Fluoroalkyl Pentacarbonylmanganese(I) Complexes as Initiators for the Radical (co)Polymerization of Fluoromonomers Roberto Morales-Cerrada, Vincent Ladmiral, Florence Gayet, Christophe Fliedel, Rinaldo Poli, Bruno Ameduri

To cite this version:

Roberto Morales-Cerrada, Vincent Ladmiral, Florence Gayet, Christophe Fliedel, Rinaldo Poli, et al.. Fluoroalkyl Pentacarbonylmanganese(I) Complexes as Initiators for the Radical (co)Polymerization of Fluoromonomers. Polymers, MDPI, 2020, 12 (2), pp.384. ￿10.3390/polym12020384￿. ￿hal-02494900￿

HAL Id: hal-02494900 https://hal.archives-ouvertes.fr/hal-02494900 Submitted on 6 Jul 2020

HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés.

Distributed under a Creative Commons Attribution| 4.0 International License polymers

Article Fluoroalkyl Pentacarbonylmanganese(I) Complexes as Initiators for the Radical (co)Polymerization of Fluoromonomers

Roberto Morales-Cerrada 1,2 , Vincent Ladmiral 1 , Florence Gayet 2, Christophe Fliedel 2 , Rinaldo Poli 2,* and Bruno Améduri 1,*

1 Institut Charles Gerhardt Montpellier, University of Montpellier, CNRS, ENSCM, Place Eugène Bataillon, 34095 Montpellier CEDEX 5, France; [email protected] (R.M.-C.); [email protected] (V.L.) 2 Laboratoire de Chimie de Coordination (LCC), Université de Toulouse, CNRS, UPS, INPT, 205 route de Narbonne, BP 44099, 31077 Toulouse CEDEX 4, France; fl[email protected] (F.G.); christophe.fl[email protected] (C.F.) * Correspondence: [email protected] (R.P.); [email protected] (B.A.); Tel.: +33-(0)5-61-33-31-73 (R.P.); +33-(0)4-67-14-43-68 (B.A.)


Received: 17 December 2019; Accepted: 4 February 2020; Published: 8 February 2020


Abstract: The use of [Mn(RF)(CO)5] (RF = CF3, CHF2, CH2CF3, COCF2CH3) to initiate the radical polymerization of vinylidene fluoride (F2C=CH2, VDF) and the radical alternating copolymerization of vinyl acetate (CH2=CHOOCCH3, VAc) with tert-butyl 2-(trifluoromethyl)acrylate (MAF-TBE) by generating primary RF• radicals is presented. Three different initiating methods with [Mn(CF3)(CO)5] (thermal at ca. 100 ◦C, visible light and UV irradiations) are described and compared. Fair (60%) to satisfactory (74%) polyvinylidene fluoride (PVDF) yields were obtained from the visible light and UV activations, respectively. Molar masses of PVDF reaching 53,000 g mol 1 were produced from the · − visible light initiation after 4 h. However, the use of [Mn(CHF2)(CO)5] and [Mn(CH2CF3)(CO)5] as radical initiators produced PVDF in a very low yield (0 to 7%) by both thermal and photochemical initiations, while [Mn(COCF2CH3)(CO)5] led to the formation of PVDF in a moderate yield (7% to 23%). Nevertheless, complexes [Mn(CH2CF3)(CO)5] and [Mn(COCHF2)(CO)5] efficiently initiated the alternating VAc/MAF-TBE copolymerization. All synthesized polymers were characterized by 1H and 19F NMR spectroscopy, which proves the formation of the expected PVDF or poly(VAc-alt-MAF-TBE) and showing the chaining defects and the end-groups in the case of PVDF. The kinetics of VDF homopolymerization showed a linear ln[M]0/[M] versus time relationship, but a decrease of molar masses vs. VDF conversion was noted in all cases, which shows the absence of control. These PVDFs were rather thermally stable in air (up to 410 ◦C), especially for those having the highest molar masses. The melting points ranged from 164 to 175 ◦C while the degree of crystallinity varied from 44% to 53%.

Keywords: fluoropolymers; manganese complexes; nuclear magnetic resonance; radicals; organometallic-mediated radical polymerization; vinylidene fluoride

1. Introduction Polyvinylidene fluoride (PVDF) [1–5] is the most produced fluoropolymer after polytetrafluoroethylene (PTFE). This specialty polymer has many relevant properties such as chemical inertness to strong acid and apolar solvents, ferroelectric, pyroelectric, and piezoelectric (when stretched) properties, as well as thermal and electrochemical resistances. It can be involved in many applications such as a binder and separator for lithium ion batteries, paints and coatings, piezoelectrical devices, water purification membranes, and more.

Polymers 2020, 12, 384; doi:10.3390/polym12020384 www.mdpi.com/journal/polymers Polymers 2020, 12, 384 2 of 17

PVDF is usually prepared by radical polymerization of vinylidene fluoride (VDF), which is known to proceed mainly by head-to-tail monomer additions to afford “head” radicals, PVDF . The H• occasional (ca. 3%–5%) head-to-head additions to afford “tail” radicals, PVDFT•, are immediately followed by a tail-to-tail addition to regenerate PVDFH• [6–8]. Reversible-deactivation radical polymerization (RDRP) of VDF has been possible via iodine transfer polymerization (ITP) [3,9–13] and, more recently, by reversible addition fragmentation chain-transfer (RAFT) polymerization [14,15] − and by organometallic-mediated radical polymerization (OMRP) [16,17]. Specifically, OMRP is based on the reversible trapping of the growing radical chain by a transition metal complex to generate an organometallic dormant species [18–21]. ITP and RAFT led to a loss of control once the inverted monomer additions led to the full conversion of the dormant species to the tail isomer, respectively PVDFT-I and PVDFT-xanthate, which are less easily reactivated than the corresponding PVDFH-I and PVDFH-xanthate species. Conversely, the OMRP technique does not suffer from this problem when the controlling complex is [Co(acac)2] because both dormant species are reactivated at similar rates [16], which is in agreement with predictions based on the density functional theory (DFT) calculations of the homolytic bond strengths [22]. Asandei et al. have demonstrated the utility of [Mn2(CO)10] in ITP in the presence of visible light irradiation because the photolytically produced [Mn(CO)5•] radicals are able to abstract the iodine atom from the less reactive PVDFT-I dormant species [3,13]. There is a question, however, about the possible direct trapping of the PVDFH• and PVDFT• chains to form (CO)5Mn-capped organometallic dormant chains and about the possible reactivation of such intermediates. Although the (CO)5Mn-PVDF bonds are predicted to be considerably stronger than the (acac)2Co-PVDF ones [22], their formation and subsequent thermal and/or photochemical reactivation may constitute an additional mechanism for controlling the polymerization, i.e., by OMRP. Therefore, we have recently embarked in synthetic and experimental bond dissociation enthalpy (BDE) determinations for a series of [Mn(RF)(CO)5] complexes where RF stands for CF3 (1), CHF2 (2), CH2CF3 (3), and CF2CH3 (4) with the purposes of (i) providing an experimental benchmark to validate the DFT calculations and (ii) testing the possible action of these complexes as initiators in OMRP [23]. This investigation has revealed that the complex with 1 the highest Mn-RF BDE is 1 for which the kinetic activation enthalpy is ∆Hexp = 53.8 3.5 kcal mol− . 1 ± · This value is very close to the DFT-calculated one (55.1 kcal mol− ). The other determined values are · 1 also in line with the DFT predictions (for 2: ∆Hexp = 46.3 1.6 vs. ∆HDFT = 48.0 kcal mol− ; for 3: 1 ± · ∆Hexp = 50.6 0.8 vs. ∆H = 50.5 kcal mol ). Compound 4 could not be isolated in a sufficiently ± DFT · − pure state for an experimental measurement of the BDE, whereas the DFT calculations suggest that it should contain a homolytically weaker bond (46.0 kcal mol 1)[22]. However, its acyl precursor, · − [Mn(COCF2CH3)(CO)5](5), could be obtained as a pure crystalline solid [23]. The inability to isolate 4 in a pure form may be related to the weaker Mn-CF2CH3 bond. All alkyl compounds were obtained by thermal decarbonylation of the corresponding acyl derivatives. However, while compounds 1, 2, and 3 are sufficiently robust and do not decompose during the decarbonylation step, compound 4 is more fragile. However, acyl derivative 5 is a suitable precursor for the generation of 4, and, thus, of CH3CF2• radicals in situ. Preliminary polymerization tests have confirmed that the Mn-CF3 complex (1) is able to initiate the VDF polymerization under both thermal and photochemical (visible, UV) conditions [23]. This article illustrates in fuller details the ability of complexes 1, 2, 3, and 5 to initiate and attempt controlling the polymerization of VDF and other monomers under thermal and photochemical conditions.

2. Materials and Methods

2.1. Materials Vinylidene fluoride (VDF) and tert-butyl 2-(trifluoromethyl)acrylate (MAF-TBE) were kindly supplied by Arkema and Tosoh Finechem Corp., respectively, and used as received. Complexes 1, 2, 3, 5, and 6 were synthesized as described in a previous contribution [23]. Dimethyl carbonate (DMC, Polymers 2020, 12, 384 3 of 17

99%, Merck KGaA), acetone (VMR Chemicals), pentane (VMR Chemicals), dimethyl formamide ≥ (VMR Chemicals), benzene-d6 (99.5%D, Eurisotop), acetone-d6 (99.5%D, Eurisotop), DMSO-d6 (99.5%D, Eurisotop) and DMF-d7 (99.5%D, Eurisotop) were used as received.

2.2. Instrumentation The Nuclear Magnetic Resonance (NMR) spectra were recorded on a Bruker Avance III HD 1 400 MHz spectrometer. The instrumental parameters were as follows. H NMR: flip angle = 30◦, acquisition time = 5.7 s, pulse delay = 2 s, number of scans = 64, pulse width = 3.05 µs. 19F NMR: flip angle = 30◦, acquisition time = 2 s, pulse delay = 2 s, number of scans = 64, and pulse width = 3.76 µs. The probe has lower background 19F signals compared to standard dual-channel probes. Size- Exclusion Chromatography (SEC) with 0.1 M LiBr/DMF as the eluent, calibrated with poly(methyl methacrylate) (PMMA) standards from Polymer Laboratories, was run with a Varian Prostar (model 210) pump at a flow rate of 0.8 mL min 1 using two 300-mm long, mixed-D PL-gel and 5-µm columns · − (molar mass range: 2 102–4 105 g mol 1 from Polymer Laboratories) thermo-stated at 70 C, connected · · · − ◦ to a Shodex (model RI-101) refractometer detector. The sample concentration was ca. 10 mg mL 1. The · − thermogravimetric analyses (TGA) of the purified and dried (co)polymer samples were performed 1 in air using a TGA 51 apparatus from TA Instruments at a heating rate of 10 ◦C min− from room temperature to 580 ◦C. Differential scanning calorimetry (DSC) analyses of these samples were carried out using a Netzsch DSC 200 F3 instrument under an N2 atmosphere. The DSC instrument was calibrated with noble metals and checked before analysis with an indium sample (Tm = 156.6 ◦C). After its insertion into the DSC apparatus, the sample was initially stabilized at 20 ◦C for 10 min. Then, 1 the first scan was made at a heating rate of 10 ◦C min− up to 200 ◦C. It was then cooled to 50 ◦C. · 1 − Lastly, a second scan was performed at a heating rate of 10 C min up to 200 C. Melting points (Tms) ◦ · − ◦ were evaluated from the second heating, taken at the maximum of the enthalpy peaks, and its area determined the melting enthalpy (∆Hm). This ensured the elimination of the thermal history of PVDFs during the first heating. The degrees of crystallinity of PVDFs were determined using Equation (1).

