Polyolefins
A polyolefin is any of a class of polymers produced from a simple olefin (also called an alkene with the general formula CnH2n) as a monomer. For example, polyethylene is the polyolefin produced by polymerizing the olefin ethylene. Polypropylene is another common polyolefin which is made from the olefin propylene.
Most polyolefin produced in the industrial scale are made via polymerization through the use of catalysts. One example is the use of Ziegler-Natta catalyst for the polymerization of ethylene to polyethylene.
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.
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.
Polyolefin surfaces are not effectively joined together by solvent welding because they have excellent chemical resistance and are unaffected by common solvents. They can be adhesively bonded after surface treatment (they inherently have very low surface energies and don't wet-out well (the process of being covered and filled with resin)), and by some superglues (cyanoacrylates) and reactive (meth)acrylate glues. They are extremely inert chemically but exhibit decreased strength at lower and higher temperatures. As a result of this, thermal welding is a common bonding technique.
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