Metal and Ligand Effects of Photoactive Transition Metal Carbonyls in

Article pubs.acs.org/Macromolecules

Metal and Ligand Effects of Photoactive Transition Metal Carbonyls in the Iodine Degenerative Transfer Controlled Radical Polymerization and Block Copolymerization of Vinylidene Fluoride Christopher P. Simpson, Olumide I. Adebolu, Joon-Sung Kim, Vignesh Vasu, and Alexandru D. Asandei* University of Connecticut Institute of Materials Science and Department of Chemistry, 97 North Eagleville Road, Storrs, Connecticut 06069-3136, United States Macromolecules Downloaded from pubs.acs.org by CENTRAL MICHIGAN UNIV on 09/10/15. For personal use only.

S Supporting Information *

ABSTRACT: The metal and ligand effect of a series of transition metal carbonyls in conjunction with alkyl and perfluoroalkyl halides was investigated in the initiation and control of the visible light, radical photopolymerizations of vinylidene fluoride (VDF) and respectively, in the synthesis of PVDF block copolymers. No polymerization was observed for CpMn(CO)3, CpCo(CO)2, Cp2Fe2(CO)4, Cp*2Cr2(CO)4, Mo(CO)6, Fe(CO)5, Cr(CO)6, Co2(CO)8, Co4(CO)12, Fe3(CO)12, Ru3(CO)12, (PPh3)2Ni(CO)2, Cp2Ti(CO)2, and Au(CO)Cl. A free radical polymerization, and respectively an iodine degenerative transfer, controlled radical polymerization was obtained for Mn2(CO)10 ∼ Re2(CO)10 ≫ Cp2Mo2(CO)6 ≫ Cp2W2(CO)6 with CH3(CH2)5−Br, CH3(CH2)5−I, CH3−I, CCl3−Cl, CCl3− Br, Br−(CF2)6−Br, and respectively with CF3(CF2)3−I and I−(CF2)4,6−I. Furthermore, while Fe(CO)5, Cp*Cr2(CO)4 and Co4(CO)12 led to ∼ CF2−I bond insertion, Re2(CO)10, Mn2(CO)10, Cp2W2(CO)6, Cp2Mo2(CO)6 and Cp2Fe2(CO)4 provided quantitative radical activation of both PVDF−CH2−CF2−I and PVDF−CF2−CH2−I chain ends, and were employed in the synthesis of well-defined ABA triblock PVDF copolymers with vinyl acetate, tert-butyl acrylate, methyl methacrylate, isoprene, styrene, and acrylonitrile.

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applicability in the CRP of side-chain fluorinated monomers (e.g., fluoroalkyl (meth)acrylates), pentafluoro styrene), remains in its early stages.2 Moreover, with the exception of some initial promising RAFT results,8−10 their suitability for the CRP of MCFMs remains unproven. Accordingly, due to the lack of convenient chemistry for MCFM−CRP and for the synthesis of the corresponding welldefined, complex architectures (blocks, graft, hyperbranched, stars, etc.), the study and understanding of their self-assembly and of the properties and applications thereby derived, lags significantly behind conventional monomers (styrene, acrylates, dienes etc.). As such, development of novel CRP procedures and methods for the synthesis of intricate fluoropolymer motifs remains a commendable, but challenging enterprise.1,11,12 In addition, unlike the propagation of most monomers except vinyl acetate, regioselectivity defects in VDF free radical polymerization (FRP) lead to about one in ten to one in 20 VDF head-to-head (HH: −CH2−CF2−CF2−CH2−CH2− CF2−) inverted units, instead of the desired heat-to-tail (HT: −CH2−CF2−CH2−CF2−CH2−CF2−) sequence. These propagation errors subsequently generate defects in the crystal packing, and improper arrangement of the units in the β-phase, required for electroactive properties.

INTRODUCTION Fluoropolymers derived from the radical (co)polymerization of main chain fluorinated monomers (MCFMs) such as tetrafluoroethylene, hexafluoropropene, trifluorochloroethylene, vinylidene fluoride (VDF, CH2CF2), etc. are a fundamental class of specialty materials endowed with wide morphological versatility, high chemical, thermal, weather and aging resistance, low flammability, surface energy, refractive index, and moisture absorption, as well as special electrical responses, including piezo- and ferroelectricity (e.g., for poly(vinylidene fluoride), PVDF).1 Such properties afford applications that range from paints and coatings, pipe linears, transmission fluids, O-rings for extreme temperatures, fuel cell membranes, Li ion batteries, or antifouling layers, to optical fibers and high power capacitors, transducers, actuators, sensors, etc.2 Thus, their precise synthesis is quite relevant. On the downside, all such monomers propagate with very reactive radicals, and are gases under normal conditions (e.g., bVDF = −83 °C). These drawbacks generate additional levels of p difficulty vs conventional monomers such as styrene or acrylates, as their polymerization typically requires high temperatures, and thus, high-pressure metal reactors.2 As such, although over the last 2 decades, remarkable progress has been realized in controlled radical polymerizations (CRPs),3−7 and atom transfer (ATRP), reversible addition− fragmentation (RAFT) and nitroxide based protocols have been reliably demonstrated for styrene and acrylates, their © XXXX American Chemical Society

Received: April 3, 2015 Revised: August 25, 2015

A

DOI: 10.1021/acs.macromol.5b00698 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Moreover, irrespective of the Y radical “protecting group” of any CRP process, such dual propagation inevitably affords PVDF containing both −CF2−CH2−Y and −CH2−CF2−Y termini. As the C−Y bond dissociation energy (BDE) in −CF2−CH2−Y is much larger than for −CH2−CF2−Y, these chain ends are de facto dead, not dormant, vs reversible activation. Consequently, they will accumulate with increasing conversion, and lead to PDI broadening, and eventual loss of control. Indeed, to date, there is no evidence of reversible activation of the “bad” PVDF−CF2−CH2−Y chain ends. In fact, even for the weaker −CH2−CF2−Y, the corresponding BDE for Y = Cl, Br13 and likely for most RAFT or nitroxide derivatives,14 are also too high for such polymerizations to proceed with an appreciable rate at mild (25−40 °C) temperatures and low monomer pressures. Consequently, while progress is being made in RAFT methods,8−10 the only approach to FM−CRP2 remains the iodine degenerative transfer4,15−20 (IDT: Pn• + Pm−I ⇌ Pn−I + Pm•),2,21,25 which evolved from earlier research on the high temperature (100−230 °C) VDF free radical telomerization22,23 with polyhalides,24 especially (per)fluorinated RF−I iodine chain transfer agents (CTAs).25 In fact, IDT is one of the oldest CRP methods, as a linear dependence of Mn on polymer yield was already shown16 in the early 80s. Likewise, taking advantage of the commercial availability of iodine CTAs25,26 and its tolerance of emulsion polymerization, IDT was the first industrially implemented CRP.16 Subsequent modeling27 and kinetic28 studies have also illustrated the effect of the structure and reactivity of the CTA (I > Br > Cl ∼ H, difunctional better than monofunctional),16,25 of the side reactions (transfer to polymer, solvent etc.) and of monomer addition mode (1,2- vs 2,1-) to the quality of the polymerization control. As a typical degenerative radical process, in addition to RF−I, VDF−IDT also requires a free radical initiator source (e.g., t butyl peroxide etc.). However, while most chains should be derived from RF−I, higher initiator levels required for higher rate inevitably lead to a decreased fraction of RF-functionalized chains. Moreover, free radical initiation is also not appropriate for the synthesis of well-defined blocks, as PVDF−CF2−CH2−I is hardly activated, and would inevitably lead to mixtures of homo- and copolymers. This is a serious disadvantage with respect to the precise synthesis of block copolymers. Thus, availability of initiation directly from RF−I and other halides, as well as quantitative activation of both −CF2−CH2−I and −CH2−CF2−I chain ends of PVDF−I, most likely mediated by transition metal catalysis, is highly desirable. Conversely, even though VDF propagates (kp) fast enough29 that polymer can be obtained even at ambient temperature,30 only very low telomers (DP = 1−3) can be obtained from redox reactions of transition metal complexes and polyhalides, even at high temperatures (T > 100 °C).2,31,32 Furthermore, while the radical perfluoroalkyl iodination of regular alkenes occurs readily with RF−I derivatives and a variety of catalysts (Cu,33 Zn,34 Pd,35 Sn,36 Cp2TiCl,37 etc.), the corresponding metal catalyzed addition of electrophilic RF• radicals to electrophilic fluorinated alkenes (MCFMs) at T < 100 °C, and especially at room temperature has never been reported. Moreover, by contrast to styrene or acrylate CRPs which can be easily sampled from Schlenk tubes on a 1 g scale, since all MCFM are gases, kinetic investigation of e.g. VDF polymerizations require many one-data-point experiments, and thus a large number of costly metal reactors, and hundreds of grams of

monomer. This is time-consuming and expensive. Therefore, development of chemistry that enables the polymerization of MCFM to proceed at room temperature and thus at a lower pressure suitable for inexpensive glass tubes, is highly desirable, as it would also enable fast, small-scale screening of multiple catalysts and conditions, as well as photocatalysis. Likewise, the ability to perform these reactions under mild conditions, would be of great synthetic use not only in MCFM−CRP and in the synthesis of their complex architectures, but also in trifluoromethylations and perfluoroalkylation reactions that are in increasing demand in organic/medicinal chemistry, as well as for postpolymerization fluoro-functionalization.38 However, to the best of our knowledge, prior to our recent work,39−44 there were no reports on metal-mediated MCFM or VDF polymerizations, let alone VDF−CRP. We are presenting herein a detailed survey and optimization of a series of transition metal carbonyl/(fluoro)alkyl halide pairs as novel photoinitiating systems for the free radical and IDT−CRP of VDF, as well as for the synthesis of well-defined PVDF block copolymers by quantitative activation of both PVDF−CF2−I and PVDF−CH2−I.

