That can react with another ethene - and so on and so on. The polymer chain gets longer and longer. The chain does not, however, grow indefinitely. Sooner or later two free radicals will collide together. That immediately stops the growth of two chains and produces one of the final molecules in the poly ethene. It is important to realise that the poly ethene is going to be a mixture of molecules of different sizes, made in this sort of random way.
Because chain termination is a random process, poly ethene will be made up of chains of different lengths. A large number of important and useful polymeric materials are not formed by chain-growth processes involving reactive species such as radicals, but proceed instead by conventional functional group transformations of polyfunctional reactants.
These polymerizations often but not always occur with loss of a small byproduct, such as water, and generally but not always combine two different components in an alternating structure. The polyester Dacron and the polyamide Nylon 66, shown here, are two examples of synthetic condensation polymers, also known as step-growth polymers.
Although polymers of this kind might be considered to be alternating copolymers, the repeating monomeric unit is usually defined as a combined moiety. Formulas for these will be displayed below by clicking on the diagram.
Condensation polymers form more slowly than addition polymers, often requiring heat, and they are generally lower in molecular weight. The terminal functional groups on a chain remain active, so that groups of shorter chains combine into longer chains in the late stages of polymerization. The presence of polar functional groups on the chains often enhances chain-chain attractions, particularly if these involve hydrogen bonding, and thereby crystallinity and tensile strength.
The following examples of condensation polymers are illustrative. Note that for commercial synthesis the carboxylic acid components may actually be employed in the form of derivatives such as simple esters. Also, the polymerization reactions for Nylon 6 and Spandex do not proceed by elimination of water or other small molecules. Nevertheless, the polymer clearly forms by a step-growth process. Some Condensation Polymers. The high Tg and Tm values for the amorphous polymer Lexan are consistent with its brilliant transparency and glass-like rigidity.
Many plastics are synthesised from hydrocarbon-containing oil or petroleum though not all plastics are: bioplastics , for example, can be made from plants or even bacteria. The process by which oil is turned into plastic typically goes something like this. First, an oil refinery cracks the oil into small hydrocarbons GLOSSARY hydrocarbons an organic compound made up of only hydrogen and carbon the monomers.
Finally, the polymers, in the form of a resin a mass of polymer chains go to a plastics factory, where additives give the plastic the desired properties. In addition polymerisation—you guessed it—monomers are simply added together in a repeating pattern. This results in no other, additional, substance being created.
The other way in which polymers can be created is called condensation polymerisation. In this process, when each monomer is added to the chain, an additional, small molecule—such as water—is created as a by-product. Nylon and polyester are made this way. Addition polymerisation relies on a monomer with a double bond connecting two carbon atoms.
A molecule called a free radical is introduced, which causes the double bond to open up and link with the next monomer molecule.
The polymer chain forms when the same basic unit is repeated over and over in a regular chain structure. This means that polymers can be made faster, cheaper, cleaner and with greater control of the final product.
Polyethylene is the simplest synthetic polymer. Other polymers can be made of two or more different monomers. Polyethylene is formed when many thousands of ethylene molecules are joined end to end. This causes it to cleave in two, creating a free radical. A free radical is a molecule with a single unpaired electron. Or, to get technical, a molecule with an unpaired electron in its outermost valence shell is an unstable molecule. Either way, the lone electron is going to want to pair up with another electron.
It attacks the double bond joining the two carbons in the ethylene molecule and swipes an electron. The other carbon, previously happily paired, now has an unpaired electron. It has become a free radical, with an unpaired electron eager to join up with another to make a pair. Finally the unit summarises the range of processing techniques that can be used to convert polymers into a vast range of different products.
Each of the other units in the Polymers section describes the manufacture, properties and uses of an individual polymer or group of polymers in more detail.
Polymers are large molecules, a type of macromolecule. Their chemical properties are similar to those of simple molecules. For example, if the polymer contains a carbon-carbon double bond, as in poly but-1,3-diene , it will undergo additions reactions with, say hydrogen or bromine. If it contains an aromatic ring, as in poly phenylethene often known as polystyrene , it will undergo substitution reactions, say with nitric acid.
