Photomediated Controlled Radical Polymerization and Block

Review pubs.acs.org/CR

Photomediated Controlled Radical Polymerization and Block Copolymerization of Vinylidene Fluoride Alexandru D. Asandei* Institute of Materials Science and Department of Chemistry University of Connecticut, 97 North Eagleville Road, Storrs, Connecticut 06269-3139, United States ABSTRACT: This review summarizes recent research on novel photochemical methods for the initiation and control of the polymerization of main chain fluorinated monomers as exemplified by vinylidene fluoride (VDF) and for the synthesis of their block copolymers. Such reactions can be carried out at ambient temperature in glass tubes using visible light. Novel, original protocols include the use of hypervalent iodide carboxylates alone or in conjunction with molecular iodine, as well as the use of photoactive transition metal carbonyls in the presence of alkyl, fluoroalkyl, and perfluoroalkyl halides. An in-depth study of the reaction parameters highlights the use of dimethyl carbonate as a preferred polymerization solvent and outlines the structure−property relationship for hypervalent iodide carboxylates and halide initiators in both the free radical and iodine degenerative transfer controlled radical polymerization (IDT-CRP) of VDF. Finally, the rational selection of metal carbonyls that are successful not only as IDT mediators but, more importantly, in the quantitative activation of both PVDF−CH2−CF2−I and PVDF−CF2− CH2−I chain ends toward the synthesis of well-defined PVDF block copolymers is presented.

CONTENTS 1. Introduction 2. Controlled Radical Polymerization of VDF 2.1. Peculiarities of VDF Free Radical and Controlled Radical Polymerization by Iodine Degenerative Transfer (IDT) 2.2. Toward Metal-Mediated VDF-CRP 2.2.1. Copper-Catalyzed VDF-ATRP 2.2.2. Degenerative Transfer with Metal-Centered Radicals: Cp2TiCl 2.3. Free and Controlled Radical VDF Photopolymerizations 2.3.1. Transition Metal Carbonyl Photoactivators in Radical Polymerizations 2.3.2. Photomediated VDF Polymerizations with Hypervalent Iodide Carboxylates 3. Metal Carbonyl Photomediated Synthesis of Well-Defined PVDF Block Copolymers 3.1. Particularities of VDF Block Synthesis 3.2. Dependence of the PVDF−CH2−CF2−I and PVDF−CF2−CH2−I Chain Ends on Conversion and Its Relevance to the Synthesis of PVDF Block Copolymers 3.3. Metal-Mediated Quantitative Activation of Both PVDF−CH2−CF2−I and PVDF−CF2− CH2−I Chain Ends for the Synthesis of Well-Defined PVDF Block Copolymers 4. Conclusions and Outlook Author Information Corresponding Author Notes © 2016 American Chemical Society

Biography Acknowledgments Abbreviations References

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1. INTRODUCTION The radical homo- and copolymerization of typical main chain fluorinated monomers (MCFM) including vinylidene fluoride (VDF, CH2CF2, boiling point (bp) = −84 °C), tetrafluoroethylene (CF2CF2, bp = −76.3 °C), vinyl fluoride (CH2 CHF, bp = −72 °C), trifluoroethylene (CFHCF2, bp = −51 °C), hexafluoropropene (CF2CF(CF3), bp = −29.6 °C), chlorotrifluoroethylene (CFClCF2, bp = −27.8 °C), perfluoromethyl vinyl ether (CF2CF(OCF3), bp = −22 C), trifluoropropene (CH2CH(CF3), bp = −18 °C), bromotrifluoroethylene (CF2CFBr, bp = −2 °C), etc. leads to a very interesting class of specialty materials that display high thermal, chemical, aging, and weather resistance, as well as low surface energy, refractive index, flammability, and moisture absorption.1,2 In addition, special electrical responses including ferroand piezoelectricity3 are observed for poly(vinylidene fluoride) (PVDF) and its random copolymers. Consequently, the resulting fluoropolymer materials have found many high-performance applications anywhere from coatings and paints, transmission fluids, pipe liners, antifouling

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Special Issue: Frontiers in Macromolecular and Supramolecular Science Received: September 14, 2015 Published: January 13, 2016 2244

DOI: 10.1021/acs.chemrev.5b00539 Chem. Rev. 2016, 116, 2244−2274

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resulting fluoromaterials genome remain commendable, but challenging, efforts.1,2,7,27 The development of novel, mild chemistry that allows gramscale polymerizations of MCFMs to be carried out at or around room temperature (rt) in inexpensive, low-pressure glass tubes would be really beneficial, as it would permit a cost-effective, rapid screening of catalyst and reaction conditions, as well as enable photocatalysis. Such procedures could easily be extended not only to the initiation and CRP of FMs, postpolymerization fluoro-functionalization, and the synthesis of complex fluoro-architectures but also toward trifluoromethylation or perfluoroalkylation reactions that are of increased synthetic use in organic/medicinal28 chemistry. CF3 is clearly distinct from CH3, in terms of both electronic structure and reactivity: CF3 has the same electronegativity as chlorine (3.2) but is similar in size to an isopropyl (i-Pr) group (van der Waals radius 2.2 Å). Accordingly, prompted by the distinctive properties imparted by the CF3- and RF-groups onto chemical entities ranging anywhere from agricultural derivatives to synthetic drugs or polymer nanostructures, perfluoroalkylation, especially trifluoromethylation (TFM), has recently become a very valuable protocol for strongly improving and expanding molecular properties and functions, and such chemistry has attracted a lot of recent interest.29−41 The vast majority of TFMs involve electrophilic29−33,36 or nucleophilic29−35 organic and organometallic protocols,29−34,37 but radical TFMs,29−33,38−40 including metal-mediated photoredox processes,42 are currently emerging as a more convenient, complementary, and synthetically powerful strategy. However, despite such efforts,38 the metal-mediated generation of CF3• from CF3−I (bp = −22.5 °C) as well as from other commercial sources43 remains expensive and inconvenient, and the development of safe, nongaseous, commercially available, inexpensive CF3• and RF• precursors is highly desirable, especially in conjunction with metal-free protocols. To address the problems above, milder means of radical generation were investigated,44−51 and the first examples of metal-mediated, controlled (IDT-CRP) and free radical (FRP) VDF photopolymerizations, carried out at 40 °C, in lowpressure glass tubes were recently reported. Such systems employ a variety of metal carbonyls such as Mn2(CO)10 as visible light photoactivators, in conjunction with perfluoroalkyl iodides (RFI) and alkyl halides.45,47,48,50 Aside from polymerization initiation, CF3−I and CF3−SO2−Cl also afforded the first examples of Mn2(CO)10-mediated radical trifluoromethylations. A complementary, metal-free photochemical approach to the iodotrifluoromethylation of alkene and VDF-IDT was developed based on the use of iodine and hypervalent iodides.46 Finally, a metal carbonyl-mediated photochemical process, which affords quantitative activation of both −CH2−CF2−I and −CF2−CH2−I PVDF iodine chain ends, was also demonstrated for the synthesis of the first examples of well-defined PVDF block copolymers.45−50 These concepts are summarized below.

layers, fuel cell membranes, and O-rings for extreme temperatures to sensors, actuators, transducers, high-power capacitors, or optical fibers.4 To date, the global fluoropolymer market is evaluated at least at $8 billion and continues to increase.5,6 Accordingly, the precise synthesis of macromolecular fluoropolymer structures remains very relevant.7 However, the corresponding monomers propagate with very reactive, sidereaction-prone radicals and are gases under normal conditions = −84 °C). These differences versus conventional (e.g., bVDF p alkene monomers (styrene, acrylates, dienes, etc.) add more complexity to the polymerizations, which become rather challenging on a laboratory scale, as they require hightemperature, high-pressure metal reactors.8 While the archetypical poly(tetrafluoroethylene) remains the most utilized fluoropolymer at ∼60% by weight of world consumption,6 the parent CF2CF2 is a delicate, explosion-prone monomer,9 which demands special safety handling precautions. This renders the even lower-boiling VDF as the second most widely used (at ∼15−20%) and second most difficult, yet more manageable fluorinated monomer for both academic as well as industrial laboratories.10 Although noteworthy advancements were made in controlled radical polymerizations (CRPs) over the last two decades,11−15 and various reversible deactivation methods based on either the persistent radical effect16 (e.g., atom transfer radical polymerization (ATRP)17 and nitroxide dissociation−combination (DC) protocols) or degenerative transfer (DT) methods (e.g., iodine degenerative transfer (IDT),18,19 reversible addition−fragmentation (RAFT),20 or the use of metal (Co, Mo, Te, Ti, etc.)21 centered capping agents) are successful for acrylates and styrene, their applicability in CRPs of their sidechain fluoro analogues (e.g., CH2CHCOO(CH2)nRF, CH2 C(RF)COORalk, and CH2CH(C6F5)) remains in its initial phases.7,13 Conversely, although the application of nitroxide or metal-mediated CRP techniques toward MCFMs has not been demonstrated, recent efforts toward RAFT-mediated copolymerization22−25 and homopolymerization26 of VDF are promising. By contrast to conventional acrylate or styrene CRPs, which can be straightforwardly sampled from Schlenk tubes on, e.g., a 1 g scale, the very low boiling point of fluorinated monomers implies time-consuming and expensive one-data-point kinetic experiments in costly metal reactors, typically with tens or hundreds of grams of monomer. The situation remains especially complicated for VDF, in view of both the low boiling point and especially the current inability to afford a metal-mediated, ATRP-like initiation directly from alkyl halides, as well as of the VDF propagation with two types of chain ends (PVDF−CF2−CH2−X and PVDF−CH2−CF2−X, X = I, RAFT reagent, etc.) with vastly different reactivities. These drawbacks render the synthesis of well-defined PVDF block copolymers and of other complex architectures a very difficult task, as detailed later. As a result, the absence of convenient chemistry for the CRP of MCFMs and for the synthesis of their corresponding welldefined and morphologically versatile, sophisticated structures (blocks, graft, hyperbranched, star (co)polymers, etc.) has resulted in a vast lag versus conventional monomers with respect to the study and understanding of their self-assembly and of the properties and applications thereby derived. Therefore, the elaboration of MCFM-CRPs, the synthesis of intricate fluoropolymer structures and the mapping of the

2. CONTROLLED RADICAL POLYMERIZATION OF VDF 2.1. Peculiarities of VDF Free Radical and Controlled Radical Polymerization by Iodine Degenerative Transfer (IDT)

Unlike most alkene hydrocarbon monomers, due to a relatively small difference in the size of the substituents, i.e., steric repulsion, VDF propagation proceeds not only via the normal head−tail (HT) arrangement but also with a significant 2245


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Scheme 1. Reversible and Irreversible Deactivation of the PVDF−CH2−CF2• and PVDF−CF2−CH2• Chain Ends in CRPs Mediated by the Persistent Radical Effect (PRE) Such As ATRP (X−Y = CuBr2), Nitroxide or Metal-Centered Radical Mediated Polymerizations (Z = TEMPO, MtLn) and in CRPs Mediated by Degenerative Transfer (DT, Q = Iodine, −S−CS−R RAFT Derivatives, TeR, Cp2TiCl, Coacac, Etc.)

polymerization tolerance, IDT is also the first industrially applied CRP.58 Subsequently, this method was extended to other nontrivial halogenated monomers, including vinyl chloride,81,82 as well as to acrylates and styrene.18 As a typical radical degenerative transfer process, VDF-IDT entails the use of an outside source of radicals, such as those provided by TBPO.2 Consequently, although most chains are derived from the RF−I CT agent, the larger amounts of initiator demanded by faster rates unavoidably increase the fraction of chains initiated directly by the free radical initiator, to the detriment of the RF−I. Even if Y−RF−I chain-transfer agents were available for Y-based postfunctionalization (Y = functional group for later chain extension, coupling, block synthesis, anchoring on surfaces, improving adhesion, etc.), this approach would still lead to mixtures of homo and block copolymers. On the other hand, functionalized Y−R−O−O−R−Y peroxides capable of initiating VDF are quite rare and not commercially available. Thus, the ability to perform a transition metalmediated, ATRP-like initiation directly from functionalized Y− RF−X initiators is highly desirable toward the quantitative incorporation of Y chain-end functionalities. Finally, as described below, the complete activation of both PVDF−CF2−I and PVDF−CH2−I chain ends toward the synthesis of blocks is extremely challenging. Because conventional IDT systems (RF−I and free radical initiator) are not appropriate for the precise synthesis of well-defined block copolymers based on MCFMs, metal-mediated processes are required. However, although VDF propagates fast enough83 that polymerization occurs even at ambient temperatures,84 redox reactions of transition metal complexes and polyhalides result only in very low telomers (degree of polymerization (DP) = 1− 3), even at T > 100 °C.1,85,86 Moreover, while radical perfluoroalkyl-iodination of regular hydrocarbon alkenes with RF−I occurs readily with many catalytic systems (Cu,87 Zn,88 Pd,89 SnCl2/CH3COOAg,90 Cp2TiCl,91 etc.), including recent photoredox systems,42 the corresponding addition of electrophilic RF• radicals to electrophilic fluoroalkenes (MCFMs) at T < 100 °C, and especially at rt, is not available under metalcatalyzed conditions.

