Question

Which of the following polymers would be stronger or more rigid? Explain your choices. a. The copolymer of ethylene glycol and terephthalic acid or the copolymer of 1,2 -diaminoethane and terephthalic acid (1,2-diaminoethane \(=\mathrm{NH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2}\) ) b. The polymer of \(\mathrm{HO}-\left(\mathrm{CH}_{2}\right)_{6}-\mathrm{CO}_{2} \mathrm{H}\) or that of c. Polyacetylene or polyethylene (The monomer in polyacetylene is ethyne.)

Short answer

The stronger and more rigid polymers are: (a) the copolymer of 1,2-diaminoethane and terephthalic acid, due to the presence of strong intermolecular hydrogen bonding from N-H groups; (b) is not possible to compare these polymers; (c) polyacetylene, because of its conjugated structure with alternating single and double bonds, resulting in strong interactions between chains.

Step-by-step solution

Comparing polymers in (a)

We are given two copolymers: one formed from the copolymerization of ethylene glycol and terephthalic acid, and the other formed from the copolymerization of 1,2-diaminoethane and terephthalic acid. In the first copolymer, there are strong covalent ester bonds between the ethylene glycol and the terephthalic acid. In the second copolymer, there are strong covalent amide bonds between 1,2-diaminoethane and terephthalic acid. Both ester and amide bonds are strong, but amide bonds tend to have a higher level of intermolecular hydrogen bonding due to the presence of the N-H group. This strong hydrogen bonding enhances the rigidity and strength of the second copolymer compared to the first one. Therefore, the copolymer of 1,2-diaminoethane and terephthalic acid is stronger and more rigid than the one with ethylene glycol and terephthalic acid.

Comparing polymers in (b)

Polymers formed from the monomer \(\mathrm{HO}-\left(\mathrm{CH}_{2}\right)_{6}-\mathrm{CO}_{2} \mathrm{H}\) have long linear chains with a carbon backbone and ester groups. The carbon backbone provides flexibility and strength, and the ester groups increase the rigidity through intermolecular forces. The second polymer mentioned is not provided, so it is not possible to compare these polymers.

Comparing polymers in (c)

We are given two polymers: polyacetylene and polyethylene. Polyacetylene is a conjugated polymer with alternating single and double bonds along the backbone between carbon atoms. The π-electrons are delocalized along the polymer backbone, which results in strong interactions between the chains, improving the strength and rigidity of the material. On the other hand, polyethylene has a completely saturated hydrocarbon backbone with only single bonds between carbon atoms. This makes the polymer highly flexible but less rigid and strong compared to polyacetylene. Thus, polyacetylene is the stronger and more rigid polymer out of the two.

Key concepts

Covalent Bonds

Covalent bonds are the fundamental connections that hold atoms together within a molecule. In the context of polymer chemistry, these bonds are incredibly significant as they determine the structure and integrity of the polymer chains.
Covalent bonds form when two atoms share a pair of electrons, creating a stable balance of attractive and repulsive forces between them. This type of bonding is critical in defining the basic properties of polymers, such as strength and flexibility.

• In step 1 of the solution, strong covalent ester and amide bonds are mentioned, with amide bonds being prominent due to their additional hydrogen bonding capabilities.
• The choice between ester and amide bonds affects the polymer's rigidity and durability.

Understanding the nature of covalent bonds helps us predict the polymer's response to physical stress and environmental conditions.

Intermolecular Forces

Intermolecular forces refer to the forces that mediate the interaction between molecules. These forces are crucial in determining the physical properties of materials, including polymers. While covalent bonds maintain the internal structure of a molecule, intermolecular forces dictate how molecules interact with each other.
In polymer systems, these forces include dipole-dipole interactions, London dispersion forces, and hydrogen bonding. Intermolecular forces influence a polymer's melting point, solubility, and mechanical strength.

• In step 1, polymers with amide bonds exhibit stronger intermolecular hydrogen bonding, enhancing rigidity.
• Strong intermolecular forces can lead to higher strength and rigidity as seen in polyacetylene, which benefits from π-electron delocalization, thereby increasing intermolecular interactions.

Recognizing these forces helps us understand how polymers behave on a macro scale, influencing their practical applications.

Polymer Structure

The structure of a polymer is defined by the arrangement of its molecular constituents. Understanding the structure is critical in predicting and manipulating a polymer's properties.
Basic polymer structures are linear, branched, or cross-linked. The specific structure impacts the polymer's mechanical properties, like flexibility, strength, and thermal resistance.

• Linear polymers, such as those made from ethylene glycol and terephthalic acid, have predictable properties due to repetitive units in a chain.
• In step 3, polyacetylene’s conjugated structure features alternating single and double bonds, which contribute to its rigidity and strength.

By tailoring the polymer structure, chemists can design materials with desired properties for various applications.

Hydrogen Bonding

Hydrogen bonding is a specific type of intermolecular force that occurs when a hydrogen atom covalently bonded to a highly electronegative atom interacts with another electronegative atom. This unique interaction is pivotal in polymer chemistry, influencing the physical characteristics of the material.
Hydrogen bonds are generally stronger than other dipole-dipole interactions but weaker than covalent bonds. They can significantly impact the polymer’s rigidity, thermal stability, and strength.

• The presence of N-H groups in amide bonds, as seen in step 1, indicates potential for hydrogen bonding, enhancing the polymer's rigidity.
• Hydrogen bonding contributes to the increased strength and stability in polymers compared to those lacking any such interactions.

Mastering hydrogen bonding concepts helps chemists improve material performance, ensuring polymers meet the necessary criteria for their intended use.