Addition and Condensation Polymerization
Polymerization joins many small molecules (monomers) into a long chain (a polymer): in addition polymerization, unsaturated monomers add to each other with no by-product, and in condensation polymerization, monomers with two functional groups join with the loss of a small molecule such as water. In brief: ethene gives polyethylene by addition (no atoms lost), whereas hexane-1,6-diamine and hexanedioic acid give nylon-6,6 by condensation with the loss of two molecules of water per repeat unit.
Plastics, synthetic fibers, rubbers, adhesives and paints are all polymers. The two main routes differ in the type of monomer, in the mechanism and in the way in which the properties of the product can be controlled. This article describes both types of polymerization, with the mechanism of free-radical addition, a table of common polymers, calculations of chain length and mass balance, and a short history.
What is addition polymerization?
The monomers are alkenes (or related compounds with a C=C bond). The π bond of each monomer opens, and the monomers join through σ bonds into a long saturated chain:
n CH2=CHX → [–CH2–CHX–]n
The polymer has the same empirical formula as the monomer: no atoms are lost, so the mass of the polymer equals the mass of the monomer used. The product is usually a mixture of chains with different lengths, so a polymer has an average molar mass. The repeat unit is the part of the polymer that repeats; in poly(chloroethene) it is –CH2CHCl–.
What is the free-radical mechanism?
- Initiation: an initiator, such as a peroxide, decomposes on heating into radicals (R·), which add to the double bond of a monomer and form a new radical: R· + CH2=CH2 → R–CH2–CH2·.
- Propagation: the radical end attacks another monomer and adds to it, and the radical moves to the end of the longer chain. This step repeats thousands of times, within a fraction of a second per chain.
- Termination: two radical chain ends combine or disproportionate, and the chain stops growing.
Other addition mechanisms use cations, anions or coordination catalysts. The Ziegler–Natta catalysts (TiCl4 with Al(C2H5)3) give linear, stereoregular chains, so they made high-density polyethylene and isotactic polypropylene possible.
Common addition polymers
| Polymer | Monomer | Repeat unit | Uses |
|---|---|---|---|
| Polyethylene (LDPE, HDPE) | Ethene, CH2=CH2 | –CH2CH2– | Bags, bottles, pipes |
| Polypropylene | Propene, CH2=CHCH3 | –CH2CH(CH3)– | Ropes, containers, car parts |
| Poly(chloroethene) (PVC) | Chloroethene, CH2=CHCl | –CH2CHCl– | Pipes, window frames, cable insulation |
| Polystyrene | Styrene, CH2=CHC6H5 | –CH2CH(C6H5)– | Foam packaging, cups |
| PTFE (Teflon) | Tetrafluoroethene, CF2=CF2 | –CF2CF2– | Non-stick coatings |
| Poly(methyl methacrylate) | Methyl 2-methylpropenoate | –CH2C(CH3)(COOCH3)– | Acrylic glass (Perspex) |
What is condensation polymerization?
The monomers have two (or more) functional groups, and each pair of groups reacts to form a link with loss of a small molecule, usually water. The repeat unit is formed from two monomers, with a diamine or a diol and a diacid:
- Polyamides: diamine + diacid → amide links. Nylon-6,6 from hexane-1,6-diamine and hexanedioic acid. Proteins are natural polyamides, made from amino acids with both an –NH2 and a –COOH group.
- Polyesters: diol + diacid → ester links. PET (polyethylene terephthalate) from ethane-1,2-diol and benzene-1,4-dicarboxylic acid.
The chain grows in steps, and the molar mass rises slowly until the conversion is very high, so exact stoichiometry of the two monomers is needed to obtain long chains.
