Polymers Natural and Synthetic

POLYMER NATURAL AND SYNTHETIC

 

The word ‘polymer’ is coined from two Greek words: poly means many and mer means unit or part. The term polymer is defined as very large molecules having high molecular mass (103-107u). These are also referred to as macromolecules, which are formed by joining of repeating structural units on a large scale. The repeating structural units are derived from some simple and reactive molecules known as monomers and are linked to each other by covalent bonds. This process of formation of polymers from respective monomers is called polymerisation. The transformation of ethene to polythene and interaction of hexamethylene diamine and adipic acid leading to the formation of Nylon 6, 6 are examples of two different types of polymerisation reactions.

 

Monomers

 

The unit molecules that combine with each other to to form a polymer is called a monomer.

 

Anionic and cationic polymerization

Anionic polymerization is a chain growth reaction that initiates with a nucleophilic species, typically an anion or a negatively charged species. Various types of initiators can be used in anionic polymerization. The polymerization process occurs in three main steps: initiation, chain propagation, and chain termination.

1.     Initiation: The polymerization reaction starts with the formation of an active species, usually an anion, which acts as a nucleophile. This nucleophilic species attacks the double bond of the monomer, leading to the initiation of the polymerization process.

2.     Chain Propagation: During chain propagation, the active species generated in the initiation step reacts with additional monomer molecules. This results in the repetitive addition of monomer units to the growing polymer chain, extending its length. The polymerization continues until there are no more monomers available or until chain termination occurs.

3.     Chain Termination: Chain termination can happen through various mechanisms. It involves the deactivation of the growing polymer chain and the formation of the final polymer product. Common chain termination processes in anionic polymerization include combination reactions, disproportionation reactions, or reaction with terminating agents.

 

 Cationic polymerization is a type of chain growth polymerization that involves the initiation of the reaction by a cation or a positively charged species. It can be categorized as a chain growth polymerization mechanism. The process begins with a cation transferring its charge to a monomer, resulting in the formation of a more reactive species known as a carbocation. This carbocation then reacts with additional monomer molecules, leading to the growth of a polymer chain.

In cationic polymerization, the choice of monomers is limited compared to other polymerization methods. Monomers that contain electron-donating substituents or heterocycles are typically suitable for cationic polymerization. This is because these groups enhance the stability of the formed carbocations and promote the initiation and propagation steps of the polymerization reaction.

During the reaction, the reactive monomer with the carbocation undergoes repetitive additions of monomer units, extending the polymer chain. The polymerization process continues until all available monomers are consumed or until chain termination occurs.

Cationic polymerization has various applications and is commonly used to produce polymers such as polyvinyl chloride, polyvinyl acetate, and polyolefins like polypropylene and polyethylene. It offers advantages such as the ability to polymerize monomers with low reactivity and the potential to control the molecular weight and microstructure of the resulting polymer.

Overall, cationic polymerization is a chain growth polymerization mechanism initiated by cations, leading to the formation of reactive carbocations that drive the polymerization process with specific monomer types.

 

 Classifications of polymers

 

On the basis of source of availability, polymers can be divided into the following types:

 

  • Natural polymers

These are the polymers that exist in nature,i.e., are found in plants and animals.

For example: Starch, cellulose, rubber, silk.

 

  • Semi-synthetic polymers

These are polymers that are prepared by making some modification in natural polymers by artificial means, in laboratory.

 

For example: Rayon, vulcanised rubber, gun cotton.

  • Synthetic polymers

These are the man-made polymers,i.e., the polymers that are prepared in laboratory.

 

For example: Bakelite, Teflon, PVC, polystyrene, nylon.

On the basis of structure, polymers can be categorized into the following types:

 

·        Linear polymers: These are the polymers in which the monomer units are linked to one another to form long and straight chains.
These chains are closely packed in space which causes the linear polymers to have high densities, tensile strength and high melting and boiling points.

For example:  High density polyethene (HDPE), PVC, nylon, polyester.

·        Branched chain polymers: These are the linear chain polymers having some branches.

