Maurizio S. Montaudo

Application of size exclusion chromatography matrix-assisted laser desorption/ionization time-of-flight to the determination of molecular masses in polydisperse polymers

The determination of molecular mass (MM) data for polydisperse polymers by size exclusion chromatography matrix assisted laser desorption/ionization time-of-flight (SEC/MALDI-TOF) involves the fractionation of samples through an analytical SEC. Selected fractions are then analysed by MALDI-TOF and the mass spectra of these nearly monodisperse samples allow the determination of Mn and Mw averages. To test the reliability of the molecular mass estimates by the SEC/MALDI-TOF method, a sample of polymethylmethacrylate (PMMA), two samples of polydimethylsiloxane (PDMS), and four samples of copolyesters, all polydisperse, were analysed. The results show that the molecular mass values of PMMA fractions obtained by MALDI-TOF are coincident with those obtained using the SEC calibration plots obtained with anionic PMMA standards. In the case of the two polydimethylsiloxanes (PDMS1 and PDMS2: linear and cyclic, respectively), two slightly differing SEC calibration plots were obtained, reflecting the different structures of the polymer chains of the two samples. The SEC traces of four copolyesters were obtained in tetrahydrofuran and CHCl3. Data on MM, MM distribution solvent effects and copolymer composition are reported.

Inter- and intramolecular ester exchange reactions in the ring-opening polymerization of (D,L)-lactide using lanthanide alkoxide initiators

Polylactide samples obtained by the ring-opening polymerization of (D,L)-lactide by using different lanthanide alkoxide initiators were examined by SEC, 13 C NMR and MALDI-TOF MS techniques. The evidence of substantial transesteriffcation was observed in the case of a polymerization initiated by lanthanum alkoxide and bimetallic aluminium-yttrium alkoxide initiators. In the case of Y and Sm alkoxide initiators, the MWD remains narrow up to high conversions for reasonable polymerization times, but the prolongation of the post-polymerization time increases the amount of ester-eschange reactions, The extent of the transesterification reactions was evaluated on the basis of the intensity of the stereosequence resonances for the methine carbon signal in the 13 C NMR spectra. MALDI-TOF MS analysis revealed the presence of add-numbered oligomers in all spectra. Thus, transesterification reactions occus from the beginning of the polymerization. Linear and cyclic oligomers were detected in some cases indicating the simultaneous occurrence of inter- and intramolecular exchange reactions. The microstructure analysis of the polymers by 13 C NMR spectroscopy indicates a preference for a syndiotactic addition during the polymerization process.

Characterization of end groups in nylon 6 by MALDI‐TOF mass spectrometry

Four samples of Ny6, each terminated by different end groups, i.e., diamino terminated, monoamino terminated (monocapped), dicarboxyl terminated, and amino-carboxyl terminated, were synthesized and analyzed by MALDI-TOF Mass Spectrometry, in order to accurately characterize their structure by direct identification of mass resolved chains. A self-calibrating method for the MALDI-TOF mass spectra of polymeric samples was used in order to distinguish the end groups existing in the four samples of Ny6. The MALDI-TOF spectra showed the presence of protonated, sodiated, and potassiated ions that were assigned to Ny6 chains containing the expecteted end groups. Furthermore, the MALDI-TOF spectra made possible the simultaneous detection of the cyclic oligomers of Ny6 present in these samples, thus achieving the full structural characterization of the molecular species present in these polyamides.

Microstructure of bacterial poly(β-hydroxybutyrate-co-β-hydroxyvalerate) by fast atom bombardment mass spectrometry analysis of the partial pyrolysis products

The partial methanolysis and the partial pyrolysis of bacterially synthesized copolyesters of β-hydroxybutyrate (HB) and β-hydroxyvalerate (HV) have been performed and the oligomers produced have been analyzed by fast atom bombardament mass spectrometry (FAB-MS). An algorithm has been developed to distinguish pure random copolymers from mixtures of random copolymers, using the relative abundances of the oligomers which can be deduced from the FAB-MS spectra.

