Free Radical Polymerization

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Yusuf Yagci - One of the best experts on this subject based on the ideXlab platform.

Nicholas J Turro - One of the best experts on this subject based on the ideXlab platform.

  • benzoin type photoinitiator for Free Radical Polymerization
    Journal of Polymer Science Part A, 2013
    Co-Authors: Duygu Sevinc Ese, Nergis Arsu, Steffe Jockusch, Jose P Da Silva, Nicholas J Turro
    Abstract:

    Benzoin, a popular photoinitiator for Free Radical Polymerization of vinyl monomers, was improved by intro- duction of two methyl thioether substituents. This new ben- zoin derivative showed an about 50 times higher light absorption in the near-UV spectral region and performed bet- ter than the unsubstituted benzoin in Polymerization experi- ments in bulk solutions or films of acrylate monomers when low initiator concentrations are used. Laser flash photolysis, low temperature luminescence experiments and photoproduct studies by mass spectrometry suggest that a slow a-cleavage mechanism (ka ¼ 2.2 � 10 5 s � 1 ) from the electronic triplet state with a quantum yield of 0.1 is the primary photoreac- tion to generate the initiating Free Radicals. V C 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 1865-1871 In the presence of vinyl monomers, these Radicals can initiate Free Radical Polymerization. The major disadvantage of benzoin and its ether derivatives is the poor light absorption in the near-UV, a spectral region which is most attractive for industrial photoPolymerizations. Here, we present a benzoin derivative, MTB, which shows the desired bathochromic shifted absorption and higher molar absorptivity compared to benzoin. We show that although the mechanism of Radical generation is different from the a-cleav- age mechanism of unsubstituted benzoin, this new photoini- tiator efficiently initiates Free Radical Polymerization.

  • mechanism of photoinitiated Free Radical Polymerization by thioxanthone anthracene in the presence of air
    Macromolecules, 2011
    Co-Authors: Demet Karaca Balta, Yusuf Yagci, Steffen Jockusch, Nergis Arsu, Arun Kumar Sundaresan, Nicholas J Turro
    Abstract:

    The mechanism of formation of initiating Radicals for Free Radical Polymerization by the thioxanthone−anthracene (TX-A) photoinitiator was investigated by laser flash photolysis, fluorescence and phosphorescence spectroscopy, and Polymerization studies. The proposed mechanism of photoinitiation involves photoexcitation of TX-A and quenching of triplet excited states of TX-A by molecular oxygen to generate singlet oxygen. Singlet oxygen reacts with the anthracene moiety of TX-A to form an endoperoxide. The endoperoxide intermediate was isolated and characterized by MS and 1H NMR. The endoperoxide undergoes photochemical or thermal decomposition to generate Radicals which are able to initiate Free Radical Polymerization. A significant deuterium effect on the Polymerization rates using CDCl3 and CHCl3 as solvents confirmed the involvement of singlet oxygen in the initiation process.

  • mechanistic studies of photoinitiated Free Radical Polymerization using a bifunctional thioxanthone acetic acid derivative as photoinitiator
    Macromolecules, 2009
    Co-Authors: Feyza Karasu, Steffen Jockusch, Nergis Arsu, Nicholas J Turro
    Abstract:

    A bifunctional photoinitiator for Free Radical Polymerization, thioxanthone catechol-O,O′-diacetic acid, was synthesized, characterized, and compared to photoinitiator parameters of the monofunctional analogue, 2-(carboxymethoxy)thioxanthone. Photophysical studies such as fluorescence, phosphorescence, and laser flash photolysis in addition to photoPolymerizations of methyl methacrylate show that the bifunctional photoinitiator is more efficient in polymer generation than the monofunctional derivative. These studies suggest that initiator Radicals are generated from a π−π* triplet state in an intramolecular electron transfer, followed by proton transfer and decarboxylation to generate alkyl Radicals, which initiate Polymerization. The initial electron transfer is faster for the bifunctional photoinitiator than the monofunctional derivative, which is based on laser flash photolysis studies. Because of the relatively fast intramolecular Radical generation from the triplet state (triplet lifetime = 490 ns), ...

