Step-Growth Polymers: Polyurethane Mechanism - Online Tutor, Practice Problems & Exam Prep

Polyurethanes are synthesized through the nucleophilic addition of a diol to toluene diisocyanate, typically in an 80:20 mixture of 2,4- and 2,6-toluene diisocyanate. The major isomer is 2,4-toluene diisocyanate, which is prioritized due to its methyl group. The formation mechanism involves two key steps, emphasizing the importance of understanding isomeric forms and reaction mechanisms in polymer chemistry.

Polyurethane Mechanism Example 1

So in this example question, it says, provide the mechanism for the reaction between toluenesocyanate and methylenediol. Alright. So here, I don't specify which version we're dealing with, but the default is 24. So we're talking about the 24 isomer of toluene diisocyanate. Now, the steps are step 1 is nucleophilic attack, and then step 2 is broken down into a and b.

In two way, we're talking about a proton transfer, and in 2b, we're talking about a protonation step. So let's get to it. Step 1, we're going to have our nucleophilic alcohol attacks the carbonyl carbon of our isocyanate. So we're going to take this lone pair, attack right here, which causes this bond to break and go to the oxygen. So what we're going to get here initially is this:

O + O+

Oxygen is making 3 bonds so it's positively charged. So there goes our structure. Now step 2a here, this is our deprotonation, which is just another type of proton transfer of the alcohol group by a second diol. So this is going to come, deprotonate this, oxygen holds on to the electrons, so we get this structure. Now this is resonance stabilized because this oxygen here could decide to resonate down and make a pi bond, causing this pi bond to break and go to the nitrogen.

N - N-

Now nitrogen is making 2 bonds with 2 lone pairs, so it's negatively charged. And again, remember that this is still here. Right there. And then finally, in 2b, we have protonation of the isocyanate nitrogen by the protonated alcohol to form our urethane at the end. Here we're going to say the urethane represents a repeating monomer that can be elongated as needed.

Okay. So that can be elongated as needed. So if we take a look here, here are the 2 resonance forms. We can say here that this nitrogen with its lone pair comes in deprotonates here, are protonated. And this here would have a hydrogen on it, which is not showing it.

It's positive here, would deprotonate it. And we have protonated the nitrogen to give us at the end our urethane molecule. Alright. So the mechanism itself isn't too bad. It's just 2 simple steps with the second step broken down into 2a and 2b.

So here, we would have our proton transfer step, and then we'd have our protonation step. Right? So just keep this in mind if you're asked to give the mechanism for a urethane formation reaction.

Determine the monomer created from the reaction between toluene diisocyanate and ethylene diol.

• A.
• B.
• C.
• D.

Provide the mechanism for the reaction between toluene diisocyanate and butane-2,3-diol.

Here, we're told to provide the mechanism for the reaction between toluene diisocyanate and butane-2,3-diol. Alright. So here, we're going to have our 2,4-isomer of toluene diisocyanate. Remember, that is the default isomer of this particular diisocyanate molecule. Alright. So we're going to have here now butane is 4 carbons. And on carbon number 2, there's an OH. And on carbon number 3, there's an OH. The first thing that happens is a nucleophilic attack. So this comes in here. It's here. This carbonyl carbonate kicks the bond here. So we're going to wind up making this structure. Negative. And then we're going to have here. Here goes my oxygen that's making 3 bonds now, so it's positively charged. It's connected to this carbon, which is still connected to a methyl, which is connected to this carbon with an OH, and then this methyl. Next, we're going to have deprotonation. So we're going to say a second mole of this diol comes in, it's going to deprotonate, remove this H, falls here to make a bond with a lone pair on the oxygen. So here, actually, I'm going to show this arrow going this way, and we're going to bring this structure down here. So let's draw it out. Okay. Okay. So we have that. And then we're going to have this neutral oxygen now. Alright. Now remember, this is resonance stabilized. So if I were to draw it again, nitrogen would have 2 lone pairs, 2 bonds, and it'd be negatively charged. We've recreated our carbonyl. So these are our 2 resonance structures. And what happens now is the alcohol that deprotonated comes back. Okay. Let's see. So now we're going to have this. This nitrogen can then deprotonate it. Oxygen holds on to the electrons. So here goes the NH group. So this will represent our final product at the end through the mechanism. Right? So we follow the 3 steps, which remember step 1 is nucleophilic attack. Step 2 a, we're talking about a proton transfer. And then in step 2 b, we're talking about our protonation in order to get to our final answer here. Right? So this would be our urethane molecule produced at the end. And if we wanted to create this and make it longer, we could. This is just again, remember, a repeating monitor.