Education
Color changes mark polymer reactions Polydiacetylenes are unusual among polymer families: They have conju gated backbones. And this property produces effects that have led one researcher working with the sub stances to foresee their value as an educational tool. The effect produced by the poly mer backbone is color change. When polydiacetylenes undergo various chemical and physical processes, they change color, making the processes visible. A group of 15 or 20 short experi ments thus could demonstrate visibly all the processes that monomers and polymers undergo. And the visible nature of the changes brought on by the processes would reinforce un derstanding of the theory. The idea of using polydiacetylenes as an educational aid comes from Gordhan N. Patel, staff chemist at Allied Chemical's corporate research center, Morristown, N.J. Allied re searchers are working on the com pounds with a number of potential commercial uses in mind. The educational use of polydia cetylenes is an offshoot of that work and not something Allied is pursuing. But Patel believes it would have ac ademic value. "What we see is what we believe," he says. Polydiacetylenes are unusual sub stances. The polymerization of diacetylene became known about 80 years ago. But nothing much hap pened until Gerhard Wegner of Al bert Ludwigs University in Freiburg, West Germany, revived interest in this area in 1969. A number of re searchers are now working on poly diacetylenes, including groups at the University of Pennsylvania, Case Western Reserve University, and Brown University in the U.S. At Allied, researchers have syn thesized about 200 polydiacetylenes having different substituent groups. So far, nine patents have been granted, 10 allowed, and filings have been made for five more. Most diacetylenes, R — C = C — C = C — R , are colorless solids. They polymerize in the solid state via 1,4trans addition reactions, either upon thermal annealing or exposure to high-energy radiation, such as ultra violet. Typically, the partially poly merized diacetylenes are either blue, 24
C&EN Aug. 4, 1980
backbone is blue, a slightly planar one is red, and a highly nonplanar one yellow. It is this feature that leads to the color changes as the polymers undergo conformational transitions in different processes. To highlight the educational use of these color changes, Patel has made a movie demonstrating a "process tree" he devised that shows most of
purple, or red. Fully polymerized polydiacetylenes are metallic gold or copper colored as a result of extensive derealization of electrons along the conjugated backbone. The polymers are poorly conductive, however. The color of the backbone—and therefore the polymer—is determined mainly by its planarity (effective conjugation length). A highly planar
Polydiacetylene "process tree" relates visible monomer, polymer processes (Red) X = -COOCu Precipitate + KCI
(Purple black) X = -COOH Precipitate + CuCI 2
HCI
(Bluish purple) X = -COOH Precipitates KOH pH>4
HCI, PH < 4
(Red) Solution
KCI
Nonsolvent Solvent
(Yellow) X = - COO- K + Solution
(Red) X = -COO-K+ =-COOH Solution
HCI, pH < 7 „ KOH,
pH > 7
Water (Blue) Precipitates
(Blue) Gel
(Red) X = -COOK Polyelectrolyte Saponification
(Yellow) Solution
(Yellow) Solution Remove solvent
Cool, crystallizatii
Heat, dissolution
KOH
i^lvft*
fi?M
Good solvent
(Metallic) [ = R C - C = C - C R = ] Polymer
> n
^
Heat, melting *
(Yellow) Polymer melt
Cool, crystallization Δ, h ν
*
Color intensifies
(Blue) Partially polymerized
Monomer dissolution * Monomer melting
Polymerization
Δ, h ν
* (Colorless) Monomer R = - (CH 2 ) 3 OCONHCH 2 X, R-C
==C-C
X = COO(CH 2 ) 3 CH 3 ==C-R
Cool
lh
(Purple) Residual polymer
the processes the polymer can undergo and the color changes that take place. The process tree is based on a polydiacetylene with butoxy carbonyl methylene urethane substituents—a polymer synthesized by Patel in 1976 that undergoes a color change in all of the processes. For example, it's possible to see the olymer melting and crystallizing, olydiacetylenes show intermolecular and intramolecular melting, Patel explains. On intermolecular melting the color changes from metallic to red, and on intramolecular melting from red to yellow. These changes, reversible, usually take place within 5° C. If polydiacetylenes can thus be used to demonstrate processes visually, other aspects lend themselves to more advanced experimentation.