contribution from regioselectivity defects, i.e., the head−head (HH) internal units. Depending on conditions, up to 1 in ∼10−20 VDF units is inverted2 to afford the −CH2−CF2− CF2−CH2−CH2−CF2− HH sequence, instead of the normal −CH2−CF2−CH2−CF2−CH2−CF2− HT enchainment. Such sequence faults impede crystallization ability52 (e.g., electroactive β-phase), solubility, etc. and, most importantly, have a strong negative impact on the ability of this monomer to undergo a controlled radical polymerization.1,2,4,7,18,19,45−48 Consequently, in any potential CRP method, irrespective of the X (or Z or Q) “radical protecting group” (Scheme 1), the mixed VDF propagation, which proceeds via destabilized primary radicals, unavoidably generates PVDF comprising both −CH2−CF2−X and −CF2−CH2−X termini, where the bond dissociation energy (BDE) of C−X in −CH2−X is always larger than that in −CF2−X.53 Therefore, the corresponding PVDF−CF2−CH2−X chain ends are much harder and slower to activate than the isomeric PVDF−CH2−CF2−X and, for all intents and purposes, behave kinetically as dead chains, as far as reversible activation is concerned. Consequently, they accumulate as the polymerization progresses and lead to broader polydispersity index (PDI) and eventual loss of control. Furthermore, even for the weaker PVDF−CH2−CF2−Y termini, the BDEs for Y = Cl, Br,54,55 RAFT, or nitroxides derivatives56 are also too large for VDF-CRP to occur at ambient temperatures and low pressures with any substantial rate. Due to the low BDE of iodine compounds, the only industrially applied approach to the CRP of main chain fluorinated monomers remains the high-temperature, highpressure iodine degenerative transfer18,57−67 (IDT: Pn• + Pm−I ⇌ Pn−I + Pm•) using perfluorinated iodides and free radical initiators such as tert-butyl peroxide (TBPO).1−7 IDT can be traced back to earlier studies on the high-temperature (100− 230 °C) VDF free radical telomerization64,65 with polyhalides,4 in particular, with (per)fluorinated RF−I iodo chain-transfer (CT) agents,18 including CF3−I,4−69 CF3−CF2−I,70 CF3− (CF2)3−I,85,71 CF3−(CF2)5−I,67,72−74 (CF3)2CF−I,85 Cl− CF2−CFCl−I,69 and I−(CF2)4−6−I,74,58,75 as well as the less active HCF2CF2CH2−I,72,73 C6F13CH2CF2−I,73,76 CH2I2,77 RF−CH2−CH2−I,78 and CH3−I.79 IDT remains one of the oldest CRP procedures, as a linear dependence of molecular weight on the conversion was already illustrated58−61 in the early 1980s using such perfluorinated alkyl iodide (RF−I) and peroxide initiators. Taking advantage of the commercial availability of RF−I derivatives18,80 and of the water/emulsion

2.2. Toward Metal-Mediated VDF-CRP

2.2.1. Copper-Catalyzed VDF-ATRP. In conventional ATRP, although the halides of R−X derivatives (X = Cl, Br) activated by ester or benzyl substituents can easily be abstracted by CuX/L (X = halide, L = ligand) to provide initiating radicals for acrylates or styrene,11−13 this process is considerably slower 2246


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3)2C(COOEt)−Br with perfluoroalkyl derivatives (k(CH /kCact8F17−Br ≈ act 2 92 10 ). Under identical conditions of ligand, solvent, and temperature, density functional theory (DFT) calculations54 VDF MA indicate that, even at 90 °C and X = Br, Kethylene ATRP /KATRP/KATRP/ MMA −7 −7 /K ≈ 2 × 10 /9 × 10 /1/6/30. Likewise, the KSt ATRP ATRP relative ATRP rates of methyl acrylate (MA) and VDF can be MA estimated54,93,94 as rateVDF/rateMA = (kVDF × KPVDF × p ATRP)/(kp −7 MA KATRP) ≈ 8.75 × 10 , which corresponds to an impractical ∼13 days (Br) and ∼317 days (Cl) of VDF-ATRP time for 1 s of MA reaction. Consistent with these trends, investigations of VDF-ATRP in low-pressure glass tubes at mild temperatures (25−60 °C)95 under a wide variety of reaction conditions (initiator, metal, ligand, solvent, etc.)96 afforded no polymerization. At mild temperatures, CuX is quite unreactive toward perfluoroalkyl bromides,92 and VDF-ATRP with X = Cl and Br is unfeasible.54 Although better results are expected with RF−I derivatives, these reactions still require high temperatures in the absence of an activating Cu ligand.97 In addition, typical CuX-ATRP ligands such as amines or phosphines11,13,17 react with perfluoroalkyl halides to form quaternary onium halide salts, charge-transfer complexes,98,99 or extended halogen-bonding associations100 that will consume both reagents and prevent initiation. Moreover, although the threshold energy for HF elimination from model compounds (e.g., F−CH2−CF2−H) may appear large (284.5 kJ/mol) and thermal elimination requires elevated temperatures,101 Lewis basic ligands in polar solvents will easily catalyze PVDF dehydrofluorination,102−104 which is promoted by the formation of conjugated double bonds and is easily noticeable by the darkening of the sample. Under these conditions, at ambient temperatures, CuX/L hardly promotes the clean homolysis of the C−I bond of −CF2−CF2−I. Thus, while modest activation may occur for the weaker PVDF−CH2−CF2−I chain ends, the clean activation of PVDF−CF2−CH2−I chain ends remains unlikely, even at higher temperatures. Likewise, although the ATRP grafting of other monomers f rom the PVDF main chain via the homolytic activation of the very strong C−H or C−F bonds with typical ATRP CuX/L catalysts was claimed in the literature105 and assigned to the direct grafting from HH linkages,106 mechanistically, this is very unlikely, and there are no known examples of radical C−H or C−F activation reactions under similar conditions with model compounds. Such polymerizations most likely proceed by the addition of the propagating chain of the other monomer to PVDF unsaturations generated by base (i.e., ligand) induced dehydrofluorination (i.e., “grafting through”),107 and only mixtures of graft and homopolymer will really form. These drawbacks render ATRP impractical for both the controlled synthesis of PVDF and that of well-defined PVDF block or graft copolymers. Nonetheless, conceptually, the ATRP-like, metal-mediated protocol employing initiation from halides or pseudohalides would be very useful in the synthesis of complex fluoro architectures. 2.2.2. Degenerative Transfer with Metal-Centered Radicals: Cp2TiCl. Because DT processes are typically characterized by lower BDEs than PRE-based DC processes,54,56 it is tempting to consider the reversible termination of the PVDF−CF2• and PVDF−CH2• growing radicals with a metal-centered radical or a neutral complex, where such BDE can be additionally modulated from the nature of the metal and its ligands. Interestingly, because RF−MtL derivatives are typically more stable than the corresponding RH−MtL,108 the

opposite BDE trend versus halides would be expected, i.e., PVDF−CF2−MtL > PVDF−CH2−MtL. However, an additional and metal-specific problem is the prevention of potential β-H and β-F eliminations,44 while enabling a reversible termination with both chain ends. Promising metal-mediated CRP systems proceeding via reversible C−Mt bond formation were thought to be based on complexes of Co, Mo, Ti, etc.11,21 which were successful for other monomers, including acrylates, styrene, and dienes. Some initial efforts were recently reported and involved the use of the Cp2TiCl44 metalloradical that, in equilibrium with its Cp2Ti{μCl2}TiCp2 dimer, can both generate and trap very reactive radicals109,110 such as those obtained from the radical ring opening of epoxides,111−125 single electron transfer (SET) reduction of aldehydes or ketones,126 halides,109,127,128 and peroxides.129 Such initiation and termination protocol was successfully employed in the initiation and CRP of both styrene111−125 and dienes,125,130,131 in the living ring-opening polymerization of cyclic esters,132−134 as well as the synthesis of graft copolymers118−124 of acrylates and fluoroacrylates,135,136 styrene and pentafluorostyrene,137−140 and caprolactone.141 However, although such destabilized radicals should add to VDF, an in-depth investigation of the effect of the reaction parameters on the polymerization met with no success, as a solvent compatible with both Cp2TiCl and PVDF could not be found.44,142 Indeed, Zn reduction of Cp2TiCl2 to Cp2TiCl requires a polar, carbonyl-free, ether-like solvent such as tetrahydrofuran (THF) or dioxane, whereas, as outlined below, VDF polymerization requires carbonyl-containing solvents, which do not behave as radical chain-transfer agents. Such conditions were found to be mutually exclusive, and no polymer was obtained under a very wide range of reaction conditions (solvent, initiator, reducing agent, Ti ligand, etc.). It is apparent that ATRP and Cp2TiCl systems, respectively, either do not provide radicals reactive enough to add to VDF, and cannot reversibly activate the PVDF−Y chain ends at rt,95 or are too reactive, and lead to side reactions with both PVDF and the solvents.44 2.3. Free and Controlled Radical VDF Photopolymerizations

2.3.1. Transition Metal Carbonyl Photoactivators in Radical Polymerizations. Because typical conventional CRP protocols failed to provide PVDF under the desired mild conditions tested, taking advantage of the amenability of glass tubes to photochemistry, alternative, photochemical means of radical generation and trapping143,144 are required. High-power, UV-promoted VDF telomerizations to low oligomers were described over 50 years ago.4,145,146 Although TBPO is a hightemperature initiator, VDF free radical polymerization (FRP) can be initiated by the UV-induced photodecomposition of TBPO under mild (T = 40−60 °C) conditions.44,147−150 However, until recently,44−50 there were no reports on the photopolymerization of VDF using low-intensity visible light. In view of the poor stability of most organometallic complexes under intense UV irradiation, the study of polymerizations mediated by photoactive, commercially available transition metal complexes, under visible light provided by low wattage (40 polymerization media were evaluated.45−49 Such an exhaustive study was deemed necessary, as a similar quest using Cp2TiCl2 failed to find solvents compatible with both Cp2TiCl• and PVDF.44 Accordingly, control polymerizations (VDF/CF3−(CF2)3−I/ Mn2(CO)10 = 25/1/0.2; VDF/solvent =1/3 v/v, 40 °C, visible light)45 indicated that, even though Mn(CO)5• radicals were relatively inert toward all solvents under consideration, the solvent chain-transfer ability played a larger role in the polymerization than PVDF solubilization. Indeed, no polymerization occurred in α,α,α-trifluorotoluene, anisole, ethyl ether, THF, dioxane, diglyme, diethylene glycol monoethyl ether, ocresol, isopropanol, trifluoroacetic anhydride, tetrametyl urea, benzonitrile, cyclopentanone, or sulfolane, even after 1−3 days.

