Chain Transfer with Dialkyl Zinc During Hafnium-Pyridyl Amido

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Chain Transfer with Dialkyl Zinc During Hafnium-Pyridyl Amido-Catalyzed Polymerization of 1-Octene: Relative Rates, Reversibility and Kinetic Models Heather C. Johnson, Eric S. Cueny, and Clark R. Landis ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00524 • Publication Date (Web): 03 Apr 2018 Downloaded from http://pubs.acs.org on April 3, 2018

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ACS Catalysis

Chain Transfer with Dialkyl Zinc During Hafnium-Pyridyl AmidoCatalyzed Polymerization of 1-Octene: Relative Rates, Reversibility and Kinetic Models. Heather C. Johnson, Eric S. Cueny, Clark R. Landis* Department of Chemistry, University of Wisconsin–Madison, 1101 University Avenue, Madison, Wisconsin 53706, United States

ABSTRACT: Kinetic modeling is applied to the polymerization of 1-octene catalyzed by Hf-pyridyl amido complex 1/[CPh3][B(C6F5)4] in the presence of chain-transfer agent ZnEt2. The model reproduces monomer consumption, end group data and the bimodal MWD (molecular weight distribution) of polyoctene. Consistent with experimental data, the model suggests chain transfer of Hf-polymeryls to ZnEt2 is an essentially irreversible process. However, to adequately fit the MWDs, slow exchange between 2 Zn(Et)Pol species to produce ZnEt2 and ZnPol2 is proposed. The model shows agreement with experimental data that approximately one polymer chain per ZnEt2 is produced. Changes in the MWDs are observed upon raising or lowering the polymerization temperature; we propose these changes arise due to increasing rates of Zn disproportionation reactions at elevated temperatures. ZnMe2, also, is examined as a chain-transfer agent using chromophore quench-labeling techniques. Qualitative comparison of RI (“bulk” polymer) and UV (catalyst-bound polymer) MWDs show that chain transfer to ZnMe2 is faster than for ZnEt2, and the bulk polymer distributions are trimodal. Furthermore, evidence for >1 chain per ZnMe2 is observed by I2 labeling of polymer chains. These results highlight the dramatic effects of the sterics of the chain transfer agent upon the MWD of polyoctene produced using catalyst 1.

Keywords: catalysis; polymerization; polyolefin; chain transfer; mechanism; kinetic models Introduction Olefin polymerization exemplifies the power of kinetic control in dictating the outcomes of chemical reactions. Single-site homogeneous living polymerization catalysts can be tuned so that specifically targeted polyolefins can be reliably produced. When polymerizations are conducted in the presence of a chain-transfer agent (CTA), e.g. a dialkyl zinc, the relative rates of chain propagation and transfer of polymeryls from the catalyst to the CTA controls the molecular weight distributions (MWDs). If chain transfer is reversible (i.e. fast interchange of polymeryls between the catalyst and CTA, relative to propagation and other termination events) narrow Poisson distributions that appear to be growing on the CTA result. This process, coordinative chain transfer polymerization,1-3 provides multiple

polymer chains per catalyst center – a more cost-effective method than using living polymerization catalysts only. The chain transfer between the polymerization catalyst and the CTA is thought to proceed via bimetallic complexes, while the actively propagating species are the monomeric catalyst alkyl fragments (Scheme 1);1,4-13 dissociation of related bimetallic complexes, e.g. [Cp2Zr(µ-Me)2AlMe2]+, has been investigated previously.10,14 The polymer MWDs therefore depend upon the interplay between the association and nondegenerate dissociation of the bimetallic complex, and the rates of propagation, as well as any additional termination events, such as β-hydride elimination. For ethylene polymerization, Gibson has assigned catalysts to several categories of product distribution (e.g. SchultzFlory, Poisson) depending upon rates of chain transfer

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and β-hydride elimination, relative to propagation.15 The chain transfer efficiency of various metal combinations of catalyst and CTAs have been summarized,1 but a strong dependence on ligand and monomer is also observed, suggesting that the steric and electronic environments around the metals are significant. Of particular relevance to this contribution, Sita has shown that Poisson product distributions can be obtained using a Hf-based catalyst and ZnEt2 with ethylene and more sterically encumbered α-olefins.12,16

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catalyst-bound species – i.e. the “active site” concentration. This method was demonstrated on a model system [(EBI)ZrMe][MeB(C6F5)3] for 1-hexene polymerization, and the MWDs were used to provide a detailed kinetic model for the polymerization.18

Figure 1 Compounds 1 and 2.

