FAD1018 W14 — Polymer Chemistry
Week 14 lecture covering polymer chemistry. Source files: W14 (1).pdf, W14 (2).pdf from lecture notes folders; Lecture Slide Polymer - 2026 ppt_v1.2.pdf (inbox).
Summary
Introduction to polymers including classification, polymerization mechanisms, important synthetic and natural polymers. This lecture also covers proteins as natural polymers (amino acids, peptide bonds, protein structure).
Part A: Proteins as Natural Polymers (W14-1)
Learning Outcomes
- Identify the general structure of amino acids.
- Identify the structures of the 20 standard amino acids.
- Name amino acids according to IUPAC rules.
- Define zwitterion and isoelectric point (pI).
- Draw the structure of a given amino acid in acidic, basic, and at pI.
- Explain the reactions of amino acids.
- Describe the formation of a peptide bond in a polypeptide.
- Explain the structure of proteins and the importance of amino acids and proteins.
1. Amino Acids
General Structure
All standard amino acids contain:
- An α-amino group (–NH₂)
- A carboxyl group (–COOH)
- A hydrogen atom
- A distinctive side chain (R group) attached to the α-carbon
N[C@@H](R)C(=O)O
Note: Glycine is the only achiral standard amino acid (R = H).
The 20 Standard Amino Acids
| Amino Acid | 3-Letter | 1-Letter | R Group | SMILES (neutral) |
|---|---|---|---|---|
| Glycine | Gly | G | –H | NCC(=O)O |
| Alanine | Ala | A | –CH₃ | CC(N)C(=O)O |
| Valine | Val | V | –CH(CH₃)₂ | CC(C)C(N)C(=O)O |
| Leucine | Leu | L | –CH₂CH(CH₃)₂ | CC(C)CC(N)C(=O)O |
| Isoleucine | Ile | I | –CH(CH₃)CH₂CH₃ | CCC(C)C(N)C(=O)O |
| Methionine | Met | M | –CH₂CH₂SCH₃ | CSCCC(N)C(=O)O |
| Phenylalanine | Phe | F | –CH₂–C₆H₅ | NC(Cc1ccccc1)C(=O)O |
| Tryptophan | Trp | W | –CH₂–indole | NC(Cc1c[nH]c2ccccc12)C(=O)O |
| Proline | Pro | P | – (cyclic) | O=C(O)C1CCCN1 |
| Serine | Ser | S | –CH₂OH | NC(CO)C(=O)O |
| Threonine | Thr | T | –CH(OH)CH₃ | CC(O)C(N)C(=O)O |
| Cysteine | Cys | C | –CH₂SH | NC(CS)C(=O)O |
| Tyrosine | Tyr | Y | –CH₂–C₆H₄OH | NC(Cc1ccc(O)cc1)C(=O)O |
| Asparagine | Asn | N | –CH₂CONH₂ | NC(=O)CC(N)C(=O)O |
| Glutamine | Gln | Q | –CH₂CH₂CONH₂ | NC(=O)CCC(N)C(=O)O |
| Aspartic acid | Asp | D | –CH₂COOH | NC(CC(=O)O)C(=O)O |
| Glutamic acid | Glu | E | –CH₂CH₂COOH | NC(CCC(=O)O)C(=O)O |
| Lysine | Lys | K | –(CH₂)₄NH₂ | NCCCC(N)C(=O)O |
| Arginine | Arg | R | –(CH₂)₃NHC(=NH)NH₂ | NC(CCCNC(=N)N)C(=O)O |
| Histidine | His | H | –CH₂–imidazole | NC(Cc1c[nH]cn1)C(=O)O |
Zwitterion and Isoelectric Point (pI)
In aqueous solution, amino acids exist predominantly as zwitterions — dipolar ions carrying both a positive and a negative charge.
[NH3+]CC(=O)[O-]
- Zwitterion: A molecule with equal positive and negative charges simultaneously.
