A photo‐oxidizable kinetic pathway of three‐component photoinitiator systems containing porphrin dye (Zn‐tpp), an electron donor and diphenyl iodonium salt

A series of kinetic experiments were conducted involving visible‐light activated free radical polymerizations with three‐component photoinitiators and 2‐hydroxyethyl methacrylate (HEMA). Three‐component photoinitiator systems generally include a light‐absorbing photosensitizer (PS), an electron donor and an electron acceptor. To compare kinetic efficiency, we used thermodynamic feasibility and measured kinetic data. For this study, 5,10,15,20‐tetraphenyl‐21H,23H‐porphyrin zinc (Zn‐tpp) and camphorquinone (CQ) were used as the PSs. The Rehm‐Weller equation was used to verify the thermodynamic feasibility for the photo‐induced electron transfer reaction. Using the thermodynamic feasibility, we suggest two different kinetic mechanisms, which are (i) photo‐reducible series mechanism of CQ and (ii) photo‐oxidizable series mechanism of Zn‐tpp. Kinetic data were measured by near‐IR spectroscopy and photo‐differential scanning calorimetry based on an equivalent concentration of excited state PS. We report that the photo‐oxidizable series mechanism using Zn‐tpp produced dramatically enhanced conversions and rates of polymerizations compared with those associated with the photo‐reducible series mechanism using CQ. It was concluded from the kinetic results that the photo‐oxidizable series mechanism efficiently retards back electron transfer and the recombination reaction step. In addition, the photo‐oxidizable series mechanism provides an efficient secondary reaction step that involves consumption of the dye‐based radical and regeneration of the original PS. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3131–3141, 2009     

Mechanism of cyclic dye regeneration during eosin‐sensitized photoinitiation in the presence of polymerization inhibitors

A visible light photoinitiator, eosin, in combination with a tertiary amine coinitiator is found to initiate polymerization despite the presence of at least 1000‐fold excess dissolved oxygen, which functions as an inhibitor of radical polymerizations. Additionally, 0.4 μM eosin is able to overcome 100‐fold excess (40 μM) 2,2,6,6‐Tetramethyl‐1‐piperidinyloxy (TEMPO) inhibitor, initiating polymerization after only a 2 min inhibition period. In contrast, 40 μM Irgacure‐2959, a standard cleavage‐type initiator, is unable to overcome even an equivalent amount of inhibitor (40 μM TEMPO). Through additional comparisons of these two initiation systems, a reaction mechanism is developed which is consistent with the kinetic data and provides an explanation for eosin's relative insensitivity to oxygen, TEMPO, and other inhibitors. A cyclic mechanism is proposed in which semireduced eosin radicals react by disproportionation with radical inhibitors and radical intermediates in the inhibition process to regenerate eosin and effectively consume inhibitor. In behavior similar to that of eosin, rose bengal, fluorescein, and riboflavin are also found to initiate polymerization despite the presence of excess TEMPO, indicating that cyclic regeneration likely enhances the photoinitiation kinetics of many dye photosensitizers. Selection of such dye initiation systems constitutes a valuable strategy for alleviating inhibitory effects in radical polymerizations. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6083–6094, 2009     

A mechanistic investigation of a three‐component radical photoinitiator system comprising methylene blue,N‐methyldiethanolamine,and diphenyliodonium chloride

Three‐component systems, which contain a light‐absorbing species (typically a dye), an electron donor (typically an amine), and a third component (usually an iodonium salt), have emerged as efficient, visible‐light‐sensitive photoinitiators. Although three‐component systems have been consistently found to be faster and more efficient than their two‐component counterparts, these systems are not well understood and a number of distinct mechanisms have been reported in the literature. In this contribution, photodifferential scanning calorimetry and in situ, time‐resolved, laser‐induced, steady‐state fluorescence spectroscopy were used to study the initiation mechanism of the three‐component system methylene blue, N‐methyldiethanolamine and diphenyliodonium chloride. Kinetic studies based upon photodifferential scanning calorimetry reveal a significant increase in polymerization rate with increasing concentration of either the amine or the iodonium salt. However, the laser‐induced fluorescence experiments show that while increasing the amine concentration dramatically increases the rate of dye fluorescence decay, increasing the DPI concentration actually slows consumption of the dye. We concluded that the primary photochemical reaction involves electron transfer from the amine to the dye. We suggest that the iodonium salt reacts with the resulting dye‐based radical (which is active only for termination) to regenerate the original dye and simultaneously produce a phenyl radical (active in initiation) derived from the diphenyliodonium salt. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2057–2066, 2000     

