What is Polyethylene?
Picture adapted from: http://study.com/academy/lesson/what-is-polyethylene-properties-uses-quiz.html
Polyethylene is an organic polymer made of several monomer subunits.
Polymerization of Polyethylene Polymer
Many different kinds of monomers have been employed in polymerization reactions. The simplest type of monomer subunit in alkene is ethylene, C2H4, which consists of a carbon-carbon double bond, with each carbon atoms bonded to two hydrogen atoms. The double bond is composed of a strong s-bond and a weaker p-bond that results from the overlap of p orbitals on the carbon centers. Ethylene has a trigonal planar molecule geometry, and the atoms around each carbon center are trigonally oriented (120o apart). When ethylene monomers polymerize, the weak p-bonds are destroyed and a long chain of –CH2–CH2– units is produced. (Fig1)
Figure 1
Picture adapted from: http://www.chemistry.wustl.edu/~edudev/Designer/session2.html
All of the carbon-carbon bonds in the polyethylene are single bonds and the groups around each carbon center has a tetrahedral structure (109.5o apart). Hence, as shown in Figure 2, the chain has a three-dimensional structure. The value of n varies with the reaction conditions but is generally 500 or more. A polyethylene polymer generally consists of a mixture of individual polymer molecules with different lengths. Therefore, it is meaningful to discuss the average chain length or molecular weight for a polymer sample.
Figure 2: Polyethylene
Picture adapted from: http://www.chemistry.wustl.edu/~edudev/Designer/session2.html
Polyethene can be produced in several different ways.
Low-density polyethylene: LDPE
One approach involves relatively high pressure (2000 atm) and high temperature (200 oC) conditions and the use of a “free radical” initiator. A free radical is a molecule with at least one unpaired electron.
Picture adapted from: https://chemistry.boisestate.edu/richardbanks/organic/alkene.html
This synthetic approach leads to polyethylene which is highly branched (Figure 3). This open structure results in a polymer with a relatively low density (usually around 0.91 g/cm3) and it is, therefore, referred to as low-density polyethylene or LDPE. LDPE melts at about 120 oC and is soft, stretchy, transparent, and not very strong. It can be found in products such as plastic cups and clear plastic bags.
Figure 3: LDPE
Picture adapted from: http://www.chemistry.wustl.edu/~edudev/Designer/session2.html
High-density polyethylene: HDPE
Picture adapted from: http://magrecycling.com.au/RecycledProducts/HDPE.aspx
HDPE, on the other hand, is made by applying intense heat to petroleum – a process known as ‘cracking’ – to produce ethylene gas. Under controlled conditions, these gas molecules link together to form long chains or polymers and produce polyethylene.
References:
- Danielle R. What is Polyethylene? – Properties & Uses – Video & Lesson Transcript. Study.com. [accessed 2017 Mar 1]. http://study.com/academy/lesson/what-is-polyethylene-properties-uses-quiz.html
- John BR, Regina FF. Session 2: Ethylene Polymerization. 2006 Jun 15 [accessed 2017 Mar 1]. http://www.chemistry.wustl.edu/~edudev/Designer/session2.html
- Banks RC. Alkene reactions. Polymerisation of Alkenes. 2000 Aug [accessed 2017 Mar 1]. https://chemistry.boisestate.edu/richardbanks/organic/alkene.html
- Ltd LP. HDPE. HDPE. [accessed 2017 Mar 1]. http://magrecycling.com.au/RecycledProducts/HDPE.aspx