Session 2:

Ethylene Polymerization


Question:

How can ethylene molecules be joined together to form polyethylene polymer?


Text:

Many different kinds of monomers have been employed in polymerization reactions. We will focus our attention on what is perhaps the simplest type of monomer subunit – alkenes or "olefins". The smallest olefin is ethylene, C2H4, which consists of two carbon atoms that are each bonded to two hydrogen atoms and linked by a double bond. The double bond is composed of a strong s-bond and a weaker p-bond that results from overlap of p orbitals on the carbon centers. Ethylene is a two-dimensional (flat) molecule, 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 (Scheme 1).

ethylene

polyethylene

Scheme 1

All of the carbon-carbon bonds in the resulting polymer ("polyethylene") are single bonds and the groups around each carbon center are tetrahedrally oriented (109.5o apart). Hence, as shown in Figure 1, the chain has three-dimensional structure. The value of n (i.e., the length of the chain) varies with the reaction conditions but is generally 500 or more. Furthermore, a typical synthetic polymer does not have a single value for n. Rather, it 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 1:

A portion of linear polyethylene, showing the three-dimensional structure. Carbon labels are omitted for clarity.

A 3D stick representation of linear polyethylene. The hydrogens are shown in white and the carbons are in green. The polymer is terminated by hydrogens.
Note: To download a pdb file for an 8-mer polyethlyene, please click on the 2D representation, the stick representation, or the words polyethylene.

 

Polyethylene can be produced in several different ways. 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. This synthetic approach leads to polyethylene which is highly branched, i.e., instead of a single long strand, a typical molecule has many side branches (see Figure 2). This open structure results in a polymer with a relatively low density (usually around 0.91 gr/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 is used in products such as flexible squeeze bottles or wash bottles and clear plastic sandwich bags.

Figure 2:

Two-dimensional representation of a portion of branched polyethylene. Hydrogen labels are omitted for clarity.

A 3D stick representation of branched polyethylene. The hydrogens are shown in white and the carbons are in green. The polymer is terminated by hydrogens.
Note: To download a pdb file for a branched polyethlyene, please click on the 2D representation, the stick representation, or the words branched polyethylene.

Prior to the 1960's, LDPE was the only form of polyethylene available. However, in the late '50's and early '60's, Karl Ziegler in Germany and Giulio Natta in Italy discovered that the use of metal-based catalysts, particularly titanium/aluminum systems, led to a new type of polyethylene. These new catalysts allowed Ziegler and Natta to make very long (>10,000 monomer units) linear polyethylene molecules under very mild conditions (1-10 atmospheres, 50 o-100 oC). Because these chains did not possess side branches, they could line up parallel to one another in an ordered structure. This efficient packing of molecules gave rise to a higher density material (density between 0.97 and 0.99 gr/cm3). In addition, this high density polyethylene or HDPE had greater strength and a higher melting point (130 oC) than the LDPE discussed above. Furthermore, it was opaque, rather than transparent like LDPE. Today, HDPE is used in applications that require strength and rigidity, e.g., the one gallon plastic milk jugs that must retain their shape when filled with milk and stack on top of one another. The plastic grocery bags that are often used as substitutes for paper are also made of HDPE.


Hands-On Activity:

Using Cochrane's Molecular Models, build a model of linear polyethylene.

 


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