Changes

Polymers are formed through the reaction of base units called [[monomer|monomers]]. These units will join together to form chains of thousands or even millions of recursions. Polymerisation 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 PolymerisationPolymerization===

''InitationInitiation''

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 polymerisation 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 (eg 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 propogation 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 polymerisation ocurspolymerization occurs.

This is when propogation 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 PolymerisationPolymerization===The basic chemistry involved in polymerisation polymerization can be concised in the reactions that take place. The main types of polymerisation polymerization reactions are:

''Addition PolymerisationPolymerization''

In this case two units "add" together to form a combined unit without any byproducts (ie i.e. loss of atoms). This is the most common process in industrial polymerisationpolymerization, 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.

''Condensation PolymerisationPolymerization''

*''Plastifiers'' - Soften the plastic or fibre fiber produced

*''StabilisersStabilizers'' - Protect the polymer against oxidation, UV decomposition etc*''Pigments'' - Colour Color the polymer (commonly used in plastics)

*''Cold Drawing'' - Individual fibres 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 nozzelnozzle, resulting in thin threads being formed. this This process is common in the formation of synthetic fibres fibers (eg rayon).

Low Density Polyethylene, made from the polymerisation polymerization of monomer ethylene or ethene units. An organis 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 fibres 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 polymerisation 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

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 intensly intensely researched is the possibility of biopolymers, carbon chains obtained from organic sources (eg 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, fertiliser 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 the field of polymer chemistry.