Article pubs.acs.org/Organometallics
Mechanistic Investigations into the Behavior of a Labeled Zirconocene Polymerization Catalyst Beth M. Moscato, Bolin Zhu, and Clark R. Landis* Department of Chemistry, University of WisconsinMadison, 1101 University Avenue, Madison, Wisconsin 53706, United States S Supporting Information *
ABSTRACT: Kinetics associated with the [(SBI)Zr(CH2SiMe2(C6H4)NMe2)][MeB(C6F5)3] (1a)-catalyzed polymerization of 1-hexene in a mixed toluene-d8/chlorobenzene-d5 solvent at −33 °C were investigated via 1H NMR and compared to the kinetics associated with the (SBI)ZrMe(MeB(C6F5)3) (1c)-catalyzed polymerization of 1-hexene under identical conditions. In the presence of 1hexene, both catalysts form an identical propagating species, (SBI)Zr(poly-1-hexyl)(MeB(C6F5)3) (1b), but the concentration of 1b during 1a-catalyzed polymerization is only ca. 40% of the anticipated value. Under reaction conditions, 1b reacts reversibly with the model complex p-TMS-C6H4-NMe2 (2) to yield the outer-sphere ion pair tentatively identified as [(SBI)Zr(poly-1hexyl)(2)][MeB(C6F5)3] (1e), which acts as an essentially dormant site during 1-hexene polymerization. Warming of 1b in the absence of additives generates the well-defined hydridoborate complex (SBI)ZrMe(HB(C6F5)3) (1d), which does not reinitiate in the presence of 1-hexene. β-Hydride elimination of 1b in the presence of additives such as 1,2-dichloroethane and 2 results in catalyst decomposition.
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INTRODUCTION The field of catalytic homogeneous olefin polymerization has advanced considerably over the past two decades.1 Modern group IV homogeneous polymerization catalysts are highly active and produce exceptionally tailored polyolefin samples with targeted molecular weights, tacticities, and levels of comonomer incorporation.1a,2 Recent developments, such as catalytic chain-shuttling polymerization, have helped to expand the range of these already versatile catalysts.1b,3 However, few catalysts have been kinetically characterized,4 largely due to the absence of rapid methods for acquiring necessary information such as active site counts. Although the kinetics of a broad range of catalysts have yet to be established, the basic elements of catalytic olefin polymerization are well-known.5 As shown in previous studies, metallocene-catalyzed α-olefin polymerization performed with the [MeB(C6F5)3]− anion involves four crucial steps: initiation into a metal−alkyl bond, propagation through subsequent insertions, and chain transfer following either a 1,2- or 2,1-αolefin insertion (Scheme 1).4a,6 Additional steps, such as catalyst dormancy, death, and other rare events, may also occur.7 A challenge currently facing researchers is how to rapidly establish the kinetics of olefin polymerization for a large number of catalysts. The measurement of active site counts, or the fraction of active catalyst, as a function of time is necessary for establishing accurate polymerization kinetics. Methods for establishing active site counts roughly can be divided into direct observation techniques and indirect label methods. Of these, direct observation techniques are by far the most informative, as, in principle, they directly monitor the concentration of active © 2012 American Chemical Society
