Synthetic Polymers

What a condensation polymer is

• Condensation polymerisation joins two different monomers, each carrying a functional group on each of its two ends
• For every link that forms between the monomers, a small molecule (usually water, sometimes HCl) is split out
• This is the key contrast with addition polymerisation:
  • Addition → polymer only (no by-product)
  • Condensation → polymer + a small molecule (e.g. water) per link

Polyesters

• The example to know is a polyester — a polymer built from many ester linkages (the −COO− group seen in topic 27)
• Two monomers are needed:
  • A dicarboxylic acid — a molecule with a carboxylic acid (−COOH) group at each end
  • A diol — an alcohol that carries one hydroxyl group at each end of its chain (two −OH groups per molecule)
• Each −COOH on one monomer reacts with an −OH on the other monomer, forming an ester linkage and splitting out a water molecule (just like the esterification in topic 27, but happening over and over down the chain)
• The chain alternates: ...diol−ester−diacid−ester−diol−ester−diacid... and so on for thousands of units

A named example: terylene

• Terylene is a polyester — its full chemical name is poly(ethylene terephthalate), often abbreviated to PET, which is the same material that drinks bottles are made from. It is built from:
  • Terephthalic acid — a benzene ring with a −COOH on opposite carbons (a dicarboxylic acid)
  • Ethane-1,2-diol — a two-carbon molecule with an −OH on each carbon (a diol)
• The two monomers react end-to-end, splitting out one water molecule per ester link, and yield a chain of alternating units suitable for being drawn into fibres or moulded into bottles

Working backward: identifying the diol and diacid from a polyester

• Given the displayed formula of a polyester section, you can find the two monomers by treating the polyester as if it were hydrolysed back into its starting materials
• A step-by-step approach:
  • Identify every ester linkage −COO− in the chain
  • Break the chain at the single bond between the carbonyl carbon and the next oxygen — the section ending in "−C(=O)−" came from the dicarboxylic acid, and the section ending in "−O−" came from the diol
  • Add an −OH to the carbonyl carbon to rebuild the carboxylic acid −COOH
  • Add an −H to the oxygen of the diol fragment to rebuild the alcohol −OH
• The two resulting molecules are the diacid and the diol from which the polyester was made

Biopolyesters

• Most synthetic polyesters (terylene, PET) are derived from petrochemicals and do not biodegrade quickly
• Biopolyesters are a newer family made from monomers obtained from plant sugars and plant oils, often with the help of microorganisms
• The chains contain ester linkages (and sometimes amide or ether linkages) that bacteria in the soil can attack, so the material gradually breaks down into harmless small molecules
• Biopolyesters are used in compostable packaging, single-use cutlery, and surgical sutures that the body can absorb after a wound heals