EthyleneâButadiene Copolymerization by Neodymocene Complexes

Research Article pubs.acs.org/acscatalysis

Ethylene−Butadiene Copolymerization by Neodymocene Complexes: A Ligand Structure/Activity/Polymer Microstructure Relationship Based on DFT Calculations Hajar Nsiri,†,‡ Islem Belaid,‡ Paolo Larini,† Julien Thuilliez,§ Christophe Boisson,*,‡ and Lionel Perrin*,† †

Université Lyon 1, CNRS, INSA, CPE, UMR 5246, ICBMS, ITEMM, 43 Bd du 11 novembre 1918, CEDEX 69622 Villeurbanne, France ‡ Université Lyon 1, CNRS, CPE, UMR 5265, C2P2, CPP, 43 Bd du 11 novembre 1918, CEDEX 69622 Villeurbanne, France § Manufacture Michelin, 23 place des Carmes, 63040 Clermont-Ferrand, France

10.1021/acscatal.5b02316 S Supporting Information *

ABSTRACT: Ethylene/butadiene copolymerization can be performed by neodymocene catalysts in the presence of an alkylating/chain transfer agent. A variety of polymerization activities and copolymer microstructures can be obtained depending on the neodymocene ligands. For a set of four catalysts, namely (C5Me5)2NdR, [Me2Si(3Me3SiC5H3)2]NdR, [Me2Si(C5H4)(C13H8)]NdR and [Me2Si(C13H8)2]NdR, we report a DFT mechanistic study of this copolymerization reaction performed in the presence of dialkylmagnesium. Based on the modeling strategy developed for the ethylene homopolymerization catalyzed by (C5Me5)2NdR in the presence of MgR2, our model is able to account for the following: (i) the formation of Nd/Mg heterobimetallic complexes as intermediates, (ii) the overall differential activity of the catalysts, (iii) the copolymerization reactivity indexes, and (iv) the specific microstructure of the resulting copolymers, including branching and cyclization. The analysis of the reaction mechanisms and the energy profiles thus relates ligand structure, catalyst activity, and polymer microstructure and sets the basis for further catalyst developments. KEYWORDS: neodymocene, ethylene, butadiene, copolymerization, mechanism, DFT

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INTRODUCTION Polyolefins are a major class of thermoplastics with a yearly production of ca. 120 million tons. They display unique chemical and mechanical properties and are used in a large range of applications in our everyday lives: films, plastic bags, packaging, household goods, pipes, and so on. The introduction of unsaturated groups in the polyolefin backbone brings additional properties. The amount and the configuration (E or Z) of the unsaturated groups are key factors that require control during the polymerization reaction.1,2 Among all the possible applications, the synthesis of tailor-made elastomers for the tire industry is one of the most prominent. Unsaturated motifs within the polymer chain can be obtained by copolymerizing an olefin with a conjugated diene. In this field, a leading material is the Ethylene Butadiene Rubber (EBR) (Scheme 1).3 The copolymerization of ethylene and butadiene is highly challenging since each monomer typically requires different polymerization conditions. Most catalysts designed for olefin homopolymerization are inefficient toward conjugated diene polymerization. Similarly, conjugated-diene polymerization catalysts do not polymerize olefins.4 This results from the involvement of two distinctly different active sites that are © XXXX American Chemical Society

Scheme 1. Ethylene Butadiene Copolymerization

obtained after ethylene or butadiene insertion, namely metal− alkyl or metal-allyl sites, respectively. The activity of group 4 metallocenes, which are very efficient ethylene polymerization catalysts, plummets when copolymerizations with butadiene are attempted, with very low overall insertion of butadiene.4 However, copolymerization of these two monomers using (η5C 5H10 )2ZrCl 2, rac-[CH2(3-tBu-1-indenyl)2]ZrCl2 or rac[CH2(3-Me-1-indenyl)2]ZrCl2 generates unique microstructures containing cyclopentane and cyclopropane units in the backbone.5,6 The mechanism of cyclic units formation is described by Scheme 2A,B as being dependent on the insertion Received: October 15, 2015 Revised: December 14, 2015

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DOI: 10.1021/acscatal.5b02317 ACS Catal. 2016, 6, 1028−1036

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Scheme 2. Cyclo-Copolymerization of Ethylene with Butadiene Demonstrating the Formation of Cyclopropane Ring (A), Cyclopentane Ring (B), and trans-1,2-Disubstituted Cyclohexane Ring (C)

