Indian Journal of Technology Vol. 26, February 1988, pp. 53-82 Recent Developments in Ziegler-Natta Catalysts for Olefin Polymerization and Their Processes V CHANDRASEKHAR, P R SRINIVASAN & S SIVARAM* Research Centre, Indian Petrochemicals Corporation Ltd, P.O. Petrochemicals, Baroda 391346, India The chemistry of Ziegler-Natta catalysts in olefin polymerization is reviewed. Factors determining catalyst activity are identified for the current generation of high activity - high selectivity catalysts. These include the nature of transition metal, its oxidation states and the ligands around it, the nature of alkylaluminium compounds, the physical state ofthe catalyst and its dependence on the method of preparation as well as activation of support and the role ofinternal and external donors. The effect of reaction parameters on catalyst performance as well as effect of nature of catalysts on polymer properties such as molecular weight, molecular weight distribution and stereoregularity are discussed. Current developments in soluble titanium! zirconium, vanadium and magnesium-titanium catalysts are reviewed. Different classes of industrial processes for production of polyethylene and polypropylene, namely slurry phase, gas phase and solution processes are discussed, The nature of contemporary catalysts is exemplified by selected patents from current literature, The various reactor types for polyolefins and salient reactor design aspects are discussed, The dependence of polymer properties on reactor design 11:1 well as kinetic modelling and simulation of polyolefin processes are briefly reviewed. I. Introduction made the dramatic discovery that not only propylene Polyolefins are a group of bulk commodity polymers could be polymerized to high molecular weight pol­ comprising of polyethylenes, polypropylene, poly ymers by Ziegler catalysts but the polymer so pro­ (butene-I) and various copolymers ofethylene, propy­ duced had a highly stereo regular structure (termed lene and higher alpha olefins. By volume used, polyo­ isotactic )resulting in improved physical properties. lefins are the leading polymers for many applications Montecatni Edison under exclusive license from Na­ such as plastics, fibres, films and elastomers. Since tta, began commercial manufacture of polypropylene their introduction in the early fifties, the polyolefin in 1957 in Italy. group of polymers have witnessed unprecedented The revolutionary discovery of Ziegler and Natta growth with the result that they have established a for the pOlymerization of ethylene and propylene was niche for themselves in every conceivable area of extended by others to produce polymers from a variety polymer applications. of other monomers such as dienes, cyclic ole fins and 1.1 Historiealoverview higher alpha olefins, leading to a large number of Polyolefins entered the commercial market with the polymers and copolymers with an endless array of discovery of high pressure low density polyethylene by properties. In view of the fact that they were based on Fawcett and Gibson in the laboratories of Imperial primary and therefore less expensive petrochemical Chemical Industries in 1934. With the increased feedstocks, they captured the commodity markets. availability of ethylene in the early fifties, low density The estimated world production of polyolefins was 4 polyethylene came into rapid prominence'. Around approximately 40 million tons in 1985 • 1955, Ziegler and his colleagues discovered a rev­ The polyolefin field has maintained its vigorous olutionary new chemistry which enabled polymeri­ growth on account of rapidly improving technology zation of ethylene at very low pressures and temp­ for process and products. In spite of thirty years of eratures2• Under these conditions, it was soon realized existence in the commercial markets, the area is being that the polyethylene produced had different prop­ subjected to frequent technological upheavels. This is erties from that of high pressure low density poly­ schematically illustrated for polyethylene and poly­ ethylene. The "Ziegler" polyethylene had a linear propylene in Fig. I. structure with very little chain branching. In 1957, the Ziegler process for polyethylene was extended to the 1.2 Scope of Ziegler-Natta catalysts polymerization of propylene by Natta in ltaly3. He Ziegler-Natta catalyst dS defined in patent descr­ iptions is a mixture of base metal alkyl ofgroup I to II l .. Present address: Polymer Chemistry Division, National Chemical metals and transition metal salt of group IV to VIII 5 Laroratory, Pune 411 008, India metals • The most commonly used transition metal 53 INDIAN 1. TECHNOL., VOL. 26, FEBRUARY 1988 CHROMIUM ZIEGLER CATALYST CATAlYST 1.... 