Living ROMP Syntheses and Redox Properties of Triblock

Article pubs.acs.org/Macromolecules

Living ROMP Syntheses and Redox Properties of Triblock Metallocopolymer Redox Cascades Haibin Gu,†,‡ Roberto Ciganda,‡ Patricia Castel,‡ Jaime Ruiz,‡ and Didier Astruc*,‡ †

Key Laboratory of Leather Chemistry and Engineering of Ministry of Education, Sichuan University, Chengdu 610065, P. R. China ISM, UMR CNRS No. 5255, University of Bordeaux, 33405 Talence, Cedex, France

‡

S Supporting Information *

ABSTRACT: A number of reports have described the synthesis by ring-opening metathesis polymerization (ROMP) of diblock metallopolymers containing ferrocene and organocobalt complexes using the very efficient Grubb’s thirdgeneration metathesis catalyst. Here the first triblock metallocopolymers are reported using this catalyst with ferrocenyl, pentamethylferrocenyl, and cobalticenyl groups in the side chains of a polynorbornene backbone. Among the six possible synthetic schemes for the successive ROMP reactions of the blocks, two of them have been successfully conducted with adequate characterization using the endgroup analysis, MALDI-TOF mass spectroscopy, and Bard−Anson’s electrochemical method. The cyclic voltammetry studies showed that the integrity of reversible redox systems was preserved in cascades of heterogeneous electron transfers in both metallocopolymers with significant adsorption and electrostatic effects. This research paves the way for the design and synthesis of multiblock metallocopolymers offering access to multiproperty nanomaterials.

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INTRODUCTION Redox cascades are involved in various area of synthesis,1 medicine,2,3 biochemistry,4,5 and nanomaterials,6−10 as for instance total synthesis,1 physiological functions and human diseases2 including oxidative-stress-induced cancer,2b regulation of gene expression,3 signaling in plants,4 biocatalysis,5 dendrimer6,7 and fullerene8 electrochemistry,6−8 sensing and catalysis,7 photoreduction of CO,2,9 gold nanocluster charging,10 and catalysis by metal clusters11 with the assistance of metal−oxo cluster cocatalysis.11b Late-transition-metal sandwich complexes12 also provide multiple redox changes13 given their delocalized electronic structures14 that allow the stabilization of several oxidation states. In particular, disandwich electron-reservoir complexes containing delocalized polyaromatic ligands provide multielectron redox cascades15 including several mixed valence species.16 Metal-containing polymers are a rich field concerning redox properties toward materials applications.17 Thus, another approach to molecular electronreservoir systems providing redox cascades is now proposed with metallopolymers synthesized by living ring-opening metathesis polymerization (ROMP) that allows the introduction of several metallopolymer blocks.18 Transition-metal sandwich-containing polymers have been abundantly published,19 and recently a number of reports have appeared with ferrocenyl,20 pentamethylferrocenyl,21 and cobaltocenyl22 units from our group20−22 and others23,24 using the very efficient and practical third-generation Grubbs’ catalyst 1 for living ROMP.18d,25 Here we report the ROMP syntheses and electrochemical properties of the first triblock metallopolymers containing three distinct redox-active metal-sandwich moieties © XXXX American Chemical Society

in the side chains, namely, the ferrocenyl (Fc), pentamethylferrocenyl (Fc*), and cobalticenyl hexafluorophosphate (CcX) groups.

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RESULTS AND DISCUSSION In the three norbornene derivatives 2, 3, and 4, the metalloredox Fc*, Fc, and CcX moieties, respectively, were connected to the norbornene group by an ethyleneamido linker,20 and these three compounds were used as monomers to prepare triblock metallocopolymers following two synthesis routes shown in Scheme 1. The controlled and living characteristics of the ROMP of these monomers were demonstrated in previous studies.20−22 These ROMP reactions provide the possibility to prepare block copolymers with these three pendant redox groups, but the feasibility of the synthetic routes depends on the features of the ROMP reactions of the corresponding homopolymers. In this respect key differences were found among these routes. For example, dichloromethane (CH2Cl2) is a perfect solvent for the ROMP of neutral monomers 2 and 3, while dimethylformamide (DMF) is the only choice for the polymerization of the cationic monomer 4. For the Fc monomer 3, its ROMP polymerization requires a prolonged reaction time depending on the size of the resulting polymers (from 15 min to several days), whereas ROMP of the Fc* monomer 2 is completed in 10 min with nearly 100% conversion whether the feed molar Received: May 18, 2016 Revised: June 8, 2016

