Polymers

Polymers

1. Definition

   •  Polymers are large molecules formed by smaller molecules called monomers which are joined together by covalent bonds.

2. Word Origin

   • Poly
     • Greek word that mean many.
   • Mer
     • Greek word that mean unit or part.

3. Classification Based on the Source

   • Natural polymers
     • Are found in plants and animals.  Example: Proteins.
   • Semi-synthetic polymers
     • Formed by treating chemically the natural polymers.  Example: Cellulose nitrate.
   • Synthetic polymers
     • These are polymers produced in laboratories or factories. Example: Plastic

4. Classification Based on Structure

   • Linear polymers
     • Polymers that have monomers arranged in large and straight chains.
   • Branched Chain Polymers
     • Polymers that have monomers arranged in linear lines that have some branches.
   • Cross-Linked or Network Polymers
     • These are polymers that have strong covalent bonds between several linear chains.

5. Classification Based on the Way of Polymerisation

   • Addition polymers
     •  These are polymers formed by the repeated addition of monomers that contain double or triple bonds. When a polymer is created from two different monomers by addition polymerisation is known as copolymer.
   • Condensation Polymers:
     • These are polymers created when a condensation reaction takes place between bi-functional or tri-functional monomeric units. Here are remove small molecules such as water.

6. Classification Based on Molecular Force

   • Elastomers
     •  In these polymers, the intermolecular force between monomers is very weak. As a result, this gives the capacity to be stretched. In other words, these are rubbers.
   • Fibers
     • In these polymers, there are strong intermolecular forces such as hydrogen bonds between the monomers. As a result, these polymers have high tensile strength and a high modulus of elasticity.
   • Thermoplastic
     • In these polymers, there are intermediate intermolecular forces between polymers. the forces are stronger than elastomers but less than fibers. As a result of these forces, it has the property that it softens when heated and hardens when cooled.
   • Thermosetting
     • These are cross-linked polymers that get hard when undergoing the molding process, but can´t be softened again. This means that are not reusable.

7. Polymerisation Reactions

   • Addition or Chain Grow Polymerisation
     • In this type, the molecules of monomers (unsaturated compounds) are added to form a polymer. There is an increase in chain length due to the formation of free radicals or ionic species. 
     • Free radical mechanism
       • This mode consists of unsaturated hydrocarbons as alkenes or dienes being polymerised in the presence of a free radical generating initiator (catalyst) to create a free radical. 
       • Process
         • 1. Chain initiating step
           • A free radical is added to the double bond of the unsaturated hydrocarbon and therefore creates a new larger free radical.
         • 2. Chain propagating step
           • As the new radical react with molecules, new bigger radicals are created. This process is repeated several times.
         • 3. Chain terminating step
           • At some point, the resulting product radical interacts with another radical to generate the polymerized product.  
     • Important Addition Polymers
       • Polythene
         • Low-density polythene
           • Is created by the addition polymerisation of ethane in the presence of trace of dioxygen or a peroxide initiator (catalyst). It has a branched structure and is not reactive. Also, it is a poor conductor of electricity is used in the manufacturing of electric wires.
         • High-density polythene
           • Is created when the addition polymerisation is carried out in a hydrocarbon solvent and in the presence of a catalyst such as s triethylaluminium. It has a linear structure and is not reactive. Also has a high density so the molecules are very close. This makes it more tough and hard, and therefore appropriate to produce buckets and bottles from it.
       • Polytetrafluoroethene (Teflon)
         •  Teflon is produced through a process in which tetrafluoroethene is heated with a free radical or persulphate catalyst at high pressure. Is not reactive and resistant to corrosion caused by reagents. Also, it is used for the manufacture of non–stick surface coated utensils.
       • Polyacrylonitrile
         • Is formed in the addition polymerisation of acrylonitrile in presence of a peroxide catalyst. Also, it is used commercially to replace wool.
   • Condensation Polymerisation
     • It is a sequence in which it is applied the division of two bi-functional monomers by condensation.  It results in the loss of simple molecules (water, alcohol…) but leads to the formation of high molecular mass condensation polymers.
     • Important condensation polymerisation reactions
       • Polyamides
         • These are examples of synthetic fibers and are termed as nylons. It is mainly prepared by the condensation polymerisation of diamines with dicarboxylic acids and also of amino acids and their lactams. Nylon is used in making sheets and mainly in textile industries.
       • Polyesters
         • These are the products of dicarboxylic acids and diols. Dacron (terylene) is its best example. It is used in blending with cotton and wool fibers or as reinforcement material in items such as safety helmets.
       • Phenol
         • These are gained by the reaction between formaldehyde and phenol by interacting with an acid or base catalyst. They are the oldest synthetic polymers. They produce some linear products such as Novolac that are used in paints. When they react with formaldehyde, they form bakelite that is used for making electrical switches and handles of various utensils.
       • Melamine
         • These are also formed by the reaction of formaldehyde but in this case with melamine. It is mostly used in the manufacture of unbreakable crockery.
   • Copolymerisation
     • A copolymer is formed by the reaction in which a mixture of more than a single monomeric species polymerise. It contains units of the monomers that participate in the polymeric chain. Their characteristics are different from homopolymers. For example, butadiene (copolymer) can be used for the creation of tires and footwear components due to its similitude to rubber.
   • Rubber
     • Polymer that has elastic properties. It is divided into natural and synthetic. 
     • Types
       • Natural
         • It is termed an elastomer and has plenty of uses. The latex is extracted from the rubber tree which can be seen in countries such as Sri Lanka, and Indonesia… Natural rubber is a linear polymer.  Natural rubber is able to absorb big quantities of water. It becomes soft at high temperatures and brittle at low temperatures. Due to its low resistivity to oxidising agents, natural rubbers pass through a process of vulcanisation in which sulfur forms links that make rubber stiffer.
       • Synthetic
         • When a force is acting on it, it has the capacity of getting stretched twice its length and when the force stops, gets to its initial form. Synthetic rubbers are homopolymers or copolymers.
         • Neoprene
           • Formed by the polymerisation of chloroprene. Used for manufacturing conveyor belts, gaskets, and hoses.
         • Buna - N
           • Formed by the copolymerisation of butadiene and acrylonitrile in the presence of a peroxide catalyst. Used in making oil seals, and tank lining.

8. Molecular Mass

   • The development of the polymer chain in the process of synthesis depends on the availability of monomers in the reaction mixture. The way of determining its molecular mass is depending on physical or chemical methods but it is expressed as a mean of the different lengths and hence in the chain.

9. Biodegradable

   • Polymers that show high resistivity to the environmental degradation processes are responsible for the accumulation of polymeric solid waste materials. Due to the problems created, some biodegradable synthetic polymers have been created. They count with a similar objective to biopolymers.
   • Poly (PHBV)
     • It is an alternative biodegradable polymer that is created by the copolymerisation of hydroxybutanoic acid and hydroxypentanoic acid. PHBV is able to undergo bacteria degradation in the environment so it is used in the controlled release of drugs, packaging, and orthopedic devices.

10. =)

    • Este increible trabajo se merece un 7! =)

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