Detection of B(C6F5)4 anions in polyethylenes made with ionic metallocene catalysts

Abstract

Polymerization of olefins with Group IVB metallocenes (Ti, Zr, Hf) activated with ionic activators such as [dimethylanilinium] [B(C6F5)4] or neutral Lewis acids such as B(C6F5)3 is widely accepted to proceed via ionic catalysts, wherein the coordinatively unsaturated cation—with metal in the+4 oxidation state—is stabilized by a weakly coordinating anion [1, 2]. Evidence for the presence of the anion(s) following metallocene activation, but prior to polymerization, is abundant [3– 6]. Theoretical calculations regarding the Group IVB active site and the olefin insertion process are invariably based on cationic structures [7, 8]. However, the fate of the anions and cations throughout any given polymerization is not generally known. This is an important question because changes in anion structure, or loss of anion during polymerization could contribute to the activity profile, depressing polymerization over time, account for time and temperature dependent changes in polymer product structure (eg. molecular weight, comonomer response), and affect product properties. In this study, the fate of a key weakly coordinating anion—[B(C6F5)4]−—following commercial production of ethylene copolymers at high temperature and pressure has been determined independently by ToFSIMS [9] and LDMS [10]. The polymers selected have narrow melting distributions, narrow composition distributions, and narrow molecular weight distributions (MWD∼2.0). These three properties lend credence to the term “single site catalyst”, and suggest that the catalyst—and anions—persist, essentially unchanged, during the time each ethylene copolymer is produced. There are notable precedents to this work. Erker and coworkers have shown that reaction of (Cp)2Zr(1,3 butadiene) (Cp=C5H5 ) with one equivalent of B(C6F5)3 generates an active olefin polymerization catalyst. These workers used laser desorption mass spectrometry (LDMS), to identify oligomer anions [R-B(C6F5)3]−, where R is (C4H6)(C3H6)n(C3H7) (n∼5–26 in the example published), in the propylene polymerization product (toluene diluent, 50 ◦C and one atm) [11]. The mass spectral signature of these anions in the polypropylene product is striking and unique. On the other hand, it is well known that these ionic catalysts are readily poisoned by trace impurities. Aluminum alkyls are commonly added to “scavenge” such impurities and so protect the active site. Bochmann and Sarsfield have shown [12] that [B(C6F5)4]− anions can react with trimethylaluminum and AlBu3 to generate Al-C6F5 species. Such reactions can lead to new catalyst species with different anions or to loss of catalytic activity. In many cases, addition of larger quantities of these alkyls can have an adverse impact on metallocene catalyzed polymerizations. The polyethylenes analyzed are produced using metallocene activated with a [dimethylanilinium][tetrakis (perfluorophenyl)borate] salt. A list of the five commercial products used for this study is provided in Table I. All mass spectra were recorded using a PHI-Evans triple sector electrostatic analyzer time-of-flight mass spectrometer equipped with a pulsed liquid metal ion source (115In) for time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis and a nitrogen laser (λ= 337 nm) for laser desorption mass spectrometry (LDMS) analysis. ToF-SIMS were obtained operating the ion gun at 15 keV and 600 pA. For LDMS, 100– 300 laser shots were used to acquire the spectrum. Laser power was ∼1011 W/m2. Mass calibration was carried out using a variety of known standards. With this instrument configuration, both ToF-SIMS and LDMS can be obtained on the same sample. Analysis results from these two measurements can be compared directly. The resin samples were prepared using two different sample preparation protocols: 1) one polymer pellet (∼30 mg) was placed in a glass vial containing 10−5 m3 (10 ml) of toluene and the mixture heated to 90 ◦C. After extraction for 3 min, approximately 10−9 m3 (1μl) of the solution is deposited on a sample substrate (clean Si wafer). The solvent was evaporated in air and the wafer with extracted material was then inserted into the instrument for mass spectral analysis. 2) The pellet is cross-sectioned to expose the interior of the pellet. A slice of the pellet cross-section is inserted into the instrument and the freshly exposed surface analyzed directly.