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DOI: 10.1021/acs.macromol.6b01046 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthesis of the Two Triblock Metallocopolymers 7 and 9

of monomer 4 at 6.30 ppm in CD2Cl2 and 6.32 ppm in acetone-d6 had disappeared. As expected, unlike in the case of the ROMP of 4 in DMF for which 30 min is required, it took only 20 min to achieve 100% conversion in the mixture of dry CH2Cl2 and DMF. In DMF, the slow reaction rate is mainly taken into account by its limitation as a solvent in ROMP due to its coordination to the Ru center that competes with the Ru initiation of the ROMP reaction. However, DMF as a solvent is useful because it keeps the cationic 4 and its homopolymer soluble. In CH2Cl2, the first Fc*-containing block endows the subsequent second cationic CcX block with good solubility, and this solvent lacks damaging coordination properties, allowing the catalyst 1 to fully play its role as highly efficient and fast ROMP initiator. The 1H NMR spectrum of the copolymer 6 is compared to that of homopolymers of 2 leading to the structure of 6 (Figure 1). Figure 1B shows the 1H NMR spectrum of diblock copolymer 6 in DMSO-d6. The peaks at 6.21, 5.90, and 5.85 ppm correspond to the characteristic substituted and free Cp protons in the CcX moiety, respectively, while the broad peak at 9.08 ppm originates from the amido proton adjacent to the [(η5-C5H4)CoCp] structure. These results clearly demonstrate the successful polymerization of the second block. Meanwhile, the characteristic double peaks arising from the olefinic protons of the polynorbornene fragment appear at 5.65 and 5.48 ppm. For the first Fc*-containing block, the amido proton close to the Fc* unit is located at 7.46−6.91 ppm, which mixes with the protons of end-phenyl group, and the characteristic methyl protons of the Fc* moiety is found at 1.98 ppm. The peaks of the substituted Cp protons are not observed, even high resolution (600 MHz) NMR. When CD2Cl2 is used as solvent, however, the two broad weak peaks at 4.48 and 4.30 ppm were assigned to the Cp protons (Figure S31). This weak intensity, if any, is taken into account by the poor relaxation of groups that are buried inside the macromolecules. In spite of this phenomenon, the integrity of the Fc and CcX units is further confirmed by the 13C NMR spectrum of copolymer 6 in CD2Cl2 (Figure S32). Indeed, the peak at 10.3 ppm is assigned

ratio is high or low. The slowest rate was found the ROMP of monomer 4 in DMF, in which 30 min (at least) is required to achieve 100% conversion. Besides these solvent and reaction rate factors, solubility difference of the prepared polymers is another problem that should not be neglected upon designing the synthesis route of the triblock metallocopolymers. Based on the above considerations and practical preliminary experiments, only two feasible synthetic routes were actually successfully conducted in the present study as shown in Scheme 1 among the six (3!) possible variations of synthetic schemes. ROMP Synthesis of Diblock Copolymer 6. The diblock copolymer 6 containing the pendant neutral Fc* and cationic CcX units was prepared by chain extension of the Fc*containing homopolymer 5 with ruthenium (Ru) end to the second monomer 4 via one-pot, two-step sequential ROMP. The order of monomer introduction in this ROMP synthesis of 6 was guided by the reaction rates and solubilities observed during the polymerization of each monomer and the satisfactory polymerization of 4 using the Ru-ended homopolymer 5 as first block. The rate of ROMP of 2 in dry CH2Cl2 was found to be higher than that of 4 in dry DMF. In the synthesis of 6, all the polymerization was carried out at room temperature (rt) using the very efficient catalyst 1.23 The feed molar ratio of [monomer 2]:[monomer 4]:[catalyst 1] was 25:25:1. The ROMP of monomer 2 was first conducted in dry CH2Cl2, and the obtained Fc*-containing homopolymer 5 with Ru end then was used as a macromolecular initiator to initiate the ROMP of the monomer 4 in a mixture of CH2Cl2 and DMF. It took 10 min to complete the ROMP of 2 with 100% conversion, then 4 in dry DMF was added, and a kinetic study was carried out to monitor the polymerization of 4. For this reaction, a small sample of the reaction mixture was taken out at different intervals, quenched by ethyl vinyl ether (EVE) and precipitated by addition of diethyl ether (Et2O). The 1H NMR spectra of the obtained precipitate and filtrate were recorded in CD2Cl2 and acetone-d6, respectively. The monomer conversion was deemed to be 100% when the signal of the olefin protons B


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Figure 1. 1H NMR spectra of the homopolymer of 2 (A), diblock copolymer 6 (B), and triblock copolymer 7 (C) in DMSO-d6.

to the characteristic methyl carbon of η5-C5Me5, while peaks at 87.0, 86.2, and 85.5 ppm correspond to the substituted Cp carbon of the CcX unit. All the other peaks of the 1H and 13C NMR spectra are suitably assigned and match well with the structure of the copolymer 6. Furthermore, the UV−vis spectrum of the copolymer shows a maximum absorption (λmax) at 416 nm in CH2Cl2 (Figure S34), which is intermediate between the λmax value of the Fc* homopolymer at 440 nm and that of the CcX homopolymer at 402 nm. So, this absorption should be attributed to the consequence of d−d* transition from Fc* and CcX units in the two blocks.

