Part 6: Monomers: Functional Groups
Part 7: The Mechanism of Condensation Polymerization
Part 8: A Simulation of Condensation Polymerization
The monomers that are involved in condensation polymerization are not the
same as those in addition polymerization. The monomers for condensation polymerization
have two main characteristics:.
Instead of double bonds, these monomers have functional groups (like alcohol, amine, or carboxylic acid groups).
Each monomer has at least two reactive sites, which usually means two functional groups.
Some monomers have more than two reactive sites, allowing for branching between
chains, as well as increasing the molecular mass of the polymer. Four examples
of these difunctional monomers were introduced in Part 2 of this tutorial. Here
they are again:
|Guess the names of each of these monomers. Give the letter that corresponds to the correct name of the structure (use each letter only once). Hints: Glycol means that a molecule has more than one alcohol (-OH) group. Amine means that a molecule has an amino (-NH2) group. Diamine (or diamino) means that a molecule contains two amino groups. Acid means that a molecule contains a carboxylic acid group (-COOH). Click the button when done.|
Let's look again at the functional groups on these monomers. We've seen three:
|The carboxylic acid group|
You might have learned in chemistry or biology class that these groups can combine in such a way that a small molecule (often H2O) is given off.
The Amide Linkage:
When a carboxylic acid and an amine react, a water molecule is removed, and an amide molecule is formed.
Because of this amide formation, this bond is known as an amide linkage.
The Ester Linkage:
When a carboxylic acid and an alcohol react, a water molecule is removed, and an ester molecule is formed.
Because of this ester formation, this bond is known as an ester linkage.
Monomers involved in condensation polymerization have functional groups. These functional groups combine to form amide and ester linkages. When this occurs, a water molecule in removed. Since water is removed, we call these reactions condensation reactions (water condenses out.). When a condensation reaction involves polymerization, we call it condensation polimarization.
Let's look at a few common examples of condensation polymers.
You know that monomers that are joined by condensation polymerization have two functional groups. You also know (from Part 6) that a carboxylic acid and an amine can form an amide linkage, jand a carboxylic acid and an alcohol can form an ester linkage. Since each monomer has two reactive sites, they can form long-chain polymers by making many amide or ester links. Let's look at two examples of common polymers made from the monomers we have studied.
A carboxylic acid monomer and an amine monomer can join in an amide linkage.
As before, a water molecule is removed, and an amide linkage is formed. Notice that an acid group remains on one end of the chain, which can react with another amine monomer. Similarly, an amine group remains on the other end of the chain, which can react with another acid monomer.
Thus, monomers can continue to join by amide linkages to form a long chain. Because of the type of bond that links the monomers, this polymer is called a polyamide. The polymer made from these two six-carbon monomers is known as nylon-6,6. (Nylon products include hosiery, parachutes, and ropes.)
A carboxylic acid monomer and an alcohol monomer can join in an ester linkage.
Because the monomers above are all joined by ester linkages, the polymer chain is a polyester. This one is called PET, which stands for poly(ethylene terephthalate). (PET is used to make soft-drink bottles, magnetic tape, and many other plastic products.)
As difunctional monomers join with amide and ester linkages, polyamides and polyesters are formed, respectively. We have seen the formation of the polyamide nylon-6,6 and the polyester PET. There are numerous other examples.
Remember: The above process is called condensation polymerization because a molecule is removed during the joining of the monomers. This molecule is frequently water.
You learned in Part 7 that condensation polymers are made from monomers that have at least two functional groups. Because of this, the polymers can grow at either end of the chain.
During the polymerization process, the monomers tend to form dimers (two linked monomers) and trimers (three linked monomers) first. Then, these very short chains react with each other and with monomers. The overall result is that, at the beginning of polymerization, there are many relatively short chains. It is only near the end of polymerization that very long chains are formed.
The simulation you will see displays this process graphically. Click on the button below to view the Simulation window. (If nothing happens, click here.)
The larger box in this window is the area where you'll see the polymerization of monomers, represented by gray balls. When you click START, you'll see a lattice of gray, or unreacted, monomers. Once they polymerize into dimers, trimers, and so on, the monomers will turn black. Polymerization will continue for a few seconds. Then the display will change into a bar graph entitled "Distribution" and show the progression of the polymerization over time. The x-axis is the number of units in the polymer (the "n" in the formula of a polymer). This is suggested graphically with the series of polymers projected into the screen. As you move to the left, the polymers are longer. The y-axis is the number of polymers. The higher the bar, the more numerous are the polymers. The graph shows dynamically the distribution of polymers in the polymerization as the reaction progresses. Notice that at the beginning of the polymerization, the distribution lies farther to the right, meaning that there are a lot of monomers, dimers, trimers, and other short chains but few long chains. As the polymerization progresses, the distribution shifts to the left, indicating that there are fewer short chains and more of the longer ones.
The smaller box is a graph that displays the average size of the polymers versus the time of the polymerization. Again, notice that at early times, there are mostly short chains, and that near the end, there are more long.
First, we assume that there is only one type of difunctional monomer, as opposed to two types, as you saw in the two examples in Part 7. If you imagine that the polymers in the simulation are polyamides (like nylon-6,6), then the monomer has one carboxylic acid group and one alcohol group (picture the dimer you saw in Example 1 in the previous section). Second, we assume that there are only 90,000 monomers in the polymerization. In real life, the number of monomers is on the order of 1023. Despite the low number of monomers in the simulation, it does show the correct, real-life distribution of polymer chains over time.
View the simulation several times. You can see at any moment a "snapshot" of the polymerization by pressing QUIT. (However, you won't be able to restart the polymerization. Click START again to start a new simulation.)
If you have looked at these tutorials in order, then you are done. But don't be fooled - there's still a lot of interesting polymer chemistry to be learned. Good luck with your studies.