Isotacticity index

Figure 11.3. Variation of tensile yield stress with melt flow index (10 kg load at 190 C) and isotactic index. (After Crespi and Ranalli )...

Figure II.5. Variation of Vicat softening point (5 kg load) with isotactic index and melt flow index.

Figure 11.6. Variation of ASTM D746 brittle point with melt flow index and isotactic index (After...

The properties of commercial polypropylene vary widely according to the percentage of crystalline isotactic polymer and the degree of polymerization. Polypropylenes with a 99% isotactic index are currently produced. 

Some studies relative to the influence of Lewis bases,124-127 1,3-diethers in particular,125-127 and the MgCl2 support have also been recently reported. The dependence of the industrially relevant isotactic indexes on the chemical structure of the 1,3-diether donor has been rationalized in the assumption that donor coordination competes with Ti catalytic species formation and xylene-insoluble (highly isotactic) and xylene-soluble (poorly isotacUc) fractions are mainly obtained by polymerization on (100) and (110) cuts, respectively.127... 

Polypropylene is characterised by isotactic index which is percentage of polymer not dissolving in boiling, n-haptane Commercial polypropylenes are having isotactic index of 95-98 per cent. 

The interaction may not be quite as strong as in the case of 2,1 insertion discussed above, but there will always be a tendency of the growing chain to arrive at an isotactic stereochemistry when 1,2 insertion occurs. One example of chain-end control leading to isotactic polymer was reported by Ewen [13] using Cp2TiPh2/alumoxane as the catalyst. The stereoregularity increased with lower temperatures at -45 °C the isotactic index as measured on pentads amounted to 52 %. The polymer contains stereoblocks of isotactic polymer. At 25 °C the polymerisation gives almost random 1,2 insertion and an atactic polymer is formed. 

ISO 9113 1986 Plastics - Polypropylene (PP) and propylene-copolymer thermoplastics -Determination of isotactic index... 

Natta first used the in-situ prepared Ziegler catalyst (TiCl4 + Et3Al), which produced polymers with low stereoselectivity324 (isotacticity index = 30 10%). 

A new class of super highly active catalysts based on an MgCl2 support have been developed since the late 1970s. These catalysts are characterised by the appearance of surface-active species of practically one type. Their productivities are up to 2000 kg of polypropylene (with an isotactic index of 95-98 %) per gram Ti/h. In addition, lower Al/Ti ratios, i.e. ratios of the trialkylaluminium activator to titanium species, are required, and the usual reciprocal correlation between polypropylene isotactic index and yield is virtually absent [28,38],... 

Ans. CD3 and H cis to each other in the polymer chain that is, they have erythro configuration with respect to each other. With Natta catalyst a high isotacticity index is expected. An alternative configuration would be threo (CD3 and H turns to each other). The observed erythro configuration means alkene insertion is cis. 

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