Online CBSE – NCERT Chemistry Notes for Class 12 Chapter 15 Polymers
A polymer (Greek poly-, “many” + -mer, “parts”) is a large molecule, or macro molecule, composed of many repeated sub units. Polymers are created via polymerization of many small molecules, known as monomers. Their consequently large molecular mass relative to small molecule compounds produces unique physical properties, including toughness, visco elasticity, and a tendency to form glasses and semi-crystalline structures rather than crystals.
Polymers can be classified in number of ways
- Classification based upon source
- Natural polymers:– These polymers are the naturally occurring polymers such as starch, cellulose, nucleic acids and natural rubber.
- Synthetic polymers:- These polymers are man-made polymers, such as polyvinyl chloride (PVC), Bakelite, nylon, polystyrene, etc
- Classification based on synthesis
- Addition polymers: These types of polymers are formed by the repeated additions of monomeric unit; the monomers involved are unsaturated compounds and are usually the derivatives of ethane.e.g.:-polypropylene, PVC,polythene,etc.
- Condensation polymers:-These types of polymers are formed by a series of a condensation reaction between two monomers, each monomer having two functional groups. During condensation simple molecule like H2O, HCl, NH3 are eliminated.The reaction is called condensation polymerization reaction. e.g. nylone66.
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- Classification based on inter molecular forces
- Elastomers:-These polymers have elasticity like rubber is an elastomer. The polymer chain in elastomers is held together by the weakest intermolecular forces. They have the ability to be elongated under stress and regain their former shapes when the stress is removed.
- Fibbers: These are thread-like polymers which can be woven into the fabric. They have the high tensile strength and high modulus. This is due to strong intermolecular forces like hydrogen bonding.e.g.:- cotton, wool, silk, nylon, and Dacron etc.
- Thermoplastic: The molecular forces of attraction in the thermoplastics polymers are intermediate between those of elastomers and fibers. As a result, thermoplastic polymers, when heated melt and can be moulded again in different forms. Example:- polyethylene, polypropylene, and polystyrene.
- Thermosetting plastic:- These polymers are prepared from low molecular mass semifluid They cannot be reshaped on heating because the polymer chain undergoes extensive cross-linking resulting in the formation of a three-dimensional network of the interconnected polymer chain. e.g.:- Bakelite, melamine etc.
- Depending on the repetition or variety of monomers, polymers are classified as:
- Homopolymer– When the polymer is formed by the same monomer throughout its chain.
- Copolymer – When the polymer is formed by at least 2 different monomers along the entire chain.
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The chemical reaction in which high molecular mass molecules are formed from monomers is known as polymerization. There are two basic types of polymerization, chain-reaction (or addition) and step-reaction (or condensation) polymerization.
- Chain-Reaction Polymerization
One of the most common types of polymer reactions is chain-reaction (addition)polymerization. This type of polymerization is a three-step process involving two chemical entities. The first, known simply as a monomer, can be regarded as one link in a polymer chain. It initially exists as simple units. In nearly all cases, the monomers have at least one carbon-carbon double bond. Ethylene is one example of a monomer used to make a common polymer.
The other chemical reactant is a catalyst. In chain-reaction polymerization, the catalyst can be free-radical peroxide added in relatively low concentrations. A free-radical is a chemical component that contains a free electron that forms a covalent bond with an electron on another molecule. The formation of a free radical from organic peroxide is shown below:
In this chemical reaction, two free radicals have been formed from the one molecule ofR2O2. The different steps in polymerization reaction are:
Step 1: Initiation
The first step in the chain-reaction polymerization process, initiation, occurs when the free-radical catalyst reacts with a double bonded carbon monomer, beginning the polymer chain. The double carbon bond breaks apart, the monomer bonds to the free radical, and the free electron is transferred to the outside carbon atom in this reaction.
Step 2: Propagation
The next step in the process, propagation, is a repetitive operation in which the physical chain of the polymer is formed. The double bond of successive monomers is opened up when the monomer is reacted to the reactive polymer chain. The free electron is successively passed down the line of the chain to the outside carbon atom.
This reaction is able to occur continuously because the energy in the chemical system is lowered as the chain grows. Thermodynamically speaking, the sum of the energies of the polymer is less than the sum of the energies of the individual monomers.Simply, we can say that the single bonds in the polymeric chain are more stable than the double bonds of the monomer.
Step 3: Termination
Termination occurs when another free radical (R-O.), left over from the original splitting of the organic peroxide, meets the end of the growing chain. This free-radical terminates the chain by linking with the last CH2ͦ component of the polymer chain. This reaction produces a complete polymer chain. Termination can also occur when two unfinished chains bond together.Other types of termination are also possible.
This exothermic reaction occurs extremely fast, forming individual chains of polyethylene often in less than 0.1 seconds. The polymers created have relatively high molecular weights. It is not unusual for branches or cross-links with other chains to occur along the main chain.
- Step-Reaction Polymerization
Step-reaction (condensation) polymerization is another common type of polymerization. This polymerization method typically produces polymers of lower molecular weight than chain reactions and requires higher temperatures to occur. Unlike addition polymerization, step-wise reactions involve two different types of di-functional monomers or end groups that react with one another, forming a chain. Condensation polymerization also produces a small molecular by-product (water, HCl, etc.). Below is an example of the formation of Nylon 66, a common polymeric clothing material, involving one each of two monomers, hexamethylenediamine, and adipic acid, reacting to form a dimer of Nylon 66.
At this point, the polymer could grow in either direction by bonding to another molecule of hexamethylenediamine or adipic acid, or to another dimer. As the chain grows, the short chain molecules are called oligomers. This reaction process can, theoretically, continue until no further monomers and reactive end groups are available. The process, however, is relatively slow and can take up to several hours or days. Typically this process breeds linear chains that are strung out without any cross-linking or branching unless a tri-functional monomer is added.
Polymer Chemical Structure
The monomers in a polymer can be arranged in a number of different ways. As indicated above, both addition and condensation polymers can be linear, branched, or cross-linked. Linear polymers are made up of one long continuous chain, without any excess appendages or attachments. Branched polymers have a chain structure that consists of one main chain of molecules with smaller molecular chains branching from it. A branched chain-structure tends to lower the degree of crystallinity and density of a polymer. Cross-linking in polymers occurs when primary valence bonds are formed between separate polymer chain molecules.
Chains with only one type of monomer are known as homopolymers. If two or more different type monomers are involved, the polymer is known copolymer can have several configurations or arrangements of the monomers along the chain. The four main configurations are depicted below:
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