Abstract: A supported solid catalyst which can be used for the polymerization and clymerization of conjugated dienes having as its basis the reaction product of:A) a solid MgCl.sub.2 support,B) an ether, preferably THF, as swelling agent for the support,C) a metal salt selected from among metals having an atomic number of between 57 and 71 or 92 in the periodic table of elements and, if the metal salt is not a halide,D) a halogenation agent selected from the group consisting of a halogenated compound of aluminum and a halogenated compound not containing aluminum, the reaction solid being free from the swelling agent, plusE) an organic derivative of aluminum which is obligatory when the halogenation agent is not a halogenated compound of aluminum and optional when the halogenation agent is a derivative of aluminum.Also, a method of preparing this catalyst.

Abstract: Disclosed is a rim intended to form a safety unit with a tire and a body of oam rubber, which gives the tire the ability to roll in a deflated state. Lateral walls of a central groove receiving the body each include an undercut portion to retain the body effectively. The groove is delimited on opposite sides by lateral walls. The groove has an orifice formed therein for communication with atmospheric pressure.