Scheme 1 Chain transfer between M–Pol and ZnR2. Chain shuttling polymerization17 utilizes reversible chain transfer to provide an economical means of producing ethylene/1-octene copolymers with desirable properties. Two catalysts, a hafnium pyridyl-amido catalyst (1, Figure 1) and a zirconium bis(phenoxyimine) catalyst, selectively produce either ‘hard’ (Zr) or ‘soft’ (Hf) copolymers. Chains are ‘shuttled’ between the catalyst centers using the chain-transfer agent ZnEt2, establishing a blocky structure. Of fundamental importance to chain shuttling polymerization is the requirement that chain transfer between the chaintransfer agent and catalyst centers is reversible. The distributions and sizes of the blocks of ‘hard’ and ‘soft’ polymer are controlled by relative rates of propagation and chain transfer events. To this end, knowledge of these rates is required to exploit chain shuttling polymerization with the goal of producing targeted block sizes and distributions. However, kinetic studies are hampered by low catalyst loadings, making direct observation of catalyst speciation by NMR spectroscopy difficult, and the inability of traditional quenching methods (e.g. quenching with D+) to distinguish between Hf-bound and Zn-bound chains. To tackle these challenges, we reported a quench-labeling strategy for olefin polymerizations in which polymerizations are quenched with concomitant labeling of the catalystbound polymeryl: a compound such as 2 (Figure 1) quenches Zr- or Hf-catalyzed polymerization via insertion of the isocyanate group into the catalyst– polymeryl bond, tagging the polymeryl with the pyrenyl group, a UV chromophore.18,19 The formerly catalystbound chains thus can be detected by UV-GPC (gel permeation chromatography with UV detection), providing (i) the MWD of the catalyst-bound polymeryls at the time of quenching (the MWD of the ‘total’ polymer sample is determined by refractive index detection (RI-GPC)) and (ii) the total concentration of

Recently, we used quench-labeling methods to study the polymerization of 1-octene using 1, the ‘soft’ precatalyst for chain shuttling polymerization.19 Addition of 2 tags Hf-bound polymeryls with the UV chromophore but, crucially, Zn-bound polymeryls are unreactive to 2. This enables differentiation between Hfand Zn-bound polymeryls. We observed that, under our conditions (toluene, 50 °C), approximately 50% of 1 becomes an active polymerization catalyst. Qualitative analysis of the “bulk” (RI) and “tagged” (UV) MWDs, suggests that chain transfer to ZnEt2 is fast relative to propagation, and effectively irreversible. Herein we examine the chain transfer processes via quantitative kinetic modeling, with the goal of extracting rate constants for the polymerization. We also show that small temperature changes and choice of chain-transfer agent induce significant changes in the MWD; the latter indicates strong steric contributions to the rate of chain transfer involving 1. Results and discussion Qualitative aspects of 1-catalyzed polymerization of 1octene: Prior work in presence and absence of ZnEt2 1) No CTA In previous work we showed that ‘preactivating’ a toluene solution of 1 and [Ph3C][B(C6F5)4] produces the methide-abstracted ion pair 3 (Scheme 2), which catalyzes the polymerization of 1-octene to form high molecular weight polyoctene (Mn = 535,400 gmol-1 at 77% conversion) with a dispersity (Đ) of ca. 1.4 (Scheme 2).19 This was classified as a controlled polymerization,20 between the limits of Poisson and Schultz-Flory distributions. Me

C6H13 503 mM

polyoctene toluene, 50 °C

Scheme 2 Polymerization of 1-octene.a


i

Hf

3 (0.083 mM)

Pr

N

N

i

Pr 3

i

Pr

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ACS Catalysis

3 is produced by activating 1 with [Ph3C][B(C6F5)4]for 3 minutes at 50 °C prior to addition to 1-octene. Reactions quenched with 2. a

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Although hafnium pyridylamido complexes have been considered catalysts for the living polymerization of olefins,21-24 we observed small amounts of vinyleneterminated polymers, presumably arising from 2,1insertions of 1-octene followed by a β-hydride elimination.25 In the polymerizations conducted at 50 °C, it was estimated that each catalyst center undergoes a βhydride elimination ca. 2 – 3 times throughout the polymerization or, equivalently, for every ca. 4,000 successful monomer insertions, one misinsertion/elimination occurs. Although termination by β-hydride elimination broadens MWDs, we note that the dispersity (Ð) is within the range 1.2 – 1.5 (i.e. greater than 1, the expected dispersity for a perfectly living system) at low conversions in which few β-hydride elimination events are expected. We speculate that additional breadth may arise from multiple active catalyst species that have different rates of propagation.26,27 Multiple species could arise from different modes of insertion of 1-octene into the Hf-aryl bond of 3, the proposed first step in polymerization (Scheme 3).22,28,29

