Constrained geometries

Other SFA studies complicate the picture. Chan and Horn [107] and Horn and Israelachvili [108] could explain anomalous viscosities in thin layers if the first layer or two of molecules were immobile and the remaining intervening liquid were of normal viscosity. Other inteipretations are possible and the hydrodynamics not clear, since as Granick points out [109] the measurements average over a wide range of surface separations, thus confusing the definition of a layer thickness. McKenna and co-workers [110] point out that compliance effects can introduce serious corrections in constrained geometry systems. 

As mentioned in Section 10.3.2, there has been recent interest in the use of the Dow constrained geometry catalyst system to produce linear low-density polyethylenes with enhanced properties based, particularly, on ethylene and oct-l-ene. 

In the next section we describe the basic models that have been used in simulations so far and summarize the Monte Carlo and molecular dynamics techniques that are used. Some principal results from the scaling analysis of EP are given in Sec. 3, and in Sec. 4 we focus on simulational results concerning various aspects of static properties the MWD of EP, the conformational properties of the chain molecules, and their behavior in constrained geometries. The fifth section concentrates on the specific properties of relaxation towards equilibrium in GM and LP as well as on the first numerical simulations of transport properties in such systems. The final section then concludes with summary and outlook on open problems. 

Structure 6. General structure of Constrained Geometry Catalysts. 

The sterically unencumbered catalyst active site allows the copolymerization of a wide variety of olefins with ethylene. Conventional heterogeneous Ziegler/Natta catalysts as well as most metallocene catalysts are much more reactive to ethylene than higher olefins. With constrained geometry catalysts, a-olefins such as propylene, butene, hexene, and octene are readily incorporated in large amounts. The kinetic reactivity ratio, rl, is approximately... 

Figure 5. Formation of cationic constrained geometry catalysts.

Scheme 1. Mechanism for formation of long-chain branching. Ti = active constrained geometry catalyst.

Constrained geometry chromium alkyls catalyzed the polymerization of ethylene however, the reaction was relatively slow, and elevated pressures (PC2H4 = 500 psi) were required to generate significant amounts of polymer. Not surprisingly then, no homopolymoization or copolymerization of a-olefins was observed. Instead, catalytic isomerization and dimerization of the alkyl-substituted olefins was found. 

OS 62] ]R 1] ]P 45] The impact of the choice of catalyst on catalyst plug-induced ethylene polymerization was analyzed [1]. A constrained-geometry catalyst (CGC) with a cyclopentadienyl moiety was about 3.6 times more active than a CGC-indenyl catalyst. 

Species concentration Capillary number Concentration of species a Computer aided design Concentration of species b Charge-coupled device Eluid specific heat Computational fluid dynamics Constrained-geometry catalyst Concentration at node i Concentration of species i Elux limiter Specific heat... 

In reactions similar to those observed with the Zr species 56 (Scheme 9), Waymouth used both chiral (e.g., 66) and achiral constrained geometry Ti complexes to desymmeterize the oxabicycloheptane 65 achieving good yields with modest enantioselectivity (Eq. 9) [29],... 

Boratabenzene analogues of commercially significant constrained geometry catalysts have also been investigated.49 For the MAO-activated copolymerization of ethylene/l-octene, the illustrated Ti(IV)-boratabenzene complex is about four times more active than the Zr(IV) complex. The level of 1-octene incorporation is significantly lower than for the corresponding Cp-derived catalysts, due perhaps to the greater steric demand of the amidoboratabenzene framework. 

The most successful examples of commercialized non-metallocene catalysts are the constrained geometry complexes such as (29) developed at Dow and Exxon.109-112 The open nature of the titanium center favors co-monomer uptake. Hence, o-oleflns such as propene, 1-butene, 1-hexene... 

Constrained geometry catalysts with alkoxide and phosphide donor arms have also been reported. The most active examples include complex (33), which polymerizes ethylene with an activity of 2,100 gmmol 1h 1 bar-1131 and (34), which exhibits an activity of 2,240 g mmol-1 h-1 bar-1 for the copolymerization of ethylene with 1-octene.132... 

The Dow corporation has recently developed constrained geometry addition polymerization catalysts (CGCT), typically Me2Si(C5Me4)(NBut)MCl2 (M = Ti, Zr, Hf) (141) activated with MAO. The homo-polymerization of a-olefins by CGCT afford atactic or somewhat syndiotactic (polypropylene rr 69%) polymers. The metal center of the catalyst opens the coordination sphere and enables the co-polymerization of ethylene to take place, not only with common monomers such as propylene, butene, hexene, and octene, but also with sterically hindered a-olefins such as styrene and 4-vinylcyclohexene [202]. 

Insite Not a process, but a range of constrained-geometry metallocene catalysts for polymerizing olefins. Developed by Dow Chemical. 

Substrates containing remote double bonds, however, are presumed to lead to preferential formation of 1,3-dienes due to coordination of the remote olefin to the metal (Equation (32)). The constrained geometry of the coordinated molecule (Figure 1) makes elimination of both Ha and Hb unfavorable thus, the weaker bond strength of the C-Hb leads to elimination of Hb and formation of the 1,3-diene as the major product. 

Constituent properties of bainite, 23 280 of martensite, 23 280-281 of pearlite, 23 280 of tempered martensite, 23 281-282 Constrained geometry catalysts, 16 81 20 193... 

Constrained-geometry catalysts for C2H4 polymerization 88 that are counterparts of well-known ansa-metallocene systems have been prepared and shown to be active, in combination with MAO, toward polymerization of ethylene the product is almost entirely polyethylene, with ca. 1% of 1-octene obtained. The titanium complex was found to be four times as active as the zirconium species.1... 

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