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A sample of poly(methy1 methacrylate) (PMMA), of which the molecular weight was known from light-scattering measurements, was subjected to gel-permeation-chromatographic analysis, and the resulting Weight versus log molecular weight (MW) plot was converted by suitable computation to a Number versus linear hZW plot. This could then be compared directly with the matrix-assisted laser desorption ionization MW distribution results obtained for the same sample. The MW distributions obtained by the two methods were significantly different. Two explanations for the discrepancy are offered.The objective of this work was to utilize the technique of matrix-assisted laser desorptionlionization (MALDI) mass spectrometry to determine the Molecular Weight (MW) Distribution of a polymer sample, and to compare the results with those obtained by the method of gel-permeation chromatography (GPC). EXPERIMENTAL The polymer sampleThe poly(methy1 methacrylate) (PMMA) had been prepared by an ionic polymerization procedure, to provide a sample with a MW dispersity close to unity. Its MW was determined as 3150 by the light-scattering technique. The sample was provided by Polymer Laboratories, Church Stretton, UK. Gel-permeation chromatographyA Waters model 441 GPC apparatus, fitted with a refractive index detector, was used. It was fitted with a universal column for separations up to a MW of 4 X lo6. The column temperature was 20 "C, and the operating solvent was tetrahydrofuran. The volume of sample solution (1 mg cm-') injected was 40 pL. Repeat runs were performed to confirm the reproducibility of the results. The MALDI apparatusThe Kratos-Kompact I11 apparatus (Kratos, Manchester, UK) is based on a time-of-flight mass spectrometer, and incorporates a nitrogen laser, of wavelength 337nm, and pulse duration 3ns. The instrument was operated in the linear mode, and other conditions used for the apparatus were those recommended by the manufacturer. The average mass spectrum obtained from 50 successive laser pulses was used, in order to minimize the effects of pulse-to-pulse irreproducibility .* Author for correspondence. Sample deposition technique for MALDIPMMA (4 mg) was dissolved in tetrahydrofuran (4 cm') in order to obtain a deposition solution of concentration 1 mg cm-', as recommended by the instrument manufacturer. The matrix solution was prepared by dissolving 10 g of 2,4-dihydroxybenzoic acid (DHB) in 1 L of trifluoroacetic acid solution (0.05% in a 50: 50 mixture of water and ethanol). 0.5 pL of this matrix solution was applied to the stainless steel sample slide, and the solvent allowed to evaporate off. The same volume of sample solution was then applied to the dry matrix, and the overall mixture allowed to dry. RESULTSThe molecular weight distribution obtained by the GPC technique employing a refractive index detector is a weight distribution on a logarithmic MW axis. This may be converted by computation to become a number distribution on a linear MW scale, in order that it may be compared directly with results from the MALD...

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Samples of a bacterial polyester, poly@-hydroxybutyrate), have been degraded thermally as a thin film on a thermocouple-controlled filament at 350 "C. In one series of these thermal experiments a short pyrolysis time (5 s) was used in order to ensure only partial pyrolysis, while in another series of thermal experiments the pyrolysis was allowed to proceed until all the sample had decomposed to volatiles. Samples from the same batch were also degraded in solution by methanolysis under acid conditions, while under reflux at 84 "C. In all cases, the yields and molecular weights of the products were assessed by gas chromatography. The number and weight distributions of the oligomeric products observed have been compared with those predicted statistically on the basis of random scissions. The results show that total thermolysis can be interpreted exclusively in terms of a random chain scission mechanism. However, the partial pyrolysis results are not consistent with random scission statistics; these results imply that there are some kinetically favored scissions occurring near the ends of the molecules. The product distributions from methanolysis are also somewhat inconsistent with those expected on the basis of random scission and may suggest that dimers hydrolyze more readily than higher molecular weight species.