Deciphering the Mechanism of Coordinative Chain Transfer

Dec 17, 2015 - Ethylene polymerizations were performed in toluene using the neodymocene complex (C5Me5)2NdCl2Li(OEt2)2 or ...
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Deciphering the Mechanism of Coordinative Chain Transfer Polymerization of Ethylene Using Neodymocene Catalysts and Dialkylmagnesium Rodolfo Ribeiro,† Rui Ruivo,† Hajar Nsiri,†,‡ Sébastien Norsic,† Franck D’Agosto,† Lionel Perrin,*,‡ and Christophe Boisson*,† †

Université de Lyon, Université Lyon 1, CPE Lyon, CNRS, UMR 5265, C2P2 (Chemistry, Catalysis, Polymers & Processes), Bat. 308F, 43 Bd du 11 Novembre 1918, 69616 Villeurbanne, France ‡ Interface Theory/Experiment: Mechanism & Modeling (ITEMM), ICBMS, UMR 5246-CNRS, Bat. 308-Curien (CPE Lyon), Université Claude Bernard Lyon 1, 43 Bd. du 11 Novembre 1918, 69622 Villeurbanne, France S Supporting Information *

ABSTRACT: Ethylene polymerizations were performed in toluene using the neodymocene complex (C5Me5)2NdCl2Li(OEt2)2 or {(Me2Si(C13H8)2)Nd(μ-BH4)[(μ-BH4)Li(THF)]}2 in combination with n-butyl-n-octylmagnesium used as both alkylating and chain transfer agent. The kinetics were followed for various [Mg]/[Nd] ratios, at different polymerization temperatures, with or without ether as a cosolvent. These systems allowed us to (i) efficiently obtain narrowly distributed and targeted molar masses, (ii) characterize three phases during the course of polymerization, (iii) estimate the propagation activation energy (17 kcal mol−1), (iv) identify the parameters that control chain transfer, and (v) demonstrate enhanced polymerization rates and molar mass distribution control in the presence of ether as cosolvent. This experimental set of data is supported by a computational investigation at the DFT level that rationalizes the chain transfer mechanism and the specific microsolvation effects in the presence of cosolvents at the molecular scale. This joint experimental/computational investigation offers the basis for further catalyst developments in the field of coordinative chain transfer polymerization (CCTP). KEYWORDS: neodymocene, ethylene, dialkylmagnesium, polymerization, kinetics, mechanism, DFT

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(CTAs) introduced control to the system.2 In such systems, the CTA is generally an organometallic species of a main-group metal: for instance, a trialkylaluminum. The polymer chains grow by inserting ethylene units on the transition-metal-based catalyst and then are transferred to the CTA on which they are dormant. If this transfer is reversible and rapid enough in comparison to propagation, each chain has an equal probability to propagate despite the large majority of chains being present in their dormant state. This polymerization technique is known as the catalyzed chain growth (CCG) process.2,3 More recently, this controlled coordination catalysis polymerization technique was

INTRODUCTION Controlling chain growth during a polymerization process can be an important factor in fine-tuning the properties of the final material. In the realm of olefin (ethylene and α-olefins) polymerization catalysis, living polymerization techniques have been developed to control the molar masses of polyolefins and to prepare block copolymers.1 The main impediment of these olefin living polymerizations, however, is that only one polymer chain is produced per transition-metal complex, making the process costly and therefore unattractive for industrial-scale production. In the case of ethylene, the first example of controlled coordination insertion polymerization under catalytic conditions was reported in the patent literature by Samsel et al. in 1993. Here, a reversible transfer reaction facilitated by the appropriate organometallic compounds acting as chain transfer agents © XXXX American Chemical Society

Received: October 15, 2015 Revised: December 15, 2015

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Figure 1. General kinetic profile for ethylene polymerization with (C5Me5)2LnCl2Li(OEt2)2/BOMAG (here shown for Ln = Nd; [Nd] = 50 μM, [MgR2] = 4 mM, [Mg]/[Nd] = 80, 80 °C, 4 bar).

CTA to provide new insight into this important polymerization process.

also named coordinative chain transfer polymerization (CCTP).4−6 On the basis of a degenerative chain transfer, this pseudo living polymerization shares similarities with the RAFT process developed for controlled radical polymerization.7 As for controlled radical polymerization, CCTP has enabled the synthesis of new, previously inaccessible specialty polymers. For example, researchers at Dow Chemicals prepared block copolymers by conducting CCTP using a combination of two catalysts to give new polyolefins called olefin block copolymers.8 Other research has focused on introducing chain end functionality to polyethylene by taking advantage of the high reactivity of chain ends which are all bound to a metal center to almost quantitatively functionalize postpolymerization.9,10 Another approach for introducing functionality to chain ends utilized new functional chain transfer agents, which allowed for the first time the synthesis of telechelic polyethylenes under catalytic conditions.11−13 The chain transfer reaction mechanism in CCTP is formally defined in the literature as a transmetalation that involves the formation of a heterobimetallic intermediate.3,5,14 One of the first reported catalytic systems meeting the requirements of CCTP of ethylene were the bis(pentamethylcyclopentadienyl) lanthanidebased metallocenes (C5Me5)2LnCl2Li(OEt2)2 (Ln = Sm, Nd) used in combination with a dialkylmagnesium (MgR2) as the CTA.15,16 The polymer chain in this system exists as bis(polyethylenyl)magnesium (MgPE2). MgR2 acts as both an alkylating agent and a CTA, with no additional activator required for the polymerization to proceed. In this system, it has been established that the active species is the neutral complex (C5Me5)2LnR and that chain transfer occurs between Nd and Mg via the formation of the aforementioned heterobimetallic intermediate.14 However, neither thorough kinetic studies nor DFT mechanistic investigations have been performed for this class of catalyst in order to determine the exact mechanism of this groundbreaking polymerization reaction. Little is known about the exact nature of the species involved during the polymerization and the underpinning mechanism that governs the kinetics. Here we report a combined kinetic study and DFT mechanistic investigation on ethylene polymerization performed in toluene with (C5Me5)2NdCl2Li(OEt2)217−19 or {(Me2Si(C13H8)2)Nd(μ-BH4)[(μ-BH4)Li(THF)]}217,20 as a precatalyst in the presence of n-butyl-n-octylmagnesium (BOMAG) as a

