Unit 26: Polymers — Long Questions

Plasticizers are chemicals added to PVC to make it softer, more flexible and easier to work with. Chemically, they work their way between the long PVC chains and reduce the strong intermolecular forces (van der Waals/dipole attractions between the polar C-Cl bonds) that normally hold the rigid chains tightly packed together. With these forces weakened, the chains can slide past one another and move more freely, lowering the polymer's rigidity.

This has a direct effect on applications. Rigid (unplasticized) PVC remains hard and stiff, chemically inert to most acids, bases and oils, a good electrical insulator, and naturally resistant to burning; it is therefore used for pipes and fittings, window frames and door profiles, and electrical insulation for wires and cables, where strength and rigidity are needed. Plasticized PVC is flexible and soft, bends and shapes easily, and still keeps good chemical and electrical properties but is less rigid; it is therefore used for cling films, bottles, blood bags and tubing/catheters, raincoats, artificial leather and inflatable products, where flexibility is essential.

Poly(alkenes) such as poly(ethene) have a backbone made up entirely of strong, non-polar carbon-carbon and carbon-hydrogen bonds. These bonds are chemically inert — they do not react with water, acids, bases, or the enzymes of soil microorganisms — so poly(alkene) chains resist being broken down and persist for a very long time in landfills, oceans and soils, accumulating as pollution and eventually fragmenting into persistent microplastics.

Polyesters and polyamides, by contrast, contain polar ester (-COO-) or amide (-CONH-) linkages built into their backbones. These linkages are susceptible to hydrolysis: soil microorganisms and water can attack and cleave these bonds, breaking the polymer down into simpler, non-toxic compounds — polyamides yield carboxylic acids and amines, while polyesters yield carboxylic acids and alcohols. This hydrolysis is the chemical process that makes polyesters and polyamides more biodegradable than poly(alkenes), which lack any such hydrolysable linkage.

Addition polymerisation monomers add to one another one by one to form long chains, and all the atoms of the original monomers become part of the polymer — nothing is lost, and no small molecule is produced as a side product. It usually proceeds as a chain reaction started by a reactive initiator that generates free radicals, under heat/pressure. Structural requirement: the monomer must contain a carbon-carbon double (C=C) or triple (C≡C) bond, which opens up to let monomers link end-to-end (e.g. ethene forming poly(ethene); chloroethene forming PVC).

Condensation polymerisation monomers join together, but each time a new bond forms, a small molecule (commonly H₂O, also HCl, CH₃OH or NH₃) is eliminated as a side product. It proceeds step by step. Structural requirement: each monomer must have at least two reactive functional groups (such as -OH, -COOH, -COCl, or -NH₂) located at different ends of the molecule, so that it can react at both ends to extend the chain. These groups may be on two different monomers (e.g. a diol with a dicarboxylic acid to give a polyester, or a diamine with a dicarboxylic acid/dioyl chloride to give a polyamide) or on a single monomer bearing two different functional groups (e.g. an amino-carboxylic acid forming a polyamide/polypeptide).

In short addition polymers have purely carbon backbones built from unsaturated monomers with no by-product, while condensation polymers have oxygen- or nitrogen-containing backbones (ester or amide links) built from bifunctional monomers with elimination of a small molecule at every link.

1) Polyester — Terylene/PET: formed from the diol ethane-1,2-diol (HO-CH₂-CH₂-OH) and the diacid benzene-1,4-dicarboxylic acid (HOOC-C₆H₄-COOH). The -OH groups of the diol react with the -COOH groups of the acid; each new ester bond formed eliminates a molecule of water:
n HO-CH₂CH₂-OH + n HOOC-C₆H₄-COOH → [-O-CH₂CH₂-O-CO-C₆H₄-CO-]ₙ + 2n H₂O.
The product contains ester (-COO-) linkages in its backbone.

2) Polyamide — Nylon 6,6: formed from the diamine 1,6-diaminohexane (H₂N-(CH₂)₆-NH₂) and the diacid hexanedioic/adipic acid (HOOC-(CH₂)₄-COOH). The -NH₂ groups of the diamine react with the -COOH groups of the acid; each new amide bond formed eliminates a molecule of water:
n H₂N-(CH₂)₆-NH₂ + n HOOC-(CH₂)₄-COOH → [-NH-(CH₂)₆-NH-CO-(CH₂)₄-CO-]ₙ + 2n H₂O.
The product contains amide (-CONH-) linkages in its backbone.

Poly(alkenes), such as poly(ethene), have a backbone made entirely of carbon-carbon and carbon-hydrogen bonds. These bonds are strong and non-polar, making poly(alkenes) chemically inert — they do not react with water, acids, bases, or soil microorganisms. As a result, poly(alkenes) are non-degradable: they persist in the environment for a very long time, accumulating in landfills and oceans, and slowly fragmenting into microplastics rather than being broken down chemically.

Polyesters contain polar ester (-COO-) linkages within their backbone. Soil microorganisms can attack these linkages through hydrolysis, splitting the polymer into simpler, non-toxic carboxylic acids and alcohols. This makes polyesters biodegradable — they break down into simpler compounds over a defined period when buried in soil, unlike poly(alkenes), which lack any such susceptible linkage and therefore remain largely unaffected by hydrolysis or microbial attack.

In summary poly(alkenes) resist degradation because of their chemically inert all-carbon backbone, whereas polyesters degrade through microbial/hydrolytic attack on their ester linkages.