higher reactivity. Successful alkyl, semifluorinated, or perfluoroalkyl R−X initiators must deliver highly reactive radicals. Likewise, the LnMt• metalloradical photoactivator needs to be reactive enough to activate/abstract such halides efficiently. Moreover, for a metal-mediated reversible deactivation, (i.e., metal-catalyzed IDT, X = I), LnMt−I should also be a great iodine donor. Representative examples143,144 of transition metal complexes that photolyze to such metalloradicals are carbonyl dimers (CO)nMt−Mt(CO)n, reminiscent of the Ti dimer described earlier for the CRP of styrene111 and isoprene,125 but which, because of side reactions, was unsuccessful for VDF.44 Several of such complexes are commercially available and were previously applied in radical reactions in both organic and polymer chemistry.144 Earlier studies have indicated that their halide-abstraction ability ranks qualitatively as Re(CO)5• > Mn(CO)5• > CpW(CO)3• > CpMo(CO)3• > CpFe(CO)2• > Co(CO)4•.151 In practice, although Re(CO)5• abstracts Cl from CCl4 ∼65 times faster than Mn(CO)5•,152 the stronger Re−Re153 BDE, and its higher price, render Mn2(CO)10154,155 the most popular and least expensive reagent in the series.144 Mn2(CO)10 is stable at rt in the dark (Keq < 2.4 × 10−19),156 but because the Mn−Mn BDE is rather low (38 ± 5 kcal/mol vs Re−Re 44.6 ± 3 kcal/mol),144,157−159 and decreased by additional ligands,160 Mn2(CO)10 easily dissociates both thermally (∼60−90 °C)161 and photolytically, even at rt. Strong UV irradiation promotes CO loss to form Mn2(CO)9 and Mn(CO)5•, but low-intensity, near-UV and visible light (λ Mn 2 (CO) 10 = 366−400 nm, λmax = 324 nm) afford the 17e− Mn(CO)5 • paramagnetic Mn(CO)5 (λmax = 780−830 nm)162 with 163−165 decent quantum efficiency. A stable, pentacoordinate Mn(0) persistent radical was also very recently synthesized.166 Mn(CO)5• is quite reactive and easily abstracts both hydrides and halides from, e.g., Bu3SnH,167 as well as from R−X derivatives with moderate-to-high BDEs (300 °C,207 and it is not a known photosensitizer. Furthermore, no photopolymerization is observed in DMC with VDF alone, VDF/Mn2(CO)10, or VDF/RF−I. The methyl groups of DMC could act as weak CT agents, but this solvent does not generate radicals under visible light irradiation. Therefore, the trends observed in the solvent effect encode the combined outcome of both chain transfer and polymer solubilization. Here, alkyl carbonates afford minimum transfer in conjunction with the highest VDF solubilization and PVDF swelling, which enhances monomer diffusion to the propagating chain. This is consistent with their use as electrolytes, which also afford the swelling of microporous PVDF membranes/separators employed in Li-ion batteries.208−211 Control experiments also confirmed that no polymer was obtained from any of the LMtp(CO)q derivatives and VDF under irradiation, or in the additional presence of halide initiators, but only in the dark.45,50 Although metal-mediated organometallic polymerizations would be of great interest for fluoromonomers,21,44 this was not the case with these transition metal carbonyls. RF−Mn(CO)5 perfluoroalkylmanganese derivatives (RF = CH2F, CF2H)227 are known, but considering their low bond dissociation energies (BDEs, kcal/mol, RF− Mn(CO)5 (34)27 < (CO)5Mn−Mn(CO)5 (38)214 < RF−I (48)212 < I−Mn(CO)5 (54)158,214), they are unstable under photopolymerization conditions. Even if Mn2(CO)10 were to accumulate, it is not a persistent radical or a radical trap versus the propagating chain under irradiation, and each radical would rather homodimerize than cross-couple. Thus, homolysis of PVDF−Mn(CO)5 to PVDF• and Mn(CO)5• cannot lead to

Scheme 2. Metal Carbonyl-Photomediated VDF Free Radical and IDT Polymerization

slow thermal decomposition of a free radical initiator, visible light promotes the slow, reversible photodissociation of Mn2(CO)10 to generate the active Mn(CO)5• metalloradical (eq 1), which, by radically activating R−X and PVDF−X (especially PVDF−CH2−CF2−I) halides, compensates for termination reactions and maintains a steady-state radical concentration. 2249


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Figure 1. Qualitative dependence of the rate of VDF photopolymerization on the nature of the metal carbonyl and halide. The PVDF−I column refers only to chain-end activation. Reprinted with permission from ref 50. Copyright 2015 American Chemical Society. to RX VDF (CT) constant to PVDF• (CTPVDF• = kPVDF• /kpropagation > RX transfer 1), the R−X initiator will also act as a chain-transfer agent (CTA) toward the propagating chains (eqs 5 and 6). However, while X = Cl or Br can only lead to telomerizations and chaintransfer-inactive PVDF−Cl or PVDF−Br, for X = I, transfer is also available between the propagating chains, and the polymerization will follow the IDT-CRP18 mechanism. In this case, the RFI initiator is quickly converted into the two iodideterminated (Pn−CH2−CF2−I and Pm−CF2−CH2−I) macromolecular PVDF CTAs.18,45−50,72,73,76 Very few CTPVDF• values have been determined (CCl3Br = RX 35, CCl4 = 0.25, CHCl3 = 0.06 at 141 °C,216,217 C6F13−I = 0.8, C6F13−Br = 0.08, and C6F13−H = 0.0002 in scCO2 at 120 °C74). However, it is noteworthy that long-chain perfluoroalkyl iodides and PVDF−CH2−CF2−I have similarly large CTPVDF• RX values (C6F13−I = 7.9, C6F13−CH2−CF2−I = 7.4 at 75 °C).73 By contrast, the reverse PVDF−CF2−CH2−I, 2,1-chain end is at least 25 times less active (e.g., CT of HCF2−CF2−CH2−I = 0.3 at 75 °C).73 Conversely, the propagating PVDF−CF2− CH2• radical is much more reactive than PVDF−CH2−CF2•. In addition, the measured CT values represent an average over the 2• two types of propagating chains, and in reality, CTPVDF−CH ≫ RX PVDF−CF2• . The reactivities of the two propagating chain ends CTRX of VDF are so different that, in a sense, they resemble the radical propagation of ethylene and respectively, that of tetrafluoroethylene. To understand the initiator effect and select the appropriate initiators for VDF−FRP and VDF−CRP, a wide structural variety of over 40 halides never previously reported in conjunction with Mn2(CO)10 and the other metal carbonyls were evaluated.45,50 The structure−property correlation in the

During the initiation sequence, irreversible191,192 halide abstraction (eq 2) from the R−X (X = Cl, Br, I, including from PVDF−X later in the polymerization), driven by the formation of the very stable (CO)5Mn−X,157,158,214 affords R•. This radical, if sufficiently reactive, adds, driven by polar effects, predominantly at the CH2 termini of CH2CF2, initiating polymerization (eq 3). Here, regioselectivity propagation defects in VDF−FRP (eq 4) lead to both 1,2-HT (90− 95%)66,196 and 2,1-HH units (5−10%). Both the R and X components of the halide initiator have a tremendous effect on the outcome of the polymerization, which is determined not only by the value of the C−X BDE but also by the combined ability of R• to add to VDF, and by that of R− X and PVDF−X to undergo chain transfer. Indeed, modeling75,215 and kinetic66,72,73 studies have illustrated the importance of the structure and reactivity of the CT agent (I > Br > Cl ≈ H and difunctional better than monofunctional),58,18 of side reactions (transfer to polymer, solvent, etc.), and of monomer addition mode (1,2- vs 2,1-) to the quality of the polymerization control. Therefore, because for a given R the BDE of R−X decreases in the order Cl > Br > I, R−Cl and R− Br initiators have very low CT values and will at best afford only VDF−FRP, including telomerizations. By contrast, R−I initiators can lead to both FRP (for high BDE, inactivated, unsubstituted alkyl iodides initiators) and IDT, in the case of the low BDE RF−I initiators. Consequently, the amount of catalyst scales with the BDE of R−X, where high BDE initiators require stoichiometric activation, but low BDE ones require only catalytic activation. With substoichiometric amounts of Mn2(CO)10, and for low BDE of R−X, i.e., large values of the initiator chain-transfer 2250


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Figure 2. Selected examples of 1H NMR spectra of PVDF photoinitiated by Mn2(CO)10 from Cl-, Br-, and I-based substrates. Adapted with permission from ref 45. Copyright 2012 American Chemical Society.

CH2−CH2−Br, and Br/I−(CH2)10−Br/I), trichloromethyl derivatives (CHCl3, CCl4, CCl3−CCl3, and CCl3−Br), semifluorinated halides (HCF2−CF2−CH2−I and Br−CF2−CH2− CF2−Br), and especially perfluorinated halides (CF 3 (CF 2 ) 2 CO−Cl, CF 3 −SO 2 −Cl, Cl−CF 2 −CClF−Cl, −(CF2−CFCl)n, EtOOC−CF2−Br/I, C6F5−CF2−I, Cl−CF2− CFCl−I, CF 3 −I, CF 3 CF 2 −I, (CF 3 ) 2 CF−I, (CF 3 ) 3 C−I, CF 3 (CF 2 ) 3 −I, Cl−(CF 2 ) 8 −Cl, Br−(CF 2 ) 4 −Br, and I− (CF2)4,6−I) are very successful initiators, not only for VDF but also for other MCFMs including CH2CFH, CF2CFCl, CF2CFBr, and CF2CCl2, as well as for random VDF copolymers with CF2CF(CF3) and CF2CF(OCF3).45,49 Following initiation, according to the principles outlined above, the polymerization outcome is governed predominantly by the effect of the CTPVDF• values (i.e., the C−X BDE of RX RX)65,74 (eqs 5 and 6), as well as by the reactivity of the fluorinated initiator radicals (more branched and more electrophilic)53 and the preferential activation of primary versus secondary or tertiary halides168 by sterically demanding metalloradicals such as Mn(CO)5•. Here, the R−X initiator BDE, i.e., CT value, determines the amount of Mn2(CO)10 required for activation, while the halide type selects between FRP and CRP. For RX species that generate radicals active enough to add to VDF, three initiators classes can be recognized depending on their CT values, where VDF undergoes telomerization or FRP for X = Cl, Br, and I and IDT−CRP only for RF−I. These distinctions become apparent by inspection of the chain-end resonances from the NMR spectra of the corresponding polymers (Figure 2). Accordingly, initiators bearing strong R−X bonds (CHCl3, RH−Br/I, and RF−Cl, i.e., CH3−I, CH3−(CH2)5−Br/I, I/Br−

R−X initiator series is as follows:45,50 halides that generate radicals more stable than PVDF• such as I2, tBu−I, CH3−SO2− Cl, CH3O−Ph−SO2−Cl, CH2Cl2/I2, Cl2CH−CHCl2, CHBr3/ I3, CBr4/I4, CH2CH−CH2−Cl/Br/I, Ph−CH2−Cl/Br/I, Ph−CH(CH3)−Br, Ph(CH 2−Br/I)2, CH3−CH(CN)−Br, CH2(CN)−I, (−CO−CH2−CH2−CO−)N−Br, Cl−(CH2)3− SiCl3, (CH3)2C(COOEt)−Br/I, and CH3−O−Ph−I, as well as inactive halides such as CH3−(CH2)5−Cl, provide none to very little polymer under a wide variety of conditions.45,50 Because the metal carbonyls, especially Mn(CO)5• and Re(CO)5•, have a very high halide affinity,144 rt abstraction is available in all cases, except perhaps with CH3−(CH2)5−Cl. Therefore, the lack of initiation results from the failure of the corresponding R• radicals to add to VDF efficiently. On the other hand, not all transition metal carbonyls are as successful, even when using the labile CF3(CF2)3−I as initiator.50 Because of a combination of poor quantum yield for the metalloradical generation under low-intensity visible light irradiation (Cr(CO)6, Co2(CO)8, and Mo(CO)6),218 fast photodecomposition (Au(CO)Cl),219 or nonradical, organometallic pathways including oxidative addition into the ∼CF2−I bond of RF−I initiators (e.g., Fe(CO)5220 and CpCo(CO)2221,222), no polymer was obtained in the presence of Fe(CO)5, Fe3(CO)12, Cp2Fe2(CO)4, Co2(CO)8, CpCo(CO)2, Co4(CO)12, Ru3(CO)12, Cr(CO)6, CpMn(CO)3, Cp*2Cr2(CO)4, Mo(CO)6, (PPh3)Ni(CO)2, Cp2Ti(CO)2, or Au(CO)Cl. Polymerizations were only observed with Mn2(CO)10, Re2(CO)10, Cp2Mo(CO)6, and Cp2W(CO)6 (Figure 1).50 By contrast, inactivated RX alkyl halides species that generate very reactive radicals (CH3−I, CH3−(CH2)5−Br/I, Ph−O− 2251


