Polyester polymers contain Group IVB metals
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both chromium and manganese bonded to the polymer's backbone. Other copolymers of styrene tricarbonyl chromium with monomers such as styrene and methyl acrylate have been made by Dr. Pittman and his associates, Dr. Paul L. Grube and Dr. Orval E. Ayers. They also have added metals to polystyrene by reacting it with chromium, molybdenum, and tungsten tricarbonyl derivatives. No degradation of the polystyrene occurs during these polymerizations. Semiconductors. Polymers containing copper atoms have semiconducting properties and, in some cases, photoconducting properties. Dr. Seiichi Kanda of Tokushima University in Japan and coworkers have produced a series of coordination polymers by reacting dithiooxamide (rubeanic acid) or iV,Af'-disubstituted dithiooxamides with cupric sulfate in aqueous solution. He, Dr. Asahi Suzuki, and Dr. Kuwako Ohkawa find that the polymers have an atactic conformation. Their studies show that the specific conductivities and activation energies of the various polymers correlate with the formula weights of the substituents—such as methyl, cyclohexyl, and benzyl groups—on the nitrogen atoms. Cyclic hysteresis of conductivity under uniaxial pressure
indicates that the macromolecules orient and relax according to plastic flow that is reversible, Dr. Kanda says. Other chemists working with organocopper polymers, Dr. Yoskiyuki Okamoto at New York University and Dr. Aleksander Golubovic at the Air Force Cambridge Research Laboratories and their associates, have prepared various arylethynyl copper polymers. These polymers have substantial back coordination from metal d orbitals to antibonding orbitals of two acetylene groups bound to the coppers. As a result, the polymers exhibit both semiconductivity and photoconductivity. Conductivity is increased if the size of the aryl group is increased by replacing the phenyl group by an anthracenyl group, Dr. Okamoto says. Electron-donating groups such as —0CH3 on the aryl rings increase conductivity, whereas electron-withdrawing groups such as —N02 reduce conductivity. Photocurrent response and decay are slow, indicating that the photocarriers are deeply trapped in the polymer. If the aryl groups have a sterically hindered structure, the photoactivity is slower. This suggests that the more complex structure provides deeper traps for the carriers, Dr. Okamoto adds.
Polymers containing two different metals can be made
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(Note: the rings are different—one monomer is a styrene and the other a cyclopentadienyl)
38 C&EN SEPT. 27, 1971
Mn(C0)s
Insect hormone use requires caution J É ^ PESTICIDES—The great expecta^ J r tions that greeted the advent of the third generation of insecticides remain undiminished. However, enthusiasm for applications of insect juvenile hormone (JH) is being tempered with caution arising from a better grasp of the practical problems of use. Field testing has begun to yield some data on the efficiency of JH, and theoreticians are zeroing in on the mechanism that governs their operation. Part of the caution expressed by several contributors to the symposium on insect juvenile hormones is due to the amount of publicity that JH has been receiving. Dr. Julius J. Menn, of Stauffer Chemical Co., tells C&EN that one of the reasons for the JH symposium in Washington was to provide an opportunity for the display of hard data. The hope was that a more practical general appreciation of the potential for JH would result. One of the extensive field testing programs for JH mimics has been conducted by Hoffmann-La Roche, Inc. Entomologist R. W. Bagley of Hoffmann-La Roche described several studies of the effects of JH mimics on a variety of insects that infest alfalfa fields in Arizona and California. Unlike the broad-spectrum insecticides, JH mimics require a better planned and longer range program of application. Mr. Bagley cites several drawbacks to JH in particular. It isn't yet certain just how stable JH may be in the environment. In some cases the effects may not be prolonged for very great periods after application. A lack of persistence could require multiple applications or better formulations than are now being used. Probably the greatest problem is timing the application of JH in the field. Mr. Bagley suggests that a much closer observation of infestation than is now employed may be necessary. The JH mimics are active only in certain well-defined stages of an insect's development, and these stages are occasionally rather short in duration. In some cases mentioned by Mr. Bagley the response may be antiproductive. If an insect's larval stage is prolonged—the general effect of JH—then the result may be higher crop damage than without application of JH. It is in the larval stages that the insects usually do most of their damage. Because JH mimics are specific rather than broad-spectrum ma-
Juvenile hormone keeps repressors (R) active ••
