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Contributors
Gozde Aktas Eken
Macromolecular Engineering Research Group (MERG), Chemistry Department, Istanbul Technical University, Istanbul, Turkey
Alexandru D. Asandei
Institute of Materials Science, Polymer Program and Department of Chemistry, University of Connecticut, CT, United States
Sabine Beuermann
Clausthal University of Technology, Institute of Technical Chemistry, Clausthal-Zellerfeld, Germany
Roberta Bongiovanni
Politecnico di Torino, Department of Applied Science and Technology, Torino, Italy
Florian Brandl
Clausthal University of Technology, Institute of Technical Chemistry, Clausthal-Zellerfeld, Germany
Vyacheslav M. Buznik
Russian Academy of Sciences, Federal State Unitary Enterprise All-Russian Scientific Research Institute of Aviation Materials State Research Center of the Russian Federation, Moscow, Russian Federation
Marco Drache
Clausthal University of Technology, Institute of Technical Chemistry, Clausthal-Zellerfeld, Germany
Abhirup Dutta
Institute of Materials Science, Polymer Program and Department of Chemistry, University of Connecticut, CT, United States
Mariya Edeleva
N. N.Vorozhtsov Novosibirsk Institute of Organic Chemistry SB RAS, Novosibirsk, Russian Federation; National Research University – Novosibirsk State University, Novosibirsk, Russian Federation
Behzad Farajidizaji
Department of Chemistry and the Marvin B. Dow Advanced Composites Institute, Starkville, MS, United States
Karen K. Gleason
Department of Chemical Engineering, MIT, Cambridge, MA, United States
Metin Hayri Acar
Macromolecular Engineering Research Group (MERG), Chemistry Department, Istanbul Technical University, Istanbul, Turkey
Ryo Honma
Department of Chemistry, Faculty of Science, Kanagawa University, Hiratsuka, Japan
Hisao Hori
Department of Chemistry, Faculty of Science, Kanagawa University, Hiratsuka, Japan
Polina Kaletina
N. N.Vorozhtsov Novosibirsk Institute of Organic Chemistry SB RAS, Novosibirsk, Russian Federation; National Research University – Novosibirsk State University, Novosibirsk, Russian Federation
Sergey A. Khatipov
Leading Researcher, Doctor of Physical and Mathematical Sciences, Federal State Budgetary Educational Institution of Higher Education “Vyatka State University”, Kirov, Russian Federation
Joon-Sung Kim
Institute of Materials Science, Polymer Program and Department of Chemistry, University of Connecticut,CT, United States
Jena M. McCollum
Department of Mechanical and Aerospace Engineering, University of Colorado Colorado Springs, CO, United States
Ganesh Narayanan
Department of Chemistry and the Marvin B. Dow Advanced Composites Institute, Starkville, MS, United States
Emanuele Nettis
Politecnico di Torino, Department of Applied Science and Technology, Torino, Italy
Hideo Sawada
Department of Frontier Materials Chemistry, Graduate School of Science and Technology, Hirosaki University, Hirosaki, Japan
Jan Schwaderer
Clausthal University of Technology, Institute of Technical Chemistry, Clausthal-Zellerfeld, Germany
Sergey A. Serov
Senior Researcher, Limited Liability Company “Research and production enterprise “Arflon”, Moscow, Russian Federation
Irene Serrano Delgado
Department of Mechanical and Aerospace Engineering, University of Colorado Colorado Springs, CO, United States
Dennis W. Smith
Department of Chemistry and the Marvin B. Dow Advanced Composites Institute, Starkville, MS, United States
Armand Soldera
Laboratory of Physical-Chemistry of Matter, Department of chemistry, Faculty of science, Université de Sherbrooke, Sherbooke, QC, Canada
Vignesh Vasu
Institute of Materials Science, Polymer Program and Department of Chemistry, University of Connecticut, CT, United States
Alessandra Vitale
Politecnico di Torino, Department of Applied Science and Technology, Corso Duca degli Abruzzi, Torino, Italy
About the editors
Dr. Bruno Ameduri (DR CNRS Senior Researcher) leads the “Fluorine” Group at the “Engineering and Macromolecular Architectures” Team of Institute Charles Gerhardt in Montpellier, France. His main interests focus on the synthesis and the characterization of fluorinated monomers (including cure site monomers, telechelics, and polyfunctional ones), telomers, and copolymers for various applications such as F-surfactants, F-elastomers, F-coatings, and F-polymers related to energy (fuel cell membranes-protonic, alkaline and quasianhydrous ones-, polymer gel electrolytes and separators for Li-ions batteries, piezo-, ferro- or electroactive films), and nanocomposites. Coauthor of three books, ca. 50 reviews or book chapters, >370 peer review publications and coinventor of more than 80 patents, he is also a member of the American and French Chemical Societies and is a member of the Editorial Boards of the Journal of Fluorine Chemistry, European Polymer Journal, Polymer Bulletin. Outside of research, Bruno enjoys cycling, soccer, tennis, and jogging and is also an active member of the “Rire” Association and visits, dressed as a clown, sick children in hospitals of Montpellier.