In order to activate the chloride or borohydride precatalysts, alkylation by an organo-magnesium, an organo-lithium, or an organo-aluminum compound is required.3,9−13 Among these, dialkylmagnesium is particularly attractive because it not only efficiently activates the catalysts but also acts as chain transfer agent (CTA) to allow coordinative chain transfer copolymerization (CCTP) of ethylene and butadiene.3 In this article, the mechanisms involved during these copolymerization reactions are explored computationally at the DFT level. After a presentation of the modeling strategy validated in our previous work using an ethylene homopolymerization system, we present and discuss the energy profile of the different insertions of ethylene and butadiene into alkyl and allyl active sites for the catalysts (η5-C5Me5)2NdR 1, [mesoMe2Si(3-Me3SiC5H3)2]NdR 2, [Me2Si(C5H4)(C13H8)]NdR 3 and [Me2Si(C13H8)2]NdR 4 in order to highlight the factors that govern the specificity of each catalyst, and contribute toward building a ligand/microstructure/activity relationship.

of butadiene in the 1,2 configuration to give dangling vinyl units.6b,7 In the search for catalysts able to copolymerize ethylene and butadiene, a significant advance was made in the development of lanthanide-based catalysts, which proved capable to efficiently homopolymerize both monomers.8 During the last two decades, our group has reported several catalytic systems based on neodymocene complexes which are able to perform ethylene/butadiene copolymerization. These catalysts were based on neodymocene chloride or borohydride precursors.9 Because the activity and selectivity of both the neodymocene chloride and borohydride were very similar,3 the present work combines the data obtained from these two systems. The first generation catalytic systems able to copolymerize ethylene and butadiene are based on the unbridged (η5C5Me5)2NdCl2Li(OEt2)210−12 and (Me3SiC5H4)2NdCl,10,11 which are efficient in ethylene homopolymerization but demonstrate low insertion of butadiene under copolymerization conditions. The second generation catalysts are characterized by ansa-dimethylsilylene-bridged complexes. They are supported by [meso-Me2Si(3-Me3SiC5H3)2],9−12 [Me2Si(C5H4)(C13H8)],9,13 or [Me2Si(C13H8)2] ligands3,11 as illustrated in Figure 1. These catalysts show higher activities and higher

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RESULTS AND DISCUSSION Modeling Strategy and Notations. The modeling of reactions involving lanthanide (III) complexes requires some comments. First, it has been shown that 4f orbitals and electrons have little to no influence on the lanthanide−ligand chemical bonds and their reactivity, provided there is no single electron transfer involved in the reaction.14−16 Because in our case, monomer insertions are concerted, there is no change in Nd oxidization state during the reaction (it remains III throughout). Thus, the population of the partially filled 4f shell remains unchanged during polymerization. As a result, the 4f shell was implicitly taken into account14 by using a 49electron quasi-relativistic large core Relativistic Effective Core Potential (RECP).17 This reinforces the validity of the DFT approach as the system turns to be a pseudosinglet. Additionally, this type of RECP drastically limits the calculation size/cost; only 11 valence electrons are explicitly computed for any lanthanide III center. Regarding the choice of the density functional, a study evaluating the performance of various density functionals and basis sets has shown that the hybrid functional B3PW91 and the basis set 6-311G(d,p) used in combination with large core RECP offers a good compromise between accuracy and computational cost for studying the reactivity of lanthanide (III) complexes.18 This strategy has been intensively used by Perrin, Maron, and Eisenstein in mechanistic studies involving lanthanide III complexes.19−21 In the present study, by default, solvation has been taken into account during optimization and dispersion has been included as a single point correction in

Figure 1. Neodymocene complexes 1−4.

butadiene/ethylene insertion ratios than the unbridged complexes at convenient temperature and pressure conditions. This family of complexes currently defines a lead of catalysts for the synthesis of ethylene/butadiene copolymers. Depending on the ligand and on reaction conditions, a large variety of copolymers, differing not only in their degree of butadiene incorporation but also in their microstructures can be synthesized by these catalysts. Unlike group 4 metallocenes which yield cyclopropane and cyclopentane motifs, catalyst 4 for example exclusively leads to the formation of trans-1,2cyclohexyl units. The mechanism of formation of this motif is depicted in Scheme 2C. Whereas the formation of cyclopropane and cyclopentane scaffolds arise from a 1,2-insertion of butadiene, these cyclohexyl units originate from 2,1-insertion of butadiene followed by ethylene insertion in the Nd−C2 allyl bond (Scheme 2C) 1029


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Figure 2. Reactivity landscape of the possible insertions during ethylene/butadiene copolymerization in the presence of BOMAG. [Nd] represents L2Nd neodymocene.

