![]() The major structural difference, then, between polyethylene (PE) and polypropylene (PP) is that PE contains no methyl groups (the high density version at least) (1,2), whereas PP clearly contains them. Note from Figure 1 that the PP repeating unit contains a CH 2 and a CH 3, and that the methyl group is not part of the polymer chain, but hangs down from it, which is why it is called a pendant group. Like PE, which is made from the ethylene monomer and where the C=C bond splits open to form chains, PP is also made from a monomer where the C=C reacts to form chains. The chemical structure, infrared spectrum, and peak assignments for polypropylene are seen in Figure 1. The infrared spectrum of polypropylene (PP) is sufficiently different from that of the polyethylenes to make sorting them easy. That is why automatic sorters have been invented that use IR spectroscopy to identify objects as they go by on a conveyor belt, and then a puff of air blows the object into the appropriate bin when it leaves the belt (4). However, not all plastic articles have these identifying numbers, and manual sorting by reading the numbers and tossing into bins is too slow for the vast amount of plastics being recycled these days. In this scheme, high density polyethylene is identified by a 2, low density polyethylene a 4, and polypropylene a 5. As you may know, if you have ever looked closely at an article made of plastic, they are often stamped with a number from 1 to 7 to denote their composition. In the recycling business, it is best to sort plastics by their chemical composition to obtain purer material that is more valuable and easier to process into new things. And although I am not sure that they are the future as a young Benjamin Braddock was advised in the movie “The Graduate” (3), plastics certainly abound, which is why they need to be recycled. Most of the us know polymers by another name-plastics. By the way, I realize PE is a hydrocarbon polymer, but I chose to not discuss this term until now. This restriction to only two chemical elements may seem to limit the substances we can study, but the beauty of carbon is that it exhibits such a rich and varied chemistry that, even with just C and H as building blocks, an amazing variety of polymeric materials can be synthesized. ![]() I define hydrocarbon polymers as polymers that are composed only of hydrogen and carbon. In this column, we move on to examine the spectra of some related materials: hydrocarbon polymers. In the first two installments (1,2) in our examination of polymer infrared (IR) spectra, the only material whose spectrum we have studied so far is polyethylene (PE). We look at the IR spectra of these materials in detail, and see how IR spectroscopy can be used to distinguish them from each other. In this installment, we look at the spectra of more hydrocarbon polymers, including polypropylene and polystyrene. We continue to examine the spectra of polymers because they are important materials, and this exercise makes for an excellent infrared (IR) spectral interpretation review.
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