Sergey Fomin is the Director of the Institute of Chemistry and Ecology of Vyatka State University (Russian Federation, Kirov). His main research interests are in the field of adhesion of polymer materials, development of new ingredients for polymer compositions, modeling, and calculations of polymer products. The main research objects are elastomers of various purposes, including fluoro-rubber, but also a great deal of attention is paid to thermoplastic fluoropolymer materials. He led more than ten research and design projects for industry (including for improving the frost resistance of rubber, the development of production technology of self-healing pneumatic tires, the development of methods for assessing the content of polyaromatic hydrocarbons in rubber, the creation of a new production of polymer monofilament, the creation of a new production of polymer materials resistant to aggressive media, etc.). He has co-authored more than 30 articles in peer-reviewed publications, 4 patents in 2019. He was the co-organizer of the the First Internation Conference on Fluoropolymers in Russsia with Dr. Ameduri. He is also a member of the Nanotechnology Society of Russia. Personal interests include music, playing electric guitar, snowboarding, and running.
Preface
New technologies are changing our daily lives. New green energy, means of communication, computers, modern medicine and prosthetics, automotive, and aerospace industries—all have been invented and created by man to ensure that as our lives undergo changes, we experience better outcomes.
But, new products for a better life require novel materials in addition to further needing new design and technology solutions. The importance of the fact that materials fundamentally change the life of mankind is reflected in the names of the eras. After all, anyone remembers that the Stone Age, Bronze Age, Iron Age and so on, have been distinguished periods in history.
What age is it now? Without doubt, we are ready to answer that now there is an era of polymers. It is difficult to imagine a modern world without plastics, rubber, and composites. After all, these materials have been used to make almost everything surrounding us.
Among many classes of macromolecules, fluoropolymers play a particular and essential role. Such a class of specialty polymers is endowed with remarkable properties, such as a wide temperature range of service, biological and chemical inertnesses, excellent mechanical and optical properties, good ionic or proton conductivity, piezoelectric characteristics, and shape memory effect. All this leads to the fact that fluoropolymers are indeed indispensable in many high-tech areas. They are used to make parts for aerospace and automotive industries, as well as membranes for fuel cells. They are involved in gas separation, water filtration, actuators and sensor devices, components of lithium-ion batteries, coatings for optical fibers, surfaces of household appliances and cookware, as well as clothing and fabrics, building structures, and many more applications.
Years have long passed when only polytetrafluoroethylene, PTFE, has encompassed fluoropolymers. Now, in these polymer material classes, tens of species and thousands of brands of materials with their own specificity have been available. But at the same time, they all have a common feature—the presence of fluorine atoms in the molecule. These atoms and their strong and short covalent bonds to carbon have supplied these materials with all their unique characteristics.
Two books from the “Fluoropolymers” series are offered to you:
BOOK 1: Opportunities of Fluoropolymers: Synthesis, Characterizations, Processing, Simulation, and Recycling
BOOK 2: Fascinating Fluoropolymers and their Applications
The first book is devoted to processes and key strategies to obtain and study various characteristics of fluoropolymers and simulation, as well as highlighting their processing and recycling.
The chapters of this book deal with the synthesis and determination of properties of various classes of fluoropolymers, such as PTFE, fluorinated aromatic perfluorocyclobutanes, polyvinylidene fluoride, and others, including composites. Tackling the kinetics of radical polymerization and controlling the molar masses and dispersities of synthesized polymers are also considered. A chapter reports the success of the use of fluoro-substituted compounds as initiators and controlling agents in various types of controlled radical polymerization (RDRP). In addition, other chapters inform about special polymerization processes such as a photoinitiated one and chemical vapor deposition polymerization in a vacuum chamber. Attention is paid to the modification of fluoropolymers (especially PTFE) by radiation whereas the use of fluoroalkanoyl peroxides to control the structure of terminal groups is also considered to induce lipophilic and superhydrophobic properties of fluoropolymers. The production and processes of fluoropolymers for membrane separation of water/oil suspensions and fluoropolymers with shape memory, as well as the modeling of the molecular structure of fluoropolymers (especially PVDF) for efficient production of their characteristics, are also discussed. One of the chapters is devoted to a detailed overview of technological approaches for processing fluoropolymers into specific items by various methods which are also industrially developped. The crucial issues of depolymerization and mineralization of different fluoropolymers as well as their recycling, which can be used to reduce negative impact on the environment, are also considered.
