Polypropylene isotacticity index

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],... 

Fig. 4. Dependence of the relative polymerization rate (R /R ) and of the polypropylene isotactic index (I.I.) on butanol additions in the propylene polymerization catalyzed by TiCl3—AA (Stauffer)/ AlEt2Cl66). Al/Ti = 3.3 [M] = 0.40 mol l-1 T = 50 °C. Butanol was added into running polymerization. Rates and I.I. s refer to the periods immediately after the butanol additions...

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. 

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

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 ... 

Unlike polyethylene, which crystallizes in the planar zigzag form, isotactic polypropylene crgtaUizes in a helical form because of the presence of the methyl groups on the chain. Commercial polymers are about 90 to 95 percent isotactic. The amount of isotac-ticity present in the chain will influence the properties. As the amount of isotactic material (often quantified by an isotactic index) increases, the amount of crystallinity will also increase, resulting in increased modulus, softening poinL and hardness. 

Titanium content was estimated quantitatively by U.V. spectrophotometric method [12]. Magnesium and chlorine contents were analyzed titrimetrically [12]. BET surface area of solid samples was measured on a Carlo Erba Sorptomatic instrument. Polypropylene samples were extracted with boiling heptane in a Soxhlet apparatus for determination of isotactic index. Isotactic index reported for each sample is the weight percentage of heptane insoluble polypropylene. 

In addition to polymerization time, the concentration of PES also influences the stereospecificity of the catalyst system (Table 4, Figure 3). The isotacticity of polypropylene obtained in absence of PES is found to be 72%. The addition of PES improves the isotactic index from 72 to 87%. Increase of Si/Al ratio up to 0.2 depresses atactic polypropylene formation by 70% and isotactic polypropylene by... 

Isotacticity level has a stronger effect on crystallinity and related properties of polypropylene. PP grades with an isotactic index (II) of 0.96-0.98 are obtainable which can offer a 20% increase in flexural modulus in comparison to standard grades (II = 0.93-0.95). This enhanced stereoregularity has very interesting consequences on the properties of bio-rientated PP films (BOPP). A 25% increase in flexural stiffness and 25%... 

Early commercial catalysts were supplied to a strict specificatioa The average particle size was usually in the range 10-100 pm with an activity exceeding 1000 g polypropylene per gram of catalyst and an isotactic index exceeding 88%. Continuous, rather than batch, operation was possible with the new, externally formed eatalyst but the efficiency was still low and the polypropylene still contained unacceptably high levels of both catalyst residues and atactic polymer. 

The magnesium supports used for polyethylene catalysts could be modified for use in polypropylene production, and magnesium chloride proved to be the most suitable when used with a Lewis base election donor. Milled magnesium chloride was known to have the same layer stracmre as a- and y-titanium trichloride with the quadrivalent titanium ion (0.068 nm diameter), being about the same size as the divalent magnesium ion (0.066 nm diameter). In 1968, Montedison and Mitsui Petrochemical Industries both disclosed the production of a highly active, very steieospecific catalyst that contained about 3% titanium on a magnesium chloride support, promoted with a Lewis base, such as ethyl benzoate. The polymer produced contained less than 1 ppm titanium with an isotactic index of more than 90%, which was improvement on the product made with previous catalysts. 

As demand for polyethylene has increased, there has been a continuous effort to improve catalyst activity. This was important to achieve high productivity at relatively short residence times. These properties have avoided the need to remove catalyst residues from the product It was also important for polypropylene to have a high isotactic index to ehminate the need for equipment for the extraction of atactic polymer. 

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