Polymers

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polymers

Polymers are the giant molecules formed by joining together of hundreds or thousands of smaller molecules. They belong to the category of macromolecules. The word polymer is derived from two Greek words; ‘poly’ meaning ‘many’, and ‘mer’ meaning ‘part’. Thus, a polymer is a large molecule made up of several identical repeating units called monomer.

A polymer needs to be made up of identical repeating units. If different types of molecules are joined together to form a larger molecule, it is simply called a giant molecule, not a polymer. 

In this section, we will have a detailed discussion on polymers, their characteristics and properties, their classification, examples and much more. 

Properties of Polymers

Polymers are identified based on their properties. The common properties that are found in all types of polymers are as follows; 

1. They are made up of repeating units

As mentioned in the introduction, the most important property of a polymer is that it is made up of identical repeating units known as monomers. The name of a polymer is also based on these monomers. Take the following examples;

  • A polysaccharide is a polymer made up of several repeating monosaccharide.
  • Polypeptide is made up of thousands of peptides (or amino acids) repeating in a particular fashion.
  • A polynucleotide is a polymer of nucleotides. 

2. Configuration of monomers

The configuration of monomers is the second property of polymers. Monomers in each polymer have a particular configuration or arrangement that is a specific characteristic of that polymer. This can be understood from the following examples;

  • In a linear polymer, all the monomers are attached in a long single chain. 
  • In a branched polymer, some monomers form short chains that are attached as a branch to the main linear chain of monomers. In this case, monomers have two configurations. 

3. Chain Length

The size of the polymer and the degree of polymerization can be identified from the chain length of the polymer. Chain length also indicates the quantity or number of monomers present in the polymer. In the case of synthetic polymers, it is easier to find the chain length as the statistical data is being reported during the process of polymerization. 

4. Morphology

Morphology indicates the final shape of the polymer it assumes after the process of polymerization. The physical properties of a polymer are highly dependent on its morphology which in turn is dependent on the interaction between the chains of monomers present in it. Following morphologies of polymer are usually seen;

  • Disordered, in which the polymer has a somewhat amorphous or glassy structure. It is formed due to a high degree of random branching chains.  
  • Linear, in which all the monomers are arranged in a single chain. The polymer behaves as a semi-crystalline solid.
  • Cross-linked, in which the chains of monomers show extensive cross-linking. These cross-links undergo decomposition when exposed to high temperatures. 

Classification of Polymers

They are classified into two broad categories.

  • Natural polymers: They are naturally present within the bodies of the living organisms. 
  • Artificial polymers: They are artificially made in industries for various commercial uses.

Polymers can also be classified based on the type of monomers present in them. This classification includes the following categories;

  • Homopolymer: A homopolymer is made up of only one type of monomer.
  • Copolymer: On the other hand, a copolymer is made up of two or more types of repeating units.

Artificial Polymers

These are made by man to fulfill several commercial and industrial needs. These are also known as synthetic polymers. We all use different synthetic polymers in our daily lives. Few examples of artificial or synthetic polymers include:

  • Nylon, used in the fabric industry. 
  • Polyvinyl Chloride (PVC), used in plastic and pipe industry.
  • Synthetic rubber, used for various purposes. 
  • Polystyrene, polyacrylonitrile, polyethylene, and many more. 

These polymers are made in industry by the process of polymerization. 

Polymerization

The method by which polymers are made artificially in the industry is known as polymerization. In this process, monomers are combined forming covalent bonds or linkages. The functional groups of monomers react with one another to form a specific covalent bond. 

There are two polymerization techniques currently used in the industry;

  • Chain-Growth: In this technique, one monomer molecule is added to the growing chain at one time. 
  • Step-Growth: In this technique, chains of monomers can combine i.e. multiple chains of monomers can be combined at one time to form a polymer.

Newer methods are also being used in polymerization industries. However, those methods are beyond the scope of our subject. 

Natural Polymers

They are present within the bodies of living organisms and carry out essential life processes. Therefore, they are also called bio-polymers. 

They are divided into three main classes

  1. Polysaccharide
  2. Polypeptides
  3. Polynucleotides

Rest of our discussion will be based on these bio-polymers. 

Polysaccharides

Simple_Polysaccharide_Hydrolysis

Polysaccharides belong to the category of carbohydrates. These are the polymers made by repeating units of monosaccharides. Several thousands of monosaccharide subunits combine via glycosidic bonds to form polysaccharides. 

Properties

Following properties are common in all polysaccharides:

  • They are amorphous solids.
  • They are tasteless and colorless.
  • Upon hydrolysis, they yield monosaccharides.
  • They are non-reducing sugars. 
  • They are insoluble in water.  

Some biologically important polysaccharides include Starch, Glycogen, and Cellulose.

Starch 

It is a polymer made up of repeating glucose subunits. Upon complete hydrolysis, starch yields glucose molecules. It may consist of branched chains of glucose as in amylopectin starch, or unbranched chains of glucose as in amylose starch. 

It can be identified by iodine test. Starch always yields blue color in the iodine test.

Starch is the main form in which the glucose is stored in plants. Humans and animals consume carbohydrates mainly in the form of starch. It is present in fruits, grains, seeds, and tubers, etc. 

Glycogen

Glycogen is also a polymer of glucose molecules and yield glucose on complete hydrolysis. It is made up of branched chains of glucose that are arranged in the form of a helix. 

It can also be identified by using the iodine test. Glycogen gives red color with Iodine. 

