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Atom Transfer Radical Polymerization with Different Halides (F, Cl, Br, and I): Is the Process “Living” in the Presence of Fluorinated Initiators? Sonia Lanzalaco,† Marco Fantin,‡,§ Onofrio Scialdone,*,† Alessandro Galia,*,† Abdirisak A. Isse,§ Armando Gennaro,§ and Krzysztof Matyjaszewski*,‡ †

Dipartimento dell’Innovazione Industriale e Digitale (DIID), Ingegneria Chimica Gestionale Informatica Meccanica, Università di Palermo, Viale delle Scienze − Ed. 6, 90128 Palermo, Italy ‡ Center for Molecular Engineering, Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States § Dipartimento di Scienze Chimiche, Università di Padova, Via Marzolo 1, 35131 Padova, Italy S Supporting Information *

ABSTRACT: Atom transfer radical polymerization (ATRP) is often used for grafting from fluorinated polymers. Nevertheless, the possibility to initiate an ATRP from a C−F functionality and the activity of the catalysts in the presence of fluoride anions are essentially unexplored. Therefore, we investigated the thermodynamics and kinetics of C−F bond activation by ATRP catalysts and compared it with other halide systems. The ATRP equilibrium constant was estimated to be small for the reaction between [CuITPMA]+ and benzyl fluoride (TPMA = tris(2-pyridylmethyl)amine). However, [CuITPMA]+ could react with the more active initiator diethyl fluoromalonate (DEFM). With DEFM as initiator and CuIBr/TPMA as catalyst, ATRP of methyl acrylate and styrene displayed initiation efficiencies of 73% and 95%, respectively. ATRP deactivation by [F−CuIITPMA]+ was slow and followed by even slower activation of newly formed C−F bonds, leading to limited conversion. Comparison with other halides indicates that Br- and Cl-based ATRP systems are more efficient that I- and F-based systems.

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radicals to fluoropolymers, and grafting of polymers “onto” or “from” a polymer backbone.3 One of the most common “grafting from” techniques is atom transfer radical polymerization (ATRP), which is based on the equilibrium between dormant species (alkyl halides initiators, RX, or halide-capped polymers, Pn−X) and active species (propagating radicals, R•) (Scheme 1). The reaction relies on the rapid and reversible catalytic activation of carbon−halogen bonds by a metal complex in its lower oxidation state, usually [CuIL]+, to generate radicals and the complex in its higher

INTRODUCTION Fluorine, the super halogen, is the most electronegative and reactive of all the elements. Its extreme reactivity resides in the very weak F−F bond (which is similar to the easily cleaved peroxide bond), whereas bonds formed between F and almost any other element are very strong.1 The strength of C−F bonds in fluorinated compounds conveys properties such as exceptional thermal and chemical stability, inertness to acids and common solvents, low inflammability, and low dielectric constant. Consequently, fluorinated compounds, including fluoropolymers, play an important role in the development of new materials for advanced applications (coatings, transmission fluids, soft lithography, high-performance lubricants, and membranes).2−10 To target specific applications, fluoropolymers should be modified by introducing new properties (hydrophilicity,11 hydrophobicity,12 or dielectric properties13), while retaining most of the advantages of the stable fluorinated backbone.14 One of the most widely used modification techniques is covalent grafting of polymeric chains from the backbone of fluoropolymers, which allows fine-tuning of the desired properties. Different synthetic pathways have been used, such as direct (co)polymerization of macromonomers, transfer of © XXXX American Chemical Society

Scheme 1. ATRP Mechanism

Received: October 20, 2016 Revised: December 1, 2016

A

DOI: 10.1021/acs.macromol.6b02286 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules oxidation state bonded to the halogen atom, [X−CuIIL]+.15,16 The equilibrium is strongly shifted to the dormant polymer (left side of Scheme 1), which ensures low radical concentration to minimize irreversible radical−radical terminations. Concurrent growth of all chains is assured by fast initiation of RX initiators (ideally faster than activation of dormant chain end, Pn−X, and comparable to the rate of radical propagation). Rapid dynamic exchange between dormant and active species is required for concomitant growth of all macromolecules.17 Vinyl monomers such as methacrylates were grafted from poly(vinylidene fluoride) (PVDF) macroinitiators and from poly(vinylidene fluoride-co-trifluoroethylene) (VDF-co-TrFE) surfaces by direct C−F bond activation.18−22 Direct initiation of the secondary fluorinated site of PVDF produced amphiphilic graft copolymers such as poly(vinylidene fluoride)-g-poly(oxyethylene methacrylate) (PVDF-g-POEM) and poly(vinylidene fluoride)-g-poly(methacrylic acid) (PVDF-gPMAA).20 When grafting from poly(VDF-co-TrFE), polymerization of tert-butyl acrylate occurred exclusively by fluorine abstraction from the TrFE units.22 It is possible, however, that PVDF grafting was not initiated by the cleavage of the stable C−F bond, but rather by activation of the less stable C−H bond of the methylene group3 or by reactive defects in the polymer structure (such as allyl fluoride groups).23 Indeed, allyl halides are active ATRP initiators, much more reactive than halogenated alkanes (such as the PVDF backbone).26 Moreover, the presence of the C−H bond reduces the chemical stability of PVDF, compared with perfluoropolymers such as polytetrafluoroethylene (PTFE) or fluorinated ethylenepropylene (FEP), suggesting the possibility of C−H activation. Abstraction of fluorine atoms in PVDF is hampered by the high energy (486 kJ/mol)20 of the C−F bond and by the poor stabilization of the radical centers produced after C−F bond cleavage.3,24 As a consequence, activation of C−F bonds could be significantly slower than activation of dormant grafted chains, thus leading to a very poor control of the polymerization process.24 To circumvent this limitation, more active ATRP-initiating sites were anchored to PVDF before grafting several monomers, such as methyl methacrylate (MMA), dimethylaminoethyl methacrylate (DMAEMA), t-butyl methacrylate (tBMA), diethylene glycol methyl ether methacrylate (MeO2MA), n-butyl acrylate (n-BA), tert-butyl acrylate (tBA), or dimethylacrylamide (DMA).24 Recently, ATRP activation of C−F bond was also proposed for perfluorosulfonic acid polymers (Nafion), but mechanistic details were not provided.25 Indeed, several reports underline the limited control of fluoropolymers modification by ATRP.14,24 Thus, the objective of this work was to examine the effect of fluorine in ATRP and the viability of fluorinated compounds as ATRP initiators. To obtain a more complete picture, the effect of fluorine was compared to that of the other halogens used in ATRP, such as the commonly employed chlorine and bromine, as well as the less studied iodine. The following key aspects of ATRP were investigated: ATRP equilibrium constants, deactivation rates of radicals by [X−CuIIL]+ (where X = F, Cl, Br, or I), activation rates of model R−X initiators, and reactivation of the dormant polymer chain ends. First, the position of the ATRP equilibrium (KATRP) for the reaction between [CuITPMA]+ and a series of benzyl halides (BnX, where X = F, Cl, Br, or I) was estimated (TPMA = tris(2-pyridylmethyl)amine). This required several thermody-

