Polymers: MDCAT Chemistry notes
Polymers for MDCAT: addition vs condensation polymers, PVC, polystyrene, Teflon, nylon-6,6 and Bakelite, homopolymers, copolymers, thermoplastics.
Basic terms
A polymer is a very large molecule built from many small repeating units called monomers. The process is polymerization, and the number of repeating units in a chain is the degree of polymerization.
- Homopolymer: made from one kind of monomer, e.g. polyethene, PVC, polystyrene, Teflon.
- Copolymer: made from two (or more) different monomers. Nylon-6,6 (a polyamide) and polyesters are built from two different monomers and are given as copolymer examples.
Addition polymers
Unsaturated monomers (containing C=C) join by opening their double bonds. Nothing is eliminated, so the polymer has the same empirical formula as the monomer.
$$\mathrm{n\,CH_2{=}CHCl \rightarrow {-}[CH_2{-}CHCl]_n{-}}$$
| Polymer | Monomer | Repeat unit | Uses |
|---|---|---|---|
| Polyethene (polythene) | Ethene | $\mathrm{-CH_2{-}CH_2-}$ | Bags, films |
| Poly(vinyl chloride), PVC | Vinyl chloride (chloroethene) | $\mathrm{-CH_2{-}CHCl-}$ | Pipes, cable insulation |
| Polystyrene | Styrene (phenylethene) | $\mathrm{-CH_2{-}CH(C_6H_5)-}$ | Disposable cups, packaging |
| Teflon (PTFE) | Tetrafluoroethene | $\mathrm{-CF_2{-}CF_2-}$ | Non-stick coatings |
| Poly(vinyl acetate) | Vinyl acetate | $\mathrm{-CH_2{-}CH(OCOCH_3)-}$ | Adhesives, paints |
Condensation polymers
Monomers with two functional groups each react so that a small molecule (usually water) is eliminated at every link.
Nylon-6,6 (a polyamide)
Made from hexanedioic acid (adipic acid, 6 C) and hexane-1,6-diamine (hexamethylenediamine, 6 C); the "6,6" refers to these carbon counts. The link is the amide group $-\mathrm{CO{-}NH-}$.
$$\mathrm{n\,HOOC(CH_2)_4COOH + n\,H_2N(CH_2)_6NH_2 \rightarrow {-}[OC(CH_2)_4CO{-}NH(CH_2)_6NH]_n{-} + 2n\,H_2O}$$
Polyester
A dicarboxylic acid (terephthalic acid) and a diol (ethylene glycol) give an ester-linked polymer (terylene, Dacron), used for fibres and bottles.
Bakelite
Phenol + formaldehyde condense, with an acid or base catalyst, to Bakelite. It is a cross-linked, hard, thermosetting resin used for electrical switches and handles.
Thermoplastics and thermosets
| Type | Behaviour on heating | Examples |
|---|---|---|
| Thermoplastic | Softens on heating, can be remoulded (linear chains) | Polyethene, PVC, polystyrene |
| Thermosetting | Sets permanently hard; does not soften again (cross-linked) | Bakelite, melamine |
Note that PVC is both a homopolymer (by monomer count) and a thermoplastic (by heat behaviour); it is not thermosetting and not a copolymer.
Natural and synthetic polymers
Polymers also occur in nature: proteins (from amino acids, by condensation), starch and cellulose (from glucose), nucleic acids and natural rubber (polyisoprene). The polymers above are synthetic. A quick way to classify an unknown polymer: if the repeat unit has the same formula as a single alkene monomer, it is an addition polymer; if the repeat unit contains linking groups such as $-\mathrm{CO{-}NH-}$ (amide) or $-\mathrm{CO{-}O-}$ (ester), water was lost and it is a condensation polymer.
Worked example: $\mathrm{-[CH_2{-}CH(CN)]_n-}$ has the formula of acrylonitrile, $\mathrm{CH_2{=}CHCN}$, with no atoms lost, so it is an addition homopolymer (polyacrylonitrile, used as acrylic fibre).
Common MDCAT traps
- PVC, polystyrene and Teflon are addition polymers; nylon, polyester and Bakelite are condensation polymers.
- The PVC repeat unit is $\mathrm{-CH_2{-}CHCl-}$ (single bond); $\mathrm{H_2C{=}CHCl}$ is the monomer.
- Nylon-6,6 is a polyamide, not a polyester.
- Hexane-1,6-diamine, not 1,5- or 1,4-, is used with adipic acid.
- Bakelite needs formaldehyde, not a carboxylic acid or ether.
Quick revision
- Addition: no by-product; condensation: small molecule lost.
- Teflon = $\mathrm{(-CF_2{-}CF_2-)_n}$.
- Polystyrene: disposable cups.
- Bakelite: phenol + HCHO, thermosetting.