Quantitative mass spectrometry of technical polymers: a comparison of several ionization methods

Matrix assisted laser desorption ionization (MALDI) was used to study the organolithium initiated ring-opening polymerization of hexamethylcyclotrisiloxane (D 3 ) in a mixed solvent system. The mass spectral peak intensities were monitored to determine the effects of polymerization time, initiator concentration, and reaction temperature on the formation of the mono, di, and trisiloxanolate initiator species and the extent of chain redistribution. The three initiator species were formed by reacting n-butyllithium and sec-butyllithium with D 3 in nonpolar solvent. The mass spectral results showed that sec-butyllithium and n-butyllithium form different populations of initiator species under the same conditions and that the measured mass spectral peak intensities do not accurately represent the population of siloxanolate initiator species prior to propagation. The changes in peak intensities were attributed to chain redistribution. . The molecular weight and product purity in these systems are difficult to control because both methods are susceptible to side reactions that broaden the polydispersity (M w /M n ). The most common side reactions are chain redistribution and cyclization shown in Scheme 1. Chain redistribution occurs when a propagating chain reacts with another chain to remove a segment of polymer without terminating the reaction. Cyclization occurs when a propagating chain reacts with itself to form cyclic species (D 3 , D 4 , D 5 , etc.). This type of side reaction is commonly referred to as chain backbiting. Lee et al. [3] first reported a new method for producing narrow molecular weight PDMS that was free of cyclic impurities. It was shown that n-butyllithium opened the D 3 ring in nonpolar solvents to form lithium siloxanolate initiators but did not propagate without the addition of a small amount (1-3%) of base promoting solvent (in this case these include polar aprotic solvents such as DMSO and THF). The control of the initiator basicity coupled with the high reactivity of the D 3 monomer significantly reduced chain redistribution and eliminated the formation of cyclic impurities.The reaction between organolithium initiators and D 3 is not well understood due in large part to the unique behavior of the initiators and the lithium counterions in solution. Organolithium compounds associate in nonpolar solvents to form aggregated species of various degrees due to the electron deficient nature of the lithium atom [4]. N-butyllithium, for example, forms a hexameric aggregate [5] whereas sec-butyllithium forms a tetrameric aggregate [6] in hydrocarbon solvents. Studies involving the reaction of organolithium species with styrene [7,8] and dienes [8,9] have shown that the degree of association directly affects the reactivity. The less associated organolithium species are found to be generally more reactive. Upon reacting with D 3 , it has been proposed that the lithium counter ions associate to form lithium siloxanolate aggregates. The degree to which these proposed structures are associated and the mech...

Section: Inear Polydimethylsiloxane (Pdms) Is Primarilysupporting

confidence: 81%

Section: Inear Polydimethylsiloxane (Pdms) Is Primarilymentioning

confidence: 99%

Evaluation of matrix-assisted laser desorption ionization mass spectrometry for studying the sec-butyllithium and n-butyllithium initiated ring-opening polymerization of hexamethylcyclotrisiloxane (D₃)

Hawkridge

Gardella

2003

“…This indicates that GPC-MALDI-TOF MS effectively reports the high-mass polymers within the MWD, however the low-mass polymers are under-reported and it is proposed to be mainly due to the mass cutoff at 550 Da for the elimination of matrix interferences. Another possibility for this mass bias is that the low mass PDMS polymers have higher volatility than that of high mass PDMS polymers as suggested in a study by Yan et al [9]. Under vacuum conditions in MALDI-TOF MS experiments, some of the low mass oligomers go into the gas phase as a result of evaporation without getting ionized through the normal MALDI process.…”

Section: Comparison Of Average Mass Values By On-line Gpc-esi and Autmentioning

confidence: 99%

“…he advent of soft ionization techniques such as matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ESI) have provided chemists the ability to evaluate polymer samples with several different mass analyzers. Specifically for polysiloxane materials the information rich data obtained from such analysis has been used qualitatively for repeat unit and end group determination [1][2][3][4][5][6][7] and quantitatively for relative concentration [8,9] and average mass determination [10,11]. Several reviews appear in the literature that highlight many of the applications of polymer mass spectrometry analysis [12][13][14][15].…”

mentioning

confidence: 99%

Comparative studies of poly(dimethyl siloxanes) using automated GPC-MALDI-TOF MS and on-line GPC-ESI-TOF MS

