Polymers with MALDI

Two relatively new techniques, matrix assisted laser desorption ionization-lime of flight mass spectrometry (MALDI-TOF) and electrospray ionization (FS1), offer new possibilities for analysis of polymers with molecular weights in the tens of thousands. PS molecular weights as high as 1.5 million have been determined by MALDI-TOF. Recent reviews on the application of these techniques to synthetic polymers include those by Ilantoif54 and Nielen.555 The methods have been much used to provide evidence for initiation and termination mechanisms in various forms of living and controlled radical polymerization.550 Some examples of the application of MALDI-TOF and ESI in end group determination are provided in Table 3.12. The table is not intended to be a comprehensive survey. 

A combination of NMR spectroscopy and MALDI-TOF MS is commonly employed in our laboratory for the characterisation of PPG polymers. Analysis of di-hydroxyl end-capped PPG (18) is initially described. The [H NMR spectrum [54] can be used to confirm the backbone structure of the polymer, as can be seen in Figure 22 (a and b are from the backbone of the polymer, with c from the methyl side chains). Peaks of low intensity, downfield of those from the backbone of the polymer, in the HNMR spectrum may be used to identify and quantify the allyl functionality in the polymer [55]. These resonances (d, e and f) are... 

Experimental considerations Sample preparation and data evaluation are similar to membrane osmometry. Since there is no lower cut-off as in membrane osmometry, the method is very sensitive to low molar mass impurities like residual solvent and monomers. As a consequence, the method is more suitable for oligomers and short polymers with molar masses up to (M)n 50kg/mol. Today, vapour pressure osmometry faces strong competition from mass spectrometry techniques such as matrix-assisted laser desorption ionisation mass spectrometry (MALDI-MS) [20,21]. Nevertheless, vapour pressure osmometry still has advantages in cases where fragmentation issues or molar mass-dependent desorption and ionization probabilities come into play. 

Liquid chromatography (LC) encompasses several techniques, which includes HPLC and SEC (sometimes referred to as GPC). Many variants of and developments from these techniques exist, and are used in the study and analysis of polymer degradation/oxidation. As discussed earlier, SEC is often coupled with MALDI-ToF-MS to facilitate the identification of the products of polymer degradation. SEC has also been coupled with mid-infrared detection and similarly used for studies of polymer degradation. SEC/GPC is discussed further below. 

Fig. 12. Analysis of the thymine-substituted polymers by MALDI-TOF mass spectrometry reveals that the polymers can be capped with two different groups...

By analysis of the ST HPMA polymers by MALDI-TOF MS in the (more sensitive) refectron mode it was possible to recognize macromolecules with... 

HPLC and the extensive complexity of ESI and MALDI spectra for multicomponent polymers with molar mass over about 10 g mol. Some applications of MALDI MS in polymer HPLC can be found for example in [300-303],... 

Although FAB has been used in polymer analysis, problems with fragmentation and the relatively low mass limit has made this less popular as new techniques have emerged. Plasma desorption has been used successfully but this too has waned in popularity with commercial spectrometers not really readily available. To a large extent polymer mass spectrometry equates to MALDI time-of-flight and the remainder of this article will bear this in mind. However, the use of electrospray ionisation (ESI) will be considered in conjunction with either quadrupole detectors or ion cyclotron resonance (ICR) N. B. ICR detectors can also be used with MALDI, as this is important and probably not as widely used as it could be. 

MALDI spectrum of a hydroxyl functional polymer prepared by GTP the level of the impurity is easily seen and the success of this reaction clearly observed. However, this is not a universally applicable technique. Perhaps the most topical living polymerisation at present is transition metal mediated radical polymerization. This typically gives a polymer with a tertiary halide terminal group. This group, as has nitroxide, has been found to be very labile in the mass spectrometer leading to fragmentation. 

For natural poly-APS alkaloids (77), matrix assisted laser desorption ionization-time-of-flight (MALDI-TOF) spectrometric analysis showed that the compounds comprised a mixture of two polymers with molecular weights centred at around 5520 Da and 18900 Da and with a low degree of polydispersity [34]. 

In order to polymerize a fluoride-type monomer 10, cellulase and xylanase were chosen for the catalyst. In an acetonitrile-acetate buffer mixed solution, the consumption of 10 was accelerated by the addition of xylanase from Trichoderma viride, giving rise to a white precipitate as the progress of the reaction in a 58% yield. From the analysis of the resulting products, it is a cellulose-xylan hybrid polysaccharide 11 and polymers with the degree of polymerization up to 12 (24 saccharide units) were detected from the MALDI-TOF/MS. 

MALDI (matrix assisted laser desorption and ionization) combined with size-exclusion chromatography is increasingly being used to study the microstructure of polymers [542-544], MALDI allows the desorption and ionization of macromolecules with molecular mass up to hundreds of kilodaltons with little or no fragmentation. The MALDI mass spectra provides structural information for identification of the polymer repeat units or copolymer sequence, end group mass and mechanism of polymerization for polymers with a narrow polydispersity (< 1.25). For samples of high polydispersity MALDI fails to provide reliable information. The separation of polydisperse samples by size-exclusion chromatography provides fractions suitable for characterization by MALDI. MALDI can also be used for mass calibration of size-exclusion columns. 

Characterization of Silicon-Containing Polymers by Coupling of HPLC-Separation Methods with MALDI-TOF Mass Spectrometry... 

MALDI-TOF-MS) has the potential to identify the fractions separated by SEC, LACCC or LAC. This method was developed by Karas et al. and by Tanaka et al. in 1988. The method was first applied for investigations of silicon-containing polymers in 1995, for example by Lorenz and by Tang. MALDI-TOF-MS represents a powerful tool not only for the determination of chemical structure but also for SEC calibration. SEC calibration for specific polymers is still problematic, since there are no commercially available standards. Therefore a calibration by the coupling of different modes of liquid chromatography with MALDI-TOF-MS is preferred. 

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