Theory

Step-growth Polymerizations and Chain-growth Polymerizations

All polymerizations fall into two categories: step-growth polymerizations and chain-growth polymerizations. Both step-growth polymerizations and chain-growth polymerizations are used to make nylons. Making nylon from a diacid and a diamine is a step-growth polymerization. So is making nylon from an amino acid. Making nylon from lactams is usually a chain-growth polymerization.

So what is the difference between the two types of polymerization? That would take a long time to explain, so if you want to know, go read the Macrogalleria page called Putting Them Together. But there are some practical differences you should know about for this experiment...

In a step-growth system, we start off with monomers. The monomers combine and grow into dimers, trimers, tetramers, and so forth. The molecules get bigger and bigger, but only when we're done (when the polymerization reaches high conversion) do we have high molecular weight polymers.

But in a chain growth system, we start off with monomers, and the monomers quickly form high molecular weight polymers. There are high molecular weight polymers present in your test tube just after you start the polymerization. What's more, you won't have dimers, trimers, and other oligomers hanging around. A growing polymer chain grows so fast that it reaches high molecular weight quickly, and it doesn't spend any real length of time as an oligomer.

Polycondensations

Please don't get confused, but there is another way to describe polymerizations other than the step-growth/chain-growth system. There is also the condensation/addition system. To know more about this system, again go visit the Macrogalleria page Putting Them Together. The most important thing you need to know about this system is that it classifies all polymerizations as polycondensations or polyadditions. Polycondensations are polymerizations in which a small molecule by-product is produced. The by-product is usually something like water, HCl, or once in awhile NaCl. Polyadditions on the other hand are polymerizations in which no by-product is produced.

We're going to talk now about using polycondensations to make nylons. The simplest polycondensation for making nylons is the polymerization of a diacid and a diamine. This reaction might not normally go to high conversions, but by removing the water by-product (usually by carrying out the reaction under vacuum so the water evaporates), we can force this reaction go to higher conversions.


Removing water makes the reaction go to high conversion thanks to LeChatlier's Principle. The self-condensation of an amino acid is a simple modification of the above reaction. Again, this reaction is forced to high conversion by removing the water formed with a vacuum. Take a look at the above two reactions. Because the first one uses two different types of monomers, one which has two amine groups and one with two acid group, we call this an AA-BB system. Think of the amine groups as A's and the acid groups as B's, and you can figure out why. Likewise, the second reaction, the one using the amino acid, is called an AB system. This is obviously because the monomer contains both an amine group (A) and an acid group (B) in the same molecule.

AB systems have an advantage over AA-BB systems. The advantage is that in an AB system, one always has the same amount amine groups and acid groups. As we all know, stoichiometric balance of amine and acid groups is absolutely critical when making nylons. With AA-BB systems, the amounts of the two monomers must be measured very carefully to ensure perfect stoichiometric balance.

Interfacial Polymerizations

Making nylon 6,6 is even easier if you use a diamine and a diacid chloride instead of a diacid. This is because acid chlorides are much more reactive than acids. The reaction is done in a two-phase system. The amine is dissolved in water, and the diacid chloride in an organic solvent. The two solutions are placed in the same beaker. Of course, the two solutions are immiscible, so there will be two phases in the beaker. At the interface of the two phases, the diacid chloride and diamine can meet each other, and will polymerize there. There is special way to do this called the "Nylon Rope Trick"⁴, and we'll show you how to do that in just a minute.

While this is a neat party trick, it isn't used commercially because, first, acid chlorides are a lot more expensive than acids, and second, acid chlorides stink horribly, and are much more toxic than acids. And third, the fibers produced by this trick aren't very strong, anyway.

