Condensation Polymerization (Step Growth Polymerization): Monomers combine with elimination of small molecules (e.g., water, HCl, methanol, ammonia).

Chain growth occurs by a series of condensation steps.
E.g., Terylene (Polyester) from ethylene glycol and terephthalic acid.
$n HO-CH_2-CH_2-OH + n HOOC-\text{C}_6\text{H}_4-COOH \xrightarrow{-nH_2O} \text{Polymer}$
Repeating units of condensation polymer do not have the same elemental composition as that of monomer.

Ring Opening Polymerization: Cyclic compounds (lactams, cyclic ethers, lactones) polymerize by ring opening. Catalyzed by strong acid/base.

E.g., Polymerization of $\epsilon$-caprolactam to Nylon 6.
Elemental composition of repeating unit is same as monomers (like addition polymerization).
Proceeds by addition of single monomer unit to growing chain (step growth).
Example: Nylon 6 from $\epsilon$-caprolactam.

$n \text{ } \epsilon\text{-caprolactam} \xrightarrow{H_2O, 533-543K} -[NH-(CH_2)_5-CO]-_n$ (Nylon 6)

2.4. Based on Intermolecular Forces

Mechanical properties (tensile strength, toughness, elasticity) vary with intermolecular forces.

Elastomers: Elastic polymers with weak van der Waals forces. Can be stretched and return to original shape. Crosslinks help retraction.
- E.g., Vulcanized rubber, Buna-S, Buna-N, Neoprene.
Fibres: Polymeric solids forming threads, high tensile strength. Strong intermolecular forces (H-bonding, dipole-dipole). Crystalline.
- E.g., Polyamides (Nylon 6,6), polyesters (Terylene).
Thermoplastic Polymers: Easily shaped/moulded. Soften on heating, harden on cooling (reversible). Moderately strong intermolecular forces (intermediate between elastomers and fibres).
- E.g., Polythene, Polystyrene, Polyvinyls.
Thermosetting Polymers: Rigid polymers. Shaped on heating, then harden irreversibly. Cannot be softened or remoulded. Extensive cross-linking by covalent bonds during hardening.
- E.g., Bakelite, Urea formaldehyde resin.

2.5. Based on Type of Monomers

Homopolymers: Only one type of repeating unit. Formed from a single monomer.
- E.g., Polythene, Polypropene, Nylon 6, Polyacrylonitrile, Nylon 6,6.
Copolymers: Two or more types of repeating units. Formed by polymerization of different monomers. Monomer units are randomly sequenced.
- E.g., Buna-S (butadiene + styrene), Buna-N.

2.6. Based on Biodegradability

Non-biodegradable Polymers: Not affected by microbes. (Most synthetic polymers). Pose pollution hazards.
Biode