PRODUCT REVIEW Second Generation Ziegler Polyolefin Processes S. Sivaram Research and Development Center. indian Petrochemicais Corporation Limited. Baroda, 391 346 India
S. Sivaram has been with the Research and Deuelopment Diuision of the Indian Petrochemicals Corporation Limited a s a Senior Research Chemist since March 1973. He received his M.S. from Indian Institute of Technology, Kanpur, India (1967), and Ph.D. from Purdue Uniuersity (1971). He did postdoctoral work a t the Institute of Polymer Science, the Uniuersity of Akron (1971-1973). His interests lie in the area of supported catalysts for polymerizations, polymer synthesis, and process development. He is a member of The American Chemical Society, Sigma-Xi, and Phi Lambda Upsilon.
Introduction Today Ziegler polyolefins are, volumewise, the leading polymer for many applications such as plastics, fibers, films, and elastomers. Further they are poised for a rapid growth in the coming years. In the United States alone a 50% capacity increase of polypropylene is planned with a predicted consumption growth rate of 13%per annum through 1980. I t is therefore natural that considerable research effort has been invested, aimed a t improving the conventional Ziegler process. The unveiling of a new generation of Ziegler-Natta catalysts with superior activity by a number of major polyolefin manufacturers around the world is evidence that these efforts have achieved fruition. Much of the useful information in this area is, however, contained in applications for patents in countries all over the world. Recently, Diedrich (1975) and Weissermel e t al. (1975) have reviewed some aspects of the second generation Ziegler catalysts focussing predominantly on the work done a t Hoechst. More recently Karol (1976) has presented a detailed review of supported catalysts. The present survey, restricted to Ziegler catalysts, is based on a literature and patent search (Table I) and attempts to systematize and critically analyze the myriad claims of these recent disclosures as applied to various polyolefins of commercial interest. T h e Conventional Ziegler Polyolefin Process The original Ziegler-Natta catalysts used in industrial polyolefin processes consist of an organometallic complex formed by the reaction of a transition metal compound with an organoaluminum compound. The catalyst cornhinations can be either heterogeneous or homogeneous depending on the specific nature of the components used. The conventional Ziegler catalysts as used today are not completely free of disadvantages. Their efficiency, defined as the amount of polymer formed per gram of transition metal catalyst used, is low, -1500. This requires the removal of catalyst residues from the polymer by a cornhination of chemical treatment and extraction in order to minimize their detrimental influence on polymer properties such as light stability, color, corrosivity, etc. Also the conventional process is basically inflexible from the point of view of producing products for a wide range of applications. Variation in molecular weights or molecular weight distribution requires additional processing steps. The mode of action of the Ziegler-Natta catalysts has been the subject of intense investigationsover the past two decades. The present status of our understanding is summarized in excellent reviews by Ketley (1967), Boor (1967), Smith (1969), Cooper et al. (1967),and Carrick (1973). It follows from these studies that the efficiency of the active catalyst center is influenced by (a) the nature, valency state, and type of ligands Ind. Eng. Chem.. Prod. Res. De"., VoI. 16, No. 2, 1977
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attached to the transition metal, (b) the type of organometallic compound, and (c) catalyst morphology. The ability to manipulate effectively these parameters has led to the successful development of the second generation Ziegler polyolefin processes.
Second Generation Ziegler Polyethylene Processes A. Supported Transition Metal Catalysts. The main objectives of research in the area of polyolefin processes have been to develop catalysts with high efficiency, so that small amounts of catalyst residues can be left unwashed without any detrimental effects on the polymer, as well as to enable one to control the basic physical parameters of the polymer such as molecular weight, polydispersity, and chain branching at the polymerization step itself. It has been found that both of these objectives can be achieved by supporting the transition metal compound on a suitable carrier. A variety of inorganic and organic supports have been cited in the patent literature, the most common being hydroxides, halides, carbonates, oxides, and alkoxides of magnesium, manganese, iron, nickel, cobalt, etc. The precise structural requirements for the support as well as its role in determining the polymer yield and properties have not been clearly defined. The degree of dehydration of the support seems to play a significant role in determining the activity of the catalyst. It also appears that an average particle diameter of 10-20 pm and specific surface area of 20-30 m2/g would yield the best results. It is possible that supports can enter into chemical bond formation with the transition metal halide (reaction 1) or form a stoichiometric compound with the transition metal halide (reaction 2) MgOH Tic14 MgOTiCls HC1 (1)
+ 2MgC12 + Tic14
-
-
+
MgClz Mg(TiC16)