Degree of crystallinity (χ) = (∆Hm/∆Hc) 100 (1) × 1 where ∆Hc (104.5 J g− ) corresponds to the enthalpy of melting of a 100% crystalline PVDF [24] while 1 ∆Hm is the heat of fusion (determined by DSC in J g− ), respectively.

2.3. VDF Polymerizations with [Mn(RF)(CO)5] The radical polymerizations of VDF were carried out in thick-walled 12 mL Carius tubes. As a representative example, a solution of [Mn(CF3)(CO)5] (128 mg, 0.48 mmol) in 5 mL of dimethyl carbonate was introduced and degassed by three freeze-pump-thaw cycles. Then, the tube was cooled to the liquid nitrogen temperature and gaseous VDF monomer (1.5 g, 23.4 mmol) was transferred using a custom-made manifold that enables the accurate measurement of the gas quantity (using a “pressure drop vs. mass of monomer” calibration curve). The tube was then sealed under static vacuum at the liquid nitrogen temperature. For the thermal activation experiments, the tubes were placed horizontally in a thermostatic shaking water bath. For the visible light activation experiments, the tubes were placed horizontally in a tube roller shaker with 3 LED bulbs (Diall 1102270698, 14 W, 1521 Lm), irradiated from above, and placed 2 cm from the tubes. The proximity of the bulbs caused the tubes to warm up to 40 ◦C. For the UV activation experiments, the tubes were equipped with a small magnetic stirring bar and the solutions were stirred vertically in a Rayonet RPR-200 UV reactor equipped with sixteen 300-nm wavelength UV-lamps of 35 W each. Despite the fan placed inside the UV chamber, the heat generated by the UV lamps caused the tubes to warm up to 50 ◦C. After a certain time (ranging from 2 to 24 h), each tube was frozen into liquid nitrogen, opened, and the solvent was evaporated in a rotary evaporator. The monomer conversion was calculated from the PVDF yields (Equation (2), where mPVDF, mVDF, and mcomplex stand for the recovered mass of PVDF, the initial Polymers 2020, 12, 384 4 of 17

Polymers 2020, 12, 384 4 of 18 mass of VDF, which was obtained from the pressure measurement, and the mass of complex 1 used to initiatemass of the complex polymerization, 1 used respectively)to initiate the since polymerization, an accurate direct respectively) measurement since of thean VDFaccurate conversion direct ismeasurement complicated of by the the VDF gaseous conversion state of is the complic monomer.ated by the gaseous state of the monomer.

ConversionConversion (%) =(%)1 = [m1 – [mPVDF/(m/(mVDF+ +m mcomplex)])] × 100100 (2) − PVDF VDF complex ×

d d d The resultingresulting polymerspolymers werewere dissolveddissolved inin DMF-DMF-d77,, DMSO-DMSO-d66, oror acetone-acetone-d66 (depending on thethe 1 19 solubility)solubility) andand characterizedcharacterized byby 1H and 19F NMR spectroscopy. Molar masses and dispersities werewere determined byby SECSEC inin DMFDMF (refractive(refractive index,index, calibratedcalibrated withwith PMMAPMMA standards).standards). 2.4. Copolymerization of VAcand MAF-TBE 2.4. Copolymerization of VAc and MAF-TBE The radical co-polymerizations of VAc and MAF-TBE were carried out in a Schlenk tube under an The radical co-polymerizations of VAc and MAF-TBE were carried out in a Schlenk1 tube under argon atmosphere. As a representative example, 93.3 mg of [Mn(CO)5(CH2CF3)] (3.35 10− mmol) were an argon atmosphere. As a representative example, 93.3 mg of [Mn(CO)5(CH2CF3)]· (3.35·10−1 mmol) introduced in a Schlenk tube. Then, 1.554 g of VAc (18.1 mmol) and 3.324 g of MAF-TBE (16.9 mmol) were introduced in a Schlenk tube. Then, 1.554 g of VAc (18.1 mmol) and 3.324 g of MAF-TBE (16.9 were added. The resulting solution was degassed by three freeze-pump-thaw cycles, and placed under mmol) were added. The resulting solution was degassed by three freeze-pump-thaw cycles, and argon. The solution was heated up to 80 C for 18 h, and the resulting dark yellow solid was dissolved placed under argon. The solution was heated◦ up to 80 °C for 18 h, and the resulting dark yellow solid in 15 mL of acetone. The polymer was precipitated by adding the solution dropwise to 200 mL of cold was dissolved in 15 mL of acetone. The polymer was precipitated by adding the solution dropwise n-pentane and then the precipitate was dried at 80 ◦C under vacuum for 16 h, which yielded 4.053 g of to 200 mL of cold n-pentane and then the precipitate was dried at 80 °C under vacuum for 16 h, which1 a yellow-brownish solid (83% yield). The polymer was dissolved in acetone-d6 and analyzed by H yielded 4.053 g of a yellow-brownish solid (83% yield). The polymer was dissolved in acetone-d6 and and 19F NMR spectroscopies. The molar masses and dispersities were determined by SEC in DMF analyzed by 1H and 19F NMR spectroscopies. The molar masses and dispersities were determined by (refractive index, calibrated with PMMA standards). SEC in DMF (refractive index, calibrated with PMMA standards). 3. Results 3. Results 3.1. Radical Polymerization of VDF with 1 as an Initiator 3.1. Radical Polymerization of VDF with 1 as an Initiator Several polymerizations of VDF using complex 1 as the initiator were carried out under thermal and photochemicalSeveral polymerizations conditions of (SchemeVDF using1). complex Among 1 all as the the fluoroalkylpentacarbonylmanganese(I)initiator were carried out under thermal complexesand photochemical synthesized conditions in previous (Scheme contributions I). Among [23 ,25all], the compound fluoroalkylpentacarbonylmanganese(I)1 possesses the strongest Mn-C BDE,complexes according synthesized to the computational in previous contributions and experimental [23,25], investigations compound 1 data. possesses Therefore, the strongest it is necessary Mn-C BDE, according to the computational and experimental investigations data. Therefore, it is necessary to apply a relatively high temperature to cleave the Mn-CF3 bond. In the case of thermal activation, to apply a relatively high temperature to cleave the Mn-CF3 bond. In the case of thermal activation, the experiment was run at 100 ◦C, which is a temperature at which the determined half-life in C6D6 the experiment was run at 100 °C, which is a temperature at which the determined half-life in C6D6 is is 37.2 minutes and considers that 99% of the complex decomposes (t99%) after 4.1 h [23]. Once the 37.2 minutes and considers that 99% of the complex decomposes (t99%) after 4.1 h [23]. Once the F3C• F3C• radicals have been generated, their addition to a monomer should be fast. The photochemical initiationradicals have was carriedbeen generated, out either bytheir visible addition light irradiation to a monomer using LEDshould bulbs be or fast. by UVThe irradiation photochemical using ainitiation monochromatic was carried 300 nmout lamp. either by visible light irradiation using LED bulbs or by UV irradiation using a monochromatic 300 nm lamp.

Scheme 1. GeneralsyntheticpathwayemployedtosynthesizePVDFusingfluoroalkylpentacarbonylmanganese(I)I. General synthetic pathway employed to synthesize PVDF using fluoroalkylpentacarbonylmanganese(I)complexes as initiators under thermal or complexes photochemical as conditions.initiators Yunder stands thermal for Mn(CO) or5 photochemicalor H or RF. conditions. Y stands for Mn(CO)5 or H or RF. Control experiments for both thermal and photochemical (visible light) activations carried out in theControl presence experiments of [Mn2(CO) for10 both] instead thermal of 1 anddid notphotochemical yield to any (visible PVDF. Sincelight) [Mnactivations2(CO)10 carried] is known out toin generatethe presence [(CO) of 5[MnMn•2(CO)] radicals10] instead under of both 1 did thermal not yield and to any photochemical PVDF. Since conditions [Mn2(CO)10 [26] is], known it can beto concludedgenerate [(CO) that [(CO)5Mn•]5 Mnradicals•] is not under capable both of thermal adding toand VDF photochemical to initiate the polymerization.conditions [26], Thisit can result be confirmsconcluded previous that [(CO) studies5Mn•] reported is not capable by Bamford of adding et al. to [VDF27] and to initiate by Asandei the polymerization. et al. [13], which This showed result thatconfirms [(CO) previous5Mn•] photoproduced studies reported from by [Mn Bamford2(CO) 10et] al. is unable[27] and to by initiate Asandei the radicalet al. [13], polymerization which showed of VDFthat [(CO) (although5Mn•] it photoproduced does initiate those from of C[Mn2F42(CO)and C10]2 Fis3 Cl)unable [27]. to initiate the radical polymerization of VDF (although it does initiate those of C2F4 and C2F3Cl) [27]. Several polymerizations were carried out with complex 1 as an initiator under thermal and photochemical activations, which stopped the reactions at different times. The reaction conditions

Polymers 2020, 12, 384 5 of 18

and analytical data of the resulting polymers are summarized in Table 1. All reactions were performed targeting a degree of polymerization of 50, expecting a molar mass of ca. 3500 g·mol−1 under the assumption of a well-controlled OMRP [28] with the pentacarbonylmanganese radical as a moderating agent. A first thermal VDF polymerization test (with protection from light) was performed at 50 °C for 24 h (entry 1). Such conditions did not lead to the formation of any PVDF, which means that this Polymers 2020, 12, 384 5 of 17 temperature is too low to homolytically cleave the Mn-CF3 bond in 1. Extrapolation of the previously determined activation parameters to 50 °C leads to a half-life of 9 × 108 s (ca. 29 years) for compound 1 atSeveral this temperature polymerizations [23]. How wereever, carried PVDF out was with produced complex in1 highas an yields initiator (up underto 70% thermal after 24 andh) at photochemicalhigher temperature activations, (100 which°C, entries stopped 2 to the7). reactionsAs mentioned at diff erentabove, times. 100 °C The leads reaction to a conditionsrelatively rapid and analyticalgeneration data of ofF3C the• radicals resulting from polymers 1 in C6D are6 (t summarized½ ≈ 37 min, t99% in Table≈ 4 h)1 .[23] All. reactionsHence, new were polymer performed chains 1 targetingshould be a generated degree of 4 polymerization h after the beginning of 50, of expecting the polymerization a molar massassuming of ca. a similar 3500 gt1/2mol in DMC/VDFunder · − theto assumptionthat determined of a in well-controlled C6D6. OMRP [28] with the pentacarbonylmanganese radical as a moderating agent. Table 1. Results for the VDF polymerization initiated by the thermal or photochemical activations of Table1. 1. Results for the VDF polymerization initiated by the thermal or photochemical activations of 1.

