Deduce the structure or repeat unit of a condensation polymer from given monomers and vice versa, limited to: (a) polyamides from a dicarboxylic acid and a diamine (b) polyesters from a dicarboxylic acid and a diol

Organic Chemistry – Polymers

Polyamides (e.g., Nylon)

Think of a polyamide as a long chain of beads linked together. Each bead is a monomer and the link between beads is an amide bond (\$-CO-NH-\$). The chain grows by a condensation reaction where a molecule of water is removed each time two monomers join.

Typical monomers

MonomerFormulaRole
Diamine\$H2N-(CH2)n-NH2\$Provides the –NH– part of the amide link.
Dicarboxylic acid\$HOOC-(CH2)m-COOH\$Provides the –CO– part of the amide link.

Example: Nylon 6,6

  1. Monomers: hexamethylenediamine (\$H2N-(CH2)6-NH2\$) and adipic acid (\$HOOC-(CH2)4-COOH\$).

  2. Condensation: each –NH– reacts with a –COOH, releasing \$H_2O\$.

  3. Repeat unit: \$[-NH-(CH2)6-NH-CO-(CH2)4-CO-]\$.

Reverse: From repeat unit to monomers

Given the repeat unit \$[-NH-(CH2)6-NH-CO-(CH2)4-CO-]\$, split it into two halves: the part ending in –NH– comes from the diamine, the part ending in –CO– comes from the dicarboxylic acid.

Exam tip 📝 – When you see a repeat unit with alternating –NH– and –CO–, remember it’s a polyamide. Count the carbon atoms in each side to identify the specific monomers.

Polyesters

Polyesters are like a chain of beads where each bead is a diol or a dicarboxylic acid, linked by an ester bond (\$-CO-O-\$). They also form via a condensation reaction, losing a water molecule each time.

Typical monomers

MonomerFormulaRole
Diol\$HO-(CH2)n-OH\$Provides the –O– part of the ester link.
Dicarboxylic acid\$HOOC-(CH2)m-COOH\$Provides the –CO– part of the ester link.

Example: PET (Polyethylene terephthalate)

  1. Monomers: terephthalic acid (\$HOOC-Ph-COOH\$) and ethylene glycol (\$HO-CH2-CH2-OH\$).

  2. Condensation: –COOH reacts with –OH, releasing \$H_2O\$.

  3. Repeat unit: \$[-CO-Ph-CO-O-CH2-CH2-O-]\$.

Reverse: From repeat unit to monomers

Given \$[-CO-Ph-CO-O-CH2-CH2-O-]\$, split at the ester bond: the part ending in –CO– comes from the dicarboxylic acid, the part ending in –O– comes from the diol.

Exam tip 📚 – Look for the pattern \$-CO-O-\$ in the repeat unit. Count the atoms between the two carbonyls to spot the diol and the acid. Remember PET is used in plastic bottles!

Quick Recap & Practice

  • Polyamides: \$-CO-NH-\$ link; monomers are a diamine and a dicarboxylic acid.

  • Polyesters: \$-CO-O-\$ link; monomers are a diol and a dicarboxylic acid.

  • Condensation reaction always releases \$H_2O\$.

  • To deduce monomers from a repeat unit, split at the heteroatom bonds.

Try drawing the repeat unit for a polymer made from 1,4-butanediol and succinic acid. Then work backwards to find the monomers. Good luck! 🚀