species as a function of time. This permits polymerization kinetics to be rapidly established in only a few experiments.8,9 However, this method is limited exclusively to species that can be studied using in situ techniques such as NMR. Alternately, the concentration of active species can be measured indirectly through labeling techniques, which introduce a label either through initiation into a label-bearing catalyst10 or by killing polymeryl-bearing catalysts with a labeled quench agent.10,11 Both of these techniques are more versatile than direct observation methods, as they can be applied even to extremely fast reactions and those with low catalyst loadings. However, both indirect techniques can potentially yield misleading results. Quench-label reagents may react with dormant, polymeryl-bearing metal centers,7f may generate a “quenched” species still capable of polymerization,12 or may fail to react equally with all active catalyst sites.11b Similarly, initial-label studies measure the quantity of initiated catalysts, even if some of those catalytic centers have died7e or are currently dormant.7c,g In all of the above cases, indirect techniques will mismeasure the number of active catalytic centers. Additionally, these techniques often suffer from sensitivity issues, due in part to the low concentrations of active sites, or from inconvenience due to the tedious nature of active site counting by NMR10,11 or the hazards of handling radioactive materials. Consequently, apparent catalytic activitywhich does not require active site countingis often reported in lieu of actual reaction kinetics. Similarly, even seemingly quantitative studies may assume complete catalyst initiation instead of directly measuring the Received: February 6, 2012 Published: February 28, 2012 2097
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Scheme 1. Typical Cycle for Catalytic Olefin Polymerization4a,8
Scheme 2. Activation of (SBI)Zr(Chrom)Me with B(C6F5)3
extent of catalyst initiation.4a,13 Recent studies by Novstrup et al. have indicated considerable catalyst death occurs during even an apparently straightforward catalytic polymerization, suggesting that such assumptions may introduce substantial systematic errors into quantitative investigations.6 Recently, we have developed a novel and convenient method for determining the fraction of initiated catalyst during olefin polymerization.14 If initiation of a well-defined single-site catalyst results in monomer insertion into a chromophorebearing alkyl group, the first polymeryl chain grown from an initiated zirconocene will contain a UV-active end group (Scheme 1; R = chromophore). This UV-active label can be readily quantified by standard GPC methods, allowing the fraction of initiated catalyst centers and the mass distribution of both bulk and first-growth chains to be rapidly established. We demonstrated this concept with our novel chromophorebearing catalyst rac-dimethylsilylbis(1-indenyl)Zr(CH2Si(Me)2-p-C6H4-NMe2)Me ((SBI)Zr(Chrom)Me) and showed that initiated site counts obtained from GPC were in good agreement with NMR data. Herein, we report the results of an in-depth NMR investigation into the kinetics of [(SBI)Zr(Chrom)][MeB(C6F5)3]-catalyzed 1-hexene polymerization and of related control studies of the kinetics of (SBI)ZrMe(MeB(C6F5)3)catalyzed 1-hexene polymerization. These studies demonstrate mechanistic complexity, including the existence of significant catalyst dormancy under some conditions and strongly differential reactivities among uninitiated catalysts, propagating species, and the hydridoborate-bearing products of chain
transfer. The overall plan of this presentation begins with characterization of chromophore-labeled catalyst species generated upon activation with borane followed by descriptions of the apparent polymerization kinetics as determined by in situ NMR. Next, we present evidence for unexpected catalyst speciation with chromophore-labeled catalysts as demonstrated by control studies of (SBI)ZrMe(MeB(C6F5)3) and probes of the reactivity of catalytically relevant species with the chromophore and with potential impurities.
