Polymer

Polymers are macro-sized molecules of relatively high molecular mass, which find extensive use in our daily life.

Polymers are large but single chain-like molecules in which the repeating unit derived from small molecules called monomers are covalently linked.

Structurally, they are characterized by many repeating molecular units which form linear chains or a cross-linked network.

Common examples of materials made from polymers are plastic dishes, cups, non-stick pans, automobile tyres, plastic bags, rain coats, television and computer cabinets, flooring materials and materials for biomedical and surgical operations.

In this chapter polymers are classified according to their structure, strength and their properties. We will also be studying about their applications.

Some Important Terms

Polymer

Polymer is a compound of high molecular mass formed by a combination of small molecules called monomers.

Monomers

Monomers are small units, which constitute the repeating units in polymers.

Polymerization

The process by which monomers are transformed into polymers is called polymerization.

Classification of polymers based on source of availability

Polymers are classified based on their source of availability as follows:

1. Naturally occurring polymers

2. Semi synthetic polymers

3. Synthetic polymers

Natural Polymers

These occur in nature in plants and animals and are very essential for life. For e.g., proteins constitute much of the animal body., nucleic acids control heredity at molecular level and cellulose provides food, clothing and shelter.

Examples: Starch, cellulose, protein, silk, wool and natural rubber are some natural polymers.

Semi Synthetic Polymers

Semi synthetic polymers are derived from naturally occurring polymers by chemical modifications.

Examples: Vulcanized rubber, gun cotton and cellulose diacetate.

Vulcanized rubber is used in making tyres. Gun cotton which is cellulose nitrate is used in making explosives. Cellulose on acetylation with acetic anhydride in the presence of sulphuric acid forms cellulose diacetate used in making threads and materials like films, glasses etc.

Synthetic Polymers

Synthetic polymers are man-made polymers which include fibers like teflon and dacron, synthetic rubbers, plastics and PVC.

Classification of polymers based on mode of polymerisation

Based on the mode of polymerization, polymers are classified as:

1. Homopolymers and copolymers

2. Addition and condensation polymers

Homopolymers and Copolymers

Homopolymer

Polymers formed from one kind of monomer is a homopolymer.

Example: Polyethylene

Copolymer

Polymer formed from more than one kind of monomer unit is a copolymer or mixed polymer.

Example: Buna-S or styrene-butadiene rubber is a copolymer formed from styrene and butadiene.

Note:

Biopolymers include various biomolecules such as carbohydrates, proteins etc.

Addition and Condensation Polymers

Addition polymers

Addition polymers are formed when monomer units are separately added to form long chains without elimination of any by-product molecules. These polymers are formed by reactions between monomer molecules possessing multiple bonds.

Example: Ethylene undergoes polymerization to form polythene.

The empirical formula of the monomer and polymer are the same.

Other examples:

Polypropylene is an addition polymer of propylene.

Styrene-butadiene rubber is an addition polymer formed by addition reactions between butadiene and styrene.

Condensation polymers

Condensation polymers are formed when the monomers containing active functional groups (generally two), which react together with the elimination of a small molecule like water, ammonia, alcohol etc.

Examples: Nylon-66, polyester, bakelite etc.

Nylon-66 is formed by condensation between hexamethylene diamine and adipic acid as shown below:

Classification of polymers based on molecular force

Elastomers

Elastomers are polymers in which the polymer chains are held by weakest intermolecular forces. These forces permit the polymers to be stretched. A few cross links are introduced between the chains to help the polymer retract to its original position after the force is released.

Example: Vulcanized rubber

Fibers

These polymers possess high tensile strength and high modulus, because of strong intermolecular forces like hydrogen bonding which operate in polyamides. These strong forces also lead to close packing of chains and thus impart crystalline nature. As a result these polymers show sharp melting points. These polymers are used for making fibers.

Examples: Nylon and terylene

Thermoplastics

In thermoplastics, the intermolecular forces are intermediate between elastomers and fibers and the polymer chain has no cross-links.

Thus thermoplastics can be moulded on heating. These polymers have no cross-linking between chains.

Examples: Polyethylene, polystyrene etc.

Thermosetting polymers

Thermosetting polymers made from relatively low molecular mass semi-fluid polymers which when heated in a mould forms an insoluble hard mass which is infusible. This is due to extensive crosslinks between the different polymer chains forming three dimensional network of bonds.

