Polymerization is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks
Classifications of polymers
Polymers can be classified in number of ways
polymers can be classified based upon source are
a)Natural polymers:- these are the naturally occurring polymers such as starch cellulose nucleic acids and natural rubber.natural rubber is a polymer of isoprene(2-methyl buta 1,3-diene) b)Synthetic polymers:- these are man made polymers such as poly vinyl chloride (PVC),Bakelite,nylon,polystyrene etc
based on the method of synthesis polymers can be classified as follows
a)Addition polymers:-these type polymers are formed by the repeated additions of monomeric unit .the monomers involved are unsaturated compounds and are usually the derivatives of ethene
eg:-polypropylene, PVC,polythene,etc belongs in this type
b)Condensation polymers:-these are formed by a series of condensation reaction between two monomers each monomers having two functional group.during condensation simple molecule like H2O,HCl,NH3 etc are eliminated.The reaction is called condensation polymerisation
based on the inter molecular forces polymers can mainly classified in to 4
a)Elastomers :-the polymers that have elasticity, like rubber are referred to as elastomers.the polymer chain in elastomers are held together by the weakest intermolecular forces.they have ability to be elongated under stress and regain their former shapes when the stress is removed
b)Fibers:-these are thread like polymers which can be woven into fabric.they have high tensile strength and high modulus.this is due to strong inter molecular forces like hydrogen bond.
eg:- cotton,wool,silk,nylon and Dacron etc
c)Thermoplastic:-The inter molecular forces of attraction in the thermoplastics polymers are intermediate between those of elastomers and fibres .As a result thermoplastic polymers when heated became melt and can be molded again in different forms.
eg:- polyethylene polypropylene and polystyrene
d)Thermosetting plastic:- These polymers are prepared from low molecular mass semifluid polymers .thy cannot be reshaped on heating because the polymer chain under go extensive cross linking resulting in the formation of three dimensional network of inter connected polymer chain
eg:- Bakelite,melamine etc
Characteristics of Polymers
The majority of manufactured polymers are thermoplastic, meaning that once the polymer is formed it can be heated and reformed over and over again. This property allows for easy processing and facilitates recycling. The other group, the thermosets, cannot be remelted. Once these polymers are formed, reheating will cause the material to ultimately degrade, but not melt.
Every polymer has very distinct characteristics, but most polymers have the following general attributes.
- Polymers can be very resistant to chemicals. Consider all the cleaning fluids in your home that are packaged in plastic. Reading the warning labels that describe what happens when the chemical comes in contact with skin or eyes or is ingested will emphasize the need for chemical resistance in the plastic packaging. While solvents easily dissolve some plastics, other plastics provide safe, non-breakable packages for aggressive solvents.
- Polymers can be both thermal and electrical insulators. A walk through your house will reinforce this concept, as you consider all the appliances, cords, electrical outlets and wiring that are made or covered with polymeric materials. Thermal resistance is evident in the kitchen with pot and pan handles made of polymers, the coffee pot handles, the foam core of refrigerators and freezers, insulated cups, coolers, and microwave cookware. The thermal underwear that many skiers wear is made of polypropylene and the fiberfill in winter jackets is acrylic and polyester.
- Generally, polymers are very light in weight with significant degrees of strength. Consider the range of applications, from toys to the frame structure of space stations, or from nylon fiber in Kevlar, which is used in bulletproof vests. Some polymers float in water while others sink. But, compared to the density of stone, concrete, steel, copper, or aluminum, all plastics are lightweight materials.
- Polymers can be processed in various ways. Extrusion produces thin fibers or heavy pipes or films or food bottles. Injection molding can produce very intricate parts or large car body panels. Plastics can be molded into drums or be mixed with solvents to become adhesives or paints. Elastomers and some plastics stretch and are very flexible. Some plastics are stretched in processing to hold their shape, such as soft drink bottles. Other polymers can be foamed like polystyrene (Styrofoam™), polyurethane and polyethylene.
- Polymers are materials with a seemingly limitless range of characteristics and colors. Polymers have many inherent properties that can be further enhanced by a wide range of additives to broaden their uses and applications. Polymers can be made to mimic cotton, silk, and wool fibers; porcelain and marble; and aluminum and zinc. Polymers can also make possible products that do not readily come from the natural world, such as clear sheets and flexible films.
- Polymers are usually made of petroleum, but not always. Many polymers are made of repeat units derived from natural gas or coal or crude oil. But building block repeat units can sometimes be made from renewable materials such as polylactic acid from corn or cellulosics from cotton linters. Some plastics have always been made from renewable materials such as cellulose acetate used for screwdriver handles and gift ribbon. When the building blocks can be made more economically from renewable materials than from fossil fuels, either old plastics find new raw materials or new plastics are introduced.
- Polymers can be used to make items that have no alternatives from other materials. Polymers can be made into clear, waterproof films. PVC is used to make medical tubing and blood bags that extend the shelf life of blood and blood products. PVC safely delivers flammable oxygen in non-burning flexible tubing. And anti-thrombogenic material, such as heparin, can be incorporated into flexible PVC catheters for open heart surgery, dialysis, and blood collection. Many medical devices rely on polymers to permit effective functioning.
They are produced in resin ducts, which can be seen in the cross-section of certain plants. They are excreted through canals or glands. Plants produce them either naturally or in response to injury.
Resins are oxidized products of various essential oils, very complex and varied in chemical composition. They mostly contain volatile terpenes. Man found the value and uses of resins very long time ago.
- Insoluble in water.
- Soluble in ether and alcohol
- Rich in hydrocarbon materials
- They are inflammable
- They produce sweet smell
- Hard resins
- Oleo resins
- Gum resins
They contain small amount of volatile oil, transparent, brittle. They lack any taste or odour. Examples are
- Copol from the plants of Caesalpinaceae family
- Damars from plants of Dipterocarpaceae and Bursaceae family
- Amber from fossils
- Lacquers from Rhus succedenia
- Shellac from Butea monosperma
They are soft compared to damars. Examples are
- Tupentine from Pinus roxburgii
- Benzoin from Styrax benzoin
- Balsam of Canada balsom
Examples of gum resins are
- Asafoetida from Ferula asafoetida
- Gukkulu from Commiphora mukul
- Resin of Boswelia serrata
- Damars are used in incense sticks
- Shellac is used for preparing moulds and used in forensic science
- Amber is used as flavoring agent and used as bindi
- Asafoetida is used as a carminative in daily food
- Turpentine and benzoin are used as antiseptics
- They are also extensively used in native medicine