Synthesis and degradation of poly (alkyl α-cyanoacrylates)

JOURNAL OF APPLIED POLYMER SCIENCIS VOL. 10, 1’1’. 259-272 (1966) Synthesis and Degradation of Poly (alkyl a-Cyanoacrylates) FRED LEONARD, R. K. KULKARNI, GEORGE BRANDES, JOSHUA NELSON, and JOHN J. CAMERON,* U . S. A r m y Medical Biomechanical Research Laboratory, Walter Reed A r m y Medical Center, Washington, D. C. Synopsis I n order to study structure-tissue reactivity relationships arid ultimately develop a less necrotizing adhesive, this laboratory undertook a study of the synthesis and degradation of the homologous series of a-cyanoacrylate monomers and polymers. A method for synthesizing high purity cyanoacrylates and some of their chemical and physical properties are presented. In vitro kinetics studies under heterogeneous and homogeneous conditions indicate that cyanoacrylate polymers degrade by hydrolytic scission of the polymer chain. The products resulting from such a scission are formaldehyde (positively identified by derivative formation) and ultimately an alkyl cyanoacetate. As the homologous series is ascended, the rate of degradation under neutral conditions decreases. In homogeneous solution, under alkaline conditions, the rate of degradation is considerably higher than under neutral conditions and the rates obtained with the methyl to the butyl derivative are of the same order. A proposed mechanism of degradation is presented. Medical evaluation has indicated that as the homologous series is ascended, the greater the tissue tolerance to the monomers and polymers. The relevance of the results of the in vitro studies to this medical finding is presented. The capability of rapidly polymerizing monomeric a-cyanoacrylates to adhere firmly to moist surfaces has evoked considerable medical interest in their potentialities as hemostatic agents and tissue adhesives for closure of wounds in place of, or as adjuncts to, the conventional surgical sutures.l Evaluation of methyl a-cyanoacrylate in such applications revealed that tissue inflammation and cell necrosis occurred in experimental animals.2n3 Research in this laboratory further showed that the P-14C-tagged methyl a-cyanoacrylate polymer was degraded gradually in vivo4 and was excreted in the urine and feces with none of the radioactive entities being retained in vital tissues or organs. Concomitantly, it was discovered4 that the polymer of methyl a-cyanoacrylate underwent degradation in contact with distilled water in vitro, giving rise to formaldehyde, analogous to the degradation of poly(viny1idene cyanide) , reported by Gilbert and COworker^.^ In order to elucidate structuretissue reactivity relationships and ultimately develop a less necrotizing adhesive, this laboratory undertook the * Preseiit address: J. Hopkiiis School of Mediciiie, Baltimore, Md. 259 260 LEONARD, KULKARNI, BKANDES, NELSON, AND CAMEIWN synthesis and evaluation of the homologous series of the alkyl a-cyanoacrylates. It was postulated that the higher honiologs niight degrade a t a slower rate, thereby perinittirig the degradation products to be more safely metabolized with the evocation of a lesser inflammatory response. I n this paper we report on a convenient technique of synthesizing the homologous series of monomers, their physical properties with respect to purity, and also the kinetics of in vitro degradation of poly(alky1 a-cyanoacrylates) in the presence of water, both in the heterogeneous and homogeneous phases. Other important findings on the mode of polymerization are given, and comparisons of in vitro with the in vivo degradation results are made. EXPERIMENTAL Preparation of Monomers The method described below for butyl a-cyanoacrylate is generally applicable to mononiers ranging from methyl to decyl based on modification of the work of Jeremias.6 The procedure adopted was as follows. Butyl-a-Cyanoacrylate. Paraformaldehyde (135 g.), 300 ml. of methanol, 100 ml. of diglyme (dimethyl ether of ethylene glycol), 2.0 ml. of piperidine, were placed in a two-liter three-necked flask, fitted with a mechanical stirrer, water-cooled condenser, Dean and Stark trap, and dropping funnel. This mixture was stirred and heated until the methanol refluxed vigorously. Then a 5-mole portion of butyl cyanoacetate (705 g.) was added through a dropping funnel a t a rate sufficient to maintain reflux, after removal of the external heat