Quantifying the Reactivity of Photolytically Generated Radicals Towards Vinylic Monomers via Electrospray Ionization-Mass Spectrometry

Exact knowledge of the reactivity of photolytically generated radicals towards initiating free radical polymerization processes is still scarce. In here, quantitative coupled size-exclusion chromatography (SEC) - electrospray ionization-mass spectrometry (ESI-MS) as well as direct infusion ESI-MS was employed to obtain the propensity of variable photolytically generated radical species to initiate macromolecular growth

Near Infrared Light Induced Radical Polymerization in Water

We introduce a gold nanorod (AuNR) driven methodology to induce free radical polymerization in water with near infrared light (800 nm). The process exploits photothermal conversion in AuNR and subsequent heat transfer to a radical initiator (here azobisisobutyronitrile) for primary radical generation. A broad range of reaction conditions were investigated, demonstrating control over molecular weight and reaction conversion of dimethylacrylamide polymers, using nuclear magnetic resonance spectroscopy. We underpin our experimental data with finite element simulation of the spatio-temporal temperature profile surrounding the AuNR directly after femtosecond laser pulse excitation. Critically, we evidence that polymerization can be induced through biological tissues given the enhanced penetration depth of the near infrared light. We submit that the presented initiation mechanism in aqueous systems holds promise for radical polymerization in biological environments, including cells

The adult murine heart has a sparse, phagocytically active macrophage population that expands through monocyte recruitment and adopts an ‘M2’ phenotype in response to Th2 immunologic challenge

AbstractTissue resident macrophages have vital homeostatic roles in many tissues but their roles are less well defined in the heart. The present study aimed to identify the density, polarisation status and distribution of macrophages in the healthy murine heart and to investigate their ability to respond to immune challenge. Histological analysis of hearts from CSF-1 receptor (csf1-GFP; MacGreen) and CX3CR1 (Cx3cr1GFP/+) reporter mice revealed a sparse population of GFP positive macrophages that were evenly distributed throughout the left and right ventricular free walls and septum. F4/80+CD11b+ cardiac macrophages, sorted from myocardial homogenates, were able to phagocytose fluorescent beads in vitro and expressed markers typical of both ‘M1’ (IL-1β, TNF and CCR2) and ‘M2’ activation (Ym1, Arg 1, RELMα and IL-10), suggesting no specific polarisation in healthy myocardium. Exposure to Th2 challenge by infection of mice with helminth parasites Schistosoma mansoni, or Heligmosomoides polygyrus, resulted in an increase in cardiac macrophage density, adoption of a stellate morphology and increased expression of Ym1, RELMα and CD206 (mannose receptor), indicative of ‘M2’ polarisation. This was dependent on recruitment of Ly6ChighCCR2+ monocytes and was accompanied by an increase in collagen content.In conclusion, in the healthy heart resident macrophages are relatively sparse and have a phagocytic role. Following Th2 challenge this population expands due to monocyte recruitment and adopts an ‘M2’ phenotype associated with increased tissue fibrosis

A qualitative and quantitative post-mortem analysis: Studying free-radical initiation processes via soft ionization mass spectrometry

The current article contains a review of the electrospray ionization-mass spectrometry characterization of polymers prepared via thermal- and photoinitiation processes. The used analysis method permits direct access to detailed endgroup information. For a qualitative and quantitative endgroup analysis, sophisticated methods have been developed which provide a detailed image of the incorporation propensity of thermally as well as photolytically generated radicals at the polymer chain termini. Such a post-mortem analysis of polymeric materials specifically allows for the quantification of the ability of radical fragments to initiate polymerization processes. Herein, the most recent progress in the field of mass spectrometric radical reactivity mapping is outlined and open questions as well as future directions are discussed. © 2012 Wiley Periodicals, Inc

Quantitative comparison of the mesitoyl vs the benzoyl fragment in photoinitiation: A question of origin

Photolytically generated radicals (at a wavelength of 351 nm) derived from the acetophenone-type photoinitiators benzoin (2-hydroxy-1,2-diphenylethanone) and 2,4,6-trimethylbenzoin (2-hydroxy-1-mesityl-2-phenylethanone, TMB) (specifically the benzoyl and mesitoyl radical) are quantified in their ability to serve as initiating species in methyl methacrylate (MMA), ethyl methacrylate (EMA), and butyl methacrylate (BMA) bulk free radical polymerizations under optimized conditions. Herein, 2,4,6-trimethylbenzoin is employed for the first time as photoinitiator in pulsed laser polymerizations (PLP) employing a high-frequency excimer laser, constituting a new source for mesitoyl radicals. The current work presents an improved method for quantifying radical efficiency of photoinitiation processes using coupled online size exclusion chromatography-electrospray ionization mass spectrometry (SEC/ESI-MS) to analyze the obtained polymers. Because of the occurrence of side reactions during the benzoin-initiated MMA polymerization, reduced laser energies (∼0.35 mJ/pulse) as well as low polymerization temperatures (∼-5 °C) were employed, which avoids side product formation. A plot of the ratio of benzoyl to mesitoyl (derived from 2,4,6-trimethylbenzoin) end groups vs the ratio of both initiators in the reaction mixture indicates that the benzoin-derived benzoyl radical is 3.0 (2.6, 2.4) times more likely to initiate the polymerization process of MMA (EMA, BMA) than the TMB-derived mesitoyl fragment. This observation is in sharp contrast to the case when mesitil is employed as a source of mesitoyl radicals (8.6 times higher likelihood of benzoyl incorporation). These results clearly support the notion that the origin of a radical species significantly determines its propensity to be incorporated at a polymer chain's terminus. The cause of such an origin dependence is tentatively assigned-at least in part-to different triplet lifetimes or intersystem crossing efficiencies (ΦISC) or both of TMB and mesitil. © 2011 American Chemical Society

