by Union Carbide to produce polyethylene. In the mid-1980s, the Unipol process was further extended to produce polypropylene. The features of the fluidized-bed process, including its simplicity and product quality, made it widely accepted all over the world. As of today, the fluidized-bed process is one of the two most widely used technologies for producing polypropylene. In the 1970s, magnesium chloride-supported Ziegler–Natta catalysts were introduced. These catalysts exhibit activities so enhanced that costly steps could be omitted from the workup. These omitted processes included deashing (removal of residual catalyst) and removal of unwanted amorphous polymer. Stereochemistry of poly-1-alkenes Natta first used polymerization catalysts based on titanium chlorides to polymerize propylene and other 1-alkenes. He discovered that these polymers are crystalline materials and ascribed their crystallinity to a special feature of the polymer structure called stereoregularity. The concept of stereoregularity in polymer chains is illustrated in the picture on the left with polypropylene. Stereoregular poly(1-alkene) can be isotactic or syndiotactic depending on the relative orientation of the alkyl groups in polymer chains consisting of units −[CH2−CHR]−, like the CH3 groups in the figure. In the isotactic polymers, all stereogenic centers CHR share the same configuration. The stereogenic centers in syndiotactic polymers alternate their relative configuration. A polymer that lacks any regular arrangement in the position of its alkyl substituents (R) is called atactic. Both isotactic and syndiotactic polypropylene are crystalline, whereas atactic polypropylene, which can also be prepared with special Ziegler–Natta catalysts, is amorphous. The stereoregularity of the polymer is determined by the catalyst used to prepare it. Classes Heterogeneous catalysts The first and dominant class of titanium-based catalysts (and some vanadium-based catalysts) for alkene polymerization can be roughly subdivided into two subclasses: catalysts suitable for homopolymerization of ethylene and for ethylene/1-alkene copolymerization reactions leading to copolymers with a low 1-alkene content, 2–4 mol% (LLDPE resins), and catalysts suitable for the synthesis of isotactic 1-alkenes. The overlap between these two subclasses is relatively small because the requirements to the respective catalysts differ widely. Commercial catalysts are supported by being bound to a solid with a high surface area. Both TiCl4 and TiCl3 give active catalysts. The support in the majority of the catalysts is MgCl2. A third component of most catalysts is a carrier, a material that determines the size and the shape of catalyst particles. The preferred carrier is microporous spheres of amorphous silica with a diameter of 30–40 mm.