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POLYMERS

Polymers are macromolecules formed by linking together of a large number of small molecules called monomers.

The polymers are giant molecules with high molecular masses. For example, the monomer ethylene gets linked with many other ethylene molecules to form polyethylene, or large number of vinyl chloride molecules combines to form polyvinyl chloride.

The single repeating unit is called as monomer, and the resultant high molecular weight compound is called as polymer

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Degree of Polymerization

The total number (n) of single monomer units combined together to form a polymer is known as degree of polymerization (DP). DP affect physical properties of polymers.

The polymers with high degree of polymerization are known as high polymers while those having comparatively low degree of polymerization are known as oligopolymers. The molecular weights of polymers are generally in the range of 5000 to 200,000. Hence, these are also known as macromolecules.

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The nature of monomer

(a) Homo-polymers: When all of the repeating units along a chain are of the same type, the resulting polymer is called a homo-polymer ie, A polymer containing identical monomers. • A-A-A-A •

(b) Co-polymers are polymer chains composed of two or more different repeat units.For example in polystyrene butadiene. One molecule of butadiene combines with one molecule of styrene. The product obtained acts as a monomer to get the polymer. • A-B-A-B-A-B •

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Functionality

The functionality is the number of bonds that a given monomer can form.
The monomers that have an active bond which may react to form two covalent bonds with other monomers forming a two-dimensional chainlike molecular structure, Such a monomer is termed bifunctional. For example, ethylene.
The monomers which have three active bonds, from which a three-dimensional molecular network structure are trifunctional. For example, monomers such as phenol–formaldehyde.

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Network

Polymers

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Classification Based on source

a) Naturally occurring polymers

The polymers that are derived from plants and animals are called Naturally occuring Polymers.
These have been used for many centuries; these materials include wood, rubber, cotton, wool, leather, and silk.
Other natural polymers such as proteins, enzymes, starches, and cellulose are important in biological and physiological processes in plants and animals

Network Polymers

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Natural Polymers

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Semi Synthetic Polymers

Semi-synthetic polymers are those that are derived from nature itself but are made to undergo chemical processes to enhance their quality.For Example: Rayon, Cellulose Nitrate.

Semi- synthetic polymers are the ideal bend of synthetic and natural polymers because it has the beneficial features of both types of polymers. that is why Rayon which is the most important polymer finds application in various medical and technological industry.

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Semi Synthetic Polymers

Cellulose Nitrate

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Synthetic Polymers

Synthetic polymers are defined as polymers that are artificially synthesized from small organic molecules in laboratories. These are also known as man-made polymers. Polymers which are. Many of our useful plastics, rubbers, and fiber materials are synthetic polymers. Polythene, polyvinyl chloride.

Synthetic polymers are used in our everyday lives from clothing, storage, to construction materials to toys for kids.

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Synthetic Polymers

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Based on Structure

Molecular structures can be categorized as: linear, branched, cross-linked, and network Polymers

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Linear Polymers: Linear polymers are those in which the repeat units are joined together end to end in single chains. These long chains are flexible and may be thought of as a mass of spaghetti, as represented schematically in Figure (a), where each circle represents a repeat unit.
For linear polymers, there may be extensive vander Waals and hydrogen bonding between the chains.
Some of the common polymers that form with linear structures are polyethylene, poly(vinyl chloride), polystyrene, poly(methyl methacrylate), nylon, and the fluorocarbons various isomeric configurations.

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Branched Chain Polymers: Branched Polymers may be synthesized in which side-branch chains are connected to the main ones, as indicated schematically in Figure (b).
The branches, considered to be part of the main-chain molecule, may result from side reactions that occur during the synthesis of the polymer.
The chain packing efficiency is reduced with the formation of side branches, which results in a lowering of the polymer density.
Polymers that form linear structures may also be branched. For example, high-density polyethylene (HDPE) is primarily a linear polymer, whereas low-density polyethylene (LDPE) contains short chain branches

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Cross-linked Polymers: In cross-linked polymers, adjacent linear chains are joined one to another at various positions by covalent bonds, as represented in Figure (c).
The process of crosslinking is achieved either during synthesis or by a nonreversible chemical reaction. Often, this crosslinking is accomplished by additive atoms or molecules that are covalently bonded to the chains.
Many of the rubber elastic materials are cross-linked. In rubbers, this is called vulcanization. The vulcanized rubber is the best known example of cross-linked polymers in which -S-S- cross links are joined irregularly. Due to cross linking polymer structure becomes three dimensional cross linked or network polymer. This makes the polymer very hard and rigid

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Network Polymers: Multifunctional monomers forming three or more active covalent bonds make three-dimensional networks Figure (d) and are termed network polymers. Actually, a polymer that is highly cross-linked may also be classified as a network polymer.
These materials have distinctive mechanical and thermal properties; the epoxies, polyurethanes, and phenol-formaldehyde belong to this group.
Polymers are not usually of only one distinctive structural type. For example, a predominantly linear polymer may have limited branching and crosslinking.

