Hexane Ziegler-Natta polymerization

Our initial idea to use borane monomers, co-alkenylboranes (7-8), in Ziegler-Natta polymerization was based on three considerations, (a) the stability of borane to transition metal catalyst. Because trialkylborane is a Lewis acid, it offers an veiy good chance for them to coexist with the catalyst. In addition, the boron atom is relatively small, steric protection can be effectively applied if needed, (b) the solubility of borane monomers and polymers in the hydrocarbon solvents (hexane and toluene) used in Ziegler-Natta polymen zation. A soluble growing polymer chain is essential to obtain high molecular weight polymer, (c) the versatility of borane groups, which can be transformed to a remarkably fruitful variety of functionalities, as shown by H. C. Brown (9). 

Slurry phase (or suspension) process. The uniquedooldng equipment in Figure 23—5 is a loop reactor. This process also takes place in a solvent (in this case, normal hexane, isobutane, or isopentane) so that the mixture can be pumped continuously in a loop while the polymerization is taking place. Feeds (the solvent, comonomer if any, ethylene and Ziegler-Natta catalyst) are pumped into the loop and circulated. Polymerization rakes place continuously at temperatures below the melting point of the polyethylene allowing solid polymer particles to form enough to form slurry. The reaction takes place at 185—212°F and 75—150 psi. A slurry of HOPE in hexane is drawn off continuously or intermittently. 

Traditional Ziegler-Natta and metallocene initiators polymerize a variety of monomers, including ethylene and a-olefins such as propene, 1-butene, 4-methyl-1-pentene, vinylcyclo-hexane, and styrene. 1,1-Disubstituted alkenes such as isobutylene are polymerized by some metallocene initiators, but the reaction proceeds by a cationic polymerization [Baird, 2000]. Polymerizations of styrene, 1,2-disubstituted alkenes, and alkynes are discussed in this section polymerization of 1,3-dienes is discussed in Sec. 8-10. The polymerization of polar monomers is discussed in Sec. 8-12. 

The Unipol process employs a fluidized bed reactor (see Section 3.1.2) for the preparation of polyethylene and polypropylene. A gas-liquid fluid solid reactor, where both liquid and gas fluidize the solids, is used for Ziegler-Natta catalyzed ethylene polymerization. Hoechst, Mitsui, Montedison, Solvay et Cie, and a number of other producers use a Ziegler-type catalyst for the manufacture of LLDPE by slurry polymerization in hexane solvent (Fig. 6.11). The system consists of a series of continuous stirred tank reactors to achieve the desired residence time. 1-Butene is used a comonomer, and hydrogen is used for controlling molecular weight. The polymer beads are separated from the liquid by centrifugation followed by steam stripping. 

The Effect of Solvents. Both toluene and hexane were used as solvents in the copolymerization reactions. Normally, toluene is the solvent for metallocene catalytic polymerization of olefins, since both catalyst and MAO are soluble in toluene. Hexane and other aliphatic hydrocarbon solvents are typical solvents for Ziegler-Natta catalytic polymerization. They are poor solvents for MAO. The original idea of using hexane as solvent for copolymerization of ethylene and p-MS was to improve the morphology of the copolymer in order to facilitate the filtration and purifi-... 

Ethylene-propylene elastomers are made by solution polymerization of ethylene and propylene in a solvent such as hexane using Ziegler-Natta catalysts. EPDM terpolymers can be similarly made by adding 3 to 9% of any of the above dienes to the monoolefin mixture. 

Solutions of complex compound cyclopentadienyl-dititana in hexane and sodium cyclopentadienyl complex have high catalytic activity. Films with a metallic luster can be obtained by slowly removing the solvent from the formed gel polyacetylene in vacuum. It is assumed that the active complex has a tetrahedral structure. Polymerization mechanism is similar to the mechanism of olefin polymerization on catalyst Ziegler-Natta. Obtained at -80°C cis-polyacetylene films after doping had a conductivity of 240 (Ohm-cm" ). 

PP is produced by a variety of processes, most of them by a diluent phase propylene polymerization utihzing a Ziegler-Natta-activated titanium trichloride catalyst in the presence of low- to high-boiling hydrocarbons. Residual catalyst removal followed by hydrocarbon slurry centrifugation is the immediate upstream operation prior to thermal drying. Hexane is the solvent used in the major PP processes in operation today. As a result these polymers are solvent wet. 

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