namic parameters to be determined by electrochemical investigation of the ternary complexes [X−CuIITPMA]+.14,20 Then, [X−CuIITPMA]+ complexes were used to evaluate the effect of X on the deactivation of polymeric radicals in reverse ATRP experiments. Next, cyclic voltammetry (CV) was used to evaluate C−F bond activation in the model initiator diethyl fluoromalonate (DEFM). Last, the living nature of ATRP of methyl acrylate (MA) and styrene (Sty) with DEFM as initiator was studied. The optimal experimental conditions to effectively activate DEFM were explored in terms of temperature and nature of the catalyst and monomer.

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RESULTS AND DISCUSSION Effect of Halide Ions on the ATRP Equilibrium. ATRP equilibrium (eq 1) depends on the thermodynamic properties of both initiator (RX) and catalyst (the [CuIL]+/[X−CuIIL]+ couple).26,27 [Cu IL]+ + RX ⇌ [X−Cu IIL]+ + R•

(1)

Equation 1 can be expressed as the combination of three reactions (eqs 2−4), i.e., the dissociative electron transfer to RX (with standard reduction potential E⊖ RX/R•+X−), the reversible electron transfer to the Cu complex (E⊖ [CuIIL]2+/[CuIL]+), and the association of the halide anion to the CuII complex (with equilibrium constant KIIX, also termed halidophilicity constant).28 RX + e− ⇌ R• + X−

⊖ • E RX/R + X−

[Cu IL]+ ⇌ [Cu IIL]2 + + e−

(2)

⊖ E[Cu II 2 + L] /[Cu IL]+

[Cu IIL]2 + + X− ⇌ [X−Cu IIL]+

KXII

(3) (4)

The set of eqs 2−4 shows that the fundamental reactions constituting an ATRP process are affected by the nature of the halogen atom bound to the dormant chain end or to the ternary catalytic complex [X−CuIIL]+.29,30 In other words, the nature of X atom affects the thermodynamic properties of the catalyst/ initiator system; herein, such effects were studied by electrochemical analysis of the system. Effect of Halide Ions on the Electrochemical Behavior of Copper Complexes. To better understand the role of the halide ions on the ATRP catalyst, the copper complexes were initially investigated in the absence of RX. CV experiments were carried out to study the effect of X− on the redox behavior of the [X−CuIITPMA]+ complexes (Figure 1a). Dimethylformamide (DMF) was selected as solvent due to the good solubility of catalyst, fluoride salts, and fluorinated compounds. The effect of temperature on the electrochemical behavior of the catalysts was also studied, in the range 25−70 °C (Figure 1; cf. more details in the Supporting Information). All examined complexes, the binary [CuIITPMA]2+ and the ternary [X−CuIITPMA]+ (X = F, Cl, Br, or I), undergo a quasireversible one-electron transfer in DMF + 0.1 M n-Bu4NPF6 (Figure 1a and Figure S1). Relevant electrochemical data are summarized in Table 1; the half-wave potential (E1/2) was calculated as the half-sum of the cathodic and anodic peak potentials, E1/2 = (Ep,c + Ep,a)/2, while peak separation was calculated as ΔEp = Ep,a − Ep,c.31 The ratio between anodic and cathodic peak currents, ip,a/ip,c ≈ 1, underlined the stability of the investigated CuI complexes within the CV time scale. Interestingly, the shape of the CV of both [F−CuIITPMA]+ and [I−CuIITPMA]+ was similar to that of the commonly used B


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complex and [I−CuITPMA] the worst. This trend could suggest the potential good activity of ATRP catalysts in the presence of F−.31 The difference between the standard potentials of the [CuIIL]2+/[CuIL]+ and [X−CuIIL]+/[X−CuIL] redox couples was used to calculate the relative halidophilicity of [CuIIL]2+and [CuIL]+ (KXII/KXI) through eq 7, which was derived by combining reactions 5 and 6 with the redox reaction of [CuIIL]2+ and [X−CuIIL]+. ⊖ ⊖ E[X = E[Cu + II 2 + −Cu IIL]+ /[X−Cu IL] L] /[Cu IL]+

