سائنس
Polymer: A Long Chain Built from One Small Molecule
A polymer is thousands of small monomer molecules joined into one long chain. Ethene boils at −104 °C; poly(ethene) is a solid. Same atoms, new material.
Polymer
A polymer is a large molecule formed when many small molecules, called monomers, join together into a long repeating chain.
- دیگر نام
- Macromolecule, Plastic, for the synthetic ones
- طلبہ کہاں پڑھتے ہیں
- Grade 9 chemistry, in organic chemistry after alkenes, and in Grade 9 biology where starch, protein and DNA appear as natural polymers.
مختصر جواب
A polymer is a very large molecule made by joining thousands of small molecules, called monomers, into a long chain. Ethene molecules join to make poly(ethene). The chain is written as a repeat unit inside brackets with an n outside, and its properties are nothing like those of the monomer.
ایک مثال
n CH₂=CH₂ → —[ CH₂—CH₂ ]ₙ—
Ethene has a carbon-to-carbon double bond. In addition polymerisation that double bond opens, and each carbon uses the freed bond to grab a neighbouring molecule, so the monomers link end to end into a chain thousands of units long. Nothing else is produced — every atom of the ethene ends up in the polymer. The repeat unit is drawn with the bonds passing through the brackets, showing that the chain continues in both directions, and n stands for however many times it repeats.
From monomer to polymer
The requirement for addition polymerisation is a carbon-to-carbon double bond in the monomer, because that is what opens up to make the joins. Alkenes have one and alkanes do not, which is why poly(ethene) exists and poly(ethane) does not.
Naming is mechanical: put poly in front of the monomer's name in brackets. Ethene gives poly(ethene), propene gives poly(propene), and chloroethene gives poly(chloroethene), which the world calls PVC. The brackets are there so that a name like poly(chloroethene) cannot be misread as something else with chlorine attached to the outside of the chain.
Why the properties change so completely
Ethene is a gas that boils at about −104 °C. Poly(ethene) is a solid tough enough to make a bottle from. The atoms are the same and even the ratio of carbon to hydrogen is the same, so the difference is entirely one of size. Forces between molecules are individually weak, but they act along the whole length of a chain, and a chain of thousands of carbon atoms accumulates enough of them to hold the material together as a solid.
Chain shape matters as much as chain length. Poly(ethene) made under conditions that give long, straight, unbranched chains packs tightly and comes out rigid and higher melting; the branched version cannot pack closely and is the soft, flexible film used for carrier bags. Same monomer, same reaction, two materials sold for different jobs.
Addition, condensation, and the ones in you
Addition polymerisation joins the monomers with no by-product at all, so every atom is kept. Condensation polymerisation is the other route: each join expels a small molecule, usually water, and it needs monomers with a reactive group at each end. Polyesters and nylon are made this way.
Nature uses condensation throughout. Starch and cellulose are polymers of glucose, proteins are polymers of amino acids, and DNA is a polymer of nucleotides — all built by removing water at each link and taken apart again by adding it back. The synthetic addition polymers have no such route back, because a carbon chain with only single C–C and C–H bonds is extremely unreactive, and that is precisely why so many of them last for centuries in landfill.
- Approximate boiling point of ethene, the gas whose molecules join to make solid poly(ethene)
- −104 °CApproximate boiling point of ethene, the gas whose molecules join to make solid poly(ethene)
عام سوالات
What is the difference between a monomer and a polymer?
A monomer is the small starting molecule, one unit on its own. A polymer is the long chain made when many monomers join. The relationship is like bricks and a wall: ethene is the brick and poly(ethene) is the wall, and the wall behaves nothing like a loose brick does.
Are all plastics polymers?
Yes, though not all polymers are plastics. Plastic is an everyday word for synthetic polymers that can be moulded when warm. Wool, silk, rubber, starch and the proteins in your muscles are polymers too, and no one calls those plastic. The chemical definition is about chain structure, not about the material's feel.
Why is poly(ethene) not biodegradable?
Because the chain is nothing but carbon-to-carbon and carbon-to-hydrogen single bonds, which are strong and give enzymes nothing to attack. Natural polymers such as starch have oxygen linkages in the chain that microorganisms can break. Poly(ethene) has no such weak point, so it is broken down only very slowly, if at all.
How is a repeat unit different from the monomer?
The monomer is drawn with its double bond intact and no bonds extending outwards. The repeat unit is drawn with the double bond opened to a single bond and a bond leaving each side through the brackets, showing where the neighbouring units attach. Same atoms, different bonding — and marks are given for exactly that difference.
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