What Is a Polymer?
The world of synthetic materials is dominated by polymers — molecules composed of long chains of repeating structural units called monomers. The word comes from the Greek poly (many) and meros (part). A simple example: polyethylene consists of thousands of –CH₂–CH₂– units linked end-to-end, with a molecular weight potentially exceeding a million g/mol.
Natural polymers — cellulose, proteins, DNA, natural rubber — have existed for billions of years. Synthetic polymers emerged in the 20th century and now permeate every aspect of modern life: the plastic in packaging, the rubber in tires, the fibers in clothing, the resins in composites.
Chain Growth vs. Step Growth Polymerization
Polymers are synthesized by one of two fundamental mechanisms:
Chain-Growth (Addition) Polymerization
A reactive intermediate (radical, anion, or cation) is generated and adds monomer units one at a time:
Initiation: I → 2R• Propagation: R• + CH₂=CH₂ → R–CH₂–CH₂• Termination: two radicals combine
Chain-growth polymerization is fast (seconds to minutes) and yields high-molecular-weight polymers. Polyethylene, polypropylene, polystyrene, PVC, polyacrylates — the vast majority of commodity plastics — are made this way.
Ziegler–Natta catalysts (titanium + aluminum alkyls) and metallocene catalysts allow control of tacticity — the spatial arrangement of side groups along the chain: - Isotactic: all side groups on the same side → crystalline, strong (isotactic polypropylene used in fibers, automotive parts) - Syndiotactic: alternating sides → semi-crystalline - Atactic: random arrangement → amorphous, rubbery
Step-Growth (Condensation) Polymerization
Bifunctional monomers react with each other at their end groups, releasing a small molecule (water, methanol, HCl):
nHO–R–COOH → HO–(R–CO–O–)ₙH + (n–1)H₂O
Step-growth polymerization is slow (hours to days) and requires nearly complete conversion to achieve high molecular weights. Polyesters (PET, PLA), polyamides (nylon), polyurethanes, polycarbonate, epoxy resins are made by step-growth.
The Three Families of Polymers
Polymers are classified by their thermal behavior into three categories, each with distinct molecular architecture and applications.
Thermoplastics
Thermoplastics are linear or branched polymers held together only by secondary intermolecular forces (van der Waals, hydrogen bonds, dipole–dipole). On heating, these forces are overcome and the material flows; on cooling, it solidifies again. This is completely reversible.
This recyclability makes thermoplastics the dominant packaging and structural plastic:
| Polymer | Abbreviation | Key Properties | Applications |
|---|---|---|---|
| Polyethylene (high density) | HDPE | Stiff, chemical-resistant | Bottles, pipes, cutting boards |
| Polypropylene | PP | Fatigue-resistant, microwave-safe | Containers, automotive parts, fibers |
| Poly(ethylene terephthalate) | PET | Strong, transparent, gas-barrier | Beverage bottles, fibers (polyester) |
| Polystyrene | PS | Rigid, transparent, low cost | Packaging, disposable cutlery |
| Polyamide | PA (Nylon) | Tough, low friction, high Tₘ | Gears, bearings, textiles, zip ties |
| Polycarbonate | PC | Very tough, transparent | Safety glasses, CDs, electrical housings |
| PTFE | Teflon | Lowest friction, chemical inert, high Tₘ | Nonstick coatings, cable insulation |
A thermoplastic's properties depend critically on temperature relative to two key transitions: - Glass transition temperature (Tg): Below Tg, the polymer is glassy and brittle; above it, rubbery and tough. Polystyrene has Tg ≈ 100°C; rubber has Tg ≈ –70°C. - Melting temperature (Tₘ): Applies only to crystalline regions; above Tₘ the crystallites melt and the polymer flows.
Thermosets
Thermosets are crosslinked polymers — their chains are covalently connected into a single giant network molecule. Unlike thermoplastics, they do not melt on heating; they degrade or char. Crosslinking produces: - Higher stiffness and hardness - Excellent dimensional stability (no creep) - Good chemical and solvent resistance - Irreversibility (cannot be remolded)
Epoxy resins cure by reaction between an epoxide monomer and a diamine or anhydride hardener, forming a dense crosslinked network. Epoxies are the matrix resin in carbon-fiber composites for aerospace and sporting goods.
Phenol–formaldehyde (Bakelite) — the first fully synthetic polymer (1907, Leo Baekeland) — is a thermoset used in circuit boards, automotive brake linings, and countertops.
Vulcanized rubber is natural rubber (polyisoprene) crosslinked with sulfur bridges. Charles Goodyear's 1844 discovery transformed natural rubber — sticky and heat-sensitive — into a tough, elastic material suitable for tires.
Elastomers
Elastomers are lightly crosslinked amorphous polymers far above their glass transition temperature. They can be stretched to several times their original length and return to their original shape on releasing the stress. This behavior requires: - Long, flexible chains (low energy barriers to rotation) - Low Tg (far below room temperature — chains are mobile) - Sparse crosslinks (enough to prevent permanent flow, but not enough to prevent extension)
Natural rubber (polyisoprene, from Hevea brasiliensis) and synthetic rubbers (styrene-butadiene rubber SBR, nitrile rubber NBR, silicone rubber PDMS) dominate applications in tires, seals, gloves, and shock absorbers.
Silicone rubber (polydimethylsiloxane, PDMS) has an unusual Si–O–Si backbone rather than C–C backbone. This gives it outstanding temperature range (–60°C to +250°C), biocompatibility, and UV stability — making it the material of choice for medical implants, food-grade seals, and high-temperature gaskets.
High-Performance Polymers
Conventional polymers lose strength and stiffness above ~100–150°C. High-performance polymers maintain properties to 200–400°C:
- PEEK (poly-ether-ether-ketone): Tg = 143°C, Tₘ = 343°C. Used in aircraft interiors, medical implants, and harsh chemical environments. Competes with metals in some structural applications.
- Kevlar (poly-para-phenylene terephthalamide): Rigid aromatic polyamide with tensile strength ~3.6 GPa — 9 times stronger than steel per unit mass. Used in body armor, cut-resistant gloves, and as reinforcement in tires and composites.
- Liquid crystal polymers (LCPs): Rigid-rod chains that align spontaneously, creating highly anisotropic properties. Used in miniaturized electronic connectors.
Conducting Polymers
Most polymers are excellent insulators. Conducting polymers are a remarkable exception. Polyacetylene, polythiophene, and polyaniline have alternating single–double bonds along the backbone (conjugated π system). Doping with electron acceptors or donors generates mobile charge carriers, making them conductive.
Alan Heeger, Alan MacDiarmid, and Hideki Shirakawa received the 2000 Nobel Prize in Chemistry for this discovery. Conducting polymers are used in organic photovoltaics, OLEDs, flexible electronics, and anti-static coatings.
Polymer Degradation and Sustainability
Conventional synthetic polymers are extremely stable — the same chemical inertness that makes them useful also makes them persist for centuries in the environment. Less than 10% of all plastic ever produced has been recycled.
Biodegradable polymers such as PLA (polylactic acid, from corn starch fermentation) and PHA (polyhydroxyalkanoates, from bacterial fermentation) can be composted under appropriate conditions. But widespread adoption requires both material development and infrastructure — composting facilities that actually process these materials.
Chemical recycling — depolymerizing plastics back to monomers — offers a route to infinite recyclability for some polymers. PET can be glycolyzed back to its monomers and repolymerized to virgin quality. Research is active in catalytic depolymerization of polyolefins, historically the most challenging class to recycle.