Mechanistic Studies of Hafnium-Pyridyl Amido-Catalyzed 1-Octene

Aug 1, 2017 - Comparison of the UV-detected MWD with the MWD of the “bulk” (all polymers, from RI-GPC analysis) provides important mechanistic inf...
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Mechanistic Studies of Hafnium-Pyridyl Amido-Catalyzed 1‑Octene Polymerization and Chain Transfer Using Quench-Labeling Methods Eric S. Cueny,† Heather C. Johnson,† Bernie J. Anding, and Clark R. Landis* Department of Chemistry, University of WisconsinMadison, 1101 University Avenue, Madison, Wisconsin 53706, United States S Supporting Information *

ABSTRACT: Chromophore quench-labeling applied to 1octene polymerization as catalyzed by hafnium-pyridyl amido precursors enables quantification of the amount of active catalyst and observation of the molecular weight distribution (MWD) of Hf-bound polymers via UV-GPC analysis. Comparison of the UV-detected MWD with the MWD of the “bulk” (all polymers, from RI-GPC analysis) provides important mechanistic information. The time evolution of the dual-detection GPC data, concentration of active catalyst, and monomer consumption suggests optimal activation conditions for the Hf pre-catalyst in the presence of the activator [Ph3C][B(C6F5)4]. The chromophore quench-labeling agents do not react with the chain-transfer agent ZnEt2 under the reaction conditions. Thus, Hf-bound polymeryls are selectively labeled in the presence of zinc-polymeryls. Quench-labeling studies in the presence of ZnEt2 reveal that ZnEt2 does not influence the rate of propagation at the Hf center, and chain transfer of Hf-bound polymers to ZnEt2 is fast and quasi-irreversible. The quench-label techniques represent a means to study commercial polymerization catalysts that operate with high efficiency at low catalyst concentrations without the need for specialized equipment.

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INTRODUCTION Homogeneous catalysts used in the synthesis of polyolefins allow for precise control over polymerization kinetics and, consequently, polymer properties.1−3 The production of block copolymers by living catalysts exemplifies precise control of polymer composition and topology but produces just one chain per catalyst.4 Chain shuttling polymerization enables the production of “blocky” 1-octene/ethylene copolymers with each catalyst center producing many chains.5 Two catalysts are used:6−8 a zirconium bis(phenoxyimine) complex polymerizes predominantly polyethylene to produce “hard” chains, and the hafnium-pyridyl amido complex 1 (Figure 1) polymerizes 1-

amount of actively propagating catalyst (often assumed to be 100%), without which, the absolute rates of propagation and chain shuttling cannot be determined.9 For efficient chain shuttling, polymer chain transfer between catalyst and zinc centers must be fast and reversible. Gibson10,11 and Sita12,13 have demonstrated that iron- or hafnium-based catalysts, respectively, undergo rapid and reversible coordinative chaintransfer polymerization14−16 in the presence of ZnEt2, leading to narrow molecular weight distributions (MWDs) with polydispersity index (PDI) values as low as 1.1 (Fe) and 1.03 (Hf). Evidence suggests that sterics and M−C bond strengths are key in determining whether chain transfer is reversible.11 The pathways of activation of 1 by different co-catalysts have been studied in detail.17 With Lewis acid activators [Ph3C][B(C6F5)4] and B(C6F5)3, methide abstraction yields I and II, respectively (Scheme 1). With Brønsted acids such as [HNMe(C18H37)2][B(C6F5)4], an alternative pathway involving protonation of the Hf−naphthyl bond followed by methane elimination forms III.17 In each case, it is postulated that the first equivalent of olefin inserts into the Hf−naphthyl bond of either I, II, or III to form the active polymerization catalyst (Scheme 2).18,19 To date, however, the effect of the activator upon the active catalyst concentrations in polymerizations conducted using 1 has not been determined. Current methods to determine the active-site counts in polymerization include the direct observation of catalyst speciation or the indirect determination of active-site counts. Direct observation of catalyst speciation by spectroscopic

Figure 1. Hafnium-pyridyl amido catalyst 1 and quench-labeling agents 2 and 3.

octene/ethylene to form “soft” chains. Growing chains are transferred between catalyst centers by transalkylation with a chain-shuttling agent, commonly diethyl zinc, to produce blocks of “hard” and “soft” polyethylene. The rate, average block lengths, and overall polymer composition are controlled by the concentrations of the catalysts and chain-shuttling agents and the rates of chain propagation and chain shuttling at each center. One often overlooked aspect of polymerization kinetics is the © 2017 American Chemical Society