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CH2−(CH2)8−CH2−Br/I, HCF2−CF2−CH2−I, and Cl− CFCl−CF2−Cl, Cl−(CF2−CFCl)3−6−Cl, CF3−(CF2)2−CO− Cl, and Cl−CF2−(CF2)6−CF2−Cl) do generate polymer, but because they do not undergo halide transfer with PVDF• PVDF• (CTRX ≪ 1) and are quickly halide-depleted by the stoichiometric amounts of Mn2(CO)10 required for activation, they will only afford PVDF with no halide chain ends and with noticeable HH defects, i.e., VDF-FRP. Such initiators, especially if containing a polymerization-inert Y group (i.e., Y−R−X), may be useful for, e.g., PVDF grafting from substrates, but they cannot enable block synthesis using the X functionality. The use of initiators with higher chain-transfer constants, i.e., substrates with weaker R−X bonds (CF3SO2−Cl, R−CCl3, and RF−Br/I), allows the use of reduced/catalytic (10%) amounts of Mn2(CO)10 or other transition metal carbonyls. Moreover, the active chain-transfer process (eqs 5 and 6) also leads to at least one or both halide-functionalized PVDF−X chain ends (i.e., −CH2−CF2−X and −CF2−CH2−X, X = Cl, Br, and I, Figure 2). However, although the initiator may be an efficient CTA, the resulting PVDF halide chain ends, particularly PVDF• PVDF−CF 2 −CH 2 −X, are less reactive (CT RX ≥ PVDF• PVDF• ≫ CTPVDF−CH2−X ) and will provide much CTPVDF−CF2−X poorer macromolecular CTAs, especially for X = Cl and Br. Therefore, as Cl- or Br-degenerative exchange does not occur, only telomerization65 or VDF-FRP is available for R−X = CF3− SO2−Cl, CCl4, CCl3Br, CCl3−CCl3, EtOOC−CF2−Br, Br− CF2−CH2−CF2−Br, and Br−CF2−CF2−CF2−CF2−Br. Consequently, unlike the case of X = I below, the absence of uncatalyzed Cl/Br−DT, i.e., of reversible chain-end activation for X = Cl and Br, also averts the accumulation of the unreactive PVDF−CH2−Cl/Br. Here, both 1,2- and 2,1propagating chain ends abstract the initiator halide to reflect the same typical ∼10/1 ratio of PVDF−CH2−CF2−X/PVDF− CF2−CH2−X as the HT/HH propagation, unless the initiator is a poorer CTA, such that only the more reactive PVDF−CH2• will abstract. Nonetheless, Mn(CO)5• may still activate the PVDF−X chain ends during polymerization, to allow for some molecular weight increase, albeit poorly. These R−Cl or R−Br initiators may provide better initiation than the preceding class, but they still cannot afford molecular weight control, compete effectively with side reactions such as solvent transfer, or suppress HH addition. Although some PVDF−Cl/Br halide chain-end functionality (CEF) is now available, it is not quantitative, and such polymers are still not apt for the synthesis of well-defined PVDF blocks via the activation of the PVDF−Cl/Br chain end. Finally, the only initiators that enable VDF−CRP and afford PVDF−X with very high CEF, appropriate for block copolymer synthesis, are perfluoroalkyl iodides, especially the difunctional I−RF−I species. Indeed, by contrast with all Cl or Br chaintransfer agents above, uncatalyzed halide DT−CRP is only possible for iodine.18,62 Here, even though HCF2−CF2−CH2−I or (CF 3 ) 3 C−I afford a less-efficient IDT due to PVDF• CTPVDF• HCF2−CF2−CH2−I ≈ CTPVDF−CF2−CH2−I ≪ 1 and, respectively, due to the slower reaction of Mn(CO)5• with bulky tertiary halides, all other perfluoroalkyl iodides (CF3−CF2−I ≈ (CF3)2CF−I < C6F5−CF2−I, EtOOC−CF2−I < Cl−CF2− CFCl−I < CF3−(CF2)2−CF2−I < CF3−I, < I−(CF2)4,6−I) in this series enable IDT. However, while these high CT RF−I initiators are the most suitable for VDF−IDT,18 in the corresponding macromolecular PVDF−I CT agents,73 the Pm−CF2−CH2−I18−76 terminal reversed unit remains ∼25

times less reactive toward IDT than the Pn−CH2−CF2−I isomer.73 Nonetheless, the very efficient competition of iodine •

•

73 PVDF degenerative transfer (CTPVDF with RF−I ≈ CTC6F13−I = 7.9), both propagation and termination, delivers not only molecular weight control but also both PVDF−CH2−CF2−I and PVDF− CF2−CH2−I iodine chain ends. Moreover, while the presence of HH units and the formation of the −CF2−H and −CH2−H chain ends by H abstraction via backbiting/branching and transfer are well-known defects in VDF−FRP,223−226 IDT also affords their significant suppression. Once all the original RF−I initiator is exhausted via chain transfer, no new PVDF−I chains can be produced, and the only relevant, thermodynamically neutral, reversible iodine degenerative exchange/transfer (IDT) is the uncatalyzed equilibrium of the propagating Pn−CH2−CF2• with the dormant Pm−CH2− CF2−I terminal 1,2-units (eq 7, Kequil (ex1) = 1). In this case, 1,2 because the exchange constant,14,76 defined as Cex = PVDF−CF2•, PVDF−CF2−I PVDF−CF2•, VDF /kpropagation,12−addition is ≫1, the reversible k1,2 exchange exchange between chains is greatly favored over propagation and termination. Unfortunately, the much stronger −CH2−I bond is not activated reversibly, and the cross-IDT of the 1,2and 2,1- units (eq 8) leads to an irreversible buildup of the Pn− CF2−CH2−I chain ends. Correspondingly, the exchange of the 2,1-terminal units (eq 9) is too slow to be of any kinetic relevance to the polymerization at ambient to moderate temperature.72,73 Nonetheless, the IDT equilibrium from eq 7 still enables the metal-mediated VDF-photo IDT under visible light, as illustrated45 (Figure 3, Mn2(CO)10) by a linear dependence of Mn on conversion for various target degrees of polymerization (i.e., [VDF]/[RFI] ratios) over a wide range of molecular weights, as well as by acceptably narrow PDI values.

Figure 3. Dependence of Mn and Mw/Mn on conversion in Mn2(CO)10-photomediated VDF-IDT-CRP. Inset: GPC of red triangle. Reproduced with permission from ref 45. Copyright 2012 American Chemical Society. 2252


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dead PVDF−PVDF and, respectively, PVDF−H, the I−PVDF• derived from difunctional I−RF−I affords dormant I− (PVDF)2−I and, respectively, I−PVDF−H (Scheme 3). The dormant chain can still be reactivated (eqs 2, 4, and 6) by Mn(CO)5• or Pn• to continue to propagate and even undergo additional dimerizations in the case or recombination. As a result, the fraction of dead −CH2−H and −CF2−H chain ends is greatly reduced, and the lifetime of the dormant/active chains is substantially enlarged. Thus, the outcome of termination is not as extensive for polymerizations initiated from I−RF−I derivatives as it is for those initiated from monofunctional RF− I. Consequently, the number of growing chains decreases at a much slower rate, and the quality of the CRP process is greatly improved (vide infra). This effect not only lowers the PDI values but, most importantly, helps retain a reasonably high iodine chain-end functionality until much higher monomer conversions, which is of tremendous importance to the synthesis of well-defined block copolymers. While this effect is obviously general, and applicable to all CRPs, the much higher reactivity of PVDF• (i.e., faster termination and transfer) dramatically amplifies the negative effect of side reactions by comparison with the more stable propagating chains of styrene, acrylates, or dienes. Finally, the demonstration of the initiation not only from polyhalides and all RF−I structures (which also enable IDT and suppression of the HH defects) but especially from the semifluorinated (HCF2−CF2−CH2−I) and inactivated (CH3− I, CH3−(CH2)5−I, and I−(CH2)10−I) alkyl iodide models of the unreactive, reverse PVDF−CF2−CH2−I chain end also suggests that the Mn2(CO)10-mediated block or graft VDF copolymerization can be directly initiated from such halides anchored on polymeric chains, surfaces, etc. 2.3.2. Photomediated VDF Polymerizations with Hypervalent Iodide Carboxylates. 2.3.2.1. Hypervalent Iodides as Radical Precursors. A large variety of free radical initiators with a wide range of half-lifetimes is commercially available, but there are, in fact, very few if any convenient and well-defined azo or peroxide CH3•, and especially CF3•, precursors. Although CH3• may be obtained in low yield alongside acetone from, e.g., the decomposition of (CH3)3C− O−O−C(CH3)3 (TBPO),72 the corresponding azo CH3−N N−CH3 or peroxy CH3−O−O−CH3 derivatives are lowboiling, unstable, and inconvenient reagents. Likewise, the generation of CF3• from CF3−I is expensive and impractical 3I = −22.5 °C), and perfluorinated azo- or peroxy230(bCF p analogues such as CF3−NN−CF3, CF3−O−O−CF3, or CF3−CO−O−O−CO−CF3231,232 are either unknown, unstable, or hazardous. In fact, except for a Mn2(CO)10 report,45 very few other CF3• precursors have ever been evaluated in the initiation of fluorinated monomers, where such radicals were

However, out of the series of the metal carbonyls investigated,50 only Re2(CO)10 yielded similar results, and, to a much lesser extent, Cp2Mo(CO)6 and Cp2Cr(CO)6.50 Unfortunately, although as described later Mn(CO)5• does activate both the initiator and the PVDF−I chain ends, control experiments45 reveal that, as for the IDT of VAc,191,192 neither Mn(CO)5−I nor any of the other metal carbonyl iodides in the series were photochemically active227,228 and suitable as a transfer agent capable of donating iodine to the growing chain end. Therefore, such activation in not reversible, i.e., the IDT process is not truly catalyzed. Conversely, IDT catalysis would have accelerated the exchange, and decreased PDI,12,57 while preventing the irreversible accumulation of the IDT-inert PVDF−CF2−CH2−I, which also leads to PDI broadening. Nonetheless, the current metal-mediated IDT still remains mechanistically distinct from conventional IDT, in that the lessactive PVDF−CF2−CH2−I chain ends can still be reactivated, especially at higher levels of Mn2CO10, which is not the case with traditional, free radical2 VDF−IDT methods. It is also interesting to note that difunctional I−RF−I initiators are the most suitable for VDF−IDT, as the presence of two iodine chain ends per chain,45,58 i.e., I−PVDF−I, still allows for the molecular weight to continue to increase, even when one of them is consumed following irreversible termination by radical coupling or transfer (Scheme 3).58 Scheme 3. Effect of Termination and Transfer Side Reactions on Polymerizations Initiated from Mono and, Respectively, Difunctional Initiators

Activation of the remaining iodine chain end by the continuously photogenerated Mn(CO)5•165,229 (Scheme 2, eq 2) compensates for termination and helps maintain a steadystate radical concentration.45 Here, by contrast with the case of monofunctional initiators, where irreversible termination of the resulting PVDF• by radical dimerization or H transfer produces

Chart 1. Commercially Available Hypervalent Iodine Carboxylates and Their [N−X−L] Designations,261 Where N = Number of Valence Electrons on Central Atom, X = Central Atom, and L = Number of Ligands on Central Atom

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Scheme 4. Free Radical and IDT-CRP of VDF Initiated by HVICs

conveniently prepared from any carboxylic acid precursor by oxidation in the presence of Ph−I and oxone.260 The vast majority of HVIC applications are oxidations,246 but as the weak I−O bond of R(CO)O−I(Ph)−O(CO)R HVICs is also predisposed to both thermal and photolytic (UV) homolysis, examples of HVIC-based radical processes are beginning to emerge.246,262,263 Aromatic HVICs such as (ArCOO)2PhI thermolyze in solution at 125−150 °C via a combination of ionic and radical processes, to form ArCOOPh, ArCOOI, and PhI and ArCOO•, which rapidly decarboxylates to Ar•.264−268 IFAB and DMPI thermolysis data is unavailable, but the thermolysis of IDAB at 127−160 °C269,270 proceeds both heterolytically (∼75%, CH3COOPh and CH3COOI) and homolytically (∼25%, PhI and CH3COO•, i.e., CH3• + CO2). The radical pathway is additionally favored by external radical sources,267,269 increasing acetate substitution271,272 and irradiation. Interestingly, while the rt thermolysis of IDAB is negligible °C ≈ 10−6 s−1, k40 °C ≈ 10−10 s−1),269,274 its rt UV photolysis (k130 therm therm C −6 mol L−1 (Hg-254, 265 nm) proceeds readily (k28 UV° ≈ 5.6 × 10 • −1 273 s ) to afford only radical products (CH3 ). Similarly, rt UV photolysis of HVICs obtained by ligand exchange of IDAB,276,280 or the more effective IFAB,280 with RCOOH carboxylic acids selectively affords the corresponding alkyl

generated either by high-temperature thermolysis or under strong UV irradiation, from gaseous and expensive CF3−Br233 and CF3−I,66,68,145,234 or from commercially unavailable CF3− SO2−SR,235,236CF3−S−(CS)−OR,237 explosive CF3−C(O)O−O(O)C−CF 3 , 231 toxic Hg(CF 3 ) 2 , Cd(CF 3 ) 2 , Te(CF3)2,238−240 or exotic structures including CF3-decorated octafluoro[2.2]paracyclophanes241 and persistent perfluoro-3ethyl-2,4-dimethyl-3-pentyl radicals.242,243 Thus, availability of a clean, safe, nongaseous, commercially available, and inexpensive source of CF3• would be highly desirable for trifluoromethylation radical reactions involving either polymerizations or organic functionalization. Interestingly, although known for over a century,244,245 hypervalent iodine(III,V) (HVI) derivatives (λ3- and λ5-iodanes), which contain iodine in the less common (+3) or (+5) oxidation states, have recently witnessed a resurgence in their applications in organic chemistry.246−259 Their very successful use as oxidants246−255 has led to such derivatives becoming inexpensively commercially available, as illustrated especially by HVI carboxylates (HVICs, Chart 1) such as (CX3COO)2IIIIPh, (X = H,250 ((diacetoxy)iodo)benzene (IDAB); X = F,254 bis(trifluoroacetoxy)iodo)benzene) (IFAB), and (CH3COO)3IV(−Ph−COO−) (Dess−Martin cyclic periodinane,255 DMPI). In addition, such HVICs can also be very 2254


