Overview

Now that you know what a polymer is, how do we make one, and how do we categorize all the many types of polymers that can be formed? We know that in order to create a polymer, we need to link together monomers, and in this lesson we will begin to discuss how that happens. We will find that there are two general pathways by which monomers can be added together: chain growth and step growth. Each mechanism is characterized by different reactions that take place. In this lesson, you will learn the general characteristics of these polymerization pathways, what is necessary for a molecule to actually be considered a monomer, and how the functionality of the monomer contributes to the skeletal structure of the polymer.

Learning Outcomes

By the end of this lesson, you should be able to:

• describe the principle of equal reactivity;
• define and compare/contrast step growth vs chain growth; condensation with addition polymerization;
• identify functionality of a monomer and how it affects the skeletal structure of polymer formed;
• name the class of polymer formed based on the chemical structure;
• When presented with a polymer, be able to suggest monomers that can be used to produce it.

Lesson Checklist

Please refer to the Canvas Calendar for specific time frames.

Questions?

If you have questions, please feel free to post them to the General Questions and Discussion forum. While you are there, feel free to post your own responses if you, too, are able to help a classmate.

Polymerization requires chemical reactions, and chemical reactions happen as a consequence of collisions of molecules; if monomers never encounter each other, they can never react! Therefore, we would expect that the rate of a reaction has dependence on collision frequency. This is an especially important consideration for polymers, because polymers can be really big which means they diffuse slowly, leading to a lower rate of collision, providing fewer opportunities for reactions. However, because the polymer is large and diffuses more slowly, each encounter between reactants actually has a longer duration, which favors a reaction. These two effects — less overall collisions but longer collision duration — are assumed to balance each other out. Analyses of polymer reaction kinetics suggest that this is a reasonable assumption in most cases. We are therefore going to make this very key assumption called the principle of equal reactivity, which is that reactivity does not vary as a function of polymer size. This assumption underlies all of our future analysis of polymerization, and is important to keep in mind.

Now to actually make a polymer in the first place, you have to have the correct degree of functionality. Clearly, for a polymer to form and grow, 1) there has to be an initial reaction between monomers, and 2) there has to be reaction between monomers and the growing polymer molecule. Consider the two molecules in Figure 2.1, where A and B are able to react together to form a bond. Because each molecule only has one A or B group, once they react, there is no more functionality left to continue the polymerization. Thus, these molecules don’t have enough functionality to form a polymer.

Figure 2.1: Reaction between monomers without functionality

Source: Lauren Zarzar

What if instead you had molecules with both an A and B group, as shown in Figure 2.2. Now, after A and B react, there is still enough functionality on the molecules to continue adding monomers to the polymer chain. There is sufficient functionality for a polymer to form.

Figure 2.2: Reaction between monomers with functionality of two

Source: Lauren Zarzar

In Figure 2.2 the monomers have a functionality of two, and so the polymer that forms is a linear polymer (We first learned about skeletal structures in Lesson 1). But what if the monomer had even more functionality? Consider Figure 2.3 where the monomer has a functionality of 4. Now we see that we can get formation of branched polymers and/or network polymers.

Figure 2.3: Reaction between monomers with functionality of four

Source: Lauren Zarzar

So identifying the functionality in a monomer — and whether it has enough to form a polymer in the first place — is going to be key. But let’s say you already know that your monomer can form a polymer — what happens next? How does a monomer actually add to another monomer, or add to a dimer, or add to an existing polymer? There are two general mechanisms for how this can happen: step growth polymerization and chain growth polymerization.

Consider this exercise: you have a bag of pop beads, and each bead represents a monomer. You reach into the bag, pick up two beads, and put them together (they “react”) to form a dimer. Then you put this dimer back into the “solution” of monomers (the bag). Reach in, and grab two more beads at random. Most likely, you pick two single beads - put them together, and throw the dimer back. After you do this a bunch of times, perhaps you reach in and grab a monomer and a dimer. Put them together, you’ve made a trimer. Throw it back! If you do this for long enough, pretty soon your bag will be full of short oligomers, and lots of them, without many monomers left. Only after a long time of doing this exercise will you actually start to piece together these short oligomers to create longer polymers.

This mechanism is characteristic of step growth polymerization. Step polymerization is characterized by:

• loss of monomer early on in the reaction (remember, you turned all those monomers into dimers, trimers, and other short oligomers);
• growth of polymers throughout the reaction;
• the average molar mass increases slowly (for the majority of the exercise, your bag had only very short oligomers in it — only after very long times did you start to get longer polymers);
• high extents of reactions are necessary to get long chain lengths (lots and lots of beads had to be combined into those dimers and trimers before you started to get longer polymers);
• no initiator is used (the bead monomers were inherently reactive with each other).

Now try this variation of the bead exercise. In the bag, put a bunch of similarly colored beads (monomers) and a few differently colored beads (which will be our initiator molecules). The difference between this and the previous exercise is that monomer can only add to the initiator or a growing polymer containing the initiator. So reach in and grab two beads until you pull out an initiator and a monomer. Put them together, throw them back! Grab two more; if you grab two monomers, they won’t react with each other. Eventually, you will grab a monomer and a growing polymer fragment that has the initiator bead; keep adding monomers to that. What you find is that, in comparison to the step polymerization exercise, here you are forming relatively few numbers of polymers but each polymer chain that does form will grow longer, faster. You will also not use up your monomer as quickly; even when your polymers grow large, you’ll still have lots of monomer beads left. This exercise is similar to chain growth polymerization. Chain growth polymerization has the following characteristics (compare to step growth polymerization!):

• growth of polymer occurs by adding monomers to relatively few polymer chains;
• monomer remains even at long reaction times;
• average molar mass increases quickly;
• initiation is required.

