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Chromophore Quench-Labeling: An Approach to Quantifying Catalyst Speciation as Demonstrated for (EBI)ZrMe2/B(C6F5)3-Catalyzed Polymerization of 1-Hexene D. Luke Nelsen, Bernie J. Anding, Julie L. Sawicki, Matthew D Christianson, Daniel J. Arriola, and Clark R. Landis ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01819 • Publication Date (Web): 22 Sep 2016 Downloaded from http://pubs.acs.org on September 22, 2016

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Chromophore Quench-Labeling: An Approach to Quantifying Catalyst Speciation as Demonstrated for (EBI)ZrMe2/B(C6F5)3-Catalyzed Polymerization of 1-Hexene D. Luke Nelsen, † Bernie J. Anding, † Julie L. Sawicki, † Matthew D. Christianson, ‡ Daniel J. Arriola, ‡ and Clark R. Landis*,† †

Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, Wisconsin 53706, United States ‡ The Dow Chemical Company, Building 1776, Midland, Michigan 48674 Keywords: Catalysis, polymerization, kinetics, active site, mechanism ABSTRACT: Chromophore-containing quench agents (2 and 3) enable quantitative active site counting and determination of the mass distribution of active catalyst polymeryls by refractive index (RI) and UV detected gel permeation chromatography (GPC) for the polymerization of 1-hexene catalyzed by (EBI)ZrMe2/B(C6F5)3. Time evolution of catalyst speciation data, along with the time profiles of monomer consumption, end-group generation, and bulk molecular weight distribution data, are analyzed by kinetic modeling to determine rate constants for initiation by insertion of hexene into a Zr-Me bond (ki), propagation (kp), chain transfer to form vinylidene (k1,2) and vinylene (k2,1) end-groups, and re-initiation from a Zr-H bond (kr). Unlike previous models that assumed fast catalyst re-initiation, this analysis reveals that kr is considerably slower than kp; catalyst speciation data are critical to making this distinction. This study demonstrates that chromophore quench-labeling with 2 and 3, enables rapid, quantitative analysis of detailed kinetic models for catalytic olefin polymerization reactions using GPC with UV and RI detectors.

Introduction Kinetics control the bulk properties – composition, microstructure, and molecular weight distribution – of polyolefins produced by transition metal catalysis. For example, the composition and block lengths that determine the special properties of the olefin block copolymers produced by chain-shuttling catalysis originate in the relative rates of propagation and chain-shuttling at dual catalyst sites.1-3 The basic elements of catalytic olefin polymerization have been firmly established over the last few decades,4,5 but robust kinetic models exist for few catalysts, in part due to the lack of accessible and sensitive methods that quantify catalyst speciation and the tedious nature of collecting the kinetic data. Accelerated innovation in, and improved understanding of, chain-shuttling technology requires more efficient methods of kinetic analysis.6 Central to the efficient kinetic analysis of chain shuttling are the active site counts and the instantaneous distribution of catalyst-polymeryl molecular weights. Numerous direct and indirect methods to estimate the total number of catalyst-polymeryls, or active site count, have been reported, generally involving quench-labeling,7-20 spectroscopic detection,21-23 or kinetic estimations based on

the growth of the number-average molecular weight at early reaction times.17,24-33 The active site counts determined by such methods are susceptible to large errors and artificially inflated values due to off-cycle species present from side reactions. More informative than the active site count is the full molecular weight distribution of catalyst-polymeryl species as a function of time. Chen and coworkers used carbodiimides as ionizable quench-label reagents. Insertion of the carbodiimide into the catalyst-polymeryl bond yielded ionizable hydrocarbon chains that could be detected by electrospray ionization mass spectrometry.34-39 Assuming that all ionizable polymers are detected, this procedure yields the entire mass distribution of activelypropagating polymeryl chains; these are quantified with high sensitivity, mass resolution, and throughput. Such data can be used to refine the rate constants for kinetic models of the polymerization reaction.9 However, despite promising initial results, it was later revealed that the observed distribution of catalyst-bound polymer chains does not quantitatively represent the true mass distributions due to mass-dependent ionization and detection efficiencies.40

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Scheme 1. a) First generation chromophore utilized previously for initial-labeling studies; b) second generation chromophore used here for quench-labeling. In order to develop a more reliable measure of catalystpolymeryl mass distributions, Moscato et al. designed a chromophore-labeling approach where the distribution of initiated polymer chains could be characterized by gel permeation chromatography (GPC) using both refractive index (RI) and UV detectors (Scheme 1a).41 With this approach the RI detector measures the mass distribution of all polymers, whereas the UV detector selects for the distribution of the first set of chains grown at the catalyst and, hence, the number of initiated catalysts. These studies use the zirconium catalyst, [(SBI)Zr(CH2SiMe2R)][MeB(C6F5)3] (R=para-N,Ndimethylanilinyl, a UV chromophore, SBI=rac-(1indenyl)2SiMe2), employ common instrumentation, provide a good approximation to the true polymer distribution, are amenable to high-throughput analysis, and feature high detector sensitivity. However, comparison of polymerization rates for labeled, [(SBI)Zr(CH2SiMe2R)][MeB(C6F5)3], and unlabeled catalyst, [(SBI)ZrMe][MeB(C6F5)3], revealed that the apparent rate of propagation is ca. 3-fold slower with the labeled catalyst.42 This, coupled with mass-balance inconsistencies, suggested that approximately 60% of the labeled catalyst lies dormant during polymerization, presumably due to binding of the N,N-dimethylanilinyl group to cationic zirconium. Such an interaction inherently changes the reaction kinetics, making this aniline-based chromophore non-ideal for kinetic studies. Furthermore, handling these species is complicated by the chemical sensitivity of the Si–phenyl bond. We sought to augment the chromophore labeling approach of Moscato et al. by using a more robust and inert chromophore for sensitive metallocene-catalyzed polymerization chemistry. Polycyclic aromatic hydrocarbon chromophores are the obvious choice, and among these, pyrene is attractive for its low cost, high extinction coefficient, and absorbance maximum located in a region (λmax = 344 nm in THF) where most solvents, particularly those used in high temperature GPC, do not ab-

sorb. Furthermore, rather than determine the distribution of first set of polymers grown at the catalyst and the initiated sites count, we sought a method that could yield a snapshot of actively propagating active sites at the time of quench. Thus, the present account describes the synthesis and application of pyrene-labeled quenching agents for the full kinetic analysis of [(EBI)ZrMe(MeB(C6F5)3)]-catalyzed hexene polymerization (Scheme 1b). Because this Zr-catalyst is wellestablished, sufficient data are available to validate the quench-labeling method.43 Furthermore, there are conflicting reports of the active site counts for this benchmark catalyst system that we sought to resolve.9,43

