Second-Generation Propylene Polymerization Catalysts

By 1960, the original preparation of brown -titanium trichloride in situ was no longer used by most producers. It was replaced by commercial y-titanium trichloride, which contained an isomorphous form of aluminum chloride in the molar ratio (STiCb.AlCls). This was ball milled to form -titanium trichloride, which increased activity and led to the formation of a higher proportion of isotactic polymer. However, even after these improvements in catalyst performance, there was still a limitation to the eatalyst activity because the aluminum trichloride in the catalyst reacted with the triethyl aluminum co-catalyst to form quantities of ethyl aluminum dichloride, which is detrimental to the polymerization reactions. Titanium trichloride ciystals were also relatively large despite the milling treatment to reduce particle size. 

A further improvement in performanee was achieved by the addition of electron donors (Lewis bases), such as esters, ethers, or amines, to the catalyst, which can form complexes with aluminum alkyls. Electron donors could be added either with the co-catalyst, or during the y-titanium trichloride milling stage, depending on the compound used or the result required. In addition to 

Refinements to the Ziegler-Natta catalysts continued, with several attempts to convert brown 8-titanimn trichloride to the active purple y-titanium trichloride at a lower temperature. The objective of this work was an increase in the activity of the catalyst by avoiding the formation of the aluminium trichloride that was isomorphous with the y-titanium trichloride. High temperatures were normally required to reduce the aluminium chloride content, and this led to a reduction in the surface area, and hence, activity. In a development by Solvay, titanium tetrachloride was reduced with aluminum alkyl at about 1°C, before extraction of the co-ciystallized aluminum chloride with either dibutyl- or di-isoamyl ether. The porous -titanium trichloride was then heated with excess titanium tetrachloride at 65-100°C to remove excess ether and to produce high-surface-area ( 75 m g ) purple a-titanium trichloride under controlled conditions.  

A feature of these catalysts, which were more active and stereospedfic in polypropylene production, was that they formed uniform 25 to 35 pm diameter spherical particles and the shape of the polymer replicated the catalyst morphology. Unfortnnately, despite the favorable effect of these catalysts on the shape of the polymer particles produced, the particles were unstable during storage and de-ashing was still necessaiy. The multistage recipes were only used where onsite production facilities were available. 

Worldwide demand for polypropylene was still only about 1-6 million tones year during the period from 1970 to 1980. As demand began to increase, more efficient catalysts, based largely on a successful range of supported polyethylene catalysts, were developed. 

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Thus, the literature from the beginning until the first half of 1982 was examined, paying also attention, expecially in the case of polyethylene, even to the patent literature available from 1968, that is, since the discovery of the so-called second generation catalytic systems. As the most important production processes involve the polymerization of ethylene and propylene, the discussion will be limited to these homopolymers. With regard to polymer MWD regulation, the most significant methods based on the choice of the catalyst system and the polymerization conditions will be examined. 

Polyethers are prepared by the ring opening polymerization of three, four, five, seven, and higher member cyclic ethers. Polyalkylene oxides from ethylene or propylene oxide and from epichlorohydrin are the most common commercial materials. They seem to be the most reactive alkylene oxides and can be polymerized by cationic, anionic, and coordinated nucleophilic mechanisms. For example, ethylene oxide is polymerized by an alkaline catalyst to generate a living polymer in Figure 1.1. Upon addition of a second alkylene oxide monomer, it is possible to produce a block copolymer (Fig. 1.2). 

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