The functionalization of fluoroelastomers: approaches, properties, and applications

Abstract

This review seeks to give an overview of different approaches to prepare functional fluoroelastomers for modern industries. These approaches can be divided into three main families of alternatives. The first extensively describes the preparation of telechelic fluoroelastomers based on chemical degradation. Carboxyl-terminated fluoroelastomers are usually achieved via a typical degradation process, which then can be changed into various functional group-terminated fluoroelastomers. The second deals with functionalization of fluoroelastomers through grafting reactions. Conventional strategies, such as radical polymerization, dehydrofluorination and sulfonation, are used to graft single or faintness functional monomers onto fluoroelastomers. Reversible deactivation radical polymerization, however, provides quantitative copolymers containing unique functional groups. The third alternative route, namely functionalized copolymerization, concerns two main types: functionalization of an initiator and functionalization of monomers. In several cases, both will be utilized to synthesize special functional fluoroelastomers. In addition to the three approaches, the properties and applications of processed fluoroelastomers are illustrated to explore their functions and influences after functionalization.

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Yu Wang

Dr Yu Wang received his bachelor degree in Polymer Material Science and Technology at Harbin Institute of Technology (HIT) in 2010. He continued to pursue a PhD degree under the supervision of Prof Yongping Bai. His main research was focused on the synthesis and applications of functional fluoropolymers.

Yongping Bai

Prof Yongping Bai received his PhD degree in Polymer Material Science and Technology at Harbin Institute of Technology (HIT) in 1996. His main research interests include the synthesis and applications of functional fluoropolymers, polyesters and polyacrylate. His research team has successfully developed more than twenty novel products under his direction, including fluorinated coatings, optical BOPET films, flexible polyesters, and special acrylic adhesives. He is also a member of the Editorial Boards of Materials and Design, Materials Letters, Applied Composite Materials and Journal of Applied Polymer Science.

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1. Introduction

Fluorinated polymers always function as specialty materials owing to their unique properties including high thermal, chemical, aging and weather resistance, excellent inertness to solvents, hydrocarbons and acids as well as low dielectric constants, flammability, and moisture absorption.^1–5 As one of the most fascinating fluorinated polymers, fluoroelastomers are desirable in many applications that deal with problems of harsh environments and the war industry.^3,6–8 These commercial applications include fuel tank hatches, joint sealants, fuel vapor seals, valve seals, hoses, coated cloth, shaft seals and O-rings for hydraulic systems in aircraft and aerospace applications.^3,6,9–11 The outstanding performance in these challenging applications inspires extensive research on the synthesis of fluoroelastomers with controlled structures and required functions.¹²

However, commercial fluoroelastomers have some drawbacks. For example, the vulcanization process of fluoroelastomers has to be separated into two steps at high temperature and meanwhile generates a lot of unsaturation sites in the polymer backbone via dehydrofluorination.¹³ Therefore, the development of simple pathways for the cure of fluoroelastomers has always been a focal point.^14–20 Due to the presence of hydrogen bonds in fluoroelastomers, only minor changes in conformational mobility and steric hindrance of C–F hand, and hence, their performances at low temperature are unsatisfactory.²¹ Moreover, quite a few studies have focused on the blend modification of fluoroelastomers,^21–27 so some functional fluoropolymer compatibilizers^28–31 should be explored to achieve compatibility³² between fluoroelastomers and other materials. The new functions and new curing chemistry of fluoropolymer materials, as well as the compatibility technologies with conventional materials, can be categorized into the functionalization of fluoroelastomers.

In the past half century, functionalized modification played a positive role in the devolvement of fluoroelastomers. The incorporation of soft segments made it possible for fluoroelastomers to be applied at even lower temperatures, i.e. −50 °C (some products can even keep elasticity at such low temperatures as −80 °C or below).^33–37 In some cases, the vulcanization systems and process have even been simplified and conducted at room temperature.^{16–18,20,38} Additionally, perfluoroelastomers take fluoroelastomers to extreme performance in medium resistance and thermostability.^39–41 Fluoroelastomers have also been used to prepare proton exchange or conduct membranes for fuel cells on a trial basis.^42–46

Major efforts in early stages focused on copolymerization with functional monomers to adjust the properties of fluoroelastomers. A series of functional fluoropolymers were produced by DuPont, Dow Corning, 3M, Solvay Special Polymers, Asahi Glass Daikin, and ShinEtsu, etc.^47–50 To date, copolymerization is still a practical approach to achieve the functionalization of fluoroelastomers in spite of high prices. To lower the cost, exploration for a cost-effective degradation pathway for functional fluoroelastomers has been on the increase since success^51,72 with the carboxyl-terminated fluoroelastomers via chemical degradation. Recently, grating reaction also exhibited its advantages in the functionalization of fluoropolymers through controlled polymerizations,⁵² radical substitution,²⁹ radiation,⁵³ and dehydrofluorination.⁵⁴

Over the past 20 years, developments in the preparation and applications of commercial fluoroelastomers and fluoroplastics have been summarized in several review articles and books.^{3,6,33,45,48,49,55–61} Our intention is not to provide another wide spanning review of fluoroelastomers, but rather to describe recent developments of fluoroelastomers from the perspective of functionalized modification. Hence, we attempt to systematically summarize the routes to incorporate functional groups in fluoroelastomers; namely, the functionalization of fluoroelastomers. In order to provide more inspiration for readers, some important references about the functionalization of important fluoropolymers containing chlorotrifluoroethylene (CTFE) and hexafluoropropylene (HFP) are included in the review as well.