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EXPERIMENTAL SECTION

Materials. Manganese carbonyl (Mn2(CO)10), rhenium carbonyl (Re2(CO)10), cyclopentadienyl molybdenum tricarbonyl dimer (Cp2Mo2(CO)6), iron pentacarbonyl (Fe(CO)5), cyclopentadienyl iron dicarbonyl dimer (Cp2Fe2(CO)4), pentamethylcyclopentadienyl chromium dicarbonyl dimer (Cp*2Cr2(CO)4), molybdenum hexacarbonyl (Mo(CO)6), chromium hexacarbonyl (Cr(CO)6), cyclopentadienylcobalt dicarbonyl (CpCo(CO)2), chlorocarbonylgold(I) (Au(CO)Cl), tetracobalt dodecacarbonyl (Co 4 (CO) 12 ), bis(triphenylphosphine)nickel dicarbonyl ((Ph3P)2Ni(CO)2), cyclopentadienylmanganese tricarbonyl (CpMn(CO)3), triiron dodecacarbonyl (Fe3 (CO)12), triruthenium dodecacarbonyl (Ru 3 (CO)12), bis(cyclopentadienyl)dicarbonyl titanium(II), (Cp2Ti(CO)2) (all from Strem Chemicals, ≥ 95%). 1,8-dichloroperfluorooctane (DCPFO, Cl(CF 2 ) 8 Cl), 1,6-dibromododecafluoro hexane (DBPFH, Br(CF2)6Br), vinylidene fluoride (VDF,99.9%), 1-iodononafluorobutane (perfluorobutyl iodide, PFBI, 98%), 1,6-diiodododecafluorohexane (DIPFH, I(CF2)6I 98%), (all from Synquest Laboratories), 4methoxybenzenesulfonyl chloride (MBSC, 99%), N,N′-dimethylacetamide, (DMAc, 99%), vinyl acetate (VAc, 99+%), acrylonitrile (AN, 99+%), styrene (Sty, 99%), methyl acrylate (MA, 99%), cyclopentadienyl tungsten tricarbonyl dimer (Cp2W2(CO)6, 97%) (all from Acros Organics); iodomethane (CH3I, ReagentPlus, 99.5%), bromotrichloromethane (BrCCl3, 99%), 1-iodohexane (CH3(CH2)5I, 98+), acetonitrile (ACN, 99%), dimethyl carbonate (DMC, ≥ 99% anhydrous), methanol (MeOH, 99%), 2-Methyl-1,3-butadiene (isoprene, Iso, ≥ 99%) cobalt carbonyl (Co2(CO)8, > 90%), 1chlorohexane (CH3(CH2)5Cl, 99%) 1-bromohexane (CH3(CH2)5Br, 98%) (all from Sigma-Aldrich); carbon tetrachloride (CCl4), (all from Fisher Scientific); and DMSO-d6, acetone-d6 (Cambridge Isotope Laboratories, Inc., D, 99.9%). All were used as received. Techniques. 1H NMR (500 MHz) and 19F NMR (400 MHz) spectra were recorded on a Bruker DRX-500 and respectively on a Bruker DRX-400 at 24 °C in acetone-d6 typically between 32 and 128 scans. 2D-19F{1H}-heteronuclear correlation (HETCOR) spectra were recorded on a Bruker DRX-400 at 24 °C with a scan set of 16 × 256 using typical Bruker pulse sequences for HETCOR. GPC analyses were performed on a Waters gel permeation chromatograph equipped with a Waters 2414 differential refractometer and a Jordi 2 mixed bed columns setup at 80 °C. DMAc (Fisher, 99.9% HPLC grade) was used as eluent at a flow rate of 1 mL/min. Number-average (Mn) and weight-average molecular weights (Mw) were determined from calibration plots constructed with poly(methyl methacrylate) standards. All reported polydispersities are those of water precipitated samples. While narrower PDIs could be obtained by MeOH B


Article


Macromolecules

−7 −7 St MMA KMA ATRP/KATRP/KATRP = ∼ 2 × 10 /9 × 10 /1/6/30. Likewise, the relative rates of the ATRP of MA and VDF can be MA × KPVDF × estimated13,48 as rateVDF/rateMA = (kVDF p ATRP)/(kp −7 MA KATRP) = ∼ 8.75 × 10 . In other words, the conversion obtained in one second of polymerization time for MA would require ∼13 days for VDF (Br) and ∼317 days (Cl). Clearly, this is unfeasible. RF−Br Likewise, while kRF−I > 1, quantitative iodine-ATRP act /kact data are not available for RF derivatives, and there are no reports on the ambient temperature activation of R−CF2− CF2−I-type derivatives by CuX. In addition, typical ATRP activating amine or phosphine ligands5 are impractical with CuX, as they alkylate RF−I derivatives to form ammonium iodide salts or charge transfer complexes.49 Thus, only modest activation is expected for the “good” PVDF−CH2−CF2−I chain end, and insignificant activation of the “bad” PVDF− CF2−CH2−I chain end. Consequently, this also explains the failure of Cu-ATRP to provide well-defined PVDF block copolymers. Similarly, CRPs based on reversible C−Mt bond formation with a transition metal complex based on Co, Ti,50 Mo, Te, etc.,351 which are successful for styrene,50 dienes50 or vinyl acetate, could be impeded by β-H and especially β-F eliminations.39 Moreover, while we have shown that the Cp2Ti{μ-Cl}2TiCp2 dimer is an great mediator for the CRPs of styrene,52 dienes,50 and even for the living ring-opening polymerization of cyclic esters53 initiated by epoxides or aldehydes, a solvent compatible with both Cp2TiCl• and PVDF could not be found.39 As such, since none of the typical CRP methods or R−CF2−I activators tested proved effective, we decided to examine photochemical means of radical generation and trapping.54,55 Indeed, high power UV induced VDF telomerizations with halides are known for over 50 years,24,56,57 and we have also shown39,58−61 that VDF−FRP occurs under UV irradiation at room temperature in the presence of TBPO. However, prior to our recent work,39−42 there were no reports on visible lightmediated VDF polymerization. Thus, while targeting a relatively low pressure in the glass tubes by carrying the polymerization at mild temperatures, and bearing in mind the instability of most organometallics under hard UV irradiation, we decided to concentrate on photopolymerizations initiated from alkyl, semifluorinated and perfluoroalkyl halides by photoactive transition metal complexes, using low wattage ( CpMo(CO)3• > CpFe(CO)2• > Co(CO)4• qualitative order (reminiscent kact in ATRP) is available in terms of their halide abstraction ability in flash photolysis.89−91 Both photo and thermal activations of the metal carbonyl proceed by either homolysis of a metal−metal bond, or by loss of a carbonyl, to eventually afford halidophilic metalloradicals. For monomeric carbonyls such as Mo(CO)6, Fe(CO)5, Cr(CO)6, CpMn(CO)3, CpRe(CO)3,92 (PPh3)Ni(CO)2,92 CpCo(CO)2, etc., UV irradiation typically leads to CO expulsion and possible rearrangement into dimers or higher species which may later photolyze to metalloradicals.93−101 Alternatively, even weak visible light photolysis may lead to ligand loss and disproportionation, as is the case for Au(CO)Cl.102 Interestingly, while Co2(CO)8103a,b was considered an inhibitor, Mo(CO6), Cr(CO)6, Mn2(CO)10,103 and Fe(CO)5104,105 were shown to mediate the radical addition of CCl4 and CCl3Br to alkenes and the subsequent polymerization

of MMA, St, and AN via either UV photolysis, or thermolysis above 80 °C. Very early reports even proposed photo or thermal Mn2(CO)10 or Re2(CO)10 mediated FRPs initiated via either H abstraction from the monomer (MMA, AN),106 addition of the metalloradical to tetrafluoroethylene to initiate its homopolymerization107,108 or MMA109,110 block copolymerization,111 as well as the addition to 1,2-disubstituted acetyls and alkenes.112 Additionally, Mn2(CO)10, Co2(CO)8, Fe(CO)5, Mo(CO)6, Cr(CO)6, and Re2(CO)10 were also tested as catalysts for the radical carbonylation of alkyl iodides under high power UV irradiation, 113 and excess Fe 2 (CO) 9 , Fe3(CO)12, Ru3(CO)12, and Co2(CO)8 were claimed to activate perfluoroalkyl halides at high temperatures and in the presence of additives.114 Nonetheless, while Re(CO)5• is a Re(CO)

•

Mn(CO)