a a bb 1−1 EntryEntry ActivationActivation Method Method ReactionReaction Time Time (h) (h) YieldYield(%) (%) MMnn (g(g·molmol )) Ɖ · − 11 ThermalThermal (50 (50◦C) °C) 2424 00 -- - 22 ThermalThermal (100 (100◦C) °C) 22 66 16,00016,000 1.42 3 Thermal (100 C) 4 15 23,000 1.48 3 Thermal (100◦ °C) 4 15 23,000 1.48 4 Thermal (100 ◦C) 6 38 20,100 1.50 4 Thermal (100 °C) 6 38 20,100 1.50 5 Thermal (100 ◦C) 12 40 ND ND 65 ThermalThermal (100 (100◦C) °C) 1812 4940 NDND ND 76 ThermalThermal (100 (100◦C) °C) 2418 6849 16,900ND 1.53ND 87 Thermal Visible light (100 °C) 224 668 16 ND,900 1.53 ND 9 Visible light 4 14 53,000 1.65 8 Visible light 2 6 ND ND 10 Visible light 6 19 48,300 1.61 119 VisibleVisible light light 244 6014 40,30053,000 1.65 1.47 1210 UVVisible (300 nm) light 26 919 11,00048,300 1.61 1.84 1311 UVVisible (300 nm) light 624 1860 19,20040,300 1.47 1.76 1412 UVUV (300 (300 nm) nm) 242 749 26,00011,000 1.84 1.63 a Determined13 from theUV mass (300 of nm) the isolated polymer. b6Calculated by SEC in18 DMF with refractive19,200 index detection1.76 (calibrated with PMMA standards). ND = not determined. Experimental conditions: 128 mg of [Mn(CF3)(CO)5], 1.5 g of VDF,14 and 5 mLUV of (300 DMC nm) in glass Carius tubes. 24 74 26,000 1.63 a Determined from the mass of the isolated polymer. b Calculated by SEC in DMF with refractive index detection (calibrated with PMMA standards). ND = not determined. Experimental conditions: 128 mg A first thermal VDF polymerization test (with protection from light) was performed at 50 ◦C for 24 h (entryof [Mn(CF 1). Such3)(CO) conditions5], 1.5 g of VDF did, and not 5 lead mL of to DMC the formationin glass Carius of anytubes. PVDF, which means that this temperature is too low to homolytically cleave the Mn-CF bond in 1. Extrapolation of the previously The most interesting result concerning the thermal3 activation is that the polymer yield keeps determined activation parameters to 50 C leads to a half-life of 9 108 s (ca. 29 years) for compound 1 increasing beyond 4 h of reaction and◦ even after 18 h (e.g., from× 49% after 18 h, entry 6, to 68% after at this temperature [23]. However, PVDF was produced in high yields (up to 70% after 24 h) at higher 24 h, entry 7). This result indicates either the intervention of a reversible PVDF• trapping by temperature (100 ◦C, entries 2 to 7). As mentioned above, 100 ◦C leads to a relatively rapid generation [Mn(CO)5•] (i.e. an OMRP mechanism) or a much longer initiator half-life in the DMC/VDF medium of F3C• radicals from 1 in C6D6 (t 1 37 min, t99% 4 h) [23]. Hence, new polymer chains should be relative to C6D6. The dispersities2 of≈ the recovered≈ polymers continuously increased as a function of generated 4 h after the beginning of the polymerization assuming a similar t in DMC/VDF to that conversion, which is consistent with a relatively slow initiator decomposition,1/2 whereas the molar determined in C D . masses initially6 increased,6 and then decreased (entries 3–7). These characteristics indicate poor The most interesting result concerning the thermal activation is that the polymer yield keeps control, although the presence of some degree of OMRP trapping cannot be excluded. increasing beyond 4 h of reaction and even after 18 h (e.g., from 49% after 18 h, entry 6, to 68% after 24 h, The polymerization initiation was also very effective under photochemical conditions. Initiation entry 7). This result indicates either the intervention of a reversible PVDF trapping by [Mn(CO) ] by visible light led to PVDF in high yields (60% after 24 h, entry 11), as well• as by UV irradiation (74%5• (i.e. an OMRP mechanism) or a much longer initiator half-life in the DMC/VDF medium relative to after 24 h, entry 14). It is worth noting that the measured reaction temperature, self-regulated by heat C D . The dispersities of the recovered polymers continuously increased as a function of conversion, released6 6 by the LED bulbs, was 40 °C during the experiment, and 50 °C in the UV reactor. However, which is consistent with a relatively slow initiator decomposition, whereas the molar masses initially entry 1 proves that this temperature is not sufficient to release radicals. Thus, it is confirmed that increased, and then decreased (entries 3–7). These characteristics indicate poor control, although the visible light and UV irradiation lead to Mn-CF3 bond cleavage to produce F3C• radicals. For these presence of some degree of OMRP trapping cannot be excluded. experiments, a tendency for a slight decrease of the polymer dispersities as a function of conversion The polymerization initiation was also very effective under photochemical conditions. Initiation was observed. However, visible light activation, contrary to the UV one, induces a decrease of the by visible light led to PVDF in high yields (60% after 24 h, entry 11), as well as by UV irradiation (74% molar masses with conversion. Their values are much greater than expected based on a controlled after 24 h, entry 14). It is worth noting that the measured reaction temperature, self-regulated by heat released by the LED bulbs, was 40 ◦C during the experiment, and 50 ◦C in the UV reactor. However, entry 1 proves that this temperature is not sufficient to release radicals. Thus, it is confirmed that visible light and UV irradiation lead to Mn-CF3 bond cleavage to produce F3C• radicals. For these experiments, a tendency for a slight decrease of the polymer dispersities as a function of conversion was observed. However, visible light activation, contrary to the UV one, induces a decrease of the molar masses with conversion. Their values are much greater than expected based on a controlled Polymers 2020, 12, 384 6 of 18 Polymers 2020, 12, 384 6 of 17

polymerization. This may be caused by a lower initiation efficiency or slower Mn-CF3 homolytic cleavage under visible irradiation than under UV or at 100 °C. Thus, it appears that generation of the polymerization. This may be caused by a lower initiation efficiency or slower Mn-CF3 homolytic {[Mn(CO)5•], PVDF•} radical pair is not followed by efficient trapping to produce an organometallic cleavage under visible irradiation than under UV or at 100 ◦C. Thus, it appears that generation of the dormant species. Rather, radicals escape from the solvent cage and subsequent uncontrolled chain {[Mn(CO)5•], PVDF•} radical pair is not followed by efficient trapping to produce an organometallic dormantgrowth and species. termination Rather, radicalsprocesses escape predominantly from the solventoccur. cage and subsequent uncontrolled chain growthThe and polymerization termination processes kinetics predominantly are close to occur.first-order and confirm that the polymerization proceedsThe polymerization with an approximately kinetics are constant close to radi first-ordercal concentration and confirm (Figures that the 1, polymerizationS1 and S2). proceeds with an approximately constant radical concentration (Figure1, Figure S1 and Figure S2).

Figure 1. First-order kinetics plot for the polymerization of VDF in the presence of [Mn(CF3)(CO)5] Figure 1. First-order kinetics plot for the polymerization of VDF in the presence of [Mn(CF3)(CO)5] initiated thermally (100 ◦C). initiated thermally (100 °C). The size exclusion chromatograms of PVDF samples obtained by thermal activation (Figure S3) exhibitThe two size di ffexclusionerent populations: chromatograms the main of PVDF one displayssamples obtained a significant by thermal molar massactivation increase (Figure at the S3) beginningexhibit two (entries different 2–3) andpopulations: then a decrease the main (entries one displays 3–7), which a signifi provescant the molar lack of mass control increase during at the the reaction.beginning The (entries negative 2–3) signal and then in the a decrease size exclusion (entries chromatograms 3–7), which proves results the from lack theof control lower refractiveduring the indexreaction. of PVDF The negative relative signal to the in DMF the eluentsize exclusio [5,29,30n chromatograms]. The signal of results the second from populationthe lower refractive at low molarindex mass of PVDF can berelative attributed to the to DMF oligomers eluent arising[5,29,30]. from The transfer signal of reactions. the second Thus, population the behavior at low of molar this polymerizationmass can be attributed is a conventional to oligomers radical polymerization.arising from transfer This isreactions. also confirmed Thus, bythe an behavior increase ofof thethis dispersitypolymerization accompanied is a conventional by a decrease radical of the polymerization. average molar massesThis is also (Figure confirmed S7), as frequentlyby an increase observed of the indispersity conventional accompanied radical polymerization. by a decrease of This the behavioraverage molar is also masses likely caused (Figure by S7), hydrogen as frequently atom observed transfer fromin conventional DMC, which radical stops polymerization. the growing macroradicals This behavior to is yield also deadlikely PVDFcaused chains by hydrogen and generates atom transfer new from DMC, which stops the growing macroradicals to yield dead PVDF chains and generates new radicals that can create new chains initiated by the H3COC(O)OCH2• radical. However, despite the lackradicals of control, that can a high create conversion new chains (68%) initiated was reachedby the H after3COC(O)OCH 24 h, which2• radical. establishes However, complex despite1 as anthe effilackcient of initiatorcontrol, ofa high VDF conversion polymerization. (68%) was reached after 24 h, which establishes complex 1 as an efficientThe size initiator exclusion of VDF chromatograms polymerization. in Figure S4 show the PVDF mass evolution during the visible light initiation.The size Asexclusion above, thechromatograms average molar in massFigure decreases S4 show (compare the PVDF entries mass 9 andevolution 13). The during plots ofthe molarvisible masses light initiation. and dispersities As above, versus the conversion average mola of ther mass polymerization decreases (compare carried out entries under 9 visibleand 13). light The activationplots of molar (entries masses 9, 10, and dispersities 11, Table1) areversus represented conversion in Figureof the S8.polymerization Molar masses carried decrease out withunder conversionvisible light such activation as in a conventional (entries 9, 10, radical and polymerization.11, Table 1) are Surprisingly,represented in it shouldFigure beS8. noted Molar that masses the dispersitydecrease slightlywith conversion decreases such with as time. in a conventional radical polymerization. Surprisingly, it should be notedThe that SEC the traces dispersity of the slightly crude polymers decreases obtained with time. by UV irradiation are represented in Figure S5. The evolutionThe SEC of traces the number of the ofcrude average polymers molar obtained masses and by dispersities UV irradiation versus are VDF represented conversion in (FigureFigure 2S5.) clearlyThe evolution shows an of increase the number of the of molaraverage mass molar and masses a decrease and dispersities of dispersity versus during VDF the conversion polymerization. (Figure Both2) clearly parameters shows evolve an increase significantly of the molar at the mass beginning and a ofdecrease the polymerization. of dispersity during This evolution the polymerization. seems less pronouncedBoth parameters at the evolve end of thesignificantly reaction. at In the addition, beginning the initially of the polymerization. present low molar This mass evolution population seems graduallyless pronounced disappears. at the The end observed of the trends reaction. in this In polymerizationaddition, the initially are suggestive present oflow a certain molar levelmass controlpopulation for an gradually RDRP. However, disappears. the The final observed polymer trends dispersity in this was polymerization relatively high are (Ð suggestive= 1.63) while of a certain level control for an RDRP. However, the final polymer dispersity was relatively high (Ð =

Polymers 2020, 12, 384 7 of 17 Polymers 2020, 12, 384 7 of 18

1 1.63)the M whilen (26,000 the gMmoln (26,000) was g·mol much−1) lower was thanmuch that lower obtained than that from obtained the visible from light the activation, visible light and · − activation,significantly and higher significantly than that higher observed than with that thermalobserved initiation with thermal (Figure initiation S6). In addition,(Figure S6). the In M addition,n growth theand M then growth dispersity and dropthe dispersity were not drop linear, were which not suggestslinear, which a certain suggests control a certain mainly control at the mainly beginning at the of beginningthe polymerization. of the polymerization.

