Ziegler-Natta catalysts.
The discovery of catalytic reactions of α-olefins to give stereo-regular polymers is among the most significant in the science and practice of catalysis. Ziegler-Natta (Z-N) catalysts and their metallocene derivatives are the only initiators that can be used to polymerize all α-olefins. With the exception of ethylene, they cannot be catalyzed in polymerization reactions by ionic or radical initiators.
From a commercial point of view, the catalysts in this group have become relevant because of the volumes of essentially linear polyethylene produced industrially from them.
Although the phenomenon of stereoisomerization is not applicable to polyethylene (due to the symmetry of the molecule), the product obtained by means of Z-N catalysts is essentially linear and, as already seen, consequently crystalline.
The stereoisomerization process is particularly important for polypropylene, since the crystalline (isotactic) forms are the most commercially valuable.
Since the behavior of Ziegler-Natta catalysts is similar for PE and PP, we will take the development of stereospecific polymerization as the base case, remembering that in the case of PE it is not necessary to pay attention to the relative location of the methyl groups (nonexistent) as in PP. In this section, we review some of the aspects already seen for Ziegler-Natta catalysts for ethylene and propylene, with the optics and particularities found by the investigations carried out for polypropylene.
First of all, it is relevant to recall the classification of isotactic, syndiotactic and atactic forms.
The above forms are differentiated by the orientation of the methyl group associated with the main chain:
Isotactic (all methyls arranged toward the same side of the main chain).
Syndiotactic (arranged alternately one to each side of the main chain).
atactic (randomly arranged on either side of the main chain).
In reality, the chain formed is not strictly linear, but adopts the conformation of a helix, with the methyl groups that define the isotacticity characteristic oriented towards the outside of the helix, and with a rotation step of the order of three monomeric units. However, the most important feature remains the orientation of the methyl groups of the original propylene all oriented on the same side of the polymer backbone, as can be seen in Figure. Since symmetry is a fundamental requirement for ordering in sheets and platelets, spherulites and crystallites, only the isotactic structure allows the generation of crystalline phases.
Isotactic polypropylene is obtained from the stereoregular polymerization of propene over catalysts of the Ziegler-Natta type or, more recently, metallocene catalysts. The former are generally the reaction product of an organometallic compound belonging to groups IA to IIIA with a transition metal compound from groups IVB to VIII, as presented when describing the catalytic mechanisms for PE.
Although patent and scientific literature report a large number of combinations of catalytic compounds that produce isotactic polypropylene, in commercial practice the most commonly used combination is based on titanium tri- and tetrachloride with monochloro-diethyl aluminum.
A list of the most common components of the Ziegler Natta catalysts used can be found in the Table:
Typical Ziegler-Natta catalysts components