  • thioxanthone anthracene a new photoinitiator for Free Radical Polymerization in the presence of oxygen
    Macromolecules, 2007
    Co-Authors: Demet Karaca Alta, Yusuf Yagci, Nergis Arsu, Steffe Jockusch, Nicholas J Turro
    Abstract:

    A novel thioxanthone−anthracene (TX-A) photoinitiator, namely 5-thia-pentacene-14-one, possessing the respective photochromic groups was synthesized. TX-A is an efficient photoinitiator for Free Radical Polymerization of acrylic and styrenic type monomers in the presence of oxygen. UV−vis, FT-IR, and fluorescence spectroscopic and Polymerization studies revealed that photoinitiation occurs through anthracene chromophore. In contrast to thioxanthone-based photoinitiators, TX-A does not require an additional hydrogen donor for the initiation.

  • mechanistic study of photoinitiated Free Radical Polymerization using thioxanthone thioacetic acid as one component type ii photoinitiator
    Macromolecules, 2005
    Co-Authors: Meral Aydin, Yusuf Yagci, Steffen Jockusch, Nergis Arsu, Nicholas J Turro
    Abstract:

    A mechanistic study concerning photoinitiated Free Radical Polymerization using thioxanthone thio-acetic acid (TX−S−CH2−COOH) as one-component Type II photoinitiator was performed. Steady-state and time-resolved fluorescence and phosphorescence spectroscopy, as well as laser flash photolysis was employed to study the photophysics and photochemistry of TX−S−CH2−COOH. The initiator undergoes efficient intersystem crossing into the triplet state and the lowest triplet state posseses π−π* configuration. In contrast to the unsubstituted thioxanthone, TX−S−CH2−COOH shows an unusually short triplet lifetime (65 ns) indicating an intramolecular reaction. From fluoroscence, phosphorescence, and laser flash photolysis studies, in conjunction with photoPolymerization experiments, we propose that TX−S−CH2−COOH triplets undergo intramolecular electron transfer followed by hydrogen abstraction and decarboxylation producing alkyl Radicals, which are the active initiator Radicals in photoinduced Polymerization. At low in...

Nergis Arsu - One of the best experts on this subject based on the ideXlab platform.

  • thioxanthone benzothiophenes as photoinitiator for Free Radical Polymerization
    Journal of Photochemistry and Photobiology A-chemistry, 2016
    Co-Authors: Nurcan Karaca, Nergis Arsu, Nuket Ocal, Steffe Jockusch
    Abstract:

    Abstract Photoinitiators for Free Radical Polymerization based on the thioxanthone chromophore that contains benzothiophene were synthesized and characterized. Compared to thioxanthone, these compounds show a bathochromic shifted absorption up to ∼460 nm. High quantum yields for intersystem crossing generate sufficient amounts of triplet states. Initiator Radicals are generated by reaction of the triplet states with tertiary amines, such as diethanolamine with high rate constants (2–6 × 10 9  M −1  s −1 ) as determined by laser flash photolysis. Photoinitiated Polymerization experiments of MMA showed efficient Polymerization with initiator concentrations as low as 0.1 mM.

  • benzoin type photoinitiator for Free Radical Polymerization
    Journal of Polymer Science Part A, 2013
    Co-Authors: Duygu Sevinc Ese, Nergis Arsu, Steffe Jockusch, Jose P Da Silva, Nicholas J Turro
    Abstract:

    Benzoin, a popular photoinitiator for Free Radical Polymerization of vinyl monomers, was improved by intro- duction of two methyl thioether substituents. This new ben- zoin derivative showed an about 50 times higher light absorption in the near-UV spectral region and performed bet- ter than the unsubstituted benzoin in Polymerization experi- ments in bulk solutions or films of acrylate monomers when low initiator concentrations are used. Laser flash photolysis, low temperature luminescence experiments and photoproduct studies by mass spectrometry suggest that a slow a-cleavage mechanism (ka ¼ 2.2 � 10 5 s � 1 ) from the electronic triplet state with a quantum yield of 0.1 is the primary photoreac- tion to generate the initiating Free Radicals. V C 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 1865-1871 In the presence of vinyl monomers, these Radicals can initiate Free Radical Polymerization. The major disadvantage of benzoin and its ether derivatives is the poor light absorption in the near-UV, a spectral region which is most attractive for industrial photoPolymerizations. Here, we present a benzoin derivative, MTB, which shows the desired bathochromic shifted absorption and higher molar absorptivity compared to benzoin. We show that although the mechanism of Radical generation is different from the a-cleav- age mechanism of unsubstituted benzoin, this new photoini- tiator efficiently initiates Free Radical Polymerization.

  • mechanism of photoinitiated Free Radical Polymerization by thioxanthone anthracene in the presence of air
    Macromolecules, 2011
    Co-Authors: Demet Karaca Balta, Yusuf Yagci, Steffen Jockusch, Nergis Arsu, Arun Kumar Sundaresan, Nicholas J Turro
    Abstract:

    The mechanism of formation of initiating Radicals for Free Radical Polymerization by the thioxanthone−anthracene (TX-A) photoinitiator was investigated by laser flash photolysis, fluorescence and phosphorescence spectroscopy, and Polymerization studies. The proposed mechanism of photoinitiation involves photoexcitation of TX-A and quenching of triplet excited states of TX-A by molecular oxygen to generate singlet oxygen. Singlet oxygen reacts with the anthracene moiety of TX-A to form an endoperoxide. The endoperoxide intermediate was isolated and characterized by MS and 1H NMR. The endoperoxide undergoes photochemical or thermal decomposition to generate Radicals which are able to initiate Free Radical Polymerization. A significant deuterium effect on the Polymerization rates using CDCl3 and CHCl3 as solvents confirmed the involvement of singlet oxygen in the initiation process.

  • photoPolymerization and photophysical properties of amine linked benzophenone photoinitiator for Free Radical Polymerization
    Journal of Photochemistry and Photobiology A-chemistry, 2011
    Co-Authors: Demet Karaca Alta, Gokha Temel, Urak Enginol, Meral Aydi, Nergis Arsu
    Abstract:

    Synthesis of amine linked type II photoinitiator (BPDEA) was achieved in high yields and photoPolymerization of mono and multiacrylate monomers was performed with this photoinitiator in the absence of a coinitiator. BPDEA is more effective than benzophenone (BP) with the coinitiator (MDEA) system under inert atmosphere in photoinduced Free Radical Polymerization of acrylates.

  • mechanistic studies of photoinitiated Free Radical Polymerization using a bifunctional thioxanthone acetic acid derivative as photoinitiator
    Macromolecules, 2009
    Co-Authors: Feyza Karasu, Steffen Jockusch, Nergis Arsu, Nicholas J Turro
    Abstract:

    A bifunctional photoinitiator for Free Radical Polymerization, thioxanthone catechol-O,O′-diacetic acid, was synthesized, characterized, and compared to photoinitiator parameters of the monofunctional analogue, 2-(carboxymethoxy)thioxanthone. Photophysical studies such as fluorescence, phosphorescence, and laser flash photolysis in addition to photoPolymerizations of methyl methacrylate show that the bifunctional photoinitiator is more efficient in polymer generation than the monofunctional derivative. These studies suggest that initiator Radicals are generated from a π−π* triplet state in an intramolecular electron transfer, followed by proton transfer and decarboxylation to generate alkyl Radicals, which initiate Polymerization. The initial electron transfer is faster for the bifunctional photoinitiator than the monofunctional derivative, which is based on laser flash photolysis studies. Because of the relatively fast intramolecular Radical generation from the triplet state (triplet lifetime = 490 ns), ...

Christopher Barnerkowollik - One of the best experts on this subject based on the ideXlab platform.

