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Polymers are one of the most important substances which have a great impact on our modern life. The manmade products which are polymerized products, have brought a r evolution in the textile industry. Nylon and terylene have almost completely replaced cotton and silk. similarly resins, plasticts, natural and synthetic rubber are a gift from the scientists to our society.
The word polymers has a greek origin where poly means many and mers stands for unit or part. Thus polymers can be defined as "the substance of a high molecular mass(103 - 107 u) formed by the combination of large no of simple molecules called monomers by chemical bonds. The process in which monomers are converted into polymers is known as polymerization." For eg :- a very widely used polymer ispolyethylene which is formed by the polymerization of ethylene molecules(monomers) by heating under pressure in the presence of oxygen.
n CH2 = CH2 --------> -(âˆ’CH2 - CH2 âˆ’)-
All the monomer units in the polymer may or may not be same. If the monomers are same then they are known as homopolymers. On the other hand if they happen to be different then they are known as copolymer. All polymers are macromolecules while all macromolecules may not be polymers in nature.
Classification of polymers
Polymers has been classified in a number of ways on account of different angles:-
Classification based upon source of origin
Classification based upon the structure
Classification based upon mode of synthesis
Classification based upon nature of molecular forces
Classification based upon source of origin
Natural or Bio-degradable polymers:- The polymers which occurs in nature mostly in plants and animals are natural polymers. These polymers are also known as bio-molecules and are easily degradable. So these polymers are known as bio-degradable polymer. A few common example are starch, cellulose , proteins, rubber nucleic acid, etc. Among them starch and cellulose are the polymers of glucose molecules. Proteins are formed from amino acid which may be linked in different ways. Natural polymer is yet another useful polymer which is obtained from the latex of the rubber tree. The monomer units are of the unsaturated hydrocarbon 2-methyl-1,3-butadiene also called isoprene.
Synthetic polymers:- the polymers which are prepared in the laboratory are known as synthetic polymers. These are also called manmade polymers and have been developed in the present century so as to meet the ever increasing demand of the modern civilization. A few common examples are:- PVC, Teflon, nylon, Bakelite, terylene, etc.
Now let us discuss further in detail about the biodegradable/natural polymers.
Biodegradable polymers are condensation polymers which indicate that solid polymeric materials or devices which break down to macromolecule degradation with dispersion in an animal body. If degraded and resorbed in vivo, they can be called the bioresorbable polymers. A wide variety of natural and synthetic biodegradable polymers have been investigated for medical and pharmaceutical applications. Natural biodegradable polymers like Collagen, Albumin, Gelatin, Hemoglobin, Chitin, Chitosan, Hyaluronic acid (HA), and Alginic acid have been studied for medical applications. The use of these natural polymers is limited due to their higher costs and questionable purity.
For the last three decades, synthetic biodegradable polymers have been increasingly used as medical, pharmaceutical and tissue-engineering products, because they are free from most of the problems associated with the natural polymers. Amongst the different classes of synthetic biodegradable polymers, the thermoplastic aliphatic poly (esters) like PLA (Polylactide), PGA (polyglycolide) and their copolymers (PLGA) have generated tremendous interest due to their favorable properties such as good biocompatibility, biodegradability, bioresorbability and mechanical strength.
Classification on the basis of occurance of biodegradable polymers :-
Occurrence in plants :- The biodegradable polymers which are present in the nature in plants comes under this category. Eg:- natural rubber,biomolecules(eg. starch, cellulose, glycogen etc) etc.
Occurrence in animals :- The biodegradable polymers which are present in the nature in animals comes under this category eg:- proteins, nucleic acids(RNA & DNA) etc.
SOME BIODEGREDABLE POLYMERS FOUND IN PLANTS
1- Natural rubber
Natural rubber is an addition polymer made up of thousands of isoprene monomer repeating units. It is obtained from the Hevea brasiliensis tree in the form of latex. The difference between natural rubber and another natural polymer, gutta-percha (the material used to cover golf balls), is the geometric form of the polyisoprene molecules. The CH 2 groups joined by double bonds in natural rubber are all on the same sides of the double bonds (cis configuration), whereas those in gutta-percha are on opposite sides of the double bonds (trans configuration). This single structural difference changes the elasticity of natural rubber to the brittle hardness of gutta-percha.
Natural rubber its extraction from tree
Biomolecules are the organic compounds present as the essential components of the living organisms in different cells. Being organic in nature they contain carbon and hydrogen as their essential constituents along with the few more elements. Biomolecules build up all the living system and are also responsible for their growth and maintenance. These may be simple or complex substances of low or high molecular masses. The common organic substances which act as Biomolecules are carbohydrates, amino acids, proteins, enzymes, lipids, nucleic acids, hormones, vitamins, etc. Each one of them has a specific role to play and its deficiency is likely to cause imbalance or disorder in the living system. Eg :- cellulose, starch, glycogen etc.