The role of diphenyl iodonium salt (DPI) in three‐component photoinitiator systems containing methylene blue (MB) and an electron donor

Three‐component initiators generally include a light‐absorbing photosensitizer, an electron donor that is often an amine, and the third component, which is usually an iodonium salt. To characterize the role of diphenyl iodonium chloride (DPI) in three‐component photoinitiator systems containing methylene blue (MB) as the photosensitizer, a systematic series of electron donors was used. The Rhem–Weller equation was used to verify the thermodynamic feasibility for photo‐induced electron transfer from the electron donors to the MB. Comparison of the photopolymerization rates of each two‐component initiator system (containing the photosensitizer and amine) to those of the corresponding three‐component system (with the addition of (DPI) allowed fundamental information regarding the role of the DPI to be obtained. It was concluded that the DPI enhances the photopolymerization kinetics in two ways: (1) it consumes an inactive MB neutral radical and produces an active phenyl radical, thereby regenerating the original methylene blue, and (2) it reduces the recombination reaction of the MB neutral radical and amine radical/cation. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5863–5871, 2004     

Photopolymerization reactions initiated by a visible light photoinitiating system: Cyanine dye/borate salt/1,3,5‐triazine

New three‐component photoinitiating systems consisting of a cyanine dye, borate salt, and a 1,3,5‐triazine derivative were investigated by measuring their photoinitiation activities and through fluorescence quenching experiments. Polymerization kinetic studies based on the microcalorimetric method revealed a significant increase in polymerization rate when the concentration of n‐butyltriphenylborate salt or the 1,3,5‐triazine derivative were increased. The photo‐induced electron transfer process between electron donor and electron acceptor was studied by means of fluorescence quenching and SrEt change of the fluorescence intensity. The experiments performed documented that an increase of the n‐butyltriphenylborate salt concentration dramatically increases the rate of dye fluorescence quenching, whereas the increasing of the 1,3,5‐triazine derivative concentration slows down the consumption of the dye. We conclude that the primary photochemical reaction involves an electron transfer from the n‐butyltriphenylborate anion to the excited singlet state of the dye, followed by the reaction of the 1,3,5‐triazine derivative with the resulting dye radical to regenerate the original dye. This reaction simultaneously produces a triazinyl radical anion derived from the 1,3,5‐triazine, which undergoes the carbon‐halogen bond cleavage yielding radicals active in initiation of a free radical polymerization chain. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3626–3636, 2007     

Heteroarm H‐shaped terpolymers through click reaction

Heteroarm H‐shaped terpolymers, (polystyrene)(poly(methyl methacrylate))‐ poly(tert‐butyl acrylate)‐(polystyrene)(poly(methyl methacrylate)), (PS)(PMMA)‐PtBA‐(PMMA)(PS), and, (PS)(PMMA)‐poly(ethylene glycol)(PEG)‐(PMMA)(PS), through click reaction strategy between PS‐PMMA copolymer (as side chains) with an alkyne functional group at the junction point and diazide end‐functionalized PtBA or PEG (as a main chain). PS‐PMMA with alkyne functional group was prepared by sequential living radical polymerizations such as the nitroxide mediated (NMP) and the metal mediated‐living radical polymerization (ATRP) routes. The obtained H‐shaped polymers were characterized by using ¹H‐NMR, GPC, DSC, and AFM measurements. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1055–1065, 2007     

Kinetic study of free‐radical polymerization photoinitiated by cyanine‐borate salts. II.

