C. S. M a w e l University
of Arizona Tucson
1
I
The Place of Pohmers in Beginning organit Chemistry
Polymer chemistry has become such an important part of chemical technology, and polymers have come to play such a role in everyday living, that no chemist can consider himself adequately trained in his science without some introduction to this field. Since most polymer-forming reactiom are the reactions of organic chemistry, the beginning courses in the field are the logical places to introduce the beginner to this extensive new area. There are two ways in which the beginner can be introduced to the concepts of polymer chemistry. The subject can be treated as a part of the study of any group of reactions which are being examined as illustrative material for organic reactions. Vmyl polymerhation would thus fall in the section dealing with reactions of olefins; the formation of polyesters wonld come under the discussion of esters; polyurethanes would come in the section dealing with reactions of isocyanates, etc. I n teaching organic chemistry I have followed a little diierent plan. About two to three weeks of the second course in organic chemistry are devoted to a discussion of the basic reactions employed to produce synthetic polymers and the monomers used to make them. This section is readily introduced a t the point where bifunctional reactions which lead to 5- and 6-membered rings are discussed. Polymer formation by condensation is a competing reaction with ring formation when one attempts to extend cyclization to rings of smaller or larger size. This then seems to he a logical place to deal with polymers in general. After showing how polymers may be formed from bifunctional condensation and addition reactions which proceed stepwise, one can demonstrate how trifunctionality or higher functionality leads to branching or crosslinking and can discuss the general properties of these large molecules. The importance of small traces of impurities which limit the size of macromolecules by terminating the growing
chains becomes apparent. The need for new tools to determine molecular size; the new idea of crystalline and noncrystalline segments in one large molecule; the fact that all macromolecules in a given polymer sample are not of the same molecular weight; and the unusual properties associated with high molecular weight which lead to high tensilestrength, toughness, etc., fall naturally into place a t this stage of the course. Once the idea of stepwise condensation reactions is understood, the unique free radical initiated polymerization reactions do not seem quite so extraordinary to the student. Here the instructor can use the modern concepts of free radical chemistry, which are so very important in mechanisms of reactions of the usual type, and show how they apply to radical polymerization. The concepts of initiation, propagation, and termination are closely related to the other radical reactions in monomer chemistry. Likewise, cationic and anionic mechanisms of polymerization are good illustrations of the usefulness of the carbonium ion and carbauion concepts in explaining organic reactions. Synthetic organic polymer chemistry does not differ markedly from any other type of synthetic chemistry. It is to be sure more important to use carefully purified starting materials in polymerizations, since trace side reactions can cause much trouble in polymer formation. A side reactiou which produces a crosslink as often as one part in a thousand may be enough to cause a polymer to be intractable. In ordinary organic syntheses this small amount of side reaction wonld not be detected. Moreover, since polymers do not lend themselves to purification by distillation, crystallization, or sublimation, it is important that they be made in a state of high purity. The treatment of configuration and conformation of stereoregular polymers is an outstanding part of stereochemistry today. Isotactic, syndiotactic and
Volume 42, Number I, January 1965
/
3
atactic polymers serve as excellent examples of the effect of asymmetric tetrahedral carbon atoms on the properties of organic materials. Similarly, cis-1,4-polyisoprene (natural rubber) and trans-1,4-polyisoprene (gutta percha) are illustrative of the effects of geometric isomerism on physical properties. The polypropylene helix is a striking example of skew conformation and its effecton the shape of a large molecule as contrasted with the more usual trans arrangement of atoms. A good grasp of the ideas and methods used in the handling of synthetic polymers is necessary for the further study of the natural polymers such as rubber. The same is true for studying the biologically important macromolecules (carbohydrates, proteins, and nucleic acid-containing substances). The study of industrial syntheses of monomers used in polymerization is valuable training to the organic chemist since many of them are reactions not usually
W. J. MacKnighf, G. E. Leroi and A. V. T O ~ O I S ~ Y Princeton University Princeton, New Jersey
II
Physical Chemistry of Crosslinked Polysvlfide Elastomers
The basic idea of polymer science is the conception of the lmear polymer chain, a few examples of which are shown:
k ~ bc&
C~HG atactic polystyrene
CHs ethylene-propylene copolymer
Atactic polystyrene, which is the product of comnmerce, differs from isotactic polystyrene in that the phenyl groups are not stereochemically regular. In atactic polystyrene the phenyl groups are randomly "up" and "down." Ethylene propylene copolymer contains random sequences of ethylene and propylene units as well as random "up" and "down" placement of methyl groups. In the case of small molecules, crystallization in the .4
/
Journal of Chemical Education
encountered in the orthodox treatment of beginning organic chemistry. For example, the ready dehydrogenation of ethyl benzene and n-butane to yield respectively styrene and l,3-butadiene, in high purity at high conversions, would be new to most beginners in the organic field. The reaction of a mixture of 1,Sdichloro3-butene and 1,4-dichloro-2-butene with hydrogen cyanide to give 1,4-dicyano-2-butene is one that is not often encountered in student texts. Many other synthetic methods for commercial monomers are equally novel. The inclusion of this body of material into the standard courses in organic chemistry will certainly not detract from a student's general training and will introduce him to one of the most rapidly growing fields in all synthetic chemistry. If he is to work in organic industrial research, there are about six or seven chances out of ten that he will be dealing with polymeric materials.
solid state is a very nearly universal tendency. However, this is by no means the case with macronlolecules, many of which may be in an amorphous condition in the solid state due to structural irregularities. Thus, polymethylene and isotactic polystyrene are semicrystalline (partly crystalline, partly amorphous) up to their melting points of 137°C and 250°C respectively. Above their melting points these polymers become wholly amorphous. By contrast, atactic polystyrene and ethylene-propylene copolymer (mole ratio ethylenepropylene in the range of 1:1 to about 2: 1) are completely amorphous at all temperatures because of their structural irregularities. All amorphous polymers possess a glass transition temperature T,which is conveniently determined as the temperature at which a plot of specific volume versus temperature shows a change of slope. Below T , an amorphous polymer is in a hard glassy state wherein the individual chain segments are fmed on the sites of an irregular quasi-lattice. They can execute vibratory motions around these fixed positions, but do not exhibit significant diffusional and translational motion from one site to another. Above T,, the polymer segments begin to exhibit longer range diffusional motions which increase in velocity and amplitude with increasing temperature. The effect of molecular structure on T, is discussed in reference (1). Lightly or moderately crosslmked amorphous polymers are in a true rubbery state at temperatures about 30 degrees above T,. Crosslinking prevents molecular flow and insures reversible elastic behavior in the
