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Article Cite This: Macromolecules XXXX, XXX, XXX-XXX

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Activation of Cp2ZrX2 (X = Me, Cl) by Methylaluminoxane As Studied by Electrospray Ionization Mass Spectrometry: Relationship to Polymerization Catalysis Scott Collins,*,†,‡ Mikko Linnolahti,§ Maricela Garcia Zamora,‡ Harmen S. Zijlstra,† María T. Rodríguez Hernández,‡ and Odilia Perez-Camacho‡ †

Department of Chemistry, University of Victoria, 3800 Finnerty Rd. Victoria, BC, Canada Centro de Investigacion en Quimica Aplicada, CP 25294 Saltillo, COAH, Mexico § Department of Chemistry, Joensuu Campus, University of Eastern Finland, P.O. Box 111 FI-80101 Joensuu, Finland ‡

S Supporting Information *

ABSTRACT: Activation of Cp2ZrCl2 and Cp2ZrMe2 by methylaluminoxane (MAO) in toluene is largely complete at Al:Zr ratios of 100:1 to 200:1 as revealed by electrospray ionization mass spectrometry (ESI MS). The anions present undergo chlorination in the case of Cp2ZrCl2. DFT calculations reveal that chlorination of MAO is favorable and involves dissociation of Me3Al, followed by association of Me2AlCl. Ethylene polymerizations were conducted using these catalyst precursors in toluene. The activity vs [Zr] data are essentially identical, while higher MW polyethylene is formed with a narrower MWD at lower Al:Zr ratios in the case of Cp2ZrMe2. The activity vs [Zr] data could be fit to a model which invokes bimolecular deactivation of growing chains. ESI MS reveals that a dinuclear Zr2 cation with m/z 557 is formed on exposure of [Cp2Zr(μ-Me)2AlMe2]+ to ethylene, in addition to other cations that are dinuclear with respect to Zr. Labeling experiments using ethylene-d4 indicate that these dinuclear cations are derived from ethylene, either through direct incorporation in the case of m/z 557, or indirectly through incorporation of deuterium following e.g. β-H elimination. These experiments shed light on the need for high Al:Zr ratios for ethylene polymerization using soluble metallocene catalysts. The active catalyst [Cp2ZrR][MAO(Me)] (R = H, Et or a higher homologue) suffers a second order deactivation, and thus activity improves upon dilution of the catalyst precursor at constant [Al].

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INTRODUCTION

chemistry has been studied by different spectroscopic techniques including NMR,8−13 UV−vis,14−18 and FTIR,19−21 spectroscopy. Recently, we reported the first study of the anions derived from MAO (which all correspond to the formula [(MeAlO)n(Me3Al)mMe]) in the presence of a variety of donors, including Cp2ZrMe2 in fluorobenzene (PhF) solvent,22 by the technique of electrospray ionization mass spectrometry (ESI MS).23−30 In another recent paper, we systematically studied activation of both Cp2ZrMe2 and Cp2ZrCl2 using MAO by ESI MS in PhF.31 The basic conclusions of these and other spectroscopic studies are summarized in Scheme 2. At low Al:Zr ratios (ca. 25−50:1) contact ion-pairs 1 and dinuclear ion-pairs 2 (m/z 485 for X = Me) are found while at higher Al:Zr ratios more loosely associated ion-pairs 3 are formed. The heterodinuclear Me3Al adduct 4 predominates at highest Al:Zr ratios with unhindered metallocenes. For X = Cl, complex 2 with m/z =

1

Since its discovery, MAO has been the activator of choice for single-site, olefin polymerization catalysts.2−4 MAO activates single-site catalysts such as metallocene complexes by alkylation and ionization−see eqs 1 and 2, Scheme 1. Although the active species, an alkylmetallocenium ion (eq 2, Scheme 1) has been widely studied,5,6 much less is known about the counteranions formed which are expected to influence ion-pairing.7This Scheme 1

Received: May 5, 2017 Revised: October 4, 2017

© XXXX American Chemical Society

A

DOI: 10.1021/acs.macromol.7b00933 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

behavior of Cp2ZrCl2 and to a lesser extent Cp2ZrMe2 upon activation with MAO has been extensively studied, and both precursors require a large Al:Zr ratio for high activity in solution.36 In this paper, we report additional ESI MS studies concerned with the effect of added Me2AlCl on the ion-pair speciation in the case of activation of Cp2ZrMe2 by MAO, DFT studies of model anion and neutral MAO chlorination, a comparison of both Cp2ZrMe2 and Cp2ZrCl2 in ethylene polymerization upon activation with various amounts of MAO, and further ESI MS experiments concerned with detection of deactivated ion-pairs formed in situ during ethylene polymerization. These studies strongly confirm early ideas that catalyst deactivation in solution37,38 is the reason why high Al:Zr ratios are necessary using soluble, MAO-activated, Cp2ZrX2 (X = Cl or Me) catalysts.

Scheme 2

505 is unstable with respect to CH4 elimination in solution forming [(Cp2Zr)2(μ-Cl)(μ-CH2)][MAO(Me)]31 which is expected to be much less active or inactive in olefin polymerization.32 In addition, the anions formed at low Al:Zr ratios in the case of Cp2ZrCl2 are extensively chlorinated by a process which involves net exchange of bound Me3Al for Me2AlCl, the byproduct of catalyst alkylation.31 It has been demonstrated in the literature that chlorination of MAO (by Me2AlCl) can be detrimental for catalyst activation,21,33 though a recent theoretical study suggests an opposite conclusion for insertion involving chlorinated anions.34 At high Al:Zr ratios, corresponding to those typically used for catalyst activation in solution (>500:1), the anions are no longer chlorinated in the case of Cp2ZrCl2,31 and the principle ion-pair present is [Cp2Zr(μ-Me)2AlMe2][MAO(Me)] (4 with m/z 307, Scheme 2), which is in equilibrium with the active catalyst 3 through dissociation of Me3Al.35 These observations led to the testable hypothesis that the different activation chemistry observed for Cp2ZrMe2 and Cp2ZrCl2 might manifest itself in significant changes to polymerization activity and polymer properties as a function of the Al:Zr ratio, especially if anion chlorination leads to changes in ion-pairing.7,34 Of course, the olefin polymerization

