1,721,594 research outputs found
Polymers with well-defined end groups via RAFT - synthesis, applications and postmodification
No full textThe control over the chain-end functionality of a polymeric chain produced by\ud
controlled/‘living’ radical polymerization is inherent to the mechanism of the re- action. Indeed, \ud
the final product contains a majority of polymeric chains showing an ω-functional end group which is \ud
used to control the molecular weight growth. The growth of the molecular weight can be mediated \ud
either via the reversible homolytic cleavage of the covalent bond between the terminal carbon and \ud
the chain-end group (e.g. halogen for transition-metal-mediated living radical poly- \ud
merization/atom transfer radical polymerization (ATRP), nitroxide for nitroxide- mediated \ud
polymerization (NMP)) or via the degenerative transfer of chain-end groups between propagating \ud
radicals and dormant species (e.g. thiocarbonylthio groups for reversible addition–fragmentation \ud
chain transfer (RAFT)). Furthermore, α-functional end groups can also be introduced via the \ud
initiator/mediator of the polymerization, for example, halogen alkyls (ATRP), alkyl nitroxides \ud
(NMP) or\ud
dithioesters (RAFT)..
Revealing Model Dependencies in “Assessing the RAFT Equilibrium Constant via Model Systems: An EPR Study”
No full textIn a recent article (W. Meiser, M. Buback, Assessing the RAFT Equilibrium Constant via Model Systems: An EPR Study, Macromol. Rapid Commun. 2011, 18, 1490-1494), it is claimed that evidence is found that unequivocally proves that quantum mechanical calculations assessing the equilibrium constant and fragmentation rate coefficients in dithiobenzoate-mediated reversible addition fragmentation transfer (RAFT) systems are beset with a considerable uncertainty. In the present work, we show that these claims made by Meiser and Buback are beset with a model dependency, as a critical key parameter in their data analysis the addition rate coefficient of the radicals attacking the C=S double bond in the dithiobenzoate induces a model insensitivity into the data analysis. Contrary to the claims made by Meiser and Buback, their experimental results can be brought into agreement with the quantum chemical calculations if a lower addition rate coefficient of cyanoisopropyl radicals (CIP) to the CIP dithiobenzoate (CPDB) is assumed. To resolve the model dependency, the addition rate coefficient of CIP radicals to CPDB needs to be determined as a matter of priority.T.J. is grateful for support from the Fonds Wetenschappelijk Onderzoek within the Odysseus program. C.B.-K. is grateful for continued support from the Karlsruhe Institute of Technology (KIT) in the context of the Excellence Initiative for leading German universities. M.L.C. gratefully acknowledges useful discussions with Dr. Ching Yeh Lin and Professor Christopher Cramer, financial support from the Australian Research Council (ARC) under their ARC Centres of Excellence program and receipt of an ARC Future Fellowship
A qualitative and quantitative post-mortem analysis: Studying free-radical initiation processes via soft ionization mass spectrometry
No full textThe 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
Critically evaluated rate coefficients in radical polymerization - 8. propagation rate coefficients for vinyl acetate in bulk
No full textPropagation rate coefficient values, <i>k</i><sub>p</sub>, reported by several groups for radical polymerization of bulk vinyl acetate are critically evaluated. All data are obtained by the combination of pulsed-laser polymerization and subsequent polymer analysis by size exclusion chromatography, as recommended by the IUPAC Working Party on Modeling of Polymerization Kinetics and Processes. Although a small (≈15%) increase in <i>k</i><sub>p</sub> is observed as laser pulse repetition rate is increased from low (25–100 Hz) to high (300–500 Hz) values, all of the data fulfill the required consistency criteria and thus are combined into a benchmark set covering the temperature range of 5–70°C. The data are fitted well by an Arrhenius relation resulting in a pre-exponential factor of 1.35 × 10<sup>7</sup> L mol<sup>−1</sup> s<sup>−1</sup> and an activation energy of 20.4 kJ mol<sup>−1</sup>, with 95% confidence ellipsoids for the parameters also presented
Design of radical sensor materials via nitrile imine-mediated tetrazole-ene cycloaddition (NITEC)
No full textDevelopment of Polymer Grafting Methodologies for Advanced Surface Engineering
No full textToday’s needs for complex functions and steady miniaturization of devices challenge materials scientists towards constant improvement. The interaction of materials is largely determined by their surface properties. The PhD thesis “Development of Polymer Grafting Methodologies for Advanced Surface Engineering” by Joachim Frieder Laun extends the toolbox of polymer grafting techniques by three contributions. Photoinduced copper-mediated radical polymerization was employed to graft polyacrylates from silicon substrates at extremely low catalyst concentrations. The controlled nature of this procedure was demonstrated via block copolymerization. Moreover, a 2D laser lithography protocol of the same technique is presented. Finally, nitrone-mediated radical coupling was used for grafting-to of short polyacrylates. Successful surface reactions were evidenced via XPS, ToF-SIMS and grazing angle ATR-FTIR. In addition, the development of a new polyzwitterionic polymer and the influence of the light intensity on radical thiol-ene reactions are presented.FW
SP-PLP-EPR Investigations into the Termination Kinetics of n-Butyl Acrylate Macromonomers
Full text linkThe definitive version is available at www3.interscience.wiley.comThe termination of model mid-chain radicals (MCRs), which mimic radicals that occur in
acrylate polymerization over a broad range of reaction conditions, has been studied by singlepulse
pulsed-laser polymerization (SP PLP) in conjunction with electron paramagnetic
resonance (EPR) spectroscopy. The model radicals were generated by initiator-fragment
addition to acrylic macromonomers that were preformed prior to the kinetic experiments, thus
enabling separation of termination from the propagation reaction, for these model radicals
propagate sparingly, if at all, on the timescale of SP-PLP experiments. Termination rate
coefficients of the MCRs were determined in the temperature range 0–60 °C in acetonitrile
and butyl propionate solution as well as in bulk macromonomer over 0–100 °C. Termination
rate coefficients slightly below those of the corresponding secondary radicals were deduced,
demonstrating the relatively high termination activity of this species, even when undergoing MCR-MCR termination. For chain length 10, a reduction by a factor of 6 is observed.