by ethylene insertion) and π-allyl (denoted B, since it is yielded by butadiene insertion). Note that the alkyl active site E can also originate from BOMAG. Monomer insertions into these two active sites can lead either to linear, branched, or 1,2-disubstituted-cyclohexyl chain ends. Starting from a B active site, insertion of either monomer at carbon C2 leads to the formation of a dangling vinyl unit (Figure 2). To differentiate this case, a “v” superscript precedes the inserted monomer (giving BvE or BvB). For butadiene insertions, two cases are distinguished: insertion leading to a πallyl active site, denoted B, and insertion leading to an alkyl active site, denoted B12. This latter insertion mode yields a dangling vinyl group. Finally, the cyclization steps are labeled with a “C”. Precoordination of a monomer to an active site is labeled with an underscore between (e.g., E_E for precoordination of an ethylene to an alkyl site). Transition states (TS) are identified with an asterisk after the inserted monomer (e.g., EE* for the TS of ethylene insertion in an alkyl chain). Unless specified, all energy variations refer to Gibbs energies in kcal mol−1 relative to the most stable dormant state of the catalytic systems and the number of monomer(s) and alkylmagnesium units required to fulfill the mass balance. The non-ansa or ansametallocene fragments will be noted [Nd] hereafter. The landscape of reactions that depicts ethylene/butadiene copolymerization is illustrated in Figure 2. It can be separated into five zones. Zone A corresponds to linear insertions into an alkyl site E; zone B to linear insertions into an allyl site B; zone C to branching insertion into an alkyl site E; zone D to branching insertion into an allyl B and finally zone E accounts for cyclization. Dormant State. In the case of ethylene homopolymerization, reversible chain transfers between neodymium and magnesium were demonstrated to be fast and efficient experimentally.12,18c,23 By analogy, we expected both alkyl and allyl chain transfer to occur during ethylene/butadiene copolymerization. These putative transfer reactions are illustrated by the following equations:

order to precisely assess reaction energetics.18b,c,21 As a structural model, growing chains are considered to contain at least four carbon units in order to avoid spurious artifacts.22 Finally, because the dialkylmagnesium plays a crucial role in this copolymerization system, this reagent has been explicitly included in the chemical model. Thermal and entropy contributions have also been included by means of frequency calculation within the harmonic approximation. Because association/dissociation between BOMAG (n-butyln-octyl-magnesium) and neodymocene complexes is known to occur in the reaction,23 knowing the most stable structure of BOMAG in toluene was important for the present work. Starting from Bu2Mg as a building block, various aggregates24 comprising up to four magnesium atoms were found to exist in our previous work.18c Among all the possible isomers, the cluster Mg4(σ-Bu)2(μ-Bu)4(μ3-Bu)2 in which two alkyl chains are terminal, four are bridging, and two are facial was calculated to be the most stable aggregate. This structure will therefore serve as a reference throughout the present study, and a 1/n ratio of Mg4Bu8 will be used when thermodynamic balance involves (MgBu2)n. We have shown in our previous work that the heterobimetallic trimer (η5-C5Me5)2Nd[(μ-Bu)2Mg]2Bu is the most stable complex resulting from the association of (η5C5Me5)2Nd-Bu and half of Mg4Bu8 in toluene. The formation of the (η5-C5Me5)2Nd-Bu from the heterobimetallic trimer (η5C5Me5)2Nd[(μ-Bu)2Mg]2Bu is endergonic by 9.7 kcal mol−1. (η5-C5Me5)2Nd-Bu was identified as the active species for ethylene insertion.18c This computational approach was successfully evaluated over ethylene homopolymerization mediated by 1 in association with BOMAG as an alkylating and chain transfer agent.18c It established the modeling strategy for the present investigation of copolymerization of ethylene with butadiene using neodymocene catalysts. Reaction Landscape of Ethylene/Butadiene Copolymerization. For convenience, insertions in the present system are described by letters, the first of which describes the nature of the initial active site, and the second of which describes the nature of the inserted monomer (Figure 2). There are two types of active site: alkyl (denoted E, since it is typically yielded

[Nd](η3 − C4 H 7) + 1/4Mg4Bu8 = [Nd](μ − Bu)2 Mg(σ − C4 H 7)

(A) 1030


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ACS Catalysis [Nd](η3 − C4 H 7) + 1/2Mg4Bu8 = [Nd]Mg 2Bu4(σ − C4 H 7)

Table 2. Gibbs Energy Profile for the Linear Insertion of Butadiene and Ethylene into an Alkyl Active Site E, Figure 2A, and Kinetic Selectivity between Butadiene and Ethylene Insertions into an Alkyl Site E, for the Set of Catalysts 1−4a

(B)

[Nd](η3 − C4 H 7) + 3/2Mg4Bu8 = [Nd]Mg 2Bu5 + Mg4Bu 7(σ − C4 H 7)

(C)

Thermodynamically, the most stable species are [Nd]Mg2Bu5 and Mg4Bu7(σ-C4H7), in which the allyl group is transferred from Nd to Mg according to Equation C. This stands for the four catalysts considered (Table 1). It is worth

