Polymers
One important class of organic structure we do not really have time to cover in class is polymers. Polymers have obviously transformed the material aspects of our lives in the last 75 years or so. The study of polymer chemistry and physics is a huge branch of science, and what we will talk about here merely scratches the surface. It is important to note that polymers are made from many different kinds of precursors, although alkenes are probably the most important precursor functional group. Importantly for us at the moment, the reactions that join the molecules are not all like those of alkenes. I am not going to even attempt to review together to make polymers are often radical reactions. Here we talk about one application of polymer chemistry that I have been somewhat involved in over the years that you may find interesting, and to introduce this we ned to talk about radical reactions in a slightly different context.
Radical polymerization has traditionally been the most important method for making polymers, so let's look at the basic principles. Phenylethylene is called styrene. If a radical (R.) can add to the C=C double bond, then a new radical is formed (see below). If this radical adds to another styrene molecule, then another radical is formed which can add to another styrene etc etc. The product of joining together many styrene molecules in this way is polystyrene, i.e. the polymer of styrene monomer. The reaction is a chain reaction. very much like the one we looked at earlier in the semester involving bromination of alkanes. Actual polystyrene consists of anywhere from 10,000 - 1000,000 styrene monomer joined into one huge polymer molecule.
Ethylene itself can be polymerized to make polyethylene, methyl methacrylate is polmerized to make polymethylmethacrylate, vinyl acetate is polymerized to make polyvinyl acetate etc.
Where do the radicals (R.) come from to start the polymerization? Most polmerization reactions are started by adding an initiator molecule to the monomer, a very common one is AIBN. Heating the AIBN causes homolytic cleavage of the CN bonds, and formation of 2 t-butyl radicals. A t-butyl radical adds to an alkene monomer, initiating polymerization. The polymerization ends when one of the radicals does something other than add to another monomer. This would be a termination reaction, again, just as in the alkane bromination reaction we studied earlier in the semester
The radicals are generated in a similar way to formation of bromine atoms when Br2 is irradiated (again as we discussed), except that light is not necessary, and it works just by heating the AIBN. Another initiator is benzoyl peroxide (otherwise known as the remover of teenage zits and angst!), which forms 2 phenyl radicals when heated (This is another way of initiating a radical chain reaction, similar to Br2 + light. This is fine if you just want to turn a load of monomer into polymer. However, if the radicals can be generated by shining light on an initiator, then the polymerization reaction can be controlled both in time (turning the light on starts the reaction, turning it off stops it), and space (where you shine the light you make polymer, where you don't shine the light you don't). In our lab, we are working on ways to make radicals with light that can be used to do various things, such as initiate polmerization.
The chemistry is a bit complicated and uses many concepts we do not discuss in general Organic Chemistry, but here is the basic idea. A dye molecule (something that is colored and thus absorbs light) is excited by the light and gives a single electron to another molecule, the initiator. In this example, the initiator is called MPF, and its structure is shown below. The MPF was originally positively charged, and when it receives the electron it becomes a neutral radical. The NO bond in the radical breaks to generate a methoxy radical, which can initiate polymerization, for example of methyl methacrylate. There are many reasons why this photochemical reaction is useful, too many to go into here, but let's look at just one, i.e. the formation of printing plates.
How do you make a printing plate containing, for example, a picture? Obtain the picture in the form of a transparent image (shown as the red sheet below), then shine light through it. Below the light passes through the cross part, but is blocked by the red part. The light that passes through the image hits a coated plate containing a mix of dye/MPF and a polymerizable monomer. Where the light shines, a polymer is formed, where the light does not shine, a polymer is not formed. In this case, polymer is formed in the shape of a cross (black image ion the coated plate). The plate is then washed. The washing removes the non-polymerized dye/MPF/monomer, but because the polymer is less soluble, it is not washed away. After washing, there remains a relief image of the cross that can be used as a printing plate.
This is a fairly simple application of "photoinduced polymerization" that I got somewhat involved in when I worked in industry, there are many others. For example, printed circuit boards are manufactured using this principle, and indeed, the circuits in electronic microchips are patterened in an analogous fashion. "Curing" (i.e. hardening) of a polymers coat on an object can be initiated this way.
In our lab we do not study the polymerization process itself, instead we study the fundamental chemistry involved in the electron transfer step and the bond breaking reaction to form the radicals. Actually, we are using the radicals to do something quite different from initiating polymerization in the lab, we are using them to cut DNA, but that is for another time :)