The copolymer 6 was obtained as a yellow-green powder after precipitation from CH2Cl2 with Et2O. Because of the combined effect of its two blocks, in general the solubility of the copolymer is larger than that of the CcX homopolymer and weaker than that of Fc* homopolymer. It is soluble in CH2Cl2, chloroform (CHCl3), and strong polar solvents such as DMF and dimethyl sulfoxide (DMSO) but not soluble in tetrahydrofuran (THF), which is good solvent for Fc* homopolymer. It cannot be dissolved either in other common organic solvents including methanol, acetone, and acetonitrile. Because of the polycationic nature of the second CcX block, it is very difficult to determine the molecular weight (MW) C


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Macromolecules Table 1. Polymerization Degrees for Each Block in the Di- and Triblock Copolymers copolymer [Mfir]:[Msec]:[Mthi]:[C]a block convb (%) np1c np2d np3e

Fc* >99 25 25 ± 22 ±

6 25:25:1 CcX >99 25 1 19 ± 2 2 23 ± 2

Fc* >99 25 25 ± 1 28 ± 3

7 25:25:25:1 CcX >99 25 22 ± 2 13 ± 2

Fc >99 25 23 ± 2 25 ± 2

Fc* >99 25 25 ± 19 ±

8 25:25:1 Fc >99 25 1 25 ± 1 2 21 ± 2

Fc* >99 25 25 ± 1 29 ± 3

9 25:25:25:1 Fc >99 25 25 ± 1 32 ± 3

CcX >99 25 25 ± 3 16 ± 2

[Mfir]:[Msec]:[Mthi]:[C]: feed molar ratio of the first, second, and third monomers to catalyst 1. bMonomer conversion determined by 1H NMR. Polymerization degree obtained from 1H NMR data using monomer conversion. dPolymerization degree determined by 1H NMR end-group analysis. eDegree of polymerization calculated using the Bard−Anson electrochemical method. a c

Figure 2. MALDI-TOF mass spectrum of diblock copolymer 6. (A) is the region of 4999−20 000 Da in the spectrum. The red dotted lines correspond to the difference between molecular peaks of a value of 620 ± 1 Da (MW of monomer 2). See the enlarged region of the spectrum from 7184 to 9855 (B) in the Supporting Information (Figure S35).

value. The relative inaccuracy of the latter value results from the large polymer size 6. The MALDI-TOF mass spectrum of the copolymer 6 (Figure 2 and Figure S35) shows the peaks for polymer fragments in the region up to about 16 000 Da, which is close to the upper limit for this technique. As expected, there are obvious and well-defined individual peaks for polymer fragments that are separated by 620 ± 1 Da, which exactly corresponds to the mass of one monomer unit 2. In the enlarged region (Figure S35B), the intervals between the peaks are 566 ± 1 Da corresponding to the MW of 4. However, peaks with this interval are not very strong because they are submerged by the strong signals from the neutral Fc* block. Even so, these results definitely show the successful formation of the copolymer 6 containing both the Fc* and CcX units. ROMP Synthesis of the Triblock Copolymer 7. The successful preparation of the diblock copolymer 6 with pendant Fc* and CcX moieties provides the opportunity to synthesize the triblock copolymer 7 containing Fc*, CcX, and Fc blocks. As shown in Scheme 1, the triblock copolymer 7 with the successively pendant neutral Fc*, cationic CcX, and neutral Fc units is prepared by chain extension of Fc* and CcX-containing copolymer 6 with Ru end to the third Fc monomer 3 via onepot, three-step sequential ROMP. The feed molar ratio of [monomer 2]:[monomer 4]:[monomer 3]:[catalyst 1] is

distribution of the diblock copolymer 6 by size exclusion chromatography (SEC). Thus, only the end-group analysis, MALDI-TOF MS, and Bard−Anson’s electrochemical method (vide inf ra) were used. First, the polymerization degree of the first Fc* block is obtained by 1H NMR end-group analysis in CD2Cl2 (Figure S5) that is conducted by comparing the intensities of the signals of the five protons of the end-phenyl group with those of the characteristic protons in the Fc* block. This polymerization degree is calculated by comparing the integration of the five protons of the end-phenyl (7.38−7.22 ppm) with those of the amido proton (6.16 ppm), olefinic protons (5.72−5.55 ppm), substituted Cp protons (4.13 and 3.87 ppm), methyl protons of C5Me5 (1.74 ppm), and linker protons (3.59−3.50 ppm). Then, using the 1H NMR spectrum of 6 in DMSO-d6 (Figure S30), the polymerization degree of the second CcX block was calculated by comparing the integration of the end-phenyl and amido proton (7.46−6.91 ppm) in the Fc* block with that of the protons of amido (9.08 ppm), substituted and free Cp (6.21, 5.90, and 5.85 ppm) in the second CcX block, respectively. The calculated polymerization degree (np2) of the first (Fc*) block is 25 ± 1 (Table 1), showing consistency with the theoretical value of 25 for the first block from the 1H NMR conversion, while that of the second (CcX) block is 19 ± 2, which is lower than the theoretical D


