Polymers - Class 12 Medical Chemistry - Extra Questions

Polymers are macromolecules or gaint molecules having high molecular masses formed by combination of a large number of small molecules called monomers and the process by which these simple molecules i.e., monomers combine to form a polymers is called polymerization. Polythene, nylon, bakalite, PVC etc., are the examples of polymers.

A $$\text {thermoplastic polymer}$$ can be repeatedly softened on heating and hardened on cooling. Hence, it can be used again and again. The examples are polythene, polypropylene, etc. 

On the basis of molecular forces present between the chains of various polymers, the classification of polymers is given as follows.
(a) Elastomers (b) Fibres (c) Thermoplastics and (d) Thermosetting plastics.

Biodegradable polymers are those polymers which get decomposed under aerobic or anaerobic conditions because of action of microorganism, enzymes. 

Polyethylene is an example of addition  polymer. The monomer is ethylene. Thus addition polymerisation of several ethylene units (monomers) give polyethylene.

(i) Increasing order of basic strength is as follows:

Linear polymers: The polymers made up of long continuous chain without any branches are called linear polymers. They have high melting points, high densities and high tensile strength. Example: Polyethene, PVC.
Branched chain polymers: The polymers made up of main chain(linear chain) with smaller chains as branches of main chain are called branched chain polymers. They have lower melting points, densities and tensile strength as compared to linear polymers. Example: Polypropylene.

Addition polymerisation	Condensation polymerisation
Different monomers add to form a polymer with same molecular formula of the repeating structural unit as that of starting monomer.	A large number of monomers combine with the loss of simple molecules (water, ammonia, HCl, alcohol) to form a polymer having molecular formula of the repeating unit different than starting monomers.
No molecule is eliminated during polymerisation	Small molecules like water, ammonia, HCl or alcohol are eliminated during polymerisation.
Usually one monomer is involved.	Two monomers are involved.
Usually unsaturated compounds are monomers.	Usually monomers contain two functional groups.

(a) The type of chemical reaction to which the given reaction belongs is called Polymerisation.

(b) The monomer in the given reaction is Vinyl chloride.

A polymer is a large single chain-like molecule in which the repeating units derived from small molecules called monomers are bound together. The process by which monomers are transformed into a polymer is called polymerisation.

For example ethylene polymerizes to form polyethylene.
$$\displaystyle nCH_2=CH_2 \xrightarrow {\text {polymerisation}} -[-CH_2-CH_2-]_n-$$

Bakelite is formed from formaldehyde and phenol.

It has been observed that polymers can be classified on the basis of different intermolecular forces and so there are four types of polymers depending upon the intermolecular force present in them.

• Elastomers are the polymers that can be stretched like elastics and also will come back to their original shape on releasing the force. They have the weakest intermolecular force (Vander Waal force). For example; Neoprene, Buna-S, Buna-N etc.
• Fibers are solids having thread like structure possessing strong intermolecular force. Due to this strong force of attraction, they have high tensile strength. For example: Nylon 66, Dacron, etc.
• Thermoplastic polymers are those polymers which contain intermolecular forces higher than Elastomers but less than Fibers. They are usually linear chain polymers and can be remolded again. Examples: Polythene, Polyvinyl Chlorides, etc.
• Thermosetting plastics are having highly branched chain molecules with extensive cross linking and if heated becomes infusible and cannot be reused. For example: melamine, Bakelite etc.

As we know that elastomers (Buna-S) have weak intermolecular forces, fibres (Nylon-$$6,6$$) have dipole-dipole interaction, thermoplastics (Polythene) have intermolecular forces intermediate between elastomers and fibres and thermosetting polymers (Bakelite) undergoes permanent change on heating.

 Thiokol is formed  from ethylene chloride [$$ClCH_2-CH_2Cl$$] and sodium polysulphide[$$Na-S-S-Na$$]

Chain growth polymerisation involves the initial formation of a free radical or an ion (from small amount of initiator such as organic peroxide) to which monomers get added up by a chain reaction. Here, the polymers are exact multiples of organic monomeric molecules and have only carbon atoms in their main chain. 

Buna-S < Polystyrene < Terylene

Addition polymers - Polythene, PVC
Condensation polymers - Nylon-6, Nylon-6,6
Copolymers - Buna-S. Buna-N. 

The cis-polyisoprene molecule consists of various chains held together by weak Van der Waals interaction and has a coiled structure. Thus, it can be stretched like a spring and exhibits elastic properties.

