GB843750A - Improvements in or relating to the polymerization of olefins, and the resulting polyolefins - Google Patents

Abstract

Solid polyolefins are made by first mixing the olefin with a catalyst, described more fully below, which comprises oxides of chromium, aluminium, and silicon, in the presence of a solvent for the polymer under such conditions of temperature and pressure that substantially no polymerization occurs and then starting the polymerization by supplying heat to the polymerizable mix. The catalyst comprising the above ingredients is made by subjecting the abovementioned oxides to a reduction treatment, at a temperature not exceeding 180 DEG C., with a saturated aliphatic hydrocarbon or a saturated cyclo-aliphatic hydrocarbon, or an aliphatic-aromatic hydrocarbon which does not contain unsaturation in the aliphatic portion of the hydrocarbon until any chemically combined chromium is at least in part in a valency state of less than 6. The above oxides may be free oxides or oxides present as chemical compounds. The hydrocarbons which may also act as solvents in the polymerization may be isooctane, propane, butane, pentane, hexane, heptane, decane, cyclohexane, toluene, or a xylene. Olefins mentioned are ethylene and propylene. The preferred condition under which substantially no polymerization occurs is at a temperature below the temperature employed in the reduction of the catalyst. Specifications 804,641 and 804,642 are referred to.ALSO:A catalyst for polymerizing olefins, is made by subjecting a mixture comprising oxides of hexavalent chromium, aluminium, and silicon to a reduction treatment at a temperature not exceeding 180 DEG C., with a saturated aliphatic hydrocarbon, or a saturated cyclo-aliphatic hydrocarbon, or an aliphatic-aromatic hydrocarbon which does not contain unsaturation in the aliphatic portion of the hydrocarbon, until any chemically combined chromium is at least in part in a valency state of less than 6. The above oxides may be free or oxides present in chemical compounds. Hydrocarbons mentioned are iso-octane, propane, butane, pentane, hexane, heptane, decane, cyclohexane, toluene, or a xylene. The starting material for the catalyst is preferably made by impregnating a composition of alumina and silica with a solution of chromic acid or chromic nitrate, ammonium chromate, or ammonium dichromate and heating the treated product at a temperature of at least 300 DEG C. for some time, in the presence of dry air or a dry inert gas. The starting material may be in the form of a powder of particle size up to 1 millimetre. Prior to impregnation the material may be (a) treated with HF or a preferably volatile fluoride; (b) heat-treated in the presence of air or an oxygen - containing gas, preferably together with water vapour, although inert gas treatment is also mentioned. In an example the starting material, a commercial cracking catalyst consisting of fine spherical particles of a gel of alumina and silica was first heated for 10 hours to 650 DEG C. in an air stream then impregnated with an aqueous CrO3 solution, air dried at 120 DEG C. for one hour, then heated in a glass tube at 500 DEG C. in the presence of dry air and the tube sealed. The catalyst was then made by breaking the tube in an autoclave in an atmosphere of nitrogen in the presence of pure iso-octane at a reduction temperature between 30 and 130 DEG C. preferably the latter temperature. Specifications 804,641 and 804,642 are referred to.