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Macromolecules 25:25:25:1. Following the previous synthesis procedure, the ROMP synthesis of copolymer 6 is conducted in a mixture of dry CH2Cl2 and DMF, and the obtained Fc* and CcXcontaining copolymer 6 with Ru end is then used as macromolecular initiator to initiate the ROMP of monomer 3 in the same solvent mixture. It took 10 min to complete the ROMP of 2 with 100% conversion and then 20 min to achieve complete polymerization of 4, and a kinetic study was carried out to monitor the polymerization of 3 in the mixture of CH2Cl2 and DMF. For this reaction, a small sample of the reaction mixture is taken out at different intervals, quenched by EVE and precipitated by adding of Et2O. The 1H NMR spectra of the obtained precipitate and filtrate are recorded in CD2Cl2 and CDCl3, respectively. The monomer conversion is deemed to be 100% when the signal of the olefin protons for monomer 3 at 6.28 ppm in CD2Cl2 and 6.25 ppm in CDCl3 disappears. As expected, a prolonged reaction time (2 h) is necessary to achieve 100% conversion of Fc monomer 3 in a mixture of dry CH2Cl2 and DMF due to the large size of the targeted copolymer and the adverse solvent of DMF that slows down the ROMP reaction due to its coordination to the Ru center. In the 1H NMR spectrum of the triblock copolymer 7 in DMSO-d6 (Figure 1C), the three peaks at 4.78, 4.31, and 4.14 ppm correspond to the characteristic substituted and free Cp protons in the Fc moiety, respectively, while the broad peak at 7.79 ppm originates from its adjacent amido proton. These peaks demonstrated undisputedly the successful polymerization of the third, Fc-containing block. Moreover, for the second CcX-containing block, the amido proton close to the CcX unit is observed at 8.92 ppm, and the three peaks at 6.17, 5.90, and 5.84 ppm are assigned to the characteristic substituted and free Cp protons of the CcX structure, respectively, which confirms the integrity of the CcX-containing block. For the first Fc*containing block, the amido proton close to the Fc* unit is located at 77.49−7.20 ppm, mixed with the protons of the endphenyl group, and the two weak broad peaks at 4.43 and 4.02 ppm are assigned to the substituted Cp protons of the Fc* moiety, while its characteristic methyl protons of η5-C5Me5 is found at 1.96 ppm. Thus, the integrity of the Fc* block is also fully proven. Furthermore, the double broad peaks at 5.64 and 5.46 ppm correspond to the characteristic olefinic protons of the polynorbornene backbone, and the other peaks are well assigned and match well with the structure of copolymer 7. The 1 H and 13C NMR spectra recorded in CD2Cl2 (Figures S40 and S41, respectively) and the IR spectrum (Figure S42) further confirm the structure of the copolymer 7. The UV−vis spectrum in CH2Cl2 of the triblock copolymer 7 (Figure 3) shows a maximum absorption (λmax) at 421 nm attributed to the d−d* transition in the Fc*, CcX, and Fc units in the three blocks. The copolymer 7 is obtained as a yellow-brown powder after precipitation from CH2Cl2 with Et2O. Because of the effect of its three blocks, in general the solubility of the copolymer is better than that of cationic CcX homopolymer and is worse than that of neutral Fc* and Fc homopolymers. Like the copolymer 6, it is not soluble in common organic solvents such as methanol, acetone, acetonitrile, THF, and water but soluble in CH2Cl2, CHCl3, and strong polar solvents including DMF and DMSO. Because of the cationic property of the second CcX block, it is very difficult to determine the MW distribution of the triblock copolymer 7 by SEC. Only end-group analysis,

Figure 3. UV−vis spectra of the triblock copolymer 7 (in CH2Cl2) and homopolymers of 2 (in CH2Cl2), 3 (in CH2Cl2), and 4 (in acetone).

MALDI-TOF MS, and Bard−Anson’s electrochemical method (vide inf ra) were used. First, the polymerization degree of the first Fc* block is calculated by 1H NMR end-group analysis in CD2Cl2 (Figure S5) that is conducted by comparing the intensities of the signals of the five protons of the end-phenyl group with those of the characteristic protons in the Fc* block, and the obtained value is 25 ± 1, showing consistency with the theoretical value of 25 for the first block from the 1H NMR conversion. Then, this value of 25 ± 1 is used for the calculation of polymer degree of the second CcX block, in which the 1H NMR spectrum in DMSO-d6 of copolymer 7 is used (Figure S39). The polymerization degree of the second CcX block is calculated by comparing the integration of the end-phenyl and amido proton (7.49−7.20 ppm) in the Fc* block with that of the protons of amido (8.91 ppm), substituted and free Cp (6.17, 5.90, and 5.84 ppm) in the second CcX block, respectively. As shown in Table 1, the calculated polymerization degree (np2) of the second (CcX) block is 23 ± 2, which is also consistent with the theoretical value of 25. Finally, the polymerization degree of the third Fc block is calculated by comparing the integration of the end-phenyl and amido proton (7.49−7.20 ppm) in the Fc* block with that of the protons of amido (7.79 ppm), substituted and free Cp (4.78, 4.31, and 4.14 ppm) in the third Fc block, respectively, and the calculated polymerization degree is 22 ± 2, which is close to the theoretical value of 25 from the 1H NMR conversion. The above results of polymerization degrees for the three blocks undisputedly demonstrate the living and controlled polymerization. The MALDI-TOF mass spectrum of the triblock copolymer 7 shows peaks for polymer fragments up to about 25 000 Da (Figure S44A). As expected, there are well-defined individual peaks for the polymer fragments that are separated by 620 ± 1 Da, which exactly corresponds to the mass of one unit of 2. In the enlarged region from 7142 to 9961 Da (Figure S44B), the intervals between the peaks are 566 ± 1 Da, corresponding to the MW of 4, and 550 ± 1 Da, assigned to the MW of 3. However, peaks with these intervals are weak because they are submerged by the strong signals from the neutral Fc* block fragments. ROMP Synthesis of the Triblock Copolymer 9. As the triblock copolymer 7, the triblock copolymer 9 is also composed of the Fc*, Fc, and CcX blocks. However, they are different copolymers because the three blocks show different relative location, and their synthesis route is also different as shown in Scheme 1. In the copolymer 7, the cationic CoE