Received: June 2, 2017 Published: August 1, 2017 11903

DOI: 10.1021/jacs.7b05729 J. Am. Chem. Soc. 2017, 139, 11903−11912

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Journal of the American Chemical Society Scheme 1. Activation of 1 Using [Ph3C][B(C6F5)4], B(C6F5)3, and[HNMe(C18H37)2][B(C6F5)4]

chromatography (GPC) with an inline UV detector (UV-GPC). Importantly, chromophore labeling and dual detection GPC distinguish the MWDs of catalyst bound polymeryls (by UVGPC) and the bulk polyhexene (by GPC with refractive index detection, RI-GPC). Such information enabled the development of a refined kinetic model for [(EBI)ZrMe][MeB(C6F5)3]catalyzed 1-hexene polymerization. In this report, chromophore quench-labeling technology is applied to the homopolymerization of 1-octene catalyzed by 1, a process related to chain shuttling polymerization. The effect of activators, B(C 6 F 5 ) 3 , [Ph 3 C][B(C 6 F 5 ) 4 ], and [HNMe(C18H37)2][B(C6F5)4], on the active-site counts is measured by quenching agents 2 and 3 (Figure 1) which are both shown to act as efficient traps of the reactive Hf-polymeryls. We show that dual detection (RI and UV) GPC analysis of the quenchlabeled polymers supports the approximately living behavior of 1 and gives qualitative insight into the reversibility of chain transfer with ZnEt2. Further analysis of rates and polymer MWDs lead to estimates of the ratio of propagation and chain transfer rate constants.

Scheme 2. Insertion of Olefin into the Hf−Naphthyl Bond of II

methods provides the most compelling data for mechanistic investigations of olefin polymerization; however, collection of these data is hampered by requirements for high catalyst concentrations or specialized equipment.20−22 Indirect methods of obtaining active catalyst concentration data have been developed such as calculations based on molecular weight data,23−27 poisoning experiments,28−34 and labeling studies.26,35−50 Disadvantages of these methods include the lack of distinction between polymers bound to the catalyst vs those bound to a main-group metal (in the presence of a chain-transfer agent51) and the lack of selectivity for active over inactive species in poisoning experiments. Recently, we reported that chromophore quench-labeling is a useful tool for determining the active-site count in 1-hexene polymerization catalyzed by [(EBI)ZrMe][MeB(C6F5)3] (EBI = rac-(C2H4(1-indenyl)2) (Scheme 3).52 Upon addition of the chromophore quenchlabeling reagent 2 to polymerization reactions, the polymerization ceased and the polyhexene chains growing from Zr centers were covalently labeled with a chromophore, allowing for quantitative analysis of active sites by gel permeation

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RESULTS AND DISCUSSION Systematic comparisons of the polymerization rate and the proportion of catalytically active sites for 1 in the presence of different activators have not been reported. We examine three different activators as co-catalysts in the homopolymerization of 1-octene using 1 as the pre-catalyst (Scheme 4): the Lewis acids B(C6F5)3 and [Ph3C][B(C6F5)4], and Brønsted acid [HNMe(C18H37)2][B(C6F5)4]. To compare each activator under identical conditions, we initiated polymerization by injecting a solution of 1 into a mixture of 1-octene, toluene, and the chosen

Scheme 3. Quench-Labeling of Polymeryls Bound to [(EBI)ZrMe][MeB(C6F5)3] Using 2

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DOI: 10.1021/jacs.7b05729 J. Am. Chem. Soc. 2017, 139, 11903−11912

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Journal of the American Chemical Society Scheme 4. Polymerization of 1-Octene (503.2 mM) with 1 (0.0829 mM) in Toluene at 50 °Ca

a Activator = B(C6F5)3, [Ph3C][B(C6F5)4], or [HNMe(C18H37)2][B(C6F5)4] (0.0912 mM). Polymerizations were quenched using 2 (0.166 mM).

activator. We refer to this protocol as in situ activation. In situ activation was selected over a pre-activation procedure (in which catalyst and activator are reacted prior to alkene addition) for these initial studies because Coates previously observed that preactivation of hafnium-pyridyl amido pre-catalysts with B(C6F5)3 results in catalyst solutions that deactivate with time.53 By not pre-activating, the reaction time course of active-site counts, monomer consumption, and MWDs provide qualitative insights into the rates of activation and initiation of 1 with the different activators under a common set of conditions. Furthermore, such conditions also mimic common batch reaction conditions used in catalyst screening. At selected time points, the reaction was quenched by the addition of 2. Consumption of 1-octene was measured by 1H NMR spectroscopy. The polyoctene was analyzed by GPC with both RI and UV detectors. Integration of the UV signal provides a concentration of pyrene-labeled polymers, which reflects the concentration of Hf-bound polymerylsi.e., concentration of active catalystat the time of the quench. In Situ Activation with [HNMe(C18H37)2][B(C6F5)4] Gives Slow Rates of Polymerization. When the Brønsted acid [HNMe(C18H37)2][B(C6F5)4] is used as the activator in the polymerization of 1-octene at 50 °C, the monomer consumption is slow compared with using B(C6F5)3 or [Ph3C][B(C6F5)4] as activators (vide inf ra), reaching 20% conversion in 15 min. Polyoctene samples analyzed by UV-GPC following the quench-labeling showed that only ∼3% of the added catalyst was active (see Supporting Information (SI)). The activation is likely slow because the Brønsted acid induces decyclometalation of the naphthyl moiety in 1 via protonation. Then, re-cyclometalation with concomitant CH4 loss forms III, as previously reported.17 Moreover, the tight binding of the amine in III presumably inhibits binding of 1-octene to the Hf center. Owing to the slow kinetics and low active-site percentages, no further studies were performed with this activator. In Situ Activation with B(C6F5)3 Yields Slow Initiation. With B(C6F5)3 used as the activator in 1-octene polymerization at 50 °C, the rate of polymerization was significantly faster (t1/2 = 60 s) than that observed using [HNMe(C18H37)2][B(C6F5)4]. UV-GPC analysis of the resulting chromophore-labeled polyoctene showed higher active-site counts (∼45%) than with [HNMe(C18H37)2][B(C6F5)4] (Figure 2). The counteranion [MeB(C6F5)3]− is weakly coordinating and presumably can be more easily displaced by alkene than the amine NMe(C18H37)2, which accounts for higher active-site counts and more rapid polymerization compared with [HNMe(C18H37)2][B(C6F5)4]-activated polymerization. It is also noteworthy that there is a rapid increase in the number of active sites using B(C6F5)3 with 29% active sites within the first 10 s, reaching a maximum of 45% at 45 s. These results indicate that initiation of polymer growth is slow with B(C6F5)3.54 We note

