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 Noriyuki K AMEDAy 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(1,3diphenyltriazenido)bis(triphenylphosphine)rhodium(III) [RhH2 (Ph2 N3 )(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 polymerization 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 (Ph2 N3 )(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 widely used techniques for producing polymers. However, in conventional radical-polymerization processes, it is rarely possible to control the molecular weight or the molecular-weight distribution of the product. Recently, the chemistry of such less-easily controlled reactions has been changed by progress in transition metal-mediated living or controlled radical polymerization systems that permit the control of molecular weights and their distribution. Such catalyst systems were first reported independently by Sawamoto and his co-workers1 and Wang and Matyjaszewsky2 in 1995. Sawamoto and his co-workers reported that methyl methacrylate (MMA) is polymerized homogeneously in toluene in the presence of a carbon tetrachloride–ruthenium complex system, (CCl4 )/ RuCl2 (PPh3 )3 , and a Lewis acid activator (methylaluminum bis(2,6-di-tert-butylphenoxide).1 Wang and Matyjaszewsky reported the bulk polymerization of styrene in the presence of a system consisting of 1phenylethyl chloride/CuCl and 2,20 -bipyridyl.2 Since then, systems based on other low-valent transitionmetal complexes of iron(II),3–16 nickel(II),17–21 rhodium(I),22–24 and palladium(II)25 have been found to be effective in similar living or controlled radical polymerizations. The transition-metal complex plays an indispensable role as a halogen carrier, through a series of consecutive reversible oxidation and reduction reactions involving single-electron transfers. y In a previous paper,26 the trivalent rhodium complex dihydrido(1,3-diphenyltriazenido)bis(triphenylphosphine)rhodium(III) [RhH2 (Ph2 N3 )(PPh3 )2 ] in conjunction with CCl4 was shown to be effective as an initiator system for the polymerization of MMA. Under the conditions used previously, the polymerization of MMA to a very high monomer conversion was not attempted and only incomplete kinetic data were obtained. There have been no previous reports on living or controlled radical polymerizations involving RhH2 (Ph2 N3 )(PPh3 )2 . This paper reports the RhH2 (Ph2 N3 )(PPh3 )2 in conjunction with CCl4 induces the living radical polymerization of MMA. This initiating system gives polymers with a controlled molecular weight, but with broad molecular-weight distributions (Mw =Mn > 2). EXPERIMENTAL Materials Methyl methacrylate (MMA) from Wako Chemicals was washed with a saturated sodium hydrogen sulfite solution to remove inhibitor and then with 10% sodium hydroxide solution. The MMA was dried over anhydrous sodium sulfate, distilled twice over calcium hydride under a reduced pressure of nitrogen, and stored at 253 K. Bromotrichloromethane (CBrCl3 ) (99+%) from Wako Chemicals and carbon tetrabromide (CBr4 ) (99%) from Aldrich were used as re- To whom correspondence should be addressed (E-mail: ). 516 Living Radical Polymerization of MMA by a Rh(III) Complex ceived without purification. Carbon tetrachloride (CCl4 ) from Wako Chemicals was dried over calcium chloride and double distilled over phosphorus pentoxide (P2 O5 ). Dimethyl sulfoxide (DMSO) from Wako Chemicals was distilled twice over calcium hydride under a reduced pressure of nitrogen. Triphenylphosphine (PPh3 ) from Wako Chemicals was purified by dissolving it in benzene and pouring the solution into ethanol to precipitate the PPh3 . 1,3-Diphenyltriazene (PhNHN=NPh) from Aldrich was recrystallized from petroleum ether. Dihydrido(1,3-diphenyltriazenido)bis(triphenylphosphine)rhodium(III) [RhH2 (Ph2 N3 )(PPh3 )2 ] was prepared according to the literature.27 Polymerization All reactions were performed under nitrogen in a glass tube equipped with a side arm, the inlet of which was fitted with a serum cap and capped by a two-way stopcock. The general procedure was as follows. The required amounts of the RhH2 (Ph2 N3 )(PPh3 )2 (2.0 mmol) and DMSO (8.9 cm3 ) were placed in the glass tube. The tube was then deaerated three times by freeze– pump–thaw cycles to remove oxygen, and nitrogen was admitted to a pressure of 1 atm. A mixture of MMA (1.0 mol) and the initiator (1.0–4.0 mmol) was then introduced through the serum cap by means of a syringe. The total volume of the reaction mixture was thus 10.0 cm3 . The reaction mixture was sealed in the glass tube under nitrogen and the tube was then heated to the desired temperature in a water bath. After the required time, the tube was opened and the contents were dried under a vacuum at 313 K. Characterization The monomer conversion was determined gravimetrically. The polymer was dissolved in THF and then the solution was vigorously stirred with activated Al2 O3 to remove the catalyst. The number- and weight-average molecular weight (Mn and Mw , respectively) and the molecular-weight distribution (Mw =Mn ) of the polymer were determined by sizeexclusion chromatography (SEC) in THF at 303 K on three styrene–divinylbenzene copolymer particle gel columns (Waters Styragel HR 0.5, HR 4E, and HR 5E) that were connected to a Waters ALC/GPC 201A liquid chromatograph and a Waters R401 refractive-index detector. PMMA standards were used for calibration. Infrared Spectra Measurement Infrared (IR) spectra were recorded on a JASCO FT/IR-8000 FT/IR spectrophotometer by the diffuse reflectance method. P (...truncated)