Common condensation polymers
| Polymer | Monomers | Link | Lost molecule | Uses |
|---|---|---|---|---|
| Nylon-6,6 | Hexane-1,6-diamine + hexanedioic acid | Amide | H2O | Fabrics, ropes, gears |
| PET | Ethane-1,2-diol + benzene-1,4-dicarboxylic acid | Ester | H2O | Bottles, polyester fibers |
| Kevlar | Benzene-1,4-diamine + benzene-1,4-dicarbonyl chloride | Amide | HCl | Bulletproof vests, ropes |
| Bakelite | Phenol + methanal | Cross-linked network | H2O | Electrical fittings, handles |
| Protein | Amino acids | Peptide (amide) | H2O | Biological structure and enzymes |
How do the two types compare?
| Feature | Addition | Condensation |
|---|---|---|
| Monomers | Unsaturated (C=C) | Two functional groups |
| By-product | None | Small molecule (H2O, HCl) |
| Polymer backbone | Carbon–carbon only | Contains links such as ester or amide |
| Chain growth | Fast; chain forms in a fraction of a second | Gradual; long chains only at high conversion |
| Biodegradability | Usually low | Often hydrolyzable |
Worked examples
Example 1: degree of polymerization
A sample of polyethylene has an average molar mass of 280,000 g/mol. The repeat unit CH2CH2 has a mass of 28.05 g/mol, so the degree of polymerization is n = 280,000 / 28.05 ≈ 10,000. The chain contains about 20,000 carbon atoms.
Example 2: mass of monomer for an addition polymer
How much ethene is needed to make 1.00 tonne of polyethylene? Since no atoms are lost, 1.00 tonne of ethene is needed (at 100 % conversion). This is 1.00 × 106 g / 28.05 g/mol = 35,650 mol, which occupies 35,650 × 24.47 L = 8.7 × 105 L (about 870 m3) as a gas at 25 °C and 1 atm. This is the reason why ethene is transported as a compressed gas or a liquid.
Example 3: water released in nylon-6,6 production
Each repeat unit forms from one diamine (116.21 g/mol) and one diacid (146.14 g/mol) with the loss of two water molecules: 116.21 + 146.14 − 2 × 18.02 = 226.31 g/mol. For 1.00 kg of nylon, n = 1000 / 226.31 = 4.42 mol of repeat units, and 8.84 mol of water, which is 159 g, is released. The condensation polymer therefore has less mass than the monomers: 262.35 g of monomers give 226.31 g of polymer.
Example 4: drawing the repeat unit
For poly(chloroethene), open the double bond of CH2=CHCl and draw the two free bonds at the ends: –CH2–CHCl–. For PET, the repeat unit is –O–CH2CH2–O–CO–C6H4–CO–, which contains one ester link from each end.
Example 5: identifying the monomers
A polymer has the repeat unit –CH2–CH(CH3)–. This comes from propene (the addition of n molecules of CH2=CHCH3), so it is polypropylene. A polymer with amide links in the backbone, –NH–(CH2)6–NH–CO–(CH2)4–CO–, is made from a diamine with six carbons and a diacid with six carbons: nylon-6,6.
How did polymer chemistry develop?
Natural polymers such as rubber and cellulose were used for ages. Leo Baekeland made Bakelite, the first fully synthetic plastic, in 1907. Hermann Staudinger proposed in 1920 that polymers consist of long chains of covalently bonded small units, against the idea that they were aggregates of small molecules, and received the Nobel Prize in Chemistry in 1953. Polyethylene was discovered by accident at ICI in 1933 by Eric Fawcett and Reginald Gibson, who found a waxy solid in a high-pressure reaction vessel. Wallace Carothers at DuPont developed nylon in 1935, and the first nylon stockings came on the market in 1940. Karl Ziegler and Giulio Natta developed their catalysts in 1953–1954, for which they shared the Nobel Prize in 1963. PET was patented by John Whinfield and James Dickson in 1941.
Which mistakes are common?
- Drawing the double bond in the polymer: it is used up in the chain.
- Forgetting the small molecule in condensation: the mass of the polymer is lower than that of the monomers.
- Confusing the repeat unit and the monomer: for addition polymers they have the same atoms, but not the same bonds.
- Miscounting the water released: a chain built from 2n monomers (n diamines and n diacids) has 2n − 1 amide links, so it releases 2n − 1 water molecules, which is about two per repeat unit.
- Assuming that all plastics are the same: structure determines hardness, melting point and recyclability.