CH 106 - Lesson 2

Polymerization of Alkenes

Process

Let's look at one particular example and then generalize from that. The following diagrams attempt to illustrate, in a step-by-step manner, the polymerization of propene. (These diagrams are also shown in Example 9 in your workbook.) In these diagrams, some of the bonds are shown in an extended form to keep the atoms spaced for easier viewing. Unfortunately, the extensions don't join and you will have to mentally provide the continuity of the bond. (They have been joined in the workbook, so you may want to follow along there.)

Step 1 depicts the addition of one propene molecule to another. The carbon atoms of the second propene molecule have been highlighted in red so that we can keep track of them through this process. The bonds broken in the dimers are highlighted in green. Since the product is one molecule made from two smaller molecules of the same type, it is called a dimer. The individual smaller molecules are called monomers. Notice that the two propene molecules did not bond to one another end-to-end. There is a methyl group hanging off to the side. If we had used 1-butene, there would have been an ethyl group off to the side. Notice also that after the addition reaction, the product still has a double bond. Therefore another molecule can be added to it. This is shown in Step 2.

The product here is made from three smaller units so it is called a trimer. It also has a double bond so it can react further, as shown in Step 3.

This process can keep on going over and over, hundreds or even thousands of times. When that happens, the product is called a polymer. The process would stop when we ran out of propene or if something else reacted with the double bond and stopped the chain reaction.

The diagram below (also shown in Example 9-d in your workbook) points out a few things about the structural formula of this polymer (actually it is still just a tetramer at this point). The structural formula shown here is the same as the one shown as the product in Step 3 above, except that it has been straightened out. Notice the location of the highlighted carbon atoms from the second propene molecule.

Let's expand our focus to include the attached hydrogen atoms of that "propene" segment of the polymer by also highlighting them in red in the structural formula shown below. Notice that there are identical groups of three carbon atoms and six hydrogen atoms on both sides of this segment. Let's highlight them, too, this time in green. Notice that we're short one hydrogen atom of being able to draw another identical segment on the far right. Also, we have an extra hydrogen atom on the left.

As the polymerization process continues, the number of identical segments we could draw would increase. When the chain reaches hundreds of these units in length, it is not only inconvenient to draw a complete structural formula, it is impossible. One simple way to show that a unit like this repeats itself a large number of times is shown below--simply use a subscript "n" to show that we have a large but unspecified number of repetitions of the formula in the brackets. Parentheses are also commonly used. Often, the bonds from the two central carbon atoms are extended through the brackets or parentheses to emphasize how the chain of carbon atoms continues on and on. This is a short-cut form for writing the structural formula of a polymer.          

It is important for you to notice that the repeating pattern is based on a two-carbon-atom sequence in the polymer chain, even though the original monomer had three carbon atoms in it. This two-carbon-atom pattern is characteristic of polymers made by addition reactions using alkenes. The two carbon atoms in the sequence are the two carbon atoms that were double bonded in the original alkene.

There are several ways in which this process can be represented. The reaction mechanism that is shown above (and in your workbook) for the polymerization of propene is not completely proper. Another approach, also not completely valid, is shown in the experiment portion of your workbook. Use whichever approach seems the best to you. You can get more information about such reactions in any organic chemistry textbook, if you wish.

Reaction Equations

We can also use a short-cut method for writing the equation for a polymerization reaction. (This is also shown in example 9-e in your workbook.) To emphasize the two-carbon-atom repetition, instead of stringing out the formula, write the formula of the alkene with the double-bonded carbon atoms on one line and everything else straight up and down from them as shown.

Then write the equation as shown below. Remember that "n" means "many". The equation for polymerization then says that many (n) alkene molecules become a long polymer chain made of many (n) repeating units.

This equation (Example 9-f in your workbook) shows the short-cut equation for the polymerization reaction of 2-pentene. Take a look at it and make sure that it makes sense to you.

A general form of the equation is shown here (and also in Example 9-g in your workbook) where W-, X-, Y-, and Z- could be any combination of carbon, hydrogen and other atoms.

Practice

Now get some practice by writing equations for the polymerization of the alkenes listed below. (They are the ones shown in Example 5 in your workbook.) Pick two or three, any that you want, figure out the equation for the reaction and then check your answers below.

Answers

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E-mail instructor: Sue Eggling

Clackamas Community College
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