Liu¹,

Maziarz²,

Heiler³

et al. 2003

In this study we compare on-line gel permeation chromatography (GPC) electrospray ionization (ESI) time-of-flight (TOF) mass spectrometry (MS) to automated GPC matrix assisted laser desorption ionization (MALDI) TOF MS for poly (dimethylsiloxane) (PDMS) analysis. Average mass values for a hydroxyl-terminated PDMS (OH-PDMS) sample were obtained and compared to traditional GPC that was calibrated with narrow polystyrene standards, by direct ESI and MALDI MS analysis, by a summation of mass spectra of all GPC fractions, and also by the recalibration method determined by both mass spectrometric methods. Quantitatively, the difference noted here between these hyphenated techniques is that GPC-ESI-TOF MS effectively reports the low-mass oligomers and underestimates the high-mass oligomers, while GPC-MALDI-TOF MS effectively reports the high-mass oligomers and underestimates the low-mass oligomers. In the GPC-ESI-TOF MS experiments, ion current suppression was observed in the high molecular weight region. The suppression effect was confirmed by repeatable sample runs and by injecting different PDMS samples. Higher chromatographic resolution was observed for GPC-ESI-TOF MS compared to GPC-MALDI-TOF MS. In fact, truly mono-disperse oligomers were observed in the low molecular weight range from GPC-ESI MS experiments. (J

“…Desorption of intact oligomers provides a direct measure of molecular weight distribution (MWD) from which a molecular weight average can be evaluated. According to the recent reports [21,22] on the comparison of mass spectrometric techniques, TOF-SIMS produced a result similar to electrospray ionization (ESI) and fast atom bombardment (FAB). They consistently showed lower MW averages compared with gel permeation chromatography (GPC)-deduced results, whereas matrix-assisted laser desorption/ionization (MALDI) and field desorption (FD) results are higher.…”

mentioning

confidence: 99%

Quantitative TOF-SIMS analysis of oligomeric degradation products at the surface of biodegradable poly(α-hydroxy acid)s

Lee

Gardella

2002

This paper reports the development of a new method for quantification of the hydrolytic surface degradation kinetics of biodegradable poly(␣-hydroxy acid)s using time-of-flight secondary ion mass spectrometry (TOF-SIMS). We report results from static SIMS spectra of a series of poly(␣-hydroxy acid)s including poly(glycolic acid), poly( L -lactic acid), and random poly( D,L -lactic acid-co-glycolic acid) hydrolyzed in various buffer systems. The distribution of the most intense peak intensities of ions generated in high mass range of the spectrum reflects the intact degradation products (oligomeric hydrolysis products) of each biodegradable polymer. First, a detailed analysis of the oligomeric ions is given based on rearrangement of the intact hydrolysis products. The pattern of ions can distinguish both degradation-generated intact oligomers and their fragment ion peaks with a variety of combinations of each repeat unit. Then, the integration and summation of the area of all ion peaks with the same number of repeat units is proposed as a measurement that provides a more accurate MW average than the typically used method which counts only the most intense peak. The multiple ion summation method described in this paper would be practical in the improvement of quantitative TOF-SIMS studies as a better data reduction method, especially in the surface degradation kinetics of biodegradable polymers. , additives in a polymer [13], and detailed structural characterization [14,15] and quantitative analysis [16 -20] of polymers. Most TOF-SIMS spectra of polymers have been obtained in the mass range m/z Ͻ10,000 [5, 18 -20].Two main processes are observed in the TOF-SIMS spectra of polymers; these are the fragmentation of polymer chains and the desorption of intact oligomers. Fragmentation under ion bombardment in ultra-high vacuum (UHV) environment generates distinct fragmentation patterns that are unique to a given polymer. From the fragmentation patterns the mass of the repeat unit, terminal groups, side groups, and functional groups in a repeat unit can be determined [14,15]. Desorption of intact oligomers provides a direct measure of molecular weight distribution (MWD) from which a molecular weight average can be evaluated. According to the recent reports [21,22] on the comparison of mass spectrometric techniques, TOF-SIMS produced a result similar to electrospray ionization (ESI) and fast atom bombardment (FAB). They consistently showed lower MW averages compared with gel permeation chromatography (GPC)-deduced results, whereas