Isotactic propene/higher α-olefin quenched copolymers at intermediate commoner contents primarily crystallize into a mesomorphic phase. Several isotactic propene-α-olefin copolymers using 1-octene, 1-dodecene and 1-tetradecene as comonomers (CiPO, CiPDD and CiPTD, respectively) have been synthesized and their structure and thermal/mechanical/viscoelastic properties have been evaluated. At intermediate comonomer molar content ranged from around 4 to 9, the mesomorphic phase is developed under rather mild quenching conditions and independently of length of the comonomer. Thermal and mechanical parameters diminish as α-olefin content increases and their dependence on composition changes as function of existence of three-dimensional monoclinic crystallites or less ordered entities.