Oligomer ions

Since the proton affinity P(Rn=) of the branched olefin Rn= is certainly greater than P(C2H4), the reaction would be endothermic a similar argument applies to the transfer of carbonium ions. This may account for the low yields observed even in the heterogeneous cationic oligomerisation of ethylene. The most likely transfer reactions are hydride ion and methide ion transfers between oligomer ions and molecules (see below), leading to conjunct polymerisation [12]. 

MALDI is the method of choice for the analysis of synthetic polymers because it usually provides solely intact and singly charged [62] quasimolecular ions over an essentially unlimited mass range. [22,23] While polar polymers such as poly(methylmethacrylate) (PMMA), [83,120] polyethylene glycol (PEG), [120,121] and others [79,122,123] readily form [M+H] or [M+alkali] ions, nonpolar polymers like polystyrene (PS) [99,100,105,106] or non-functionalized polymers like polyethylene (PE) [102,103] can only be cationized by transition metal ions in their l-t oxidation state. [99,100] The formation of evenly spaced oligomer ion series can also be employed to establish an internal mass calibration of a spectrum. [122]... 

The ions in a plasma under a powder forming condition, shown in Fig. 19, exhibit a clear contrast with those shown in Fig. 18. Few oligomer ion peaks and many fragment peaks are seen in this case. It may be considered that a powder forming reaction occurs very rapidly, thus a small amount of oligomer ions are left in the gas phase. In a plasma forming a membrane of an inferior quality... 

K+ ionization of desorbed species (K+IDS) spectrometry, linear 1-butoxide-initiated, acetate-capped chloral oligomers, ion source pressure < 10 Torr, source temperature = 200°C Amine-initiated chloral oligomers, K+IDS mass spectrometry and... 

FAB-MS analysis also has been used to characterize low mass commercial polyethylene- and polypropylene-glycols, by detecting the oligomer ion abxmdances in the mass spectra of the samples. However, the relative peak intensities of the lower mass oligomers are distorted by the fragmentation processes of the oligomers at higher molar masses. Therefore it is not possible to estimate molar mass distributions by FAB-MS. ... 

Figure 46.11 contains the LDI mass spectra of PE500. The inset of Figure 46.11 highlights the isotope distribution of a silver-attached PE oligomer ion which matches that from the theory. The masses of the major peaks correspond to [alkane -r Ag]+. Minor peaks, labeled X, with masses matched with silver attached alkene, are also observed. 

Van der Flage and co-workers [280] combined MALDI and Fourier transform ion cyclotron mass spectrometry (FT-ICR-MS) for the characterisation of polyoxyalkyleneamines. MALDI FT-ICR-MS was used to resolve intact, sodium ion cationised oligomer ions in the mass range from m/z 500 to 3500. NMR was used to measure the average end-group distribution to provide insight into conformational differences. In this respect, FT-ICR-MS and NMR data were complementary. Combined results yielded detailed information about chemical composition distributions of polyalkyleneamines that hitherto it was not possible to obtain with either technique separately. Merits and limitations of the data produced with MALDI FT-ICR-MS are discussed and compared with those of H and C NMR data [280]. 

A disadvantage of TOP-MS, however, is the inability to mass select oligomer ions and subsequently cause them to fragment inside the mass spectrometer for tandem mass spectrometry (MS/MS) experiments that provide detailed structural information for individual ions. In such cases, MS/MS can only be achieved by placing a mass selective filter between the ionization source and the drift tube the development of the quadrupole-TOP (QTOP) mass spectrometer (19), typically coupled to ESI, was a response to the need for enabling MS/MS with TOP. 

Using PMMA samples, some of the causes of mass discrimination encountered in the MALDI-TOF analysis of polydisperse polymers were investigated. It was found that detector saturation could be a major instrumental factor causing mass discrimination. This was due to the strong signals of matrix-related ions and low-mass oligomer ions. The matrix effect as another source of mass discrimination was analysed quantitatively. 34 refs. 

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