Metallocene hydride complexes

The dual function of the precatalysts 4 opened the way to well-controlled block polymerization of ethylene and MMA (eq. (5)) [89, 90]. Homopolymerization of ethylene (Mn = 10000) and subsequent copolymerization with MAA (Mn 20000) yielded the desired linear AB block copolymers. Mono and bis(alkyl/silyl)-substituted flyover metallocene hydride complexes of type 8 gave the first well-controlled block copoymerization of higher a-olefins with polar monomers such as MMA or CL [91]. In contast to the rapid formation of polyethylene [92], the polymerization of 1-pentene and 1-hexene proceeded rather slowly. For example, AB block copolymers featuring poly( 1-pentene) blocks (M 14000, PDI = 1.41) and polar PMMA blocks (M 34000, PDI = 1.77) were obtained. Due to the bis-initiating action of samarocene(II) complexes (Scheme 4), type 13-15 precatalysts are capable of producing ABA block copolymers of type poly(MMA-co-ethylene-co-MMA), poly(CL-co-ethylene-co-CL), and poly(DTC-co-ethylene-co-DTC DTC = 2,2-dimethyltrimethylene carbonate) [90]. 

Allylic activation represents a possible evolution path for the intermediate which is obtained after unimolecular /3-hydrogen transfer to the metal, that is, the olefinic chain end coordinated to the metallocene hydride complex. The DPT calculations of Ziegler and co-workers on the geometrically costrained H2Si(Cp)(NH)Ti-R+ system and of Brintz-inger and Prosenc on the Cp2Zr(methallyl)+ system ... 

Hydride complexes of Group IV metallocenes have been implicated as catalysts and as important intermediates in olefin hydrogenahon and polymerization reactions [232, 233]. 

The polymerization of ethylene was also qualitahvely inveshgated by pulse injec-hons of ethylene into helium flowing over thorium (67) and uranium (86) metallocene hydrocarbyl complexes supported on 7-AI2O3.950 at 25 °C, both revealing similar achvihes [171, 173]. Supported thorium half-sandwich complexes 65 exhibited higher achvity than surface species, resulhng from coordinatively more saturated tris(cyclopentadienyl) and metallocene U/Th-alkyl/hydride complexes, that is, 77, 79, 82, 90 and 91 [171]. C CP MAS NMR spectra revealed no clear evidence of ethylene insertion into [Th-CHs] or [AL5-CH3] moiehes of material... 

Although metallocene silyl and hydride complexes are the active species, many researchers have sought to develop more convenient precatalysts. The original dimethyltitanocene system reported by Harrod is reasonably easy to prepare, but Corey140 and others have shown that in situ catalyst generation from metallocene dichlorides (Ti, Zr, and Hf) and "BuLi is both simpler and equally effective. However, Harrod and Dioumaev have shown... 

Manganese complex, 46 Metallocene derivatives, Group 4, 18 Metallocene hydride, 14 Metallocenes, Group 4, mechanism proposed by Tilley, 14... 

The bis(i7-pentamethylcyclopentadienyl)zirconium dinitrogen complex [tj-C5(CH3)5kZrj(N2)3 (22), in solution reversibly releases its dinitrogen in vacuo over several hours (Section III,E). The solutions remaining are said to consist mainly of [T7-C5(CH3)5][T7-C5(CH3)4CH2]ZrH (23) (33). Permethylzirconocene [tj-C5(CH3 )5l,Zr (24) may exist in these solutions in equilibrium with this hydride tautomer (as in the case of titanium). However, thus far permethylzirconocene has not been isolated as a discrete compound or otherwise characterized. It is very likely that permethylzirconocene is less stable and more prone to form complex metallocene hydrides than permethyltitanocene (16). 

S-Hydride transfer after a primary insertion (Scheme 16) produces vinylidene-terminated, n-pro-pyl-initiated PP and follows the rate law Rt = (/5-h)q[Q. when it is unimolecular (yS-hydride transfer to the metal), and Rt = /C( -h)i[D] [M], when it is bimolecular (either by concerted /1-hydride transfer to the coordinated monomer or by associative displacement). Both neutral group 3 and cationic group 4 metallocene alkyl complexes readily undergo spontaneous /S-hydride transfer to the corresponding metal hydrides and alkenes, as shown by Ber-... 

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