Thursday, August 25, 2011

Sex, Blogs, and Videotapes

Most any lengthy composition has a sequence, including things as disparate as polymers, novels, films, or even a blog. All these different things are sequentially pieced together from smaller subunits. For polymers, the subunits are monomers, for novels they are alphabet characters, for films and videotapes, they are "frames," and for blogs, they are separate posts (which themselves comprise a smaller sequence of characters).

It might be fun to explore the mechanics of stringing together novels, films, and blogs, etc. For polymers, I already mentioned "step growth polymerization" back here, describing how Wallace Carothers mastered the art of making long-chain synthetic polymers like nylon and neoprene. One way for a non-chemist to visualize step-growth polymerization is to imagine a very large group of unattached people, each willing to reach out and join hands with another to begin forming a human chain. Imagine how this must work at the beginning of the process. All are separate. In a first step, two people get together, each joining just one hand to make a pair, but each leaving one hand free. Now that pair could get together with another like pair to make a chain of four, but it's unlikely to do so because in a sea of singletons it's far more likely that the first pair will just hook up with another single person to make a trio. Again, that's not a choice thing -- it's a statistical thing because the number of available singletons far exceeds the number of available pairs -- at first. Look at the chain growth profile labeled "step-growth" in this chart (the red curve): 

For step-growth polymerization, not until the very end of the coupling orgy do the relative amounts of already linked members far outnumber the available remaining singletons and the daisy-chaining really takes off because having consumed all the singletons, the short chains must join hands with the ends of other chains. In the graph, the step-growth mechanism is contrasted with the living chain growth mechanism (straight line) which grows steadily. An example of the living growth chain mechanism is the Zeigler-Natta mechanism mentioned back here. Comment threads on blogs are also like "living" polymers.

Taking a closer look at the handholding analogy, it's a sanitized whitewash of what's really going on. The monomers in nylon are really "gendered" i.e. there are two types of monomers being joined: there is a "male" monomer, 1,6-diaminohexane which looks like this:

and a female monomer, 1,6-dicarboxyhexane (also called adipic acid) which looks like this:
I've already called attention to the "male" nature of amines, and the "female" nature of acids here. If that analogy bothers you, think instead of plugs and sockets or hands and gloves.

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