[Cl−CuIITPMA]+ and [Br−CuIITPMA]+, suggesting that complexes with similar and well-defined structures were present in solution. [CuIITPMA]2+/[CuITPMA]+ and [X−CuIITPMA]+/[X− CuITPMA] were the main redox couples in the absence and in the presence of halide ions, respectively.32 In the presence of X−, equilibria 5 and 6 hold:

I

+

−

I

[Cu L] + X ⇌ [X−Cu L]

KXI

KXII

(7)

where R is the gas constant, F the Faraday constant, and L = TPMA. KIIX/KIX ratios for the association of Cu/TPMA with the halide ions are presented in Table 1; KIIX/KIX increases following a trend opposite to redox potential of [X−CuIIL]+, with F > Cl > Br > I. The ratio KIIX/KIX appears to depend mostly on KIIX. Indeed, it has been observed for different halogens, copper complexes,34 and solvents35 that KIIX varies over several orders of magnitude, while KIX is roughly constant. Therefore, the higher the KIIX/KIX ratio, the stronger the CuII−X affinity (KIIX) and the higher the stabilization of the deactivating species, [X−CuIITPMA]+. As a consequence, [F−CuIITPMA]+, with the highest KIIX/KIX (Table 1), is the most stable complex and a potentially less efficient deactivator. Another CV parameter is the separation between anodic and cathodic peak potentials (ΔEp), which depends on the extent of molecular rearrangements between the two oxidation states upon electron transfer.33 ΔEp increased with KIIX/KIX (Table 1) because stronger binding of the halide ions to [CuIIL]+ (but not to [CuIL]+) caused larger geometrical and electronic rearrangements between the CuII and CuI coordination spheres. It is worth making a precise distinction between the role of the redox potentials of binary and ternary complexes, [CuL]2+/+ and [X−CuL]+/0, respectively. [CuIL]+ is the ATRP activator complex;32 therefore, the more negative E⊖ [CuIIL]2+/[CuIL]+, the higher the catalyst activity in reducing the C−X bond. Instead, [X−CuIIL]+ is the deactivator complex; in this case, the more II negative E⊖ [X−CuIIL]+/[X−CuIL], the higher the halidophilicity of Cu (eq 5), which facilitates the C−X bond cleavage. Hence, the redox potentials of both binary and ternary complexes must be sufficiently negative to promote efficient activation of the alkyl halides initiators. As a result, [F−CuIITPMA]+, with the most negative reduction potential, appears as relatively the most active ATRP catalyst. However, the contribution of R−X bond energy must also be considered, because in the presence of fluorinated initiators ATRP requires the cleavage of the very strong C−F bond. Therefore, to complete the thermodynamic picture of the ATRP equilibrium, R−X bond dissociation energy was taken into consideration to estimate relative KATRP values, as described in the next section. Regarding [I−CuIITPMA]+, unexpectedly KIIX/KIX was no halide ligand (binary complex) > Br > Cl > F. In other words, [F−CuITPMA] is the best reducing

KATRP

[Cu ITPMA]+ + BnX HoooooI [X−Cu IITPMA]+ + Bn• C

(8)


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Table 1. Electrochemical and Thermodynamic Properties of [X−CuITPMA]+ Complexes at Different Temperatures in DMFa complex II

[Cu TPMA]

T (°C)

ip,a/ip,cb

E1/2c (V vs SCE)

ΔEpd (mV)

25 40 55 70 25 40 55 70 25 40 55 70 25 40 55 70 25 40 55 70

1.00 0.99 1.05 1.00 1.06 1.09 1.09 1.13 0.87 0.84 0.71 0.68 1.01 0.99 0.91 0.92 1.06 1.07 1.04 1.06

−0.127 −0.104 −0.081 −0.057 −0.439 −0.426 −0.414 −0.388 −0.330 −0.308 −0.285 −0.269 −0.213 −0.198 −0.178 −0.151 −0.092 −0.083 −0.068 −0.053

71 73 78 84 241 190 178 160 79 78 93 102 69 71 76 87 70 73 77 81

2+ f

[F−CuIITPMA]+ g

[Cl−CuIITPMA]+ h

[Br−CuIITPMA]+ h

[I−CuIITPMA]+ h

KIIX/KIXe

1.9 × 2.8 × 4.3 × 3.9 × 2.7 × 2.8 × 2.8 × 3.8 × 28 39 44 39 0.26 0.44 0.60 0.86

105 105 105 105 103 103 103 103

CV recorded at a scan rate of 0.1 V s−1 on a glassy carbon (GC) disk electrode. CCu = 2 × 10−3 M. bThe ratio between anodic (ip,a) and cathodic (ip,c) peak currents, estimated as described in ref 33. cE1/2 = (Ep,c + Ep,a)/2. dΔEp = Ep,a − Ep,c. eCalculated from eq 6. fCuII(OTf)2/TPMA = 1/1. g CuIIF2/TPMA = 1/1. hCuII(OTf)2/TPMA/n-Bu4NX = 1/1/1. a

which is the main energetic contribution to the value of ⊖ E RX/R • +X − ) were previously reported for various systems.26,31,38−40 To estimate KATRP through eq 10, KIX was considered to be roughly constant and independent of the nature of the halogen. This allowed estimating the relative ATRP equilibrium constants, as KATRP,X/KATRP,Br (KATRP of the most active system, BnBr + [CuITPMA]+, was used as a reference; detailed calculations are in the Supporting Information). KATRP,X/KATRP,Br values (Table 2) were useful to compare the activity of ATRP systems with different halide atoms, as discussed hereafter. The estimated KATRP,X/KATRP,Br for reaction 8 are displayed in Table 2 and Figure 2, together with BDE of the Bn−X bond (BDEBn−X) and KIIX/KIX of the copper complex. BDEBn−X and KIIX/KIX represent the strength of the C−X bond and the relative affinities of CuII and CuI for X−, respectively. Both values increase in the order I < Br < Cl < F. When progressing from C−I to C−F, the carbon−halogen bond becomes more difficult

