CHEMISTRY: POLYMERS

CHEMISTRY: POLYMERS

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Overview

• A polymer is a large molecule consisting of repeating structural units
• The repeating units are usually connected by covalent chemical bonds
• Polymers can be of two types
  • Natural polymers: shellac, amber, rubber, proteins etc
  • Synthetic polymers: nylon, polyethylene, neoprene, synthetic rubber etc
• Synthetic polymers are commonly referred to as plastics
• The first plastic based on a synthetic polymer to be created was Bakelite, by Leo Baekeland(Belgium/USA) in 1906
• Vulcanization of rubber was invented by Charles Goodyear (USA) in 1839. Vulcanization is the process of making rubber more durable by addition of sulphur
  
• The first plastic to be created was Parkesine (aka celluloid) invented by Alexander Parkes (England) in 1855

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Synthesis of polymers

• The synthesis of polymers – both natural and synthetic – involves the step called polymerization
• Polymerization is the process of combining many small molecules (monomers) into a covalently bonded chain (polymer)
• Synthetic polymers are created using of two techniques
  • Step growth polymerization: chains of monomers are combined directly
  • Chain growth polymerization: monomers are added to the chain one at a time
• Natural polymers are usually created by enzyme-mediated processes, such as the synthesis of proteins from amino acids using DNA and RNA

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Categories of polymers

• Organic polymers are polymers that are based on the element carbon. Eg: polyethylene, cellulose etc
• Inorganic polymers are polymers that are not based on carbon. Eg: silicone, which uses silicon and oxygen
• Copolymer is one that is derived from two or more monomeric units. Eg: ABS plastic
• Fluoropolymers are polymers based on fluorocarbons. They have high resistance to solvents, acids and bases. Eg: teflon

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TYPES OF BIOPOLYMERS

DNA as a biopolymer

1. Structural proteins
   1. Structural proteins are proteins that provide structural support to tissues
   2. They are usually used to construct connective tissues, tendons, bone matrix, muscle fibre
   3. Examples include collagen, keratin, elastin
2. Functional proteins
   1. Proteins that perform a chemical function in organisms
   2. Usually used for initiate or sustain chemical reactions
   3. Examples include hormones, enzymes
3. Structural polysaccharides
   1. They are carbohydrates that provide structural support to cells and tissues
   2. Examples include cellulose, chitin
4. Storage polysaccharides
   1. Carbohydrates that are used for storing energy
   2. Eg: starch, glycogen
5. Nucleic acids
   1. Nucleic acids are macromolecules composed of chains of nucleotides
   2. Nucleic acids are universal in living beings, as they are found in all plant and animal cells
   3. Eg: DNA, RNA

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TYPES OF SYNTHETIC POLYMERS

1. Thermoplastics
   1. Thermoplastics are plastics that turn into liquids upon heating
   2. Also known as thermosoftening plastic
   3. Thermoplastics can be remelted and remoulded
   4. Eg: polyethylene, Teflon, nylon
   5. Recyclable bottles (such as Coke/Pepsi) are made from thermoplastics
2. Thermosetting plastics
   1. Thermosettings plastics are plastics that do not turn into liquid upon heating
   2. Thermosetting plastics, once cured, cannot be remoulded
   3. They are stronger, more suitable for high-temperature applications, but cannot be easily recycled
   4. Eg: vulcanized rubber, bakelite, Kevlar
3. Elastomers
   1. Elastomers are polymers that are elastic
   2. Elastomers are relatively soft and deformable
   3. Eg: natural rubber, synthetic polyisoprene

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IMPORTANT NATURAL POLYMERS AND THEIR APPLICATIONS

Polymer		Application	Notes
Collagen		Connective tissue Gelatine (food)	Most abundant protein in mammals
Keratin		Hair, nails, claw etc
Enzymes		Catalysis
Hormones		Cell signalling
Cellulose		Cell wall of plants Cardboard, paper	Most common organic compound on Earth
Chitin		Cell wall of fungi, insects
Starch		Energy storage in plants	Most important carbohydrate in human diet
Glycogen		Energy storage in animals
DNA		Genetic information
RNA		Protein synthesis

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IMPORTANT SYNTHETIC POLYMERS AND THEIR APPLICATIONS

Polymer	Developed by	Constituent elements	Application	Notes
Parkesine	Alexander Parkes (Britain, 1855)	Cellulose	Plastic moulding	First man-made polymer
Bakelite	Leo Baekeland (USA, 1906)	Phenol and formaldehyde	Radios, telephones, clocks	First polymer made completely synthetically
Polyvinylchloride (PVC)	Henri Regnault (France, 1835)	Vinyl groups and chlorine	Construction material	Third most widely used plastic
Styrofoam	Ray McIntre (USA, 1941)	Phenyl group	Thermal insulation	Brand name for polystyrene
Nylon	Wallace Carothers (USA, 1935)	Amides	Fabric, toothbrush, rope etc	Family of polyamides First commercially successful synthetic polymer
Synthetic rubber	Fritz Hoffman (Germany, 1909)	Isoprene	Tyres, textile printing, rocket fuel
Vulcanized rubber	Charles Goodyear (USA, 1839)	Rubber, sulphur	Tyres	Vulcanized rubber is much stronger than natural rubber
Polypropylene	Karl Rehn and Guilio Natta (Italy, 1954)	Propene	Textiles, stationary, automotive components	Second most widely used synthetic polymer
Polyethylene	Hans von Pechmann (Germany, 1898)	Ethylene	Packaging (shopping bags)	Most widely used synthetic polymer
Teflon	Roy Plunkett (USA, 1938)	Ethylene	Cookware, construction, lubricant	Brand name for polytetrafluoroehtylene (PTFE) Very low friction, non-reactive

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DEGRADATION OF POLYMERS

Ozone cracking in natural rubber tubing

• Degradation of polymers can be desirable as well undesirable: desirable when looking for biological degradation, undesirable when faced with loss of strength, colour etc
• Polymer degradation usually occurs due to hydrolysis of covalent bonds connecting the polymer chain
• Polymer degradation can happen because of heat, light, chemicals and galvanic action
• Ozone cracking is the cracking effect of ozone on rubber products such as tyres, seals, fuel lines etc. Usually prevented by adding antiozonants to the rubber before vulcanization
• Chlorine can cause degradation of plastic as well, especially plumbing
• Resin Identification Code is the system of labelling plastic bottles on the basis of their constituent polymers. This Code helps in the sorting and recycling of plastic bottles
• Degradation of plastics can take hundreds to thousands of years

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Biodegradable plastics

• Biodegradable plastics are plastics than can break down upon exposure to sunlight (especially UV), water, bacteria etc
• Biopol is a biodegradable polymer synthesized by genetically engineered bacteria
• Ecoflex is a fully biodegradable synthetic polymer for food packaging

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Bioplastics

• They are organic plastics derived from renewable biomass sources such as vegetable oil, corn, starch etc

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Oxy-biodegradable plastics

• Plastics to which a small amount of metals salts have been added
• As long as the plastic has access to oxygen the metal salts speed up process of degradation
• Degradation process is shortened from hundreds of years to months

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JOHAR