The supported catalysts are prepared by either of the following two techniques. (I) Slurry technique: in this method the support and the transition metal compound are slurried with or without a hydrocarbon solvent at -130 "C and filtered and the support is washed free of excess transition metal compound using the same hydrocarbon solvent and dried in vacuo. (2) Ball milling technique: in this method the carrier and the transition metal compound are ball milled, if necessary at an elevated temperature to yield a catalyst of specific surface area; the method appears especially suitable for supporting a solid transition metal halide on an inorganic carrier. Compared to conventional Ziegler-Natta catalysts which yield about 1.5-3.0 kg of polyethylene/g of transition metal, the supported catalysts yield as high as 6000 kg of polymer. In the conventional process as much as 300-1700 ppm of residual metal is found in the product, whereas in the newer processes only 2-10 ppm is found. This eliminates completely catalyst removal steps from the polymer. Control of polydispersity of the polymer in the conventional process requires additional processing steps or a change in the basic nature of catalyst itself, Le., from heterogeneous to homogeneous. Supported catalyst seems to offer unique advantages in this respect. It is claimed that polydispersity can be changed from 4-5.7 to -16 simply by doping the magnesium hydroxide support with 20% by weight sodium peroxide. A change in the support also causes a change in the polydispersity of polymer. The second generation processes also employ hydrogen for molecular weight control and copolymerization with cy olefins for control of density. It is generally observed that the presence of support yields polymer with high MFI. B. Nonsupported High Efficiency Titanium Catalysts. Boucher et al. (1974) and Duck et al. (1974) have shown that 122
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in situ reduction of titanium tetrachloride with magnesium or an organomagnesium compound (Grignard reagent) yields in conjunction with organoaluminum compound a new class of high activity Ziegler catalyst for ethylene polymerization. An inorganic support is also sometimes used, but is not essential for achieving high activity. A tenfold increase in activity was noted between unreduced titanium chloride and titanium chloride reduced by phenyl magnesium bromide in conjunction with triisobutylaluminum. The same catalyst can also be used with advantage for the preparation of random copolymer of ethylene and propylene. The copolymers prepared exhibit the same elastomeric properties as those produced by soluble VOCl3-EASQ catalysts. The yields, however, are superior amounting to -10 kg/g of T i (h atm of CzH4).
Second Generation Ziegler Polypropylene Processes The twin objectives of high activity and high stereospecificity for polypropylene has proved to be a more elusive goal. According to experience high activity and high stereospecificity usually run countercurrent; i.e., high activity propylene catalysts produce high percentage of undesirable atactic polypropylene and vice versa. In the absence of a clearer understanding of the factors which control these two parameters only empirical progress has been possible by either modifying the transition metal component by special treatment or by using stereoregulators, namely, addition of special compounds which improve stereospecificity without significantly retarding the rates. In general three types of second generation catalysts have been specifically reported for polypropylene. (1) In situ preparation of highly active titanium trichloride by reducing titanium tetrachloride with aluminum powder or an organoaluminum compound and further activated by (a) dry grinding or (b) by addition of activators such as 0-dichlorobenzene, ethers, phosphorus compound, or aluminum chloride. The active titanium trichloride thus prepared is used in conjunction with organoaluminum compounds to prepare >96% isotactic polypropylene. Under .these conditions polymer yields are not significantly lowered. Duck et al. (1974) found that reduction of titanium chloride by organomagnesium compound yields only 35% isotactic polypropylene. (2) Supported titanium tetrachloride catalysts prepared by milling titanium tetrachloride with magnesium hydroxide or magnesium chloride and used in conjunction with organoaluminum compounds. Supported catalysts do increase activity but may result in decreased stereospecificity. (3) Conventional Ziegler catalysts wherein a modifier is added to achieve high stereospecificity. Cyclic triolefins such as 1,3,5-cycloheptatriene, diamine such as TMDA, aromatic and oxygenated organic compounds, and sulfur and phosphorus compounds all serve as effective modifiers. However addition of a modifier reduces catalyst activity as was originally observed by Cooper et al. (1967). Though claims of highly stereospecific catalysts for polypropylene are replete in patent literature, there is hardly any unequivocal demonstration of high activity, of the order generally achieved in the second generation polyethylene processes. Weissermel et al. (1974) state that by properly choosing the organoaluminum compound and, if necessary, stereoregulators, and by carefully selecting the polymerization conditions, catalyst efficiencies of the order of 40 kg/g of T i and isotacticities of >97% can be achieved. From a study of the effect of support on the rates and tacticity of propylene polymerizations, Soga et al. (1973) concluded that (a) the supported catalyst had longer lifetime compared to carrier-free titanium tetrachloride, (b) the closer the ionic radii of the metal in the support to Ti+4the higher the tacticity, and (c) the crystal structure of the support plays
Twenty years after the discovery of the first stereospecific pokymerization of olefins, the Ziegler process has once again emerged as a subject of considerable interest in the current chemical literature. The key to this renewed interest is primarily due to new developments in catalyst research which has led to disclosures of highly active and stereospecific Ziegler-Natta catalysts. The review attempts to systematize and analyze the myriad claims of these recent disclosures as applied to polyethylene, polypropylene, and elastomeric copolymers of ethylene and propylene, as well as high-cis polydienes. Possible reasons for the higher activity of the second generation catalysts are discussed in terms of the present understanding of the mechanism of conventional Ziegler-Natta catalyst systems. The implications of these developments to the existing Ziegler polyolefin technology are discussed.