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RESULTS AND DISCUSSION ESI MS Studies of MAO and Anion Chlorination. In earlier work, we showed that extensive anion chlorination was observed upon activation of Cp2ZrCl2 by MAO at lower Al:Zr ratios vs a basically unchanging anion distribution in the case of Cp2ZrMe2.31 The anions that are formed correspond to the simple formula [(MeAlO)n(Me3Al)m(Me2AlCl)oMe], and for the discussion which follows, a short-hand notation will be used to refer to specific anions as [n,m,o]−. Control experiments suggested chlorination was through formation of Me2AlCl, formed in situ during alkylation of Cp2ZrCl2. This material is known to modify MAO through displacement of bound Me3Al.21 These earlier studies were conducted with MAO supplied by Sigma-Aldrich as a 10 wt % solution. During the course of this work, Sigma-Aldrich stopped supplying pure MAO, supplying customers with MAO that contained higher n-alkylaluminums, and even trace levels of an α-olefin as revealed by high field 1H NMR spectra (see Supporting Information). While this material (now sold as MMAO-12 by Sigma-Aldrich) can be used for activating metallocene catalysts its use for ESI MS

Figure 1. Positive ion mass spectra of a mixture of (a) Cp2ZrMe2 and MAO (Al:Zr = 100, [Zr] = 0.5 mM) and (b) Cp2ZrCl2 and MAO (Al:Zr = 100, [Zr] = 0.5 mM). Cone voltage = 8.0 V; collision voltage = 2.0 V. Negative ion mass spectra of (c) Cp2ZrMe2 and MAO (Al:Zr = 100, [Zr] = 0.5 mM) and (d) Cp2ZrCl2 and MAO (Al:Zr = 100, [Zr] = 0.5 mM). Cone voltage = 16.0 V; collision voltage = 6.0 V. B


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Macromolecules experiments is complicated by the lower volatility of the R3Al components, as well as more complex (due to permutation of Me and R groups) and less intense negative ion spectra. As a result, we switched to a new supplier of MAO and all ESI MS work reported here involved the use of 10 wt % solutions supplied by Albemarle Corp. This material was analyzed by NMR spectroscopy, and was found to contain 13− 14 mol % Me3Al and 1.0 mol % [Me2Al(THF)2]+.39 The Me3Al values are similar to MAO previously supplied by SigmaAldrich, though the amount of the cationic activator [Me2Al(THF)2]+40 was about double that typically found. As a result, in the case of activation of Cp2ZrMe2 by this material, catalyst activation is complete at Al:Zr ratios of 100:1 or greater, and activation of Cp2ZrCl2 is nearly complete at these ratios - see Figure 1a) vs 1b). Surprisingly, the anion distribution is markedly different for the Albemarle material, even though the same anions are present in both it and Aldrich samples.41 In particular, for freshly received material (which was stored in a glovebox freezer at −20 °C) the negative ion spectra are dominated by a single anion with m/z 1375 [16,6,0]−. Depending on instrumental conditions, another anion with m/z 1303 is also present and is related to m/z 1375 through loss of Me3Al ([16,5,0]− Figure 1c). This can be deliberately effected by increasing the cone voltage (in source CID42,43) or alternately the collision voltage with the collision gas on. Another difference is that the anion distribution is sensitive to the Al:Zr ratio in the case of Cp2ZrMe2, with higher MW anions (e.g., m/z 1853 [23,7,0]−)22 more prominent at higher Al:Zr ratios (see Supporting Information). Even though the anion distribution of the Albemarle material is different, it responds to chlorination by Cp2ZrCl2 in a manner that is similar to that observed earlier.31 The anions with m/z 1303 and 1375 are progressively chlorinated to form anions with the formula (in the case of m/z 1375) [(MeAlO)16(Me3Al)6‑n(Me2AlCl)nMe] with n = 1 (m/z 1395), n = 2 (m/z 1415) and so on (Figure 1d). What is also interesting is that the resulting (chlorinated) anion distribution is somewhat independent of how it is generated. Shown in Figure 2 are negative ion spectra of a mixture of Cp2ZrCl2 and MAO (100:1, 2 mol % Cl, Figure 2a), a mixture of Cp2ZrMe2 + MAO (100:1) treated with 2.5 mol % Me2AlCl (Figure 2b), and a mixture of Cp2ZrMe2 + MAO (100:1) where the MAO had been previously modified by the addition of 2.5 mol % Me2AlCl (Figure 2c). There are subtle differences in the extent of chlorination; in particular the close correspondence between Figure 2a) and b) suggests that anion chlorination by Me2AlCl (i.e., either added or formed in situ during activation) is significantly more effective than when MAO is previously modified by the addition of Me2AlCl (Figure 2c). This might be anticipated based on prior studies of MAO chlorination by this additive,21 where displacement of bound Me3Al appears highly favored; consequently, free [Me2AlCl] is reduced and anion chlorination is less pronounced. The similarity of these spectra, especially of parts a and b of Figure 2, suggest the resulting anion distribution is thermodynamic (it does not change noticeably with time) and that the equilibrium is established more or less independently of metallocene catalyst activation (the corresponding positive ion spectra are all similarsee Supporting Information). The results suggest two processes, one which involves direct chlorination of the ion-pairs by Me2AlCl vs an indirect process

Figure 2. Negative ion mass spectra of (a) Cp2ZrCl2 and MAO (Al:Zr = 100, [Zr] = 0.5 mM). Cone voltage = 16.0 V, collision voltage = 6.0 V. (b) Cp2ZrMe2 and MAO (Al:Zr = 100, [Zr] = 0.5 mM) treated with 2.5 mol % Me2AlCl. (c) Cp2ZrMe2 and MAO (Al:Zr = 100, [Zr] = 0.5 mM) with MAO previously modified by addition of 2.5 mol % Me2AlCl. The corresponding positive ion spectra are shown in Figure S-2.