•

better activator (kact,CCl4 5 /kact,CCl4 5 ∼ 65),115 the stronger Re− Re bond116 and its higher price, render the relatively inexpensive Mn2(CO)10117,118 the preferred reagent in the series.55 Yet, to the best of our knowledge, aside from our40 and other work87,119 with Mn2(CO)10, there is no precedent for the radical activation of CH 3 (CH 2 ) 5 Cl, CH 3 (CH 2 ) 5 Br, CH3(CH2)5I, CH3I, CCl4, CCl3Br, CF3(CF2)3I Cl(CF2)8Cl, Br(CF2)6Br, and I(CF2)6I in conjunction with Re2(CO)10, Cp 2 W 2 (CO) 6 , Cp 2 Mo 2 (CO) 6 , Fe(CO) 5 , Cp 2 Fe 2 (CO) 4 , Cp*2Cr2(CO)4, Co2(CO)8, Mo(CO)6, Cr(CO)6, or Au(CO) Cl under low power (≤30 W) visible light irradiation. We thus decided to perform a quantitative evaluation of the scope and limitations of these complexes in the initiation and mediation of VDF−FRP and VDF−IDT−CRP form R−Cl, R−Br and respectively RF−I substrates, and in the synthesis of welldefined PVDF block copolymers following quantitative activation of both PVDF−CH2−I and PVDF−CF2−I iodine chain ends. These investigations are described below. Polymerization Mechanism and Overview of the Trends Previously Observed with Mn2CO10. In terms of reaction media, while typical VDF “solution” polymerizations are performed in AN,23 (a weak CTA nonsolvent), little is known120−122 on the solvent effect in VDF polymerizations, and there is no data for photopolymerizations. However, investigations of PVDF solution properties123−128 have suggested that the best solvents are polar aprotic and capable of H-bonding123 between the polar YO solvent group (Y C, S, P, e.g., DMAc), with the weakly acidic Hs of −CF2−CH2− CF2−. Accordingly, noting that minimization of solvent side reactions (chain transfer, or reactions with the catalyst) outweigh monomer/polymer solubility considerations, we previously scanned40 > 40 polymerization media, and highlighted the superiority of alkyl carbonates, especially dimethyl carbonate (DMC), a green solvent.129 Indeed, even though DMC does not dissolve PVDF until ∼80 °C, and like ACN, affords a heterogeneous polymerization,130 it provides by far the fastest reaction rates, at least thrice those obtained in ACN, and most essentially, minimum chain transfer. Thus, the best solvents (alkyl carbonates) encompass the combined outcome of very low transfer to PVDF•, and of highest VDF solubilization and PVDF swelling, enabling better monomer diffusion to the propagating center. These results are consistent with PVDF swelling by carbonate solvent electrolytes for Li-ion batteries131 containing PVDF microporous membranes.132 Although alkyl halides, especially iodides may photodissociate, control experiments revealed no polymerization in the dark, or under irradiation in the absence of Mn2(CO)10.40 D


Article

Macromolecules Polymerizations were tested at T = 0−100 °C, but T = 40 °C was eventually selected for all experiments, as a reasonable compromise between rate and a safe pressure inside the tube. In fact, positioning the bottom part of the fluorescent light bulb inside the oil bath helped maintain such temperature, with minimal assistance from the hot plate.40 The general polymerization mechanism is described in Scheme 1 and exemplified for Mn2(CO)10. The reversible

I, CF3CF2−I, (CF3)2CF−I, (CF3)3C−I, CF3(CF2)3−I and I− (CF2)4,6−I which all afford polymer, not only for VDF, but also for CF2CFCl, CF2CCl2, CF2CFBr, CH2CFH as well as VDF random copolymers with CF2CF(CF3) and CF2 CF(OCF3).40 It is interesting to note that initiation occurs not only from polyhalides and all RF−I structures (which also enable IDT and elimination of HH defects), but even from semifluorinated H− CF2−CF2−CH2−I models of the “bad” PVDF chain end, and even from inactivated alkyl iodides such as methyl or hexyl iodide. These results suggest that Mn2(CO)10-mediated VDF block or graft copolymerization initiated directly from all such halides anchored on polymeric chains or surfaces is also feasible. Following initiation with catalytic amounts of Mn2(CO)10,40 if the value of initiator chain transfer (CT) constant to PVDF•, to RX VDF (CTPVDF• = kPVDF• /kpropagation), is large enough, excess RX RX transfer may act as a chain transfer agent (CTA) toward the propagating chains (eqs 5 and 6). Unfortunately, very few CTPVDF• values are known (CCl3Br = 35, CCl4 = 0.25, CHCl3 RX = 0.06 at 141 °C,134 C6F13−I = 0.8, C6F13−Br = 0.08, C6F13−H = 0.0002 in scCO2 at 120 °C135). For IDT−CRP,25 high CT values (CTPVDF• > 1) and X = I (i.e., RF−I derivatives) are RX required, and such initiators lead to macromolecular PVDF− CH2−CF2−I and PVDF−CF2−CH2−I40 CTAs.25,28,40 However, while typical perfluoroalkyl iodides and PVDF−CH2− CF2−I have similarly large CT values (C6F13−I = 7.9, C6F13− CH2−CF2−I = 7.4),28 the “bad” PVDF−CF2−CH2−I chain end25 is about 25 times less reactive (CT of HCF2−CF2− CH2−I = 0.3 at 75 °C). Likewise, PVDF−CF2−CH2• is correspondingly much more reactive than the isomeric PVDF− CH2−CF2•.28 Thus, following initiation, the polymerization type (FRP or CRP) is determined by the combined effect of the CTPVDF• RX values (i.e., the C−X BDE of RX),23,28 (eqs 5 and 6), the reactivity of fluorinated radicals (more branched, more electrophilic),136 and by the preferential activation of primary vs secondary or tertiary halides82 with Mn(CO)5•. Consequently, depending of the amount of Mn2(CO)10 required for activation (i.e., the CT values) and the nature of the halide, three initiators classes can be identified, where VDF undergoes FRP or telomerization for R-X (X = Cl, Br, I) and IDT−CRP only for RF−I. These distinctions are easily identifiable from the features of corresponding PVDF NMRs, especially the halide chain ends and HH/HT units.40 Accordingly, halides with strong R−X bonds (Ralk−I, CHCl3, and RF−Cl, i.e., Cl−CFCl−CF2−Cl, Cl−(CF2−CFCl)3−6−Cl, CHCl 3 , CF 3 (CF 2 ) 2 −CO−Cl, Cl−CF 2 −(CF 2 ) 6 −CF 2 −Cl, CH3−I, CH3(CH2)4−CH2−I and I−CH2−(CH2)8−CH2−I) afford only FRP initiation. They do not undergo noticeable CT with PVDF•, require stoichiometric Mn2(CO)10 for activation, and afford PVDF with no halide chain ends, and with visible HH defects. The lack of polymer halide termini stems from the


Scheme 1. Mechanism of the Mtx(CO)y-Photomediated VDF−IDT

photodissociation of the metal carbonyl (eq 1), is followed by irreversible87,40 halide abstraction from R−X (and later from PVDF−X), which is driven by the formation of high BDE Mt(CO)n−X,70,71,133 Mt = Mn, Re, Mo, etc., X = Cl, Br, I (eq 2). The resulting R•, if reactive enough, adds to VDF, thus initiating polymerization (eq 3). Because of regioselectivity propagation defects, both 1,2- and 2,1-units, (eq 4, HT, ∼ 95%40,121 and respectively, HH) occur in FRP (eq 4). Mt(CO)n• is slowly but continuously photogenerated throughout the polymerization, and, by activating R−X and later PVDF−X (especially PVDF−CH2−CF2−I), compensates for termination reactions and maintains a steady state radical concentration. In order to select the appropriate initiators for VDF−FRP and respectively VDF−IDT, we evaluated40 over 40 halide structures never previously reported with Mn2(CO)10. As such, no initiation was observed40 from I2, tBu−I, CH3−SO2Cl, CH3O−Ph−SO2Cl, CH2Cl2, CH2I2, CHCl2−CHCl2, CHBr3, CHI3, CBr4, 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, (CH3)2C(COOEt)−Br/I, I−Ph−O−CH3, and NBS under a wide variety of conditions. Since Mn(CO)5• has a very high halide affinity,55 abstraction is available in all cases. Thus, the lack of initiation stems from the corresponding radicals being more stable than the propagating PVDF•, and thus failing to add to the monomer at moderate temperatures. By contrast, very reactive alkyl, polyhalide, as well as semi- and perfluorinated radicals are obtained from CHCl3, CCl4, CCl3− CCl3, CF3(CF2)2CO−Cl, CF3−SO2−Cl, Cl−CF2−CClF−Cl, Cl−(CF2)8−Cl, -(CF2−CFCl)n-, CCl3−Br, EtOOC−CF2−Br, Br−(CH2)10−Br, Br−CF2−CH2−CF2−Br, Br−(CF2)4−Br, CH3−I, CH3(CH2)5−I, I−(CH2)10−I, C6F5−CF2−I, H− CF2−CF2−CH2−I, EtOOC−CF2−I, Cl−CF2−CFCl−I, CF3−

•

very low initiator CT value (CTPVDF ≪ 1) and from the RX stoichiometric amount of Mn2(CO)10 required, leading to VDF−FRP. Second, substrates with weaker R−X bonds (CF3SO2−Cl, R−CCl3, RF−X, X = Br, I; i.e., CF3−SO2−Cl, CCl4, CCl3Br, CCl3−CCl3, Br−CF2−CH2−CF2−Br, Br−CF2−CF2−CF2− CF2−Br, EtOOC−CF2−Br), do undergo CT (eqs 5 and 6), require reduced (10−20%) amounts of Mn2(CO)10, and thus, provide at least one or both (−CH2−CF2−X and −CF2−CH2− E


Article

Macromolecules Table 1. Effect of Initiator and Cp2W2(CO)6 on VDF Photopolymerizationa


a

ex.

initiator

catalyst

[VDF]/[I]/[Cp2W2(CO)6]

Mn

PDI

time (h)

convn (%)

kpapp (h‑1)

init. effic.