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RESULTS AND DISCUSSION Characterization of [(SBI)Zr(Chrom)][MeB(C6F5)3]. As previously reported,14 addition of excess (1.1 equiv) B(C6F5)3 to a pale yellow solution of (SBI)Zr(Chrom)Me in a mixture of 0.7 mL of toluene-d8/0.2 mL of chlorobenzene-d5 at −33 °C results in clean catalyst activation, generating the orange monomeric uninitiated catalyst [(SBI)Zr(Chrom)][MeB(C6F5)3] (1a) (Scheme 2). This catalyst has been characterized by 1H and heteronuclear (11B and 19F) NMR. Spectral data indicate displacement of the [MeB(C6F5)3]− counteranion by coordination of the anilinyl ring to the zirconium center. As seen by 1H NMR at −33 °C, the anilinyl ring is desymmetrized, yielding four distinct signals in the aromatic region, two with significant upfield shifts relative to the expected values for the free chromophore (free chromophore δ 6.59, 7.43 ppm; observed δ 4.63, 6.19, 6.43, 7.29 ppm). In addition, the [MeB(C6F5)3]− resonance is located at δ 1.34 ppm, significantly upfield of the expected shift 2098
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for a bound anion but in the expected range for a [MeB(C6F5)3]− anion located in the outer coordination sphere.15 Finally, by 19F NMR, the chemical shift difference between the m- and p-fluorines is Δδ = 2.5 ppm, indicating that the anion is displaced from the metal center.16 At higher temperatures, the phenyl resonances observed by 1H NMR broaden, consistent with dynamic processes. EXSY1D experiments were conducted to probe the rate at which diastereotopic anilinyl protons interconvert (Table 1). Three of the four anilinyl resonances could be cleanly irradiated Figure 1. Growth of the polymeryl-bound chromophore 3, as seen by 1 H NMR during [(SBI)Zr(Chrom)][MeB(C6F5)3] (1a)-catalyzed 1hexene polymerization. Reaction conditions are described in Table 2, entry 1. Each spectrum is separated by 16 min 48 s. Legend: (★) 1a; (□) 3; (*) (SBI)ZrPol(MeB(C6F5)3) (1b); (○) p-TMS-C6H4-NMe2 (2).
Table 1. EXSY1D Analysis of Rotation Exchange around the Anilinyl Ring temp (°C) −33 −1
kex (s ) a
2.61
−22 5.38
−11 10.36
0 22.31
10 a
50.27b
b
Average of two resonances. Based on a single resonance.
adjacency of the chiral polymeryl group. The characterization of 3 has previously been reported.14 Low temperature in situ NMR analysis of olefin polymerizations often permits catalyst and substrate concentrations to be quantified throughout a reaction (Figure 2).7f,8,17 Modeling these observed concentrations enables kinetic rate laws and
and their exchange monitored. Reported rate constants are first order in 1a. As anticipated, exchange is rapid even at low temperatures and accelerates significantly upon warming. Exchange is substantially faster than that of catalyst initiation at all temperatures, even at high 1-hexene concentrations (vide infra). If exchange occurs by phenyl ring decoordination followed by rotation, this process is not rate controlling for initiation. The synthesis of (SBI)Zr(Chrom)Me is not an entirely clean process. The chromophore-bearing Grignard reagent used to synthesize the catalyst precursor (SBI)Zr(Chrom)Cl always contains some free chromophore (p-TMS-C6H4-NMe2; 2), which cannot be completely removed during the reaction workup. Similarly, the methylation of (SBI)Zr(Chrom)Cl generates a small amount of (SBI)ZrMe2 (ca. 90% of the anticipated catalyst concentration, demonstrating that, in the absence of external effects, catalyst is conserved (cf. Figure 3). No other species are visible by 1H NMR, although the observation of trace
Scheme 4. Simple Reaction Scheme Used To Model [(SBI)Zr(Chrom)][MeB(C6F5)3] (1a)-Catalyzed 1-Hexene Polymerization
nation; cf. Scheme 1) are visible at the end of polymerization, rates of 2,1-chain transfer were not established due to overlap with resonances from 1-hexene and 1a. As catalyst reinitiation is typically fast, chain transfer was assumed to have little effect upon other reaction kinetics (vide infra).4a,6 The low rates of initiation and propagation permitted apparent polymerization rate constants to be measured directly at temperatures ranging from −34 to 6 °C. These are reported in Table 2. The activation entropy and enthalpy of initiation and propagation were computed from these data (initiation, ΔH⧧ = 34.7 kJ/mol, ΔS⧧ = −0.16 kJ/(mol K); propagation, ΔH⧧ = 22.5 kJ/mol; ΔS⧧ = −0.17 kJ/(mol K)). Overall, as reported previously, 1a-catalyzed 1-hexene polymerization is superficially well-behaved. Apparent rates of catalyst initiation and propagation were computed through standard NMR techniques, and 1a itself was readily characterized in situ. With limited exceptions, all rate constants obtained at −34 °C are in good agreement with each other. Evidence for Low Active Catalyst Concentrations. According to the simple mechanism shown in Schemes 2 and 3, catalyst initiation is expected to yield equal amounts of two distinct species: 3, the polymeryl-bound chromophore, and 1b, the propagating species. However, as shown in Figure 1, the concentration of 1b is only approximately 40% that of 3. Thus, some of the initiated catalyst appears to be missing. GPC traces suggest the presence of a single propagating species. As we have previously shown, polyhexene produced from 1a-catalyzed polymerizations yields both an RI trace, which reflects the total concentration of bulk polymer as a function of molecular weight, and a UV−vis trace, which reflects the concentration of the polymeryl-bound chromophore (3) as a function of molecular weight. If two noninterconverting catalytic species were generated upon initiation, the distribution of the chromophore-bound polymer and/or the bulk polymer would yield multimodal GPC traces.