involving chlorinated MAO. The latter process is evidently slower than the former Scheme 3.Shown in Figure 3 are the negative anion spectra of Cp2ZrMe2 + MAO modified by 2.5, 5.0, 10.0, and 20.0 mol % Me2AlCl (parts a−d, respectively, of Figure 3). There is a progressive increase in chlorination and as one exceeds the total amount of Me3Al present (ca. 14 mol %) there is a change in the anion distribution where those anions derived from chlorination of m/z 1375 are largely absent, and a new cluster of anions (which are all chlorinated) is present, centered at m/z 1567 [16,1,6]−. The latter series of anions are related to those formed directly from m/z 1375 by the addition of another Me2AlCl molecule but without release of Me3Al. That is, their formula is consistent with [(MeAlO)16(Me3Al)6‑n(Me2AlCl)n+1Me] where n = 3 is the first ion detected in this series with m/z 1527 [16,3,4]−, and which could be formed from m/z 1435 [16,3,3]− by the addition of Me2AlCl. From theoretical work on the addition of Me3Al to large neutral MAO clusters it is possible this involves a change in structure from cage-like (closo) to raft-like (nido).44 C


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Macromolecules Evidently, it is possible, through addition of Me2AlCl, to modify the anion distribution without affecting catalyst activation, at least at lower levels of this additive, as the corresponding positive ion spectra are largely similar to those shown in Figure 1 (see Supporting Information). We have studied both theoretical aspects and experimental consequences of this anion chlorination reaction in the following two sections. DFT Studies of MAO and Anion Chlorination. Earlier theoretical work studied a possible structure of the anion with m/z 1375, as well as a neutral precursor (with MW 1360), and two possible, monochlorinated anions with m/z 1395.31,41 The structure of 1360 was located by carefully simulating the synthesis of MAO by (gas phase) hydrolysis of Me3Al, as described in our previous papers,44−46 and most recently through use of the M06-2X functional47 at the TZVP level of theory48 using Gaussian 09.49 A brief consideration of the most stable monochlorinated anion (Figure 4) shows that it is possible to substitute an Scheme 3

additional, five bridging methyl groups C(1)−C(5) in this structure by chlorine without altering the basic structure of this cage. The most stable di-, tri-, tetra-, penta-, and hexachlorinated anions feature successive substitution of bridging methyls by chlorine in the order that these carbon atoms are numbered. In Table 1 are summarized the energy and free energy changes associated with these substitution processes, for two different chlorinating agents Cp2ZrCl2 and Me2AlCl. Of the two reagents, Cp2ZrCl2 is more effective on a thermodynamic basis for fully chlorinating m/z 1375 than Me2AlCl. The reason for this difference relates to the dimerization of Me2AlCl vs Me3Al which is calculated to be endoergonic by about 25 kJ mol−1 monomer. In other words, for each chlorination step involving Me2AlCl there is a free energy penalty of this magnitude for forming Me6Al2 from Me4Al2Cl2. The same basic conclusions also apply to a neutral precursor of this anion (see Supporting Information for these structures), though chlorination is less exothermic overall for this precursor (Table 1). This indicates that the equilibria depicted earlier favor chlorination of the anions, regardless of how approached. Thus, whether one directly chlorinates the anions vs the neutral precursors, by either Cp2ZrCl2 or Me2AlCl, the end result is similar from a thermodynamic perspective. These findings are largely consistent with the experimental work. Close examination of Figure 3c and 3d illustrates that anions derived from m/z 1375 never feature more than five

Figure 3. Negative ion spectra of a mixture of Cp2ZrMe2 + MAO (100:1 Al:Zr [Zr] = 0.5 mM, Cone voltage 16 V, Collision voltage 6 V after adding (a) 2.5, (b) 5.0, (c) 10.0, and (d) 20.0 mol % Me2AlCl. The corresponding positive ion spectra are shown in Figure S-3.

Figure 4. Model for anion with m/z 1395 [16,5,1]− featuring a terminal Me2AlCl group, and bridging Me groups labeled C(1)−C(5). Carbon atoms are gray, Al atoms pink, O atoms red, and the Cl atom green with H atoms omitted for clarity.

chlorine atomsi.e., the highest observable mass ion in this series has m/z 1475 [16,1,5]− and even this ion is very weak in intensity compared to m/z 1455 [16,2,4]− (Figure 3d). Even at this level of chlorination, modifications to basic anion structure to form m/z 1567 etc. are much more prominent (Figure 3d vs 3c). D


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Macromolecules Table 1. Chlorination of Model Anionic [16,6,0]− or Neutral 16,6,0 Methylaluminoxane by Me2AlCl or Cp2ZrCl2a anion chlorination by Me2AlCl

neutral chlorination by Me2AlCl

[16,m,o]− + 1/2Me4Al2Cl2 → [16,m−1,o+1]− + 1/2Me6Al2

16,m,o + 1/2Me4Al2Cl2 → 16,m-1,o+1 + 1/2Me6Al2

ΔE (kJ mol−1) [16,6,0]− [16,5,1]− [16,4,2]− [16,3,3]− [16,2,4]− [16,1,5]− [16,6,0]−

→ [16,5,1]− → [16,4,2]− → [16,3,3]− → [16,2,4]− → [16,1,5]− → [16,0,6]− + 3Me4Al2Cl2 → [16,0,6]− + 3Me6Al2 anion chlorination by Cp2ZrCl2

−20.4 −24.4 8.4 −1.7 10.0 −1.6 −29.6

ΔG (kJ mol−1) −18.1 −21.6 12.2 −1.6 9.6 −4.2 −23.8

16,6,0 → 16,5,1 −21.2 16,5,1 → 16,4,2 −7.6 16,4,2 → 16,3,3 7.3 16,3,3 → 16,2,4 −0.7 16,2,4 → 16,1,5 10.2 16,1,5 → 16,0,6 −1.2 16,6,0 + 3Me4Al2Cl2 → 16,0,6 + 3Me6Al2 −13.1 neutral chlorination by Cp2ZrCl2