A series of cyanine butyltriphenylborate salts were prepared and tested as initiators of free‐radical polymerization photoinitiated via a photoinduced electron‐transfer process. For the majority of the tested series, the highest rate of photoinitiated free‐radical polymerization was observed when sec‐butyl radicals were formed. Essentially, there was no influence of the quantum yield of the free‐radical formation on the rate of the free‐radical polymerization initiated by the cyanine‐borate salts. The experimental data revealed that the relationship between the rate of polymerization and the free energy change for the electron transfer displayed typical Marcus region kinetic behavior. The photoreduction of the cyanine butyltriphenylborate salts produced colorless products. The efficiency of the bleached‐dye formation had no effect on the overall efficiency of photoinitiated polymerization. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2365–2374, 2000     

Light‐Regulated Polymerization under Near‐Infrared/Far‐Red Irradiation Catalyzed by Bacteriochlorophyll a

Photoregulated polymerizations are typically conducted using high‐energy (UV and blue) light, which may lead to undesired side reactions. Furthermore, as the penetration of visible light is rather limited, the range of applications with such wavelengths is likewise limited. We herein report the first living radical polymerization that can be activated and deactivated by irradiation with near‐infrared (NIR) and far‐red light. Bacteriochlorophyll a (Bachl a) was employed as a photoredox catalyst for photoinduced electron transfer/reversible addition–fragmentation chain transfer (PET‐RAFT) polymerization. Well‐defined polymers were thus synthesized within a few hours under NIR (λ=850 nm) and far‐red (λ=780 nm) irradiation with excellent control over the molecular weight (M_n/M_w<1.25). Taking advantage of the good penetration of NIR light, we showed that the polymerization also proceeded smoothly when a translucent barrier was placed between light source and reaction vessel.     

Photopolymerization of vinyl ether networks using an iodonium initiator—The role of photosensitizers

Photo‐differential scanning calorimetry (photo‐DSC) and UV‐visible spectrometry were used to investigate the photocuring kinetics of visible light initiated cationic photopolymerization of triethyleneglycol divinyl ether with a diphenyl iodonium salt and three photosensitizers, 1‐chloro‐4‐propoxy‐9H‐thioxanthen‐9‐one (CPTXO), acridine orange (AO), and camphorquinone (CQ). Although all photosensitizers were effective in causing photopolymerization, CPTXO and AO photo‐reacted during the irradiation; whereas CQ was not significantly consumed during the timescale of the photo‐DSC experiment. This difference in photo‐reactivity has not been reported previously, and indicates that the reaction of the iodonium ion with CPTXO and AO results in the formation of the photosensitizer radical cation, whereas the CQ mechanism involves the reduction of the CQ excited state to a ketyl radical by a H‐donor (monomer) followed by the oxidation of the ketyl radical by the iodonium salt and thus regeneration of the CQ. For the CPTXO and AO systems, the photopolymerization rate was retarded by a radical inhibitor but the CQ system was unaffected, which confirms that different mechanisms are involved. The cure rate was found to be proportional to the concentrations of CPTXO and CQ but appeared to follow an approximately square root dependence on the AO concentration. Mechanisms to explain these differences were presented. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5474–5487, 2009     

Antimicrobial polymers containing melamine derivatives. II. Biocidal polymers derived from 2‐vinyl‐4,6‐diamino‐1,3,5‐triazine

Poly(2‐vinyl‐4,6‐diamino‐1,3,5‐triazine) (PVDAT) and a series of poly(styrene‐co‐2‐vinyl‐4,6‐diamino‐1,3,5‐triazine) (PS‐co‐VDAT) copolymers were synthesized via conventional free‐radical polymerizations. The polymer structures were confirmed by Fourier transform infrared, NMR, and elemental analysis. The molecular weights were determined by gel permeation chromatography studies, and the thermal properties were characterized by differential scanning calorimetry and thermogravimetric analysis. After treatment with chlorine bleach, PVDAT and PS‐co‐VDAT provided potent antimicrobial functions against multidrug‐resistant Gram‐negative and Gram‐positive bacteria. The antimicrobial functions were durable for longer than 3 months and rechargeable for more than 50 times. The structure–property relationship of the polymers was further discussed. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4089–4098, 2005     

A mechanistic investigation of the three‐component radical photoinitiator system Eosin Y spirit soluble,N‐methyldiethanolamine,and diphenyliodonium chloride