Figure 3. Concentrations of 1-hexene and polyhexene (left) and catalyst species (right) during (SBI)ZrMe(MeB(C6F5)3) (1c)catalyzed olefin polymerization, as quantified via 1H NMR and modeled with COPASI. Reaction conditions are described in Table 3, entry 2. Legend: (left) (yellow squares) polyhexene, (blue triangles) 1hexene; (right) (orange squares) (SBI)ZrMe(MeB(C6F5)3) (1c), (dark red diamonds) (SBI)ZrPol(MeB(C6F5)3) (1b), (light purple diamonds) vinylene. Lines represent the best-fit model in COPASI.
concentrations (5 h) at −33 °C in the presence of as little as 10 mM 2; vinylidene is present following decomposition, suggesting that 2 may facilitate β-hydride elimination. Although reactions of B(C 6 F 5 ) 3 with amines have been reported at higher concentrations and temperatures,20 no significant coordination of B(C6F5)3 to 2 was observed by 19F NMR under our conditions, even after 5 h reaction time. The data in Table 4 reveal that 2 mildly inhibits polymerization by coordinating to the polymeryl 1b. Inhibition is mild because the kinetics of coordination are not competitive with propagation at high 1-hexene concentrations and the apparent equilibrium constant (Keq = kc/k−c ≈ 70 M−1) is modest. Applying these data to the polymerization of 1-hexene as initiated by the chromophore-labeled catalyst [(SBI)Zr(Chrom)][MeB(C6F5)3] (1a) does not account for all the 2102
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Scheme 7. (SBI)ZrMe(MeB(C6F5)3) (1c)-Catalyzed 1Hexene Polymerization Performed in the Presence of Dimethylaniline Derivative 2 Can Be Modeled Assuming Reversible Coordination of 2 to (SBI)ZrPol(MeB(C6F5)3) (1b)
Figure 6. Concentrations of species during 1c-catalyzed 1-hexene polymerization in the presence of free chromophore (p-TMS-C6H4NMe2, 2). Lines represent the fit by COPASI. Conditions are identical with those reported in Table 4, entry 2. Legend: (left) (red diamonds) 1-hexene; (right) (pink diamonds) (SBI)ZrPol(MeB(C6F5)3) (1b), (orange squares) (SBI)ZrMe(MeB(C6F5)3) (1c), (blue triangles) [(SBI)ZrPol(Chrom)][MeB(C6F5)3] (1e), (light yellow squares) free chromophore 2.
