Manufacture of olefin polymers

A process for the manufacture of olefin by polymerization of C.sub.2-8 monoolefins at temperatures of from 60.degree. to 160.degree. C and olefin pressures of from 0.5 to 40 bars using a silicic xerogel/chromium trioxide catalyst which has been obtained by (1) synthesizing a particulate silicic xerogel having a particle diameter of from 10 to 2,000.mu.m, (2) doping said xerogel with from 0.1 to 10% by weight, based on the xerogel and calculated as chromium, of chromium trioxide or a chromium compound capable of converting to chromium trioxide under the conditions at stage (3), from a solution with evaporation of the solvent, and (3) finally maintaining the resulting product at a temperature of from 400.degree. to 1,100.degree. C for from 10 to 1,000 minutes in an anhydrous stream of gas containing oxygen in a concentration of more than 10% by volume, wherein the silicic xerogel/chromium trioxide catalyst is one which has been obtained by

• 1. synthesizing the silicic xerogel in the first stage by
• 1.1. taking a silicic hydrogel having a solids content, calculated as silicon dioxide, of from 10 to 25%,
• 1.2. extracting water from said hydrogel by means of an organic liquid selected from the series of C.sub.1-4 alkanols and/or C.sub.3-5 alkanones until said organic liquid absorbs no further water,
• 1.3. extracting said organic liquid from stage (1.2) from the resulting dehydrated gel saturated with said organic liquid from stage (1.2) by means of hydrocarbon or oxohydrocarbon having a boiling point at 1 bar from -20.degree. to +40.degree. C and a latent heat of vaporization of less than 7 kcal/mole and used in liquid form, until said hydrocarbon or oxohydrocarbon absorbs no further organic liquid from stage (1.2) and
• 1. 4. drying the resulting dehydrated gel saturated with said hydrocarbon or oxohydrocarbon at product temperatures of from 30.degree. to 140.degree. C and pressures of from 1 to 760 mm of Hg until no further loss of weight occurs at a temperature of 120.degree. C under vacuum of 10 mm Hg in a period of 30 minutes, thereby producing a xerogel, and
• 2. doping said xerogel in the second stage with the desired amount of chromium from a 0.05-5% w/w solution of chromium trioxide in a C.sub.3-5 alkanone or of a chromium compound capable of converting to chromium trioxide under the conditions of stage (3) in a C.sub.1-4 alkanol or C.sub.3-5 alkanone with evaporation of the solvent, the solution of said chromium component not containing more than 20% by weight of water.