Current Trends in Matrix-Assisted Laser Desorption/Ionization of Polymeric Materials:

In the last few years, mass spectrometry has rapidly become indispensable in polymer analysis and complements, in many ways, the structural data provided by nuclear magnetic resonance. Mass spectrometry of polymers is emerging as a revolutionary technique, capable of challenging the techniques and protocols established for years for the characterization of synthetic polymers. Matrix-assisted laser desorption/ionization (MALDI) has become a widely applied method for the structural characterization of synthetic polymers. The primary aim of this review is to illustrate some recent advances in the study of macromolecular systems by MALDI. MALDI allows the identification of repeat units and end groups, the structural analysis of linear and cyclic oligomers and the estimate of composition and sequence for co-polymers. MALDI is also quite useful for the measurement of molar mass and bivariate distributions in polymers and for the detection of self-association in macromolecules, performed by coupling MALDI with size exclusion chromatography (SEC). Recently MALDI has been applied, with remarkable success, to the study of thermal and oxidative processes in polymers and to the characterization of co-polymers obtained by reactive polymer blending. Selected applications of MALDI to polymers are illustrated herewith.

Molecular Weight Determination and Structural Analysis in Polydisperse Polymers by Hyphenated Gel Permeation Chromatography/Matrix-Assisted Laser Desorption Ionization—Time of Flight Mass Spectrometry

Abstract A method for the determination of molecular weight distribution data for polydisperse polymer samples by MALDI-TOF is reported that involves the fractionation of polydisperse samples through analytical GPC columns and collection of fractions. Selected fractions are then analysed by MALDI-TOF and the mass spectra of these nearly uniform samples allow the determination of Mn and Mw averages. The GPC tracing, calibrated against molecular weight values obtained by MALDI, was used to compute molecular weight distribution data of the unfractionated sample. To test the reliability of the molecular weight estimates by the GPC/MALDI-TOF method, a sample of poly(methyl methacrylate) and two samples of poly(dimethyl siloxane), both with wide polydispersity, were analyzed. The results show that the molecular weights of PMMA fractions obtained by MALDI coincide with the GPC calibration plots obtained with anionic PMMA standards. In the case of the two poly(dimethyl siloxanes), two slightly different GPC calib...

Determination of sequence and composition in poly(butyleneadipate-co-butyleneterephthalate) by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) has been used here for the analysis of an unfractionated copolymer sample of poly(butyleneadipate-co-butyleneterephthalate) (PBA/PBT). This is our first report on unfractionated samples, since previous studies by MALDI-TOF MS have concerned copolymer fractions. Mass-resolved peaks are seen up to 8000 Da. The peaks are due to oligomers which have a methylester and an hydroxy as end groups (CH3O/OH), which desorb by uptake of a sodium cation. We compared the intensity of mass spectral peaks with the theoretical intensities generated using a Bernoullian distribution of sequences. The composition which gives the best match was searched by the MACO4 program and a value of 49/51 in favor of terephthalic units was found, in excellent agreement with the composition derived from the 1H NMR data.

Analysis of Melt Copolymers

Melt copolymer chains are the main (most abundant) reaction product obtained when heating a blend of two (or more) condensation polymers (such as polyester + polycarbonate or polyester + polyamide or polyester + polyester) in which exchange reactions occur. In fact, during the melt-mixing reaction, an AB copolymer is formed and, as a consequence, the sample is a complex mixture made of three components or simply “parts”, referred to as Z1, Z2 and Z3, where Z1 and Z2 are the parts for unreacted homopolymers (A and B), whereas Z3 is the part for the copolymer. In this paper, it is shown that matrix-assisted laser desorption/ionization mass spectrometry (and mass spectrometry in general) can be used to monitor the yield of the reactive blending reaction, YR, by measuring the amount of unreacted homopolymer (Z1 and Z2). In order to allow for comparisons, the paper also discusses conventional methods for measuring Z1 and Z2, such as liquid chromatography and nuclear magnetic resonance.

Some Comments on a Recent Article Dealing with Three Aliphatic-Aromatic Polyesters and Their Spectral Identification

This work discusses the results appearing in Wu et al.’s (2014) article dealing with three aliphatic-aromatic polyesters based on terephthalic acid and their spectral identification. Using an instrument equipped with a switch that allows ions with masses below 5 kDa to be discarded (ion deflector), the authors report the MALDI spectrum of a commercial poly(ethylene terephthalate) with signals up to 80 kDa. I plot the spectral intensities and fit them with a Gaussian curve. The result is carefully examined. Thereafter, attention is moved to the MALDI spectra obtained when the ion deflector is turned off. A computation that gives the number-average and the weight-average molar mass (Mn and Mw) was performed, but the result was Mn = 1362 g/mol and Mw = 1416 g/mol. These values are much too low and they cannot be used. In order to explain this failure, I modified a model developed for poly(pentylene adipate). This hypothesis is discarded and the correct explanation is found. I also discuss other types of instrumentation, for instance, Py-GC/MS combined with library-search and also NMR spectroscopy, with specific reference to the chemical shifts of the -CH2- protons in aliphatic-aromatic polyesters.