Fluoropolymers display a wide range of remarkable properties and are used in a number of applications including high performance elastomers, thermoplastics, coatings for optical fibers, and hydrophobic and lipophobic surfaces. The set contains two books, Fluorinated Polymers: Synthesis, Properties, Processing and Simulation and Fluorinated Polymers: Applications. The first book covers the fundamentals of fluoropolymers including the kinetics of homopolymerisation and copolymerization, process chemistry, and controlled radical co-polymerisation techniques. The second book discusses the recent developments in the uses of fluoropolymers. Examples include materials for energy applications such as fuel cell membranes, lithium ion batteries and photovoltaics, as well as high-tech areas such as aerospace and aeronautics, automotives, building industries, textile finishings and electronics. Written by internationally recognized academic and industrial contributors, the book will be of interest to those in industry and academia working in the fields of materials science, polymer chemistry and energy applications of polymers. Together these two books provide a complete overview of different fluorinated polymer materials and their uses.
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Shanghai University, China
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Volume 1: Synthesis, Properties, Processing and Simulation
By Bruno Ameduri, Hideo Sawada
The Royal Society of ChemistryCopyright © 2017 The Royal Society of Chemistry
All rights reserved.
Fluorinated Peroxides as Initiators of Fluorinated Polymers
SHOHEI YAMAZAKI AND HIDEO SAWADA
Department of Frontier Materials Chemistry, Graduate School of Science and Technology, Hirosaki University, Hirosaki 036-8561, Japan *Email: firstname.lastname@example.org
It is in general well known that alkanoyl peroxides [R-C(=O)–O–O–(O=)C-R; R = alkyl group] decompose homolytically via a stepwise radical fission to produce an acyloxy radical [R-C(=O)O] and finally an alkyl radical (R), as shown in Scheme 1.1. However, interestingly, fluoroalkanoyl peroxides [R-C(=O)–O–O–(O=)C–RF; RF = fluoroalkyl group] can decompose homolytically through three-bond radical fission to afford fluoroalkyl radicals (RF; see Scheme 1.2). This unique decomposition mechanism has already been applied as a radical initiator for fluoroolefins such as tetrafluoroethylene to produce thermally stable fluorinated polymers. The thermal stability of the fluorinated polymers thus obtained is due to the direct introduction of fluoroalkyl segments (RF) related to the peroxide into the fluorinated polymer end-chains (RF-CFC2F2~) during the radical polymerization process of fluoroolefins initiated by fluoroalkanoyl peroxides. The thermal decomposition of fluoroalkanoyl peroxides selectively affords the corresponding coupling products (RF-RF) in good yields, indicating the formation of RF radicals during the decomposition process (see Scheme 1.2), although the corresponding non-fluorinated alkanoyl peroxide affords the ester products [R–C(=O)OR] through stepwise radical decomposition fission (see Scheme 1.1).
Another specific characteristic of fluoroalkanoyl peroxides is that they are useful electron acceptors even from well-known relatively poor electron-donor aromatic compounds such as benzene, chlorobenzene and heteroaromatic compounds such as thiophenes and furan to proceed via a single electron transfer reaction from these aromatic compounds to the peroxide. As shown in Scheme 1.3, this single electron transfer reaction permits the direct introduction of a fluoroalkyl group (RF) related to the peroxide into the corresponding aromatic compounds in good yields. In this way, fluoroalkanoyl peroxides can exhibit considerably different properties from those of the corresponding non-fluorinated compounds.
It is of great interest how fluorination improves the reactivity of alkanoyl peroxides so substantially. For the understanding of the effects of fluorination, it is useful to elucidate the electronic structures of alkanoyl and fluoroalkanoyl peroxides, because the alkyl and fluoroalkyl groups exhibit significantly different electronic properties, especially a much higher electro-negativity of the fluorine atom than the hydrogen atom. In this chapter, the electronic structure of alkanoyl/fluoroalkanoyl peroxides is studied computationally using ab initio molecular orbital methods. In 1990, Sawada et al. performed a computational study of these peroxides using a semi-empirical molecular orbital method, in which some parameters were determined from experimental data. Ab initio methods, on the other hand, use no empirical parameters in the calculation of molecular electronic structure. The advantage of ab initio over semiempirical methods is that one can systematically improve the accuracy of the molecular wavefunction, in principle, towards the exact solution of Schrodinger equation. An ab initio method involves a much larger computational cost than a semiempirical method. Owing to recent progress in computer technology, however, today it is not difficult to carry out ab initio calculations on alkanoyl/fluoroalkanoyl peroxides when the size of the alkyl/fluoroalkyl group is relatively small.