Results Application of Pyrene-Labeled Quenching Agents and UV-GPC Analysis. Three pyrene-labeled molecules with different reactive functional groups were investigated for quench-labeling: an isocyanate (2), an aldehyde (3),


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Figure 1: Labeled active sites determined by integration of the UV-GPC signal for polymerization reactions with [hexo ene]0 = 1.0 M and [(EBI)ZrMe2/B(C6F5)3]0 = 2.17 mM at 0 C in toluene solvent as quenched at 120 s reaction time with quenching agents 2-4.

and a nitrile (4). We hypothesized that these quenchlabel agents could trap actively propagating species, successively halting catalysis, and labeling the catalystbound polymeryls with a chromophore. Demonstration of quantitative kinetic trapping behavior requires that quenching and labeling are independent of the nature of the trap and the amount of excess trap. For catalytic polymerizations with [hexene]0 = 1.0 M and [(EBI)ZrMe2/B(C6F5)3]0 = 2.17 mM at 0°C using 1, 2, and 10 equivalents of quench relative to catalyst, monomer consumption rates reveal that 2-4 were all effective quenching agents at ≥ 2 equivalents of quench relative to catalyst (Figure S1, Supporting Information). Quantitative integration of the quench labeled polymer by UVGPC reveals 60-90% active

Figure 2. Monomer consumption with time for reactions at 0 oC using 2 or 3 as quenching agents. Initial hexene concentration was varied between 0.5 and 1.5 M at [catalyst]0 = 2.17 mM (top) and initial catalyst concentration was varied between 0.58 mM and 2.91 mM at [hexene]0 = 1.0 M (bottom).


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tions; the nitrile is a poison but not a quench-label reagent.

Figure 3. Vinylene group formation with time for reactions o at 0 C using 2 or 3 as the quenching agent. Initial hexene concentration was varied between 0.5 and 1.5 M at [catalyst]0 = 2.17 mM (top) and initial catalyst concentration was varied between 0.58 mM and 2.91 mM at [hexene]0 = 1.0 M (bottom).

sites for reactions quenched with 2 or 3, but not 4 (Figure 1). The percent active sites are slightly less using 2 vs. 10 equivalents quench. Precisely why the active site counts differ slightly when the quench equivalents are increased five-fold is not known because we don’t know if, for example, the insertion of quench into the alkenes generates a species that may bind to an unlabeled catalyst-polymeryl and render it less reactive to labeling. Polymerization is quenched by 4, but the quenched samples show no absorption due to pyrene in the UVGPC trace following workup, demonstrating that 4 does not permanently label the polymeryls. Presumably, the nitrile group coordinates strongly to the propagating catalyst inhibiting further polymerization but does not insert into the metal-polymeryl bond under these condi-

Figure 4. Vinylidene group formation with time for reactions o at 0 C using 2 or 3 as the quenching agent. Initial hexene concentration was varied between 0.5 and 1.5 M at [catalyst]0 = 2.17 mM (top) and initial catalyst concentration was varied between 0.58 mM and 2.91 mM at [hexene]0 = 1.0 M (bottom).

Data Collection Using Quench-Labels 2 and 3. Having established that 2 and 3 act as quantitative quenchlabels, these agents were applied to the kinetic investigation of [(EBI)ZrMe2/B(C6F5)3]-catalyzed hexene polymerization at various catalyst and monomer concentrations. As in previous studies, the data required for kinetic analysis include the time evolution of monomer and the vinylidene and vinylene end-group concentrations (Figures 2-4) and the bulk polymer MWD as determined by GPC with a refractive index detector (RI-GPC). The qualitative trends of these data are consistent with previous studies from Landis and Abu-Omar.9,43 Most notably, monomer consumption rate is roughly first order


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with respect to 1-hexene and catalyst concentrations, the rate of vinylene formation is monomer dependent whereas the rate of vinylidene formation is independent of monomer concentration, and the average molecular weight grows approximately linearly at early reaction times but appears to approach a steady state at longer reaction times (Figure S2, Supporting Information).

elimination event, the catalyst will begin to grow a new chain, resulting in a gradual broadening of the live MWD. Repeated cycles of chain-transfer and re-initiation events

240s 180s 120s

60s

60s 120s 180s 240s

Figure 5. An overlay of the UV-GPC data acquired for reactions quenched at 60 s, 120 s, 180 s, and 240 s ([hexene] = 1.0 M; [(EBI)ZrMe2] = [B(C6F5)3] = 2.17 mM, 0 °C). The baseline corrected RI response (red) and UV response (blue) signals correspond to bulk MWDs and live MWDs, respectively.

Data unique to this study are the time evolutions of the active site count and the distributions of polymeryl chains bound to active sites, referred to here as the “live MWDs” (Figures 5-6). Live MWD data are collected by GPC analysis with the UV detector (UV-GPC, 344 nm band) and bulk MWD data are obtained from the refractive index detector (RI-GPC). In analyzing the data, it is important to note that the refractive index (RI) signal for a given molecular mass polymer scales according to the product of that polymer’s concentration with its molecular mass whereas the UV signal depends only on the concentration of the labeled polymer end-group. As a result, for any given sample the RI signal intensities increase at higher molecular weight (early retention volumes) relative to the UV signal and the GPC traces from the two detectors have different shapes. A couple of key characteristics highlight the fact that the live MWD is distinctly different than the bulk MWD, even when the different molecular weight dependencies of the two detectors are taken into account (Figure 5; for a comparison of bulk and live MWD with the mass dependence of the RI detector removed, see the Figure S3 in the Supporting Information). First, whereas the bulk MWD is narrow (PDI 200s represent single runs where as all other data points are averages over two or more runs.

result in the live MWD flatly distributed across the mass range. If monomer were fed in continuously, the PDI of the live MWD will tend to a large value (>10) whereas PDI of the bulk MWD will tend toward a value of 2. Second, while the area under the bulk MWD curves increase in correspondence with monomer consumption, the area under live MWD traces are relatively constant. The integrated area of the live MWD represents the active site counts, and the data reveal that the active site


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counts are high (60-90%) (Figure 6). For initial concentrations of catalyst >2 mM in the first 200 s of reaction, conditions for which active site counts should be most reliable the average active site count by quench labeling is 84%. Active site counts increase rapidly at the start of the reaction, and at late reaction times, a slight decrease