Branching causes these polymers to be loosely packed in space due to which thay have low densities, low tensile strength as well as low melting and boiling points. For example: Low density polyethene (LDPE), amylopectin, glycogen.

·        Cross-linked or network polymers:  These are the polymers formed of various linear polymers connected to each other by strong covalent bonds.

These are polymers hard, rigid and brittle.

For example: Bakelite, formaldehyde polymer, glyptal, melamine-formaldehyde polymer.

On the basis of mode of polymerization, polymers are of following two types:

·        Addition polymers: These are the polymers formed by the repeated addition of monomer molecules containing multiple bonds.

 

Homopolymers: These are the polymers derived from the polymerisation of only one kind of monomers.

For example:

 

Copolymers: These are the polymers obtained by the polymerisation of two or more kind of monomers.
For example:

 

       Condensation polymers: These polymers are formed by the repeated condensation reaction of different bifunctional or trifunctional monomers, with the elimination of small molecules like H2O,HCl, CH3OH. 

For example:

 

On the basis of the molecular forces, the polymers can be classified as:

·        Elastomers: These polymers are held together by weak van der Waals, forces and have low elasticity. For example: Buna-S, buna-N, neoprene.

·        Fibres: These are the polymer held together by strong hydrogen bonds. They have high tensile strength and sharp melting point. For example: Nylon, polyster, silk, wool, orlon, rayon.

·        Thermoplastics polymers: They are the polymers having intermolecular forces of attraction intermediate between elastomers and fibers. They can be made soft and remoulded by heating. For example: Polythene, PVC, polystrene, polypropene.

·        Thermosetting polymers: These are the hard, rigid, cross-linked polymers. They cannot be remoulded once set into a desired shape.  For example: Melamine, and bakelite.

Types of Polymerisation Reactions

There are two general methods of polymerisation:

(i) Addition polymerisations or chain growth polymerisation

(ii) Condensation polymerization or step growth polymerization

(iii) Copolymerisation

 

Addition Polymerisation or Chain Growth Polymerisation

Molecules of the same monomer or different monomers simply add together to form a polymer.

The monomers used are mainly the unsaturated compounds.

 

Mechanism of Addition Polymerisation

Free radical mechanism

It involves formation of reactive intermediate such as free radical, a carbocation or a carbanion.

This is a three step process:

(i) Chain initiating step

(ii) Chain propagating step

(iii) Chain terminating step.

For example, the ethene is converted to polythene by free radical polymerization as follows:

o Chain initiating step:

 

o Chain propagating step

 

o Chain terminating step

 

Some Important Addition Polymers

Polythene

The polythene is of two types:

  • Low density polyethene (LDPE)
  •  High density polyethene (HDPE)

 

Low density polythene

It is obtained by polymerization of ethene at 350 to 750 K and 1000 to 2000 atm pressure.

It is chemically inert and tough but flexible and a poor conductor of electricity.

It is used in the insulation of electricity carrying wires and manufacture of squeeze bottles, toys and flexible pipes.

 

High density polythene

It is obtained by the addition polymerisation of ethene at 330 to 350 K at atmospheric pressure.

It is tough and hard with high tensile strength.

It is used in the manufacture of plastic containers, house wares, pipes.

 

Polytetrafluoroethene (Teflon)

• It is obtained by the free radical polymerisation of tetrafluoroethene at high pressures.

 

• It is chemically inert and resistant to attack by corrosive reagents.

• It is used in the manufacture of oil seals and gasket and non-stick kitchen wares.

 

Polyacrylonitrile

• It is obtained by the addition polymerisation of acrylonitrile in presence of a peroxide catalyst.

 

• It is used in the formation of substitute for wool as orlon or acrilan.

 

Condensation Polymerisation or Step Growth Polymerisation

Such polymerisation involves a repetitive condensation reaction between two bi-functional monomers.

It occurs in a stepwise manner with elimination of some smaller molecules like H2O, NH3, HCI, ROH, etc., therefore it is also named as step Growth Polymerisation.