In the case of the introduction of fluoroalkyl groups into aromatic compounds (Scheme 1.3), previous computational studies revealed how fluorination significantly improves the reactivity. According to molecular orbital calculations, the LUMO (lowest unoccupied molecular orbital) of a fluoroalkanoyl peroxide exhibits considerably lower energy than that of the corresponding non-fluorinated alkanoyl peroxide. This stabilization significantly reduces the energy difference from the HOMO (highest occupied molecular orbital) of aromatic compounds. As a result, the HOMO–LUMO interaction of fluoroalkanoyl peroxides is much larger than that of nonfluorinated alkanoyl peroxides, which leads to a higher efficiency of electron transfer from aromatic compounds to fluoroalkanoyl peroxides.
With respect to thermal decomposition (Schemes 1.1 and 1.2), some computational studies with ab initio methods have been performed for the dissociation of alkanoyl/fluoroalkanoyl peroxides and acyloxy radicals. However, it is unclear why fluoroalkanoyl and alkanoyl peroxides exhibit quite different decomposition mechanisms.
In the present work, we performed computational studies of the thermal decomposition of alkanoyl/fluoroalkanoyl peroxides. Our goal was to clarify the effect of fluorination on the mechanism of decomposition at the molecular level. In particular, our aim was to elucidate what determines whether the decomposition occurs via a stepwise or a concerted mechanism. As the first step in achieving this goal, this chapter focuses on the properties of O–O and C–C bonds and the energetics of thermal decomposition.
1.2 Computational Methods
Ab initio electronic structure calculations were performed for alkanoyl/ fluoroalkanoyl peroxides [RC(O)O–OC(O)R] (hereafter R denotes both alkyl and fluoroalkyl groups and the double bond in the chemical formula is omitted) and their fragments including RC(O)O and R radicals. Electronic energies of these compounds were calculated with the second-order M0ller-Plesset (MP2) method, a perturbation method for evaluating electron correlation energies using the Hartree–Fock wavefunction as reference. The restricted Hartree–Fock (RHF) and restricted open-shell Hartree–Fock (ROHF) methods were applied for the calculation of the reference wave-function of closed-shell systems (peroxides, etc.) and open-shell systems (radicals), respectively. The Sapporo-DZP-2012 basis set, recently developed by Noro et al., was employed for all calculations (DZP is the abbreviation for double zeta with polarization), hereafter denoted DZP. The molecular structure in the electronic ground state was optimized at the MP2/DZP level, where symmetry of the C2 point group was assumed for RC(O)O–OC(O)R peroxides and no symmetry constraint was imposed for other compounds. Normal-mode analysis was also performed for the optimized structures, in order to confirm that they are minimum-energy structures with no imaginary-frequency mode and to estimate the zero point energy (ZPE) of each compound. All calculations were carried out using the GAMESS program package.
Results and Discussion
For alkanoyl/fluoroalkanoyl peroxides [RC(O)O–OC(O)R], it was proposed that non-fluorinated alkanoyl peroxides decompose in a stepwise manner, whereas fluoroalkanoyl peroxides decompose in a concerted manner, as discussed in the Introduction. In the case of alkanoyl peroxides, the decomposition is likely to occur in two steps (see also Scheme 1.1). In the first step, the RC(O)O–OC(O)R peroxide undergoes homolytic dissociation of the central O–O bond to produce an RC(O)O radical:
RC(O)O–OC(O)R -> 2RC(O)O (1.1)
In the second step, the RC(O)O radical decomposes to the R radical and a CO2 molecule by dissociation of the C–C bond:
RC(O)O -> R + CO2 (1.2)
The intermediate RC(O)O leads to the formation of ester products (see the Introduction), which are thermally less stable. In the case of fluoroalkanoyl peroxides, on the other hand, the decomposition into an R radical and a CO2 molecule occurs in a single step, that is, by concerted dissociation of the O–O bond and two C–C bonds of RC(O)O–OC(O)R (see also Scheme 1.2):
RC(O)O–OC(O)R -> 2R + 2CO2 (1.3)
Direct formation of the R radical results in the selective production of thermally stable fluorinated polymers that do not have an ester group.