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radicals, as decarboxylation of RCOO • is extremely fast.262,274,275,296 Such thermal or photoderived radicals subsequently add to alkenes 276−279 or alkylate aromatics.262,280−283 Although no data is available on the visible light photolysis of IDAB, IFAB, and DMPI, it is reasonable to assume that it also proceeds only by homolysis. Because HVICs can alkylate or ligand exchange with many H-labile species,246 molecular iodine also promotes the hypoiodite reaction246,284,285 of alcohols,284,286−294,309 carboxylic acids,287,295−302 and amines.303−308 This constitutes another unusual source of radicals, as the resulting transient R−Y−I309−311 iodinated species photolyzes under UV−vis irradiation312 to generate the corresponding R−Y• (Y = O, COO, and NH) radicals. However, while the related diaryliodonium hypervalent iodide salts are known cationic polymerization photoinitiators312,313 and photoacid generators in photolithography,246,312 even though an early attempt of their use in thermal polymerization was disclosed over 60 years ago,314 the thermolysis or photolysis of HVICs including IDAB and IFAB to produce radical initiators, or the photolysis and use of the Dess−Martin reagent in radical reactions, remains largely overlooked.315−320 Moreover, the use of such HVICs as convenient and clean CX3• or CX3I precursors (X = H and F) for methylation (IDAB and DMPI), trifluoromethylation (IFAB), or iodotrifluoromethylation (IFAB/I2) and as radical initiators for both conventional or fluorinated monomers has not been recognized until very recently.46 Nonetheless, a structural comparison of these unstable peroxides with the solid, stable, high-melting (Tm > 120 °C), hypervalent iodide carboxylates (e.g., CF3−CO−O−O−CO− CF3 vs CF3−CO−O−I(Ph)−O−CO−CF3) exposes a remarkable similarity, where the central hypervalent iodide (I(III)Ph) acts as a “protecting group” that dramatically increases the stability of these structures, while still enabling the safe generation of the corresponding alkyl radicals. It is apparent that the thermolysis or the metal-free, rt, visible-light photodecarboxylation of commercially available or easily RCOOH-derived246−248 (RCOO)2IPh, readily, cleanly, and safely delivers the corresponding R• (RF•, CX3•, etc.). This renders typical HVICs as convenient, stable, and safe protected synthetic equivalents of their corresponding expensive, hazardous, or inaccessible azo or peroxide analogues radicals. The inexpensive IDAB, DMPI, and IFAB are therefore recommended herein as the most practical and green321 CX3• and CX3I (X = F, H) precursors. Their use is exemplified below as visible-light VDF photoinitiators and as precursors for CX3−I or RF−I iodine chain-transfer agents for VDF-IDT, but such methodologies are general and likewise appropriate for the radical addition of CX3• to any other alkenes, for the IDT-CRP of other alkene monomers, and for radical (iodo)trifluoromethylations of arenes or other substrates. 2.3.2.2. HVIC-Initiated VDF Free Radical Polymerizations. As described earlier for the metal carbonyl polymerizations, balancing polymerization rate and safe tube pressure, all investigations were carried out at 40 °C.46 The mechanism of the free radical VDF photopolymerization and of two methods for VDF-IDT in the presence of HVICs is illustrated in Scheme 4. The free radical initiation and propagation is illustrated in panel (a). The rt thermolysis of IDAB is negligible,269,273 and the UV rt photolysis of IDAB273 and of its congeners derived from IDAB or IFAB ligand exchange with RCOOH262 readily and selectively generates R• via diffusion-controlled RCOO•

decarboxylation.262,275 Therefore, it is reasonable to assume that the low-intensity, visible-light photolysis of IDAB, IFAB, and DMPI, at or around rt, is also homolytic. Control experiments reveal no dark reaction between VDF and all HVICs (X = F and H). Under irradiation, the radical generation in the FRP mechanism is prompted by the photolysis of the weak322−324 I−O bond to produce CX3COO• (Scheme 4, eqs 1 and 1′).246 Fast (k ≈ 109 s−1)275,284 subsequent decarboxylation affords CX3•, which adds to VDF, thereby initiating the polymerization, which propagates as before, with both 1,2- and 2,1-units (eq 2). 1H- and 19F-NMR confirm46 the decarboxylation and selective, regiospecific, CX3• initiation, i.e., predominant formation of CX3−CH2−CF2− PVDF not CX3−CF2−CH2−PVDF, and the absence of CX3COO−PVDF (eq 2′). Moreover, as indicated by the absence of any PVDF−I chain ends, no IDT derived from the potential trapping268,315 of the growing chain with the photolytically unstable246,315 iodanyl (9-I-2)325−328 radicals (e.g., R−I • −Ph or RI • (−Ph−COO−), R = CX 3 −, CX3COO−, and PVDF−) or by chain transfer with the in situ generated PhI is occurring. Thus, decarboxylation is faster than addition to VDF, and the cogenerated Ph−I or HOOC− Ph−I are not effective iodine CT agents at 40 °C. Detailed kinetic investigations (Figure 4) revealed only free radical polymerizations, as evidenced by the independence of

Figure 4. Dependence of Mn and PDI on conversion in the HVIC photomediated VDF-FRP. All polymerizations are in DMC at 40 °C, except otherwise noted. [VDF]/[IFAB] = 100/1 (red solid circle), 100/1 (red open circle) in (CF3CO)2O, 200/1 (red solid square), 500/1 (red diamond), [VDF]/[IDAB] = 100/1 (blue solid circle), 100/1 (blue open circle) in (CH3CO)2O, [VDF]/[DMPI] = 50/1 (green triangle), 100/1 (green solid circle), 150/1 (green open circle) in (CH3CO)2O, 200/1 (green solid square). Reproduced with permission from ref 46. Copyright 2013 Wiley.

Mn on conversion in all cases. Here, for the same [VDF]/ [CX3COO−] ratios, all HVICs provide similar initiator efficiencies, indicating that all acetate groups are released, and that IDAB, IFAB, and DMPI are difunctional and trifunctional initiators, respectively (i.e., they release 2 and 3 radicals/ molecule, respectively). These reactions demonstrated the first examples of the use of IFAB as a trifluoromethylating reagent, as well as the use of the Dess−Martin periodinane as a radical initiator. 2255


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similar to that of Ph−I and PVDF−CF2−CH2−I.18,45 In control experiments, excess CH3• abstracts 50% activation of PVDF−CF2−I and PVDF−CH2−I, respectively.46 Therefore, such VDF-IDT-CRPs are supported by the continuous influx of CX3• radicals provided by the slow273 photodecomposition of HVICs, which, by activating primarily the PVDF−CH2−CF2−I chain end (Scheme 4, eqs 4e and 4f) or by adding to VDF, compensate for termination reactions (eqs 8a and 8b) and help maintain a steady-state radical concentration. Subsequently, the I−(CF2)6−I chain-transfer agent is rapidly consumed18,45,73 to produce the macromolecular I−PVDF−I chain-transfer agents (eq 4c). As seen earlier in the Mn2(CO)10-mediated VDF-IDT,45 the control of the polymerization is based on the thermodynamically neutral (KIDT equil = 1) IDT of the propagating and dormant Pm−CH2−CF2−I and Pn−CH2−CF2• species (eq 7a). Again, the large reactivity difference between Pn−CH2−CF2−I and Pm−CF2−CH2−I chain ends73 shifts the cross-IDT of 1,2- and 2,1-units (eq 7b) toward the irreversible accumulation of the unreactive Pn− CF2−CH2−I, as described in detail in section 3.2. The IDT of such units (eq 7c) is too slow to be kinetically irrelevant at 40 °C, and their unavoidable accumulation leads to PDI broadening. Nonetheless, the IDT of the macromolecular PVDF−CH2−CF2−I transfer agents (eqs 7a and 7b) does enable VDF-CRP as indicated (Figure 6) by the linear dependence of molecular weight on conversion, reasonable polydispersity values (∼1.5), and linear kinetics in all cases. A CRP process occurs either when using a constant amount of IFAB and varying the target degrees of polymerization (DP = [VDF]/[I(CF2)6I] = 50, 200, 500, 1000, and 2500 to afford PVDF with a wide range of molecular weights (Figure 6a)) or when maintaining a constant DP = [VDF]/[I(CF2)6I] = 200, to obtain the same Mn profile (Figure 6b) regardless of the structure (IFAB, IDAB, and DMPI) and relative ratio (0.1, 0.25, 0.5, and 1) of the HVIC. However, the amount, and especially the HVIC type does affect the rate of the polymerization. The faster decomposition of the more efficient IFAB246−284,322,368 affords polymerizations that are almost 1 order of magnitude faster than those initiated by IDAB and DMPI under the same conditions (Figure 6c). These results confirm that, in accordance with the IDT mechanism,14 Mn scales only with [VDF]/[I(CF2)6I] not [VDF]/[HVIC], as well as that the relative amount of HVIC only controls the polymerization rate. I(CF2)6I is the sole chain-transfer agent, and the HVICs serve only as typical free radical initiators, as in any conventional IDT process. 2.3.2.4. VDF-IDT with HVICs and I2: In Situ Generated CF3−I and Macromolecular Iodotrifluoromethylations. While the [I−(CF2)6−I]/[HVIC] protocol described above does afford VDF-CRP, as seen from Figure 5, the trifluoromethylation of the PVDF chain ends is 108 kcyclohexane* ≈ 1010 L mol−1 s−1)331−333 and, by intercepting

The solvent effect was also studied for the three HVICs, and out of the 28 solvents tested, dimethyl carbonate (DMC) followed by anhydrides provided complete or partial solubilization of IFAB and, respectively, of PVDF, IDAB, and DMPI, and was again optimum.46 Here, additional possible side reactions (ligand exchange/oxidation/hydrolysis) of HVICs with solvents containing labile Hs such as alcohols or water should be taken into consideration. On the other hand, acetic anhydride and trifluoroacetic anhydride are good solvents for polymerizations initiated by IDAB or DMPI and by IFAB, respectively, as ligand exchange with solvent would compensate for trace-hydrolyzed HVIC. 2.3.2.3. VDF-IDT with HVICs and RF−I External ChainTransfer Agents. Because IDAB, DMPI, and IFAB are only capable of initiation but afford no polymerization control, IDT can be established50 by the addition of external chain-transfer agents such as RF−I derivatives (Scheme 4, panel b1). Acetoxylation of RalkI368,373 derivatives by HVICs may take place under dark conditions, but the UV photolysis of IDAB/ RalkI mixtures is a radical process that results in iodide abstraction and formation of Ralk•284,368,373 and not esterification (eq 3). Indeed, 1H and 19F-NMR studies46 confirm the presence of PVDF−I iodine chain ends and the lack of RFI or PVDF−I acetoxylation. Therefore, the photogeneration of CX 3 • (eqs 1 and 1′) is much faster than potential esterification,373 and RF−I derivatives act exclusively as iodine chain-transfer agents. Accordingly, NMR kinetic investigations of the dependence of the nature of the initiator chain ends on conversion (Figure 5) reveal that, consistent with the IDT mechanism,2,4,7,12,14,15

