Liquid propylene polymerization polypropylene

FIGURE 7.14 Distal ligand elaboration impacts catalyst performance for the production of syndiotactic polypropylene. Shown are precatalysts diphenylmethylene(9-fluorenyl)(cyclopentadienyl)zirconium dichloride (4) and diphenylmethylene(octamethyloctahydrodibenzofluorenyl)(cyclopentadienyl)zirconium dichloride (5). (Liquid propylene polymerization at 20 °C, Zr Al 1 1000 and 1 2000, methylaluminoxane cocatalyst. 

We have a choice of four major polymerization techniques by which to manufacture polypropylene using Ziegler-Natta catalysts slurry, liquid propylene, solution, and gas phase. Regardless of which technique is employed, all polymerization plants must accomplish the same basic goals they must... 

Another method of manufacturing polypropylene employs the liquid monomer as the polymerization solvent. This process, known as the liquid propylene or bulk-phase process, has a major advantage over the slurry method in that the concentration of the monomer is extremely high. The high concentration increases the rate of the reaction relative to that seen... 

UNIPOL [Union Carbide Polymerization] A process for polymerizing ethylene to polyethylene, and propylene to polypropylene. It is a low-pressure, gas-phase, fluidized-bed process, in contrast to the Ziegler-Natta process, which is conducted in the liquid phase. The catalyst powder is continuously added to the bed and the granular product is continuously withdrawn. A co-monomer such as 1-butene is normally used. The polyethylene process was developed by F. J. Karol and his colleagues at Union Carbide Corporation the polypropylene process was developed jointly with the Shell Chemical Company. The development of the ethylene process started in the mid 1960s, the propylene process was first commercialized in 1983. It is currently used under license by 75 producers in 26 countries, in a total of 96 reactors with a combined capacity of over 12 million tonnes/y. It is now available through Univation, the joint licensing subsidiary of Union Carbide and Exxon Chemical. A supported metallocene catalyst is used today. 

Liquid monomer Polymer swollen with monomer Precipitation or slurry polymerization Polypropylene in a pool of liquid propylene... 

In 1963, liquid polymerization was introduced in which liquid propylene, catalysts, and hydrogen were pumped continuously into the reactor while polypropylene slurry was transferred to a cyclone separator. The unconverted... 

Investigations of the bis(benzamidinate) dichloride or dialkyl complexes of Group 4 metals show that these complexes, obtained as a racemic mixture of c/s-octahedral compounds with C2 symmetry, are active catalysts for the polymerization of a-olefins when activated with MAO or perfluoroborane cocatalysts [29-41]. As was demonstrated above, polymerization of propylene with these complexes at atmospheric pressure results in the formation of an oily atactic product, instead of the expected isotactic polymer. The isotactic polypropylene (mmmm>95%, m.p.=153 °C) is formed when the polymerization is carried out at high concentration of olefin (in liquid propylene), which allows faster insertion of the monomer and almost completely suppresses the epimerization reaction. 

The polymerization of propylene using complex 14 activated by MAO (Al Zr ratio=500, solvent toluene, 25 °C) yielded 80 g polymer-mol Zrl-hrl with a molecular weight Mw= 115,000 and polydispersity=2.4 [119]. The reaction was carried out in liquid propylene to avoid, as much as possible, the epimerization of the last inserted monomer unit and to allow rational design of the elastomeric polymer. The formation of elastomeric polypropylene is consistent with the proposed equilibrium between ds-octahedral cationic complexes with C2 symmetry inducing the formation of the isotactic domain, and tetrahedral complexes with C2v symmetry responsible for the formation of the atactic domain (Scheme 7). The narrow polydispersity of the polypropylene obtained supports the polymerization mechanism in which the single-site catalyst is responsible for the formation of the elastomeric polymer. 

Polymerization occurs at pressures usually less than 50 atm and at temperatures below 110°C (to avoid dissolving the polymer) to form a slurry of about 20% polymer in an aliphatic liquid diluent. The diluent can be liquid propylene itself in the manufacture of polypropylene. 