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RESULTS Experimental Investigation. Figure 1 shows the typical kinetic profile for the (C5Me5)2LnCl2Li(OEt2)2/BOMAG catalytic system. After a short period during which the activity drops (phase 1), a constant activity corresponding to the steady state is observed (phase 2) until the formed MgPE2 reaches its limit of solubility and precipitates. In conjunction with this precipitation, an increase in activity is observed (phase 3), which is followed by the deactivation of the catalyst. This general kinetic profile was also found for the Nd system (C5Me5)2NdCl2Li(OEt2)2/BOMAG examined here when polymerizing ethylene. Hereby, aliquots were regularly withdrawn from the polymerization medium in the time period before the precipitation of MgPE2 took place and analyzed by hightemperature SEC (Table 1). The theoretical values of the Table 1. Characteristic Values of the Molar Mass Distributions and Productivities during Ethylene Polymerization Performed with (C5Me5)2NdCl2Li(OEt2)2/ BOMAG as a Catalytic Systema sample 1 2 3 4 5 6

time (min)

productivityb (kg mol−1)

Mntheo c (g mol−1)

Mnd (g mol−1)

Đ

10 20 40 60

186 262 343 581 660 817

580 817 1070 1815 2061 2553

780 980 1320 1740 2370 2870

2.2 1.4 1.4 1.3 1.3 1.3

120

Conditions: T = 80 °C; 4 bar; V(toluene) 200 mL; [(C5Me5)2NdCl2Li(OEt2)2] 50 μM; [BOMAG] 8 mM; [Mg]/[Nd] = 160. bMeasured using the solid content method for the samples. c Mntheo = productivity/(2*[Mg]/[Nd]). dDetermined by SEC in trichlorobenzene at 150 °C using a PE calibration. a

number-average molar mass (Mn) were calculated by assuming that each R group of MgR2 initiated a polyethylene chain. In agreement with the characteristics of a pseudo living polymerization, Mn increased linearly with the productivity and a good agreement with theoretical values was observed. The dispersity was below 1.4, except for the first sample taken at the beginning 852

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Figure 2. SEC analyses of the different samples (Table 1) withdrawn during ethylene polymerization performed with (C5Me5)2NdCl2Li(OEt2)2/ BOMAG as a catalytic system (T = 80 °C; 4 bar; V(toluene) = 200 mL; [(C5Me5)2NdCl2Li(OEt2)2] 50 μM; [MgR2] 8 mM; [Mg]/[Nd] = 160).

heterobimetallic neodymium−magnesium intermediates and will be further commented in the Discussion. In order to assess the activation energy of this polymerization reaction, four apparent polymerization constants kpapp were measured in the temperature range from 70 to 85 °C using standard polymerization conditions (Table S2 in the Supporting Information). The Arrhenius plot (Figure 3) of the kpapp values led to the determination of an activation energy of 17 kcal mol−1. One should note that the used kpapp value and its associated activation energy account for both (i) the dissociation of the catalyst from its dormant to its active state and (ii) the insertion of ethylene. The fast exchange between chains prevents the determination of the exact fraction of active sites (C*), so that it was not possible to deconvolute kpapp in polymerization propagation rate constant (kp) and C*.21 Though the reaction under scrutiny combines two steps, the overall enthalpy and entropy of activation have been quantified as ΔH⧧ = 16.5 kcal mol−1 and ΔS⧧ = −6 cal K−1 mol−1 by means of an Eyring plot (Figure S2 in the Supporting Information). These activation parameters compare well to the computed ΔH⧧ value of 17.4 kcal mol−1 and the ΔS⧧ value of −9 cal K−1 mol−1. The sign of the entropy of activation shows that the entropy gain of dissociation of the dormant heterobimetallic complex in the active site and CTA is overcompensated by the entropy loss associated with ethylene insertion. Cosolvent Effect. Since the chain transfer step most likely involves heterobimetallic intermediates, which can be considered as dormant species, a cosolvent was added to the polymerization medium with the aim of disturbing the equilibrium between active and dormant species. Although the use of THF as a cosolvent did result in an increase of activity, the concomitant formation of saturated chain ends was also observed (not shown). However, when di-n-butyl ether was used as an alternative cosolvent ([Bu2O]/[Mg] = 10), the activity of the catalyst increased and an additional improvement in molar mass control was observed, as shown in Table 3. In all polymerizations, a good agreement between experimental and expected Mn values along with low dispersity values (Đ = 1.1−1.2) were found. Influence of the Metallocene Ligand. The bis(fluorenyl)silylene-bridged catalyst {(Me2Si(C13H8)2)Nd(μ-BH4)[(μBH4)Li(THF)]}2/BOMAG had already revealed a remarkable activity for ethylene/butadiene cyclocopolymerization and enabled the synthesis of new elastomers.20 This catalytic system

of the polymerization. Figure 2 shows the evolution of the SEC traces for the different samples. Their shift toward higher molar mass values with time confirmed that the polymerization was well controlled by a CCTP mechanism. It is worth mentioning that the final sample was obtained after MgPE2 precipitation. It displays a bimodal distribution with a low molar mass fraction corresponding to the polyethylene stemming from MgPE2 (Mn = 3400 g mol−1, Đ = 1.5) and a high molar mass polymer fraction (Mn = 109000 g mol−1) showing a profile similar to a Schulz− Flory distribution (Đ = 2.15). To further highlight the efficiency of the control, a series of polymerizations were performed using varying concentrations of magnesium and [Mg]/[Nd] ratios to target different molar masses. All polymerizations were stopped before the precipitation phase of MgPE2 was reached. In all cases, a very good agreement between experimental and expected molar masses was observed and dispersity values of 1.2−1.4 were attained (Table S1 in the Supporting Information). Kinetic Investigation. The influence of the [Mg]/[Nd] ratio on the polymerization kinetics was investigated. Although all the kinetic profiles were similar to that reported in Figure 1, the increase of the [Mg]/[Nd] ratio from 20 to 160 led to a net decrease in activity (Figure S1 in the Supporting Information). In order to determine the apparent propagation rate constant kpapp, a steady state period was considered during which the polymerization is controlled (phase 2 in Figure 1). Plot of the productivity (n(PE)/n(Nd)) against the polymerization time allowed determining the apparent propagation rate coefficient kpapp (Table 2). When the [Mg]/[Nd] ratios increased, kpapp decreased. This is in agreement with the formation of dormant Table 2. Determination of kpapp for Different [Mg]/[Nd] Ratios Employed When Polymerizing Ethylene with (C5Me5)2NdCl2Li(OEt2)2/BOMAG as a Catalytic Systema

a

[BOMAG] (mM)

[Mg]/[Nd]

kpapp (L mol−1 s−1)

1 2 3 4 8

20 40 60 80 160

45.6 25.0 20.6 16.0 10.1

Conditions: T = 80 °C; 4 bar; V(toluene) = 400 mL; [Nd] = 50 μM. 853

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Figure 3. Arrhenius plot obtained by determining kpapp for polymerizations of ethylene performed with (C5Me5)2NdCl2Li(OEt2)2/BOMAG as a catalytic system at 70, 75, 80, and 85 °C. The ethylene pressure had been adjusted at different temperatures to ensure a constant concentration of monomer (V(toluene) = 400 mL; [Nd] 50 μM; R = [Mg]/[Nd] = 80).