source. After the addition, methanol was distilled off until the vapor temperature reached 88°C. Benzene (250 ml.) was added, and water was removed from the reaction mixture by azeotropic distillation. The apparatus was converted to a conventional distillation set up, and a total of 3.5 mole water was removed. An essential characteristic of the setup was the short path through the condenser providing a small glass area in contact with the monomer vapors. At this stage, 15 g. of phosphorus pentoxide was added, benzene was removed under water aspirator, and residual benzene and diglyme were removed a t 3 mni. Hg. The vacuum distillation was continued until the pot temperature reached 160°C. to remove any residual unreacted butyl cyanoacetate. At this point, cracking began to occur, and a receiver with small amounts of pyrogallol and phosphorus pentoxide was attached to the apparatus, and the monomer was collected. Some polymer may tend to form a t this stage on the sides of the distilling adaptors; however, the replacement of the adaptors and addition of another charge of 1 5 g. of phosphorus pentoxide t o the reaction mixture minimizes this c*ondition. Over 500 g. of crude mononier collected was redistilled through a 6-in. Vigreux column. A sulfur dioxide bleed wits introduced through a capillary tube as an inhibitor to prevent anionic polymerization atid also bumping during distillation. POLY(ALKYL a-CYANOACRYLATES) 261 The mononiers obtained after redistillatioil were found to have average 1)urities of 98.Sy0 and better, based OII the pe:tk areas 011 the gas chromatograms. In many instances the lwrities were 99% or higher. Gas Chromatography. The gas chromatograms 011 the monomer samples were run on a Model F & M 700 chromatograph, with a silicone gum nitrile column (6 ft.) with Diatoport a s solid support, in dual column operation a t column temperatures of 150°C. and 170°C. The monomers were dissolved in spectrograde nitromethane or methylene chloride, to make 10% solutions prior to inject,ion in the columns to obviate the difficulties of clogging the injection syringe and needle experienced when pure monomers were used. The detailed results of the chromatographic study will be published elsewhere. Refractive indices were determined by using a Bausch & Lomb dipping refractometer a t 20°C. with sodium light. Densities and surface tensions were determined on a Westphal balance and du Nouy tensiometer, respectively, both a t 20°C. The molecular constants, parachor, and molar refraction were calculated. Preparation of Polymer Samples The poly(alky1 a-cyanoacrylates) used for the study of degradation kinetics were prepared by polymerizing the respective monomer samples by anionic initiation with methanol. Later the polymers formed were precipitated in the powder form by diluting with water and small amounts of sodium chloride. The samples were thoroughly washed with water and methanol and dried under high vacuum a t 40°C. to constant weight. Degradation in Heterogeneous Phase Poly a-cyanoacrylates degrade slowly in the presence of distilled water, producing formaldehyde as one of the products. The following general technique was used t o study this degradation. Cyanoacrylate polymer (1 g.) was placed in a Soxhlet thimble and extracted with 100 ml. of water for 4 hr. This treatment solubilizes a portion of the polymer. The water was then distilled away and the procedure was repeated with fresh water, and the quantities of formaldehyde determined on each run were plotted cumulatively against time of extraction. The amount of formaldehyde contained in the samples was estimated by the method of Bricker and Johnson,' in which formaldehyde was allowed to react with 1,8-dihydroxy-3,6-disulfonic acid (chromotropic acid) in presence of sulfuric acid. The color developed was determined by measuring the absorption a t 570 mp on a Beckman spectrophotometer. The method is as follows. A 0.5-ml. sample was placed in a 100-ml. Kjeldahl digestion flask, along with 0.5 ml. of 10% chromotropic acid reagent in water. Concentrated sulfuric acid (5 ml.) was added with continuous shaking. After digesting the reaction mixture on a boiling water bath for l/z hr., the flask was cooled, the mixture was diluted with 30 ml. of cold water and made to 50 262 LEONARD, KULKARNI, BRANDES, NELSON, AND CAMERON ml. in a volumetric flask. The absorbency of