1 2 3 4

Table 1. Thermodynamics of Putative Allyl Chain Transfer from Nd to Mg Sites According to Equations A, B, and C for Catalysts 1−4a

a

equation

1

2

3

4

A B C

3.4 1.6 −2.1

4.2 2.3 −9.6

3.3 1.2 −0.7

1.4 −0.3 −4.6

E_B

EB*

EB

E_E

EE*

14.6 13.6 12.1 15.6

22.6 21.2 18.7 22.6

−18.5 −21.4 −19.4 −17.3

13.7 15.7 12.2 12.4

20.6 22.4 22.1 22.6

−3.6 −2.7 −4.1 −2.1

EE

EB*EE*

(−12.6)b (−12.3)b (−14.4)b (−13.6)b

2.0 −1.2 −3.4 0.0

Gibbs free energies in kcal mol−1. bThermochemistry including association with 1/2 Mg4Bu8 to yield [Nd]Mg2Bu4Hex.

a

Table 3. Gibbs Energy Profile for the Linear Insertion of Butadiene and Ethylene into an Allyl Active Site B, Figure 2B, and Kinetic Selectivity between Butadiene and Ethylene Insertions into an Allyl Site B, for the Set of Catalysts 1−4a

Gibbs free energies kcal mol−1.

B_E 1 2 3 4

12.5 2.9 3.7 7.7

BE* 21.1 18.1 17.0 15.9

BE c

0.3 (−9.4) −2.3 (−11.9)c −3.8 (−14.1)c −6.2 (−17.7)c

BB*b

BB

BE*-BB*

30.4d 19.6 18.2 24.9d

−9.7 −17.7 −14.4 −12.8

−9.3 −1.5 −1.2 −9.0

a Gibbs free energies in kcal mol−1. bNo butadiene adduct to [Nd](η3C4H7) could be located on the potential energy surface (PES). c Extrapolated thermochemistry including association with 1/2 Mg4Bu8 to yield [Nd]Mg2Bu4 ((CH2) 3-CHCH−CH3 ) based on the thermodynamic balance of [Nd]Bu + 1/2 Mg4Bu8 = [Nd]Mg2Bu5. d Insertion into a σ-allyl.

ditions were adapted to provide low molar mass copolymers i.e. by using a high [Mg]/[Nd] ratio and low productivity (Mn = 2000 g mol−1, Đ = 1.7 and Mn = 2500 g mol−1, Đ = 1.4 respectively for each catalyst). The 1H and 13C NMR spectra of the copolymers (which contained 20 mol % and 38 mol % of inserted butadiene units, respectively) showed no allylic (−CHCH−CH3) or vinyl terminal groups (CH2−CH CH2). This demonstrates that the transfer of an allyl group between Nd and Mg does not occur after the insertion of a butadiene monomer unless it is after a B12 insertion (Figure 2). A high activation barrier for allyl transmetalation, at least higher than those involved in propagations and alkyl chain transfer, seems to prevent the allyl transfer from occurring, even though this transmetalation is thermodynamically favorable. As a consequence, in the following, the reference Gibbs f ree energy state is composed of [Nd]Mg2Bu5 and [Nd](η3-C4H7) that respectively account for the most stable alkyl and allyl active sites. In other words, insertions into an alkyl chain will involve [Nd]Mg2Bu5 and its subsequent dissociation in [Nd]Bu/ Mg4Bu8, and insertions into an allyl site will use [Nd]allyl. Linear Propagation and Kinetic Model. In this section, we report the mechanisms and the associated energy profiles of linear insertions occurring in zones A and B of Figure 2. The profiles are detailed in Figures 3 and 4. EB insertion is exergonic by 17 to 22 kcal mol−1 irrespective of the ligand. It is more exoergic by ca. 15 kcal mol−1 than the EE sequence if the recombination with the Grignard reagent is not taken into account; this energy difference is lowered to 5 kcal mol−1 if the recombination is included (Table 2). The driving force which favors butadiene insertion rather than ethylene insertion into an alkyl site is the formation of an allyl ligand, which is a strong ligand to lanthanide(III) center.26 Allyl groups also show a larger interaction with neodymium

Figure 3. General profile for the formation of EE and EB motifs. The energetics are reported in Table 2.

Figure 4. General profile for the formation of BE and BB motifs. The energetics are reported in Table 3.

mentioning that in all allyl magnesium clusters/complexes considered, the allyl ligand is σ-bonded to magnesium, with the η3-binding mode not found as a minima on the potential energy surface. This is consistent with the reported X-ray structures of bis-allyl-magnesium compounds.25 As an experimental support, two ethylene/butadiene copolymers were prepared using {(Me 2 Si(C 13 H 8 ) 2 )Nd(μ-BH 4 )[(μ-BH 4 )Li(THF)]} 3b and [Me2Si(C5H4)(C13H8)Nd(BH4)2][Li(THF)]9 activated by BOMAG as described previously. The polymerization con1031


