Abstract
C-13-Labeled ethylene polymerization (pre)-catalysts [kappa(2)-(anisyl)(2)P,O]Pd((CH3)-C-13)(L) (1-(CH3)-C-13-L) (L = pyridine, dmso) based on di(2-anisyl)phosphine benzenesulfonate were used to assess the degree of incorporation of (CH3)-C-13 groups into the formed polyethylenes. Polymerizations of variable reaction time reveal that ca. 60-85% of the C-13-label is found in the polymer after already 1 min polymerization time, which provides evidence that the pre-equilibration between the catalyst precursor 1-(CH3)-C-13-L and the active species 1-(CH3)-C-13-(ethylene) is fast with respect to chain growth. The fraction of 1-(CH3)-C-13-L that initiates chain growth is likely higher than the 60-85% determined from the C-13-labeled polymer chain ends since (a) chain walking results in in-chain incorporation of the C-13-label, (b) irreversible catalyst deactivation by formation of saturated (and partially volatile) alkanes diminishes the amount of (CH3)-C-13 groups incorporated into the polymer, and (c) palladium-bound (CH3)-C-13 groups, and more general palladium-bound alkyl(polymeryl) chains, partially transfer to phosphorus by reductive elimination. NMR and ESI-MS analyses of thermolysis reactions of 1-(CH3)-C-13-L provide evidence that a mixture of phosphonium salts ((CH3)-C-13)(x)P+(aryl)(4-x) (2-7) is formed in the absence of ethylene. In addition, isolation and characterization of the mixed bis(chelate) palladium complex [kappa 2-(anisyl)(2)P,O]Pd[kappa(2)-(anisyl)((CH3)-C-13)P,O] (11) by NMR and X-ray diffraction analyses from these mixtures indicate that oxidative addition of phosphonium salts to palladium(0) species is also operative. The scrambling of palladium-bound carbyls and phosphorus-bound aryls is also relevant under NMR, as well as preparative reactor polymerization conditions exemplified by the X-ray diffraction analysis of [kappa(2)-(anisyl)(2)P,O]Pd[kappa(2)-(anisyl)(CH2CH3)P,O] (12) and [kappa(2)-(anisyl)(2)P,O]Pd-[kappa(2)-(anisyl)((CH2)(3)CH3)P,O] (13) isolated from pressure reactor polymerization experiments. In addition, ESI-MS analyses of reactor polymerization filtrates indicate the presence of (odd- and even-numbered alkyl)(anisyl)phosphine sulfonates (14) and their respective phosphine oxides (Is). Furthermore, 2-(vinyl)anisole was detected in NMR tube and reactor polymerizations, which results from ethylene insertion into a palladium-anisyl bond and concomitant beta-hydride elimination. In addition to these scrambling reactions, formation of alkanes or fully saturated polymer chains, bis(chelate)palladium complexes [kappa(2)-P,O](2)Pd, and palladium black was identified as an irreversible catalyst deactivation pathway. This deactivation proceeds by reaction of palladium alkyl complexes with palladium hydride complexes [kappa(2)-P,O]Pd(H)(L) or by reaction with the free ligand H[P,O] generated by reductive elimination from [kappa(2)-P,O]Pd(H)(L). The model hydride complex 1-H-(PBu3)-Bu-t has been synthesized in order to establish whether 1-H-(PBu3)-Bu-t or H[P,O] is responsible for the irreversible catalyst deactivation.
However, upon reaction with 1-(CH3)-C-(13)-L or 1-CH2CH3-PPh3, both 1-H-(PBu3)-Bu-t and H[P,O] result in formation of methane or ethane, even though H[P,O] reacts faster than 1-H-(PBu3)-Bu-t. DFT calculations show that reductive elimination to form H[P,O] and (alkyl)[P,O] from 1-H/(allkyl)-(PBu3)-Bu-t is kinetically accessible, as is the oxidative readdition of the P-H bond of H[P,O] and the P-anisyl bond of (alkyl)[P,O] to [Pd((PBu3)-Bu-t)(2)]. These calculations also indicate that for a reaction sequence comprising reductive elimination of H[P,O] from 1-H-(PBu3)-Bu-t and reaction of H[P,O] with 1-CH3-(PBu3)-Bu-t, 1-CH3-dmso, or 1-CH2CH3-PPh3 to form methane or ethane, the rate-limiting step is reductive elimination of H[P,O] with a barrier of 124 kJ mol(-1). However, a second reaction coordinate was found for the reaction of 1-H-(PBu3)-Bu-t with 1-CH3-(PBu3)-Bu-t or 1-CH3-dmso, which evolves into bimetallic transition-state geometries with a nearly linear H-(CH3)-Pd alignment and which exhibits a barrier of 131 or 95 kJ mol(-1) for the formation of methane.