Atactic or amorphous poly-alpha-olefins containing 100%-65% propylene and optionally up to about 35% ethylene and/or up to about 15% of a C₄-C₈ alpha-olefin are prepared by polymerizing the monomer(s) at 180-450 F. and at a pressure sufficient to substantially maintain the monomer(s) in the liquid phase. The polymerization is carried out in the presence of catalyst system consisting of (a) a transition metal halide selected from the group consisting of (i) a titanium trihalide and an aluminum alkyl, (ii) a titanium halide on a comminuted magnesium halide support, and (iii) a titanium halide sandwich compound and (b) aluminum alkyl as a co-catalyst. No stereoregulator or only minor amounts of a stereoregulator are used.