Example: Bakelite and melamine

General method of polymerisation

Two major methods generally used for preparing polymers are:

a) Addition polymerization

b) Condensation polymerization.

Addition Polymerization

When the molecules of same monomer or different monomers simply add together to form a polymer,the process is called addition polymerization. The monomers commonly used are unsaturated compounds such as alkenes, alkadienes and their derivatives. This mode of polymerization can take place through formation of either radicals or ionic species such as carboanions and carbocations. This process is also called chain growth polymerization. It involves the addition of monomer units of the growing chain by a chain mechanism. At each stage a reactive intermediate is produced for use in the next stage of growth of the chain.

Addition polymerization reactions are of two types. They are:

a) Free Radical and

b) Ionic Addition Polymerization.

Free radical addition polymerization

For chain growth polymerization to occur by a radical mechanism, a radical initiator must be added to the monomer to convert it into a radical. The initiator breaks homolytically into radicals and each radical adds to an alkene monomer, converting it into a radical. The radical site reacts with another monomer which further reacts and thus the process keeps on going, propagating the chain. The process is repeated over and over.

t - Butyl peroxide is a commonly used initiator because it decomposes under mild conditions to form t - butoxide radical.

The free radical formed adds to a monomer molecule to form a new free radical of larger size.

Vinyl polymerizations

Most of the commercial addition polymers are vinyl polymers obtained from alkenes and their derivatives.

This type of polymerization is performed by heating the monomer with only a very small amount of the initiator or by exposing the monomer to light. The process of polymerization starts with the addition of the radical formed from the initiator to the alkene double bond generating a new radical and these steps are called chain initiating steps.

As this radical reacts with alkene, another bigger sized radical is generated.

The repetition of this sequence with new and bigger radicals propagates the reaction and these steps are termed as chain propagating steps.

Ultimately, at some stage the product radical thus formed reacts with another radical to form the polymeric product. This step is called chain-terminating step. One mode of termination of chain is shown below.

In vinylic polymerizations, various other reactions of free radicals with some other compounds present may compete with the parent addition chain reactions. One such reaction takes place with molecules that can react with the growing chain to interrupt the further growth of the original chain. But again the product of such a reaction may initiate its own chain growth. This leads to the lowering of the average molecular mass of the polymer. Such reagents are called chain transfer agents and include carbon tetrachloride, carbon tetrabromide etc.

For e.g., in the presence of tetrachloride, styrene polymerizes to form polystyrene of a lower average molecular mass which also contains some chlorine.

The growing polystyrene radical which normally would add on to a monomer reacts with the chain transfer agent to end the original chain and produces a new radical. The latter initiates a new polymerization chain and thereby forms a new polymer as shown below.

If the chain transfer agent forms a radical which is highly unreactive, the reaction chain gets terminated. Such a compound thus inhibits polymerization. Many amines, phenols, quinines act as inhibitors. So even traces of certain impurities which can act as chain transfer agent or an inhibitor can interfere with the original polymerization chain reaction. Hence the monomers should be free from such inhibitors.

Formation of polythene is an example of vinylic polymerization.

Polyethene formation

It is obtained by polymerization of ethylene under a high pressure of 1000 to 2000 atmospheres at a temperature of 350 to 570 K in the presence of traces of oxygen or a peroxide which initiates the polymerization. The mode of formation is shown below:

The polymer obtained by this procedure of free radical addition and H atom abstraction has a highly branched structure and is called Low Density Polythene (LDP).

LDP is chemically inert, tough but flexible and a poor conductor of electricity. Hence it is used in insulation of electric wires, manufacturing of squeeze bottles, toys and flexible pipes.

When polymerization of ethylene is carried out in the presence of a catalyst at about 330 to 350 K, at atmospheric pressure, the polymer obtained has a linear structure and is called high density polythene. It is also chemically inert but relatively tough and hard with high tensile strength and is used in the manufacture of containers, house wares, bottles, pipes etc.

Conjugated diene polymerization

1, 3 butadiene, a conjugated diene can be polymerized like a simple alkene but there are two modes by which this process can take place:

1, 4 Polymerisation

When the polymerization takes place at C₁ and C₄ of butadiene, an unbranched polymer is formed. This product is different from that formed from an alkene in having a double bond which at each of its carbons is substituted by different groups. Due to this the product can exist either as trans-polybutadiene or cis-polybutadiene or a mixture as shown below:

1, 2 - Polymerization

1, 3 - butadiene can also undergo polymerization at C₁ and C₂ to yield the polymeric product poly vinyl polythene.

