THE STORY : Rubber & Yale & You

Polymer chemistry began on a January day almost 160 years ago, when Charles Goodyear, a 39-year-old New Haven native, accidentally let a dollop of natural rubber and powdered sulfur fall onto a hot stove in Woburn, Massachusetts. He had spent five years, since going bankrupt in hardware, trying to make something useful from natural rubber, which is gooey in the summer and brittle in the winter. His idea had been to coat it with a powder to make it less sticky, but by dropping the mixture onto the stove he had invented "vulcanization" (Vulcan, Roman god of fire) and created the elastic form of rubber upon which we increasingly depend. That October he took a sample to Yale to seek endorsement by the dean of American science, Benjamin Silliman (whose statue is less than 25 yards to your left). Over the next five years, while perfecting his process, Goodyear hocked his watch and furniture, was jailed for inability to pay a hotel bill, and, being unable to afford a funeral for his infant son, borrowed a wagon to carry the body to burial. He got his first patent (#3,633 Improvements in India-Rubber Fabrics) in 1844. By 1852 he had enough money to pay Secretary of State Daniel Webster $15,000 (the highest legal fee in America to that time) to help him win "The Great India Rubber Suit", one of 32 cases involving infringement of his patent.

Life never ran smoothly for Goodyear. In 1855 he was awarded the Cross of the Legion of Honor by Napoleon III, but he received it while residing in Clichy, the debtors prison of Paris. He died in New York in 1860 with debts of $200,000.

[Exactly 100 years ago, when Frank Seiberling established a company in Akron to make rubber bicycle tires, he emphasized science by naming it Goodyear. Historians doubt that he intended to create confusion with B. F. Goodrich, Akron's principal industry, but he certainly did so.]

Charles Goodyear lies buried in Grove Street Cemetery, less than 550 yards from where you are now taking this exam, and only 100 yards from J. Willard Gibbs. At the time of Goodyear's death, Gibbs was a second-year Yale graduate student designing gear wheels. Probably Goodyear and Gibbs never met. Too bad, because in the 1870s they would have enjoyed discussing vulcanized rubber. Rubber has fascinated thermodynamicists since Joule and Kelvin. Now it's your chance.

Here are some relevant Web Sites:

1. Draw the idealized chemical structure for natural rubber.

2. Rubber is "isoprenoid", meaning that Nature makes it from CH₂=C(CH₃)-CH₂CH₂-OPP (isopentenyl pyrophosphate, IPP).

3. What happens to rubber molecules during vulcanization, and how does this make rubber more elastic? (You need not present specific reaction mechanisms)

5. What should happen on heating a rubber band supporting a heavy weight, and why?

In a recent organic seminar Pavel Kocovsky (University of Leicester) described the following transformation that apparently occurs in the crystalline solid state. In the solid state it proceeds slowly at room temperature (and in 15 min at 150°C), but it does not occur at all in xylene solutions, even on refluxing. (Smrcina, M. et al., J. Am. Chem. Soc. 1996, 118, 487-488)

[Note that the propagation steps involve reactive allylic pyrophosphates]

Answer to Question 2b:

Terpenes, Carotenoids, etc.

Answer to Question 3:

Vulcanization creates sulfide and disulfide bonds that cross-link the network of linear polymers. These cross-links prevent the long-range sliding of the chains past one another that constitutes plastic flow and destroys the elastic behavior.

[Think of the chains as long coil springs all jumbled together. If you deformed the bulk, the chains would begin by deforming. If you let go immediately, they could regain their original shape and arrangement (elastic deformation), but if you stretched too hard for too long the chains would move relative to one another and define a new equilibrium structure (plastic deformation). The cross-links allow local elastic deformation, but not long-range plastic flow. Proposing the type of chain motion involved in plastic deformation - snake-like "reptation" - is one of the claims to fame of P.-G. de Gennes, who gave the Silliman Lectures two weeks ago.]

Answer to Question 4:

Stretching the balloon causes it to release heat and become hot. Allowing it to contract causes it to absorb heat and become cool. The cooling is particularly striking and surprising. [Incidentally John Gough in 1802 tried to rationalize this in terms of there being less room in the stretched rubber for heat - which he regarded as caloric, a material fluid. This was a good, imaginative idea, although wrong.]

One might have thought that allowing the molecules to regain their natural shape would result in a release of heat as they achieve lower internal energy. In fact the opposite is the case. The molecules have lower internal energy (that is they lie in deeper, narrower potential energy minima) in the stretched balloon than in the relaxed one!

Of course free energy must fall in a spontaneous process like relaxation of the stretched balloon.

What drives contraction must be an increase in entropy, large enough to overwhelm the increase in internal energy. For the isolated (adiabatic) balloon the source of the increased internal energy is the vibration that represents heat, so the sample cools.

The structural rationale is clear. The stretched form is low in energy for two reasons:

Conformation. Extending the chain converts gauche conformations to anti ones, which are lower in energy.

Intermolecular packing. The extended chains can pack more tightly together and give local crystallization, which also lowers the energy.

Of course the stretched form is also low in entropy, since it is so highly ordered both in terms of conformation and in terms of packing.

Contraction of stretched rubber (and of other important biological or artificial polymers) belongs to the class of spontaneous processes that offend a chemist's intuition by absorbing, rather than releasing, heat. All of these processes, which include evaporation of a liquid, expansion of a gas, and dissolution of many solids, are driven by entropy.

Incidentally, stretching a metal spring, where deformation increases the internal energy, does cause absorption of heat. This is a normal case dominated by internal energy rather than by entropy.

Answer to Question 5:

In the tightly stretched rubber band, the molecules are highly trans-configured and locally crystallized. Warming it causes the crystallites to melt and gauche conformations to become increasingly populated. The resulting bent, coiled, higgledy-piggledy molecules favored by entropy are shorter, and the rubber band contracts upon heating.

Kelvin was probably carelessly extrapolating from a spring when he wrote in 1857, "For it is certain that an india-rubber band with a weight suspended by it will expand in length if the temperature is raised."

As in observing the temperature change, I've found that a balloon works better than a rubber band. It is important that the weight be heavy enough to cause dramatic stretching. Early stretching involves mostly change in molecular conformation, which is not so dramatically affected by heat. The latter stages of stretching are dominated by crystallization of the straightened molecules, and this is where most of the change in internal energy comes in. You will note that slight stretching of the balloon doesn't cause much heating; the last bit is the most effective. Evolution of heat is highly non-linear.

Kocovsky and his collaborators prepared separate ¹⁸O labeled and CD₃-labeled samples and resolved their enantiomers using a Chiralpak AD column on a small scale (and by aminolysis with (S)-phenethyl amine on a larger scale).

They prepared racemic solid by mixing equal parts of (R) ¹⁸O labelled material with (S) CD₃ material.

Mass spectroscopy of the product from this solid showed molecules labeled either with ¹⁸O or with CD₃, never with both or neither (except as would be expected for incomplete labeling).

Thus reaction must have occurred only between molecules of the same handedness, which is consistent with reaction of screw-related molecules in the crystal.

Of course this kind of diastereoselection might also occur in solution, but this was impossible to test, since there was no reaction in solution.