Table 3. Influence of the Addition of Di-n-butyl Ether on the Kinetics and on the Control of the Polymerization of Ethylene Performed in the Presence of (C5Me5)2NdCl2Li(OEt2)2/BOMAG as a Catalytic Systema

a

[Bu2O]/[Mg]

time (min)

yield (g)

av activity (kg mol−1 h−1)

Mntheo,b (g mol−1)

Mn (g mol−1)

Đ

chains/Mg

1 10

115 50 35

11.3 13.4 16.10

295 804 1380

1766 2094 2516

1850 2250 2570

1.2 1.2 1.1

1.9 1.9 2.0

Conditions: T = 80 °C; V(toluene) = 400 mL; [(C5Me5)2NdCl2Li(OEt2)2] 50 μM; [BOMAG] 8 mM. bMntheo = yield/(2nMg).

Table 4. Polymerization of Ethylene with the {(Me2Si(C13H8)2)Nd(μ-BH4)[(μ-BH4)Li(THF)]}2/BOMAG Catalytic Systema

a

time (min)

yield (g)

av activity (kg mol−1 h−1)

Mntheo (g mol−1)

Mn (g mol−1)

Đ

chains/Mg

26 65 30b

2.67 4.97 8.19

308 229 819

839 1574 2573

2140 2380 3850

2.1 2.1 1.6

0.79 1.33 1.33

Conditions: T = 80 °C; V(toluene) = 400 mL; [Nd] 50 μM; [BOMAG] 4 mM. bIn the presence of 10 equiv of Bu2O per magnesium atom.

the (C5Me5)2NdR (C5Me5 = Cp*) and Me2Si(C13H8)2NdR (C13H8 = Flu) based systems. The results will be discussed first for decamethylneodymocene, (C5Me5)2Nd. Differences for the bis-fluorenyl catalyst will be highlighted afterward. Modeling Strategy. The Nd(III) center features a high-spin open 4f3 shell that requires specific attention. It has been shown that this shell has little to no influence on the nature of the lanthanide−ligand bonds and their reactivity as long as no singleelectron transfer is involved in the reaction.22−24 Consequently, in the present study, this shell has been implicitly taken into account22 by using a quasi-relativistic 49-electron large-core relativistic effective core potential (RECP).25,26 The use of this pseudopotential not only reduces the computational cost but also strengthens the DFT approach, since the systems are defined by a pseudosinglet state. Regarding the choice of the density functional, a study evaluating the performance of various density functionals and basis sets showed that the hybrid functional B3PW91 and the basis set 6-311G(d,p) used in combination with large-core RECP offers a good compromise for studying the reactivity of lanthanide(III) complexes.27,29 This computational strategy has been intensively and fruitfully used by Perrin, Maron, and Eisenstein in mechanistic studies involving lanthanide(III) complexes, including polymerization reaction studies.28−32 However, the experimental kinetic data determined in this study offered an additional opportunity for density functional benchmarking. The influence of the density functional on the computed energy profiles is presented and discussed in Table S3 in the Supporting Information. Once again, a comparison

was also evaluated in the present study for ethylene homopolymerization and compared to (C5Me5)2NdCl2Li(OEt2)2/BOMAG. Kinetic profiles similar to those depicted in Figure 1 were observed for various [Mg]/[Nd] ratios (Figure S3 in the Supporting Information). These results indicate that the polymerization is controlled by a reversible chain transfer between neodymium and magnesium. The experimental molar masses were higher than the expected values calculated on the basis of the yield and the concentration of BOMAG (Table 4), while the molar mass distributions were broader (Đ = 2.1) than in the case of (C5Me5)2NdCl2Li(OEt2)2/BOMAG (Đ < 1.3). The number of chains per Mg was below the theoretical value of 2 and increased over time. All these data are consistent with the fact that a reversible chain transfer does take place during the polymerization, but the transfer reaction is not fast enough with respect to the chain propagation. As previously shown, the addition of di-n-butyl ether highly increased the activity of (C5Me5)2NdCl2Li(OEt2)2/BOMAG toward monomer insertion and led to better control over the molar masses, narrowing their distribution. The same trends were observed for the catalyst {(Me2Si(C13H8)2)Nd(μ-BH4)[(μ-BH4)Li(THF)]}2/BOMAG (Table 4). This leaves room for further improvements of this catalytic system for controlling the polymerization of ethylene and other olefins, which, however, exceeds the scope of the present paper and will be the topic of forthcoming communications. Computational Mechanistic Investigations. Computational mechanistic investigations have been conducted for both 854

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Figure 4. Schematic and 3D representations of the most stable Nd(III) and Mg(II) species in (C5Me5)2NdR/dialkylmagnesium mixtures: t, terminal; b, bridging; f, facial. Some hydrogen atoms have been omitted for clarity.

Bu4Mg2 (Mg2). The Mg−C bond length averages to 2.148 Å for the terminal sites t, to 2.433 Å for the top sites f, and to 2.303 Å for the bridging sites b. A similar speciation has been conducted for the heterobimetallic complexes formed between Cp*2NdBu (1) and Mg4a. For this purpose, heterobimetallic (NdMg) and heterotrimetallic (NdMg2a and NdMg2b) species have been considered. In NdMg, only one structure in which two of the three butyl groups are bridging between Nd and Mg and the remaining one is terminal at Mg was found as a minimum. In heterotrimers, two isomers in which metals and bridging carbon atoms form either a fused polytetrahedron or a six-membered ring were optimized (Figure 4B). The heterodimer Cp*2Nd(μ-Bu)2MgBu is 6.2 kcal mol−1 more stable than the dissociated form [Nd]Bu (1) and Mg4a. Relative to the same reference, the heterotrimer Cp*2Nd[(μBu)2Mg]2Bu (NdMg2a) was found to be the most stable complex. Its formation is 9.7 kcal mol−1 exergonic in reference to 1 and Mg4a. The heterotrimer NdMg2b, in which metal atoms and bridging carbon atoms form a six-membered ring, is 32.6 kcal mol−1 less stable than NdMg2a and is thus irrelevant in the current context. Additional sampling did not reveal complexes more stable than NdMg2a; this complex will be used hereafter as the energy reference for the Nd complexes under the employed polymerization conditions. In the following section, we will explore the reactivity of [Nd]Bu (1), NdMg, and NdMg2a toward ethylene in the context of its homopolymerization. The Gibbs energy profile of the three chemical steps involved in this reaction were computed: namely, monomer insertion, β-H elimination, and H transfer to monomer (Figure 5). Only the front-side insertion has been considered since it had been shown to be the lowest in energy in a previous computational study.29 Ethylene Insertion. Considering the presumed coexistence of heterotri- and heterobimetallic species in equilibrium with a monometallic Nd alkyl complex, the key question is to determine which metal−alkyl bond leads to the lowest energy barrier for ethylene insertion. Insertions into bridging Mg−C bonds can be rapidly ruled out, since such transition states are high in energy (higher than 35 kcal mol−1). We then focused on the insertion into terminal metal−carbon bonds (Nd or Mg). Figure 6 presents the energy profile for ethylene insertion into the terminal M−Bu bonds in NdMg2a, NdMg, and 1.

