What are polymers?
Polymers in chemistry are big molecules made by the repeated joining of simpler molecules by covalent bonding. They are macromolecules and complex molecules having a very high molecular weight of the order of 103 to 106. When two compounds have the same empirical formula but differ in molecular weight, the more complicated compound is called a polymer of a parent monomer. Rubber, protein, cellulose, nitrocellulose, nylon, bakelite, perspex are common examples of polymers. The terms polymer and polymerization are used mainly for high molecular weight compounds. Polymers may be classified into three groups like natural, semi-synthetic, synthetic polymers.
What is Polymerization?
As the investigation of polymerization reaction progressed, it is found that many compounds formed by a large number of monomer molecules with high molecular weight do not contain exactly the same empirical formula of parent monomer. It led to a modification of the definition of polymer and polymerization. According to Carothers (1931) polymerization is defined as intermolecular combinations that are functionally capable of proceeding indefinitely. There are two main types of polymerization processes like addition and condensation polymerization.
Addition polymerization occurs between molecules containing double or triple bonds. In certain cases, it also occurs between bifunctional compounds that have ring structures like ethylene oxide. There is no liberation of small molecules during addition polymerization.
Condensation polymerization occurs when polymers are formed by the combination of monomers with the elimination of simple molecules like water, ammonia, methanol, etc. Terylene, nylon-6,6 are examples of polymers formed by the condensation polymerization process.
Natural polymers are substances that are obtained from naturally occurring materials like living organisms. These are found mainly in nature. Proteins, gums, resins, silk, rubber, cellulose are common examples of natural polymers.
Synthetic polymers are man-made polymers or materials obtained from petroleum oil. Polyethylene, polyvinyl chloride (PVC), Teflon, polystyrene, terylene, nylon, neoprene, styrene-butadiene are examples of synthetic polymers which we used commonly in our daily life. Production and use of some synthetic polymers are given below,
Polyethylene is a synthesis polymer made by polymerization of ethylene. Low-density polyethylene has chemically inert, flexible, and properties of insulation. Therefore, low-density polyethylene is used for making squeeze bottles, toys, flexible pipes, electric wires cover, etc.
High-density polyethylene has thermally stable, tough, and high tensile strength. Therefore, it is used for making bottles, pipes, inner insulation of coax cable, plastic bags, etc.
Nylon or nylon-6,6 is an important synthetic polymer prepared by heating adipic acid with hexamethylene diamine. It can be used widely in textile industries and different types of consumer items.
Neoprene or polychloroprene rubber can be obtained by polymerization of chloroprene. This rubber is resistant to chemical action. Therefore, neoprene can be used for making containers for corrosive chemicals.
Styrene-butadiene rubber is a copolymer made of styrene and 1,3-butadiene. It is also called Bunna-S or SBR. Styrene-butadiene rubber is used mainly in making tires, shoes, gaskets, coated papers, cutting boards, binders in lithium-ion battery electrodes.
Plastics form a group of high polymers which has a fair range of deformability and mouldability. In plastics, polymers formed do not have the same molecule weight and they have different types of applications. The average molecular weight of plastic varies from 20,000 for nylon to several hundred thousand for polyvinyl chloride. Plastics are of main two types like thermoplastic and thermosetting plastics.
- Thermoplastics are linear polymers that are soluble in organic solvents. They are softened on hating and rigid on cooling. Typical examples of thermoplastics are cellulose acetate, nitrocellulose, polyethylene, polyvinyl chloride (PVC), perspex, etc. All of these are used mainly for the manufacture of electrical wires, windows, gutters, pipes, television cabinets, food containers, cosmetics bottles, plastic cups, toys, etc.
- Thermosetting plastics are three-dimensional polymers that are insoluble in any kind of solvent. They can be heat-treated before they are set. Typical examples of thermosetting plastics are phenol formaldehyde, urea formaldehyde, melamine formaldehyde, silicones, etc. Due to thermosetting properties, they are highly useful to produce permanent parts in a wide range of industries. They are used mainly for the production of construction equipment panels, electrical components, insulators, cell tower tops, heat shields, circuit breakers, motor components, etc.
How polymer is made?
Commercially important vinyl polymers like polyethylene, polystyrene, polyvinyl chloride, polyacrylonitrile polymer are made by a chain mechanism. There are three types of growth processes for polymerization like free radical, cationic and anionic processes. The formation of a polymer by cationic, anionic, or free radical mechanism involves normally three steps,
- Initiation process
- Propagation process
- Termination process
The polymerization reaction is initiated with the help of free radicals which are made by using a suitable chemical catalyst or initiator. Tetramethyllead, α-methyl acrylate, benzoyl or substituted benzoyl peroxides, azo and diazo compounds are common catalysts for the initiation process. We also used ultraviolet or x-rays for the initiation step in polymerization. The polymerization reaction producing free radical R· by a catalyst C is represented as C → R·.
The radical R· formed in the chain initiation step adds to the double bond of the vinyl monomer to form the next higher radical. The addition process is continued to produce polymerized radicals.
The chin is terminated when the chin carrier free radicals are destroyed. There are two different possibilities of the chain termination process.
- Free radical recombination: The free radical of a growing polymer chain can react with the free radical of another chain to form either one or two inactive polymer molecules.
- Chain transfer: It is another process that occurs to stop the growth of the chain but does not stop the polymerization process. Therefore, it generates a new polymer.
Molecular Weight of Polymers
Generally, we used osmotic pressure, viscosity, light scattering, and sedimentation methods for the determination of the molecular weight of polymers. The molecular weight of a single polymer will be equal to the molecule weight of repeated units multiplied by the degree of polymerization. But some polymer substances may not contain the same monomers. Therefore, these polymers cannot be assigned definite molecular weights. Hence, we take the average molecular weight of such types of polymers. Two types of average molecular weight are taken in our calculation.
Number Average Molecular Weight of Polymers
The number average molecular weight of the polymer is equal to the weight of the macromolecular substance divided by the total number of moles present.
Mathematically it is defined as, Mn = (n1M1 + n2M2 + n3M3 + …)(n1 + n2 + n3 …). Here n1, n2, n3, … be the number of the moles of particular macromolecules having molecular eight M1, M2, M3, … respectively.
Weight Average Molecule weight of Polymers
The weight average molecular weight of the polymer is defined as,
Mw = (w1M1 + w2M2 + w3M3 + …)/(w1 + w2 + w3 + …)
Here w1, w2, w3, … are the weight of the polymers having molecular weight M1, M2, M3, … respectively.