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Based on Molecular Sources

Elastomers

Elastomers are polymers that possess the elastic properties of natural rubber.
They are lightly cross-linked and amorphous.
They have a glass transition temperature below room temperature.
They can also be envisaged as one very huge molecule of macroscopic size.
The forces between the molecules within the polymer chains are comparatively weaker.
The crosslinks fully suppress irreversible flow. The chains are very flexible at temperatures above the glass transition, and a small force leads to very large deformation. Hence, proper precautions must be taken while handling elastomers.

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Synthetic Fibres
Synthetic fibers are also a very common type of Synthetic polymer. There are man-made textile fibers which include fibers and other materials that are made from natural materials such as rayon, acetate which is a derivative of cellulose, or even regenerated protein fibers from zein. They can also be fully synthetic fibers such as nylon or acrylic fibers.
Advantages: Synthetic polymers are an important part of the modern world. They have always made life easier and more convenient in hundreds of different ways.
Disadvantages: The raw materials used to produce them can get extinct, and disposing of synthetic polymers is a very difficult and time-consuming task. If proper care is not taken, it can result in environmental degradation.

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The response of a polymer to mechanical forces at elevated temperatures is related to its dominant molecular structure. In fact, one classification scheme for these materials is according to behavior with rising temperature.
Thermoplastics
Thermoplastics soften when heated (and eventually liquefy) and harden when cooled. This processes is totally reversible and may be repeated.
On a molecular level, as the temperature is raised, secondary bonding forces are diminished (by increased molecular motion) so that the relative movement of the adjacent chains is facilitated when a stress is applied.
Irreversible degradation results when a molten thermoplastic polymer is raised to too high a temperature. In addition, thermoplastics are relatively soft.
Most linear polymers and those having some branched structures with flexible chains are thermoplastic. These materials are normally fabricated by the simultaneous application of heat and pressure. Examples of common thermoplastic polymers include polyethylene, polystyrene, poly(ethylene terephthalate), and poly(vinyl chloride). �
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Examples of thermoplastics are given below:
Polystyrene
Teflon
Acrylic
Nylon

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Thermosetting Polymers:
Thermosetting polymers are network polymers.They become permanently hard during their formation and do not soften upon heating. Network polymers have covalent crosslinks between adjacent molecular chains. During heat treatments, these bonds anchor the chains together to resist the vibrational and rotational chain motions at high temperatures. Thus, the materials do not soften when heated. Crosslinking is usually extensive, in that 10 to 50% of the chain repeat units are crosslinked. Only heating to excessive temperatures will cause severance of these crosslink bonds and polymer degradation. Thermoset polymers are generally harder and stronger than thermoplastics and have better dimensional stability. Most of the crosslinked and network polymers, which include vulcanized rubbers, epoxies, and phenolics and some polyester resins, are thermosettinThermoplastics usually melt on heating and most of the time contain little or no crosslinking. They can be recycled more easily compared to Thermosets, and they can withstand heating and reforming. Linear polymers are examples of thermoplastic materials. They are polymers that can be manipulated into molds by the factor of heat and solidify when cooling.
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Examples of thermosetting polymers are as follows:
Vulcanized Rubber
Bakelite
Polyurethane
Epoxy Resin
Vinyl Ester Resin

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Parameter	Thermoplastic	Thermosetting Plastic
Synthesis	The synthesis process of thermoplastic can be done using addition polymerization	The synthesis process of thermosetting plastic can be done using condensation polymerization.
Tensile Strength	Their tensile strength is low.	They have high tensile strength.
Melting Point	The melting point of these plastics are low.	The melting point of thermosetting plastic is high.
Bonds	Thermoplastics have secondary bonds between the molecular chains and they are joined with weak bonds.	Thermosetting plastics have primary bonds between molecular chains and they are joined with strong cross-linked bonds.
Molecular Weight	They have lower molecular weight as compared to thermosetting plastics.	They have high molecular weight than thermoplastics.
Processing	Thermoplastics are processed by injection moulding, extrusion process, blow moulding, thermoforming process, and rotational moulding.	Thermosetting plastics are processed by reaction injection moulding and compression moulding.

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Based on Mode of Polymerization

The synthesis of large molecules (polymers) is termed polymerization. It is simply the process by which monomers are linked together to generate long chains composed of repeat units. The reactions by which polymerization occur are grouped into two general classifications according to the reaction mechanism:
Addition Polymerization
Condensation Polymerization

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Addition Polymerization

Addition polymerization (sometimes called chain reaction polymerization) is a process by which monomer units are attached one at a time in chainlike fashion to form a linear macromolecule. The composition of the resultant product molecule is an exact multiple of that of the original rea.
Three distinct stages—initiation, propagation, and termination—are involved in addition polymerization.

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Initiation Stage

During the initiation step, an active center capable of propagation is formed by a reaction between an initiator (or catalyst) species and the monomer unit.

Where R. represents the active initiator, and “.” is an unpaired electron.

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Chain Propagation

Propagation involves the linear growth of the polymer chain by the sequential addition of monomer units to this active growing chain molecule.
Chain growth is relatively rapid; the period required to grow a molecule consisting of, say, 1000 repeat units is on the order of
This may be represented , for polyethylene, as follows:

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Chain Termination

Propagation may end or terminate in two different ways.
The active ends of two propagating chains may link together to form one molecule according to the following reaction:

This type of termination reaction is referred to as combination

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