Photo-living radical polymerization of methyl methacrylate using alkoxyamine as an initiator

The photoradical polymerization of vinyl acetate was performed using 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl (MTEMPO) as the mediator in the presence of bis(alkylphenyl)iodonium hexafluorophosphate (BAI). The MTEMPO/BAI system using 2,2’-azobis(isobutyronitrile) or 2,2’-azobis(4-methoxy-2,4-dimethylvaleronitrile) as the initiator did not succeed in controlling the molecular weight and produced polymers that showed a bimodal gel permeation chromatography with the broad molecular weight distribution. On the other hand, the polymerization using 1-(cyano-1-methylethoxy)-4-methoxy-2,2,6,6-tetramethylpiperidine and BAI proceeded by the living mechanism based on linear increases in the first order time–conversion and conversion–molecular weight plots. The molecular weight distribution also increased with the increasing conversion due to cloudiness of the solution as the polymerization proceeded. It was found that the polymerization had a photolatency because the propagation stopped by interruption of the irradiation and was restarted by further irradiation.     

Photo-living radical polymerization of methyl methacrylate by 2,2,6,6-tetramethylpiperidine-1-oxyl in the presence of a photo-acid generator

The novel photo-living radical polymerization of methyl methacrylate (MMA) was determined using 2,2’-azobis(4-methoxy-2,4-dimethylvaleronitrile) (AMDV) and 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl (MTEMPO) in the presence of bis(alkylphenyl)iodonium hexafluorophosphate (BAI). The polymerization provided a comparatively narrow molecular weight distribution in the range of 1.4–1.7. The resulting PMMA contained no BAI fragments in its structure and had the 1-cyano-1,3-dimethyl-3-methoxybutyl radical and MTEMPO at the 1:1 molar ratio. The experimental molecular weight was in close agreement with the theoretical one when the initiator efficiency was taken into consideration. The plots of ln([MMA]₀/[MMA]) vs. time and the molecular weight of PMMA vs. the conversion and vs. the reciprocal of the initial concentration of AMDV showed linear correlations, indicating that the polymerization proceeded in accordance with a living mechanism. It was found that the polymerization had a photo-switching ability, because the polymerization was interrupted by turning off the irradiation, and then restarted by the irradiation again.     

Photo-living radical polymerization of methyl methacrylate by a nitroxide mediator

The novel photo-living radical polymerization was determined using 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl (MTEMPO) and bis(alkylphenyl)iodonium hexafluorophosphate (BAI) as the photo-acid generator. The polymerization of methyl methacrylate was performed using azobisisobutylonitrile as an initiator in the presence of MTEMPO and BAI at room temperature by irradiation with a high-pressure mercury lamp to produce poly(methyl methacrylate) with a comparatively narrow molecular weight distribution (M _w/M _n?=?1.3–1.7). The polymerization proceeded by a living mechanism based on the fact that the first-order time-conversion plots linearly increased. A linear increase in the plots of the molecular weight versus the conversion also supported the living nature of the polymerization. It was found that MTEMPO had an interaction with the propagation chain end to control the molecular weight, while BAI weakened the interaction of MTEMPO with the propagation chain end to reduce the molecular weight distribution and polymerization time.     

Effect of azoinitiators on nitroxide-mediated photo-living radical polymerization of methyl methacrylate

In order to clarify the initiator factor dominating the molecular weight distribution of the resulting polymer, the nitroxide-mediated photo-living radical polymerization of methyl methacrylate was performed using eight different kinds of azoinitiators: i.e., 2,2′-azobisisobutyronitrile, 2,2′-azobis(2-methylbutyronitrile), 2,2′-azobis(2,4-dimethylvaleronitrile), 1,1′-azobis(cyclohexane-1-carbonitrile), racemic-(2RS,2′RS)-azobis(4-methoxy-2,4-dimethylvaleronitrile), meso-(2RS,2′SR)-azobis(4-methoxy-2,4-dimethylvaleronitrile), dimethyl 2,2′-azobis(2-methylpropionate), and 2,2′-azobis(N-butyl-2-methylpropionamide). The bulk polymerization was carried out at room temperature for 3 h using 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl (MTEMPO) as the mediator in the presence of bis(alkylphenyl)iodonium hexafluorophosphate as the photo-acid generator. All the initiators provided a molecular weight distribution below 1.7 for the MTEMPO/initiator ratio of 2, although at the ratio of unity, about half of the initiators produced the molecular weight distribution around 2.3–3.4. The UV analysis revealed that the initiators having a higher ε value tended to more strictly control the molecular weight and provide a higher initiator efficiency. The half-lives of the initiators had little effect on the molecular weight control and initiator efficiency.     

