Polymers

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your MDCAT exam.

What are Polymers? Polymers are large molecules made of repeating structural units called monomers, joined together by polymerisation reactions. They have high molecular masses (10³–10⁷ g/mol). In MDCAT, expect questions on classification, properties, and uses of major polymers.

Classification of Polymers:

By Structure:

• Addition polymers: Formed by addition of monomer units without elimination (e.g., polythene, PVC, PTFE). Usually from unsaturated monomers (alkenes).
• Condensation polymers: Formed with elimination of small molecules like H₂O or HCl (e.g., nylon, terylene, bakelite). Usually from bifunctional monomers.

By Origin:

• Natural: Starch, cellulose, proteins, natural rubber
• Synthetic: Polythene, nylon, PVC, Teflon

By Molecular Forces:

• Elastomers: Weak intermolecular forces, elastic (e.g., natural rubber)
• Fibres: Strong hydrogen bonding, linear, thread-like (e.g., nylon)
• Thermoplastics: Soften on heating, can be remoulded (e.g., polythene)
• Thermosetting: Permanently hard, cannot be remoulded (e.g., bakelite)

Key Polymers to Memorise for MDCAT:

Polymer	Monomer	Uses
Polythene (PE)	Ethene (C₂H₄)	Plastic bags, containers
PVC	Vinyl chloride	Pipes, insulation
Teflon (PTFE)	Tetrafluoroethene	Non-stick cookware
Nylon 6,6	Hexamethylenediamine + adipic acid	Ropes, fabrics
Terylene (PET)	Terephthalic acid + ethylene glycol	Textile fibres
Bakelite	Phenol + formaldehyde	Electrical switches

⚡ MDCAT Tip: MDCAT often asks “which polymer is used for X?” — know the major uses. Also watch for biodegradable polymers as an environmental science question.

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🟡 Standard — Regular Study (2d–2mo)

For students who want genuine understanding.

Addition Polymerisation: Mechanism: Free radical polymerisation — initiator (e.g., benzoyl peroxide) produces free radicals that attack the double bond of an alkene, opening it and creating a new free radical that continues the chain.

General mechanism:

1. Initiation: $I_2 \rightarrow 2I^\cdot$
2. Propagation: $I^\cdot + M \rightarrow IM^\cdot$, then $IM^\cdot + M \rightarrow IM_2^\cdot$ (continues)
3. Termination: Two radicals combine to end the chain

Copolymers: When two different monomers polymerise together. Example: SBR (styrene-butadiene rubber) — used in car tyres.

Condensation Polymerisation: Each monomer must have two reactive functional groups. The reaction proceeds by step-growth polymerisation with elimination of water or HCl.

Example: Nylon 6,6 synthesis: $$\text{H}_2\text{N-(CH}_2)_6\text{-NH}_2 + \text{HOOC-(CH}_2)_4\text{-COOH} \rightarrow \text{…-NH-(CH}_2)_6\text{-NH-CO-(CH}_2)_4\text{-CO-…} + \text{H}_2\text{O}$$

Molecular Mass and Degree of Polymerisation: Number-average molecular mass $M_n = \sum N_i M_i / \sum N_i$ Degree of polymerisation $n = M_{\text{polymer}} / M_{\text{monomer}}$

Physical Properties and Structure:

• Linear polymers (low density PE): Pack well, high density, high melting point
• Branched polymers (LDPE): Irregular packing, lower density, softer
• Cross-linked polymers (rubber vulcanisation): 3D network, hard, infusible

Biodegradable Polymers:

• Polylactic acid (PLA): From corn starch, used in medical implants
• Polyhydroxyalkanoates (PHA): Produced by bacteria, fully biodegradable

⚡ MDCAT Tip: Vulcanisation of rubber — adding sulfur at ~140°C creates cross-links between polymer chains, making rubber harder and more durable. Unvulcanised rubber is sticky and soft.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Mechanism of Free Radical Polymerisation — Step by Step:

Step 1 — Initiation: $$R-O-O-R \xrightarrow{\Delta} 2R-O^\cdot \quad \text{(homolytic bond cleavage)}$$ The radical $R-O^\cdot$ attacks a double bond in the monomer (e.g., ethene): $$R-O^\cdot + CH_2=CH_2 \rightarrow R-O-CH_2-CH_2^\cdot$$

Step 2 — Propagation: Each radical adds monomer units rapidly: $$R-O-CH_2-CH_2^\cdot + CH_2=CH_2 \rightarrow R-O-CH_2-CH_2-CH_2-CH_2^\cdot \quad \text{(continues rapidly)}$$

Step 3 — Termination: Two radicals combine: $$R-(CH_2-CH_2)_n^\cdot + ^\cdot(CH_2-CH_2)m-R \rightarrow R-(CH_2-CH_2){n+m}-R$$

Or by disproportionation: $$R-(CH_2-CH_2)_n^\cdot + ^\cdot(CH_2-CH_2)_m-R \rightarrow R-(CH_2-CH_2)_n-H + R-(CH_2-CH_2)_m-CH=CH_2$$

Molecular Mass Distribution: Real polymers have a distribution of chain lengths. $M_w$ (weight average) is always ≥ $M_n$ (number average). The polydispersity index $M_w/M_n$ indicates uniformity — a value close to 1 means uniform chain lengths.

Elastomers — Natural and Synthetic Rubber: Natural rubber (polyisoprene): Has cis-configuration at the double bonds, giving it flexibility. The double bonds also allow vulcanisation.

Synthetic rubber types:

• Buna-S (SBR): Copolymer of styrene and butadiene, used in tyres
• Buna-N (NBR): Copolymer of butadiene and acrylonitrile, oil-resistant
• Neoprene: Polymer of chloroprene, used in chemical-resistant gloves

Environmental Aspects of Polymers:

• Non-biodegradable: Most synthetic polymers persist for hundreds of years
• Source of microplastics: UV degradation breaks plastic into microplastics
• Solution: Biodegradable polymers, recycling, polymer degradation research

MDCAT Common Mistakes:

1. Confusing addition and condensation polymers — condensation ALWAYS produces a small molecule (H₂O, NH₃, HCl)
2. Forgetting that condensation monomers need TWO functional groups per molecule
3. Mixing up thermoplastics and thermosetting plastics — thermoplastics can be remelted, thermosets cannot
4. Not knowing the monomer of common polymers (e.g., PTFE = tetrafluoroethene, PVC = vinyl chloride)
5. Forgetting that Teflon is non-stick due to C-F bonds being very strong and unreactive

Priority Order for MDCAT: Polymer classification → Addition vs condensation → Key polymers and uses → Polymerisation mechanism → Environmental aspects

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