The second book focuses on the most important and advanced applications of these remarkable materials. Much attention is paid to the electroactive (relaxor, multiferroic ferro- and piezoelectric) characteristics of polymers, which allow them to be used as sensors and actuators. Several sections are also devoted to membrane applications, either for ionic and proton conductions for fuel cells or for purification or separation of various substances. Another section reviews the optical properties of fluoropolymers and methods to improve optical fibers and waveguides. In addition, a chapter deals with the medical use of fluoropolymers, particularly in artificial blood circulation systems while two other ones report the development of new organic electronics using fluoropolymers and their redox properties. The use of fluoropolymer-based textiles in architecture is also discussed
whereas a chapter summarizes the production of frost-resistant rubber containing fine particles of PTFE.
These two books were initiated from the First International Conference on Fluoropolymers in Kirov (Russia) in October 2019, entitled “Fluoropolymers: Research, Production Problems, New Applications” and we would like to thank all contributors who kindly extended their talks into such book chapters, as well as authors who could not attend that meeting but accepted to contribute in such books.
We are confident that these two books, written by internationally recognized scientists involved in cutting-edge research projects, will be highly informative for researchers, engineers, and practitioners from industry or academia working in materials science, chemistry, polymer physics, engineering and energy, developing or facing synthesis or applications of fluoropolymers in various aspects. They will also serve as excellent reference sources of information for graduate students and scientists at all levels.
Bruno Ameduri Sergey Fomin
CHAPTER 1
Semi-fluorinated aromatic ether polymers via step-growth polymerization of fluoroalkenes
Ganesh Narayanan, Behzad Farajidizaji, Dennis W. Smith, Jr. Department of Chemistry and the Marvin B. Dow Advanced Composites Institute, Starkville, MS, United States
1 Introduction
Ever since its discovery by Plunkett et al. [1], at the Dupont de Nemours and company, Teflon has found niche applications where properties such as thermal, chemical, and solvent resistance are required along with outstanding electrical resistance. Impeccable success with Teflon led to the invention of various fluorine containing homo-, co-, and terpolymers such as polyvinylidene fluoride (PVDF), P(VDF-hexafluoropropylene) (PVDF-HFP), P(VDF-co-tetrafluoroethylene) (PVDF-TFE), and P(VDF-TFE-chlorotrifluoroethylene) (PVDF-TFE-CTFE) [2,3]. Two common features among all the commodity fluoropolymers include the free-radical polymerization of fluoro-olefins [4] along with higher crystallinity leading to higher melt processing cost, and poor solution processing capability [5].
To overcome drawbacks associated with traditional commodity fluoropolymers, various amorphous fluoropolymers containing fluoroalkene groups have been reported in the past 30 years. Among these, the most extensively studied fluoroalkene-containing polymer is perfluorocyclobutyl (PFCB) aromatic ether polymer, which is obtained by the radical mediated, [2 + 2] cyclodimerization of trifluorovinyl ether (TFVE)-bearing monomers (Scheme 1.1A). Originally conceptualized by Babb et al., at The Dow Chemical Company, who demonstrated the feasibility of 1,2- bisaryloxysubstituted perfluorocyclobutane containing polymers for high performance aerospace and electronics applications, has now been studied for optical, opto-electronic, gas separation, and proton-exchange membranes. In addition to PFCB aromatic ether polymers, two additional, complementary classes of fluoroalkene-containing polymers, namely: perfluorocycloalkenes (PFCA) (Scheme 1.1B) and fluorinated arylene vinylene ether
Scheme 1.1 Three semi-fluorinated aromatic ether polymers discussed in this review. (A) PFCB, (B) PFCA, (C) FAVE.
(FAVE) (Scheme 1.1C) have been reported in the past decade. The aim of this chapter is to review the synthesis, characterization, and various applications of these three fluoroalkene-containing polymers, and to further present recent advancements reported in the literature.
2 Synthesis of perfluorocyclobutyl (pfcb) aromatic ether polymers from trifluorovinyl ether-bearing monomers
2.1 Synthesis of trifluorovinyl aromatic ethers (TFVE)
First report of synthesizing TFVE aromatic ether from the reaction of alkali metal phenoxide with tetrafluoroethylene was reported by Wall et al. [6]. Even though aromatic trifluorovinyl ether was obtained albeit at lower yields, two additional bi-products, namely, tetrafluoroethylphenyl ether and 1,2- diphenoxy-1,2-difluoroethylene, were also present preventing isolation of highly pure TFVE monomers [6]. Furthermore, TFVE monomers did not undergo radical mediated polymerization under light or heat but did undergo polymerization under gamma radiation [6].