Photo-controlled/living radical polymerization of tert-butyl methacrylate in the presence of a photo-acid generator using a nitroxide mediator

The photo-controlled/living radical polymerization of tert-butyl methacrylate was performed using a (2RS,2′RS)-azobis(4-methoxy-2,4-dimethylvaleronitrile) initiator and a 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl (MTEMPO) mediator in the presence of a (4-tert-butylphenyl)diphenylsulfonium triflate photo-acid generator. The bulk polymerization was carried out at 25 °C by irradiation with a high-pressure mercury lamp. Whereas the polymerization in the absence of MTEMPO produced a broad molecular weight distribution, the MTEMPO-mediated polymerization provided a polymer with a comparatively narrow molecular weight distribution around 1.4 without elimination of the tert-butyl groups. The living nature of the polymerization was confirmed on the basis of the linear correlations for the first-order time–conversion plots and conversion–molecular weight plots in the range below 50% conversion. The block copolymerization with methyl methacrylate also supported the livingness of the polymerization based on no deactivation of the prepolymer.     

Nitroxide-mediated photo-controlled/living radical dispersion polymerization of methyl methacrylate

The nitroxide-mediated photo dispersion polymerization of methyl methacrylate (MMA) was performed by irradiation at room temperature using (2RS,2′RS)-azobis(4-methoxy-2,4-dimethylvaleronitrile) as the initiator, 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl (MTEMPO) as the mediator, (4-tert-butylphenyl)-diphenylsulfonium triflate as the photo-acid generator, and polyvinylpyrrolidone (PVP) as the surfactant in a mixed solvent of methanol/water = 3/1 (v/v). The MTEMPO-mediated photo dispersion polymerization produced spherical particles of PMMA, while the uncontrolled photo dispersion polymerization without MTEMPO provided nonspherical particles. The size distribution of the spherical particles decreased as the PVP concentration increased. The spherical particles showed a comparatively narrow molecular weight distribution of ca. 1.6. The livingness of the polymerization was confirmed on the basis of the linear correlations of the first-order time–conversion plots and conversion–molecular weight plots. The simultaneous control of the size distribution and molecular weight was possible as long as the light penetrates into the particles.     

Photo-controlled/living radical polymerization mediated by 2,2,6,6-tetramethylpiperidine-1-oxyl in inert atmospheres

The photo-controlled/living radical polymerization of methyl methacrylate using a nitroxide mediator was established in an inert atmosphere. The bulk polymerization was performed at room temperature using 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl as the mediator and (2RS,2′RS)-azobis(4-methoxy-2,4-dimethylvaleronitrile) as the initiator in the presence of (4-tert-butylphenyl)diphenylsulfonium triflate as the accelerator by irradiation with a high-pressure mercury lamp. The photopolymerization in a N₂ atmosphere produced a polymer with a comparatively narrow molecular weight distribution; however, the experimental molecular weight was slightly different from the theoretical molecular weight. The Ar atmospheric polymerization also provided a polymer with the molecular weight distribution similar to that of the polymer obtained by the N₂ atmospheric polymerization. These inert atmospheric polymerizations more rapidly proceeded to produce polymers with narrower molecular weight distributions than the vacuum polymerization. The livingness of the Ar atmospheric polymerization was confirmed on the basis of the first-order time–conversion plots and conversion–molecular weight plots.     

Nitroxide-mediated photo-living radical polymerization of methyl methacrylate using (4-tert-butylphenyl)diphenyl-sulfonium triflate as a photo-acid generator

The photo-living radical polymerization of methyl methacrylate (MMA) was performed at room temperature using (2RS,2′RS)-azobis(4-methoxy-2,4-dimethylvaleronitrile) (r-AMDV) as the initiator, 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl (MTEMPO) as the mediator, and (4-tert-butylphenyl)diphenylsulfonium triflate ( ^t BuS) as the photo-acid generator. The livingness of the polymerization was confirmed on the basis of linear increases in the ln([MMA]₀/[MMA]_t) vs. time and in the molecular weight vs. the conversion. The molecular weight distributions of the resulting polymers were around 1.45. The polymerization rate was dependent both on the ^t BuS/MTEMPO and MTEMPO/r-AMDV molar ratios. Furthermore, it was found that the polymerization had a photo-latency because the polymerization was retarded by the interruption of the irradiation; however, it was accelerated again by further irradiation without deactivation of the growing polymer chain ends.     

