Probing mechanistic features of conventional, catalytic and living free radical polymerizations using soft ionization mass spectrometric techniques

Progress on the development of a metal salt-assisted ionization source for the mass spectrometric analysis of polymers

The mass spectrometric analysis of polymers has been addressed as a challenging research topic due to poor ionization and complicated analysis using conventional mass spectrometry. The ionization source has demonstrated a promising future in rapid mass spectrometric analysis. Soft ionization techniques, such as electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI) are the most ionization sources appeared to be a powerful tools for polymer characterization when combined with MS. However, they always need metal salts to be introduced during the ionization protocol for polymers due to the crucial role played by their ions (cations and anions). The current review focuses on the progress in the development of metal ion-assisted-ionization sources for the mass spectrometric analysis of polymers. Different ionization systems are comprehensively reviewed. The application of metal ion-assisted ESI, nanoESI, PSI, and MALDI-MS for polymer sample analyses is systematically discussed. The future research trends and challenges in this cutting-edge research field are summarized. It also aims to provide the current state-of-the-art of metal salts as a platform for ionization systems for the mass spectrometric characterization of polymers and offers the current challenges and perspectives on the promising future to improve analytical performance in this field. Finally, this mini-review provides a comprehensive handbook to researchers from different research backgrounds wishing to work in this area.

On the identification and quantification of proton-initiated species in the synthesis of poly(2-alkyl-2-oxazoline)s by high resolution liquid chromatography

Hydrophilic poly(2-oxazoline)s represent a promising alternative to replace poly(ethylene glycol) in the biomedical field. For that purpose, reliable analytical protocols to confirm identity and quantity of impurities are required. In particular, side products deriving from chain transfer reactions occurring during the cationic ring-opening polymerization and incomplete end-capping processes may be present. The analytical approach must hence be capable of separating polymers according to minor changes regarding their end group. We demonstrate that liquid chromatography, relying on a monolithic C18-modified silica column and isocratic as well as gradient elution using water / acetonitrile mixtures and varying detectors, can accomplish such demanding high resolution separations. Poly(2-ethyl-2-oxazoline)s (PEtOx) with acetyl, hydroxyl, and phthalimide ω-end groups were investigated. Identification of side products was achieved through coupling with electrospray ionization mass spectrometry. UV / Vis detection was applied to quantify chain transfer products in PEtOx comprising biphenyl moieties. In addition, gradient elution enabled the separation of PEtOx into macromolecules according to their specific degrees of polymerization in molar mass ranges around 2,000 g mol^-1.

Enhanced Anti-Fouling Behavior and Performance of PES Membrane by UV Treatment

An easy method to prepare hydrophilic PES membranes with anti-fouling properties was developed by UV-polymerization of poly vinyl pirrolidone (PVP) on membrane surfaces. The modified membrane surfaces were analyzed by ATR-FTIR, and the new hydrophilic nature of the membranes was determined by contact angle measurements. The novel membranes were prepared using Rhodiasolv® Polarclean as a green solvent and compared with a control PES membrane, without the exposure at the hydrophilization procedure. The influences of the UV lamp distance (15 and 30 cm) and the exposure time (0 cm to 60 cm) were evaluated. All membranes were characterized in terms of surface morphology, porosity, pore size, and pure water permeability (PWP). The treated membranes resulted in an increase in hydrophilicity and in improved performances in terms of PWP and foulant rejection. In particular, an anti-fouling test was performed using a solution of 100 mg/L of humic acid (HA) as a model foulant. The UV-treated membrane efficiency, compared with a commercial PES membrane, showed a recovery of about 97%, confirming that these membranes can be applied in wastewater treatment.

Atom Transfer Radical Polymerization (ATRP) Catalyzed by Visible Light-Absorbed Small Molecule Organic Semiconductors

Atom transfer radical polymerization is successfully conducted with an organic semiconductor (OS)-based visible light activated photocatalyst via a precisely designed catalytic cycle. The living nature of the polymerization by the OS type photocatalyst is proved by the linearly-increasing molecular weight with consumption of the monomer, the expected molecular weight from MALDI-TOF measurements, and the chain extending reaction, as well as block copolymer synthesis. Benefitting from clear and measurable reductive potential as well as the oxidative potential of the OS-type photocatalyst, the intrinsic demand for "living radical polymerization" is discussed and examined, which indicates the diversity and great potential of OS-type photocatalysts for organic synthesis.