1 2 3 4 5 6 7 8 9

Cl−(CF2)8−Cl CCl3Br CCl3Br CCl4 Br−(CF2)6−Br CF3(CF2)3−I CF3(CF2)3−Ib CF3(CF2)3−I I−(CF2)6−I

Cp2W2(CO)6 Cp2W2(CO)6 Cp2W2(CO)6 Cp2W2(CO)6 Cp2W2(CO)6 Cp2W2(CO)6 Cp2W2(CO)6 Cp2W2(CO)6 Cp2W2(CO)6

50/1/0.4 25/1/0.2 25/1/0.2 25/1/0.2 50/1/0.4 25/1/0.2 25/1/0.1 50/1/0.4 50/1/0.4

− − − − 4700 − 700 1750 1700

− − − − 3.37 − 1.35 1.98 1.28

38.5 38.5 66.5 38.5 38.5 19.5 16.5 40.0 64.3

0 0 0 0 25 0 8 25 25

0.0000 0.0000 0.0000 0.0000 0.0075 0.0000 0.0051 0.0072 0.0045

− − − − 0.27 − 0.68 0.65 0.79

All experiments in DMC except. bIn acetone, at 40 °C under visible light irradiation.

CRP. This was demonstrated40 by the linear dependence of Mn on conversion over a wide range of [VDF]/[RFI] ratios and by reasonably low PDI < 1.5 values. While the above work outlined the first example of metal and visible light mediated VDF−CRP, mechanistically, with the exception of the PVDF−I chain end reactivation by Mn(CO)5• to compensate for termination, the process remains a conventional IDT. Indeed, while IDT catalysis would decrease PDI,4,15 control experiments40 show that, similar to the IDT of VAc,87 Mn(CO)5−I is photochemically inactive137a and unable to donate iodine. Conversely, Mn−alkyls photolyze easily,118 and would not be effective “protecting groups” for the propagating radical, and likewise, Mn(CO)5• did not initiate VDF.40 Thus, although RF−Mn(CO)5 perfluoroalkyl manganese derivatives (RF = −CH2F, −CF2H)137 are available, potential organometallic CRP mediation via reversible C−Mn bond homolysis in PVDF−Mn(CO)5 was discounted based on the observed iodine not −H or −Mn(CO)5 chain ends, the successful CRP with only catalytic Mn2(CO)10 vs RF−I, and in view of the relative order of the BDEs (kcal/mol) of RF−Mn(CO)5 (34)27 < (CO)5Mn−Mn(CO)5 (38) < RF−I (48)138 < I−Mn(CO)5 (54).40 Finally, difunctional I−RF−I initiators are most suitable for VDF−IDT. Thus, even if one of two resulting polymeric iodine chain ends is consumed in side reactions, the other one can still be reactivated by the continuously photogenerated Mn(CO)5•139b (eq 2), to compensate for termination, maintain a steady state radical concentration40 and enable a continuous increase in molecular weight.40 Indeed, as radical dimerization provides I−(PVDF)2−I, while H transfer affords I−PVDF−H, termination is significantly decreased by comparison with polymerizations initiated from monofunctional RF−I. As a result, the remaining iodines can be reactivated by Mn(CO)5• to continue to propagate and even undergo additional dimerizations. Thus, the lifetime of the dormant/growing chains is greatly extended while amount of −CH2−H or −CF2−H chain ends is greatly reduced. This simple initiator selection significantly improves the quality of the polymerization, lowers the PDI values and helps maintain a reasonably high iodine CEF. Metal, Ligand, and Initiator Effect in Photomediated VDF Polymerization with Metal Carbonyls. While their known reactivity trends are based on high power laser flash photolysis,90,93 little is actually known about the radical reactions of Mtx(CO)y under low power (≤30 W) visible light irradiation. We thus decided to comparatively evaluate an extended set of 17 commercially available monomeric and dimeric metal carbonyls including Re2(CO)10, Mn2(CO)10, CpMn(CO)3, Cp2Mo2(CO)6, Mo(CO)6, Cp2Fe(CO)4, Fe-

X; X = Cl, Br, I) halide functionalized PVDF−X chain ends. In this case, the RX initiators are efficient CTAs, but the resulting • PVDF−X halide chain ends are less reactive (CTPVDF >1> RX •

•

PVDF PVDF ≫ CTPVDF−CH ). Thus, as Cl- or BrCTPVDF−CF 2 −X 2 −X degenerative exchange cannot occur, only telomerization or VDF−FRP is available.11 However, excess Mn(CO)5• could still activate PVDF−X (X = Cl, Br) throughout the polymerization, and some molecular weight increase could occur, although in a poorly controlled fashion. This lack of exchange also prevents the accumulation of the “bad” PVDF−CH2−X. As a result, both types of propagating radicals abstract the initiator halide to provide the same typical ∼10/1 ratio of “good”/“bad” halide chain ends as seen for HT vs HH • propagation. However, if the CTPVDF is very low, only the RX more reactive PVDF−CH2• may abstract. Finally, while Cl- and Br-based CTAs can at most provide efficient telomerizations,23 uncatalyzed halide DT−CRP occurs solely for iodine.21−25 Thus, the only appropriate initiators for VDF−IDT−CRP and thus for obtaining PVDF−I with high chain end functionality (CEF), suitable for the synthesis of well-defined PVDF block copolymers or chain end derivatizations, are semi- and perfluoroalkyl iodides. Accordingly, although HCF2−CF2−CH2−I or (CF3)3C−I afford a less • • efficient IDT due to CTPVDF ∼ CTPVDF R−I PVDF−CH2−I ≪ 1 and respectively, the slower reaction of Mn(CO)5• with tertiary halides, all other activated 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) are excellent CTAs for IDT. Thus, they afford not only PVDF−I with high CEF, but moreover, the very efficient competition of degenerative transfer with both propagation and termination enables a dramatic suppression of HH propagation defects, and of the PVDF−H chain ends to below a few %. Following complete consumption of the RF−I initiator, no new PVDF−I chains can be generated, and the only effective IDT process, is the thermodynamically neutral, uncatalyzed equilibrium of equally reactive, propagating and dormant Pn− CH2−CF2• and Pm−CH2−CF2−I terminal 1,2-units (eq 7, Kequil (ex1) = 1). In this case, since the exchange constant, C1ex = • • 2 , PVDF−CF2−I 2 , VDF kPVDF−CF /kPVDF−CF 1,exchange propagation,12−addition is ≫1, the exchange is favored over propagation and termination. Conversely, due to the much stronger −CH2−I bond, the cross−IDT of the 1,2and 2,1- units (eq 8) is shifted toward an irreversible buildup of dead Pn−CF2−CH2−I chain ends, whereas the IDT of the 2,1terminal units becomes kinetically inconsequential (eq 9).40−43 Nonetheless, the IDT equilibrium from eqs 7 and 8 still enables an efficient Mn2(CO)10 photomediated VDF−IDT−

F


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Macromolecules


Chart 1. Qualitative Dependence of the Rate VDF Photopolymerization on the Nature of the Metal Carbonyl and Halidea

a

The PVDF−I column refers only to chain end activation.

generation under low intensity visible light irradiation103a,b as such reactions were previously conducted only with high power UV−vis light sources, for example, Cr(CO)6, Co2(CO)8, Mo(CO)6,89−97 and conversely, the high photosensitivity of the complex, e.g., Cp2Ti(CO)2140 or (CO)AuCl).102 Indeed, while polymer was obtained in certain cases with Au(CO)Cl, the fast photodecomposition to CO, Au(0) and AuCl3, made this reaction less reliable. Furthermore, some metalloradicals may have low propensity toward abstracting halides, while certain metal complexes would prefer nonradical, oxidative additions into the ∼CF2−I bond of the RF−I initiators, (e.g., Fe(CO)5,99 CpCo(CO)2141). Indeed, while monomeric, dimeric, and trimeric Fe(CO)5, Cp2Fe2(CO)4, Fe3(CO)12 iron carbonyls as well as the related Ru3(CO)12 and Co4(CO)12 were evaluated, no polymer was obtained regardless of the reaction conditions. These results are most likely an outcome of a faster oxidative addition of RF−I derivatives99 by comparison with slow photolysis under visible light. Conversely, strong UV also produces the unreactive and insoluble Fe2(CO)9,142 while upon heating Fe3(CO)12 can be obtained.143 However, such cluster carbonyls including Ru3(CO)12144 are more resistant to photolysis. Nonetheless, for all remaining complexes, polymer could be obtained as follows:

(CO)5, Fe3(CO)12, Cp*2Cr2(CO)4, Cr(CO)6, Co2(CO)8, CpCo(CO) 2 , Co 4 (CO) 1 2 , Cp 2 W 2 (CO) 6 , Ru 3 (CO) 1 2 (PPh3)2Ni(CO)2, and Au(CO)Cl) in conjunction with a representative set of alkyl and perfluoroalkyl halides (CH3(CH2)5−Cl, CH3(CH2)5−Br, CH3(CH2)5−I, CH3−I, CCl4, CCl3−Br, CF3(CF2)3−I, Cl−(CF2)8−Cl, Br−(CF2)6− Br, I−(CF2)6−I), which we previously demonstrated40 as successful VDF initiators with Mn2(CO)10. The results are summarized in Tables 1−4 and S1, and Chart 1. An initial scan of all catalysts was performed using primarily CF3−CF2−CF2−CF2−I as an initiator already proven successful with Mn2(CO)1040 in DMC. As expected, no reaction was observed in the dark at 40 °C for over >24 h, with either catalytic or stoichiometric amounts of any of the complexes investigated. However, no polymer was obtained under visible light in the presence of Co2(CO)8, CpCo(CO)2, Co4(CO)12 Cr(CO)6, CpMn(CO)3, Cp*2Cr2(CO)4, Mo(CO)6, Fe(CO)5, Fe3(CO)12, Cp2Fe2(CO)4, Ru3(CO)12, (PPh3)2Ni(CO)2, and Cp2Ti(CO)2 (Table S1) regardless of reaction conditions, initiator or reagent ratios, while (CO)AuCl gave inconsistent results. Thus, with the exception of CCl4, other halides, were not explored further with these catalysts. Depending on the nature of the catalyst, this was likely due either to the poor quantum yield for the metalloradical G


