Cationic Polymerization - Online Tutor, Practice Problems & Exam Prep

Cationic polymerization involves alkenes with electron donating groups and is initiated by an electrophile, typically a Lewis acid like boron trifluoride. The mechanism consists of three steps: initiation, where an adduct forms and donates an H⁺ ion; propagation, where the cation reacts with another monomer to form a new cation; and termination, which can occur through H⁺ removal or nucleophilic attack. This process is crucial for creating addition polymers, showcasing the importance of electrophiles and cation stability in polymer chemistry.

Cationic Polymerization Concept 1

Hey, everyone. In this video, let's talk about cationic polymerization. Here, we're going to say that alkenes with electron donating groups undergo cationic polymerization. Now here, we're going to say that the electrophile, so an electrophile, or H⁺ ion initiates the reaction. And we're going to say the electrophile can be a Lewis acid such as boron trifluoride.

Now, here we have our alkene, connected to it is an electron donating group. We have our electrophile and we have our H⁺ catalyst. As a result of this, we can create our polymer. Here, we're just showing one of the repeating units that comprises this chain of monomers to create this polymer. Right?

So this is what we can take when it comes to cationic polymerization. And now that we've talked about the overview of it, let's take a look at the steps involved.

Cationic Polymerization Concept 2

In our discussion of Cationic Polymerization, we're going to take a look at the mechanism. The mechanism is comprised of our 3 steps which are step 1 initiation, step 2 propagation, and step 3 termination. Here, let's take a look at step 1. So with step 1, we have initiation. We're going to say the Lewis acid, typically boron trifluoride, reacts with water to form an adduct.

We're going to say that the adduct donates an H⁺ ion to the monomer. So here we have our water molecule. It's our Lewis base so it's going to attack the boron which is the Lewis acid. It's going to create our adduct. So here we have our BF₃ boron gained electron, so it's negatively charged.

Oxygen is making 3 bonds. It has one lone pair. So again, it's positive. So here it is. In this adduct state, it can now donate an H⁺ to our monomer, alkene.

So this comes in, removes this H. Oxygen holds on to the electrons. Now boron is still making 4 bonds, so it's still negatively charged. Oxygen now though is neutral. And we would say here that this becomes a CH₃ because it gained an H⁺.

And we use the pi bond to grab that H, so this carbon here becomes positive. Alright. This will represent the initiation step of this cationic polymerization mechanism.

Cationic Polymerization Concept 3

In step 2 of cationic polymerization, we now take a look at the propagation step. Here, we're going to say the monomer cation reacts with a monomer molecule and forms a new cation through a head-to-tail addition. Right. So here, we have the tail end, and we're going to use this pi bond to attach to this carbocation. So here goes the pi bond that we used.

Here's the tail that attached. Here we have the head portion. Here, we use that pi bond to attach to that carbocation initially. So now, the new carbocation is here. Again, remember it's stabilized because OCH₃ represents an electron-donating group.

Alright? So this is what we make in step 2, the propagation step of cationic polymerization.

Cationic Polymerization Concept 4

Now, we take a look at step 3, which is termination when it comes to Karyonic Polymerization. Here, step 3 is termination, and it can occur by either 1, removal of an H⁺ or 2, nucleophilic attack. Considering the removal of H⁺, we're going to say loss of H⁺ is similar to beta elimination. Here, we would have this bond breaking, and we'd use it to grab this H. The bond would break.

Carbon would use it to make a double bond here. So, at the end, we make an alkene. We reform our boron trifluoride, and we just created water. We could also do a nucleophilic attack. Here, the counter ion from the initiation step can attack the cation.

Here again, this bond can be used. It comes in, attacks this carbocation here. So there goes the OH that attached. Boron is no longer making 4 bonds, so it's neutral. Thus, we've recreated boron trifluoride.

These are the two possibilities that can exist; having our nucleophilic attack as one option and the removal of H⁺ as the other option.

Cationic Polymerization Example 1

Hey everyone. So, here in this example question it says, arrange the following monomers from highest to lowest reactivity towards cationic polymerization. Remember, we said that electron donating groups help to promote cationic polymerization. So if we take a look here, here are our alkenes, and we just have to see what's attached to them. Here, we have an electron donating group and benzene also acts as an electron donating group.

Here, we have 2. We're I'll put them as blue. These are electron withdrawing groups, the nitrile and the carbonyl. This oxygen has lone pairs. It is not as electron donating as in one because the carbonyl does siphon off some of this energy.

Alright? And then finally, we have here another carbonyl. So this is another electron withdrawing group. So we wanna go from highest to lowest reactivity. 1 would be the highest because one has electron donating groups, pretty good ones.

Followed by 3 because although oxygen here is not as good at electron donating, it's still 1. And then 4 would be next because we only have 1 electron withdrawing group. And then finally, 2 would be the slowest because it's connected to 2 electron withdrawing groups in the form of Nitrile and the carbonyl group. Right? So again, remember, we say that electron donating groups attached to our alkene carbons help to foster or promote cationic polymerization.

Many heterocyclic compounds can undergo polymerization under acidic conditions. Draw the mechanism of the propagation step for cationic polymerization of thietane (four-membered saturated sulfur heterocycle).

In this question, we're told that many heterocyclic compounds can undergo polymerization under acidic conditions. The propagation step for cationic polymerization of thiatane, which is a four-membered saturated sulfur heterocycle, is discussed. The thiatane structure features sulfur in a four-membered ring. Under acidic conditions, the sulfur atom is protonated, resulting in a structure similar to protonated oxygen when forming three bonds, thereby carrying a positive charge. During the polymerization process, another thiatane molecule approaches and attacks the positively charged sulfur, leading to the opening of the ring. This attack results in the formation of a new bond extending from the initially attacked sulfur to the sulfur of the incoming molecule. The principle ring thus extends, and the newly attached sulfur, now making three bonds, becomes positively charged as well, ready for the next incoming thiatane molecule. This mechanism illustrates the propagation step in the cationic polymerization of thiatane under acidic conditions. It emphasizes continuous attack by incoming thiatane molecules, drawing attention to the dynamic and regenerative nature of the polymer chain growth facilitated by the protonated sulfur in an acidic environment.