Question 13 of 13: Poly(ester amide) Monomer Identification

Question 13: Poly(ester amide) Monomer Identification (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Approach. Trace the backbone bond-by-bond, classifying every carbonyl-heteroatom linkage as an amide (C(=O)–N) or an ester (C(=O)–O), and identify each stretch of backbone between amide/ester junctions as a distinct monomer residue.

1. Diacid residue –C(=O)CH₂CH₂C(=O)– forms TWO amide bonds ⇒ succinic acid, HOOC–CH₂CH₂–COOH. Both ends of this four-carbon diacyl fragment terminate in –NH– on the backbone, so this monomer contributes no ester linkage at all — it is a simple saturated diacid, the shortest possible one that still separates its two carbonyls by a flexible –CH₂CH₂– spacer.
Monomer 1: succinic acid (diacid)
Monomer 2: leucine (amino acid)
2. –NH–CH(CH₂CH(CH₃)₂)–C(=O)–O–, an isobutyl-bearing α-amino-acid residue whose amine forms an amide (with succinic acid) and whose carboxyl forms an ester (with the diol) ⇒ leucine, (CH₃)₂CHCH₂CH(NH₂)COOH. The –CH₂CH(CH₃)₂ side chain hanging off the backbone α-carbon is leucine's diagnostic isobutyl group; using leucine's own –COOH as an ester partner (rather than a second amide) is exactly what turns this from a simple polyamide (nylon-type) into a poly(ester amide).
3. –O–CH₂CH₂–O– bridges two ester carbonyls ⇒ ethylene glycol, HOCH₂CH₂OH. A simple diol, the shortest one available, supplying the second oxygen needed to complete the leucine ester on one side and a second amino-acid ester on the other.
4. The final backbone stretch — a long –NH–(CH₂)₄–CH(–NH–)– chain with a pendant benzyl ester (–C(=O)–O–CH₂C₆H₅) hanging off its own α-carbon — is lysine, used "in reverse". Lysine, H₂N–(CH₂)₄–CH(NH₂)–COOH, is the one natural amino acid with two amine groups (α and ε). Here both amines are used as backbone amide partners (the ε-NH₂ condenses with the second succinic-acid diacyl unit; the α-NH₂ continues the backbone onward), while lysine's own α-carboxyl is capped as a benzyl ester (a protecting/pendant group, not a backbone linkage) — this pendant hydrophobic benzyl group is exactly the kind of tunable side chain that lets formulation chemists adjust the coating's hydrophobicity/degradation rate.
Monomer 3: ethylene glycol (diol)
Monomer 4: lysine (amino acid, used "in reverse")