Phosphorus-bridged racemic salt metathesis route outlined in Scheme 278.917 The PhP-bridge has been extended to ansa-bis(Flu) zirconocene dichloride and ansa- Flu-Gp) zirconocene dichloride complexes. The pure rac-1186 was isolated, but a 1 2 rac/ meso-mixture of 1187 was obtained due to the inability to separate the two diastereomers by repeated recrystallization. Nevertheless, when activated with a large excess of MAO, the 1 2 rac/meso-mYxX.uxo of the 2,4-disubstituted // -zirconocene 1187 polymerizes liquid propylene at 50 °C to highly isotactic polypropylene with > 98%. 

Rexene Co. and Philips Petroleum Co. first developed the bulk polymerization process with the first-generation TiCU catalyst [8,11,70]. It was then commercialized by Dart Industries in 1964. The reactor feed contains 10-30% propylene in the liquid phase. A mixture of hexane and isopropanol was employed for the removal of catalyst residue as well as the amorphous polypropylene. The process step of removing residual catalyst was later eliminated after the high-efficiency catalyst was adopted, constituting the so-called liquid pool process. Subsequently, Philips and Sumitomo companies further developed the liquid-phase polymerization process. This process enhances the reaction rate, catalyst efficiency, monomer conversion, and therefore results in high productivity. It also eliminates the need for solvent recovery and reduces environmental pollution. However, the process is somewhat complicated by the unreacted monomer, which has to be first vaporized and then liquefied before it is reused. The reaction vessel must be designed to operate under high pressures. In most cases, this process employs autoclaves for batch operation and tubular reactors for continuous operation. 

The propylene polymerization was carried out in a 2.0-L stainless steel autoclave. In the presence of a small amount of n-heptane, Al(C2Hs)3 (1.32 mmol) and an external donor, cyclohexylmethyldimethoxysilane were placed in the autoclave, and then the catalyst (2.6 pmol-Ti) was introduced at room temperature. After hydrogen (2.0 L) was charged, liquid propylene (740 g) was introduced and prepolymerization was conducted at 20 °C for 5 min. The temperature was then raised to 70 °C, and polymerization was conducted at 70 °C for 60 min. Typically, about 300 g of polypropylene powder were obtained. The results were summarized in Table 2. 

A very efficient alternative for heat removal is to use overhead condensers. This modification uses the latent heat of evaporation of the monomer to remove the heat of polymerization. Monomer is evaporated in the reactor, condensed in the overhead condenser, and the cooled liquid monomer is returned to the reactor. This design works well for propylene polymerization, but it is not a good option for ethylene because of its much lower boiling point. Overhead condensers are used in the El Paso bulk polypropylene process [72]. 

The dominant process in this market segment is the Spheripol process by Basell. Similar to the dominance achieved by the Phillips process in HOPE, roughly one-third of the world s polypropylene is produced using the Spheripol process. The Spheripol process uses loop reactors. A small loop reactor is used to prepolymerize the catalyst the main polymerization, for homopolymer or random copolymer, takes place in one or two loop reactors. For impact copolymer production, a gas-phase reactor is required after the loop reactor because of the limited solubility of ethylene in liquid propylene. A typical flow diagram of the Spheripol process is shown in Figure 2.40. 

FIGURE 2.20 Metallocenes used for the preparation of isotactic-hemiisotactic polypropylene. The [m] dyad fractions are for MAO-cocatalyzed polymerizations performed in liquid propylene at 0 °C. Hafnocene [m] values are given in parentheses. 

To further probe these two hypotheses of Ewen and Razavi, Bercaw prepared singly bridged Ci-symmetric 34 (Figure 4.14) and tested it for propylene polymerization. Combination of 34 with MAO in liquid propylene at 0 °C provided polypropylene that was essentially atactic (55% r dyads)." °... 

The result is that the driving force at the wall must increase by a factor of about 10 when scaling with S = 512 and constant power per unit volume. This may be acceptable when the pilot unit operates with a AT of 2°C but becomes problematic when the pilot plant operates with a 20°C AT. There are many solutions. In a CSTR, use cold feed. Some processes for PMMA use a 40°C feed to control the reaction exotherm. Diluents and low per-pass conversions can also be used this approach is typical of solution polyolefin processes. Reflux boiling can be used it is common in styrenic polymerizations where the reflux solvent is normally returned as a liquid. In some polypropylene processes, the returning propylene is flashed into the first reaction vessel. Finally, the external loop shown in Figure... 

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