Polymer Simulator: Build Polymers from Monomers
Addition & condensation polymerization, chain length, structure and properties — from polyethylene to nylon, Kevlar and DNA
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Polymer Simulator: addition and condensation polymerization you can watch
This free polymer simulator shows how real polymers form from their monomers, atom by atom. Pick a monomer and press Play mechanism: the monomers are drawn as displayed formulas and the reaction plays step by step — the C=C double bond opening as a free radical attacks it in addition polymerization, or two functional groups meeting and a water or hydrogen chloride molecule leaving in condensation. The repeating unit is drawn in brackets as it forms, and the chain-length averages (Xn, Mn, Mw and the dispersity) are measured from the simulated flask and compared with the Carothers and Flory equations. A structure bench then heats and stretches the polymer to show why a thermoplastic melts, a thermoset chars and an elastomer springs back.
How to use the simulator
- Watch the mechanism. Choose one of 21 reactions: eight addition polymers (including your own monomer, built by choosing the four groups on C=C), six condensation polymers, two thermoset resins and four natural polymers. The canvas draws the monomers as displayed formulas and plays the reaction step by step: curly arrows show the electrons moving in a free-radical addition, the atoms that are about to react glow, the new bond appears in green, and in a condensation the water (or HCl) molecule visibly leaves. The last step brackets the repeating unit. Use Back and Next (or the arrow keys) to go at your own pace; every atom you see at the start is still on the screen at the end.
- Many chains. Switch to Many chains to see thousands of molecules react at once. In step growth (pink and blue beads for the two monomers) dimers and short chains form first; in chain growth (grey monomer, one colour per chain) long chains appear at once. Set the number of molecules, the ratio of the two monomers, or the kinetic chain length and termination mode; the rail draws the histogram of chain lengths, with Flory's distribution in gold.
- Structure & properties. Put a polymer on the hot plate. Heat it past its glass transition and melting point, or stretch it to three times its length. Compare branched LDPE, linear HDPE and cross-linked PEX, or raw and vulcanized rubber. The 3D view turns the chains so you can see the cross-links.
- Full screen. The Fullscreen button (or F11) fills the window with the canvas, its controls and the rail; Esc leaves.
- Explore, Practice, Quiz. Explore has a card for each of 21 polymers and 11 concept cards; Practice has 12 problem types with worked answers; the Quiz draws ten questions from a pool of 22.
Addition vs condensation polymerization
Addition polymers are made from monomers with a carbon–carbon double bond. The double bond opens, the monomers join end to end and nothing else is produced, so the repeating unit contains exactly the atoms of the monomer: ethylene CH2=CH2 gives –[CH2–CH2]n–. Condensation polymers are made from monomers with two reactive groups at their ends. Each link releases a small molecule: an alcohol and a carboxylic acid make an ester link and water (PET, PLA), an amine and a carboxylic acid make an amide link and water (nylon-6,6, proteins), and an acid chloride gives hydrogen chloride instead (the nylon rope trick, Kevlar).
Chain growth, step growth and the Carothers equation
In chain growth a free radical or a catalyst site adds one monomer after another, so each chain reaches its full length in moments while most of the monomer waits; ideal termination by disproportionation gives a dispersity of 2, by combination 1.5. Ziegler–Natta and metallocene catalysts make linear HDPE and stereoregular (isotactic or syndiotactic) polypropylene, which free radicals cannot. In step growth any two molecules with matching groups can react, so long chains appear only at the very end. Carothers’ equation, Xn = 1/(1 − p), gives the number-average chain length from the extent of reaction p, and Flory’s distribution gives Đ = 1 + p. A small excess of one monomer (r = NA/NB < 1) caps the chain at (1 + r)/(1 − r). The simulator bonds groups on different molecules only, as both theories assume, so its measured Xn equals Carothers’ exactly and its Xw scatters around Flory’s.
