Effects of Illuminance and Heat Rays on Photo-Controlled/Living Radical Polymerization Mediated by 4-Methoxy-2,2,6,6-Tetramethylpiperidine-1-Oxyl
The effects of illuminance and heat rays released from the light source on the photo-controlled/living radical polymerization of methyl methacrylate were investigated with the aim of strict control of molecular weight. The bulk polymerization was performed at room temperature using 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl as the mediator and (2RS,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 polymerization by the direct irradiation from the light source yielded polymers containing an uncontrolled high-molecular-weight polymer and having the molecular weight distribution over 3. On the other hand, the polymerization by the indirect irradiation with reflective light using a mirror produced polymers with controlled molecular weights with comparatively narrow molecular weight distribution at ca. 1.4. Too high an illuminance caused an increase in the molecular weight distribution. During the polymerization, the monomer conversion increased as the illuminance increased. It was found that the elimination of heat rays from the illuminating light was indispensable for the molecular weight control by the photo-controlled/living radical polymerization.
Light is a desirable stimulus to manipulate the properties and functions of materials and living organs without damage by heat, such as thermal expansion and deactivation. Photo-controlled systems include the environmental advantage in utilizing solar energy, the significance of local applications, and the use of photo-specific reactions. A significant variety of photo-controlled systems has been created using reversible and also irreversible photoreactions. Examples include the photo-controlled mechanical motion of crystals through the azobenzene photoisomerization , the photo-controllable changes in surface morphology of salt crystals by enantio-specific and enantioselective photocyclization of a benzophenone derivative , the photo-induced wetting properties on an ultrathin ZnO-coated surface , the photo-responsive loading and release of drugs on nanoparticle  and nanofiber surfaces , self-assembly induced by photo irreversible reactions of photolysis [6–8], photo-rearrangement , and photo onium salt formation  for block copolymers, size change of core-shell nanogel particles through the photodimerization and photocleavage of coumarin , magnetization of CdS-modified nanoparticles by photo-induced electron transfer from CdS to Prussian blue , DNA cleavage by the combination of the photoactive Zn(II) cooperation and the azobenzene photoisomerization , the inhibition of telomerase activity by photo-cross-linking , and the photoswitch to induce paralysis in a living organism using the photocyclization of bis(pyridinium)-dithienylethene .
The photo-controlled/living radical polymerization is also a photo-controllable system that can regulate the molecular weight of a polymer. Photo-living radical polymerization systems have been discovered using various mediators; dithiocarbamate derivatives [16–18], -tetraethylthiuram disulfide , dibenzyl trithiocarbonate , 4-thiobenzoyl sulfanylmethyl benzoate , bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide, bis(4-methoxybenzoyl)diethylgermanium , and manganese complex . In recent years, the photo-controlled/living radical polymerization mediated by 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) has been established for methacrylate monomers that could not be applied to the thermal TEMPO-mediated polymerization [24–36] due to the disproportionation termination at high temperature. This TEMPO-mediated photopolymerization was accelerated by the photo-acid generators of the diaryliodonium salts [24, 25] and triarylsulfonium salts  and produced polymers with the comparatively narrow molecular weight distributions of ca. 1.4 even at a high conversion. However, there is no report concerning the effects of illuminance and heat released from the light source on the photo-controlled polymerization. It was found that the illuminance and heat from the light source affected the polymerization rate and molecular weight control. This paper describes the influences of illuminance and heat from the light source of a high-pressure mercury lamp on the TEMPO-mediated photo-controlled/living radical polymerization of methyl methacrylate (MMA).
The photopolymerization was carried out using an Ushio optical modulex BA-H502, an illuminator OPM2-502H with a high-illumination lens UI-OP2SL, and a 500 W super high-pressure UV lamp (USH-500SC2, Ushio Co. Ltd.). The illuminance was measured using a Topcon IM-5 illuminance meter. Gel permeation chromatography (GPC) was performed using a Tosoh GPC-8020 instrument equipped with a DP-8020 dual pump, a CO-8020 column oven, and a RI-8020 refractometer. Three polystyrene gel columns, Tosoh TSKGEL , , and were used with tetrahydrofuran as the eluent at 40°C.
4-Methoxy-TEMPO (MTEMPO) was prepared as reported previously . (2RS,2′RS)-Azobis(4-methoxy-2,4-dimethylvaleronitrile) (r-AMDV) was obtained by separation from a mixture of the racemic and meso forms of 2,2′-azobis(4-methoxy-2,4-dimethylvaleronitrile) . Commercial grade MMA was washed with 5 wt.% sodium hydroxide solution and water and distilled over calcium hydride. (4-tert-Butylphenyl)-diphenylsulfonium triflate (tBuS) was purchased from Sigma-Aldrich and used as received. A heat ray absorbent filter, HA30 and a neutral density filter, ND-50 were purchased from Hoya Candeo Optronics Corporation.
