The photochemistry of aliphatic disulfides is presented. The photolysis products are photoionized with coherent vacuum ultraviolet radiation and analyzed by time-of-flight mass spectrometry. With 248-nm excitation, the predominant dissociation pathway is S-S bond cleavage. With 193-nm excitation, S-S bond cleavage, C-S bond cleavage, and molecular rearrangements are all observed as primary processes. The branching ratio for S-S bond cleavage relative to C-S bond cleavage is typically 1-2 orders of magnitude greater at 248 run than 193 run. This wavelength dependence cannot be explained readily by photodissociation from the ground electronic state. The ground state S-S bond energy, ∼ 280 kJ/mol, is much larger than the C-S bond energy, ∼ 235 kJ/mol. If dissociation occurred from the ground state, higher wavelength radiation would be expected to favor the lower energy process, but the opposite effect is observed. Thus, excited state photochemistry is indicated. These results are discussed with respect to the differences between low and high energy collision-induced dissociation of peptides that contain disulfide linkages and to the possibility of achieving bond-selective photodissociation of such ions.

Rapid microanalysis of a wide variety of polyolefins was performed using pyrolysis-photoionization mass spectrometry (py-PI-MS). Solid samples (∼10 µg) were pyrolyzed on a heated probe in the source region of a timeof-flight mass spectrometer. Pyrolysates were "softly" ionized using coherent vacuum ultraviolet radiation (118 nm). The resulting mass spectra were clearly different for low-density polyethylene, high-density polyethylene, and several ethylene/r-olefin copolymers. A combination of principal component analysis and linear discriminant analysis was used to classify polyolefin samples directly from their photoionization mass spectra. The compositions of ethylene-butene and ethylene-octene copolymers were predicted using partial least-squares analyses. The values obtained using py-PI-MS were in good agreement with the measured 13 C NMR values. Samples of ethylene-octene containing 30 wt % carbon black and ethylene-butene containing 20 wt % silica were correctly classified and compositionally analyzed using py-PI-MS. Samples containing these additives are typically not amenable to study by solution-state 13 C NMR or IR.

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry of ozonolysis degradation products was used to determine the microstructures of several butadiene copolymers. A random styrene−butadiene copolymer containing 45 wt % styrene was distinguished from a block (ABA) styrene−butadiene copolymer containing 38 wt % styrene. In addition, several acrylonitrile−butadiene copolymers with acrylonitrile contents ranging from 21 to 51 wt % were analyzed. The microstructures for the acrylonitrile−butadiene copolymers were confirmed to be fairly random. Quantitatively, the acrylonitrile compositions determined by ozonolysis/MALDI-MS were close to the reported values for these copolymers (typically within 5 wt %). The discrepancy between the reported and experimentally obtained compositions may be attributed, in part, to a composition bias arising from the ozonolysis process. A simple model for ozonolysis of a random copolymer was developed to investigate the effects of ozone exposure time on the oligomer distributions observed in the MALDI mass spectra.

The saturation of polydienes with deuterium produces labeled polyolefins that, when studied with the identical hydrogenated polydienes, have proven quite useful for the understanding of many of the basic properties of these materials. However, until now the only way to determine the amount of deuterium substitution on the labeled molecules has been to compare the densities of the hydrogenated and deuterated polymers, which gives no information on how the deuterium is distributed along the chains. The information is important, since it is clear from the density results that with some saturation catalysts (Pd) there must be some H-D exchange since there are more than two deuterium atoms per diene monomer on the labeled species, while for others (Wilkinson's) there appears to be no exchange. In this work, pyrolysis-photionization mass spectrometry has been used to examine the structure of the hydrogenated and deuterated polydienes in more detail. The deuterium distributions were determined using a modified Bernoullian model that takes into account the first two D's that were added by the saturation event. The most revealing data come from the tetramer C 4H8-xDx, which confirm the average level of deuteration as measured by density. For polymers saturated with Pd catalysts, the pyrolysisphotionization mass spectra show that the deuterium was randomly distributed by the H-D exchange. Perdeuteriopolyisobutylene was also examined, as was its blend with hydrogenous polyisobutylene, which helped to confirm that exchange had occurred during the saturation of the polydienes using the catalyst rather than during the measurement step. The data for the polydiene deuterated using Wilkinson's catalyst confirmed that there was little, if any, exchange.