#2009 The Society of Polymer Science, Japan Polymerization of Methyl Methacrylate with Radical Initiator Immobilized on the Inside Wall of Mesoporous Silica By Kenji IKEDA, Mariko KIDA, and Kiyoshi ENDOÃ Polymerization of methyl methacrylate (MMA) with a radical initiator covalently immobilized 4,40-azobis(4-cyanopetanoyl) (ACP) group on the inside wall of MCM-41 (ACP-MCM-41) was investigated. The polymerization of MMA with ACP- MCM-41 proceeded to give high-molecular weight polymers. The relationship between the Mn of the resulting polymer and the polymer yield gave a straight line, and the line passed through the origin, which is in contrast to that such relationship was not observed in the polymerization of MMA with dimethyl 2,20-azobis(2-methyl propionate) (MAIB) even in the cavity of MCM-41. The results demonstrates that the termination reactions of the propagating radicals were extremely suppressed by movement restriction of the polymer chains covalently immobilized on the inside wall of MCM-41 in the polymerization of MMA. The presence of super long-lived propagating radical on the inside wall of MCM-41 was confirmed by ESR measurement and post-polymerization. KEY WORDS: Immobilized Radical Initiator / MCM-41 / Mesoporous Silica / Long Lived Propagating Radical / Termination Reaction / Poly(methyl methacrylate) / ESR Measurement / Mesoporous silica has been of great interest in the past on the inside wall of the mesoporous silicate cavity has been decade for a large internal surface area, a well-defined pore investigated to synthesize polymer-silica hybrid materials with size, and a capability of including relatively large molecules well-defined mesoporosity.21 The encapsulations of organic such as oligomers and polymers in the cavity. Properly polymers in the cavity of the mesoporous silica have been also functionalized and structurally ordered nanoporous materials studied.22,23 such as MCM-type silica with a tunable pore size and a narrow Although controlled polymerizations of vinyl monomers pore-size distribution have attracted much attention for their with a radical initiator bonded to the surface of silica particle potential applications as new catalysts for synthesizing desired have been achieved by many workers,24–33 a radical polymer- products.1–8 Among many type of mesoporous silicas synthe- ization confined reaction space like a mesoporous silica is also sized, MCM-41 belonged to a family of mesoporous silicates an useful approach for the controlled reactions. Because a disclosed by Mobil researchers in 1992.2 termination of growing radicals may be suppressed signifi- In the application of mesoporous silicas, they give a cantly by the restriction of the growing chain movement. Aida characteristic polymerization spaces for transition metals and his coworkers reported that a long lived polymer-radicals catalyzed olefin polymerizations. Polymerization of ethylene was confirmed by the polymerization of MMA in the MCM-41 with half-titanocene/MAO catalyst in the cavity of MCM-41 cavity in the presence of 2,20-azobisisobutyronitrile or benzoyl was reported to give a high performance polymer with a fiber peroxide.34 9 structure. Polymerization of propylene with Et(ind)2ZrCl2/ However, polymerizations of vinyl monomers with radical MAO catalyst in the cavity of mesoporous silica afforded a initiators covalently immobilized on the inside wall of MCM- high molecular weight polymer.10,11 Moreover, rac-ethylene- 41 were not found in the literatures to our knowledge. If such bis(1-indenyl)ZrCl2 in combination with covalently anchoring initiators are used for the polymerization of vinyl monomers, a MAO12 and borate13 on the inside wall of MCM-41 and long lived propagating radical will be expected to be produced polymerizations of olefins with organometallic compounds This leads to the molecular weight control of the polymer. To such as Cr,14 Mo,15 Rh,16 and Ni17,18 catalysts on the inside wall elucidate these points, we synthesized the radical initiator of MCM-41 were reported. including 4,40-azobis(4-cyanopetanoyl) group (ACP), which is Polymerizations in the cavity of mesoporous silicas are covalently immobilized on the inside wall of MCM-41(ACP- influenced by the wall of host materials. Moller and his MCM-41), and polymerization of MMA was investigated. coworkers reported that a large amount of poly(MMA) was adsorbed in the pore volume of host material in the cavity of EXPERIMENTAL mesoporous silica in the radical polymerization of MMA.19 The weight of encapsulated poly(MMA) in the cavity of MCM- Materials 41 increased with decreasing pore diameter.20 Radical poly- MMA and dimethyl 2,20-azobis(2-methyl propionate) merization induced with the initiator immobilized covalently (MAIB) (Wako Pure Chemical) were purified by distillation Department of Applied Chemistry and Bioengineering, Graduate School of Engineering, Osaka City University, Sugimoto, Sumiyoshi-ku, Osaka 558- 8585, Japan ÃTo whom correspondence should be addressed (Tel: +81-6-6605-2697, Fax: +81-6-6605-2697, E-mail: [email protected]). 