Many reactions (such as those in polymer formation and nuclear fission) are involved in processes called chain reactions. A chain reaction can be shown by standing LEGO®s (i.e. 2 x 4 peg bricks) on end in patterns and knocking them down like dominos. To make the LEGO®s easier to knock down a long, narrow LEGO® (i.e. 1 x 6 peg brick) is added to the top of each brick. This not only makes the standing LEGO® bricks top-heavy, but when they fall they can cover a larger area, allowing branching chain reactions to be shown.
A chain reaction can be subdivided into smaller reactions, or steps. These steps can be classified as initiation steps, propagation steps, and termination steps. An initiation step begins the chain reaction; the reactants involved in this step are called initiators. One or more products of the initiation reaction become reactive species that then move on to react with another substance in the system. This is the first propagation step. The products of this reaction go on to react with more of the substance in the system. These propagation steps continue for as long as more reactants are available or if a termination reaction step occurs. If no more reactants are available, the chain reaction stops.
To halt a chain reaction with reactant remaining, substances called terminators must be included in the system. These substances will themselves react with the reactive species in the chain reaction, but then do not become reactive themselves. The remaining reactant in the chemical system may stay unreacted or may be consumed by another chain reaction.
The quantity of terminators in a polymerization chain reaction controls the average chain length of the polymer chains and thus the polymer physical properties. Many terminators tend to result in formation of many short polymer chains; relatively few terminators tend to result in formation of a few long chains.
Chain reactions do not have to be linear processes. Some chain reactions branch. That is, one propagation step triggers more than one concurrent propagation step. This sort of chain reaction occurs during sustained nuclear fission reactions. For example, when a uranium nucleus splits it releases two or three neutrons, each capable of splitting other uranium nuclei. This geometric progression of nuclear reactions, each capable of releasing energy, results in the tremendous energy output of nuclear reactors and bombs
The template pattern for setting up LEGO®s to demonstrate the branching chain reaction may be downloaded here as an Adobe Acrobat (.PDF) file. When the template pattern is printed on a transparency the demonstration may be set up on a level overhead projector.
Copyright 2001 by the University of Wisconsin - Madison Materials Research Science and Engineering Center for Nanostructured Materials and Interfaces, Board of Trustees of Bradley University. All rights reserved. This project was supported, in part, by the National Science Foundation. Opinions expressed are those of the authors and not necessarily those of the Foundation. LEGO and DACTA are trademarks of the LEGO group and are used with special permission. © 2001 the LEGO group. The illustrations in these experiments are used here with special permission.