an important role in determining stereospecificity. However, maximum isotacticity of only 60% could be achieved with a supported titanium tetrachloride catalyst and triethylaluminum. Recently Chien and Hsieh (1976) have studied supported organotitanium compounds in conjunction with diethylaluminum chloride as catalysts for propylene polymerizations. However, stereospecificities were only moderate and could be increased by the addition of electron donors, but only a t the cost of reduction in activity. Also with these catalyst systems (tetrabenzyltitanium and diethylaluminum chloride), the increase in activity upon using a support was very modest.
Second Generation Ziegler Process for Elastomers A. Ethylene-Propylene Copolymers. Amorphous copolymers of ethylene and propylene can be produced in high yields using second generation Ziegler catalysts. The copolymers thus prepared exhibit the same elastomeric properties as the random ethylene-propylene copolymers produced by conventional soluble Ziegler catalysts such as vanadium oxychloride and ethylaluminum sesquichloride. An ethylene reactivity ratio of 14 has been estimated for supported catalyst, which is very close to that observed with conventional Ziegler catalysts. This is evidence of a basic similarity in mechanism between the supported and unsupported catalysts. B. High-Cis Tactic Polydienes. High cis-l,4-polybutadiene and high cis-1,4-polyisoprene are two polydienes of commercial importance which are produced by Ziegler-Natta processes. Natta and Porri (1969) have reviewed in detail the chemistry of Ziegler diene polymerizations. Stereospecific polymerization of butadiene can be achieved using both the heterogeneous and homogeneous catalytic systems. The polymer microstructure (cis-1,4, trans-1,4, 1,2-syndio) can be varied depending on the catalyst system chosen. It is reported that a catalyst comprising a soluble organic complex of cobalt and diethylaluminum chloride yields polymers with high conversions (97-98%) and yields as high as 300 kg of polymer/g of cobalt. The cobalt requirement for polymerization is very small, as little as 0.5 to 0.006 mmo1/100 g of monomer. With these catalysts cis contents of 9698% can be achieved. Recently Bruzzone et al. (1974) have described a a-allyluranium catalyst which yields a polymer with a cis content of 99%. Consequently there appears to be little incentive for further improvement of catalyst for diene polymerizations which could be responsible for the absence of any reference in the current literature regarding the applicability of second generation Ziegler catalysts to dienes. However, Lasky et al. (1962) have reported that supporting a heterogeneous vanadium chloride catalyst on a support increased the efficiency of polymerization of isoprene to trans-1,4-polyisoprene by a factor of ten. More recently Ermakov et al., (1975) have examined the effect of carriers on butadiene polymerization by a-allyl complexes of nickel and chromium. They observed that the nature of carrier had dramatic effects on the stereospecificity of polymerization, a-allylnickel on silica giving 96% cis while
on alumina giving 98% trans. However, such systems do not fall into the same class of catalysts as Ziegler catalysts. Very little is known about the nature of active sites responsible for stereoselectivity in diene polymerizations. Therefore theory is of little use in predicting what effects supported Ziegler catalysts would have on the course of diene polymerizations. Only experimentation could provide the answers.
Mechanistic Aspects of High Efficiency Catalysts Any speculations regarding the reasons behind the high activity of second generation Ziegler catalysts are tentative and limited in scope at the present time, especially when one considers the fact that many aspects of the conventional Ziegler-Natta catalysts have yet not been fully delineated. However, some broad generalizations are possible. Because of the specificity of the catalyst used for different monomers, the mechanistic aspects are discussed with particular reference to each monomer-polymer system. A. Ethylene-Polyethylene. The heterogeneous ZieglerNatta catalysts can be regarded as a complex in which the organoaluminum compound is chemisorbed on the surface of the crystalline transition metal halide. As proposed by Cossee (1964), Cossee and Arlman (1964), and Arlman (1966), the polymer growth reaction can be viewed as occurring from the surface of the titanium catalysts, utilizing the crystal vacancies that exist on the catalyst surface as active sites. An increase in the number of active sites would be expected to increase the rate of polymerization. It was shown by Natta (1959) and Natta and Pasquon (1959) that commercial titanium catalyst possess