1

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is common but does not typically decrease below 60%. These data suggest persistently higher active site counts throughout the time course than calculated by the partial active site model (PASM),9 where all of the catalyst initiates but only 57% of the catalyst is active. It is noteworthy that

2

3n

4b

4a

Scheme 2. Model of olefin polymerization catalyzed by (EBI)ZrMe2/B(C6F5)3. Catalyst-polymeryls resulting from 2,1misinsertion (k2,1) undergo rapid β-hydride elimination to give vinylene-terminated polymers; this process is not depicted above. Species 4a and 4b are assumed to be in rapid equilibrium with the equilibrium favoring the observed species 4a. reactions with lower initial catalyst concentrations afforded lower active site counts, as expected if minor impurities are present. The dependence of active sites on the absolute catalyst concentration and the nature of the quench is addressed in the supplementary information. Kinetic Modeling. Monomer consumption, end-group formation, active site count, bulk MWD, and live MWD data were modeled using the general kinetic scheme proposed in previous studies43,22,9 (Scheme 2) except that Zr activation, the reaction of the borane with the dimethyl zirconium catalyst precursor, is not included in the kinetic model. Methide abstraction by B(C6F5)3 is very rapid, estimated at two orders of magnitude faster than initiation.43 Thus, the model includes five adjustable rate constants ki, kp, k1,2, k2,1, and kr and nine sets of differential equations (eq. 1-9) for the kinetic steps. The step not depicted in Scheme 2, but represented as k2,1 in the kinetic model, corresponds to β-hydride elimination chain transfer following a 2,1-misinsertion of hexene. Unlike chain transfer following the normal 1,2-insertion, 2,1-misinsertion is treated as a bimolecular step comprising both the insertion and elimination events; further propagation following a misinsertion is negligibly slow as shown by a lack of enchained 2,1-regioerrors.22,43

1– 1 1– 1– 2 , , 1– 1– 3 , , 1– 1– 4 !"# $%&#'' , 1– 5 !"# $%&#'' , 1– 6 !"# $%&#%*'' , 7 !"# $%&#%*'' , 8 1– , , 1– 9


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Two approaches to conversion of the discrete polymer concentrations obtained from the kinetic model to a simulated GPC chromatogram were used.44 One approach converts the concentrations of each polymer mass provided by the kinetic simulation to the fractional differential concentration, %dw/dV, of polymer as a function of eluent volume. Optimization of the kinetic model involved minimization of the square deviations of the discrete values of %dw/dV calculated from the kinetic model with the measured %dw/dV taken from the continuous distribution obtained by GPC. The implicit assumption is that the discrete points obtained by kinetic modeling approximate points along the continuous distribution observed in GPC analysis. We will refer to this as the unit conversion method. The second approach involves the convolution of the discrete concentrations at each polymer weight obtained from the kinetic model with an exponentially-modified Gaussian (EMG) function to obtain a continuous simulated GPC that was compared directly with the experimental GPC.45 This approach will be referred to as the EMG method. With both approaches, manipulation of the raw data was minimized and the concentrations of each polymer chain obtained from the kinetic model were converted %dW/dV distributions that simulate the experimental data. Rate constants optimized by kinetic modeling are shown in Table 1 and compared with the results of previous studies and the comparison of fitted values with experiment are shown in Figure 7. Values for ki, kp, k1,2, and

k2,1 are reasonably similar to values previously reported. We note that the differences in propagation rate constant reported here compared with the value of Abu-Omar et al. is almost solely due to differences in active site counts. Unlike previous studies however, the re-initiation rate constant, kr, was estimated using experimental data. The best-fit value for this rate constant is much smaller than previously assumed values (in previous studies, reinitiation was assumed to be orders of magnitude faster than propagation). However, slow re-initiation was conclusively demonstrated for the closely related siliconbridged indenyl (SBI) zirconium catalyst by stopped-flow NMR spectroscopy. In the stopped-flow NMR studies the Zr product of β-hydride elimination of Zr-polymeryl could be detected and its concentration monitored throughout the reaction. Error associated with the fitted value of kr is relatively high in the GPC studies reported here, and this seems to be the result of two combined factors: (1) error inherent with the experimental catalyst speciation data and (2) relatively low sensitivity of the model to changes in kr. Overall, these data were sufficient to simultaneously fit five rate constants with a modest number of experiments. These results demonstrate the intrinsically high kinetic information contained within the dual-detection GPC analysis of quench-label polymerization experiments. Furthermore, these data indicate that catalyst re-initiation is approximately four orders of magnitude slower than previously assumed. Such estimates are consistent with stopped-flow NMR data.

Table 1. Optimized rate constants obtained in this and prior studies. -1 -1

-1 -1

-1

-1 -1

-1 -1

Study

ki (M s )

kp (M s )

k1,2 (s )

k2,1 (M s )

kr (M s )

Landis

0.033

2.2

0.00066

0.0016

>10kp

Abu-Omar

0.031

3.7

0.0024

0.014

kp × 10

c 3c

This work

a

0.029 ± 0.008

2.9 ± 0.3

0.0027 ± 0.0006

0.010 ± 0.001

0.11 ± 0.07

This work

b

0.028 ± 0.006

2.6 ± 0.4

0.0027 ± 0.0007

0.010 ± 0.002

0.13 ± 0.16

Rate constants optimized here represent an average of rate constants obtained using 1/variance weighting for 6 different initial concentration conditions. For conditions with [cat]0 = 0.583 and 0.897 mM, initial catalyst concentrations were adjusted by a 0.015 mM to reflect catalyst deactivation prior to the reaction (vide infra). Rate constants optimized using EMG band broadb c ening. Rate constants optimized using unit conversion treatment. Assumed values.


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Figure 7. Data compared with the kinetic model used in this work (EMG band broadening) and the PASM with the relative rate constants shown in Table 1 ([hexene]0 = 1.0 M; [(EBI)ZrMe2]0 = [B(C6F5)3]0 = 2.17 mM, 0 oC).9 Depicted MWD distribution data represents the 60 s and 240 s time points. Error bars represent 95% confidence for monomer consumption and one standard deviation in all other graphs.


