There are disclosed new catalyst-forming components and catalysts prepared therefrom which, used in the polymerization of olefins, result in high yields of polyolefins having a narrow molecular weight distribution and which do not require special purifying after-treatments for the removal of catalyst residues. More particularly, the new catalyst components and catalysts prepared from them are useful in the polymerization of ethylene or alpha-olefins CH.sub.2 .dbd.CHR, in which R is an alkyl or aryl radical of from 1 to 8 carbon atoms, or mixtures of ethylene with said alpha-olefins.