How Are Polymers Made?
from
GROSBERG, et al - Giant Molecules: Here, There, and Everywhere... / Academic Press 1may1997
We have talked about different types of molecules. Now let's explore how all these various types are actually made, ranging from the simplest linear polymer chain to a polymer network of a complex, densely entangled structure. In a living cell, chains of biopolymers are "built" by special systems in a process called
biosynthesis, and there are also natural ways to prepare even more complex polymer systems.
But how are artificial polymers synthesized? This is a major task of polymer chemistry. This book, however, is meant to concentrate on physics, so we shall not discuss this question in any great detail. Nevertheless, it might help to have some general idea of the methods of polymer synthesis. It would let us understand physical properties of polymers better and more profoundly.
Long polymer chains are synthesized from low molecular weight compounds that are monomers. There are two main methods of synthesis: polymerization and polycondensation.
During polymerization, monomers are joined successively to the main chain, according to the rule AN + A ---> AN+1. For example, polyethylene (Figure 2.1a) is obtained through polymerization of ethylene, CH2 = CH2. Under some conditions, two ethylene molecules can form butylene:
CH2 = CH2 + CH2 = CH2 CH3 — CH2 — CH = CH2. (2.1)
Then a butylene molecule can react with another molecule of ethylene to make a six-carbon chain, and so on. "Capturing" more and more ethylene molecules, the chain becomes longer and longer and eventually grows into a macromolecule of polyethylene (Figure 2.1a).
From this example we can discern the main features of the polymerization process. First, to enable this kind of synthesis, a monomer molecule has to have at least one double (or triple) chemical bond. Second, the whole process is merely a rearrangement of chemical bonds between the molecules (e. g., a double bond transforms into two single ones). This is why no byproducts are normally created during polymerization, and the growing molecule consists of exactly the same atoms as the initial compounds.
It would be natural to ask here what conditions are needed for a chain to start growing. And when does the process stop? Apparently, a reaction like (2.1) cannot begin of its own accord. A lot of energy is required to create an active center of polymerization, which may be a free radical or an ion. The energy can come from heat, light, or radioactive radiation. Alternatively, one could sprinkle around some so-called initiators—special substances that can easily form free radicals (hydrogen peroxide is an example). As soon as an active center has formed, polymerization continues by itself with no outside help.
If an active center at the end of a chain ceases to exist (say, a free radical becomes a molecule, or an ion becomes an atom, etc.), then the chain stops growing. It is said in this case that a break in the chain occurs. This can happen for natural reasons (if, for example, two free radicals come together at the end of the chain), but it can also be deliberately stimulated by special substances called inhibitors. Obviously, the chain will also stop growing if the supply of monomers becomes exhausted.
Polycondensation is rather different. Segments of a polymer chain, with free radicals at the ends, gradually join on to each other: AN + AM AN+M. "Esterification" is an example of such a process:
R—COOH + R'—OH <---> RCOOR' + H2O,
where R and R' are two segments of a polymer chain. The chain, of course, will grow further if the piece of the chain RCOOR' is able to attach itself to another similar piece; obviously, this can only happen if there are functional groups (like COOH or OH) at the ends of it.
During polycondensation low molecular weight substances are normally produced. (For instance, in the reaction (2.2) that substance happens to be water.) This is why, in contrast to polymerization, the content of a growing molecule changes compared to that of the initial compounds. Another special feature of polycondensation is the reversibility of reactions like (2.2). Creation of longer chains and their destruction are both happening at the same time. The latter, in fact, is mainly caused by low molecular weight products. So if one is aiming to obtain reasonably long polymer chains it is a good idea to get rid of the low molecular weight components during the reaction.
It would seem that polymer chains constructed from a mixture of monomers as a result of random chemical reactions should have a rather wide distribution in their lengths. This is indeed true, and the name for this phenomenon when chains of various lengths coexist in a polymer substance is polydispersity. Polydispersity has to be considered when analyzing polymer properties. In practice there are ways to reduce polydispersity by separating chains of different lengths.
Heteropolymers can also be synthesized as described above, but, of course, there must be a few different types of monomer in the mixture.
Now let's talk briefly about branched polymers. If, say, polycondensation is going on and the initial monomers have only two functional groups each, then we shall end up with linear polymer chains (with a small proportion of ring chains). However, if the monomers have three or more functional groups, a branched macromolecule can be synthesized (see Figure 2.7c). Given plenty of "multifunctional" monomer units at the start, one can even obtain a polymer network (Figure 2.7d).
Branched macromolecules and polymer networks can also be formed by the cross-linking of linear chains. There are various chemical ways of cross-linking. Sometimes chemically active cross-linking agents are used; they establish covalent bonds between different chain strands. Alternatively, ionization in a polymer system can be stimulated by radiation, etc. The simplest everyday-life example of cross-linking is vulcanization—during this process viscous natural rubber becomes a highly elastic polymer network (see Chapter 6 for more details).
Giant Molecules: Here, There, and Everywhere...
Alexander Yu. Grosberg
Massachusetts Institute of Technology
Institute of Chemical Physics, Russian Academy of Sciences
Alexei R. Khokhlov
Moscow State University
Institute of Organoelement Compounds, Russian Academy of Sciences
Academic Press
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