Benzyl halides are common ATRP initiators, and a complete set of thermodynamic data on their redox properties and C-X bond energy is available in the literature36,37 (relevant thermodynamic data for the atom transfer reaction 7 between the four BnX and [CuITPMA]+ are presented in Table 2). Based on the Table 2. Thermodynamic Properties of the Atom Transfer Reaction between Benzyl Halides and [CuITPMA]+ at 25 °C entry 1 2 3 4

X F Cl Br I

BDEBn−X (kJ mol−1) a

416.7 299.9c 239.3c 187.8c

E⊖ RX/R•+X− (V vs SCE)

KIIX/KIX

−0.88 −0.67c −0.39c −0.38c

1.9 × 10 2.7 × 103 28 0.26

b

KATRP,X/KATRP,Br 5

4 × 10−5 2 × 10−3 1 2 × 10−2

a

From ref 42. bCalculated as described in the Supporting Information following the procedure reported in ref 37. cFrom ref 37.

thermodynamic reaction cycle in eqs 2−4, KATRP can be calculated as ln KATRP = ln KXII +

F ⊖ ⊖ • (E RX/R + X− − E[Cu IIL]2 + /[Cu IL]+) RT (9)

⊖ II 2+ where E⊖ and RX [CuIIL]2+/[CuIL]+ and ERX/R•+X− are [Cu TPMA] standard redox potentials, respectively. However, since values of KIIX were not available in the literature, eq 9 was rearranged into the following equation by substituting KIIX from eq 7 (detailed calculations in the Supporting Information):

ln KATRP = ln KXI +

F ⊖ ⊖ • (E RX/R + X− − E[X − Cu IIL]+ /[X − Cu IL]) RT (10)

If KIX is roughly constant, as suggested by some literature values,17,18 plots of ln KATRP vs E⊖ [X−CuIIL]+/[X−CuIL] are expected to be linear. Indeed, linear correlations between ln KATRP and E⊖ [X−CuIIL]+/[X−CuIL] or R−X bond dissociation energy (BDERX,

Figure 2. Bn−X bond dissociation energy, KIIX/KIX ratio of X−Cu/ TPMA complexes, and KATRP,X/KATRP,Br for the reaction between [CuITPMA]+ and BnX (X = F, Cl, Br, I). All calculations were performed at 25 °C. D


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Figure 3. Effect of the presence of different ternary [X−CuIITPMA]+ complexes (X = F, Cl, Br, or I) on (a) conversion and (b) Mn evolution during reverse ATRP of MA and comparison with FRP of MA. Conditions for reverse ATRP: molar ratios CuII(OTf)2:TPMA:n-Bu4NX = 1:1:1 for X = I, Br, or Cl; CuIIF2:TPMA = 1:1; MA 10% v/v, C[X−CuIIL]+ = CAIBN = 10−3 M. Conditions for FRP: MA 10% v/v; CAIBN = 10−3 M. T = 65 °C.

to cleave, but concurrently the stronger CuII−X interaction (I < Br < Cl < F)36 facilitates halogen abstraction. The net result of these two opposite trends, higher BDEBn−X and stronger CuII− X interaction, is shown in Figure 2: KATRP,X reaches a maximum for X = Br. This result is in agreement with previous literature, which reports that ATRP with RBr initiators is fast and well controlled.41 To test the validity of KATRP estimation, a second different set of E⊖ RX/R•+X− values available in the literature was used to determine KATRP,X/KATRP,Br (Figure S3). Although KATRP,Cl or KATRP,I was somewhat underestimated, the general trend of KATRP did not change using either set of E⊖ RX/R•+X− values, confirming the reliability of the estimates. KATRP,F has the lowest value because BDEBn−F is exceptionally high (416.7 kJ mol−1 for BnF vs 299.9 kJ mol−1 for BnCl).37,42,43 The remarkable “fluoridophilicity” of the CuII complex (KIIX/KIX = 1.9 × 105) is not sufficient to compensate for the high BDEBn−F, which led to the low KATRP,F. This result points out that ATRP activation of the C−F bond should be rather difficult, especially in the case of less reactive F as in PVDF or other polymer backbones. On the other hand, C−H activation from a polymer backbone appears more unlikely than C−F activation. Even if the C−H bond is slightly weaker than the C−F bond,36 Cu−H interaction is much weaker than Cu−X affinity. In essence, C− F cleavage is promoted by the strong Cu−F interaction, whereas C−H cleavage is hampered by the absence of a significant contribution from Cu−H interaction.36 C−H activation would result in the formation of an unstable CuII− hydride complex, a thermodynamically unfavorable step.44 Instead, a more favorable ATRP pathway could be the activation from defects on the polymer backbones, such as allylic fluoride groups. Such defects could react similarly to traditional ATRP initiators (e.g., ethyl α-haloisobutyrates).23 Estimated KATRP,I was lower than KATRP,Br because of the weak CuII−I bond (i.e., low “iodophilicity”). This result is corroborated by previous works, which reported lower KATRP for RI than for RCl.17,26 The intrinsic instability of CuI2 is a further proof of the weak CuII−I interaction. Therefore, ATRP in the presence of R−I should be ineffective: KATRP,I is low even if the C−I bond is very weak (BDEBn−I = 187.8 kJ mol−1), which can favor polymerization through other pathways, such as degenerative iodine transfer polymerization.45,46

