LIST OF FIGURES

We have had a string of fantastic post-docs in the Bercaw group who have shared a wealth of knowledge and a diverse set of skills over the years. Professor Yen Nguyen has been a confidant and close friend for seemingly as long as I have been at Caltech.

LIST OF TABLES

Polymerization Overview

In the simplest sense, Ziegler-Natta polymerization is the polymerization of -olefins using homogeneous or heterogeneous catalysts with group 4 metals (Scheme 1.1), giving polyolefins, which are produced on the order of 108 tons per year. As would be expected for any process of such great commercial interest, the field of Ziegler-Natta polymerization is very broad and has been thoroughly reviewed.1-10 As such, the present introduction will be limited to a general overview of what is known as homogeneous polymerization. and where knowledge is still limited.

Stereocontrol of Olefin Insertion

The incoming monomer is oriented so that interaction between the polymeryl chain and the methylene of the incoming olefin is minimized. For C2-symmetric precatalysts, the two conformations of the insertion transition state with the polymeryl group on either side of the wedge are identical (Figure 1.3), resulting in insertion of the same enantioface of the olefin.

Figure 1.1: Transition state model for insertion of propene into zirconocene alkyl

Activation and Initiation

Activation with strongly Lewis acid boranes or alanes results almost exclusively in a zirconocene methyl cation with a closely associated methylborate anion24-25 (Figure 1.4), while at low borane to Zr ratios another species is formed due to the incomplete activation of zirconocene to give a methyl-bridged dinuclear monocation corresponding to the type II complex. Initially, a type II {Zr-Me+} species is formed, which is stabilized by coordination of another equivalent of the neutral zirconocene through a Me bridge.

Figure 1.4: Cations formed with boron based activators as modeled by {(SBI)Zr}

Direct Observation of Catalytic Species

AlMe3.36 Further addition of AliBu3 or smaller amounts of HAliBu2 to the (SBI)ZrCl2/MAO systems gives rise to a new peak at –1.95 ppm, which is believed to be due to a hydride species, but its identity could not be determined at the time.47 Simultaneous UV -vis spectroscopy showed a decrease in the absorbance of the AlMe3 adduct (490 nm) and the growth of a shoulder at 380 nm. In contrast, Landis49, 53 and Klamo52 report no decrease in total Zr concentration from expected values.

Kinetics of Polymerization

Analysis of all these data led to a kinetic model depicted in Scheme 1.2 together with a set of rate constants consistent with the data demonstrating that propagation (kp) was faster than initiation (ki) by almost 2 orders of magnitude of size.58 Further analysis of the data by Abu-Omar and Caruthers leads to the rather surprising conclusion that inclusion of the full molecular weight distribution of the polymer resulted in a good fit only if a dormant Zr species was included in the kinetic model. 59 Full treatment of the data resulted in a much better fit to the data when only 57% of the catalyst was considered active for polymerization (Table 1.1). The identity of the inactive/dormant species could not be ascertained from the data present, but several possibilities could be ruled out.

Dormant Species

While insertion into the hydride (ki,H = 2.06(2) M-1/s-1) was an order of magnitude faster than insertion into the initial methyl (ki M-1/s-1), insertion into the hydride was still an order of magnitude slower than insertion into a polymeryl group (kp M-1/s-1) agreeing with the observation of the accumulation of Zr hydrides during the course of the reaction. Two problems persist with the dormancy of hydride species: (1) the identity of such a hydride in the presence of alkylaluminum species would certainly be different from that in the Landis system with only B(C6F5)3 as initiator, and (2) the hydride is observed only in some cases, while in others it is absent.

Aims of this Thesis

Landis saw the hydride as the Zr-containing end product from the polymerization of 1-hexene with (EBI)ZrMe2/B(C6F5)3.53 Fitting the concentration data showed that the hydride product was [(EBI)ZrMe][HB(C6F5) 3 ] reacted much more slowly with 1-hexene than [(EBI)ZrMe][MeB(C6F5)3], suggesting that the hydride may indeed be an elusive quiescent species. Kinetic models previously used by Landis58 and Abu-Omar/Caruthers59 assumed that reinitiation by hydride insertion was extremely fast compared to all other processes (Scheme 1.2), which has now been shown to be incorrect.