Synthesis of Waterborne Polyurethane by the Telechelic α,ω-Di(hydroxy)poly(n-butyl acrylate)

A key for the preparation of polyacrylate-based polyurethane is the synthesis of hydroxyl-terminated polyacrylate. To our knowledge, exactly one hydroxyl group of every polyacrylate chain has not been reported. The hydroxyl-terminated poly(butyl acrylate) (PBA) has been successfully synthesized by degenerative iodine transfer polymerization (DITP) of the n-butyl acrylate (n-BA) using 4,4′-azobis(4-cyano-1-pentanol) (ACPO) and diiodoxylene (DIX) as initiator and chain transfer agent, respectively, and subsequently substituted reaction of the iodine-terminated PBA with β-mercaptoethanol in alkaline condition. The latter reaction was highly efficient, and the terminal iodine at the end of polymer chains were almost quantitatively transformed to a hydroxyl group. 2,2′-Azobis(isobutyronitrile) (AIBN) and ACPO were used as initiators in the DITPs of n-BA. The results demonstrated that they had a significant influence on the terminal groups of the formed polymer chains. The structure, molecular weight, and molecular weight distribution of the hydroxyl-terminated PBA have been studied by ¹H, ¹³C NMR, and GPC results. The components of hydroxyl-terminated PBA were determined by MALDI-TOF MS spectra, and their formation is discussed. The broad molecular weight distribution of the PBA and the difference in the polymerization behaviors from typical living radical polymerization are explained based on the results of ¹H NMR and MALDI-TOF MS spectra. The hydroxyl-terminated PBA has been successfully used in the preparation of PBA-based polyurethane dispersions (PUDs). The aqueous PUDs were stable, and based on the DSC results it can be said that the miscibility of hard segments with PBA chains was improved.

MALDI-LID-ToF/ToF analysis of statistical and diblock polyacrylate copolymers

We report the use of MALDI-LID-ToF/ToF utilising the laser induced dissociation (LID) fragmentation technique, which has been almost exclusively applied to protein/peptide analysis to date.

Allylthioketone Mediated Free Radical Polymerization of Methacrylates

By combination of high trapping free radical efficiency of the thioketone and resonance of the allylic radical, a new type of mediating agent, 1,3,3-triphenylprop-2-ene-1-thione (TPPT) has been successfully synthesized, and then is used to study controlled free radical polymerization of methacrylates. Very stable TPPT radicals at the end of poly(methyl methacrylate) (PMMA) are detected in the polymerization of MMA using TPPT and AIBN as the control agent and initiator. The MALDI-TOF MS spectra are used to identify terminal groups of the resultant poly(glycidyl methacrylate) (PGMA), and major component of the obtained polymer has the structure, (CH₃)₂(CN)C-PGMA-C₇H₉O₃. Chain extension reaction tests ascertain formation of the dead polymers during the polymer storage and purification process of the polymers. Owing to very slow fragmentation reaction of the TPPT-terminated polymethacrylate radical and addition reaction of this radical with a primary radical, the growing chain radicals are difficult to be regenerated, leading to an unobvious change of the molecular weight with monomer conversion. The molecular weights of polymers can be controlled by the ratios of monomer/initiator and TPPT/initiator. However, the first order kinetics of the polymerization and the polymers with narrow polydispersity are obtained, and these phenomena are discussed. This study provides useful information on how to design a better controlling agent.

Metal-Free Atom Transfer Radical Polymerization of Methyl Methacrylate with ppm Level of Organic Photocatalyst

It is well known that the recently developed photoinduced metal-free atom transfer radical polymerization (ATRP) has been considered as a promising methodology to completely eliminate transition metal residue in polymers. However, a serious problem needs to be improved, namely, large amount of organic photocatalysts should be used to keep the controllability over molecular weights and molecular weight distributions. In this work, a novel photocatalyst 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) with strong excited state reduction potential is successfully used to mediate a metal-free ATRP of methyl methacrylate just with parts per million (ppm) level usage under irradiation of blue light emitting diode at room temperature, using ethyl α-bromophenyl-acetate as a typical initiator with high initiator efficiency. The polymerization kinetic study, multiple controlled "on-off" light switching cycle regulation, and chain extension experiment confirm the "living"/controlled features of this promising photoinduced metal-free ATRP system with good molecular weight control in the presence of ppm level photocatalyst 4CzIPN.

