Radical Polymerization - Online Tutor, Practice Problems & Exam Prep

Radical polymerization involves three key steps: initiation, propagation, and termination. The process begins with radical initiators, such as peroxides, activated by heat or UV light, leading to homolytic cleavage and the formation of radicals. During propagation, these radicals react with monomers, creating a growing polymer chain through head-to-tail addition. Termination occurs when two radicals combine or through disproportionation, resulting in stable products. This method typically produces branched polymers due to chain transfer reactions, which can be intramolecular (short branches) or intermolecular (long branches), affecting the polymer's physical properties.

Radical Polymerization Concept 1

Now we're going to say radical initiators catalyze the radical polymerization of alkenes. We're going to say that it is heat or UV light that can start the reaction. Now if we take a look here, we have our alkene, and we're going to introduce our peroxide. Underneath it, we have a radical initiator in the form of either heat or UV light. This will allow polymerization to occur, and this alkene will be transformed into repeating units that are strung together to form our polymer.

Here is our repeating unit. We're not specifying how many there are, so we're using the variable n. Remember, the generic form of a peroxide is ROOR. R here could be hydrogens, or it could be a carbon group. Typically, you might see benzoyl peroxide, which is here.

We have these representing our R groups, and, again, we have our two oxygens. Now the reaction mechanism is similar to that of free radical halogenation of alkanes. So it's going to be similar in terms of the steps that we are used to seeing with those types of reactions, so this shouldn't be completely new to all of you. So, just remember here, we're talking about the beginnings of radical polymerization. Now let's click on the next video and take a look at the steps involved.

Radical Polymerization Concept 2

Hey, everyone. So let's take a look at the radical polymerization mechanism. Here, we're going to say that it's comprised of 3 steps. So we have our initiation step, our propagation step, and then our termination step. In this video, let's take a look at the initiation step.

So step 1 here, we're going to have our radicals form when the initiator undergoes homolytic cleavage. So we have our peroxide here. We're going to introduce our radical initiators in form of either heat or UV light. Remember, in homolytic cleavage, we have equal breakage of the bond. So one group keeps one electron and the other one keeps its electron.

This will create 2 radicals. Now one of these radicals can then react with a monomer to form monomer radicals. So what we can say here is this would come out to form a connection with this one, and this electron will come here. So at the end, we've connected our R_o group to this monomer to create our monomer radical. Alright.

So this is step 1, initiation. Click on the next video to take a look at step 2, propagation.

Radical Polymerization Concept 3

In step 2, we talk about propagation. Now here, the monomer radical reacts with a monomer molecule and forms a new radical through a head to tail addition. Now the chain grows at the propagating site. So if we take a look here, here's our radical, our monomer radical. It'll come out here to form a connection with this CH₂.

So we take this electron and move it here, We take this and move it here. So the head of this monomer radical is reacting with the tail of our monomer molecule. So now we're going to say that they form a connection here. So that's the new bond that we created. And then this one electron that lands here is right here.

And this is our propagating site. So this will be our chain propagation or step 2 of this entire process. Now click on the next video to take a look at step 3, termination.

Radical Polymerization Concept 4

Combination, we say any 2 radicals can combine to form a sigma bond, thereby joining them together. So if we take a look at 1 a and 1 b, this radical would combine with this radical. And here's their connection now. Or we could have this radical combined with this radical, and here's their connection now. Under 2, we have disproportionation.

We can say a radical can abstract hydrogen forming another radical. So if we take a look here now, we're gonna say that this radical comes out here and this hydrogen decides to join up with this, so they connect together. This would become a CH₂ in the process. We can say that this electron then comes down here and this one here meets it, thereby forming a pi bond with each other. So in this termination here, they have basically gotten rid of our radicals and we created an alkene and we created this alkyl group at the end.

Right? So these are just some examples of termination or chain termination reactions that are possible.

Radical Polymerization Example 1

Hey, everyone. So here it says rank the following monomers from the highest to the lowest ability to undergo radical polymerization. So if we take a look here, we have our alkene portions in all of these. We can say right off the bat that one would have the lowest ability because it is the least substituted out of all the options here. So ethane is out.

Then we have 2, 3, and 4. In 2 and 4, we have our alkene structure connected to benzene, which is capable of a lot of resonance, a lot of movement of electrons. But the difference between the 2 is that in 2, we also have the addition of this hydroxyl group, which acts as an electron donating group. And this increases the ability to undergo radical polymerization even more. It allows for more resonance to occur.

So we'd say that 2 would have the highest ability. And then 4 would be next because it still has the benzene ring. We have this alkyl group here attached so hyperconjugation can get involved, which could help with our radical polymerization. That is an allylic position as well. So we'd say here 3 would be next.

