Introduction of Organic reactions, Polymerization: free-radical, ionic and condensation mechanisms; degree of polymerization

Organic Reactions & Polymerization Mechanisms, Degree of Polymerization

1. Basic organic reaction types (primer)

• Substitution: one group replaces another. $CH_3Cl + OH^- \to CH_3OH + Cl^-$.
• Addition: across double bond. $CH_2=CH_2 + Br_2 \to CH_2Br-CH_2Br$.
• Elimination: loss of small molecule to form double bond. $CH_3CH_2OH \to CH_2=CH_2 + H_2O$.
• Rearrangement: skeleton reshuffles (carbocation shift).

Bond breaking: homolysis (one electron each → free radicals) vs heterolysis (both electrons to one fragment → carbocation + carbanion). Homolysis drives radical polymerization; heterolysis drives ionic polymerization.

2. Free-radical polymerization (e.g. polyethylene, PS, PVC)

Initiator (benzoyl peroxide) splits on heating: $I \to 2R^\bullet$.

1. Initiation: $R^\bullet + CH_2=CHX \to R-CH_2-\dot{C}HX$.
2. Propagation: radical adds monomer repeatedly, chain grows: $\sim CH_2-\dot{C}HX + CH_2=CHX \to \sim CH_2-CHX-CH_2-\dot{C}HX$.
3. Termination: coupling ($2\sim^\bullet \to$ dead chain) or disproportionation (H-transfer → one saturated + one unsaturated end).

Features: needs vinyl monomer, tolerant of moisture, broad molecular-weight distribution, branching by chain transfer.

3. Ionic polymerization

• Cationic: initiator $BF_3 + H_2O \to H^+[BF_3OH]^-$; $H^+$ adds to alkene giving carbocation; monomers: isobutylene, styrene (electron-donating groups stabilize $C^+$). Killed by water/bases — needs dry, low T.
• Anionic: initiator $n$-BuLi gives carbanion; monomers: styrene, acrylonitrile, butadiene (electron-withdrawing groups stabilize $C^-$). Living polymers possible (no natural termination) → block copolymers.

4. Condensation (step-growth) polymerization

Monomers with two functional groups join with loss of small molecule (water, HCl). Example nylon-6,6:

• Any two oligomers can join at any time (vs chain-growth where only monomer adds to active end).
• Needs exact 1:1 stoichiometry for high molecular weight; $M_n$ rises slowly then steeply at >98% conversion.

5. Degree of polymerization (DP)

where $M_n$ = number-average molecular weight, $M_0$ = monomer weight.

Numerical: polystyrene $M_n = 104{,}000$, styrene $M_0 = 104$ → $DP = 1000$. Higher DP → higher strength, $T_g$, viscosity, but harder to process.

Exam Points

• Homolysis vs heterolysis + one example each of substitution/addition/elimination (2–3 marks).
• Three steps of free-radical mechanism with equations (5 marks favourite); contrast cationic vs anionic monomer choice.
• Condensation example (nylon/terylene) + DP definition and one numerical (3 marks).