the solution was determined on a Beckman DK-2A spectrophotometer. The concentration of fonnaldehyde was estimated from an accurately determined calibration curve of absorbency against known solutions of formaldehyde. The method was found to be accurate in estimation of formaldehyde down to 2-3 ppm. Degradation in Homogeneous Phase After preliminary investigation, aqueous acetonitrile was selected as the medium for use in the study of homogeneous degradation of these polymers. Although other systems, like dioxane-water or dimethylformamide-water, could be used, these solvents interfered with the technique of determinabion of formaldehyde by the method of Bricker and Johnson.? The general method followed for the various polymers is as follows. The polymer (0.02 basal mole) was dissolved in 100 ml. of aqueous acetonitrile containing 5.04 ml. of water (0.28 mole) and refluxed under a water-cooled condenser. Aliquots (10 ml.) were pipetted out a t definite intervals and treated with 100 ml. of saturated salt solution. The precipitated polymer was filtered through a sintered glass funnel, and the filtrate was distilled. The aliquots (0.5 ml.) from the distillate were subjected to formaldehyde estimation in each case. The amount of formaldehyde obtained per mole of polymer was calculated. Identifkation of Formaldehyde That formaldehyde was produced by the hydrolytic degradation of polycyanoacrylates was proven by the preparation of two derivatives of the 2,4-dinitrophenylhydrazine dimethylcyclohexanedione (methone). The and melting points of the derivatives made from the unknown, of derivatives of a known formaldehyde sample, and mixed melting points were determined. (Table I). TABLE I Proof of Presence of Formaldehyde Derivative 2,4Dinitrophenylhydrazone Methylene bisdimedone M.P. known M.P. formaldehyde unknown sample, sample, "C. "C. 164-165 190 160-161 191 Mixed melting point, "C. M.P. M.P. (lit.) acetalformalde- dehyde hyde, (lit.), "C. "C. 163-164 190.5 166-167 191.4 168.5 140 Average Molecular Weights The number-average molecular weights were determined in acetonitrile by using the Mechrolab 301A vapor phase osmometer. I’OLY( ALK YL a-CYANOACRYLATES) 263 RESULTS AND DISCUSSION Synthesis of Monomers Although details of preparative procedures for the synthesis of monomers is available in patent literature,8*9 our experience, it was found necessary in to modify the procedures to obtain high purity products. The pyrogallol and tricresyl phosphate used in the cracking stage were found to introduce pyrolytic impurities in the distilling monomers, necessitating further fractional distillation. However, when these two ingredients were eliminated and diglyme was used as a diluent, pure monomers could be obtained in a single redistillation. The parachor and molar refraction determined on the monomer samples prepared showed a high degree of consistency within the homologous series and compared favorably with the calculated values obtained from the theoretical constants for the individual atoms, groups and bonds already established. These data are summarized in Table I1 and in Figures 1 and 2. The close agreement between calculated and theoretical values along with the data on purity obtained through the use of gas chromatography are further indications of the purity of the monomers. I NUMBER OF A L K Y L C A R B O N ATOMS Fig, 1. Parachor values of alkyl cyanoacrylates: (0) found; (--) theoretical. 264 LEONARD, KtJLKARNI, BRANDES, NELSON, AND CAMERON TABLE I1 1'hyhic::il Propert ics of Cyai ioacqilate E;.;I.era ~ -~ ~~ 13oiliiig Cyaiioniolecular acrylate ester weight Methyl Ethyl Propyl Biityl Amy1 Hexyl Hept.yl Octyl Ally1 111 125 130 153 167 181 195 209 137 1)ciisiIy "C./ mm. Hg. (20°C.), 55/4 60/3 80/6 68/1.8 113/5.4 90/1.6 12.5/1.2 117/1.8 74/4.2 1.1044 1,040 1 ,001 poiiil,, g./c.c:. 0 ,989 0.972 0.958 0,942 0.931 1 ,066 Iiidex of refwctioii (ZOOC.) Srirface tension, dynes/cm. 1 .4459 1,4391 1 .4408 1.4424 1.4440 1 .4458 1,4466 1.4489 1.4586 37.41 34.32 32.80 31.11 30.25 29.98 30.28 29.18 35.38 N U M B E R O F A L K Y L C A R B O N ATOMS found; (-) theoretical. Fig. 2. Molar refraction of alkyl cyanoacrylat,es: (0) POLY(ALKYL a-CYANOACRYLATES) 265 Fig. 3. Infrared spect,ra of poly(methy1 a-cyanoacrylate), 10% in acetonitrile: ( a ) prepared in methanol; ( b ) prepared in water; (c) prepared in 1% pyridine (as.). Polymerization Mechanism The proposed mechanism of polymerization in accordance with Coover and co-workers is as shown in eq. (1). 