Chain-Growth Polymers - Online Tutor, Practice Problems & Exam Prep

Chain growth polymers, also known as addition polymers, are formed through addition reactions where pi bonds in monomers break to create sigma bonds, resulting in a repeating unit represented by n. There are three types of chain growth polymerization: radical, cationic, and anionic. Radical polymerization involves free radicals initiating the reaction, while cationic polymerization uses positively charged ions, and anionic polymerization involves negatively charged ions. Each mechanism stabilizes intermediates through resonance or hyperconjugation, crucial for understanding polymer synthesis and properties.

Chain-Growth Polymers Concept 1

In this video, let's take a look at chain growth polymers. Now, chain growth polymers are also known as addition polymers because they form by addition reactions. Here, if we take a look, we have styrene. By stringing them together through polymerization, we make our polystyrene. Here, we're going to say that the pi bonds and the monomers break and the sigma bonds form between monomer molecules to produce the polymer.

So, if we take a look, these pi bonds here are what we can use to make these new sigma bonds here, these ones in green. And we can say that we see a repeating pattern. Here is our styrene, these are the carbons involved, and we can see that here. We can see that here, and we can see that here. So, we'd say that each one of these represents our repeating unit.

And here we'll put n because the number could be numerous, so we'll just use the variable n to show that. Alright. So just remember, we're talking about chain growth polymers. We're discussing the utilization of our pi bonds in order to make new sigma bonds that string together these monomers to create a polymer.

Chain-Growth Polymers Concept 2

In this video, let's take a look at the types of chain growth polymerization. We're going to say depending on the mechanism of the reaction, chain growth polymerization can occur in 3 ways. Now we're going to say there's radical polymerization, there's cationic polymerization, and then there's anionic polymerization. With radical polymerization, we're going to say monomers add to the chain via a free radical reaction or free radical addition reaction. So we come here, we have our radical because here's our lone electron.

We're going to say that this lone electron is going to come out here. Remember, we don't use a full arrow, we use a hook arrow. One of the electrons from this pi bond will come out and meet it, and then this electron comes here. As a result of this, what do we make? We're going to make our connection between this CH and the CH₂.

So here it goes. It's connected to the CH, which is now the radical, it has the lone electron, and it's still connected to CH₃. Now if we're talking about these structures, we can say that our alkene, this is represented, and we say that this is the tail, and this is the head. So what we saw here is the head of this, the head which is a radical, is interacting with the tail of this. And that's how we're able to connect them together.

Now, let's go to cationic polymerization. Remember, a cation is a positively charged ion. So here we're going to say the monomer acts as a nucleophile and attacks the positively charged chain. So here we're going to say that this pi bond comes here and attaches to this positive carbon. So we're attaching there.

So what effect does that have? Well, we're attaching to that. So we're going to have our CH₂ connected to now our CH. We had to move both of these electrons over here, which means this carbon lost its electrons so it's positive now. So this is our new carbocation, but it is okay because it is resonance stabilized.

Remember, we have lone pairs on this oxygen, which could resonate here to make a double bond, thereby stabilizing that carbocation. So it's resonance stabilized here. Now, here we're going to say anionic polymerization. Remember, an anion is a negatively charged ion. We're going to say the monomer acts as an electrophile.

And the negatively charged chain attacks it. Alright. So what we're going to say here now is we're going to have this carbon which is negatively charged because it has the lone pair. It's going to come out and attack. It's going to attack this CH₂ carbon, which is going to cause this pi bond to break and come here.

So at the end, what do we have? We have our connection to CH₂, CH, which has a lone pair now, which is negative now, and then we have CN. Remember CN, our nitrile group here represents an electron withdrawing group, if you remember your meta directors in the benzene chapters of our EAS reactions. So this here could stabilize this negative charge because it is electron withdrawing. It's going to minimize the negativity of that carbon atom.

Now here we have head to tail addition. Remember, this produces our stable intermediates. It's stabilized through these factors. We have hyperconjugation, which remember deals with alkyl groups. And then we have our resonance, which is our aromatic rings, our electron donors, and our electron withdrawing groups.

We saw here that this route will be stabilized by hyperconjugation because of this alkyl group. This carbocation here is stabilized by this electron-donating group. And then this carbanion is stabilized by this electron-withdrawing group. Right? So these are just our different types of polymerizations that can happen.

Just remember, we have our radicals, our cationic, and our anionic polymerization reactions.

Chain-Growth Polymers Example 1

In this example question, it asks you to predict the most plausible chain growth polymerization mechanism for each of the following monomers. Now, in the first one, we're dealing with acrylamide. You should realize that the alkene carbons here are connected to a carbonyl group, which is an electron-withdrawing group. So, they're taking electron density away. Remember, when we have a meta director attached to this alkene portion, which can act as the bridging portion where it can use its pi bonds to connect different monomers together, remember, that is indicative of an anionic polymerization reaction.

For the next one, we have Cl with its lone pairs, which can act as an electron-donating group. Remember, our meta directors or ortho pair directors from our benzene reactions, halogens, although they can be electron-withdrawing by the inductive effect, they can be electron donating by the resonance effect, which has a greater impact. So, because of it acting as an electron donor, it would work best under a cationic polymerization. And then finally, here we have a methyl group, and then we have OCH₃ with its oxygen lone pairs. This is a strong electron-donating group.

Although we have an alkyl group which can be stabilizing in terms of hyperconjugation, we say that we have this strong OCH₃ electron-donating group. So this would be best for cationic polymerization. Alright? So, these would be the different types of chain growth polymerizations that are possible with each of the following monomers.

Which of the following monomers will show the greatest selectivity for head-to-tail addition?

• A. prop-1-ene
• B. (E)-but-2-ene
• C. 2-methylprop-1-ene
• D. None of these.

Which of the following monomers could polymerize through all three chain-growth mechanisms?

• A. styrene
• B. methyl 2-cyanoacrylate
• C. vinyl acetate
• D. 2-methoxyprop-1-ene