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four complexes considered here, the η3-binding mode of the allyl ligand is always energetically favored over the η1-binding mode, driven by the strong positive charge carried by the metal center. Copolymerization Statistics. The calculated activation barriers of the linear propagation steps were employed in a kinetic model of copolymerization based on Markov chain statistics.30 In this model, a reactivity ratio r is defined for each type of active site. Ethylene and butadiene reactivity ratios are defined as r1 = kEE/kEB and r2 = kBB/kBE, respectively. Ratio of kinetic constants are computed according to the Eyring equation ki/kj = exp((ΔrG#j − ΔrG#i)/RT) evaluated at T = 298 K. For the four catalysts studied, r1·r2 tends to 0 (Table 4), which is characteristic of alternating copolymerization systems.30 With an r1 significantly higher than 1, catalyst 1 strongly favors consecutive insertion of ethylene and corresponds to the insertion of the most reactive monomer toward the most reactive active site. The parameters calculated for catalyst 1 agree with experimental results. In copolymerization conditions, with an input of 5% of butadiene, catalyst 1 showed a significant drop in activity compared to its activity in ethylene homopolymerization. In addition, a very small proportion of butadiene units were contained in the resulting copolymer.10,11 This behavior originates from the formation of small amount Nd-allyl active sites that are less reactive than Nd-alkyl site toward ethylene insertion. For catalyst 2, 13C NMR spectroscopy revealed that the butadiene and ethylene units are mostly alternating,12 which implies that kEE < kEB and kBB < kBE. This result is verified by the calculations which show r1 and r2 to be both less than 1. 13C NMR spectroscopy of copolymer obtained with catalyst 3 demonstrated a strong alternating character with a predominance of BEB units over BEE units; no BEEE units were found.9,13,31 This result means that the EB insertion is favored over EE insertion. The calculated rate constants support this observation, i.e. kEB ≫ kEE. For catalyst 4, the butadiene percentage in the copolymer was proportional to the concentration of butadiene in the feed stream, at least up to 30% of butadiene feed.3 This result suggests an r1 near 1, which is in agreement with the calculated value. Finally, for the catalysts 2 and 3, r1 and r2 have already been determined experimentally in our previous work9,11 and could be used to estimate the difference between the activation energies, as shown in Table 4. The calculated values are in excellent agreement with experiment, with our modeling strategy affording relative kinetics parameters within 1 kcal mol−1 of experimental values. This precision is comparable to that attained in state-of-the-art molecular modeling of transition metal reactivity.21,32−34 Branching Propagation. In this section, we report the mechanisms and the associated energy profiles of branching

compared to two bridging alkyl chains shared with a magnesium center. Kinetically, the non-ansa catalyst 1 (C5Me5)2NdR is more efficient toward the EE insertion than the EB insertion, whereas catalysts 2 and 3 are more selective toward the EB insertion, as shown in Table 2. The kinetics of these reactions will be further discussed when describing the microstructure of the polymers. For catalysts 1 to 4, the calculated activation barrier for the BB insertion is higher than for the BE insertion (Table 3). However, based on the expected precision of the computational level used, the computed selectivity for catalysts 2 and 3 is not significant enough. For these two catalysts, the small energy difference between transition states BB* and BE* does not allow the assignment of a selectivity trend for these two insertions (Table 3). Mechanistically, insertion in an allyl group is fully concerted and does not require a haptotropic shift from η3 to η1 or σ to proceed, unless the steric bulk of the ligand demands a partial decoordination of the allyl group in order to allow the incoming monomer to interact with the metal center.27−29 This is illustrated in Figure 5. BB insertion proceeds without a

Figure 5. 3D-representation of transition states BE* and BB*, respectively, for the insertion of ethylene and butadiene in [Nd](η3C4H7) for complexes 1 and 3. Bond distances are in Å. Hydrogen atoms of the ligand have been omitted for clarity.

precoordination of the butadiene. The BB* activation barrier of catalyst 1 is the highest of the four catalysts. This is due to the η3 to η1 haptotropic shift required for the binding of the incoming monomer (see Supporting Information). In contrast, the allyl ligand remains η3-bonded in the monomer adducts and insertion transition states for catalysts 2, 3, and 4. For the set of

Table 4. Kinetic Statistic Model Based on Reactivity Ratios r1 = kEE/kEB and r2 = kBB/kBE, under Linear Chain Growth Assumption, and Experimental vs Computed Kinetic Selectivity in Ethylene Butadiene Copolymerization Performed with Complexes 2 and 3a r1 1 2 3 4 a

r1·r2

r2

2.48 × 10 1.32 × 10−1 3.79 × 10−3 1.00 × 100 1

1.50 7.93 1.32 2.49

× × × ×

−7

10 10−2 10−1 10−7

3.72 1.04 4.99 2.49

× × × ×

EB*- EE*b −6

10 10−2 10−4 10−7

d

n.a. −0.95 ± 0.05 −1.9 ± 0.2 n.a.d

EB*- EE*c 2.0 −1.2 −3.4 0.0

BE*-BB*b d

n.a. −1.8 ± 0.3 −1.45 ± 0.05 n.a.d

BE*-BB*c −9.3 −1.5 −1.2 −9.0

Gibbs free energies in kcal mol−1. bFrom experimental results.9,11 cFrom DFT level of calculation. dn.a.: data not available. 1032