Figure 2. Plots of the number of average molar masses and dispersities vs. conversion of VDF Figure 2. Plots of the number of average molar masses and dispersities vs. conversion of VDF polymerization initiated by UV irradiation in the presence of [Mn(CF3)(CO)5]. polymerization initiated by UV irradiation in the presence of [Mn(CF3)(CO)5]. 3.2. NMR Characterization of PVDFs 3.2. NMR Characterization of PVDFs The 1H NMR spectra of the recovered polymers after 24 h for the three different initiation methods (FigureThe3 ,1 FigureH NMR S9, spectra Figure of S11 the and recovered Figure S13) polymers exhibit after the characteristic 24 h for the PVDFthree signalsdifferent centered initiation at methods2.4 (minor) (Figure and 3.0 3, (major)S9, S11 and ppm, S13) assigned exhibit to the the ch reversearacteristic -CF2C PVDFH2-C Hsignals2CF2- (tail-to-tail)centered at and2.4 (minor) regular 2 -CHand 23.0CF 2(major)-CH2CF ppm,2- monomer assigned additions, to the reverse respectively -CF2 [C3H,62,13-C,H312CF,322].- The(tail-to-tail) triplet ( andJHF =regular55 Hz) -CH of triplets2CF2- 3 2 3 C( HJHH2CF=2- 7monomer Hz) at δ additions,6.44 ppm respectively is attributed [3,6,13,31,32]. to the proton The in triplet the -CH ( J2HF-CF = 552H Hz)chain-ends of triplets generated ( JHH = 7 Hz)by hydrogen at δ 6.44 ppm transfer is attributed from the solvent,to the proton or by backbiting,in the -CH2-CF which2H chain-ends both involve generated the main by (-CH hydrogen2-CF2•) • transfermacroradical from [the33]. solvent, Similarly, or this by hydrogenbackbiting, transfer which to both the minorinvolve -CF the2-CH main2• macroradical (-CH2-CF2 ) macroradical is responsible 3 • [33].for the Similarly, observed this triplet hydrogen ( JHF = 18transfer Hz) centered to the atminorδ 1.85 -CF ppm,2-CH which2 macroradical is assigned to is the responsible methyl end-group for the 3 observedin -CF2-C tripletH3 [6, 13( J,HF31 ].= 18 From Hz) thecentered relative at δ signal 1.85 ppm, integrals, which it is seems assigned that to the the hydrogen methyl end-group transfer rates in - CFare2-C lowerH3 [6,13,31]. for thepolymerization From the relative carried signal out integrals, under visible it seems light that and the UV hydrogen light activation transfer compared rates are lowerto the for thermal the polymerization initiation, according carried to out the under different visible measured light and molar UV light masses activation (entries compared 11, 14, and to 7the in 1 thermalTable1,M initiation,n = 40300, according 26,000, andto the 16,900 different g mol measured, respectively). molar masses The transfer(entries side-reaction11, 14, and 7 in leads Table to 1, a · − Mdecreasen = 40300, of the26,000, number and 16,900 of average g·mol molar−1, respectively). mass and to The an increasetransfer side-reaction of dispersity, asleads observed to a decrease in entries of the3–7. number In addition, of average two relatively molar mass intense and resonances to an increase at δ 4.38 of dispersity, and 3.79 ppm as observ in an approximateed in entries 2:33–7. ratio In addition,may be assigned two relatively to the –CFintense2-CH resonances2-CH2-OC(O)O-C at δ 4.38H and3 chain 3.79ends, ppm whichin an approximate results from 2:3 a chain ratio transfermay be assignedto the DMC to the solvent –CF2 [-CH14] because2-CH2-OC(O)O-C their integratedH3 chain intensities ends, which are consistentresults from with a chain that of transfer the CH 2toCF the2H DMCsignal. solvent This is [14] simultaneously because their generated integrated by intensities the same are process consistent (1:2:3 with for the thatδ 6.44, of the 4.38, CH and2CF2 3.79H signal. ppm Thisresonances, is simultaneously respectively) generated [6,14]. by the same process (1:2:3 for the δ 6.44, 4.38, and 3.79 ppm resonances, respectively) [6,14].

Polymers 2020, 12, 384 8 of 17 Polymers 2020, 12, 384 8 of 18

Figure 3. 1H NMR spectrum (400 MHz, DMF-d ) between 0 and 10 ppm of the PVDF obtained by Figure 3. 1H NMR spectrum (400 MHz, DMF-d7) between 0 and 10 ppm of the PVDF obtained by thermalthermal activationactivation ofof 11 (entry(entry 77 ofof TableTable1 1).). The The full full spectrum spectrum is is shown shown in in the the SI SI (Figure (Figure S9). S9). TheThe starredstarred resonancesresonances areare duedue toto thethe solvent.solvent. The 19F NMR spectra of the PVDF obtained from the three activation methods (Figures S10, S12, The 19F NMR spectra of the PVDF obtained from the three activation methods (Figures S10, S12, and S14) also exhibit all the expected resonances of PVDF [6,13,14,31,32,34,35]. The characteristic signal and S14) also exhibit all the expected resonances of PVDF [6,13,14,31,32,34,35]. The characteristic centered at 91.9 ppm (a in Figure4) is assigned to the regular -CH 2CF2-CH2CF2- (head-to-tail, H-T) signal centered− at −91.9 ppm (a in Figure 4) is assigned to the regular -CH2CF2-CH2CF2- (head-to-tail, sequences. The head-to-head (H-H, -CH2CF2-CF2CH2-) sequences, which are systematically followed H-T) sequences. The head-to-head (H-H, -CH2CF2-CF2CH2-) sequences, which are systematically by tail-to-tail (T-T, -CF2CH2-CH2CF2-) sequences, give rise to the resonances at 113.9 and 116.2 followed by tail-to-tail (T-T, -CF2CH2-CH2CF2-) sequences, give rise to the resonances− at −113.9− and ppm (c and d, respectively). The percentage of H-H additions, obtained from the integrals of the 19F −116.2 ppm (c and d, respectively). The percentage of H-H additions, obtained from the integrals of NMR signals (for the 24-hour-reaction, the fractions are 3.9%, 3.5% and 3.9% for the thermal, visible the 19F NMR signals (for the 24-hour-reaction, the fractions are 3.9%, 3.5% and 3.9% for the thermal, light and UV initiations, respectively) is typical for a PVDF produced by radical polymerization [36]. visible light and UV initiations, respectively) is typical for a PVDF produced by radical The F3C• radicals generated from 1 should preferentially attack the VDF tail (CH2), as described in polymerization [36]. The F3C• radicals generated from 1 should preferentially attack the VDF tail previous studies [32,34,37–39]. The head-end attack was reported to occur at less than 3% frequency (CH2), as described in previous studies [32,34,37–39]. The head-end attack was reported to occur at at 150 C[37]. Thus, the use of this radical should not significantly affect the percentage of inverted less than◦ 3% frequency at 150 °C [37]. Thus, the use of this radical should not significantly affect the monomer additions. The CF3-CH2-CF2- chain-end produced by the initiation of the F3C• radical could percentage of inverted monomer additions. The CF3-CH2-CF2- chain-end produced by the initiation be shown by a small signal at δ –60.8 ppm, which confirms the previous studies [32,34,39], but only of the F3C• radical could be shown by a small signal at δ –60.8 ppm, which confirms the previous for the PVDF obtained by UV activation despite the high molar mass of the polymer. On the other studies [32,34,39], but only for the PVDF obtained by UV activation despite the high molar mass of hand, the expected signals for the chain ends generated by the transfer processes are always visible, the polymer. On the other hand, the expected signals for the chain ends generated by the transfer as displayed in the spectra (g and h in Figure4 and Figure S14, h and i in Figure S12). In addition, processes are always visible, as displayed in the spectra (g and h in Figures 4 and S14, h and i in several unidentified small signals are present at δ 150.0, 99.0, 98.3, and 85.0 ppm in the case of Figure S12). In addition, several unidentified small− signals− are present− at δ −−150.0, −99.0, −98.3, and the thermal activation, and an even smaller one at δ 85.0 ppm for the UV activation, which suggests −85.0 ppm in the case of the thermal activation, and− an even smaller one at δ −85.0 ppm for the UV that side-reactions occurred during the VDF polymerization carried out under these conditions. These activation, which suggests that side-reactions occurred during the VDF polymerization carried out signals were not observed for the polymers obtained by visible light initiation. It is important to under these conditions. These signals were not observed for the polymers obtained by visible light underline that the 19F NMR resonance of 1 (expected at δ 5.65 ppm) [23] was not observed in any of initiation. It is important to underline that the 19F NMR resonance of 1 (expected at δ 5.65 ppm) [23] the isolated PVDF products, which indicates full consumption of this compound. was not observed in any of the isolated PVDF products, which indicates full consumption of this compound.

Polymers 2020, 12, 384 9 of 17 Polymers 2020, 12, 384 9 of 18 Polymers 2020, 12, 384 9 of 18

19 Figure 4. F19 NMR spectrum (376.5 MHz, DMF-d7) between −75 and −125 ppm of the PVDF obtained FigureFigure 4.4. 19F NMR spectrum (376.5 MHz,MHz, DMF-DMF-dd77)) betweenbetween −75 and −125 ppm of the PVDF obtainedobtained by thermal activation of 1 (entry 5 in Table 1). The full spectrum− is shown− in Figure S10. The starred byby thermalthermal activationactivation ofof 11 (entry(entry 55 inin TableTable1 ).1). The The full full spectrum spectrum is is shown shown in in Figure Figure S10. S10. The The starred starred resonances could not be attributed to any expected signal of possible products. resonancesresonances couldcould notnot bebe attributedattributed toto anyany expectedexpected signalsignal ofof possiblepossible products.products. 3.3.3.3. Thermal Thermal Properties Properties of ofthe the Resulting Resulting PVDFs PVDFs 3.3. Thermal Properties of the Resulting PVDFs TheThe thermal thermal properties properties of of the the PVDF PVDF samples samples ob obtainedtained by by thermal or photochemicalphotochemical activationsactivations of The thermal properties of the PVDF samples obtained by thermal or photochemical activations ofcompound compound1 after1 after a 24 ah-reaction 24 h-reaction was studiedwas studied by thermogravimetric by thermogravimetric analysis analysis (TGA) and (TGA) differential and of compound 1 after a 24 h-reaction was studied by thermogravimetric analysis (TGA) and differentialscanning calorimetryscanning calorimetry (DSC). Figure (DSC).5 displays Figure the 5 displays TGA thermograms the TGA thermograms of three PVDFs of achievedthree PVDFs from differential scanning calorimetry (DSC). Figure 5 displays the TGA thermograms of three PVDFs achievedvarious initiationsfrom various while initiations Table2 summarizes while Table the 2 summarizes temperatures the at temperatures which 2%, 5%, at and which 10% 2%, weight 5%, lossesand achieved from various initiations while Table 2 summarizes the temperatures at which 2%, 5%, and 10%are weight noted. losses are noted. 10% weight losses are noted.

FigureFigure 5. 5.TGATGA thermograms thermograms in in the the air air of of PVDFs PVDFs obtained obtained by by polymerization polymerization of VDF in the presencepresence of Figure 5. TGA thermograms in the air of PVDFs obtained by polymerization of VDF in the presence of [Mn(CF3)(CO)5] after a 24 h-reaction and initiated thermally (full line), by visible (dashed line) or [Mn(CF3)(CO)5] after a 24 h-reaction and initiated thermally (full line), by visible (dashed line) or UV of [Mn(CF3)(CO)5] after a 24 h-reaction and initiated thermally (full line), by visible (dashed line) or UV(dotted (dotted line) line) light. light. UV (dotted line) light.