with some catalyst loss and possibly with some isomerization to 1c through unknown mechanisms22 (Scheme 9a). Alternately, 1d can be generated somewhat more cleanly through reaction of 1b with H2 at −33 °C, which reacts according to the secondorder rate equation d[1d]/dt = kHyd[H2][1b] (kHyd = 0.044 M−1 s−1) (Scheme 9b). Either method generates a well-defined compound which has been characterized via 1H, 19F, and 11B NMR and by comparison with related compounds. These data are presented in the Supporting Information. As discussed in the Supporting Information, EXSY/NOESY experiments reveal that 1c,d undergo extremely rapid stereospecific exchange. Does 1d reinitiate immediately upon addition of 1-hexene? To test this, 1-hexene polymerization was followed in samples containing a mixture of 1b, 1c, and 1d. In all cases, reinitiation of 1d (indicated by a decrease in [1d]) was slow relative to both propagation and initiation into 1c, demonstrating that 1d is dormant under some conditions. However, as documented in the Supporting Information, kinetics obtained following 1d formation were irreproducible and yielded rates of initiation and propagation significantly greater than those obtained in the absence of 1d. Could 1d be a dormant catalyst center during [(SBI)Zr(Chrom)][MeB(C6F5)3] (1a)-catalyzed 1-hexene polymerizations? To test if 1d accumulates during standard 1a-catalyzed 1hexene polymerizations, we used 19F NMR to study catalyst speciation. Due to the high concentrations of 1-hexene and polyhexene under standard polymerization conditions, it is difficult to detect low levels of distinct catalyst species using standard 1H NMR techniques. In contrast, only cocatalyst speciation is visible by 19F NMR, and so it is a highly sensitive method for detecting even low concentrations of trace species. However, even with the greater sensitivity of 19F NMR, no evidence for 1d accumulation during the polymerization
dormant sites. Thus, the dormant sites are not entirely due to binding of aniline-terminated polymer or small amounts of 2 to the propagating species. (SBI)ZrMe(HB(C6F5)3) Reacts Slowly with Alkene. The effects of chain transfer upon catalyst speciation were also studied. Until recently, it was assumed that any form of chain transfer from (SBI)ZrPol(MeB(C6F5)3) (1b) generates an unsaturated polymeryl chain and the putative (SBI)ZrH(MeB(C6F5)3) (1f), which rapidly reinitiates in the presence of monomer, regenerating 1b (cf. Scheme 1). However, recent studies of (EBI)ZrMe(MeB(C6F5)3)-catalyzed 1-hexene polymerization by stopped-flow NMR have shown that, at room temperature, the transient species (EBI)ZrH(MeB(C6F5)3) may isomerize rapidly to generate the hydridoborate (EBI)ZrMe(HB(C6F5)3), which only slowly reinitates and effectively acts as a dormant site during polymerization (Scheme 8).21 This implies that the hydridoborate complex (SBI)ZrMe(HB(C6F5)3) (1d) may be one of the dormant sites during 1acatalyzed 1-hexene polymerization. To investigate this possibility, control experiments were again performed using (SBI)ZrMe(MeB(C6F5)3) (1c)-catalyzed 1-hexene polymerizations. The hydridoborate complex 1d can be readily synthesized from the propagating species 1b through either β-hydride elimination or hydrogenolysis. Warming a sample of 1b to room temperature for as little as 10 min results in almost complete conversion of 1b to 1d, albeit
