No abstract

The oxidative degradation of PMR (for polymerization of monomeric reactants) polyimides at elevated temperatures was followed by cross-polarized magic angle spinning (CP-MAS) NMR. Labeling of selected sites in the polymers with 13C allowed for direct observation of the transformations arising from oxidation processes. The formation of several degradation products has been proposed to be occurring in the cross-links derived from the nadic end caps on oxidation. Model compounds have been synthesized and characterized by CPMAS NMR with both normal and delayed decoupling to distinguish between protonated and unprotonated carbons. Comparison of these spectra to predicted chemical shifts of proposed products for the aged polymer provides further insight to degradation occurring in the cross-linked moieties.

No abstract

We consider possible replacements for the norbornenyl end cap in addition polyimides that should favor bridge degradation and lead to lower weight loss in the resin system. Our preliminary evaluation demonstrates that molding powder made with 7-hydroxynorbornene-2,3-dicarboxylic acid, 17, as the end cap can be fully imidized at 200 °C. By differential scanning calorimetry (DSC), onset of cross-linking occurs approximately 50 °C lower than that for the unsubstituted end cap. The hydroxy-bearing carbon on 17 is more easily oxidized to carbonyl on aging in the polymer than the parent end cap. Therefore, the new end cap more highly favors bridge degradation. However, processing studies, long-term weight loss, and careful evaluation of degradation mechanisms must be carried out to fully assess 17 as a more thermo-oxidatively stable replacement for the parent norbornenyl end cap.

No abstract

Three reductones, 2,3-dihydroxy-4,4-diphenyl-2,5-cyclohexadien-l-one (11), 3,4-dihydroxycoumarin (35), and 3,4-dihydroxyspiro[5.5]undecan-3-en-4-one (64), were prepared and subsequently reacted with superoxide anion radical (02*-), generated from K02 and 18-crown-6 polyether. The reactions were carried out in aprotic media and quenched with methyl iodide which facilitates the trapping of the various oxyanions formed. While a plethora of products were formed in each case [2-hydroxy-2-methyl-4,4-diphenyl-5-cyclohexene-l,3-dione (17), dimethyl 4,4-diphenylglutaconate (18), methyl 4,4-diphenyl-3-butenoate ( 19), phenylcinnamaldehyde (20), methyl 3-phenylcinnamate ( 21), and benzophenone (22) from 11; 3-hydroxy-2-methoxycoumarin (39), 2-carbomethoxy-2-hydroxy-3coumaranone (40), 2-hydroxy-2-methyl-3-coumaranone (41), methyl o-hydroxyphenylglyoxylate (42), methyl salicylate (43), and catechol (44) from 35; and 2,4-dihydroxyspiro[5.5]undeca-l,4-dien-3one (66), 2-hydroxyspiro[4.5]dec-l-en-3-one (70), dimethyl 1,1-cyclohexanediacetate ( 73), and dimethyl a-keto-l-[(methoxycarbonyl)methyl]cyclohexane-l-propionate) (75) from 64], an overall analysis of the product distribution indicates that the basic elements of the reaction sequence are the same. The first step involves facile deprotonation and the concomitant generation of the reductone monoanion, a process which lends support to the suggestion of Afanas'ev and co-workers (Afanas'ev, I. B.; Grabovetskii, V. V.; Kuprianova, N. S. J. Chem. Soc. Perkin Trans. 2 1987, 281-285). Oxidation of this monoanion yields the corresponding triketone. Of the various options available to this polyketone, superoxide attack at the most electrophilic central carbonyl followed by oxidative cleavage and/or benzylic acid rearrangement are clearly the most prominent. These are followed by a variety of base catalyzed autoxidative processes which are highly dependent on the nature of the substrate.

for optimum performance. The use of immobilized whole organisms of this type, or of crude extracts from them, would be the most efficient method for preparation of the required enzymatic activities.

Bicyclic cyclopropenes 14a, 14b, and 26 were prepared by various synthetic routes. Polymer rose Bengal (p-RB) photosensitized oxygenation of bicyclooctenes 14a,b in CDCl(3) proceeded sluggishly (variable O(2) uptake of ca. 0.35-0.75 equiv in 8 h) and was accompanied by sensitizer bleaching. Preparative gas chromatography of the complex product mixtures from 14a and 14b yielded both dienes (Z- and E-29, 30, and 31) and enones (E- and Z-12, 32, 34). By contrast, p-RB photosensitized oxidation of bicyclononene 26 in CDCl(3) proceeded somewhat more rapidly (O(2) uptake of ca. 1 equiv in 2.5 h) yielding enones (20, 42-45) exclusively upon GC separation. The diene products, observed in the case of 14, result from the thermolysis of the remaining unreacted cyclopropenes, while the enones are the oxygenation products. The oxygenation was slowed by radical inhibitors, but not by (1)O(2) quenchers; nor were any oxidation products observed when these cyclopropenes were reacted with triphenylphosphine ozonide, a chemical (1)O(2) source. The data indicates that a photosensitizer-initiated free radical autoxidative process is involved. Likely intermediates in this oxygenation are epoxide 27 or 37 and hydroperoxide 28 or 38, for the bicyclooctene (14) and bicyclononene (26) systems, respectively. The absence of (1)O(2) product in these cyclopropene systems, in contradistinction to their higher homologues, may be attributable to either the relatively long C(alpha)-H(allylic) distance in alkylcyclopropenes, which places the abstractable allylic hydrogen "out of reach", or their relatively high IP. Either, or both, of these factors may have slowed the rate of the singlet oxygenation of the cyclopropenes to a point where free radical processes compete favorably. In the course of this study, we also explored the singlet oxygenation (DABCO inhibited) of enones 12a,b and 20. These generated, respectively, a mixture of peroxides identified as alpha-keto hydroperoxides 51/54 and hemiperketals 52/55 (the cyclic form of beta-keto hydroperoxides 53/56). Phosphine reduction of these peroxides yields the corresponding alcohols 33/43 and 32/42.