- Isoelectric point (pI): The pH at which the amino acid has no net charge (exists mainly as the zwitterion).
pH Dependence of Charge
| Medium | Carboxyl Group | Amino Group | Net Charge | Movement in Electrophoresis |
|---|---|---|---|---|
| Acidic (pH < pI) | –COOH (protonated) | –NH₃⁺ (protonated) | Positive (+1) | Moves toward negative electrode |
| pI | –COO⁻ | –NH₃⁺ | Neutral (0) | Does not move |
| Basic (pH > pI) | –COO⁻ (deprotonated) | –NH₂ (deprotonated) | Negative (–1) | Moves toward positive electrode |
Key pKa values: α-COOH ≈ 2; α-NH₃⁺ ≈ 9–10.
Electrophoresis
At any pH other than the pI, amino acids migrate in an electric field:
- pH < pI → net positive charge → migrates to the cathode (–).
- pH > pI → net negative charge → migrates to the anode (+).
2. Peptides
Peptide Bond Formation
Amino acids are linked by peptide bonds (amide bonds) formed via a condensation reaction that eliminates water.
NCC(=O)O.NCC(=O)O>>NCC(=O)NCC(=O)O
Generic peptide bond between two amino acid residues:
N[C@@H](R1)C(=O)N[C@@H](R2)C(=O)O
- The bond forms between the α-carboxyl group of one amino acid and the α-amino group of the next.
- Not between the side chains (R groups).
Peptide Classification by Size
| Class | Number of Amino Acid Residues |
|---|---|
| Dipeptide | 2 |
| Tripeptide | 3 |
| Oligopeptide | 2 – 9 |
| Polypeptide | 10 – 100 |
| Protein | > 40 (typically > 100) |
N- and C-Termini
Every peptide chain has:
- A free N-terminal amino group (–NH₂ or –NH₃⁺) at one end.
- A free C-terminal carboxyl group (–COOH or –COO⁻) at the other end.
Nomenclature
Peptides are named from the N-terminus to the C-terminus.
- Full name: All residues except the C-terminal one use the -yl suffix.
- Three-letter abbreviation: e.g., Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg
- One-letter abbreviation: e.g., RPPGFSPFR
Examples
Ala-Gly-Ala (tripeptide):
CC(N)C(=O)NCC(=O)NC(C)C(=O)O
Ala-Cys (dipeptide):
CC(N)C(=O)NC(CS)C(=O)O
3. Proteins
Definition
- Proteins are polyamides containing 40 to several thousand amino acid residues.
- Each protein has a unique amino acid sequence that determines its shape and function.
Hydrolysis
Proteins can be hydrolyzed back to amino acids by:
- Boiling with dilute mineral acid (e.g., HCl)
- Boiling with aqueous alkali (e.g., NaOH)
- Enzymatic action (~40 °C)
Types of Proteins
| Type | Description | Examples |
|---|---|---|
| Simple | Hydrolyze to amino acids only | Albumins, globulins |
| Fibrous | Long, stringy filaments; insoluble; structural support | Collagen, elastin, keratin |
| Globular | Folded spherical shape; enzymes, hormones, transport | Hemoglobin, myoglobin, lysozyme |
| Conjugated | Bonded to a non-protein group (prosthetic group) | Nucleoproteins, glycoproteins, mucoproteins |
Levels of Protein Structure
| Level | Description | Key Interactions | Examples |
|---|---|---|---|
| Primary | Linear sequence of amino acids; location of disulfide bridges | Peptide bonds (covalent) | Insulin |
| Secondary | Local folding into α-helices or β-pleated sheets | Hydrogen bonds between backbone C=O and N–H | Keratin (α-helix), silk fibroin (β-sheet) |
| Tertiary | Overall 3-D folding of a single polypeptide | Ionic bonds, H-bonds, disulfide bridges, van der Waals forces | Myoglobin, lysozyme |
| Quaternary | Association of two or more polypeptide subunits | Same as tertiary (inter-subunit) | Hemoglobin (4 subunits), collagen (triple helix) |
Part B: Synthetic and Natural Polymers (W14-2)
Learning Outcomes
- Define and explain terminologies, condensation and addition polymerization
- Classify polymers and describe their usage
Definitions
- Monomer: The large molecule from which a polymer is synthesized
- Polymer: The large molecule which is made up of many repeating units of monomers
- Repeating Unit: The basic structure of a polymer; by repetition it produces a long polymer chain
Example: Polyethene
C=C
Classification
By Number of Monomer Types
Homopolymer
Polymers made up from only one type of monomer.