between density functionals supports the use of B3PW91 for this type of system, since it offers the best compromise among cost, accuracy, and optimization convergence. In addition, it is now well accepted that solvation (here toluene) and dispersion corrections have to be included in the theoretical model in order to precisely account for reaction thermodynamics and kinetics.24,28 By default, solvation was taken into account during optimization and dispersion was included as a single-point correction.28,33,34 Within the structural model, growing chains contain at least four carbon atoms in order to avoid spurious artifacts.35 Finally, dialkylmagnesium oligomers have been explicitly included in the chemical model. Thermal and entropic contributions have also been included by means of frequency calculations within the harmonic approximation. The fully detailed computational procedure is available in the Supporting Information. Speciation. As the dialkylmagnesium species plays a crucial role in the studied coordinative chain transfer polymerization, the most stable structures of BOMAG in toluene were identified computationally, in addition to the Nd/Mg heterobimetallic complexes. Starting from Bu2Mg as a building block, various aggregates comprised of up to four magnesium atoms have been optimized.36−40 Among all the possible isomers, the cluster Mg4(σ-Bu)2(μ-Bu)4(μ3-Bu)2 (Mg4a) (Figure 4A) in which two alkyl chains are terminal (t), four are bridging (b), and two are facial (f) was determined to be the most stable aggregate. Cubane-like (Mg4b), polytetrahedral (Mg4c), and envelope (Mg4d) structures are less stable than Mg4a by 1.5, 2.2, and 5.0 kcal mol−1 (ΔrG° values), respectively. Dissociation of Mg4a into two BuMg(μ-Bu)2MgBu (Mg2) dimers or four MgBu2 (Mg) monomers is endergonic by 17 and 38 kcal mol−1, respectively. As a result, the Mg4a cluster will be considered as the energy reference for free magnesium structures in toluene solution; the 1/n ratio of Mg4a will be used in thermodynamic balances that involve (MgBu2)n. In Mg4a, the magnesium atoms define a rhomboid in which the two short (long) distances are 2.875 and 2.869 Å (2.964 and 2.869 Å). In this structure, the larger angle averages to 125° and the smaller angle to 55°. As a result, the closest Mg atoms are opposite and not vicinal; the shortest Mg···Mg distance measures 2.753 Å, i.e. 0.03 Å longer than in the dibutylmagnesium dimer 855

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Figure 5. Elementary steps explored in the homopolymerization of ethylene catalyzed by Cp*2NdR in the presence of R1 MgR2, M being a Nd or Mg center.

The lowest insertion profile involves the dissociation of NdMg2a in 1 and 1/2 equiv of Mg4a, followed by the insertion of the monomer in the Nd−alkyl bond of 1. The dissociation between the Nd site and the alkylmagnesium is 9.7 kcal mol−1 endergonic. From 1, the overall barrier comprising ethylene binding to Nd followed by its insertion into the Nd−C bond is calculated to be 10.9 kcal mol−1. This value is in agreement with previous studies.29 The geometry of the transition state will not be further discussed here, since it has already been reported.29 With respect to the most stable state (dormant) of the system, the energy barrier for ethylene insertion is 20.6 kcal mol−1. Insertion reactions into the terminal Mg−Bu bond of NdMg2a and NdMg complexes were found to be higher in energy than the overall sequence involving dissociation of Nd and Mg fragments. The energy differences are 2.0 and 6.5 kcal mol−1 for NdMg2a and NdMg, respectively. Hence, Cp*2NdR appears to be the active complex for monomer insertions. The overall free enthalpy of reaction was calculated to be ΔrG° = −12.6 kcal mol−1. This value is in excellent agreement, without any scaling of the entropy contribution,41 with the experimental value of −12.2 kcal mol−1 reported by Jessup in 1948 for ethylene homopolymerization.42 H Transfer to Monomer. Transition states for transfer to monomer between ethylene and NdMg2a, NdMg, and 1 have been optimized (Figure 7). Surprisingly, the reactivity trend obtained for the insertion reaction is not followed for this transfer reaction. Here, relative to the most stable complexes, the energy

Figure 7. Transition states of H transfer to monomer and β-H elimination in mono- and bimetallic complexes.

barrier for transferring from 1 is 30.6 kcal mol−1 (TS 6), 26.7 kcal mol−1 for the transfer from NdMg (TS 7), and 25.6 kcal mol−1 for the transfer from NdMg2a (TS 8). The lowest energy pathway for transfer to monomer is thus at the Mg site in the heterotrimer NdMg2a. However, considering the difference in energy between the transition states of monomer insertion and transfer to monomer, termination by chain transfer is the least likely in the presence of neodymocene and dialkylmagnesium. An alternative chain end mechanism is the β-H elimination. β-H Elimination. Similarly to the considerations above, the energy for β-H elimination was calculated for the terminal alkyl chains of NdMg2a, NdMg, and 1 (Figure 7). Elimination at Mg sites is kinetically and thermodynamically disfavored, since the energy barriers are higher than 34.0 kcal mol−1 (TS 9) and the reaction is endergonic by 12 kcal mol−1. At the Nd center, the transition state for β-H elimination is lower in energy (29.0 kcal mol−1, TS 10) than at the Mg center, but this step is highly

Figure 6. Free energy profile in kcal mol−1 for CCTP ethylene polymerization by the bimetallic Cp*2NdR/MgR2 catalytic system. 856

