Chapter Guide
Step-Growth Polymerization and Step-Growth Polymers
Step-growth polymerization builds chains by reaction between functional groups. High molecular weight normally requires very high conversion, accurate stoichiometry, low monofunctional impurity, and removal of small-molecule byproducts when the chemistry produces them.
How Step-Growth Differs From Chain-Growth
| Feature | Step-Growth | Chain-Growth |
|---|---|---|
| Reactive species | Functional groups on monomers, oligomers, and chains. | Active centers such as radicals, ions, or coordination sites. |
| Molecular-weight development | High molecular weight forms late, near high conversion. | High molecular weight can form early. |
| Stoichiometry | Functional-group balance is critical. | Monomer-to-initiator ratio often matters more. |
| Common products | Polyesters, polyamides, polyurethanes, polyimides, polycarbonates. | Polyolefins, styrenics, acrylics, vinyls, dienes. |
High Molecular Weight Requirements
Step-growth polymers are sensitive to small errors because chain extension depends on functional groups continuing to find partners. Monofunctional impurities cap chains, stoichiometric imbalance limits growth, and water or oxygen can interfere depending on chemistry.
- Use high-purity monomers and known functionality.
- Control stoichiometric balance for A-A plus B-B systems.
- Drive equilibrium reactions by removing water, alcohol, acid, or other small molecules when relevant.
- Control viscosity and heat transfer as oligomers grow into high-MW materials.
- Use end-group analysis when molecular weight is closely tied to functional-group conversion.
Major Step-Growth Polymer Classes
| Class | Chemistry | Property Notes |
|---|---|---|
| Polyesters | Diols plus diacids or diesters, or related transesterification routes. | Crystallinity, hydrolysis resistance, Tg, Tm, and processing window vary widely. |
| Polyamides | Diamines plus diacids, acid chlorides, lactams, or salt routes. | Hydrogen bonding increases strength and moisture sensitivity. |
| Aromatic polyamides | Aromatic diacids and diamines. | High stiffness and thermal resistance, often difficult processing. |
| Polyimides | Dianhydrides plus diamines, often via polyamic acid intermediates. | High thermal performance and strong dependence on cure history. |
| Polyurethanes | Isocyanates plus polyols and chain extenders. | Foams, elastomers, coatings, adhesives, and segmented hard-soft architectures. |
| Polycarbonates | Carbonate linkages from bisphenols and carbonate-forming reagents. | Transparent engineering plastics with molecular-weight and additive sensitivity. |
| Polysulfones and aromatic ethers | Aromatic nucleophilic substitution or condensation-style routes. | High Tg, chemical resistance, and engineering performance. |
Polyurethane Special Case
Polyurethanes deserve special attention because the same chemistry can make flexible foams, rigid foams, elastomers, fibers, adhesives, sealants, and coatings. The final behavior depends on isocyanate type, polyol functionality, chain extender, catalyst, water content, hard-segment content, and phase separation.
For any polyurethane data sheet, ask whether the property belongs to the neat resin, formulated system, cured part, foam, elastomer, coating, or adhesive.
Qualification Workflow
- Identify functional groups, monomer pair, and linkage type.
- Record molecular weight, end groups, residual monomer, residual catalyst, and byproduct history.
- Check moisture sensitivity, hydrolysis risk, and conditioning conditions.
- Record whether the material is linear, branched, crosslinked, thermoplastic, thermoset, or segmented.
- For engineering polymers, compare Tg, Tm, heat deflection, chemical resistance, and processing window together.