Chromocene catalysts for ethylene polymerization: Scope of the polymerization

Abstract

Chromocene deposited on silica supports of high surface area forms a highly active catalyst for polymerization of ethylene. Polymerization is believed to occur by a coordinated anionic mechanism previously outlined. The catalyst formation step liberates cyclopentadiene and leads to a new divalent chromium species containing a cyclopentadienyl ligand. The catalyst has a very high chain‐transfer response to hydrogen which permits facile preparation of a full range of molecular weights. Catalyst activity increases with an increase in silica dehydration temperature, chromium content on silica, and ethylene reaction pressure. The temperature‐activity profile is characterized by a maximum near 60°C, presumably caused by a deactivation mechanism involving silica hydroxyl groups. A value of 72 was estimated for the ethylene–propylene reactivity ratio (r1). Linear, highly saturated polymers are normally prepared below 100°C. By contrast with other commercial polyethylenes, the chromocene catalyst produces polyethylenes of relatively narrow molecular weight distribution. Above 100°C, unsaturated, branched polymers or oligomers are formed by a simultaneous polymerization–isomerization process.

[(TptBu, Me)CrR]: A New Class of Mononuclear, Coordinatively Unsaturated Chromium(II) Alkyls with cis‐Divacant Octahedral Structure

“…For catalysts carrying a cyclopentadienyl group on the chromium (e.g. the "Union Carbide catalyst", Cp,Cr on Si0,), [2] we have built a case based on the reactivity of model compounds that chromium(II1) alkyls (A, R = alkyl) are responsible for the polymerization activity.r31 One of the arguments against catalytic activity in the lower oxidation state was '"] However, the extreme coordinative unsaturation of the hypothetical "Cp*CrR" fragment, combined with the limited steric protection offered by the pentamethylcyclopentadienyl ligand, resulted in the formation of metal-metal bonded dinuclear complexes. It might be argued that the role of the heterogeneous support of commercial catalysts includes site isolation of the catalytic centers and prevention of metal-metal bonding, which might adversely affect the reaction with olefins.…”

“…Synthesis of hydrotris(3-trrt-butyl-5-methylpyrat.olyl)borate (Tp'B",Me) complexes of chromium 0; or NO;) . [' I The coordination environment around chromium is close to trigonal bipyramidal, with N(5) of the Tp ligand and N(8) of the coordinated pyrazole serving as the axial ligands (N(8)-Cr [l)-N(5), 169.8 (2)"); the remaining ligands (N(I), N(3), Cl(1)) define the equatorial planc (sum of angles, 359.6"), although the N(l)-Cr(l)-N(3) angle is constrained to 101.4 (2)" by the nature of the Tp ligand. In a notable deviation from established structural principles for trigonal bipyramidal metal complexes," 'I thc most electronegative substituent of 1 (i.e., Cl(1)) occupies an equatorial position, and the axial Cr-N bonds are…”

Metallocenes

“…Other nontraditional bridges include R 2 E (E = Ge, Sn) (35), as well as an alkyl boryl (36), and one with nitrogen that can bind directly to the transition metal (37). The bridge shown in 6 employs a Lewis acid-Lewis base interaction between P and B atoms; the metallocene gives highly isotactic polypropylene (iPP) at low temperatures, indicating the bridge can be robust under polymerization conditions (38,39 (44), which is proposed (45) to have a half-sandwich chromium center as shown in equation 7. This catalyst exhibits facile chain transfer to H 2 , poor comonomer incorporation, and polymers of fairly broad MWD.…”

Catalytic Olefin Polymerization

“…increases by 2-3 orders of magnitude if the same compounds are supported on Si0 2 or Al 2 0 3 (ef Table 2 [21,23]). The course of reaction between organometallic compounds and support depends on the ratio between the reagents and on the thermal activation of the support.…”