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ACS Catalysis endergonic (ΔrG° = 19.9 kcal mol−1). As a result, termination by β-H elimination will occur neither at the Nd nor at the Mg sites, which is consistent with the found pseudo livingness of the polymerization system. An alternative mechanism in which β-H elimination is assisted by the coordination of ethylene was also considered. However, the steric hindrance induced by the Cp* ligands and the edge angle of the metallocene prevent the formation of Cp*2Nd(η2-C2H4)H(η2-C4H8) as an intermediate; no stable minimum could be optimized for this structure. Chain Exchange. The mechanism by which polyethylene chains are exchanged between Nd and Mg is formally a transmetalation reaction in which an intermediate is a heterobimetallic trimer complex (e.g., NdMg2a). As a result, the exchange of chains is thermoneutral and the intermediate heterobimetallic complex is 10 kcal mol−1 lower in energy (Figure 8). Despite our effort, no transition state could be

Figure 9. 3D representations of the heterobimetallic (C5Me5)2NdMg2Et5 complex (top) and the transition state TS_CCTP for Mg2Et4 insertion in [Nd]−Et which is involved in the chain exchange reaction. Some hydrogen atoms have been omitted for clarity.

Figure 8. Mechanism of chain exchange between Mg and Nd centers. Asterisks (*) indicate that the energy of the transition state was calculated for (C5Me5)2NdMg2Et5.

as indicated by short Nd···H(Cα) [Mg···H(Cα)] bond lengths of 2.76 Å [2.19 Å] (Figure 9). This transient binding mode between the two partners is thus governed by weak interactions between the terminal methylene group of the growing polymer chain and the two different metal centers. Following the reaction coordinate leads to the bis-alkylbridged complex. Despite our effort, no other transition state corresponding to this reaction could be located on the potential energy surface. Cosolvent Effect. The influence of the use of dialkyl ethers (Bu2O or THF) was assessed by computing the thermodynamics of eqs 1−5, in which L is an ether molecule, namely Bu2O or THF (second values).

optimized for the insertion of a terminal Bu−Mg bond in the Nd−Bu bond to yield the Nd(μ-Bu)2Mg scaffold. All attempts ended with spurious transition states corresponding to methyl rotation within C5Me5 or Bu ligands. However, on the basis of the simplified model (C5Me5)2NdEt and Mg2Et4, we were able to correctly optimize the transition state TS_CCTP. The imaginary frequency is remarkably low (i22 cm−1). For this model system, an energy barrier of 13.1 kcal mol−1 is computed relative to Cp*2Nd{(μ-Et)2Mg}2Et (NdMg2a’). This barrier constitutes an upper limit with respect to the experimental system. The combination of this energy barrier and the relative stability of 1 and NdMg2a reported previously afford a Gibbs energy profile for the chain exchange mechanism (Figure 8). In this profile, the two transition states involved in the chain exchange reaction are formally degenerate. The energy profile shows that the chain transfer mechanism is essentially controlled by the thermodynamics of the dissociation of the heterobimetallic trimer. In terms of overall geometry, transition state TS_CCTP shares several features with the dormant NdMg2a′ complex. Figure 9 shows a representation as well as the key geometrical features of these two stationary points optimized for Et groups. The main difference lies in the nature of the interactions between the Nd− Et and Et−Mg bonds. Whereas this interaction can be described in NdMg2a′ by classical bridging of two electron-poor metal centers by alkyl groups, at the insertion/dissociation transition state TS_CCTP, the methylene hydrogen atoms of the Nd−Et [Mg−Et] groups transiently interact with the Mg [Nd] centers,

Mg4 a + 4L = 2Mg 2Bu4(L)2 Δr G°(1) = −6.7/− 20.7 kcal mol−1

(1)

NdMg 2 a + 3L = 1·L + Mg 2Bu4(L)2 Δr G°(2) = 6.3/− 5.8 kcal mol−1

(2)

1·C2H4 + L = 1·L + C2H4 Δr G°(3) = −4.0/−9.2 kcal mol−1

(3)

NdMg 2 a + C2H4 = 1·C2H4 + 1/2Mg4 a Δr G°(4) = 13.7 kcal mol−1 857

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ACS Catalysis

barrier for TS_CCTP can be transferred from the decamethylneodymocene to the ansa-silylene-bis-fluorenyl complex, the barrier of energy for chain transfer in the latter system can be estimated to be 15 kcal mol−1.

NdMg 2 a + 2L + C2H4 = 1·C2H4 + Mg 2Bu4(L)2 Δr G°(5) = 10.4 kcal mol−1

(5)

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Eq 1 shows that both THF and Bu2O alter the aggregation state of the Mg4a cluster to the solvated Mg2Bu4(L)2 dimers that become the reference structure of the dialkylmagnesium complex in a mixed ether/toluene solution. Interestingly, eq 2 highlights a strongly different behavior of Bu2O and THF. Starting from the heterodimer NdMg2a and 3 equiv of ether, dissociation into Cp*2NdBu(L) and Mg2Bu4(L)2 is exergonic by 5.8 kcal mol−1 for THF but endergonic by 6.3 kcal mol−1 for Bu2O. In addition, according to eq 3, Bu2O has an affinity to Cp*2NdBu that is merely higher than that of ethylene, whereas THF is a much stronger ligand. As a consequence, the ground state of the Nd complex in the presence of THF is Cp*2NdR(THF), whereas it remains NdMg2a in the presence of Bu2O. Kinetically, with respect to these most stable states, the transition state for ethylene insertion has a relative energy of 16.1 kcal mol−1 [17.3 kcal mol−1] in the presence of THF [Bu2O]. In other words, the barrier of ethylene insertion is reduced by 3 to 4 kcal mol−1 in the presence of ether as a cosolvent. We then considered possible side reactions between the ether molecule and the alkyl complex. A possible side reaction is the metalation of the ether molecule in its α position. The transition states for α-C−H activation of THF and Bu2O with Cp*2NdBu were optimized. Relative to the ether adduct 1·L, the transition states for the metalation reaction have energies of 23.4 and 24.1 kcal mol−1 for THF and Bu2O, respectively. With respect to the dormant state of the systems in the presence of ethers, the energies of these barriers are 18.6 and 30.4 kcal mol−1, respectively. Hence, due to the differential solvation effect between THF and Bu2O, metalation of THF is a competing pathway to ethylene insertion (ΔrG⧧inser = 16.1 kcal mol−1), whereas it remains a high-energy process in the presence of Bu2O. Ligand Effects. In order to highlight the influence of the catalyst on the CCTP of ethylene, the computational study reported above was repeated for the ansa complex Me2Si(C13H8)2NdR. In the absence of any ether molecules, the most stable complex formed between Me2Si(C13H8)2NdBu (1_Flu) and BOMAG remains the heterobimetallic trimer Me2Si(C13H8)2NdMg2Bu5 (NdMg2a_Flu). The formation of this heterobimetallic complex is 1.8 kcal mol−1 more exergonic than the formation of the respective decamethylneodymocene complex. Considering NdMg2a_Flu, the transition state for ethylene insertion possesses a relative energy of 22.6 kcal mol−1. In comparison to the decamethylneodymocene system, the energy barrier is raised by 2.0 kcal mol−1. This effect essentially originates from the overstabilization of the bimetallic heterotrimer. The energy difference between the ethylene adduct and the transition state of insertion for the decamethylneodymocene 1 and the fluorenyl 1_Flu complexes does only differ by 0.6 kcal mol−1. As a consequence, a lower activity is expected for the ansabis(fluorenyl) catalyst relative to the decamethylneodymocene catalyst. In terms of chain transfer, the transition state in which the Mg−R bond of Mg2R4 bridges with the Nd−R bond of Me2Si(C13H8)2NdR (R = Bu, Et) could not be fully elucidated. However, the dissociation of Me2Si(C13H8)2NdMg2Bu5 to Me2Si(C13H8)2NdBu and 1/2 equiv of Mg4Bu8 is endergonic by 11.7 kcal mol−1: i.e., 1.8 kcal mol−1 more than for the decamethylneodymocene complex. If we assume that the energy