The KATRP (thermodynamic) trend shown in Figure 2 is expected to influence the activation and deactivation kinetics of an ATRP process. Indeed, both activation and deactivation are slower for the less reactive RCl compared to RBr.47,48 RF is expected to be less reactive than RCl; therefore, this trend could be worsened in the case of RF, with activation impeded both thermodynamically and kinetically by the large C−F bond energy. In conclusion, ATRP conducted with RF initiators is expected to be rather slow, due to a significantly low KATRP that leads to low radical concentration. Moreover, both activation and deactivation processes should be quite slow and therefore unfavorable to ATRP control. This can be correlated with the observed difficulties in synthesis of welldefined polymers bearing a C−F functionality. The obtained results also pointed out that CuL/RBr is the most active ATRP system. Deactivation of Polymer Chains in the Presence of Different Ternary CuII Complexes. The catalysts [X− CuIITPMA]+ (with X = F, Cl, Br, or I) were used to study how the kinetics of radical deactivation was influenced by the nature of the halide ion. Experiments were performed by reverse ATRP, which differs from normal ATRP in the initiation process by using 2,2′-azobis(2-isobutyronitrile) (AIBN) as source of free radicals. In reverse ATRP, free radicals In•, generated by thermal decomposition of the conventional radical initiator In2, abstract the halogen atom X• from the oxidized catalyst [X− CuIITPMA]+, thus forming a reduced transition-metal species ([CuITPMA]+) and the dormant species (In−X). In the case of slow deactivation, In• can add to several monomer units before being deactivated to Pn−X. The dormant species can then be activated again by [CuITPMA]+ and thus continue the controlled polymerization.49 The initially formed MW can be used to estimate deactivation rate constants for relatively slow deactivation.50 Figure 3 and Table S2 present the results of the polymerizations of methyl acrylate (MA) by reverse ATRP and by free radical polymerization (FRP). Conventional FRP in the absence of deactivating CuII complexes was carried out as a reference: fast conversion of MA was observed, with Mn decreasing with conversion as expected for a FRP process. Similarly, the reverse ATRP reaction carried out in the presence of [F−CuIIL]+ led to fast polymerization with Mn decreasing E


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Macromolecules with conversion. However, lower Mn and slower polymerization kinetics indicated the occurrence of some radical deactivation in the presence of [F−CuIIL]+. The other [X−CuIIL]+ catalysts (X = Cl, Br, or I) produced poly(methyl acrylate) (PMA) with lower conversions and Mn increasing with conversion, suggesting much faster radical deactivation. Long induction periods were observed in the presence of [Br−CuIIL]+ and [Cl−CuIIL]+, indicating quick trapping of the thermally generated In•. Shorter induction period was observed with [I−CuIIL]+ and no induction period for [F−CuIIL]+. In particular, in the presence of [F−CuIIL]+ the slope of the conversion vs time plot was similar to that obtained in FRP of MA (in comparison to the plots obtained with the other deactivating complexes). This indicated only minimal radical deactivation and confirmed that the CuII−F bond is strong and difficult to break.36 Overall, the obtained results indicated that deactivation of radicals by [F−CuIIL]+ was difficult. Conversely, [I−CuIIL]+ may be used as effective deactivator for the polymerization of acrylates, but it is not possible to exclude the occurrence of other reactions, such as iodine transfer.51 Activation of a Model Fluoride Initiator. Grafting from fluoropolymers by ATRP traditionally assumed direct activation of the C−F bond in the presence of an ATRP catalyst.19−21,24,52−54 However, the successful direct activation of less ATRP-active fluoropolymers, such as PVDF, should be challenging due to the high energy of the carbon−fluorine bond (486 kJ/mol in PVDF).36 PVDF has been recently studied as macroinitiator for ATRP of hydroxyethyl methacrylate.55 Electrochemical analyses showed that C−F activation required a very active catalyst, such as Cu/Me6TREN. Herein, activation of a model alkyl fluoride initiator (diethyl fluoromalonate, DEFM) by [CuITPMA]+ and [Cu I Me 6 TREN] + catalysts was probed by CV. These complexes were chosen because they are among the most active and used ATRP catalysts.26 In the absence of DEFM, the voltammetric response of [CuIIMe6TREN]2+ exhibited a reversible wave with E1/2 = −0.274 V vs SCE, peak separation ΔEp = 75 mV, and ip,a/ip,c = 1.04, which underlined the quasi-reversible character of the electron transfer involving the [CuIIL]2+/[CuIL]+ couple (where L = Me6TREN). After addition of DEFM at a ratio γ = CDEFM/CuIIL = 1 (Figure 4, CDEFM = 2 × 10−3 M), an electrochemical catalytic process was established (see Scheme 2) and the voltammetric pattern changed, with the appearance of a new reversible peak at a more negative potential (E1/2 ≈ −0.55 V vs SCE). First, [CuIL]+ was electrogenerated by reduction of [CuIIL]2+ (Scheme 2, step I). Then, [CuIL]+ irreversibly reacted with DEFM in the presence of the radical scavenger 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO). TEMPO quantitatively trapped radicals generated by the atom transfer reaction,56−58 forming relatively stable alkoxyamines (Scheme 2, step II).48 This permitted the evaluation of the activation reaction without contributions from radical deactivation. The CuII species generated from the activation reaction was reduced again at the electrode surface, closing the electrocatalytic cycle (Scheme 2, step III).59,60 The reversible peak at E1/2 ≈ −0.55 V vs SCE was attributed to the heterogeneous reduction of the deactivator [F−Cu II L]+ produced by ATRP activation. Further additions of DEFM up to 5 × 10−3 M and 10−2 M (corresponding to γ = 2.5 and γ = 5, respectively) additionally increased the [F−CuIIL]+ reduction peak and decreased the