Animals store glucose in their bodies in the form of glycogen. It is present in every animal cell. However, large stores of glycogen are found in liver and muscle cells. It is also sometimes called animal starch.

Both glycogen and starch are digestible in the human intestinal tract.

Cellulose

Cellulose is a branched polymer of glucose subunits that are linked via glycosidic bonds. These glycosidic bonds are different from those in starch and glycogen in a way that they cannot be broken in the human body. That is why cellulose is not digestible by the human digestive system. 

Another factor that differentiates cellulose from other polysaccharides is its reaction with the iodine solution. The iodine test of cellulose is negative as it does not give any color with the iodine solution. 

It is the most abundant carbohydrate present in nature. Cellulose is the essential component of plant cell walls and is thus present in every plant cell. However, it is not present in animal cells. 

Polypeptides

Polypeptides are the polymers of amino acids. Several amino acids are linked together via peptide bonds to form long chains called polypeptides. These chains then undergo different structural arrangements resulting in the formation of functional proteins.

Properties

Following properties are shared by all polypeptides;

  • They are unbranched chains of amino acids. 
  • Each polypeptide has an amino acid with a free amino group at N-terminal and an amino acid with a free carboxylic group at C-terminal. 
  • Upon proteolysis, they yield different amino acids. 
  • Depending on the nature of amino acids, they may or may not be soluble in water. 
  • They are synthesized by ribosomes within the cells. 

Functions

Polypeptides undergo different structural arrangements to form proteins. Thus, the functions performed by polypeptides in the human body are the same as performed by proteins. These include the following;

  • They form proteins that are an essential component of all types of membranes. 
  • Polypeptides from proteins that function as enzymes.
  • They form transport proteins such as hemoglobin.
  • They are present in hair, nails, bones, and cartilage, etc. 
  • They are essential for muscle contraction. 
  • Some polypeptides function as hormones in the human body such as insulin made up of two polypeptides. 
  • Neuropeptides in the human body act as neurotransmitters. 
  • They can also be attached to a lipid molecule to form a lipopeptides. These lipopeptides are the components of cell membranes and perform several functions essential for the growth and survival of the cell. 
  •  They have a role in cell signaling.

These polymers of amino acids have several other functions that will be discussed somewhere else in detail.

Polynucleotides

These are the polymers of nucleotides that are joined together via phosphodiester bond. A polynucleotide is a single chain containing 13 or more nucleotides attached via phosphodiester bonds. They yield individual nucleotides when exposed to the nuclease enzymes that break the phosphodiester bonds. 

DNA and RNA are biologically most important polynucleotides.

DNA

Deoxyribonucleic acid (DNA) is a polymer of deoxyribonucleotides. It is a double polymer i.e. it consists of two polymeric chains of nucleotides. The two chains of nucleotides are attached together via hydrogen bonds to form a DNA double helix. 

DNA is present in the nucleus and nucleolus of all living cells. It is also present in chloroplast of animal cells as well as mitochondria of both animal and plant cells. All the structural and functional information of a cell is stored in the form of DNA.  This information is also passed onto the next generation via DNA. 

DNA undergoes degradation by nucleases that cleave the phosphodiester bond between nucleotides. This enzyme is also present in the human digestive tract that digests the nucleic acid taken in the form of diet into nucleotides that can be absorbed. 

RNA

Ribonucleic acid (RNA) is another example of polynucleotide. It is a polymer of ribonucleotides. Contrary to the DNA, it consists of only a single long chain of nucleotides. The nucleotides in RNA are also linked together via the phosphodiester bonds. 

RNA is essential for passing information from the nucleus into the cytoplasm and also for the synthesis of proteins in the cell. 

RNA taken in diet is digested by nuclease of the digestive tract into nucleotides that are then absorbed into the blood. 

Summary

Polymers are the macromolecules formed when several identical repeating units combine to form long chains as a result of chemical bonding. 

A compound must have the following properties to be a polymer; 

  • It must be made up of identical repeating units called monomers. 
  • Monomers can have linear or branched configuration.
  • The size of the polymer depends on its chain length.
  • In morphology, it may have chains that are disordered, linear, or cross-linked. 

Two broad categories of polymers include;

  • Artificial or Synthetic Polymers
  • Natural Polymers

Depending on the nature of monomers forming a polymer, they are classified as;

  • Homopolymer (only one type of monomer)
  • Copolymer (two or more types of monomers)

Artificial polymers made for industrial and commercial uses include artificial rubber, PVC, nylon, etc. 

Natural polymers are made within the living organisms. These include;

  • Polysaccharide
  • Polypeptides
  • Polynucleotides

Polysaccharides are polymers of monosaccharides that are tasteless and odorless amorphous solids. 

Starch, glycogen, and cellulose are the most important polysaccharide. All these are the polymers of glucose. 

Starch is the storage form of glucose in plants while glycogen is the storage form of glucose in animals. 

Cellulose is present in plant cell walls

Polypeptides are the polymers made up of single, unbranched chain of amino acids linked via peptide bonds. 

These polypeptides undergo different spatial organization to form complex structural and functional proteins.

The functions performed by polypeptides are also the same as performed by proteins. 

Polynucleotides are the polymers of nucleotides and include nucleic acids like DNA and RNA.  

They are also single unbranched chains consisting of 13 or more nucleotides. 

The phosphodiester bond between the individual nucleotides is cleaved by the nuclease enzymes that are present in the cells as well as the digestive tract of animals. 

References

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