The major differences between smaller molecules and polymers lie not with their chemical properties but with their physical ones. Their larger sizes lead to much stronger intermolecular forces leading in turn to much higher melting points, and the characteristic properties of hardness and flexibility.
These intermolecular forces are even stronger when the polymer chains pack together in a regular way as in HPDE high density poly ethene and have regions of crystallinity.
When heated, it melts and the crystallinity is lost. As it does not have a sharp melting point, the temperature at which this occurs is termed the melt transition temperature , T m.
Above this temperature, the polymer is amorphous. Some polymers are hard and amorphous, having no regions of crystallinity, for example, poly methyl 2-methylpropenoate. The temperature at which they become soft and pliable is termed the glass transition temperature , T g. Figure 1 These crystallites have order in which the zigzag polymer chains are held together in a regular pattern by intermolecular forces.
There are many examples of polymers that occur naturally, for example, starch, cellulose and proteins. Over the last 70 years, synthetic polymers have been invented, often mimicking nature and they are now manufactured in millions of tonnes a year and are one of the most essential materials we use. Many are used as fibres. Others are moulded into required shapes and when they are used in this way, they are often termed plastics.
There are several ways in which polymers can be characterised: a how they are made, by addition or by condensation b whether they are homopolymers or heteropolymers co-polymers c whether they are themoplastics, thermosets, elastomers or fibres d by their steric structure. In addition polymerization , the polymer has the same empirical formula as the monomer but a higher molecular mass Table 1. An example is the polymerization of chloroethene vinyl chloride to form poly chloroethene , PVC:.
In condensation polymerization , polymerization of one or more monomers is accompanied by the elimination of small molecules such as water or ammonia Table 2. For example, in producing polyamide 6,6, two monomers are used.
Another type of condensation polymer is said to be formed if the polymer chain contains rather than appended to the chain a functional group such as an ester, amide or urethane Table 2. Another way of characterising polymers is to divide them into homopolymers and heteropolymers.
Many of the well known polymers such as poly chloroethene are produced from a single monomer and so are referred to as homopolymers Table 1 :. A heteropolymer, or as they are more commonly known, a co-polymer , is produced from two or more monomers. There are several types of co-polymer.
One type is produced when two or more monomers are mixed and polymerized together. Depending on the reactivities of the monomers, they may form polymers with different arrangements of the monomer units Figure 2.
SBS is an example of a block co-polymer. First, phenylethene is polymerized. Buta-1,3-diene is then added and adds on to both the reactive ends of the poly phenylethene molecules to form SBS:. Another type of co-polymer is known as a graft co-polymer. An example is ABS. A is Acrylonitrile, the trivial name for propenonitrile. The backbone of the polymer is formed from phenylethene styrene and buta-1,3-diene. Propenonitrile acrylonitrile is added to the system and forms a grafted side chain onto the backbone.
The nitrile adds to the double bond on the butadiene unit:. Co-polymers are very useful as they have the properties of the constituent polymers and thus can be produced for specific purposes.
For example, poly phenylethene polystyrene is brittle but when it is co-polymerized with buta-1,3-diene, the latter gives the polymer resilience and strength. He had no idea what he had made, so he turned the analysis of the material over to Eugen Bamberger and Friedrich Tschirner, who found long chains of -CH 2 -, which they called "polymethylene".
They were trying very hard to make an explosive gas ethylene react with a much larger molecule benzaldehyde , by forcing them together under high pressure. What they got was a useless, so they thought!
How wrong they were, but nothing much more was done with this "polyethylene" until the start of the Second World War. Suddenly there was a need for a flexible, non-reactive insulator to go around the cables of a new invention - radar. The British firm Imperial Chemical Industries re-discovered polyethylene and put it into production in Small molecules of the odorless gas ethylene were then, and now, transformed into a polymer called polyethylene by uniting the ethylene monomers into a long chain.
Some of these chains can be as long as 10, units. In some forms these chains branch, and they all coil and fold. Modern manufacturing methods start with ethylene gas which is heated under very high pressure until it becomes what is known as low-density polyethylene.
This material is a crystalline, transluscent thermoplastic which softens when heated.
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