Q.What are condensation polymers? Write the chemical equations for the preparation of bakelite and polyester by condensation.

Terylene, the polyester fibre also known as Dacron or PET, is manufactured by condensing many molecules of ethylene glycol with terephthalic acid, catalysed and heated so that the two monomers link up repeatedly through ester bonds, releasing water at each step.

Terylene (polyethylene terephthalate, PET) is a condensation (step-growth) polymer prepared by heating a mixture of ethylene glycol and terephthalic acid (or, industrially, dimethyl terephthalate, which releases methanol instead of water) in the presence of a catalyst such as zinc acetate and antimony trioxide, at a high temperature (typically $420$–$460\ K$): $n\,HOCH_2CH_2OH + n\,HOOC-C_6H_4-COOH \xrightarrow[\text{Zn acetate/}Sb_2O_3]{\Delta} \left[-OCH_2CH_2-OOC-C_6H_4-CO-\right]_n + 2n\,H_2O$ The two monomers repeatedly undergo ester-forming condensation, eliminating a small molecule of water (or methanol, in the dimethyl ester route) at each linking step, building up the long-chain polyester. Terylene is widely us …

Condensation polymers are built up by successive condensation reactions between two different multifunctional monomers, each step releasing a small molecule such as water — nylon-6,6 (from a diamine and a diacid) and terylene (from a diol and a diacid) are the classic textbook examples.

A condensation polymer is formed by repeated condensation reactions between two different bifunctional (or polyfunctional) monomer molecules, in which each successive step involves the elimination of a small molecule such as water, an alcohol, ammonia, or hydrogen chloride. Because a small molecule is lost at each linking step, the molecular formula of the repeating unit in the polymer is not simply the sum of the monomer formulae (unlike addition polymers).

Example 1 — Nylon-6,6: formed by the condensation polymerisation of hexamethylenediamine and adipic acid, with loss of water at each amide-bond-forming step: $n\,H_2N(CH_2)_6NH_2 + n\,HOOC(CH_2)_4COOH \rightarrow [-NH(CH_2)_6NHCO(CH_2)_4CO-]_n + 2n\,H_2O$

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Condensation polymers grow by monomers joining with loss of a small molecule (e.g. water); Nylon-6,6 is the classic example, made from a diamine and a diacid.

Condensation polymers

These are polymers formed when two different bi-functional (or polyfunctional) monomers react repeatedly with each other, with the elimination of a small molecule such as water, ammonia, or an alcohol at each linking step. The repeat unit is joined by functional linkages (ester, amide, etc.) rather than by simple addition of double bonds. Examples include polyesters (e.g. Terylene/Dacron) and polyamides (e.g. Nylon-6,6, Nylon-6).

Preparation of Nylon-6,6

Nylon-6,6 is a polyamide prepared by the condensation polymerization of two 6-carbon bifunctional monomers:

• Hexamethylenediamine: $H_2N-(CH_2)_6-NH_2$
• Adipic acid: $HOOC-(CH_2)_4-COOH$

When heated together under high pressure and temperature, the $-NH_2$ groups of the diamine react with the $-COOH$ groups of the diacid, forming amide linkages ($-CO-NH-$) at each junction, with elimination of a water molecule per linkage:

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Condensation polymers form when bifunctional monomers join repeatedly, each step expelling a small molecule ($H_2O$/$HCl$); bakelite = phenol + formaldehyde, polyester = glycol + terephthalic acid.

Condensation polymers: These are polymers formed by the repeated condensation reaction between two bifunctional or polyfunctional monomers, with the elimination of small molecules such as water, alcohol or $HCl$ at each linkage. Common examples: nylon-6,6, terylene (polyester), and bakelite.

Bakelite (phenol–formaldehyde resin): phenol condenses with formaldehyde (HCHO) first to o-/p-hydroxymethylphenol, which then undergoes further condensation on heating to give a hard, cross-linked network resin:

$n\,C_6H_5OH + n\,HCHO \xrightarrow{\Delta,\ H^+/OH^-} [\,\text{–}C_6H_3(OH)\text{–}CH_2\text{–}\,]_n + n\,H_2O$

Polyester (Terylene / Dacron): ethylene glycol condenses with terephthalic acid, eliminating water at each ester linkage: …