We will be considering step and chain polymerization independently, and in great depth, in the coming lessons, but for now you should have a good idea of the similarities and differences between these two mechanisms.

We will first consider polymerization in which a linear polymer is formed, and the mechanism is step growth, i.e., linear step polymerization. We already have seen an example of what this could look like (Figure 2.2). Linear step polymerization occurs from polymerization of either bifunctional or difunctional monomers, which have a functionality of two. A difunctional monomer has two of the same reactive groups capable of forming bonds (e.g. two “A” groups), while a bifunctional monomer has two reactive groups, although they are not the same moieties (but can still react to form bonds, e.g. an “A” and “B” group) (Figure 2.4).  We have terminology to describe each of these various combinations of reactions, as depicted in Figure 2.4. For example, if we have monomers that are bifunctional (A-B) and A reacts with B,  we call this type of polymerization “ARB” type. (The “R” represents whatever chemical structure is between the reactive functional groups of the monomer). If we have difunctional monomers that react with themselves (A-A reacts with A-A) we call this “RA2”. And if we have difunctional monomers (A-A and B-B) where A reacts with B, then this is an “RA2+RB2” type reaction.

Figure 2.4 Polymerization of bifunctional and difunctional monomers

Source: Lauren Zarzar

PROBLEM

Consider polycarbonate, formed from the reaction between bisphenol A and phosgene. What “type” of polymerization is this?

Bisphenol A combines with phosgene to form polycarbonate

Source: wikimedia

1. RA₂ + RB₂
2. RA₂
3. RB₂
4. ARB

Click here for the answer.

ANSWER

A. RA₂ + RB₂

So what exactly are A and B? What functional groups can we use to create polymers, and what sorts of polymers do they produce?

Polymers are often distinguished by the structure of the linkages that are produced. Table 3.1 in the text provides seven common classes of polymers that can be produced by step growth polymerization. We will consider each of these in depth, and identify the functional groups that are needed in the monomers to produce the desired polymers. Be conscious of whether the reactions are polycondensation or polyaddition reactions (refer back to Lesson 1 - What is a Polymer).

Polyesters

Polyesters can be produced from the reaction between carboxylic acids or acid halide groups with alcohols (Figure 2.5). Notice that when a carboxylic acid is used, water is a product; when an acid halide is used, acid is a product.

Figure 2.5 Polyesters

Source: Lauren Zarzar

A very common polyester is polyethylene terephthalate, or PET (Figure 2.6). If you see recycling symbol #1 on your container, it's PET! PET is the most common thermoplastic polymer and is frequently used in synthetic fibers. Even though there are many different polyesters, PET is so common - 18% of world polymer production - that it is often just identified by the general name, "polyester". So if you look on your clothing label and find that it says "polyester", now you know that it's PET. The inclusion of aromatic groups in the backbone lends the polymer more mechanical and thermal stability. University of Liverpool - Chem Tube 3D.

Figure 2.6 Polyethylene terephthalate (PET)

Source: Lauren Zarzar

Polyamides

Polyamides are produced from the reaction between carboxylic acids or acid halides with amine groups that creates amide bonds (Figure 2.7).

Figure 2.7 Polyamides

Source: Lauren Zarzar

Nylon is a great example of a polyamide. Nylon 6,6 is shown in Figure 2.8; the "6,6" part comes from the fact that each monomer has 6 carbons; there are many other kinds of nylon as well, depending on the specific monomers used. Nylons have extensive hydrogen bonding between polymer chains which generates a relatively high degree of order (and crystallinity) contributing significantly to nylon's strength and rigidity. This ordering also makes nylon great for fibers. Nylon 6,6 can be easily made from interfacial polymerization, because the adipic acid (or sebacoyl chloride) is water soluble and the hexamethylenediamine is oil soluble. (VIDEO/www.youtube.com/watch?v=VtCBarLbHRM)

Figure 2.8 Nylon 6,6

Source: Lauren Zarzar

Other important examples of polyamides are polypeptides or proteins. Proteins are polypeptides, produced from the polymerization of amino acid monomers. The generic structure of an amino acid is shown in Figure 2.9. Do you see how the polymerization of an amino acid would create an ARB type polymer?

Figure 2.9 Polypeptides or proteins

Source: Wikipedia - Amino acid ball.

Polyethers

Polyethers are formed from reactions between diols in an RA₂ type polymerization.

Figure 2.10 Polyethers

Source: Lauren Zarzar

Polyurethanes

Polyurethanes are formed from the reaction between diisocyanates with diols (Figure 2.11).

Figure 2.11 Polyurethanes

Source: Lauren Zarzar

Polyurea

Polyurea is formed from the reaction between diisocyanates with diamines (Figure 2.12)

Figure 2.12 Polyurea

Source: Lauren Zarzar

Directions: Drag the polymer diagrams into the box indicating the proper polymer type. When you have placed all of the polymers, click on the Check button to see how you did.

Hint: You should end up with three polymers in each group.

Note: You may want to zoom out to help you see the whole activity window. Ctrl + – on Windows or Command + – on a Mac. To return to the default zoom level, use Ctrl + 0 on Windows or Command + 0 on a Mac.

Summary

Hopefully, you now feel comfortable identifying classes of polymers based on the linkages in the backbone and suggesting monomers with appropriate functionality to make polymers of a targeted type and skeletal structure. The various classes of polymers we learned (polyester, polyether, etc.) and the specific reactions we discussed to produce them are all examples of step polymerizations. In Lesson 3, we will continue to dive deeper in step growth mechanisms and especially start to consider the kinetics of polymerization.

Reminder - Complete all of the Lesson 2 tasks!

You have reached the end of Lesson 2! Review the checklist on the Lesson 2 Overview / Checklist page to make sure you have completed all of the activities listed there before you begin Lesson 3.