| Although the Phillips catalyst for ethylene polymerization has been used in industry and intensively studied for more than 50 years, the nature of its active sites remains unconfirmed. Several possibilities have been proposed, including tetravalent, cationic chromium alkyl sites and neutral alkylidenes. In this project, we aim to develop a uniform, silica-supported cationic organochromium(IV) complex using a well-defined borane co-catalyst, to correlate its structure with its polymerization performance, and to investigate the nature of the interaction between silica and supported chromium(IV) complexes, especially those containing an alkylidene ligand.;This thesis is divided into three sections. The first focuses on the reaction of B(C6F5)3 with the surface of anhydrous but fully hydroxylated silica (thermally pretreated at 200 °C) and a partially dehydroxylated silica (thermally pretreated at 500 °C). B(C6F5)3 reacts with silica when a trace of H2O is present. The water can either be added intentionally or formed in situ by borane-induced dehydration of adjacent silanol pairs. The adduct (C6F5)3B*OH2 hydrolyzes to afford C6F5H and (C6F5) 2BOH. The latter reacts with a surface hydroxyl group of silica to yield ≡SiOB(C 6F5)2 and regenerate H2O. When excess B(C6F5)3 is present, the resulting grafted borane appears to be completely dry, due to the eventual formation of [(C 6F5)2B]2O. The grafted borane was characterized by solid-state 11B and 19F MAS NMR, IR, elemental analysis, and C5H5N-TPD. Its ability to activate organometallic complexes for ethylene polymerization was explored.;The second section deals with the reaction between B(C6F 5)3 and silica-supported tris(neopentyl)chromium(IV) (≡SiOCrNp3). In this reaction, a methyl group is transferred to the borane following alpha-methyl elimination from a neopentyl ligand. The resulting silica-supported ion pair, analyzed by IR as well as solid-state 11B, and 19F MAS NMR techniques, is suggested to be [≡SiOCrNp2]+[MeB(C 6F5)3]-. The borate anion can be extracted from the solid by ion exchange, using either Ph3CCl or MePPh3Br. While the activity of the cationic catalyst is comparable to that of neutral ≡SiOCrNp3, the polydispersity of the resulting polyethylene is much broader.;The third section contains a structural analysis of silica-supported bis(neopentyl)chromium (≡SiO)2CrNp2 and its thermolysis product, (≡SiO)2Cr=CHC(CH3) 3), using Cr K-edge EXAFS. The majority of sites in the starting material are five-coordinate via interaction with a siloxane ligand derived from the silica surface, while all of the product is four-coordinate with a siloxane ligand. |