Photo‐DSC and in situ, time‐resolved, laser‐induced, steady‐state fluorescence spectroscopy were used to study the initiation mechanism of the three‐component system: Eosin Y spirit soluble (EYss), N‐methyldiethanolamine, and diphenyliodonium chloride. Kinetic studies based on photo‐DSC revealed that the fastest polymerization occurred when all three components were present (the next fastest was with the dye/amine pair, and the slowest was with the dye/iodonium pair). However, the laser‐induced fluorescence experiments showed that the pairwise reaction between the eosin and iodonium bleaches the dye much more rapidly than does the reaction between the eosin and amine. We concluded that although a direct eosin/amine reaction can produce active radicals in the three‐component system, this reaction is largely overshadowed by the eosin/iodonium reaction, which does not produce active radicals as effectively. We proposed that the amine reduces the oxidized dye radical formed in the eosin/iodonium reaction back to its original state as well as the simultaneous production of an active initiating amine‐based radical. Because of the difference in the pairwise reaction rates for eosin/amine and eosin/iodonium, it is likely that this regeneration reaction was the primary source of active radicals in the three‐component eosin/amine/iodonium system. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 715–723, 2001     

Outer‐sphere electron transfer metal‐catalyzed polymerization of styrene using a macrobicyclic ligand

The CuBr‐catalyzed polymerizations of styrene in the presence of a macrobicyclic mixed donor (N and S) encapsulating ligand, NH₂capten, were carried out in toluene at 60 and 100 °C. The macrobicyclic nature of the ligand ensures that a transition metal ion is effectively encapsulated (caged) within the three‐dimensional cavity, resulting in activation of radicals through an outer‐sphere electron transfer mechanism. The kinetic data showed that the polymerizations were uncontrolled with little “living” behavior. The external orders of reaction in [CuBr], [NH₂capten], and [CuBr₂] were 0.5, 0.5, and close to zero, respectively, in agreement with the postulated mechanism of little or no deactivation of polymeric radicals and a significant amount of bimolecular termination. Although “living” behavior was not found using the cage ligand, it was decided that it would provide an ideal method for radical coupling experiments to make high‐molecular weight multiblock copolymers from a difunctional PSTY (Br‐PSTY‐Br, PDI = 1.11). The coupling reaction of Br‐PSTY‐Br using CuBr/NH₂capten and excess Cu(0) in toluene at 100 °C gave no loss of the starting Br‐PSTY‐Br. Changing the solvent to the aprotic DMSO resulted in a significant increase in the rate of consumption of starting Br‐PSTY‐Br, with over 87% being used in under 10 min at 60 °C. In addition, higher molecular weight species were also formed, suggesting that OSET gives little or no side reactions on this time scale. It was initially thought that to get such high rates of reaction that the SET‐LRP disproportionation mechanism (2Cu(I) → Cu(0) + Cu(II)) was at play. However, UV–Vis experiments of the CuBr/NH₂capten showed little or no disproportionation products. This important result suggests that DMSO catalyzes the OSET reaction through the stabilization of the radical‐anion intermediate, which then rapidly fragments to a polymeric radical. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 146–154, 2008     

Thermal‐initiating potentialities of poly(methyl methacrylate) peroxide: Metamorphosis of block‐into‐block copolymer and comparative studies on surface texture and morphology

This is the first report concerning the use of vinyl polyperoxide, namely, poly(methyl methacrylate) peroxide (PMMAP), as a thermal initiator for the synthesis of active polymer PMMAP‐PS‐PMMAP by free‐radical polymerization with styrene. The polymerizations have been carried out at different concentrations of macroinitiator PMMAP. The active polymers have been characterized by ¹H NMR, DSC, thermogravimetric analysis, and gel permeation chromatography. PMMAP‐PS‐PMMAP is further used as the thermal macroinitiator for the preparation of another block copolymer, PMMA‐b‐PS‐b‐PMMA, by reacting the active polymers with methyl methacrylate. The block copolymers have been synthesized by varying the concentrations of the active polymers. The mechanism of block copolymers has been discussed, which is also supported by thermochemical calculations. Studies on the surface texture and morphology of the block copolymer of polystyrene (PS) and PMMA material have been carried out using scanning electron microscopy. Furthermore, in this article, a blend of the same constituent materials (PS and PMMA) in proportions (v/v) similar to that contained in block copolymers has been formulated, and the morphology and surface textures of these materials were also investigated. A comparative microscopical evaluation between two processing methods was done for a better understanding of the processing route dependence of the microstructures. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 546–554, 2001     

The effect of oxygen on the three‐component radical photoinitiator system: Methylene blue,N‐methyldiethanolamine,and diphenyliodonium chloride