Table 4. Measured Kinetic Rates for (SBI)ZrMe(MeB(C6F5)3) (1c)-Catalyzed 1-Hexene Polymerizations Performed in the Presence of Free Chromophore 2 at −33 °Ca entry
[1c] (M)
[2] (M)
[hexene] (M)
ki (M−1 s−1)
kp (M−1 s−1)
kc (M−1 s−1)
k−c (s−1)
1 2 3 4 5 6
0.0067 0.0079 0.0079 0.0079 0.0050b 0.0053b
0.0103 0.0103 0.0206 0.0206 0.0085 0.0204
0.8 0.8 0.8 0.8 n/a n/a
0.0022(2) 0.0043(1) 0.0067(6) 0.0043(1) n/a n/a
1.267(3) 1.352(2) 1.677(8) 1.206(3) n/a n/a
0.109(7) 0.136(5) 0.136(7) 0.095(3) 0.140(6) 0.14(1)
0.0016(4) 0.0021(1) 0.0015(1) 0.0014(1) 0.00213(2) 0.0016(1)
a
Reaction conditions: toluene-d8/chlorobenzene-d5 (0.7 mL/0.2 mL), 20 mM 1,2-dichloroethane internal standard. b2 was added following polymerization; the concentration given is that of the polymeryl 1b. 2103
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Scheme 8. Recently Proposed Model for Reactivity of the Transfer Product during (EBI)ZrMe(MeB(C6F5)3)-Catalyzed 1Hexene Polymerization
tions performed using diphenylmethane as an internal standard were notably faster than those performed in the presence of 4 (under standard conditions (8 mM 1a, 0.8 M 1-hexene), t1/2 ≈ 15−20 min instead of the anticipated 30 min). Kinetic evaluations of time courses performed using diphenylmethane instead of 4 yielded rates of initiation identical with those obtained in the presence of 4 but with rates of propagation ca. 70% greater than previous values (Table 5). Consequently, it is clear that 4 has a considerable effect upon 1a-catalyzed 1hexene polymerization. In contrast, the effects of 4 upon (SBI)ZrMe(MeB(C6F5)3) (1c)-catalyzed 1-hexene polymerization are small, even at concentrations as high as 125 mM 4.
Scheme 9. Alternate Methods of (SBI)ZrMe(HB(C6F5)3) (1d) Formation: (a) β-Hydride Elimination Induced through Sample Warming; (B) Hydrogenolysis at Low Temperatures
timecourse was observed (cf. Figure 4). Control experiments performed with (SBI)ZrMe(MeB(C6F5)3) (1c) yielded identical results. It is therefore likely that, in the absence of trapping agents (vide infra), formation of (SBI)ZrH(MeB(C6F5)3) (1f) is followed by immediate reinitiation in the presence of all but the lowest concentrations of 1-hexene, conditions under which isomerization to 1d may be competitive. Effects of 1,2-Dichloroethane. Late in the course of this work, investigations into the process and products of β-hydride elimination demonstrated that the complex previously used as an internal standard, 1,2-dichloroethane (4), rapidly reacts with one of the products of chain transfer. Instead of the anticipated (SBI)ZrMe(HB(C6F5)3) (1d), β-hydride elimination in the presence of 4 yields a complex mixture of unidentified catalyst species. This reaction is remarkably robust and occurs during both β-hydride elimination at −33 °C overnight and at room temperature. Consequently, subsequent studies were conducted using an alternative internal standard, diphenylmethane, which does not appear to react with 1d.23 To our surprise, we found that subsequent [(SBI)Zr(Chrom)][MeB(C6F5)3] (1a)-catalyzed 1-hexene polymeriza-
Table 5. Polymerization Kinetics of [(SBI)Zr(Chrom)][MeB(C6F5)3] (1a)-Catalyzed Olefin Polymerization in the Presence and Absence of 1,2Dichloroethanea entry b
1 2c
[1a] (mM)
[hexene] (M)
init (%)
ki (M−1 s−1)
kp (M−1 s−1)
8 8
0.8 0.8
26.6 23.4
0.000 17(1) 0.000 127(1)
0.40(1) 0.682(7)
a
Reaction conditions: toluene-d8/chlorobenzene-d5 (0.7 mL/0.2 mL), −33 °C. b In the presence of 20 mM 4. cWith 5.45 mM diphenylmethane employed as an internal standard.