[16,m,o]− + Cp2ZrCl2 → [16,m−1,o+1]− + Cp2ZrMeCl

[16,6,0]− [16,5,1]− [16,4,2]− [16,3,3]− [16,2,4]− [16,1,5]− [16,6,0]− a

→ [16,5,1]− → [16,4,2]− → [16,3,3]− → [16,2,4]− → [16,1,5]− → [16,0,6]− + 6Cp2ZrCl2 → [16,0,6]− + 6Cp2ZrMeCl

ΔE (kJ mol−1)

ΔG (kJ mol−1) −21.8 −2.3 6.5 0.3 9.3 −1.5 −9.5

16,m,o + Cp2ZrCl2 → 16,m−1,o+1 + Cp2ZrMeCl

ΔE (kJ mol−1)

ΔG (kJ mol−1)

−28.7 −32.6 0.1 −10.0 1.8 −9.9 −79.3

−28.5 −32.0 1.8 −12.0 −0.8 −14.6 −86.1

16,6,0 → 16,5,1 16,5,1 → 16,4,2 16,4,2 → 16,3,3 16,3,3 → 16,2,4 16,2,4 → 16,1,5 16,1,5 → 16,0,6 16,6,0 + 6Cp2ZrCl2 → 16,0,6 + 6Cp2ZrMeCl

ΔE (kJ mol−1)

ΔG (kJ mol−1)

−29.4 −15.9 −1.0 −9.0 2.0 −9.5 −62.8

−32.2 −12.7 −3.9 −10.1 −1.1 −11.9 −71.9

Energies are reported per mole of neutral or anionic MAO.

Figure 5. Possible routes to the monochlorinated anions with m/z 1395 starting from m/z 1375.

With reference to the model structure of m/z 1395 it is evident that some of these substitution processes may be kinetically more accessible than others, despite being energetically less favorable. For example, in order to form the most stable dichlorinated anion featuring substitution of C(1) by Cl, one must first disrupt the Al−C(1)H3−Al interaction involving this C atom, dissociate Me3Al from the resulting structure, associate Me2AlCl to the resulting vacancy at O, and establish a Al−Cl−Al interaction. Alternately, a lower energy pathway might involve dissociation of Me3Al from either of the basal Me5Al2 moieties located

at the other end of the anion, and association of Me2AlCl to the resulting vacancy. It is evident this basic process might occur three more times so as to produce a penta-chlorinated anion, the highest degree of chlorination observed experimentally. Since our experimental work as well as other studies certainly confirm that Me2AlCl is an active chlorinating agent, we performed some additional computations focusing on this mechanistic aspect. Starting with the structure of m/z 1375, we examined the energy changes associated with two possibilities; dissociation of Me3Al from the terminal O-AlMe3 site, following by association of Me2AlCl to this site (so as to give E


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Macromolecules

Figure 6. Possible routes to dichlorinated anions with m/z 1415.

Table 2. Ethylene Polymerization Using Either Cp2ZrCl2 or Cp2ZrMe2 and MAO Activationa entry 1 2 3 4 5 6 7 8 9 10 11 12c 13c 14c 15d 16d

catalyst Cp2ZrCl2 Cp2ZrCl2 Cp2ZrCl2 Cp2ZrCl2 Cp2ZrCl2 Cp2ZrCl2 Cp2ZrCl2 Cp2ZrCl2 Cp2ZrMe2 Cp2ZrMe2 Cp2ZrMe2 Cp2ZrMe2 Cp2ZrMe2 Cp2ZrMe2 Cp2ZrMe2 Cp2ZrMe2

+ + + + +

Me2AlCl Me2AlCl Me2AlCl Me2AlCl Me2AlCl

[Al] (M) 0.015 0.015 0.017 0.017 0.045 0.091 0.009 0.007 0.015 0.015 0.013 0.015 0.015 0.015 0.015 0.015

[Zr] (M) 2.86 7.33 7.45 6.43 7.45 7.45 1.48 4.23 7.39 3.70 7.43 3.69 3.69 3.69 7.34 7.34

× × × × × × × × × × × × × × × ×

−4

10 10−5 10−5 10−5 10−5 10−5 10−5 10−6 10−5 10−5 10−6 10−5 10−5 10−5 10−5 10−5

Al:Zr

t (min)

wt (g)

Ab

Mw (×103)

PDI

52 200 230 260 625 1250 600 1580 200 400 1800 400 400 400 200 200

20 30 30 30 30 30 30 20 30 26 30 26 26 26 30 30

7.8 4.8 5.5 4.5 5.1 8.9 4.4 4.1 4.9 5.1 4.1 4.3 4.3 3.9 7.3 8.6

403 645 728 689 692 1210 2958 8270 653 1569 5467 1323 1323 1200 975 1150

237 228 190 165 143 59 372 489 317 353 425 262 − 227 248 213

4.3 3.7 3.8 3.4 3.8 2.6 1.8 1.5 2.0 2.4 1.6 1.9 − 2.0 5.5 7.7

a Conditions: Toluene solvent (200−215 mL), 40 °C, 2.9 bar C2H4, 500−1000 rpm. bActivity in kg PE/mol Zr h. cEntries 12−14 involve polymerization with Cp2ZrMe2 (activated with 100 equiv of MAO) in the presence of 4 mol % Me2AlCl with respect to total MAO dEntries 15−16 involve polymerization with Cp2ZrMe2 (activated with 100 equiv of MAO) in the presence of 8 mol % Me2AlCl with respect to total MAO.

