Ziegler-Natta catalyst, polypropylene

Before the development of the Ziegler-Natta catalyst systems (Section 6 21) polymer ization of propene was not a reaction of much value The reason for this has a stereo chemical basis Consider a section of polypropylene... 

In the commercial process for the production of polypropylene by Ziegler-Natta catalysts, hydrogen is added to terminate the reaction, so neither of these reactions is pertinent to this process. 

The primary use of TiCl is as a catalyst for the polymerisa tion of hydrocarbons (125—129). In particular, the Ziegler-Natta catalysts used to produce stereoregular polymers of several olefins and dienes, eg, polypropylene, are based on a-TiCl and A1(C2H3)3. The mechanism of this reaction has been described (130). SuppHers of titanium trichloride iaclude Akso America and Phillips Petroleum ia the United States, and Mitsubishi ia Japan. 

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. 

Ziegler-Natta catalysts currently produce linear polyethylene (non-branched), stereoregular polypropylene, cis-polybutadiene, and other stereoregular polymers. 

The three different stereochemical forms of polypropylene all have somewhat different properties, and all can be made by using the right polymerization catalyst. Propylene polymerization using radical initiators does not work well, but polymerization using Ziegler-Natta catalysts allows preparation of isotactic, syndiotactic, and atactic polypropylene. 

Polypropylene grades manufactured using Ziegler-Natta catalysts are predominandy isotactic, with a comparatively broad molecular weight distribution (Afw/Mn 3.5). Ziegler-Natta catalyzed polypropylene comprises the majority of commodity grade resins. 

Manufacturing Polypropylene with Ziegler-Natta catalysts... 

We have a choice of four major polymerization techniques by which to manufacture polypropylene using Ziegler-Natta catalysts slurry, liquid propylene, solution, and gas phase. Regardless of which technique is employed, all polymerization plants must accomplish the same basic goals they must... 

Ziegler-Natta catalysts are used commercially for the production of stereoregular polymers, especially isotactic polypropylene and high-density linear polyethylene. The resultant polymers have number and weight average molecular weights (Mn and Mw, respectively) that are defined as... 

Both syndiotactic and isotactic polypropylene have been made using Ziegler-Natta catalysts. 

The world market for polypropylene is currently over 30 x 106 tons, and more than 80% of the global polymer is obtained with heterogeneous Ziegler-Natta catalysts. Over the years, these catalysts have evolved from simple TiCl3 crystals into the current systems based on MgCl2 as a support for TiCLt. 

One recent development in Ziegler Natta catalysts was in producing catalyst particles that expanded as the polymerization reaction occurred. In this polymer the catalyst remains dispersed throughout the polymer, retaining its activity. This led to the development of fluidized bed processes to make polyethylene and polypropylene in which a sphere of polymer formed around each initial catalyst particle, and the polymer remained sohd as the reaction proceeded, rather than requiring a liquid solution. A major class of these catalysts and fluidized bed reactor was developed by Union Carbide and by Shell Oil and called the Unipol process. In this process a very active solid catalyst is introduced into the reactor, and reaction occurs on the catalyst particles, which expand to maintain active sites on the growing polymer sphere. 

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. 

Soluble Ziegler-Natta catalysts can exhibit unique stereochemical properties. Group IV metallocenes in combination with methylaluminoxanes produce isotactic polypropylene with two different isotactic microstructures. The usual enantio-morphic site control is characteristic of enantiomeric racemic titano- and zirco-nocene complexes (e.g., ethylene-bridged indenyl derivatives279,349). In contrast, achiral titanocenes (e.g., [Cp2TiPh2]) yield isotactic polypropylene with microstructure 49, which is consistent with a chain end control mechanism 279,349-351... 

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