Principles of Polymer Chemistry

The initial proof of the existence of very large organic molecules was supplied by Raoult [1] and van’t Hoff [2], who carried out cryoscopic molecular weight determinations on rubber, starch, and cellulose nitrate. By the methods developed by Raoult and by van’t Hoff and by the formulation of solution laws, molecular weights of 10,000–40,000 were demonstrated. Unfortunately, chemists of that day failed to appreciate this evidence and refused to accept it. The main reason for such a response was the inability to distinguish macromolecules from colloidal substances that could be obtained in low molecular weights. The opinion of the majority of that day was that “Raoul’s solution does not apply to materials in colloidal state.”

For a very large proportion of polymeric materials in commercial use, mechanical properties are of paramount importance, because they are used as structural materials, fibers, or coatings and these properties determine their usefulness. Properties that also determine their utilization are compressive, tensile, and flexural strength, and impact resistance. Hardness, tear, and abrasion resistance are also of concern. In addition, polymers may be shaped by extrusion in molten state into molds or by deposition from solutions on various surfaces. This makes the flow behaviors in the molten state or in solution, the melting temperatures, the amount of crystallization, as well as solubility parameters important.

Polymerizations by free-radical mechanism are typical free-radical reactions. That is to say, there is an initiation, when the radicals are formed, a propagation, when the products are developed, and a termination, when the free-radical chain reactions end. In the polymerizations, the propagations are usually chain reactions. A series of very rapid repetitive steps follow each single act of initiation, leading to the addition of thousands of monomers.

Ionic polymerization can be either cationic or anionic. This difference stems from the nature of the carrier ions on the growing polymeric chains. If in the process of growth, the chains carry positive centers, or carbon cations, the mechanism of chain growth is designated as cationic. On the other hand, if the growing chains carry negative ions, or carbanions, then the polymerization is designated as anionic.

Formation of polymers through ring-opening reactions of cyclic compounds is an important process in polymer chemistry. In such polymerizations, chain-growth takes place through successive additions of the opened structures to the polymer chain:

Polyethylene is produced commercially in very large quantities in many parts of the world. The monomer can be synthesized from various sources. Today, however, most of ethylene comes from petroleum by high temperature cracking of ethane or gasoline fractions. Other potential sources can probably be found, depending upon the availability of raw materials.

There are many naturally occurring polymeric materials. Many are quite complex. It is possible, however, to apply an arbitrary classification and to divide them into six main categories. These are:

In consideration of various chemical reactions of macromolecules, the reactivity of their functional groups must be compared to those of small molecules. The comparisons have stimulated many investigations and led to conclusions that functional groups exhibit equal reactivity in both large and small molecules, if the conditions are identical. These conclusions are supported by theoretical evidence [1, 2]. Specifically, they apply to the following situations [1]:

Supports are materials that are used for immobilization of various reagents, catalysts, drugs for release. Many of them are specially prepared macromolecules. Reagents and catalysts on support find applications in organic syntheses, biochemical reactions, special separations, and analyses. They also find uses in medicine for drug release, etc. An advantage of immobilized polymeric reagents in chemical reactions is that they can be separated, often easily by filtration, from the products of these reaction. Cross-linked polymeric reagents have an additional advantage in that several different polymeric reagents can be used simultaneously without the functional groups being accessible to each other for interaction. Reactions of some compounds in solution require high dilutions. Immobilization, however, may permit the same reactions to be carried out at relatively high concentrations. Immobilization can also be very useful in syntheses that consist of many steps, where the undesired by-products from each step can simply be washed away. This avoids lengthy isolation and purification procedures [2]. Also, by immobilizing on a polymer, the macromolecule may provide microenvironmental effects to the attached species for the reactions. These may include special electronic and steric conditions that could be different from those existing in bulk or in solution.