9.3 · Properties & polymers

Polymers

Giant molecules made from small repeating units.

By the end you should be able to:

Relate monomers to repeat units, and distinguish addition from condensation polymers
Predict how chain length, branching, cross-linking and crystallinity change a polymer's properties

Key idea

Monomers, repeat units and addition polymers

A polymer is a very large molecule built from many linked repeat units; the small molecules that join up are the monomers. The number of repeat units, $n$ , is often in the thousands to hundreds of thousands.

Addition polymers form from monomers with a C=C double bond: the π bond opens and the monomers join end to end. No atoms are lost, so the repeat unit has the same formula as the monomer:

n\,\ce{CH2=CHCl} \longrightarrow \left[\,\ce{CH2-CHCl}\,\right]_n

Polymer	Monomer	Repeat unit	Uses
polyethylene (PE)	$\ce{CH2=CH2}$	$\left[\,\ce{CH2-CH2}\,\right]_n$	bags, bottles, film
polypropylene (PP)	$\ce{CH2=CH-CH3}$	$\left[\,\ce{CH2-CH(CH3)}\,\right]_n$	containers, rope, carpet
poly(vinyl chloride) (PVC)	$\ce{CH2=CHCl}$	$\left[\,\ce{CH2-CHCl}\,\right]_n$	pipes, window frames
polystyrene (PS)	$\ce{CH2=CH-C6H5}$	$\left[\,\ce{CH2-CH(C6H5)}\,\right]_n$	foam cups, packaging
PTFE (Teflon)	$\ce{CF2=CF2}$	$\left[\,\ce{CF2-CF2}\,\right]_n$	non-stick coatings

Key idea

Condensation polymers

Condensation polymers form from monomers that each carry two functional groups. Every new link releases a small molecule, usually $\ce{H2O}$ (or $\ce{HCl}$ when an acid chloride is used).

Polyesters (ester link –CO–O–): carboxylic acid + alcohol. PET is made from terephthalic acid and ethylene glycol; PLA from lactic acid, which carries both groups in one molecule.
Polyamides (amide link –CO–NH–): carboxylic acid (or acid chloride) + amine. Nylon-6,6 comes from adipic acid and hexamethylenediamine (releasing $\ce{H2O}$ ); Kevlar from terephthaloyl chloride and p-phenylenediamine (releasing $\ce{HCl}$ ).
Natural examples: proteins (amide links between amino acids) and cellulose (glucose units joined with loss of $\ce{H2O}$ ).

Because atoms leave, repeat unit = monomers − small molecules. For nylon-6,6, with two amide links per repeat unit:

\ce{C6H16N2} + \ce{C6H10O4} - 2\,\ce{H2O} = \ce{C12H22N2O2}\quad (226.3\ \text{g/mol})

Method

From repeat unit back to monomer

Addition polymer (backbone of carbon atoms only):

Take the two backbone carbons of the repeat unit.
Put a C=C between them and keep the substituents. So $\left[\,\ce{CH2-CH(CN)}\,\right]_n$ comes from acrylonitrile, $\ce{CH2=CH-CN}$ .

Condensation polymer (ester or amide links in the backbone):

Find each ester (–CO–O–) or amide (–CO–NH–) link.
Break the C–O or C–N bond of the link.
Add –OH to the carbonyl side (giving –COOH) and –H to the O or N side (giving –OH or –NH₂).
Count the pieces: two different monomers (nylon-6,6, PET, Kevlar) or one monomer carrying both groups (PLA, proteins).

Key idea

Structure–property rules

Change	Effect	Why
longer chains	higher melting point, stronger	more contact (total dispersion) and more tangling between chains
more branching (LDPE vs HDPE)	lower density, crystallinity and melting point	branches stop chains packing closely
extensive cross-linking	rigid; does not melt (a thermoset)	covalent cross-links stop chains sliding past each other
light cross-linking (vulcanized rubber)	elastic; snaps back after stretching	a few sulfur bridges pull the chains back into place
more crystallinity	denser, stiffer, higher melting, more opaque	ordered, closely packed chains attract more strongly
hydrogen bonds between chains (nylon, Kevlar)	stronger, higher melting	N–H···O=C hydrogen bonds link neighbouring chains
plasticizer added (soft PVC)	softer, more flexible	small molecules sit between chains and weaken their attraction

LDPE (low-density, branched, about 0.92 g/cm³) is soft and used for bags and film; HDPE (high-density, linear and more crystalline, about 0.95 g/cm³) is stiff and used for jugs and pipes. Thermoplastics (no cross-links) soften on heating and can be remoulded; thermosets cannot.

Key idea

Polymers and sustainability

Addition polymers such as PE and PP have all-carbon backbones that almost nothing in the environment can break, so they persist for decades or longer.
Polyesters and polyamides have backbone links that water can split (hydrolysis, the reverse of condensation). PLA, made from lactic acid fermented from corn starch or sugar cane, is a renewable, compostable polyester, although it needs industrial composting conditions to break down quickly.
Recycling codes: 1 PET, 2 HDPE, 3 PVC, 4 LDPE, 5 PP, 6 PS, 7 other (including PLA). Thermoplastics can be melted and reformed; thermosets cannot.

Common mistake

Polymer traps

Wrong: the repeat unit of polyethylene is $\ce{CH2=CH2}$ . Right: the C=C is used up in making the chain, so the repeat unit $\left[\,\ce{CH2-CH2}\,\right]_n$ is saturated. (A diene keeps one C=C: natural rubber, from isoprene, is $\left[\,\ce{CH2-C(CH3)=CH-CH2}\,\right]_n$ .)
Wrong: a condensation polymer's repeat unit is the sum of its monomers. Right: subtract the small molecule released at each link.
Wrong: condensation always releases water. Right: Kevlar, made from an acid chloride, releases $\ce{HCl}$ .
Wrong: PVC is a condensation polymer because it contains Cl. Right: its monomer $\ce{CH2=CHCl}$ has a C=C and no second functional group; it is an addition polymer and releases nothing.

Worked example

Worked example: identifying polymers

(a) $\left[\,\ce{O-CH2CH2-O-CO-C6H4-CO}\,\right]_n$ (PET). The backbone contains ester links, so it is a condensation polymer. Breaking each ester C–O bond and adding –OH and –H gives ethylene glycol, $\ce{HOCH2CH2OH}$ , and terephthalic acid, $\ce{HOOC-C6H4-COOH}$ ; $\ce{H2O}$ is released at each link. Repeat unit: $\ce{C2H6O2} + \ce{C8H6O4} - 2\,\ce{H2O} = \ce{C10H8O4}$ .

(b) $\left[\,\ce{CH2-CH(CH3)}\,\right]_n$ . A two-carbon, all-carbon backbone: an addition polymer of propylene, $\ce{CH2=CH-CH3}$ , with no byproduct.

(c) How many repeat units are in a polyethylene chain of molar mass $2.8 \times 10^5$ g/mol? The repeat unit $\ce{C2H4}$ is 28.05 g/mol, so $n = 2.8 \times 10^5 / 28.05 \approx 1.0 \times 10^4$ .

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