A polymer is any of numerous natural and synthetic compounds of usually high molecular weight sometimes consisting of millions of repeated linked units, each a relatively light and simple molecule. Within each living organism, polymers (biopolymers) such as DNA, RNA, proteins, and polysaccharides perform specific functions that enable the organism to survive, grow, and reproduce. In addition, natural polymers—such as cotton, flax, jute, silk, and wool—have long been used for the production of clothing, rope, carpeting, felt, insulation, and upholstery.
Polymers are formed through the reaction of base units called monomers. These units will join together to form chains of thousands or even millions of recursions. Polymerization tends to occur under intense heat and pressure required to supply the activation energy for the reaction, this can be monitored and changed to affect the final properties of the polymer. The products or monomers can also be altered, a polymer consisting of more than one type of base unit is called a copolymer.
Steps of Basic Polymerization
There are three steps in the formation of a polymer.
A chemical (called the initiator) is added to a substance containing the monomers. It activates one of the monomers (in many cases when alkenes are used the double bond is split producing two free electrons which can react with other molecules) allowing polymerization to occur. It is important to note that the initiator is not a catalyst as in most cases it is absorbed into the final product. Common initiators include organic peroxides (used to produce LDPE).
The active monomer activates another unit (e.g. by causing its double bond to split), as there are now four free electrons a covalent bond can form between the two units and two electrons (one on each atom) will be left to combine with other units. This process continues and the chain grows in size with every unit added. During propagation side branching can occur, in which monomers start a secondary chain leading off the first. These chains can affect the properties of the end polymer, and can be controlled depending on the conditions in which polymerization occurs.
This is when propagation of a chain or a section of the chain is halted because of a reaction with a unit which does not produce a free electron (and hence cannot react with other units). An example of a terminator is oxygen, which will form a covalent bond with the chain but will not have any free electrons to continue reacting.
Reactions Involved in Basic Polymerization
The basic chemistry involved in polymerization can be compacted in the reactions that take place. The main types of polymerization reactions are:
In this case two units "add" together to form a combined unit without any byproducts (i.e. loss of atoms). This is the most common process in industrial polymerization, and is most commonly applied to alkene monomer units. In this case the double bond within the alkene unit is opened up, resulting in two free electrons which can then react with other units to form a chain.
In this process two monomers react and form a larger unit and a byproduct. A common condensation reaction occurring in nature is the reaction between glucose units to produce the polymer cellulose, the two monomers are fermented with yeast and combine to form the larger chain, expelling a water molecule in the reaction (C6H12O6 + C6H12O6 - C12H22O11 + H2O, as the number of each type of atom is the same the law of conservation of mass is adhered to).
Once the polymer has been formed additives can be mixed in to change its properties.
- Plastifiers - Soften the plastic or fiber produced
- Fillers - Reinforce a particular property of the polymer (e.g. boiling point, hardness)
- Stabilizers - Protect the polymer against oxidation, UV decomposition etc.
- Pigments - Color the polymer (commonly used in plastics)
Extraction of Polymer After Formation
Once the polymer is formed it needs to be extracted from the container in which it was made. In many cases this is simplified because the polymer will be in a liquid form (due to the intense heat). This can be achieved through several processes.
- Cold Drawing - Individual fibers are drawn from the vessel and wrapped around a rod. This is not used extensively in industry as it is a slow process (similar to winding thread around a thimble).
- Extrusion - The polymer in liquid form is forced through a nozzle, resulting in thin threads being formed. This process is common in the formation of synthetic fibers (e.g. rayon).
- Molding - The liquid polymer is set into a particular shape and left to cool. This is used when clearly defined shapes are required (e.g. in making microscopic tools the polymer can be poured into a cast and left to solidify, where it will then form the exact shape).
- Calendering - The polymer is rolled into sheets where the chains are compacted against each other to give the impression of a solid object, this is the practice commonly used to produce plastics.
Examples of Polymers and Their Uses
Polymers are commonly used both in synthetic threads and in plastics (where they can be either thermosetting polymers which have high cross branching - side branches actually connecting the chains - and will keep their shape during heating, or thermoplastics which have low cross branching and can be reshaped by applying heat). Common polymers and their uses are:
For a more detailed treatment, see Bakelite.
Bakelite is one of the earliest synthetic polymers.
Low Density Polyethylene, made from the polymerization of monomer ethylene or ethene units. An organic peroxide is used as the catalyst, and high temperates and pressure is used resulting in extensive side branching off the chain (about one chain every 50 monomer units). Because of these chains the polymer fibers are unable to be compacted against each other resulting in lower density and increased flexibility. LDPE is commonly used in cellophane.
High Density Polyethylene, undergoes the same process as LDPE except lower temperatures are used and a Zeiger Natta catalyst is added (this has a large surface area on which polymerization occurs, reducing possibility of side branching). The chains of HDPE can be more closely packed together, resulting in higher density and a more rigid structure. HDPE is used extensively in the modern world, including lunch boxes, computer cases etc.
Polychloroethylene, aka vinyl chloride, contains a chlorine atom which has substituted a hydrogen atom (in a substitution reaction one atom replaces another). This chlorine atom has a higher atomic mass, increasing the density of the polymer and increasing its durability in use. The polymer will decompose under intense heat to form hydrochloric acid (reaction between hydrogen and chlorine atom) which is a fire retardant, causing vinyl chloride to see applications in the building sector.
Polyphenylethylene, also known as polyethylbenzene or polystyrene. In its natural state is very hard and brittle, the phenyl group (C6H5 replacing a hydrogen atom) increases hardness (due to increased dispersion forces) but decreases density. However, gas can be blown through the liquid polymer during its formation to produce a foam, which is an excellent insulator (the phenyl group does not conduct heat very well) allowing it extensive application in disposable cups.
For a more detailed treatment, see Nylon.
Nylon fibers are used in carpets, guitar strings, racket strings, fishing lines and pantyhose.
Future of Polymers
The base unit for most industrial polymers is ethylene or an alkene allotrope, these in turn are commonly extracted from crude oil either directly (through fractional distillation or through cracking of the larger carbon chains. Oil reserves are being depleted, and may last anywhere between 20 and 50 years. Although the petrochemical industry (the market producing polymers) only uses 5% of the fossil fuels extracted it will still be affected by the decreasing supplies, and as such scientists are looking to alternative produces which can be used to produce polymers. The alternative most intensely researched is the possibility of biopolymers, carbon chains obtained from organic sources (e.g. trees and plants such as sugarcane). These would have the benefit of being renewable (able to be replaced in a few years, compared to the millions of years needed to reform crude oil), biocompatible (able to be used in the human body, could see new applications of plastics in the medical sector) and biodegradable (a major problem with current plastics is that they don't break down and can remain in landfill for hundreds of thousands of years). However, current biopolymers tend not to be as durable as their industrial counterparts, and they are less economically viable (large areas of land, fertilizer and energy are required to produce and extract the monomer units). That said, with increasing advances in technology biopolymers may become the norm, and new possibilities may open up the field of polymer chemistry.