  • the role of mid chain Radicals in acrylate Free Radical Polymerization branching and scission
    Journal of Polymer Science Part A, 2008
    Co-Authors: Thomas Junkers, Christopher Barnerkowollik
    Abstract:

    The past 5 years have seen a significant increase in the understanding of the fate of so-called mid-chain Radicals (MCR), which are formed during the Free Radical Polymerization of monomers that form highly reactive propagating Radicals and contain an easily abstractable hydrogen atom. Among these monomers, acrylates are, beside ethylene, among the most prominent. Typically, a secondary propagating acrylate-type macroRadical (SPR) can easily transfer its Radical functionality via a six-membered transition state to a position within the polymer chain (in a so-called backbiting reaction), creating a tertiary MCR. Alternatively, the Radical function can be transferred intramolecularly to any position within the chain (also forming an MCR) or intermolecularly to another polymer strand. This article aims at providing a comprehensive overview of the up-to-date knowledge about the rates at which MCRs are formed, their secondary reactions as well as the consequences of their occurrence under variable reaction conditions. We explore the latest aspects of their detection (via electron spin resonance spectroscopy) as well as the characterization of the polymer structures to which they lead (via high resolution mass spectrometry). The presence of MCRs leads to the formation of branched polymers and the partial formation of polymer networks. They also limit the measurement of kinetic parameters (such as the SPR propagation rate coefficient) with conventional methods. However, their occurrence can also be used as a synthetic handle, for example, the high-temperature preparation of macromonomers. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7585–7605, 2008

  • the role of mid chain Radicals in acrylate Free Radical Polymerization branching and scission
    Institute for Future Environments; Science & Engineering Faculty, 2008
    Co-Authors: Thomas Junkers, Christopher Barnerkowollik
    Abstract:

    The past 5 years have seen a significant increase in the understanding of the fate of so-called mid-chain Radicals (MCR), which are formed during the Free Radical Polymerization of monomers that form highly reactive propagating Radicals and contain an easily abstractable hydrogen atom. Among these monomers, acrylates are, beside ethylene, among the most promi-nent. Typically, a secondary propagating acrylate-type macro-Radical (SPR) can easily transfer its Radical functionality via a six-membered transition state to a position within the polymer chain (in a so-called backbiting reaction), creating a tertiary MCR. Alternatively, the Radical function can be transferred intramolecularly to any position within the chain (also forming an MCR) or intermolecularly to another polymer strand. This article aims at providing a comprehensive overview of the up-to-date knowledge about the rates at which MCRs are formed, their secondary reactions as well as the consequences of their occurrence under variable reaction conditions. We explore the latest aspects of their detection (via electron spin resonance spectroscopy) as well as the characterization of the polymer structures to which they lead (via high resolution mass spectrometry). The presence of MCRs leads to the formation of branched polymers and the partial formation of polymer networks. They also limit the measurement of kinetic parameters (such as the SPR propagation rate coefficient) with conventional methods. However, their occurrence can also be used as a synthetic handle, for example, the high-temperature preparation of macromonomers. © 2008 Wiley Periodicals, Inc.

  • probing the reaction kinetics of vinyl acetate Free Radical Polymerization via living Free Radical Polymerization madix
    Polymer, 2006
    Co-Authors: Alexander Theis, Martina H. Stenzel, Thomas P. Davis, Christopher Barnerkowollik
    Abstract:

    Abstract Living Free Radical Polymerization technology (macromolecular design via the interchange of xanthates (MADIX)) was applied to give accesses to chain length and conversion dependent termination rate coefficients of vinyl acetate (VAc) at 80 °C using the MADIX agent 2-ethoxythiocarbonylsulfanyl-propionic acid methyl ester (EPAME). The kinetic data were verified and probed by simulations using the PREDICI® modelling package. The reversible addition-fragmentation transfer (RAFT) chain length dependent termination (CLD-T) methodology can be applied using a monomer reaction order of unity, since VAc displays significantly lower monomer reaction orders than those observed in acrylate systems (ω(VAc, 80 °C)=1.17±0.05). The observed monomer reaction order for VAc is assigned to chain length dependent termination and a low presence of transfer reactions. The α value for the chain length regime of log(i)=1.25−3.25 (in the often employed expression k t ( i , i ) = k t 0 i − α ) reads 0.09±0.05 at low monomer to polymer conversion (10%) and increases significantly towards larger conversions (α=0.55±0.05 at 80%). Concomitantly with a lesser amount of midchain Radicals, the chain length dependence of kt is significantly less pronounced in the VAc system than in the corresponding acrylate systems under identical reaction conditions. The RAFT(MADIX)-CLD-T technique also allows for mapping of kt as a function of conversion at constant chain lengths. Similar to observations made earlier with methyl acrylate, the decrease of kt with conversion is more pronounced at increased chain lengths, with a strong decrease in kt exceeding two logarithmic units from 10 to 80% conversion at chain lengths exceeding 1800.

  • probing the reaction kinetics of vinyl acetate Free Radical Polymerization via living Free Radical Polymerization madix
    Institute for Future Environments; Science & Engineering Faculty, 2006
    Co-Authors: Alexander Theis, Martina H. Stenzel, Thomas P. Davis, Christopher Barnerkowollik
    Abstract:

    Living Free Radical Polymerization technology (macromolecular design via the interchange of xanthates (MADIX)) was applied to give accesses to chain length and conversion dependent termination rate coefficients of vinyl acetate (VAc) at 80 °C using the MADIX agent 2-ethoxythiocarbonylsulfanyl-propionic acid methyl ester (EPAME). The kinetic data were verified and probed by simulations using the PREDICI® modelling package. The reversible addition-fragmentation transfer (RAFT) chain length dependent termination (CLD-T) methodology can be applied using a monomer reaction order of unity, since VAc displays significantly lower monomer reaction orders than those observed in acrylate systems (ω(VAc, 80 °C)=1.17±0.05). The observed monomer reaction order for VAc is assigned to chain length dependent termination and a low presence of transfer reactions. The α value for the chain length regime of log(i)=1.25-3.25 (in the often employed expression kt(i,i)=kt0i-α) reads 0.09±0.05 at low monomer to polymer conversion (10%) and increases significantly towards larger conversions (α=0.55±0.05 at 80%). Concomitantly with a lesser amount of midchain Radicals, the chain length dependence of kt is significantly less pronounced in the VAc system than in the corresponding acrylate systems under identical reaction conditions. The RAFT(MADIX)-CLD-T technique also allows for mapping of kt as a function of conversion at constant chain lengths. Similar to observations made earlier with methyl acrylate, the decrease of kt with conversion is more pronounced at increased chain lengths, with a strong decrease in kt exceeding two logarithmic units from 10 to 80% conversion at chain lengths exceeding 1800. © 2005 Elsevier Ltd. All rights reserved.

  • critically evaluated termination rate coefficients for Free Radical Polymerization experimental methods
    Progress in Polymer Science, 2005
    Co-Authors: Christopher Barnerkowollik, Atsushi Goto, Takeshi Fukuda, Philipp Vana, Michael Buback, Mark Egorov, Oskar Friedrich Olaj, Gregory T. Russell, Bunichiro Yamada, Per B. Zetterlund
    Abstract:

    The knowledge of accurate rate coefficients for individual steps of Free-Radical Polymerization (FRP) is of scientific interest and of application-oriented importance. For a wide variety of homoPolymerizations and for many coPolymerizations, reliable propagation rate coefficients, kp, are accessible via the IUPAC-recommended method of PLP-SEC (pulsed laser Polymerization—size-exclusion chromatography). For termination rate coefficients, kt, the situation is less favorable. Even for very common monomers, no kt benchmark data sets are available. Moreover, instead of having one recommended technique for measuring kt, there are a plethora of such methods. Seventeen of the most prominent approaches for measuring kt are here reviewed, including innovative ones that have emerged over the last decade. The methods have been subdivided into two categories: (i) ‘Kinetic methods’, in which analysis of the time dependence of concentrations is essential, and (ii) ‘MWD methods’, in which the analysis of the molecular weight distribution plays the dominant role. The methods are evaluated with respect to their potential for providing routine access to measuring kt as a function of monomer conversion and of Free-Radical chain length. Moreover, it has been considered whether expensive instrumentation or highly demanding analysis is required for a particular method and whether a method is applicable to many types of monomers. A table summarizes all these evaluations in a readily accessible form. The use of kinetic methods appears to be generally preferable over MWD-based methods. The largest potential is currently seen for methods in which Polymerization is induced by a single laser pulse and where the subsequent time evolution of either monomer concentration or Free-Radical concentration is measured.

