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Living Radical Polymerization of Methyl Methacrylate with a Rhodium(III) Complex--Organic Halide System in Dimethyl Sulfoxide

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Living Radical Polymerization of Methyl Methacrylate with a Rhodium(III) Complex--Organic Halide System in Dimethyl Sulfoxide Polymer Journal, Vol. 38, No. 6, pp. 516–522 (2006) Living Radical Polymerization of Methyl Methacrylate with a Rhodium(III) Complex–Organic Halide System in Dimethyl Sulfoxide y Noriyuki KAMEDA College of Science and Technology, Nihon University, Narashinodai, Funabashi 274-8501, Japan (Received November 24, 2005; Accepted January 19, 2006; Published May 17, 2006) ABSTRACT: The polymerization of methyl methacrylate (MMA) with the rhodium(III) complex dihydrido(1,3- diphenyltriazenido)bis(triphenylphosphine)rhodium(III) [RhH2(Ph2N3)(PPh3)2] as a catalyst and an organic halide (CCl4, BrCCl3, or CBr4) as an initiator in dimethyl sulfoxide (DMSO) was studied. For the CCl4 initiator system, a kinetic study of MMA polymerization indicated that polymerization follows first-order kinetics with respect to the monomer and that the number-average molecular weight (Mn) of the polymers produced increases in direct proportion to the monomer conversion. Monomer-addition experiments showed that after addition of further MMA, the Mn of the polymers continues to increase in direct proportion to the monomer conversion. These results confirmed that the poly- merization of MMA in the CCl4-initiated system proceeds in a living radical manner. In contrast, the systems involving the bromo compounds BrCCl3 or CBr4 did not show such a living radical nature. For all these initiator systems, the polymers produced had broad molecular-weight distributions. The catalytic activities are discussed in relation to the reaction product between RhH2(Ph2N3)(PPh3)2 and DMSO. [doi:10.1295/polymj.PJ2005176] KEY WORDS Living Polymerization / Free Radical Polymerization / Methyl Methacrylate / Rh(III) Complex / Halomethane / Dimethyl Sulfoxide / Molecular Weight / Free-radical polymerization is one of the most In a previous paper,26 the trivalent rhodium com- widely used techniques for producing polymers. How- plex dihydrido(1,3-diphenyltriazenido)bis(triphenyl- ever, in conventional radical-polymerization process- phosphine)rhodium(III) [RhH2(Ph2N3)(PPh3)2]in es, it is rarely possible to control the molecular weight conjunction with CCl4 was shown to be effective as or the molecular-weight distribution of the product. an initiator system for the polymerization of MMA. Recently, the chemistry of such less-easily controlled Under the conditions used previously, the polymeriza- reactions has been changed by progress in transition tion of MMA to a very high monomer conversion was metal-mediated living or controlled radical polymer- not attempted and only incomplete kinetic data were ization systems that permit the control of molecular obtained. There have been no previous reports on liv- weights and their distribution. Such catalyst systems ing or controlled radical polymerizations involving were first reported independently by Sawamoto and RhH2(Ph2N3)(PPh3)2. 1 2 his co-workers and Wang and Matyjaszewsky in This paper reports the RhH2(Ph2N3)(PPh3)2 in con- 1995. Sawamoto and his co-workers reported that junction with CCl4 induces the living radical polymer- methyl methacrylate (MMA) is polymerized homo- ization of MMA. This initiating system gives poly- geneously in toluene in the presence of a carbon mers with a controlled molecular weight, but with tetrachloride–ruthenium complex system, (CCl4)/ broad molecular-weight distributions (Mw=Mn > 2). RuCl2(PPh3)3, and a Lewis acid activator (methyl- aluminum bis(2,6-di-tert-butylphenoxide).1 Wang and EXPERIMENTAL Matyjaszewsky reported the bulk polymerization of styrene in the presence of a system consisting of 1- Materials phenylethyl chloride/CuCl and 2,20-bipyridyl.2 Since Methyl methacrylate (MMA) from Wako Chemi- then, systems based on other low-valent transition- cals was washed with a saturated sodium hydrogen metal complexes of iron(II),3–16 nickel(II),17–21 rho- sulfite solution to remove inhibitor and then with dium(I),22–24 and palladium(II)25 have been found to 10% sodium hydroxide solution. The MMA was dried be effective in similar living or controlled radical over anhydrous sodium sulfate, distilled twice over polymerizations. The transition-metal complex plays calcium hydride under a reduced pressure of nitrogen, an indispensable role as a halogen carrier, through a and stored at 253 K. Bromotrichloromethane (CBrCl3) series of consecutive reversible oxidation and reduc- (99+%) from Wako Chemicals and carbon tetra- tion reactions involving single-electron transfers. bromide (CBr4) (99%) from Aldrich were used as re- yTo whom correspondence should be addressed (E-mail: [email protected]). 