2.2 Synthesis of multifunctional trifluorovinyl ether (TFVE)-bearing monomers
To overcome the drawbacks associated with the use of this technique for preparing TFVE-monomers, various alternatives were proposed. The most common and popular route for preparing TFVE monomers is via twostep process starting with fluoroalkylation of phenolic derivatives by dibromotetrafluoroethane, followed by metal (mostly Zn or Mg) mediated dehalogenation resulting in TFVE-bearing monomers (Scheme 1.2) [7].To date, TFVE-bearing monomers have been synthesized from bis-, tris-, and tetra functional phenolic derivatives (Scheme 1.3). Some of the prominent phenols that have been converted this route into its corresponding TFVE include: 4,4′-dihydroxybiphenyl (1), 4,4’-(9H-fluorene-9,9-diyl)diphenol (2) 4,4′-(hexafluoroisopropylidene) diphenol (3), 4,4’-(prop-1-ene-1,2diyl)diphenol (4), 2,2-bis(4-hydroxyphenyl)propane, naphthalene-1,5-diol (5), hydroquinone (6).
In addition to these well known, industrial bisphenols, more recently, our group has utilized lesser known bisphenols from polycyclic aromatic hydrocarbons such as acenaphthenequinone and phenanthrenequinone toward the synthesis of TFVE monomers. In addition to bis functional phenolic derivatives listed in Scheme 1.3, both tri-, and tetrafunctional monomers such as 4,4’,4”,4’”-methanetetrayltetraphenol (7) and (R)-3,3,3’,3’-tetramethyl2,2’,3,3’-tetrahydro-1,1’-spirobi[indene]-5,5’,6,6’-tetraol (8). Unlike the bis functional TFVE monomers which afford thermoplastic PFCB polymers, tri- and tetra functional TFVE affords thermally cross-linkable PFCB polymers. For example, spirophenol 9 upon treatment with fluoroalkylation agent (1-(bromomethyl)-4-(bromotrifluoroethyloxy)benzene) and subsequent zinc-mediated dehalogenation led to tetra TFVE monomer [8]. Cross-linked polymer was then obtained by thermal polymerization of tetraTFVE monomer transforming the monomer into a cross-linked network, which was insoluble in common solvents such as THF, DMF, chloroform, common solvents for solubilizing linear, thermoplastic PFCB polymers [8]
Although fluoroalkylation/dehalogenation still remains the choice of method for obtaining TFVE monomers from phenolic derivatives, the yield
Scheme 1.2 A commonly used two-step process for preparing TFVE monomers from phenolic derivatives.
Scheme 1.3 Di-, tri-, and tetra functional trifluorovinyl aromatic ether monomers obtained from phenolic precursors. Monomers 8 and 9 adapted with permission from Ref. [2]. Published by The Royal Society of Chemistry.
is abysmal in starting materials containing electron withdrawing groups. Additionally, fluoroalkylation and subsequent Br F elimination is cumbersome in some phenolic precursors [9] and coupled this with the lack of abundant phenolic precursors [7] and the occurrence of the impurity, tetrafluorovinyl ether, resulting in the low molecular weight of the polymers, necessitating investigation of other strategies for preparing TFVE monomers.
Alternatively by using intermediate TFVE-bearing monomer, p-bromo(trifluorovinyloxy)benzene, first reported by Smith et al. [10], closely followed by organometallic (aryl lithium) reagents obtained via metal-halogen exchange reactions affording novel functional fluoro-monomers containing silicon, phosphorous, or other monomers with intact trifluorovinyl ethers [11]. Subsequently, remarkable tolerance of the aromatic TFVE group to functional group transformation at p-substitution was utilized to create diversified portfolio of TFVE-monomers bearing, for example, aldehyde, hydroxymethyl, carboxylic acid, and hydroxymethyl groups (Scheme 1.4) [12]. Moreover, p-substitution by various functional groups had an impact on the cyclodimerization rate, with general trend being the
Scheme 1.4 Representative TFVE-bearing monomers 11-16 obtained from p-transformation of bromotrifluorovinyloxy benzene 10. Adapted and redrawn with permission from Ref. [1]. Copyright John Wiley and Sons 2014.
higher the electronegativity in the substituent group resulting in higher activation energy [13]. Despite the obvious differences in the dimerization activation energies attributed to the substituent types, two types of diastereomers of the PFCB were formed and that too at almost equal amounts, resulting in amorphous PFCB polymers [12]
Scheme 1.4 illustrates some of the derivatives obtained from the p-bromotrifluorovinyloxy benzene resulting in useful TFVE-bearing monomers for accessing various functional monomers. For example, presence of boronic acid in 11 affords the possibility of preparing various TFVE monomers via Suzuki coupling [14]. Likewise, dimethyl(4-((1,2,2-trifluorovinyl) oxy)phenyl)(vinyl)silane (14) and 1-allyl-4-((1,2,2-trifluorovinyl)oxy)benzene (15) facilitated hydrosilation reaction and incorporation of siloxanes in the fluoropolymer chemistry. Scheme 1.5 illustrates double nucleophilic addition of Grignard reagent, p-MgBr-C6H4-OCF = CF2, to acenapthenequinone resulting in a diol (17), which was subsequently converted to a ring-retained acenaphthene (18) and ring-cleaved (19) trifluorovinyl ether (TFVE) monomers by treating with TiCl4 [15]. This one-pot, seemingly facile procedure afforded two unique TFVE monomers, 18 and 19,
Scheme 1.5 Synthetic procedure for deriving ring-fused (18) and ring-open (19) acenaphthylene bearing-TFVE monomers from acenapthenequinone. Adapted with permission from Ref. [9]. Copyright John Wiley & Sons 2019.
