Some polymerizations join entire monomers together, whereas others join only portions of monomers and create "leftover" materials, or by-products. Co-polymers can be formed using two or more different monomers. And two or more polymers can be combined to produce an alloy, or blend, that displays characteristics of each component.
For an example, let's consider the common plastic polyethylene, which is found in such items as grocery bags, toys and bottles. The monomer ethylene is composed of two carbon atoms, each bonded to two hydrogen atoms and sharing a double bond with one another.
Polyethylene consists of a chain of single-bonded carbon atoms, each still carrying its two hydrogen atoms. To begin, we need to add a catalyst to our supply of ethylene. A common catalyst is benzoyl peroxide, which when heated has the habit of splitting into two fragments, each with one unpaired electron, or free radical.
These fragments are known as initiator fragments. The unpaired electron naturally seeks another and finds a convenient target in the double bond between the carbon atoms in the ethylene molecule. Taking an electron from the carbon bond, the initiator fragment bonds itself to one of the monomer's carbon atoms. The radical is now happy, but this initiating reaction creates another free radical associated with the ethylene molecule's other carbon atom.
Remember to show only six carbon atoms in the chain if three monomers are used. Also, remember to show the 'open' bonds at each end of the polymer section. If the structure of the polymer is given then the structure of the monomer can be worked out. The example below shows the repeating unit, ie the part of the molecule that is repeated.
Step one - A section of a polymer chain. Step two - The repeating unit. Following initiation, the process continues with the successive addition of monomer units to the chains. This is known as propagation. Propagation occurs because it lowers the overall energy of the system. In other words, there is a driving force for the reaction in that the energy sum of the produced polymer chains is lower than the energy sum of the individual monomers that produced them.
The third and final stage in the polymerisation process is known as termination. Termination of the reaction process can occur through the addition of a terminating free radical or when two chains combine. In condensation polymerisation, or stepwise growth, two different kinds of molecules react to form a larger molecule.
Styrofoam materials do not break down quickly under exposure to the elements. When buried in a landfill, styrofoam will remain intact for a long time. There is no good way to recycle Styrofoam at present, but in the future, a creative scientist may change that. Polymers are very different from the other kinds of organic molecules that you have seen so far. Whereas other compounds are of relatively low molar mass, polymers are giant molecules of very high molar mass. Polymers are the primary components of all sorts of plastics and related compounds.
A polymer is a large molecule formed of many smaller molecules covalently bonded in a repeating pattern.
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