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Plastics and polymersthe chains that changed the world

A bottle, a fleece, a tire — and also wood, silk and DNA — are all made of polymers: giant molecules built from thousands of repeating pieces. Link beads into a chain, stretch a coil, cross-link rubber, recognize the codes on packaging, sort plastic flakes in water, and follow what really happens to a bottle after you throw it away.

Monomer → chain → structure → properties. The same chemistry can give a soft bag or a hard pipe — it all depends on the shape of the chains.

Addition polymerization: n C₂H₄ → (–CH₂–CH₂–)ₙ

0 mers in chains
ethylene C₂H₄ (hydrogens omitted)polyethylene chainactive end (radical R•)

Reaction mixture

160

2

Chains
0
Longest
0 mers
M of longest
0 g/mol
Average mers
0
Monomer used
0%
Active ends
0

Simplified model: a flat box, each bead is a carbon atom, hydrogens omitted. Real ethylene polymerization needs high pressure or a catalyst, and a single chain takes a fraction of a second.

A coil that winds itself up

grab an end and pull

Chain length and mass

60

M = n · 28.05 g/mol = 1683 g/mol end groups omitted
Ends: R
0.0
Average √⟨R²⟩
—
Theory √n
7.7
Contour length
60
Extension
0%
Force (model)
0.00

Distances in lengths of one link. Model: an ideal chain of freely jointed links, jostled by random “kicks” from the surrounding molecules. No spring pulls it together — the coil returns because there are vastly more coiled shapes than straight ones (entropy).

How long is the chain in a plastic bag?

In polyethylene films a chain usually has from several thousand to tens of thousands of mers (molar mass on the order of 105–106 g/mol), and in UHMW polyethylene for ropes and joint implants — over a hundred thousand. Stretched out, such a chain would be a few micrometers long, yet coiled up it fits in a few tens of nanometers.

Different chains in one sample have different lengths — that's why chemists give an average molar mass.

Addition vs. condensation

Addition polymerization (chain-growth): the C=C double bond “opens up” and a monomer joins the active end. The chain grows very fast and nothing is released. This is how PE, PP, PVC, PS and Teflon are made.

Polycondensation: two different monomers with reactive groups at both ends join anywhere, and each link releases a small molecule — usually water. This is how nylon, PET and proteins are made.

Molecules and bonds →Reaction rates →

The nylon rope trick

A classic lab demonstration: a solution of an acid chloride in an organic solvent is carefully poured onto a solution of a diamine in water. A film of nylon forms instantly at the boundary between the two layers — with tweezers you can pull a long “rope” out of it. Here the by-product is hydrogen chloride, not water.

Only as a demonstration with a teacher: acid chlorides and solvents are corrosive and irritating. Don't try this at home.

How does it work?

Polymer, monomer, mer

A polymer is a very large molecule (a macromolecule) built from many repeating units — mers. The compound a polymer is made from is the monomer. The number of mers in a chain is the degree of polymerization n. The molar mass of a chain is simply n times the mass of a mer:

M = n · Mmer    for polyethylene: Mmer = 2 · 12.011 + 4 · 1.008 ≈ 28.05 g/mol

Two ways to build a chain

In addition polymerization a radical (a particle with an unpaired electron, here R•) attacks the C=C double bond, opens it and passes its activity to the new end of the chain. Less initiator means fewer growing chains, and so longer chains. In polycondensation the functional groups of two different monomers join, and each bond releases a molecule of water. Long chains only form at the very end — this is described by the Carothers equation:

Xn = 11 − p Xn — average number of units per molecule, p — fraction of groups that have already reacted

At p = 0.9 (90% of groups) a molecule has on average only 10 units; to get 100, 99% of the groups must react.

The coil and entropy

A chain in a melt or solution takes the shape of a random coil — like the path of a random walk. The average distance between its ends grows as the square root of the number of links:

√⟨R²⟩ ≈ b · √n b — link length; for real PE with a correction for bond stiffness

A stretched chain has very few possible shapes, a coiled one — an enormous number. That's why rubber springs back, and why its force grows with temperature: this is entropic elasticity.

Structure and properties

Linear chains pack tightly and form crystallites (PE-HD: stiff, milky). Branched ones pack worse (PE-LD: soft, flexible). Thermoplastics soften and melt when heated, so they can be remelted. Elastomers (rubber) have a loose network of cross-links — sulfur bridges from vulcanization. Thermosets (Bakelite, epoxy resins) have a dense network: when heated they don't melt, they char. Below the glass transition temperature Tg chain segments cannot move, and the plastic is hard and brittle.

Sorting and recycling

An object floats when its density is lower than the liquid's: ρplastic < ρliquid. This is how polyolefins (PP, PE) are separated from the heavier PS, PET and PVC. PET and PVC have almost the same density, so they are separated by sensors (NIR, or X-rays that detect chlorine).

Simplifications: the simulations are two-dimensional, and the chains are shorter than in reality. The tensile curves, plastic breakdown rates and recycling yields are models with rough figures.

Challenges

  1. Build a 50-mer polyethylene chainChain of beads: in addition polymerization, grow one chain to at least 50 mers. Hint: little initiator.not completed
  2. Cross-link rubber and compare the tensile curvesStructure: stretch raw rubber, vulcanize it and stretch it again — both curves must be on the graph.not completed
  3. Sort 5 kinds of plastic by floatingPlastics around us: test the flakes in the tanks and correctly name all five.not completed
  4. Recognize the recycling code of 8 itemsPlastics around us: get the code right on the first try for at least 8 items.not completed
  5. Take a PET bottle through the whole recycling processRecycling: go through all five steps — from the machine to a new product.not completed
  6. Tell compostable from biodegradable plasticMicroplastic: put all seven cards in the right cells of the table.not completed

Did you know…?

  • Polyethylene was discovered by accident: in 1933, in a lab of the ICI company, Eric Fawcett and Reginald Gibson were squeezing ethylene under enormous pressure and found a white, waxy deposit in the apparatus. As it turned out later, the reaction was started by a trace of oxygen that had accidentally got into the apparatus.
  • DNA is a polymer too: the nucleus of a single human cell holds about 6 billion nucleotide pairs, and the whole strand stretched out would be about 2 meters long.
  • A spider's dragline silk is, weight for weight, several times stronger than steel — and very stretchy as well. It is a protein that the spider “spins” at room temperature from a water solution.

Stay safe

  • Don't burn or heat plastics at home or in a campfire. Smoke from burning plastics is toxic (PVC, for example, releases hydrogen chloride, and poor combustion produces dioxins).
  • The nylon rope trick and liquid nitrogen are lab demonstrations, done with a teacher and safety goggles.
  • You can do sorting by floating in the kitchen with water and salt — cut only clean packaging and watch out for sharp edges. Don't drink the alcohol, and keep it away from flames.
Combustion →

Keys

Q W E R T Y
switch modes
Space
start / pause: polymerization, tensile test, recycling line, time by the sea
N
restart the current mode
I
chain: add initiator
V
structure: vulcanize rubber
B
recycling: next bottle step
← →
on a slider or switch: change the value
Esc
stop everything
Molecules →DNA →Kitchen chemistry →Reaction rates →Solutions →States of matter →Combustion →Power plant →