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CONCLUSIONS
Our studies have demonstrated the complexities associated with mechanistic investigations into catalytic olefin polymerization. While GPC and superficial NMR studies suggest that [(SBI)Zr(Chrom)][MeB(C6F5)3] (1a)-catalyzed polymeriza2104
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by 5 are mixed. In a number of cases, at least partial N,Ndimethylaniline (DMA) coordination to the catalytic center has been reported7g,30 and, in one instance, the coordinated complex was isolated and fully characterized.31 Indeed, partial DMA coordination to uninitiated catalysts has been proposed as a probe for catalytic activity.32 In some cases, however, DMA coordination has not been observed.33 A detailed study by Brintzinger, for example, found that while DMA partially coordinates to B(C6F5)3-activated bis(cyclopentadienyl)dimetylzirconocenes, it does not coordinate to B(C6F5)3activated bis(indenyl)dimethylzirconocenes.19 However, possible interactions between the propagating, polymeryl-bearing catalyst and the aniline center have rarely been directly studied. Multiple studies found the activities of catalysts activated with the trityl borate salt ([CPh3][B(C6F5)4]) were similar to34 or less than35 those of catalysts activated by 5; on this basis, it was concluded that DMA and related amines have little effect upon polymerization. However, trityl-activated catalysts have been reported to be unstable,13,36 possibly due to the lack of coordination to the highly electrophilic metal center. Thus, if the activities measured for trityl-activated catalysts are artificially low due to partial catalyst decomposition, the comparison between trityl- and 5-activated catalysts may not be a valid probe of the interaction between DMA and olefin polymerization catalysts in solution. More recent findings suggest that DMA may in fact have a detrimental effect upon polymerization. In 2005, Baird et al. found that the addition of 1 equiv of DMA to B(C6F5)3activated Cp2ZrMe2, (Indenyl)2ZrMe2, or (SBI)ZrMe2-catalyzed propene polymerizations at 0 °C in toluene systematically reduced catalyst activities by a factor of 2, although the effects on molecular weight distributions were erratic.37 In 2003, Schrock et al. investigated the behavior of DMA as a potential inhibitor for a living polymerization reaction. Although it did inhibit polymerizations at high concentrations, a competitive C−H activation process precluded it from use as a reaction inhibitor. Subsequent investigations with the uninitiated catalyst showed that DMA reacted slightly more slowly with the uninitiated complex than with the propagating species.25 Our experiments with 3 imply a similar resultnot only may DMA coordinate to catalyst centers, it may coordinate selectively with polymeryl-bearing catalysts. Reactivity of Hydridoborate Complexes. Previously, Landis et al. assumed that both the zirconocenium hydride (EBI)ZrH(MeB(C6F5)3) and its more stable isomer (EBI)ZrMe(HB(C6F5)3) reinitiated quickly in the presence of excess monomer.4a,9a Later kinetic modeling postulated that the hydridoborate is a dormant site in rapid equilibrium with the more reactive zirconocene hydride.21 Our studies with SBIligated zirconium differ. Although no evidence for hydridoborate accumulation is observed during [(SBI)Zr(Chrom)][MeB(C6F5)3] (1a)- or (SBI)ZrMe(MeB(C6F5)3) (1c)-catalyzed 1hexene polymerization, considerable evidence indicates that the transient species (SBI)ZrH(MeB(C6F5)3) (1f) reacts with complexes other than 1-hexene, including 1,2-dichloroethane and possibly N,N-dimethylaniline derivatives. Once the hydridoborate complex is generated, however, reinitiation is slow or even nonexistent, in agreement with some previous studies.7a Implications of these and related findings will be further explored in subsequent publications. In conclusion, the kinetics of polymerization of 1-hexene by the labeled catalyst 1a has been investigated, revealing a complex process. Control experiments performed with
