Condensation Polymers, Biodegradability & Disposal (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching chemistry, and this video is about condensation polymers, their biodegradability and how they are disposed of.

These parts go together because condensation polymers are joined by ester or amide links, and it is those same links that let the polymer be broken down again.

Condensation polymerisation is a reaction in which a polymer is produced by repeated condensation reactions between monomers.Those monomers end up linked by ester or amide bonds. Those links are what separates these polymers from addition polymers: esters and amides can be hydrolysed, so polyesters and polyamides are biodegradable, while polyalkenes are inert because of the carbon to carbon bonds between their monomers.

We start with how condensation polymers form and what links them. Then their uses, and the properties those links give them. Then biodegradability, and finally the three ways polymers are disposed of.

Condensation polymerisation produces a polymer by repeated condensation reactions between monomers. The process involves the elimination of a small molecule like water or HCl. Natural condensation polymers are all formed by the elimination of water.

Condensation polymers can be identified because the monomers are linked by ester or amide bonds. A polyester forms from dicarboxylic acid monomers and diol monomers, linked by ester bonds. A polyamide forms from a diamine and a dicarboxylic acid, linked by amide links, and a diacyl dichloride can be used instead of the acid, which is more reactive but more expensive. Amino acids can also polymerise, because each one carries both an amine group and an acid group.

That ester or amide link is the bond that defines this whole family of polymers.

Terylene, also known as polyethylene terephthalate or PET, is a polyester and a thermoplastic. It can be extruded to form fine fibres for artificial fabrics, or moulded into fizzy drinks bottles and containers.

The best known artificial polyamide is nylon. Nylon-6,6 contains a diamine and a dicarboxylic acid, each of which contains six carbon atoms. It proved a cheap substitute for silk, and is used to make ropes, twines and Velcro, and is often added to natural fibres in clothing and carpets to make them last longer.

Aromatic polyamides are known as aramids, and include Nomex and Kevlar. They are very tough and lightweight, and are used to make bulletproof vests and fireproof suits. These polymers are drawn out into fibres, and as the linear molecules align they become increasingly linked by hydrogen bonds between adjacent chains, which increases the strength of the fibre. Those hydrogen bonds form on the amide links themselves.

Both polyesters and polyamides can be broken down using hydrolysis reactions, and that is a major advantage over the polymers produced using alkene monomers. Simple esters and amides are hydrolysed by aqueous acids or bases into carboxylic acids and alcohols, or into carboxylic acids and amines.

Condensation polymers are linked by those same ester or amide linkages, so under acid catalysts or biological enzymes the links can be hydrolysed into smaller fragments. Polyalkenes are inert and non-biodegradable, because of the carbon to carbon bonds between the monomers in addition polymers. When polyesters and polyamides are taken to landfill they can be broken down easily, and their products used for other applications.

There are three main ways of disposing of polymers: landfill, incineration and recycling.

Landfill buries the waste at a specific contained site, and biodegradable polymers, the polyesters and polyamides, will slowly degrade there. However, the land available is limited, degradation of plastics is slow, toxic compounds can leach out, and gas emissions including methane are released.

Incineration uses less space than landfill, the energy released from burning can generate electricity, and it prevents the build up of polymers in the environment. It does increase carbon dioxide, and releases toxic gases and particulates into the atmosphere.

Recycling allows the reuse and conservation of finite hydrocarbon resources and reduces the waste going to landfill. Collection, sorting, separating and remoulding all require energy, and it is time consuming and expensive.

Draw the simplest repeating unit, not a double unit, because examiners penalise that. Always show trailing bonds extending from the repeating unit, so it is clear the unit is part of a chain rather than a standalone molecule, and do not attach those trailing bonds to hydrogen atoms.

Condensation polymers form by repeated condensation reactions, eliminating water or HCl, and their monomers end up joined by ester or amide links.

Polyesters such as Terylene, and polyamides such as nylon-6,6 and Kevlar, take their strength from hydrogen bonds between adjacent chains.

Those ester and amide links can be hydrolysed by acid, base or enzymes, so these polymers are biodegradable, while polyalkenes with their carbon to carbon bonds are not.

Polymers are disposed of by landfill, incineration or recycling, each with its own advantages and disadvantages. The link that builds the polymer is the link that lets it be broken down.