516 Living Radical Polymerization of MMA by a Rh(III) Complex ceived without purification. Carbon tetrachloride (CCl4) from Wako Chemicals was dried over calcium RESULTS AND DISCUSSION chloride and double distilled over phosphorus pentox- ide (P2O5). Dimethyl sulfoxide (DMSO) from Wako Polymerization of MMA with the RhH2(Ph2N3)- Chemicals was distilled twice over calcium hydride (PPh3)2–Organic Halide System under a reduced pressure of nitrogen. Triphenylphos- The polymerization of MMA was first attempted in phine (PPh3) from Wako Chemicals was purified by DMSO solution at 323 K with RhH2(Ph2N3)(PPh3)2 in dissolving it in benzene and pouring the solution into the absence of an organic halide: in this case the con- ethanol to precipitate the PPh3. 1,3-Diphenyltriazene version reached only 5% in 24 h. In the absence of (PhNHN=NPh) from Aldrich was recrystallized from organic halide, RhH2(Ph2N3)(PPh3)2 was found to petroleum ether. Dihydrido(1,3-diphenyltriazenido)- be almost inactive for the polymerization of MMA. bis(triphenylphosphine)rhodium(III) [RhH2(Ph2N3)- Under similar conditions, the polymerization of MMA 27 (PPh3)2] was prepared according to the literature. was carried out with RhH2(Ph2N3)(PPh3)2 as the cat- alyst and one of three organic halides (CBr4, CBrCl3, Polymerization or CCl4) as the initiator in a homogeneous mixture. All reactions were performed under nitrogen in a Figure 1 shows plots of the monomer conversion with glass tube equipped with a side arm, the inlet of which time. All three organic halide initiating system in- was fitted with a serum cap and capped by a two-way duced a smooth polymerization without an induction stopcock. phase. The CCl4 initiating system caused rapid poly- The general procedure was as follows. The required merization and the monomer conversion reached amounts of the RhH2(Ph2N3)(PPh3)2 (2.0 mmol) and 94% after 24 h. The CBrCl3 initiating system pro- 3 DMSO (8.9 cm ) were placed in the glass tube. The duced rapid polymerization similar to that of the CCl4 tube was then deaerated three times by freeze– initiating system in the early stages, but the polymer- pump–thaw cycles to remove oxygen, and nitrogen ization slowed down in the later stages and the mono- was admitted to a pressure of 1 atm. A mixture of mer conversion reached 89% in 24 h. Similarly, poly- MMA (1.0 mol) and the initiator (1.0–4.0 mmol) was merization with the CBr4 initiating system preceded then introduced through the serum cap by means of relatively quickly in the early stages, but leveled off a syringe. The total volume of the reaction mixture at a monomer conversion of around 50%. Figure 2 3 was thus 10.0 cm . The reaction mixture was sealed shows the Mn and Mw=Mn values of the PMMA sam- in the glass tube under nitrogen and the tube was then ples obtained with RhH2(Ph2N3)(PPh3)2 in the pres- heated to the desired temperature in a water bath. Af- ence of the three organic halide initiating systems. ter the required time, the tube was opened and the With the CCl4 initiating system, the Mn increased in contents were dried under a vacuum at 313 K. direct proportion to the monomer conversion. In the CBrCl3 initiating system, the Mn increased Characterization with increasing monomer conversion in the early The monomer conversion was determined gravi- stages, but approached a constant value at higher metrically. The polymer was dissolved in THF and monomer conversions. Thus, the polymerization is then the solution was vigorously stirred with activated ill controlled under these conditions. Al2O3 to remove the catalyst. The number- and In the CBr4 initiating system, the Mn is independent weight-average molecular weight (Mn and Mw, re- of the monomer conversion: this result is in agreement spectively) and the molecular-weight distribution with a conventional free-radical polymerization mech- (Mw=Mn) of the polymer were determined by size- anism. When the halides of bromo-derivatives RBr exclusion chromatography (SEC) in THF at 303 K (CBr4 or CBrCl3) used as the initiator, Rh(I) complex on three styrene–divinylbenzene copolymer particle catalyzes the activation of one of the carbon-bromine gel columns (Waters Styragel HR 0.5, HR 4E, and bonds in RBr to form the one-electron oxidized spe- HR 5E) that were connected to a Waters ALC/GPC cies, a Rh(II)Br complex and free radicals (R:CBr3 201A liquid chromatograph and a Waters R401 re- or CCl3). In general, bromides are much more reac- fractive-index detector. PMMA standards were used tive than the corresponding chlorides. This Rh(II)Br for calibration. complex may be reacts with RBr in preference to MMA and converts into irreversible Rh(III)Br com- Infrared Spectra Measurement plex. Therefore, living radical polymerization is only Infrared (IR) spectra were recorded on a JASCO possible with CCl4 as the initiator. FT/IR-8000 FT/IR spectrophotometer by the diffuse reflectance method. Powdered KBr was used as the Effects of the [CCl4]/[RhH2(Ph2N3)(PPh3)2] Ratio dispersant. To clarify the role of CCl4 in the polymerization of Polym. J., Vol. 38, No. 6, 2006 517 N. KAMEDA Figure 1. Time-conversion curves for the polymerization of MMA with RhH2(Ph2N3)(PPh3)2–organic halide system in DMSO at 3 3 3 323 K; [MMA]: 1.0 mol/dm ; [RhH2(Ph2N3)(PPh3)2]: 2.0 mmol/dm ; [Organic halide]: 2.0 mmol/dm ; Organic halide: CCl4 ( ), CBrCl3 ( ), CBr4 ( ).
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