Condensation polymerisation: monomers and repeating units
Organic chemistry • Synthetic and natural polymers
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Key concepts
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Definition and core principle
Condensation polymerisation is a step-growth process in which two functional groups react to form a new covalent bond and a small molecule is lost. The reaction occurs between monomers that each contain at least two reactive functional groups, allowing chain extension in both directions. Each bond-formation step produces a repeating unit and releases a small molecule such as water or HCl, so mass balance excludes the small molecule from the polymer repeating unit.
Functional groups and bond types
Carboxylic acid (–COOH) and alcohol (–OH) react to form an ester (–COO–) plus water, so diacid + diol monomers produce a polyester with ester linkages. Carboxylic acid (–COOH) and amine (–NH2) react to form an amide (–CONH–) plus water, so dicarboxylic acid + diamine monomers produce a polyamide with amide linkages. Amino acids contain both amine and carboxyl groups and polymerise to form polypeptides with amide (peptide) bonds and water elimination.
Repeating units and how they form
Each condensation step removes atoms that become the small molecule, so the repeating unit is the original monomer fragments joined by the new bond with the lost atoms omitted. For a polyester from HO–R–OH and HOOC–R'–COOH, the repeating unit is –[O–R–O–CO–R'–CO]–, shown in simplified form as –[R–O–CO–R']–. For a polyamide from H2N–R–NH2 and HOOC–R'–COOH, the repeating unit is –[R–NH–CO–R']–. The repeating unit indicates the functional groups that previously reacted.
Small molecule elimination and equilibrium
Polymer formation proceeds by many successive condensation reactions that are reversible in principle. Removal of the small molecule (for example by heating, reduced pressure, or use of a dehydrating agent) shifts the equilibrium toward polymer formation and higher molecular mass. Incomplete removal of the small molecule limits chain length and produces oligomers rather than high-molecular-mass polymer.
Examples: polyesters, polyamides and peptides
Polyethylene terephthalate (PET) forms from ethane-1,2-diol (a diol) and benzene-1,4-dicarboxylic acid (a dicarboxylic acid) with ester linkages and water elimination. Nylon-6,6 forms from hexane-1,6-diamine and hexanedioic acid with amide linkages and water elimination. Polypeptides form from amino acids by amide (peptide) bond formation and water elimination. Each example follows the same functional-group pairing principle and produces characteristic repeating units.
Practical factors affecting polymer formation
Temperature, catalysts, stoichiometry and removal of the small molecule control polymer molecular mass and reaction rate. Exact stoichiometric balance of the two monomers maximises chain length because any excess of one monomer terminates growth at that end. Catalysts or acid/heat accelerate bond formation but do not change the type of bond formed, which depends on the reacting functional groups.
Key notes
Important points to keep in mind