The identity of the putative hydride has been investigated through a study of the reactivity of zirconocene chlorides with HAliBu2 (Chapter 2). Further work investigated the effect of cationization reagents on the hydrides to identify cationic zirconocene hydrides that could be formed under conditions closer to MAO-activated homogeneous polymerization (Chapter 3).

Abstract

Introduction

Recently, however, zirconocene hydride complexes deviating from the commonly observed {ZrH3} coordination pattern have been encountered in MAO-activated precatalytic zirconocene systems upon addition of HAliBu2 or AliBu3.28-29. The 1H NMR spectra of these species show only a single hydrogen. resonance with an integral corresponding to two hydride units per Zr center, indicating the presence of a {ZrH2} unit with two equivalent H ligands.29 To characterize the species responsible for this NMR signal and the factors governing the formation of {ZrH2} species, in comparison to zirconocene hydride complexes of the {ZrH3} type, a re-evaluation of some previously described reactions of unreduced zirconocene complexes with diisobutylaluminum hydride and the subsequent study of the corresponding reactions of some ansa-zirconocene dichlorides was carried out. For simplicity, diisobutylaluminum hydride is written here, and in the following, as a monomer, although it is obviously present mainly as a trimer in solution under our experimental conditions. dimeric in solution.33.

Figure 2.1: {ZrH 3 } coordination geometries proposed for alkylaluminum-complexed zirconocene hydride complexes in references 21-27

Results and Discussion

A gCOSY analysis (Figure 2.10 B), which shows the coupling of the signal at 2.65 ppm with both high-field fields. Reaction of the fluorine analog of complex 18, (EBTHI)ZrF2, with 1 or 2 equiv of HAliBu2 is found to give, in agreement with previous studies,7 the dihydride 24 as the sole zirconocene product, with no indication of an intermediate of the type (EBTHI)ZrF(μ-H)2AlBu2.

Figure 2.2: 1 H NMR spectrum of a 50 mM solution of (C 5 H 5 ) 2 ZrCl 2 (1) in benzene-d 6 at 25 °C after addition of 3 equiv of HAl i Bu 2 (A) and blowups of the hydride regions of the same (B) as well as a 71 mM solution of (C 5 H 5 ) 2 Z

1/ {ppm-1}

Conclusions

Binuclear complexes {ZrH(μ-H)2AliBu2}, containing a Zr-H instead of a Zr-Cl unit, are formed when HAliBu2 is reacted with a zirconocene dihydride or with a zirconocene dichloride carrying highly plugged ring ligands. However, a terminal Zr-Me group rather than a Zr-Cl or Zr-H unit appears to predispose otherwise analogous dinuclear dihydrides to decay, possibly due to reductive CH4 elimination.44-46.

Experimental

CAUTION: Alkyl aluminum compounds are pyrophoric and must be handled with special precautions (see, for example, Shriver, D.F. The Manipulation of Air-sensitive Compounds;). The mixture was allowed to warm to room temperature, during which time a warm pink color developed. the reaction was stirred overnight. The reaction was filtered and washed 3x to give an orange solution.

Abstract

Introduction

Apart from such neutral species, alkylaluminum complex zirconocene hydrides can also give rise to cationic species, especially in zirconocene-based reaction systems containing methylalumoxane (MAO) or other "cationization" reagents typically used for olefin polymerization catalysis.4 Cationic zirconocene hydride species have been crystallographically characterized either as discrete ion pairs 5-8 or with stabilizing Lewis bases 9-15. Numerous other examples of cationic zirconocene hydrides, in ion pairs16-21 and with stabilizing Lewis bases,17, 22-25 have been identified by NMR. Three alkylaluminum complex hydrides have recently been observed by NMR26-28 and three borane-complex zirconocene hydrides have been structurally characterized29-32 (Figure 3.1).