2. Functionalization via chemical degradation

2.1 Carboxyl-terminated fluoroelastomers

The chemical degradation of fluoroelastomers is conducted in two main steps: dehydrofluorination reaction and chain scission reaction.^62–66 Fluoroelastomers show poor resistance to the alkaline medium,⁶⁴ which provides a solid foundation for their degraded functionalization. Fluoroelastomers, as one of the halohydrocarbons, tend to eliminate β-H and fluorine atoms in the backbone,⁶⁷ and then unsaturated bonds are introduced into fluoroelastomers under the alkali's attack. In this process, Hofmann's rule is followed (this is different from other hydrochloric ethers⁶⁸). After π bonds in unsaturated structures are broken by oxidants, carbonyl groups (with weak oxidants) and carboxyl groups (with strong oxidants) are obtained.⁶⁹ The mechanism of the oxidative degradation reaction is illustrated in Fig. 1.³⁸

Fig. 1 Mechanism of chemical degradation reaction. Reprinted with permission from ref. 38.

It should be noted that not all the vinylidene fluoride fluoroelastomers can be used as raw materials for preparation of carboxyl-terminated fluoroelastomers via chemical degradation. Schmiegel realized that only those fluoroelastomers whose backbones involve sensitive groups (as shown in the following part) to alkali are qualified:⁷⁰

The molecular weight and carboxyl end group value of carboxyl-terminated fluoroelastomers are significantly dependent on alkaline strength, solvent solubility, and oxidant species. Oxidants can be chosen from H₂O₂, KMnO₄, OsO₄, K₂Cr₂O₇–H₂SO₄, O₃, etc. Various alkalis mainly include hydroxide and amines. And solvents such as acetone, butanone, THF, and DMF can dissolve fluoroelastomers.⁷¹

Degradable methods can be separated into two alternatives: one-step and two-steps. The first type of method adds both alkalis and oxidants at the same time to achieve carboxyl termination. Mizuide et al.⁶² and Li et al.^38,51 took this pathway so as to degrade commercial fluoroelastomers into low-molecular weight productions. The maximum COOH percentage can reach 3.6% during the process of degradation. But, so far there is no evidence to prove that carboxyl end groups can be found in every fluoropolymer. Carbonyl or aldehyde groups are also distributed at the ends of polymers.

The second alternative is carried out in the following steps: (i) add alkali for dehydrofluorination; (ii) separate fluoride copolymers and add oxidants to accomplish the oxidation reaction. The oxidants used in these types of methods are stronger, compared to the one-step degradation. Cluff et al.⁷² heated THF mixtures of fluoroelastomers and amines, and then separated and oxidized the products with KMnO₄. Similarly, Mizuide⁷³ prepared carboxyl-terminated fluoroelastomers (COOH% = 0.2–3.75%) through the oxidation reaction of KMnO₄ or HNO₃. Because the Mn element in KMnO₄ could cause pollution in the environment, such processes should be well controlled to avoid pollution. So the one-step degradation methods are relatively more moderate, simpler, and more environmentally friendly.

2.2 Other activated group-terminated fluoroelastomers

Different types of telechelic fluoroelastomers can be prepared by virtue of esterification and reduction of a carboxyl (as shown in Fig. 2). Qi's team^38,74–79 devoted a good deal of effort to obtain a series of telechelic fluoroelastomers (Fig. 3). Activated pentafluorophenol (PFP),⁷⁷ silane,⁷⁸ carborane,⁷⁷ sulfonyl,⁷⁴ hydroxyl^74,79 and amino⁷⁴ were introduced via such reactions as the Steglich reaction and Mitsunobu reaction. Different approaches for the same products always result in different conversion percentages. For example, both direct amidation and indirect methods by the Steglich reaction are alternatives to prepare silane-terminated fluoroelastomers, but the carboxyl conversion of direct esterification produces just 71.0%.³⁸ Meanwhile, the carboxyl conversion of indirect esterification using mica ester produces 88%, which is attributed to an easy reaction between PFP and AMEO.^38,78 AlLiH₄ and NaBH₄ are both effective reductants in the conversion reaction to hydroxyl-terminated fluoroelastomers; however, different conversions will be observed. Although AlLiH₄ displays such strong reducibility that even the carbonyl of a rearrangement reaction can be reduced to a hydroxyl, no hydration process is required.⁷⁴ Hence, the application of AlLiH₄ in large-scale production is limited. On the other hand, because NaBH₄'s reducing capacity is rather weak, it can only exhibit its reducing capacity for carboxyl in the presence of a Lewis base.^74,79 But the prerequisites for the reaction of borohydride complex systems are rather milder, compared to AlLiH₄.

Fig. 2 Various routes to preparation of function-terminated fluoroelastomers.^38,75–79

Fig. 3 Grafting mechanism of an aromatic-containing amine onto the poly(VDF-co-HFP) copolymer. Reprinted with permission from ref. 111.

Combinations of chemical degradation and conversion reactions have been proven to be a simple way to obtain a wide range of functional fluoroelastomers. But, some problems still arise. For instance, how to eliminate unsaturated C=C double bonds thoroughly in an oxidation process remains a great challenge in degradation methods. After degradation, functional fluoroelastomers show a low tensile strength. This drawback will reduce their lifetime in their application in shock absorbers.

3. Functionalization via grafting reaction

Grafting reaction is a significant pathway to functionalize fluoropolymers by hanging functional groups such as carboxyl,⁸⁰ hydroxyl,⁵³ sulphonic acid,⁸¹ and carbonyl⁵² onto their backbones. Many methods have been adopted to achieve a large variety of fluoride grafts, including chemical oxidation,^54,82–84 free radical polymerization (FRP),^29,30 high-energy inducement (HE)⁵³ and several novel chemical modifications.⁸⁵ Table 1 shows the utilized monomers and corresponding methods for functionalization of different fluoropolymers.

