Sonja Krause Rensselaer Polytechnic Institute Troy, New York 12181
Macromolecular Solutions as an Integral Part of Beginning Physical Chemistry
About twenty years ago, when I first began to work in the polymer field, I was amazed to discover that many chemists felt that polymers followed their own peculiar laws of nature, somewhat different from those adhered to by ordinary small molecules. This feeling seemed akin to the awe with which organic molecules were viewed before Woehler's synthesis of urea from inorganic molecules. Unfortunately, in spite of the surge of polymer education in recent years, polymers and other macromolecules are still viewed by many scientists as peculiar sorts of molecules which may well ohey scientific laws but such laws as are somehow outside the mainstream of scientific thought. One reason for this view is quite probably the fact that macromolecules are still ignored in the basic courses taken by all chemistry students. Since I am a physical chemist, I shall take the usual undergraduate course in physical chemistry as my example. The problem begins when choosing an available textbook for such a course. Some of the more recent physical chemistry texthwks have added a chapter on macromolecules, to be sure, hut this is generally the last chapter in the hook, almost an afterthought. Other recent textbooks do not acknowledge the existence of macromolecules even to that extent. I t is only in physical chemistry textbooks written for students in the life sciences that more or less successful attempts are made to integrate at least some kinds of macromolecules into some of the topics covered. It is easy to think of a number of reasons why the physical chemistry of macromolecules should form an integral part of the first course in physical chemistry. First of all, it is well known that most chemists spend their working lives in the field of macromolecules, predominantly in the subfield of synthetic polymers hut also in the area of hiopolymers. Secondly, although macromolecules ohey the same physical laws as do small molecules, the application of these laws to macromolecules sometimes leads to unique results that are not even surmised when only small molecules are being considered. Pedagogically, it is important toshow some of theseresults to our students early in their careers. The subject of macromolecules in physical chemistry is a large one, parts of which have been covered by other papers in this Journal recently and will not be repeated here. For example, Wuuderlich,' in a discussion on the place of macromolecules in freshman chemistry, used some illustrations, including the changes in entropy when a rubber is stretched and the kinetics of polymerization as an example of a chain reaction, that are equally applicable to physical chemistry. Morton2 also discussed polymerization as a model chain reaction. Here I shall confine my remarks to the subject of macromolecules in solution and to some of the topics from heginning courses in physical chemistry into which this subject should he integrated. The examples given here are not exhaustive and reflect my own particular background and my own opinions of the important general properties of macromolecules that are fairly easily integrated into such beginning courses. What is a Macromolecule? At some time early in the physical chemistry course, it will be necessary to state a simple definition of a macromolecule. This definition should note that any macromolecule is prepared hy polymerization reactions involving only a small numher, one to twenty, of chemically distinct monomers per 174 1 Journal of Chemical Education
macromolecule. These monomers may he amino acids as in proteins, nucleotides as in RNA and DNA, sugars as in cellulose and its derivatives, vinyl compounds and dienes as in many synthetic rubbers and plastics (polystyrene and polyhutadiene, for example), diols plus dicarboxylic acids as in most polyesters (polyethylene terephthalate, Dacron, for example), and so on. Each macromolecule thus contains repeat groups, one derived from each monomer, linked by generally covalent chemical honds. Each reoeat . "eroun. occurs manv times in the macromolecule, leading to molecular weights in the ranee 1O4-108. Molecules with molecular weiahts below 5 x 1o"are nor usually considered macromuler~~les, and abwe 1 0 h o s t ex~~erimental maninulati~nsbreak the molec~~le into smaller pieces. One should then note that macromolecules can occur in a numher of different shapes. Some macromolecules can he considered rigid even in solution; such molecules may he rodlike, such as collagen (a protein), roughly 15 .&indiameter and 2800 A long with molecular weight 3 X 105. In contrast, the small rodlike carhon dioxide molecule has diameter about 1 A, length about 3 A, and molecular weight 44. Other rigid macromolecules are roughly spherical or ellipsoidal in shape, such as many globular proteins, i.e., hemoglobin, while still others have more irregular shapes, such as the immunoglobulins, which are roughly Y-shaped. Most svnthetic . oolvmers . and manv "denatured" naturallv occurring macromolecules cannot he considered rigid and may have manv different conformations in solution. This occurs because there is relatively free rotation about many honds in the macromolecule. so that manv conformations are possible. A discussion of s&h coiling (nonrigid) polymer; should probably follow a discussion of possihle conformations in the ethane molecule, i.e., staggered versus eclipsed conformations. In ethane, the three staggered conformations are equally probable, and rotation through the eclipsed conformations is possible and increases with increasing temperature. In high mblecu~arweight polyethylene, with thousands or even millions of singly bonded carhon atoms all in a row, an enormous numher of different conformations becomes nossihle. Simplist~cally,if thr conformation at each mrtam atom is w n ot i~
