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Polyethylene

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Polyethylene or polyethene is one of the simplest and most inexpensive polymers. It is a waxy, chemically inert, rather soft plastic.

It is named thus because it is obtained by the polymerization of ethylene (ethene).

It is also known as polythene–a contraction of the name–and even more simply as PE.

The ethene molecule, C2H4 is CH2 = CH2, Two CH2 connected by a double bond, thus:

    H   H
    |   | 
    C = C

    |   |
    H   H

(The angles are somewhat different than shown in ASCII: The C-C-H bonds should all be approximately 120 degrees.)

In the polymerization process of ethene, the double bonds of the monomers are broken to allow additional single bonds to form amongst the carbon atoms, thus a macromolecule is created.

     H   H   H
     |   |   |
 R - C - C - C - R
     |   |   |
     H   H   H

(R-CH2-CH2-CH2-R)

R indicates that the chain consisting of the same compound CH2 continues.

Types of polyethene

Their are two main types of polyethene: high density poly(ethene) (HDPE) and low density poly(ethene) (LDPE). The main difference between these is the extent of branching[?] in the polymer chains. LDPE has many branches which means that the chains do not fit well together. It has therefore less strong intermolecular forces as the instantaneous-dipole induced-dipole attraction is less. This results in a lower density and tensile strength, increased malleability and faster biodegradation. LDPE is created by free radical polymerization.

HDPE has virtually no branching and thus stronger intermolecular forces and tensile strength. The lack of branching is ensured by the the aid of a Zieglar-Natta catalyst, where the polymer chain actually grows out from the catalyst.

The most common household use of LDPE is in plastic bags[?]; the most common household use of HDPE is in containers for milk, liquid laundry detergent[?], etc.



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