Polymerization could be considered to be perhaps the main chemical reaction associated with modern living. It is a reaction that is common in nature for the formation of “biopolymers” such as celluloses and proteins. It can also be thought of as the fundamental reaction type for the formation of DNA, making polymerization essential for life as it exists on this planet. In general terms, polymerization is a reaction process in which the molecules of an appropriate material, termed the 'monomer', become bonded together to form a much larger linear chain or a three-dimensional network. A polymer is thus a material that consists of very large molecules consisting of repeated sub-units of an identifiable monomeric compound (Atul, 2012). Polymers, which are also commonly known as plastics, have greatly impacted the daily lives of people, and of the entire world, through their various applications and uses. The range of applications in which polymers are used is greater than any other types of materials that are available to mankind. Polymers are used in adhesives, coatings, packaging materials, electronic devices, composites, and in many other applications. It is not an understatement to say that the use of polymers can be observed in every aspect of the daily lives of individuals (Birley, 1992).
The principal industrial application of the polymeriaztion reaction is for the production of plastics such as polyethylenes and polyamides. The simplest of these is plain polyethylene, produced by the polymerization of ethylene (H2C=CH2) alone. Variants of this basic polyethylene are produced by the use of monomeric compounds that have other atoms or groups of atoms attached in place of one or more of the hydrogen atoms. Styrene, for example, has a benzene ring attached in place of one of the hydrogen atoms, and produces polystyrene when it is polymerized. Vinyl chloride has a chlorine atom in place of one of the hydrogen atoms and produces polyvinylchloride (also known as PVC). Polypropylene is produced by the polymerization of propene, which has a methyl group (-CH3) in the place of one of the hydrogen atoms of ethylene. If the side chain also contains a functional group that can take part in the polymerization process then the reaction can cross over into other polymer chains. This is called cross-linking, and forms a three-dimensional network instead of long linear molecules.
Polymers such as polyamides and polyesters also form linear chains and cross-linked networks, but they do this through reaction between two different types of monomeric compounds that have reactive groups on each end of their respective molecules. Their repeating subunits are therefore made up of groups of two identifiable compounds rather than just one as it is in polyethylenes.
Plastics fall into two categories termed thermoplastic and thermosetting. Thermoplastic polymers become softer and more malleable with heat, which lends them to applications such as injection molding and heat forming (Holden et al, 2004). They seldom melt at a specific temperature like pure compounds do, but instead they melt over a broad range of temperatures. This enables thermoplastics to be readily recycled or reformed and put to new uses instead of being put into the environment as garbage. As temperatures become colder, thermoplastics become harder and more brittle, and are thus more susceptible to breaking or shattering. At a certain temperature called the 'glass transition temperature', thermoplastics lose their malleability and will break like glass. Thermosetting polymers, on the other hand, are generally liquid or semi-liquid mixtures that become solids as they undergo polymerization while being heated (Jarecki, 1979). They never regain malleability, and because they are typically three-dimensional polymers rather than linear polymers, they are also typically resistant to being broken down by various solvents as well as retaining of their hardness (Birley, 1992). Thermosetting polymers are typically formed by reactions that eliminate some simple small molecule such as HCl or H2O that is subsequently removed from the system to make the reaction irreversible. Thermosetting plastics are also called resins, and are used in adhesives such as epoxy cements, in electronics circuit boards, and in the advanced composite materials used to construct modern aircraft, to name but a few applications. They are typically much stronger than thermoplastics, but also more brittle (Atul, 2012).
Plastics of many different kinds are used almost everywhere in the modern world. One large consumer of plastics such as polyethylenes is the packaging industry. A very common packaging material is known as Saran (as in Saran Wrap), which is made of polyvinylidenechloride. Vinylidene chloride (H2C=CCl2) is similar to vinyl chloride (H2C=CHCl), but the presence of a second chlorine atom results in a polymer with greater suppleness and 'clinginess'. Where PVC forms a fairly rigid thin, tough sheet, Saran forms a highly flexible sheet that can be stretched by as much as one-third (Hawley, 1981). Saran is very useful in the food packaging industry because it is tasteless, odorless, non-toxic, resists abrasion, and it is impermeable to flavours and moisture. It provides these advantages with very low weight due to the thinness of the sheet, and its very low permeability to vapours also makes it very useful as an oxygen barrier for the protection of meats and other food products.
