Anja Baumgaertel

Tandem mass spectrometry of synthetic polymers

The detailed characterization of macromolecules plays an important role for synthetic chemists to define and specify the structure and properties of the successfully synthesized polymers. The search for new characterization techniques for polymers is essential for the continuation of the development of improved synthesis methods. The application of tandem mass spectrometry for the detailed characterization of synthetic polymers using the soft ionization techniques matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) and electrospray ionization mass spectrometry (ESI-MS), which became the basic tools in proteomics, has greatly been increased in recent years and is summarized in this perspective. Examples of a variety of homopolymers, such as poly(methyl methacrylate), poly(ethylene glycol), as well as copolymers, e.g. copolyesters, are given. The advanced mass spectrometric techniques described in this review will presumably become one of the basic tools in polymer chemistry in the near future.

Recent developments in the detailed characterization of polymers by multidimensional chromatography

Synthetic polymers as well as biopolymers reveal complex structures, such as variations in functionality, chain length and architecture. Therefore, combinations of different chromatographic techniques are a prerequisite for a detailed characterization. One possible approach is the combination of high performance liquid chromatography at critical conditions (LCCC) and size-exclusion chromatography, also named as two-dimensional chromatography, which allows the separation of the polymers according to different properties, like molar mass, chemical composition or functionality. In addition, LCCC hyphenated with different mass spectrometry techniques, e.g. MALDI-TOF or ESI-TOF, leads to additional information about molecular details of the polymeric structure. We summarize in this article the recent developments in two-dimensional chromatography of synthetic polymers and biopolymers since 2005.

Preparation of Methacrylate End-Functionalized Poly(2-ethyl-2-oxazoline) Macromonomers

Three methods for the functionalization of 2-ethyl-2-oxazoline (EtOx) oligomers with a methacrylate or methacrylamide unit are compared to identify the best suitable route to obtain such macromolecules. In the first method, a functional initiator, namely methacryloyl chloride, was used for the cationic ring-opening polymerization of EtOx, which proceeded in a living manner. However, the formation of a large amount of hydrogen initiated chains resulted in a low degree of functionalization (21%). The second method was based on the termination of active oligo(EtOx) chains with an aqueous sodium carbonate solution yielding hydroxyl-terminated oligomers. These end-groups were subsequently modified by an esterification reaction with methacryloyl chloride yielding oligomers with a large variety of different end-groups, as demonstrated by MALDI-TOF-MS. In the last method, the living chain ends were reacted with in situ formed triethyl ammonium methacrylate yielding macromonomers with a high degree of functionalization (>80%). Besides, MALDI-TOF-MS analysis revealed only a single oligomer distribution with the desired end-groups. Furthermore, these directly end-capped oligomers revealed narrow molar mass distributions with PDI values below 1.2 making this the best method for macromomonomer synthesis.

Characterization of different poly(2-ethyl-2-oxazoline)s via matrix-assisted laser desorption/ionization time-of-flight tandem mass spectrometry

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) coupled with CID (collision-induced dissociation) has been used for the detailed characterization of two poly(2-ethyl-2-oxazoline)s as part of a continuing study of synthetic polymers by MALDI-TOF MS/MS. These experiments provided information about the variety of fragmentation pathways for poly(oxazoline)s. It was possible to show that, in addition to the eliminations of small molecules, like ethene and hydrogen, the McLafferty rearrangement is also a possible fragmentation route. A library of fragmentation pathways for synthetic polymers was also constructed and such a library should enable the fast and automated data analysis of polymers in the future.

Characterization of poly(2-oxazoline) homo- and copolymers by liquid chromatography at critical conditions.

In this study liquid chromatography at critical conditions for poly(2-ethyl-2-oxazoline)s (PEtOx) has been performed for the first time in order to analyze functional PEtOx homopolymers and block copolymers. Besides the verification of the critical point of adsorption with two series of ester end group functionalized PEtOx homopolymers, to evaluate the effect of both the chain length dependence and the end group polarity, using a cyano column with a solvent combination of 2-propanol and water, also two-dimensional liquid chromatography (2D-LC) has been applied for a poly(2-oxazoline) block copolymer. The combined characterization techniques provided further information about the polymerization procedure with regard to the formation of side-products by separation of the block copolymer from the corresponding homopolymer impurities. In addition, hyphenation of LCCC with MALDI-TOF MS and ESI-Q-TOF tandem mass spectrometry verified the obtained results.