Figure 2. (Top) Consumption of 1-octene vs reaction time measured by 1H NMR spectroscopy. (Bottom) Active catalyst percentage vs time, measured by UV-GPC analysis of polyoctene quenched by 2. In both plots, points represent separate experiments quenched with 2, and error bars show the standard deviation obtained from duplicates. Activators used: B(C6F5)3 (red squares), [Ph3C][B(C6F5)4] (green triangles), and [Ph3C][B(C6F5)4], pre-activated (blue diamonds).

that prior NMR studies of alkene polymerization by Macchioni et al. suggested that only a “small fraction” of the B(C6F5)3activated catalyst underwent initiation when 3−6 equiv of 1hexene was added.19 It seems reasonable that the much higher active-site counts obtained herein reflects the effects of higher temperature and higher monomer/catalyst ratios. Injecting a pre-mixed solution of 1 and B(C6F5)3i.e., pre-activation of the catalyst precursorinto a toluene solution of 1-octene caused decreased rate of monomer consumption and active-site counts (see SI), similar to the observations by Coates et al.53 In Situ Activation with [Ph3C][B(C6F5)4] Exhibits an Induction Period for Polymerization. Activation of 1 by the Lewis acid [Ph3C][B(C6F5)4] also gave much higher active-site counts (∼20%) and more rapid monomer conversion (t1/2 = 150 s) than activation by [HNMe(C18H37)2][B(C6F5)4]. However, both conversion and active sites were lower compared with B(C6F5)3 as the activator, and an induction period for monomer consumption of ∼20−30 s was observed. The induction period was accompanied by a slow growth in the number of active sites (Figure 2). We hypothesize that the induction period in monomer consumption and slow increase in active-site counts result from slow activation of 1 using [Ph3C][B(C6F5)4].55,56 The length of the induction period was not significantly altered using an excess (5 equiv) of [Ph3C][B(C6F5)4] (see SI). Pre-activation with [Ph3C][B(C6F5)4] Prior to Monomer Addition Eliminates the Induction Period. Mixing 1 and [Ph3C][B(C6F5)4] in toluene for 3 min at 50 °C quantitatively generates I, as shown by 1H NMR spectroscopy.17 Injection of this solution into a toluene solution of 1-octene results in rapid polymerization without an induction period. Following this procedure, 1-octene consumption is faster (t1/2 = 30 s) and the first measured active-site counts are higher (∼58% at 10 s) than for activation with B(C6F5)3 (Figure 2). Likely, these differences result from poorer coordination of the [B(C6F5)4]− anion to the Hf center as compared with [MeB(C6F5)3]−.54,57−59 It is plausible that slow activation of 1 with [Ph3C][B(C6F5)4] could be the result of a binuclear cationic complex formation; related 11905

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Journal of the American Chemical Society Scheme 5. Formation of Vinylene Groups

