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: 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

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.

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.

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.

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.

Key concepts

step-growth polymerization

In the world of polymers, there are different mechanisms that determine how monomers link up to form intricate structures. Step-growth polymerization is one such mechanism. Unlike chain-growth polymerization, where monomers add to an active site one at a time, step-growth polymerization involves the gradual build-up of polymer chains through the reaction of bi-functional or multi-functional monomers.
In the case of polyethylene terephthalate (PET), the two main monomers involved are ethylene glycol and terephthalic acid. They react together to form long chains. Each of these monomers has functional groups that can react with others to build up the chain.
Here’s what's interesting:

• It’s a slow process and typically requires that the polymer chains grow from both ends.
• The molecular weight of the polymer increases steadily as the reaction progresses.
• The process often involves the formation and elimination of small molecules, like water, which are byproducts of the reaction.

Overall, step-growth polymerization is a key process for creating high-performance materials like PET, and understanding it is essential for anyone studying polymer chemistry.

ester linkage formation

In chemistry, the formation of ester linkages is a crucial step in creating many synthetic materials such as polymers. In PET synthesis, ester linkages are formed between ethylene glycol and terephthalic acid.
Here’s how it happens: the hydroxyl group ( -OH ) in ethylene glycol reacts with the carboxyl group ( -COOH ) in terephthalic acid. This -OH group attacks the carbon of the carbonyl group ( C=O ) creating a tetrahedral intermediate, which then loses a hydroxyl group creating an ester linkage.
What results is a powerful bond between the two molecules, with the simultaneous release of a water molecule as a byproduct. This ester bond (-C(O)-O-) is a covalent bond that holds the polymer chains together, acting like molecular super glue.

nucleophilic attack

A nucleophilic attack is a basic concept in organic chemistry that plays a central role in the mechanism of PET polymerization. Simply put, it involves a nucleophile, a molecule or ion with a pair of lone electrons, attacking a positively charged or electron-deficient center in another molecule.
In the case of PET formation, the hydroxyl group (-OH) of ethylene glycol acts as the nucleophile. It attacks the carbonyl carbon ( C=O ) of terephthalic acid, which is electron-deficient. This results in the formation of a tetrahedral intermediate. This reaction step is crucial as it sets the stage for forming the stable ester linkage.

• It’s like a game of molecular chess, where electron-rich parts of molecules strategically move to react with electron-poor parts.
• This attack occurs repeatedly during the polymerization process, leading to the formation of long polymer chains.

Understanding nucleophilic attack helps in grasping the complex yet systematic world of organic reactions.

ethylene glycol

Ethylene glycol, often referred to in shorthand as EG , is a simple diol—that is, an alcohol with two hydroxyl groups. Its chemical formula is C_2H_6O_2 . It plays a pivotal role in forming PET.
This molecule's two hydroxyl groups allow it to act as a monomer that easily reacts with other monomers, like terephthalic acid, to form extended polymer chains.
Some key points about ethylene glycol:

• It serves as the backbone structure of many polyesters, including PET.
• Its reactivity is due to the hydroxyl groups, making it versatile for various chemical reactions.
• It is a clear, viscous liquid, widely used in antifreeze and as a precursor to polymers.

Ethylene glycol's characteristics make it perfect for forming stable and long chains in polymer production.

terephthalic acid

Terephthalic acid is a dicarboxylic acid and is a crucial monomer in the manufacture of PET. With the chemical formula C_8H_6O_4 , it consists of a benzene ring flanked by two carboxyl groups ( -COOH ).
This acidic compound is particularly reactive due to these carboxyl groups, which facilitate the esterification process with ethylene glycol.
Here are some important facts about terephthalic acid:

• It contributes rigidity and thermal stability to the polymer when integrated into PET.
• The benzene ring adds to the hydrophobic nature of the polymer, making it suitable for various applications like bottles and textiles.
• It is usually derived from the oxidation of p-xylene.

Terephthalic acid’s structure and properties are foundational to the attributes of PET, contributing to its widespread industrial application.