Question

Poly(ethylene terephthalate) (PET) can be prepared by this reaction. Propose a mechanism for the step-growth reaction in this polymerization.

Short answer

Question: Explain the mechanism for the step-growth reaction (polymerization) of poly(ethylene terephthalate) (PET). Answer: The mechanism for the step-growth reaction (polymerization) of poly(ethylene terephthalate) (PET) involves the following steps: 1. Ethylene glycol (HO-CH2-CH2-OH) reacts with terephthalic acid (HOOC-C6H4-COOH) to create an ester linkage between them, forming a water molecule in the process. 2. The ester-linked reaction product now has an unreacted hydroxyl group on one end and an unreacted carboxylic acid group on the other end. This molecule can react with another molecule of ethylene glycol or terephthalic acid, forming another ester linkage and another water molecule. 3. This second step occurs repeatedly (n times), producing a longer and longer polymer chain, with the repeating unit: -(CH2-CH2-O-C(O)-C6H4-CO)-. 4. The final mechanism is represented by the following equation: n(HO-CH2-CH2-OH) + n(HOOC-C6H4-COOH) -> { -(CH2-CH2-O-C(O)-C6H4-CO)- }_n + (n-1)H2O

Step-by-step solution

Identify the monomers

The two monomers for this reaction are ethylene glycol (HO-CH2-CH2-OH) and terephthalic acid (C6H4(COOH)2).

Examine the reaction mechanism

To understand how PET forms, it's important to recognize that there's an esterification reaction taking place. During the reaction, one hydrogen atom from the hydroxyl group of ethylene glycol and one hydroxyl group from terephthalic acid come together to create a water molecule. As this process occurs repeatedly, the ester linkage between the two monomers forms, creating the polyester chain.

Show the polymerization mechanism

Let's break down the mechanism for the step-growth reaction of PET polymerization: 1. One molecule of ethylene glycol reacts with one molecule of terephthalic acid. This creates an ester linkage between them, and a water molecule is formed in the process. The reaction is as follows: HO-CH2-CH2-OH + HOOC-C6H4-COOH -> HO-CH2-CH2-O-C(O)-C6H4-COOH + H2O 2. The ester-linked reaction product from step 1 now has an unreacted hydroxyl group on one end and an unreacted carboxylic acid group on the other end. This molecule can react with another molecule of ethylene glycol or terephthalic acid (respectively), forming another ester linkage and another water molecule: HO-CH2-CH2-O-C(O)-C6H4-COOH + HO-CH2-CH2-OH -> HO-CH2-CH2-O-C(O)-C6H4-CO-O-CH2-CH2-OH + H2O This second step occurs repeatedly (n times), producing a longer and longer polymer chain, with the repeating unit: -(CH2-CH2-O-C(O)-C6H4-CO)- Finally, the polymerization process might be thermodynamically controlled and catalyzed by an acid or base.

Write the final mechanism

Based on the above steps, we can now write the mechanism for the step-growth reaction (polymerization) of poly(ethylene terephthalate) (PET): n(HO-CH2-CH2-OH) + n(HOOC-C6H4-COOH) -> { -(CH2-CH2-O-C(O)-C6H4-CO)- }_n + (n-1)H2O Here, the PET polymer is represented by the repeating unit { -(CH2-CH2-O-C(O)-C6H4-CO)- }_n.