Table 1 Functional monomers and grafting technologies for functionalization of different fluoropolymers

Fluoropolymer	Functional monomer	Grafting technologies	Reference
VDF-HFP		FRP	29
VDF-HFP		FRP	30
VDF-CTFE		ATRP	52
VDF-CTFE		ATRP	52
VDF-HFP		O3/O2 treatment	53
VDF-HFP		Dehydrofluorination	54
VDF-HFP		Gamma radiation	80
VDF-CTFE-TBPAC	CH2=CF2	FRP (macroinitiator)	86 and 87
VDF-HFP-TFMAA		Mitsunobu reaction	88
VDF-HFP		Dehydrofluorination	89
VDF-HFP		Gamma radiation	90–96
VDF-CTFE		ATRP + sulfonation reaction	60, 85 and 97
VDF-CTFE		ATRP	98
VDF-CTFE		ATRP	99
VDF-HFP		Dehydrofluorination	100
VDF-HFP		Dehydrofluorination	101 and 102
VDF-HFP		FRP	103–106

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3.1 Grafting via radical polymerization

Grafting silicones from fluoroelastomers via radical grafting is one effective approach for preparing fluorinated silicone materials.^107,108 Guo et al. proposed a grafting process which is separated into three sections: the formation of macro radicals, grafting VTEO, and radical transform. After capturing the free radical of VTEO, KFM-g-VTEO is produced and the new KFM macro radical begins to graft new VTEO. Similarly, the same team^30,109 also grafted γ-methacryloxypropyltrimethoxysilane to the fluoroelastomers. These two grafted polymers can be used as compatibilizers of a KFM and MVQ blend. But no adequate evidence was presented for the proposed mechanism and grafting ratios were not given in their studies either.

In addition to silicones, grafting maleic anhydride via radical copolymerization is another typical modification of fluoroelastomers. Dong et al.^105,106 investigated the effects of reaction conditions on the grafting ratio. The highest grafting ratio (1.9%) was obtained when the conditions were set as 0.4 wt% DCP, 10 wt% MAH, and 150 °C. KFM-g-MAH can be utilized to increase compatibilization of fluoroelastomers with acrylic rubber (ACM)^103,110 or thermoplastic polyurethane (TPU).¹⁰⁵

3.2 Grafting via chemical oxidation

In order to graft functional monomers onto fluoroelastomers via chemical oxidation, expected groups and other groups, such as amino or hydroxyl groups, should be involved in grafted monomers. The grafting process via chemical oxidation contains three steps: (i) eliminate HF in the poly(VDF-co-HFP) copolymer, thus creating double bonds; (ii) subsequently rearrange the dehydrofluorinated poly(VDF-co-HFP) copolymer; and finally (iii) substitute the phenol (or amido) onto double bonds.⁸⁹

Taguet et al.^82,111 grafted a series of commercially available amines bearing aromatic rings onto poly(VDF-co-HFP) copolymers. With no methylene spacer, grafting will not be successful. The authors claimed that the higher the molar percentage of HFP, the higher the maximum amount of grafted benzylamine. This may be attributed to the rearrangement of created CF=CH bonds (Fig. 3). The synthetic amine salt (H₂N–CH₂–CH₂–S–CH₂–CH₂–C₆H₄–SO₃Na)⁵⁴ was also suspended onto unsaturated VDF-HFP chains. In this work, [mercaptan]₀/[monomer]₀ initial molar ratio should be controlled as with the one above (Fig. 4). Taguet et al.⁸⁹ also grafted (C₁₀H₂₁)₂(CH₃)₂N⁺–O–C₆H₄–SO₃–⁺N(CH₃)₂(C₁₀H₂₁)₂ salt onto fluoropolymers. However, these molar percentages of grafted phenolates were lower than those of grafted H₂N–CH₂–CH₂–S–CH₂–CH₂–C₆H₄–SO₃Na. Compared to the amine (H₂N–CH₂–CH₂–S–CH₂–CH₂–C₆H₄–SO₃Na)-grafted copolymers, the phenate sulfonate seems slightly less thermostable. The grafted poly(VDF-co-HFP) copolymers have found their potential applications in conductive membranes.

Fig. 4 Grafting mechanism of 4-hydroxybenzenesulfonic acid onto the poly(VDF-co-HFP) copolymer, previously dehydrofluorinated. Reprinted with permission from ref. 89.

Phenolic hydroxyl groups also have similar effects on amino groups. Moorlag et al.^101,102 in their two patents reported a method to form a silane functionalized fluoropolymer by the reaction between organic grafts and fluoropolymer chains. The grafts include a phenol end group, a linking group, and at least one silane end group.

The dehydrofluorination is a necessary step in grafting via chemical oxidation for providing grafting sites. But unsaturated structures have a negative influence on thermodynamic stability and solvent resistance of products.

3.3 Grafting via reversible deactivation radical polymerization

So far, the graft copolymerization from commercial polymers has been mainly accomplished through free radical routes, which unfortunately often produce many homopolymers, resulting in a low grafting density. Reversible deactivation radical polymerization opened a new door for the preparation of functional grafts with well-defined structures.^112,113 Recently, grafting copolymerization from fluoropolymers via RDRPs, including ATRP, NMP, and RAFT, has received more and more attention.^55,114–116 Considering that alkyl bromides and alkyl chlorides offer the best controlled consequence, pendant secondary halogen atoms help poly(VDF-co-HFP) copolymers and poly(VDF-co-CTFE) copolymers become potential ATRP initiators.

The sulfonic group and corresponding sodium salts have been introduced onto fluoropolymers through styrene-controlled grafting polymerization and sulfonation. Zhang and Russell⁵² performed the synthesis of poly(VDF-co-CTFE)-g-PS and poly(VDF-co-CTFE)-g-PBA graft copolymers with ATRP. Using BPy as ligand, the degree of polystyrene grafting and time exhibited a nice linear relationship (hence first-order polymerization kinetic was observed). The resulting polymer has a polydispersity index of 1.60, which is higher than that of linear polymers synthesized from typical ATRP. Unfortunately, the authors provided no direct evidence to support the viewpoint that grafting started from the cleavage of the C–Cl bond of CTFE.