DISCUSSION The temperature-dependent kinetics led to an estimation of the activation energy of 17 kcal mol−1 for the polymerization of ethylene by (C5Me5)2LnR/BOMAG. Considering the complexity of the system and the expected accuracy of the calculations, this value compares well to the computational value of 20.6 kcal mol−1 for a mechanism that involves the heterotrimer NdMg2a as a dormant species and (C5Me5)2LnR as the active complex. In addition, the difference of 3 kcal mol−1 between measured and calculated activation energies corresponds to the best match among the set of functionals tested and fits with the expected precision of this methodology.43−45 In addition, the mechanism is in line with a previously postulated mechanism.16,19 In this mechanism, the active species is ca. 10 kcal mol−1 less stable than the dormant heterobimetallic trimer species. The endergonicity of the dissociation reaction of the heterobimetallic species, which yields the active catalyst, can be related to the increase of activity in the steady state phase 2 (Figure 1) while decreasing the [Mg]/ [Nd] ratio as shown in Figure S2 in the Supporting Information. Additionally, the low dispersity values ranging from 1.2 to 1.4 for polymers produced during phase 2 show that termination reactions by β-H elimination and H transfer to monomer are disfavored. This coincides well with the computed energy barriers of the β-H elimination or monomer transfer at both Nd or Mg sites which are at least 5 kcal mol−1 higher than the transition state for monomer insertion. For the (C5Me5)2NdR catalyst, the number of chains per magnesium atom approaches a value of 2 for all samples. This indicates a fast and reversible chain transfer between Nd and Mg. Computationally, the two exchange mechanism steps show an overestimated energy barrier of 13 kcal mol−1 (ΔG value) with respect to the dormant heterobimetallic trimer. The exchange reaction is thus the fastest step of the whole polymerization reaction. As the transition state lies ca. 3 kcal mol−1 above the active complex, the chain exchange mechanism is governed by the thermodynamics of the association of active site and CTA, at least when alkyl sites are exchanged. As a consequence, raising the association energy between the CTA and the active site will disfavor chain exchange. This is consistent with the results from the polymerization carried out in the presence of the {(Me2Si(C13H8)2)Nd(μ-BH4)[(μ-BH4)Li(THF)]}2/BOMAG catalytic system. In this case, the polymerization occurs via a CCTP mechanism, although the control of the polymerization is less efficient than with (C5Me5)2NdCl2Li(OEt2)2/BOMAG and broadly distributed molar masses were obtained as mentioned above (Table 4). Further inspection of the molar mass distribution reveals a second population at higher molar masses that most likely corresponds to polymer chains produced by (C5Me5)2NdR in the absence of CTA and stems from the third phase of polymerization in which precipitation of MgPE2 gives rise to a high increase in activity. The presence of the high molar mass population means that there is no more interaction between MgPE2 and the active species in the solid phase. In the energy profile shown in Figure 6, precipitation of MgPE2 corresponds to a switch of the energy reference from NdMg2a to 1. Under these conditions, all of the energy barriers are decreased by ca. 10 kcal mol−1. Ethylene polymerization is then controlled by the two 858

DOI: 10.1021/acscatal.5b02316 ACS Catal. 2016, 6, 851−860

Research Article

ACS Catalysis lowest transition states: i.e. that for ethylene insertion (ΔG⧧ = 10.9 kcal mol−1) and that for β-H elimination (ΔG⧧ = 19.3 kcal mol−1). Although the selectivity between these two events remains high, the β-H elimination process is then thermally accessible. In addition, in phase 3, a change of state and transient kinetics makes barrier comparisons intricate. Overall, a large increase of activity leading to higher molar masses with a dispersity of approximately 2 was expected. The kinetics and polymer characteristics are in perfect agreement with this mechanistic view of the system (Figures 1 and 2), and are supported by the report of the high activity of ethyl yttrium metallocene analogue complex toward olefin polymerization and β-H elimination.46 It remained difficult to rationalize the initial higher activity observed in phase 1. However, in a speciation study, the stability of the Mg4a isomer was assessed for butyl chains as a model. We suspect that the stability of the dialkylmagnesium may change when the size of the alkyl groups is increased. This is the case while the polymerization takes place, leading to a slightly different equilibrium after a few minutes, analogous to RAFT polymerization for which a pre-equilibrium involving the starting molecular chain transfer agent and a main equilibrium involving macromolecular chain transfer agents are the fundamental polymerization control steps.47 The addition of di-n-butyl ether proved to be a valuable method for increasing the rate of polymerization for the catalysts (C5Me5)2NdCl2Li(OEt2)2/BOMAG while also improving the control over the polymerization. Computational investigations of this cosolvent effect revealed that Bu2O is not a strong enough Lewis base to provoke the dissociation of the dormant heterobimetallic trimer, but it solvates the Mg4a and triggers its dissociation into dimers. This results in a decreased Nd−Mg interaction that consequently lowers the monomer insertion barrier. This decreased Nd−Mg interaction also favors chain exchange, in full agreement with experimental findings. A similar effect cannot be exerted by THF, since it solvates both the Mg and Nd centers. Switching from the non-ansa (C5Me5)2NdR metallocene to ansa-Me2Si(C13H8)2NdR, computational investigations show that the lower activity of the ansa complex is related to a higher interaction between the CTA and the active site, since the barrier of monomer insertion relative to the active species is almost unchanged. The higher affinity of the organomagnesium fragment to the neodymocene can be related to the edge angle of the metallocene that is higher in the ansa complex. The lower activity of the bis-fluorenyl complex and the less efficient chain transfer are due to stronger interactions between the CTA and the active site.

