Q.Which one of the following is a thermosetting plastic?

Thermosetting plastics are heavily cross-linked and set permanently on heating; Bakelite is a classic example.

Plastics are of two kinds:

• Thermoplastics: soften on heating and can be remoulded repeatedly (e.g. PVC, polyethene, PVA). …

Bakelite is a thermosetting plastic — option (C).

Thermosetting polymers become permanently hard on moulding because they form heavily cross-linked, three-dimensional networks that cannot be remelted or reshaped. Bakelite (phenol–formaldehyde resin) is the classic example, with methylene cross-links between phenol units.

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With no polar groups anywhere in the chain, only weak van der Waals attractions hold the polymer chains together, giving it elastomer-like (rubbery) rather than fibre-like properties.

Isoprene ($CH_2=C(CH_3)-CH=CH_2$) and propene ($CH_2=CH-CH_3$) are both purely hydrocarbon monomers, with no polar functional groups (no $-OH$, $-COOH$, $-NH_2$, etc.) anywhere on either monomer.

A copolymer built from these two monomers is therefore also a purely non-polar hydrocarbon chain. The only intermolecular forces possible between such chains are weak van der Waals (dispersion) forces; there is no possibility of hydrogen bonding or strong dipole-dipole interaction.

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