Condensation Polymerization
Condensation polymerization, also known as step-growth polymerization, builds polymers by reacting bifunctional or multifunctional monomers with one another. Each reaction step forms a new bond between two monomer units and typically releases a small molecule — most often water, but sometimes methanol, HCl, or another byproduct. Unlike addition polymerization, where monomers add one at a time to a growing chain, condensation polymerization involves reactions between any two species in the mixture: monomer with monomer, monomer with oligomer, oligomer with oligomer.
The Step-Growth Mechanism
In step-growth polymerization, every molecule in the reaction mixture is reactive. Consider a mixture of a dicarboxylic acid (HOOC-R-COOH) and a diamine (H₂N-R'-NH₂). Any acid group can react with any amine group to form an amide bond (-CO-NH-) and release water.
At first, monomers react to form dimers. Then dimers react with other dimers or with remaining monomers to form tetramers. Tetramers react with trimers, hexamers with octamers, and so on. Molecular weight builds slowly and steadily. High molecular weight polymer appears only at very high conversions — typically above 99% of the functional groups must react to achieve DPs above 100. This is a fundamental difference from addition polymerization, where high-molecular-weight chains form early.
Wallace Carothers derived the relationship between conversion (p) and degree of polymerization: DP = 1 / (1 - p). At 95% conversion, DP = 20; at 99%, DP = 100; at 99.5%, DP = 200. This equation reveals why purity and stoichiometric balance are critical — even a slight excess of one monomer caps the chains and limits molecular weight.
Polyesters
Polyesters form when diacids react with diols (dialcohols), creating ester linkages (-COO-). The most commercially important polyester is poly(ethylene terephthalate) (PET), made from terephthalic acid and ethylene glycol:
- PET is the material in plastic beverage bottles, polyester clothing fibers, and food packaging film.
- Global PET production exceeds 30 million tons annually.
- PET is one of the most widely recycled plastics, identified by recycling code #1.
Other notable polyesters include polylactic acid (PLA), a biodegradable polyester derived from corn starch or sugarcane, and polycarbonate (PC), a tough transparent plastic used in eyeglass lenses and bulletproof glass.
Polyamides (Nylon)
Polyamides are condensation polymers linked by amide bonds (-CO-NH-). The most famous polyamide is nylon-6,6, synthesized by reacting hexamethylenediamine (a 6-carbon diamine) with adipic acid (a 6-carbon diacid). Each condensation step releases one molecule of water.
Nylon was first produced by Wallace Carothers at DuPont in the 1930s. Its initial application — women's stockings — demonstrated that synthetic fibers could match or exceed natural silk in strength and elasticity. Today nylon is used in textiles, carpets, automotive parts, and engineering plastics.
Nylon-6, produced by ring-opening polymerization of caprolactam rather than condensation of two monomers, is another widely used polyamide. Despite the different synthesis route, the resulting polymer contains the same amide linkage and exhibits similar properties.
Polyurethanes
Polyurethanes form by reacting diisocyanates (OCN-R-NCO) with diols (HO-R'-OH), producing urethane linkages (-NH-COO-). Unlike classic condensation, no small molecule is released; all atoms from both monomers are incorporated into the polymer. For this reason, polyurethane formation is sometimes classified as an addition step-growth reaction.
By varying the diisocyanate and diol components, chemists produce an extraordinary range of polyurethane materials:
- Flexible foams — mattresses, car seats, shoe soles
- Rigid foams — building insulation with R-values exceeding 6 per inch
- Coatings — durable floor finishes and automotive clear coats
- Elastomers — skateboard wheels, industrial rollers
- Adhesives — structural bonding in aerospace and construction
Global polyurethane production exceeds 25 million tons per year, making it one of the most versatile polymer families.
Other Important Condensation Polymers
- Phenol-formaldehyde resins (Bakelite) — the first fully synthetic plastic (1907), still used in electrical insulators and cookware handles.
- Melamine-formaldehyde resins — hard, heat-resistant materials for laminates (countertops) and dinnerware.
- Silicones (polysiloxanes) — technically polycondensation products of silanols, used in sealants, medical implants, and high-temperature lubricants.
Controlling Molecular Weight
Because high DP requires near-perfect conversion, condensation polymerization demands:
- High purity — impurities with only one functional group act as chain stoppers.
- Stoichiometric balance — equal numbers of each reactive group.
- Efficient removal of byproduct — driving the equilibrium toward polymer by removing water (vacuum, azeotropic distillation, or molecular sieves).
Failing to meet any of these conditions results in low-molecular-weight, brittle material unsuitable for most applications.
Condensation vs. Addition: Key Differences
Understanding the distinction between the two major polymerization mechanisms is essential for polymer chemists:
| Feature | Addition (Chain-Growth) | Condensation (Step-Growth) |
|---|---|---|
| Monomer type | Unsaturated (C=C) | Bifunctional (diols, diacids, diamines) |
| Byproduct | None | Small molecule (H₂O, HCl, etc.) |
| MW growth | Rapid; high MW from the start | Gradual; high MW only at high conversion |
| Kinetics | Chain reaction (fast propagation) | Step-wise (all molecules react) |
| Typical PDI | 1.5-2.0 | ~2.0 (Flory distribution) |
This distinction has practical consequences for industrial production. Addition polymerization can produce useful polymer at moderate conversions (50-70%), while condensation polymerization requires driving the reaction to extreme conversions (99%+) to obtain mechanically useful material. Reactor design, reaction time, and purification strategies differ accordingly.