Natta-Ziegler catalyst 1400 INDEX

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

Heterogeneous catalysts are suspended in the solvent. The Ziegler catalyst TiCl A1(C2H5)3 affords polypropene with very low stereospecifity (compare Table 14). One criterion for the determination of stereospecifity is the isotacticity index, which is defined as the percentage of polymer that is insoluble in boiling heptane [10]. Natta achieved a substantial increase in stereoselectivity by using TiCls instead of TiCl4 [240]. 

Propylene and 4-methyl- 1-pentene were copolymerized by Colin et al. (1) using a Ziegler-Natta catalyst and the product characterized as having at least one fraction obtained by that had a block index greater than about 0.3 and up to about 1.0 with a polydispersity greater than 1.3. 

Metallocene catalysis is an alternative to the traditional Ziegler-Natta vanadium-based catalysis for commercial polyolefin production, e.g. the use of metallocene-catalyzed ethylene alpha-olefin copolymers as viscosity index modifiers for lubricating oil compositions [23]. The catalyst is an activated metallocene transition metal, usually Ti, Zr or Hf, attached to one or two cyclopentadienyl rings and typically activated by methylaluminoxane. Metallocene catalysis achieves more stereo-regularity and also enables incorporation of higher alpha-olefins and/or other monomers into the polymer backbone. In addition, the low catalyst concentration does not require a cleanup step to remove ash. 

Equation 5.1 is applicable to polyolefins made with single-site catalysts, such as metallocenes, and predicts a polydispersity index of 2.0. It is discussed later how this equation can also be used to model the CLD of polyolefins made with multiple-site catalysts, such as heterogeneous Ziegler-Natta and Phillips catalysts. Despite its simplicity, this equation can be used to predict the complete CLD of single-site polyolefins instantaneously using an easy-to-estimate parameter, t. 

One polydispersity index which measures the width of the distribution function is the ratio /. For many addition polymers, this polydispersity index lies in the range 1 5-2 0 but other polymers, especially those prepared by Ziegler-Natta catalysts, may have a polydispersity index that is an order of magnitude greater. The polydispersity index is 1-0 for a monodisperse (>ol3mier. Anionic polymerizations can lead to polydispersity indices close to unity. 

Ethylene-propylene copolymers can be made with narrower polydispersity index and chemical composition distribution than those from conventional Ziegler-Natta catalysts. Similiarly, LLDPE s prepared with Cp ZrClj/MAO catalyst have lower melt temperatures, for the same level of incorporation of 1 -butene, than copolymers made from a heterogenous TiCl - triethylaluminum system. This indicates that the metallocene derived LLDPE has a more uniform distribution of 1 -butene in the copolymer, i.e., it has a narrower chemical composition distribution. 

Zeichner and coworkers [33,34] developed a measure of the breadth of the molecular weight distribution that is based on the curves of storage and loss moduli versus frequency. Based on data for a series of polypropylenes made by Ziegler-Natta catalysts and degraded by random chain scission, they found that the polydispersity index, i.e., the ratio Af /M , was related to the crossover modulus, G, as shown in Eq. 5.8... 

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