demonstrating the feasibility of accessing polycyclic aromatic hydrocarbon cores resulting in PFCB aryl ether polymers [15]. Not only were the polycyclic aromatic hydrocarbon (PAH) cores accessible to TFVE moieties, the resulting PFCB polymers formed stable, mechanically strong, creasable films and also showed high glass transition temperatures (>200 °C) and thermal stabilities under N2 and air environments (Td 5% >500 °C) [15]
Prior to this work, very few PAH cores, for example, hexa-peri-hexabenzocoronene (HBC) [16] and rylene diimide [17], have been successfully made into processable TFVE-bearing monomers, which were subsequently converted into PFCB polymers. Two different hexaphenylbenzene compounds were synthesized from p-bromotrifluorovinyloxy benzene (10) first by Sonagashira coupling, and then by cobalt-catalyzed cyclotrimerization of diphenylacetylenes affording compound 20 (Scheme 1.6 left) [12]. Hexakis intermediate 20 underwent FeCl3-mediated oxidative ring fusion affording 21 (Scheme 1.6 right). Both 20 and 21 showed propensity towards thermal cyclodimerization to form PFCB networks, with polymer from 20 demonstrating excellent solubility in common organic solvents [12]. Although solubility is still a concern with these aromatic systems, 20 and 21 underwent thermal copolymerization with commercial TFVEmonomers 42 and, copolymer of 3 and 20 showed HBC luminescence, in addition to the emission (360 nm) attributed to the homopolymer poly20, providing access to potentially processable optical materials from HBC [12].
Scheme 1.6 TFVE-bearing hexaphenylbenzene 20 and preparation of fused ring hexaperi-hexabenzocoronene 21. Monomers 20 and 21 adapted with permission from Ref. [10]. Copyright American Chemical Society 2004.
Like HBC, a new structural fluoropolymer, polynaphthalene-containing fluoropolymer (PFN) (23) was obtained via oxidative coupling of corresponding monomer (22) in tetrachloroethane using FeCl3 as oxidative coupling agent (Scheme 1.7) [18]. The monomer 22, 1,2-Bis(1-naphthoxy) hexafluorocyclobutane was obtained by heating 1-(1,2,2-trifluorovinyloxy) naphthalene at 180 °C for 36 h under in argon atmosphere (Scheme 1.7) and subsequent recrystallization from methanol and ethanol (1:1 V/V) [18]. More interestingly, PFN polymers showed extremely low dielectric constant of 2.33 with very low dielectric loss (<1.2 × 10 3 at 30 MHz) along with superior mechanical properties (Young’s modulus and bonding strength between the thin film and a silicon wafer of 17.13 and 13.46 GPa, respectively). In addition, PFN films also showed high hydrophobicity (contact angle of 101 °) and low surface roughness implying possible applications in electronics industry [18].
Another way for preparing PAH-enchainment in PFCB technology is to couple tetrabromide containing PAH moieties via a modified Suzuki–Miyaura cross-coupling reaction with anhydrous potassium phosphate (24, Scheme 1.8) [17]. Similarly, tetraaryloxy monomer can also be synthesized by a nucleophilic substitution reaction between tetrabromide containing PAH moieties with TFVE-bearing hydroxyl group 25 as shown in Scheme 1.8. Swager research group synthesized compounds 24 and 25 from tetrabromide containing-terrylene diimides using established procedures [17]. Although, 24 and 25 did not elicit optical properties that were drastically different
Scheme 1.7 Chemical structure and schematic representation of PFN 23 synthesis via oxidative coupling, beginning with the fluoroalkylation of 1-naphthol, followed by Znmediated dehalogenation, and thermal mediated cyclodimerization of 1,2-bis(1-naphthoxy)hexafluorocyclobutane 22. Adapted with permission from Ref. [12]. Copyright John Wiley & Sons 2013.