Additionally, Zhang et al.⁹⁷ prepared proton exchange membranes by sulfonating fluoropolymer grafted PS (Fig. 5). Resulting membranes showed notably higher proton conductivity than Nafion 112 conditioned upon higher RH (RH > 60%) or low temperature (T < 120 °C).

Fig. 5 Preparation of PVDF-g-SPS graft copolymers. Reprinted with permission from ref. 97.

Tsang et al.⁸¹ compared poly(VDF-co-CTFE)-g-SPS and poly(VDF-co-CTFE)-b-SPS, which were synthesized by ATRP. Authors found that graft copolymers yielded membranes tolerating much higher ionic contents. The authors also demonstrated that nanoarchitectured morphology plays an essential role in determining the properties of proton conducting membranes.

Differing from the above-mentioned approaches, Kim et al.⁹⁸ grafted PSSA from fluoropolymers via ATRP (Fig. 6) and then crosslinked the poly(VDF-co-CTFE)-g-PSSA under UV irradiation. Interestingly, after introducing Na⁺, the resulting grafts can be utilized as proton conducting electrolyte membranes, instead of proton exchange membranes. The same team prepared another similar membrane based on the poly(VDF-co-CTFE)-g-PHEA graft copolymer. The membrane's maximum conductivity achieved 0.015 S cm⁻¹, which was lower than the former one. This may be attributed to an incomplete esterification reaction.

Fig. 6 Graft copolymerization of poly(VDF-co-CTFE)-g-PSSA. Reprinted with permission from ref. 98.

Nowadays, atom transfer radical polymerization (ATRP) has been applied to introduce functional groups into fluoropolymers, especially poly(VDF-co-CTFE) copolymers. Although the research on ATRP is not completely robust, the graft ratio and solid content are satisfactory. Therefore, there is tremendous value in the functionality of special fluoropolymers containing fluoroelastomers.

3.4 Grafting via high energy methods

Fluoropolymers are relatively susceptible to high energy methods, despite their high chemical stability.^117,118 Functionalized modification of fluoroelastomers by these high energy methods has enhanced some properties.¹¹⁹

Plasma treatment is a commonly used method to change the properties of fluoropolymers by introducing functional groups onto their surfaces. Urushihara and Nishino¹²⁰ modified thermoplastic fluoroelastomers (TPFEs) with O₂ plasma treatment. The TPFE is hydrophobic (θ = 96°), and it turned hydrophilic (θ = 36°) after plasma treatment. More importantly, a high hydrophilic surface can be maintained during the stretching/recovering process. After O₃/O₂ treatment, Rahimabady⁵³ performed thermally-initiated graft copolymerization of DMA (dopamine methacrylamide) from the ozone-pretreated poly(VDF-co-HFP) copolymers (graft ratio = 5.9%) in order to be applied in high energy density storage (E = 33 J cm⁻³). This work demonstrated that grafting an appropriate amount of polymer with polar groups is an effective strategy for improving polymer dielectrics. On the contrary, Gao et al.¹²¹ grafted a great number of hydrophobic groups (most of which were CF₃, CF₂, and CF) onto the surface of a fluoroelastomer by direct fluorination with fluorine gas. According to the contact angle measurement, no apparent improvement was found in the hydrophobic property of fluoroelastomers.

Similar to plasma treatment, irradiation of fluoroelastomers results in a grafting reaction or the formation of a three-dimensional network through the union of the macro radicals generated. Dutta et al.¹²² investigated the influence of some polyfunctional monomers including tripropylene glycol diacrylate (TPGDA), trihydroxymethyl propane triacrylate (TMPTA), trimethylolpropane trimethacrylate (TMPTMA), tetramethylene maleic anhydride (TMMA) and triallyl cyanurate (TAC) on fluorocarbon elastomers with the presence of an electron beam. The results showed that 50 kGy is a key radiation dose (beyond this value, oxidation, dehydrofluorination, grafting, crosslinking, and scission change marginally). Machado's group¹²³ discovered that electron beam (EB) irradiation could improve the tensile strength, hardness, and stiffness of fluoroelastomers. A micrograph of the sample, which is irradiated with the highest applied dose (250 kGy), presented a homogeneous surface (as shown in Fig. 7). Recently, swift heavy ion irradiation was used to functionalize poly(VDF-co-HDF) nanohybrid membranes for fuel cell applications.⁴⁶ After radiation treatment, the bulk dc conductivity of the functionalized membrane increased 13 orders of magnitude, and activation energies also increased to 32 kJ mol⁻¹.

Fig. 7 SEM micrographs of fractured surfaces from: (A) non-irradiated sample and (B) sample after EB irradiation processing up to 250 kGy. Reprinted with permission from ref. 123.

4. Functionalization via copolymerization

4.1 The initiator functionalization

4.1.1 Iodine transfer radical polymerization. Iodine transfer radical polymerization (ITRP) is a well-established strategy for copolymerization of perfluoroalkyl monomers (mainly including VDF and HFP) and it has been reported by many reviews.^55,124–126 Most relevant telomeres are synthesized from fluorinated telechelic monoiodides (R_F–I)^126,127 or diiodides (I–R_F–I)^128,129 which generate stable R_F radicals, induced by UV light, thermal initiation, or radical abstraction of iodides using conventional initiators.^130,131

Ameduri's group made a big contribution to the synthesis of fluoropolymers (especially fluoroelastomers) by iodine transfer radical polymerization in the presence of C₆F₁₃I. Boyer et al.¹³² in this team performed the synthesis of poly(VDF-co-PMVE) copolymers using C₆F₁₃I as CTA, which controlled the molecular weight of products, ranging from 2000 to 4450 g mol⁻¹. These copolymers showed an amorphous behavior and potential use for elastomer applications.

