Introduction to Polymers

• Copolymerization is used to create polypeptides which can contain as many as 20 different amino acids.

Polymers

• Polymers are vital to daily life and industry, used in plastics, elastomers, fibers, paints and varnishes.
• The term "polymer" comes from the Greek words "poly" (many) and "mer" (unit or part).
• Polymers are large molecules with high molecular masses ranging from 103 to 107u, and are also called macromolecules.
• Macromolecules are formed when repeating structural units called monomers join on a large scale, linked by covalent bonds through a process called polymerization.

Classification of Polymers

• Polymers are classified based on things like source and there are three subcategories.

Natural Polymers

• Natural polymers are sourced from plants and animals.
• Examples of natural polymers include proteins, cellulose, starch, resins, and rubber.

Semi-synthetic Polymers

• Semi-synthetic polymers are cellulose derivatives.
• Examples include cellulose acetate (rayon) and cellulose nitrate.

Synthetic Polymers

• Synthetic polymers are man-made.
• Examples of synthetic polymers are plastics (such as polythene), synthetic fibers (such as nylon 6,6) and synthetic rubbers (such as Buna-S).

Polymerization Types

• There are 2 types of polymerisation reactions: addition and chain growth, and condensation or step growth.

Addition Polymerization

• Addition polymerization is where same or different monomers add together on a large scale to form a polymer.
• Monomers used are unsaturated compounds like alkenes, alkadienes, and their derivatives which can increase chain length via free radicals or ionic species.
• Free radical governed addition is the most common mode, which can use alkenes or dienes and their derivatives.
• A free radical generating initiator (catalyst) such as benzoyl peroxide, acetyl peroxide, or tert-butyl peroxide, is required.
• Ethene polymerizes into polythene by heating or exposing a mixture of ethene with a small a benzoyl peroxide initiator.
• The process initiates with a phenyl free radical addition from the peroxide to the ethene double bond.
• Chain initiating is the name of the creation a new and larger free radical. This radical reacts to form an even bigger radical which is called chain propagating.
• The product radical then reacts with another radical to form the polymerised product, which is called chain terminating.

Addition Polymers

• Addition polymers formed from a single monomeric species are known as homopolymers, such as polythene.
• Polymers made from two different monomers are copolymers, such as Buna-S (made from buta-1,3-diene and styrene).

Polythene

• Polythenes are linear or slightly branched chains, that soften when heated & harden when cooled, making them themoplastic polymers.

Low Density Polythene

• Low density polythene is obtained by polymerizing ethene under high pressure (1000 to 2000 atmospheres) at 350 K to 570 K.
• Traces of dioxygen or peroxide are used as an initiator/catalyst.
• It is made through free radical addition and H-atom abstraction.
• Has a highly branched structure, is chemically inert and tough but flexible, a poor conductor of electricity.

High Density Polythene

• High density polythene is formed when ethene undergoes addition polymerization in a hydrocarbon solvent.
• A catalyst such as triethylaluminium/titanium tetrachloride (Ziegler-Natta catalyst) is used at 333 K–343 K and 6-7 atmospheres.
• High density polythene (HDP) has linear molecules, high density from close packing, and is chemically inert.
• It is tougher and harder than other forms of polythene, used for manufacturing buckets, dustbins, bottles, and pipes.

Polytetrafluoroethene (Teflon)

• Teflon is manufactured by heating tetrafluoroethene with a free radical or persulphate catalyst at high pressures.
• It is chemically inert, resistant to corrosive reagents, and used in oil seals, gaskets, and non-stick surface coatings.

Polyacrylonitrile

• Adding polymerizing acrylonitrile in the presence of a peroxide catalyst yields polyacrylonitrile.
• It is used as substitute for wool in commercial fibers like orlon or acrilan.

Condensation Polymerization

• Condensation involves repetitive reaction between two bi-functional or trifunctional mono-meric units.
• This may result in loss of molecules and lead to high molecular mass condensation polymers.
• Each step produces a bifunctional species, therefore each step creates a functional species in a process that is called step growth polymerisation.
• An example of condensation polymerization includes the production of terylene.

Polyamides

• Polyamides are polymers with amide linkages, are important synthetic fibers known as nylons, and are created through condensation polymerization.