these polymerizations, 0.5 equiv of 2 (relative to [Hf]0) was added to the mixture of toluene and 1-octene as a “poison” that could react with activated catalyst before (or soon after) polymerization began. Next, I was added to the mixture to initiate polymerization. After 10 s, the polymerization was quenched with exogenous 2 (1 equiv relative to [Hf]0). The initial rate of this poisoned reaction was approximately half that of a polymerization without poison (see SI), indicating that 2 quenches rapidly and competitively with monomer insertion. To expand the selection of quench-labeling reagents, 1pyrenyl isonitrile (3) was synthesized from 1-aminopyrene. Isonitriles have been shown to insert into M−alkyl bonds (M = Ti, Zr, and Hf) to form η2-iminoacyls,62−64 suggesting that 3 could act as a quench-labeling reagent by insertion of the pyrene chromophore into Hf−polymeryl bonds to generate catalytically inactive η2-iminoacyls. When 3 is used as a quench the plots of monomer consumption reproduce those obtained with quenching by 2. Although 2 and 3 are fast quenching agents in this system, quantitative active-site information requires quantitative labeling of actively propagating chains. Application of 2 as a quench agent over the range of 1−4 equiv, relative to [Hf]0, results in active-site counts of ∼50−60% (see SI). These results indicate that only 1 equiv of quench-label is required for complete labeling and support the idea that 2 rapidly and quantitatively labels all actively propagating polymeryls. In contrast, 6 equiv of 3 is required to effect active-site counts of 50−60% (see SI); using 10 equiv of 3 has no effect on the observed active sites, suggesting saturation of active-site labeling. It is unclear why 6 equiv of 3 is required for complete labeling. Nonetheless, these data show that two independent reagents give, within error, the same active-site counts for 1-catalyzed 1-octene polymerization and both exhibit saturation behavior in the observed active-site counts. Is the 1/[Ph3C][B(C6F5)4-Catalyzed Polymerization of 1Octene “Living”? Although hafnium-pyridyl amido catalysts have been considered living,53,65 under these experimental conditions ([1]0 = 0.083 mM, [1-octene]0 = 503 mM, toluene, 50 °C) deviation from strictly living behavior is observed. On examination of the polyoctene by 1H NMR spectroscopy, small signals at δ 5.34, corresponding to vinylene end groups,43 are observed. Such chain-transfer events likely arise from a 2,1misinsertion of 1-octene, followed by β-hydride elimination (Scheme 5). The concentration of vinylenes (on the order of 1 vinylene per 4500 monomer insertions) was too small to be seen by 13C NMR spectroscopy, nor could we detect any enchained 2,1-insertions in the polymer.43 It is therefore possible that some 2,1-insertions yield Hf-secondary polymeryl species that lie dormant in the polymerization, prior to eventual β-hydride elimination. No evidence for vinylidene end groups (as would

dimers [(L2ZrMe)2(μ-Me)][B(C6F5)4] (L = Cp or L2 = SBI) and [(Cp2Hf)2(μ-Me)][B(C6F5)4] have been observed previously,60,61 although dimer formation has not been observed with 1. All further polymerizations in this study were conducted using [Ph3C][B(C6F5)4] as the co-catalyst in conjunction with this pre-activation protocol. We attempted to identify the active catalyst by 1H NMR spectroscopy using the pre-activation conditions. Mixing 1 (9 mM) and 1.0 equiv of [Ph3C][B(C6F5)4] in toluene for 3 min at 50 °C generates the cationic complex I. Upon addition of either 5 or 50 equiv of 1-octene, rapid formation of polyoctene was observed by 1H NMR spectroscopy, but no change in the resonances for I was observed (see SI). This result suggests that little of the catalyst initiates to form small amounts of highly active propagating species, in stark contrast with the high activesite counts revealed by GPC analyses for polymerization reactions performed at much lower catalyst concentrations (vide supra). It seems reasonable that, under the NMR reaction conditions, the high catalyst concentration, low monomer:catalyst ratios, and mixing limitations might preclude substantial initiation of the catalyst. Several studies attempted to elucidate the nature of the active polymerization species when pre-catalyst 1 is used. The reaction of 1/B(C6F5)3 with doubly 13C-labeled ethylene at low temperature (−20 °C) resulted in just two new 13C NMR resonances, leading the authors to conclude that ethylene underwent a single insertion into the Hf−naphthyl bond without inserting into the Hf−methyl bond.18 A large-scale (2.05 g of 1) reaction of 1/B(C6F5)3 with 50 equiv of 4-methyl1-pentene at room temperature resulted in 30% of the ligand being appended with 4-methyl-1-pentene (from GC/MS analysis of the reaction after methanol workup), suggesting that formation of the active catalysts may involve modification of the pyridyl amido ligand.18 The reaction of 1/B(C6F5)3 with 170 equiv of 1-hexene at −56 °C resulted in polyhexene formation and the insertion of 1-hexene into the Hf−naphthyl bond with the Hf−Me bond still intact (Scheme 2), but no Hf-polymeryl species were identified.19 These empirical studies, along with computational results,18,19 provide important information about the reactivity of 1, especially the insertion of monomer into the Hf−naphthyl bond. However, these studies do not unequivocally establish the nature of the actively propagating catalyst nor its concentration under more common polymerization conditions. Quench-Labeling with 2 Is Efficient and Quantitative. Only 1 equiv of 2 (relative to [Hf]0) is required to halt polymerizations; identical monomer consumption plots are obtained when reactions are quenched with 1−4 equiv of 2. To further demonstrate that 2 is a fast and efficient quench for polymerizations conducted using 1, a two-step poisoning experiment was designed and performed. In the first step of 11906