In addition, the authors¹² adopted a similar strategy to successfully introduce a SF₅ group into (VDF-ter-HFP-ter-SF₅M) terpolymers. When using CF₂=CFSF₅ as a third monomer, it did not terpolymerize under ITP conditions, leading to poor yields. Many other iodine transfer agents, such as C₂F₅(CF₂CF₂)I and (F₃C)₂CFI, have been utilized in some literature¹³⁵ to prepare various fluoropolymers via telomerization of VDF, HFP and other fluorinated monomers (Fig. 8).

Fig. 8 Direct or sequential co-telomerization of vinylidene fluoride (VDF) or 3,3,3-trifluoropropene (TFP) in the presence of 2-iodoperfluoroisopropane. Reprinted with permission from ref. 135.

Compared to R_FI, fluorinated telechelic diiodides (I–R_F–I) have been used more frequently to design well-architectured thermoplastic fluoroelastomers. The synthesis of block fluoropolymers from the I–R_F–I was pioneered by Tatemoto's group^133,134 and Ameduri's group.^{55,132,135,136} In the past five decades, iodine transfer polymerization of fluoroalkenes has contributed to extensive research on the synthesis of fluorinated hard-b-soft-b-hard triblock copolymers. In this method, fluoroalkenes were usually copolymerized with ethylene to form hard block chains, such as poly(E-alt/co-TFE),^137,138 poly(E-co-CTFE),¹³⁹ poly(E-co-HFP-co-TFE)¹⁴⁰ and poly(E-co-HFP-co-PMVE).¹⁴¹

Recently, in the synthesis of telechelic diiodopoly(VDF-co-PMVE) polymers^35,132,142 using IC₆F₁₂I and IC₄F₈I, experimental M_n increases linearly with monomer conversion, revealing the controlled or “pseudo-living” character of ITP, VDF, and PMVE. Boyer et al.³⁵ synthesized fluorinated thermoplastic elastomers via a two-step procedure. In the second step, TFE was introduced to establish the hard chain of fluoroelastomers. IC₆F₁₂I was utilized again by Sawada et al.¹⁴³ to act as a chain transfer agent in the ITP of α-trifluoromethacrylic acid with VDF and HFP.

Although iodine-terminated fluoroelastomers have become important telechelic fluoropolymers, terminal iodine is not useful for crosslinking of polymers; it is expected to be transformed into other active groups. DuPont researchers transformed terminated iodines to telechelic bis(cyanate)s, bis(azido)s,¹⁴⁴ bis(ester)s, or bis(trialkoxysilane)s.¹⁴⁵ And Kostov et al.^146,147 provided some strategies to synthesize photo-cross-linkable telechelic diacrylate poly(VDF-co-PMVE) co-oligomers by changing iodines into “acrylate groups” (Fig. 9). When using acryloyl chlorides as acrylating agents, the transformation was most successful. Some reviews report various approaches through which transfer agent R_FI is transformed into useful derivatives, which are useful for producing innovative telechelic fluoropolymers (Fig. 10). However, it is still worthwhile to make many efforts to simplify post-processing and improve the yield of final products.

Fig. 9 Overall strategies to synthesize photo-cross-linkable telechelic diacrylate poly(VDF-co-PMVE) co-oligomers. Reprinted with permission from ref. 146.

Fig. 10 Various fluorinated products from perfluoroalkyl ethyl iodide. Reprinted with permission from ref. 126.

4.1.2 Peroxide functional polymerization. Some researchers focused their sights on functionalization of peroxides to design telechelic fluoropolymers (Table 2 and Fig. 11). As early as 1969, Rice obtained some functional fluoroelastomers by the reaction of a liquid mixture of VDF and HFP with a bis-(omega carboxyl ester perfluoroacyl) peroxide.^155–157 In their patents, functional peroxide initiators were prepared from anhydride, acyl chloride, or their ramifications via esterification reaction (Fig. 11b). Thirty years later, Ameduri's team¹⁵¹ initiated radical co-polymerization of VDF and HFP with hydrogen peroxides, and then obtained original telechelic fluoroelastomers (mechanism of reaction was shown in Fig. 12). Compared with commercial fluoroelastomers, these products showed lower glass transition temperatures, ranging from −80 °C to −40 °C. Carboxyl groups were reduced to hydroxyl groups in a two-step procedure. Based on previous research, functional peroxides were utilized by Wang's group to prepare various telechelic fluoropolymers with VDF and CTFE.¹⁴⁸

Table 2 Functional peroxide initiators for fluoropolymers

Peroxide initiator	Initiated monomer	Reference
	VDF/CTFE	148
	VDF/CTFE	148
H2O2	VDF/HFP	149–151
	VDF/CTFE
	VDF/CTFE	148 and 152
	VDF/CTFE	148, 153 and 154
	VDF/PSFVE
	VDF/HFP	155–157
	VDF/CTFE	158
	VDF/HFP/PS	159–163
	VDF/PS
	VDF/MAA
	VDF/BMA

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Fig. 11 Synthesis of BPO-based functional initiators. Reprinted with permission from ref. 149.

Fig. 12 Radical co-polymerization of VDF and HFP by hydrogen peroxide. Reprinted with permission from ref. 151.