F/C(F)=C(F)/F
Example — Teflon (PTFE):
- Monomer: Tetrafluoroethylene
- Polymer: Polytetrafluoroethylene (Teflon)
Copolymer
Polymers made up from two or more different monomers.
| Type | Pattern | Description |
|---|---|---|
| Random | -A-B-B-A-B-A-A-B- | Repeating units in purely random fashion |
| Regular (alternating) | -A-B-A-B-A-B-A-B- | Regularly alternating units |
| Block | -A-A-A-A-B-B-B-B- | Occurs in blocks of different length |
| Graft | A-A-A-A-A… with B-B-B branches | Chain of one repeating unit grafted onto backbone of another |
Example — Saran®:
- Monomers: 1,1-dichloroethene + 1-chloroethene
- Use: Film for wrapping food
By Structure
i) Linear Polymers
Linear or straight-chain polymer consisting of monomers linked in a straight and long continuous chain without any lateral linkage or branching.
ii) Branched Polymers
Polymers with branches at regular intervals along the polymer chain. These branches make it difficult for polymer molecules to pack in a regular array → less crystalline.
iii) Cross-linked Polymers
Formed when linear or branched polymer chains are joined together by covalent bonds through cross-linked process.
- Adding cross-links makes polymer more elastic
- When number of cross-links is relatively larger, polymer becomes more rigid
By Thermal Behavior
| Type | Behavior | Structure |
|---|---|---|
| Thermoplastics | Flow when heated; can be molded into variety of shapes; retain shape when cool. Can be melted repeatedly. | Linear and branched polymers (no cross-linking). Weak forces between chains broken by heating. |
| Thermosets | Once cross-linked, shape cannot be changed without destroying plastic. Cannot be melted. | Heavy cross-linked polymers. Strong covalent bonds between chains. |
Polymerization Mechanisms
I) Addition Polymerization
- Addition reaction in which unsaturated monomers (monomers with double bond) are joined together by covalent bonds to form a polymer without elimination of a small molecule
- Empirical formula of polymer is the same as empirical formula of monomer
- Polymers obtained are called addition polymers
- Peroxide (CH₃OOCH₃) is used as an initiator
Examples: Polyethylene, polyvinyl chloride, polystyrene, polyisoprene, teflon
Example 1: Polyethene (PE)
C=C
Low-Density Polyethylene (LDPE) — Recycling code 4
- Discovered 1933 by ICI
- Conditions: 1200 atm, 200°C, O₂ — free radical mechanism
- Properties: Highly branched, cannot be packed closely together. Low mp (150°C), low density (0.92 g cm⁻³). Easily deformed and softens in boiling water.
- Uses: Plastic bags, wrapping sheet, bottles, electrical insulation
High-Density Polyethylene (HDPE) — Recycling code 2
- Manufactured 1953 by Karl Ziegler and Giulio Natta
- Conditions: 1 atm, 60°C, TiCl₄ + (C₂H₅)₃Al — Ziegler-Natta mechanism
- Properties: Mostly linear chains, closely packed, ordered structures. Density 0.96 g cm⁻³, mp 130–140°C. Stronger and harder.