tion of 1-hexene proceeds straightforwardly, in-depth NMR and kinetic experiments demonstrate that this system exhibits extremely complex behavior with substantial catalyst dormancy. Further investigations, including detailed control experiments, have failed to elucidate the actual cause of catalyst dormancy. This study serves as a cautionary tale against overinterpertation of available evidence. Dormant Site Formation. Evidence indicates that [(SBI)Zr(Chrom)][MeB(C6F5)3] (1a)-catalyzed 1-hexene polymerization contains at least two distinct dormant sites which form early on during polymerization and persist throughout the reaction. Control experiments performed with the unlabeled catalyst (SBI)ZrMe(MeB(C6F5)3) (1c) yield a rate constant for propagation 2−3 times the apparent rate constant measured during 1a-catalyzed polymerization. Furthermore, direct NMR evidence indicates that only approximately one-third of all initiated catalyst exists as the propagating species at any given time. Two dormant sites are also visible by 13C{1H} NMR during 1a-catalyzed polymerization of 1-13C-1-hexene; at least one of these sites persists following injection of unlabeled 1hexene, indicating that its rate of propagation is significantly slower than that of the active species (SBI)ZrPol(MeB(C6F5)3) (1b). Little evidence for the identity of the dormant sites is available. On the basis of 2JCH coupling constants, the two observable species are sp3 hybridized and hence cannot be allyls. Although chromophore adducts can be generated with 1b, the species does not appear to exist in significant enough concentrations to explain the large fraction of dormant catalytic sites. A surprising contribution to catalyst dormancy appears to arise from the hydride complex 1f. Although on its own it will react with 1-hexene to regenerate 1b, in the presence of 4 it decomposes. This appears to be responsible for a significant fraction of dormant catalyst, although it is not responsible for any of the dormant sites observed by 13C{1H} NMR. It is possible that the dormant sites observed in situ are metallacycles15,24 or products of C−H activation,7b,19,25 but no direct evidence supports these hypotheses. Reactivity of Zirconocene−Polymeryl and Zirconocene−Methyl Complexes. Over the past decade, accumulating evidence has indicated that methyl−methylborate complexes react very differently from polymeryl−methylborate complexes, for both steric and electronic reasons. It is recognized, for example, that rates of [MeB(C6F5)3]− anion exchange increase with increased alkyl steric bulk26 and that rates of monomer insertion into methyl-bearing catalysts are much slower than into polymeryl-bearing ones.4,8 This has inspired the synthesis and study of novel model complexes bearing methylene trimethylsilyl substituents,27 long-chained alkyls,28 neopentyl groups,29 and others.26 Few studies have examined directly the reactivity of authentic polymeryl-bearing species. To the best of our knowledge, this is the first study to directly compare the reactivity of a catalyst precursor with that of a propagating (or even model propagating) species. We have found that the aniline derivative 2 reacts rapidly and reversibly with (SBI)ZrPol(MeB(C6F5)3) (1b), even though it does not significantly react with (SBI)ZrMe(MeB(C6F5)3) (1c). These findings about the reactivity of 2 raise questions concerning the behavior of the common cocatalyst [PhNMe2H][B(C6F5)4] (5). [PhNMe2H]+ is a strong Brønsted acid which quickly protonates a single alkyl or benzyl group from a catalyst. Reports on the products of catalyst activation 2105
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standard-decoupled 13C{1H} NMR (array of 10 spectra composed of 256 transients, each using an uncalibrated 30° pulse width, 1 s acquisition time, and 2.5 s relaxation delay). (SBI)ZrMe(MeB(C6F5)3) (1c)-Catalyzed 1-Hexene Polymerization. A typical reaction used the procedure employed for 1a-catalyzed 1hexene polymerization, albeit with 3.3 mg (8 μmol; 8 mM) (SBI)ZrMe2 instead of (SBI)Zr(Chrom)Me. Activation was performed as described above. 1c-Catalyzed Control Experiments with Free Chromophore. NMR samples were prepared as described above, but contained an appropriate amount of a 1.03 M stock solution of free chromophore in toluene-d8 in toluene-d8. Activation was performed as described above. Alternately, to directly test the reaction of (SBI)ZrPol(MeB(C6F5)3) (1b) with the additives, a polymerization was performed as described above. After all 1-hexene was consumed (typically