?
For example, Patel notes that there are no experiments in laboratory manuals for determining important energetic parameters such as activation energy of polymerization, heat of polymerization, heat of dissolution of polymers, and heat of conformational transitions. Since most monomers are liquids or gases, determining polymerization energetic parameters is an involved procedure. But since diacetylenes are solids, such parameters are easily determined by differential scanning calorimetry. As for heat of dissolution, some polydiacetylenes dissolve abruptly— within a few degrees C and in less than a minute. Thus, differential scanning calorimetry can be used for determining this parameter as well. James H. Krieger, Washington
Process
Color transition
Origin of the color transition
Polymerization
Colorless—» Blue (or red)
Formation of conjugated backbone
Polymer conversion increases
Blue or red color intensifies
Concentration of polymer chains increases
Extraction of unreacted monomer
Blue
> Red
Planar nonplanar
Phase change in a partially polymerized diacetylene
Pink
> Blue
Slightly planar Planar
Melting of polymer
Metallic
> Yellow
Planar — > Nonplanar
Crystallization
Yellow
> Metallic
Nonplanar—> Planar
Dissolution Precipitation, gelation Drying
Metallic Yellow Blue
> Yellow > Blue > Metallic
Planar—> Nonplanar
> Slightly >
Nonplanar—> Planar Planar—> Planar (reflection)
POLYELECTROLYTES Hydrolysis (hydrophobic to hydrophilic) Salting out
Yellow > Red (solution) (precipitate)
Nonplanar planar
Yellow
Nonplanar > Slightly planar
> Red
Decrease pH (10 >2)
Yellow > Red Dark red > Purple > Blue
Increase pH (2 >10)
Blue—» Red—» Yellow
Exchange of H+ by K+(Planar > Nonplanar)
Ion exchange
Yellow
> Red
• Add CuCI2 • Add HCI
Exchange of K+ by Cu+2 (Nonplanar > Slightly planar)
Red
> Blue (black)
Exchange Cu+2 by H+ (Slightly planar > Planar)
• Add KOH
>
> Slightly
Exchange of K+ by H+ (Nonplanar > Planar)
Biue (black) —» Yellow Exchange H+ by K+ (Planar > Nonplanar)
Science Mushroom toxins yield to analysis Mushrooms deserve their notoriety for producing peculiar poisons. Recently, several previously undescribed mushroom toxins, including one family of compounds that's not dangerous to man and another compound that caused a small outbreak of poisonings and several fatalities in Poland during the 1950's, have yielded to chemical analysis. The task apparently isn't easy. According to one chemist who is an avid amateur mycologist, the challenge of doing good chemistry on rare mushrooms often results in a mismatch of talents. Few chemists know much about mushrooms, and so their analyses often may be wasted on improperly identified specimens. And the reverse case creates its own set of problems wherein expert mycologists find themselves at a loss when fine chemical analysis is needed. Fortunately there are exceptions to these rules. And the recent (and long-term) efforts of Theodor Wieland, Heinz Faulstich, and their colleagues at the Max Planck Institute for Medical Research in Heidelberg, West Germany, to analyze and describe the toxic constituents of several varieties of poisonous Amanita mushrooms are a good example. "Theif work represents German research in the finest tradition," one chemist tells C&EN. The Heidelberg group recently has characterized a group of toxins, named "virotoxins" after the species Amanita virosa from which they come (Biochemistry, 19,3334 (1980)]. Though not poisonous to man as such, these toxins are potent biochemical inhibitors and poisons once they reach certain constituents in human cells. Ordinarily, the virotoxins are prevented from reaching those target constituents, and thus information about their behavior is more an object lesson in natural chemistry than a description of something to avoid during nature walks. The virotoxins are peptides, containing amino acids having both D and L configurations. The virotoxins also contain two amino acids, 2,3trans3,4-dihydroxy-L-proline and 2/-(methylsulfonyl)-L-tryptophan, that had not been found in nature before, according to the German group. These toxins are similar in several ways to the phallotoxins, which are toxic peptides from another Aug. 4, 1980 C&EN
25
