Explain all types of chemical bonds existing in polymers.

Polymers contain primary (covalent) bonds along the chain and secondary (intermolecular) forces between chains. Together they decide the strength, melting point, solubility and elasticity.

A. Primary bonds

1. Covalent bonds: atoms in the backbone (C–C, C–H, C–O, C–N, Si–O) are joined by strong covalent bonds (about 300–400 kJ/mol). They form the long chain, and they also form cross-links between chains in thermosets and vulcanised rubber. Cross-links make the polymer rigid, insoluble and infusible. Example: –CH₂–CH₂– in polyethylene; S–S bridges in vulcanised rubber.
2. Ionic bonds: found in ionomers, e.g. ethylene–methacrylic acid copolymers neutralised with Na⁺/Zn²⁺ (Surlyn), where ionic clusters act as physical cross-links.
3. Coordinate bonds: found in coordination and metal-containing polymers.

B. Secondary bonds (intermolecular forces)

1. Van der Waals / London dispersion forces: weak forces (about 1–10 kJ/mol) between non-polar chains. Example: polyethylene and polypropylene, which are soft and have low melting points.
2. Dipole–dipole interactions: between polar groups such as C–Cl and C≡N on adjacent chains. Example: PVC and polyacrylonitrile, which are stronger and more rigid.
3. Hydrogen bonds: the strongest secondary force (about 10–40 kJ/mol), between –NH and C=O or –OH groups. Example: nylon-6,6, cellulose and Kevlar, which give high strength, high melting point and fibre-forming ability.

  Nylon chains held by H-bonds:
   ...-C(=O)-NH-...         chain 1
         O ..... H           (hydrogen bond)
   ...-NH-C(=O)-...         chain 2

Stronger intermolecular forces give higher tensile strength, crystallinity, T_g and T_m, and lower solubility.