Figure 4. CV of 2 × 10−3 M (a) CuII(OTf)2/Me6TREN or (b) CuII(OTf)2/TPMA in DMF + 0.1 M n-Bu4NPF6 in the presence of 2, 5, or 10 mM DEFM, recorded on a GC working electrode at υ = 0.1 V s−1 and T = 25 °C.

[CuIL]+ oxidation peak, in agreement with the catalytic cycle represented in Scheme 2. The same behavior was observed for the reduction of [CuIITPMA]2+ in the presence of DEFM (Figure 4b), but a smaller increase of cathodic current was recorded due to the lower activity of this complex. The higher activity of [CuIMe6TREN]+ was observed also for the activation of commonly used alkyl chlorides or bromides.61 Both experiments confirmed that the C−F bond in the model initiator DEFM could be cleaved by [CuIL]+ complexes. Activation of the DEFM initiator was also tested by 19F NMR (Figure 6). The doublet centered at −196.1 ppm (1J(F,H) = 47 Hz) corresponds to the fluorine of the DEFM initiator. The initiator was consumed after a few hours. The new peaks arising at ca. −166 ppm were associated with the formation of PMA-F oligomers with DP 1−10 (see Figures S5 and S6). From the rate of DEFM consumption, a rate constant kact = 5.9 × 10−3 M−1 s−1 was obtained for the reaction between CuIBr/TPMA and DEFM at 25 °C (full calculations in the Supporting Information). While this kact value is small, it is comparable to the activation of some typical alkyl chloride initiators (e.g., kact = 5.5 × 10−3 M−1 s−1 for the activation of BnCl by CuICl/ PMDETA in MeCN at 35 °C).61 Defining the “Livingness” of ATRP in the Presence of a Fluoride Initiator. After confirming that DEFM could be effectively activated, a series of ATRPs were conducted with this model initiator. The aim was to study the initiator efficiency and the “living” character of the polymerization in the presence of the C−F moieties (i.e., how easily the chain-end could be reactivated). “Normal ATRP” of MA in the presence of DEFM as initiator and 10−3 M CuIBr/TPMA as catalyst were performed at 65 °C. Under these conditions, most of the chains were capped with a C−F chain end because of the much F


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Macromolecules Scheme 2. ATRP Electrocatalytic Cycle in the Presence of DEFM and TEMPO

Figure 5. 19F NMR spectra recorded during the reaction of DEFM with [CuITPMA]+ in 20% MA (v/v) in DMF-d7. Spectra recorded 0, 0.3, 0.6, 2, and 3.6 h after the addition of CuIBr.

Table 3. ATRP of MA with DEFM as Initiatora entry

catalyst

monomer

T (°C)

conv (%)

Mn

Mn,th

Đ

Ieffb

1 2 3 4 5 6 7 8

CuIBr/TPMA CuIBr/TPMA CuIBr/TPMA CuIBr/TPMA CuIBr/PMDETAc CuIBr/TPMA CuIBr/Me6TREN

MA MA MA MA MA MA sty sty

65 65 25 40 80 65 65 65

63.0 26.1 7.8 12.5 41.4 24.1 22.1 32.9

42400 7900 5300 6900 4900 15100 2400 22500

2300 700 1100 3600 2100 2300 3400

1.98 1.48 1.37 1.42 1.76 1.43 1.57 2.44

0.29 0.13 0.16 0.73 0.14 0.95 0.15

Conditions: MA:DEFM:CuIBr:L (where L = TPMA or Me6TREN or PMDETA) = 100:1:1:1 (1.11 × 10−2 M DEFM). bIeff = Mn,th/Mn. cPMDETA = N,N,N′,N″,N″-pentamethyldiethylenetriamine. a

stronger C−F bond compared to C−Br.62 The low KATRP calculated for the reaction in the presence of fluoride initiators (Table 2) required a rather active catalyst, such as CuIBr/ TPMA. Normal ATRP afforded PMA chains with moderately low dispersity (Đ < 1.5) but with a value of Mn 3.5 times higher than the theoretical one (Mn,th, calculated as Mn,th = conversion × C0MA/C0DEFM, where C0MA and C0DEFM are the initial monomer and initiator concentrations, respectively). This result confirmed that [CuITPMA]+ activated the fluorinated initiator under normal ATRP conditions, but the initiation efficiency (defined as Ieff = Mn,th/Mn) was low, indicating incomplete consumption of DEFM. The slow rate of addition

of the stable malonate radical to MA monomer could be responsible for the poor initiation efficiency. Polymerization rate was initially fast, but then the rate of monomer consumption suddenly diminished (Figure 6). A similar result was obtained when monitoring the reaction by 19 F-NMR (Figure 5). Once the initiator was consumed, the polymerization essentially stopped because PMA-F chain ends were much less reactive than the DEFM initiator. Comparison of ATRP with FRP carried out under similar conditions (T = 65 °C, 10−3 M AIBN as radical source) confirmed the occurrence of radical deactivation by [F− CuIITPMA]+. ATRP was slower, with lower Mn and lower dispersity (Figure 6 and Table 3, entries 1, 2). G