Results and Discussion

The addition of relatively small amounts of AlMe3 to a solution of the cation [(SBI)Zr(-H)3(AliBu2)2]+ in benzene-d6 results in the appearance of additional signals near the signals of [(SBI)Zr(-H)3(AliBu2)2 ]+. The broadening of the Zr-hydride signals of these mixed alkyl aluminum species, hereafter referred to as [(SBI)Zr(-H)3(AlR2)2]+, is probably due to the statistical nature of it. The sensitivity of the Zr-H signals to the nature of the R groups bound to Al in cations of the type [(SBI)Zr(-H)3(AlR2)2]+ is evident.

Figure 3.2: 1 H NMR spectrum of the cationic hydride [(SBI)Zr(-H) 3 (Al i Bu 2 ) 2 ] + in benzene-d 6 solution, obtained by treating a 4 mM solution of (SBI)ZrCl 2 first with 5 equiv of HAl i Bu 2 and then with 1 equiv of [Ph 3 C

Conclusions

Experimental

The resulting solution was added to another vial containing 2.2 mg of [Ph3C][B(C6F5)4] (2.4 mol), resulting in a solution of the hydride cation. The slurry was added to a J-Young tube and 0.9 L of ClAliBu2 (5 mol) was added to the side of the tube without mixing. After measuring the UV-vis spectrum of each sample (Figure 3.9), enough HALiBu2 was added to reach 10 equiv. in relation to Zr.

Figure 3.14: 1 H NMR of 4.1 mM solution of [(SBI)Zr(-H) 3 (AlMe x i

Abstract

Introduction

Results

The additional signal at 3.59 ppm increases during the initial phase of the reaction and then disappears when all the hydride is consumed. The formation of propane, in addition to methane, was observed by 1H NMR (Figure 4.8) 11, which confirms the formation of nPrAlR2 during the process. The appearance of such a peak during the initial phase of the reaction when HAlMe2 is used as the aluminum hydride is consistent with the formation of the trialkylaluminum species in Figure 4.5.

Figure 4.1: 1 H NMR spectra of a toluene-d 8 solution of the {Al i Bu 2 }-complexed trihydride cation [(SBI)Zr(μ-H) 3 (Al i Bu 2 ) 2 ] + immediately, 49 and 99 min after warming to –30 ºC in the presence of 20 equiv of propene

Conclusions

Experimental

The tube was heated in a dry ice/acetone bath to 78 ºC and the contents mixed at this temperature. The NMR tube was heated to 78ºC in a dry ice/acetone bath and the contents mixed at this temperature. For this signal, as for another signal observed at 20.03 ppm, no obvious assignments are currently apparent.

APPENDIX A

Abstract
Introduction
Adduct Formation
Exchange Reaction
Conclusion
References

We can use the same derivation as for equation A.9, except that [AX] is now replaced by [B]. Therefore, a plot of the left-hand side of equation A.18 against [Zr]TOT/[Al2Me6] should give a straight line through the origin with a slope of 1/K, which should thus be independent of [Zr]TOT . The equilibrium constant for two species in rapid equilibrium can be obtained by monitoring the change in the average NMR chemical shift of the mixture as a compound is added to the solution.

APPENDIX B

Abstract
Introduction
Results and Discussion
Conclusions
Future Directions
Experimental
References

Hydroboration of the mixture with HB(C6F5)2 resulted in the disappearance of the vinyl resonances, but that was it at this point. Attempts to crystallize the pendant borate were hindered by the complex's high solubility in aromatic solvents. This site epimerization would be expected to result in more insertions from one side of the wedge, giving a more isotactic polymer.

Figure B.1: Complexes used as inspiration for current work

APPENDIX C X-ray Crystallographic Data

Data Collection

Structure Solution and Refinement

The weighted R-factor (wR) and goodness of fit (S) are based on F2, conventional R-factors (R) are based on F, with F set to zero for negative F2. All esds (except the esd at the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in the cell.

Table C.2: Atomic coordinates (x 10 4 ) and equivalent isotropic displacement parameters (Å 2 x 10 3 ) for SMB04 (CCDC 776421)

Structure solution and Refinement

The toluene solvent was constrained to be flat and the ADPs to simulate isotropic behavior. The difference Fourier map revealed the hydride positions as the six largest peaks near Zr. The positions of the six hydrides (H1H-H6H) were refined during least squares with temperature factors limited to 1.2 times the Ueq of the corresponding metal atom.