEs and HFEs decrease continuously to a plateau that approximately coincides with the constant V sp of PFPEs.This behavior can also be explained on the basis of the differences between terminal groups.Moving from -CF 3 to -CF 2 H and to -CH 3 , the molecular weight of the groups diminishes; on the contrary, the variation of volume is less noticeable because the Van der Waals radii of fluorine and hydrogen are similar [34]: 0.147 and 0.12 nm, respectively.Consequently, the difference in specific volume between the three series is mainly due to the variation of molecular weight, explaining the tendencies of V sp observed in Figure 11.Furthermore, as the weight fraction of the chain ends decreases with N, it is logical that the specific volume of HFEs and DM-FPEs tends to the V sp values of PFPEs when the chain is sufficiently long.

Vapor Pressure and Vaporization Enthalpy
The vapor pressure data, plotted as Clausius-Clapeyron equation in Figure 12, show a deviation from linearity at low temperatures, which is common for many oligomeric substances.Vaporization enthalpies (ΔHv) are also derived from the Clausius-Clapeyron plot in the range of temperatures in which the function is linear.
The ΔH v data of DM-FPEs, reported as a function of number of chain atoms, N, are compared with those of HFEs and PFPEs in Figure 13.These data indicate the highest values for DM-FPEs, intermediate for HFEs and the lowest for PFPEs.As the effect of the polar terminal groups (-OCF 2 H and -OCH 3 ) diminishes with the length of the chain, ΔH v also decreases and tends to the values of PFPEs when N is above 15 [4].

Refractive Index
The refractive indices of the series DM-FPEs and HFEs plotted as a function of the number of chain atoms, N, indicate two opposed behaviors (Figure 14).In particular the refractive index n increases for DM-FPEs and decreases for HFEs.In both cases, increasing the chain length, the refractive indices tend to a plateau value, close to the characteristic refractive index of linear PFPEs (1.290-1.295)[11].The tendencies of DM-FPEs and HFEs refractive indices are mainly due to the terminal groups that contribute to n in an opposite way.As reported in the literature, the group influence of -OCH 3 is higher than that of -OCF 3 , while that of -OCF 2 H is lower [35].

Atmospheric Chemistry of DA-FPEs
The absence of chlorine atoms in HFEs makes them possess a negligible ozone depleting potential (ODP), but the presence of C-F and C-O bonds may absorb in the terrestrial infrared radiation region (800-1200 cm −1 ), and could play a significant role as a greenhouse gas.Therefore, considerable attention has been paid in recent years to perform both experimental and theoretical studies on atmospheric chemistry of HFEs.However, surprisingly, only relatively little information about DA-FPEs has been reported [36,37].Since the only available data on DA-FPEs compounds are related to DM-FPEs with structure CH 3 O(CF 2 CF 2 O) n CH 3 that only contain the -(CF 2 CF 2 O)-units, the discussion has been focused prevalently on the atmospheric chemistry of these compounds.

Atmospheric Lifetimes and Decomposition Mechanism of α,ω-Dialkoxyperfluoropolyethers
In 2004, the first understanding of the atmospheric chemistry for DM-FPEs was achieved by studying the kinetics and mechanism of CH 3 O(CF 2 CF 2 O) n CH 3 (n = 1-3) oxidation which is initiated by Cl atoms and OH radicals in the atmosphere; the impact of their degradation products on the climate were also discussed [1].Like most organic compounds, the primary tropospheric degradation of CH 3 O(CF 2 CF 2 O) n CH 3 is mainly dominated by the attack of OH radicals [38].Understanding of reaction kinetics is essential for the evaluation of the atmospheric lifetime of these molecules.In addition to the reaction with hydroxyl radicals (OH), organic compounds can also be removed from the atmosphere via photolysis, wet deposition, and reaction with NO 3 radicals, Cl atoms, and O 3 [38][39][40].For saturated compounds such as CH 3 O(CF 2 CF 2 O) n CH 3 , reactions with NO 3 radicals and O 3 are typically too slow to be of importance [1].The average concentration of Cl atoms in the troposphere is orders of magnitude less than that of OH radicals [41].Reaction with Cl atoms will be a less significant atmospheric loss mechanism.Since ethers do not absorb at UV wavelengths > 200 nm, photolytic sink will be not be an important degradation pathway [42] either.Highly fluorinated molecules, such as CH 3 O(CF 2 CF 2 O) n CH 3 , are hydrophobic and wet deposition is unlikely to be of importance.In conclusion, the atmospheric lifetime of CH 3 O(CF 2 CF 2 O) n CH 3 is determined by its reaction with OH.
Smog chambers equipped with FTIR spectrometers were used to study the Cl atom and OH radical initiated oxidation of CH 3 O(CF 2 CF 2 O) n CH 3 in 720 Torr of air at 296.3 K. Relative rate methods were used to measure k(Cl+CH 3 O(CF 2 CF 2 O) n CH 3 )) = 3.7 (±0.7) × 10 −13 and k(OH+CH 3 O(CF 2 CF 2 O) n CH 3 ) = 2.9 (± 0.5) × 10 −14 cm 3 molecule −1 •s −1 .The relative rate method is widely used to measure the reactivity of OH radicals with organic compounds [43].The hydrogen abstraction reaction from the terminal groups has been assumed, according to established literature data, as the rate-determining step for the decomposition of the entire molecule [44].It is well known that -CF 2 CF 2 -units are unreactive toward Cl and OH radicals.Thus, the reactivity of CH 3 O(CF 2 CF 2 O) n CH 3 is confined to the C-H bonds on either end-groups of the molecule.Indeed, appropriate studies showed that the reactivity of the methyl group did not change noticeably by increasing the number n of -(CF 2 CF 2 O)-units from 1 to 3 [45].Nevertheless, the average concentration of Cl atoms in the troposphere is orders of magnitude less than that of OH radicals [41].Thus, knowing the k(OH+CH 3 O(CF 2 CF 2 O) n CH 3 ) s rate constant, the DM-FPEs atmospheric lifetime has been evaluated to be two years [1].[1].Concerning the impacts of fluoroformates on the climate, since fluorinated esters are known to be easily hydrolyzed [46,47], it is usually assumed that they can be easily removed though wet deposition; therefore Andersen et al. [1] deduced that formate CH 3 O(CF 2 CF 2 O) n C(O)H is not expected to be persistent or pose any significant environmental hazard.However, Bravo et al. [48] assumed that the highly fluorinated nature of fluoroformates may decrease their solubility in water and could contribute a significant indirect GWP.Up to now, due to the lack of experimental data like gas-to-water equilibrium and solubility test of fluoroformates, it is still unclear how rapidly fluoroformates will be removed via uptake and hydrolysis in rain/cloud/seawater [49].Further experimental examinations are needed for the determination of the atmospheric implication of fluoroformats.

