(a) (i) Explain the various steps involved in benzoyl peroxide-initiated polymerization of ethylene to give polyethylene. (10…

(a) (i) Benzoyl peroxide-initiated polymerization of ethylene

The polymerization of ethylene to polyethylene using benzoyl peroxide is a free-radical chain reaction that proceeds through three distinct stages: initiation, propagation, and termination.

Initiation: The process begins with the thermal homolysis of the weak peroxide bond in benzoyl peroxide. This generates two benzoyloxy radicals, which can further decarboxylate to form phenyl radicals. These primary radicals attack the double bond of the ethylene monomer to form a new, larger carbon-centered radical.

Propagation: The newly formed radical is highly reactive and adds to another ethylene monomer. This step repeats rapidly, with the radical center moving along the growing chain. The structure of the growing radical is a long alkyl chain with an unpaired electron at the terminal carbon.

Termination: The chain growth stops when two growing radicals react with each other. This occurs via either coupling (where two chains join to form a single, longer molecule) or disproportionation (where one radical abstracts a hydrogen atom from another, creating one saturated chain and one unsaturated chain).

Chain Transfer: In industrial settings, chain transfer agents or back-biting mechanisms can occur, leading to the formation of short branches. This is the primary reason for the structural difference between Low-Density Polyethylene (LDPE) and linear forms.

(a) (ii) Physical properties of polystyrenes

The physical properties of polystyrene are dictated by its tacticity, which determines its ability to pack into a crystal lattice.

• Atactic Polystyrene: Due to the random orientation of phenyl groups, the chains cannot pack efficiently. It is amorphous, transparent, and glassy (rigid and brittle) at room temperature. It has a glass transition temperature (T_g) of approximately 100 °C but no distinct melting point.
• Isotactic Polystyrene: All phenyl groups are on the same side of the chain. This regularity allows for high crystallinity, resulting in a rigid, high-melting-point material with high density.
• Syndiotactic Polystyrene: Phenyl groups alternate sides. Like the isotactic form, it is crystalline and rigid with a high melting point, though it often exhibits different thermal stability and density compared to the isotactic version.

Typical values: isotactic PS Tm ≈ 240 °C (crystallises slowly, 3₁ helix); syndiotactic PS Tm ≈ 270 °C (crystallises fast, planar zig-zag; an engineering plastic made with metallocene catalysts); atactic PS (ordinary radical polystyrene) Tg ≈ 100 °C, amorphous, transparent and soluble in common solvents.

(b) Mechanism for the formation of B and C from A

The transformation of compound A into B and C upon photoirradiation is a classic example of the Norrish Type I reaction. This process involves the homolytic cleavage of a carbon-carbon bond adjacent to a carbonyl group.

Step 1: Excitation and Cleavage Upon absorption of light, the cyclopentanone in A undergoes an n → π^* excitation. This leads to the homolytic cleavage of the C–C bond at the quaternary ring-junction carbon (the more substituted α-carbon). This produces a 1,5-biradical intermediate consisting of an acyl radical and a tertiary alkyl radical.

Step 2: Formation of B (Spiro Lactone) In one pathway, the tertiary alkyl radical abstracts a hydrogen atom from the methylene group adjacent to the acyl radical. This disproportionation yields a ketene intermediate. The tertiary hydroxyl group then performs an intramolecular nucleophilic attack on the electrophilic carbon of the ketene. This cyclization results in the formation of the spiro lactone B.

Step 3: Formation of C (Cyclic Hemiacetal) In the competing pathway, the acyl radical abstracts a hydrogen atom from the methyl group adjacent to the tertiary radical centre. This yields an aldehyde and an exocyclic methylene group. The tertiary hydroxyl group then adds intramolecularly to the aldehyde carbonyl, forming a cyclic hemiacetal (lactol), which is compound C.

The two products represent the two possible directions of hydrogen transfer from the initial biradical, followed by the trapping of the resulting carbonyl derivatives by the hydroxyl group.

(c) Identification of A, B, C, and D

Identification of Compounds:

• A: Cyclohexanone.
• B: Cyclohexene.
• C: NBS (N-Bromosuccinimide).
• D: 3-bromocyclohexene oxide (2-bromo-7-oxabicyclo[4.1.0]heptane).

Reaction Sequence Analysis:

1. Reduction: Cyclohexanone (A) is reduced by sodium borohydride (NaBH₄) in methanol to yield cyclohexanol.
2. Dehydration: Cyclohexanol undergoes acid-catalyzed dehydration (H⁺) to form cyclohexene (B).
3. Allylic Bromination: Cyclohexene reacts with NBS (C) to produce 3-bromocyclohexene. NBS provides a low, steady concentration of Br₂, which favors allylic substitution over addition to the double bond.
4. Epoxidation: 3-bromocyclohexene reacts with mCPBA to form the epoxide D.

Mechanism of the First Step (Reduction of A): The reduction of cyclohexanone by NaBH₄ proceeds via a nucleophilic hydride transfer.

1. Hydride Attack: A hydride ion (H⁻) from the BH₄⁻ anion attacks the electrophilic carbonyl carbon of cyclohexanone. This occurs through a four-center transition state where the C=O π-bond breaks and the C–H bond forms.
2. Alkoxyborate Formation: The oxygen of the carbonyl group becomes negatively charged and coordinates with the boron atom, forming a tetrahedral alkoxyborate intermediate.
3. Protonation: The methanol solvent (MeOH) acts as a proton source. The oxygen of the alkoxyborate abstracts a proton from the methanol, yielding the final product, cyclohexanol, and regenerating the borate species.