2.3. Polymerization by Indirect Irradiation
MMA (936.0 mg, 9.35 mmol), r-AMDV (14.0 mg, 0.0454 mmol), MTEMPO (9.0 mg, 0.0483 mmol), and tBuS (11.0 mg, 0.0235 mmol) were placed in an ampoule. After degassing the contents, the ampoule was sealed under vacuum. The bulk polymerization was carried out at room temperature for 7 h with irradiation at 5.0 × 105 lux by reflective light using a mirror with a 500 W high-pressure mercury lamp. The product was dissolved in dichloromethane (10 mL). The solution was concentrated by an evaporator to remove the dichloromethane and unreacted monomer and was freeze-dried with benzene (20 mL) at 40°C to obtain the product as white powder (545.8 mg). The monomer conversion was estimated gravimetrically. The product was dissolved in dichloromethane (3 mL) and poured into hexane (500 mL). The precipitate was collected by filtration and dried in vacuo for several hours to be subjected to GPC analysis.
3. Results and Discussion
The photo-controlled/living radical polymerization of MMA was performed at room temperature using the r-AMDV initiator and the MTEMPO mediator in the presence of the tBuS accelerator. The bulk polymerization was carried out at different illuminances by direct irradiation in a water bath to avoid a rise in the temperature of the reaction system by the direct irradiation (Figure 1). The results are shown in Table 1. The polymerization rapidly proceeded by the direct irradiation and provided very broad molecular weight distributions of Mw/Mn > 3. The GPC analysis revealed that the resulting polymers contained uncontrolled high-molecular-weight polymers. As can be seen in Figure 2, the proportion of the high-molecular-weight polymer was reduced as a result of decreasing the illuminance. In addition, no polymerization occurred by the direct irradiation through a heat ray absorbent filter, HA30, that can exclude the rays with the wavelengths over ca. 900 nm and a neutral density filter, ND-50, that reduces the illuminance to 50%.
In order to avoid the influence of heat rays on the polymerization, the polymerization was performed with the indirect irradiation by a reflective light using a mirror. This indirect irradiation can exclude heat rays of wavelengths around 1,100 nm included in the light from the mercury lamp because the heat rays are not reflected by a mirror. A schematic of the polymerization with the indirect irradiation is shown in Figure 3. The results of the polymerization are shown in Table 2.
The indirect irradiation polymerization produced polymers with controlled molecular weights without any uncontrolled high-molecular-weight polymers. The experimental molecular weights, Mn (obs), were in good agreement with the theoretical molecular weights, Mn (theor). The GPC profile of the resulting polymer obtained at a 5.0 × 105 lux-illuminance is shown in Figure 4. Figure 5 shows the plots of the monomer conversion versus the illuminance. An increase in the illuminance accelerated the polymerization. However, too high an illuminance increased molecular weight distribution and produced a deviation in the molecular weight from the theoretical one. The polymerization at too high an illuminance may be influenced by the heat released from the light source because the distance from the light source to the reaction vessel was quite short (12.5 cm). The elimination of heat rays using HA30 decelerated the polymerization. This deceleration can be accounted for by the fact that this heat ray absorbent filter eliminates not only rays over 900 nm, but also rays below 270 nm. When it is taken into consideration that tBuS has a UV absorption at nm, tBuS was not excited by the irradiation through HA30, resulting in the fact that tBuS should not have served as the accelerator. The irradiation through HA30 also caused an increase in the molecular weight distribution probably due to the deceleration of the initiation. This implies that the excited tBuS also accelerates the decomposition of the initiator. The proposed mechanism throughout the polymerization is shown in Figure 6. It can be deduced that the elimination of only heat rays that cause the uncontrolled polymerization is significant for controlling the molecular weight of a polymer.
This is the first study clarifying the influence of heat rays on the photo-controlled/living radical polymerization mediated by MTEMPO. The heat rays caused uncontrolled polymerization during the MTEMPO-mediated photopolymerization. A decrease in the illuminance reduced the proportion of a polymer with an uncontrolled molecular weight. The exclusion of heat rays is indispensable for the molecular weight control. However, the use of a heat ray absorbent filter is ineffective for controlling the molecular weight because the filter also eliminated the rays below 270 nm when the accelerator with the UV absorption below this wavelength was used for the polymerization. The indirect irradiation by reflective light using a mirror effectively controlled the molecular weight by the TEMPO-mediated photopolymerization.
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