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Macromolecules


Table 2. Effect of Initiator and Mo Carbonyls on VDF Photopolymerizationa

a

ex.

initiator

catalyst

[VDF]/[I]/[Mo]

Mn

PDI

time (h)

convn (%)

kpapp (h‑1)

init. effic.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

CH3I CF3(CF2)3−I CF3(CF2)3−I CF3(CF2)3−I CH3(CH2)5−Cl CH3(CH2)5−Br CH3(CH2)5−I CH3I CCl3Br CCl3Br Cl−(CF2)8−Cl CF3(CF2)3−Ib CF3(CF2)3−Ic I−(CF2)6−Ib I−(CF2)6−Ic I−(CF2)6−Ic CCl4 Br−(CF2)6−Br CF3(CF2)3−I CF3(CF2)3−I CF3(CF2)3−I CF3(CF2)3−I CF3(CF2)3−I I−(CF2)6−I I−(CF2)6−I I−(CF2)6−I I−(CF2)6−I I−(CF2)6−I I−(CF2)6−I I−(CF2)6−I I−(CF2)6−I

Mo(CO)6 Mo(CO)6 Mo(CO)6 Mo(CO)6 Cp2Mo2(CO)6 Cp2Mo2(CO)6 Cp2Mo2(CO)6 Cp2Mo2(CO)6 Cp2Mo2(CO)6 Cp2Mo2(CO)6 Cp2Mo2(CO)6 Cp2Mo2(CO)6 Cp2Mo2(CO)6 Cp2Mo2(CO)6 Cp2Mo2(CO)6 Cp2Mo2(CO)6 Cp2Mo2(CO)6 Cp2Mo2(CO)6 Cp2Mo2(CO)6 Cp2Mo2(CO)6 Cp2Mo2(CO)6 Cp2Mo2(CO)6 Cp2Mo2(CO)6 Cp2Mo2(CO)6 Cp2Mo2(CO)6 Cp2Mo2(CO)6 Cp2Mo2(CO)6 Cp2Mo2(CO)6 Cp2Mo2(CO)6 Cp2Mo2(CO)6 Cp2Mo2(CO)6

50/1/0.15 25/1/0.2 50/1/0.3 25/1/0.15 50/1/0.5 50/1/0.5 50/1/0.5 25/1/0.2 25/1/0.2 25/1/0.2 50/1/0.4 50/1/0.5 25/1/0.2 50/1/0.1 50/1/0.1 50/1/0.2 25/1/0.2 50/1/0.4 25/1/0.2 25/1/0.2 100/1/1 100/1/1 100/1/1 50/1/0.2 50/1/0.2 50/1/0.2 50/1/0.2 50/1/0.5 100/1/1 100/1/1 100/1/1

− − − − − − − − − − − − − − − − 3400 1500 800 600 2000 2000 1900 700 750 800 1000 900 3100 3900 3800

− − − − − − − − − − − − − − − − 1.16 3.03 1.28 1.34 2.08 1.89 1.92 1.42 1.34 1.33 1.38 1.19 1.82 1.85 1.79

94.0 67.5 26.0 100.0 60.0 60.0 60.0 22.5 19.0 66.5 24.0 20.0 91.0 23.0 94.5 94.5 17.8 20.8 15.0 23.3 23.3 49.5 67.0 15.5 28.5 51.0 75.0 94.5 20.8 46.5 67.0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 5.0 18.3 20.0 25.2 20.0 21.0 21.0 14.0 16.0 19.5 23.0 12.0 21.0 24.0 30.0

0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0029 0.0097 0.0192 0.0097 0.0096 0.0048 0.0035 0.0097 0.0061 0.0043 0.0040 0.0014 0.0114 0.0059 0.0053

− − − − − − − − − − − − − − − − 0.02 0.39 0.44 0.65 0.64 0.67 0.71 0.64 0.68 0.78 0.83 0.43 0.43 0.39 0.51

All in DMC at 40 °C under visible light irradiation except. bNo light 80 °C. cNo light 40 °C.

Group 6: Cr, Mo, W. Tungsten. While no polymer was obtained with either Cr(CO)6 or Cp*2Cr(CO)4, both W and Mo congeners proved more reactive under visible light. Thus, for tungsten, Cp2W2(CO)6, no polymer was obtained (Table 1) from halides with high and medium BDE (Cl−(CF2)8−Cl, CCl3−Cl, and CCl3−Br), but only from Br−(CF2)6−Br, CF3(CF2)3−I, and I−(CF2)6−I. Even so, the polymerizations still required at least twice the catalyst amount vs. Mn2(CO)10 (i.e., [RF−I]/[Cp2W2(CO)6] = [1]/[0.4]). This is partially attributable to the poor solubility of Cp2W2(CO)6 in DMC, where no polymer is obtained with 20 mol % catalyst. However, in acetone, even 10 mol % Cp2W2(CO)6 affords PVDF, although still with very low conversion. Thus, such polymerizations were not investigated kinetically. Molybdenum. Mo(CO)6 and Cp2Mo2(CO)6: While no polymerizations were obtained for Mo(CO)6, Cp2Mo2(CO)6 proved to be a more active system. Thus, while Cp2Mo2(CO)6 is a weaker halide abstractor than Cp2W2(CO)6,89,90 it is more soluble in DMC. However, even in up to stoichiometric amounts, no polymer was obtained under irradiation from halides with higher BDE (CH3(CH2)5−Cl, CH3(CH2)5−Br, CH3(CH2)5−I, CH3−I, CCl3−Br, and Cl−C8F16−Cl), or in the dark at 40−80 °C, from RF−I derivatives (Table 2). However, Cp2Mo2(CO)6 did afford PVDF from CCl4, Br−(CF2)6−Br, CF3(CF2)3−I, and I−(CF2)6−I, although very poorly. Nonetheless, Cp2Mo2(CO)6 was the first carbonyl in this series to

promote VDF−IDT (Figure 1). These results are explained by the particular differences between this catalyst and Mn2(CO)10.

Figure 1. Cp2Mo2(CO)6-mediated VDF−IDT: (a) Dependence of Mn and Mw/Mn on conversion and (b) kinetics. [VDF]/[DIPFH]/ [Cp2Mo2(CO)6] = [50]/[1]/[0.2] (red ●, ○) and [100]/[1]/[1] (blue ■, □).

Indeed, similarly to Mn2(CO)10, visible light (λ > 400 nm) readily promotes Cp2Mo2(CO)6 homolysis145 to the 17e− CpMo(CO)3•, which can also undergo recombination or hydride and halide abstraction. However, by contrast to Mn(CO)5•, which reacts faster with primary over secondary or tertiary halides,82 CpMo(CO)3• abstraction rates follow146 the expected I > Br > Cl and benzyl > allyl >3° > 2° > 1° > H


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Macromolecules


Table 3. Effect of Initiator and Re2(CO)10 on VDF Photopolymerizationsa

a

ex.

initiator

catalyst

[VDF]/[I]/[Re2(CO)10]

Mn

PDI

time (h)

convn (%)

kpapp (h‑1)

init. effic.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

CH3(CH2)5−Cl CH3(CH2)5−Br CH3(CH2)5−I CH3I CCl3Br Cl−(CF2)8−Cl CCl4 Br−(CF2)6−Br CF3(CF2)3−Ib CF3(CF2)3−Ib I−(CF2)6−Ic CF3(CF2)3−I CF3(CF2)3−I CF3(CF2)3−I I−(CF2)6−I I−(CF2)6−I I−(CF2)6−I I−(CF2)6−Id I−(CF2)6−Id I−(CF2)6−Id I−(CF2)6−I I−(CF2)6−I I−(CF2)6−I I−(CF2)4−I I−(CF2)4−I I−(CF2)4−I

Re2(CO)10 Re2(CO)10 Re2(CO)10 Re2(CO)10 Re2(CO)10 Re2(CO)10 Re2(CO)10 Re2(CO)10 Re2(CO)10 Re2(CO)10 Re2(CO)10 Re2(CO)10 Re2(CO)10 Re2(CO)10 Re2(CO)10 Re2(CO)10 Re2(CO)10 Re2(CO)10 Re2(CO)10 Re2(CO)10 Re2(CO)10 Re2(CO)10 Re2(CO)10 Re2(CO)10 Re2(CO)10 Re2(CO)10

50/1/0.5 50/1/0.5 50/1/0.5 50/1/0.2 25/1/0.2 50/1/0.5 25/1/0.2 50/1/0.4 25/1/0.2 50/1/0.5 50/1/0.1 25/1/0.2 25/1/0.2 25/1/0.2 50/1/0.2 50/1/0.2 50/1/0.2 50/1/0.1 50/1/0.1 50/1/0.1 200/1/0.4 200/1/0.4 200/1/0.4 50/1/0.2 50/1/0.2 50/1/0.2

− 5700 7500 15 000 3000 9300 6400 8200 − − − 800 1250 1400 1000 2900 4100 700 1800 2500 3000 6100 7200 1100 1300 2100

− 2.03 1.62 1.82 1.37 2.01 1.60 1.69 − − − 1.27 1.38 1.55 1.31 1.37 1.50 1.35 1.36 1.39 1.43 1.61 1.63 1.56 1.57 1.60

48.0 88.7 48.0 22.0 20.0 88.7 20.0 20.8 91.0 20.0 23.0 1.7 3.0 6.0 2.0 4.0 8.0 2.5 8.5 16.5 2.0 6.0 12.0 3.3 5.0 7.0

0.0 26.0 42.0 26.0 37.0 17.0 43.9 68.5 0.0 0.0 0.0 23.0 58.0 78.0 22.0 56.0 77.0 12.0 38.0 65.0 13.0 30.0 41.0 41.0 58.0 84.0

0.0000 0.0034 0.0113 0.0137 0.0231 0.0021 0.0289 0.0557 0.0000 0.0000 0.0000 0.1537 0.2892 0.2524 0.1242 0.2052 0.1837 0.0511 0.0562 0.0636 0.0696 0.0892 0.0352 0.1584 0.1735 0.2618

− 0.17 0.09 0.07 0.44 0.10 0.24 0.27 − − − 0.46 0.71 0.89 0.70 0.62 0.60 0.85 0.86 0.96 0.55 0.63 0.73 1.19 1.43 1.28

All in dimethyl carbonate at 40 °C under visible light irradiation except. bNo light, 40 °C. cNo light, 80 °C. d65 °C.