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Figure 6. General profile for the formation of EB12 motif (A); and BvE and BvB motifs (B). The energetics are reported in Tables 5 and 6

insertions occurring in zones C and D of Figure 2. Detailed profiles for these insertion are depicted in Figure 6. For catalysts 2, 3, and 4, the activation barrier EB12* is the highest of all the possible insertions involved in ethylene/butadiene copolymerization (Table 5). Although the EB12 insertion does

Table 6. Gibbs Energy Profile for the Branching Insertion of Butadiene and Ethylene into an Allyl Active Site B, Figure 2D, and Kinetic Selectivity between Butadiene and Ethylene Insertions into an Allyl Site B for the Set of Catalysts 1−4a 1 2 3 4

Table 5. Gibbs Energy Profile for the Branching Insertion of Butadiene into an Alkyl Active Site E, Figure 2C, for the Set of Catalysts 1−4a E_B12 1 2 3 4

b

/ 12.6 11.1 13.1

EB12*

EB12

26.7 28.1 26.4 27.1

0.2 0.7 0.8 2.6

B_vE

BvE*

BvE

BvB*b

BvB

BvE* - BvB*

5.7 4.2 4.7 3.3

22.9 21.2 20.5 17.0

−0.2 −0.4 −0.7 −3.3

25.4 21.8 19.6 21.5

−7.3 −13.9 −13.2 −8.9

−2.5 −0.6 0.9 −4.5

a Gibbs free energies in kcal mol−1. bNo butadiene adduct to [Nd](nbutyl) could be located on the potential energy surface (PES).

and trans-1,4-butadiene units (26%) for a copolymerization using 25% of butadiene in the feed performed at 80 °C and 4 bar.3a The calculated selectivity derived from energy profiles is in agreement with this experimental data.3 Catalyst 1 shows the same reactivity pattern as catalyst 4. 13C NMR spectroscopy showed that the butadiene units were isolated in the copolymer, with no butadiene/butadiene sequences identified.11 Additionally, experimental data reveal the presence of a small amount of vinyl units in the copolymer.10,11 As discussed in the previous section, this is in line with the disfavored BvE insertion behavior displayed by catalyst 1. Within the precision of the computational level used, no selectivity toward BvE or BvB insertion sequences can be assigned to catalysts 2 and 3 (Table 6). However, these two transition states are ca. 3 kcal mol−1 higher in energy than the transition state BE* meaning the formation of vinyl units is not favorable for the catalysts 2 and 3. Microstructure analysis by NMR showed a low percentage of 1,2 units in the copolymer, accounting for ca. 3% of the inserted butadiene.3,9 Mechanistically, for catalysts 1 to 4, BvB insertions proceed without precoordination of the butadiene, and a η3 to η1 haptotropic shift of the allyl occurs to allow the monomer to approach (Table S1). Cyclization. According to zone E of Figure 2, which describes the formation of cyclohexyl units within the ethylene/butadiene copolymer, we explored the mechanism of this ring formation in order to highlight the influence of the ancillary ligand. Starting from the allyl complex [Nd](η3-C4H7) B, a first insertion of ethylene in the Nd−C3 side yields BvE, which is characterized by a dangling vinyl group. A second ethylene insertion generates the BvEE unit. At this stage,

a Gibbs free energies in kcal mol−1. bNo butadiene adduct to [Nd](nbutyl) could be located on the potential energy surface (PES).

not possess the highest barrier for 1, the energy required to realize this insertion remains high relative to the experiment conditions. Mechanistically, no precoordination of butadiene was found while following the reaction coordinate, meaning that EB12 insertion is therefore direct for catalyst 1. As shown in the Scheme 2, EB12 insertion is the first insertion that leads to the formation of cyclopropane and cyclopentane rings in ethylene/butadiene copolymerization using group 4 metallocenes. Considering the energy barrier of this sequence for the Nd catalysts in the present study, however, EB12 insertions are kinetically too unfavorable to occur. Experimentally, this result is verified by the absence of these cycles in the NMR analyses,3,11 highlighting the fundamental difference in reactivity of group 4 and Nd metallocenes with butadiene. As a result, the formation of vinyl branching originates from the BvE insertion. Catalyst 4 shows the lowest activation barrier for the BvE insertion and has also a high selectivity toward the BvE sequence compared to the BvB insertion (Table 6). As depicted in Scheme 2, BvE is the preliminary insertion that leads to the formation of cyclohexane rings. Experimentally, only 4 yields copolymers composed of trans-1,2-cyclohexyl units, with this polymer product also displaying the highest content of vinyl units.3,11 The mechanism of cyclohexyl formation will be discussed in the next section. NMR spectroscopy revealed that the inserted butadiene was distributed in cyclohexane rings (51%), vinyl units (23%), 1033