Nitroxide-mediated photo-living radical polymerization of methyl methacrylate in the presence of (η6-benzene)(η5-cyclopentadienyl)FeII hexafluorophosphate

The photoradical polymerization of methyl methacrylate (MMA) was performed at room temperature using (2RS,2’RS)-azobis(4-methoxy-2,4-dimethylvaleronitrile) as the initiator and 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl (MTEMPO) as the mediator in the presence of (η⁶-benzene)(η⁵-cyclopentadienyl)Fe^II hexafluorophosphate (BzCpFe^II). The bulk polymerization provided narrower molecular weight distributions (Mw/Mn = 1.4 − 1.5) than the solution polymerization in acetonitrile, although BzCpFe^II was insoluble in MMA. The polymerization rate was retarded by an increase in the amount of BzCpFe^II. BzCpFe^II, which had no ability to control the molecular weight by itself, could control it through the interaction with MTEMPO. The interaction of BzCpFe^II and MTEMPO was attributed to the electron transfer involving the MTEMPO–aminoxy anion redox system and the iron redox system. The polymerization was confirmed to occur in accordance with a living mechanism because linear correlations were obtained for both the plots of the first order time–conversion and the conversion–molecular weight.     

Synthesis of a well-defined polybromostyrene by living radical polymerization with a nitroxyl radical

Radical polymerization of p-bromostyrene was investigated with benzoyl peroxide (BPO) as an initiator in the presence of 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl (MTEMPO). The polymerization was performed in bulk for 3.5 h at 95°C and then continued for another 48 h at 125°C to afford the corresponding polybromostyrene with a narrow molecular weight distribution in high yield. ¹H NMR study revealed that the polymer obtained had BPO and MTEMPO moieties at its head and tail, respectively. It was confirmed that the polymerization proceeded in accordance with living mechanism, because the molecular weight linearly increased with an increase of the conversion, and it was directly proportional to the reciprocal of the initial concentration of BPO. Furthermore, the polystyrene obtained in the present study could quantitatively act as the initiator for the polymerization of p-bromostyrene in the living radical manner to afford the corresponding block copolymer, and vice versa. © 1996 John Wiley & Sons, Inc.     

Nitroxide-mediated photo-living radical polymerization of methyl methacrylate in solution

The photoradical polymerization of methyl methacrylate (MMA) was performed in an acetonitrile solution at room temperature using (2RS,2′RS)-azobis(4-methoxy-2,4-dimethylvaleronitrile) as the initiator, 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl as the mediator, and (4-tert-butylphenyl)diphenylsulfonium triflate as the photo-acid generator. This solution polymerization showed a non-steady-state during the very early stage followed by a steady-state. The polymerization produced oligomers with several thousand molecular weights at a very low conversion under the non-steady-state. It was confirmed that the polymerization proceeded in accordance with a living mechanism under the steady-state based on the linear correlations for both the first-order time-conversion plots and the conversion–molecular weight plots. The molecular weight distributions of the polymers obtained in the steady-state were approximately 1.8. The block copolymerization with isopropyl methacrylate ( ⁱ PMA) demonstrated that the growing polymer chain ends of the MMA prepolymer were stabilized even at a high conversion and efficiently initiated the ⁱ PMA polymerization.     

Photo-controlled/living radical polymerization of 2-(dimethylamino)ethyl methacrylate using 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl as a mediator

The photo-controlled/living radical polymerization of 2-(dimethylamino)ethyl methacrylate (DMAEMA) was attained using 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl as the mediator and (2RS,2′RS)-azobis(4-methoxy-2,4-dimethylvaleronitrile) (r-AMDV) as the initiator. The bulk polymerization of DMAEMA produced a polymer with a comparatively narrow molecular weight distribution below 1.6. The first-order time conversion plots showed a linear increase. The molecular weight of the resulting polymer also increased with an increase in the monomer conversion. The molecular weights of the resulting polymers were in good agreement with the theoretical molecular weights. A linear correlation was also obtained for the plots of the molecular weight vs. the reciprocal of the initial concentration of r-AMDV. The GPC analysis demonstrated the living nature of the polymerization based on the fact that the curves were shifted to the higher molecular weight side without deactivation as the conversion increased.     