Benzoyl peroxide (BPO) has demonstrated its efficiency in radical polymerization; therefore, synthesis of BPO-based functional initiators is a convenient strategy for the chain-end functionality of fluoroelastomers. Wang's group developed routes to chain-end functionalization of fluoropolymers with BPO-functional initiators. The team prepared a series of BPO-functional initiators by utilizing functional benzoic acids and benzoyl chlorides. Zhao et al.¹⁴⁹ took advantage of BPO-functional initiators to initiate the copolymerization of VDF and CTFE. The molecular weight of resulting products was 48 000 or 56 000 g mol⁻¹, while the PDI ranged from 1.5 to 1.6. If BPO is functionalized with halogen, this traditional co-polymerization can be combined with ATRP for preparation of novel block fluoropolymers. Most importantly, these functionalized approaches provide new possibilities for the self-assembly of inorganic nanoparticles in a fluoropolymer matrix to form hybrid materials.¹⁴⁸

4.1.3 Other functional polymerization.
Controlled by functional borane. The success of reversible deactivation radical polymerization by functional borane initiators propelled Chung et al.^164,165 to explore the functionality of fluoropolymers. Using this controlled manner, Chung's team¹⁶⁶ synthesized poly(VDF-co-CTFE) copolymer and then introduced silicon-containing monomers to form terpolymers which resulted in a fair yield. Five years later, Zhang et al.¹⁶⁷ in Chung's team prepared silane-terminated PVDF by silane/borane-terminated initiators. The resulting products can function as an effective polymeric surfactant, which exhibited high interfacial activity in exfoliated fluoropolymer/clay nanocomposites. Nowadays, borane/oxygen radical initiators have been proven to be effective for these fluoromonomers, such as VDF, CTFE and TrFE.
Controlled by functional xanthate. Co-polymerization of VDF and HFP by this method was carried out for the first time by Severac¹⁶⁸ in 2003. Two years later, an original fluorinated xanthate was also synthesized by Sauguet.¹⁶⁹ Poly(VDF-co-HFP) copolymers were terminated by CH₃OCOCH (CH₃)SC(S)OEt. When the proportion of VDF and HFP changed at the second step, fluoropolymers can exhibit different chain properties. Some reviews⁵⁵ also have summarized controlled polymerizations by xanthates, so we didn't present the application of xanthates in the functionality of fluoroelastomers.

4.2 Co-monomer functionalization

In general, except for Viton, the fluorine content of fluoroelastomers ranges from 66.6% to 75.0%, which limits their surface properties. Therefore, it is necessary to incorporate more special groups to optimize fluoroelastomers' properties.^170,171 The pentafluorosulfanyl (SF₅) group shows outstanding properties, such as a good lubricant, insulating properties, and oil-resistance.^172,173 Ameduri's team¹² reported the terpolymerization of both SF₅-monomers (i.e., F₂C=CFSF₅ and H₂C=CHSF₅) with VDF and HFP in the presence of C₆F₁₃I (see the earlier discussion on functional initiators).

To satisfy the demand of fuel cell technology, novel proton-conducting fluoropolymers were prepared by incorporating aryl sulfonic acids via direct or indirect copolymerization. Through the terpolymerization with VDF and HFP, trifluoromethacrylic acids were introduced into a chemical structure, and then were changed into aryl sulfonic acids via the Mitsunobu reaction.^88,143 Sulfonated triblock fluorocopolymers can also be obtained by virtue of the ATRP of fluorinated monomers (mainly including VDF, HFP, and PMVE) and sulfonated monomers.^174,175 But researchers prefer to obtain sulfo groups through the sulfonation of styrenes.^92,97,159

Interestingly, a few grafted fluoroelastomers can be achieved from radical copolymerization of fluorinated monomers. Ameduri's team prepared fluorinated graft copolymers bearing perfluoropolyether via the terpolymerization of VDF, HFP, and a ω-allyl amide type perfluoropolyether.^176,177 Unfortunately, the incorporated amount of allylic perfluoropolyether (PFPE) is poor, so it is necessary to increase the reactivity of this allyl derivative. Similar methods also have been used with pendant ester groups or nitrile groups as crosslinking points of fluoroelastomers. In this way, it is feasible to cure fluoroelastomers at low temperature.^19,178

In the last century, fluorosilicone elastomers were prepared by many researchers and companies^48,179,180 by using fluorinated groups to replace grafted methyl of polysiloxane macromers. These “fluoroinorganic” elastomers retained good tensile properties at high temperature, and showed exceptional low-temperature flexibility. Shin-Estu pendanted fluorinated chains into main or side chains and synthesized a series of hybrid fluorinated homopolymers.^37,151,181 Some novel fluoropolymers were obtained by the radical polymerization of fluorocarbons and silicones in scCO₂.^20,182–186

In contrast to other commercial polymers, polyurethane exhibited lower T_gs, ranging from −50 to −100 °C. So, the family of polyurethane fluoroelastomers makes up for the shortcoming of KFM's low-temperature resistance. Some articles and patents reported a class of PFPE elastomers.^33,187,188 After functionalizing polyurethane fluoroelastomers^189,190 with reactive groups, vulcanization is much easier. For instance, Turri's team³⁶ introduced allyl type functions –CH₂–CH=CH₂ into polyurethane fluoroelastomers, so vulcanization can be more easily conducted by the destruction of double bonds in the presence of peroxides (reaction process is shown in Fig. 13).

Fig. 13 Reaction process for the preparation of polyurethane fluoroelastomers. Reprinted with permission from ref. 36.

In addition to fluorosilicone and polyurethane fluoroelastomers, F-phosphazenes also exhibit a low glass transition temperature (T_g = −70 °C). These fluorinated phosphazene elastomers are mainly obtained by sequential polycondensation of phosphoranimines or by hydrosilylation reaction.^191,192

5. The properties influence of functionality

5.1 Vulcanization

A hot research topic has always been to vulcanize fluoroelastomers by a simpler process or at a lower temperature.^193–197 In order to make fluoroelastomers better curable by peroxides, bromine-containing monomers have been introduced to provide curing sites.¹⁹⁸ The progress in curing chemistry of fluoroelastomers has been triggered by the synthesis of various telechelic polymers.^38,75,76,80 In this way, terminated carboxyl³⁸ and hydroxyls⁷⁴ of fluoroelastomers can be vulcanized by isocyanate at room temperature. Epoxy is another effective curing agent for carboxyl-terminated fluoroelastomers, but it requires a higher temperature than 100 °C.³⁸ Under UV radiation, the network can be established from the double bonds of telechelic poly(VDF-co-PMVE) polymers.¹⁴⁶ Wang's group¹⁴⁸ induced the cross-linking of the amine-terminated fluoropolymers by trimesic acid trichlorides at a relatively low temperature. More conveniently, silane-terminated fluoroelastomers can be cured at room temperature in the presence of DBTDL/TEOS.^20,75,80 RGO, MWNT, and POSS have been incorporated for modifying vulcanization behavior or other properties of fluoroelastomers.

