Overview

In this Lesson, we are going to learn how free radical polymerization works, what the actual mechanism of the reaction is like, and why it happens. Free radical polymerization is a type of chain growth polymerization, but not the only one – in coming lessons we will cover other kinds of chain growth mechanisms, including ionic polymerization and ring opening polymerization. For now, in Lesson 4 and 5, we are going to focus on free radical, which is one of the most common types of polymerization. We are breaking down the topic of free radical polymerization into two lessons: Lesson 4 focuses on the reaction mechanisms and Lesson 5 focuses on the reaction kinetics.

Learning Outcomes

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

• Identify monomers that can undergo free radical polymerization.
• Draw arrow pushing mechanisms for polymerization by free radical polymerization, including initiation, propagation, termination (by combination or disproportionation), and chain transfer.
• Name and describe the stages of the free radical polymerization mechanism.
• Identify the head and tail of a monomer and whether a reaction occurred in a head to tail, head to head, or tail to tail fashion in a polymer and why
• Describe the effects of chain transfer on polymer skeletal structure

Lesson Checklist

Please refer to the Canvas Calendar for specific timeframes.

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.

Recall from Lesson 2 that we learned about two types of polymerizations: step growth and chain growth. In Lesson 3, we explored in depth the monomers that undergo step growth polymerization and Carothers theory for linear step polymerization. In Lesson 4, we now begin to examine the details of chain growth polymerization. More specifically, in this lesson, we are going to focus on free radical polymerization, which is a specific type of chain growth polymerization.

First, we should probably refresh our memory of what a radical actually is! A radical species is characterized by having an unpaired electron. From general chemistry, we recall that it is much more stable to have electrons paired in orbitals rather than unpaired — so as you may expect, radicals tend to be highly reactive and relatively short-lived. Radicals are denoted by a single dot (representing the electron), such as in Figure 4.1.

Figure 4.1: Various ways of representing a radical in a chemical structure.

Source: Section 4.1 from Young, Robert J., and Peter A. Lovell.
Introduction to Polymers, Third Edition, CRC Press, 2011.

The radical is shown as the single dot in Figure 4.1. In (i) the geometry and the hybridization of the central carbon is emphasized. The carbon is sp² hybridized, and the radical exists in the unhybridized p orbital that is perpendicular to the plane containing the substituents. Usually, the orbital is not drawn, as in (ii) or (iii).

Free radical polymerization makes use of this unstable radical by sequentially adding unsaturated monomers to the active center (radical end) of a growing polymer. Let's break that idea down further. As a refresher, recall that unsaturated molecules are characterized by not having the greatest possible number of hydrogens based on the carbon content. Usually, this means that the molecule contains a double or triple bond. Monomers that are most frequently used for free radical polymerization have a double bond, and more specifically, often a vinyl group or acryloyl group (Figure 4.2). Examples of a few common monomers are shown in Figure 4.3.

Figure 4.2: Common functional groups that can undergo free radical polymerization

Source: Lauren Zarzar

Figure 4.3: Examples of monomers that can be polymerized with free radical polymerization

Source: Lauren Zarzar

We already stated that we are going to use the radical to react with unsaturated monomers and this is going to create a polymer — but how? We can represent this reaction, and where the electrons go within the molecules, by using arrow pushing mechanisms which you learned in organic chemistry. In free radical polymerization, monomers are sequentially added together and the reactive radical end (the active center) attacks double bonds of monomers as shown in Figure 4.4.

Figure 4.4: Mechanism of free radical polymerization

Source: Lauren Zarzar

There is actually a lot going on in Figure 4.4. First of all, you may want to refresh your memory regarding the conventions of arrow pushing mechanisms. The arrows show where the electrons start (at the arrow tail), and where they go (the arrow head); single headed arrows represent the movement of one electron, while double-headed arrows represent movement of an electron pair. Here, we will be using lots of single headed arrows because we are dealing with radicals, which are unpaired electrons. As shown in Figure 4.4, we have broken down the "overall reaction" into two steps for clarity. Let's look in detail at Step 1 in Figure 4.4. Note that the π bond of the double bond is broken (the σ bond still remains) and that the π bond is broken homolytically. By homolytically, we mean that the two electrons in that bond are split evenly between the two carbons in the bond, one electron going to each carbon. Then, one of those new radicals that is generated can react with the radical on the other monomer, forming a new carbon-carbon bond (step 2). In the future, when writing reaction mechanisms, all you would need to show is the "overall reaction" as shown in bottom of Figure 4.4. Notice that after this reaction takes place, our product now has one additional monomer linked to the polymer chain, and our radical is still at the end of the molecule (this is called the active center). Building upon this general mechanism of free radical polymerization, we move on to more in depth understanding of the process. For example, how do we even get these radicals in the first place? How do we get rid of the radicals if we want to stop our reaction to make polymers of a specific length? These are the sorts of questions that we will soon address.