Light-triggered NO release from a nanofibrous non-woven

Light-controlled NO delivery systems promise new applications in phototherapies. For this purpose, a ruthenium nitrosyl complex [(2)Ru(NO)(Cl)] with the novel ligand N,N′-(1,2-phenylene)bis(1-methyl-1H-imidazole-2-carboxamide) (2) has been synthesised and characterised in detail. The photoactive {Ru–NO}6 nitrosyl released NO in DMSO upon exposure to low-intensity UV-A light (λ = 366 nm). In order to create an applicable system the water-insoluble [(2)Ru(NO)(Cl)] was embedded into poly(L-lactide-co-D/L-lactide) nanofibrous non-wovens by electrospinning. Exposure of a 25 wt% non-woven of [(2)Ru(NO)(Cl)] to UV-A light resulted in a continuous release of NO into an aqueous solution. The cytoxicity of the non-woven against 3T3 mouse fibroblasts was very low.

Dual hydrophilic polymers based on (meth)acrylic acid and poly(ethylene glycol) – synthesis and water uptake behavior

Synthesis and characterization of dual hydrophilic random and block copolymers of acrylic acid (AA) or methacrylic acid (MAA) with poly(ethylene glycol) (PEG) via different controlled radical polymerization techniques are discussed. Initially, reversible addition fragmentation chain transfer (RAFT) polymerization was employed to synthesize homo, random and block copolymers of AA and MAA in ethanol. The polymers were characterized in detail by means of size exclusion chromatography (SEC), 1H NMR spectroscopy, matrix assisted laser desorption ionization time of flight (MALDI-TOF) mass spectrometry as well as MALDI-TOF MS coupled with collision induced dissociation (CID) to identify the end groups and the repeating units. Following that, atom transfer radical polymerization (ATRP) and RAFT polymerization were employed for the preparation of block copolymers using a PEG macroinitiator and a PEG macro chain transfer agent. Moreover, graft copolymers that contain oligo(ethyleneglycol) pendant groups and AA or MAA have been prepared using the RAFT polymerization process. Additionally, selected homo or block copolymers were tested for their water-uptake properties using a thermal gravimetrical analyzer with a controlled humidity chamber. An advantageous behavior of the copolymers compared to the related homopolymers was reached with the obtained ability to absorb moisture over the complete humidity range as well as to a very high absolute water uptake.

Analysis of different synthetic homopolymers by the use of a new calculation software for tandem mass spectra

The manual interpretation of tandem mass spectra of synthetic polymers is very time-consuming. Therefore, a new software tool was developed to accelerate the interpretation of spectra obtained without requiring any further knowledge about the polymer class or the fragmentation behavior under high-energy collision-induced dissociation (CID) conditions. The software only requires an alphabetical list of elements and a peak list of the measured substance as an xml file for the evaluation of the chosen mass spectrum. Tandem mass spectra of different homopolymers, like poly(2-oxazoline)s, poly(ethylene glycol) and poly(styrene), were interpreted by the new software tool. This contribution describes a fast and automated software tool for the rapid analysis of homopolymers.

Determination of the relative ligand-binding strengths in heteroleptic IrIII complexes by ESI-Q-TOF tandem mass spectrometry

An electrospray ionization quadrupole time-of-flight mass spectrometer has been utilized to investigate the relative ligand-binding strengths in a series of heteroleptic-charged iridium(III) complexes of the general formula [(C^N)(2) Ir(III) (S-tpy)](PF(6) ) by using variable collision energies. Collision-induced dissociation experiments were performed in order to study the stability of the Ir(III) complexes that are, for instance, suitable phosphors in light-emitting electrochemical cells. The ratio of signal intensities belonging to the fragment and the undissociated complex depends on the collision energy applied for the tandem mass spectra (MS/MS) analysis. By defining the threshold collision energy and the point of complete complex dissociation, it is possible to estimate the relative complex stabilities depending on the nature of the coordinated ligands [i.e. type of cyclometalating ligand (C^N), substituents on the S-shaped terpyridine (S-tpy)]. The collision energy values differed as a function of the coordination sphere of the Ir(III) centers.