Table 4. Effect of Mn2(CO)10 and Initiator on the VDF Photopolymerizationa

a b

ex.

initiator

catalyst

[VDF]/[I]/ [Mtx(CO)y]

Mn

PDI

time (h)

convn (%)

kpapp (h‑1)

init. effic.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

CF3(CF2)3−I CH3(CH2)5−Cl CH3(CH2)5−Br CH3(CH2)5−I CH3−I CCl3−Br Cl−(CF2)8−Cl CCl4 Br−(CF2)6−Br CF3(CF2)3−Ib CF3(CF2)3−Ic CF3(CF2)3−I CF3(CF2)3−I CF3(CF2)3−I CF3(CF2)3−I CF3(CF2)3−I CF3(CF2)3−I I−(CF2)4−I I−(CF2)4−I I−(CF2)4−I I−(CF2)6−I I−(CF2)6−I I−(CF2)6−I I−(CF2)6−I I−(CF2)6−I I−(CF2)6−I

CpMn(CO)3 Mn2(CO)10 Mn2(CO)10 Mn2(CO)10 Mn2(CO)10 Mn2(CO)10 Mn2(CO)10 Mn2(CO)10 Mn2(CO)10 Mn2(CO)10 Mn2(CO)10 Mn2(CO)10 Mn2(CO)10 Mn2(CO)10 Mn2(CO)10 Mn2(CO)10 Mn2(CO)10 Mn2(CO)10 Mn2(CO)10 Mn2(CO)10 Mn2(CO)10 Mn2(CO)10 Mn2(CO)10 Mn2(CO)10 Mn2(CO)10 Mn2(CO)10

25/1/0.2 50/1/0.5 50/1/0.5 50/1/0.5 50/1/0.2 25/1/0.2 50/1/0.4 25/1/0.2 50/1/0.4 25/1/0.2 25/1/0.2 50/1/0.2 25/1/0.2 25/1/0.2 25/1/0.25 25/1/0.25 25/1/0.25 50/1/0.2 50/1/0.2 50/1/0.2 50/1/0.2 50/1/0.2 50/1/0.2 200/1/0.2 200/1/0.2 200/1/0.2

− − 3100 5700 4900 2500 4800 3600 3500 − − 4200 950 1200 1550 1700 1900 1500 1800 2300 1200 1700 2200 6800 8200 9400

− − 1.81 1.89 1.8 1.70 3.04 2.22 2.33 − − 1.56 1.31 1.56 1.62 1.85 1.85 1.63 1.46 1.51 1.48 1.39 1.40 1.55 1.56 1.67

48.0 50.0 50.0 50.0 13 22.5 24.0 22.5 20.8 93.0 93.0 18.5 10.0 23.3 3.0 6.0 9.0 6.5 22.0 30.0 4.0 8.0 20.0 1.0 4.0 15.0

0.0 0.0 15.3 26.7 11.0 35.0 5.0 43.0 69.3 0.0 0.0 60.9 22.7 72.4 50.0 64.0 75.0 44.0 64.0 80.0 34.0 50.0 63.0 28.0 35.0 47.0

0.0000 0.0000 0.0033 0.0062 0.0090 0.0191 0.0021 0.0250 0.0569 0.0000 0.0000 0.0508 0.0250 0.0554 0.2310 0.1700 0.1540 0.0892 0.0464 0.0536 0.1387 0.0990 0.0910 0.3285 0.1077 0.0423

− − 0.16 0.15 0.07 0.28 0.14 0.19 0.63 − − 0.46 0.52 0.97 0.47 0.60 0.67 0.94 1.14 1.11 0.91 0.94 0.92 0.53 0.55 0.64

All in dimethyl carbonate at 40 °C under low power (≤30 W) visible light irradiation with a compact fluorescent bulb unless otherwise stated. Dark, 40 °C. cDark, 75 °C.

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Macromolecules CH3 trend, and its recombination rate constant is about twice that of Mn(CO)5• (kMo = 3.1 × 109, kMn = 1.9 × 109 M−1 s−1).147,148 In addition, as for Mn(CO)5−I,40 the quantum yield of Mo−X bond photolysis is negligible (Φ = 9 × 10−4).149 Thus, it is also unlikely that CpMo(CO)3−I can reversibly transfer iodine. Moreover, in solution, Cp2Mo2(CO)6 is based on a fluxional equilibrium of the dominant trans with the lower reactivity gauche conformer.150 Interestingly, following visible light irradiation, in-cage recombination instantly converts ∼30% of the trans to the higher energy gauche, where the thermal rotation about the Mo−Mo bond back to trans (k = 2 × 102 s−1),150 is 7 orders of magnitude slower than recombination.151 As such, under continuous visible light irradiation, a buildup of the gauche conformer occurs, causing a rapid decrease in the ability of Cp2Mo2(CO)6 to homolyze and abstract halides. Thus, Cp2Mo2(CO)6 visible light homolysis to radical active species, is in a sense, autoinhibited. Conversely, while UV irradiation (λ < 400 nm) could promote gauche homolysis, it would also cause CO loss before Mo−Mo bond scission.152 This is consistent with the observed trends, i.e., a fast increase in monomer conversion (higher initial kpapp), followed by a rapid decrease in polymerization rate (Table 2, Figure 1b) yielding nonlinear first order kinetic. Nonetheless, a linear dependence of Mn on conversion can still be observed, indicating that some degree of IDT−CRP is in operation. Group 7, Mn, Re. Rhenium. Re2(CO)10: As for all carbonyls herein, control experiments (Table 3) revealed that at both 40 and 80 °C there is no polymerization in the dark, and thus, photoactivation is required. However, by contrast with all the other carbonyls except Mn, the very active Re2(CO)10 afforded PVDF with all initiators except the inactivated C6H13−Cl. Conversely, C6H13−Br, C6H13−I, CH3−I, Cl−(CF2)8−Cl, Br−(CF2)6−Br and CCl4 displayed poor initiator efficiency (up to ∼25%), and required stoichiometric activator. While better results were obtained with CCl3Br, as expected, perfluoroalkyl iodides provided the fastest rates, and highest initiator efficiencies. Manganese. CpMn(CO)3 and Mn2(CO)10: While CpMn(CO)3 afforded no polymerization, in order to obtain a fair comparison under the same conditions between the very closely related Mn2(CO)10 and Re2(CO)10 systems, a new set of halide activations complementary to the previously reported ones40 was finally conducted with Mn2(CO)10. Again control experiments (Table 4) confirmed that at both 40 and 75 °C there is no polymerization in the dark, and thus photoactivation is required. As such, under irradiation, similarly to Re2(CO)10, Mn2(CO)10 afforded polymer with all initiators except the inactivated C6H13Cl. Likewise, while C6H13−Br, C6H13−I, CH3−I, CCl3−Br, Cl−(CF2)8−Cl, and CCl4, yielded polymer, but with poor initiator efficiency ( CTPVDF−CH > 1 and kp,21 RX RX > kp,12). Thus, the reverse propagating unit is very efficiently intercepted by chain transfer to RF−I, and most of the HH units of VDF−FRP are observed in IDT as the terminal, inactive PVDF−CF2−CH2−I.40 Indeed, since PVDF−CH2−I is rather inert in IDT or in other metal or organic mediated radical processes,28,40 its activation will demand stronger halide abstractors than the PVDF−CF2−I cogener.40 Conversely, such activators will form an even stronger bond with the halide, and the process will not be reversible. Thus, while IDT catalysis by Mn(CO)5•/ Mn(CO)5−I would have prevented accumulation of Pn− CF2−CH2−I, this was not observed.40 Nonetheless, some reactivation if using excess Mt(CO)y•, or much slower by IDT with ∼PVDF−CF2−CH2•, as well as normal propagation, may still provide an internal HH unit. Consequently, as the concentration of the IDT-unreactive PVDF−CH2−I chains increases continuously with conversion, they accumulate,40 and already dominate at medium to high conversions.41 Consequently, such pseudodead species also represent a lower molecular weight population than the dormant/propagating PVDF−CF2−I, and as seen for VAc− IDT,87,153 contribute to PDI broadening. However, while the concentration of the “good” −CH2−CF2−I chain ends decreases, and that of the “bad” −CF2−CH2−I increases with conversion,40,41 the total (−CH2−CF2−I + −CF2−CH2−I) iodine chain functionality (CEF) remains at least 90%, even at larger levels of Mn2(CO)10,40 and especially in IDT using J


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Macromolecules

Figure 3. Effect of metal carbonyls on the photoactivation of PVDF−CH2−I and PVDF−CF2−I chain ends. (a) PVDF−I, (b) no metal carbonyl, (c) Co2(CO)8, (d) Mo(CO)6, (e) Au(CO)Cl, (f) (PPh3)2Ni(CO)2, (g) Fe(CO)5, (h) Co4(CO)12, (i) Cp*2Cr(CO)4, (j) Mn2(CO)10, (k) Re2(CO)10, (l) Cp2Mo2(CO)6, (m) Cp2W2(CO)6, and (n) Cp2Fe2(CO)4. [PVDF−I]/[activator] = 1/2, T = 40 °C, visible light, minimum of 12 h, 500 MHz 1H NMR spectra in d6-acetone, ∗ = acetone, ◀ = H2O.