unaffected by using ZnEt2 as a chain-transfer agent.19 Quench-labeling experiments suggest that exchange of a polymeryl chain with an ethyl group from ZnEt2 is fast and effectively irreversible. This hypothesis was based on GPC traces of polyoctene obtained via quenchlabeling (Figure 2). At low conversions (~19%), the RI MWD is broad (Ð = 2 – 3) and relatively low molecular weight (Mn 8,500 gmol-1, vs 259,400 gmol-1 in the absence of ZnEt2). This suggests fast, relative to propagation, exchange of growing chains from the Hf catalytic center to a Zn center; a Mayo-type analysis31 showed that kex/kp ~ 7 (kex = rate constant for exchange, kp = rate constant for propagation). At higher conversions (~70%, Figure 2b), the bulk MWD is bimodal (Ð > 8). We hypothesize that, as chain transfer continues, the zinc sites are loaded with chains, ultimately reaching saturation. This forms the lower molecular weight mode. Once the available zinc sites are saturated, the polymer chains continue to grow from Hf, forming the higher molecular weight mode. The UV data support this: comparing the RI and the mass-scaled UV distributions in Figure 2, it is evident that the formerly Hf-bound polymeryls have a monomodal distribution that overlaps with the higher molecular weight portion of the RI signal, suggesting that the higher molecular weight material was Hf-bound at the time of quenching. This scenario requires that chain transfer to zinc is essentially irreversible.32 If chain transfer were reversible (i.e. polymeryls readily swapped between Hf and Zn centers), the bimodality would not be expected, and the RI and UV distributions would resemble each other more closely. C NMR Analysis of polyoctene supports essentially irreversible chain transfer. Through 13C NMR microstructure analysis, Stevens and Busico have shown that chain shuttling of polypropylene chains between a Hf-pyridyl amido catalyst and AlMe3 produces polypropylene with mx(r)my stereoblocks.33 The authors concluded that these blocks arise from shuttling the polymer chain between different enantiomers of the catalyst, and increased concentration of AlMe3 resulted in a higher probability of stereoblocks. In our system, therefore, rapid reversible chain transfer should produce polyoctene with stereoblocks. However, we observe predominantly isotactic polyoctene ( 10,000), this simplification provides a reasonable compromise between a sufficiently detailed MWD and computational time demands. Termination by β-hydride elimination. 2,1-insertion/βhydride elimination events are rare compared with chain transfers to ZnEt2 (vide supra), but are included in the model since even rare events impact the MWDs (for


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ACS Catalysis

simulations showing the effect of k2,1 upon the MWDs, see ESI). This is modeled as a single kinetic step that is first-order in monomer (Figure 3a). It is approximated that the hafnium hydride species generated by βhydride elimination is equivalent to the species Hf-Ins (i.e. the analogous hafnium-methyl complex) and so rapidly re-enters catalysis. M–H complexes are generally considered to insert olefins faster than M–alkyl complexes,38,39, but there are exceptions.18 Catalyst dormancy/death. The temporal profiles of 1octene consumption deviate, especially at longer reaction times, from a strictly first order decay of [1octene]. This implies some catalyst death or dormancy occurs during polymerization. Since there is no significant change in the ‘active sites’ obtained via quench-labeling throughout the reaction, it is likely that, whatever the nature of the dead/dormant species, the polymeryl remains bound to the metal center in a form that would allow it to undergo quench-labeling. One possibility for such dead/dormant species is the formation of allyl species during polymerization;40 metal–allyl groups have been shown to undergo insertion reactions with isocyanates41 or CO2.42 Alternatively, transfer of a [C6F5]– group from the anion to the metal center43-47 may attenuate polymerization while retaining a metal-bound polymer chain that can be labeled. [C6F5]– transfer has been speculated by Macchioni and co-workers for complexes closely related to 3.37,48 We also cannot discount that catalyst death/dormancy may arise from the reaction of the catalyst with an exogenous impurity. Despite the origin being unclear, the process is represented in the model as Hf-Pol[n] undergoing a first order transformation (rate constant kd) to form Hf-Inact[n].

first transfer. This step in the model therefore considers only one chain transfer event, with rate constant kex1. Zn-alkyl equilibria. Incorporating a step involving disproportionation of EtZnPol improves the fit of the model to the data. Each redistribution between two EtZnPol releases ZnEt2, which then undergoes chain transfer with Hf-Pol. Even relatively slow redistributions can induce a noticeable effect on the MWDs (see ESI for details). Equilibria between various ZnR2 mixtures (R = alkyl, alkenyl) were previously measured by calorimetry, although the rates of equilibration were not clearly elucidated.51 We observed that mixtures of ZnMe2 and ZniPr2 at 50 °C in d8-toluene as a model system52 yielded an equilibrium mixture favoring ZnMeiPr (Keq = 4.1) as measured by 1H NMR spectroscopy. This suggests that ZnEtPol is likely favored during polymerization reactions. Equilibrium is reached within several minutes of reaction; due to long relaxation delays required for quantitation of the Zn-species by 1H NMR spectroscopy, we were unable to probe distributions at earlier reaction times. Nonetheless, a rate constant of 0.34 M-1s-1 (as optimized for kex2, vide infra) would be expected to reach equilibrium within minutes (see ESI), suggesting that the model experiment does not invalidate the optimized kex2.

