ESSENTIALS OF POLYMER CHEMISTRY
Polymers are materials whose molecules are made up of many (poly~) small subunits (~mers). The most familiar of these are plastics, which are actually a broad variety of different compounds. Many natural materials are also polymers; for example, rubbers are polymers, and the main structural component of wood, cellulose, is also a polymer.
A couple of definitions are important when talking about polymer chemistry. The precursor compound for a polymer is referred to as a monomer. Monomers are the individual molecules which react to form the compound. Segmers are what monomers turn into once the polymer is formed. Oligomers are polymers that only have a smaller number of individual subunits, on the order of at most tens of segmers.
Addition polymers are a class of polymers in which bonds between doubly bonded carbons are formed to link together individual subunits. The consequence is either the loss or movement of the double bonds. The simplest case is that of polyethylene, a very common plastic. Polyethylene is a polymer of ethene (ethylene is a trivial name for ethene). Figure 3.13 depicts this transformation. Arrows indicate the movement of one of the bonds from a double bond into a new bond between the monomers
Figure 3.13. Addition polymerization
Here the double bond of an ethene molecule is reduced to a single bond, as it makes a connection to the next ethene molecule in line. This causes a chain reaction, as the new bond forces the next ethene molecule to lose its double bond, and form a bond to a third ethene. This chain reaction propagates until thousands of units of ethenes are bound into a chain.
Other commonly encountered plastics, such as polyvinylchloride (PVC) and polystyrene, are also formed by addition polymerizations. Both of these plastics have a subunit that can be seen as an ethene with a substituent replacing one of its hydrogens. For PVC, the substituent is chlorine, and for polystyrene, the substituent is a benzene ring (the technical term for this is a phenyl substitution). This concept is illustrated in Figures 3.14 and 3.15.
Figure 3.14. Formation of polyvinylchloride (PVC)
Figure 3.15. Formation of polystyrene
Rubbers are also formed from an addition polymerization, which is slightly more complex. The basic subunit here is an isoprene, which has two double bonds. The isoprene loses the double bonds at the two ends of the molecule, but gains one in the middle, for a net loss of one double bond and the formation of a polymer chain as shown in Figure 3.16.
Figure 3.16. Complex addition polymerization in the formation of rubber