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Figure 4. 1H NMR spectrum of the triblock copolymer 9 in DMSO-d6.

100% in a short time, for example, in 1 or 2 h. Overnight stirring is necessary to achieve 100% monomer conversion, although the homopolymer of 4 with a polymerization degree of 25 is obtained in 30 min. The slow reaction rate is attributed not only to the mediocre efficiency of DMF as a ROMP solvent in the mixture solvent but also to the larger size of the prepared triblock copolymer. The triblock copolymer 9 shows almost the same 1H NMR spectrum (Figure 4) as the triblock copolymer 7 in DMSO-d6. The peaks at 6.17, 5.90, and 5.84 ppm correspond to the characteristic substituted and free Cp protons in the CcX moiety, respectively, whereas the broad peak at 8.92 ppm originates from the amido proton adjacent to the [(η5C5H4)CoCp] unit. These results clearly demonstrate the successful polymerization of the third, CcX-containing block. Moreover, for the Fc-containing block, the amido proton close to the Fc unit is observed at 7.79 ppm, and the three peaks at 4.78, 4.31, and 4.14 ppm are assigned to the characteristic substituted and free Cp protons of Fc structure, respectively, which confirms the integrity of the Fc block. For the first Fc*containing block, the signal of the amido proton close to the Fc* unit is located at 7.50−7.21 ppm, mixing with the protons of the end-phenyl group. The two weak broad peaks at 4.44 and 4.02 ppm are assigned to the substituted Cp protons of the Fc* moiety, while the characteristic signal of the methyl protons of η5-C5Me5 is found at 1.97 ppm. Hence, the integrity of the Fc* block is also fully demonstrated. Furthermore, the double broad peaks at 5.64 and 5.46 ppm correspond to the characteristic olefinic protons of the polynorbornene backbone, and the other peaks are suitably assigned and matched well with the structure of copolymer 9. The 1H NMR (Figure S58) and 13C NMR (Figure S59 and S60) spectra recorded in CD2Cl2 and the FTIR spectrum (Figure S61) can further confirm the structure of the copolymer. The UV−vis spectrum of 9 (Figure S62) also shows a maximum absorption (λmax) at 421 nm in CH2Cl2, attributed to the d−d* transition of the Fc*, Fc, and CcX units in the three blocks. In conclusion, the two triblock copolymers, 7 and 9, which are prepared by different synthesis routes, show almost same NMR, IR, and optical characteristics, although

containing block is embedded between neutral Fc* and Fc blocks, namely, the second block. However, in the copolymer 9, the cationic Co-containing block is the third block which is located at one end of the triblock copolymers, and the first two block are composed of the adjacent neutral Fc* and Fc units. This difference in block arrangement results in an obvious difference in synthesis (Scheme 1). First, the diblock copolymer 8 with pendant Fc* and Fc units is prepared by chain extension of Fc*-containing homopolymer 5 with Ru end to the second monomer 3 via one-pot, two-step sequential ROMP. The overall ROMP is carried out at rt using the initiator 1, and the feed molar ratio [monomer 2]:[monomer 3]:[catalyst 1] is 25:25:1. The kinetic study showed that it took 15 min to complete the ROMP of 3 with 100% conversion. The successful preparation of the diblock copolymer 8 paves the way to synthesize the triblock copolymer 9 in which the third block contains the cationic CcX moieties. As shown in Scheme 1, the triblock copolymer 9 with the successive pendant neutral Fc*, Fc, and cationic CcX units is prepared by chain extension of Fc* and Fc-containing copolymer 8 with Ru end to the third cationic CcX monomer 4 via one-pot, three-step sequential ROMP. The feed molar ratio of [monomer 2]:[monomer 3]: [monomer 4]:[catalyst 1] is 25:25:25:1. The synthesis of the first two blocks is carried out in dry CH2Cl2, and the obtained Fc* and Fc-containing copolymer 8 with Ru end is then used as macromolecular initiator to initiate the ROMP of monomer 4 in the mixture of CH2Cl2 and DMF. It took 10 min to complete the ROMP of 2 with 100% conversion and then 15 min to achieve complete polymerization of 3. Then monomer 4 in dry DMF was added, and a kinetic study was carried out to monitor the ROMP process of monomer 4 in the mixture of CH2Cl2 and DMF. For this reaction, a small sample of the reaction mixture is taken out at different intervals, quenched by EVE, and precipitated by the addition of Et2O. The 1H NMR spectra of the obtained precipitate and filtrate are recorded in CD2Cl2 and CDCl3, respectively. The monomer conversion is deemed to be 100% when the signal of the olefin protons for monomer 4 at 6.30 ppm disappears. The monomer conversion of 4 does not reach F