There are three general stages of free radical polymerization: 1) initiation, 2) propagation, 3) termination. Let's consider these stages in the order which they occur.

1. Initiation

This step involves the creation of the radical, i.e., creation of the active center. Usually, we need to add a special molecule called an initiator to our reaction to generate these radicals in the first place. By using a trigger like an input of light (hv) or heat (Δ), we induce the initiator to homolytically decompose into fragments containing radicals that can be used to initiate polymerization. Common molecules that are used as initiators include peroxides (containing a peroxide bond, -O-O-) and azo compounds (containing R-N=N-R'). Examples of common initiators include benzoyl peroxide, with is thermally triggered, and benzophenone, which is UV triggered. Note that for each bond that is broken, you get two radicals formed.

Figure 4.5: Initiation mechanism for radical generation

Source: Lauren Zarzar

Figure 4.6: Examples of common radical initiators

Source: Lauren Zarzar

Once we generate the radical, it has to be transferred to a monomer to create an active center. It's worth pointing out that not all radicals generated during initiation actually get transferred to the monomer. Some molecules with radicals may further decompose, and the radicals recombine, before they ever get a chance to react with the monomer. But assuming we do have transfer to monomer, there are actually two ways that a radical can add to the double bond of a monomer. There are two ways because the two carbons involved in the double bond may not be equivalent (i.e., they may have different substituents). Figure 4.7 depicts the two ways that the initiator radical can add to a monomer.

Figure 4.7: A radical can add to a monomer in two different ways where "X" represents any substituent (except hydrogen, in which case these pathways would be equivalent)

Source: Lauren Zarzar

The first reaction (Mode I) typically predominates in free radical polymerization. Why do you think this might be the case? Comparing Mode 1 and Mode 2, we notice that difference is whether the initiator radical forms a bond with the carbon attached to the "X" group or not. Remember that for a reaction to occur, two reagents have to be able to get close to each other, which is why consideration of steric effects is important. Hopefully you learned some about steric hindrance in organic chemistry, but to quickly summarize, the basic idea is that bulky substituent groups (i.e., bulky, large "X" groups) prevent the radical from being able to approach the other molecule closely enough to react. Well, in Mode 1, the only substituents on the reacting carbon are hydrogens (since it is a methylene group), which are small, so there's really not much steric hindrance. For Mode 2, "X" could be anything —  imagine a tert-butyl group that's big and bulky — the radical will have a harder time approaching that carbon. Thus, Mode 1 tends to be favored based just on steric considerations. But there is another reason as well, which is that the radical in Mode 1 ends up on the carbon with more substituents, which can help to make that radical more stable than the radical on the methylene group.

2. Propagation

Propagation involves the growth of the polymer chains, where monomers are sequentially added to the active center. The mechanism of propagation looks like what we already discussed in Figure 4.4, so refer back to that if you get confused about the movement of the electrons. Similarly to initiation, there are actually multiple ways in which a monomer can add to the active center (again, because the two carbons in the vinyl group of the monomer may be non-equivalent). Figure 4.8 depicts the two ways in which the monomer can add to the active center. We name these configurations depending on how the "head" or "tail" of the monomer is connected to the active center. The "head" of the monomer is the side where the substituent is attached, while the "tail" is the methylene. Because of the reasons discussed in the section on initiation (sterics, and stabilization of the radical), we expect that the active center is usually found on the "head" group and preferentially reacts with the "tail" of another monomer (i.e., "head to tail"). Less frequently, we would see head to head addition; if head to head addition occurs, it will most likely be followed immediately by a tail to tail addition, such that the active center again ends up on the head side.