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Journal of the American Chemical Society arise from β-hydride elimination of 1,2-inserted 1-octene) was observed (Scheme 5). The concentration of vinylenes of ∼0.13 ± 0.3 mM suggests that each catalytic site (assuming 50% of the catalyst is active, as indicated through quench-labeling) undergoes approximately three 2,1-insertion/β-hydride elimination events over 120 s. This is consistent with a rate constant of 0.2 M−1 s−1, assuming a first-order dependence upon [1-octene]. Although these events are rare (compared with ∼4500 insertions required to form a polymer chain with Mn = 500 000 g mol−1), the molecular weights are strongly affected; at 89% conversion, the polyoctene has Mn = 520 000 g mol−1, vs a predicted Mn value of 1.2 × 106 g mol−1 at this conversion (based on the Natta equation, vide inf ra). The PDI ranges from 1.2 early in the reaction to 1.5 at the later conversions, indicating that this is a controlled polymerization,66 between the limits of strictly Poisson (PDI = 1) and Flory−Schulz (PDI = 2).67 In contrast, (EBI)ZrMe2/B(C6F5)3catalyzed 1-hexene polymerization undergoes vinylene formation at a frequency of ∼1 per 300 1,2-insertions of 1-hexene, estimated from the optimized rate constants for propagation and 2,1-insertion rate.52,68 Natta Analysis Provides Independent Assessment of Active-Site Counts. Additional evidence in support of activesite counts determined by quench-labeling comes from observations of the polymer number-average molecular weight (Mn) and the Natta equation (eq 1). The Natta equation [catalyst]0 = conversion ×

([1]0 = 0.166 mM) and 1-octene (127 mM) quenched with 2 at ∼40% conversion (within the range simulated to resemble living behavior, see SI) have a calculated active-site percentage of 54 ± 7% (eq 1), whereas the active-site counts observed via quenchlabeling were 51 ± 10%i.e., the same within error. It is noteworthy however that polymers produced in this regime exhibit broader than expected MWDs, with PDI ≈ 1.5. It has been suggested that multiple catalytically active sites are formed from 118,19,70 and closely related (pyridylamide)hafnium systems.8,71 Multiple species, potentially arising from differing insertions of the first equivalent of olefin into the Hf−naphthyl bond could be responsible for the broadened PDI.4,72,73 GPC Analysis of Quench-Labeled polymers. In addition to providing values for Mn, Mw, and active-site concentration, the MWDs obtained by RI-GPC and UV-GPC contain a wealth of mechanistic information. The RI traces are monomodal throughout the reaction (Figure 3a), with PDI values of ∼1.2− 1.5. Figure 3b shows RI and mass-scaled UV chromatograms for polyoctene quenched by 2 at 55% conversion (30 s). Recall that the RI detector samples all polymers whereas the UV detector senses only the quench-labeled polymers. The RI response is dependent upon both the concentration and the mass of the polymers, whereas the UV signal depends only upon the concentration. To directly compare the two analyses, a “massscaled” UV signal is shown, obtained by multiplying the UV signal for polymer i by the molecular weight of polymer i. The RI and mass-scaled UV distributions are nearly identical, indicating that those polymeryls bearing a UV labeli.e., those bound to Hf prior to quench-labelingare generally representative of the “bulk” polymer. This is expected for a living polymerization. Since end-group analysis reveals a low frequency of β-hydride transfer events in the 1-catalyzed system, the overlaps of the RI- and UV-detected GPC chromatograms are not sensitive to such rare termination events. Influence of ZnEt2 on Polymerization. It is well established that growing polymer chains may be transferred to a main-group metal center via transmetalation (Scheme 6),11 thought to occur via a polymeryl-bridged heterobimetallic complex.74,75 Related Zr/Al bimetallic complexes, e.g., [Cp2Zr(μ-Me2)AlMe2]+, have been characterized previously, and rate constants determined for their cleavage.76,77 If chain transfer is fast, relative to propagation, and reversible (n.b.: “reverse” chain transfer refers to polymeryl exchange between [M]-pol and [E]-

[monomer]0 × MW (monomer) Mn (1)

calculates catalyst concentration under the assumption that the polymerization is living.25,69 While the observation of 2,1misinsertions/β-hydride elimination suggests that our polymerizations are not strictly living but controlled, kinetic simulations (see SI) indicate that behavior resembling living polymerization should be observed at low conversions using lower initial concentrations of 1-octene (127 mM). Therefore, at these low conversions, the Natta equation can be used to compare calculated catalyst concentrations to active-site counts obtained via quench-labeling to provide an estimate of the accuracy of the experimentally obtained active-site counts. For example, several samples of polyoctene produced using 1/[Ph3C][B(C6F5)4]

Figure 3. GPC chromatograms for the homopolymerization of 1-octene catalyzed by 1/[Ph3C][B(C6F5)4] and quenched by 2. [1-octene]0 = 503 mM, [1]0 = 0.083 mM, toluene, 50 °C. (a) RI traces (scaled to reflect yield of polymer) of polymers formed on quenching at 5, 10, 20, 30, 60, and 120 s. (b) RI (blue) and mass-scaled UV (red) of polymers formed on quenching at 30 s. 11907

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Journal of the American Chemical Society Scheme 6. Chain Transfer of a Polymer Chain to Chain-Transfer Agent, ERna

a

Irreversible chain transfer is shown here.

