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Polyolefin properties range from liquidlike to rigid solids, and are primarily determined by their molecular weight and degree of crystallinity. Polyolefins are generally described as semicrystalline polymers, with degrees of crystallinity ranging from 0% (liquidlike) to 60% or higher (rigid plastics). Low degrees of crystallinity (0-20%) are associated with liquidlike-to-elastomeric properties. Intermediate degrees of crystallinity (20-50%) are associated with ductile thermoplastics, and degrees of crystallity over 50% are associated with rigid and sometimes brittle plastics.

Chemically, the degree of 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 decreases with increasing content defects or comonomers (which don't fit on the crystal lattice.)