Article

Macromolecules Initiation efficiency of DEFM increased with temperature: Ieff was 0.13, 0.16, 0.29, and 0.73 when the temperature was raised from 25 to 40, 65, and 80 °C, respectively (Table 3, entries 2− 5). CV of [CuIITPMA]2+ catalyst in the presence of DEFM as initiator (Figure 7) showed a remarkable enhancement of the

Me6TREN catalyst (Table 3, entry 8) was faster but poorly controlled, suggesting excessive termination or occurrence of side reaction (e.g., slow CuI disproportionation).63 Overall, these results indicate that DEFM initiator is reactive enough for ATRP of Sty, which forms less active dormant species (compared to those formed in the polymerization of MA).

Figure 6. Monomer conversion and evolution of dispersity for the FRP and ATRP of MA. Conditions of FRP (Table 3, entry 1): 10% v/ v MA in DMF + 10−3 M AIBN at 65 °C. Conditions of ATRP (Table 3, entry 2): molar ratios MA:DEFM:CuIBr:TPMA = 100:1:1:1 (MA 10% v/v in DMF) at 65 °C. Filled symbols: monomer conversion; empty symbols: dispersity.

Figure 7. CV of 10−3 M CuII(OTf)2/TPMA in DMF + 0.1 M nBu4NPF6 in the presence of 5 × 10−3 M DEFM and 2 × 10−2 M TEMPO at different temperatures recorded on a GC disk electrode, υ = 0.01 V s−1.

Figure 8 presents a comparison between the Mn values obtained during polymerization under different conditions: (i) reverse ATRP of MA with [F−CuIITPMA]+ as catalyst, (ii) normal ATRP of MA, and (iii) normal ATRP of Sty with CuIBr/TPMA as catalyst. Normal ATRP provided lower Mn, confirming the efficiency of DEFM as initiator. When Sty was used, a better initiation efficiency was obtained at 65 °C.

cathodic peak current when increasing the temperature from 25 to 55 °C, suggesting faster reaction between the catalyst and DEFM. Therefore, increasing temperature allowed achieving fast and almost complete activation of the fluoride initiator. At low temperature, oligomers with very low DP were produced (see Figure S5). The oligomers consumed initiator without generating polymer, resulting in low initiation efficiency, as calculated from the Mn,th/Mn ratio. Initiation efficiency and conversion increased with T. On the other hand, dispersity of the obtained PMA also increased, indicating poor deactivation of the growing chains (Table 3, entries 2−5). With the aim of increasing the deactivation efficiency, ATRP of MA was also carried out with the catalyst CuIBr/PMDETA, which is characterized by a high deactivation rate constant.26 Evolution of Mn with conversion was linear (Figure S4), but low initiation efficiency was still observed (Table 3, entry 6). This could be due to the lower activity of CuIBr/PMDETA than CuIBr/TPMA in activating the C−F bond. Styrene (Sty), a monomer with a lower propagation rate constant, kp, was used to enhance the ratio between deactivation and propagation rates, rdeact/rp. Sty is characterized by a significantly higher kdeact/kp ratio than acrylates (kdeact/kp = 20 for Sty vs kdeact/kp = 6 for MA at 90 °C).50 Indeed, the more nucleophilic styryl radical has higher affinity for halogen abstraction, and therefore radical deactivation should be faster than in the case of the more electrophilic acrylate radicals. On the other hand, a styryl halide end-group is less reactive, and therefore the activity of the polystyrene dormant species becomes more comparable to that of the unreacted initiator, and the problem of poor initiation efficiency could be decreased. Indeed, an increased Ieff was observed for the polymerization of Sty in the presence of CuIBr/TPMA, with Mn matching the theoretical values (95% initiation efficiency, Table 3, entry 7). Polymerization of Sty with the more active CuIBr/

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CONCLUSIONS Thermodynamic and kinetic aspects of the Cu/TPMA catalyst in the presence of different alkyl halides were investigated. The properties of the deactivator [X−CuIIL]+ were strongly influenced by the nature of X. Standard reduction potentials

Figure 8. Mn vs conversion plots for reverse ATRP of MA with [F− CuIITPMA]+ as catalyst and normal ATRP of MA and Sty in the presence of DEFM as initiator. Conditions for normal ATRP of MA and Sty were monomer:DEFM:CuIBr:TPMA = 100:1:1:1 (1.11 × 10−2 M DEFM) and T = 65 °C. Conditions for reverse ATRP of MA: CuII(OTf)2/TPMA/KF = 1:1:2, CMA = 1.105 M, C[X−CuIIL]+ = CAIBN = 10−3 M, and T = 65 °C. H


Article

Macromolecules *E-mail: [email protected] (O.S.).

of the ternary complexes decreased in the order [I−CuIIL]+ > [Br−CuIIL]+ > [Cl−CuIIL]+ > [F−CuIIL]+, while affinity of CuII for X− (halidophilicity) had the opposite trend. KATRP for the reaction between BnX and [CuITPMA]+ was estimated to follow the order KATRP,F < KATRP,Cl < KATRP,I < KATRP,Br, indicating the high stability and weak reactivity of the C−F functionality under ATRP conditions. This suggested that ATRP activation of a fluorinated polymer might proceed from reactive defects in the polymer backbone, such as allyl fluoride groups. Scheme 3 illustrates the general characteristics of an ATRP in the presence of fluoride initiators. Reverse ATRP in the

ORCID

Abdirisak A. Isse: 0000-0003-0966-1983 Armando Gennaro: 0000-0002-7665-7178 Krzysztof Matyjaszewski: 0000-0003-1960-3402 Author Contributions

S.L. and M.F. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.G. and S.L. acknowledge Università di Palermo for FFR 2012−2013 grant. Fondazione Ing. Aldo Gini is gratefully acknowledged for a research visiting grant (M.F.). Support from NSF (CHE 1400052) is also acknowledged. Gayathri Withers and Roberto R. Gil are acknowledged for assistance in recording the NMR spectra and for helpful discussions.