PolymersPolymers 20202020, ,1212, ,384 384 1010 of of 18 17

Table 2. Temperatures of decomposition (in air) of PVDFs synthesized with [Mn(CF3)(CO)5] after a 24Table h-reaction 2. Temperatures (2%, 5%, and of decomposition10% wt. loss). (in air) of PVDFs synthesized with [Mn(CF3)(CO)5] after a 24 h-reaction (2%, 5%, and 10% wt. loss). Activation Method Mn (g·mol−1) T2% T5% T10% 1 Activation Method Mn (g mol ) T2% T5% T10% Thermal (100 °C) 16,900· − 343 391 424 ThermalVisible (100 ligh◦C)t 40 16,900,300 412 343 444 391 459 424 Visible light 40,300 412 444 459 UVUV (300 (300 nm) nm) 26,000 26,000 364 364 414 414 441 441

As shown in Table 2, the higher the molar mass of PVDF is, the higher its thermal stability is. As shown in Table2, the higher the molar mass of PVDF is, the higher its thermal stability is. The polymer synthesized under visible light, with the highest molar mass (Mn = 40,300 g mol−1), 1 exhibitsThe polymer a 2% weight synthesized loss at under the highest visible temperat light, withure the (412 highest °C), whereas molar massthat obtained (Mn = 40,300 from gthermal mol− ), exhibits a 2% weight loss at the highest temperature (412 C), whereas that obtained from thermal initiation, with the lowest molar mass (Mn = 16,900 g mol−1) shows◦ the same weight loss at the lowest 1 temperatureinitiation, with (343 the °C). lowest molar mass (Mn = 16,900 g mol− ) shows the same weight loss at the lowest temperatureOn the other (343 ◦hand,C). as expected, all DSC thermograms of the synthesized PVDFs (Figure 6) did On the other hand, as expected, all DSC thermograms of the synthesized PVDFs (Figure6) did not not exhibit any glass transition (Tg is expected at −40 °C) [1,4]. However, the thermograms revealed exhibit any glass transition (Tg is expected at 40 C) [1,4]. However, the thermograms revealed the the crystalline phase melting at higher temperatures− ◦ for the higher molar masses (reaching 175 °C, whichcrystalline is in good phase agreement melting at with higher previous temperatures reports for [4] the for higher the PVDF molar obtained masses by (reaching visible 175light◦C, initiation; which is seein goodTable agreement 3). The highest with previous molar mass reports polymer [4] for also the PVDF showed obtained the highest by visible degree light of initiation; crystallinity see Table(53%),3). whereasThe highest the lower molar molar mass polymermass polymers also showed obtained the by highest thermal degree or UV of activations crystallinity were (53%), less whereas crystalline the (44%).lower molar mass polymers obtained by thermal or UV activations were less crystalline (44%).

Figure 6. DSC thermograms of PVDFs synthesized in the presence of complex 1 after a 24 h-reaction Figure 6. DSC thermograms of PVDFs synthesized in the presence of complex 1 after a 24 h-reaction with different activation methods. with different activation methods. Table 3. Molar masses and thermal characteristics of PVDFs obtained by polymerization of VDF in the Tablepresence 3. Molar of complex masses1 fromand thermal different characteristics activation methods. of PVDFs obtained by polymerization of VDF in the presence of complex 1 from different activation methods. Activation Mn Melting Point Enthalpy of Degree of 1 1 ActivationMethod (gMnmol ) Melting(◦ PointC) EnthalpyFusion (J g of ) CrystallinityDegree *of (%) · − · − ThermalMethod (100 ◦C)(g·mol 16,900−1) (°C) 164 Fusion 46.1 (J·g−1) Crystallinity 44 * (%) ThermalVisible (100 light °C) 16,900 40,300 164 175 46.1 55.3 53 44 UV (300 nm) 26,000 171 45.9 44 Visible light 40,300 175 55.3 53 * Assessed from Equation (1). UV (300 nm) 26,000 171 45.9 44 * Assessed from Equation (1). 3.4. Radical Polymerization of VDF with 2, 3, and 5 as Initiator 3.4. Radical Polymerization of VDF with 2, 3, and 5 as Initiator Compound [Mn(CHF2)(CO)5](2) has a weaker Mn-C bond dissociation energy (BDE) than 1 Compound [Mn(CHF2)(CO)5] (2) has a weaker Mn-C bond dissociation energy (BDE) than 1 and and even weaker than [Mn(CH2CF3)(CO)5](3), as confirmed by both the experimentally determined even weaker than [Mn(CH2CF3)(CO)5] (3), as confirmed by both the experimentally determined and the calculated BDEs [23]. Thus, the radical generation should require less energy compared to

Polymers 2020, 12, 384 11 of 17 and the calculated BDEs [23]. Thus, the radical generation should require less energy compared to compound 1. Therefore, 80 ◦C was chosen as the reaction temperature to initiate the polymerization by the thermal activation mode (t 1 46 min and t99% 5 h for 2 at 80 ◦C in C6D6)[23]. This temperature 2 ≈ ≈ was also selected for complex [Mn(COCF2CH3)(CO)5](5), which is expected [23] to undergo thermal decarbonylation and to form the alkyl complex [Mn(CF2CH3)(CO)5](4) in-situ before generating CH3CF2• radicals. In the case of complex 3, 90 ◦C was chosen as the reaction temperature due to its slightly higher BDE compared to complex 2 (t 1 21.6 min and t99% 2.4 h at 90 ◦C in C6D6)[23]. 2 ≈ ≈ The experimental conditions and the results are presented in Table4. The thermal activation of 2 at 80 ◦C for 24 h (entry 1 in Table4) led only to a small amount of a brown-yellowish powder. However, this material does not contain any PVDF, as shown by the absence of observable resonances in the 19F NMR spectrum. Photochemical initiation, on the other hand, led to PVDF formation, which is only in very low yields (entries 2 and 3 of Table4). This is much lower than for compound 1 under the same conditions. Similar low yields were obtained by thermally initiating the polymerization with complex 3 (entries 4–6). Photoactivation experiments were not carried out with this complex. In this case, the polymers were analyzed by SEC and found to have high molar masses, which is essentially independent of the monomer/initiator ratio and of the activation temperature (90 or 100 ◦C), which are quite similar to those of the polymers obtained in much higher yields using 1 as the initiator. However, these polymers surprisingly show low dispersities likely because of the very small quantities of the polymer that could be injected in the SEC apparatus (the signal/noise ratios were very close to unity in these cases). The polymerizations initiated by complex 5 led to slightly greater polymer yields, but still much lower than those initiated by complex 1. Thermal activation gave only a 7% yield after 24 h (entry 7, Table4), while visible light (entries 8–11) and UV light (entries 12–15) irradiation gave yields that increased with irradiation time up to 23% or 21%, respectively, after 24 h. The molar masses of these polymers, like those obtained from initiation with 1, are high and the dispersities are moderate. Once again, this suggests the absence of a controlled chain growth of the type expected from OMRP. Since we have established that the Mn-RF bond is homolytically weaker in compounds 2, 3, and 4 than in compound 1 [22,23], the lower polymer yield obtained with 2, 3, and 5, at least under thermal activation conditions, cannot be attributed to a reduced primary radical flux. Rather, it appears that the CHF2•, CF3CH2•, and CH3CF2• radicals (the latter being generated after decarbonylation of 5 to form 4 in situ) are not able to efficiently add to VDF. The 19F NMR spectra of the recovered PVDF products display the usual features in terms of the monomer addition mode. Those obtained with initiation by photolytic activation of 2 (visible light, entry 2, Table4, Figure S15, UV light, entry 3, Figure S16) show the presence of residual 2 in the polymer (resonance at δ 71.6 ppm), [23] particularly for the visible light irradiation experiment, − which indicates a slow Mn-CHF2 bond photocleavage. Clearly, the efficiency of this process does not correlate with the BDE because the Mn-RF bond is weaker for 2 than for 1, whereas compound 1 is a quite efficient photo-initiator (vide supra). A relatively intense resonance at δ 114.4 ppm is assigned − to the CHF2 chain-ends. Integration of this signal relative to the main PVDF resonance indicates that the obtained product is an oligomer with 8 monomer units. This result is similar for entries 2 and 3. This chain end likely corresponds to the primary radical produced by the initiator because there is no reason why the H atom transfer from DMC onto the PVDFH macroradical, which leads to the same function, would be more frequent for the 2-initiated polymerization than for that initiated by 1 (cf. with the intensity of the same resonance in Figures S12 and S14). In addition, two more resonances were detected in the sample and resulted from the visible light activation (Figure S15) at δ 75.9 and − 74.7 ppm. These may correspond to a PVDF-CH = CF chain-end (cf. δ 72.2 and 73.4 ppm vs. − 2 − − acetone-d6 according to a previous report) [40]. Polymers 2020, 12, 384 5 of 18

and analytical data of the resulting polymers are summarized in Table 1. All reactions were performed targeting a degree of polymerization of 50, expecting a molar mass of ca. 3500 g·mol−1 under the assumption of a well-controlled OMRP [28] with the pentacarbonylmanganese radical as a moderating agent. A first thermal VDF polymerization test (with protection from light) was performed at 50 °C for 24 h (entry 1). Such conditions did not lead to the formation of any PVDF, which means that this temperature is too low to homolytically cleave the Mn-CF3 bond in 1. Extrapolation of the previously determined activation parameters to 50 °C leads to a half-life of 9 × 108 s (ca. 29 years) for compound 1 at this temperature [23]. However, PVDF was produced in high yields (up to 70% after 24 h) at higher temperature (100 °C, entries 2 to 7). As mentioned above, 100 °C leads to a relatively rapid generation of F3C• radicals from 1 in C6D6 (t½ ≈ 37 min, t99% ≈ 4 h) [23]. Hence, new polymer chains should be generated 4 h after the beginning of the polymerization assuming a similar t1/2 in DMC/VDF

Polymers 2020, 12, 384 to that determined in C6D6. 12 of 17

Table 1. Results for the VDF polymerization initiated by the thermal or photochemical activations of Table 4. Experimental conditions1. and results of the polymerization of VDF (1.5 g) in the presence of [Mn(RF)(CO)5] initiators.

a a b b 1 −1 Entry Complex Activation MethodEntry ReactionActivation Time Method (h) YieldReaction(%) Time [VDF] (h)/[Mn(CO) Yield5 R (%)F] MMn n (g (g·molmol ) ) Ɖ · − 1 Thermal (50 °C) 24 0 - - 1 2 Thermal (80 ◦C) 24 0 50 - - 2 2 hν (visible light)2 Thermal 24 (100 °C) 22 506 ND16,000 1.42 ND 3 2 hν (UV 300 nm)3 Thermal 24 (100 °C) 54 5015 ND23,000 1.48 ND 4 3 Thermal (90 ◦C)4 Thermal 24 (100 °C) 26 5038 32,20020,100 1.50 1.11 5 3 Thermal (90 C) 24 3 100 32,200 1.16 ◦ 5 Thermal (100 °C) 12 40 ND ND 6 3 Thermal (100 ◦C) 24 3 100 31,600 1.15 6 Thermal (100 °C) 18 49 ND ND 7 5 Thermal (80 ◦C) 24 7 50 16,200 1.38 8 5 hν (visible light)7 Thermal 4 (100 °C) 324 5068 12,60016,900 1.53 1.48 9 5 hν (visible light)8 Visible 8 light 82 506 22,500ND 1.44ND 10 5 hν (visible light)9 Visible 12 light 134 5014 24,00053,000 1.65 1.40 11 5 hν (visible light) 24 23 50 23,000 1.45 10 Visible light 6 19 48,300 1.61 12 5 hν (UV 300 nm) 4 16 50 11,100 1.57 13 5 hν (UV 300 nm)11 Visible 8 light 1824 5060 10,00040,300 1.47 1.69 14 5 hν (UV 300 nm)12 UV 12(300 nm) 202 509 950011,000 1.84 1.81 15 5 hν (UV 300 nm)13 UV 24(300 nm) 216 5018 700019,200 1.76 1.94 a Determined from the mass of the obtained polymer. b Calculated14 by SECUV in (300 DMF nm) by a refractive index (calibrated24 with PMMA74 standards). ND26 stands,000 for “not1.63 determined.” a Determined from the mass of the isolated polymer. b Calculated by SEC in DMF with refractive index detection (calibrated with PMMA standards). ND = not determined. Experimental conditions: 128 mg of [Mn(CF3)(CO)5], 1.5 g of VDF, and 5 mL of DMC in glass Carius tubes.

