Free Radical Polymerization - Online Tutor, Practice Problems & Exam Prep

Radical polymerization involves a continuous chain reaction where alkenes react with radicals, producing long hydrocarbon chains, essential for creating plastics. The process begins with an initiation step using a radical initiator, typically peroxide, which reacts with a double bond. This leads to propagation, where the radical continues to react with more alkenes, forming a polymer without a termination step. The resulting polymers, like polypropylene, have diverse applications, including in tires and synthetic materials, highlighting their significance in industry and environmental considerations.

General features of Radical Polymerization.

When radicals are exposed to alkanes in excess, they can do something really weird. And what that is is they can do something called polymerization. Polymerization is where you basically have a chain reaction that never stops. It never has a termination step because it keeps propagating forever. Okay? So what you wind up getting is these hydrocarbon chains that could be 10,000, 20,000 or thousands of units long. Okay? And that's actually what plastic is. A lot of synthetic materials come from polymerization. It is radical intermediated. Basically, like I said, radical polymerization reactions use alkenes in excess to extend the propagation step. And this is the way that industries use petroleum, petroleum just from underground. They use it and they convert it into plastic through this method. Okay? And all of this is just a big chain reaction that just keeps on going and that's how you get stuff like tires and shopping bags or whatever. All these different things that are made out of weird plastics, all these synthetic materials, it's basically petroleum just linking up and cross-linking.

So basically, here's a really common byproduct of petroleum, propylene. Believe me, like they wind up, they mine tons of this stuff. They drill it and you get tons of that stuff from underground. If you do a polymerization reaction using some kind of radical, so I'm just going to put here a radical initiator like OR•. What you wind up getting is a polymer. The general formula for a polymer is basically n , which is just the number of units. Okay? And we don't really know how many units there are, so n is just going to stay like that. Okay? What it means is that you have these repeating subunits that just keep going on forever. Okay? In this case, this repeating subunit would have basically 2 carbons and then another carbon here. Okay? The way that works is that the 2 carbons in between are always the ones that are made out of the double bond. Okay? So in this case, that would represent these 2 right here. Okay? This extra CH₃ at the top, what's that got to do with? Well, that's this CH₃ right here. Okay? All the other H's that are around are the other H's that are sticking out of this thing. So there's an H, H and H, and these three H's are the three H's right there. Okay? This just keeps extending and extending. Actually, polypropylene is used to make like astroturf and car tires and ropes and stuff. Okay? So it's kind of cool how it actually has a real-life application.

The mechanism of Radical Polymerization.

Let's just get into the mechanism though, because I know that's what you're interested in. So, the general mechanism is, of course, we need an initiation step, so we're going to go ahead and use peroxide as it’s commonly used in the industry. We would get 2 equivalents of my peroxide radical. Okay? And that's going to react with my double bond. Okay? Now, in this case, the radical is going to react directly with the double bond because the double bond is a good source of electrons. So what I wind up getting is, I'm just going to show you guys right here: three arrows once again. I'm going to get this going out into the middle of nowhere. Then, I'm going to get the double bond attacking. Now, I just have to ask myself where would the radical, the extra radical, be most stable? On the primary carbon, the red one? Or on the secondary carbon, the blue one? The answer is secondary, so my radical would go ahead and jump there.

What I would get for this first step is I would now get OR and, well, there’s no more double bond anymore—it's just a single bond with a radical. Okay? So that's my first step. Now, how is that going to propagate? Well, it’s going to attach to another radical. So what winds up happening is, this is now going to do the same thing again: 1, 2, 3. And this is the whole idea behind polymerization. It's going to keep doing the same thing over and over. So what I wind up getting here now is I'm going to draw the blue part just the way it was before, but watch; I'm going to twist it a little bit. So now, this goes here and that _CH3 that I had basically there that's like not participating in blue, I'm going to move that one up. Okay? So, that means that now, what is that attached to? Well, there's a new single bond. I'm going to make that black. That represents the new single bond that was made by the radical reaction. And that's going to be attached to a new 3 carbon subunit that now has a radical there. Okay? And this is going to keep on going and going and going.

So what you see is that now we have our general formula starting to pop up. This is going to be the repeating subunit is going to look like this. Okay? Where basically I have this thing that just keeps going over and over and over again. Okay? So what about the termination step? I know you're curious about that. We're just going to put here NA. Why? Because that's the whole point of polymerization, that there is no termination step. It just doesn't terminate. It just keeps on going forever until you run out of alkenes. Okay? And what you wind up getting are these really, really, really long polymers that have unusual properties like being elastic, and never biodegrading, and stuff like that. Being terrible for the earth and killing animals.