Additionally, functionality via a grafting reaction and functional monomers copolymerization graft ester groups onto fluoroelastomers. The grafts can provide active posts for polyamide to complete the curing process at room temperature.^16–18 Nowadays, click reactions using azide–nitrile cycloaddition also have vastly increased in the vulcanization field.^19,174

5.2 Thermal properties

Fluoroelastomers exhibit outstanding resistance to high temperature but poor resistance to low temperature due to their strong fluorinated moiety. Van Cleeff pointed out that the higher the hydrogen contents, the lower the T_gs of Viton elastomers (all their T_gs are higher than −29 °C).^199,200 Thermal properties of functional fluoropolymers are dependent on molecular weights and introduced functional groups. The T_g of classical carboxyl-terminated fluoroelastomers, degraded by Qi's team, is about −25 °C,³⁸ which is lower than that of original VDF-HFP fluoroelastomers. Meanwhile, the initial degradation temperature of these liquid fluoroelastomers appears to be at 250 °C.^38,80 The thermostability of grafted fluoroelastomers reduces because of fewer fluoride groups and lower molecular weights. The melting points of the CEFRAL® poly(VDF-co-CTFE)-g-PVDF suggested by Central Glass Company Ltd. were 155–160 °C.^86,87 The content ratio of soft and hard chains also plays a decisive role in determining thermal properties. For instance, fluoropolymers initiated by H₂O₂ exhibited lower T_gs (−41 to −77 °C).¹⁵¹ The T_g values of poly(VDF-ter-HFP-ter-CF₂CF(SF₅)) terpolymers, which were prepared by Kostov et al., ranged from −52 to −42 °C.¹² This fluoropolymer showed potential applications as an elastomer because of its amorphous behavior. Furthermore, compared to other functional fluoroelastomers, the super flexibility of PFPE^33,36 and silane chains^20,48 enables T_gs of polyurethane fluoroelastomers and fluorosilicone to decrease to almost −100 °C.

5.3 Surface properties

In general, the surface properties of telechelic fluoropolymers mainly depend on the molecular weight, not the groups at the end of polymers. The contact angles of water on saline-terminated fluorinated elastomer membranes is 91–114°,^75,80 which indicates that their hydrophobicity is similar to the original fluoroelastomers' hydrophobicity. On the other hand, the surface energy of grafted fluoroelastomers is mainly dependent on the introduced groups. By direct fluorination, the introduced fluorinated groups enabled fluoroelastomers to increase from 97° to 107°.¹²¹ On the contrary, O₂ plasma treatments changed hydrophobicity of fluoroelastomers into hydrophilicity (the water contact angle decreased from 96° to 36°).¹²⁰ When functionalizing via copolymerization with other functional monomers, the surface energy of fluoroelastomers decreases with the presence of hydrophobic monomers. On fluorosilicone coatings²⁰ prepared by Baradie and Shoichet, the copolymerized PDMSMA led to an increase of the water contact angle from 105° to 125°. Enriched SF₅ groups exert similar hydrophobic effects on the surface of fluoroelastomers.

5.4 Mechanical properties

Table 3 shows the mechanical properties of fluoroelastomers after functionality. In general, the tensile strength and elongation of KFM are 9.0–18.6 MPa and 100–500%, respectively. After degradation, most functional fluoropolymers keep a good elongation but exhibit a lower tensile strength. Especially, the tensile strength of carboxyl-terminated fluoropolymers is just 1.7 MPa. This is attributed to the decrease of precursors' molecular weight (functional grafting and copolymerization also have these drawbacks if the number of suspended activated points is too few or the molecular weight is too low). DuPont has designed a low molecular weight fluoroelastomer using VDF, HFP, and fluorinated acrylate. When the molecular weight of this fluoroelastomer is 4000, its tensile strength is just 1.3 MPa. Grafted fluoroelastomers can be used as compatibilizers in a KFM and VMQ blend. Composited elastomers can only show their best mechanical properties when adding an appropriate content (about 10 phr). In a family of polyurethane fluoroelastomers, diol extender plays a positive role in elongation. In Turri's work, the biggest elongation of polyurethane fluoroelastomers is 490%, which nearly reaches the maximum of KFM's elongation. Although fluoroelastomers show lower surface energy after treatment with fluorine gas, they keep their original mechanical properties.

Table 3 Mechanical properties of some functional fluoropolymers^a

Functional fluoropolymer	Tensile strength (MPa)	Elongation at break (%)	Shore A hardness	Reference
a a and b: mechanical properties correspond to the FKM and MVQ blend.
	1.3–2.5	210–300		18
TFE-ter-VAc-ter-PDMSMA	86	350		20
	8.1–9.0	394–427	62–64	29
	5.2–8.8	440–520		30
		81–492	42–62	36
	1.77	440	52	38
	0.22–1.35	170–270	43–50	74
	1.44–4.45	50–300	—	75
	7.32–9.60	50–75	—	77
	28/30	520/563		86 and 87
	7.6–81.6	8.7–26.6	—	98
	26.2	23.4		99
Direct fluorination with fluorine gas	13 ± 2	209 ± 20	68 ± 1	121

---

6. Applications

Commercial fluoroelastomers are usually utilized as seals,²⁰¹ rings, coatings,²⁰² gaskets, hoses, expansion joints, etc. in aircraft, aerospace, and car industries. Functional fluoroelastomers overcome some shortcomings of commercial fluoroelastomers, so they are used in a wide range of applications.

