Polyolefin

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A polyolefin is a type of polymer with the general formula (CH2CHR)n where R is an alkyl group. They are usually derived from a small set of simple olefins (alkenes). Dominant in a commercial sense are polyethylene and polypropylene. More specialized polyolefins include polyisobutylene and polymethylpentene. They are all colorless or white oils or solids. Many copolymers are known, such as polybutene, which derives from a mixture of different butene isomers. The name of each polyolefin indicates the olefin from which it is prepared; for example, polyethylene is derived from ethylene, and polymethylpentene is derived from 4-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 as butadiene and isoprene yield polymers that contain double bonds (polybutadiene and polyisoprene) and are usually not considered polyolefins. Polyolefins are the foundations of many chemical industries. [1]

Contents

Industrial polyolefins

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

Traditionally, Ziegler-Natta catalysts are used. Named after the Nobelists Karl Ziegler and Giulio Natta, these catalysts are prepared by treating titanium chlorides with organoaluminium compounds, such as triethylaluminium. In some cases, the catalyst is insoluble and is used as a slurry. In the case of polyethylene, chromium-containing Phillips catalysts are used often. Kaminsky catalysts are yet another family of catalysts that are amenable to systematic changes to modify the tacticity of the polymer, especially applicable to polypropylene.

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 during polymerization. [2] Examples include adding a small percentage of comonomer like 1-hexene or 1-octene during the polymerization of ethylene, [3] or occasional irregular insertions ("stereo" or "regio" defects) during the polymerization of isotactic propylene. [4] 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. [5]

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. [6] They are extremely inert chemically but exhibit decreased strength at lower and higher temperatures. [7] As a result of this, thermal welding is a common bonding technique.

Practically all polyolefins that are of any practical or commercial importance are poly-alpha-olefin (or poly-α-olefin or polyalphaolefin, sometimes abbreviated as PAO), a polymer made by polymerizing an alpha-olefin. An alpha-olefin (or α-olefin) is an alkene where the carbon-carbon double bond starts at the α-carbon atom, i.e. the double bond is between the #1 and #2 carbons in the molecule. Alpha-olefins such as 1-hexene may be used as co-monomers to give an alkyl branched polymer (see chemical structure below), although 1-decene is most commonly used for lubricant base stocks. [8]

1-hexene, an example of an alpha-olefin 1-Hexene.svg
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 numerous conformations, make it very difficult for the polymer molecules to align themselves up side-by-side in an orderly way. This results in lower contact surface area between the molecules and decreases the intermolecular interactions between molecules. [9] Therefore, many poly-alpha-olefins do not crystallize or solidify easily and are able to remain oily, viscous liquids even at lower temperatures. [10] Low molecular weight poly-alpha-olefins are useful as synthetic lubricants such as synthetic motor oils for vehicles and can be used over a wide temperature range. [8] [10]

Even polyethylenes copolymerized with a small amount of alpha-olefins (such as 1-hexene, 1-octene, or longer) are more flexible than simple straight-chain high-density polyethylene, which has no branching. [7] The methyl branch groups on a polypropylene polymer are not long enough to make typical commercial polypropylene more flexible than polyethylene.

Uses


Related Research Articles

A Ziegler–Natta catalyst, named after Karl Ziegler and Giulio Natta, is a catalyst used in the synthesis of polymers of 1-alkenes (alpha-olefins). Two broad classes of Ziegler–Natta catalysts are employed, distinguished by their solubility:

<span class="mw-page-title-main">Petrochemical</span> Chemical product derived from petroleum

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<span class="mw-page-title-main">Polypropylene</span> Thermoplastic polymer

Polypropylene (PP), also known as polypropene, is a thermoplastic polymer used in a wide variety of applications. It is produced via chain-growth polymerization from the monomer propylene.

In polymer chemistry, living polymerization is a form of chain growth polymerization where the ability of a growing polymer chain to terminate has been removed. This can be accomplished in a variety of ways. Chain termination and chain transfer reactions are absent and the rate of chain initiation is also much larger than the rate of chain propagation. The result is that the polymer chains grow at a more constant rate than seen in traditional chain polymerization and their lengths remain very similar. Living polymerization is a popular method for synthesizing block copolymers since the polymer can be synthesized in stages, each stage containing a different monomer. Additional advantages are predetermined molar mass and control over end-groups.