Another interesting observation in the thermal decomposition of alkanoyl/ fluoroalkanoyl peroxides is that the decomposition rate depends strongly on the substituent R. In particular, the rate of decomposition of fluoroalkanoyl peroxides is much higher than that of non-fluorinated alkanoyl peroxides.
In this section, we computationally examine the molecular structure of RC(O)O–OC(O)R peroxides (Section 1.3.1), the molecular structure of RC(O)O radicals produced by homolytic dissociation according to eqn (1.1) (Section 1.3.2), the bond dissociation energy (BDE) of peroxides and radicals (Section 1.3.3) and the heat of reaction for thermal decomposition according to eqn (1.3) (Section 1.3.4), focusing on the peroxides that have a methyl/ fluoromethyl group or ethyl/fluoroethyl group as R. To elucidate the effects of fluorination on molecular properties in more detail, we studied partially fluorinated compounds in addition to non-fluorinated and perfluorinated compounds.
1.3.1 Molecular Structure of Alkanoyl/Fluoroalkanoyl Peroxides
Figure 1.1 shows the equilibrium geometry of RC(O)O–OC(O)R peroxides whose R substituent is a methyl or fluoromethyl group (R = CH3, CHF2, CHF2 and CF3), optimized with the MP2/DZP method. The length of the central O1-O1' bond (see Figure 1.1a for the atom labeling of peroxides) is around 1.45 Å for all compounds, a typical value for the O–O single bond of peroxides. The C2–O1–O1'–C2' group is largely twisted, with dihedral angles of 77.5, 79.1, 80.4 and 80.3° for R = CH3, CH2F, CHF2 and CF3, respectively. These dihedral angles are much smaller than the H-O–O–H dihedral angle of hydrogen peroxide, calculated to be 115.0° by geometry optimization at the MP2/DZP level. The small dihedral angle of the C–O–O–C group is supported by previous theoretical and experimental studies, and is also found in peroxides with other electron-withdrawing groups, e.g. FO–OF and ClO–OCl.
The optimized structures in Figure 1.1 indicate that the C2–C4 bond, i.e. the C–C bond adjacent to the central O–O group and assumed to dissociate by thermal decomposition of peroxide, becomes substantially longer with fluorination of the R group, whereas the O–O and O– C bond lengths are very similar for all compounds. The C2–C4 bond distances optimized at the MP2/DZP level are 1.507, 1.520, 1.534 and 1.545 Å for R = CH3, CH2F, CHF2 and CF3, respectively. The C2'–C4' bond of each peroxide exhibits the same length as the C2–C4 bond due to the C2 symmetry of the molecular structure. The present results suggest that the introduction of F atoms on the R group substantially weakens the bond between the R group and the adjacent C atom.
As shown in Figure 1.1, the length of C2–O3 and C2'–O3' bonds is 1.20 Å for all four compounds, reflecting the formation of a C–O double bond. On the other hand, the C2–O1 and C2'–O1' bonds exhibit a distance of about 1.4 Å, which indicates that they form a single bond.
The CC(O)O part of each RC(O)O moiety exhibits a nearly planar structure, that is, O1, C2, O3 and C4 atoms (or O1', C2', O3' and C4' atoms) are located almost in the same plane. Each of the C2–O3 and C2'–O3' bonds is synperiplanar to the O1–O1 bond, that is, O1, O1, C2 and O3 atoms are located almost in the same plane, and this also applies for O1, O1', C2' and O3 atoms.