Figure 4.8: Configurations in which a monomer can add to an active center during the propagation step of free radical polymerization

Source: Lauren Zarzar

CASE STUDY

Consider the polymerization of vinyl fluoride (Figure 4.9). Do you think this monomer will still preferentially add in a head to tail fashion?

Figure 4.9: Vinyl fluoride

Source: Introduction to Polymers Section 4.1

The tail (the methylene) is less sterically hindered, so in terms of steric effects, we still expect head to tail addition. But, you may recall from organic chemistry, that fluorine is strongly electron withdrawing which would actually destabilize a radical on the head carbon (which is electron deficient). So, we have competing effects - which wins out? Turns out that 90% of the linkages in poly(vinyl fluoride) are still head to tail, emphasizing the importance of steric considerations.

3. Termination

Termination involves destruction of the radical active centers, thus preventing any further propagation. But, what happens to those radicals then? There are few different possibilities for where they go. Often, termination occurs via combination of radicals (Figure 4.10).

Figure 4.10: Termination by combination

Source: Lauren Zarzar

Also notice in Figure 4.10 that upon termination by combination, the final polymer has a final degree of polymerization of x+y, and there are two initiator fragments on each end. Termination by combination is not the only pathway for termination. In termination by disproportionation, an H atom is extracted from the end of a growing chain and the leftover electron combines with another radical, generating a terminal π bond (Figure 4.11).

Figure 4.11: Termination by disproportionation

Source: Lauren Zarzar

In termination by disproportionation, we have created two polymers, one with an unsaturated end, and both have one end with an initiator fragment.

Chain transfer is a process that occurs during polymerization, in which the active center is transferred from one species to another (Figure 4.12).

Figure 4.12: Chain transfer
TA can be initiator, monomer, polymer, the solvent, or something you add to purposefully transfer the radical (i.e., a transfer agent). Often, T is a hydrogen atom or halogen atom

Source: Lauren Zarzar

In Figure 4.12, we use T and A to just represent fragments of a molecule that are linked by a single bond. TA can be initiator, monomer, polymer, the solvent, or something you add to purposefully transfer the radical (i.e., a transfer agent). Often, T is a hydrogen atom or halogen atom. Notice that breakage of the single bond in TA happens by homolytic cleavage, as we have seen before, such that the radical has been transferred from one species to another. Chain transfer - the rates at which it happens, and the species between which the radical is transferred - can have dramatic effects on the structure of the resulting polymer. First, we will consider the impact of chain transfer to polymer.

Chain transfer to polymer

Chain transfer to polymer means that the radical is transferred to somewhere in or on a polymer - it can be the same polymer where the radical was initially (intramolecular chain transfer, or backbiting), or the active center can be transferred to a different polymer chain (intermolecular chain transfer). Both of these processes are shown in Figure 4.13.

Figure 4.13: Intramolecular and intermolecular chain transfer to polymer. Intramolecular chain transfer to polymer, sometimes called backbiting, most often leads to short branches. Intermolecular chain transfer to polymer tends to lead to longer branches.

Source: Lauren Zarzar

If we were to polymerize a monomer with a functionality of two (e.g., a monomer with a vinyl group) we expect to create a linear polymer, without branching. But if there is chain transfer to polymer, then we get a very different polymer skeletal structure that does have branching. If there is intramolecular chain transfer to polymer, sometimes called backbiting, the active center is transferred to somewhere else along the same polymer chain where the active center originated. Usually, it's transferred to a position pretty close to the polymer end resulting in the formation of a short branch. If there is intermolecular chain transfer to polymer, then the active center is transferred to another polymer chain. It could be transferred anywhere along the polymer; usually, this mechanism leads to branches that are longer.

Summary

Free radical polymerization is one of the most important types of polymerizations, and now you should feel comfortable indenting monomers that will undergo free radical polymerization, drawing the free radical polymerization mechanism, and describing how chain transfer affects skeletal structure. It is very key that you understand how the monomers react to give the polymer so that you can draw an accurate repeat unit for any monomer(s) that you start with. Next, we are going to be building upon the foundation we learned in Lesson 4 and apply that to the kinetics of free radical polymerization in Lesson 5.  

Reminder - Complete all of the Lesson 4 tasks!

You have reached the end of Lesson 4! Review the checklist on the Lesson 4 Overview / Checklist page to make sure you have completed all of the activities listed there before you begin Lesson 5.