Ziegler Natta catalysts isotactic/atactic polymer

Erom 1955—1975, the Ziegler-Natta catalyst (91), which is titanium trichloride used in combination with diethylaluminum chloride, was the catalyst system for propylene polymerization. However, its low activity, which is less than 1000 g polymer/g catalyst in most cases, and low selectivity (ca 90% to isotactic polymer) required polypropylene manufacturers to purify the reactor product by washing out spent catalyst residues and removing unwanted atactic polymer by solvent extraction. These operations added significantly to the cost of pre-1980 polypropylene. 

Following their introduction in 1953, Ziegler-Natta catalysts revolutionized the field of polymer chemistry because of two advantages the resultant polymers are linear, with practically no chain branching, and they are stereochemical ly controllable. Isotactic, syndiotactic, and atactic forms can all be produced, depending on the catalyst system used. 

Alkene polymerization can be carried out in a controlled manner using a Ziegler-Natta catalyst. Ziegler-Natta polymerization minimizes the amount of chain branching in the polymer and leads to stereoregular chains—either isotactic (substituents on the same side of the chain) or syndiotactic (substituents on alternate sides of the chain), rather than atactic (substituents randomly disposed). 

As polystyrene obtained by free radical polymerisation technique is atactic it is therefore non-crystalline. The isotactic polystyrene is obtained by the use of Ziegler-Natta catalysts and n-butyl lithium. Isotactic polystyrene is having a high crystalline Melting point of 250°C. It is transparent. It is more brittle than the atactic polymer. 

Free radical polymerisation is largely atactic while polymerisation with Ziegler-Natta catalysts can result in isotactic or syndiotactic polymers. 

The versatility of Ziegler-Natta catalysis is shown in the polymerization of butadiene. Polybutadiene may have either a 1,2 or 1,4 configuration. The 1,4 polymer has a double bond as part of the main chain and this can be atactic, isotactic, or syndiotactic. Thus many different polybutadienes can be made and all of them have been made with the aid of Ziegler-Natta catalysts. 

Polypropylene (PP) is a semicrystalline commodity thermoplastic produced by coordination addition polymerization of propylene monomer [197]. Most frequently, stereospecific Ziegler-Natta catalysts are used in industrial processes to produce highly stereospecific crystalline isotactic (iPP) and syndiotactic (sPP) polymer with a small portion of amorphous atactic PP as a side product. Polymerization of non-symmetrical propylene monomer yields three possible sequences however, the steric effect related to the methyl side group highly favors the head-to-tail sequence. The occurence of head-to-head and tail-to-tail sequences produces defects along the PP chain [198]. Presence of such defects affects the overall degree of crystallinity of PP. 

Huels and Mobil developed technologies473 to manufacture isotactic poly (1-butene), a less important and more expensive polymer by Ziegler-Natta catalysts. The Mobil process474 is carried out in excess 1-butene and produces highly isotactic polymer. The Huels technology475 is a slurry operation and requires removal of the atactic isomer. 

It may be of interest that isotactic polystyrene formed by styrene polymerisation with Ziegler Natta catalysts [13] did not appear to be a polymer that could exhibit significantly better usable properties compared with atactic polystyrene produced in free radical styrene polymerisation processes. 

The exact make-up and architecture of the polymer backbone will determine the ability of the polymer to crystallize. Figure 15.7 shows the different types of microstructure that can be obtained for a vinyl polymer. Isotactic and syndiotactic structures are considered stereospecific polymers, and their highly regular backbone structure allows them to crystallize. The atactic form is irregular and would produce an amorphous material. This nature of the polymer microstructure can be controlled by using different synthetic methods. As will be discussed below, the Ziegler-Natta catalysts are capable of controlling the microstructure to produce different types of stereospecific polymers. 

Ziegler-Natta catalysts form polymers without significant branching. Polymers can be isotactic, syndiotactic, or atactic depending on the catalyst. Polymers prepared from 1,3-dienes have the or Z configuration depending on the catalyst (30.4, 30.5). 

Tacticitv. The Ziegler-Natta catalyst chosen for the polymerization reaction is Et3Al/TiCl4, a catalyst that has been reported to give a 50/50 mixture of atactic and isotactic polymer when used to polymerize 1-octadecene or 1-propene. It was reasonable to expect that this catalyst would give similar results with the polymers prepared in this work. However, the isotactic content of our hydrocarbon polymers... 

Chain-growth polymerization exhibits a preference for head-to-tail addition. Branching affects the physical properties of the polymer because linear unbranched chains can pack together more closely than branched chains can. The substituents are on the same side of the carbon chain in an isotactic polymer, alternate on both sides of the chain in a syndiotactic polymer, and are randomly oriented in an atactic polymer. The structure of a polymer can be controlled with Ziegler-Natta catalysts. Natural rubber is a polymer of 2-methyl-l,3-butadiene. Synthetic rubbers have been made by polymerizing dienes other than isoprene. Heating mbber with sulfur to cross-link the chains is called vulcanization. 

Three different polypropylene (PP) modifications can be distinguished the atactic, the syndiotactic and the isotactic modification. The atactic modification is an amorphous polymer with a Tg(onset)-value of -21°C. The syndiotactic modification, made with a stereospecific homogeneous metallocene catalyst, is a semi-crystalline polymer (crystallinity about 25 %wt.) with a Tm-value of about 133°C [10]. The isotactic modification, made with a stereospecific heterogeneous Ziegler Natta catalyst is also a semicrystalline polymer (crystallinity about 50 %wt.) with a Tm-value of about 160°C and contains nearly always 2 %wt. - 5 %wt. of atactic material. 

Stereoregular polymers synthesized with (A and B) and without (C) the Ziegler-Natta catalyst A) isotactic polypropylene B) syndiotactic polypropylene C) atactic polypropylene... 

Isotactic PP (iso-PP), the most common commercial form of PP, is synthesized using Ziegler-Natta catalysts under controlled conditions of temperature and pressure. The placement of the methyl groups all on the same side of the polymer backbone provides a structure that readily yields a highly crystalline material, resulting in good solvent and heat resistance. Industrial processes are designed to minimize the production of atactic PP, a lower-value, noncrystalline, tacky byproduct that is used mainly in adhesives. 

Of the three stereoisomers, atactic polypropylene was the first to be synthesized. This is because it is the atactic structure that is obtained by conventional polymerization of the monomers when no optically active catalyst is used. It was only when the Ziegler-Natta catalysts were introduced in the 1950s that the production of stereoregular polymers became possible. It is interesting to note that polypropylene did not find any commercial use until the Ziegler-Natta catalysts became available. In fact, the atactic structure that was originally produced leads to a viscous liquid at room temperature of limited use. Isotactic polypropylene instead is one of the most important commercial polymers. 

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