Polymer isotactic index

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. 

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. 

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

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. 

Yields Polymer yields of 40,000-60,000 kg/kg of supported catalyst are obtained. The polymer has a controlled particle size distribution and an isotactic index of 90-99%. 

Description The process, with a combination of the most advanced high-yield and high-stereospecificity catalyst, is a nonsolvent, nondeash-ing process. It eliminates atactic polymers and catalyst residue removal. The process can produce various grades of PP with outstanding product quality. Polymer yields of 20,000 to 100,000 kg/kg of supported catalyst are obtained, and the total isotactic index of polymer can reach 98% to 99%. 

The kinetic curve would then be the result of two curves, one representing the 1st order decay attributed to isospecific polymerization centers, and the other representing a stationary state attributed to the less stereospecific centers. This expression can be credited with taking into consideration a stationary state and, furthermore, it is in agreement with the inverse correlation between productivity and isotacticity of the polymer found experimentally. In fact, assuming Is to be the isotacticity of propylene produced by the isospecific centers, unstable with time, and IA the isotacticity of polypropylene produced by the less specific centers, stable with time, the total isotactic index IIt is given by the expression ... 

Fig. 28. Plot of isotactic index versus Rwt/P(. Rw stationary poymerization mte Pt polymer yield at time t (A, ) 1,5 atm. 70 °C modified cocatalyst (O, ) 7 atm. 70 °C modified cocatalyst ( , O) 7 atm. 60 °C modified cocatalyst (A) 7 atm. 80 °C modified cocatalyst ( ) 7 atm. 50 °C modified cocatalyst. 100>. By permission of Huthig Wepf Verlag.

For example, Soga et al. > found, by examining propylene polymerization in toluene in the temperature range 0-65 °C with an apparently soluble catalyst such as tetrabenzylzirconium, that the isotactic index and the polydispersity of the polymer increased as the polymerization temperature increased. Furthermore, the value of Q > 30 was much greater than that predicted for homogeneous active centres. From these results the authors concluded that the catalyst should be made up of small invisible colloid-type particles. The same interpretation is valid for the surprising results (Q 40) obtainedfor polyethylene, in Isopar solution at 200 °C, with the originally soluble tetrabenzylzirconium-water catalyst. 

It is well established that effective external donors not only increase the isotactic index of the polymer (the proportion of polymer insoluble in boiling heptane or in xylene at 25°C), but they can also increase in absolute terms the amount of isotactic polymer formed. This has been demonstrated by Kashiwa for the catalyst system MgCl2/TiCl4-AlEt3. An increase in the molecular... 

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