A Xanthate-Derived Photoinitiator that Recognizes and Controls the Free Radical Polymerization Pathways of Methyl Methacrylate and Styrene

W 7 1. 6606 J. Am. Chem. Soc.. Vol. 121, N0. 28, 1999 the case of MMA, MAX acts as a photoinitiator, leading to linear polymers with controlled molecular weights and poly- dispersities. On the other hand, MAX plays the dual role of a photoinitiator and a co~monomer during the photopolymerization of styrene, leading to branched and cross-linked polymers. The observed twist in the photopolymerization of styrene from that of MMA is explained on the basis of a weak donor-acceptor interaction between styrene and MAX. This could be the first report of a smart photoinitiator that can sense the electron availability around the rnonomer’s double bond, thereby facili- tating the formation of different macromolecular architectures. We believe that the concept of ‘smart’ photoinitiators will add a new dimension to the field of free radical mediated controlled synthesis of novel macromolecular materials such as hyper- branched and dendrigraft polymers. Experimental Section Infrared (IR) and electronic spectra were recorded on a Perkin—Elmer model 880 and Shimadzu 2100 A spectrophotometers, respectively. Electronic spectra were recorded on a Shimadzu 2100 A spectropho- tometer. Mass spectra were recorded on a Hew1ett~Packard mass spectrometer model 5791, attached to 5890 series 11 gas chromatography setup, attached with an OV 101 (25 m long and 0.2 mm i.d.) or with MP-FFAP (25 m long and 0.2 mm id) capillary column, and a FID detector. Nuclear magnetic resonance (NMR) spectra were recorded on a Joel EX 90 or a Brucker DPX 300 spectrometer using CDCI3 as the solvent and tetramethylsilane as the internal standard. Size—exclusion chromatography (SEC) was performed on a Shimadzu LC—6A system equipped with three serially connected Shimpac columns and a refractive index detector. Calibration was performed with polystyrene standards. THF was used as the mobile phase at a flow rate of 1 mL min” at 28 °C. Methyl methacrylate (MMA), methyl acrylate (MA), and styrene (St) were purified by distillation under reduced pressure after washing with 5% aqueous N aOH solution. All solvents were dried and distilled before use. Preparation of S-Methacryloyl 0-Ethyl Xanthate (MAX). To a stirred suspension of potassium 0—ethyl xanthate (16.0 g, 0.1 mol) in dichloromethane (100 IILL), maintained at 0 °C, was gradually added a solution of methacryloyl chloride (10.4 g, 0.1 mol) in dichloromethane (100 mL). The reaction mixture was stirred for an additional period of l h and allowed to warm gradually to room temperature. The reaction mixture was washed several times with water, and the organic layer was dried over anhydrous sodium sulfate. Removal of the solvent under reduced pressure gave 17.0 g (90%) of MAX as a yellow liquid. IR Ajayaghosh and Francis 1/ma, (neat) 1720, 1640, 1240, 1050 cm”; UV firm, (CHC13), 280 (E, 10400), 395 (90) nm; 1H NMR (CDCI3. 90 MHz) 5 5.9-6.5 (2 H, in, CH2), 4.7 (2 H, q, OCHz), 2.0 (3 H, s, CH3), 1.45 (3 H, t, CH3); ‘3C NMR (CDCI3, 22.4 MHZ) 6 204, 186, 144, 126, 71, 18, 14. Mass spectrum m/z 191 [M1 + 1], 147, 172, 155, 103, 87, 73, 69. Photopolymerization of MMA Using MAX. A stock solution of MMA in dry benzene (5 M) containing MAX (5 X 10’3 M) was prepared, and 10 mL each of this solution was transferred into different Pyrex glass tubes (1.4 cm diameter and 15 cm long). They were stoppered with rubber septa and purged with argon for 15 min followed by irradiation in a Rayonet photochemical reactor (RPR) containing eight 350-nrn fluorescent lamps for known periods of time. After irradiation, the contents were poured into excess methanol, and the precipitated polymers were collected by filtration and washed with methanol. Purification of the polymers was achieved by reprecipitation from their chloroform solutions by methanol. After several reprecipi— rations, the polymers were filtered and dried in a vacuum oven (50 °C) for 24 h. IR Vmax (neat) 1721, 1452, 1249, 1148, 1043 and 993 cm“; 1H NMR (CDCI3, 300 MHZ) 6 6.2 (m, CH2=C), 4.65 (s (broad), OCH2), 3.6 (s, OCH3), 1.8-0.8 (m, aliphatic). Photoinduced Block Copolymerization of Methyl Acrylate (MA) with Macroinitiator 7. The macroinitiator 7 (50 mg) with a molecular weight of 1.6 X 10“ g/mol was dissolved in MA (4 M in benzene) (10 mL) and placed in a quartz reaction vial, which was closed with a rubber septum. The polymerization mixture was purged with dry argon for 15 min and irradiated in an RPR with eight 300—nm fluorescent lamps. After 90 min, the reaction mixture was diluted with chloroform, and the contents were precipitated with methanol. The precipitated polymer was purified by redissolving in THF and reprecipitating from hexane. Yield, 25%. Mn = 2.1 X 105, MW/Mn = 1.2. Photopolymerization of Styrene with MAX. A solution of styrene (4.5 M) and MAX (4.5 X 10‘1 M) in dry benzene (10 rnL) was taken in a Pyrex glass tube and purged with argon for 15 min. The solution was irradiated for 7 h as described in the case of the polymerization of MMA. Yield 29%. IR vmax (neat) 1700, 1600, 1492, 1451, 1242, 1040, 757, 709, and 541 crn"1; ‘H NMR (CDCI3, 300 MHZ) 6 7.1-6.3 (m, aromatic), 4.5 (m, OCH2), 2.1-1.2 (m, aliphatic); M,, = 2.5 X 104, MW/Mn = 1.6. Acknowledgment. We thank the Council of Scientific and Industrial Research, Government of India and the Regional Research Laboratory, Trivandrum for financial support. This paper is dedicated to Professor M. V. George on the occasion of his 70th birthday. JA983239C