Constrained-geometry initiators

The anra-cyclopentadienyl-amido initiators (CpA), often referred to as constrained-geometry initiators, retain one cyclopentadienyl ring of the metallocenes, but replace the other ring with a nitrogen substituent that coordinates with the metal center, usually a group 4 metal... 

Lindenberg K, Romero AH, Sancho JM. (1998) Nonclassical kinetics in constrained geometries Initial distribution effects. International Journal of Bifurcation and Chaos 8 853-868. 

A rare example of isospecific 3,4-polymerization of isoprene mediated by a constrained-geometry rare-earth metal initiator was reported by Z. Hou [270]. Binuclear silyl-linked cyclopentadienyl phosphido lanthanide dialkyl complexes were synthesized in good yields and activated with an equimolar amount of [Ph3C] [B(C6Fs)4] (Scheme 68). Cationic alkyl species were proposed as intermediates and an activation scenario was presented based on DFT calculations [270]. 

Although lanthanocene catalysts initially developed for aminoalkene hydroami-nation are highly sensitive and not readily available, their catalytic activity remains unsurpassed as of now and only a few postmetallocene rare-earth metal complexes can reach comparable levels of catalytic efficiency. Besides constrained-geometry (Fig. 15) [118, 119] and other half-sandwich [102, 120, 121] rare-earth metal complexes, a large number of cyclopentadienyl-free catalyst systems have been... 

We will initially discuss SAW in random geometries, and then SAW in constrained environments. In the former case, many of the studies have used methods other than series analysis, typically Monte Carlo, and so our discussion of this topic will be briefer than that of SAW in constrained geometries. 

Taking into account that thermal conversion of diamido to imido species is not accessible to BnjCyclam zirconium complexes, the cyclization reaction is likely to occur by 1,2-insertion of the olefin moiety in the Zr-N bond (Scheme 25.4). This pathway was initially proposed by Tobin Marks for lanthanocene and constrained geometry zirconium complexes and was more recently also suggested for other group 4 metal catalysts [15, 16]. The activation of the olefin toward insertion may... 

Alexandre et al. [98] reported the preparation of PE/clay nanocomposite using Ti-based constrained geometry catalyst (CGC) catalyst. The MMT and hectorite were initially treated with TMA-depleted MAO and then intercalated with the CGC catalyst. Finally, addition of ethylene resulted in the formation of high molecular weight and exfoliated PE/clay nanocomposites. The vinyl groups were chemically linked to the sDicate surface when copolymerized was carried out with ethylene inside the clay galleries using the nickel catalyst (Fig. 9). This method not only results in the effective exfoliation of the layered silicate but also in polyethylene chains that are chemically bonded to silicate surface. 

Rg. 9.8. Mixed-metallocene polymerization of ethylene in a semibatch reactor branching (constrained geometry) catalyst CGC-Ti linear catalyst Et[lnd]2ZrCl2. Reactor and kinetic data initial concentration CGC-Ti 8 x 10 kmol m initial concentration Et[lnd]2ZrCl2 3.2 X 10 kmol m monomer molar feed... 

All the calculated structures and their relative energies on the potential energy surface of the initial B1(A7) + H2 and B11(A3) + H2 reactions are shown in Figures 6 and 7 (where A7 will be called Bl), and in Figures 8 and 9 (where A3 will be called Bll), respectively. Below we will mainly discuss the geometries and energetics calculated with the Zr-P bonds constrained at... 

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