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Macromolecules their three blocks containing Fc*, Fc, and CcX units have been synthetically introduced in different orders. The copolymer 9 was obtained as a yellow-brown solid powder after precipitation from CH2Cl2 with Et2O, and it shows solubility properties similar to those of copolymer 7. It is soluble in CH2Cl2, CHCl3, and strong polar solvents such as DMF and DMSO but not soluble in other common organic solvents such as methanol, acetone, acetonitrile, and water. The end-group analysis, MALDI-TOF MS, and Bard− Anson’s electrochemical method (vide inf ra) were used to determine the MW distribution of the triblock copolymer 9. As for copolymer 7, the polymerization degree of the first Fc* block of 9 is calculated by 1H NMR end-group analysis in CD2Cl2 (Figure S5) that is conducted by comparing the intensities of the signals of the five protons of the end-phenyl group with those of the characteristic protons in the Fc* block. The obtained polymerization degree for the Fc* block is 25 ± 1, which is in excellent agreement with the theoretical value of 25 (Table 1). Then, using the 1H NMR spectra of 8 in CD2Cl2 (Figure S49), the polymerization degree of the second Fc block is calculated by comparing the integrations of the signals of the amido proton (6.50 and 6.14 ppm) and substituted Cp protons (4.69, 4.32, and 3.87 ppm). These calculations show that the number of Fc units is equal to that of Fc* units, namely, the polymerization degree for the second block is 25 ± 1 (Table 1), too. Finally, the polymerization degree of the third CcX block was calculated by comparing the integration of amido protons (7.79 and 8.92 ppm), substituted Cp protons (4.78 and 6.17 ppm), substituted and free Cp protons (4.31 and 4.14 ppm, 5.90 and 5.84 ppm) in the second and third blocks, respectively. As shown in Table 1, the calculated polymerization degree (np2) of the third (CcX) block is 25 ± 3, which is also consistent with the theoretical value of 25. These results further confirm the living and controlled characteristics for ROMP synthesis of copolymer 9. The MALDI-TOF mass spectrum of the triblock copolymer 9 in the region up to about 25 000 Da (Figure S63) shows welldefined individual peaks for polymer fragments that are separated by 620 ± 1 Da, which exactly corresponds to the mass of one unit of monomer 2. Similarly, the signals arising from the neutral Fc and cationic Co blocks are weak, so that it is necessary to amplify the spectrum to observe them. In the enlarged region from 7830 to 10 580 Da (Figure S63B), the interval shown by blue dotted lines corresponding to the difference between molecular peaks of a value of 550 ± 1 Da is equal to the MW of monomer 3, while the interval peaks that are separated by 566 ± 1 Da (pink dotted lines) are assigned to the MW of monomer 4. Thus, these MS results further demonstrate the presence of the expected three blocks in the copolymer 9. Electrochemical Properties of Copolymers. The electrochemical properties of all the obtained di- and triblock copolymers were investigated by cyclic voltammetry (CV) using decamethylferrocene, [FeCp*2], as the internal reference26 and [n-Bu4N][PF6] as the supporting electrolyte and compared with those of the corresponding homopolymers (Figure 5). Dry DMF was used as solvent for the homopolymer of 4 and copolymers 6, 7, and 9, and dry CH2Cl2 was used as the solvent for the copolymer 8 and homopolymers of 2 and 3. The E1/2 data measured vs [FeCp*2] are gathered in Tables S3, S7, S10, and S13. Cyclic voltammetry of the Fc+/Fc,13,27−29 Fc*+/Fc*,13,30 and CcX/Cc+/Cc31 redox systems is well-known. In general, the

Figure 5. Compared cyclic voltammograms of the homopolymers of 3 (a, in CH2Cl2), 2 (b, in CH2Cl2), 4 (c, in DMF), the diblock copolymers 6 (d, in DMF) and 8 (e, in CH2Cl2), and triblock copolymers 7 (f, in DMF) and 9 (g, in DMF). Internal reference: [FeCp*2]; reference electrode (0.0 V): Ag; working and counter electrodes: Pt; scan rate: 0.4 V/s; supporting electrolyte: [nBu4N][PF6].

