It is well known to prepare poly-.alpha.-olefines, which can be used e.g. as a lubricant or as a component thereof, by using a complex of BF.sub.3 and a co-catalyst as a catalyst, as described e.g. in U.S. Pat.  Nos.  3,780,128 and 4,434,309.  A catalyst complex is separated by distillation from the poly-.alpha.-olefine to be obtained as a reaction product, and the catalyst complex can thereafter be reused for catalyzing a similar polymerization reaction (see, e.g. FI Patent No.  80 891). 
When separating the catalyst complex from the reaction product by distillation, a small portion (less than 10%) of this complex decomposes and the gaseous BF.sub.3 is released, causing a chemical loss.  Due to this chemical loss, this compound (BF.sub.3) which is hazardous to environment, has to be recovered e.g. by means of a washer. 
In order to avoid the decomposition of the catalyst complex, the distillation is performed in a reduced pressure, and thereby in a lower temperature.  The underpressure can be achieved by means of a fluid ring pump or a torque pump (a fluid ejector pump).  Usually, the fluid ring pump, which can be a piston pump, a centrifugal pump, rotary sealed pump etc.,  is sealed with a fluid ring, wherein the fluid is water or an inert fluid with respect to the gases to be removed.  The efficiency of the fluid-ring and/or torque pump may be further increased by placing a mechanical blaster (e.g. a rotary piston pump) in series with said pump.