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Atoms are basic unit of any matter which are composed of certain sub-atomic particles; electrons, protons and neutrons. These particles are arranged in a certain way and balancing of charges and masses of them provide stability to atom. Out of these three sub-atomic particles, electrons are negatively charged particles whereas protons have positive charge and neutrons are neutral or charge-less.

Electrons are arranged in certain energy levels called as atomic orbitals whereas protons and neutrons are placed at the center of atom and called as nucleus.  Atoms share or transfer their outermost electrons (valence electrons) with same or different atoms to form molecules in which atoms are bonded through chemical bonds.  The formation of chemical bonds provides stability to atoms. Today we know a large number of chemical compounds with different structures and masses.

They can be small like methane or large like polymers. A name suggested, poly stands for many and –mers for repeating units. It means polymers are large molecules which are composed of many small repeating units. These repeating units are called as monomers. Monomers are combined together with certain chemical bonds in a fix pattern to form large molecules. 


Polythene is made of a polymer polyeththylene which is formed by ethylene molecules. So we can say that polyethylene is polymer which is composed of a large number of ethylene molecules. Each ethylene molecule has two carbon atoms bonded with double bond. These monomer units are bonded together through single covalent bonds and the process of formation of polymers from monomer units are called as polymerization. 

On the basis of mechanism, polymerization can be classified as condensation and addition polymerization. For example; polyethylene is formed by addition polymerization of ethylene molecules in which the multiple bond of each monomer unit involves in the formation of polymer. In other words there will be no more multiple bonds in polymer. 

Another type of polymerization is called as condensation polymerization. As name suggested, condensation polymerization involves the elimination of some small molecules specially water molecule during the combination of monomer units. For example hexamethylenediamine combines with adipic acid through condensation process and form Nylon polymer. Each step of combination involves elimination of water molecule and form an amide linkage between carboxylic group and amine group of monomer units. 


On the basis of formation of intermediate as cation, anion and free radicals, polymerization can also classify as;
  • Anionic polymerization
  • Cationic polymerization
  • Free radical polymerization


Polystyrene Definition

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Additional polymerization is the process of polymerization in which monomer units with multiple bonds polymerize through addition reactions. No small molecule eliminate like condensation polymerization. In other words we can say that addition polymerization is only possible with molecule having multiple bonds. It is also called as chain-growth polymerization or chain polymerization.

Monomers like alkenes or other unsaturated molecules with multiple bonds react by addition to their unsaturated double bonds. Here electrons of multiple bonds involves in bonding between two monomer units. For example formation of polyethylene from ethylene is a chain growth polymerization. Since the multiple bonds involve in bonding with other molecules so there will be no more double or triple bond in polymer. These additional polymers can be represented as -[A-A-A-A-A]- because they are formed by same type of monomer units. The arrangement of monomer units determines the physical and chemical properties of chain growth polymers. 

For example highly branched polymer of ethylene is called as Low Density Polyethylene or LDPE. It is manufactured in the presence of peroxide catalysts at a temperature of 500°C and 1000 atm of pressure. Due to high level of branching, polymeric chains do not pack together well that reduces density of polymer. It is a flexible and recyclable plastic which is widely used for packaging and trash bags. 

Another polymer of ethylene is High Density Polyethylene or HDPE which is prepared in the presence of Ziegler-Natta aluminum titanium catalysts. It has very little branching so polymeric chains can be packed closely that increases the density of polymer. It is used for crinkly plastic bags to pack groceries.

Another well known example of chain growth polymer is PVC or polyvinyl chloride which is a polymer of vinyl chloride molecules (H2C=CHCl). It is present in plastic wrap, simulated leather, water pipes, and garden hoses. Polypropylene is also a chain growth polymer which is made of propylene units. Remember in chain growth polymerization the monomer units are always bonded through double or triple bonded carbon atoms and remaining carbon chain will arrange as branch of polymer chain. Some other chain growth polymers are listed below. 

 Monomer   Polymer   Uses 
 F2C=CF2  Polytetrafluoroethylene 
 For non-stick coating, manufacturing  of plastic parts, Gore-Tex 
 H2C=CCl2  Polyvinylidene dichloride 
 For Clinging food wrap
 H2C=CH(CN)   Polyacrylonitrile  As fibers for textiles, carpets, upholstery
 H2C=CH(OCOCH3)   Polyvinyl acetate  As Elmer's glue
 H2C=CH(OH)  Polyvinyl alcohol  For ghostbusters Demo
 H2C=C(CH3)COOCH3  Polymethyl methacrylate  To make stiff plastic sheets, tubing, and other shapes
 Polyisoprene  As rubber
 H2C=CH-CH=CH2  Polybutadiene  As rubber
 Polychloroprene  As cshemically-resistant rubber

One of most important chain growth polymer is polystyrene. Polystyrene can be defined as the polymer of styrene molecules. Styrene is an aromatic compound in which the vinyl group is bonded with aromatic ring. The presence of double bond in vinyl group of styrene is responsible for chain growth polymerization of molecules. The double bonded carbon atoms of vinyl groups bonded with each other to form long polymeric chain of polystyrene as given below. 