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Discussion Addressing Discrepancies with Previous Studies. As shown in Table 1, results from the present study highlight some differences from those of the previous two major investigations on Zr(EBI)Me2/B(C6F5)3-catalyzed 1hexene polymerizations. These discrepancies are due most likely to different active site counts, the method of aliquot sampling, and the method of simulating GPC data. Abu-Omar and co-workers found a better fit to their data with the PASM, wherein catalyst deactivates via an undefined step after initiation and reduces the active sites to a modeling-based value of 57%, versus a model for which all sites were active.9 This prompted us to compare the data predicted by PASM with our experimental data in attempt to identify distinguishing features. Remarkably, application of the rate constants of Abu-Omar et al. and the reduced initial catalyst concentration to our kinetic model revealed a fit that is similar to that obtained with the optimized rate constants described herein (Figure 7). The primary distinction between the two models is in active site counts; differences between 57% active sites proposed by PASM and the 70-90% values shown here may be due to small differences in catalyst and solvent purity, which can have a significant effect at these low catalyst concentrations (vide infra). The optimized kp and k21 reported by Abu-Omar and coworkers are approximately 20% larger than those found here, warranting some discussion. The value of kp correlates with active site concentration; the smaller kp found here reflects the larger number of active sites. The origins of differences in the termination rate constant are less clear, but we speculate that the increased termination rate constant determined previously by AbuOmar and coworkers may be due to brief temperature increases as aliquots are removed by syringe from a 0 °C reactor and then quenched at room temperature. The rate of 2,1-termination is known to be more rapid at high temperatures.43 Non-constant temperatures were circumvented in the present method by quenching individual reactions, without removing sample, while maintaining 0 oC temperatures. Major discrepancies exist between the model originally proposed by the Landis group43 with the models presented here and by Abu-Omar et al. As shown by AbuOmar et al., the rate constants proposed in 2001 produce MWD data that is far too narrow and sharp to accurately represent the data. Two factors seem to be at play. First the rate constants reported in the 2001 study made no use of mass distribution data from actual polymerizations. Instead the rate constants were derived from a combination of end-group analyses and monomer consumption data. Upon re-examination of the data, it is clear that the absolute concentrations of vinylene and vinylidene end-groups were in error. The source of error is unknown to us but the result is that the originally reported termination rate constants are systematically too small. Thus, the polymer molecular

weight distributions computed with these rate constants certainly are incorrect. A second factor relates to the simulation of the GPC data. The polymer mass distributions computed from the kinetic model in our original kinetic analysis did not use any band-broadening correction nor the correct conversion from molecular weight based concentrations (dw/dM) to retention volume based distributions (dw/d log(M)). Thus, we were incorrect in attributing the poor match between computed and observed polymer mass distributions to calibration of the GPC data for poly-1-hexene. As subsequently shown by Abu-Omar et al., more careful consideration of the observed polymer mass distributions would have revealed errors in the absolute rate constants and the conversion from concentrations to RI-based molecular weight distributions. Furthermore, as shown in this study, errors would have been more easily identified if we had had access to live MWD data (Figure 7). Information-rich Kinetic Data. The data presented herein demonstrate that selective labeling of catalystpolymeryls with strong UV chromophores followed by GPC analysis of the labeled polymers enables efficient catalyst active site counting and detailed kinetic analysis. This strategy works because the insertion of isocyanates and aldehydes, but not nitriles, into Zr-polymeryl intermediates is rapid, quantitative, and irreversible, yields quenched catalysts and polymeryls covalently attached to the quenching agent. Furthermore, the time evolution of the propagating polymeryl mass distributions represents exacting kinetic data that generally is not available. Compared with quench-ionization methods in which putative metal-polymeryls are quenched to give an ionizable polymer, GPC analysis with UV detectors does not require that all polymers are ionized and detected with equal efficiencies. Potential limitations of the quench-label strategy described here may arise due to issues with selectivity and sensitivity. It is possible that other catalysts may not cleanly insert the quench-label functional group. In the present case, the demonstration that the catalyst active site count is independent of both the nature of the inserting functional group and it excess concentration, indicates that the agents act as quantitative traps. Different behavior may be seen with other catalysts. Another possibility is that other reactive species present during polymerization, such as methylalumoxane (MAO) or diethylzinc, might also react with the quench-label reagent. For these reasons it will be useful to develop a broad selection of quench-label reagents. Because the quench-label installs just one chromophore per catalyst, sensitivity limitations may arise at low catalyst concentrations. The molar absorptivity of pyrene is large (ca. 50,000 L mol-1 cm-1) and other detection methods, such as fluorescence, are expected to yield a 10100 fold enhancement of sensitivity. Because the effective signal-to-noise ratio depends on the broadness of the live chain distribution it is not possible to determine an absolute detection limit in terms of catalyst concentration. For the studies reported here, catalyst concentrations of 394 µM yield high signal-to-noise ratios even at


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reaction times that yield the broadest distribution of live polymers (Figure S24, supplementary information).

cient elucidation of additional kinetic data by GPC analysis.

Conclusion

Experimental

Ultimately, robust kinetic models are required for deep understanding of how any catalyst works, for the design of reactors and catalytic processes, or in the case of catalytic polymerization, for rationally tailoring the polymer mass distributions. Unfortunately, obtaining sufficient data to create a robust kinetic model for catalytic alkene polymerization commonly is arduous and timeconsuming. This work describes the application of chromophore quench-label agents to the kinetic investigation of 1-hexene polymerization catalyzed by [(EBI)ZrMe2/B(C6F5)3]. We demonstrate that a single GPC analysis with UV and RI detection rapidly provides the polymer bulk MWD, the live MWD (i.e. the distribution of polymeryls attached to the catalyst at the time of quench), and the catalyst active site count. By quenching at different reaction times, the time evolution of the active site counts, live polymer MWD, and bulk polymer MWD as the distributions approach the steady-state are obtained. In this way, performing just a few reactions followed by analysis with common polymer laboratory equipment yields a data set that is rich in kinetic information. The significance of this approach is that detailed kinetic analyses can be performed in a practical, time efficient manner without unusual equipment or reagents. Furthermore, the resulting kinetic models are sufficiently robust to compute the entire mass distribution of the polymer product under any set of conditions for which the model is valid. For the specific example of polymerization of 1-hexene as catalyzed by [(EBI)ZrMe2/B(C6F5)3], active site counts, polymer end-group analysis, and the kinetic information intrinsic to live and bulk polymer MWDs provide sufficient data for resolution of five rate constants by kinetic modeling in a small number of experiments (e.g., as few as six). The data show conclusively that the active site counts are high (60-88%) for this catalyst under most of the reaction conditions. Although previous studies had assumed that reaction of alkene with Zr-H produced by β-hydride elimination is fast, this kinetic analysis suggests a re-initiation rate constant, kr = 0.11 ± 0.07 M-1s-1, that is slower than propagation by more than one order of magnitude. This finding is supported by stopped-flow NMR analysis of a closely related catalyst. In the study reported here, determination of the live MWD (catalyst speciation) as a function of time enables estimation of the re-initiation rate constant. The UV-GPC chromophore-labeling methods described here add critical new data for kinetic modeling of catalytic alkene polymerization and should be applicable to many catalysts. Analysis of such data with the full kinetic modeling of MWDs demonstrated by Abu-Omar and coworkers should enable practical, time-efficient mechanistic studies of commercially-relevant, high performance catalysts. Furthermore one can envision logical extensions of these methods, such as UV-labeled chain shuttling reagents, co-monomers, etc. that will enable effi-