0 L GAS SLURRY GAS GAS L HIGH PHASE PHASE 0 FLUID BED YiElD LlDPE lLDPE P E 1980 CHROMIUM CHROMIUM CATALYST CATALYST 19~L-______________________________________________________ LOW PRESSURE POLYETHYLENE POlY PROPYLE ME Fig. I - Progress tree of polyethylene and polypropylene technology Table I-Chronology of Development in Ziegler-Natta "2"(HX_F4 - H X Catalysts Polyethylene Polyethylene H X ISOTACTIC 1 Year Catalyst Yield Catalyst Yield 1.1. kg/gTi kg/gTi % " ",~ H X 1953-55 TiCI.+EhAI 10-15 TiCh+Et2AICI 1-3 90 Fxq- X H 1960 TiQ3+EtlAICI+ 1-10 92 IH X Electron donor SYNOIOTACTIC 1915 to Supported 1000 Supported 300 96 present TiCI.+Et3AI Ti+EhAI+ Electron donor - X H Et3AI: Triethylaluminium; E12AICl: Diethylaluminium chloride H X ATACTIC t Fig. 2 - Confiuration of isotactic, syndiotactic and atactic Beginning early seventies, Ziegler-Natta catalysts polyproplenes underwent revolutionary modifications, whereby high salts are halides and alkoxides ofTi, V, Cr, Co and Ni activity catalysts requiring no catalyst removal from salts. Organometallic compounds of aluminium are polymer appeared on the scene. Table I shows a the preferred cocatalysts. Using these class of catalysts chronology of development for polyolefin catalysts. a wide variety of acyclic olefins, dienes, cyclic olefins Rapid developments in propylene catalysts further and vinyl monomers have been polymerized and improved catalyst performance as shown in Table 27. copolymerized to products with useful properties and A number of recent reviewss-18 and a monographl9 substantial commercial interest. provide extensive coverage of the chemistry of Ziegler­ The distinctive feature of the Ziegler-Natta catalyst Natta catalyst systems. is its extraordinary stereospecificity. Unlike ethylene, 1.3 Commercial development of processes which is achiral, propylene is prochiral and therefore Subsequent to the initial discovery of Ziegler-Natta upon polymerization can give two distinct config­ catalysts, many processes were developed to exploit urations, namely isotactic and syndiotactic, depending the catalysts for the commercial manufacture of on the position of the methyl group (Fig.2t Ziegler­ polyolefins. Initially slurry and solution phase pro­ Natta catalysts are highly specific in the sense that they cesses were developed, and later processes operating in produce exclusively either one or the other confi­ gas phase or in low boiling diluents came into prom­ guration depending on the nature of the catalysts. inence. 54 CHANDRASEKHAR et aL: ZIEGLER-NATIA CATALYSTS FOR OLEFIN POLYMERIZATION Table 2 - Relative Performance of High Activity Catalysts Table 3-Commercial Heterogeneous Catalysts for Propylene Polymerization" TiCb+ High activity catalysts No. Trade name Commercial firm Presumed catalyst E12AICI introduced during composition Stauffer A Stauffer Chemical TiCh Co., USA 1975-80 1980­ 2 Stauffer AA Stauffer Chemical TiCh.0.3 AICh todate Co., USA Catalyst productivity, 3 300 300 3 Toho 140 Series Toho Titanium TiCh.O.3 AICh kg PPlg Ti Co., Japan Catalyst productivity, 0.3 5 > 12.5 4 Solvay Solvay et Cie, TiCh.O.3 AICI; kg PPI g catalyst Belgium diisoamylether 1.1. of polymer, % 90 92 94-98 5 Toho 144 Toho Titanium TiCh.O.3 AICI; Bulk density of polymer, 0.2-0.3 0.5-0.54 0.50-0.54 Co., Japan electron donor g/mL 6 Lynx-900 Catalyst Resources Unsupported Flowability, S 25·30 16-20 13-16 Inc. titanium chloride 2 modified 'Slurry process in heptane: 70"C and 7 kg/cm ;see Ref. 7. 7 foIl KT Series Himont Inc., USA Supported Mitsui Petrochemi- catalyst cals Co., Japan An important aspect of the current stage of dev­ elopement of polyolefin processes is that they appear to have almost reached the end point in terms of Table 4-Homogeneous Ziegler-Natta Catalysts process engineering. Large single line capacities and simplified processes have reduced to bare minimum No. Catalyst Olefin Structure the capital and utility costs. Further simplification I TiCL. + E12AICI Ethylene Linear PE such as elimination of extrusion, pelletization is al­ 2 Cp2TiCh+Et2AICI Ethylene Linear PE 3 VCL.+ Et2AICI Propylene Syndiotactic PP ready a reality in case of polyethylene and appears below -45°C imminent for polypropylene also. Target character­ 4 VCL,+Et2AICI Ethylene Random copolymer istics of catalysts required to produce high bulk and propy­ density polymers in spherical form with sufficient lene porosity for absorption of stabilizers and additives 5 Cobalt octoate+ Butadiene cis-I.4-polybuta­ Et2AICI diene have been defined and appear close to commercial 6 TiCL.+EtJAI ISoprene ciS-I,4-polyiso­ availability'o. prene 1.4 Scope of the review In spite of three decades of existence, Ziegler-Natta catalysts and processes continue to be the object of an alloy with a metal halide, e.g. TiCh x-AlCb, the innumerable studies in both industrial and