Olefin polymerizations with

A key question remains how is the olefin formed in the overall process Molecular tantalum complexes are known to undergo facile a- and transfer processes, leading to tantalumalkylidene and tantalum tt-olefin complexes, respectively (mechanism 9, Scheme 29) [98]. Moreover, olefin polymerization with tantalum complexes belongs to the rare case in which the Green-Rooney mechanism seems to operate (Eq. 10, Scheme 29) [102]. Finally, intramolecular H-transfer between perhydrocarbyl ligands has been exemplified (Eq. 11, Scheme 29) [103,104]. 

Moreover, the molecular catalysts have provided systematic opportunities to study the mechanisms of the initiation, propagation, and termination steps of coordination polymerization and the mechanisms of stereospecific polymerization. This has significantly contributed to advances in the rational design of catalysts for the controlled (co)polymerization of olefinic monomers. Altogether, the development of high performance molecular catalysts has made a dramatic impact on polymer synthesis and catalysis chemistry. There is thus great interest in the development of new molecular catalysts for olefin polymerization with a view to achieving unique catalysis and distinctive polymer synthesis. 

Scheme 4 Mechanism of chain growth for a all Pd(II) polymerizations and ethylene polymerizations with Ni(II), and b a-olefin polymerizations with Ni(II). Specific kinetic data shown for Ni catalyst 1.15b [63]...

Table 6 Olefin polymerizations with Ni(II) cyclic a-diimine catalysts3...

Figure 12. Scheme of stereospecific 1-olefins polymerization with generic C2 and Cs symmetric metallocenes. In the framework of a regular chain migratory mechanism, the C2 and Cs symmetric catalysts lead to iso- and syndiotactic polymers, respectively. In fact, multiple insertions of the same enantioface occur with C2 symmetric metallocenes, while multiple insertions of alternating enantiofaces occur with Cs metallocenes. 

Fig. 3.49. Types of product arising from olefin polymerization with metathesis catalysts.

Unique and versatile olefin polymerizations with group 4 bis(phenoxy-imine) complexes have been disclosed by scientists at Mitsui. " In a series of Ti complexes with fluorinated iV-phenyl groups (Figure 10, complexes FlO-7-FlO-14), Mitani et al. observed a substantial difference in ethylene polymerizations between complexes having... 

J.-C. Liu, Olefin polymerization with pyridine moiety-containing single-site catalysts, US Patent 7 091272, assigned to Equistar Chemicals, LP (Houston, TX), August 15,2006. 

The published values of related to olefin polymerization with soluble and heterogeneous catalysts are in the range from several seconds to hours 7). It should be noted that the value of is strongly dependent upon the polymerization conditions since the rates of chain-terminating reactions Rt are functions of the temperature and concentrations of chain-terminating reagents. In a living polymerization the value of is infinite. 

In the preceding chapter it has been shown that the DFT methods currently available can be used to reproduce relative trends in both reactivities and transition-metal NMR chemical shifts. Thus, NMR/reactivity correlations can be modeled theoretically, at least when relative reactivities are reflected in relative energies on the potential energy surfaces (activation barriers, BDEs). It should in principle also be possible to predict new such correlations. This is done in the following, with the emphasis on olefin polymerization with vanadium-based catalysts. 

II. Mechanism of Olefin Polymerization with Polar Ligands. 61... 

The traditional catalysts for olefin polymerization were invented by Hogan et al. (their catalyst is the Phillips catalyst) (13-15) in 1952 and Ziegler et al. (16, 17) and Natta and Corradini (18, 19) in 1953. Commercialization of olefin polymerization with these catalysts provided the first linear polyethylene and the first isotactic polypropylene. Before these innovations,... 

The cocatalyst has various functions. The primary role of MAO as a cocatalyst for olefin polymerization with metallocenes is alkylation of the transition metal and the production of cation-like alkyl complexes of the type Cp2MR+ as catalytically active species (91). Indirect evidence that MAO generates metallocene cations has been furnished by the described perfluorophenyl-borates and by model systems (92, 93). Only a few direct spectroscopic studies of the reactions in the system CP2MCI2/MAO have been reported (94). The direct elucidation of the structure and of the function of MAO is hindered by the presence of multiple equilibria such as disproportionation reactions between oligomeric MAO chains. Moreover, some unreacted trimethylaluminum always remains bound to the MAO and markedly influences the catalyst performance (77, 95, 96). The reactions between MAO and zirconocenes are summarized in Fig. 8. 

Brintzinger HH, Fischer D, Miilhaupt R, Rieger B, Waymouth RM, Stereospecific Olefin Polymerization with Chiral Metallocene Catalysts, Angew Chem Int Ed Engl 34, 1143 (1995), and refs Therein... 

H. Kawamura-Kuribayashi, N. Koga, and K. Morokuma, J. Am. Chem. Soc., 114, 8687 (1992). An Ab Initio MO and MM Study of Homogeneous Olefin Polymerization with Silylene-Bridged Zirconocene Catalyst and Its Regio- and Stereoselectivity. 

A four-step mechanism for olefin polymerization with the participation of a carbene and metalcyclobutane complexes has been proposed to describe both a-olefm... 

Values in the literature relate to olefin polymerization with heterogeneous titanium based catalysts where the necessary information has been acquired, and the data are given in Table 9. It would not appear to... 

This olefin polymerizes with TiCl4/Al(i-Bu)3 (Al/Ti = 1/2) to form low molecular weight polymers [185]. Rates are first order in monomer concentration and from the initial values the apparent propagation rate coefficient is ca. 6 x 10 1 mole sec at 50°C, the activation energy being 9.5 kcal mole . This is very similar to the rates observed with propene and butene-1, and suggests that fep has a comparable magnitude. 

Studies on ct-olefin polymerization with Cp-amido titanium complexes have been performed. 

The thermodynamics of the reaction of the ethylene molecule with the Cp2TiClMe/AlClMe2 system, as a model for olefin polymerization with homogeneous Ziegler-Natta catalysts, has been investigated using DFT calculations.1237... 

Olefin Polymerizations with Group IV Metal Catalysts... 

Figure 4 (a) Modified Cossee mechanism for olefin polymerizations with group 4 transition metals (b) modified mechanism in the presence of an anionic counterion. 

---