dissociation of Me4Al2Cl6. The overall process is moderately unfavorable with a free energy change of +12.4 kJ mol−1. On the other hand, as shown in Figure 6, once substitution occurs at one basal site, a second substitution by Me2AlCl at the same site is favorable (ΔG = −8.4 kJ mol−1), while a second substitution at the other basal site is only slightly unfavorable (ΔG = +5.5 kJ mol−1). Given that anions such as m/z 1375 are prone to multiple losses of Me3Al in the gas phase due to CID, it is plausible that anion chlorination in solution does involve the latter substitution processes, even if the resulting anions are not as stable as those which are formed by more energetically demanding pathways. Indeed, it is possible that in the (contact) ion-pair formed from Cp2ZrMe2 and MAO, there is a bridging interaction of the

the most stable monochlorinated anion) vs the other processes involving one or both of the two basal Me5Al2 moieties. In the first instance, dissociation of Me3Al from the terminal site in the gas phase is strongly disfavored with ΔG = +67.3 kJ mol−1 for the first reaction shown in Figure 5. The strong OAlMe3 interaction in m/z 1375 is not offset by forming weaker Al−Me−Al interactions in Me6Al2. On the other hand, association of Me2AlCl is strongly favored, relative to dissociation of Me4Al2Cl2, leading to the energy change reported in Table 1. In our view, dissociation of the terminal Me3Al is very unlikely in solution or even the gas phase, despite the favorable overall free energy change. In contrast, dissociation of Me3Al from a basal site is spontaneous, whereas coordination of Me2AlCl to the resulting vacancy results in an unfavorable energy change, relative to F


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Macromolecules [Cp2ZrMe]+ cation with the terminal Me3Al group of this anion, as determined by Zurek and Ziegler in their study of smaller aluminoxane cages.4 Thus, it is possible that chlorination by Me2AlCl of the ion-pair involves initially, exclusive substitution at the basal (or more remote) sites of the anion. If so, the theoretical studies of Kuklin et al. suggest that reactivity might only be subtly affected compared to chlorination at a site close to the active catalyst.34 In the section which follows we examine the consequences of this in ethylene polymerization. Ethylene Polymerization in Toluene Slurry. From the previous results, it is evidently possible to chlorinate the anions formed upon activation of Cp2ZrMe2 by MAO at Al:Zr ratios of ca. 100:1, without affecting cation speciation, through addition of Me2AlCl. If anion chlorination is important for catalysis, then the polymerization behavior of Cp2ZrMe2 and Cp2ZrCl2 should differ significantly at sufficiently low Al:Zr ratios and moreover, the behavior of the latter should be mimicked by activation of Cp2ZrMe2 by MAO, and polymerization in the presence of small amounts of Me2AlCl. To address these hypotheses, ethylene polymerization was conducted in toluene slurry at 40 °C and 2.9 bar of ethylene pressure using both catalyst precursors and also in the presence of Me2AlCl. To relate the results to the ESI MS work, activation of each catalyst precursor by MAO was conducted in the same manner (e.g., addition of 1.0 mL of a 0.015 M stock solution of Cp2ZrCl2 in toluene to 1.0 mL of 1.5 M MAO at room temperature), and the resulting ion-pairs were delivered to a well-conditioned, reactor in 200 mL total of toluene solvent, containing additional MAO as scavenger. Ethylene was then introduced with stirring at 500−1000 rpm on demand. These relatively mild conditions were chosen such that large exotherms were avoided (typically exotherms of 500:1) which our ESI MS work (as well as other studies8−13) show that catalyst activation is complete and the extent of anion chlorination is minimal.31 In contrast, it is evident that polymer MW is affected by choice of catalyst precursor, particularly at lower Al:Zr ratios. Cp2ZrMe2 produces significantly higher MW polymer with a more narrow MWD at the lowest ratios investigated (200:1 entries 9−11). Both catalysts produce higher MW polymer as the catalyst is diluted (Figure 7b) and indeed within experimental error, the polymer MW for either precursor is the same as the catalyst is diluted, while MWD becomes narrow (PDI ∼ 2) for both precursors under these conditions. G


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experiment at 6.9 bar under conditions corresponding to Table 1, entry 1 provided PE at an activity of 942 kg PE mol Zr1− h−1 with Mw = 446 000 and PDI = 3.6 with a largely unimodal MWD. This behavior is not what one would expect for the scenario just described and so the origin(s) of the broad and bimodal MWD in the presence of high levels of Me2AlCl or at low Al:Zr ratios (for Cp2ZrCl2) is unclear. In discussing the basic trend observed in activity, there are obviously many explanations that might account for the increase in activity on dilution of the catalyst.36 Diffusion effects are undoubtedly important54 but it is not obvious that they would conspire to produce a relationship, which is clearly hyperbolic (Activity ∝ [Zr]−0.87 in Figure 7a) and inversely related to [Zr].55 Similarly, materials like Me3Al are known to inhibit polymerization activity through reversible binding to the active catalyst,56,57 and though one expects dissociation of [Cp2Zr(μMe)2AlMe2][MAO(Me)] to be favored upon dilution of the catalyst (at constant [Al]) this behavior should lead to an inverse, linear relationship between activity and [Zr] vs that seen. It has been suggested that a large excess of Cp2ZrMe2 will inhibit the polymerization activity of the ion-pair [Cp2ZrMe][B(C6F5)4] through reversible formation of the dinuclear adduct [(Cp2ZrMe)2(μ-Me)][B(C6F5)4].56,58 This clearly would result in an inverse dependence of activity on [Zr] like that observed but it should only be observed at much lower Al:Zr ratios than those used here for catalyst activation by MAO; both our ESI MS and other spectroscopic work concur that the analogous dinuclear complex [(Cp2ZrMe)2(μ-Me)][MAO(Me)] is not present in meaningful amounts for Al:Zr > 100:1 (see Figure 1a). In fact, the work of Brintzinger and co-workers suggest the presence of excess dialkyl is mainly beneficial when it comes to scavenging of impurities.58 We chose to model the observed behavior by invoking irreversible second order catalyst deactivation. This was first proposed by Fischer and Mulhaupt to explain the kinetics of propylene polymerization using MAO-activated Cp2ZrCl2.59 This process, the nature of which is not at all well understood chemically,60 has been proposed by other workers to explain why supported metallocene catalysts are frequently more stable toward catalyst deactivation than their soluble counterparts,61,62 and often produce higher MW polymer under the same conditions. The simplest version of this model assumes the propagating catalyst [Zr*] is otherwise stable, is instantaneously and efficiently activated (i.e., [Zr*] = [Zro]), chain growth follows first order kinetics in monomer and [Zr*] with propagation rate constant kp, while [Zr*] decomposes only through an irreversible second order process involving two propagating chains with rate constant kd. Plotted in Figure 8 is catalyst activity (in [PE]/[Zr] s) vs the product of [Zr]t (M s) for the data summarized in Table 2. The curve had to be fit to the formula indicated using nonlinear, least-squares regression by using reasonable initial estimates for both a = kp[M]o/2kd and b = kd to ensure convergence for these correlated constants.63 We obtain kd = 2.185 × 103 M−1 s−1 with the product of kp[M]o = 216 s−1. Given that [M]o = 0.28 M, then kp ∼ 771 M−1 s−1. This value is in good agreement with those recently reported by Ahmadi et al. in a detailed kinetic modeling study at a somewhat higher temperature (860−980 M−1 s−1 at 50 °C).64 Since no other assumptions were made with this simplified