Unusually high activation energies were found for the termination rate coefficient in these
systems, with 35 kJ mol⁻¹ being determined for bulk macromonomer
Development of Polymer Grafting Methodologies for Advanced Surface Engineering
No full textToday’s needs for complex functions and steady miniaturization of devices challenge materials scientists towards constant improvement. The interaction of materials is largely determined by their surface properties. The PhD thesis “Development of Polymer Grafting Methodologies for Advanced Surface Engineering” by Joachim Frieder Laun extends the toolbox of polymer grafting techniques by three contributions. Photoinduced copper-mediated radical polymerization was employed to graft polyacrylates from silicon substrates at extremely low catalyst concentrations. The controlled nature of this procedure was demonstrated via block copolymerization. Moreover, a 2D laser lithography protocol of the same technique is presented. Finally, nitrone-mediated radical coupling was used for grafting-to of short polyacrylates. Successful surface reactions were evidenced via XPS, ToF-SIMS and grazing angle ATR-FTIR. In addition, the development of a new polyzwitterionic polymer and the influence of the light intensity on radical thiol-ene reactions are presented.FW
Utilization of Nitrones in Radical Polymerizations and Polymer Conjugations
No full textMuch research has been performed in the last 10 years on the use of nitrones in synthetic polymer chemistry. Nitrone polymer chemistry represents an additional tool to the “polymer toolbox”, which contributes to the advancement of modern polymer science and technology. In this present thesis, four interconnecting themes centered on the use of nitrones in radical polymerizations and coupling have been investigated. (1) Synthesize nitrones with various functionalities. (2) Utilize controlled radical polymerization techniques to synthesize macroinitiators with well-defined end-groups and molecular weights. (3) Use ESCP and NMRC to couple the macroinitiators from CRP techniques to synthesize new architectures. (4) Study nitrones in the context of ESCP and NMRC in ESR. One of these central themes is the synthesis of various nitrones. Most nitrones mentioned have been previously been synthesized and utilized by E.H.H. Wong et al. in ESCP or NMRC reactions. The addition of a linear type nitrone was subjected to limited ESCP conditions resulting in imperfect control of the reaction. With the development of N-hydroxysuccinimide (NHS) functionalized aldehydes and nitrones, a new use in bioconjugation reactions of nano-particles was investigated. In addition, UV-active nitrone degradation experiments were performed, with a 100% loss in functionality in one hour for PBN and a pyrene containing nitrone exhibiting the formation of oxaziridine rings. The study of nitrones was further extended to the use of cyclic nitrones in ESCP and NMRC reactions. The best cyclic candidate of three investigated was 2-isopropyl-2,3-dimethyl-1-oxy-2,3-dihydro-imidazol-4-one, it resulted in a spin trapping CSC value of 12.2 and an addition rate of macroradicals to nitrone of 8.0·103 L·mol−1·s−1 at 80 °C. This addition rate represents a factor of 10 higher than that observed in the conventionally employed nitrone PBN. Furthermore, nitrone mediated radical coupling (NMRC) reactions with a polystyrene (PS) macroinitiator were achieved with an almost perfect coupling with a precise doubling in molecular weight and a slight reduction of dispersity. NMRC conditions were then further extended to iodine capped polystyrene (PS) and poly(methyl methacrylate) (PMMA) macroinitiators, synthesized using reverse iodine transfer polymerization. A halide exchange mechanism was required to promote a coupling to occur, as iodine is not sufficiently reactive for NMRC reactions. PMMA represents a monomer that has not been successfully coupled without a promoting agent. The addition of styrene resulted in a coupling efficiency of 0.84 with an additional 10 units of styrene built into the chain to result in a PMMA-b-PS-b-PMMA triblock copolymer. The NMRC of PS-I resulted in a coupling efficiency of 0.75. The investigation of suitable