Polyolefin

Apolyolefinis a type ofpolymerwith the general formula (CH₂CHR)_nwhere R is analkylgroup. They are usually derived from a small set of simple olefins (alkenes). Dominant in a commercial sense arepolyethyleneandpolypropylene.More specialized polyolefins includepolyisobutyleneandpolymethylpentene.They are all colorless or white oils or solids. Manycopolymersare known, such aspolybutene,which derives from a mixture of differentbuteneisomers. The name of each polyolefin indicates the olefin from which it is prepared; for example, polyethylene is derived fromethylene,and polymethylpentene is derived from4-methyl-1-pentene.Polyolefins are not olefins themselves because the double bond of each olefin monomer is opened in order to form the polymer. Monomers having more than one double bond such asbutadieneandisopreneyield polymers that contain double bonds (polybutadieneandpolyisoprene) and are usually not considered polyolefins. Polyolefins are the foundations of many chemical industries.

Industrial polyolefins

Most polyolefin are made by treating the monomer with metal-containing catalysts. The reaction is highly exothermic.

Traditionally,Ziegler-Natta catalystsare used. Named after the NobelistsKarl ZieglerandGiulio Natta,these catalysts are prepared by treatingtitanium chlorideswithorganoaluminium compounds,such astriethylaluminium.In some cases, the catalyst is insoluble and is used as a slurry. In the case of polyethylene, chromium-containingPhillips catalystsare used often.Kaminsky catalystsare yet another family of catalysts that are amenable to systematic changes to modify thetacticityof the polymer, especially applicable topolypropylene.

Thermoplastic polyolefins

low-density polyethylene (LDPE),

linear low-density polyethylene (LLDPE),

very-low-density polyethylene (VLDPE),

ultra-low-density polyethylene (ULDPE),

medium-density polyethylene (MDPE),

polypropylene (PP),

polymethylpentene (PMP),

polybutene-1 (PB-1);

ethylene-octene copolymers,

stereo-block PP,

olefin block copolymers,

propylene–butane copolymers;

Polyolefin elastomers (POE)

polyisobutylene (PIB),

poly(a-olefin)s,

ethylene propylene rubber (EPR),

ethylene propylene diene monomer (M-class) rubber (EPDM rubber).

Properties

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 duringpolymerization.Examples include adding a small percentage ofcomonomerlike1-hexeneor1-octeneduring the polymerization ofethylene,or occasional irregular insertions ( "stereo" or "regio" defects) during the polymerization ofisotacticpropylene.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 bysolvent weldingbecause they have excellentchemical resistanceand are unaffected by common solvents. They inherently have very lowsurface energiesand don't wet-out well (the process of being covered and filled withresin). They can be adhesively bonded after surface treatment, and by some superglues (cyanoacrylates) and reactive (meth)acrylateglues.They are extremelyinertchemically but exhibit decreased strength at lower and higher temperatures.As a result of this, thermal welding is a common bonding technique.

Practically all polyolefins that are of any practical or commercial importance arepoly-Alpha-olefin(or poly-α-olefin or poly Alpha olefin, sometimes abbreviated asPAO), a polymer made by polymerizing anAlpha-olefin.AnAlpha-olefin(or α-olefin) is analkenewhere the carbon-carbondouble bondstarts at the α-carbon atom, i.e. the double bond is between the #1 and #2 carbons in themolecule.Alpha-olefins such as1-hexenemay be used as co-monomers to give analkylbranchedpolymer(seechemical structurebelow), although1-deceneis most commonly used for lubricant base stocks.

1-hexene, an example of an Alpha -olefin

Many poly- Alpha -olefins have flexible alkyl branching groups on every other carbon of their polymer backbone chain. These alkyl groups, which can shape themselves in numerousconformations,make it very difficult for the polymermoleculesto align themselves up side-by-side in an orderly way. This results in lower contactsurface areabetween themoleculesand decreases theintermolecular interactionsbetween molecules.Therefore, many poly- Alpha -olefins do not crystallize or solidify easily and are able to remain oily,viscousliquids even at lowertemperatures.Low molecular weight poly-Alpha-olefins are useful as syntheticlubricantssuch assynthetic motor oilsfor vehicles and can be used over a wide temperature range.

Evenpolyethylenescopolymerizedwith a small amount of Alpha -olefins (such as1-hexene,1-octene,or longer) are more flexible than simple straight-chain high-density polyethylene, which has no branching.Themethylbranch groups on apolypropylenepolymer are not long enough to make typical commercial polypropylene more flexible than polyethylene.

Uses

• Polyethylene:
  • HDPE:used for film (wrapping of goods), blow molding (e.g. bottles), injection molding (e.g., toys, screw caps), extrusion coating (e.g., coating on milk cartons), piping for distributing water and gas, insulation for telephone cables. Wire and cable insulation.
  • LDPE:mainly (70%) used for film.

• Polypropylene:injection molding, fibers, and film. Compared to polyethylene, polypropylene is stiffer but less prone to breaking. It is less dense but shows more chemical resistance.
• Synthetic base oil(by far the most used one): industrial and automotive lubricants.

External links

• MSDS (Material Safety Data Sheet)