Figure 5. Dependence of the −CH3 (IDAB = △, DMPI = ○), −CF3 (IFAB = □), −(CF2)6− (IDAB = ◮, IFAB = ◨, DMPI = ◑) initiator fragments, and of the % HH units (IFAB = ■, IDAB = ▲, DMPI = ●) on conversion, and on the [VDF]/[I(CF2)6I]/[HVIC] = 200/1/0.25 (green), 200/1/0.5 (blue), and 200/1/1 (red) ratios. Reprinted with permission from ref 46. Copyright 2013 Wiley.

regardless of the extent of the reaction, only a very small fraction (10 h and a Mn intercept of >10 000 (i.e., poor control) are observed. This is because here, the kinetics of the formation of the chain-transfer agent (CF3−I) are not controlled by the slow decomposition of a free radical initiator (AIBN) but by the very fast CF3COOI formation and its subsequent photodecarboxylation. The CTA is generated very fast and directly from the initiator, not from high molecular weight polymer species, and the Mn of PVDF−I is controlled from the beginning of the polymerization. This mechanism is consistent also with the reaction color sequence (Figure 8), where the initial violet I2 is quickly ( b) are equivalent with direct IDT polymerizations where [VDF]/[CX3I]/[HVIC] = a/2b/(c − b) for IDAB or IFAB and, respectively, with a/2b/(1.5c − b) for

shown that the photolysis of RCOO−I is a two-step process,309,310,336,369 where the rate-determining O−I homolysis is independent of R. Subsequent fast R Alk COO • decarboxylation (k Adlekc a r b ≈ 10 9 s − 1 , k Adre c a r b ≈ 10 4 s−1)262,274−276,285,296 provides Ralk• which is intercepted by I2 into RalkI276,284,295,312 in the solvent cage,369 whereas the more stable ArCOO• can be trapped again as ArCOOI372 or cyclize in aromatic lactonizations.296 Consequently, RCOOI intermediates have found use in Hunsdieker-like photohalodecarboxylative reactions, or in halogenations with IDAB, IFAB, or their ligand-exchanged HVIC congeners in the presence of R−I346,368,373 or I2.246,276−284,287−296,312,369 However, they have never been employed in trifluoromethylations or polymerization reactions. Accordingly, it is reasonable to assume that instead of external RFI CTAs, I2 could be employed for the in situ CX3I and CH3I generation (Scheme 4 panel b2). Indeed, when using, e.g., [VDF]/[I2]/[HVIC] = 50/0.25/1, NMR confirmed the selective, VDF initiation by CX3•.46 However, only free radical polymerizations (Figure 7) with broad PDI (2.5−3) and no iodine chain ends were observed for IDAB and DMPI. By contrast, IFAB cleanly afforded CF3−PVDF−I via a VDF-IDT process (Figure 7a) characterized by a linear dependence of molecular weight on conversion, relatively narrow (∼1.6) PDI, and a three times faster polymerization, without any induction time (Figure 7b). These trends indicate that all I2 is rapidly depleted in a fast reaction with the HVICs246−312,336 (Scheme 4, eqs 5 and 5′) to swiftly and quantitatively afford CX3COOI. CX3COOI could be considered a chain-transfer agent for the polymerization, but its stability under irradiation is quite low, and it is subsequently consumed by photo-276,284,309−312,369 and by CX3•-induced decarboxylation to generate CX3I (eqs 4d and 6). The photolysis of the HVIC/I2-derived CX3COOI to CX3I is faster (k ≈ 10−4 s−1)369 than the photolysis of the parent HVIC (k ≈ 5.5 × 10−6 mol/s).273 Under these conditions, all CX3I accumulates as a chain-transfer agent before CX3• (which is slowly273 released from the photolysis of remaining HVIC) starts initiating VDF. Conversely, assuming that the slower273 photolysis of HVICs were rate-determining, CX3I would have been slowly generated, either by I2 trapping CX3COO• to 2258


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Figure 9. Comparison of the dependence of Mn and Mw/Mn on conversion in VDF-IDT polymerizations carried out in the presence of IFAB and iodine (blue symbols) and RFI (orange symbols) at equivalent ratios. (a) CF3−(CF2)3−I used instead of I(CF2)6I. Reprinted with permission from ref 46. Copyright 2013 Wiley.

generated CF3−I and of the CF3−CF2−CF2−CF2−I systems versus I(CF 2 ) 6 I experiments (∼1.6), as explained for Mn2(CO)10 above, are due to the beneficial effect of di- vs monofunctional chain transfer agents.45,58−61 Although RF−I initiators generate RF−PVDF•, which recombines into dead RF−PVDF−RF chains, recombination of I−RF−I-derived I− PVDF• affords dormant I−PVDF−I, which can still undergo IDT or be activated by CX3•.46 Here, bidirectional growth from difunctional propagating chains, supported by the continuously photogenerated CX3•,273,276,280 compensates for termination by radical coupling and affords a decreased fraction of dead chains, i.e., lower PDI.

Figure 8. Experimental setup (top) and close-up (bottom) of IFAB/I2mediated VDF-IDT photopolymerizations at 40 °C in glass tubes, and an example of the evolution of the polymerization color with time: (a) initial dark violet I2 color following defrosting (t = 0−5 min). (b) Emergence of CF3COOI dark orange color; (c) complete disappearance of CF3COOI (30 min, 2 h); (d) polymerization at later stages. VDF/I2/IFAB = 50/0.25/1 (i.e., VDF/CF3I/IFAB = 100/1/1.5). Reprinted with permission from ref 46. Copyright 2013 Wiley.

3. METAL CARBONYL PHOTOMEDIATED SYNTHESIS OF WELL-DEFINED PVDF BLOCK COPOLYMERS 3.1. Particularities of VDF Block Synthesis

DMPI. The I2 ↔ 2CF3I equivalence and the IDT mechanism are supported (Figure 9) by the obtainment of the same Mn dependence and initiator efficiency at constant [VDF]/[I2] ratios, irrespective of IFAB levels (i.e., [VDF]/[I2]/[IFAB] = 250/0.25/{0.5; 0.75; 1} ↔ [VDF]/[“CF3I”]/[IFAB] = 500/1/ {0.5; 1; 1.5}), and by production of different linear Mn profiles, which scale only with [VDF]/[I2], for a constant [VDF]/ [IFAB] ratio (i.e., [VDF]/[I2]/[IFAB] = {50; 100; 250; 500}/ 0.25/1 ↔ [VDF]/[“CF3I”]/[IFAB] = {100; 200; 500; 1000}/ 1/1.5). The final demonstration of the generality of this concept is gratifyingly obtained (Figure 9) from the superposition of VDF-IDT kinetics carried out with IFAB/I2 and with IFAB/ RFI, which satisfy the [VDF]/[“CF3I”]/[IFAB] = [VDF]/ [RFI]/[IFAB, IDAB, DMPI] relationship, where RFI = CF3(CF2)3−I and I(CF2)6I are used as models for the gaseous CF3I. The slightly higher PDI values (∼1.8) of the in situ

As stated in the Introduction, the lack of simple, clean, and reliable chemistry for the synthesis of well-defined, “pure” PVDF block copolymers has vastly impeded the study of such structures and the exploration of their properties, by comparison with the well-understood field of conventional block copolymers. Although many methods for PVDF block copolymer synthesis can be envisaged,374 the specifics of VDF polymerizations render many traditional approaches inefficient. Aside from chain extension from initiator functionalized PVDF (e.g., CCl3−PVDF),375 previous attempts have included VDF initiation from macromolecular RF−I species and free radical initiators76,376 (which unavoidably produce PVDF homopolymer) or incorrect assumptions such as the fact that both PVDF−I377−379 chain ends could be activated toward the initiation of another monomer by either CuX/ATRP377−379 or thermal IDT,375 or that they could both be converted to azides. 380−382 As seen earlier, CuX/L barely activates 2259


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Figure 10. Dependence of PVDF−I chain ends (mole fractions) on conversion: PVDF−CH2−CF2−I (a and d), PVDF−CF2−CH2−I (b and e), and total PVDF−CH2−CF2−I + PVDF−CF2−CH2−I (c and f) chain-end functionality in VDF-IDT initiated by [VDF]/[CF3(CF2)3I]/[Mn2(CO)10] = 25/1/0.05 (red circle), 25/1/0.1 (red square), 25/1/0.25 (red triangle), [VDF]/[I−(CF2)6−I]/[Mn2(CO)10] = 50/1/0.1 (blue circle), 50/1/0.2 (blue square), 50/1/0.4 (blue triangle) (a, b, and c, top) and, respectively, by [VDF]/[I(CF2)6I]/[IFAB; IDAB; DMPI] = 200/1/0.25 (black square), 200/1/0.5 (black triangle), and 200/1/1 (black circle) (d, e, and f, bottom). Adapted from refs 45 and 46. Copyright 2012 American Chemical Society and 2013 Wiley, respectively. 3)2C(COOEt)−Br 8F17−Br perfluoroalkyl halides (k(CH /kCabstr ≈ 102),383 abstr and, even if ignoring the known side reactions of Cu ligands with alkyl iodides,98−102 only minimal initiation would occur from PVDF−CH2−CF2−I and none from the unreactive −CF2−CH2−I. In addition, radical ethyleneation80,384,385 or SN2 azidations386 only proceed selectively from the PVDF− CH2−CF2−I and PVDF−CF2−CH2−I chain ends, respectively, at high temperature (150−200 °C) or under microwave irradiation. However, the most important weakness of previous approaches was their failure to measure, study, and acknowledge the dependence of the iodine chain-end functionality on conversion. Indeed, except for one case,375 this aspect was ignored; no details on the chain ends were given, or the fact that PVDF−CF2−CH2−I/PVDF−CH2−CF2−I mixtures are produced during IDT was recognized and understood. Thus, the study of the PVDF−I chain-end functionality and the optimization of its dependence on the nature of the initiator, catalyst, reagent ratios, etc. toward maximum possible values is of paramount importance for the ability to synthesize PVDF blocks.

VDF-FRP. Similar features are observed in the VDF-IDT carried out with the HVIC/RFI or HVIC/I2 systems.46 Clearly, IDT is not a coordination polymerization and has no mechanism for the 1,2- vs 2,1-regioselectivity control of VDF propagation. In reality, the inverted units still exist, and the HH addition is just apparently prevented, as they are kinetically trapped by chain transfer to RF−I as the unreactive iodide chain ends. The suppression of the internal HH defects, and of the terminal PVDF−CF2−CH2−H and PVDF−CH2−CF2−H units is simply due to the faster chain transfer to RF−I species by comparison to propagation, dimerization, or H abstraction. As such, the HH units observed in VDF-FRP become the terminal, inactive PVDF−CF2−CH2−I in VDF-IDT. Although some internal 2,1-propagation may still occur, due to the much higher reactivity of PVDF−CF2−CH2• vs PVDF− 2• ≫ CH2−CF2• radicals (i.e., kp,21 > kp,12, and CTPVDF−CH RX PVDF−CF 2 • ), the vast majority of PVDF−CF 2 −CH 2 • CT RX propagating chains are intercepted via transfer with RF−I and PVDF−CH2−CF2−I to form PVDF−CF2−CH2−I, which is at least 25 times less reactive45,73 in IDT than the PVDF−CH2− CF2−I chain end. Thus, it will not be reactivated via chain transfer with PVDF−CH2−CF2• to any relevant extent. While some reactivation by Mn(CO)5•, CX3•, or, considerably slower, by IDT with PVDF−CF2−CH2• could still afford an internal HH unit, unless stoichiometric amounts of Mn2(CO)10 are used, the PVDF−CF2−CH2−I chain ends are, for all intents and purposes, kinetically dead, not dormant, as far as propagation is concerned. As these chains are dead and IDT-inert, they can no longer propagate, and they continuously generate a lower molecular

3.2. Dependence of the PVDF−CH2−CF2−I and PVDF−CF2−CH2−I Chain Ends on Conversion and Its Relevance to the Synthesis of PVDF Block Copolymers

A detailed analysis of the features of the NMR spectra of PVDF obtained from various RX initiators in the presence of Mn2(CO)10 (Figure 2), indicates that, in VDF-IDT, the internal HH defects (peak a′, δ = 2.3−2.4 ppm), as well as ∼CF2−H chain ends (peak d, δ = ∼6.3 ppm) derived from chain transfer to solvent, are dramatically decreased by comparison with 2260


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Figure 11. 1H NMR (500 MHz) spectra of PVDF−I, PVDF−H, and various PVDF block copolymers. ◀ = H2O, * = acetone, ⧫ = DMAC. Adapted from refs 43, 45, and 50. Copyright 2015 Elsevier, 2012 American Chemical Society, and 2015 American Chemical Society, respectively.

