The Stages of Free Radical Polymerization

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.