Addition polymers are usually made from a monomerOne of the units that joins with other units to form a polymer. containing a double bondAttraction between two atoms (nuclei and core electrons) that results from sharing two pairs of electrons between the atoms; a bond with bond order = 2.. We can think of the double bond as "opening out" in order to participate in two new single bonds in the following way:
Thus, if ethene is heated at moderate temperatureA physical property that indicates whether one object can transfer thermal energy to another object. and pressureForce per unit area; in gases arising from the force exerted by collisions of gas molecules with the wall of the container. in the presence of an appropriate catalystA substance that increases the rate of a chemical reaction but that undergoes no net change during the reaction., it polymerizes:
Some Common Addition Polymers.
The result is the familiar waxy plastic called polyethylene, which at a molecular level consists of a collection of long-chain alkaneA hydrocarbon containing only single bonds between carbon atoms. molecules, most of which contain tens of thousands of carbon atomsThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume.. There is only an occasional short branch chain.
Polyethylene is currently manufactured on a very large scale, larger than any other polymer, and is used for making plastic bags, cheap bottles, toys, etc. Many of its properties are what we would expect from its molecular composition. The fact that it is a mixtureA combination of two or more substances in which the substances retain their chemical identity. of molecules each of slightly different chain length (and hence slightly different melting pointThe temperature at which a solid becomes a liquid. Also called freezing point.) explains why it softens over a range of temperatures rather than having a single meltingThe process of a liquid forming from a solid. point. Because the molecules are only held together by London forces, this melting and softening occurs at a rather low temperature. (Some of the cheaper varieties of polyethylene with shorter chains and more branch chains will even soften in boilingThe process of a liquid becoming vapor in which bubbles of vapor form beneath the surface of the liquid; at the boiling temperature the vapor pressure of the liquid equals the pressure of the gas in contact with the liquid. water.) The same weak London forces explain why polyethylene is soft and easy to scratch and why it is not very ‘strong mechanically.'
The table above lists some other well-known addition polymers and also some of their uses. You can probably find at least one example of each of them in your home. Except for Teflon, all these polymers derive from a monomer of the form
The resulting polymer thus has the general form
By varying the nature of the R groupThose elements that comprise a single column of the periodic table. Also called family., the physical properties of the polymer can be controlled rather precisely.