General Considerations: All reactions were performed under an Ar or purified N2 atmosphere using standard glovebox and Schlenk techniques. Unless otherwise specified, all chemicals were purchased as reagent grade and used without further purification. Zr(EBI)Me2 was prepared by the literature procedure.46 THF was distilled from Na/benzophenone prior to use. Toluene was deoxygenated by freeze-pump-thaw and dried over activated alumina, which was removed by filtration immediately before use. DMF was dried over molecular sieves and degassed by freeze-pump-thaw. General NMR spectra were collected using Bruker Avance-400 MHz and Avance-500 MHz instruments fitted with a SmartProbe and DCH cryoprobe, respectively. Quantitative NMR spectra measuring monomer consumption or end-group analysis were collected using a relaxation delay of 25 seconds. End-group analyses were conducted with 64 scans rather than the customary 16 scans used for all other analyses. GPC analyses were performed using a Viscotek GPCmax/VE 2001 instrument fitted with PolyPore columns (2x, 300 x 7.5 mm) featuring 5 µm particle size from Polymer Laboratories. Samples were eluted with THF at a flow rate of 1 mL/min at 45 oC. Polymers were characterized by differential refractive index (RI) and UV (λ = 344 nm) detection using Viscotek Model 302-050 Tetra Detector Array. Absolute molecular weight calibrations were conducted previously using low-angle and right-angle light-scattering detectors.41 Omnisec software (Viscotek, Inc.) was used for initial data processing such as positioning the baseline, setting limits, and applying the molecular weight calibration. Further manipulations such as extracting the data corresponding to discrete molecular weights was accomplished using Microsoft Excel. General Polymerization Procedure. This procedure is adapted from that in previous kinetic studies.9 Under glovebox atmosphere, stock solutions of Zr(EBI)Me2 (0.0312 M), B(C6F5)3 (0.0135 M), and the chromophore quench (ca. 0.1 M) were prepared in toluene. Four separate GC vials fitted with mini magnetic stir bars were each charged with B(C6F5)3 (58, 80, 198, or 266 µL to load 0.783, 1.08, 2.67, or 3.59 µmol), hexene (67, 134, or 201 µL to load 0.536, 1.071, or 1.607 mmol, respectively), and the appropriate amount of toluene so that the final reaction volume is 1.071 mL (598-859 µL). Each vial was closed using a cap fitted with a septum. Meanwhile, a flask was charged with Zr(EBI)Me2 (ca. 500 µL) and covered with a septum. If applicable, a third flask was also charged with labeled quenching agent RCHO or RNCO (ca. 500 µL) and covered with a septum. All materials were removed from the glovebox. Each vial was placed into an ice-bath under Ar pressure equalization via syringe and stirred magnetically. Catalyst and quench solutions were also vented with Ar. Reactions were initiated by injecting Zr(EBI)Me2 (20, 30, 74, or 100 µL to load 0.624, 0.937, 2.31, or 3.12 µmol, respective-


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ACS Catalysis

ly). At the desired time, between 60 and 600 seconds, individual vials were quenched with 2.0 equivalents of CD3OD, RNCO, or RCHO. With RNCO and RCHO quenches, solutions were allowed to warm to ambient temperature, forming vibrant red/orange solutions. Then a secondary quench was added to cleave Zr from the labeled polymeryl; MeOH (20 µL) was charged to quenched solutions of RNCO and 0.1 M NaOH in MeOH (20 µL) was added to quenched solutions of RCHO. To maintain consistency, reaction concentrations within each set of four solutions were held constant, and the time of quench was varied. Quenched solutions were first analyzed for monomer consumption; each vial was charged with a standard (1.00 × 102 µL or 0.959 M diphenylmethane in CDCl3), and approximately 50 µL of the resulting solution was analyzed in CDCl3. After 1H NMR analysis, reaction solutions including NMR aliquots were re-collected and prepared for UV-GPC analysis by diluting each quenched reaction to 10.0 mL in THF. Aliquots (2.00 × 102 µL) of the resulting solutions were further diluted to 1.600 mL in THF and filtered using disposable syringe filters with 0.2 µm pore size. After UV-GPC, each entire reaction solution was collected and concentrated to a thick clear gel. Samples were treated with a second NMR standard (2.00 × 10 µL or 0.803 M mesitylene in CDCl3) and analyzed in CDCl3. Kinetic Modeling. Fits were performed using two different modeling suites. The first utilized COPASI software version 4.14 (build 89) (www.copasi.org). Inputs for the software were generated on Mathematica, con-

verted to SBML files using math.sbml, and the resulting SBML files were imported into COPASI, which allowed for rapid generation of models that featured 1950 insertions and thousands of equations. Time-courses were calculated by deterministic(LSODA) simulation. Rate constants were optimized by Levenberg-Marquardt gradient descent method. This software was also applied in the unit conversion treatment of band broadening, but the implementation of EMG broadening was not possible. RI and UV curves were fit using discrete insertion values at 5, 10, 20, 30, 50, 70, 90, 120, 150, 180, 230, 280, 350, 450, 550, 650, 800, 950, 1100, 1250, 1500, 1750, and 1950 insertions. A more versatile kinetic fitting program was written using Excel software using a dynamic linked library. Integration was conducted using the backward differentiation method for 0.001 second step size and a 0.000001 error tolerance. Parameter optimization was executed using the Excel Solver function with GRG nonlinear solving method. Rate constant deviance was determined using the Excel Solvstat.xls macro. RI and UV traces were recreated by individual broadening of each polymeryl species by retention volume dependent EMG,47 calculation of each species’ signal along the retention volume axis, and summing all inputs. For optimizations of four time points, this required eight (RI and UV) 1000×1000 matrices to re-calculate for each trial, thereby significantly increasing computation time. This Excel fitting program was also used to resolve the rate constants using the unit conver-

Scheme 3. Synthesis of RNCO (2).