Research Article

ACS Catalysis

isoenergetic, meaning that the formation of vinyl branching via BvE is possible for these two catalysts. The second critical event that determines the formation of cyclohexyl units is the competition between intramolecular insertion of the vinyl group and intermolecular insertion of free ethylene. For catalyst 1, starting from the BvEE complex, the intramolecular insertion requires 9.8 kcal mol−1 to proceed, whereas the intermolecular insertion of ethylene requires 10.9 kcal mol −1 , as deduced from ethylene insertion in Cp*2NdBu.18c For catalyst 4, starting from the most stable complex BvEE_C, the energy barrier to overcome for cyclization is 12.1 kcal mol−1, whereas intermolecular insertion of ethylene needs 15.3 kcal mol−1, as computed for the BvEE sequence. As a result, catalyst 1 is expected to form fewer 6member rings than complex 4. The underlying reason for this trend is the energetics of the rebinding of the dangling vinyl group. This result agrees with NMR analyses: the signals corresponding to the cyclohexyl motif are only found as trace in the 13C NMR spectrum of the polymer yielded by catalyst 1,10,11 whereas catalyst 4 is highly selective toward the formation of cyclohexyl units.3,11 For catalyst 4, the rebinding of vinyl group is in this case exergonic by 6 kcal mol−1; therefore, catalyst 4 is the most capable of the four catalysts to form cyclohexane ring, in full agreement with the experimental results. Indeed, 13C NMR analysis of the copolymer obtained with catalyst 4 showed up around 50% of inserted butadiene units are contained in 6membered rings when synthesized under 4 bar of monomer pressure.3 Note that the percentage of cyclohexane rings can be regulated by changing the monomer concentration. Because the cyclization is an intramolecular mechanism, the higher the concentration, the lower the formation of cyclohexane rings, which leads to a higher content of vinyl units in the copolymer. Structural and Electronic Properties. In an attempt to gain a better understanding of the ligand effects on key insertions, we explored the structural (Tables S1 to S14) and electronic properties (Tables S15 and S16) of a family of complexes and transition states. Structurally, the main difference between the complex 1 and ansa-dimethylsilylene-bridged complexes 2, 3, and 4 is the edge angle between the mean planes of the two πbonded Cp-rings. Complex 1 has an edge angle of ca. 44°, whereas ansa-dimethylsylene-bridged complexes have an edge angle of ca. 78° (Table S14). This can be related to the haptotropic shift accompanying the insertion in a B site of complex 1. For complexes 2, 3, and 4, this angle is maintained in each stationary point, and no meaningful variation could be related to kinetic patterns (Table S14). Aside from the structural effect discussed in the previous sections, electronic effects were investigated both through natural bonding orbital (NBO) and noncovalent interaction (NCI) analyses. The charge on the Nd metal center is hardly affected by the nature of the ligand and the insertion sequence; variations were too small to be confidently discussed except when comparing non-

reinsertion of the vinyl group in the Nd−C bond yields the cyclic BvEEC scaffold, as shown in Figure 7. Tables 6 and 7 report the energy of each stationary point for catalysts 1 to 4.

Figure 7. General profile for formation of the trans-1,2-cyclohexyl motif during ethylene/butadiene copolymerization. The energetics are reported in Tables 6 and 7

In this sequence, the kinetic limiting step is the BvE insertion, whose transition state BvE* is the highest in energy of the whole ring formation sequence (Tables 6 and 7). This observation stands for the entire set of catalysts 1−4. Though transition states BvEE* and BvEEC* are lower in energy than BvE*, a second determining step was identified. Indeed, after the second insertion of ethylene the interaction between the Nd and the vinyl group is lost. In BvEE, Nd···vinyl bond distances range from 5.3 to 6.1 Å across the four systems. Rebinding the vinyl group to the Nd center to yield BvEE_C is thereof mandatory for the cyclization to occur. In order to build a ligand/microstructure relationship, these two critical points of the cyclization sequence will be discussed for each neodymocene complex. The formation of cyclic unit requires the preliminary formation of a vinyl branch. This is kinetically controlled by the difference in energy between transitions states BE* and BvE*. For the four catalysts, the activation barrier is higher for the BvE insertion compared to the BE insertion. Considering catalysts 2 and 3, BE insertion is favored by ca. 3 kcal mol−1 (Table 3 and 6). As the BE insertion is more likely to occur than the BvE insertion, no cyclohexyl ring was therefore expected with these two catalysts. Experimentally this expectation is verified by an absence of cyclohexane rings as analyzed by 13C NMR spectroscopy.11,13 Concerning catalysts 1 and 4, although BE insertion is still favored, the computed selectivity is not significant relative to the precision of the computational level. The transition state BE* and BvE* are essentially