initiator. Moreover, even at low Mn2(CO)10 levels, the total iodine chain-end functionality (ICEF = PVDF−CH2−CF2−I + PVDF−CF2−CH2−I) is much lower for mono- than for the difunctional I−PVDF−I. Second, for both VDF/I−RF−I/ Mn2(CO)10 and VDF/I−RF−I/HVIC systems, while the ∼CF2−I fraction decreases and the ∼CH2−I fraction increases with conversion, the total ICEF remains at least ∼90% even at high conversions. This value is reasonably high for the synthesis of PVDF block copolymers by the quantitative activation of both these PVDF−I chain ends, as is the case with stoichiometric Mn(CO)5• and other metal carbonyls, discussed later.

weight PVDF population than the propagating/dormant PVDF−CF2−I. Similarly with VAc-IDT,191,387 their buildup leads to PDI broadening. The PDI values are still reasonably low (∼1.5, Figure 3 and Figure 6), but they remain larger than those obtained from typical CRPs of acrylates and styrene.13,17 As stated earlier, Mn(CO)5−I is inert in this polymerization and does not reversibly donate iodine, and although the Mn(CO)5•/Mn(CO)5−I catalysis of the IDT12,14,15,57,388,389 of PVDF−CF2−CH2−I chains (eq 9) would have prevented their current accumulation at the expense of Pn−CH2−CF2−I, this was not observed. These chain ends are unreactive toward most free radical initiators or metal complexes under mild conditions72 and require much stronger halide abstractors than PVDF−CH2−CF2−I.45 As a result, such activators would irreversibly form an even stronger bond with the halide. Consequently, as VDF-IDT progresses, the mole fraction of the PVDF−CH2−CF2−I chain ends decreases, and that of the PVDF−CF2−CH2−I increases, to the extent that, at high conversions, the dead chains are the dominant species (Figure 10).45−48,50,390,391 This effect is general and is observed with both VDF/I−RF−I/Mn2(CO)10 (Figure 10 top, a, b, and c) and VDF/I−RF−I/HVIC (Figure 10, bottom, d, e, and f) systems, as well as in recently reported VDF-RAFT polymerizations.26 There are several remarkable features in these plots. First, as outlined above, difunctional initiators afford an IDT process with significant suppression of termination and transfer. Indeed, a comparison of polymerizations carried out with mono- (CF3(CF2)3I) and difunctional (I−(CF2)6−I) initiators under identical conditions (i.e., same [VDF]/[initiator] and [Mn2(CO)10]/[∼CF2−I] group ratios, respectively) reveals that, upon increasing the Mn2(CO)10 loading, the fraction of the ∼CH2−CF2−I chain ends decreases much more strongly with conversion for PVDF−I initiated from the monofunctional

3.3. Metal-Mediated Quantitative Activation of Both PVDF−CH2−CF2−I and PVDF−CF2−CH2−I Chain Ends for the Synthesis of Well-Defined PVDF Block Copolymers

As the concentration of the inactive PVDF−CF2−CH2−I chain end increases with conversion,45 it was the major component of all high-conversion IDT-derived PVDF−I samples used in prior attempts at block synthesis. In retrospect, it is now obvious that, due to the inability to activate the dominant PVDF−CF2− CH2−I termini, all previous efforts were ineffectual and incomplete and that, on a mole fraction basis, all resulting “blocks” were, at best, poorly defined mixtures of predominantly PVDF−CH2−I with a minor PVDF-block copolymer component, presumably initiated from PVDF−CH2−CF2−I. Therefore, to cleanly synthesize well-defined PVDF block copolymers, a total activation of both PVDF chain ends, especially of ∼CF2−CH2−X, is required. Interestingly, while the mole fraction of the dormant PVDF− CH2−CF2−I chains decreases, and that of dead PVDF−CF2− CH2−I chains increases with conversion,45 their sum, the total iodine chain functionality (I−CEF) (i.e., −CH2−CF2−I + 2261


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Table 1. Characterization of PVDF Block Copolymersa PVDF-I exp

M

Mn

PDI

metal carbonyl

[M]/[PVDFI]/ [Mt carbonyl]

composition [M]/[VDF]

Mn

PDI

1 2 3 4 5 6 7 8 9 10

NSS St BD IP VAc VCl MA MMA AN t BA

2 800 1 500 1 400 2 100 2 100 1 800 2 300 2 100 2 100 2 100

1.29 1.38 1.47 1.28 1.28 1.29 1.52 1.28 1.28 1.28

Mn2(CO)10 Cp2Mo2(CO)6 Mn2(CO)10 Cp2W2(CO)6 Re2(CO)10 Mn2(CO)10 Mn2(CO)10 Mn2(CO)10 Cp2Fe2(CO)4 Mn2(CO)10

55/1/1 100/1/1 200/1/1 200/1/1 100/1/1 100/1/1 75/1/4 100/1/1 175/1/1 50/1/1

80/48 58/42 62/38 62/38 56/44 77/23 72/28 67/33 66/34 64/36

11 500 5 900 4 700 4 300 5 400 20 100 9 000 7 800 5 200 8 400

1.53 1.49 2.00 1.48 1.52 1.52 2.46 1.65 1.85 1.75

All polymerizations in DMAC, except VCl in dioxane and NSS in DMC. Polymerization temperatures are T = 40 °C (VAc, VCl, MA, and AN), 60 °C (tBA), 90 °C (NSS, St, and MMA), 100 °C, (IP), and 110 °C (BD). Data from refs 45, 49, and 50.

a

Figure 12. 19F{1H} heteronuclear correlation spectra of PMMA-b-PVDF-b-PMMA (Table 1, exp 8). Adapted from ref 390.

−CF2−CH2−I), remains, as noted in Figure 10, at least 90%, even at larger levels of Mn2(CO)10.45 This CEF value is reasonable for the synthesis of PVDF block copolymers, if both iodide chain ends can be converted to initiating radicals. Earlier examples of halide initiation in the presence of Mn(CO)5• have included not only active RF−CF2−I species but also the inactive CH3−I, CH3−(CH2)5−I, I−(CH2)10−I, and HCF2−CF2−CH2−I models of the unreactive PVDF− CF2−CH2−I chain end.45 Because both ∼CF2−CH2−I and ∼CH2−CF2−I are more prone to radical activation than regular, unsubstituted alkyl iodides, Mn2(CO)10 should be capable of providing quantitative activation of both PVDF chain ends. Consequently, irrespective of the VDF-IDT polymerization conversion, i.e., of the ratio of the active/inactive iodine chain ends, they can now both initiate another monomer in the

presence of Mn2(CO)10. However, even though PVDF−CH2− CF2−I is a great chain-transfer agent, very similar in reactivity with typical RF−I initiators,72 and would require only catalytic amounts of Mn2(CO)10, the inactive PVDF−CF2−CH2−I demands stoichiometric activation. The PVDF−I ICEF determines the minimum amount of Mn2(CO)10 required, but excess Mn2(CO)10 vs the total iodine functionality should provide complete activation of all chain ends, regardless of the sample chain-end composition, i.e., the PVDF-IDT polymerization conversion. The NMR demonstration of the quantitative activation of both types of PVDF−I chain ends by a series of photoactive transition metal carbonyls45,50 is outlined in spectra (a) and (b) at the top of Figure 11. Here, in the spectrum of PVDF−I, and as detailed in Figure 2, the HT and HH units, i.e., −CF2− 2262


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−124.1 ppm, whereas the first VDF unit is seen as resonance bF, at δ = −92.7 ppm. Likewise, on the vertical H axis, in addition to the PVDF peaks previously described, the l1, l2, and l3 resonances denote the PMMA block. The major cross-peak A (−92.3 × 2.9 ppm) indicates the CH2−CF2 linkage along the PVDF main chain. The connectivity of the initiator with the first VDF unit is evidenced by cross-peak B (b1 × bH, −112.7 ppm × 3.2 ppm). The proof of VDF−MMA block formation from the activation of both PVDF−I chain ends is established first by the complete disappearance of the PVDF−CH2−CF2−I and PVDF−CF2−CH2−I iodine resonances originally observed at δ = −38.6 ppm and δ = −108.33 ppm, respectively,45 and by their conversion into the cross-peaks C1 (aF′ × l2′, −95.7 ppm × 2.1 ppm), C2 (aF″ × aH″, −117 ppm × 2.1 ppm), and C2′(aF‴ × aH‴, −114.7 ppm × 3.0 ppm), indicating the VDF− MMA linkages from the more active and less active PVDF−I chain ends, respectively, i.e., PVDF−CH2−CF2−CH2−C(CH3)(COOCH3)− and PVDF−CF2−CH2−CH2−C(CH3)(COOCH3)−. Finally, because Mn2(CO)10 and other metal carbonyls are used at stoichiometric levels, all iodine from the PVDF−I chain ends is now gradually, but irreversibly, lost as Mn(CO)5I. Thus, the final blocks are devoid of halide chain ends, and the block copolymerization is mostly free radical. However, block length, i.e., copolymer composition and molecular weight, can still be manipulated, by using increasing amounts of the second monomer under otherwise identical block copolymerization conditions. While the block PDIs are broader than for the starting I−PVDF−I, this may still lead to interesting properties, especially in view of the unexpected consequences of the slightly broader PDIs in the self-assembly of triblock copolymers.394 Conversely, even though IDT is not possible here, control of the second polymerization can be envisaged by using other CRP methods, on condition that they are compatible with the Mn2(CO)10 chain-end activation.

[CH2−CF2]n−CH2− (a) and −CF2−CH2−CH2−CF2− (a′) are observed at δ = 2.8−3.1 ppm and δ = 2.3−2.4 ppm,80,392 respectively, with acetone and water seen at δ = 2.05 and 2.84 ppm. The RF−CH2−CF2−(CH2−CF2)n− initiator connectivity with the first VDF unit is represented by resonance b (δ = 3.25 ppm), while the PVDF−CH2−CF2−I (c) and PVDF−CF2− CH2−I (c′) iodine chain ends are observed45,72 at δ = 3.62 ppm and δ = 3.87 ppm, respectively. In addition, trace termination by H transfer to PVDF• (eqs 11 and 12) (i.e., PVDF−CH2− CF2−H and PVDF−CF2−CH3, peaks d and d′) is detected as minor resonances at δ = 6.30 ppm and δ = 1.80 ppm.392 Control experiments indicate no iodine chain-end loss under irradiation at 40 °C for at least 24 h in the absence of Mtx(CO)y. However, the use of stoichiometric or excess amounts of Mn2(CO)10 vs PVDF−I in a good H transfer PVDF solvent such as DMAC results in quantitative PVDF−CF2− CH2• and PVDF−CH2−CF2• radical generation from both PVDF−I chain ends, and their subsequent quenching by H abstraction, to the corresponding PVDF−CF2−CH2−H and PVDF−CH2−CF2−H. This is validated by the disappearance of the iodide c and c′ resonances, and by the corresponding increase in the d and d′ H chain-end resonances, as well as a more pronounced −CH2−CF2−CH2−CF2−H d″, δ = 2.77 ppm.44,392 As this reaction no longer involves the VDF gaseous monomer, it can be safely performed under a variety of conditions, including other solvents, temperatures, and with metal complexes that were not necessarily successful in the radical polymerization of VDF initiated from halides. Out of the transition metal carbonyl series investigated,50 although none were as reactive as Mn2(CO)10 or Re2(CO)10, some interesting trends were also noticed as follows:50 Co2(CO)8 and CpCo(CO)2 and Cp2Ti(CO)2 were inert, and only trace or partial activation of the PVDF−CF2−I chain end was seen for Mo(CO)6, (CO)AuCl, and (PPh3)2Ni(CO)2; although Fe(CO)5, Cp2Fe2(CO)4, Cp*2Cr2(CO)4, and Co4(CO)12 did not afford PVDF from RF−I initiators at catalytic levels, such carbonyls, as well as Cp2Mo2(CO)6 and Cp2W2(CO)6, completely activated both PVDF−I chain ends when used at stoichiometric amounts. Finally, for Fe(CO)5, Cp*2Cr2(CO)4, and Co4(CO)12, an organometallic pathway (metal insertion followed by β-F elimination) also led to terminal PVDF−CF CH2 unsaturations in addition to PVDF−H.50 Carrying out the quantitative radical activation of PVDF−I or I−PVDF−I in the presence of Mn2(CO)10,45,49 Re2(CO)10, Cp2Mo2(CO)6, Cp2W2(CO)6, or Cp2Fe2(CO)4,50 and radically polymerizable alkenes lead to the first examples of well-defined, AB or ABA-type PVDF block copolymers (Figure 11 and Table 1) with neopentyl styrenesulfonate (PNSS, e1−e5), styrene (PSt, f1−f4), butadiene (PBD, g1−g5), isoprene (PIP, h1−h5), vinyl acetate (PVAc, i1−i3), vinyl chloride (PVC, j1, j2), methyl acrylate (PMA, k1−k3), methyl methacrylate (PMMA, l1−l3), acrylonitrile (PAN, m1, m2), and tbutyl acrylate (PtBA, n1− n3).45,49,50 Likewise, Mn2(CO)10-mediated synthesis of PVDF block copolymers with methyl 2-(trifluoromethyl)acrylate, 2,2,2-trifluoroethyl methacrylate,46 and with chlorotrifluoroethylene393 was also recently reported. A further demonstration of block formation is available from the inspection of the 2D 19F{1H} heteronuclear correlation spectra (HETCOR) of PMMA-b-PVDF-b-PMMA block copolymer from Figure 12.390 Here, on the horizontal F-axis, the b1, b2, and b3 −CF2− resonances derived from the I− (CF2)6−I initiator are observed at δ = −112.6, −122.2, and