- Uses: Molding rigid articles, agricultural/municipal/industrial pipes
Example 2: Polyvinyl chloride (PVC)
C=CCl
- Monomer: Chloroethene
- Uses: Pipes
Example 3: Polystyrene (PS)
c1ccccc1C=C
- Monomer: Phenylethene (styrene)
- Uses: Food packaging (foam containers)
Example 4: Polytetrafluoroethylene (PTFE, Teflon)
F/C(F)=C(F)/F
- Monomer: Tetrafluoroethylene
- Uses: Non-stick coatings, thread seal tape
Example 5: Polypropylene (PP)
C=CC
- Monomer: Propene
- Due to methyl groups, polymeric chains can have different structures
- Presence of CH₃ increases intermolecular van der Waals forces but makes chain difficult to pack → lower density but higher melting point
- Uses: Ropes, moulds, bottles, kitchenware, carpets, battery containers
Example 6: Synthetic Rubber — Neoprene
C=C(Cl)C=C
- First synthetic rubber produced by polymerization of 2-chloro-1,3-butadiene
- Resistant to most chemicals
- Uses: Hoses for petrol, containers for corrosive liquids
Example 7: Styrene-butadiene rubber (SBR)
- Copolymer between styrene (phenylethene) and 1,3-butadiene in ratio 1:3
- Can be vulcanized like natural rubber
- Uses: Car tyres, footwares, carpetbacking
II) Condensation Polymerization
- Process that combines monomers with elimination of a small molecule such as water, methanol, hydrogen chloride, or ammonia
- Monomers must have at least two identical or different functional groups
- Polymers obtained are called condensation polymers
2 major classes:
- Polyamide — formed when carboxylic acid with two -COOH reacts with amine with two -NH₂
- Polyester — formed when carboxylic acid with two -COOH reacts with alcohol with two -OH
(A) Polyamides
Example 1: Nylon 6,6
NCCCCCCN
O=C(O)CCCCC(=O)O
- Monomers: Hexane-1,6-diamine + hexane-1,6-dioic acid
- By-product: nH₂O
Example 2: Nylon 6
NCCCCCC(=O)O
- Monomer: 6-aminohexanoic acid
- By-product: nH₂O
Example 3: Kevlar
Nc1ccc(N)cc1
O=C(O)c1ccc(C(=O)O)cc1
- Monomers: 1,4-diaminobenzene + terephthalic acid
- Properties: Very strong and flexible
- Uses: Bulletproof vests
(B) Polyester
- Repeating functional groups: ester
- Most familiar polyester: polyethylene terephthalate (PET), known as Dacron and Terylene
Example 1: Dacron (PET)
COC(=O)c1ccc(C(=O)OC)cc1
OCCO
- Monomers: Dimethyl terephthalate + ethylene glycol
- By-product: Methanol (nCH₃OH)
- Uses: Clothing, tyre cords, carpets
Example 2: Terylene
O=C(O)c1ccc(C(=O)O)cc1
OCCO
- Monomers: Terephthalic acid + ethane-1,2-diol
- By-product: Water (nH₂O)
Other Terminologies in Polymer Chemistry
Crystallites
- The large size of polymers means that they experience greater van der Waals forces than small molecules.
- Because these forces operate only at small distances, they are strongest if the polymer chains can line up in an ordered, closely packed array.
- The regions of the polymer in which the chains are highly ordered with respect to one another are called crystallites.
- The more crystalline (more ordered) the polymer is, the denser, harder, and more resistant to heat it is.
Elastomers
- An elastomer is a polymer that stretches and then reverts to its original shape.
- It is a randomly oriented amorphous polymer, but it must have some cross-linking so that the chains do not slip over one another.
- When elastomers are stretched, the random chains stretch out. The van der Waals forces are not strong enough to maintain them in that arrangement; therefore, when the stretching is removed, the chains go back to their random shapes.
- Rubber is an example of an elastomer.
Fibers
- Fibers are thin threads produced by passing a molten polymer through small holes in a die.
- When the fibers are cooled and drawn out, the crystalline regions orientate themselves along the axis of the fiber which adds considerable tensile strength.
- Examples of fibers are nylon, Dacron, and polyethylene.
Plasticizers
- A plasticizer is an organic compound that dissolves in the polymer, lowering the attractions between the polymer chains, which allows them to slide past one another.
- This makes the polymer more flexible.
- Example: Dibutyl phthalate (added to PVC to make flexible flooring).
