Step-Growth Polymers: Polyurethane Mechanism: Videos & Practice Problems

Polyurethanes are synthesized through a nucleophilic addition reaction involving a diol and toluene diisocyanate (TDI). The most common mixture of TDI used in this process consists of 80% 2,4-toluene diisocyanate and 20% 2,6-toluene diisocyanate. This means that when discussing TDI, the default reference is typically to the 2,4 isomer unless specified otherwise. The structure of toluene indicates that the methyl group is the priority substituent, leading to the designation of the 2 and 4 positions for the isocyanate groups.

In the formation mechanism of polyurethanes, two key steps are involved. The first step is the nucleophilic attack of the hydroxyl group from the diol on the electrophilic carbon of the isocyanate group. This reaction results in the formation of a carbamate intermediate. The second step involves the further reaction of this intermediate, leading to the formation of the polyurethane polymer. Understanding these steps is crucial for grasping how polyurethanes are formed and their applications in various industries.

In the reaction between toluene diisocyanate (specifically the 24 isomer) and methylenediol, the mechanism involves a series of well-defined steps leading to the formation of a urethane. The process begins with a nucleophilic attack, where the alcohol from methylenediol targets the carbonyl carbon of the isocyanate group. This interaction results in the breaking of the carbon-oxygen bond, allowing the oxygen to form three bonds, which gives it a positive charge.

Following this initial step, the mechanism progresses to a two-part process. The first part, termed deprotonation (Step 2a), involves the alcohol group from a second diol molecule removing a proton from the positively charged oxygen. This transfer stabilizes the structure through resonance, where the oxygen can shift its electrons to form a pi bond with the nitrogen, resulting in the nitrogen carrying a negative charge due to its two bonds and two lone pairs.

In the second part of Step 2 (Step 2b), the nitrogen of the isocyanate undergoes protonation by the protonated alcohol, culminating in the formation of the urethane. This final product can serve as a repeating monomer, allowing for further elongation as needed in polymer synthesis.

Overall, the mechanism is straightforward, consisting of a nucleophilic attack followed by a proton transfer and a protonation step, which are essential for understanding urethane formation reactions. The resonance stabilization during the process is crucial, as it influences the reactivity and stability of the intermediates formed.