Table 7. Energy of the Insertion Involved in the Formation of 1,2-Cyclohexane Ring and Selectivity of the Limiting Stepa 1 2 3 4 a

BvE_E

BvEE*

BvEE

BvEE_C

BvEEC*

BvEEC

BE*-BvE*

BvEE_C−BvEE

9.6 8.1 6.7 4.6

15.3 15.8 16.4 12.0

−11.7 −10.3 −8.1 −11.3

−7.7 −7.3 −11.6 −17.3

−1.9 −2.6 −2.5 −5.2

−30.7 −30.9 −27.5 −31.2

−1.8 −3.1 −3.5 −1.1

4.0 3.0 −3.5 −6.0

Gibbs free energy in kcal mol−1. 1034


Research Article

ACS Catalysis ansa and ansa complexes (see Supporting Information). Additional analysis are currently underway and will be reported in due time.

the extrema (minimum or transition state). Gibbs energies were computed within the harmonic approximation and estimated at 298.15 K, 1 atm. Criteria for SCF convergence and geometry optimization, and the integration grid were set to default values. All these computations were performed with the Gaussian 09 suite of programs.44

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CONCLUSIONS For a set of four neodymocene catalysts, a mechanistic study of the ethylene/butadiene copolymerization reaction in the presence of an alkylating/transfer agent has been performed at the DFT level. The energy profile of all the key steps involved in this copolymerization reaction have been computed, including alkyl chains transfer, linear and branched chain growth, and ring formation. The low activity and low amount of butadiene obtained in copolymerization with (η5C5Me5)2NdR is explained by the bulkiness of the ligand. In contrast, catalysts [meso-Me2Si(3-Me3SiC5H3)2]NdR, [Me2Si(C5H4)(C13H8)]NdR, and Me2Si(C13H8)2]NdR efficiently insert butadiene during the copolymerization. A kinetic model based on computed activation barriers was able to reproduce the alternating behavior of the copolymerization, and an excellent correlation was obtained between experimental and computed reactivity indexes for metallocene catalysts supported by [Me2Si(3-Me3SiC5H3)2] and [Me2Si(C5H4)(C13H8)] ligands. We also determined the limiting steps leading to the formation of unique ethylene/butadiene rubber containing trans-1,2-disubstituted cyclohexane rings. To form this type of cycle, the catalyst must be not only able to form dangling vinyl units but also allow the rebinding of the closest dangling vinyl group to the Nd center before cyclization. All those requirements were met with the catalyst {(Me2Si(C13H8)2)Nd(μ-BH4)[(μ-BH4)Li(THF)]}2/BOMAG and computationally justified. The present work is the first to depict, at a DFT level, the reactions occurring during copolymerization of ethylene with butadiene while taking into account heterometallic intermediates. This study also offers a survey of the ligand effects on the activity of catalysts and the microstructure of the resulting copolymers. The analysis of the reaction mechanisms and energy profiles offers a relationship between ligand structure, catalyst activity and polymer microstructure and sets the base for further catalyst developments in this intricate copolymerization system.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.5b02317. Three-dimensional representation of key transition states; geometrical parameters and population analysis; Cartesian coordinates and associated energies (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: 33 472 448 168. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Manufacture Michelin is acknowledged for scientific and financial support of this project. H.N. and L.P. thank CCIR of ICBMS and P2CHPD of Université Lyon 1 for providing computational resources and technical support. The authors thank Vincent Monteil for fruitful scientific discussions. The authors thank the reviewers for their detailed reports and both helpful and fruitful comments.

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REFERENCES

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COMPUTATIONAL DETAILS Geometry optimization of the reactants and transition states were carried out at a DFT level of theory using the hybrid functional B3PW9135,36 without any symmetry restrictions. Neodymium was represented by a 49-electron Stuttgart− Dresden−Bonn quasi-relativistic large Effective Core Potential, which includes the 4f electrons in the core, and the associated basis set37 completed with a polarization f function (ζf = 1.0). Silicon was represented by a Stuttgart−Dresden−Bonn pseudopotential and its adapted basis set38 augmented by a d function (ζd = 0.284). Magnesium was represented by a polarized all electron triple-ζ 6-311G(d,p) basis set and supplemented by a diffuse function.39−41 Carbon and hydrogen atoms were represented by a polarized all electron triple-ζ 6311G(d,p) basis set.39,40 Solvation by toluene was implicitly represented during optimization using the SMD method.42 The Grimme empirical correction with the original D3 BJ damping function was used to include the dispersion correction as a single point calculation at the optimized geometry.43 Analytical frequency calculations were carried out to verify the nature of 1035


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EthyleneâButadiene Copolymerization by Neodymocene Complexes

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EthyleneâButadiene Copolymerization by Neodymocene Complexes