Common polymers: monomers, Tg, Tm, density and recycling codes
| Polymer | Monomer(s) | Type | Tg (°C) | Tm (°C) | Density (g/cm³) | Code |
|---|---|---|---|---|---|---|
| Low-density polyethylene (LDPE) | ethylene | Addition (free radical, branched) | −130 to −100 | 100 to 115 | 0.91–0.93 | 4 |
| High-density polyethylene (HDPE) | ethylene | Addition (Ziegler–Natta, linear) | −130 to −100 | 125 to 135 | 0.94–0.96 | 2 |
| Cross-linked polyethylene (PEX) | ethylene, then cross-linked | Cross-linked polyethylene | −130 to −100 | 125 to 135 | 0.94–0.96 | — |
| Polypropylene (PP) | propylene | Addition (Ziegler–Natta) | −20 to 20 | 160 to 165 | 0.90–0.91 | 5 |
| Polyvinyl chloride (PVC) | vinyl chloride | Addition (free radical) | 80 to 90 | none | 1.38–1.55 | 3 |
| Polystyrene (PS) | styrene | Addition (free radical) | 80 to 105 | none | 1.05 | 6 |
| PTFE (Teflon) | tetrafluoroethylene | Addition (free radical) | 120 to 130 | 325 to 335 | 2.13–2.23 | 7 |
| PMMA (acrylic, Perspex) | methyl methacrylate | Addition (free radical) | 105 | none | 1.15–1.19 | 7 |
| PET (polyester) | ethylene glycol + terephthalic acid | Condensation (polyester, loses water) | 70 to 85 | 245 to 260 | 1.33–1.45 | 1 |
| Nylon-6,6 | hexamethylenediamine + adipic acid | Condensation (polyamide, loses water) | 65 to 90 | 225 to 265 | 1.13–1.16 | 7 |
| PLA (polylactic acid) | lactic acid | Condensation (polyester, loses water) | 45 to 65 | 150 to 160 | 1.21–1.43 | 7 |
| Kevlar (aramid) | p-phenylenediamine + terephthaloyl chloride | Condensation (polyamide, loses HCl) | — | none | 1.44 | — |
| Natural rubber (raw) | isoprene | Natural elastomer | −72 to −55 | none | 0.91–0.93 | — |
| Vulcanized rubber | natural rubber + sulfur | Elastomer, cross-linked | −72 to −55 | none | 0.91–0.93 | — |
| Bakelite (phenol–formaldehyde) | phenol + formaldehyde | Thermoset, condensation cure (loses water) | 70 to 120 | none | 1.40–1.80 | — |
| Epoxy resin (cured) | epoxy resin + amine hardener | Thermoset, ring-opening cure (nothing lost) | 50 to 200 | none | 1.15 | — |
Temperatures and densities are the ranges in the NETZSCH polymer property tables (polymers.netzsch.com), Kevlar from the Avient para-aramid datasheet; a range means real grades differ. PEX shares the crystalline phase of HDPE, so its crystallites melt in the same range, but its cross-links stop it flowing. Codes are the resin identification codes of ASTM D7611.
Linear, branched and cross-linked polymers
Linear chains such as HDPE and nylon pack side by side into crystalline regions, which makes the material denser, stiffer and higher-melting. Side branches, as in LDPE, keep the chains apart, so LDPE is less dense and softer. Cross-links are covalent bonds between chains: sulfur bridges in vulcanized rubber, a three-dimensional network in Bakelite and epoxy, or the cross-linked polyethylene of PEX pipe. A cross-linked polymer cannot flow, however hot it gets.
Thermoplastics, thermosets and elastomers
Below its glass transition temperature Tg the amorphous part of a polymer is a rigid glass; above it the chains can move. Only crystalline regions have a melting point Tm. A thermoplastic softens above Tg and melts above Tm (or flows well above Tg if it is amorphous, like polystyrene), so it can be remoulded and recycled. A thermoset never melts: heated far enough it decomposes and chars. An elastomer is a lightly cross-linked network above its Tg; stretched, its chains uncoil, and on release they pull it back. The pull is entropic, which is why a stretched rubber band pulls harder when it is warmed (σ = G(λ − 1/λ²) with G = ρRT/Mc).
Natural polymers
Starch and cellulose are both made of glucose joined by condensation, losing water at each link: α-glucose gives starch, which coils into helices, and β-glucose gives cellulose, whose alternate flipped units make straight chains that hydrogen-bond into fibres. Proteins are polyamides of amino acids joined by peptide links, and DNA is a chain of nucleotides joined by phosphodiester links. Cells build proteins and DNA on templates (messenger RNA and a DNA strand), so each protein molecule has one exact length; starch and cellulose are made by enzymes with no template, and their chains come in many lengths.