In this article, we extend our mechanistic study of the three‐component radical photoinitiator system, consisting of methylene blue (MB), N‐methyldiethanolamine, and diphenyliodonium chloride, by investigating the influence of oxygen on the rate of the consumption of MB dye. The mechanism involves electron transfer/proton transfer from the amine to the dye as the primary photochemical reaction. Oxygen quenches the triplet state of the dye, leading to retardation of the reaction. We used time‐resolved steady‐state fluorescence monitoring to observe the MB concentration in situ in both a constant oxygen environment and a sealed reactor as the dye is consumed via photoreaction. In the sealed reactor, we observed a retardation period (attributed to the presence of oxygen) followed by rapid exponential decay of the MB fluorescence after the oxygen was depleted. On the basis of the impact of the amine and iodonium concentrations on the fluorescence intensity and the duration of the retardation period, our proposed mechanism includes an oxygen‐scavenging pathway, in which the tertiary amine radicals formed in the primary photochemical process consume the oxygen via a cyclic reaction mechanism. The iodonium salt is an electron acceptor, acting to reoxidize the neutral dye radical back to its original state and allowing it to reenter the primary photochemical process. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3336–3346, 2000     

SET‐RAFT of MMA mediated by ascorbic acid‐activated copper oxide

In this work, cupric oxide (CuO) or cuprous oxide (Cu₂O) was used as the catalyst for the single electron transfer‐reversible addition‐fragmentation chain transfer (SET‐RAFT) polymerization of methyl methacrylate in the presence of ascorbic acid at 25 °C. 2‐Cyanoprop‐2‐yl‐1‐dithionaphthalate (CPDN) was used as the RAFT agent. The polymerization occurred smoothly after an induction period arising from the slow activation of CuO (or Cu₂O) and the “initialization” process in RAFT polymerization. The polymerizations conveyed features of “living”/controlled radical polymerizations: linear evolution of number‐average molecular weight with monomer conversion, narrow molecular weight distribution, and high retention of chain end fidelity. From the polymerization profile, it was deduced that the polymerization proceeded via a conjunct mechanism of single electron transfer‐living radical polymerization (SET‐LRP) and RAFT polymerization, wherein CPDN acting as the initiator for SET‐LRP and chain transfer agent for RAFT polymerization. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Universal xanthate‐mediated controlled free radical polymerizations of the “less activated” vinyl monomers

The living free radical polymerizations of three “less activated” monomers (LAMs), vinyl acetate, N‐vinylcarbazole, and N‐vinylpyrrolidone, were successfully achieved in the presence of a disulfide, isopropylxanthic disulfide (DIP), using 2,2′‐azoisobutyronitrile (AIBN) as the initiator. The living behaviors of polymerizations of LAMs are evidenced by first‐order kinetic plots and linear increase of molecular weights (M_ns) of the polymers with monomer conversions, while keeping the relatively low molecular weight distributions, respectively. The effects of reaction temperatures and molar ratios of components on the polymerization were also investigated in detail. The polymerization proceeded with macromolecular design via interchange of xanthate process, where xanthate formed in situ from reaction of AIBN and DIP. The architectures of the polymers obtained were characterized by GPC, ¹H NMR, UV–vis, and MALDI‐TOF‐MS spectra, respectively. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Acid‐treated g‐C3N4‐Cu2O composite catalyst with enhanced photocatalytic activity under visible‐light irradiation

Acid‐treated g‐C₃N₄‐Cu₂O was prepared by hydrothermal reduction followed by high temperature calcination and acid exfoliation. The structures and properties of as‐synthesized samples were characterized using a range of techniques, such as X‐ray photoelectron spectroscopy, scanning electron microscopy, Photoluminescence Spectroscopy and the Brunauer–Emmett–Teller (BET) theory. The photocatalytic activity was evaluated by measuring the photodegradation of methyl orange under visible‐light irradiation. Based on the results of TEM, XPS, EPR and other techniques, it was verified that a heterojunction was formed. The acid treatment process can increase the specific surface area to form abundant heterojunction interfaces as channels for photo‐generated carrier separation, thereby enhancing its light utilization and quantum efficiency. Results indicate that acid‐treated g‐C₃N₄‐Cu₂O possesses a large specific surface area, which provides plentiful activated sites for heterojunctions to form; in addition, it showed a high visible light effect and the minimum charge‐transfer resistance. Furthermore, the g‐C₃N₄‐Cu₂O material exhibits high levels of effectiveness and stability. Electron paramagnetic resonance and a series of radical trapping experiments demonstrate that the holes and ?O₂^? could be the main active species in methyl orange photodegradation. This work could provide new insights into the fabrication of composite materials as high‐performance photocatalysts, and facilitate their application in addressing environmental protection issues.     

