Abstract
The simple picture of molecule as a tiny particle that interacts with its neighbors while being buffeted by thermal motion has proved valuable. However, when molecules become very large, they gain certain properties distinct from their smaller cousins that make them an interesting group to look at in their own right. “Very large” is intentionally a vague definition but if we take it to mean molecular weights in the tens of thousands and greater, then all of the molecules in this group are polymers. Polymers are formed from the combination of a series of smaller molecules (i.e., monomers) to form a chain. Thus, the fundamental requirement for a molecule to be able to polymerize is that it needs at least two reactive groups. If a molecule has one reactive group, then it can react with a second molecule to form a dimer but this blocks the reactive sites on both molecules and prevents further polymerization. If a molecule with two reactive groups forms a dimer, it blocks one reactive site but still has another available to continue the reaction and lengthen the chain. If a monomer has more than two reactive groups, it can form a branched chain.
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Notes
- 1.
In many cases, steric hindrance and other factors mean the chain is much less flexible than suggested here. In these cases, the chain can be described as equivalent to another, ideally flexible, chain made up of a number of Kuhn segments where the length of each segment, the Kuhn length, is greater than one monomer.
- 2.
Note thatχis large because of the hydrophobic effect “repelling” water and nonpolar molecules. As we saw in Chap. 2, the hydrophobic effect is largely due to entropy changes resulting from ordering of water molecules. We have talked aboutχas a purely enthalpy term but it is quite straightforward to treat it as having an entropic component as well.
- 3.
The structural requirements for polymer helix formation will be discussed below in the context of polysaccharides. In anticipation of this, it is worth noting that collagen has an unusually simple and repetitive primary structure.
- 4.
Be careful here—polymer crystallization was used to calculate the protein structure in Fig. 7.7. In that case, a crystal was made by packing many polymer molecules together with each molecule serving as a unit of the crystal lattice. Protein crystals can be grown in the laboratory but it takes a lot of patience and special conditions. In polysaccharide crystallization, the crystal structure occurs over part of the chain length and can occur spontaneously in food and in nature.
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Coupland, J., Ettelaie, R. (2014). Polymers. In: An Introduction to the Physical Chemistry of Food. Food Science Text Series. Springer, New York, NY. https://doi.org/10.1007/978-1-4939-0761-8_7
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DOI: https://doi.org/10.1007/978-1-4939-0761-8_7
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