Geometry optimization at the MP2/DZP level was also performed for RC(O)O–OC(O)R peroxides with R being an ethyl or fluoroethyl group. Table 1.1 gives the O1–O1' and C2–C4 (or C2'–C4') bond lengths and the C2–O1–O1–C2 dihedral angles of peroxides when R is an ethyl/fluoroethyl group and also a methyl/fluoromethyl group. For comparison, Table 1.1 also gives the O–O bond lengths and C–O–O–C dihedral angles of HC(O)O–OC(O)H (R = H) and the O–O bond length and H–O–O–H dihedral angle of hydrogen peroxide, where the molecular structures of HC(O)O–OC(O)H and hydrogen peroxide were optimized with the MP2/DZP method.
As can be seen in Table 1.1, the O1–O1 bond distance seems to be nearly independent of the number of F atoms in the R substituent in the case of an ethyl/fluoroethyl group, as in the case of a methyl/fluoromethyl group. The length of the O1–O1' bond is around 1.45 Å for all ethyl and fluoroethyl groups and the lengths differ in the order of 0.001 Å from one another (the same applies for methyl/fluoromethyl groups; see also Figure 1.1). HC(O)O–OC(O)H and hydrogen peroxide exhibit slightly larger lengths of the O–O bond, 1.454 and 1.460 Å, respectively, but the difference from those of alkanoyl/fluoroalkanoyl peroxides is still very small.
On the other hand, the C2–C4 (and C2–C4) bond distance of peroxides with R being an ethyl/fluoroethyl group exhibits a clear dependence on the number of F atoms in the methylene bridge at the C4 position (CH2, CHF or CF2). The length of the C2–C4 bond determined at the MP2/DZP level is 1.510–1.514 Å for the CH bridge, 1.521–1.528 A for the CHF bridge and 1.539–1.542 Å for the CF2 bridge (see Table 1.1). For the R group with the same methylene/fluoromethylene bridge, the bond length is likely to be almost independent of the number of F atoms in the terminal methyl/ fluoromethyl group (CH3, CH2F, CHF2 or CF3). The C2–C4 bond lengths differ by less than 0.01 Å among CH3, CH2F, CHF2 and CF3. In addition, no significant correlation is found between the bond length and the number of F atoms in the terminal group.
Table 1.1 also shows bond order of O1–O1' and C2–C4 for alkanoyl/ fluoroalkanoyl peroxides, calculated at the RHF/DZP level at the MP2-optimized geometry. As expected from the bond distance, the bond order of O1–O1' exhibits very similar values for all compounds, whereas the bond order of C2–C4 depends significantly on the number of F atoms. In the case of methyl/fluoromethyl groups, the O1–O1' bond order is 0.931, 0.934, 0.934 and 0.938 and the C2–C4 bond order is 1.069, 1.036, 0.998 and 0.990 for R = CH3, CH2F, CHF2 and CF3, respectively. The latter result suggests that the C2–C4 bond is weakened by fluorination of the methyl group. In the case of ethyl/fluoroethyl groups, the C2–C4 bond order is largely reduced by fluorination of the methylene bridge at the C4 position. Fluorination of the terminal methyl group is also likely to reduce the bond order, but the extent of the reduction is smaller than for the methylene bridge. For example, the C2–C4 bond order is 1.043 for R = CH2CH3, but 0.984 and 1.007 for R = CF2CH3 and CH2CF3, respectively.
As in the case of methyl/fluoromethyl groups, the C2–O1–O1'–C2' part exhibits a dihedral angle of about 80° in the case of ethyl/fluoroethyl groups (see Table 1.1). This dihedral angle is less likely to correlate with the number of F atoms. It should be noted that HC(O)O–OC(O)H also exhibits a very similar dihedral angle, whereas hydrogen peroxide exhibits a much larger dihedral angle of H–O–O–H, as mentioned above. The C–O–O–C dihedral angle of HC(O)O–OC(O)H and the H–O–O–H dihedral angle of hydrogen peroxide are 80.9° and 115.0°, respectively. This finding suggests that the small C–O–O–C dihedral angle of alkanoyl/fluoroalkanoyl peroxides can be attributed to the carbonyl group (C2–O3 group), which is largely electron withdrawing.