Graeme Moad - One of the best experts on this subject based on the ideXlab platform.

  • living Free Radical Polymerization with reversible addition fragmentation chain transfer the life of raft
    Polymer International, 2000
    Co-Authors: Graeme Moad, John Chiefari, Roshan T. A. Mayadunne, Yen K Chong, Julia Krstina, Almar Postma, Ezio Rizzardo, San H Thang
    Abstract:

    Free Radical Polymerization with reversible addition-fragmentation chain transfer (RAFT Polymerization) is discussed with a view to answering the following questions: (a) How living is RAFT Polymerization? (b) What controls the activity of thiocarbonylthio compounds in RAFT polymeriza- tion? (c) How do rates of Polymerization differ from those of conventional Radical Polymerization? (d) Can RAFT agents be used in emulsion Polymerization? Retardation, observed when high concentra- tions of certain RAFT agents are used and in the early stages of emulsion Polymerization, and how to overcome it by appropriate choice of reaction conditions, are considered in detail. Examples of the use of thiocarbonylthio RAFT agents in emulsion and miniemulsion Polymerization are provided. # 2000 Society of Chemical Industry

  • living Free Radical Polymerization by reversible addition fragmentation chain transfer the raft process
    Macromolecules, 1998
    Co-Authors: John Chiefari, Roshan T. A. Mayadunne, Catherine L. Moad, Yen K Chong, Frances Ercole, Julia Krstina, Justine Leigh Jeffery, Tam P T Le, Gordon Francis Meijs, Graeme Moad
    Abstract:

    mechanism involves Reversible Addition-Fragmentation chain Transfer, and we have designated the process the RAFT Polymerization. What distinguishes RAFT Polymerization from all other methods of controlled/living Free-Radical Polymerization is that it can be used with a wide range of monomers and reaction conditions and in each case it provides controlled molecular weight polymers with very narrow polydispersities (usually <1.2; sometimes <1.1). Living Polymerization processes offer many benefits. These include the ability to control molecular weight and polydispersity and to prepare block copolymers and other polymers of complex architecturesmaterials which are not readily synthesized using other methodologies. Therefore, one can understand the current drive to develop a truly effective process which would combine the virtues of living Polymerization with versatility and convenience of Free-Radical Polymerization.2-4 However, existing processes described under the banner “living Free-Radical Polymerization” suffer from a number of disadvantages. In particular, they may be applicable to only a limited range of monomers, require reagents that are expensive or difficult to remove, require special Polymerization conditions (e.g. high reaction temperatures), and/or show sensitivity to acid or protic monomers. These factors have provided the impetus to search for new and better methods. There are three principal mechanisms that have been put forward to achieve living Free-Radical Polymerization.2,5 The first is Polymerization with reversible termination by coupling. Currently, the best example in this class is alkoxyamine-initiated or nitroxidemediated Polymerization as first described by Rizzardo et al.6,7 and recently exploited by a number of groups in syntheses of narrow polydispersity polystyrene and related materials.4,8 The second mechanism is Radical Polymerization with reversible termination by ligand transfer to a metal complex (usually abbreviated as ATRP).9,10 This method has been successfully applied to the Polymerization of various acrylic and styrenic monomers. The third mechanism for achieving living character is Free-Radical Polymerization with reversible chain transfer (also termed degenerative chain transfer2). A simplified mechanism for this process is shown in

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