<span class="mw-page-title-main">Synthetic oil</span> Lubricant consisting of artificially made chemical compounds

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Butene, also known as butylene, is an alkene with the formula C4H8. The word butene may refer to any of the individual compounds. They are colourless gases that are present in crude oil as a minor constituent in quantities that are too small for viable extraction. Butene is therefore obtained by catalytic cracking of long-chain hydrocarbons left during refining of crude oil. Cracking produces a mixture of products, and the butene is extracted from this by fractional distillation.

A post-metallocene catalyst is a kind of catalyst for the polymerization of olefins, i.e., the industrial production of some of the most common plastics. "Post-metallocene" refers to a class of homogeneous catalysts that are not metallocenes. This area has attracted much attention because the market for polyethylene, polypropylene, and related copolymers is large. There is a corresponding intense market for new processes as indicated by the fact that, in the US alone, 50,000 patents were issued between 1991-2007 on polyethylene and polypropylene.

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<span class="mw-page-title-main">Straight-chain terminal alkene</span>

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James Carl Stevens, a chemist, was the first Distinguished Fellow, at the Dow Chemical Company, retiring in January 2015. His area of expertise is organometallic chemistry and his primary field of research is in the area of polyolefin catalysis, particularly in the area of polyethylene, polypropylene, ethylene/styrene copolymers, and the combinatorial discovery of organometallic single-site catalysts. Stevens major contributions have come in the discovery and commercial implementation of single-site polyolefin catalysts. He invented and led the commercialization of constrained geometry catalyst for the polymerization of olefins. These have been commercialized by Dow as a number of polymers, elastomers and plostomers.

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References

  1. 1 2 Whiteley, Kenneth S.; Heggs, T. Geoffrey; Koch, Hartmut; Mawer, Ralph L.; Immel, Wolfgang (2000). "Polyolefins". Ullmann's Encyclopedia of Industrial Chemistry . Weinheim: Wiley-VCH. doi:10.1002/14356007.a21_487. ISBN   978-3527306732.
  2. ↑ Tashiro, Stein, Hsu, Macromolecules 25 (1992) 1801-1810
  3. ↑ Alizadeh et al., Macromolecules 32 (1999) 6221-6235
  4. ↑ Bond, Eric Bryan; Spruiell, Joseph E.; Lin, J. S. (1 November 1999). "A WAXD/SAXS/DSC study on the melting behavior of Ziegler-Natta and metallocene catalyzed isotactic polypropylene". Journal of Polymer Science Part B: Polymer Physics. 37 (21): 3050–3064. Bibcode:1999JPoSB..37.3050B. doi:10.1002/(SICI)1099-0488(19991101)37:21<3050::AID-POLB14>3.0.CO;2-L.
  5. ↑ A. J. Kinloch, R. J. Young, The Fracture Behaviour of Polymers, Chapman & Hall, 1995. pp. 338-369. ISBN   0 412 54070 3
  6. ↑ "Properties and Applications of Polyolefin Bonding" " Master Bond Inc." Retrieved on June 24, 2013
  7. 1 2 James Lindsay White, David D. Choi (2005). Polyolefins: Processing, Structure Development, And Properties. Munich: Hanser Verlag. ISBN   1569903697.[ page needed ]
  8. 1 2 R. M. Mortier, M. F. Fox and S. T. Orszulik, ed. (2010). Chemistry and Technology of Lubricants (3rd ed.). Netherlands: Springer. ISBN   978-1402086618.[ page needed ]
  9. ↑ "Properties of Alkanes Archived 2013-01-07 at the Wayback Machine ." Retrieved on June 24, 2013
  10. 1 2 L. R. Rudnick and R. L. Shubkin, ed. (1999). Synthetic Lubricants and High-performance Functional Fluids (2nd ed.). New York: Marcel Dekker. ISBN   0-8247-0194-1.[ page needed ]
  11. ↑ "Comparison of PE and PP".
  12. ↑ "Polyalphaolefin (PAO) Lubricants Explained". www.machinerylubrication.com. Retrieved 2022-06-26.
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