essentially controlled by the affinity of the CTA to the active site where the monomer insertion takes place; high interaction energies disfavor the chain transfer. Hence, metallocene catalysts that favor the association of dormant species, e.g. ansa-bisfluorenyl, showed a lower activity and a lower efficiency for chain transfer in ethylene homopolymerization. Conversely, we demonstrated that specific solvation of the CTA by a cosolvent significantly reduces the dissociation energy between the dormant bimetallic complex and the active site, resulting in higher activities and narrower molar mass distributions for the same catalytic system. This experimental study supported by computational investigations provides the basis for the development of further novel catalysts.

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EXPERIMENTAL SECTION General Conditions. Toluene was degassed by argon purging and purified with an SPS800 MBraun solvent purification system. Di-n-butyl ether (Sigma-Aldrich) was purchased dry, degassed by argon purging, and stored over 3 Å molecular sieves. n-Butyl-n-octylmagnesium ((n-C4H9)1.5(nC8H17)0.5Mg, called BOMAG in this paper) was supplied by Chemtura as a 20 wt % solution in heptane. Polymerization Procedure. Polymerization runs were carried out in a 500 mL glass reactor equipped with a stainless steel blade stirrer and an external water jacket for temperature control. Consumption of ethylene was monitored by the pressure drop in an ethylene cylinder equipped with a pressure gauge. A solution of BOMAG was diluted with toluene (400 mL). The resulting solution was transferred to the reactor under an argon atmosphere. An antechamber was then charged with a solution of (C5Me5)2NdCl2Li(OEt2)2 in toluene (10 mL). The reactor was heated to the desired temperature and then charged with an ethylene atmosphere at a pressure of 4 bar. The precatalyst solution was then added to the reactor and the consumption of ethylene recorded. After the desired consumption of ethylene, the reaction was stopped by venting the gaseous ethylene through reactor exhaust and cooling the reactor to 20 °C. The reactor contents were added to methanol, and the suspension was filtered. The polyethylene recovered was washed three times with methanol and dried. When the complex {(Me2Si(C13H8)2)Nd(μ-BH4)[(μ-BH4)Li(THF)]}2 was used, it was solubilized in 10 mL of BOMAG solution before injection. Size Exclusion Chromatography (SEC). High -temperature size exclusion chromatography (HT-SEC) analyses were performed using a Viscotek system (from Malvern Instruments) equipped with three columns (PLgel Olexis 300 mm × 7 mm i.d. from Agilent Technologies). Portions (200 μL) of sample solutions with concentrations of 5 mg mL−1 were eluted in 1,2,4trichlorobenzene using a flow rate of 1 mL min−1 at 150 °C. The mobile phase was stabilized with 2,6-di-tert-butyl-4-methylphenol (200 mg L−1). The OmniSEC software was used for data acquisition and data analysis. The molar mass distributions were calculated with a calibration curve on the basis of narrow poly(ethylene) standards (Mp = 170, 395, 750, 1110, 2155, 25000, 77500, 126000 g mol−1) from Polymer Standards Service (Mainz, Germany).

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CONCLUSION The kinetic investigation of ethylene polymerization mediated by (C5Me5)2NdCl2Li(OEt2)2 in the presence of dialkylmagnesium (BOMAG) as chain transfer agent allowed us to elucidate and rationally interpret the underlying mechanism governing both catalyst activity and chain transfer exchange. Targeted molar masses with a narrow dispersity of their distribution could be obtained under these CCTP conditions. Varying the [Nd]/[Mg] ratio not only demonstrated the efficiency of the chain transfer but also revealed the equilibrium between the dormant state and the active insertion site. The good match between the experimentally determined activation parameters of the propagation step and the computational energy profile supports the overall mechanistic interpretation. We show that chain transfer is