For example: Dacron is obtained by the condensation polymerization of ethylene glycol and terephthalic acid.

 

 

Polyamides

These polymers possessing amide linkages are important examples of synthetic fibres and are termed as nylons. The general method of preparation consists of the condensation polymerisation of diamines with dicarboxylic acids and also of amino acids and their lactams.

 

Nylons

Nylon 6,6

It is prepared by the condensation polymerisation of hexamethylenediamine with adipic acid under high pressure and at high temperature.



Nylon 6, 6 is used in making sheets, bristles for brushes and in textile industry.

 

Nylon 6

It is obtained by heating caprolactum with water at a high temperature.

 



 

Nylon 6 is used for the manufacture of tyre cords, fabrics and ropes.

 

Polyesters

 These are the polycondensation products of dicarboxylic acids and diols. Dacron or terylene is the best known example of polyesters. It is manufactured by heating a mixture of ethylene glycol and terephthalic acid at 420 to 460 K in the presence of zinc acetate-antimony trioxide catalyst as per the reaction given earlier. Dacron fibre (terylene) is crease resistant and is used in blending with cotton and wool fibres and also as glass reinforcing materials in safety helmets, etc.

 

Phenol – formaldehyde polymer

Phenol - formaldehyde polymers are the oldest synthetic polymers. These are obtained by the condensation reaction of phenol with formaldehyde in the presence of either an acid or a base catalyst.

 

Bakelite

Novolac on heating with formaldehyde undergoes cross linking to form an infusible solid mass called bakelite. It is used for making combs, phonograph records, electrical switches and handles of various utensils.

Novolac

The reaction starts with the initial formation of o-and/or p-hydroxymethylphenol derivatives, which further react with phenol to form compounds having rings joined to each other through –CH2 groups. The initial product could be a linear product – Novolac used in paints.



Melamine — formaldehyde polymer

• It is obtained by the condensation polymerisation of melamine and formaldehyde.

 

• It is used in the manufacture of unbreakable crockery.

 

Elastomers

These are rubber – like solids with elastic properties. In these elastomeric polymers, the polymer chains are held together by the weakest intermolecular forces. These weak binding forces permit the polymer to be stretched. A few ‘crosslinks’ are introduced in between the chains, which help the polymer to retract to its original position after the force is released as in vulcanised rubber. The examples are buna-S, buna-N, neoprene, etc.

 

 

Polymer Fibres

Fibres are the thread forming solids which possess high tensile strength and high modulus. These characteristics can be attributed to the strong intermolecular forces like hydrogen bonding. These strong forces also lead to close packing of chains and thus impart crystalline nature. The examples are polyamides (nylon 6, 6), polyesters (terylene), etc.

 

 

Thermosetting and thermoplastic polymer

Thermoplastics

Polymers that flow when heated; therefore, simply reshaped and recycled. This property is due to presence of long chains through limited or no crosslinks. In a thermoplastic material the very long chain-like molecules are held together via comparatively weak Vander Waals forces. When the material is heated the intermolecular forces are weakened so that it becomes soft and flexible and eventually, at high temperatures, it is a viscous melt (it flows). When the substance is permitted to cool it solidifies once more. For example, polyethylene (PE), polypropylene (PP), poly (vinyl chloride) (PVC), polystyrene (PS), poly (ethylene terephthalate) (PET), nylon (polyamide), unvulcanized natural rubber (polyisoprene)

Thermosetting

Decompose when heated; therefore, can't be reformed or reprocessed. Presence of extensive crosslinks between long chains induces decomposition upon heating and renders thermosetting polymers brittle. A thermosetting polymer is created through a chemical reaction that has 2 stages. The 1st stage consequences in the formation of long chain-like molecules similar to those present in thermoplastics, but still able of additional reaction. The second stage of the reaction (crosslinking of chains) takes place during moulding, generally under the application of heat and pressure. During the 2nd stage, the long molecular chains have been interlinked via strong covalent bonds so that the substance can't be softened again through the application of heat. If excess heat is applied to such materials they will char and degrade.