pol′; see Scheme 8), with no other termination events, the system approaches the theoretical limit of a Poisson distribution (PDI = 1).11,15 This is the basis of Sita’s coordinative chaintransfer polymerization method. We chose to study the influence of ZnEt2 on 1-octene polymerization using quench-labeling techniques. ZnEt2 has been used in various olefin polymerization systems,11,78,79 including chain shuttling polymerization.5 The interpretation of quench-label polymer MWDs in the presence of diethyl zinc requires that we know which polymeryls are labeledZnpolymeryls, Hf-polymeryls, or both. Ideally selective labeling of Hf-polymeryls occurs, enabling (1) active-site count determination in the presence of ZnEt2 and (2) a comparison of the MWDs between Hf- and predominantly Zn-bound polymeryls. Upon mixing 2 or 3 with ZnEt2 at 50 °C for 5 min in toluene (simulating catalytic conditions), no reaction was observed by 1 H NMR spectroscopy (see SI); these control reactions suggest that 2 and 3 are unreactive toward Zn-polymeryls under the conditions used herein (vide inf ra). Previously reported reactivity between isocyanates80−83 or isocyanides84 with zinc alkyls has required elevated temperatures and/or long reaction times. For polymerizations conducted in the presence of ZnEt2, 1 was pre-activated with [Ph3C][B(C6F5)4] (1.1 equiv) to avoid induction periods (vide supra) and reaction between ZnEt2 and [Ph3C][B(C6F5)4].85 Activated catalyst solution then was added to a toluene solution of 1-octene and ZnEt2 to start polymerization; concentrations of ZnEt2 ranged from 0.15 mM (1.8 equiv relative to [Hf]) to 1.5 mM (18 equiv relative to [Hf]) (Scheme 7). Reactions were quenched at selected time points with 2 or 3.

Figure 4. Plots of [1-octene] vs time for the polymerization of 1-octene in the presence of 0 (blue diamonds), 0.15 (red squares), and 1.5 (green triangles) mM ZnEt2. [Hf]0 = 0.083 mM, [1-octene]0 = 503 mM, toluene, 50 °C. Each point represents the mean of two experiments, in which the reaction is quenched by 2. [1-octene] was determined by 1H NMR spectroscopy, relative to an internal standard.

to the values measured for polymerizations in the absence of ZnEt2. The RI-based MWDs of bulk polyoctene produced in the presence of 1.5 mM ZnEt2 (Figure 5a) appear monomodal at low conversions, with lower Mn and greater PDI compared with polymers produced in the absence of ZnEt2 (see Table 1). These data suggest that transfer of polymer chains from Hf to Zn centers is fast relative to propagation and that exchange of Hf-polymeryl with Zn-polymeryl′ is slow.67 The UV-detected MWD (mass-scaled) is narrower than, and shifted to higher molecular weight than, the RI trace. Because 2 selectively labels Hf-polymeryls, the Zn-polymeryls present at quenching are invisible to the UV detector. This means that the shorter polymer chains pool on zinc centers in the low conversion experiments (Figure 5b). As monomer conversion increases, the RI GPC trace broadens (to PDI = 8.8 at 70% conversion) and becomes bimodal (Figure 5a). These observations are consistent with saturation of exchangeable zinc sites over the course of the reaction. Such saturation is expected if Hf-polymeryl exchange with Zn−Et bonds is much faster than exchange with βbranched polymeryl′−Zn bonds. The low-molecular-weight mode corresponds to short Hf-polymeryls that exchanged with ZnEt2 early in the reaction. As the zinc sites become unavailable for exchange with Hf-polymeryls, longer chains grow on Hf sites, and a bimodal RI distribution results. For the bimodal RI distribution, the high-molecular-weight mode corresponds to chains that grow at Hf after exchange with Zn centers has shut down (Figure 5c). Similar mechanistic scenarios have been modeled using molecular weight simulations.67 In further support of this hypothesis, the mass-scaled UV trace has a monomodal distribution that overlaps only with the highermolecular-weight mode of the RI trace, showing that highermolecular-weight polymers are Hf-based. These data are consistent with quasi-irreversible86 chain transfer to zinc relative to the polymerization time scalei.e. any exchange of polymeryls between Hf-Pol and Zn-Pol′ (“reverse” chain transfer, Scheme 8) is slow relative to

Scheme 7. General Scheme for the Polymerization of 1Octene by 1a

a I = product obtained after mixing 1 and 1.1 equiv of [CPh3][B(C6F5)4] for 3 min at 50 °C. [1-octene]0 = 503 mM. [ZnEt2] = 0, 0.15, and 1.50 mM.

Plots of [1-octene] vs time show that the rate of 1-octene polymerization is essentially unaffected by ZnEt2 (Figure 4), indicating that under these conditions ZnEt2 does not inhibit or enhance the rate of polymerization, although complexes of I with ZnR2 (R = Me, Et) have been observed previously at lower temperatures.74,75 These data also demonstrate that 2 remains an effective quenching agent in the presence of ZnEt2. Moreover, the active-site counts measured throughout the polymerization were ∼40−50% in the presence of ZnEt2, similar 11908

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Journal of the American Chemical Society

Figure 5. (a) RI vs retention volume for the polymerization of 1-octene (503 mM) in the presence of ZnEt2 (1.5 mM) at various time points. Comparison between the RI and mass-scaled UV signals at (b) 30% conversion and (c) 70% conversion are also shown. [Hf] = 0.083 mM. Conversions determined by 1H NMR spectroscopy. GPC traces are scaled to reflect the yield of polymer.

