The word polymer has been derived from the Greek term that denoted "many parts" Polymers are defined as large molecules created by chemically linking (bonding) a succession of building blocks. Scientists refer to each of these parts that are chemically bond together via chemical bonding as a monomer (in Greek a monomer denotes "one part"). So you can imagine a polymer chain comprising numerous monomers linked together through chemical bonding. These monomers can be of two varieties - simple (comprising only one or two to three atoms) and complicated ring-shaped arrangements comprising one dozen or even more atoms.
Polymers can also be created artificially and the link of each chain in such polymers is similar to its neighbours. However, in DNA, proteins as well as other natural polymers, the chain’s links are different from their neighbours.
Among all molecules, the molecular weight of polymers is the highest and they may comprise several billion atoms. In fact, the human DNA is also a polymer and it has more than 20 billion atoms that comprise it. In addition, proteins, which comprise amino acids, as well as several other molecules that make up a living organism are also polymers. Among all known molecules, these are not only the largest, but also the most diverse types of molecules. This even holds true for plastics.
Typically, monomers are defined as molecules having 4 to 10 atoms each. They are reactive as they bond readily with other monomers in the process known as polymerization. Both polymers as well as the polymerization processes are extremely diverse. In fact, there are a variety of dissimilar systems to categorize them. One major such system is known as condensation polymerization, wherein water is released by the reacting molecules as a by-product. Protein is formed by this particular polymerization process.
It is worth mentioning here that it is not always that polymers comprise straight chains of regular molecules that are identical to their neighbours. At times, polymers comprise chains of a variety of lengths or may even contain chains that branch out in various different directions. Often residual monomers are present along with the polymers created by them contributing to the additional properties of the molecules. A variety of catalysts, which are basically secondary molecules that accelerate the molecules' reactions, are needed to persuade the monomers to link with others in specific arrangements. In fact, catalysts form the basis in nearly all processes that produce synthetic polymer.
In the process known as copolymerization, chains containing two or more dissimilar monomers are formed. Compared to other polymers, the larger and more complex polymers usually have higher melting points and also greater tensile strengths due to the intermolecular forces working between their constituent monomers. Then again, some molecules are so extremely complex that one cannot identify them easily. Therefore, different techniques like small angle X-ray scattering, wide angle X-ray scattering and small angle neutron scattering are used to identify them.
Incidentally, the majority of the polymers are organic in nature as carbon bonds are employed as their main strength. The remaining polymers employ silicon as their backbone. Since polymers are very diverse, scientists are yet to discover many of them. This leaves much scope for further fruitful research as well as development.
In a number of instances, polymers actually form branching networks instead of single straight chains. Irrespective of the shape and form of the polymers, their constituent molecules are especially big. In fact, they are so large that scientists have categorized these molecules as macromolecules. It has been found that one polymer chain can comprise several hundred or thousands of molecules. Sometimes, a polymer may even contain millions of molecules. In simple terms, the longer a polymer chain is, the heavier it will be. In addition, the longer a polymer chain is, materials made from it will have higher boiling temperatures and melting points. At the same time, longer polymer chains mean that their viscosity will be higher. In other words, they will not flow as freely as liquids. This is because their surface area is greater enabling them to stick to the molecules in their neighbourhood.
Some of the materials that are naturally polymer-based include cotton, wool and silk, which have been used by people since prehistoric times. Cellulose, which is the primary constituent of paper and wood, is also a natural polymer. Some other such natural polymers include starch prepared by the plants. It is interesting to note that cellulose as well as starch comprises the same monomer, which is sugar glucose. Despite this, the properties of starch and cellulose are very different. As starch dissolves in water it can be digested very easily. On the other hand, cellulose does not dissolve in water and, hence, humans cannot digest it. This is mainly because though both are made of glucose monomers, they are chemically linked together in different ways in starch and cellulose.
All living things make proteins, which is a specific variety of polymer. Proteins are made from monomers known as amino acids. So far, scientists have identified around 500 diverse types of amino acids, but plants and animals employ only 20 of these amino acids to make the proteins needed by them. In experiments undertaken in laboratories, chemists have several options because they design as well as produce polymers. Chemists can also use natural ingredients to construct synthetic polymers. Alternatively, they may also use amino acids to make synthetic proteins, which are different from those found naturally. However, generally chemists build polymers using various compounds they prepare in the laboratories.
Chemical reactions called polymerization result in the creation of all polymers. Most polymers are produced by two types of fundamental reactions. One form of polymerization reaction is called a condensation polymerization, while the other type of reaction is called the chain-growth polymerization.
The first type of polymerization - condensation polymerization, is also known as step-growth polymerization and this chemical reaction takes place when two monomers react to produce an identical and smaller molecule - for instance, water. An excellent example of this kind of chemical reaction is the production of polymers from nylon monomers using basic amines and carboxylic acids. This reaction leads to the creation of a link between both monomers and yields water, which is a by-product of this chemical reaction. On the other hand, nylon fibers are used for making clothes.
Chain growth polymerization works differently. This chemical reaction takes place when a monomer creates a free radical that is highly reactive. It may also occur when any molecule contains an unpaired electron. Free radicals actually react quickly with monomers and produce a repeat unit having another free radical. In this way, as the chain reaction carries the process of polymerization forward, the length of the chain continues to be longer. Production of polystyrene is an ideal example of chain growth polymerization. The polymer polystyrene can be found in throwaway drinking cups.
As several types of polymers comprise long and flexible chains, they easily become intertwined - something similar to cooked spaghetti in a bowl. When polymer chains become tangled in such a disorderly manner, it leads to the creation of what is called an amorphous structure. Typically, amorphous structures are transparent and they melt easily to make things such as kitchen cling film.
However, polymer chains do not organize themselves into amorphous configurations all the time. In normal conditions, for instance when they are stretched, polymer chains can form straight lines alongside one another to form systematic, crystallized configurations. In addition, polymers can form crystalline configurations when they are slowly cooled, wherein each polymer chain double over on themselves.
Moreover, polymers can be employed to build gigantic three-dimensional arrangements. Such networks are usually built when the monomers react with in excess of two potential sites for polymerization to take place. These multiple sites for reaction allow the different polymer chains to bond with one another, thereby forming cross-lined chains. When such cross-lined chains occur due to multiple boding of each chain, it results in a three-dimensional solid structure, which is basically a very large molecule.
In our everyday life we come across a wide array of polymers and most of these are referred to as plastics. These plastics, also known as thermoplastics, are basically polymers which become soft when they are heated and subsequently molded into different desired forms. It is worth mentioning here that plastics are versatile and can be employed for making a wide variety of things that range from soda bottles to simple buckets to picnic cutlery.
Polymers have another specific application – when they form long strands they are called fibers. There are several types of such fibers, including synthetic rope or yarn. These are essentially produced from amorphous substances like the polymers. Similarly, crystallized polymers can be employed to produce fibers. Of the different types of fibers, the most popular one is the fiber that is used in bullet resistant clothing.
