Polypropylenes isoblock

Isoblock polypropylene is characterised by the appearance of stereosequences of m diads bridged by pairs of r diads [20,21], Such polypropylene, containing a small number of long blocks, is usually referred to as isotactic polypropylene. Note that the polymer chains in this case contain helix sequences of the same handedness (right or left) in the chain, which is due to the same relative configuration of tertiary carbon atoms in these sequences. Stereoblock polypropylene, which is also called block-isotactic polypropylene,... 

During the last decade, a variety of new catalysts have been presented for the stereospecific polymerisation of a-olefins, based on non-bridged metallocene or stereorigid ansa-metallocene as the procatalyst and a methylaluminoxane activator [29,30,37,105-107,112-114,116-135], Apart from isotactic [118,119,124, 131,132] and syndiotactic [23,118,124,133] polypropylenes and other poly(a-olefin)s [121], hemiisotactic [112,121,124], isoblock [131,132,134], syndioiso-block (stereocopolymer) [127], stereoblock isotactic [135] and stereoblock isotactic atactic [116,128,129] polypropylenes have been obtained using these new catalysts. 

In the case of propylene polymerisation with class III catalysts based on rac.-dimethylsilylenebis[l-(3-methylcyclopentadienyl)]zirconium dichloride [rac.-Me2Si(MeCp)2ZrCl2] (Table 3.1) or rac.-dimethylsilylenebis[l-(3-methyl-cyclopentadienyl)]hafnium dichloride [rac.-Me2Si(MeCp)2HfCl2] as a procatalyst, isoblock polypropylene is obtained [132]. 

Polypropylenes with low amounts of ohgomers atactic, isotactic, isoblock, stereoblock, and syndiotactic... 

Metallocenes with diastereotopic sites for monomer coordination show quite an interesting polymerization behavior introduction of a methyl group in position 3 of the cyclopentadienyl ring in (21) disturbs the stereospecificity at this site, giving rise to hemiisotactic polypropylene [53], while a f-butyl group at the same position inverts the preferred mode of coordination (22) thus an isotactic polymer is generated [54]. Metallocene (8) has one nonspecific and one stereo-specific site, too at low temperature, hemiisotactic polypropylene is produced while at high temperatures site isomerization without insertion facilitates the formation of isoblock polypropylene. 

Chiral titanocenes, zirconocenes, and hafnocenes in combination with methylalu-minoxane [A1(CH3)—0] , can lead to highly isotactic propylene. Nonchiral metallocenes like (Cp)2ZrCl2 or other similar compounds produce only pure atactic polypropylenes. Molecular mass of 590,000 for atactic polypropylenes can be achieved by low polymerization r. The activities of these hydrocarbon soluble catalysts are extremely high. Different structures of polypropylenes are obtained when the rr-bonded ligand of the transition metal is varied (Fig. 1). With no other catalyst can atactic, isotactic, stereoblock, isoblock, and syndiotactic polypropylene of such purity be produced. 

The propene units are bonded mainly head to tail. A small part (0.7%) is incorporated in 1,3 enchainment . MW of the polypropylene depends strongly on reaction T. At T between -20 and 60°C, MW range from 300,000 to 1500 . Hafnocenes give polypropylenes with a higher MW °. Stereoblock and isoblock polypropylenes were obtained by bis(neomenthyl) ZrCl2 or dimethyl silyl(cyclopentadienylindenyl)ZrCl2. 

By this it can be demonstrated in which way mo-difi-cations of the molecular structure of the metallocenes are reflected in the polymer microstructure Shortening of the bridge leads to a new microstructure the "isoblock" polypropylene. 

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