# What purpose do chain reactions serve?

I just learned what chain reactions are, but I can't seem to wrap my head around them because it seems utterly useless and doesn't accomplish anything. At least that's how I understood it because the textbook doesn't have any real world applications for it. So what I am asking is do chain reactions serve any purpose. If so, it would be great if you could support your answer with a real world example or application.

• Would you mind to give an example for an utterly useless chain reaction? – Klaus-Dieter Warzecha Mar 25 '14 at 11:53
• Are you joking, open your textbook and see nuclear reactor:). I was just joking, nuclear reactor is one of the major application of chain reaction, it is common in all textbooks. – Immortal Player Mar 25 '14 at 12:13
• I imagine you mean to say it's very hard to control the products of typical radical reactions. While true, it is also true that radical mechanisms can operate on molecules which are otherwise almost inert and very hard to derivatize, such as alkanes. C-H bond activation is a major issue in organic chemistry today, yet we've known for over 100 years that radical reactions can easily substitute hydrogen atoms linked to carbon. It just happens that radical C-H activation usually comes at the cost of low chemical selectivity and the formation of many byproducts. – Nicolau Saker Neto Mar 25 '14 at 12:49

[...] it would be great if you could support your answer with a real world example or application.

Free radical polymerizations of monomers with $\ce{C=C}$ double bonds are not completely useless ;)

Let me give some examples:

• Polymethyl methacrylate (PMMA) is obtained by polymerization of methyl methacrylate (MMA). Think acrylic glass.

• Styrene can be polymerized to polystyrene (PS). Think Styrodur and food containers.

• Vinyl chloride is polymerized to polyvinyl chloride (PVC). Think pipes, insulation for electric cables, flooring.

Nuclear reactors and warheads, addition polymers (all hail Parylenes, too, and linear low density polyethylene, epoxy resins, truck liner beds, synthetic rubbers), genomes into proteomes, fire...to "gray goo." Now, some footnotes.

http://en.wikipedia.org/wiki/Living_polymerization

CONTROL. A free radical polymerization or classical Ziegler-Natta zips until it dies. Big molecular weight spread puts physical properties all over the map. Living polymerization is an non-terminating polymerization. The hot end(s) reversibly cap, allowing blocks to be assembled such that component pieces and the whole are hard by identical molecule to molecule. This is called "low dispersivity." The polymer soles of your shoes are filled Kraton thermoplastic elastomers (anionic living polymerization). They are an identical short block of polystyrene, identical long block of disordered hydrocarbon, identical short block of polystyrene. It flows when hot, the polystyrene end segments being soluble in the melt. It reversibly crosslinks as it cools, polystyrene domains coming out of solution and crystallizing. There is no manufacturing waste. Trimmings and rejects are remelted. Physical properties are remarkably controllable and absolutely reproducible. The polymer is inexpensive.

Polyethylene was a miracle, though an uncooperative one. Dow's Insite catalysts changed the world with linear low density polyethylene. The molecules now can be dialed into compositional anything, with molecular weight control. Price/performance plummeted. A plastic shopping bag weighs about five grams. Less hair-thickness film has astounding strength. On the other end Dyneema and Spectra ultra-high molecular weight linear polyethylene, especially Plasma crystallized. Much stronger than Kevlar/weight, does not hydrolyze, and it floats.

Chain reactions, their engineering and control, construct civilization on huge bulk scales.

For extra credit: Why is there exactly one silicon atom in every molecule of Kraton triblock polymer regardless of molecular weight? Where is it within the molecule?