Quantifying photoinitiation efficiencies in a multiphotoinitiated free-radical polymerization

Free to read Online size exclusion chromatography-electrospray ionization-mass spectrometry (SEC/ESI-MS) is employed for quantifying the overall initiation efficiencies of photolytically generated radical fragments. In a unique experiment, we present the first quantitative and systematic study of methyl-substituted acetophenone-type photoinitiators being employed in a single cocktail to initiate the free-radical polymerization of methyl methacrylate (MMA) in bulk. The photoinitiators are constituted of a set of two known and four new molecules, which represent an increasing number of methyl substituents on their benzoyl fragment, that is, benzoin, 4-methylbenzoin, 2,4-dimethylbenzoin, 2,4,6-trimethylbenzoin, 2,3,5,6-tetramethylbenzoin, and 2,3,4,5,6-pentamethylbenzoin. The absolute quantitative evaluation of the mass spectra shows a clear difference in the initiation ability of the differently substituted benzoyl-type radical fragments: Increasing the number of methyl substituents leads to a decrease in incorporation of the radical fragments. The first systematic study of six (methyl substituted) acetophenone-type photoinitiators employed in a single cocktail experiment is presented. PLP synthesis in conjunction with coupled online SEC/ESI-MS is the method of choice to obtain absolute quantitative information regarding the initiation ability of the derived radical fragments. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

UV-triggered end group conversion of photo-initiated poly(methyl methacrylate)

The analysis of photo-initiated poly(methyl methacrylate) via electrospray ionization-mass spectrometry (ESI-MS) (synthesized by pulsed laser polymerization (PLP, at λ = 351 nm) of methyl methacrylate (MMA) and benzoin as photoinitiator at 6 mJ/pulse laser energy) evidences the presence of unidentified species. The determination of the origin of these species requires a detailed investigation via size exclusion chromatography-electrospray ionization-mass spectrometry (SEC/ESI-MS) and chemically induced dynamic nuclear polarization-nuclear magnetic resonance spectroscopy (CIDNP-NMR). It was found that post-irradiation of benzoin-initiated poly(methyl methacrylate) leads to α-cleavage of the benzoyl fragment leading to a sequence of cascade reactions, including the formation of an additional double bond within the polymer chain as evidenced via ESI-MS. Furthermore, the reaction products of the benzoyl radical post α-cleavage (e.g., benzaldehyde, phenyl methyl ketone, methyl formate, or methane) as well as the formed macroradical can be followed by CIDNP-NMR, which allows establishing a reaction mechanism for the UV-induced cleavage process. The study thus evidence that-if the integrity of UV initiated polymers is to be kept intact during their synthesis-very low irradiation energies need to be employed. © 2012 American Chemical Society

Fluorescent polymers from non-fluorescent photoreactive monomers

A facile, fast and ambient-temperature avenue towards highly fluorescent polymers is introduced via polymerizing non-fluorescent photoreactive monomers based on light-induced NITEC chemistry, providing a platform technology for fluorescent polymers. The resulting polypyrazolines were analyzed in depth and the photo-triggered step-growth process was monitored in a detailed kinetic study. © 2014 The Royal Society of Chemistry

Radical addition fragmentation chain transfer (RAFT) polymerization of ferrocenyl (Meth)acrylates

We report on the controlled free radical homopolymerization of 1-ferrocenylethyl acrylate as well as of three new ferrocene bearing monomers, namely 4-ferrocenylbutyl acrylate, 2-ferrocenylamido-2-methylpropyl acrylate, and 4-ferrocenylbutyl methacrylate, by the RAFT technique. For comparison, the latter monomer was polymerized using ATRP, too. The ferrocene containing monomers were found to be less reactive than their analogues free of ferrocene. The reasons for the low polymerizability are not entirely clear. As the addition of free ferrocene to the reaction mixture did not notably affect the polymerizations, sterical hindrance by the bulky ferrocene moiety fixed on the monomers seems to be the most probable explanation. Molar masses found for 1-ferrocenylethyl acrylate did not exceed 10,000 g mol-1, while for 4-ferrocenylbutyl (meth)acrylate molar masses of 15,000 g mol-1 could be obtained. With PDIs as low as 1.3 in RAFT polymerization of the monomers, good control over the polymerization was achieved. © 2011 Wiley Periodicals, Inc

Two Photon Induced Pulsed Laser Polymerization with Near Infrared Light

We introduce two-photon (2P) pulsed laser polymerization (PLP) at 800 nm, demonstrating its working principle even through biological tissue. We show that 2P PLP is reliable in determining propagation rate coefficients on the example of the free radical polymerization of methyl methacrylate (MMA) at frequencies ranging from 10 to 100 Hz.</p