expected redox waves that are known for the monomers are found in the CVs of the metallocopolymers.32 In spite of the high molecular weights of all the metallopolymers, the redox waves are chemically reversible;27−29 i.e., ia and ic are relatively close, although rarely identical intensities. A strong characteristic in all the CVs for the redox centers of the copolymers is the weak Epa − Epc value, signifying strong adsorption28 onto the electrode due to the large molecular weight and important solubility changes upon variations of large change of charge state. This adsorption, and thus also the Epa − Epc value varies from one center to the next, however, depending on the charge of the species involved at the electrode redox process that greatly influences the solubility. These weak Epa − Epc values of the polymer redox centers clearly contrast in all the CVs of these metallocopolymers with the standard Epa − Epc value of 0.06 V (theoretically 0.059 V at 25 °C) of the monomeric Fc* G


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electrochemically reversible and match well the CV waves obtained for the homopolymers of 2 and 3, no significant adsorption being found for the two waves. As for the triblock copolymer 7, the triblock copolymer 9 shows four reversible redox waves. The CoIII/CoII wave of the third cationic CcX block is located at E1/2 = −0.700 V vs [FeCp*2] with Epa − Epc = 0.050 V and ic/ia = 1.29, and the CoII/CoI wave is observed at −1.625 V vs [FeCp*2] with Epa − Epc = 0.120 V and ic/ia = 1.40. Decreased reversibility for this CoII/CoI wave in 9 compared to that in 7 probably results from a superior exposure of the CoCp2 groups toward the periphery of the macromolecule in 9. Indeed in 9, the CoCp2 groups are introduced last in the metallocopolymer, whereas they are in the middle of the macromolecule in 7 (Scheme 1). For the first neutral Fc* block, the FeIII/FeII wave at E1/2 = 0.380 V vs [FeCp*2] is fully chemically (ic/ia = 1.0) and essentially electrochemically reversible, although the Epa − Epc value is low (0.020 V). For the second Fc block, the FeIII/FeII wave is observed at 0.670 V vs [FeCp*2] (Epa − Epc = 0.050 V, ic/ia = 1.23), and essentially chemically (ia = ic) and electrochemically reversible. Determination of the Number of Monomer Units in the Metallocopolymers Using the Bard−Anson Electrochemical Method. Diblock Copolymer 6. The Bard−Anson method32 usually is a reliable method for the determination of the number of monomer units in the metallopolymers in the cases of redox-robust groups in polymers of relatively modest size such as those involved here. This method has already been successfully used for Fc, Fc*, and CcX-containing polymers. In the CV measurement, the total number (np) of electrons transferred in the oxidation wave of each redox center in the copolymer 6 should be the same as that of the corresponding monomer units in the copolymer, as only one electron is transferred from the cathode for each redox center. Thus, this electron number np for each block is estimated using Bard− Anson’s empirical equation that was previously derived for conventional polarography.

reference that is not marred by this problem of large molecular weight. Another strong trend is the breath of the polymeric redox waves that is due to the fact that all the identical redox centers of the polymers do not transfer electrons at exactly the same potential due to the electrostatic effect.16c,33,34 This phenomenon is much more marked than with metallostars34 and metallodendrimers7,30 because these molecules can rotate more rapidly than the electrochemical time scale and present in turn analogously all the peripheral redox groups close to the electrode.35 On the other hand, with metallopolymers, the polymer conformation differentiates the redox centers among one another,36 so that some of them are more buried than others inside the macromolecule. For the diblock copolymer 6 (Figure 5d), three reversible waves are observed in addition to the fully chemically (ia = ic) and electrochemically reversible FeIII/FeII wave of the internal reference [FeCp*2] for which the difference Epa − Epc between the anodic (Epa) and cathodic (Epc) peak potentials is 0.06. The FeIII/FeII wave of the poly-Fc* block at 0.367 V vs [FeCp*2] is essentially chemically (ic/ia = 1.06) and electrochemically reversible, although the Epa − Epc value is lower (0.010 V) than that expected for a fast redox process in the case of a single Fc* unit at 25 °C (0.059 V). The CoIII/CoII wave of the cobalticinium block is nearly chemically reversible (ic/ia = 1.06) and as the CV wave of the poly-Fc* block shows a minute potential difference between the anodic and cathodic wave (Epa − Epc = 0.020 V). Notably, this CoIII/CoII CV wave at −0.711 V vs [FeCp*2] is very large (0.2 V at half-height). The CoII/CoI wave that is close to the solvent discharge on the cathodic side is located at −1.667 V vs [FeCp*2] and is essentially chemically reversible (ic/ia = 1.14), the 20-electron CoI species being apparently relatively stable in DMF at the electrochemical time scale. The electrochemical reversibility of this CV wave is relatively good, however, since the Epa − Epc value is 0.050 V, close to the expected 0.060 V for a fully electrochemically reversible wave. No apparent cooperativity was observed between the heterogeneous electron transfer processes of the various redox units of this copolymer. For the triblock copolymer 7 (Figure 5f), four reversible redox waves are observed. As expected, the FeIII/FeII wave of the third polyferrocene block is found at E1/2 = 0.633 V vs [FeCp*2], and it is mostly chemically reversible (ic/ia = 1.24) with some some adsorption. However, this CV wave shows fully electrochemical reversibility because the Epa − Epc value is 0.060 V. For the first Fc* block, its FeIII/FeII oxidation potential is found at 0.365 V vs [FeCp*2], and this redox wave is essentially chemically (ic/ia = 1.06) and electrochemically reversible, although the Epa − Epc value is low (0.020 V). For the second CcX block, the first reduction wave corresponding to the reduction of cobalticenium to the 19-electron cobaltocenyl species (CoIII/II)31−33 is located at −0.700 V (Epa − Epc = 0.070 V; ic/ia = 1.10); the second wave corresponding to the reduction of cobaltocene to the 20electron cobalt sandwich anion (CoII/I) appears at −1.647 V (Epa − Epc = 0.110 V; ic/ia = 1.0). These two waves are essentially chemically and electrochemically reversible, and no significant adsorption is observed. The CV of the diblock copolymer 8 (Figure 5e) compares with that of triblock copolymer 9 (Figure 5g). For the diblock copolymer 8, the FeIII/II oxidation potential of the Fc* redox center is observed at E1/2 = 0.38 V versus [FeCp*2], whereas for the Fc redox center the potential appears at E1/2 = 0.64 V vs [FeCp*2]. These two CV waves are essentially chemically and