Ring-Opening Polymerization

The ring opening-polymerization of lactams is a chain-growth polymerization. It is also a polyaddition reaction, that is, no byproducts are produced. The thermodynamic driving force for ring-opening polymerizations is ring strain. Cyclic molecules polymerize in order to relieve the strain. Take a look at Table 3, and you'll see that 5- and 6-membered rings don't have very much ring strain, so they don't polymerize well. But 7-membered rings, like e-caprolactam, are much more strained and polymerize easily. As you can see in Table 3, so do many larger cyclic monomers.

Table 3

Polymerizability of Lactams⁵

Type of	Order of Polymerizability
Polymerization	(ring sizes)
anionic (strong base)	7 > 5 > 6
hydrolytic (water initiated)	7 > 8 > 9 >> 5 > 6
cationic	8 > 7 > 11 > 5 > 6

There are two ways to carry out a ring-opening polymerization of e-caprolactam. Down at the nylon factory, nylon 6 is made using a water-initiated process. Read about it on the Macrogalleria page Making Nylon 6

The second way to make nylon 6 is to use a strong base as an initiator.⁶ How strong a base? Very strong. A normal strong base like NaOH isn't going to work here. We'll need an extra strong base like sodium hydride (NaH). The hydride anion is an incredibly strong base, and when it sees caprolactam, it runs straight to the amide hydrogen and pulls it right off, as you can see in Figure 1.

Figure 1



Now the leftover sodium cation can form a salt with the negatively charged nitrogen that is left behind when the hydrogen is extracted. But we don't want that. We want the nitrogen to be a free anion, because it will be more reactive that way. So we're going to throw some poly(ethylene oxide) into the reaction mixture. This will complex the sodium cation, keeping it from associating with the anionic nitrogen. You can see this in Figure 2.
Figure 2



So the nitrogen, free to react, will donate an unshared pair of electrons to the carbonyl carbon of another caprolactam molecule. (Remember, carbonyl carbons are electron deficient, and are easily attacked by anions.) After some electron-shuffling in which the electrons in the bond between the carbonyl carbon and the amide nitrogen shift to the nitrogen, the second caprolactam molecule ring-opens, as you can see in Figure 3.

Figure 3



The new molecule formed also has a negatively-charged nitrogen, an amine anion. This is an unstable species, so the activation energy is high for this step. This makes this the slow step of the reaction. For this reason, there is an induction period at the beginning of the reaction before polymerization begins.

Being unstable, that anionic nitrogen will abstract a proton from another molecule of e-caprolactam, as Figure 4 clearly shows.

Figure 4

We have an anion of e-caprolactam once again, and the negatively charged nitrogen attacks the ring carbonyl carbon of the species we just formed. Take a look at Figure 5 and you'll see what's happening.

Figure 5



This step is faster than the that first ring opening step, the one in Figure 3, because the starting species involved this time is much more reactive. You see, the reacting compound this time is an imide, a compound with a nitrogen atom bonded to two carbonyl carbons. The last time we were reacting an amide, which is a compound with a nitrogen bonded to only one carbonyl carbon. Imides are much more reactive than amides.



Part of the reason why imides are more reactive than amides is the fact that when an imide ring-opens we get an amide anion, whereas the product of an amide ring-opening is an amine anion. Amine anions are very unstable, but amide anions are more stable because the negative charge is stabilized by the carbonyl group bonded to the nitrogen atom.



Of course, this gives us another unstable nitrogen negative charge, and it takes a proton from another e-caprolactam molecule, which then adds to the growing chain in the same way as we saw before. This keeps happening over and over until we get high molecular weight nylon 6.

Just one more thing...remember that the first ring-opening step was so slow? We can get around this slow step by throwing a little bit of N-acetylcaprolactam into the reaction mixture. Not only is this is a reactive imide, just like the rings in our growing chain, but it produces an amide anion when it ring-opens rather than an unstable amine anion. It takes the place of e-caprolactam in the first step, so the slow step is eliminated, and so is that annoying induction period.

