Amorphous polymers of alpha-olefins, particularly of propylene, having high molecular weights and narrow molecular weight distributions, in which the isotactic sequences are more abundant than the syndiotactic ones, can be obtained in high yields at temperatures of industrial interest by carrying out the polymerization reaction in the presence of metallocene catalysts comprising particular bridged bis-indenyl compounds substituted in the 3-position on the indenyl groups. The obtained amorphous polymers are particularly useful for the preparation of miscible compositions with substantially isotactic alpha-olefins.