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Scheme 4. Synthesis of RCHO (3). A more direct route was attempted by brominating pyrene to use as a nucleophile after metal-bromide exchange, but nucleophilic addition with the pyrenyl nucleophilic was not clean, generating numerous byproducts that were difficult to remove. sion treatment. These results matched those found with the COPASI package. Attempts to simplify the EMG broadening by other mathematical convolution/deconvolution operations were not successful. The synthetic routes to the quench-label reagents 3pyrenyl-1-isocyanatopropane (2), 8-pyrenyloctanal (3), and 8-pyrenyloctanenitrile (4) are summarized in Schemes 3, 4, and 5, respectively. 4-Oxo-4-(pyren-1-yl)butanoic acid (5). Compound 5 was synthesized by a modification of a literature procedure.48 In a flame-dried flask, pyrene (2.00 g, 9.89 mmol) and succinic anhydride (1.58 g, 15.8 mmol) were dissolved in CH2Cl2 (12.0 mL). The solution was chilled to -10 oC with ice/NaCl and treated with TiCl4 in small portions over 5 minutes (1.95 mL, 17.8 mmol). After addition, the ice bath was allowed to warm naturally to ambient temperature, and the mixture was stirred for 24 hours. The resulting dark purple mixture was acidified with dilute aqueous HCl. The crude material was extracted with ethyl acetate (3 × 30 mL), washed with water (2 × 30 mL) and brine (30 mL), dried over anhydrous MgSO4, and concentrated in vacuo. Purification by column chromatography on silica gel using 9:1 acetone to CH2Cl2 eluent afforded the product as a pale yellow crystalline solid (1.20 g, 40.0% yield). Characterization data are consistent with those previously reported.

4-(pyren-1-yl)butanoic acid (6). Compound 6 was synthesized by a modification of a literature procedure.48 A flask charged with 5 (3.00 g, 9.92 mmol), KOH (4.45 g, 79.4 mmol), and ethylene glycol (12.0 mL) was heated to 110 oC. It was treated with hydrazine monohydrate (4.81 mL, 99.2 mmol) dropwise over 5 minutes and heated to 120 oC for 2 hours. The temperature was increased to 180 oC for 50 minutes. The hot reaction solution was poured into an ice bath (30 mL), and neutralized with dilute HCl. The crude product was extracted with ethyl acetate (3 × 5 mL), washed with water (2 × 5 mL), dried over anhydrous MgSO4, and concentrated to dryness in vacuo. Although the previous authors purified by column chromatography, we obtained an acceptable product by recrystallization with CH2Cl2/hexanes (3:5). Pale yellow crystals were obtained (2.0 g, 70% yield). Spectra match previous reported specifications. Characterization data are consistent with those previously reported. 3-Pyrenyl-1-isocyanatopropane (2). In a flame-dried flask, 6 (3.96 g, 13.8 mmol) was dissolved in dry THF (77 mL) and chilled to -10 oC in an ice/NaCl bath. The solution was treated with ethyl chloroformate (2.00 mL, 21.0 mmol) and triethylamine (6.70 mL, 48.1 mmol) slowly.

Scheme 5. Synthesis of RCN (4). The suspension was stirred for 30 minutes before treating with sodium azide solution, which was prepared by dissolving sodium azide (4.48 g, 68.9 mmol) in deionized water (16.5 mL). After allowing the solution to warm to ambient temperature over 1.5 hours, the product was extracted with ethyl acetate (2 × 40 mL), washed with water (20 mL), washed with brine (20 mL), and dried over anhydrous MgSO4. The crude material was con-

centrated to dryness in vacuo. CAUTION: Despite being relatively large molecular weight, great care must be taken when handling acyl azides as they are explosive. Maintaining the blast shield safeguard, the solid was redissolved in dry toluene (55 mL) and heated to reflux for 3 hours. The sample was concentrated to a brown oil, and recrystallization via dry toluene/hexanes under glovebox atmosphere rendered a brown viscous oil (3.6


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ACS Catalysis

g, 92% yield). Despite appearing >97% pure by 1H NMR spectroscopy, diminishing quench-labeling at higher quench loadings suggested that significant impurities remain in this sample that led to unlabeled-quenching. Successive recrystallizations with toluene/hexanes eventually furnished a light brown semi-soft solid that performed more favorably (2.2 g, 56% yield). This highlights the challenge associated with 2; since sample stability precludes chromatography, consistent and reliable purification will require further optimization. 1H-NMR (500 MHz, CDCl3): δ 7.92-7.87 (m, 3H), 7.84-7.80 (m, 2H), 7.78-7.72 (m, 3H), 7.47 (d, J = 7.7 Hz, 1H), 2.92 (t, J = 7.6 Hz, 2H), 2.62 (t, J = 6.5 Hz, 2H), 1.55 (p, J = 6.6 Hz, 2H). 13C{1H}-NMR (125 MHz, CDCl3): δ 134.79, 131.50, 130.93, 130.20, 128.72, 127.66, 127.55, 127.35, 126.94, 126.03, 125.21, 125.17, 125.04, 124.98, 124.95, 123.03, 122.26, 42.57, 32.95, 30.28. ESI-MS calculated for C20H15NO 285.1154, found 285.1146. Spectra data are consistent with those previously reported.49 1-Pyrenylheptene (9). Pyrene-1-carbaldehyde (7) and 1-methylpyrene (8) were synthesized as described previously.50 In a flame-dried flask, a stirring solution of 8 (8.26 g, 38.2 mmol) in dry THF (38 mL) was chilled to 0 o C and treated with n-BuLi (16.8 mL, 2.5 M solution in hexanes, 42.1 mmol). After stirring for 1 hour, the solution was cooled to -78 oC with dry ice/acetone and treated with 6-bromo-hexene (6.13 mL, 45.9 mmol). For convenience, the solution was allowed to warm to ambient temperature overnight. Excess base was neutralized by the addition of dilute aqueous ammonium chloride. Crude product was extracted with ether (3 × 50 mL), washed with brine (50 mL), dried over anhydrous MgSO4, and concentrated to a vibrant yellow solid in vacuo. Recrystallization in hot hexanes afforded white to pale-yellow crystals (7.98 g, 74% yield). If further purification is required, good separation has been achieved by column chromatography with 1:40 ethyl acetate to hexanes eluent. Characterization data are consistent with those previously reported. 8-Pyrenyloctanal (3). In a glovebox, a pressure tube was charged with 9 (113.2 mg, 0.379 mmol), (acetylacetonato)dicarbonylrhodium(I) (29 µL of a 2.0 × 10 mM stock solution in THF, 5.8 × 10-4 mmol), a Dow proprietary linear-selective phosphorus-based ligand (14 µL of a 82 mM stock solution in acetone, 1.2 × 10-3 mmol), toluene (4 mL), and a magnetic stir bar. The tube was fitted with a regulator and charged with 145 psi of synthesis gas (H2/CO), which included purging the nitrogen atmosphere by pressurizing with synthesis gas to 145 psi and venting to ambient pressure (3x). The vessel was heated to 80 oC and agitated with lively stirring overnight. The crude material was concentrated to dryness in vacuo and purified by column chromatography on silica gel using CH2Cl2: hexanes (1.1 : 1) eluent to afford a fluffy white solid (85.1 mg, 0.259 mmol, 68% yield). The branched isomer was not detectable by NMR. 1H-NMR (400 MHz, CDCl3): δ 9.75 (s, 1H), 8.27 (d, J = 9.3 Hz, 1H), 8.18-8.09 (m, 4H), 8.05-7.97 (m, 3H), 7.86 (d, J = 7.8 Hz, 1H), 3.34 (t, J = 7.8 Hz, 2H), 2.41 (t, J = 7.1 Hz, 2H), 1.86 (p, J = 7.7 Hz, 2H), 1.63 (p, J = 7.2 Hz, 2H), 1.52-1.32 (m, 6H). 13C{1H}-NMR (100 MHz, CDCl3): δ