An alternative to irreversible catalyst death is that the catalyst lies dormant following a 2,1-misinsertion prior to the β-hydride elimination of a vinylene.49 This possibility was explored through kinetic modeling (see ESI) but does not lead to any significant improvement in the fits of the MWDs. Although the data do not require dormancy following 2,1-misinsertion, it cannot be discounted. Chain transfer to ZnEt2. In each model, the exchange of a hafnium-bound polymeryl with an ethyl group of ZnEt2 forms Et-Zn-Pol[n] and Hf-Ins. The presence of ZnEt2 does not change the propagation rates, so it is assumed that Hf-Ins (which contains a Hf–Me bond) behaves in the same manner as the Hf–Et species that results from chain transfer.50 We previously determined through labeling experiments that each zinc center receives 1.1 ± 0.1 chains, so, if a second chain transfer occurs, it is expected to be significantly slower than the


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As shown in Figure 3b, the fit of simulated [1-octene] vs time to the experimental data, using the rate constants given above, is excellent. The MWDs are also fit well generally at a range of reaction times and conditions (Figure 3 and ESI). Some systematic deviations in the MWDs at lower molecular weights (logMW ~ 4.6) are observed in which the model overestimates the amount of polymer. This is a consequence of the lowered resolution effected by incorporating 200 monomers per propagation step: at these relatively low MWs, features of the MWDs are lost since each ‘step’ corresponds to 22448 gmol-1. The calculated number of polymer chains per Zn with this model is 1.04, within experimental values of 1.1 ± 0.1. [Vinylene]calc (at 120 s) is 0.12 mM, similar to the experimentally obtained values of 0.11 ± 0.3 mM. As an independent assessment, the UV distributions from the model can be simulated and compared with experimental UV distributions as obtained via quench-labeling. These distributions match well (see ESI). The rate constants suggest that chain transfer to Zn is faster than propagation with a chain transfer constant (kex1/kp) of 8.4 ± 0.3, similar to the value of 7 obtained by a Mayo-like analysis. Any Zn redistribution equilibria are slow compared with propagation and polymer chain transfer to Zn (rate constants of 0.34 M-1s-1, 892 M-1s-1, and 7493 M-1s-1 for redistribution, propagation, and chain transfer, respectively) and, as a result, ca. 1 chain per Zn is expected.

Figure 3. (a) Kinetic model used to simulate chain transfer with ZnEt2. kp200 = kp/200 (kp = rate constant for a single propagation). (b) Monomer consumption vs time (line = simulation, points = data). RI vs logMW experimental data (blue circles) and simulations (red diamonds) at (c) 20 s, (d) 30 s, and (e) 120 s. Experimental conditions: [1-octene]0 = 0.503 M; [1]0 = 0.083 mM (amount added), 0.037 mM (used in simulations, assuming 45% active sites).

Optimized rate constants. The rate constants that achieve the fits shown in Figure 3b are: kp = 892 ± 18 M-1s1, k2,1 = 0.26 ± 0.01 M-1s-1, kex1 = 7493 ± 95 M-1s-1, kd = 0.0156 ± 0.0009 s-1, kex2 = 0.34 ± 0.03 M-1s-1. Several variables were fixed (ki = 105 M-1s-1; [Hf] = 0.037 mM, i.e. 45% active sites, as is reasonable from quench-labeling experiments). The equilibrium constant K2 (= kex2/kex-2) is set to 0.24, the inverse of the K value determined for ZnMeiPr formation (vide supra). The same fits were produced for models with values of K2 fixed from 0.2 to 10 showing that, within this range, determining the exact K2 is not important; the time to reach equilibrium is slow relative to the polymerization timescale.