preferentially after an ethylene insertion, for steric reasons.87 This supports the hypothesis that exchange of β-branched polymeryls between Hf and Zn centers is slow. Interestingly, chain shuttling of polypropylene has been shown to operate in polar solvent with a catalyst closely related to 1 and AlMe3.88 Contrasting our system, polymerizations of α-olefins including 1-octene conducted using Sita’s (Cp*)Hf(Me)2{N(Et)C(Me)N(Et)} pre-catalyst and ZnEt2 lead to rapid and reversible coordinative chain-transfer polymerization.12,13 Does saturation of exchangeable Zn sites occur when one ethyl group of ZnEt2 has been swapped for a β-branched polymeryl or when both ethyl groups have been swapped? The number of polymer chains in solution can be estimated by quenching Zn-polymers with I2 and integrating the resulting ICH2-Pol resonances in the 1H NMR spectrum (see SI). These results suggest that ∼1.75 ± 0.15 μmol of iodine-labeled polyoctene chains is produced when 1.7 μmol of ZnEt2 is used. That is, there is one polymer chain per Zn. A separate analysis of Mn values from polyoctene produced also supports one chain per Zn (see SI). The data suggest that, under the conditions used for these experiments, only one of the two ethyl groups of ZnEt2 exchanges rapidly and the subsequent exchange is much slower. These results likely are dependent on the reaction temperature. Further Support for Quasi-irreversible Chain Transfer to Zn. A two-step polymerization experiment was devised to test the hypothesis that chain transfer to Zn is quasi-irreversible. In the first step, polymerizations were initiated in the absence of ZnEt2 and run for 10 s (∼27% conversion). This step should create a pool of Hf-polymeryls with moderate average molecular weights. In the second step, ZnEt2 (1.5 μmol) was added to the reaction mixture. Reactions were quenched with 2 after a further 5, 10, or 50 s (total reaction times 15, 20, or 60 s), denoted as

Table 1. Molecular Weight Data for Polyoctene Produced at Various Conversions with and without ZnEt2a conversion (%)

[ZnEt2] (mM)

Mn (g mol−1)

Mw (g mol−1)

PDI

19 77 19 70

0 0 1.5 1.5

259 400 535 400 8 500 22 200

351 500 753 000 22 600 194 500

1.4 1.4 2.7 8.8

a [1-octene]0 = 0.503 M, [Hf]0 = 0.083 mM, toluene, 50 °C. All reactions quenched with 2. Conversion measured by 1H NMR spectroscopy. Mn and Mw measured by GPC relative to polystyrene standards. PDI = Mw/Mn.

Scheme 8. Generalized “Reverse” Chain-Transfer Process between Catalyst- and Zn-Bound Polymerylsa

a

Pol1 and Pol2 = polymer chains; R′ and R″ = alkyl groups or H.

propagation. If “reverse” chain transfer were fast relative to propagation, the Zn-based chains would exchange with those on Hf, and upon quenching, the whole distribution of polymeryls would be UV-labeled. Presumably, “reverse” chain transfer is sterically hindered by the hexyl β-branches of polyoctene. Previously reported deuterium labeling studies upon 1-catalyzed copolymerization of 1-hexene/ethylene, with Al(oct)3 as the chain-transfer agent, have concluded that chain transfer occurs 11909

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Figure 6. Polymerization of 1-octene with ZnEt2 added after 10 s of reaction. (a) RI GPC traces (scaled to reflect the yield of polymer). Run A: quench after 10 s. Other runs: addition of ZnEt2 at 10 s and then quenching at a total reaction time of 15 s (Run B), 20 s (Run C), and 60 s (Run D). (b) RI and UV GPC traces for Run C.

Runs B, C, and D, respectively (Figure 6). For comparison, Run A shows a polymerization quenched at 10 s with no ZnEt2. The RI traces of Runs B−D are bimodal, consisting of a highmolecular-weight mode and a broader, lower-molecular-weight mode. On comparing the RI traces for Runs A and B (Figure 6a), the high-molecular-weight components are almost identical. This suggests negligible growth of these high-molecular-weight chains after ZnEt2 addition. Instead, these high-molecularweight polymers are transferred from Hf to Zn rapidly and effectively irreversibly such that this distribution is fixed throughout the remainder of the reaction. Polymerization of the remaining 1-octene continues, but there is an excess of ZnEt2 relative to the catalyst, so these “new” polymer chains undergo transfer to zinc, resulting in the broad lower-molecularweight component in Run B. Over time, the leading edge of this component shifts toward higher molecular weights as the ZnEt2 becomes depleted (Run C). Eventually, baseline separation between the two modes is lost as exchangeable Zn sites are saturated and longer chains grow from Hf (Run D). The observation of these distinctly bimodal RI traces supports quasiirreversible chain transfer. A comparison between the RI and UV trace for Run C is shown in Figure 6b. The UV signal is concentrated in the lower-molecular-weight mode, showing that the chains in the higher-molecular-weight mode are Zn-based, and thus further supporting quasi-irreversible chain transfer. This experiment further highlights the power of the quenchlabel technique in discriminating between Hf- and Zn-bound polymeryls and the mechanistic information that can be gained as a result. Relative Rates of Propagation and Chain Transfer. The determination that chain transfer is quasi-irreversible suggests that an approximate Mayo-like analysis of the ratio of chain transfer and propagation rates is appropriate.89 Polymerizations were conducted in the presence of various amounts of ZnEt2 and quenched with 2 at ∼10% conversion, prior to saturation of exchangeable ZnEt2 sites. A plot of 1/Mn vs [Zn]0/(112.24[octene]0) are shown in Figure 7. The gradient, which represents the ratio of the rate constant of bimolecular exchange to the rate constant of bimolecular propagation (kex/kp), shows that the rate of chain transfer is 7-fold higher than the rate constant for propagation, consistent with experimental observations of fast chain transfer.