The most interesting result concerning the thermal activation is that the polymer yield keeps increasing beyond 4 h of reaction and even after 18 h (e.g., from 49% after 18 h, entry 6, to 68% after 24 h, entry 7). This result indicates either the intervention of a reversible PVDF• trapping by [Mn(CO)5•] (i.e. an OMRP mechanism) or a much longer initiator half-life in the DMC/VDF medium relative to C6D6. The dispersities of the recovered polymers continuously increased as a function of conversion, which is consistent with a relatively slow initiator decomposition, whereas the molar masses initially increased, and then decreased (entries 3–7). These characteristics indicate poor control, although the presence of some degree of OMRP trapping cannot be excluded. The polymerization initiation was also very effective under photochemical conditions. Initiation by visible light led to PVDF in high yields (60% after 24 h, entry 11), as well as by UV irradiation (74% after 24 h, entry 14). It is worth noting that the measured reaction temperature, self-regulated by heat released by the LED bulbs, was 40 °C during the experiment, and 50 °C in the UV reactor. However, entry 1 proves that this temperature is not sufficient to release radicals. Thus, it is confirmed that visible light and UV irradiation lead to Mn-CF3 bond cleavage to produce F3C• radicals. For these experiments, a tendency for a slight decrease of the polymer dispersities as a function of conversion was observed. However, visible light activation, contrary to the UV one, induces a decrease of the molar masses with conversion. Their values are much greater than expected based on a controlled

Polymers 2020, 12, 384 13 of 17

For the PVDF generated by thermal initiation with compound 3, no initiator resonances (e.g., the 19F NMR resonance, which is expected at δ 52.3 ppm) [23] were evident in the spectra of the − isolated polymer (Figures S17 and S18). Thus, the initiator was fully activated under these conditions. No photoactivation experiments were carried out with this compound. The CF3CH2- chain end was highlighted by a small resonance at δ 60.8 ppm in the 19F NMR spectrum (Figure S18). Compound 5 − was also fully activated under thermal conditions since 19F NMR resonances attributable to either 5 or to its decarbonylation product 4 (expected at δ 91.5 and 28.9 ppm, respectively) [23] were not − − detected (Figure S19).

3.5. Copolymerization of Vinyl Acetate with tert-Butyl 2-(Trifluoromethyl)acrylate Initiated by Complexes 3 and 6 Polymers 2020, 12, 384 14 of 18 The aim of these investigations was to check the initiation capacity by using more reactive monomersof this study, of thein addition complexes to thatcomplex do not 3, e thefficiently acyl complex initiate the [Mn(COCHF VDF polymerization.2)(CO)5] (6), For which the purpose generates of thiscomplex study, 2 in by addition thermal to complex decarbonylation3, the acyl complexin situ [Mn(COCHFwas used instead2)(CO) 5](of6 ),the which isolated generates 2. Four complex co- 2polymerizationsby thermal decarbonylation of vinyl acetate in situ (VAc) was usedand insteadtert-butyl of the2-(trifluoromethyl)acrylate isolated 2. Four co-polymerizations (MAF-TBE) of vinylwere acetatecarried (VAc)out using and tert compounds-butyl 2-(trifluoromethyl)acrylate 3 and 6 as initiators (MAF-TBE)(Scheme II). were This carried copolymer out using was compoundspreviously 3synthesizedand 6 as initiators by conventional (Scheme 2[41]). This OMRP copolymer using cobalt(II) was previously acetylacetonate synthesized as a controlling by conventional agent [[42]41] OMRPand RAFT using polymerization cobalt(II) acetylacetonate [43]. All these as four a controlling co-polymerizations agent [42 led] and to RAFTcopolymers, polymerization which display [43]. All these four co-polymerizations led to copolymers, which display a perfectly alternating structure, a perfectly alternatingPolymers 2020 structure,, 12, 384 in agreement with the measured reactivity ratios that are both close5 of 18 into agreementzero [41]. The with copolymerizations the measured reactivity were ratioscarried that out are only both with close thermal to zero initiation [41]. The and copolymerizations the results are werecollected carried in Table outand only analytical5. with thermal data of initiationthe resulting and polymers the results are are summarized collected in in Table Table5. 1. All reactions were performed targeting a degree of polymerization of 50, expecting a molar mass of ca. 3500 g·mol−1 under the assumption of a well-controlled OMRP [28] with the pentacarbonylmanganese radical as a moderating agent. A first thermal VDF polymerization test (with protection from light) was performed at 50 °C for 24 h (entry 1). Such conditions did not lead to the formation of any PVDF, which means that this temperature is too low to homolytically cleave the Mn-CF3 bond in 1. Extrapolation of the previously determined activation parameters to 50 °C leads to a half-life of 9 × 108 s (ca. 29 years) for compound 1 at this temperature [23]. However, PVDF was produced in high yields (up to 70% after 24 h) at

Scheme 2.higherAlternatingII. Alternating temperature copolymerization (100copolymerization °C, entries of VAc 2 andto 7). MAF-TBE ofAs mentionedVAc initiated and above, byMAF-TBE the 100 [Mn(CH °C leadsinitiated2CF to3)(CO) a relatively 5](by3 ) the rapid • [Mn(CHand2 [Mn(COCHFCF3)(CO)generation5] 2()(CO)3) andof 5F](3 C[Mn(COCHF6) radicals complexes. from2 R)(CO) F1stands in C5]6 D(6 for6) (tcomplexes. fluoroalkyl½ ≈ 37 min, group tR99%F stands≈ 4 and h) X[23] for represents. fluoroalkylHence, Mn(CO)new polymergroup5 or and chains should be generated 4 h after the beginning of the polymerization assuming a similar t1/2 in DMC/VDF X representsH or RF. Mn(CO)5 or H or RF. to that determined in C6D6. Table 5. Experimental conditions and results of the copolymerization of VAc and MAF-TBE thermally Table 5. Experimental conditions and results of the copolymerization of VAc and MAF-TBE thermally initiated with compoundsTable 1. Results6 and for the3. VDF polymerization initiated by the thermal or photochemical activations of initiated with compounds 6 and 3. 1. Mn:VAc: Temperature Reaction [Mn(RF)(CO)5] Mn:VAc:a Yield b Mn −1 Entry Initiator TemperatureEntry Activation Reaction Method [Mn(RReactionF)(CO) Time5] (h)MAF-TBE Yield (%) YieldMn (g·molM1)n Ɖ Entry Initiator (◦C) Time (h) (mg) MAF-TBE (%) (g mol− ) Ɖ 1(°C) ThermalTime (50 (h) °C) (mg) 24 Molar Ratio0 (%) -· (g·mol−1) - 1 6 2 70Thermal (100 72 °C) 53.02 Molar 1:50:50 Ratio6 1716, 58,300000 1.421.53 2 6 85 3 50.5 1:50:50 38 41,300 1.43 1 6 3 70 Thermal (10072 °C) 53.0 4 1:50:5015 17 23,00058,300 1.48 1.53 2 3 6 3 85 70 3 72 22.550.5 1:50:501:50:50 2138 700041,300 1.69 1.43 4 3 4 80Thermal (100 18 °C) 93.36 1:50:5038 8320, 76,000100 1.502.12 3 3 5 70 Thermal (10072 °C) 22.5 12 1:50:5040 21 ND 7000 ND 1.69 4 3 6 80 Thermal (10018 °C) 93.3 18 1:50:5049 83 ND 76,000 ND 2.12 The poly(VAc-alt-MAF-TBE)7 Thermal copolymer (100 °C) was obtained24 in moderate68 to high yields16,900 depending 1.53 on the reactionThe poly(VAc- temperature,alt-MAF-TBE)8 whichVisible proves copolymer light that compoundswas obtained2 3 andin moderate6 can generate6 to high a yields fluxND of depending radicalsND able on tothe initiate reaction a radicaltemperature, polymerization.9 whichVisible proves It light is that noted compounds that the temperatures4 3 and 6 can usedgenerate14 for thesea flux53 polymerizations,000 of radicals1.65 able areto initiate equivalent a radical or lower polymerization.10 than thatVisible of It the lightis noted corresponding that the temperatures6 VDF polymerizations used19 for these (i.e.,48 up,polymerizations300 to 80 ◦1.61C with 1 19 compoundare equivalent3 in or entry lower 4,11 vs. than 100 that◦VisibleC forof the thelight corresponding VDF polymerization).24 VDF polymerizations The H60 and F (i.e., NMR40 up,300 spectra to 80 (Figures°C1.47 with S20compound and S21, 3 in respectively) entry 4,12 vs. 100 of theUV°C for (300 isolated the nm) VDF copolymer polymerization). (Table2 5, The entry 1H 4) and9 are 19F in NMR agreement11 ,spectra000 with (Figures1.84 the previouslyS20 and S21, published respectively) ones13 [ 42of, 44theUV] and (300isolated clearly nm) copolyme show ther perfect(Table6 alternation5, entry 184) ofare comonomers in agreement19,200 since with1.76 no the1H 14 UV (300 nm) 24 74 26,000 1.63 NMRpreviously signal published centered atonesδ 4.9 [42,44] ppm (theand typicalclearly chemicalshow the shiftperfect ofVAc-VAc alternation dyads) of comonomers was observed. since no a Determined from the mass of the isolated polymer. b Calculated by SEC in DMF with refractive index 1H NMR signal centered at δ 4.9 ppm (the typical chemical shift of VAc-VAc dyads) was observed. detection (calibrated with PMMA standards). ND = not determined. Experimental conditions: 128 mg of [Mn(CF3)(CO)5], 1.5 g of VDF, and 5 mL of DMC in glass Carius tubes. 4. Discussion The combinedThe results most interestingof this investigation result concerning show thatthe thermalcompound activation 1 is an is efficientthat the polymerinitiator yieldfor the keeps increasing beyond 4 h of reaction and even after 18 h (e.g., from 49% after 18 h, entry 6, to 68% after polymerization of VDF under both thermal (100 °C) and photochemical (visible or UV light 24 h, entry 7). This result indicates either the intervention of a reversible PVDF• trapping by irradiation) conditions. This occurs by homolytic cleavage of the Mn-CF3 bond and generation of CF3• [Mn(CO)5•] (i.e. an OMRP mechanism) or a much longer initiator half-life in the DMC/VDF medium 2 radicals, whichrelative efficiently to C6D add6. The onto dispersities the VDF of tail the (CH recovered), as reported polymers in continuously previous studies increased [32,34,37–39]. as a function of The head-endconversion, attack was which reported is consistent to occur with at lessa relatively than 3% slow frequency initiator decomposition,at 150 °C [37]. whereasOn the theother molar hand, the CHFmasses2• and initially CF3CH increased,2• radicals, and generated then decreased from compounds (entries 3–7 ).2 and These 3 , characteristicsrespectively, indicateand, to a poor certain extent,control, the CH although3CF2• radical the presence released of some from degree compound of OMRP 4 (whichtrapping is, cannot in turn, be excluded. generated in situ from compound 5The) do polymerization not lead to efficient initiation initiation was also very of the effective VDF underpolymerization. photochemical A lowerconditions. ability Initiation of compounds 2by, 3 visible, and light5 to ledproduce to PVDF enough in high yieldsradical (60% flux after can 24 be h, entryexcluded 11), as as well a possibleas by UV irradiationalternative (74% explanation foraft erthe 24 observedh, entry 14). low It is PVDFworth notingyields that (Table the measured4) because reaction experimental temperature, and self computational-regulated by heat released by the LED bulbs, was 40 °C during the experiment, and 50 °C in the UV reactor. However, studies have shown that the Mn-RF bond is homolytically stronger for 1. Therefore, fluoroalkyl entry 1 proves that this temperature is not sufficient to release radicals. Thus, it is confirmed that radicals can be generated from compounds 2, 3, and 5, and are able to initiate the VDF visible light and UV irradiation lead to Mn-CF3 bond cleavage to produce F3C• radicals. For these experiments, a tendency for a slight decrease of the polymer dispersities as a function of conversion was observed. However, visible light activation, contrary to the UV one, induces a decrease of the molar masses with conversion. Their values are much greater than expected based on a controlled

