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l-lactide or meso-lactide are polymerized either at 120 °C where the polymerization process of l-lactide is accompanied by crystallization, or at 180 °C where poly(l-lactide) remains in the molten state. Polymerizations at 120 °C initially yield even-numbered chains (with respect to lactic acid units) having relatively low dispersity, but the fraction of odd-numbered chains increases with time and the entire molecular weight distribution changes. Traces of cyclics are only formed after 7 d. Polymerizations at 180 °C yield equilibrium of even and odd-numbered chains from the beginning, but at low monomer/initiator ratios and short reaction times (<4 h) cyclics are again not formed. They appear at longer reaction times and entail higher dispersities. The results are discussed in terms of five different transesterification mechanisms.
L-Lactides were polymerized in bulk at 120 or 160 °C with cyclic dibutyltin catalysts derived from 1,2-dimercaptoethane or 2-mercaptoethanol. Only linear chains having one benzyl ester and one OH-end group were obtained when benzyl alcohol was added. When L-lactides were polymerized with neat dibutyl-2-stanna-1,3-dithiolane, exclusively cyclic polylactides were formed even at 120 °C. The temperature, time and monomer/catalyst ratio (M/C) were varied. These results are best explained by a combination of ring-expansion polymerization and ring-extrusion of cyclic oligo- or polylactides with Elimination of the cyclic catalyst. Neither syntheses of linear polylactides nor of cyclic lactides involved racemization up to 20 h at 160 °C.
Matrix-assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF MS) is a well-implemented analytical technique for the investigation of complex biological samples. In MS, the sample preparation strategy is decisive for the success of the measurements. Here, sample preparation processes and target materials for the investigation of different pollen grains are compared. A reduced and optimized sample preparation process prior to MALDI-TOF measurement is presented using conductive carbon tape as target. The application of conductive tape yields in enhanced absolute signal intensities and mass spectral pattern information, which leads to a clear separation in subsequent pattern analysis. The results will be used to improve the taxonomic differentiation and identification, and might be useful for the development of a simple routine method to identify pollen based on mass spectrometry.
Die MALDI-TOF Massenspektrometrie stellt eine wichtige Methode zur simultanen Bestimmung von Molmassen, Molmassenverteilungen und Endgruppen von synthetischen Polymeren dar. In dem Vortrag wird gezeigt, welche zusätzlichen Informationen über die Art der Polymerisation, der Nebenreaktionen und der Topologie der gebildeten hochmolekularen Polylaktid-Homo- und Copolymere im Verlauf ihrer Synthese erhalten werden können.
Alcohol-initiated ring-opening polymerizations (ROPs) of L-Lactide (LA) were conducted in bulk at 130 °C catalyzed by Zn acetate (ZnAc2), lactate (ZnLac2), or caprylate (ZnCap2). 11-undecenol (UND), 1-hydroxymethylnaphtalene (HMN), and 4-nitrobenzylalcohol (4NB) were used as alcohols. Whereas variation of the alcohols had little effect, the usefulness of the catalysts increased in the order: acetate < lactate < caprylate. Hence, further alcohol-initiated polymerizations were performed with ZnCap2 alone and with variation of the lactide/catalyst (LA/Cat) ratio. With increasing LA/Cat ratio higher fractions of cyclic poly(L-lactide) (PLA) were found, so that the measured degree of polymerization (DP) is considerably lower than the theoretical value (i.e., 2 x LA/alcohol). With neat ZnCap2 cyclic PLAs were the largely prevailing reaction products. For these cyclic PLAs weight average molar masses (Mw) up to 134,000 were obtained and an optical purity around 99% was indicated by 13C NMR spectroscopy and DSC measurements even after 48 h at 150 or 160 °C.