Effects of initiators and photo-acid generators on nitroxide-mediated photo-living radical polymerization of methyl methacrylate

The effects of the structure of initiators and photo-acid generators on the nitroxide-mediated photo-living radical polymerization of methyl methacrylate were explored. The bulk polymerization was performed at room temperature using nine different initiators in the presence of (4-tert-butylphenyl)diphenylsulfonium triflate as the photo-acid generator. 2,2′-Azobisisobutyronitrile, 2,2′-azobis(2,4-dimethylvaleronitrile), dimethyl 2,2′-azobis(2-methylpropionate), and 2,2′-azobis(N-butyl-2-methylpropionamide) produced the polymers with a molecular weight distribution (MWD) around 1.6, while the racemic- and meso-(2RS,2′RS)-azobis(4-methoxy-2,4-dimethylvaleronitrile) provided a 1.4 MWD. 2,2′-Azobis(2-methylbutyronitrile), 1,1′-azobis(cyclohexane-1-carbonitrile), and 1-(cyano-1-methylethoxy)-4-methoxy-2,2,6,6-tetramethylpiperidine produced a broad MWD over 4.0. The structure of the photo-acid generator also had an influence on the molecular weight control. The photo-acid generator of sulfonium salts supporting the alkyl, methoxy, phenoxy, methylthio, and tert-butoxycarbonylmethoxy groups, coupled with halogens with the exception of the iodide had no effect on the MWD. On the other hand, the salts containing the iodide, phenylthio, and naphthyl groups produced polymers with broad MWDs and with uncontrolled high molecular weights.     

Living radical polymerization of methylstyrenes by a stable nitroxyl radical,and stability of the aminoxy chain end

Radical polymerization of 2-, 3-, and 4-methylstyrenes (MeSts) was investigated with benzoyl peroxide (BPO) as an initiator, in the presence of 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl (MTEMPO). The polymerization was performed in bulk for 3.5 h at 95°C, and then continued for a defined time at 125°C, to give the corresponding poly(MeSt)s with narrow polydispersity in high yield. It was found that the polymerization proceeded in accordance with a living mechanism, because the molecular weight of the resulting polymers was proportional to the conversion, and to the reciprocal of the initial concentration of MTEMPO. It was found that steric hindrance between the methyl group of 2-MeSt, and the tetramethyl ones of MTEMPO, significantly contributed to the rate of polymerization, and to the stability of the growing polymer chain end. The stability decreased in the order of 2- > 3- > 4-MeSt, by occurrence of decomposition, which was caused by disproportionation of the growing chain end. However, the steric hindrance had no effect on the tacticity of the resulting polymer. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36 , 269–276, 1998     

Synthesis of well-defined polychlorostyrenes by living radical polymerization with 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl

Radical polymerization of 2-, 3-, and 4-chlorostyrenes (ClSts) was investigated with benzoyl peroxide (BPO) as an initiator, in the presence of 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl (MTEMPO). The polymerization was performed in bulk for 3.5 h at 95°C and then continued for a defined time at 125°C to give the corresponding poly(ClSt)s with narrow polydispersity in high yield. It was found that the polymerization proceeded in accordance with a living mechanism in all cases, because the molecular weight of the resulting polymers was proportional to the conversion, and inversely proportional to the initial concentration of MTEMPO. Furthermore, the polymers obtained from 2- and 3-ClSts quantitatively act as initiators for the polymerization in the living radical manner, of styrene to give the corresponding block copolymers, except for poly(4-ClSt). The thermal stability of the living poly(ClSt)s was found to decrease in the order of 2- > 3- > 4-ClSt on the basis of the results of their postpolymerizations. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 2371–2378, 1997     

Synthesis of poly(methyl methacrylate)-<Emphasis Type="Italic">block</Emphasis>-poly(tetrahydrofuran) by photo-living radical polymerization using a 2,2,6,6-tetramethylpiperidine-1-oxyl macromediator

The synthesis of a poly(methyl methacrylate)-block-poly(tetrahydrofuran) (PMMA-b-PTHF) diblock copolymer was attained by the photo-living radical polymerization of methyl methacrylate using 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) supported on the chain end of poly(tetrahydrofuran) (PTHF) as the macromediator. The polymerization was performed at room temperature by 2,2′-azobis(4-methoxy-2,4-dimethylvaleronitrile) as an initiator in the presence of bis(alkylphenyl)iodonium hexafluorophosphate as a photo-acid generator to produce the diblock copolymer consisting of poly(methyl methacrylate) (PMMA) and PTHF blocks connected through the TEMPO. The polymerization was confirmed to proceed in accordance with a living mechanism based on linear correlations for three different plots of the first order time-conversion, the molecular weight of the copolymer versus the monomer conversion, and the molecular weight versus the reciprocal of the initial concentration of the initiator. The molecular weight distribution of the block copolymer was dependent on the molecular weight of the macromediator based on the miscibility of PMMA and PTHF.     