Polymers 2020, 12, 384 14 of 17

4. Discussion The combined results of this investigation show that compound 1 is an efficient initiator for the polymerization of VDF under both thermal (100 ◦C) and photochemical (visible or UV light irradiation) conditions. This occurs by homolytic cleavage of the Mn-CF3 bond and generation of CF3• radicals, which efficiently add onto the VDF tail (CH2), as reported in previous studies [32,34,37–39]. The head-end attack was reported to occur at less than 3% frequency at 150 ◦C[37]. On the other hand, the CHF2• and CF3CH2• radicals, generated from compounds 2 and 3, respectively, and, to a certain extent, the CH3CF2• radical released from compound 4 (which is, in turn, generated in situ from compound 5) do not lead to efficient initiation of the VDF polymerization. A lower ability of compounds 2, 3, and 5 to produce enough radical flux can be excluded as a possible alternative explanation for the observed low PVDF yields (Table4) because experimental and computational studies have shown that the Mn-RF bond is homolytically stronger for 1. Therefore, fluoroalkyl radicals can be generated from compounds 2, 3, and 5, and are able to initiate the VDF polymerization. However, the yields of the resulting PVDF were rather low. Furthermore, the acyl precursor 6 (which yields 2 in situ by thermal decarbonylation) and 3 were shown to efficiently initiate the radical alternating copolymerization of VAc and MAF-TBE. The effect of the radical and alkene substituents on the rate of the addition reaction has received some attention [45,46]. As pioneered by Tedder and Walton [37,45,46], Sokolov et al. [47] reported the reactivity of F3C• and other perfluorinated radicals onto several fluorinated olefins and ethylene, concluding that the reactivity of F3C• decreases with the number of fluorine atoms on the monomer (e.g., addition of F3C• onto ethylene is almost 15 times faster than on VDF). However, to the best of our knowledge, there is no quantitative information on the rate of addition of differently substituted radicals such as CF3•, CHF2•, CF3CH2•, and CH3CF2• onto the same monomer. The results of our studies were very unexpected because the PVDF-CH2CF2• and PVDF-CF2CH2• macroradicals can rapidly add onto the VDF molecule, preferentially to the tail end, during the VDF radical propagation step. We can, therefore, conclude that the rate of radical addition onto CH2 = CF2 is strongly affected by the radical substitution and, most importantly, that the CHF2•, CF3CH2•, and CH3CF2• radicals cannot be considered as sufficiently good models for the reactivity of the PVDF head and tail macroradicals.

5. Conclusions Fluoroalkylpentacarbonylmanganese(I) complexes are able to initiate the polymerization of VDF and the copolymerization of VAc and MAF-TBE by the homolytic bond cleavage of the Mn-RF bond, which releases RF• radicals. In the case of the VDF homopolymerization, the results are different depending on the nature of the RF group and on the activation method, which yields PVDF in almost all cases but with different conversions, molar masses, and dispersities. The best results in terms of activation were achieved with [Mn(CF3)(CO)5](1), with yields up to 74% from UV irradiation, 68% by thermal activation, or 60% by visible light activation. Although this complex has the strongest BDE among all investigated complexes [22,23], the resulting CF3• radical is the most efficient one for the addition onto the VDF monomer. The second best result was surprisingly obtained with [Mn(COCF2CH3)(CO)5](5), which generates the CH3CF2• radical, following prior decarbonylation and [Mn(CF2CH3)(CO)5](4) formation. Despite the fact that the latter complex has the lowest predicted BDE [22], it can initiate the polymerization of VDF in a relatively low yield under visible light and UV irradiation (21% and 23%, respectively). Nevertheless, this complex has a low efficiency in thermal activation at 80 ◦C. Complexes [Mn(CHF2)(CO)5](2) and [Mn(CH2CF3)(CO)5](3) are not efficient initiators under any of the tested activation conditions. However, these compounds are efficient initiators for the radical alternating copolymerization of VAc with MAF-TBE. Hence, the effect of the substituents on the radicals and their reactivity toward VDF are subtle. While the CF3• radical is efficient, CHF2•, CF3CH2•, and CH3CF2• radicals have shown a lower reactivity to be considered as suitable models for initiating the polymerization of VDF. Polymers 2020, 12, 384 15 of 17

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4360/12/2/384/s1, Figure S1: First-order kinetics plot for the polymerization of VDF initiated by visible light in presence of [Mn(CF3)(CO)5]. Figure S2: First-order kinetics plot for the polymerization of VDF initiated by UV irradiation (300 nm) in the presence of [Mn(CF3)(CO)5]. Figure S3: SEC traces of PVDF samples obtained by thermal radical polymerization of VDF with [Mn(CF3)(CO)5] in DMF normalized with conversion (entries 2, 3, 4, and 7 of Table1). Figure S4: SEC traces of PVDF samples initiated by visible light (entries 9 to 11 of Table1) in DMF normalized with conversion. Figure S5: SEC traces of PVDF samples initiated by UV irradiation (entries 12 to 14 of Table1) in DMF normalized with conversion. Figure S6: SEC traces of PVDF samples (entries 7, 11, and 14 of Table1) in DMF after 24 h-reaction by various initiation methods. Figure S7: Plot of the number of average molar mass and dispersity vs. conversion of VDF polymerization initiated thermally (100 ◦C) by complex 1. Figure S8: Plot of the number of average molar mass and dispersity vs. conversion of VDF polymerization initiated by complex 1 under visible 1 light irradiation. Figure S9: H NMR spectrum (400 MHz, DMF-d7) of the PVDF obtained by thermal activation of 19 1 (entry 7 of Table1). Figure S10: F NMR spectrum (376.5 MHz, DMF-d7) of the PVDF obtained by thermal 1 activation of 1 (entry 5 in Table1). Figure S11: H NMR spectrum (400 MHz, DMSO-d6) of the PVDF obtained by 19 visible light activation of 1 (entry 11 of Table1). Figure S12: F NMR spectrum (376.5 MHz, DMSO-d6) of the PVDF obtained by visible light activation of 1 (entry 11 of Table1). Figure S13: 1H NMR spectrum (400 MHz, 19 DMF-d7) of the PVDF obtained by UV light activation of 1 (entry 14 of Table1). Figure S14: F NMR spectrum 19 (376.5 MHz, DMF-d7) of the PVDF obtained by UV light activation of 1 (entry 14 of Table1). Figure S15: F NMR 19 spectrum (376.5 MHz, DMF-d7) of the PVDF obtained by visible activation of 2 (entry 2 of Table4). Figure S16: F NMR spectrum (376.5 MHz, DMF-d7) of the PVDF obtained by UV activation of 2 (entry 3 of Table4). Figure S17: 1 H NMR spectrum (400 MHz, acetone-d6) of the PVDF obtained by thermal activation of 3 (entry 6 of Table4). 19 Figure S18: F NMR spectrum (376.5 MHz, acetone-d6) of the PVDF obtained by thermal activation of 3 (entry 6 19 of Table4). Figure S19: F NMR spectrum (376.5 MHz, DMSO-d6) of the PVDF obtained by thermal activation 1 of 5 (entry 7 of Table4). Figure S20: H NMR spectrum (400 MHz, acetone-d6) of the poly(VAc-alt-MAF-TBE) obtained by thermal activation of 3 (entry 4 of Table5). The starred resonance is due to the solvent. Figure S21: 19 F NMR spectrum (376.5 MHz, acetone-d6) of the poly(VAc-alt-MAF-TBE) obtained by thermal activation of 3 (entry 4 of Table5). Author Contributions: Conceptualization, R.P. and B.A. Methodology, V.L. and B.A. Investigation R.M.-C. Writing-original draft preparation, R.M.-C. and B.A. Writing-review and editing, R.M.-C., V.L., F.G., C.F., R.P., and B.A. Visualization, R.M.-C. Supervision, V.L., F.G., R.P., and B.A. Project administration, R.P. Funding acquisition, R.P. All authors have read and agreed to the published version of the manuscript. Funding: The Agence Nationale de la Recherche (ANR, French National Agency) through the FLUPOL project (grant No. ANR-14-CE07-0012) funded this research. We also gratefully acknowledge additional financial support from the Centre National de la Recherche Scientifique (CNRS). Acknowledgments: We thank P.Falireas for his assistance and Arkema (Pierre-Bénite, France) and Tosoh Finechem Corp. (Shunan, Japan) for supplying free samples of VDF and MAF-TBE, respectively. Conflicts of Interest: The authors declare no conflict of interest.

References

1. 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. [CrossRef] † 2. Martins, P.; Lopes, A.C.; Lanceros-Mendez, S. Electroactive phases of poly(vinylidene fluoride): Determination, processing and applications. Prog. Polym. Sci. 2014, 39, 683–706. [CrossRef] 3. Asandei, A.D. Photomediated Controlled Radical Polymerization and Block Copolymerization of Vinylidene Fluoride. Chem. Rev. 2016, 116, 2244–2274. [CrossRef] 4. Goldbach, J.T.; Amin-Sanayei, R.; He, W.; Henry, J.; Kosar, W.; Lefebvre, A.; O’Brien, G.; Vaessen, D.; Wood, K.; Zerafati, S. Chapter 6 Commercial Synthesis and Applications of Poly(Vinylidene Fluoride). In Fluorinated Polymers: Volume 2; Applications, A.B., Sawada, H., Eds.; The Royal Society of Chemistry: London, UK, 2017; Volume 2, pp. 127–157. 5. Golzari, N.; Adams, J.; Beuermann, S. Inducing β Phase Crystallinity in Block Copolymers of Vinylidene Fluoride with Methyl Methacrylate or Styrene. Polymer 2017, 9, 306. [CrossRef] 6. Guiot, J.; Ameduri, B.; Boutevin, B. Radical homopolymerization of vinylidene fluoride initiated by tert-butyl peroxypivalate. Investigation of the microstructure by 19F and 1H NMR spectroscopies and mechanisms. Macromolecules 2002, 35, 8694–8707. [CrossRef] Polymers 2020, 12, 384 16 of 17