Scheme 1.8 Synthesis of representative terrylene dyes-bearing TFVE moieties 24 and 25 and thermal polymerization into PFCB polymers.Monomers 24 and 25 adapted and reprinted with permission from Ref. [11]. Copyright American Chemical Society 2011.
from parent dye (terrylene diimide), both showed unique photophysical properties compared to linear TFVE-bearing monomers obtained from dibromo-containing terrylene diimides (not shown) [17]. For example, not only the fluorescent quantum yields were significantly lower (0.08 and 0.17 for 24 and 25, respectively compared to other diimide-bearing monomers which ranged from 0.61 to 0.88, but the excited state lifetimes were considerably shorter (5.7 and 2.4 ns for 24 and 25, respectively versus 8.7 and 9.8 ns for two other diimide monomers), illustrating possible mechanisms to modulate the photophysical properties of the PFCB-polymers [17]. More interestingly, 24 and 25 were synthesized into chromophore nanoparticles and subsequently polymerized/crosslinked into PFCB polymers.
An identical method was more recently reported by Fang’s research group, who first synthesized bromine-containing bisphenols from fluorene (4,4’-(2-bromo-9H-fluorene-9,9-diyl)diphenol) (26) [19]. Subsequently, using standard fluoroalkylation and dehalogenation techniques (dibromotetrafluoroethane and Zn), bisphenol was converted into trifluorovinyloxy groups with intact Br in the fluorene skeleton, which was subsequently eliminated by Suzuki coupling affording tetrafunctional TFVE-bearing monomer 28 [19]. Polymer 29 was then obtained by thermal cyclodimerization of 28 affording crosslinked polymer that was insoluble in most organic solvents (THF, chloroform, chlorobenzene, etc) (Scheme 1.9).
Scheme 1.9 Synthesis and transformation of fluorene-based bisphenols into PFCBbased molecular glass. Monomers 26-28 and polymer 29 adapted and reprinted with permission from Ref. [19]. Copyright Royal Society of Chemistry 2016.
While the photophysical properties were not extensively studied like those studies from Swager [17] or from our group [16], dielectric and mechanical properties showed low dielectric constant (<2.5) and dissipation factor (2.0 × 10–2) between the frequencies 0 to 30 MHz [19]. These values are significantly lower than commercial low k materials such as, for example, polyimides, polysiloxanes, and poly(aryl ether). Not only did these fluorine-based PFCB polymers show low k values, they also demonstrated extremely low surface roughness, high hydrophobicity, and mechanical properties (strength and Young’s modulus), making them suitable for applications in electronics/optical industry [19].
As can be seen from limited examples of PAH-enchainment in PFCB technology, combination of semi-fluorination with PAH moieties lead to improvements in optical, thermal, surface, and mechanical properties, while also addressing processing issues (poor solubility in solvents) in commonly plaguing PAH chemistries. However, PFCB aryl ether technology needs to be evaluated in higher aromatic systems like corannulene, buckybowl, buckminister fullerene-like molecular structures for their influence on the photophysical, mechanical, and thermal properties of PFCB polymers.
2.3 PFCB containing monomers for classical polymerization
Despite broad spectrum of possible PFCB polymers, TFVE monomers are susceptible to nucleophilic fluoride addition in the presence of stronger base resulting in (-O-CHF-CF3) side products, which inhibit increase in the molecular weight [17]. Also, in some cases, functional groups present in the aryl monomers are less tolerant of the polymerization processes, necessitating alternative strategies for expanding the pool of PFCB aryl ether polymers [20]. One way is to utilize preformed PFCB oligomer or monomer (from TFVE) and subsequently polymerize the end groups via coupling chemistry, which have shown excellent selectivity, reactivity, and reliability [21,22]. Qing’s research group at Donghua University reported the first click chemistry (copper (I) catalyzed Huisgen’s 1,3-dipolar [3 + 2] cycloaddition) reaction between compounds containing polyethylene glycol-bearing diazides and PFCB-bearing dialkynes, leading to the formation of series of aromatic ether polymers containing PFCB and triazole units [23].
In subsequent study, the same group studied Huisgen’s 1,3-dipolar [3 + 2] cycloaddition reaction between 1,2-bis(4-azidomethylphenoxy) perfluorocyclobutane (30) and PFCB-bearing bisethynyl compounds (31) (Scheme 1.10) [24]. Both 30 and 31 were obtained in three straightforward steps from phenolic precursors, beginning with fluoroalkylation/dehalogenation of monofunctional phenols by BrCF2CF2Br / Zn, respectively [24].
Scheme 1.10 A schematic illustrating the synthesis of aromatic ether polymers containing perfluorocyclobutyl and triazole units via click chemistry. Monomers 30-31 and polymer 32 adapted and reprinted with permission from Ref. [18]. Copyright Elsevier Ltd 2008.
