Polyolefin properties range from liquidlike to rigid solids, and are primarily determined by their molecular weight and degree of crystallinity. Polyolefin degrees of crystallinity range from 0% (liquidlike) to 60% or higher (rigid plastics). Crystallinity is primarily governed by the lengths of polymer's crystallizable sequences established during polymerization. Examples include adding a small percentage of comonomer like 1-hexene or 1-octene during the polymerization of ethylene, or occasional irregular insertions ("stereo" or "regio" defects) during the polymerization of isotactic propylene. The polymer's ability to crystallize to high degrees decreases with increasing content of defects.