7. Mladenov, G.; Ameduri, B.; Kostov, G.; Mateva, R. Synthesis and characterization of fluorinated telomers containing vinylidene fluoride and hexafluoropropene from 1,6-diiodoperfluorohexane. J. Polym. Sci. Part A Polym. Chem. 2006, 44, 1470–1485. [CrossRef] 8. Pladis, P.; Alexopoulos, A.H.; Kiparissides, C.; Pladis, P.Mathematical Modeling and Simulation of Vinylidene Fluoride Emulsion Polymerization. Ind. Eng. Chem. Res. 2014, 53, 7352–7364. [CrossRef] 9. Tatemoto, M. In Proceedings of the First Regular Meeting of Soviet-Japanese Fluorine Chemists, Tokyo, Japan, 14–15 February 1979. 10. Boyer, C.; Valade, D.; Sauguet, L.; Ameduri, B.; Boutevin, B. Iodine Transfer Polymerization (ITP) of Vinylidene Fluoride (VDF). Influence of the Defect of VDF Chaining on the Control of ITP. Macromolecules 2005, 38, 10353–10362. [CrossRef] 11. Beuermann, S.; Imran-Ul-Haq, M. Homogeneous Phase Polymerization of Vinylidene Fluoride in Supercritical CO2: Surfactant Free Synthesis and Kinetics. Macromol. Symp. 2007, 259, 210–217. [CrossRef] 12. Imran-ul-haq, M.; Förster, N.; Vukicevic, R.; Herrmann, K.; Siegmann, R.; Beuermann, S. Chapter 15: Iodine Transfer Radical Polymerizations of Vinylidene Fluoride in Supercritical Carbon Dioxide and Polymer Functionalization via Click Chemistry. In Controlled/Living Radical Polymerization: Progress in RAFT, DT, NMP & OMRP; Matyjaszewski, K., Ed.; ACS: Washington, DC, USA, 2009; Volume 1024, pp. 233–243. 13. Asandei, A.D.; Adebolu, O.I.; Simpson, C.P. Mild-Temperature Mn2(CO)10-Photomediated Controlled Radical Polymerization of Vinylidene Fluoride and Synthesis of Well-Defined Poly(vinylidene fluoride) Block Copolymers. J. Am. Chem. Soc. 2012, 134, 6080–6083. [CrossRef] 14. Guerre, M.; Campagne, B.; Gimello, O.; Parra, K.; Ameduri, B.; Ladmiral, V. Deeper Insight into the MADIX Polymerization of Vinylidene Fluoride. Macromolecules 2015, 48, 7810–7822. [CrossRef] 15. Guerre, M.; Rahaman, S.M.W.; Améduri, B.; Poli, R.; Ladmiral, V. Limits of Vinylidene Fluoride RAFT Polymerization. Macromolecules 2016, 49, 5386–5396. [CrossRef] 16. Banerjee, S.; Ladmiral, V.; Debuigne, A.; Detrembleur, C.; Poli, R.; Améduri, B. Organometallic-Mediated Radical Polymerization of Vinylidene Fluoride. Angew. Chem. Int. Ed. 2018, 57, 2934–2937. [CrossRef] 17. Falireas, P.G.; Ladmiral, V.; Debuigne, A.; Detrembleur, C.; Poli, R.; Ameduri, B. Straightforward Synthesis of Well-Defined Poly(vinylidene fluoride) and Its Block Copolymers by Cobalt-Mediated Radical Polymerization. Macromolecules 2019, 52, 1266–1276. [CrossRef] 18. Debuigne, A.; Poli, R.; Jérôme, C.; Jerôme, R.; Detrembleur, C. Overview of cobalt-mediated radical polymerization: Roots, state of the art and future prospects. Prog. Polym. Sci. 2009, 34, 211–239. [CrossRef] 19. Poli, R. New Phenomena in Organometallic-Mediated Radical Polymerization (OMRP) and Perspectives for Control of Less Active Monomers. Chem. Eur. J. 2015, 21, 6988–7001. [CrossRef] 20. Debuigne, A.; Jérôme, C.; Detrembleur, C. Organometallic-mediated radical polymerization of ‘less activated monomers’: Fundamentals, challenges and opportunities. Polymer 2017, 115, 285–307. [CrossRef] 21. Fliedel, C.; Poli, R. Homolytically weak metal-carbon bonds make robust controlled radical polymerizations systems for “less-activated monomers. ” J. Organomet. Chem. 2019, 880, 241–252. [CrossRef] 22. Poli, R.; Rahaman, S.; Ladmiral, V.; Améduri, B. Effect of α- and β-H/F substitution on the homolytic bond strength in dormant species of controlled radical polymerization: OMRP vs. ITP and RAFT. J. Organomet. Chem. 2018, 864, 12–18. [CrossRef] 23. Morales-Cerrada, R.; Fliedel, C.; Daran, J.-C.; Gayet, F.; Ladmiral, V.; Améduri, B.; Poli, R. Fluoroalkyl Radical Generation by Homolytic Bond Dissociation in Pentacarbonylmanganese Derivatives. Chem. Eur. J. 2019, 25, 296–308. [CrossRef] 24. Nakagawa, K.; Ishida, Y. Annealing effects in poly(vinylidene fluoride) as revealed by specific volume measurements, differential scanning calorimetry, and electron microscopy. J. Polym. Sci. Part A-2 Polym. Phys. 1973, 11, 2153–2171. [CrossRef] 25. Morales-Cerrada, R.; Fliedel, C.; Gayet, F.; Ladmiral, V.; Améduri, B.; Poli, R. Thermal Decomposition of Fluoroalkyl Pentacarbonylmanganese(I) Derivatives by α-Fluorine Elimination. Organometallics 2019, 38, 1021–1030. [CrossRef] 26. Fawcett, J.P.; Poe, A.; Sharma, K.R. Reaction mechanisms of metal-metal bonded carbonyls. X. Thermal decomposition of decacarbonyldimanganese and decacarbonylmanganeserhenium in decalin. J. Am. Chem. Soc. 1976, 98, 1401–1407. [CrossRef] 27. Aliwi, S.M.; Bamford, C.H.; Mullik, S.U. Recent studies in photoinitiated polymerization. J. Polym. Sci. Part C 1975, 50, 33–50. [CrossRef] Polymers 2020, 12, 384 17 of 17

28. Allan, L.E.; Perry, M.R.; Shaver, M.P. Organometallic mediated radical polymerization. Prog. Polym. Sci. 2012, 37, 127–156. [CrossRef] 29. Destarac, M.; Matyjaszewski, K.; Silverman, E.; Ameduri, B.; Boutevin, B. Atom Transfer Radical Polymerization Initiated with Vinylidene Fluoride Telomers. Macromolecules 2000, 33, 4613–4615. [CrossRef] 30. Girard, E.; Marty, J.-D.; Ameduri, B.; Destarac, M. Direct Synthesis of Vinylidene Fluoride-Based Amphiphilic Diblock Copolymers by RAFT/MADIX Polymerization. ACS Macro Lett. 2012, 1, 270–274. [CrossRef] 31. Duc, M.; Ameduri, B.; Boutevin, B.; Kharroubi, M.; Sage, J.M. Telomerization of vinylidene fluoride with methanol. Elucidation of the reaction process and mechanism by a structural analysis of the telomers. Macromol. Chem. Physic. 1998, 199, 1271–1289. [CrossRef] 32. Boschet, F.; Ono, T.; Ameduri, B. Novel Source of Trifluoromethyl Radical As Efficient Initiator for the Polymerization of Vinylidene Fluoride. Macromol. Rapid Commun. 2012, 33, 302–308. [CrossRef] 33. Pianca, M.; Barchiesi, E.; Esposto, G.; Radice, S. End groups in fluoropolymers. J. Fluor. Chem. 1999, 95, 71–84. [CrossRef] 34. Ameduri, B.; Ladaviere, C.; Delolme, F.; Boutevin, B. First MALDI-TOF Mass Spectrometry of Vinylidene Fluoride Telomers Endowed with Low Defect Chaining. Macromolecules 2004, 37, 7602–7609. [CrossRef] 35. Kim, J.-S.; Dutta, A.; Vasu, V.; Adebolu, O.I.; Asandei, A.D. Universal Group 14 Free Radical Photoinitiators for Vinylidene Fluoride, Styrene, Methyl Methacrylate, Vinyl Acetate, and Butadiene. Macromolecules 2019, 52, 8895–8909. [CrossRef] 36. Mattern, D.E.; Lin, F.T.; Hercules, D.M. Laser mass spectrometry of poly(fluoroethylenes). Anal. Chem. 1984, 56, 2762–2769. [CrossRef] 37. Tedder, J.M.; Walton, J.C. The kinetics and orientation of free-radical addition to olefins. Acc. Chem. Res. 1976, 9, 183–191. [CrossRef] 38. Dolbier, W.R. Structure, Reactivity, and Chemistry of Fluoroalkyl Radicals. Chem. Rev. 1996, 96, 1557–1584. [CrossRef] 39. Dolbier, W.R.; Duan, J.-X.; Abboud, K.; Ameduri, B. Synthesis and Reactivity of a Novel, Dimeric Derivative of Octafluoro[2.2]paracyclophane. A New Source of Trifluoromethyl Radicals. J. Am. Chem. Soc. 2000, 122, 12083–12086. [CrossRef] 40. Otazaghine, B.; Sauguet, L.; Ameduri, B. Synthesis and copolymerisation of fluorinated monomers bearing a reactive lateral group Part 21. Radical copolymerisation of vinylidene fluoride with 2-hydroperfluorooct-1-ene. J. Fluor. Chem. 2005, 126, 1009–1016. [CrossRef] 41. Banerjee, S.; Ladmiral, V.; Totée, C.; Améduri, B. Alternating radical copolymerization of vinyl acetate and tert-butyl-2-trifluoromethacrylate. Eur. Polym. J. 2018, 104, 164–169. [CrossRef] 42. Banerjee, S.; Ladmiral, V.; Debuigne, A.; Detrembleur, C.; Rahaman, S.M.W.; Poli, R.; Ameduri, B. Organometallic-Mediated Alternating Radical Copolymerization of tert -Butyl-2-Trifluoromethacrylate with Vinyl Acetate and Synthesis of Block Copolymers Thereof. Macromol. Rapid Commun. 2017, 38.[CrossRef] 43. Banerjee, S.; Guerre, M.; Ameduri, B.; Ladmiral, V. Syntheses of 2-(trifluoromethyl)acrylate-containing block copolymers via RAFT polymerization using a universal chain transfer agent. Polym. Chem. 2018, 9, 3511–3521. [CrossRef] 44. Banerjee, S.; Bellan, E.V.; Gayet, F.; Debuigne, A.; Detrembleur, C.; Poli, R.; Améduri, B.; Ladmiral, V. Bis (formylphenolato)cobalt(II)-Mediated Alternating Radical Copolymerization of tert-Butyl 2-Trifluoromethylacrylate with Vinyl Acetate. Polymers 2017, 9, 702. [CrossRef] 45. Tedder, J.M.; Walton, J.C. The Importance of Polarity and Steric Effects in Determining the Rate and Orientation of Free-Radical Addition to Olefins-Rules for Determining the Rate and Preferred Orientation. Tetrahedron 1980, 36, 701–707. [CrossRef] 46. Tedder, J.M. Which Factors Determine the Reactivity and Regioselectivity of Free Radical Substitution and Addition Reactions? Angew. Chem. Int. Ed. 1982, 21, 401–410. [CrossRef] 47. Serov, S.I.; Zhuravlev, M.V.; Sass, V.P.; Sokolov, S.V. Reactions of fluoro-containing free radicals in solution. II. Kinetics of the addition of perfluoroalkyl radicals to olefins in heptane. J. Org. Chem. USSR 1981, 17, 53–58.

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).