The CC(O)O part of peroxides with ethyl/fluoroethyl groups exhibits similar features to those with methyl/fluoromethyl groups. The lengths of the C2–O3 and C2–O1 bonds are about 1.2 and 1.4 Å, respectively. Each CC(O)O part is in a nearly planar structure and the C2–O3 bond is synperiplanar to the O1–O1' bond. 1.3.2
Molecular Structure of Alkanoyl/Fluoroalkanoyl Radicals
Figure 1.2 shows the MP2-optimized equilibrium geometry of RC(O)O radicals, produced by homolytic dissociation of RC(O)O–OC(O)R peroxides [eqn (1.1)] with R being a methyl or fluoromethyl group. As in the case of the parent peroxides, the C2–C4 bond length becomes larger as the number of F atoms in the methyl group increases (see Figure 1.2a for the atom labeling of radicals). The bond distance is 1.514, 1.524, 1.538 and 1.548 Â for R = CH3, CH2F, CHF2 and CF3, respectively. This result suggests that weakening of the C–C bond by fluorination occurs in RC(O)O radicals as well as in RC(O)O–OC(O)R peroxides.
Table 1.2 summarizes the C2–C4 bond lengths of RC(O)O radicals with R being a methyl/fluoromethyl or ethyl/fluoroethyl group. For the latter, the C2–C4 bond distance becomes longer with fluorination of the methylene bridge: the bond length is 1.519–1.522 Å for the CH group, 1.528–1.540 Å for the CHF group and 1.543–1.548 Å for the CF2 group. Within each methylene/ fluoromethylene bridge, the bond length exhibits very similar values for all terminal methyl/fluoromethyl groups.
Excerpted from Fluorinated Polymers by Bruno Ameduri, Hideo Sawada. Copyright © 2017 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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Table of Contents
Fluorinated Polymers: Synthesis, Properties, Processing and Simulation; Fluorinated Peroxide as an initiator of Fluorinated Polymers; Fluoroalkylated Styrene Dimers: Synthesis, Properties, and Applications; Anionic Polymerization of Fluorinated Vinyl Monomers; Polyaddition of Fluorinated Vinyl Monomers; Semi-fluorinated Aromatic Polymers and their Properties; Synthesis of Fluoro-functional Conjugated Polymers by Electrochemical Methods; Supercritical Carbon Dioxide as Reaction Medium for Polymer Synthesis and Kinetic investigations; Structure-property Relations in Semifluorinated Poly(Methacrylate)S; Amphiphilic Fluoropolymer: Synthesis and Self-assembly; Tailoring the Melt-Processing Characteristics of Fluoropolymers; Molecular Simulation on Fluorotelomers and Polymers; Fluorinated Polymers: Applications; Fluorinated Oligomers and Polymers; Fluoroacrylate Polymers and its Applications; Structural Diversity in Fluorinated Polyphosphazenes. Exploring the Change From Crystalline Thermoplastics to High Performance Elastomers; Fluoroplastics and Fluoroelastomers－Basic Chemistry and High Performance Applications; Fluorinated Specialty Chemicals - Fluorinated Copolymers for Paint and Perfluoropolyethers for Coating; PVDF industrial Synthesis and Applications; the Role of Perfluoropolyethers in the Development of Polymer Electrolyte Membrane Fuel Cells; Fluorinated Ionomers and Ionomer Membranes -Monomer and Polymer Synthesis and Applications; Recent Advances in F-Polymers for Fuel Cell Membranes; Chlorotrifluoroethylene Copolymers for Energy-Related Materials; Fabrication of Flexible Transparent Nanohybrid With Heat-Resistant Property By Fluorinated Crystalline Polymer; Creation of Superamphiphobic, Superhydrophobic/Superoleophilic and Superhydrophilic/Superoleophobic Surfaces By Using Fluoroalkyl End-capped Vinyltrimethoxysilane Oligomer as a Key intermediate;