Further, PFCB monomer was obtained by cyclodimerization and subsequently brominated via NBS (N-bromosuccinimide) under IR lights. Finally, azidization of bromophenoxy PFCB compound was achieved by sodium azide, affording compound 30 [24]. Upon polymerization via click chemistry, polymers (32) showed Mn values ranging from 30 to 70, 000 and M w values ranging from 36 to 86,000, making the PDI values significantly low (1.19–1.24) [24]. Despite high molecular weights obtained in this study, T g and Td (5% decomposition) values ranged from 90 to 150 °C and from 330 to 370 °C [24]. More interestingly, first 1,3 cycloaddition study which investigated the click chemistry between polyethylene glycol-bearing diazides and PFCB-bearing dialkynes reported low Mn and M w values that ranged from 7 to 20,000 and 10 to 25,000 (PDI from 1.2 to 1.7), but reported higher Td values (>420 °C) [24]
Recently, bisethynyl compounds from biphenol, bisphenol-A, 4,4’-sulfonyldiphenol (bisphenol-S) without PFCB linkage have been reported for step-growth polymerization with bisazide-bearing PFCB monomer [25].
Surprisingly, both the Mn (>50,000 in 3 of 4 polymers) and Mw (>65,000 in 3 of 4 polymers) were higher compared to the previous studies [23,24] which first reported the click chemistry for synthesizing aryl ether polymers bearing PFCB and triazole units. While the Tg and Td values were largely dependent on the starting material and were in general low (Td ranged from 106 to 156 °C and Td 5% ranged from 330 to 370 °C), all the polymers showed very low surface energy (14.11–14.38 mJ/m2) [25].
Scheme 1.11 Synthesis of triazine and benzocyclobutene containing PFCB linkage by the step-growth polymerization of dichloro-s-triazine-bearing benzocyclobutene (33) and bisphenols-bearing PFCB monomer 34. Monomers 33-34 and polymer adapted and reprinted with permission from Ref. [20]. Copyright Royal Society of Chemistry 2017.
More recently, step-growth polymerization was carried out between dichloro-s-triazine-bearing benzocyclobutene (33) and bisphenol-containing PFCB units (34) (Scheme 1.11) in the presence of phase transfer catalyst (PTC) [26]. Presence of PTC and the PTC type played a key role in the molecular weight and distribution, and total yield, although no particular relationship was established between the PTC and the polymer characteristics [26]. Having benzocyclobutene in the reactive monomer 33 is well known to produce a reactive intermediate o-quinodimethane, which is also known to participate as a diene in the Diels–Alder reaction, providing thermocrosslinkable sites [26]. Using identical approach, triazine-based polymers have been derived from triazine-modified anethole containing thermocrosslinkable benzocyclobutene and bisphenols-bearing PFCB monomers [27]. In addition to step-growth polymerization, presence of benzocyclobutene was found to initiate the Diels–Alder polyaddition reaction resulting in thermoset PFCB-bearing triazine polymers [27]. Unlike the previous studies that utilized azide-based click chemistry [23,24], all the triazine-based polymers showed much improved thermal stability (Td 5% >430 °C compared to <350 °C for aryl ether polymers-bearing triazole units), thermomechanical stability (stable storage modulus >6 GPa even at 300 °C), and extremely low dielectric constants (as low as 1.5 DK at up to 30 MHz) [27].
Another advantage with the presence of PFCB monomer units in either the main or in the side chain is the possibility of radical-mediated polymerization leading to the formation of well controlled copolymersbearing PFCB units. Huang’s group demonstrated the first atom transfer radical polymerization (ATRP) of PFCB-containing methacrylate-based monomer, 4-(4′ -p-tolyloxyperfluorocyclobutoxy)benzyl methacrylate (35) initiated by methyl 2-bromopropionate, and copolymer of 35 with polyethylene glycol (PEG) segments (also synthesized by ATRP) acting as macroinitiators [28]. 1H-, 13C-, 19F-NMR experiments on homopolymer from 35 clearly showed disappearance of double bonds attributed to the methacrylates, presence of PFCB units between 105.0 and 115.2 ppm, further corroborated by peaks between 127 and 132 ppm, respectively. Addition of PFCB-units further increased the T g and thermal stability of the methacrylate homopolymer while retaining the optical clarity.
The same group pursued a different approach for synthesizing polymethacrylate-bearing PFCB units in the main chain via free radical polymerization using 2,2′-azobis(isobutyronitrile) (AIBN) or benzoyl peroxide (BPO) as radical initiators [29]. Key differences between the studies include using simple and directly relevant starting materials such as p-substituted phenol, tetrafluoroethylene and methacryloyl chloride (precursor for methacrylate) for synthesizing PFCB-containing methacrylate monomers [29]. Substituting groups in the phenols had a significant influence in the Tg values of the polymer. For example, methyl substitution resulted in a very low T g of 160 °C; whereas, cumyl substitution resulted in a Tg of 213 °C. Succeeding these studies, various homo-, and copolymers (including triblock) such as polyacrylic acid (PAA) [30], PEG and poly(methacrylic acid) (PMAA) [31], polyisobutylene (PIB) [32], poly(2-(diethylamino)ethyl methacrylate) (PDEAEMA) [33], polymethoxylmethyl acrylate (PMOMA) [34]- bearing PFCB units in the polymer main or side chain via ATRP or reversible addition–fragmentation chain-transfer (RAFT) techniques. One general advantage of combining PFCB units with other monomers is the formation of polymers bearing hydrophobic and hydrophilic moieties capable of self-assembly, resulting in the formation of micelles, which can have profound effects in various applications [28-34].