Scheme 3. General Characteristics of ATRP in the Presence of C−F Initiators

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(1) Banks, R. E.; Smart, B. E.; Tatlow, J. C. Organofluorine Chemistry: Principles and Commercial Applications; Springer: New York, 1992. (2) Dinoiu, V. Fluorine Chemistry: Past, Present and Future. Rev. Roum. Chim. 2006, 51, 1141−1152. (3) Bruno, A. Controlled Radical (Co)Polymerization of Fluoromonomers. Macromolecules 2010, 43, 10163−10184. (4) Ameduri, B.; Boutevin, B.; Kostov, G. Fluoroelastomers: Synthesis, Properties and Applications. Prog. Polym. Sci. 2001, 26, 105−187. (5) Ameduri, B.; Boutevin, B. Update on Fluoroelastomers: From Perfluoroelastomers to Fluorosilicones and Fluorophosphazenes. J. Fluorine Chem. 2005, 126, 221−229. (6) Moore, A. L. Fluoroelastomers Handbook: The Definitive User’s Guide and Databook; William Andrew Publishing: New York, 2006. (7) Williams, S. S.; Hampton, M. J.; Gowrishankar, V.; Ding, I.-K.; Templeton, J. L.; Samulski, E. T.; DeSimone, J. M.; McGehee, M. D. Nanostructured Titania− Polymer Photovoltaic Devices Made Using PFPE-Based Nanomolding Techniques. Chem. Mater. 2008, 20, 5229− 5234. (8) Gratton, S. E. A.; Pohlhaus, P. D.; Lee, J.; Guo, J.; Cho, M. J.; DeSimone, J. M. Nanofabricated Particles for Engineered Drug Therapies: A Preliminary Biodistribution Study of Print Nanoparticles. J. Controlled Release 2007, 121, 10−18. (9) DeSimone, J. M.; Guan, Z.; Elsbernd, C. S. Synthesis of Fluoropolymers in Supercritical Carbon Dioxide. Science 1992, 257, 945−947. (10) Tumbleston, J. R.; Shirvanyants, D.; Ermoshkin, N.; Janusziewicz, R.; Johnson, A. R.; Kelly, D.; Chen, K.; Pinschmidt, R.; Rolland, J. P.; Ermoshkin, A.; Samulski, E. T.; DeSimone, J. M. Continuous Liquid Interface Production of 3D Objects. Science 2015, 347, 1349−1352. (11) Zhao, X.; Cheng, J.; Chen, S.; Zhang, J.; Wang, X. Hydrophilic Modification of Poly(Vinylidene Fluoride) (PVDF) by in Situ Polymerization of Methyl Methacrylate (MMA) Monomer. Colloid Polym. Sci. 2010, 288, 1327−1332. (12) Kang, G.-d.; Cao, Y.-m. Application and Modification of Poly(Vinylidene Fluoride) (PVDF) Membranes − a Review. J. Membr. Sci. 2014, 463, 145−165. (13) Thakur, V. K.; Lin, M.-F.; Tan, E. J.; Lee, P. S. Green Aqueous Modification of Fluoropolymers for Energy Storage Applications. J. Mater. Chem. 2012, 22, 5951−5959. (14) Sauguet, L.; Boyer, C.; Ameduri, B.; Boutevin, B. Synthesis and Characterization of Poly (Vinylidene Fluoride)-G-Poly (Styrene) Graft Polymers Obtained by Atom Transfer Radical Polymerization of Styrene. Macromolecules 2006, 39, 9087−9101. (15) Matyjaszewski, K.; Davis, T. P. Handbook of Radical Polymerization; Wiley: Hoboken, NJ, 2002.

presence of [X−CuIITPMA]+ complexes indicated that the CuII−F bond is strong and does not readily transfer the fluorine atom to propagating or initiator radicals. Indeed, slow deactivation hampered control of reverse ATRP in the presence of fluoride anions. However, 19F NMR highlighted that deactivation produced well-defined C−F chain ends. CV and 19F NMR proved that the C−F bond in the model initiator DEFM could be activated by both [CuITPMA]+ and [CuIMe6TREN]+, the latter being more active, similarly to the trend observed for commonly used chloride- or bromide-based initiators. Normal ATRP of MA and Sty was successful with DEFM as initiator and CuIBr/TPMA as catalyst. For ATRP of MA, increasing the temperature from 25 to 80 °C led to an increase in initiation efficiency up to 73%. An initiation efficiency of 95% was achieved with Sty at 65 °C, suggesting that reactivity and stability of radicals is fundamental to regulate the reaction kinetics. However, Mn did not increase with conversion. This suggests that slow deactivation of radicals was followed by even slower reactivation of the chain-end, limiting conversion and control over polymer growth in the presence of the C−F chain-end functionality.

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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.6b02286. Experimental procedures, effect of temperature on the redox properties of the copper complexes, estimation of ⊖ KATRP and ERX/R • +X −, and additional polymerization results (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.M.). *E-mail: [email protected] (A.G.). I


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