Polymerization in liquid propylene

The results mentioned above, referring to polymerization in a slurry, can be further improved by polymerizing in liquid propylene (Figs. 55, 56, 57). 

Fig. 55. Super active 3rd generation catalyst activity vs. polymerization time. Polymerization in liquid propylene at 70 °C...

The first stage of the Spheripol process consists of polymerization in liquid propylene. Usually, two loops are used in series to narrow the residence-time distribution of the catalyst particles. For the ethylene-propylene copolymer (EPR) stage, the Spheripol process (Fig. 2.33) utilizes a gas phase fluidized bed reactor (FBR). The liquid propylene/ polymer suspension from the first reactor is flashed to gas/solid conditions prior to entering the second stage. The second stage operates at pressures of 15-35 atm, which is often close to the dew point of the gas. Elevated temperatures of approximately 80°C are used to provide a reasonable amount of copolymer contents in the final product. 

Table 12 Performance Data of Metallocenes in Polymerization of Liquid Propylene...

In heterogeneous polymerizations in bulk, the formed polymer is insoluble in its monomer and the polyreaction is performed below the softening point of the polymer. On an industrial scale, this type of process is especially utilized for chain polymerizations, for example, the radical polymerization of liquid vinyl chloride, the polymerization of liquid propylene with Ziegler-Natta or with metallocene catalysts, and the polymerization of molten trioxane. 

Polymerization in liquid monomer was pioneered by Rexall Drug and Chemical and Phillips Petroleum (United States). Gas-phase polymerization of propylene was pioneered by BASF, who developed the Novolen process which uses stirred-bed reactors, Eastman Chemical has utilized a unique, high temperature solution process for propylene polymerization. In the 1970s, Solvay introduced an advanced TiCl3 catalyst with high activity and stereoregularity. 

After an induction period, the polymerization rate reaches a maximum and then becomes almost constant for over 20 hours. The constant rate of polymerization of the homogeneous system indicates that living polymers are present in this case. Indeed, block copolymers of propylene and ethylene could be obtained with this homogeneous system when ethylene was dissolved in liquid propylene [see also, related experiments with the heterogeneous system (3/)]. 

Description In the Spheripol process, homopolymer and random copolymer polymerization takes place in liquid propylene within a tubular loop reactor (1). Heterophasic impact copolymerization can be achieved by adding a gas-phase reactor (3) in series. 

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. 

Description In the Spheripol process, homopolymer and random copolymer polymerization takes place in liquid propylene within a loop tubular reactor (1). Heterophasic impact copolymerization is done by adding a gas-phase reactor (3) operated in series. Removal of catalyst residue and amorphous polymer is not required. Unreacted monomer is flashed in a two-stage pressure system (2, 4) and recycled back to the reactors. This improves yield and minimizes energy... 

Metallocenes immobilized on solid support materials have been successfully introduced in industry as polymerization catalysts for the production of new application-oriented polymer materials. Industrial polymerizations, which are carried out either as a slurry process in liquid propylene or as a gas-phase process (Section 7.2.3), require that catalysts are in the form of solid grains or pellets soluble metallocene catalysts thus have to be supported on a solid carrier. 

An important phenomenon occurs in ethylene-propylene co-polymerization, especially in liquid propylene polymerization addition of small amounts (< 30 mol%) of ethylene causes a strong decrease in PP molecular masses. This... 

These ( -symmetric zirconocenes are made from relatively inexpensive ligands, but have some limitations their synthesis produces an isomer ratio racjmeso < 1 and their catalytic activities are not very high. In addition, the elastomeric PP cannot be produced with controlled morphology in liquid propylene and, as is the case of the bis(2-arylindenyl) systems mentioned above, do not give sufficiently high molecular masses in solution at industrial polymerization temperatures. 

Some examples are shown in Table 14, together with a selection of polymerization results. It is worth noting the higher syndioselectivity of the Zr complex 74 compared to that of 73, and the even higher syndioselectivity of the Zr complex, which bears the expanded octamethyloctahydrodibenzofluorenyl ligand (>99% rrrr, 7m 165 °C at —15 °C in liquid propylene).942 The very low activities and sPP molecular masses of 74/MAO however render this complex of little interest. 

Liquid monomer Polymer swollen with monomer Precipitation or slurry polymerization Polypropylene dispersed in liquid propylene... 

It could also be demonstrated that a very rapid passing of these stages occurs when the polymerization is carried out in liquid propylene [45 7] this means that the industrially important bulk polymerization can also be exactly described by this polymer growth and particle expansion model [41-43, 48]. 

Fig. 8 Kinetic curves of propene polymerization with MgCl2/Di/TiCl4/D2-AlEt3, 70°C. Open circles) polymerization of propylene in n-heptane pcsHs = 2.5 atm, Kh = 0.325 mol/(l atm) closed circles) polymerization in liquid propene [CaHg] = 10.5 mol/L open squares) gas-phase polymerization of propylene, pcjHs = 2.5 atm, = 0.13mol/(l atm) closedtriangles)gas-...

In this type of process, polymerization takes place in liquid propylene without the use of an inert diluent. This is a significant simplification over the traditional diluent slurry process, as propylene can be separated from the polymer by flashing, and there is no need for the extensive diluent recovery system. 

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. 

By far the most commonly used PP-process is Montell s Spheripol Process. The first reaction stage consists of one or two tubular loop reactors where bulk polymerization of homopolymers, random and terpol5nners is carried out in liquid propylene. The prepolymerized catalyst, liquid propylene, hydrogen for controlling molecular weight and eventually comonomers are continuously fed into the reactor in which polymerization takes place at temperatures of 60-80°C and pressures of 35-40 bar. The tubular configuration enables a perfect heat transfer and control of the reaction temperature. 

Polymerization productivity hexane slurry, 70°C, 0.7 MPa, 4 h, with hydrogen for molecular weight control. Values in parentheses are polymerizations performed in liquid propylene at 70°C for 2 h with hydrogen. 

In comparison to bis(indenyl) anM-metallocenes, there are considerably fewer examples of C2-symmetric isoselective metallocene catalysts on the basis of bridged bis(cyclopentadienyl) ligand framework. Representative complexes 14-17 are illustrated in Figure 1.13. Many of the reported chiral bis(cyclopentadienyl) flni fl-zirconocenes produce highly stereoregular iPP, but with polymerization activies much lower than those of the structurally optimized bis(indenyl)-based catalysts. For example, in liquid propylene at 70 °C, 16 produces iPP with mmmm > 94%,... 

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)." °... 

Unless otherwise indicated, polymerizations were performed in liquid propylene. 

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