This reaction won't work well without poly(ethylene oxide) or N-acetylcaprolactam. But add both of them and you can get high molecular weight nylon 6 after heating the mixture for 2-3 minutes. Just don't heat it longer, because your polymer will thermally degrade. The molecular weight will drop, and then you won't be able to draw good fibers from your polymer. Your fibers will be hard to draw and they will also be brittle, so don't let it cook for too long!

Nylon 6,10 Experimental

Materials:

hexamethylene diamine (1g)

sebacoyl chloride (1g)

hexane (25ml)

two 100 ml beakers

glass stirring rod

balance

Procedure:

1. Dissolve about 1 g of hexamethylene diamine in 25 ml of water in a 100 ml beaker.

2. Make solution of about 1 g of sebacoyl chloride in 25 ml hexane.

3. Gently pour the sebacoyl chlorie/hexane solution on top of the hexamethylene diamine/water solution in the beaker, using a glass rod to pour down. A film will form at the interface.

4. Draw a thread out of this interface using a glass rod, and draw the thread out of the beaker. Using a second 100 ml beaker as a spool, slowly wind up the thread as you draw it out.

5. After all the polymer has been collected, wash it thoroughly with water, dry it superficially with a towel, then let it air dry.

6. Unwind the dry thread and let the students examine its physical properties. Rarely will this material display any significant strength.

Nylon 6 Experimental

Materials:

disposable test tube, 18 x 150 mm or larger

test tube holder

Bunsen burner

disposable Pasteur pipette

balance

e -caprolactam (recrystalized from cyclohexane)

sodium hydride (NaH), 60% dispersion in oil

polyoxyethylene [also called poly(ethylene glycol)]

N-acetylcaprolactam

Procedure:

Weigh out the following:

caprolactam (10g)

polyoxyethylene (POE) (molecular weight = 2000-7500) (0.2 g)

N-acetylcaprolactam (3-5 drops)

1. Mix the above in a test tube. Hold the tube with a test tube holder. (Don't hold it with your hands; it's going to get HOT!)

2. Light your Bunsen burner and adjust the flame to about 1 inch. Heat the tube in the flame, moving it around, passing it in and out of the flame. This is to ensure even heating. Uneven heating can cause an explosion, which would mean you'd lose your reactants when they shoot out of the tube, and the hot reactants wil splatter all over your lab partner causing severe burns.

3. When the mixture in the tube melts, add a spatula tip of NaH (move NaH from large contaner to smaller vials to prevent the whole can of NaH from burning up). Make sure all of your NaH reacts by carefully tilting your test tube so that any NaH stuck to the sides of the tube comes into contact with the reaction mixture. Be careful not to spill the contents when you tilt the tube!

4. Keep heating the test tube for 2-4 more minutes. Be careful to keep moving the tube to ensure even heating. If you don't, you can get hot spots, which will boil. This will lead to "bumping", which is when your reaction mixture shoots out of the tube. The liquid is very hot (220-230 ^o C) and will cause severe burns.

5. The reaction should be done about the time the reaction mixture starts to reflux, or simmer. You will know it has polymerized because the mixture will become much more viscous. If polymerization does not occur within several minutes, cool to just above the solidification temperature, add more NaH, and reheat.

Caution! Adding too much initiator or heating too long will lead to low molecular weight and brittle fibers. Also, heating too slowly allows the active species (the sodium salt of caprolactam) to react with moisture in the air, andit will no longer be abel to initiate polymerization.

You may not get it right on the first try. After several attempts, you will figure out just how much NaH to add and how long to heat it (Note 3).

The most exciting part of the experiment involves pulling fibers from the molten polymer. The polymer is ready when the viscous mass no longer bubbles freely and barely flows at the polymerization temperature. Fibers are drawn by dipping a glass rod into the polymer and rapidly drawing out the solidifying material. It may be necessary to let the polymer cool some to get just the right combination of viscosity and strength. With two students working together and one of them walking down the hallway trailing barely-visible fiber behind him, thin strands 75-100 feet long have been made with this method. These fibers will stretch under tension to complete polymer orientation and are then very tough. You should be able to make material similar to commercial nylon thread or fishing line (Note 4).