203.01, 137.27, 131.58, 131.06, 129.85, 128.72, 127.66, 127.37, 127.26, 126.65, 125.91, 125.22, 125.19, 124.94, 124.91, 124.77, 123.60, 44.03, 33.70, 31.98, 29.69, 29.46, 29.27, 22.19. ESI-MS calculated for C24H24O 328.1827, found 328.1826. 7-Iodoheptylpyrene (10). 9-Borabicyclo[3.3.1]nonane (9-BBN) was generated in situ by treating BH3-THF (1.0 M, 1.60 mL, 1.60 mmol) with cyclooctadiene (2.00 × 102, 1.63 mmol) in a dried Schlenk flask and heating gently to 30-40 oC for 2 hours. Immediate bubbling was noted upon addition of cyclooctadiene. The solution was then treated with a solution of 9 (473 mg, 1.59 mmol) in THF (2 mL) and stirred overnight at ambient temperature. A small amount of methanol (5 µL) was added to quench excess hydride, and then solid I2 (221 mg, 1.74 mmol) was added in a single portion. The resulting purple solution was treated with sodium methoxide (0.8 M solution in methanol) dropwise until the purple color dissipated; in this instance, ca. 800 µL was added over 10 minutes. Excess iodine was quenched by pouring the reaction into aqueous sodium thiosulfate (1.0 M, 10 mL). Crude product was extracted by washing with diethyl ether (3x, 10 mL) and dried over anhydrous magnesium sulfate. Purification by column chromatography on silica gel using CH2Cl2: hexanes (1:15) eluent afforded a white fluffy solid (145.6 mg, 22% yield). 1H-NMR (500 MHz, CDCl3): δ 8.28 (d, J = 9.2 Hz, 1H), 8.16 (dd, J = 7.3, 4.8 Hz, 2H), 8.11 (dd, J = 8.5, 3.3 Hz, 2H), 8.06 – 7.96 (m, 3H), 7.87 (d, J = 7.8 Hz, 1H), 3.34 (t, J = 7.8 Hz, 2H), 3.18 (t, J = 7.0 Hz, 2H), 1.93 – 1.78 (m, 4H), 1.53 – 1.46 (m, 2H), 1.46 – 1.37 (m, 4H). 13C{1H}-NMR (125 MHz, CDCl3): δ 137.25, 131.59, 131.07, 129.87, 128.74, 127.67, 127.39, 127.28, 126.66, 125.92, 125.23, 125.20, 124.95, 124.92, 124.78, 123.61, 33.69, 33.66, 31.95, 30.60, 29.70, 28.64, 7.42. ESI-MS calculated for C23H23I 426.0844, found 426.0840. 8-Pyrenyloctanenitrile (4). A dry flask was charged with 10 (244.7 mg, 0.574 mmol), potassium cyanide (47.8 mg, 0.734 mmol), and magnetic stir bar. After dissolving the solids in DMF (5.0 mL), the solution was heated to 70 oC for 6 hours. Product was precipitated by addition of water (10 mL) and collected by vacuum filtration. Purification by column chromatography on silica gel with hexanes/CH2Cl2 (2:1) eluent afforded a pale yellow to white crystalline solid (140.1 mg, 0.430 mmol, 75% yield). 1H-NMR (400 MHz, CDCl3): δ 8.27 (d, J = 9.3 Hz, 1H), 8.19-8.10 (m, 4H), 8.08 – 7.95 (m, 3H), 7.86 (d, J = 7.8 Hz, 1H), 3.38 (t, J = 7.8 Hz, 2H), 2.28 (t, J = 7.1 Hz, 2H), 1.85 (p, J = 7.6 Hz, 2H), 1.62 (p, J = 7.2 Hz, 2H), 1.54 – 1.31 (m, 6H). 13C{1H}-NMR (100 MHz, CDCl3): δ 137.05, 131.54, 131.01, 129.84, 128.68, 127.62, 127.33, 127.26, 126.64, 125.89, 125.18, 125.14, 124.93, 124.88, 124.76, 123.51, 119.94, 33.58, 31.82, 29.50, 28.83, 28.73, 25.42, 17.19. ESI-MS calculated for C24H23N 325.1830, found 325.1834.

ASSOCIATED CONTENT Supporting Information. More details on the quench efficiency, time evolution weight-average molecular weight, RI and UV trace comparisons, error analysis, catalyst death verses unlabeled quenching, data fits with optimized rate


ACS Catalysis

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constants, and NMR spectra is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected]

Author Contributions The manuscript was written through contributions of all authors.

Funding Sources This research was supported by Dow Chemical Company.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We thank Drs. Charlie Fry and Dr. Anna Kiyanova for assistance with data analysis as well as Dr. Heather Johnson and Mr. Eric Cueny for helpful discussions.

ABBREVIATIONS GPC, gel-permeation chromatography; MWD, molecular weight distribution; EMG, exponentially modified Gaussian; RI, refractive index; UV, ultraviolet; PASM, partial active site model; EBI, rac-ethylene bis indenyl (rac-(C2H4(1-indenyl)2); THF, tetrahydrofuran.