Figure 7. Approximate Mayo plot for the polymerization of 1-octene with 1/[Ph3C][B(C6F5)4] and various [ZnEt2]0 (0, 0.16, 0.8, 1.6, 2.4, and 3.2 μmol). Data points shown are the average of duplicate runs; error bars represent the standard deviations between each run. Reactions were quenched by 2 at ∼10% conversion.

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CONCLUSIONS This study demonstrates the utility of the quench-labeling method in the polymerization of 1-octene by 1 in the presence of diethyl zinc, a process pertinent to chain shuttling polymerization. For this catalyst system, quench-labeling with isocyanate- or isonitrile-functionalized pyrenes selectively traps reactive Hf-polymeryl intermediates in the presence of Znpolymeryls, allowing for quantitative active-site counts independent of the trap concentration and the nature of the trap. The active-site counts and apparent rates of 1-octene polymerization for the activators B(C6F5)3, [Ph3C][B(C6F5)4] and [HNMe(C18H37)2][B(C6F5)4] in the absence of ZnEt2 reveal strong activator effects. Under the conditions of these studies (50 °C, ∼0.5 M 1-octene, 83 μM 1), [Ph3C][B(C6F5)4] yields clean reaction profiles with rapid initiation and consistent activesite counts so long as the catalyst and activator are allowed to react prior to mixing with monomer. These conditions were used for the remainder of the studies. Quench-labeling applied to 1-octene homopolymerization in the absence of diethyl zinc yields active-site counts of 40−60%. It is not certain why 100% active-site counts are not observed in the quench-label experiments. It is possible that the insertion of octene into the Hf−naphthyl bond of activated 1, which has been proposed as an essential step in creating active, stereoselective polymerization catalyst produces isomers (such 11910

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acknowledge the NSF through the University of Wisconsin Nanoscale Science and Engineering Center (DMR-0832760 and 0425880) for funding the GPC and Soft Materials Laboratory. We acknowledge the NSF (CHE-1048642) and the Paul and Margaret Bender Fund for support of the NMR facility.

as 2,1-insertion products) that are inactive in polymerization. Observation of similar MWDs for “bulk” polymer (RI trace) and the hafnium-bound polymeryls (UV trace) following quenchlabeling imply that the catalytic homopolymerization is a controlled process. NMR evidence for 2,1-insertion followed by β-hydride elimination, combined with molecular weight data, suggest that the polymerization is not living, and that quenchlabeling is not sensitive to rare termination events at low catalyst concentrations. In addition, when this termination route is suppressed by changing conditions, the broader PDI value of 1.5, rather than 1 as expected for a living process, might reflect the presence of isomeric catalyst sites with slightly different rates. The unreactivity of ZnEt2 with the quench-label agents enables more detailed examination of the homopolymerization of 1-octene in the presence of ZnEt2. The combined information inherent to the RI- and UV-detected MWDs reveal the following critical attributes of chain transfer to Zn under the conditions studied: (1) chain transfer between Hf-polymeryls and ZnEt2 is fast (relative to propagation) with a ratio of rate constants of kex:kp = 7 estimated by Mayo analysis, (2) chain transfer appears to be quasi-irreversible, meaning that Zn-polymeryl/Hfpolymeryl exchange is slow, and (3) there appears to be one polymeryl chain attached to the Zn centers at the end of the reaction. Of course, these conclusions apply only to this particular catalyst system at these temperatures and concentrations. This work illustrates that the chromophore quench-label strategy enables efficient extraction of unique information about the mechanism of metal-catalyzed polymerization reactions. The results reported here, along with our recent demonstration of quantitative kinetic analyses of the evolution of RI- and UVdetected MWDs for metallocene-catalyzed alkene polymerization,52 indicate that development of a quantitative kinetic model for the chain-shuttling process is feasible.

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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/jacs.7b05729. Experimental details, synthesis of 3, and kinetic simulations (PDF) X-ray crystallographic data for 3 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Clark R. Landis: 0000-0002-1499-4697 Author Contributions †

E.S.C. and H.C.J. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by The Dow Chemical Company. We thank Dr. Matthew Christianson and Dr. Daniel Arriola for helpful discussions. We thank Dr. Ilia Guzei (UW-Madison Molecular Structure Facility) for the X-ray structure of 3. We thank Dr. Anna Kiyanova (UW-Madison Soft Materials Characterization Laboratory) for help with the GPC. We 11911

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