Highly Isotactic PPs

The Tj, and the isospecificity obtained with these FI catalysts represent some of the highest values for iPPs ever synthesized. The isospecific propylene polymerization proceeds via a 1,2-insertion with a site-control mechanism. 

The production of highly isotactic PPs with Zr- and Hf-FI catalysts//-Bu3Al/ Ph3CB(C6F5)4 (phenoxy-amine complexes site-controlled polymerization with 1,2-insertion) is in sharp contrast to that of highly syndiotactic PPs with Ti-FI cata-lysts/MAO (phenoxy-imine complexes chain-end controlled polymerization with 2,1-insertion), which will be described later [64]. 

6 Stereo- and Regioirregular High Molecular Weight Poly (Higher (XrOlefin)s 

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In general, Group 4 benzamidinates show poor activities as olefin polymerization catalysts.158-162 However, bis(benzamidinate) complex (52) affords isotactic PP (>95% mmmm) at >7 atm propylene pressure 163 at ambient pressure atactic PP is produced.164 An unsymmetrical tris (benzamidinate) zirconium complex has also been shown to afford highly isotactic PP.165... 

Looking at the structure of these crack tip plastic zones in more detail, it is found that the individual crazes are less straight compared to the low temperature crazes (Fig. 21). This indicates a more pronounced influence of the crystalline microstructure on craze formation. Figure 21a and b demonstrate for fine spherulitic, highly isotactic PP the interaction between the crazes and the microstructural features. Most of the... 

In addition to the described role of shear bands in crack initiation, the influence of spherulitic boundaries on crack propagation has to be considered, especially in coarse spherulitic, highly isotactic PP. Cracks which may have been formed at a particular point of a shear band according to the mechanism described above can leave their shear plane with further growth in order to move into an adjacent spherulite boundary (Fig. 36, Point I). This is especially probable when the boundaries are cut by the shear plane under a small angle. The selection of the particular crack path is determined by the partial fracture mechanical properties of the shear band and the spherulite boundary, respectively, as well as by the local geometrical conditions... 

The schematic layout of an iPP production plant based on violet TiCls catalysts in the 1960s is shown in Fig. 10 [11]. Notably, a large part of the plant was for operations other than polymerization, such as catalyst alcoholysis and neutralization and separation of the highly isotactic PP part from a less tactic part by filtration. The latter made it necessary to work in a low-boiling aliphatic hydrocarbon diluent, because the less tactic PP fraction is partly insoluble in liquid propene. 

Fig. 14 Three-site model proposed by Busico et al. [63] (reproduced frran [75]). Active site models relevant to the productirai of (a) syndiotactic (or atactic), (b) isotactoid (or isotactic), and (c) highly isotactic PP. A growing chain and propylene monomer occupy the chained squares. M = Ti, Mg, or Al L = Cl, donor, or alkylaluminum moiety [64]...

Kaminsky et al. [16] reported the production of highly isotactic PP from the activated zirconium-based metallocene [ethylene bis(4,5,6,7-tetrahydro-l-indenyl) zirconium(IV) dichloride] [ etheryZiCl, (Fig. 3.2) MAO serves as a co-catalyst. 

Eltex P Polypropylenes, PP also highly isotactic PP Solvay Cie SA... 

Kettles Coffee machines Toasters Deep fat fryers Irons Surface finish/gloss Stiffness (flexural modulus) Heat resistance (HDT) Temperature stability Processability (MFI) Colorability Heat stabilized homopolymers Mineral reinforced PP High isotacticity PP... 

Upper housing Canister Aesthetic surface High gloss Colorability Mineral reinforced PP High isotacticity PP... 

Highly isotactic PP crystallizes in the a-modification less stereoregular polypropylenes can also crystallize in the g-30f31)... 

Many metallocenes carrying alkyl substituted Cp ligands bridged with a Me2Si group were prepared, whose prochiral face selectivity would be superior to catalyst 4. It was found that the C2-symmetric catalyst 5 produces highly isotactic PP (T = 163°C) under a moderate polymerization condition (30°C) [70]. 

A different type of Ci symmetric catalyst (catalyst 15) produces highly isotactic PP (mmmm 99%) also under a mild polymerization condition (0°Q [85]. Their performance-relevant structural features are illustrated in Table 5. 

Highly isotactic PP can be produced with C2-sym-metric metallocene catalysts. In such isotactic propagation, monomer insertion is to take place on two active sites on the metal center alternately, and the stereochemistry on one site must be the same as that on the other site. Highly syndiotactic PP has been... 

In addition to living 1-hexene polymerization, Harney et have shown that 21b activated with [PhNMe2H][B(C6F5)4] in a stoichiometric ratio furnished highly isotactic PP ([mmmm = 0.71) in a living fashion Interestingly, activa-... 

By referring to the symmetries of 6.12 and 6.13, the effects of the catalyst structures on the microstructures of PP could be explained. Complex 6.12 gives highly isotactic PP, whereas with 6.13 highly syndiotactic PP is obtained. It is important to note that during chain propagation, the polymer chain and the alkene keep exchanging places between two adjacent coordination sites (compare 6.26 with 6.28). 

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