In a plot of reciprocal number-average degree of polymerization vs [Al] for polymerizations involving Cp2ZrCl2 at constant or at least similar [Zr] a reasonably good linear correlation is observed (see Supporting Information). This behavior strongly suggests that polymer MW is limited by chain transfer to Al at this lower T, and since no attempt was made to minimize or control free Me3Al concentration in these experiments, this result is not unexpected given that Me3Al is a potent chain transfer agent.35 As for the effect of added Me2AlCl on the polymerization behavior of Cp2ZrMe2, we investigated two different levels − 4.0 mol % with respect to [Al], and an excess approaching total Me3Al present corresponding to 8.0 mol %. In both cases, the Me2AlCl was first added to MAO used for scavenging and then the activated catalyst (i.e., Cp2ZrMe2 + MAO Al:Zr = 100:1) was then added to this mixture in 200 mL of toluene. On the basis of the results shown in Figures 1-3 one can anticipate significant anion chlorination, at least at the higher level of Me2AlCl but without significant effects on cation speciation (see Figures S-2 and S-3). At low levels, the results in Table 2 (entries 12−14 and Figure 7a) show that polymerization activity is hardly suppressed, while polymer MW is significantly lowered in the presence of this additive compared to its absence (entry 10). In fact, the MW values observed nearly coincide with the curve observed for Cp2ZrCl2 vs PE MW depicted in Figure 7b). We suggest this reflects displacement of bound Me3Al by Me2AlCl so that free Me3Al concentrations are increased as a result, leading to more effective chain transfer. Finally, the breadth of the MWD of the resulting polymer (relative to Cp2ZrMe2) is largely unaffected by the addition of this material in small amounts to MAO. On the other hand at the higher levels, as well as at higher [Zr] (i.e., lower overall Al:Zr) activity slightly improves (entries 15−16 vs entry 9) but now the polymer formed has a broad and bimodal MWD with average MW depressed from that seen in the absence of this material (Table 2, entry 9). The broader MWD is in part related to the fact that these two experiments (entries 15−16) were definitely not isothermal with exotherms of 8 °C. In comparison to the sample prepared from Cp2ZrCl2 at the lowest Al:Zr ratio (Table 1, entry 1) a similar MWD is also observed (see Supporting Information) though the maximum amount of Me2AlCl that could have been present in that experiment is about 3.8 mol %; on the other hand our ESI MS experiments indicate anion chlorination is most effective when generating Me2AlCl during catalyst activation vs prior addition to MAO (Figure 2, parts a and b vs part c). The similarity in MWD for these two experiments suggests that high levels of “free” Me2AlCl may interfere with catalyst activation (at low Al:Zr ratios). One might also generate a broad and bimodal MWD simply due to differing chain transfer constants to “free” Me3Al vs Me2AlCl, at least at sufficiently high levels of both compounds. Alternately, extensive anion chlorination might lead to propagating ion-pairs differing in their chain transfer characteristics. Different propagating ion pairs would lead to a broad or bimodal MWD only if the lifetime of a given ion-pair is comparable to the chain lifetime75 and the ion-pairs differ in their chain transfer vs propagation rates. Since the latter process involves monomer insertion while chain transfer to Al does not, and appears to be rate determining under these conditions, it might be possible to influence the MWD through changes to ethylene pressure.75 An H


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exposure and diluted 4:1 with PhF and then filtered through a 0.2 μ PTFE syringe filter to remove any suspended polymer prior to analysis. At high conversion of the starting ion-pairs, all Zr-containing ions were found in greatly reduced amounts after dilution and filtration. In one case, the solid PE was observed to be pale yellow in color, even after washing with PhF, and the filtrate contained only trace amounts of Zr-containing ions (see Supporting Information). Thus, in agreement with an earlier kinetic study,55 ethylene polymerization using metallocene catalysts starts off as a homogeneous process but is at some point converted to a heterogeneous polymerization, involving polymer-encapsulated catalyst. At lower conversion, a typical spectrum is shown in Figure 9b). There are significant amounts of ions that do not contain

Figure 8. Catalyst activity vs the product of [Zro]t using data from Table 2 (entries 2−4 and 7−11). The formula shown was fit to the observed data using nonlinear regression analysis.