REFERENCES [1] Arriola, D. J.; Carnahan, E. M.; Hustad, P. D.; Kuhlman, R. L.; Wenzel, T. T. Science 2006, 312, 714. [2] Zintl, M.; Rieger, B. Angew. Chem. Int. Ed. 2007, 46, 333. [3] Tynys, A.; Eilertsen, J. L.; Seppälä, J. V.; Rytter, E. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 1364. [4] Resconi, L.; Cavallo, L.; Fait, A.; Piemontesi, F. Chem. Rev. 2000, 100, 1253. [5] Bochmann, M. Organometallics 2010, 29, 4711. [6] Sita, L. R. Angew. Chem. Int. Ed. 2009, 48, 2464. [7] Shen, X.; Hu, J.; Fu, Z.; Lou, J.; Fan, Z. Catal. Commun. 2013, 30, 66. [8] Zhang, L. T.; Fu, Z. S.; Fan, Z. Q. Macromol. Res. 2010, 18, 696. [9] Novstrup, K. A.; Travia, N. E.; Medvedev, G. A.; Stanciu, C.; Switzer, J. M.; Thomson, K. T.; Delgass, W. N.; Abu-Omar, M. M.; Caruthers, J. M. J. Am. Chem. Soc. 2010, 132, 558. [10] Kuhlman, R. L.; Wenzel, T. T. Macromolecules 2008, 41, 4090. [11] Yaluma, A. K.; Tait, P. J. T.; Chadwick, J. C. J. Polym. Sci. Pol. Chem. 2006, 44, 1635. [12] Vatamanu, M.; Boden, B. N.; Baird, M. C. Macromolecules 2005, 38, 9944. [13] Bukatov, G. D.; Zakharov, V. A.; Barabanov, A. A. Kinet. Catal. 2005, 46, 166. [14] Zakharov, V. A.; Bukatov, G. D.; Barabanov, A. A. Macromol. Symp. 2004, 213, 19. [15] Liu, Z.; Somsook, E.; Landis, C. R. J. Am. Chem. Soc. 2001, 123, 2915. [16] Marques, M. M.; Tait, P. J. T.; Mejzlík, J.; Dias, A. R. J. Polym. Sci. Pol. Chem. 1998, 36, 573.

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[17] Han, T. K.; Ko, Y. S.; Park, J. W.; Woo, S. I. Macromolecules 1996, 29, 7305. [18] Fan, Z. Q.; Feng, L. X.; Yang, S. L. J. Polym. Sci. Pol. Chem. 1996, 34, 3329. [19] Chien, J. C. W.; Song, W.; Rausch, M. D. Macromolecules 1993, 26, 3239. [20] Bukatov, G. D.; Goncharov, V. S.; Zakharov, V. A. Makromol. Chem. 1986, 187, 1041. [21] Chen, C.-H.; Shih, W.-C.; Hilty, C. J. Am. Chem. Soc. 2015, 137. [22] Christianson, M. D.; Tan, E. H. P.; Landis, C. R. J. Am. Chem. Soc. 2010, 132, 11461. [23] Landis, C. R.; Rosaaen, K. A.; Sillars, D. R. J. Am. Chem. Soc. 2003, 125, 1710. [24] Hong, D. S.; Jeong, D. W.; Cho, H. Y.; Cui, L.; Tarte, N. H.; Woo, S. I. Polymer 2006, 47, 184. [25] Liu, B.; Matsuoka, H.; Terano, M. Macromol. Rapid Commun. 2001, 22, 1. [26] Busico, V.; Cipullo, R.; Esposito, V. Macromol. Rapid Commun. 1999, 20, 116. [27] Woo, T. W.; Woo, S. I. J. Catal. 1991, 132, 68. [28] Kim, II; Woo, S. Polym. Bull. 1990, 23, 35. [29] Chien, J. C. W.; Wang, B.-P. J. Polym. Sci., Part A: Polym. Chem. 1989, 27, 1539. [30] Keii, T.; Terano, M.; Kimura, K.; Ishii, K. Makromol. Chem. Rapid Commun. 1987, 8, 583. [31] Chien, J. C. W.; Kuo, C.-I. J. Polym. Sci. Pol. Chem. 1985, 23, 731. [32] Mejzlík, J.; Lesná, M.; Majer, J. Makromol. Chem. 1983, 184, 1975. [33] Coover, H. W.; Guillet, J. E.; Combs, R. L.; Joyner, F. B. J. Polym. Sci. Pol. Chem. 1966, 4, 2583. [34] Dietiker, R.; di Lena, F.; Chen, P. J. Am. Chem. Soc. 2007, 129, 2796. [35] Quintanilla, E.; di Lena, F.; Chen, P. Chem. Commun. 2006, 4309. [36] di Lena, F.; Quintanilla, E.; Chen, P. Chem. Commun. 2005, 5757. [37] Wehrli, D.; Chen, P. Chimia 2003, 57, 354. [38] Hinderling, C.; Chen, P. Int. J. Mass Spectrom. 2000, 195–196, 377. [39] Hinderling, C.; Chen, P. Angew. Chem. Int. Ed. 1999, 38, 2253. [40] Moscato, B.; Landis, C. Chem. Commun. 2008, 5785. [41] Moscato, B. M.; Zhu, B.; Landis, C. R. J. Am. Chem. Soc. 2010, 132, 14352. [42] Moscato, B. M.; Zhu, B.; Landis, C. R. Organometallics 2012, 31, 2097. [43] Liu, Z.; Somsook, E.; White, C. B.; Rosaaen, K. A.; Landis, C. R. J. Am. Chem. Soc. 2001, 123, 11193. [44] Popovici, S. T.; Kok, W. T.; Schoenmakers, P. J. J. Chromatogr. A 2004, 1-2, 237. [45] Wolpers, A.; Russell, G. T.; Vana, P. Macromol. Theory Simul. 2011, 20, 667. [46] Diamond, G. M.; Jordan, R. F.; Petersen, J. L. J. Am. Chem. Soc. 1996, 118, 8024. [47] Yossen, M. M.; Vega, J. R.; Chang, T.; Meira, G. R. J. Liq. Chromatogr. Related Technol. 2012, 35, 79. [48] Hahma, A.; Bhat, S.; Leivo, K.; Linnanto, J.; Lahtinen, M.; Rissanen, K. New J. Chem. 2008, 32, 1438. [49] Werner, F.; Schneider, H.-J. Helv. Chim. Acta 2000, 83, 465. [50] Yamato, T.; Miyazawa, A.; Tashiro, M. J. Chem. Soc., Perkin Trans. 1 1993, 3127.

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