Sometimes pendant groups, instead of hanging loose from one polymer chain, actually connect two chains together. Think of this as looking like a rung that stretches between the legs of a ladder.
Chemists refer to these ties as crosslinks. They tend to strengthen a material such as a plastic made from this polymer. They also make the polymer harder and more difficult to melt. The longer the crosslinks, however, the more flexible a material becomes. A chemical bond is what holds atoms together in a molecule and some crystals.
Once oxygen forms two bonds, it ibecomes stable. None are left to hold a pendant group. Some polymers are flexible. Others are very stiff. Just think of the many types of plastics: The material in a flexible soda bottle is very different from that in a rigid pipe made from polyvinyl chloride PVC. Sometimes materials scientists add other things to their polymers to make them flexible.
Known as plasticizers PLAA-stih-sy-zurs , these take up space between individual polymer chains. Think of them as acting like a molecular-scale lubricant. They let the individual chains slide across each other more easily. It has become a free radical, with an unpaired electron eager to join up with another to make a pair. A second ethylene molecule is introduced. The newly created free radical breaks the carbon-to-carbon bond, swiping an electron, and creating a new free radical with a single unpaired electron on the end.
This continues, as a chain reaction, with a long chain forming as more ethylene molecules are added. The process keeps going until free radicals meets another free radical, completing the chain. Now we have our polymer, polyethylene, made up of the monomer repeating unit ethylene. Some other examples of polymers formed in this way are polychloroethylene PVC , used to make things like plumbing pipes and insulation for electrical cables, and polypropylene, used in products such as rubber ducks and other toys and, when processed into fibres, carpets.
The way molecules are arranged gives different polymers different properties. Polyethylene, for example, has long polymer chains side by side. When they cool down, the chains interact and become entangled.
Polyethylene can be melted and reformed into a new shape over and over again. These meltable, reshapable polymers are known as thermoplastics.
Other examples include polystyrene and polypropylene. The strength of polymers also varies depending on how the molecules are arranged. To use our paperclip analogy, you may decide to have some paperclips branching off your main line. They result in a polymer with a lower density. Low-density polyethylene LDPE —the squishy material that plastic bags and wrap like the kind you might wrap your sandwich in —is an example. The resulting polymer is stronger and has a higher density.
An example is high-density polyethylene HDPE , used to make things like plastic bottles, food containers and plumbing pipes. In contrast to thermoplastic polymers are thermosetting polymers. It is useful, though, for things like car tyres, since a tyre that melts in the heat is going to make for a pretty interesting drive to the beach.
Glues and electrical components are also thermosetting polymers. As well as the arrangement of molecules, the properties of a polymer are also determined by the length of the molecular chain. They are often water-based. Examples of naturally occurring polymers are silk, wool, DNA, cellulose and proteins.
In our previous section on network polymers, we mentioned vulcanized rubber and pectin. Vulcanized rubber is a synthetic man-made polymer, while pectin is an example of a natural polymer. Rubber can be found in nature and harvested as a latex milky liquid from several types of trees. Natural rubber coming from tree latex is essentially a polymer made from isoprene units with a small percentage of impurities in it.
Rubber can also be made synthesized by man. Synthetic rubber can be made from the polymerization of a variety of monomers, including isoprene. Macromolecules are involved in processes such as food digestion, information storage, energy manipulation and metabolism. They are complex, huge associations of molecular subunits that appear impossibly difficult to understand. Fortunately they are all built using the same construction principle. Monomers are small molecules, mostly organic, that can join with other similar molecules to form very large molecules, or polymers.
All monomers have the capacity to form chemical bonds to at least two other monomer molecules. Polymers are a class of synthetic substances composed of multiples of simpler units called monomers. Polymers are chains with an unspecified number of monomeric units. Homopolymers are polymers made by joining together monomers of the same chemical composition or structure.
Heteropolymers are polymers composed of more than one kind of monomer. Artificial Polymers and Special Properties. One of the first humans to discover, and make, an artificial polymer, was the German chemist Hans von Pechmann.
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