672 Polymer Journal, Vol. 41, No. 8, pp. 672–678, 2009 doi:10.1295/polymj.PJ2008332 Polymerization of MMA with Immobilized Radical on MCM-41 H3C CN CN O O H3C OH PCl / Benzene Cl N N 5 N N HO r.t., Stirring Cl H C CN O O 3 H3C CN 4,4'-Azobis(4-cyanopentanoyl chloride) (ACPC) 4,4'-Azobis(4-cyanopentanoic acid) MCM-41 OH O Si Dehydrated MCM-41 O O (CH CH ) N / CH Cl N 3 2 3 2 2 O Si O O CH3 CN O Si OH 2 ACP-MCM-41 Figure 1. Synthetic method for ACP-MCM-41. over calcium hydride before use. ACP-MCM-41 was synthe- Synthesis of ACP-MCM-41 sized as mentioned below, and used as a radical initiator. ACP-MCM-41 used as a radical initiator was synthesized Other reagents were used after purification by conventional by the method depicted in Figure 1. MCM-41 (2.0 g) was methods. dehydrated under vacuum at 200 C for 2 h in a Schlenk tube before use. After cooling it to room temperature, MCM-41 was Synthesis of MCM-41 suspended in CH2Cl2 (20 mL) under argon atmosphere. To a MCM-41 was synthesized by a similar method reported in solution, ACPC (160 mg, 0.5 mmol) in CH2Cl2 (20 mL) was the literature.35 Sodium silicate (24.7 g) was added to 133.3 added and stirred vigorously for 30 min in the presence of a mL of H2O and stirred for 15 min. A solution of 10.6 g of small amount of triethylamine. Then, the mixture was stirred hexadecyltrimethylammonium bromide in 66.7 mL of H2O overnight at room temperature, and the reaction products were was prepared by warming the mixture up to 45 C to dissolve separated by a filtration, washed with an excess of CH2Cl2 to the template. Then, the surfactant solution was added to the remove unreacted ACPC, and dried under vacuum at room silicate slurry and stirred for 30 min. The pH of the mixture temperature. The reaction product was identified to be MCM- 2 was adjusted to 11 during an addition of 2.8 g of H2SO4 41 by X-ray diffraction and IR spectroscopy. In the IR (95%) in 20 mL of H2O. The mixture was transferred into a spectrum of the reaction product after extraction with CH2Cl2, static autoclave and heated to 100 C. After 24 h, the reaction new bands emerge at 2989, 2250, 1730, 1400–1480, and À1 vessel was allowed to cool down to 65 C and the pH was 1380 cm assigned to be C-H, -CN, C=O, and –CH2- and 24 adjusted to 10 by an addition of 0.69 g of H2SO4 (95%) in –CH3 groups, respectively, but the peak based on C=O of À1 13.3 mL of H2O. The reaction vessel was again sealed and CO-Cl group (1810 cm ) disappeared completely. This dem- the mixture was heated at 100 C for 24 h. The white solid onstrates that the covalently immobilized radical initiator on products were recovered by a filtration, washed with arge the inside wall of MCM-41, i.e., ACP-MCM-41 was synthe- amounts of a distilled water, and dried at 25 C for 24 h under sized. The amount of the initiator attached to the inside wall of reduced pressure. The product was calcined at 540 C for 6 h MCM-41 was determined from TG/DTA measurement. From in air to remove the surfactant of the framework, and TG/DTA analysis, 0.44 mmol ACPC per 1 g MCM-41 was identified to be MCM-41 by the methods described in the attached covalently on the inside wall of the MCM-41. literature.35,36 Polymerization Procedure Synthesis of ACPC ACP-MCM-41 was placed into a y-shaped glass tube with a 4,40-Azobis(4-cyanopetanoyl chloride) (ACPC) was synthe- rubber septum, and the glass tube was connected to a vacuum sized by a similar method described in the literature.37 4,40- system. After the inside of the tube was kept high vacuum, Azobis(4-cyanopentanoic acid) (1.5 g, 5.4 mmol) in 15 mL of MMA was charged in the tube by a syringe. The tube was benzene was chilled in ice bath and treated with PCl5 (3.0 g, subjected to several freeze-to-thaw cycles, and allowed to stand 14.4 mmol). Stirring was continued for a few minutes at 0 C, overnight at 5 C. Then, MMA-including ACP-MCM-41 was followed by stirring at room temperature for about 2 h. The obtained by a removal of MMA which presents outside of the solution was then filtered and concentrated to a pasty solid.