model, the good correspondence between kp obtained for this study vs a careful modeling study suggest that this catalyst deactivation process may be significant. ESI MS Analyses of Ethylene Polymerization. ESI MS has been used to study ethylene polymerization under a variety of conditions, and oligomeric cations are formed in situ when the activated catalysts are exposed to ethylene monomer.23−30 Most recently, and using a new type of electrospray technique,65 oligo-alkylzirconocenium cations were directly detected in PhF solution following exposure of MAO-activated Cp2ZrCl2 to ethylene. None of these studies have provided evidence for catalyst deactivation in solution, though the short mixing and analysis times may have precluded observation of such species. On the basis of these results, as well as the results of the polymerization experiments just presented, we decided to monitor a conventional ethylene polymerization by ESI MS with a view to detecting dinuclear products resulting from catalyst deactivation. Since we did not have a temperature controlled, efficiently stirred pressure reactor at the University of Victoria, we elected to conduct these experiments in magnetically stirred vials or Schlenk flasks and inside the glovebox adjacent to the Micromass QTOF mass spectrometer and at low ethylene pressures (0.5−1.0 bar). Thus, these polymerizations are undoubtedly mass transfer limited and not isothermal. An additional complication relates to an appropriate concentration range for polymerization in toluene vs analysis by ESI MS in PhF: ESI MS analysis of MAO solutions in PhF feature effective ion suppression66 under all conditions we have examined and this normally sensitive technique is rendered relatively insensitive as a result. Also, depending on the water content of the PhF solvent, hydrolysis of Zr product ions is a significant problem at [Al] < 0.01 M. These features necessitate preparation of solutions in PhF with final [Zr] in the 10−4 M range for adequate signal intensity and with spectra largely free of artifacts due to hydrolysis at Al:Zr ratios of about 100:1. However, this [Zr] is certainly at the upper limit of concentrations typically used for ethylene polymerization while ESI MS experiments are not practical in toluene solution per se.67 In practice, we had to adopt higher initial [Zr] concentrations (i.e., 10−4 to 10−3 M) in toluene followed by dilution of aliquots with PhF. Exploratory experiments revealed that efficient mixing at these elevated concentrations was a significant problem even at static, low pressures of ethylene. Under these conditions, solid PE was observed to rapidly form as a pale yellow suspension. Aliquots were removed, typically after just a few minutes

Figure 9. Positive ion ESI MS of (a) mixture of Cp2ZrMe2 and MAO (Al:Zr 200:1) with [Zr] = 2.0 mM diluted with PhF to 0.4 mM and (b) the filtered reaction mixture following exposure of the same toluene solution with [Zr] = 2.0 mM to ethylene at 25 °C and 0.75 bar for 5 min and dilution with PhF. Peaks for major non-Zr containing ions are marked with an asterisk.

Zr in this spectrum. Compared with the starting mixture (Figure 9a), the concentration of Zr-containing ions is a fraction of that initially present, as many of the non-Zr containing ions are also present in this initial mixture. Nevertheless, in addition to starting materials with m/z 235 and 307, there a major product ion present with m/z 557 and based on its isotope pattern (Figure 9b inset) it is obviously contains two Zr atoms. In an attempt to better control the product distribution as well as conversion, experiments were conducted with ethylene diluted by nitrogen. Highest conversions to m/z 557 (ca. 70% conversion of the starting ion-pairs) were seen in concentrated solution, and while bubbling diluted ethylene through the solution, using a balloon to maintain a static pressure of gas over a short period of time (5−10 min). Experiments at higher initial [Zr] show significantly increased amounts of this product ion relative to starting material consistent with a bimolecular process involved in its formation. I


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Macromolecules This ion was also observed to form from m/z 235 and 307, during ethylene polymerization in PhF solution (see Supporting Information), using the technique of pressurized sample infusion (PSI).68 This experiment proves that this dinuclear ion forms in situ during polymerization (vs after consumption of monomer is complete) and is evidently much less reactive toward ethylene insertion than m/z 235 and 307. The structure of m/z 557 is not immediately evident from its nominal mass, MS−MS experiments (see Supporting Information), nor isotope pattern, other than that it contains two Zr atoms with the rest monoisotopic elements. Of relevance is that on occasion an ion with m/z 591 is formed concomitantly with m/z 557 and contains one chlorine atom, presumably arising from adventitious chlorination of this precursor in the source compartment of the mass spectrometer (see Supporting Information Figure S-10). This is suggestive of one Zr−H or Al−H group reactive toward chlorination. To distinguish between possible formulas, the reaction of the starting ion-pairs with ethylene-d4 (99.8% D) was examined. Under these conditions, several dinuclear ions were generated with a quasi-statistical isotope distribution (see Figure 10b). This is a surprising result, since it implies H-D scrambling of the label prior to or following ethylene incorporation with either excess MAO or solvent.69

