31.4 Step-Growth Polymers

Step-Growth Polymers

Step-growth polymerization builds polymers through reactions between monomers that each carry two (or more) functional groups. Unlike chain-growth polymerization, where monomers add one at a time to a growing chain, step-growth proceeds by coupling any two reactive species in the mixture. Dimers form first, then dimers react with monomers or other dimers to give trimers and tetramers, and so on until long chains develop. Most of these coupling steps release a small molecule (water, HCl, etc.), which is why step-growth is closely associated with condensation reactions.

Formation of Step-Growth Polymers

For a step-growth polymer to reach high molecular weight, two conditions matter most:

1. Bifunctional monomers — each monomer must have a reactive group on both ends so the chain can keep growing in both directions.
2. Stoichiometric balance — the two monomers must be present in nearly equal molar amounts. Even a small excess of one monomer caps chain ends and limits molecular weight.

Because any two reactive molecules can combine at any time, molecular weight builds up slowly at first and rises steeply only at very high conversion (>95%). This is a key difference from chain-growth polymerization, where high-molecular-weight chains appear early.

Nylon 6,6 forms by condensation of hexamethylenediamine (a six-carbon diamine) with adipic acid (a six-carbon dicarboxylic acid). Each coupling step releases one molecule of water. The "6,6" name reflects the six carbons in each monomer. A related material, nylon 6, is made by ring-opening polymerization of caprolactam rather than by condensation of two different monomers.

Polyethylene terephthalate (PET) forms by condensation of ethylene glycol (a diol) with terephthalic acid (a dicarboxylic acid), again releasing water. PET is the polymer in most plastic beverage bottles, polyester clothing fibers, and food packaging films.

The degree of polymerization ($n$) is the number of repeat units in a chain. Higher $n$ generally means greater tensile strength and a higher melting point, so controlling conversion and stoichiometry is critical during manufacturing.

Polycarbonates vs. Polyurethanes

These two families illustrate how different linkage chemistry produces very different material properties.

Polycarbonates

• Formed by reacting bisphenol A with phosgene ($\text{COCl}_2$) or diphenyl carbonate. The repeat unit contains bisphenol A segments connected by carbonate (–O–CO–O–) linkages.
• The rigid bisphenol A backbone and the carbonate group together give polycarbonates their hallmark combination: optical transparency, high impact resistance, and good heat resistance.
• Common uses: eyewear lenses, electronic housings, automotive headlight covers, and safety shields.

Polyurethanes

• Formed by reacting polyols (molecules with multiple –OH groups) with diisocyanates (molecules with two –N=C=O groups). The coupling creates urethane (–NH–CO–O–) linkages, and no small molecule is lost, so this is technically an addition step-growth process.
• By varying the polyol chain length, the diisocyanate structure, and additives, manufacturers can tune polyurethanes from soft foams to rigid plastics to tough elastomers.
• Common uses: insulation panels, mattress foam, shoe soles, coatings, and adhesives.

Production and Uses of Polyurethanes

The versatility of polyurethanes comes from adjusting just a few variables: polyol chain length, crosslink density, and whether a blowing agent is used.

• Flexible foams — Long-chain polyols reacted with diisocyanates in the presence of a blowing agent (often water or a volatile liquid) produce open-cell foams that are soft and resilient. These go into mattresses, upholstered furniture, and automotive seating.
• Rigid foams — Short-chain polyols with higher functionality (more –OH groups per molecule) and a blowing agent yield closed-cell foams with excellent thermal insulation. Used in building wall panels, refrigerator linings, and protective packaging.
• Elastomers — Long-chain polyols and diisocyanates reacted without a blowing agent produce solid, elastic materials with strong abrasion resistance. Applications include shoe soles, skateboard wheels, conveyor belts, and industrial rollers.
• Coatings and adhesives — Tailored polyol/diisocyanate formulations with additives (catalysts, UV stabilizers, pigments) provide durable protective films and strong bonding layers. Examples include wood floor finishes, automotive clear coats, and laminating adhesives.

Polymer Characteristics and Properties

Molecular weight distribution — Real polymer samples contain chains of many different lengths. The breadth of this distribution affects processability and mechanical performance. Narrower distributions tend to give more uniform properties.

Crosslinking — When monomers with three or more functional groups are included, covalent bonds form between chains, creating a network. Crosslinked polymers are stronger, more heat-resistant, and less soluble than their linear counterparts. Increasing crosslink density pushes the material from flexible toward rigid.

Thermoplastics vs. thermosets — This is one of the most important distinctions in polymer science:

• Thermoplastic step-growth polymers (e.g., nylon, PET, polycarbonate) can be melted and reshaped repeatedly because their chains are held together only by intermolecular forces.
• Thermoset step-growth polymers (e.g., heavily crosslinked polyurethanes, epoxy resins) form permanent covalent networks during curing. Once set, they cannot be remelted, which gives them superior dimensional stability at high temperatures but makes recycling more difficult.