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.5b02316. 859

DOI: 10.1021/acscatal.5b02316 ACS Catal. 2016, 6, 851−860

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ACS Catalysis

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Budzelaar, P. H. M.; Busico, V.; Cipullo, R.; Macchioni, A. J. Am. Chem. Soc. 2010, 132, 13651−13653. (22) Maron, L.; Eisenstein, O. J. Phys. Chem. A 2000, 104, 7140−7143. (23) Castro, L.; Kefalidis, C. E.; McKay, D.; Essafi, S.; Perrin, L.; Maron, L. Dalton Trans. 2014, 43, 12124−12134. (24) Perrin, L.; McKay, D.; Carr, K. J. T.; McMullin, C. L.; Eisenstein, O.; Macgregor, S. A. Struct. Bonding (Berlin, Ger.) 2015, DOI: 10.1007/ 430_2015_176. (25) Dolg, M.; Stoll, H.; Savin, A.; Preuss, H. Theor. Chim. Acta 1989, 75, 173−194. (26) Dolg, M.; Stoll, H.; Preuss, H. Theor. Chim. Acta 1993, 85, 441− 450. (27) Roca-Sabio, A.; Regueiro-Figueroa, M.; Esteban-Gomez, D.; de Blas, A.; Rodriguez-Blas, T.; Platas-Iglesias, C. Comput. Theor. Chem. 2012, 999, 93−104. (28) Castro, L.; Kirillov, E.; Miserque, O.; Welle, A.; Haspeslagh, L.; Carpentier, J.-F.; Maron, L. ACS Catal. 2015, 5, 416−425. (29) Perrin, L.; Maron, L.; Eisenstein, O. New J. Chem. 2004, 28, 1255− 1259. (30) Perrin, L.; Bonnet, F.; Visseaux, M.; Maron, L. Chem. Commun. 2010, 46, 2965−2967. (31) Perrin, L.; Bonnet, F.; Chenal, T.; Visseaux, M.; Maron, L. Chem. Eur. J. 2010, 16, 11376−11385. (32) Perrin, L.; Kirillov, E.; Carpentier, J.-F.; Maron, L. Macromolecules 2010, 43, 6330−6336. (33) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. J. Chem. Phys. 2010, 132, 154104. Grimme, S.; Ehrlich, S.; Goerigk, L. J. Comput. Chem. 2011, 32, 1456−1465. (34) Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. J. Chem. Theory Comput. 2009, 5, 2447−2464. (35) Talarico, G.; Blok, A. N. J.; Woo, T. K.; Cavallo, L. Organometallics 2002, 21, 4939−4949. (36) Menoret, S.; Carlotti, S.; Fontanille, M.; Deffieux, A.; Desbois, P.; Schade, C.; Schrepp, W.; Warzelhan, V. Macromol. Chem. Phys. 2001, 202, 3219−3227. (37) Gallegos, C.; Tabernero, V.; Garcia-Valle, F. M.; Mosquera, M. E. G.; Cuenca, T.; Cano, J. Organometallics 2013, 32, 6624−6627. (38) Harrison-Marchand, A.; Mongin, F. Chem. Rev. 2013, 113, 7470− 7562. (39) Mongin, F.; Harrison-Marchand, A. Chem. Rev. 2013, 113, 7563− 7727. (40) Jimenez-Halla, J. O. C.; Bickelhaupt, F. M.; Sola, M. J. Organomet. Chem. 2011, 696, 4104−4111. (41) Morales, G.; Martínez, R.; Ziegler, T. J. Phys. Chem. A 2008, 112, 3192−3200. (42) Jessup, R. S. J. Chem. Phys. 1948, 16, 661. (43) Bremond, E.; Kalhor, M. P.; Bousquet, D.; Mignon, P.; Ciofini, I.; Adamo, C.; Cortona, P.; Chermette, H. Theor. Chem. Acc. 2013, 132, 1− 12. (44) Harvey, J. N. Annu. Rep. Prog. Chem., Sect. C: Phys. Chem. 2006, 102, 203−226. (45) Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120, 215−241. (46) MacDonald, M. R.; Langeslay, R. R.; Ziller, J. W.; Evans, W. J. J. Am. Chem. Soc. 2015, 137, 14716−14725. (47) Barner-Kowollik, C. Handbook of RAFT Polymerization; WileyVCH: Weinheim, Germany, 2008.

Additional polymerization assays, temperature-dependent kinetics and kinetic profiles, 3D representation of key transition states, geometrical parameters and population analysis, and Cartesian coordinates and associated energies (PDF)

AUTHOR INFORMATION

Corresponding Authors

*L.P.: e-mail, [email protected]. *C.B.: e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Manufacture Michelin is acknowledged for financial support of the Master’s thesis of H.N. H.N. and L.P. thank the CCIR of ICBMS and P2CHP of Université Lyon 1 for providing computational resources and technical support. The authors thank Maria Rosário Ribeiro (Instituto Superior Técnico of Lisbon, Lisbon, Portugal) (C2P2) for fruitful scientific discussions and Manel Taam (C2P2) for SEC analyses. The authors thank the reviewers for their thorough reports and both helpful and fruitful comments.

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REFERENCES

(1) Coates, G. W.; Hustad, P. D.; Reinartz, S. Angew. Chem., Int. Ed. 2002, 41, 2236−2257. (2) Samsel, E. G. Patent EP539876A2, 1993. (3) Britovsek, G. J. P.; Cohen, S. A.; Gibson, V. C.; van Meurs, M. J. Am. Chem. Soc. 2004, 126, 10701−10712. (4) Kempe, R. Chem. - Eur. J. 2007, 13, 2764−2773. (5) Valente, A.; Mortreux, A.; Visseaux, M.; Zinck, P. Chem. Rev. 2013, 113, 3836−3857. (6) Hou, Z. M.; Wakatsuki, Y. Coord. Chem. Rev. 2002, 231, 1−22. (7) D’Agosto, F.; Boisson, C. Aust. J. Chem. 2010, 63, 1155−1158. (8) Arriola, D. J.; Carnahan, E. M.; Hustad, P. D.; Kuhlman, R. L.; Wenzel, T. T. Science 2006, 312, 714−719. (9) Briquel, R.; Mazzolini, J.; Le Bris, T.; Boyron, O.; Boisson, F.; Delolme, F.; D’Agosto, F.; Boisson, C.; Spitz, R. Angew. Chem., Int. Ed. 2008, 47, 9311−9313. (10) Mazzolini, J.; Espinosa, E.; D’Agosto, F.; Boisson, C. Polym. Chem. 2010, 1, 793−800. (11) Norsic, S.; Thomas, C.; D’Agosto, F.; Boisson, C. Angew. Chem., Int. Ed. 2015, 54, 4631−4635. (12) German, I.; Kelhifi, W.; Norsic, S.; Boisson, C.; D’Agosto, F. Angew. Chem., Int. Ed. 2013, 52, 3438−3441. (13) Makio, H.; Ochiai, T.; Mohri, J.; Takeda, K.; Shimazaki, T.; Usui, Y.; Matsuura, S.; Fujita, T. J. Am. Chem. Soc. 2013, 135, 8177−8180. (14) Bogaert, S.; Chenal, T.; Mortreux, A.; Carpentier, J. F. J. Mol. Catal. A: Chem. 2002, 190, 207−214. (15) Olonde, X.; Mortreux, A.; Petit, F.; Bujadoux, K. J. Mol. Catal. 1993, 82, 75−82. (16) Pelletier, J.-F.; Mortreux, A.; Olonde, X.; Bujadoux, K. Angew. Chem., Int. Ed. Engl. 1996, 35, 1854−1856. (17) Llauro, M. F.; Monnet, C.; Barbotin, F.; Monteil, V.; Spitz, R.; Boisson, C. Macromolecules 2001, 34, 6304−6311. (18) Barbotin, F.; Monteil, V.; Llauro, M. F.; Boisson, C.; Spitz, R. Macromolecules 2000, 33, 8521−8523. (19) Boisson, C.; Monteil, V.; Ribour, D.; Spitz, R.; Barbotin, F. Macromol. Chem. Phys. 2003, 204, 1747−1754. (20) Thuilliez, J.; Ricard, L.; Nief, F.; Boisson, F.; Boisson, C. Macromolecules 2009, 42, 3774−3779. (21) Busico, V.; Cipullo, R.; Esposito, V. Macromol. Rapid Commun. 1999, 20, 116−121. Song, F.; Cannon, R. D.; Bochmann, M. J. Am. Chem. Soc. 2003, 125, 7641−7653. Ciancaleoni, G.; Fraldi, N.; 860

DOI: 10.1021/acscatal.5b02316 ACS Catal. 2016, 6, 851−860