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

Short Answer

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Question: Describe the step-growth mechanism involved in the polymerization of poly(ethylene terephthalate) (PET). Answer: The step-growth mechanism in the polymerization of PET involves four main steps: 1) Identification of the two monomers - ethylene glycol and terephthalic acid. 2) Formation of ester linkages between the monomers through nucleophilic attack and elimination of water molecules. 3) Chain growth as more monomers react with the growing polymer chain, resulting in a step-growth polymerization process. 4) Termination and the production of high molecular weight polymers, sometimes facilitated by a vacuum application to remove water byproducts and drive the equilibrium in favor of ester linkage formation.

Step by step solution

01

Identify the monomers

Poly(ethylene terephthalate) (PET) is a polymer formed from two monomer units: ethylene glycol (HO-CH2-CH2-OH) and terephthalic acid (HO-C6H4-CO-OH). The polymerization reaction involves the formation of ester linkages between the monomers, with the elimination of a water molecule for each linkage formed.
02

Formation of ester linkage

The polymerization mechanism involves the nucleophilic attack of the hydroxyl group (-OH) from ethylene glycol on the carbonyl carbon (C=O) of terephthalic acid. This attack forms a tetrahedral intermediate (an unstable species with four bonds to the carbon atom) that subsequently loses a hydroxyl group to form the ester linkage (-C(O)-O-) between the two monomers, releasing a water molecule as a byproduct.
03

Chain growth

The ester formation reaction can continue as more ethylene glycol and terephthalic acid monomers react with the growing polymer chain. The reaction can occur at both ends of the chain, as each ethylene glycol has two hydroxyl groups to react with the terephthalic acid carbonyl groups. This allows the chain to grow in both directions, leading to a step-growth polymerization process.
04

Termination and high molecular weight polymers

The step-growth polymerization of PET continues until most of the hydroxyl and carbonyl groups have reacted to form ester linkages. This results in the formation of high molecular weight polymers. Sometimes, in order to increase the molecular weight and mechanical properties of the product even more, the reaction can be continued with a vacuum applied to the system, which removes the water formed as the byproduct and drives the equilibrium in favor of the formation of more ester linkages. In conclusion, the mechanism for the step-growth reaction in the polymerization of poly(ethylene terephthalate) (PET) involves the formation of ester linkages between the monomers through nucleophilic attack and elimination of water molecules, leading to chain growth and high molecular weight polymers.

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