Some of the Biomolecules are discussed further:-
Cellulose is the most abundant organic compound on Earth, and its purest natural form is cotton. The woody parts of trees, the paper we make from them, and the supporting material in plants and leaves are also mainly cellulose. Like amylose, it is a polymer made from glucose monomers. The difference between cellulose and amylose lies in the bonding between the glucose units. The bonding angles around the oxygen atoms connecting the glucose rings are each 180° in cellulose, and 120° in amylose. This subtle structural difference is the reason we cannot digest cellulose. Human beings do not have the necessary enzymes to break down cellulose to glucose. On the other hand, termites, a few species of cockroaches, and ruminant mammals such as cows, sheep, goats, and camels, are able to digest cellulose.
Starch is a condensation polymer made up of hundreds of glucose monomers, which split out water molecules as they chemically combine. Starch is a member of the basic food group carbohydrates and is found in cereal grains and potatoes. It is also referred to as a polysaccharide, because it is a polymer of the monosaccharide glucose. Starch molecules include two types of glucose polymers, amylose and amylopectin, the latter being the major starch component in most plants, making up about three-fourths of the total starch in wheat flour. Amylose is a straight chain polymer with an average of about 200 glucose units per molecule.
Glycogen is an energy reserve in animals, just as starch is in plants. Glycogen is similar in structure to amylopectin, but in a glycogen molecule a branch is found every 12 glucose units. Glycogen is stored in the liver and skeletal muscle tissues. A typical amylopectin molecule has about 1,000 glucose molecules arranged into branched chains with a branch which occur every 24 to 30 glucose units. Complete hydrolysis of amylopectin yields glucose; partial hydrolysis produces mixtures called dextrins, which are used as food additives and in mucilage, paste, and finishes for paper and fabrics.
SOME BIODEGREDABLE POLYMERS FOUND IN ANIMALS
All proteins are condensation polymers of amino acids. An immense number of proteins exists in nature. For example, the human body is estimated to have 100,000 different proteins. What is amazing is that all of these proteins are derived from only twenty amino acids. In the condensation reaction whereby two amino acids become linked, one molecule of water forming from the carboxylic acid of one amino acid and the amine group of the other is eliminated. The result is a peptide bond; hence, proteins are polypeptides containing approx fifty to thousands of amino acid residues.
The primary structure of a protein is the sequence of the amino acid units in the protein. The secondary structure is the shape that the backbone of the molecules(containing peptide bonds) assumes. The two most common structures are the Î± -helix and the Î² -pleated sheet. An Î± -helix is held together by the intramolecular hydrogen bonds that form between the N-H group of one amino acid and the oxygen atom in the third amino acid down the chain from it.The Î± -helix is the basic structural unit of hair and wool, which are bundles of polypeptides called Î± -keratins. STRUCTURES OF PROTEIN
2- Nucleic acids(RNA & DNA)
Nucleic acids are condensation polymers. Each monomer unit in these polymers is composed of one of two simple sugars, one phosphoric acid group, and one of a group of heterocyclic nitrogen compounds that behave chemically as bases. Nucleic acids are of two types: deoxyribonucleic acid ( DNA ), the storehouse of genetic information, and ribonucleic acid (RNA), which transfers genetic information from cell DNA to cytoplasm, where protein synthesis takes place. The monomers used to make DNA and RNA are called nucleotides. DNA nucleotides are made up of a phosphate group, a deoxyribose sugar, and one of four different bases: adenine , cytosine , guanine , or thymine . The nucleotides that polymerize to produce RNA differ from DNA nucleotides in two ways: they contain ribose sugar in place of deoxyribose sugar and uracil instead of thymine.
RNA & DNA
The advantages of these biodegradable polymers are that they offer favorable properties such as good biocompatibility, biodegradability, bioresorbability and mechanical strength. They possess processing facility, great variety, adaptability and reliability.
Natural rubber is used as a major component in our day to day life
Biomolecules are helpful to human beings in a number of ways.
Starch is a major source of carbohydrates which provides energy to the body.
Cellulose acts as a source of shelter and clothing to the human beings.
Glycogen converts to glucose when energy is needed by the body.
Proteins play a very important role in the structure and metabolism of the body. It helps in growth and maintenance of life.
Nucleic acids are required for the storage of the genetical information and helps in transfer of hereditary information.
They possess low or negligible toxicity of degradation products, in terms of both local tissue response and systemic response.
Adequate Mechanical properties address short-term function and do not interfere with long-term function.
They have the capability to form the polymer into the final product design and make it easier to control lot-to-lot uniformity and mechanical properties.
Drug delivery compatibility in applications that release or attachment of active compounds.
These polymers have been consistently approved numerous use in humans for medical and pharmaceutical applications.
Bibliography & reference websites:-
Dinesh companion chemistry by S.K. Malhotra 2009 edition, volume 2.
Joesten, Melvin D., and Wood, James L. (1996). The World of Chemistry , 2nd edition. Fort Worth, TX: Saunders College.
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