0.275 (idp/Cp) ⎛ M p ⎞ np = ⎟ ⎜ (idm /Cm) ⎝ M m ⎠

The id, M, and C are the CV wave intensity of the diffusion current, molecular weight, and concentration of the monomer (m) and polymer (p), respectively. The intensities in the CV of the copolymer and monomer are compared, and consequently the electron numbers np are determined (Table 1). The estimated values of electron numbers (np3) are 22 ± 2 for the first Fc* block and 23 ± 2 for the second CcX block, consistent with the theoretical value 25 of the polymerization degrees obtained from the 1H NMR conversion data. Therefore, in the absence of electrostatic effect (neutral FeII redox centers) the electrochemical measurements clearly confirm the controlled characteristics of the one-pot, two-step sequential ROMP of the Fc* monomer 2 and CcX monomer 4. The CcX centers appear perturbed in all CVs due to a significant electrostatic effect. This effect strongly decreases the intensity of the CV wave related to these redox sites, so that much lower np values are obtained. Triblock Copolymer 7. As for 7, the intensities in the CV of the copolymer and monomer are compared, and consequently the electron numbers np are determined. As shown in Table 1, the estimated values of electron numbers (np3) are 28 ± 3 for the first Fc* block, 13 ± 2 for the second CcX block, and 25 ± 2 for the third Fc block. The values of the neutral blocks show H


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consistency with the theoretical values of 25 from NMR conversion, whereas that of the cationic CcX block is much lower than 25 due to the electrostatic effect that flattens the CoIII/CoII redox wave. Triblock Copolymer 9. The intensities in the CVs of 9 and monomer are compared, and consequently the electron numbers np are determined; the results are listed in Table 1. The estimated values of electron numbers (np3) are 29 ± 3 for the first Fc* block, 32 ± 3 for the second Fc block, and 16 ± 2 for the third CcX block. The values of the first and second neutral blocks are somewhat higher than the theoretical values of 25 from NMR conversion due to some adsorption, whereas that of the cationic CcX block is much lower than 25. This deviation is again due to the electrostatic effect flattening the CcX CV wave in triblock copolymer 9.

CONCLUSION The remarkable efficiency of Grubbs’ third-generation catalyst opened the ROMP routes to two triblock metallocopolymers containing 25 units of each of the ferrocenyl, pentamethylferrrocenyl, and cobaltocenyl redox groups in the side chains of the polynorbornene backbone. Starting in both cases from the most soluble Ru-ended homopolymer containing the Fc* units, the two successful metallocopolymer syntheses involved either introduction of the ferrocenyl-, then the cobalticenyl-containing blocks, or the reverse. Among the classic spectroscopy and analytical characterization data (in particular, 1H and 13C NMR, FT-IR, and UV−vis spectroscopy, MALDI TOF MS, and cyclic voltammetry), some unusual behaviors were noted: (i) very weak 1H NMR signals were observed for the substituted Cp ring of the Fc* units best taken into account by relaxation problems signifying that these groups were buried with restricted motion inside the macromolecules; (ii) in cyclic voltammetry reversibility and adsorption were clear for all the waves, but electrostatic effects were selectively observed for the cationic cobalticinium groups in the polar DMF solvent whereas this effect was observed neither with the ferrocene nor with the pentamethylferrrocene groups. The consequence is that for both triblock metallocopolymers the Bard−Anson method yields values for the numbers of cationic cobaltcontaining redox centers that are much lower than the actual values due to flattening of the related CV waves. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01046. Syntheses, kinetic studies, MALDI-TOF mass, 13C NMR, SEC, IR, and UV−vis spectra of the copolymers (PDF)

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from Sichuan University, the Centre National de la Recherche Scientifique (CNRS), the Universities of Bordeaux, San Sebastian, and the Basque Country grant (RC) is gratefully acknowledged. I


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