4. CONCLUSIONS AND OUTLOOK Main chain fluorinated monomers (CFX = CYZ, X, Y, Z = H, Cl, Br, I, OR, etc.) share a series of commonalities that set them apart from common monomers such as acrylates, styrene, and dienes. First, their gaseous nature and low reactivity typically confine their polymerization to high-temperature, high-pressure metal reactors. Second, the limitations of current CRP methods and the particularities of MCFMs render peroxide-initiated IDT the only industrially suitable method for their CRP. However, due to the limited selection of commercial RF−I chain-transfer agents and the unavailability of functional Y−RF−I derivatives (Y = functional group HO, HOOC, NH2, etc.), typical peroxide/RF−I-initiating systems afford at best I−PVDF−I, but more often than not RF−PVDF−I with poor iodine chainend functionality. Thus, the introduction of novel perfluorinated radical initiator precursors, which facilitate IDT with concomitant chain-end functionalization, is highly desirable. Likewise, the ability to perform a metal-mediated, ATRP-like initiation of MCFMs directly from R−X, and especially from functionalized Y−R−X alkyl halides, would be very helpful in the synthesis of, e.g., block, graft, or surface-attached copolymers. Therefore, the use of photochemical reactions, carried out under mild conditions, is a suitable avenue toward both fast and low-cost reaction optimization and elaborating novel chemistry for the synthesis of complex fluorinated architectures. Especially for VDF, the ability to quantitatively activate both PVDF−CH2− 2263


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CF2−I and PVDF−CF2−CH2−I chain ends is of tremendous importance toward the synthesis of well-defined block copolymers initiated from the PVDF chain ends. The research summarized above introduces novel photochemical methods for the initiation and control of MCFM polymerizations and for the synthesis of their block copolymers. Such reactions can be carried out at or around rt, in glass tubes, and using visible light from commercial fluorescent bulbs. These original protocols include the use of hypervalent iodide carboxylates alone or in conjunction with molecular iodine and, respectively, the use of photoactive transition metal carbonyls in the presence of alkyl, fluoroalkyl, and perfluoroalkyl halides. An in-depth study of the reaction parameters highlights the use of dimethyl carbonate as a preferred solvent and outlines the structure−property relationship for the hypervalent iodides and for the halide initiators for both free radical and IDT-CRP of VDF, as well as the structural features of the successful transition metal carbonyl photoactivators. By contrast to their typical use as oxidants, the metal-free, radical photodecarboxylation of HVICs under visible light at ambient temperatures enables their use as convenient synthetic equivalents of the corresponding unstable azo or peroxide analogues. Here, the inexpensive CX3COOH feedstock renders the commercially available IFAB, IDAB, and DMPI as the least expensive and most convenient CX3•/CX3I sources for radical (trifluoro) (iodo)methylations of alkenes. Moreover, the straightforward synthesis of HVICs by the oxidation of PhI in the presence of a wide set of commercially available RCOOH precursors provides a very simple avenue for the photogeneration of libraries of radical R• initiators and iodine CTAs with applications in both organic chemistry and the photomediated synthesis of architecturally complex fluoromaterials. The practicality of HVICs in VDF polymerizations was illustrated by the first examples of their use as CX3•photoinitiators for the metal-free VDF-CRPs under very mild conditions (40 °C, glass tubes), with either external (I(CF2)6I) or in situ generated (CF3I) CTAs, via the first instance of I2− VDF-IDT. This protocol is likewise suitable for the radical trifluoromethylation or perfluoroalkylation of other alkenes and arenes and for the IDT-CRP of other monomers. However, while such methods display polymerization control and reasonable iodine chain-end functionality, a metal-mediated initiation of VDF and other MCFMs directly from alkyl halides is required for the synthesis of complex fluorinated architectures, including block, graft, hyperbranched, or star copolymers. A vast overview of many transition metal complex/alkyl halide combinations and optimization of reaction conditions reveals that several photoactive metal carbonyls, especially Mn2(CO)10 and Re2(CO)10, and to a much lesser extent Cp2Mo(CO)6 and Cp2Cr(CO)6, are particularly suitable in the initiation of VDF polymerization from a wide range of alkyl, semifluorinated, and perfluoroalkyl halides (R−X, X = Cl, Br, and I) that can generate radicals reactive enough to add to VDF at rt, under visible light irradiation in dimethyl carbonate. Perfluoroalkyl iodides, especially difunctional ones (I−RF−I), also afford VDF-IDT-CRP with a linear dependence of molecular weight on conversion, and a dramatic minimization of side reactions, including HH additions and chain transfer. While VDF-IDT proceeds with formation of PVDF−I with high iodine chain-end functionality, the different reactivity of the two isomeric propagating radicals leads to two types of iodine chain ends. However, even though the fraction of the

reactive PVDF−CH2−CF2−I decreases, and that of the unreactive PVDF−CF2−CH2−I increases with monomer conversion, the total iodine chain-end functionality remains higher than 90%. Very reactive photoactive transition metal complexes such as Mn2(CO)10, Re2(CO)10, Cp2Mo(CO)6, Cp2W2(CO)6, and Cp2Fe2(CO)4 are thus required for the quantitative activation of both types of iodine termini toward the synthesis of well-defined PVDF block copolymers, as demonstrated by the synthesis of various AB or ABA type of PVDF blocks with styrenes, acrylates, dienes, etc. The use of such a photochemical metal-mediated approach toward VDF alkyl halide initiation, VDF-IDT-CRP, and the quantitative activation of PVDF−I chain ends enables the reliable and unprecedented synthesis of well-defined, architecturally complex, novel fluoromaterials. Consequently, not only can MCFMs now be easily initiated from alkyl halides to enable grafting from various surfaces, nanoparticles, or other polymer chains, but likewise, other monomers can also be initiated quantitatively from the halide chain ends of such fluoropolymers toward block formation. Similarly, multifunctional halide initiators could be utilized in the preparation of star and hyperbranched fluoropolymers. The metal-mediated photoactivation of alkyl halides, especially of RF−I derivatives, is equally useful in radical trifluoromethylation/perfluoroalkylation reactions, which are of increasing interest in organic/ pharmaceutical chemistry. Nonetheless, synthetic challenges remain. The most important one, as far as VDF is concerned, is the development of a catalyst that would enable IDT for both VDF−I chain ends and prevent the accumulation of PVDF−CF2−CH2−I termini. This would lead to a dramatic decrease in the PDI values and put VDF-IDT on par with the CRP of other monomers. Second, by contrast to the inexpensive and ubiquitous, functionalized ATRP initiators or their precursors (e.g., bromoisobutyryl bromide Br−C(CH3)2−CO−Br), which allow the facile synthesis of complex architectures based on styrene and acrylates, the availability of analogous structures suitable for the IDT of MCFM (e.g., HOOC−CF2−I, HO− CH2−CF2−I, etc.) is very scarce and pricey. Third, the development of other methods for MCFM-CRP is also highly desirable. It is likely that rational structural design will eventually enable VDF-CRPs to occur via the reversible deactivation not only with iodine but also with transition metalloradicals (Mn, Te, Co, Ti, etc.), more labile nitroxides, or novel RAFT reagents under either thermal or photoirradiation conditions. Ideally, such novel CRP chemistry should also enable not only the easy and quantitative activation of the PVDF chain ends toward block synthesis but also the control of the polymerization of the second monomer.

AUTHOR INFORMATION Corresponding Author

*Phone: 860-486-9062. E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biography Prof. Alex Asandei obtained his Ph.D. in 1998 from Case Western Reserve University, Cleveland, OH, working on the stepwise synthesis of liquid crystalline macrocyclics and polymers with complex architectures in the group of Prof. Virgil Percec. Following postdoctoral work in 1999−2001 on the controlled radical polymer2264


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ization of vinyl chloride at the University of Pennsylvania, Philadelphia, PA, he was appointed Assistant Professor (2001) and Associate Professor (2007) in the Department of Chemistry at the University of Connecticut. His main research interest is polymer chemistry, especially controlled radical and ring-opening polymerizations, as well as organometallic chemistry, with a focus on fluorinated polymers.

ACKNOWLEDGMENTS Financial support from the National Science Foundation, Grants NSF-CHE-1309769, NSF-CHE-1058980, and NSFCHE-1508419 is gratefully acknowledged. ABBREVIATIONS ATRP atom transfer radical polymerization CEF chain-end functionality CRP controlled radical polymerization CTA chain-transfer agent DMPI Dess−Martin cyclic periodinane HVIC hypervalent iodide carboxylate IDAB ((diacetoxy)iodo)benzene IDT iodine degenerative transfer IFAB (bis(trifluoroacetoxy)iodo)benzene MCFM main chain fluorinated monomer RAFT reversible addition−fragmentation transfer rt room temperature VDF vinylidene fluoride REFERENCES (1) Bruno, A. Controlled Radical (Co)polymerization of Fluoromonomers. Macromolecules 2010, 43, 10163−10184. (2) Ameduri, B. From Vinylidene Fluoride (VDF) to the Applications of VDF-Containing Polymers and Copolymers: Recent Developments and Future Trends. Chem. Rev. 2009, 109, 6632−6686. (3) Sun, F. C.; Dongare, A. M.; Asandei, A. D.; Alpay, S. P.; Nakhmanson, S. Temperature Dependent Structural, Elastic, and Polar Properties of Ferroelectric Poly(vinylidene Fluoride) (PVDF) and Trifluoroethylene (TRFE) Copolymers. J. Mater. Chem. C 2015, 3, 8389−8396. (4) Ameduri, B.; Boutevin, B. Well Architectured Fluoropolymers: Synthesis, Properties and Applications; Elsevier: Amsterdam, 2004; pp 1−99. (5) Global Fluoropolymer Market Report 2013−2018: PTFE, PVDF, FEP, Fluoroelastomers in Fluoropolymer Market by Types, by ApplicationsGlobal Trends and Forecasts to 2019, http://www. researchandmarkets.com/reports/3144243/fluoropolymer-market-bytypes-by-applications. (6) Chemical Economics Handbook Fluoropolymers; IHS Inc.: Englewood, CO, 2012; https://www.ihs.com/products/ fluoropolymers-chemical-economics-handbook.html. (7) Ameduri, B. Recent Advances in the Controlled Radical (Co) Polymerization of Fluoroalkenes and Applications Therefrom. J. Taiwan Inst. Chem. Eng. 2014, 45, 3124−3133. (8) Dohany, J. E. Poly(Vinylidene Fluoride). In Kirk−Othmer Encyclopedia of Chemical Technology, 4th ed.; Kirk, R. E., Othmer, D., Eds.; John Wiley and Sons: New York, 1998. (9) Ferrero, F.; Zeps, R.; Kluge, M.; Schröder, V.; Spoormaker, T. The Explosive Decomposition of Tetrafluoroethylene: Large Scale Tests and Simulations. Chem. Eng. Trans. 2013, 31, 817−822. (10) Humphrey, J. S.; Amin-Sanayei, R. Vinylidene Fluoride Polymers. In Encyclopedia of Polymer Science and Technology, 3rd ed.; Mark, H. F., Ed.; Wiley: New York, 2004; Vol. 4, pp 510−533. (11) di Lena, F.; Matyjaszewski, K. Transition Metal Catalysts for Controlled Radical Polymerization. Prog. Polym. Sci. 2010, 35, 959− 1021. (12) Handbook of Radical Polymerization; Matyjaszewski, K., Davis, T. P., Eds.; Wiley: New York, 2002. 2265


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