Instead of PFCB-bearing monomer units, PFCB polymers can be directly accessed for developing perfluorocyclobutyl aromatic ether-based copolymers. For example, an ABA-type (A: polystyrene block; B: PFCB block) have been reported via ATRP polymerization technique using 2-bromo-1-(p-trifluorovinyloxy)phenylpropan-1-one as an ATRP initiator
Scheme 1.12 Representative examples of PFCB-based radical monomer, 4-(4′ -p-tolyloxyperfluorocyclobutoxy)benzyl methacrylate 35 and TFVE-based ATRP initiator, 2-bromo-1-(p-trifluorovinyloxy)phenylpropan-1-one 36. Monomers 35 and 36 adapted and reprinted with permission from Ref. [22] and [29]. Copyright Elsevier Ltd 2008 and American Chemical Society 2005.
36 (Scheme 1.12) [35]. At all studied mole ratios (monomer: initiator) and reaction times, the PDI was low (<1.3). Furthermore, with reaction times, an increase in the molecular weight was observed. Since the TFVE do not undergo radical polymerization and exclusively polymerizes via [2 + 2] mechanism, using a transformational strategy such as forming PFCB monomer or polymeric units would therefore facilitate conventional radical polymerization techniques with other monomers that undergo radical polymerization [35]
2.4 Postpolymerization modification on PFCB aryl ether polymers
An advantage of PFCB polymer over TFVE monomer is their capability to undergo postpolymerization reactions such as sulfonation shown in Scheme 1.13. Taking cues from studies that investigated fluorene-based polymer blends [36], PFCB-based blends were selectively sulfonated for
Scheme 1.13 Synthesis of PFCB from biphenol and sulfone-based TFVE and sulfonation of the PFCB homopolymers/blends. Scheme adapted and reprinted with permission from Ref. [31]. Copyright Elsevier Ltd 2013.
imparting hydrophilic properties and nanophase-separated morphologies for improving proton conductivity and performance [37].
While it would be more attractive to polymerize functionalized monomer, that is, sulfonated TFVE, dehalogenation step using Zn/CuCl2 resulted in an abysmal 5% yield, with the major products being organo-zinc salts and hydrogenated species [37]. Therefore, postpolymerization route of first synthesizing PFCB polymers and subsequent sulfonation is a preferred route as fewer steps are required to achieve target product and at higher yields (>65%) [37]. Another added advantage of polymer blending (BP-PFCB with SO2-PFCB) is greater control in ion exchange capacity (1.36 and 1.37 for sBP/BP-PFCB and sBP/SO2-PFCB vs 0.82 mmol/g for Nafion®) and proton conductivity at all studied temperatures (25, 50, and 80 °C) [37]. This result suggests postpolymerization route to be yet another route for effectively using PFCB-based polymers as high performance materials (additional references 50 and 51 in Table 1.1).
2.5
Characterization of TFVE monomers and PFCB aromatic ether polymers
The aromatic TFVE monomer and polymeric PFCBs are best characterized by Fourier transform infrared (FT-IR) and Raman spectroscopy, thermal analysis (in particular, differential scanning calorimetry) and NMR spectroscopic (13C-, and 19F-NMR) techniques.
2.5.1 Infrared and Raman spectroscopy of TFVE monomers and PFCB aromatic ether polymers
As a preliminary characterization technique, FT-IR of TFVE exhibits a characteristic, but faint band at ∼1830 cm–1, which vanishes and a new sharp band in the IR near 960 cm–1emerges, which is diagnostic band for the presence of the hexafluorocyclobutane group [10] (Fig. 1.1). As most characteristic functional groups do not elicit any response in these ranges of IR spectrum (1830 or 960 cm–1), these easily resolvable absorption bands have proven to be a useful analytic tool for TFVE monomers and PFCB linkage [10].
While a weak signal is observed for TFVE group in FT-IR as seen in Fig. 1.1 [10], Raman spectroscopy provides enhanced signal intensity for these fluoro-olefins [38]. Due to enhanced sensitivity, quantification of the signal intensity provides accurate cure kinetic information, which is not feasible with FT-IR [38]. As an example, Raman spectroscopy of conversion of tris(trfluorovinyloxyphenyl)ethane monomer 39 into cross-linked PFCB