Alternative kinetic models. Several other kinetic models were trialed to find the best fit to the data. Excluding the zinc redistribution equilibria still yielded satisfactory MWD fits, but overestimated the [vinylene] required to produce these fits (see ESI). As an alternative to the zinc equilibria, we instead included a step for ‘reverse’ chain transfer, i.e., polymeryl for polymeryl exchange between Zn and Hf (reaction 1, Scheme 4); this is a necessary step in chain shuttling polymerization. This model fit the data comparably well with the model shown above, with rate constants for exchange to kex1 = 7473 ± 92 M-1s-1, k2,1 = 0.26 ± 0.01 M-1s-1, kex3 = 13.1 ± 1.0 M1s-1. In this regime, it is notable that kex1/kex3 > 500, showing that reverse chain transfer is very slow compared with chain transfer to ZnEt2. Despite both models showing similar SSDs, we favor the model involving Zn redistribution equilibria, as this provides a route for >1 chain per zinc (as may be required at higher temperatures or with ZnMe2, vide infra). Interestingly, optimizations involving direct exchange of the 2nd ethyl group (reaction 2, Scheme 4) provided poorer fits.


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ACS Catalysis (1) reverse chain transfer

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Hf-Pol[n]

+

EtZnPol[m]

kex3

(2) exchange of 2nd Zn-Et group kex4 Hf-Pol[n] + EtZnPol[m]

Hf-Pol[m] +

Hf-Ins

+

EtZnPol[n]

Pol[m]ZnPol[n]

Scheme 4. Alternative chain transfer events considered in kinetic model Kinetic model: conclusions Quench-labeling has provided data used to produce kinetic models that adequately reproduce the MWDs. The best fits incorporate alkyl exchange between dialkylzinc species as well as initiation, propagation, termination (to Zn or by β-hydride elimination) and catalyst death steps. However, we cannot exclude reversible chain transfer (kex3) as an alternative to, or in combination with, alkyl exchange of dialkylzinc species in this system. Nonetheless, all models examined share several important common factors: (1) The chain transfer to propagation rate constant ratio (kex1/kp) is similar across all models (9±1), and close to the empirical estimate of 7 that was determined using an approximate Mayo analysis. This shows chain transfer to Zn is kinetically competitive with propagation at common concentrations of monomer and CTA.

Effect of temperature upon chain transfer with ZnEt2. To determine the effects of temperature upon chain transfer and propagation, the polymerizations were conducted at 40 °C and 60 °C. In each case, 1 and [Ph3C][B(C6F5)4] first were reacted at 50 °C for three minutes to ensure that the catalyst composition was consistent throughout all runs. At each temperature, the monomer consumption curves are similar (Figure 4a) and the active sites counts are ca. 50% (Figure 4b), suggesting comparable rates of propagation at these temperatures. At later time points, however, catalyst death (or dormancy) increases with increasing temperature, illustrated by a greater deviation from first order behavior at 60 s and 120 s upon increasing the temperature (in Figure 4a the dashed line represents calculated first order decay of [1-octene]). Although the rates of polymerization appear relatively insensitive to temperature within this range, the molecular weight distributions (MWDs) are affected strongly. The RI distributions for polyoctene produced after 120 s reaction (within the regime in which Zn sites are expected to be effectively saturated) at each temperature are shown in Figure 5.

(2) Any additional exchange events that broaden the MWDs (e.g. ‘reverse’ chain transfer, transfer of the 2nd Zn–Et group) are not kinetically competitive with propagation. (3) All models support experimental observations that chain transfer is effectively irreversible (i.e. even when ‘reverse’ chain transfer is explicitly included in the kinetic model, the rate of chain transfer to ZnEt2 is over 500 times higher than the rate of ‘reverse’ chain transfer). (4) Although minor aspects of the model remain to be resolved, each model adequately fits the entire MWD across a range of times and concentration regimes. In light of the necessary approximations in the modeling of a complex system, it is surprising how well multiple sets of heterogeneous data are fit to extract well-defined rate constants. A special feature of the Hf-pyridyl amido catalyst (1)/ZnEt2 system is that chain transfer is fast but essentially irreversible. Gibson has proposed that key requirements for efficient reversible chain transfer include well-matched Cat–C and Zn–C bond strengths, and low steric hindrance.4 Since kex1 is fast, and this catalyst undergoes efficient chain shuttling under Dow’s conditions, the match of bond strengths for this system appears favorable. Apparently, sterics encumber the formation of the proposed 4-membered intermediates (Scheme 1) that would enable reversible chain transfer, or transfer of the 2nd Zn-Et group.

Figure 4. Polymerization of 1-octene at different temperatures. (a) [1-octene] vs time obtained and (b) active sites vs time for polymerizations performed at 40 °C (blue squares), 50 °C (green diamonds) and 60 °C (orange triangles). Conditions: [1-octene]0 = 0.503 M, [1]0 = 0.083 mM (preactivated with 1.1 eq. [Ph3C][B(C6F5)4] at 50 °C), polymerizations quenched with 2 at selected time points. Dashed line represents calculated first order


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monomer consumption: rate = k[cat][monomer] with k[cat] = 0.034 s-1.

