D15.2 Addition Polymers

Addition polymers are made by addition reactions, where two molecules combine to form a single product molecule (we have already seen examples of addition reactions involving alkenes). Typical monomers for addition polymerization have at least one C=C double bond.

The figure below shows polyethylene, an addition polymer, forming from ethylene (ethene, H2C=CH2) monomers.

Figure: Example of addition polymerization. The polymerization of ethylene to polyethylene is shown. The newly formed σ bonds between ethylene monomer units are highlighted in red (right); they are formed from the two electrons in the ethylene π bond (shown in red, left). Green carbon-atom numbers in the polymer correspond to the numbering in the ethylene monomer.

The addition polymerization reaction can be initiated by a molecule with an unpaired electron—a free radical. An example initiator is an organic peroxide, which can form two free radicals when the relatively weak O-O bond breaks:

Activity: Analyzing Peroxide Decomposition

When a radical encounters an ethylene monomer, the ethylene π bond breaks. One electron from the π bond goes to pair with the electron from the radical and form a σ bond. The other electron from the π bond remains a radical, and can go on to react with another ethylene monomer.

Figure: Polymerization of ethylene. Move the slider at the bottom to show steps in polymerization of ethylene. The curved arrows represent movement of each π-bond electron in the Lewis structure (that is, rearrangement of electron density). One π-bond electron pairs with another electron forming a new σ bond and the other becomes an unpaired electron in a new free radical.

The process illustrated in the figure above repeats with many, many more monomers, sometimes as many as 100,000 units, yielding a long polymer chain of carbon atoms.

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Chemistry 109 Fall 2021 Copyright © by John Moore; Jia Zhou; and Etienne Garand is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.