When using ethylene-d4, and comparing this spectrum to that of unlabeled material (see Figure 10a), it is apparent that dinuclear ions with m/z 529, 557, and 585, differing in mass by 28 Da, are derived from ethylene or at least have variable amounts of deuterium incorporated into their structure. In the presence of ethylene-d4 the most abundant isotopomer for each ion is m/z 531, 561 and 592, indicating on average, incorporation of 2, 4, or 7 deuterium atoms, respectively. These results suggest the corresponding unlabeled ions, have zero, one and two equivalents of ethylene incorporated as such, though the quasi-statistical labeling makes this conclusion somewhat tentative. Taken together, the ESI MS, chlorination and labeling studies suggest m/z 557 has a partial formula of (Cp2Zr)2H(C2H4) with an overall charge of +1 or +3 for this fragment depending on how the ethylene is incorporated. The nominal mass of this fragment is 469 leaving a remainder of 88 Da. If we assume this ion is not oxygenated nor hydrolyzed, which seems reasonable given the precautions taken to purify the ethylene and solvents, as well as the high [Zr] used,76 there are actually only four possible formulas for this ion, assuming it incorporates C, H and Al. They are (Cp2Zr)2H(C2H4)(C5HAl), (Cp2Zr)2H(C2H4)(C2H10Al2), (Cp2Zr)2H(C2H4)(C4H13Al), and (Cp2Zr)2H(C2H4)(C6H16). The first can be discarded, while the last seems somewhat implausible if C6H16 is not derived from ethylene; a maximum of one molecule is incorporated as such based on the labeling study. Of the two remaining formulas, the most plausible is (Cp2Zr)2H(C2H4)(C4H13Al) in that if we invoke MAO, Me3Al or Cp2ZrMe2 as the carbon source (i.e., methyl groups) we can write a sensible formula that results in a net charge of +1 for this ione.g., [(Cp2Zr)2(H)(C2H5)(CH3)Al(CH3)3]+. As to possible structures for this ion, we did investigate these mixtures by 1D and 2D NMR techniques (see Supporting Information). We were able to relate the intensity of four broadened, signals of roughly equal intensity, corresponding to Cp protons, to the formation of the ion with m/z 557. At high conversion of the starting ion pairs, this ion is quite dominant in these mixtures (ca. 80−90% of the total product ion intensity) and yet the presence of four Cp signals implies that this ion has at least two different structures, in which the Cp2Zr moieties are inequivalent. This is largely consistent with the result of the labeling studies. We were unable to correlate these Cp signals to each other, nor conclusively identify any other resolved signals present. In particular, we found no evidence for a Zr−Et or Al−Et group, bridging or terminal, in these spectra which suggests ethylene is incorporated into the structure(s) of m/z 557 as a bridging ligand. This structural motif is actually quite common in group 4 metal chemistry.70−72 Dinuclear complexes of this type form in situ from unhindered zirconocene complexes and higher aluminum alkyls. In one especially relevant case (Et3Al + Cp2ZrCl2), the resulting complex was characterized by crystallography;71 this structure can be viewed as a [Cp2Zr(μCH2CH2)ZrCp2]2+ dication stabilized by coordination of two ClAlEt3 anions (one at each metal center). Also, for [Cp″HfEt2(η6-toluene)][EtB(C6F5)3] {Cp″= η51,3-(Me3Si)2C5H3)} complexes, dimerization and alkane elimination occurs to furnish dicationic [{Cp″Hf(η6-toluene)Et}2(μ-C2H4)][EtB(C6F5)3]2 complexes. Formation of this product was thought to involve β-H elimination followed by

Figure 10. Positive ion ESI MS of mixtures of Cp2ZrMe2 and MAO (Al:Zr 200:1) with [Zr] = 3.0 mM diluted with PhF to 0.4 mM (a) after exposure to ethylene and (b) after exposure to ethylene-d4. In the first spectrum, the vertical axis has been expanded and shifted to show the less intense product ions with the m/z values indicated (m/z 585 is overlapped with m/z 591 in this spectrum).

Alternately, it is possible that there is more than one ion with the same nominal mass but differing in structure, and thus the amount of deuterium incorporated. We favor the latter explanation as the observed isotope distribution could be modeled assuming that the labeled material had the following composition: d4 0.4076, d3 0.1607, d2 0.0207, d1 0.2853, h4 0.1257, with a total deuterium incorporation of 61% (see Supporting Information). It is unclear how random scrambling of the label could give rise to this distribution. J


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be achieved in practical experimental work in the absence of a scavengeri.e., a larger excess of MAO.

alkane elimination (Scheme 4) and was suggested as a model for bimolecular metallocene catalyst deactivation.72

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Scheme 4

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00933. Experimental details, additional ESI MS and MS-MS spectra and their interpretation, polymerization data, NMR spectra and tables of atomic coordinates for the structures examined by computational techniques (PDF)

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AUTHOR INFORMATION

Corresponding Author

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

CONCLUSIONS Though an inverse dependence for polymerization activity on [Zr]−1 has been noted in several prior kinetic studies of ethylene polymerization, and a second order, deactivation reaction is often proposed to account for this feature,37,59 we believe this is the first time unreactive, dinuclear Zr2 complexes have been detected on exposure of a MAO-activated, metallocene complex to ethylene under polymerization conditions. This process is evidently distinct from reversible formation of [Cp2ZrMe(μ-Me)ZrMeCp2][WCA] from a neutral and cationic metallocene complex; the work of Brintzinger and coworkers in particular, demonstrates that such complexes (WCA = B(C6F5)4) are viable catalyst precursors.58 Further, as mentioned previously, the analogous complex is not present in meaningful amounts at Al:Zr ratios ≥100:1 in MAOactivated systems. Finally, though the complex [Cp2ZrMe(μCl)ZrMeCp2][MAOMe] is unstable in solution at room temperature,31 the μ-Me analogue is quite stable under these conditions toward methane elimination.31,73 One possible interpretation of our results, is that at higher absolute Zr concentration, unhindered ion-pairs present in MAO-activated systems, may reversibly associate into ion quadruples or other aggregates,7,73 and it is during this association that irreversible bimolecular deactivation can proceed at a meaningful rate. Unfortunately, neither the ESI MS nor our NMR results shed definitive light on the chemical nature of this deactivation process. We suspect the process involves transient formation of a dinuclear dication (by analogy to published work72), followed by reaction with one of the MAO-based anions (a potential source of Al(CH3)4−) to furnish a mixture of dinuclear monocations. − e.g. a [(Cp2ZrH)2(μ-C2H4)]2+ dication, partnered with a coordinating and reactive [Al(CH3)4]− anion or an isomeric formulation. The polymerization results described here conclusively demonstrate that it really is absolute [Zr] in a reactor that dictates polymerization behavior, not the Al:Zr ratio (within limits), and that at least according to ESI MS and other spectroscopic techniques, catalyst activation is largely complete at much lower Al:Zr ratios than can be practically employed at [Zr] ∼ 10−6 M or lower concentrations in solution. For example, at a 200:1 Al:Zr ratio and with [Zr] = 10−6 M, [Al] = 2 × 10−4 M with [Me3Al] ∼ 2 × 10−5 M. In our experience, this amount of TMA would be just sufficient to remove [H2O] ∼ 2 × 10−6 M from dissolved monomer and solvent. This corresponds to a moisture level of

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