difunctional I−RF−I initiators.40 Such CEF is satisfactory for block copolymer synthesis, on condition that both halide chain ends can be activated. However, aside from e.g. ATRP initiation from telomerization-derived CCl3−PVDF,154 earlier attempts at the synthesis of PVDF blocks155 included either VDF initiation from macromolecular RF−I CT agents using free radical initiators47,154,156 (which inherently produce PVDF homopolymers), or the flawed assumption that both PVDF−I154,157 chain ends could be radically activated (e.g., Cu/ATRP,157 or thermal IDT154) for the initiation of another monomer, or nucleophilically converted to other groups (e.g., azide etc.)158−160 for subsequent couplings. Indeed, as described earlier, CuX/L (CH3)2C(COOEt)−Br hardly activates perfluoroalkyl halides (kabstr / C8F17−Br 2 161 kabstr ∼ 10 ), and thus, would barely initiate from −CF2−CF2−I, let alone from −CH2−CF2−I, and especially from the unreactive −CF2−CH2−I chain end. Conversely, substitution at semi- and perfluoroalkyl halides does not proceed readily, as RF substituents deactivate both SN1 and SN2 pathways, and nucleophiles also induce dehydrofluorination.162 Thus, PVDF−CF2−CH2−I prefers SN2, whereas PVDF− CH2−CF2−I favors SRN1 substitutions.163,164 As such, these

methods can only lead to ill-defined, inseparable mixtures of homo and block copolymers. Most importantly, prior to our work,40 the conversion dependence of the iodine CEF was never studied and acknowledged. Thus, with one exception,154 no details were ever provided on the PVDF−X halide chain ends, or the fact that mixtures are actually produced, was recognized. Indeed, as the concentration of inactive PVDF−CF2−CH2−I increases with conversion,40 all PVDF−I samples previously used likely contained predominantly “bad” chain ends. Moreover, as they were initiated from monofunctional RF−I CTAs, their total CEF (unreported) was likely also very poor. It is thus clear that, due to the failure to activate the stronger and dominant −CF2− CH2−X termini, all previous endeavors were fundamentally incomplete, and that the so-called “blocks” prepared by such approaches were in fact always ill-defined mixtures of homo and block copolymers. Thus, clean synthesis of “pure”, well-defined PVDF block copolymers requires complete activation of both PVDF chain ends, especially of ∼CF2−CH2−X. This is where Mn(CO)5• demonstrated to be a useful substoichiometric activator not only for IDT, but especially as a stoichiometric activator for block copolymer synthesis. Indeed, Mn(CO)5• enables40,41 the activation not only of ∼CF2−I K


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Figure 4. 500 MHz 1H NMR spectra of PVDF block copolymers synthesized using metal carbonyls for the complete activation of PVDF−I chain ends. (a) Cp2Mo2(CO)6, (b) Cp2W2(CO)6, (c) Re2(CO)10, (d, e) Mn2(CO)10, (f) Cp2Fe2(CO)4. ◀ = H2O, ∗ = acetone, ◆ = DMSO. All in acetone-d6 except PAN in DMSO-d6.

−CH2−CF2−H and −CF2−CH3, peaks d, d′) is seen at δ = 6.30 ppm and δ = 1.80 ppm.40 Subsequently, upon using catalytic Mn2(CO)10, selective activation of only PVDF∼CH2− CF2−I (disappearance only of δ ∼ 3.65 ppm) is observed,43 while stoichiometric Mn2(CO)10 activates both chain ends (disappearance of both δ ∼ 3.65 ppm, and δ ∼ 3.85 ppm ∼CH2−CF2−I and ∼CF2−CH2−I).40 Conversely, treatment of PVDF−I with a stoichiometric amount of a strong Mt(CO)y• activator, in a solvent prone to H chain transfer (e.g., DMAC), should result in the complete radical activation of both iodide chain ends, and in the deactivation of the PVDF• radicals to the corresponding PVDF−H, by H abstraction from the solvent. This is demonstrated (Figure 3, e.g. spectra j−n), by the disappearance of the c and c′ peaks, by the dramatic increase in the d and d′ peaks, and by the more resolved −CH2−CF2−CH2−CF2−H d″, δ = 2.77 ppm.39,40 Selected examples of the metal carbonyl mediated chain end activations are presented in Figure 3. Control reactions indicate that there no iodine chain end loss under irradiation at 40 °C for at least 24 h, in the absence of Mtx(CO)y. Conversely, under irradiation, Co2(CO)8 and CpCo(CO) 2 appear inert, and only trace activation of the good chain end is obtained for Mo(CO)6. Likewise, (CO)AuCl appears to activate only the PVDF−CF2−I chain end, before undergoing fast decomposition to Au(0).102 Likewise, (PPh3)2Ni(CO)2 affords complete activation of PVDF−CF2−I, and even some partial activation of PVDF−CH2−I. However, although Fe(CO)5, Cp2Fe2(CO)4,

based initiators, but also of the stronger C−I bonds of CH3−I, CH3−(CH2)5−I, and H−CF2−CF2−CH2−I models of the PVDF−CF2−CH2−I chain end. Thus, as ∼CF2−CH2−I and ∼CH2−CF2−I are easier to activate than regular alkyl iodides, Mn(CO)5• was also able to afford quantitative activation of both types of PVDF−I chain ends. As such, regardless of the VDF−IDT conversion, i.e., the ratio of the two types of iodine chain ends, they are both viable initiating sites in the presence of Mn(CO)5•. However, while PVDF∼CH2−CF2−I is a very good chain transfer agent that can be activated even with catalytic Mn(CO)5•, PVDF∼CF2−CH2−I requires stoichiometric activation. In addition, while VDF homopolymerizations are performed at 40 °C, block copolymerizations no longer involves pressurized tubes, and the activation and polymerization can be thermally assisted. Thus, it would be interesting to evaluate the performance of the above metal carbonyls in the synthesis of PVDF blocks. The quantitative activation and the block copolymer synthesis can be demonstrated by NMR40 (Figure 3). Here, in the spectrum of the starting PVDF−I, besides acetone and water (δ = 2.05 and 2.84 ppm), the HT −CF2−[CH2−CF2]n− CH2− (a) and HH −CF2−CH2−CH2−CF2− (a′) PVDF sequences40,42,165 are seen at δ = 2.8−3.1 ppm and δ = 2.3−2.4 ppm. Resonance b (δ = 3.25 ppm) indicates the initiator RF− CH2−CF2- connectivity with the first polymer unit, while the 1,2−CH2−CF2−I (c) and 2,1−CF2−CH2−I (c′), iodine chain ends are observed40 at δ = 3.62 ppm and δ = 3.87 ppm. Trace termination by H transfer to PVDF• (eqs 11 and 12), (i.e., L


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Macromolecules Table 5. Characterization of PVDF Block Copolymersa I−PVDF−I


a

exp.

monomer

Mn

PDI

[M]/[IPVDFI]/ [Mn2(CO)10]

temp (°C)

convn (%)

composition M/VDF

Mn

PDI

1 2 3 4 5 6

styrene isoprene VAc MMA t Bu acrylate acrylonitrile

1500 2100 2100 2100 2100 2100

1.38 1.28 1.28 1.28 1.28 1.28

100/1/1 200/1/1 100/1/1 100/1/1 50/1/1 175/1/1

90 100 40 90 60 40

56 12 58 28 40 30

58/42 62/38 56/44 67/33 64/36 66/34

5900 4300 5400 7800 8400 5200

1.49 1.48 1.52 1.65 1.75 1.85

All polymerizations in DMAC.