Degradable multisegmented polymers synthesized by consecutive radical addition‐coupling reaction of α,ω‐macrobiradicals and nitroso compound

A consecutive radical addition‐coupling reaction induced by spin‐trapping agent is applied to produce degradable multisegmented polymer using α,ω‐dibromo polymer as a precursor. The macroradical generated by single electron transfer process catalyzed by Cu/PMDETA from α,ω‐dibromo polymer can be efficiently captured by 2‐methyl‐2‐nitrosopropane (MNP), which results in nitroxide radical. The in situ formed nitroxide radical immediately undergoes cross‐coupling reaction with polymeric radical, generating block polymer bridged with alkoxyamine moiety. The consecutive radical addition‐coupling reaction generates multisegmented polymer via step‐growth mechanism. Different multisegmented polymers have been prepared from α,ω‐dibromo‐PS, PtBA, and PtBA‐PS‐PtBA. The block number of multisegmented polymers can be tailored by varying the feed ratio of α,ω‐dibromo precursor to MNP. The multisegmented polymer can be degraded in the presence of hydrogen atom donor or air, and the molecular weight distribution transformed back into shape of its original precursor as it is conjugated by alkoxyamine moieties. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Emulsion and controlled miniemulsion polymerization of the renewable monomer γ‐methyl‐α‐methylene‐γ‐butyrolactone

The kinetics of free‐radical emulsion polymerization of γ‐methyl‐α‐methylene‐γ‐butyrolactone (MeMBL), a renewable monomer related to methyl methacrylate, are presented in detail for the first time, and stable polymer latices are prepared. The effects of different reaction parameters on free‐radical emulsion polymerization of MeMBL are presented. Homogeneous nucleation is asserted to be the dominant path for particle formation. Miniemulsion copolymerization of MeMBL and styrene is also reported. In this case, the homogeneous nucleation process appears limited when using an oil soluble initiator. Both the RAFT miniemulsion polymerizations and RAFT bulk polymerizations are well controlled and narrow polydispersity copolymers are produced. Rate retardation is observed in the RAFT miniemulsion polymerizations compared with the free‐radical polymerization and RAFT bulk polymerizations and the possible causes of the retardation are discussed. The reactivity ratios of MeMBL and styrene in RAFT bulk copolymerization are also determined. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5929–5944, 2008     

Mechanism and kinetics of the imidazolidinone nitroxide‐mediated free‐radical polymerization of styrene

Styrene radical polymerizations mediated by the imidazolidinone nitroxides 2,5‐bis(spirocyclohexyl)‐3‐methylimidazolidin‐4‐one‐1‐oxyl (NO88Me) and 2,5‐bis(spirocyclohexyl)‐3‐benzylimidazolidin‐4‐one‐1‐oxyl (NO88Bn) were investigated. Polymeric alkoxyamine (PS‐NO88Bn)‐initiated systems exhibited controlled/living characteristics at 100–120 °C but not at 80 °C. All systems exhibited rates of polymerization similar to those of thermal polymerization, with the exception of the PS‐NO88Bn system at 80 °C, which polymerized twice as quickly. The dissociation rate constants (k_d) for the PS‐NO88Me and PS‐NO88Bn coupling products were determined by electron spin resonance at 50–100 °C. The equilibrium constants were estimated to be 9.01 × 10^?11 and 6.47 × 10^?11 mol L^?1 at 120 °C for NO88Me and NO88Bn, respectively, resulting in the combination rate constants (k_c) 2.77 × 10⁶ (NO88Me) and 2.07 × 10⁶ L mol^?1 s^?1 (NO88Bn). The similar polymerization results and kinetic parameters for NO88Me and NO88Bn indicated the absence of any 3‐N‐transannular effect by the benzyl substituent relative to the methyl substituent. The values of k_d and k_c were 4–8 and 25–33 times lower, respectively, than the reported values for PS‐TEMPO at 120 °C, indicating that the 2,5‐spirodicyclohexyl rings have a more profound effect on the combination reaction rather than the dissociation reaction. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 327–334, 2003