Global Warming Potential
In 1995, Pinnock et al. outlined a method to determine the instantaneous forcings (IF) from the IR absorption spectra such as those reported in Figure 14.As the values of GWP can be estimated on the basis of the IF, in Table 3 GWP relative to carbon dioxide of DM-FPEs at 100 year time horizons has been reported, and the comparison with other greenhouse gases, such as CF 3 Cl (CFC-11), CF 2 HOCF 3 (HFE125), CH 3 OCF 3 (HFE143a) and other HFEs, is reported in Table 3.It is evident from Table 3 that DM-FPEs have relatively high instantaneous forcing (IF) but much lower GWPs, if compared with CFC11 and HFE compounds.In addition, the higher the number of -(CF 2 CF 2 O)-units, the higher the absorption in the 800-1200 cm −1 range, that is responsible for the high IF.Thus the reason of the mitigation of DM-FPEs GWP is mostly due to their short atmospheric lifetimes and the emission of DM-FPEs into the atmosphere should not contribute significantly to the climate change.
On the basis of the data of DM-FPEs which contain only -(CF 2 CF 2 O)-units and the studies on series of HFPE with -CF 2 H end-groups [51], it seems reasonable to expect that the reactivity of C-H bonds is independent from the number of -CF 2 O-units in the DM-FPE molecule.However, there are no available kinetic data for DM-FPEs with structure CH 3 O(CF 2 CF 2 O) n (CF 2 O) m CH 3 to confirm this assumption.
In addition, DM-FPE commercial products are mixtures of homologous copolymers with similar lengths but different unites, i.e., different n and m; further experimental studies are most likely necessary for a better understanding of the effects of the (CF 2 CF 2 O) n /(CF 2 O) m ratio on atmospheric chemistry of DM-FPEs.

Figure 8 .
Figure 8. Reaction between DAF-PFPEs and alkyl fluoroformate with metal fluoride as source fluorine ions.

Figure 9 .
Figure 9. Reaction mechanism for the synthesis of DM-FPEs.For clarity, here using R f -C(O)F to represent DAF-PFPEs.

Figure 14 .
Figure 14.Refractive index (n) at 293 K as a function of the number of chain atoms, N, for DM-FPEs (×) and HFEs (□), and the range of refractive index of linear PFPEs.
Tuazon et al. claimed that in air the oxidation of HFEs with difluoromethoxy ending groups, OCF 2 H, like HCF 2 OCF 2 OCF 2 CF 2 OCF 2 H, HCF 2 OCF 2 CF 2 OCF 2 H, and HCF 2 OCF 2 OCF 2 H, gives C(O)F 2 as the only carbon-containing product [46].For DM-PFEs which end with methoxy-groups, fluoroformates CH 3 O(CF 2 CF 2 O) n C(O)H were recognized as the only oxidation decomposition products with a 100% yield; the formed fluoroformates can be further oxidized to di-formates H(O)CO(CF 2 CF 2 O) n C(O)H

Table 3 .
Atmospheric lifetimes and global warming potential (GWP relative to carbon dioxide) of DM-FPEs, CFC and HFE.