Modification of Polymer Properties
Polymer properties can be deliberately modified by:
- Changing the length of the polymer chain
- Varying the chemical composition of the monomer units
- Changing the branching of the polymer chains
- Cross-linking the polymer chains
- Varying the arrangement of the chains in the solid
- Modifying the orientation of the monomer units within the chains
Polymer Research Fields
Fiber Reinforced Composites
- Used in building materials, vehicle parts, etc.
- Use of natural fiber vs synthetic fiber
- Waste to wealth, reduce manufacturing cost
- Sustainable, environmentally friendly
Drawbacks:
- Natural fiber has many hydrophilic –OH groups
- HDPE is entirely hydrophobic
- Weak adhesion between fiber and HDPE matrix
- Weak physical and mechanical properties
- Poor resistance to moisture
Research solutions:
- Use a compatibilizer to address the weaknesses (improves interface adhesion)
Sustainable Applications
- Sustainable GunPla made from eggshells (injection moulding)
- From crab shell to solar cell: gel polymer electrolyte based on N-phthaloylchitosan
- Self-healing coatings: UV-curable alkyd coatings for buildings and industrial structures
- Sustained drug delivery materials: nanofiber capsules that slowly release drugs into the body (less strain on kidney compared to immediate-release capsules)
Natural Polymers
Natural Rubber
- Polymer of 2-methyl-1,3-butadiene (isoprene)
- Isoprene undergoes addition polymerization to give poly(isoprene)
- Poly(isoprene) exists in two forms: cis and trans depending on relative spatial arrangement of the two -CH₂ groups
CC(=C)C=C
Vulcanization
- Natural rubber is soft, sticky, not strong or elastic
- To improve properties, it undergoes vulcanization process
- Soft rubber transformed to harder cross-linked polymer by heating rubber with sulphur atoms
- Long chains of polyisoprene are cross-linked by sulphur atoms
- Too much vulcanization makes rubber hard and brittle
- Vulcanized rubber uses: Tyres, footware, gloves, elastic bands, tubings, toys
Related Topics
- Amines & Amino Acids — Proteins as natural polymers
- Carboxylic Acids & Derivatives — Polyesters and polyamides
- Polymer Chemistry — General polymer concepts
Study Notes
[!note] Modern materials science Polymer chemistry connects organic chemistry to materials science. Know the major polymer types, their monomers, polymerization mechanisms, and applications.
[!tip] Exam focus Be able to draw monomers and repeating units for: PE, PVC, PS, PTFE, PP, nylon 6,6, nylon 6, Kevlar, Dacron, Terylene. Know conditions for LDPE vs HDPE synthesis. Understand cis/trans isoprene and vulcanization.
Related Course Page
References
- Nasrulzamani B. Mohd. Rodzi & Dr Ahmad Danial, Prof Madya Dr. Norbani Abdullah, Dr. Hazar Bebe Mohd Ismail. (2015). Comprehensive College Chemistry (Upgraded). SAP Publications.
- Wade, L. G. (2012). Organic Chemistry (8th Ed). Pearson Education.
- Brown, W. H., Iverson, B. L., Anslyn, E., & Foote, C. S. (2018). Organic Chemistry (8th Ed). Cengage Learning.
- Favre, H. A., & Powell, W. H. (2014). Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013. Royal Society of Chemistry.
- Salleh, F. M., et al. (2014). Improvement in the mechanical performance of kenaf fiber reinforced high density polyethylene composites. Journal of Polymer Research, 21, 1-11.
- Yusuf, S. N. F., et al. (2016). From crab shell to solar cell: A gel polymer electrolyte based on N-phthaloylchitosan. RSC Advances, 6(33), 27714-27724.
- Saman, N. M., et al. (2019). UV-curable alkyd coating with self-healing ability. Journal of Coatings Technology and Research, 16, 465-476.
- Mazlan, M., et al. (2021). The impact of substitution of two hydrophobic moieties on the properties of guar gum based hydrogels. Pigment & Resin Technology, 50(6), 485-495.