Figure 5. RI-GPC traces for polyoctene produced after 120 s of polymerization at 40 °C, 50 °C and 60 °C.

The RI-GPC traces appear distinctly bimodal at 40 °C. At 50 °C, the bimodality is somewhat ‘smoothed’ and the leading edge of the higher mass mode has a lower molecular weight. At 60 °C, the polyoctene produced is almost monomodal. Interestingly, the lower molecular weight edges of each distribution overlap, suggesting that rate of initial chain transfer to ZnEt2 is similar at each temperature. Consistent with this, polyoctene isolated after 5 s (ca. 15% – 20% conversion) has Mn values of 7900 ± 600 g mol-1 and 7600 ± 200 g mol-1 at 40 °C and 60 °C, respectively – i.e. the same within error. The differences in the MWDs therefore arise from other events that act to broaden the distributions. If β-hydride elimination events were more frequent with increasing temperature, a ‘smoothed’ distribution would result. However, the vinylene end group concentrations for polymers produced at each temperature (obtained by quenching the reaction with stearic acid at 120 s and measuring the [vinylene] by 1H NMR spectroscopy) are similar (the same within error) across this temperature range, consistent with previous reports of vinylene formation being relatively insensitive to temperature.25 Thus, it is unlikely that 2,1-misinsertions/β-hydride eliminations are responsible for the changed MWDs with reaction temperature. Thus, the temperature effect does not originate in different active site counts, increased propagation rates, or increased frequency of termination events.

To obtain the chains per zinc, polymerizations were quenched with 2 (1.1 eq. relative to [1]0) after 120 s reaction (ca. 80 – 85% conversion in all cases), then by I2. bFor [vinylene], the polymerizations were quenched with stearic acid. In each case, the polymers were isolated and analyzed by 1H NMR spectroscopy relative to CH2Ph2 internal standard. a

Why does temperature affect the MWDs? We favor an explanation that increased rates of EtZnPol disproportionation with increasing temperature effects an increase in the steady-state concentration of ZnEt2, especially in the reaction regime after the initial charge of ZnEt2 has been converted to EtZnPol. This increases the frequency of exchange of the 2nd ethyl group of ZnEt2 which is expected to cause (1) lowered average polymer molecular weight (2) polymer MWDs that appear more monomodal (3) mass-scaled UV-based MWDs that look more similar to the RI-based MWDs, and (4) an increasing number of polymer chains per Zn as the temperature is increased. As the data in Figures 5 and 6 and Table 1 demonstrate, these expectations largely are met. As the reaction temperature is increased, all MWDs shift to lower average MWt and show a smoother more monomodal shape (Figure 5). At the highest temperature of 60°C the mass-scaled UV trace broadens and more closely resembles the RI trace (Figure 6b). The influence of increasing temperature on the number of polymeryl chains per Zn (Table 1, obtained by labeling the zincbound polymers with iodine and measuring the resulting PolCH2I resonances by 1H NMR spectroscopy), is consistent with increasing numbers as the temperature is increased. However, the low precision of the measured chains per Zn limits the confidence with which this result can be interpreted.

Table 1. Chains per Zna and [vinylene]b of polyoctene produced in the presence of ZnEt2 Temperature / °C

Chains per Zn

[Vinylene] / mM

40

1.1 ± 0.1

0.115 ± 0.015

50

1.1 ± 0.1

0.105 ± 0.015

60

1.2 ± 0.1

0.125 ± 0.005

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ACS Catalysis

Figure 6. RI and mass-scaled UV traces for polyoctene produced after 120 s of polymerization, followed by quenching with 2, at 40 °C and 60 °C. Chain transfer is faster using ZnMe2 as chain transfer agent. To assess the importance of the nature of the chain-transfer agent in the polymerization of 1-octene by 1, reactions were conducted using ZnMe2 as the CTA. The Zn–C bonds in ZnMe2 are stronger than those in ZnEt2,53 and this was proposed to explain slower chain transfer using ZnMe2 as the CTA than when using ZnEt2 with Tonks’ nickel catalyst.54 However, in Gibson’s Fecatalyzed coordinative chain transfer polymerization, ZnMe2 and ZnEt2 behaved similarly as CTAs.4 Under our conditions, polymerizations with ZnMe2 were conducted under conditions analogous to those with ZnEt2: a prereacted solution of 1 and [Ph3C][B(C6F5)4] was added to a toluene solution of 1-octene (0.503 M) and ZnMe2 (0.0015 M) stirring at 50 °C (Figure 7). Polymerizations were quenched with 2 at selected time points to enable active site counting.