The double bonds in these initial polymers can be linked by further treatment with chemicals to modify the properties of the polymers.

Ionic addition polymerization

Vinylic monomers can undergo addition polymerization through the formation of ionic intermediates instead of free radicals. Here the initiator is an ion source and not a free radical source. Ionic polymerization may be:

a. Cationic addition polymerization

When the initiator is cationic in nature, on addition to the double bond, it would generate a cationic intermediate for propagating the addition chain process and is called cationic addition polymerization.

The process is initiated by an acid, which adds on the double bond to form a cation.

Chain initiation step

On further addition to the double bond, a bigger cation is formed and the sequence of steps propagates the chain to the polymeric cation.

Chain propagating step

Chain termination step

The chain termination can take place by loss of the proton.

Cationic polymerization is facilitated in monomers containing electron releasing groups.

Thus isobutylene undergoes cationic polymerization easily as it has two electron releasing - CH₃groups that can stabilize the intermediate cation.

b. Anionic addition polymerization.

An anionic initiator will generate a carbanionic intermediate and thus the polymerization is of anionic addition type. Here the active centre of the propagating species is negatively charged. Hence it occurs easily with monomers containing electron withdrawing groups such as phenyl, nitrile etc., which are able to stabilize the propagating species.

Initiation can be brought about by reagents such as n-butyl lithium or potassium amide. In the initiation step, the base adds to a double bond to form a carbanion.

In the chain propagation, this carbanion adds to the double bond and the process repeats to form a polymeric carbanion.

The chain reaction can be terminated by addition of an acid.

The formation of polystyrene from styrene in the presence of potassium amide is another example of this type of polymerization.

Copolymerization

If a mixture of more than one monomeric species is allowed to polymerise, a copolymer is formed and it contains multiple units of each monomer used in the same polymeric chain.

The composition of the polymer depends not only on the proportion of the monomers but also on their reactivity. Some monomers as such do not polymerize at all but copolymerize. Maleic anhydride does not polymerize but copolymerizes with styrene in a highly symmetrical manner to form styrene maleic anhydride copolymer.

Examples of copolymers are styrene butadiene rubber.

Chain Growth Polymerization

Step Growth Polymerization

Polyolefins

Polydienes

Polydienes are polymers that have double bonds in their polymer chains. They are obtained from dienes. Some examples are given below:

Poly halo olefins

Commercially Important Condensation Polymers

Polyesters

Polymers with ester linkage are called polyesters.

Terylene or Dacron

Terylene or Dacron is manufactured from ethylene glycol and terephthalic acid. The reaction is carried out at 420 - 460 K in the presence of catalyst zinc acetate and antimony trioxide.

Preparation

Properties

Terylene forms strong fibers. It is crease resistant, has high moisture absorption and has a high tensile strength.

Uses

i) Making wash and wear garments.

ii) In seat belts and sails.

Glyptal or Alkyl Resin

Preparation

Property

It is a thermoplastic. It dissolves in suitable solvent and the solution on evaporation leaves a tough but non flexible film.

Uses

It is used in the manufacture of paints and lacquers.

Polyamides

Polyamides are polymers with amide linkage (-NH-CO-).

Nylon-66

In Nylon-66 both monomers have 6 carbon atoms each and hence the name.

Preparation of Nylon-66

Here the polyamide Nylon-66 is formed by heating the reactant mixture under pressure and the process has been developed so that the molecular mass of the polymer is controlled in the range of 12,000 to 20,000 amu.

Properties of Nylon-66

High tensile strength, tough, abrasion resistant and elastic.

Uses of Nylon-66

It is fabricated into sheets, bristles for brushes and in textiles as crinkled nylon fibers that are used for making elastic hosiery.

Nylon-6 or (perlon)

Preparation

Nylon-6 is prepared from the monomer caprolactum which is obtained from cyclohexane (petrochemical). Since caprolactum is more easily available, it is used for polymerization which is carried out in the presence of water that first hydrolyses the caprolactum to amino acid. Subsequently the amino group of the amino acid can react with caprolactum to form the polyamide polymer.

Filaments of Nylon-6 are obtained by melt-spinning of the polymer. The fibers are cooled by a stream of air.

Uses of Nylon-6

i) Nylons are insoluble in common solvents, have good strength and absorb little moisture.

ii) It is used for tyre cords, fabrics, ropes, carpets and manufacture of garments.