monomer, control of the block copolymerization can still be envisioned by other CRP methods. Finally, while a comprehensive catalyst comparison is not possible due to the lack of all the possible structure/ combinations, it is still interesting to qualitatively evaluate the effect of the nature of the metal and its oxidation state, ligand, as well as the type of the complex (monomeric, dimeric or higher), on the VDF polymerization and PVDF chain end activation results. These trends are modulated by the stability of the complex toward visible light photolysis and by the reactivity of the irradiation derived species toward alkyl halides. As such, based on a combination of polymerization rates and chain end activation ability, the overall qualitative order in the activity of these complexes is Mn2(CO)10 ∼ Re2(CO)10 ≫ Cp 2 Mo 2 (CO) 6 > Cp 2 W 2 (CO) 6 ∼ Cp 2 Fe 2 (CO) 4 ∼ Cp*2Cr2(CO)4, ∼ Co4(CO)12, > Fe(CO)5 ≫ Mo(CO)6 ∼ Cr(CO)6, Co2(CO)8, CpMn(CO)3, CpCo(CO)2, Fe3(CO)12, Ru3(CO)12, (PPh3)2Ni(CO)2, Cp2Ti(CO)2 and Au(CO)Cl. These results do qualitatively parallel the Re(CO)5• > Mn(CO)5• > CpW(CO)3• > CpMo(CO)3• > CpFe(CO)2• > Co(CO)4•89,90 flash photolysis halide abstraction data, as well as previous thermal and UV-polymerization investigations.103 Thus, while Mn2(CO)10 and Re2(CO)10 remain by far the most active complexes, as seen for Cp2Mo2(CO)6 > Mo(CO)6, Cp2Fe2(CO)4 > ∼ Fe(CO)5 ≫ Fe3(CO)12, Cp*2Cr2(CO)4 > Cr(CO)6, dimers containing a Mt(+1) stabilized by electron donating Cp or Cp* ligands are more active than the corresponding Mt(0) monomers, which in turn may appear more reactive than their dimers or higher carbonyl oligomers, i.e. Cp2Mt(+1)2(CO)2x > Mt(0)(CO)y ≫ Mt(0)n(CO)z. In fact, trimeric or tetrameric oligomers such as Fe 3 (CO) 12 , Ru3(CO)12, and Co4(CO)12, structures should indeed be less reactive and more photolytically stable, as they are typically derived by the CO expulsion and oligomerization following photolysis and thermal rearrangement of the corresponding Mt(0)(CO)y monomer.95−102 However, Cp-stabilized monomers are not better than the CO dimers (Mn2(CO)10 ≫ CpMn(CO)3 and Co4(CO)12 > CpCo(CO)2 ∼ Co2(CO)8. Conversely, as it would appear that Mt (0) (CO) X > LMt(+1)(CO)y (e.g., Fe(CO)5 > Mo(CO)6 > Cr(CO)6 ≫ CpMn(CO)3, CpCo(CO)2, Au(CO)Cl) and respectively Mt (0) 2 (CO)2y > Cp 2 Mt (+1)2 (CO)2y (e.g., Re 2 (CO)10 ∼ Mn 2 (CO) 1 0 ≫ Cp 2 Mo 2 (CO) 6 > Cp 2 W 2 (CO) 6 , > Cp2Fe2(CO)4 > Cp*2Cr2(CO)4), it would be tempting to generalize that Cp 2 Mt (+1) 2 (CO) 2x > Mt (0) 2 (CO) 2x ≫ CpMt(+1)(CO)y > Mt(0)(CO)y ≫ Mt(0)n>2(CO)z. However, this trend is only tentative and qualitative due to the lack of relevant examples in all cased and the obvious dependence of reactivity on the metal. Indeed, while this might suggest that Cp2Mn2(CO)n > Mn2(CO)10, unfortunately, Cp2Mn2(CO)n -like species are not stable.168

Cp*2Cr2(CO)4 and Co4(CO)12 failed to generate polymer in conjunction with RF−I initiators at catalytic levels, these carbonyls, as well as Cp2Mo2(CO)6, Cp2W2(CO)6, afford the complete activation of both PVDF−I chain ends while at stoichiometric ([PVDF−I]/[Mtx(CO)yLz] = 1/1) levels. Nonetheless, all these carbonyls are of much lower activity than the most successful Mn2(CO)10 and Re2(CO)10. Interestingly, in addition to PVDF−H, a terminal PVDF− CFCH2 unsaturation, most likely derived from metal insertion followed by β-F elimination (d‴ at δ = 5.25 ppm), is observed for Fe(CO)5, Cp*2Cr2(CO)4, and Co4(CO)12. Indeed, e.g., Fe(CO)5 is known to oxidatively insert into the CF2−I bond of RF−I derivatives to yield a relatively stable RF− Fe(CO)4−I,99,100 thus preventing the formation of the required metalloradical at 40 °C, and explaining the lack of initiation in both VDF homopolymerization, as well as in block synthesis. Moreover, in line with the higher stability of perfluoroalkyl organometallics vs semi and perhydrogenated analogues,166 i.e. R−CF2−CF2−Mt(CO)x−I > R−CH2−CF2−Mt(CO)x−I ≫ R−CF2−CH2−Mt(CO)x−I, it is very likely that PVDF−CF2− CH2−Mt(CO)x−I quickly undergoes β-fluoride elimination to a metal fluoride and ∼ CFCH2 (d‴, δ = 5.20 ppm), whereas the more stable ∼CH2−CF2−Mt(CO)x−I persists until acidic aqueous work-up hydrolyses it to ∼CH2−CF2−H, thus affording the same chain (d″, δ = 2.77 ppm) as H radical chain transfer. The insertion mechanism is also consistent with the block copolymerization results. Indeed, Fe(CO)5 and Cp*2Cr2(CO)4 are the only metal carbonyls that activate both iodide chain ends, but do not enable block copolymerization. Thus, it appears that here, the PVDF−CH2−Mt chain ends eliminate faster than potential radical homolysis which would have enabled the initiation of a second monomer, whereas the PVDF−CF2−Mt chain ends do not homolyze at 40 °C. Nonetheless, such PVDF−Mt(CO)y−I organometallics may well represent the first examples of metal insertion into semifluorinated macromolecular alkyl iodides, and could be used in chain end derivatizations (e.g., electrophilic additions to carbonyl substrates).167 As such, carrying out quantitative I−PVDF−I chain end activations in the presence of Cp2Mo2(CO)6, Cp2W2(CO)6, Cp2Fe2(CO)4, Re2(CO)10, Mn2(CO)10, and radically polymerizable alkenes leads to further examples of well-defined, ABAtype PVDF block copolymers (NMR Figure 4, Table 5) with styrene (f, f′), isoprene (g, g′, g″), vinyl acetate (h, h′, h″), methyl methacrylate (i, i′HH, i″), t-butyl acrylate (j, j′, j″), and acrylonitrile (k, k′) initiated from both PVDF halide chain ends (Figure 4a−f). While the stoichiometric amounts of metal carbonyls needed for activation of the dominant and poorly reactive PVDF− CF2−CH2−I also lead to complete and irreversible iodine consumption and thus prevent the IDT of the second M


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Notes

CONCLUSIONS The metal and ligand effect of a series of 18 transition metal carbonyl complexes (Re2(CO)10, Mn2(CO)10, CpMn(CO)3, Cp2Mo2(CO)6, Mo(CO)6, Cp2Fe(CO)4, Fe(CO)5, Fe3(CO)12, Cp* 2 Cr 2 (CO) 4 , Cr(CO) 6 , Co 2 (CO) 8 , CpCo(CO) 2 , Co4(CO)12, Cp2W2(CO)6, Ru3(CO)12 (PPh3)2Ni(CO)2 and Au(CO)Cl) was investigated in the initiation and control of vinylidene fluoride (VDF) visible light radical photopolymerization initiated from alkyl and perfluoroalkyl halides (CH3(CH2)5Cl, CH3(CH2)5Br, CH3(CH2)5I, CH3I, CCl4, CCl3Br, Cl(CF2)8Cl, Br(CF2)6Br, CF3(CF2)3I, I(CF2)4I, I(CF2)6I) in dimethyl carbonate at 40 °C, as well as in the synthesis of well-defined PVDF block copolymers by quantitative activation of both PVDF−CH2−CF2−I and PVDF−CF2−CH2−I chain ends. Accordingly, no polymerization was observed in the presence of CpMn(CO)3, CpCo(CO)2, Cp2Fe2(CO)4, Cp*2Cr2(CO)4, Mo(CO) 6, Fe(CO)5, Cr(CO)6, Co2(CO)8 , Co 4(CO) 12, Fe3(CO)12, Ru3(CO)12, (PPh3)2Ni(CO)2, Cp2Ti(CO)2 and Au(CO)Cl, regardless of reaction conditions. However, a free radical polymerization (FRP), and respectively an iodine degenerative transfer controlled radical polymerization (IDT− CRP) was obtained for Mn2(CO)10 ∼ Re2(CO)10 ≫ Cp 2 Mo 2 (CO) 6 ≫ Cp 2 W 2 (CO) 6 and alky halides (CH3(CH2)5−Br, CH3(CH2)5−I, CH3−I, CCl3−Cl, CCl3−Br, Br−(CF2)6−Br), and respectively perfluoroalkyl iodides (RFI = CF3(CF2)3−I, I−(CF2)4−I and I−(CF2)6−I). A selection of the above metal complexes was further evaluated in the radical activation of the PVDF−I chain ends. Here, while Fe(CO)5, Cp*Cr2(CO)4 and Co4(CO)12 also led to chain end unsaturations from eliminations following metal insertion in the C−I bond, Re 2 (CO) 10 , Mn 2 (CO) 10 , Cp2W2(CO)6, Cp2Mo2(CO)6, and Cp2Fe2(CO)4 all provided quantitative radical activation of both PVDF−CH2−CF2−I and PVDF−CF2−CH2−I, and were subsequently employed in the synthesis of well-defined, ABA triblock PVDF copolymers with vinyl acetate, tert-butyl acrylate, methyl methacrylate, isoprene, styrene, and acrylonitrile. As such, including the suitability of the metal complex for both IDT and block copolymerization, the overall trend in activity is Mn2(CO)10 ∼ Re2(CO)10 ≫ Cp2Mo2(CO)6 > Cp2W2(CO)6 ∼ Cp2Fe2(CO)4 ∼ Cp*2Cr2(CO)4, > Fe(CO)5 ≫ Mo(CO)6 ∼ Cr(CO)6, Co2(CO)8, CpMn(CO)3, CpCo(CO)2, Co4(CO)12, Fe3(CO)12, Ru3(CO)12, (PPh3)2Ni(CO)2, Cp2Ti(CO)2 and Au(CO)Cl. While this may suggest that Cp2 Mt (+1) 2(CO) 2x > Mt(0) 2(CO) 2x > CpMt(+1) (CO) > Mt(0)(CO)y ≫ Mt(0)n(CO)z, this relationship is at best uncertain, due to the unavailability of pertinent examples for all metals.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Science Foundation, Grants NSF-CHE-1309769 and NSF-CHE-1058980, is gratefully acknowledged.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b00698. Table S1, detailing the effect of initiator and Co, Cr, Fe, Au, Ni, and Ti carbonyls on VDF photopolymerizations (PDF)

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*(A.D.A.) E-mail: [email protected]. N


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