So what I want you guys to do now, that sounded really jaded. What I want you guys to do now is just figure out what the general subunit would look like if we were to polymerize this molecule. The best way to do that would just be to kind of draw the first parts of the mechanism and see if you guys can figure out what that repeating subunit is going to look like there. That's called the general formula of the polymer, which would be in those brackets. So, I'm going to give you guys some time to do it. Go ahead and use the OR radical to start the reaction and then kind of just continue it and see if you can get that repeating subunit, then I'll give you guys the answer.

Use the polymerization mechanism to draw the polymer formula for the following compound.

So let's just draw this from the beginning. What I would have is that, I'll just draw it a little bit closer. I would get 3 arrows. 1, 2, 3. In this case, notice that both carbons were exactly the same in terms of their substituents, so it didn't matter where I put the radical. Okay? So now what I'm going to do is I'm going to draw something that looks like this. I'm going to have OR with a single bond, with a single bond, and the radical would be over on this side because it's on the opposite side to wherever the OR attached to. And those are the 2 carbons. Basically, this is let's say this is carbon 1 and this is carbon 2. This is carbon 1 and this is carbon 2. Now I just have to figure out what goes on each of these and it looks like it's chlorine. So 1 is going to get a chlorine. 2 is going to get a chlorine. Okay? Now I have that radical there. Let's react it again. So now I'm going to react it with another alkene. So I'm going to get this, this, and that. And what that's going to give me is now I have the same exact thing as before. Cl, Cl. But now that's attached to what? Well, that's going to have a new bond and that new bond is going to be attached to a 2 carbon chain with Cl and Cl. Are we starting to see a repeating subunit? Yes. So what that means is that my repeating subunit would look like this. So it'd just be basically just one equivalent of this. So it would be that I let's say I take my this portion of it right here, okay, and I say that I have a Cl at the top and a Cl at the bottom, but I also have an H at the top and I also have an H at the bottom. Then I would just go ahead and draw the lines going out of it and that just means that there's other stuff attached to it. Okay. And that's really it. We're done. That's all you needed to do. Okay? And then you just keep adding these subunits and this would make its own polymer that has its own unique synthetic qualities. All right? So I hope that made sense. It's not a terrible mechanism, but it's just something interesting that radicals do and it is obviously something that might be fair game for your exam.

Here’s what students ask on this topic:

What is free radical polymerization and how does it work?

Free radical polymerization is a process where alkenes react with radicals to form long hydrocarbon chains, creating polymers. The process starts with an initiation step, typically using a radical initiator like peroxide. The initiator reacts with a double bond in the alkene, forming a radical. This radical then reacts with another alkene, propagating the chain reaction. The reaction continues without a termination step, producing long polymers. These polymers, such as polypropylene, are used in various applications like tires and synthetic materials.

What are the steps involved in the mechanism of free radical polymerization?

The mechanism of free radical polymerization involves three main steps: initiation, propagation, and termination. In the initiation step, a radical initiator (e.g., peroxide) generates radicals. During propagation, these radicals react with alkenes, forming new radicals and extending the polymer chain. This step repeats, creating long polymer chains. Termination is typically absent in free radical polymerization, allowing the reaction to continue until the monomer is depleted. The resulting polymers have unique properties and are used in various industrial applications.

What are some common applications of polymers produced by free radical polymerization?

Polymers produced by free radical polymerization have numerous applications. For example, polypropylene is used in making astroturf, car tires, and ropes. Other polymers are used in producing plastic bags, bottles, and various synthetic materials. These polymers are valued for their durability, elasticity, and resistance to biodegradation, making them essential in many industries. However, their environmental impact is a concern due to their non-biodegradable nature.

Why is there no termination step in free radical polymerization?

In free radical polymerization, there is typically no termination step because the reaction is designed to continue propagating until the monomer is exhausted. The absence of a termination step allows the formation of very long polymer chains. This continuous chain reaction is driven by the availability of alkenes and the presence of radicals, which keep reacting to extend the polymer. The lack of termination contributes to the unique properties of the resulting polymers, such as their elasticity and durability.

How does the choice of radical initiator affect the polymerization process?

The choice of radical initiator significantly impacts the polymerization process. Common initiators like peroxides decompose to form radicals that start the chain reaction. The efficiency, rate of decomposition, and stability of the initiator influence the rate of polymerization and the molecular weight of the resulting polymer. A suitable initiator ensures a controlled and efficient polymerization process, leading to polymers with desired properties for specific applications.