Olefins polymerizations

The polymerization of ethylene or a-olefins to give polyolefins represents a major industrial process with a worldwide production of more than 103 million tons reported in 2005 [98). While early transition metals have led the way for metal-mediated polymerization processes, the great potential of late transition metal 

These reactions include the Phillips process, i.e. the polymerization of ethylene at Tk 105 °C and pressures aroimd 30 bar [4, 5]. Of course, this procedure attracted our special attention. In spite of its worldwide use, which makes more than 1/4 of the world production of HDPE, and after almost four decades of research the mechanism of the reaction is still a matter of controversial discussions [1, 2]. The varying results are partially due to different experimental starting points (e.g. different supports), occasionally also to the use of inadequate measurements, and the neglect of involved parameters. 

If we consider the case of ethylene polymerization, the main experimental obstacles are the necessary application of pressure and the insolubility of the high molecular product. A REM picture (Fig. 18.5) shows the growing polyethylene worms [58, 59]. We tried to avoid these drawbacks studying the polymerization of higher (liquid) 1-olefins, preferentially 1-n-octene [60, 61]. With nc 4 the polymers are easily soluble in alkane solvents. This allows to apply high concentration of the monomer at normal pressure, to analyse the reaction mixture at any wanted time, and to use classical procedures for the study of the products [61]. 

According to these experiments the polymerization of olefins is caused by a member (all members ) of the A species, while the B type centres cause some isomerization of the monomer. With catalysts of the same preparative history the specific polymerization activity is not dependent on the Cr concentration up to 1% [61, 62]. The reaction rate (as measured by the decrease of the monomer) is decreasing with tic from ethylene to 1-hexene [22], 

Higher olefins polymerize practically with the same rate as Ce- The comparison was carried out at deep temperatures to allow the gas phase olefins to be present in solution in the same starting concentration as the liquid members. This offers the chance of copolymerizing different olefins with n 6 without discrimination [60], As to the products, the 1-olefins form comb-like polymer molecules with a practically unbranched backbone. The broad distribution of the molecular weight (MW) around 10 shows at low polymerization temperatures a three-modal profile that collapses at higher reaction temperatures to a mono-modal distribution with a lower MW value for its centre [61]. The chains contain one double bond per molecule [61]. 

The use of the new system at ambient pressure and at moderate temperatures allows to follow the reaction kinetics by analysing the concentration of educts and products as a function of time. Batch experiments showed that there is a marked induction period of slow reaction progress (Fig. 18.6) [22, 60, 63]. The systems exhibit the characteristics of an auto-catalytic reaction. Addition of new monomer up to the original starting concentration during the conversion of the monomer does not result in a new induction period but shortly after completion of the reaction the system answers to another addition of monomer with a new induction period [14, 64]. In steady state experiments the polymerization was proceeding over days with constant reaction rate, i. e. without decrease of the catalyst s activity. The re- 

In the following we shall take a closer look at supported catalysts for the polymerization of olefins [T22]. Oxides of Cr and Ti on various support materials have high activities for the polymerization of ethylene to linear chains (HDPE). The processes operate at relatively low ethylene pressure (20-30 bar) in the temperature range 130-150°C (solution polymerization) or 80-100°C (suspension and gas-phase polymerization). 

The Phillips catalysts are manufactured by impregnating amorphous silica gel with chromates up to a metal loading of ca. 1 %. The material is then dried and calcinated at 500-1000 °C. The surface silanol groups react with the chromate groups to give a disperse monolayer of chromate and dichromate esters (Eq. 8-10). 

However, it is assumed that the active centers are coordinatively unsaturated Cr or Cr centers that are generated by reaction with ethylene (Eq. 8-11). It is also possible to convert the chromate deposited on the silica surface to an active form by high-temperature reduction with CO. In an alternative method of catalyst production, low-valent organochromium compoimds such as chromocene and tris( -allyl)-chromium are used as catalyst precursors. 

Similar to the polymerization of ethylene on Ziegler catalysts, the first reaction step is the coordination of an ethylene molecule at a Cr center. The initiator of the polymerization reaction is thought to be a Cr-H group, into which ethylene inserts to form an ethyl ligand (Eq. 8-12). It was shown that only isolated Cr centers on the surface are catal5h ically active. 

Coordinatively unsaturated transition metal centers are the prerequisite for olefin polymerization in both Phillips and Ziegler-Natta catalysts, and this makes it possible to simultaneously bind the monomer and the growing chain. This does not occur 

Due to the importance of group 4 metallocenes to the development of the field, we include here a brief outline of some of their key features. The majority of this section, however, is devoted to advances in non-metallocene catalyst systems. Where necessary, catalyst activities have been converted into the units gmmol-1 h-1 bar-1 for gaseous monomers such as ethylene and propylene, and gmmol-1 h-1 for reactions carried out in liquid a-olefins such as 1-hexene. Activities are classified as very high ( 1,000), high (1,000-100), moderate (100-10), low (10-1) and very low ( 1).8 

When a transition metal alkyl or a metal hydride reacts with olefin molecules to undergo successive insertions, chain growth of a polymer attached to the transition metal takes place. If -hydrogen elimination occurs from the polymer chain, a transition metal hydride coordinated with the olefin derived from the polymer chain will be produced. By displacement of the coordinated olefin from the transition metal by the other monomer olefin, the polymer with an unsaturated terminal bond is liberated with generation of a transition metal hydride coordinated with the olefin. New chain growth will follow from the hydride, with the net result of control of the molecular weight without termination of the polymerization process. The process is in fact a chain transfer process. 