266 LEONARD, KULKARNI, BRANDES, NELSON, AND CAMERON Fig. 4. Infrared spectra of poly(methy1 a-cyanoacrylate), 10% in acetonitrile: ( a ) prepared in cysteine (pH 7); ( b ) prepared in palanine (pH 7); (c) prepared in glycine (PH 7). CN CN further -COOR e LOOR reaction Polymer (1) POLY(ALKY1, a-CYANOACRYLATES) 2 6 4 1 0 8 1 2 1 4 267 1 6 I8 3 TIME IN H O U R S Fig. 5. Heterogeneous degradation of a-cyanoacrylate polymers. 4 8 I2 1 6 20 24 28 32 Time (Days) Fig. 6. I n vivo degradation of a-cyanoacrylate polymers. The presence of the attacking nucleophile brings about strong electromeric ( - E ) effects which make the nitrile and the alkoxy carbonyl group highly electronegative, thereby causing polarization of the double bond. Even weak bases, such as water or alcohol, can apparently induce such effects and initiate polymerization.8-10 This mechanism requires that the base initiating species be present as an endgroup in the polymer. That such is the case is demonstrated by the infrared spectra plotted in r'1g ures ' 3-6. The polymer which is prepared in water shows the presence of the 268 LEONARD, KIJLKARNI, BRANDES, NELSON, AND CAMEROV OH group a t 3600 cm.-'. The polymer I)rcparcd in undried methailol shows a suppression of the band a t 3G00 cm.-l and the appeatraim of the OCH, absorption band a t 1100 cm.-l. These data indicate an apparent competition between the methoxy arid hydroxy nucleophiles for initiation of polymerization. Polymers prepared in aqueous solutions of pyridine (Fig. 3c) and in aqueous solutions of cysteine, alanine, and glycine (Fig. 4) show substantial suppression of the OH absorption band. These data suggest that nucleophiles other than the OH may be preferentially involved in the initiation of polymerization in the presence of these amino acids, such as the NHz groups for example. If such is the case, then the NHz groups of protein molecules in the tissue may possibly be involved in initiating polymerization of the monomers when used in vivo. Such initiation could lead t o primary chemical bonding of the adhesives t o the tissue substrate, thereby resulting in strong bonding. Some further evidence indicative of this possibility is produced by the fact that efforts to extract the in vivo polymer adherent t o the tissues, by the usual polymer solvents, such as nitromethane, dimethylformamide, and acetonitrile, were not successful. Polymer Degradation The polymers of alkyl a-cyanoacrylates degrade in the presence of water, giving rise to formaldehyde as one of the end products of the process. This has been proven by the preparation of derivatives of 2,4-dinitrophenylhydrazine and dimedone (Table I) from the aqueous extracts of the pure polymer prepared under standard conditions. Concomitantly, the production of formaldehyde is accompanied by a decrease in number-average molecular weight of the polymer. This was shown by the change in molecular weight of poly(ethy1 a-cyanoacrylate), 1427 to 990, when the polymer dissolved in 95y0 acetonitrile-5% water solution was heated at 80°C. for 24 hr. The same treatment converted a poly(buty1 a-cyanoacrylate sample from a solid powder to an oil. Heterogeneous Degradation Water-insoluble poly-a-cyanoacrylate powders are degraded in the presence of water with the formation of formaldehyde. The process of degradation reaches an equilibrium state, at which the amount of formaldehyde produced remains constant. This equilibrium value of formaldehyde is reached slowly a t pH 7 a t 25°C. but faster in neutral boiling water or in cold alkaline dispersion. The equilibrium can be shifted however and more formaldehyde produced by using fresh quantities of water. The results of such experiments on polymers in the homologous series presented in Figure 5, show that the rate of aqueous degradation is considerably slower for the polymers of the higher alkyl esters. Poly(methyl a-eyanoacrylate) degrades much faster than others, and the diminution in the degradation rate becomes smaller in the higher members of the homologous series. These data may be compared with the results POLY(ALKYL a-CYANOACHY LATES) 269 obtained from experiments in which the disappearance of radioactivity P-14C-tagged methyl and butyl a-cyanoacrylate, implanted in rats, was measured as a function of time. In Figure 6 are plotted the per cent radioactivity remaining in the implanted polymer versus time. The data show that the disappearance of the butyl polymer from the implanted sit,e is slower than for that of the methyl polymer. Homogeneous Degradation The degradation of cyanoacrylate polymers in aqueous acetonitrile, (a good solvent) is presented graphically in Figure 7 as the quantity of for- T I M E IN H O U R S Fig. 7. Horiiogeiieoua degradatioii of a-cyaiioacryliLte polymers. 