Laccase-catalyzed controlled radical polymerization of N-vinylimidazole

Laccase from Trametes versicolor catalyzes the controlled radical polymerization of N-vinylimidazole, yielding narrowly dispersed, metal-free polymers.

Metal-free photoinduced electron transfer–atom transfer radical polymerization (PET–ATRP) via a visible light organic photocatalyst

This work developed the first example of PET-ATRP using a reductive pathway, which provides new opportunities for the synthesis of well-controlled polymer architectures through a photochemical approach.

MALDI of synthetic polymers with labile end-groups

Mass spectrometry is increasingly used in the field of synthetic polymers as a fast and accurate technique for end-group analysis. More particularly, matrix-assisted laser desorption/ionization (MALDI) has gained much popularity because it allows quite simple mass spectra to be obtained, displaying a single distribution for each polymeric species present in the sample, in contrast to electrospray ionization (ESI) which readily promotes multiple charging for most polymers. A soft ionization process, ensuring the integrity of the species upon transfer into gas phase ions, is however mandatory for polymer end-group analysis since information about the chain terminations mainly rely on the m/z values measured for polymer adducts. As compared to ESI, MALDI is sometimes suspected to be a quite "hard" ionization technique, leading to spontaneous dissociation of ionized species either in the source or during their flight time. This issue is of particular concern for polymers carrying so-called fragile end-groups arising from their mode of synthesis. In particular, controlled radical polymerization (CRP) processes, one of the most important advances in the field of polymer science during the last 20 years, allow the production of polymers with well-defined molecular distribution and low polydispersities, but they are all based on the low dissociation energy of the chemical bond between the last monomer and the terminating group. As a result, if macromolecules are activated while being ionized, this end-group is prone to fragmentation and ions measured in the mass spectra do no longer reflect the original chain composition. However, different results are reported in the literature about the ability of MALDI to generate intact ions from CRP synthetic polymers. This article reviews MALDI MS data reported for synthetic polymers produced by atom transfer radical polymerization (ATRP), reversible addition-fragmentation transfer polymerization (RAFT), and nitroxide-mediated polymerization (NMP), the three most studied CRP techniques. The general principle of each polymerization process, which defines the structure of the end-groups in both targeted macromolecules and species arising from eventual side-reactions, is first briefly presented. An overview of MALDI data reported for samples obtained upon polymerization of different monomers are then commented for each polymerization techniques with regards to the success of the ionization method to generate intact cationic adducts and its propensity to distinguish in-source fragments from polymerization side-products.

End-group cleavage in MALDI of ATRP-made polystyrene: a silver-catalyzed reaction during sample preparation

Cleavage of the labile halide termination upon matrix-assisted laser desorption/ionization (MALDI) has always been reported as a major concern in mass analysis of polystyrene prepared by atom transfer radical polymerization (ATRP). By studying this issue using nuclear magnetic resonance (NMR) and electrospray ionization-mass spectrometry, we evidence here that the ionization step is not involved in this deleterious process. Instead, removal of the halogen was shown to readily occur upon interaction of the silver salt (AgTFA) used as the cationizing agent in mass spectrometry, either in solution or in the solid-state when performing solvent-free sample preparation. In solution, this silver-induced reaction mostly consists of a nucleophilic substitution, leading to polystyrene molecules holding different terminations, depending on relative nucleophilicity of species present in the liquid-phase solution composition. In chloroform supplemented with AgTFA, trifluoroacetate-terminated PS were evidenced in ESI-MS spectra but experienced end-group cleavage in MALDI. In contrast, the major methoxy-terminated PS macromolecules formed when the silver-catalyzed nucleophilic substitution was performed in methanol were generated as intact gas-phase ions using both ionization techniques. This controlled and fast modification could hence be advantageously used as a rapid sample pretreatment for safe MALDI mass analysis of ATRP-made polystyrene.

Synthesis of well-defined PPV containing block polymers with precise endgroup control by a dual-initiator strategy

The initiator efficiency in PPV polymerization was tested and PPV-b-(P-tBuA) block copolymers were synthesized. Via hydrolysis amphiphilic materials undergoing micelle formation were obtained.