Nylon 6,6

• Nylon 6,6 is prepared by condensation polymerization of hexamethylenediamine with adipic acid, under high pressure and temperature.
• It is a fiber-forming solid with high tensile strength from strong intermolecular forces like hydrogen bonding, leading to close chain packing and crystalline nature.
• It is used to make sheets and bristles, and in the textile industry.

Nylon 6

• Nylon 6 is obtained by heating caprolactum with water at high temperature.
• It is used to make tire cords, fabrics, and ropes.

Polyesters

• Polyesters are polycondensation products of dicarboxylic acids and diols.
• Dacron/terylene is manufactured by heating ethylene glycol mixed with terephthalic acid at in the presence of zinc acetate-antimony trioxide catalyst.
• It is crease resistant, used in cotton, wool fiber blends, and reinforcing material for safety helmets.

Phenol-Formaldehyde Polymer

• Phenol-formaldehyde polymers are obtained by reacting phenol with formaldehyde with an acid or base catalyst.
• It starts with creation of o-/p-hydroxymethylphenol derivatives, which react with phenol to form rings joined by –CH2 groups. Novolac, is used in paints.
• Novolac heats with formaldehyde and undergoes cross linking to create bakelite, a thermosetting polymer resistant to remoulding or reuse.
• Bakelite manufactures combs, phonograph records, electrical switches, and utensil handles.

Melamine-Formaldehyde Polymer

• Melamine-formaldehyde polymer is formed by condensation polymerisation of melamine and formaldehyde.
• It is used in unbreakable crockery.

Copolymerization

• Copolymerization is polymerising more than one monomeric species to form a copolymer, via either chain growth or step growth polymerisation.
• The copolymer contains multiple units of each monomer that are used in the polymeric chain.
• A mix of buta-1,3-diene & styrene can form a copolymer.
• Butadiene-styrene copolymer has properties that different from homopolymers.
• It is tough, for autotyres, cable insulation

Rubber

• Possesses elastic properties. Also known as elastomeric polymer
• Elastomeric polymers have chains held by the weak permitting stretching. Crosslinks help it retract to its original position after force is released.
• Made from rubber latex.

Natural Rubber

• Harvested from rubber trees.
• It is considered a linear polymer of isoprene (2-methyl-1,3-butadiene) and cis-1,4-polyisoprene.
• Cis-polyisoprene has chains held by the weak van der Waals interactions and exhibits it's coiled structure, which is can be stretched like a spring

Vulcanization of Rubber

• Natural rubber is soft, loses temperature, is more absorbent and isnt resistant for oxidising agents.
• Vulcanization requires heating mixture of rubber and sulphur and to vulcanize it.
• Sulphur cross at the reactive sites of double bonds and rubber is stiffened.
• 5% is an example of the ammount of sulphur in the rubber.

Synthetic Rubber

• Any vulcanisable rubber like polymer.
• Capable of stretching to twice its length, but returns to its original form.

Neoprene

• Neoprene or polychloroprene is made of chloroprene.
• It is resistant to the action of petrol, lubricating oil and organic solvents.
• Making of the oil seals and linings etc.

Buna-N

• Buna-N is obtained by copolymerisation of 1,3-butadiene & acrylonitrile.
• Resistant to vegetable and mineral oils.
• Used for belts, gaskets and hoses.

Molecular Mass of Polymers

• Polymer properties relate to molecule mass,size.
• Mass effects availability of mass and length, it is always an average

Biodegradable Polymers

• A number of polymers are quite resistance to degradation.
• Solid wastes cause acute environmental problems and stay for too long.
• New biodegradable synthetic have similar structures to biopolymers

Poly β-hydroxybutyrate - co-β-hydroxy valerate(PHBV)

• Created by copolymerisation
• Used in packaging, orthopaedic applications
• PHBV is naturally degraded when exposed to bacteria.

Nylon 2-Nylon 6

• Copolymer of glycine (H2N-CH2-COOH) and amino caproic acid [H2N (CH2)5 COOH] is biodegradable.
• Used to identify monomers in polymeric structures.

Commercial Importance of Polymers

• A variety of polymers, as shown in Table 15.1, includes polypropylene, polystyrene, polyvinyl chloride, urea-formaldehyde resin, glyptal, and bakelite.

Explore the basics of polymers: their molecular mass range, etymology, and formation. Learn about the industries relying on polymers. Understand the significance of bonds and macromolecular structure.