270 LEONARD, KULKARNI, BHANDES, NELSON, AND CAMERON maldehyde produced, plotted against time, a t both pH 7 and 8. Analysis of the results shows that the degradation of polymers in large excess of water (1 : 14 molar ratio) obeys first-order kinetics, indicating a pseudounimolecular reaction. The data do not fit bimolecular reaction kinetics in the presence of a large excess of water. The rate constants calculated on the basis of the data are given in Table 111, along with the M , of the polymers concerned. It was attempted to study the kinetics of the reaction using equimolar proportions of water, but it was found that formaldehyde produced in the initial 4 hr. was very low and remained constant. Nevertheless, when large quantities of water are used, the concentration of formaldehyde is much larger, and the kinetics of the initial reaction before it reaches equilibrium could be studied. A control experiment run under the same conditions with anhydrous pure acetonitrile did not produce formaldehyde. TABLE I11 Rates of Homogeneous llegradatiori of Cyaiioacrylate Polymers Numberaverage molecular K , hr. -1 Polymer of weight PH 7 PH 8 Methyl ester Ethyl ester Butyl ester 2204 1533 2054 3.0 x 10-3 2 . 0 x 10-4 1.0 x 10-5 1 . 0 x 10-2 1.5 X 2 . 0 x 10-2 It is seen from these data that the rates of degradation in the homogeneous phase, as in the heterogeneous system, also are much larger for methyl polymer and are greatly diminished in case of the polymers of higher esters at p H 7. The rates of degradation however, remain high in alkaline solutions (pH 8) for all polymers and differ only slightly from each other (Table 111). The polymer solutions a t pH 8 showed a distinct yellowing tendency on refluxing, which may be indicative of a chemical reaction, perhaps intramolecular cyclization, through the cyano groups under alkaline conditions, similar to the behavior of polyacrylonitrile, which was noted by 3lcCartney.ll Degradation Mechanism The aqueous degradation process is analogous to that reported by Gilbert These authors preand co-workers in case of poly(viny1idene ~ y a n i d e ) . ~ sume the random addition of water molecules to the polymer chain, which further degrades into a polymer fraction and formaldehyde. The same mechanism may hold for poly(alky1 a-cyanoacrylate) due to the similarity in molecular structure. However, the fact that the slow degradation in neutral water is highly accelerated in the alkaline inediuin (see homogeneous degradation) suggests that the degradation is started by the initial attack POLY(ALKYL a-CYANOACRYLA'I'ES) 271 of the hydroxyl ion, leading to the reverse Knoevenagel reaction as shown in eqs. (2)-(4). CN CN I -CHz-LcHz-cw CN + OH- I -+ CN -cHz-c-cHzo~~ + ecI L o c=o I D O L o OR OR OR I (2) dR I CN CN + HOH I -* HC- L=o L OR + OH8 OR I I O CN I *CHz-C--CHzOH C I I (3) + OH- +. *CHz- 4 8"+ 8 I c=o I OR OR /OH CHz (4) O 'H (Formaldehyde) The carbarlions formed according to this mechanism can recombine with the formaldehyde product, making the whole process reversible. Medical Implications Medical evaluation of methyl a-cyanoacrylate monomer as a tissue adhesive has indicated that the polymer as well as the monomer elicits acute inflammatory responses in tissues. Further medical studies with the homologous series of a-cyanoacrylates prepared in this laboratory have indicated that as the homologous series is ascended, the inflammatory response is decreased. The butyl derivative and higher homologs appear to be well tolerated by the tissues. These data are in general accord with the in vitro studies presented in this paper. If indeed, the implanted polymers degrade in vivo as has been demonstrated in vitro, then in the vicinity of the implanted polymer particles, one may expect to find formaldehyde and an alkyl cyanoacetate (if the degradation proceeds to the ultimate stoichiometric products). Both these compounds are toxic and can elicit acute inflammatory responses. The fact that the butyl derivative is tissue-tolerated and degrades at a considerably slower rate than the methyl derivative, which is not well tolerated by the tissues, leads to the implication that the tissues can metabolize more easily the lower concentration of degradation products present at a given time in the case of the butyl derivative. References 1. Physiological Tissue Adhesive Review, Methyl-2-Cyanoacrylate Monomer, Bulletin from Medical Research Department, Ethicon, 11ic. Arbrook, Somerville, N. J. Nov. 1964. 