Tandem mass spectrometry of poly(ethylene imine)s by electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI)

In this contribution, linear poly(ethylene imine) (PEI) polymers, which are of importance in gene delivery, are investigated in detail by using electrospray ionization-quadrupole-time of flight (ESI-Q-TOF) and matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectrometry (MS). The analyzed PEIs with different end groups were synthesized using the polymerization of substituted 2-oxazoline via a living cationic ring-opening polymerization (CROP) and a subsequent hydrolysis under acidic conditions. The main goal of this study was to identify linear PEI polymers in a detailed way to gain information about their fragmentation pathways. For this purpose, a detailed characterization of three different linear PEIs was performed by using ESI-Q-TOF and MALDI-TOF MS in combination with collision-induced dissociation (CID) experiments. In ESI-MS as well as MALDI-MS analysis, the obtained spectra of PEIs resulted in fitting mass distributions for the investigated PEIs. In the tandem MS analysis, a 1,2-hydride shift with a charge-remote rearrangement via a four-membered cyclic transition state, as well as charge-induced fragmentation reactions, was proposed as the main fragmentation mechanisms according to the obtained fragmentation products from the protonated parent peaks. In addition, heterolytic and homolytic cleavages were proposed as alternative fragmentation pathways. Moreover, a 1,4-hydrogen elimination was proposed to explain different fragmentation products obtained from the sodiated parent peaks.

High temperature synthesis of vinyl terminated polymers based on dendronized acrylates: a detailed product analysis study

The article focuses on the combination of dendrons with high temperature acrylate polymerization for the generation of dendronized macromonomers.

Structural identification of hindered amine light stabilisers in coil coatings using electrospray ionisation tandem mass spectrometry

Hindered amine light stabilisers (HALS) are the most effective antioxidants currently available for polymer systems in post-production, in-service applications, yet the mechanism of their action is still not fully understood. Structural characterisation of HALS in polymer matrices, particularly the identification of structural modifications brought about by oxidative conditions, is critical to aid mechanistic understanding of the prophylactic effects of these molecules. In this work, electrospray ionisation tandem mass spectrometry (ESI-MS/MS) was applied to the analysis of a suite of commercially available 2,2,6,6-tetramethylpiperidine-based HALS. Fragmentation mechanisms for the [M + H](+) ions are proposed, which provide a rationale for the product ions observed in the MS/MS and MS(3) mass spectra of N-H, N-CH(3), N-C(O)CH(3) and N-OR containing HALS (where R is an alkyl substituent). A common product ion at m/z 123 was identified for the group of antioxidants containing N-H, N-CH(3) or N-C(O)CH(3) functionality, and this product ion was employed in precursor ion scans on a triple quadrupole mass spectrometer to identify the HALS species present in a crude extract from of a polyester-based coil coating. Using MS/MS, two degradation products were unambiguously identified. This technique provides a simple and selective approach to monitoring HALS structures within complex matrices.

Obtaining kinetic information from the chain-length distribution of polymers produced by RAFT

We describe a simple model for the kinetics and chain-length distribution of polymers made by living radical techniques. Living radical methods give good control over the molecular weight of a linear polymer by capping the growing end and forming a dormant chain. The polymer is predominantly capped, and occasionally decaps to form a radical that propagates for a short period before recapping. Our model uses this mechanism to describe the chain-length distribution of polymers made by living radical methods. We focus on oligomers made by reversible addition-fragmentation chain transfer (RAFT) polymerization as model systems. Our model can determine optimal reaction conditions for desired polymer properties and test hypotheses about reaction schemes by using only two parameters, with each parameter related to the kinetics. The first parameter is the mean number of monomers added when a chain decaps. A broad distribution results if many monomers are added upon decapping. The second parameter is the mean number of times a polymer decaps. Many decapping events indicate high monomer conversion. Our model gives kinetic information by directly fitting to an experimental chain-length distribution, which is the reverse of other kinetic models that generate the distribution from rate coefficients. Our approach has also the advantage of being simpler than previously published kinetic schemes, which use many rate coefficients as inputs. Our model was tested against three monomers (acrylic acid, butyl acrylate, and styrene) and two RAFT agents. In each case, we successfully describe the chain-length distribution, and give information about the kinetics, especially the probability of propagation versus deactivation by the RAFT mechanism. This excellent agreement with a priori expectations and quantum calculations makes our model a powerful tool for predicting the structure of polymers obtained by living radical polymerization.