And again, one is the least substituted out of the 4 options. It's going to be the least likely to undergo radical polymerization. It's hard to make a radical on a double bonded carbon, not stable at all.

So, this would be the order that we look in terms of highest ability to undergo radical polymerization with option 2 to the lowest which is option 1.

Write a mechanism for the peroxide-catalyzed radical polymerization of vinyl acetate.

Hey, everyone. So in this practice question, it says, write a mechanism for the peroxide catalyzed radical polymerization of vinyl acetate. Alright. So vinyl acetate. Vinyl just means we have an alkene and then acetate would be this portion here. So this is what vinyl acetate resembles. Now here, we're going to do the mechanism. So remember, we're starting out with initiation as our first step. So we have our peroxide interacting with either UV light or heat. We'd have homolytic cleavage where this bond breaks evenly between the two to give us our 2 radicals. Now this radical could then react with a mole of this vinyl acetate. If we do it like this, we can say that it decides to come here. One of the electrons decides to come here, and this one comes here. And that'll give us here initially this. We'd have RO attached to our CH2, which is connected to this CH, which is a radical, connected to the rest of this. Alright. So we have this structure here, then that can react with another mole of this. So this comes out here, this meets up with it, and this comes here. And we're gonna continue this. So if we come over here now, so this CH connects now to this CH2, which connects to this CH. And here I'm just gonna orient this stuff up here just so it's easier to track. Okay. And then this becomes a radical. Now if we want to do a termination, we could do different things. Here, I could bring another copy of this so that they can interact and our 2 radicals become 1. So if we have the same exact compound CH CH2 and then OR, This can interact. This can interact. What we get at the end is just this long structure. And here, we're just gonna say disconnected. And this is the connection they have to each other. Giving us this final polymer at the end.

Radical Polymerization Concept 5

Hey, everyone. So, in our continued discussion of radical polymerization, we now take a look at branching during chain growth. Here, we're going to say that radical polymerization always produces branched polymers. We're going to say branching occurs due to a chain transfer reaction. Now, when we say chain transfer, if we look on the right, that's just the change in the active site during radical polymerization.

Now, here we can look at it as an intramolecular process or an intermolecular process. In the intramolecular process, this results in short branches. And we're going to say here that our hydrogen abstraction occurs at the fifth carbon from the propagation site. So if we take a look here, we have here an example of intramolecular which happens within the same molecule. We're going to say here that this electron comes out here.

This one here follows it up, so they connect together. And then we can say here that this one sits right here. Now we see that this is called backbiting as a term. And what effect will this have? Well, we're going to say here as a result of this backbiting, we would get this CH₂CH₂CH₂, and then this gained a hydrogen so it becomes a CH₃.

And then there's this one electron right here. This electron that moved and sat on top of this carbon. We can say here this chain that's created has short branches. So, you have these short branching groups off the main polymer chain. Now, in intermolecular, so between molecules, this is going to result in long branches.

So we take a look here. We now have two different molecules interacting with each other. Here we can say that this one comes here. This electron from hydrogen meets it. And this electron here sits right on this carbon.

So what effect will that have? So here we're going to say that this carbon gained an H, so here it is right here. And then we're going to say that this is going to have a one electron here. And then we're still connected to the rest of this. So CH₂, CH₂, and then it's connected to some larger thing.

This is going to create our long branches. So we have our long branches here. Now here we're going to say branching affects the physical properties of the polymer itself. So just remember, when we're talking about our chain growth here, we're going to have to deal with branching. The branching could be intramolecular, which is within the same molecule, which creates short branches, or it could be intermolecular between two different molecules, and this will create long branches.

Radical Polymerization Example 2

In this example, it asks which one of the following statements is true about radical polymerization? Impurities in the reaction mixture induce branching during polymerization. Here, we never mentioned that impurities are necessary for branching to occur during polymerization, so this is not true. Chain branching occurs through either an intermolecular or an intramolecular chain transfer reaction. This statement is true.

Remember, intermolecular reactions create long branches, while intramolecular reactions create short branches. Thus, this is true. Chain branching occurs if the termination step is slow. Once again, this is something we never mentioned when discussing branching, so this is not true. The initiator determines the amount of branching in the polymer chain.

Again, we never discussed initiators, such as peroxide, UV light, and heat, in terms of determining the amount of branching. We framed our discussion in terms of inter versus intramolecular reactions. So here, out of our four choices, only option B is correct.

Why does back-biting result in the formation of short branches?

• A. Back-biting results in short branches because it is faster than intermolecular chain transfer.
• B. Intramolecular chain transfer is a controlled reaction and results in a uniform distribution of branches.
• C. Back-biting only takes place in short chains, and therefore produce shorter branches.
• D. Intramolecular H abstraction at the 5^th carbon produces a stable transition state.