macroinitiators for coupling and chain extension was expanded to the use of bromine functionalized poly(ethylene glycol) (PEG-Br). Upon NMRC a coupling efficiency of ≥99 % was observed. The preparation of ABA-type block copolymers was achieved using a combination of NMRC and NMP resulting in a PEG-NO-PS-PEG triblock copolymer with a 12 200 g・mol-1 block of PS built in. The resulting polymer contained some dead chain ends and they were removed by performing recycling GPC. A second method was investigated termed simultaneous coupling and chain extension whereby an in situ formation of an ABBA-triblock copolymer occurs. Reactions were successfully performed with a high level of control with a minimum of 100 equivalents of n-butyl acrylate or styrene. The NMRC system, utilized in PEG, was further investigated by electron spin resonance (ESR) spectrometry by looking at the disproportionation of copper(I)bromide (Cu(I)Br) to copper(II)dibromide (Cu(II)Br2) and copper(0) (Cu(0)). This was performed to gain an insight into the exact interplay of the included copper species. It was confirmed that the N-ligand/solvent system strongly affects the rates of disproportionation observed. The highest level observed after 200 seconds was observed for a Cu(I)Br/PMDETA/ACN solution with a 2.75 times higher concentration of Cu(II) formed than the next closest copper system of Cu(I)Br/TPMA/ACN. The NMRC system investigation was limited by the low stability of solutions and the varying concentration of Cu(0) in each solution tested. Future research must be performed to optimize the copper system for this reaction. The use of ESR was further extended to an ESR-based spin-trapping protocol to estimate fragmentation rate coefficients in the RAFT pre-equilibrium. It was found to be a highly reliable tool that confirmed the slow fragmentation nature of dithiobenzoate-derived tert-butyl adduct radicals. A fragmentation rate coefficient at 20 ºC was found to be close to 0.042 s-1 with the activation parameters for the fragmentation reaction as EA = 82 ± 13.3 kJ·mol-1 and A = (1.4 ± 0.25) · 1013 s-1. The obtained fragmentation rate coefficient represents an upper limit for dithiobenzoates. This method proved to be a suitable tool for evaluating the fragmentation rate coefficients for a wide range of RAFT adduct radicals.De laatste 10 jaar is er veel onderzoek verricht naar het gebruik van nitronen in de synthetische polymeerchemie. De vereniging van deze twee methodes zorgt voor een nieuwe strategie wat een nieuwe impuls betekent voor onderzoek binnen de moderne polymeerwetenschap en -technologie. In dit proefschrift worden vier nauwverwante thema’s rondom nitronen in de polymeerchemie behandeld: 1: De synthese van nitronen met verschillende functionele groepen. 2: Het gebruik van gecontroleerde radicaalpolymerisatie technieken omwille van het verkrijgen van macroinitiatoren met specifieke molecuulgewichten en eindgroepen. 3: Het koppelen van deze gedefinieerde macroinitiatoren met behulp van ESCP en NMRC ter vorming van nieuwe polymeerarchitecturen. 4: Het bestuderen van nitronen in ESR met betrekking tot ESCP en NMRC. Ten eerste werden er nitronen met verschillende functionele groepen gesynthetiseerd. Veel van de genoemde nitronen werden reeds gesynthetiseerd door E.H.H. Wong et al. en zijn gebruikt in ESCP en NMRC reacties. Het toevoegen van een lineair nitron en het toepassen van milde ESCP condities leidde tot een onvoldoende controle over de reactie. Met de ontwikkeling van N-hydroxysuccinimide (NHS) gefunctionaliseerde aldehydes en nitronen was het mogelijk om deze voor het eerst te gebruiken in bioconjugatie reacties op het oppervlak van nanodeeltjes. Tevens is er gekeken naar de degradatie van het UV-actieve nitron PBN en pyreen-bevattende nitronen, waarbij is vastgesteld dat er 100% verlies aan UV-activiteit optreedt binnen het uur. Dit werd gekenmerkt door de vorming van oxaziridine ringen. Het onderzoek naar nitronen werd verder uitgebreid met het gebruik van cyclische nitronen voor toepassing in ESPC en NMRC reacties. Van drie onderzochte cyclische nitronen bleek 2-isopropyl-2,3-dimethyl-1-oxy-2,3- dihydro-imidazol-4-one de beste optie, mede dankzij de ‘spin-trapping’ waarde voor Csc = 12.2 en een waarde voor de additiesnelheid van de macroradicalen van 8.0・103 L・mol-1・s-1 bij T=80°C. Deze waarde voor de additiesnelheid ligt een factor 10 hoger dan gemeten waardes voor PBN. Bovendien, het gebruik van polystyreen (PS) macroinitiatoren in ‘nitrone mediated radical coupling’ (NMRC) reacties leidde tot perfecte koppeling gebaseerd op een exacte verdubbeling van de oorspronkelijk massa van de PS macroinitiatoren en een kleine afname in de polydispersiteit. NMRC condities werden vervolgens verder uitgebreid naar macroinitiatoren van polystyreen en poly(methyl methacrylaat) (PMMA) met de functionele groep jood als ketenuiteinde. Deze konden worden gesynthetiseerd met behulp van ‘reverse iodine transfer polymerization’, waarbij gebruik gemaakt werd van een halide-uitwisseling om zo de koppelingsreactie te promoten. Dit wordt hoofdzakelijk veroorzaakt door de lage reactiviteit van het jood m.b.t NMRC reacties. Met PMMA is het voor het eerst mogelijk om een koppelingsreactie uit te voeren zonder daarbij gebruik te maken van een promotie-agens. Het toevoegen van styreen resulteerde in een koppelingsefficiëntie van 0.84 waarbij 10 eenheden styreen werden ingebouwd resulterend in een PMMA-b-PS-b-PMMA triblock copolymeer. Het gebruik van jood getermineerd PS in NMRC resulteerde in een koppelingsefficiëntie van 0.75. Vervolgens werd er verder onderzoek gevoerd naar bruikbare macroinitiatoren door gebruik te maken van broom-gefunctionaliseerd polyethyleenglycol (PEGBr). Onder invloed van NMRC werd een koppelingsefficiëntie gemeten ≥ 99%. Het synthesiseren van ABA-type block copolymeren werd bereikt door het gebruik van een combinatie van NMRC en NMP ter vorming van een PEG-NO-PSPEG triblock copolymeer met een blocklengte van PS van 12 200 g・mol-1. Het gevormde polymeer bevatte echter kleine hoeveelheden aan dode ketens (zonder de gewenste broom-eindgroepen) welke verwijderd zijn met behulp van de zogenoemde ‘recyling GPC’. Een alternatieve methode werd getest waarbij gebruik werd gemaakt van het simultaan laten verlopen van de koppelingsreactie en het verlengen van de ketens (hier genoemd ‘simultaneous coupling and chain extension’). Hiertoe werd in-situ een ABBA-triblock copolymeer gevormd. De reacties werden succesvol uitgevoerd met een hoge mate van controle met een minimum van 100 equivalenten aan styreen of nbutyl acrylaat. Het NMRC systeem, zoals gebruikt in combinatie met PEG, werd verder onderzocht door gebruik te maken van ESR spectrometrie. Hierbij werd gekeken naar de disproportionering van koper(I)bromide (Cu(I)Br) tot koper(II)bromide (Cu(II)Br2) en elementair koper (Cu(0)). Dit onderzoek was nodig om beter inzicht te krijgen in het samenspel van de verschillende koper componenten. Uit dit onderzoek kwam naar voren dat het N-ligand/solvent systeem een sterke invloed uitoefent op de snelheid van de disproportioneringsreactie. De hoogste mate van disproportionering na 200 seconde werd gemeten voor het Cu(I)Br/PMDETA/ACN systeem waarbij de concentratie aan koper(II) 2.75 maal hoger was dan voor het op-één-na beste systeem te weten Cu(I)Br/TPMA/ACN. Echter werd het onderzoek naar het NMRC gelimiteerd door de lage stabiliteit van de oplossingen en door de wisselende concentraties aan Cu(0) en de geteste oplossing. Verder onderzoek is nodig voor het optimaliseren van het koper systeem voor dit type reacties. Het gebruik van ESR werd verder uitgebreid naar een ESR-gebaseerd ‘spintrapping’ voorschrift ten behoeve van het bepalen van snelheidsconstantes in het RAFT pre-equilibrium met betrekking tot fragmentatie. Het bleek een zeer betrouwbare methode te zijn waarbij het trage fragmentatiekarakter werd bewezen voor butyl adduct radicalen afkomstig van dithiobenzoaat. Er werd een waarde voor de fragmentatiesnelheidscoëfficiënt gemeten van 0.042 s-1 bij 20°C en waarbij de activeringsparameters voor fragmentatie zijn gegeven als EA = 82 ± 13.3 kJ・mol-1 and A = (1.4 ± 0.25)・1013 s-1. De verkregen fragmentatiesnelheid kan worden gezien als de bovengrens voor dithiobenzoaten. Deze methode blijkt een geschikte methode te zijn voor het evalueren van het fragmentatiegedrag van verschillende soorten RAFT adduct radicalen
- …