The role of aluminoxane in polymerization with transition metal complexes has not been completely clarified. It certainly serves as the alkylating reagent of the transition metal halides to produce metal dialkyls. Methylaluminoxane, having Lewis acidity, can abstract one of the two alkyl groups at the metal center and creates a cationic metal alkyl species with a readily accessible coordination site for the incoming olefin molecules. As described previously, a cationic monoalkyl complex having a vacant coordination site for the monomer is suitable in accepting the monomer and initiate the polymerization. Abstraction of one of the methyl 

Another important issue in olefin polymerization is copolymerization of different types of monomers. If one can freely produce copolymers of non-polar and polar monomers, which are difficult to copolymerize with conventional initiators, it would provide useful polymer materials. The Ziegler type catalysts using trialkylaluminum is not suitable for polymerizing polar monomers, whereas late transition metal catalysts are more tolerant of polar monomers. Recently catalysts using late transition metal catalysts have been intensively studied [89]. Because of the obvious importance of these polymeric materials in industrial use, further studies are expected on the applicability of late transition metal complexes for polymerization. 

3-Dienes such as butadiene and isoprene are important feedstock for production of polymer materials as well as low molecular weight compounds. Of particular synthetic importance is manufacturing synthetic rubbers using transition metal catalysts. Diene polymers can be prepared by successive insertions of dienes into transition metal alkyls or metal hydrides. 

Butadiene may insert into the M-R bond by 1,2-insertion or 1,4-insertion mode. The latter mode giving the poly-cA-1,4-butadiene is particularly important for producing materials for synthetic rubber tires. For isoprene polymerization the situation is further complicated because of the presence of the methyl substituent. Since the physical properties of poly-cw-l,4-isoprene are quite similar with those of natural rubber, knowing the means to control the polymerization behavior is of particular industrial importance. 

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The growth of polyolefin fibers continues. Advances in olefin polymerization provide a wide range of polymer properties to the fiber producer. Inroads into new markets are being made through improvements in stabilization, and new and improved methods of extmsion and production, including multicomponent extmsion and spunbonded and meltblown nonwovens. 

Other applications of zirconium tetrafluoride are in molten salt reactor experiments as a catalyst for the fluorination of chloroacetone to chlorofluoroacetone (17,18) as a catalyst for olefin polymerization (19) as a catalyst for the conversion of a mixture of formaldehyde, acetaldehyde, and ammonia (in the ratio of 1 1 3 3) to pyridine (20) as an inhibitor for the combustion of NH CIO (21) in rechargeable electrochemical cells (22) and in dental applications (23) (see Dentalmaterials). 

Organic titanates perform three important functions for a variety of iadustrial appHcations. These are (/) catalysis, especially polyesterification and olefin polymerization (2) polymer cross-linking to enhance performance properties and (J) Surface modification for adhesion, lubricity, or pigment dispersion. 

Olefin Polymerization. Titanates having a carbon—titanium bond are extensively kivolved ki Ziegler-Natta and metallocene polymerization of... 

Titanium bromide [7789-68-6] TiBr, is claimed as a catalyst for olefin polymerizations (18). Chromous bromide [10049-25-9] CrBr2, is used in chromizing. Chromic bromide [10031-25-1], CrBr, and tungsten bromide [13701 -86-5], are catalysts for polymerizing olefins (19). Manganese... 

These siUca-supported catalysts demonstrate the close connections between catalysis in solutions and catalysis on surfaces, but they are not industrial catalysts. However, siUca is used as a support for chromium complexes, formed either from chromocene or chromium salts, that are industrial catalysts for polymerization of a-olefins (64,65). Supported chromium complex catalysts are used on an enormous scale in the manufacture of linear polyethylene in the Unipol and Phillips processes (see Olefin polymers). The exact stmctures of the surface species are still not known, but it is evident that there is a close analogy linking soluble and supported metal complex catalysts for olefin polymerization. 

Polymerization. Supported catalysts are used extensively in olefin polymerization, primarily to manufacture polyethylene and polypropylene. Because propylene can polymerize in a stereoregular manner to produce an isotactic, or crystalline, polymer as well as an atactic, or amorphous, polymer and ethylene caimot, there are large differences in the catalysts used to manufacture polyethylene and polypropylene (see Olefin polymers). 