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terials, there may be a tendency to promote damage by one type of in sect while eradicating another type as desired. Insufficient work has been done in measuring field effects. Model. On the theoretical front, the "father" of JH, Dr. Carroll M. Wil liams, Benjamin Bussey Professor of Biology at Harvard, has proposed a new model for J H activity that in cludes a manual for building an in sect. The manual is divided into three chapters, one each for the egglarva, larva-pupa, and pupa-adult transformations. There is a gene-set for each transition. Although the mechanisms operative within the gene-sets are extremely complicated and not always understood, Dr. Wil liams suggests that understanding them may not be necessary to con trol insect development. The suspicion is that JH has little to do with specifying detailed instruc tions to particular cells. Rather, al most everything known about JH in dicates that it is involved in gene switching on a massive scale: turn ing on one set of genes and turning off another at the appropriate time. In short, it appears that J H functions in the control of entire gene-sets rather than in the internal operation of a given set. By virtue of events that begin in the embryo and continue at a di minishing rate throughout the life of an insect, cells are programed for the roles they play in development of the insect and its progeny. Dr. Wil liams and his coworkers offer no mo lecular explanation of these events but they don't believe explanation is necessary to an understanding of JH. There is no evidence, they feel, that J H plays any part in cell programing or prepatterning of an insect as a whole. To explain the action of JH, Dr. Williams proposes a model in which three gene-sets are presumed to be under control of a master regulatory gene, one for each of the transitions in his insect building manual. He fur
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ther suggests that the operator of the gene is subject to inhibition by a re pressor that is active only in the presence of a specific concentration of JH. According to this model JH is believed to be in negative control of transcription by the master reg ulatory genes. The model proposed by Dr. Wil liams can account, in his view, for nearly all of the developmental aber rations that can be induced by ad ministering JH. However, one phe nomenon that cannot be explained is the ability of JH to trigger the depo sition of yolk in eggs of certain adult species. Another is the reversal of metamorphosis—the occasional re sidua of juvenile characteristics in pu pae or adults that are caused to molt in the presence of JH. Although he recognizes that his model isn't fool proof, Dr. Williams believes that its worth should be measured in terms of its ability to trigger new thinking about insect metamorphosis. Site. One line of further inquiry encouraged by the new model is a search for the site of action of JH, which has become possible with ra diolabeled JH. If injected hormone reacts directly with the hypothetical repressors in Dr. Williams' model, a significant portion of the label should be recovered in the nuclear fraction of the homogenates. The nuclear la bel should be tightly bound to a pro tein fraction, and the JH-protein com plex should b e beneficially retained on the insect-DNA columns. If, on the other hand, the label were pref erentially bound to the nonnuclear fraction, this would argue in favor of indirect action by JH. Interest in JH, Dr. Williams notes, has already resulted in a multimil lion-dollar industry for producing JH. The JH originally isolated by Dr. Wil liams was obtained from male cecropia moths and is the methyl ester of the epoxide of a previously unknown fatty acid derivative. The moth also provides a second form of JH differ ing by only a methyl group. Both
have been synthesized, along with hundreds of hormonally active ana logs. In the living insect, J H is synthe sized by two tiny head glands that are also responsible for regulating the hormone's flow into the blood. At certain stages the hormone must be secreted and at other stages it must not. The period when J H must be absent is the weak point in the in sect's development. Contact at this time causes lethal derangement of the metamorphosis. The result is that immature larvae fail to form viable pupae or adults. The effects of JH are not restricted to the insect larva. Dr. Lynn M. Riddiford, of Harvard's biological laboratories, has found that JH ap plied to an egg-bearing female will block further development of the eggs. This ovicidal effect offers a promising means of selective insect control. In some cases, apparently, the effects may not result in ovicide but in abnormalities during larval life. These studies are continuing.
Chemists find keys to fluorine in drugs FLUORINE—Medicinal chemists have long had confidence that fluorine chemistry was their Land of Cockaigne, if they could only find the keys. In a symposium on fluorinated compounds in medicinal chemistry, a few keys were forthcoming. • Photofluorination with trifluoromethyl hypofluorite emerges as a general, safe, selective direct fluorination method for organic compounds, including bioactive molecules. • Elemental fluorine gives 5-fluorouracil and other compounds of phar maceutical interest. • A mechanism proposed for fluorine elimination from organic compounds may finally explain why biological sys tems cleave C—F bonds in some mole cules but not in others. Such a finding would leave the way open to predict ing the fate of fluorine in pharma ceuticals yet to be made. Photofluorination. Dr. Janos Kollonitsch, working with Dr. Louis Barash and George A. Dolduras at Merck & Co., Rahway, N.J., irradiated compounds with trifluoromethyl hy pofluorite (CF3OF) reagent under ul traviolet light at - 7 8 ° C. in fluorotrichloromethane, liquid hydrogen fluo ride, or trifluoroacetic acid solvents. L-Isoleucine in liquid hydrogen fluo ride, for example, gives δ-fluoro-L-isoleucine in 39% yield. Dr. Kollonitsch postulates a free radical mechanism initiated by photolSEPT. 27, 1971 C&EN
39