Another common application of plastics is in construction (Jarecki 1979). Many kinds of pipes are made from plastics instead of metals. Plastics are not affected by water and so do not corrode like metal pipes would. Also, plastics do not conduct electricity well, whereas metal pipes are excellent conductors of electricity. So pipes for many common uses such as water lines and drains are made of PVC and another type of plastic called ABS, which stands for Acrylonitrile-Butadiene-Styrene. These three monomers are combined in different proportions in the polymerization process to produce different grades of ABS plastics. Some of these are used to produce conduit pipes for electrical wires. The wires themselves typically have a coating of PVC over a copper conductor, which acts as an electrical insulator. In addition, PVC is used to make 'vinyl siding' that is commonly used to cover the outside of houses and other buildings in a decorative manner. The PVC and many other such polymers are naturally colourless, so different coloured compounds are added to the plastic blend to achieve a desired colour, such as the red, orange, black, brown, blue, yellow and green that are used as standard colours for wires, and the many other colours that are available in vinyl siding. One of the most common uses for PVC is in the production of molded plastic toys such as those given away with childrens' fast food meals or sold as souvenirs in tourist areas. PVC is extremely useful in this application.
The automobile industry is making ever greater use of plastics in the construction of vehicles of all kinds (Crawford 1985). As plastics have been developed with high strength and lower weight than steel, many auto parts such as bumpers and various body components have been replaced by their plastic counterparts. This is not a new idea, however, as Henry Ford had demonstrated a car with an almost indestructible plastic fiber body in November of 1940 (Fenichell 1996). It did not become common then, however, because of the demands for certain materials in fighting World War II, and the low cost of steel after that war was over.
Plastics are considered to be one of the fastest growing types of materials in regard to new applications and its disposition throughout the world. The disposal of used plastics, however, is a major problem or issue of concern with the use of the material. Plastics are considered to be non-biodegradable, and they remain in the environment for a very long time (Atul, 2012). The properties that make them so useful, such as their resistance to reaction with water and other chemicals and their lack of reactivity with oxygen, also prevents them from breaking down into their component atoms and smaller molecules in the environment, and so they remain for many years. Large amounts of plastics are disposed around the world, and plastic waste can be found in large quantities in the oceans, which is an issue of great environmental concern. Landfills also contain large quantities of plastics. The toxicity of some additives used to alter the basic properties of a plastic material is both an environmental and a medical concern. A compound called bis-phenol A, or BPA, is commonly blended into PVC and other plastics to make the resulting polymer more flexible. This has been linked to health problems because the BPA molecule can leach out of the polymer and into food products. The BPA molecule has a structure like some steroid hormones and has been linked to some medical problems. Another additive is di-iso-octyl terephthallate, which is added to some kinds of rigid plastics like PVC to make them very flexible and somewhat stretchy. A common example of this application id the Tygon tubing used in chemistry and medical laboratories. This compound is readily leached from the plastic by different solvents and can contaminate the environment very easily.
Polymers can be considered one of the most important inventions of modern times. Plastics are types of polymers that have a wide range of applications. The physical and chemical properties of plastics depend on the structure of the polymer molecule, whether it is linear or cross-linked, and the identities of the monomers used in its production. There are two major types of polymers, known as thermoplastic and thermosetting. Plastics are used in almost every application and industry, but especially in packaging and construction applications. However, this imposes disposal problems such as physical pollution and contamination by toxic additives leached from the plastics into the environment. Improvements in the composition of plastics are needed in the future in order to maximize their potential.
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