Olefin polymerization catalysts are unique in their utilization of supported catalysts in that the catalysts that have been developed are so highly active that the spent catalysts are intentionally left in the polymer where, at the extremely low parts per million concentrations used, they pose no threat to the properties of the polymer or to the well-being of the end user. 

Volume 56 Olefin Polymerization Catalysts. Proceedings of the International Symposium on Recent Developments in Olefin Polymerization Catalysts, Tokyo,... 

The reaction section consists of the high pressure reactors filled with catalyst, and means to take away or dissipate the high heat of reaction (300-500 Btu/lb of olefin polymerized). In the tubular reactors, the catalyst is inside a multiplicity of tubes which are cooled by a steam-water condensate jacket. Thus, the heat of reaction is utilized to generate high pressure steam. In the chamber process, the catalyst is held in several beds in a drum-type reactor with feed or recycled product introduced as a quench between the individual beds. 

The next major commodity plastic worth discussing is polypropylene. Polypropylene is a thermoplastic, crystalline resin. Its production technology is based on Ziegler s discovery in 1953 of metal alkyl-transition metal halide olefin polymerization catalysts. These are heterogeneous coordination systems that produce resin by stereo specific polymerization of propylene. Stereoregular polymers characteristically have monomeric units arranged in orderly periodic steric configuration. 

Gas Phase Olefin Polymerization Process with Recovery of Monomers from Reactor Vent Gas by Absorption, U.S. Patent 5,521.264, May 28, 1996. 

Absorption Process for Rejection of Reactor Byproducts Recovery of Monomers from Waste Gas Streams in Olefin Polymerization Process, U.S. Patent 5.681.908. Oct. 28. 1997. 

In the depropanizer tower the propane and lighter gases are taken overhead to become feed to the ethylene and propylene recovery facilities. Separation is accomplished at a relatively low overhead temperature of -25°F to minimize reboiler fouling by olefin polymerization. 

Allison, M. and Bennet, A., Novel, Highly Active Iron and Cobalt Catalysts for Olefin Polymerization, CHEMTECH, July, 1999, pp. 24-28. 

Enikolopyan NS, Diachkovsky FS, Novokshonova LA (1982) In Complex organ-ometallic catalysts for olefin polymerization Publication IKRF 9 174... 

Semikolenova NV, Zakharov VA (1982) Complex organometallic catalysts for olefin polymerization, IKhF AN SSSR Publication 9 51... 

Carrick, W. L. The Mechansim of Olefin Polymerization by Ziegler-Natta Catalysts. Vol. 12, pp. 65-86. 

Kennedy, J. P. and Trivedi, P. D. Cationic Olefin Polymerization Using Alkyl Halide — Alkyl-Aluminium Initiator Systems. I. Reactivity Studies. II. Molecular Weight Studies. Vol. 28, pp. 83-151. 

Inhibition of olefin polymerization occurred when its basicity was not sufficient to produce an appreciable displacement of initiator from the aldehyde-acid complex isoprene, cyclopentadiene and styrene were in this category. 

The propagation centers of the catalysts of olefin polymerization contain the active transition metal-carbon propagation center two-component and one-component.1... 

The terms one-component and two-component for the catalysts of olefin polymerization were used in the review by Berger et al. (1). 

Some results obtained have already been reviewed (104-107),2 so only the data of general interest in the problem of olefin polymerization by one-component catalysts will be touched upon here. 

In the propagation centers of chromium oxide catalysts as well as in other catalysts of olefin polymerization the growth of a polymer chain proceeds as olefin insertion into the transition metal-carbon tr-bond. Krauss (70) stated that he succeeded in isolating, in methanol solution from the... 

In essence the active centers for catalytic polymerization of olefins are organometallic complexes of transition metals. For this reason a search for individual organometallic compounds that would possess catalytic activity in olefin polymerization is of great interest. The first attempts to use organometallic compounds of transition metals as catalysts for olefin polymerization were made long ago [e.g. CH3TiCl3 as a catalyst for polymerization of ethylene 116). However, only in recent years as a result of the application of relatively stable organometallic compounds of transition... 

To determine the number of propagation centers in one-component catalysts, in principle the same methods used to study two-component catalysts of olefin polymerization may be applied Qsee (18, 160, 160a) ]. The most widely used methods for the determination of the number of propagation centers in polymerization catalysts are ... 

In studying two-component polymerization catalysts, beginning with Feldman and Perry (161), a radioactive label was introduced into the growing polymer chain by quenching the polymerization with tritiated alcohols. The use of these quenching agents is based on the concept of the anionic coordination mechanism of olefin polymerization occurring... 

However, in olefin polymerization by two-component catalysts during polymerization not only active transition metal-polymer bonds are formed, but also inactive aluminum-polymer ones, as a result of the transfer process with the participation of a co-catalyst (11, 162-164). The aluminum-polymer bonds are quenched by tritiated alcohol according to the scheme (25), so an additional tagging of the polymer occurs. The use of iodine (165, 166) as a quenching agent also results in decomposing inactive metal-polymer bonds. 

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