Formation of Polystyrene

Polystyrene Structure

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Polystyrene is a recyclable plastic in which the backbone of polymeric chain is composed of benzene rings. A large number of styrene monomer units polymerized together to form long polystyrene chain. Since the terminal molecules have free valance at carbon atom so chain can further extends at both the left and right ends.

Styrofoam is an expanded form of polystyrene in which blow of fine gas bubbles into liquid polystyrene expanded the polymer and solidification produces expanded polystyrene. The cross-linking between polymeric chains can be done with p-divinylbenzene (DVB) which has vinyl groups $(-CH=CH_{2})$ at each end of its molecule that can take part in polymerization with other styrene molecules.

On the basis of branching, the polystyrene can be classified as

Linear polystyrene – This form of polymer can form a solid with over 90% crystallinity. It has a planar zig-zag structure. 
Atactic polystyrene - It is one of the regular structure of polystyrene which does not have any regular order in the position of the aromatic rings along the chain. The random arrangement of polymeric chains does not allow the close packing that effects the chemical and physical properties of polymer. It is amorphous and soft polymer with low melting point. 

Atactic Polystyrene

Syndiotactic polystyrene - In this structure of polystyrene chain, the aromatic rings are arranged on alternate sides of the chain. Due to alternate arrangement of aromatic rings, the polymeric chain can pack easily. It is a rigid, crystalline plastic with high melting point.  Due to close packed structure solvent molecules cannot penetrate the structure. 

Syndiotactic Polystyrene

Polystyrene Properties

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Polystyrene is a rigid, inexpensive, amorphous, glassy polymer. Crystal polystyrene is unfilled polystyrene with sparkle appearance. It is also known as general purpose polystyrene (GPPS). Addition of rubber or any other copolymer produces high impact polystyrene grades with more toughness and impact strength. At temperature below degradation, this polymer possesses good flow properties.  That is the reason it can easily be extruded, injection molded, or compression molded. 

Foamed polystyrene can be produced with the help of some blowing agents. Some other properties of polystyrene are high impact resistance, flame retardant, food contact acceptable, good processability, high heat resistance, expandable, high flow, fast molding cycle and good flow rate. At the same time there are few limitations of this polymer such as poor solvent resistance, brittleness of Homopolymer, poor thermal stability and environmental cracking. Some other properties are listed below.
  • A clear, amorphous, nonpolar commodity thermoplastic.
  • Easy to process.
  • One of the largest volume commodities plastic.
  • Very good electrical insulator.
  • Excellent optical clarity due to the lack of crystallinity.
  • Good chemical resistance to diluted chemicals.
  • Easy to fabricate into a large number of finished goods.
  • A viscose liquid above its glass transition temperature.
  • Attacked by hydrocarbon solvents.
  • Poor oxygen and UV resistance.
  • Poor impact strength due to the stiffness.
  • Can be copolymerized with methylmethacrylate.
Polystyrene uses
Polystyrene is widely used as packaging material, for the manufacturing of sheet, for insulation, manufacturing of toys, household goods, electrical and electronic equipments etc. Some other uses of polystyrene are listed below. 

 1. Packaging One of the most common uses of polystyrene is as packaging material for eggs and dairy products, meat, fish and poultry, cold drinks or carry-out meals as it prevents the spoilage of foods. Polystyrene is a versatile and cost-effective solution for rigid packaging of any material and also for food service disposables.
 2. Appliances Polystyrene resins is used in injection-molding, extrusion and thermoforming applications for refrigerators and air conditioners, ovens, microwaves, vacuum cleaners, blenders tec.
They are safe and cost effective, with easy-processing and excellent appearance. 
 3. Consumer electronics  Polystyrene is one of the major component in almost all IT components like computer, TV, CD’s ad DVD’s as it is the best option with high performance and cost effective.
 4. Construction Polystyrene resins are also used as building materials and for the manufacturing of construction applications such as insulation foam, roofing, siding, panels, bath and shower units, lighting, plumbing fixtures. They have some properties such as price effective, good processability and bets performance so they are most likely used substances.
 5. Medical Polystyrene resins are used for disposable medical applications such as diagnostic components, tissue culture trays, test tubes and petri dishes etc.
 6. Other Polystyrene is also used for variety of consumer goods like toys, electric lawn and garden equipment, kitchen and bath accessories etc.

Environmental Impact of Polystyrene

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To overcome the limitations of polystyrene, many copolymers are added to form different polystyrene derivatives. Some commercially produced derivatives of polystyrene are α-methylstyrene, o-, m-, and p-methylstyrene, divinylbenzene, methoxystyrene, chlorostyrene and p-divinylbenzene. Like other polymers, polystyrene also effects our environment. It is a non biodegradable plastic which exhibits resistant to photolysis.

That is the reason; polystyrene is a major contributor to the debris in the ocean. No doubt it can be recycled but does not recycled in many parts of the world. The expandable polystyrene (EPS) is the major environmental issue as landfills of it takes up large area due to its low density. Therefore now a day food packaging industry has developed alternative insolating plastics such as Versalite that is expanded polypropylene. It can be recycled in the general recycle stream with other polypropylene objects.
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