After degradation, functional fluoroelastomers turn into liquids from solids so that they can adhere readily to many substrates. They are used as protective coatings on elastomers, metals, and other surfaces that need to be protected from harsh service conditions. Interestingly, the decrease of molecular weight, increase in the pressure sensitivity of fluoropolymers, and the resulting fluoropolymers may become novel PSAs. Degradative fluoroelastomers can find applications in many other fields, such as aerospace, automotive, dry cleaning, and high-speed industrial printing industries, plus environmental protection.

Additionally, introduction of silicon increases the flexibility of fluoroelastomers so that they can be used as seals at low temperatures. But, they are less stable at high temperatures than commercial fluoroelastomers. Polyurethane fluoroelastomers also can achieve materials with low temperature or high working pressure, as in the case of a new generation of diesel engines.

After grafting silicone or acrylate, functional fluoroelastomers can act as a compatibilizer of KFM with other elastomers such as MVQ, ACM, or TPU. For instance, by using grafted fluoroelastomers to enhance compatibility of KFM/MVQ, blend rubbers can show similar performances to fluorinated polysiloxanes but at a lower cost.

Some fluoropolymers in the form of VDF-HFP and VDF-CTFE can be developed as proton conducting membranes by incorporating ionogenic functions. And the successful introduction of sulfo groups enables them to function as proton exchange membranes.

7. Conclusions

As the most charming materials, fluoroelastomers have shown their irreplaceable status in many applications. But fluoroelastomers have some drawbacks, such as high price, complex vulcanization, poor resistance at low temperatures and poor compatibility with other components. In order to overcome these drawbacks, more and more attention has been paid to functionalized approaches for obtaining novel structures and better properties.

All the functionalized approaches can fall into three main categories: (i) functionalized degradation. Various functional groups are introduced at the end of polymer when the molecular weight decreases. (ii) Functionalized grafting. FRP, oxidation, dehydrofluorination, sulfonation, ATRP, etc. are involved in this method to graft functional groups or functional monomers into the backbone of fluoropolymers. (iii) Functionalized copolymerization. Functional monomers and initiators are used for the preparation of novel functional fluoroelastomers. In fact, these three categories are interrelated. Sometimes, the same functional fluoropolymers can be prepared via different functionalized pathways.

After the introduction of curable groups, quite a few fluoroelastomers can be vulcanized at a relatively low temperature, even at room temperature. A decrease in molecular weight enhances the compatibility of fluoroelastomers with other polymers, and then some nanocomposites can be prepared. Furthermore, embedding soft chains into fluoropolymers improves their resistance at low temperatures. The functionalization of fluoroelastomers enables them to be more widely applied.

However, there are some shortcomings in these functionalized approaches. When using functionalized degradation, too many double bonds and unexpected groups will be introduced into the backbone of a fluoropolymer, which could reduce fluoropolymers' heat resistance. Grafting via dehydrofluorination also faces the same challenge. Grafting via FRP will produce many homopolymers, and grafting via ATRP is not suitable for most fluoropolymers in the form of VDF-HFP. Functionalized copolymerization is expensive, because functional monomers or functional initiators have to be prepared.

Further works are expected to improve functional degrees, decrease unexpected structures, and reduce costs of functional fluoroelastomers. These goals are so attractive that many researchers will devote themselves to this interesting and challenging area.

Abbreviations

ACM	Acrylic rubber
AlLiH4	Lithium aluminium hydride
AMEO	Aminopropyltriethoxysilane
ATRP	Atom transfer radical polymerization
BMA	Butyl methacrylate
BPO	Benzoyl peroxide
CTFE	Chlorotrifluor ethylene
DBTDL	Dibutyltin dilaurate
DCP	Dicumyl peroxide
E	Ethylene
EB	Electron beam
FRP	Free radical polymerization
HDI	Hexamethylene diisocyanate
HFP	Hexafluoropropylene
ITP	Iodine transfer polymerization
MAA	Methacrylic acid
MAH	Maleic anhydride
MVQ	Methyl vinyl silicone rubber
MWNT	Multiwalled carbon nanotube
NaBH4	Sodium borohydride
NMP	Nitroxide mediated polymerization
PFP	Pentafluorophenol
PFPE	Perfluoropolyether
PFSVE	Perfluoro(4-methyl-3,6-dioxane-7-ene) sulfonyl fluoride
PMVE	Perfluoromethyl vinyl ether
POSS	Polyhedral oligomeric silsesquioxane
PS	Polystyrene
PSSA	Poly(styrene sulfonic acid)
RAFT	Reversible addition–fragmentation chain transfer
RDRP	Reversible deactivation radical polymerization
RGO	Reduced graphene oxide
RF	Perfluorinated group
RFI	Perfluoroalkyl iodide
scCO2	Supercritical carbon dioxide
TAC	Triallyl cyanurate
TBPAC	tert-Butyl peroxyallyl carbonate
TEOS	Tetraethyl orthosilicate (or ethyl silicate or tetraethoxysilane)
TFE	Tetrafluoroethylene
TFMAA	2-(Trifluoromethyl) acrylic acid
THF	Tetrahydrofuran
TMMA	Tetramethylene maleic anhydride
TMPTA	Trihydroxymethyl propane triacrylate
TMPTMA	Trimethylolpropane trimethacrylate
TPFE	Thermoplastic fluoroelastomers
TPGDA	Tripropylene glycol diacrylate
TPU	Thermoplastic polyurethane
TrFE	Trifluoroethylene
VDF	Vinylidene fluoride
VTEO	Vinyltriethoxysilane

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