For example, epoxy, unsaturated polyesters, phenol-formaldehyde resins, vulcanized rubber. 

    Table: Characteristics of Thermoplastic and Thermosetting Polymers

 

Copolymerisation

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

Copolymer contains multiple units of each monomer.

For example: 1, 3-Butadiene and styrene can undergo copolymerization to form butadiene –styrene copolymer.

Rubber

Rubber is a versatile material that exhibits elastic properties, allowing it to stretch and then return to its original shape. It is commonly used in various applications due to its unique characteristics.

Rubber can be classified into two main types: natural rubber and synthetic rubber.

 

Natural rubber

It is a natural polymer possessesing elastic properties.

It is a linear 1, 4-polymer of isoprene (2-methyl-1, 3-butadiene).

 

Drawbacks of Natural Rubber:

•   It becomes soft at high temperature and brittle at low temperatures.

•   It is non-resistant to the attack of oxidizing agents.

 

Vulcanisation of rubber

The process of adding sulphur to rubber to improve its physical properties is called vulcanisation of rubber.

 

Vulcanisatlon is carried out by adding sulphur (3-5%) and zinc oxide to the rubber, and then heating the object at about 110°Cfor about 20-30 minutes.

Zinc oxide accelerates the rate of vulcanisation

Sulphur forms cross links at the reactive sites of double bonds and thus the rubber gets stiffened.

Thus about 5% sulphur is used for making tyre rubber and 30% of it for making battery case rubber.           

The improved properties of vulcanised rubber are:

(i) High elasticity.

(ii) Low water-absorption tendency

(iii) Resistance to oxidation.

 

Synthetic rubbers

Synthetic rubber is any vulcanisable rubber like polymer, which is capable of getting stretched to twice its length. However, it returns to its original shape and size as soon as the external stretching force is released. Thus, synthetic rubbers are either homopolymers of
1, 3 - butadiene derivatives or copolymers of 1, 3 - butadiene or its derivatives with another unsaturated monomer.

 

Neoprene

     o  It is a homopolymers of chloroprene (2-Chloro-1,3-butadiene).

 

     o  Uses: It is used for manufacturing conveyor belts, gaskets and hoses.

 

Buna – N

     o  It is a copolymer of 1,3-Butadiene with acrylonitrile (Buns-N) carried out in the presence of a peroxide catalyst.

 

      o  Uses: It is used in making conveyor belts and printing rollers.

 

Molecular Mass of Polymers

Biodegradable Polymers

These are the synthetic polymers that are hydrolysed by enzymes and to some extent degrade by oxidation.

These polymers contain functional groups similar to the functional groups present in biopolymers.

 

Poly β-hydroxybutyrate – co-β-hydroxy valerate (PHBV)

•   Preparation: It is a copolymer of 3-hydroxy butanoic acid and 3-hydroxy pentanoic acid in which the monomeric units are connected by ester linkages.

 

•   Uses: It is used in packaging, orthopaedic devices and even in controlled drug release.

 

Nylon 2–Nylon 6

It is a copolymer of (H2N–CH2–COOH) and amino caproic acid [H2N (CH2)5 COOH]  and is biodegradable.

 

Polymers of Commercial Importance 

Some Commercially Important Polymers, their monomers and uses are given below:

Polymer

Monomer

Uses

Polyvinyl chloride (PVC)

CH2=CHCl

Vinyl Chloride

Manufacture of rain coats, hand bags, vinyl flooring, and water pipes.

Polyethylene

CH2=CH2

Ethylene

Manufacture of coats, wire insulators, bags

Polystyrene

CH2=CH‒C6H5

Vinylbenzene (Styrene)

As insulator, wrapping material, manufacture of toys, radio and television cabinets

Polyvinyl acetate (PVA)

CH2=CH‒OCOCH3

Vinyl acetate

Making latex, paint

BUNA rubber

CH2=CH‒CH=CH2

1,3-Butadiene

Manufacture of tyres and hoses