Figure 7 Polymerization of 1-octene in the presence of ZnMe2 and plots for (a) [1-octene] vs time (by 1H NMR spectroscopy) and (b) active site counts (by UV-GPC) vs time for polyoctene obtained upon reaction of 1-octene in the presence of ZnMe2 catalyzed by [1]/[Ph3C][B(C6F5)4], and quenched with 2 (1.1 eq).

The rate of consumption of 1-octene by 1 in the presence of ZnMe2 is similar to that observed in the presence of ZnEt2, or no chain-transfer agent (Figure 7a). Macchioni and co-workers previously observed a bimetallic adduct of 3 and ZnMe2,13,55 and demonstrated a lack of reactivity of this complex with 1-hexene in the presence of excess ZnMe2 at low temperature. While these results may imply that this bimetallic complex is not formed here, the different polymerization conditions in our system (temperature, concentrations) cannot rule out some bimetallic complex formation. Analysis of the resulting polyoctene by UV-GPC analysis indicates that, similar to reactions with ZnEt2, ca. 50% ‘active sites’ are observed throughout the time course (Figure 7), where ‘active sites’ reflect propagating polymeryls, or dormant/dead sites that retain a polymeryl that can be labeled with the chromophore quench. Consistent with the monomer consumption profile, this suggests that ZnMe2 does not inhibit the active sites. The difference in the relative rate of chain transfer with ZnEt2 and ZnMe2 can be quantified using a Mayo analysis for the reaction at low monomer conversions, which provides a rate constant ratio, kex/kp.31 With ZnMe2, the kex/kp ratio is 20 ± 4 (Figure 8).56 In contrast,


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this ratio is approximately 7 for ZnEt2 as the CTA. These observations contrast data obtained by Tonks54 for Nicatalyzed polymerization that indicate slower chain transfer with ZnMe2, presumably due to stronger Zn–Me vs Zn–Et bonds.53 For the Hf catalyst studied here it appears that relative rates of chain transfer are highly dependent upon the sterics of the chain-transfer agent.

Figure 8. Mayo-like octene]0*112.24).

plot

of

1/Mn

vs

([Zn]0/([1-

Closer examination of the RI and UV GPC traces reveals further differences between the behavior of ZnMe2 and ZnEt2 as CTAs. As discussed previously, the RI distribution using ZnEt2 appears bimodal: the lower molecular weight mode corresponds to short chains transferred onto Zn that lie dormant, and the higher molecular weight mode corresponds to longer chains that can grow from Hf once the exchangeable Zn sites are saturated (vide supra). In contrast, the RI molecular weight distributions obtained using ZnMe2 are trimodal (Figure 9) and broader (Ð > 15 observed at ca. 50%

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conversion). There are two possibilities for the differences in reactivity between ZnMe2 and ZnEt2. The direct second exchange of the Me group of ZnMePol with Hf-Pol, or a faster redistribution reaction (vide supra) occurs. We favor the direct second exchange reaction as this could give rise to a trimodal MWD. A trimodal MWD can arise from the following scenario: 1) Early in the reaction, Hf–Pol chains are rapidly exchanged with a methyl group in ZnMe2, forming ZnMePol, and these Pol chains form the low molecular weight mode in the RI distribution. 2) The next mode arises from slower transfer of the methyl group in ZnMePol, to form ZnPolPol’. Since the ZnMePol is more sterically congested than ZnMe2, this exchange event is expected to be slower than that of the initial chain transfer due to increased sterics. Therefore, chains grow longer from Hf before transfer to Zn, resulting in the second mode in the RI distribution with higher molecular weight. 3) Eventually, the Zn sites approach saturation and chains growing from Hf become longer still; this forms the 3rd, highest molecular weight, mode. As shown in Figure 9d, this mode shifts to higher molecular weights as the reaction time (and so conversion) increases, while the two lower modes remain essentially unchanged with increasing conversion. Throughout the reaction, the mass-scaled UV-GPC distribution is monomodal and resembles this third mode in the RI-GPC, supporting the hypothesis of the highest molecular weight component consisting of Hf-bound polymeryls.

Figure 9. RI and mass-scaled UV distributions at (a) 10 s, (b) 30 s and (c) 120 s. Plot (d) shows the progression of RI values at various time points sampled during the polymerization.