What this polymer simulator models
The simulator is computed, not drawn by hand. Every repeating unit is derived from its monomers minus the small molecules lost, and the drawn formula is checked to contain exactly those atoms; every step of every mechanism is checked to keep every atom and to give every atom its correct number of bonds; molar masses use the IUPAC 2021 atomic weights. Step growth is a Monte Carlo flask of up to 4,000 molecules in which every bond joins two different molecules; chain growth is an ideal free-radical or coordination model with a set kinetic chain length. Stated limits: no reaction rates in real time, no ring formation, no gel-point calculation for the thermosets (their network is a picture), and amorphous thermoplastics are taken to flow from about 100 °C above Tg.
Frequently Asked Questions
What are the 4 types of polymers?
Polymers are usually sorted four ways by how they behave: thermoplastics, which soften and melt when heated and can be remoulded (polyethylene, PVC, PET); thermosets, which are cross-linked networks that never melt and char instead (Bakelite, epoxy); elastomers, which are lightly cross-linked chains that stretch and spring back (vulcanized rubber); and fibres, whose chains are drawn out and packed side by side for strength (nylon, Kevlar). They are also sorted by origin, natural (starch, cellulose, proteins, DNA) or synthetic, and by how they are made, addition or condensation.
What is the difference between addition and condensation polymerization?
In addition polymerization monomers with a C=C double bond join end to end and nothing else is made, so the repeating unit has exactly the same atoms as the monomer, as in polyethylene and PVC. In condensation polymerization monomers with two reactive groups such as –OH, –COOH or –NH2 join, and every new link releases a small molecule, usually water (or hydrogen chloride from an acid chloride), as in polyesters like PET and polyamides like nylon and Kevlar.
What is the difference between a thermoplastic and a thermoset?
A thermoplastic is made of separate chains held together only by intermolecular forces, so heating lets the chains slide past each other: it softens, melts and can be reshaped and recycled. A thermoset is a single cross-linked network joined by covalent bonds; heat cannot break those cross-links, so it never melts and stays rigid until it decomposes and chars.
What is a repeating unit in a polymer?
The repeating unit is the smallest part of the chain that repeats along its whole length, written in brackets with n outside, for example –[CH2–CH2]n– for polyethylene. For an addition polymer it has the same atoms as the monomer with the C=C turned into C–C. For a condensation polymer it has fewer atoms than the monomers that made it, because the small molecule was lost.
Why do long chains only appear at the end of a step-growth polymerization?
In step growth any two molecules with matching groups can react, so monomers first make dimers, then short oligomers. The Carothers equation, Xn = 1/(1 − p), shows that the average chain length depends on p, the fraction of groups that have reacted: half the groups reacted gives chains only 2 units long, and 99 % reacted gives 100. Any excess of one monomer caps the length at (1 + r)/(1 − r).
Is DNA a polymer?
Yes. DNA is a natural condensation polymer of nucleotides joined by phosphodiester links, with one water molecule lost at each link. Proteins are natural polyamides of amino acids, and starch and cellulose are natural polymers of glucose. Cells build proteins and DNA on templates (messenger RNA and a DNA strand), so every copy of a protein has one exact length; starch and cellulose are made by enzymes with no template, so their chains come in many lengths.
What do the recycling codes 1 to 7 mean?
The resin identification codes of ASTM D7611 tell recyclers what a plastic is: 1 is PET, 2 is HDPE, 3 is PVC, 4 is LDPE, 5 is polypropylene, 6 is polystyrene and 7 is every other plastic, including PLA, nylon and acrylic. Thermoplastics can be melted and remoulded; thermosets cannot.
Explore Related Simulators
See how the atoms in a monomer share electrons in the Chemical Bonds Simulator, build the atoms themselves in Build Your Atom, and pack chains and atoms into crystals in Build Your Crystal. Balance the condensation equations in Balancing Chemical Equations, then pull a polymer specimen in the UTM Virtual Lab and compare its curve in the Stress-Strain Curve simulator.
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