2 . Nathaii, H. S., et al., Ann. Surg., 152, ti48 (l9tiO). 272 LEONARD, KULKARNI, BlANDES, NELSON, AND CAMERON 3. Woodward, S. C., et al., Ann. Surg., 162,114 (1965). 4. Cameron, J. L., et al., Surgery, 58,424 (1956). 5. Gilbert, H., et al., J. Am. Chem. SOC., 76,1074 (1954). 6. Jeremias, C. G., (to Eastman Kodak), U. S. Pat. 2,763,677. 7. Bricker, C. E., and H. It. Johnson, Ind. Eng. Chem., 17,400 (1945). 8. Coover, H. W., Jr., F. B. Joyner, N. H. Shearer, and T. H. Wicker, SPE J.,15, 413 (1959). 9. Coover, H. W., Jr., in The Handbook of Adhesives, I. Skeist, Ed., Reinhold, New York, 1962, pp. 409-414. 10. Gould, E. S., Mechanism and Structure in Organic Chemistry, Holt, Reinhart, arid Winston, New York, 1959, p. 217. 11. McCartney, J. R., in Polymer Degradation Mechanisms, National Bureau of Staridards, Circular 525, Washington, D. C., 1953, pp. 123-132. R6sum6 En vue d’6tudier les rapports entre la structure et la rdactivitk des tissus e t en dernkre analyse de developper un adhksif manifestant moins de nCcrose, ce laboratoire a entrepris 1’Qtudede la synthhse et l’dtude de la degradation d’une serie homologue de monomere a-cyanoacryliques et de leurs polymkres correspondants. Une methode de synthhse de cyarioacrylate de haute puret6 et de certaines de leurs proprieths physiques et chimiqiies sont present6es. I n vitro, des Ptudes cinetiques daris des conditions h6tCrogenes et homogknes indiquent que les polymhres cyanoacryliques d4gradent par scission hydrolytique de la chaine polymerique. Les produits resultant de cette scission sont le formaldehyde (identifie avec certitude par formation de ses di.rivc!s) et finalement un cyanoacetate d’alcoyle. mesure que la sCrie homologue croft, la vitesse de degradation daris des conditions neutres diminue. E n solution homogene dam des solutions alcalines, la vitesse de degradation est considerablement plus ClevCe que dans des conditions neutres, e t la vitesse obtenue avec des derives methyliques jusque butyliques sont du meme ordre de grandeur. Un mecanisme est propose pour la degradation. L’6valuation medicale indique que, B mesure que 1’011 monte dam la serie hornologrie, la tolerance des tissus s’accroit pour les monomeres et les polymkres. On prCsente Cgalement les rapports existant entre ces resultats in vitro et les donnkes medicales A Zusammenfassung Um die Beziehungen fur die Strukturgewebe reaktivitat z i i untersuchen und schliesslich ein weniger nekrotisch wirkendes Klebemittel zu entwickeln, wurde in unserum Laboratorium die Synthese und der Abbau der homologen Iteihe der a-Cyanoacrylab monomeren und -polymeren untersucht. Eine Methode zur Synthese von Cyanoacrylaten hoher Reinheit wird angegeben, und einige ihrer chemischen und physikalischen Eigenschaften werden beschrieben. Kinetische Untersuchungen in vitro unter heterogenen und homogenen Bedingungen zeigen, dass Cyanoacrylatpolymere durch hydrolytische Spaltung der Polymerkette abgebaiit werden. Die bei eiiier solchen Spaltung eritstehnden Produkte sind Formaldehyd (durch Ilerivatbildiing identifiziert) und schliesslich ein Alkylcyanoacetat. Bei Aufsteigen in der homologeti Iteihe nimmt die Abbaugeschwindigkeit uiiter iieutralen Bedingungen ab. In homogerier Lijsutig ist die Abbaugeschwiridigkeit uiiter alkitlischen Bedingungen betriichtlich hoher als unter neutralen Bedingiingeri, iitid die Gesrhwitidigkeileti der Methyl- bis Biit,ylderivate siiid von gleichgrosser Ordtiiing. Ein Abhatimechanismiis wird vorgesvhlageir. Die medizinische Auswertiitig zeigte, dass heim Aiifsteigeii iti der homologeti lieilie der Gewehevertraglirhkeit, der NIotiomereti uiid Polyrnereti ziitiimml.. Die Bedeutiitig der Ergebiiisse der in-vitro-Uiitersuchutigetifiir dieseii medixitiischeti Befiiiid wird gezeigt. Received October 22,1965 Prod. No. 1301