GB938069A - Process for the production of polypropylene - Google Patents

Abstract

Solid polypropylene is produced by polymerizing propylene in an inert hydrocarbon solvent at a temperature of 90 DEG to 200 DEG C. in the presence of a catalyst comprising titanium dichloride or trichloride and an aluminium trialkyl, aluminium alkyl chloride or aluminium alkyl hydride to obtain a solution of polypropylene in the solvent, adding to the solution an adsorbant consisting of silica gel as the sole effective purification additive, separating the solution from the adsorbant, at least the major part of the catalyst residues being removed with the adsorbent; by filtration while retaining the polypropylene in solution, and recovering the polypropylene from the filtrate. Preferably the mole ratio range of the titanium chloride to the alkyl aluminium compound is between 2:1 and 1:2 and the preferred titanium chloride is the trichloride and the preferred aluminium compound is aluminium triethyl. Preferably the titanium chloride is ground in an inert atmosphere prior to use. Generally 0,002 to 0,02 g. of catalyst are used for each gram of propylene to be polymerized. The catalyst components can be premixed and if desired heated to the polymerization temperature, and then added to the polymerization reactor, or they can be added simultaneously to the reactor in separate streams, the titanium component preferably being added as a slurry in the solvent to be used in the polymerization. The premixing is generally carried out in the solvent that is to be used in the polymerization reaction. The polymerization pressure is preferably in the range 0,07 to 70 atmospheres. The effluent from the polymerization reactor, comprising a slurry consisting of the hydrocarbon solvent, dissolved propylene polymer, dissolved unreacted propylene and the catalyst residues, is preferably passed to a flash tank to flash off the unreacted propylene. The silica gel can be added under pressure in the flash vessel simultaneously with the flashing operation, or after the flashing operation the slurry can be passed to a vessel to which the silica gel is added under pressure, e.g. under the pressure of an inert gas. The slurry containing silica gel is filtered at a temperature sufficient to keep the polymer in solution and at a pressure sufficient to keep the solvent liquid, e.g. 90 DEG to 180 DEG C. and 7 to 35 atmospheres and the dissolved polymer is recovered from the filtrate.