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Further support for this scenario arises from the analysis of the number of polymer chains per Zn as determined by labeling with I2. While these experiments indicate that, with ZnEt2 at 50 °C, ca. 1 polymer chain per zinc is produced, under analogous conditions using ZnMe2, >1.5 chains per zinc are formed.57 This shows that the second group is exchangeable under these conditions, consistent with studies by Macchioni and coworkers that shows ZnMe2 is converted to ZnPol2 when 1-hexene is added to a closely related bimetallic complex of 3 and ZnMe2.55 Direct comparison (see ESI for plots) between the RI distributions of polyoctene produced in the presence of ZnMe2 and ZnEt2 suggests that this postulated 2nd transfer, while ‘slow’ relative to the 1st chain transfer, is expected to have a comparable rate of exchange with the 1st chain transfer with ZnEt2 (i.e. the kex1 step in Figure 3a), since ‘saturation’ behavior is observed earlier with ZnMe2 than with ZnEt2. Qualitatively, the mass-scaled UV and the RI traces for ZnMe2 at 30 s reaction time resemble the ZnEt2 reactions at 60 °C and 120 s. Thus, the rate of exchange of the 2nd Zn-alkyl is increased either by raising the temperature (increasing the rate of ZnEtPol disproportionation) or decreasing the steric bulk of the CTA (increasing the rate of the R group exchange in ZnRPol). Conclusion This study provides a detailed, quantitative analysis of the polymerization of 1-octene by 1 in the presence of ZnEt2. Consistent with experimental observations, the kinetic model predicts that exchange of Hf–polymeryl chains with a Zn–Et group is fast relative to propagation. However, the exchange of the 2nd ethyl group of ZnEt2 appears to be significantly slower and only small amounts of ZnPolPol′ are formed, likely via solution equilibria between ZnEtPol and ZnEtPol′ species. Any reversible chain transfer (i.e. Hf–Pol and Zn–Pol′ chain exchange), a process required for chain shuttling polymerization, is significantly slower. This is supported experimentally by the mismatch between RI and UVGPC traces. Qualitatively, we observe that increasing the polymerization temperature by only 10 °C broadens the MWDs and decreases the average molecular weight primarily because of increased rates of exchange of the 2nd ethyl group of ZnEt2. The 2nd exchange of an ethyl group on ZnEt2 likely occurs via an increased rate of ZnEtPol disproportionation and subsequent chain transfer between the generated ZnEt2 and Hf. Chain transfer with ZnMe2 is faster than with ZnEt2 (Mayo analysis suggests a threefold difference in the chain transfer constants between ZnMe2 and ZnEt2). Evidence

for exchange of the 2nd methyl group was also observed through iodine labeling experiments. As with the ZnEt2 system, significant mismatches between the RI and UV MWDs of polyoctene produced in the presence of ZnMe2, suggest that any reversible chain transfer events in this system are slow relative to propagation. Our observations underscore the importance of sterics on chain transfer rates. One chain per Zn is produced when using ZnEt2, presumably because of steric barriers to transfer of the Et group from ZnEtPol. Since the steric requirement of exchanging an ethyl group is lower than exchanging a polyoctene chain that contains hexyl βbranches, it is consistent that reverse chain transfer, if occurring, is slow. Polyethylene blocks close the metal centers reduce steric hindrance, which may facilitate fast and reversible chain transfer in other systems.35 Further support for the importance of sterics in this system is the significantly faster first chain transfer event using the less sterically encumbered ZnMe2, and the accessibility of the 2nd Zn-Me group for exchange. This contrasts other reports, in which chain transfer with ZnMe2 is slower than or comparable to chain transfer with ZnEt2.4,54 It therefore appears that 1 is sensitive to its steric environment. This work illustrates the utility of quench-labeling in studying industrially relevant polymerization catalysts. Kinetic models that reproduce MWDs for these complex processes can be produced using data from relatively few experiments. Moreover, qualitative analysis of the GPC traces of quench-labeled polymers is a convenient means of assessing of the effect of changing experimental conditions (e.g. temperature, choice of CTA) upon important factors such as the MWD, amount of active catalyst, and reversibility of chain transfer.

AUTHOR INFORMATION Corresponding Author * [email protected]

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org Experimental details, kinetic modeling details

ACKNOWLEDGMENT This research was supported by The Dow Chemical Company. We thank Dr. Matthew Christianson and Dr. Daniel Arriola for helpful discussions, and Dr. Bernie Anding for assistance with kinetic modeling. We thank Dr. Anna Kiyanova (UW-Madison Soft Materials Characterization Laboratory) for help with the GPC. We acknowledge the NSF through the University of Wisconsin


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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.

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Group Alkyls during Ethylene Polymerization. ACS Catal. 2014, 4, 4223-4231. (55) Rocchigiani, L.; Busico, V.; Pastore, A.; Talarico, G.; Macchioni, A., Unusual Hafnium–Pyridylamido/ERn Heterobimetallic Adducts (ERn=ZnR2 or AlR3). Angew. Chem., Int. Ed. 2014, 53, 2157-2161. (56) A major source of the error is likely to be sampling error: to achieve low conversions (

Chain Transfer with Dialkyl Zinc During Hafnium-Pyridyl Amido

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