The addition polymers formed by the polymerization of a single monomeric species are known as homopolymers,

This is a type of chain growth polymerization.

The polymers made by addition polymerization from two different monomers are termed as copolymers, e.g., Buna-S, Buna-N, etc.

On the basis of source polymers are classified as:

• Natural polymers (Example: Starch, Cellulose, Portions, etc.)
• Synthetic polymers (Example: PVC, -Nylon Teflon, etc.)
• Semi-synthetic polymers (Example.- Rayon, Cellulose nitrate)

Carothers in $$1928$$ classified polymers on the basis of the polymerization process into two groups:
1. Addition polymers
2. Condensation polymers.

1. Addition polymers:
A polymer formed by direct addition of repeated monomer molecules possessing double or triple bond, without the elimination of any small molecule is called addition polymer.
Example: PVC, Teflon, Neoprene, etc.

2. Condensation polymers:
These polymers are formed through a series of independent reactions. These reactions involve the bond formation condensation between two difunctional monomer molecules to form dimers, which in turn, produce tetramers and so on (with the loss of simple molecules like $$NH_3$$,$$H_2O$$, etc.).
Example: Bakelite, Nylon –$$6, 6$$, etc.

Dacron is fromed by reaction of Ethylene glycol and terepthalic acid.

Type of polymers on the basis of molecular forces operating between different polymer chains, the polymers are of the following types: 

• Elastomers: (Example: vulcanized rubber, natural rubber, etc)
• Fibers: (Example: nylon, Orlon, polyesters) 
• Thermoplastics Polymers: (Example: polypropylene, polythene, etc) 
• Thermosetting polymers: (e.g. Bakelite, phenol-formaldehyde resin, etc)

Addition polymers: Addition polymers are formed by repeated addition of monomer molecules possessing double or triple bonds.

Condensation polymers: Condensation polymers are formed by repeated condensation between two different bi-functional or tri-functional monomeric units. In these polymerisation reactions, the elimination of small molecules such as water, alcohol, hydrogen chloride, etc., takes place.

Homopolymer is polymer made of only one type of monomer.

Copolymerization is a polymerisation reaction in which a mixture of more than one monomeric species is allowed to polymerise and form a copolymer.

1. Buna-S is addition and copolymer.

2. Buna-N is an addition and copolymer.

3. Polythene is homopolymer and addition polymer.

4. Nylon-6,6 is a copolymer and condensation polymer.

B. Natural rubber is a natural polymer and it contains long chains (linear polymer).

In $$\text{addition polymerization}$$, the molecules of the same or different monomers add together to form a large polymer molecule. 

All these molecules are expected to be planar.Their stereochemistry is identical to that of the corresponding chloroethanes.Also, (a) can exist as cis and trans isomers and show enantiomeism.Only one compound exists in the case of (b) and (c),
A llenes are compounds whose molecules contain the following double bond sequence.
The planes of $$\pi$$-bonds of allenes are perpendicular to each other.
This geometry of $$\pi$$-bonds causes the groups attached to the end carbon atoms to lie in perpendicular planes.Due to this, allenes with different substitutes on the end carbon atoms are chiral as shown in the figure given below.(allenes do not show cis-trans isomerism).
These two molecules are non-superimposable mirror images of each other and are,therefore,chiral.However , they do not possess a tetrahedral atom with four different groups.

The polymerisation of acetonitryl gives $$\underline{Orion}$$.

In the manufacture of tyre rubber, $$5\%$$  of sulfur is used as a crosslinking agent. The probable structure of vulcanized rubber molecules are depicted below:

Rubbers: In rubber, the polymer chain is together by the weakest intermolecular forces. These weak binding forces permit the polymer to be stretched. A few crosslinks are introduced in between the chain, which helps the polymer to react to its original position after the force is released as in vulcanized rubber.
Plastics: These are the linear or slightly branched long-chain molecules capable of repeatedly softening on heating and hardening on cooling. These polymers possess intermolecular forces of attraction intermediate between elastomers and fibers.

In order to have an idea about the homogeneity of a polymer, we define the term polydispersity index. It is the ratio of mass average molecular mass to the number average molecular mass.

                                    $$PDI=\dfrac{\overline M_w}{M_n}$$

• For natural polymers, PDI is usually equal to one which means that they are monodisperse. Such polymers are more homogeneous.
• Synthetic polymers on the contrary generally have PDI > 1 which means that they were less homogeneous.