Extension Options

As the ratio of NaH to monomer increases, the molecular weight of the polymer decreases. The experiment can be extended by having the same or different students use varying ratios of NaH to caprolactam. The polymers synthesized can then be tested in one of two ways. A rough test is based on the fact that as the molecular weight of the polymer decreases, the fibers become weaker and finally the polymer cannot be drawn into fibers at all. A more quantitative approach is to make a capillary viscometer^9,10. Either method should show an inverse relationship between amount of NaH used and the molecular weight of the polymer synthesized.

Safety Considerations

Do not point the test tubes at a fellow student when heating it since heating too rapidly can cause splattering.

The test tube and the molten polymer are hot. Don't touch the test tube or the fiber with their hands until they are cool.

The NaH must be kept away from water. Hydrolysis will generate hydrogen gas which will burn or explode!

Both caprolactam and N-acetylcaprolactam are slightly toxic chemicals. Don't eat them (duh!) or touch them with your hands.

Wear safety goggles or glasses at all times.

Notes

1. 18-Crown-6 may be used instead of POE in the procedure. With the crown ether, the reaction is even faster and the final polymer is lighter in color. However, 18-crown-6 is expensive when compared to the linear polyether. Also, poly(ethylene oxide) is very safe, while crown ethers are toxic and easily absorbed through the skin!

2. Although weighing out the reactants is good practice, this procedure is versatile enough to work even with rough estimates of the reactant amounts.

3. If repeated attempts at polymerization fail, the caprolactam may be impure. Caprolactam may be recrystallized from cyclohexane.⁸

4. The experiment can be made more challenging for a by including the synthesis of caprolactam as the first part of the experiment. Caprolactam can be synthesized be the Beckman rearrangement of cyclohexanone oxime. An excellent lab experiment for this synthesis has been reported⁸. Final purification by recrystallization followed by polymerization makes this a sequence of experiments representing the complete commercial synthesis of nylon, and giving a product with excellent "hands-on" properties.

References

1.a. B. F. Greek, Chem. Eng. News, May 30, 1983, p.14; b. Chem. Eng. News, April 18, 1983, p. 6.

2. R. B. Fox, J. Chem. Educ., 1974, 51, 41 and 113.

3. B. Odian, Principles of Polymerization, 2nd edition, John Wiley & Sons, Publishers, New York, 1981, p. 12.

4. P. W. Morgan and S. L. Kwolek, J. Chem. Educ., 1959, 36, 1982.

5. Ref. 3, p. 542

6. H. R. Allcock and F. W. Lampe, Contemporary Polymer Chemistry, Prentice-Hall, Inc. Englewood Cliffs, N.J., 1981, pp. 128-129.

7. B. Z. Shakhashiri, Chemical Demonstrations, Vol. 1, University of Wisconsin Press, Madison, Wisconsin, p. 213.

8. Procedures and References cited in L. J. Mathias, J. Chem. Educ., 1983, 60, 990.

9. E. Pearce, C. E. Wright and B. K. Bordolo, Laboratory Experiments in Polymer Synthesis and Characterization, Pennsylvania State University Press, University Park, PA, 1982, pp. 133-152.

10. Carrothers, W.H. (to E.I. DuPont de Nemours and Co.), U.S. Patent 2,130,523 (1938).

11. Schlack, P., German Patent 748,253 (1938), U.S. Patent 2,241,321 (1941).

Self-Test

1. Name five common uses for Nylons in general and Nylon 6 in particular.

2. Name the following polyamides with Nylon, source and structure names.







3.Why would you expect the following two polyamides (aramids) to have better thermal and mechanical properties than aliphatic nylons?





4. Which of the above aramids would you expect to have better properties? Why? (Hint: What is Kevlar and what is it used for?)

5. Why are nylon ropes better in many applications than cotton, jute, and metal cables? (A good example is on boats, barges, and ships.)

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