Nitroxide-mediated photo-controlled/living radical polymerization of ethyl acrylate

The nitroxide-mediated photo-controlled/living radical polymerization of ethyl acrylate was attained using (2RS,2′RS)-azobis(4-methoxy-2,4-dimethylvaleronitrile) as the initiator, 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl as the mediator, and (4-tert-butylphenyl)diphenylsulfonium triflate as the photo-acid generator. The photopolymerization was performed in acetonitrile at room temperature by irradiation with a high-pressure mercury lamp. The molecular weight distribution of the resulting polymer decreased as the monomer concentration decreased. It was confirmed that the polymerization was controlled on the basis of the linear correlations for the first-order time-conversion plots and the plots of the molecular weight vs. the reciprocal of the initial concentration of the initiator, although the conversion–molecular weight plots did not show a completely linear correlation. The block copolymerization with methyl methacrylate accompanied by no deactivation of the growing polymer chain end supported the livingness of the polymerization.     

Graft copolymerization of methyl methacrylate on polystyrene backbone through nitroxide-mediated photo-living radical polymerization

Polystyrene-graft-poly(methyl methacrylate) (PSt-graft-PMMA) was prepared by the nitroxide-mediated photo-living radical polymerization using poly(4-vinylbenzyl-4-oxy-2,2,6,6-tetramethylpiperidine-1-oxyl-ran-styrene) (P(VTEMPO-r-St)) as the macromediator. The bulk polymerization of methyl methacrylate was performed at room temperature by irradiation using a high-pressure mercury lamp with P(VTEMPO-r-St) as the mediator having the molar ratio of VTEMPO/St unit = 0.40/0.60 and the molecular weight of Mn = 21,700 and the (2RS,2′RS)-azobis(4-methoxy-2,4-dimethylvaleronitrile) as the initiator in the presence of the (4-tert-butylphenyl)diphenylsulfonium triflate as the photo-acid generator. The polymerization proceeded via a controlled polymerization mechanism because both the first-order time-conversion plots and the conversion-molecular weight plots showed linear increases. It was found that all the VTEMPO units supported the controlled PMMA chains by ¹H NMR analysis because the molar ratio of the VTEMPO at the terminal chain end to the 1-cyano-3-methoxy-1,3-dimethylbutyl group at the initiation chain end of the PMMA was unity.     

Stability of growing polymer chain ends for nitroxide-mediated photo-living radical polymerization

The stability of the growing polymer chain ends for the nitroxide-mediated photo-living radical polymerization of methyl methacrylate (MMA) was explored through block copolymerization with isopropyl methacrylate ( ⁱ PMA). The block copolymerization of ⁱ PMA was performed with the PMMA prepolymer prepared by the photopolymerization of MMA using the racemic-(2RS,2′RS)-azobis(4-methoxy-2,4-dimethylvaleronitrile) (r-AMDV) as the initiator, 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl (MTEMPO) as the mediator, and (4-tert-butylphenyl)-diphenylsulfonium triflate ( ^t BuS) as the photo-acid generator. When the polymerization of MMA was carried out for 6.5 h, the resulting block copolymer showed a bimodal GPC due to the deactivation of part of the growing chain ends of the prepolymer. On the other hand, when the MMA polymerization was shortened to 5 h, the unimodal block copolymer was obtained without deactivation of the prepolymer.     

Tertiary amine-functionalized polymers by atom transfer radical polymerization

The quantitative synthesis of tertiary amine-functionalized polymers by atom transfer radical polymerization is reported. Tertiary amine-functionalized polystyrene was prepared with the adduct of 1-(bromoethyl)benzene with 1-(4-dimethyl-aminophenyl)-1-phenylethylene as an initiator in the atom transfer radical polymerization of styrene in the presence of a copper (I) bromide/2,2′-bipyridyl catalyst system. The polymerization proceeded via a controlled free-radical polymerization process to afford quantitative yields of the corresponding tertiary amine-functionalized polystyrene with predictable number-average molecular weights (1600–4400), narrow molecular weight distributions (1.09–1.31), and an initiator efficiency of 0.95. The polymerization process was monitored by gas chromatographic analysis. The tertiary amine-functionalized polymers were characterized by thin-layer chromatography, size exclusion chromatography, potentiometry, and spectroscopy. All experimental evidence was consistent with quantitative functionalization via the 1,1-diphenylethylene derivative. Polymerization kinetic measurements showed that the polymerization reaction followed first-order-rate kinetics with respect to monomer consumption and that the number-average molecular weight increased linearly with monomer conversion. © 2001 John Wiley & Sons, Inc. J Polym Sci A Part A: Polym Chem 39: 2058–2067, 2001