Abstract: The conversion is increased in the preparation of polyisobutylene by polymerization of isobutylene in a polymerization zone at from 0.degree. to -130.degree. C. in the presence of from 0.01 to 1.0% by weight of a boron trifluoride catalyst, from 0.001 to 1.0% by weight of a molecular weight regulator and from 30 to 90% by weight of a low-boiling solvent which is inert under the reaction conditions, by a process in which the content of but-1-ene in the monomeric isobutylene is kept constant at from 1.9 to 2.1, in particular 2.0, % by weight.

Abstract: Isobutylene polymers are prepared by polymerizing the monomers at from 0.2 to 20 bar and from -130.degree. and +60.degree. C. in the presence of from 0.01 to 20 mmol of a boron halide catalyst system which contains as cocatalyst a carboxylic anhydride or carbonyl halide, preferably an aliphatic or aromatic, unsubstituted or halogen-substituted C.sub.4 -C.sub.18 -carboxylic anhydride of C.sub.2 -C.sub.12 -carbonyl halide and where appropriate also a customary polymerization accelerant in customary amounts, in the presence or absence of an inert solvent with dissipation of the heat of reaction.

Abstract: A process for the preparation of polyisobutylene by polymerizing isobutylene in a polymerization zone which is at from 0.degree. to -130.degree. C., by means of from 0.01 to 1.0 percent by weight of boron trifluoride catalyst, in the presence of a primary or secondary alcohol as polymerization accelerator and in the presence of from 30 to 90% by weight of a low-boiling solvent which is inert under the reaction conditions, wherein the solvent vaporized during the polymerization is continuously drawn off, liquefied and recycled, in the liquid state, to the polymerization zone, and wherein the boron trifluoride catalyst used contains less than 0.3, in particular less than 0.01, percent by volume of silicon tetrafluoride and the solvent, which is preferably ethylene, is recycled not less than twice but not more than 36 times during the continuous polymerization.