D6.2 Addition Polymers

Addition polymers are made by addition reactions, where two molecules combine to form a single product molecule. 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

Suppose you were asked to provide evidence that the O-O bond in an organic peroxide is relatively weak. You are told by another student that the O-O bond in, for example, diethyl peroxide, CH3CH2O-OCH2CH3, breaks at a lower temperature than other bonds in the molecule. Write a description of how you could verify (or refute) this student’s statement.

Write in your notebook, then left-click here for an explanation.
A reasonable approach to this problem is to look up bond energies. The weakest bond (lowest bond energy) can be broken at a lower temperature. Average bond energies are O-O, 146 kJ/mol; C-C, 346 kJ/mol, C-O 358 kJ/mol, and C-H, 416 kJ/mol. Thus, the C-C and C-O bonds are more than twice as strong, and the C-H bonds almost three times as strong, as the O-O bond. Breaking the O-O bond can occur at a lower temperature compared to the other bonds in the molecule and so the organic peroxide can form two free radicals with unpaired electrons on the O atom relatively easily.

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.

Comments
Please use this form to report any inconsistencies, errors, or other things you would like to change about this page. We appreciate your comments. 🙂 (Note that we cannot answer questions via the google form. If you have a question, please post it on Piazza.)

License

Icon for the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License

Chem 104 Summer 2024 Copyright © by Jia Zhou; John Moore; and Etienne Garand is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.