ABSTRACT:An ESR study on the radical polymerization of triphenylmethyl methacrylate was carried out in 2-methyltetrahydrofuran over a temperature range from -90oC to 30°C. A wellresolved 9-line spectrum was observed at -90°C, a 5-line spectrum at -50°C, and a 13-line spectrum at ooc. When the observation temperature was raised from ooc to 30°C, a 5-line spectrum was recorded again. This change in the spectrum with temperature was explained on the basis of two stable conformations and the interchange between them through the free rotation of the radical end. The existence of these two conformations was supported by the calculation of conformation energy.KEY It was previously reported that the solvent effect on the kP could be explained in terms of the donoracceptor complex of the propagating radicals with aromatic solvents. 1 -5 The ESR study of the polymerization of methyl methacrylate (MMA) and its homologs in the solvents was undertaken in an effort to confirm the presence of the complex. A number of papers have been published on the ESR study of the poly(methyl methacrylate) radical, which gives 9 (5+4)-line, 13 (5+8)-line or 5-line spectrum according to the measurement conditions. However, the origin of these spectra is still not completely elucidated.We previously reported an ESR study on the propagating radical of methyl methacrylate (MMA), deuterated-MMA (a-CD3 or f3-d2) and triphenylmethyl methacrylate (TPMA) in aromatic rigid glass, and discussed the origin of the 9 (5+4)-line spectrum of the poly(MMA) radical. 6 When the ESR spectrum of the poly(MMA) radical was measured in 2-methyltetrahydrofuran (MTHF) rigid glass, a well-resolved 13 (5+8)-line spectrum was observed at -150°C instead of 9 (5+4)-line spectrum, suggesting that the properties of the glassy matrix influenced the conformation of the propagating radica,L 6 In order to confirm this a more detailed ESR study on the poly(MMA) radical was attempted in MTHF. No ESR spectrum, however, was observed above -130°C in the polymerization system, since the poly(MMA) radical is too unstable to be observed in a fluid medium. Instead, we found that the ESR spectrum of the poly(triphenylmethyl methacrylate) [poly(TPMA)] radical with a bulky side group could be observed in MTHF even at room temperature. Temperature dependence of the ESR spectrum of the poly(TPMA) radical was investigated over a wide temperature region in MTHF; this spectrum was explained on the basis of two conformations and the interchange between them through free rotation of the radical end. The existence of the two conformations was supported by the calculation of conformation energy.

ESR studies on the radical polymerization of vinyl ethers were performed from −50°C to room temperature using di‐tert‐butylperoxide as a photoinitiator. Well resolved ESR spectra were assigned to propagating radicals of vinyl ethers. Their hyperfine splitting constants due to α‐proton were about 16 G, being smaller than those of ethyl acrylate and vinyl acetate. The smaller constants is ascribed to a deviation of the propagating radicals from sp2 hybrid structure. The reason why high polymers are not obtained from vinyl ethers by radical polymerization is discussed on the basis of information from the ESR studies.

ESR spectroscopy has been known to be potentially the best method for a clear understanding of the nature of propagating radicals in the radical polymerization of vinyl compounds and for the determination of the rate constants, because it is possible in principle to measure directly the structure, the electronic state, and the concentration of the radical. Unfortunately, the concentration of the propagating radical was too low to be detected by commercial ESR spectrometers. Thus, ESR studies on the propagating radical were performed under special conditions such as the frozen state 1 -3 and the crystalline state, 4 · 5 or by the use of such a special technique as the flow technique. 6 -Jo Since these conditions for the measurement are far from the usual conditions of radical polymerization, these cannot be used for the quantitative estimation of the kinetic rate constants for the radical polymerization.Recently, we succeeded in the detection of ESR spectra of the propagating radicals of methacrylates in solution polymerization at room temperature by using a new cavity designed specifically for photoreaction.U· 12 Although Bresler et a/. 13 • 14 detected the propagating radicals in a bulk polymerization system of vinyl compounds by using a special ESR spectrometer, they have not yet reported the ESR spectra of the propagating radicals in solution polymerization. We have observed more highly resoluted ESR spectra than Bresler's of the propagating radical of vinyl acetate (VAc) in solution polymerization by using the new cavity. We will report here the ESR observation of the propagating radicals of V Ac in bulk and solution at room temperature, and the results of estimation of propagation rate constants (kp) in the polymerization.The purification of V Ac, solvents, and benzoyl peroxide (BPO) was described previously.15 A solution of BPO (0.010 g) in 1 ml of a mixture ofVAc and benzene (volume 1: 1) was charged under nitrogen into a flat quartz cell (3.5 em x 3.0 em, 1.0 mm thickness). In bulk polymerization the initiator (0.003 g) was dissolved in 1 ml of V Ac. The polymerization was carried out at 32.5°C in a cavity of the ESR spectrometer by being irradiated through the slotted opening of the cavity with a 500 w high pressure mercury lamp (USHIO 500).The extent of polymerization was determined by the precipitation method in which the formed polymer was precipitated with hexane, 541

Considerable data on the propagation and termination rate constants (kp and k,) in the radical polymerization of vinyl compounds have been reported so far. 1 These data were obtained primarily by the rotating sector method, non-stationary measurements (pre-effect or after-effect), the viscosity method, and the spatial interference method. The estimation of ·kp and k, from these indirect measurements is inevitably accompanied by large error. This explains the considerable scattering of data obtained by various authors. 1 Moreover, the data was obtained on the assumption that the rate of polymerization is described by the normal kinetic ESR spectroscopy is known to be possibly the best method for determining rate constants, since in principle stationary free radical concentration can be measured directly. Unfortunately, the stationary concentration of propagating radicals was too low to be detected by a commercial ESR spectrometer. Thus, ESR studies on propagating radicals were performed under special conditions such as the frozen state 2 • 3 and the crystalline state, 4 • 5 or by use of a special technique such as the flow technique. 6 -8 Since these conditions for measurement are quite different from those ordinarily used in radical polymerization, they are not applicable to the quantitative estimation of kinetic rate constants in radical polymerization.In 1972, Bresler et a/. 9 • 10 detected propagating radicals in the bulk polymerization system of vinyl compounds using a sensitive ESR spectrometer equipped with "a balance resonator," and estimated rate constants directly.We have observed more highly resolved ESR spectra than Bresler's for the propagating radicals of methacrylates in solution using an ESR spectrometer equipped with a specially designed TM 110 cavity. In the following, we report the ESR observation of the propagating radicals of methyl methacrylate (MMA) and its homologs in benzene at room temperature.The purification of MMA, 2 triphenylmethyl methacrylate (TPMA) 12 and benzene 11 has already been described. Benzyl methacrylate (BzMA), and isobutyl methacrylate (iBMAl were purified by the same method as MMA. 2 Benzoyl peroxide was repeatedly recrystallized from ethanol and dried in vacuo.

An electron spin resonance (ESR) spectrum of the propagating radical in the photopolymerization of isopropenyl acetate (IpAc) in bulk at 32.5 °C was observed. The propagation rate constant ftp for the bulk polymerization of IpAc at 32.5 °C was determined to be 280 M*1 s"1 by ESR, comparable to that of methyl methacrylate (MMA). The cross propagation rate constants for the copolymerization of IpAc with MMA and with vinyl acetate (VAc) were estimated from ftp's for each monomer and the monomer reactivity ratios. The rate constants suggest that reactivities of IpAc and its propagating radical are large enough to give high-molecular-weight polymers in the homopolymerization. The chain-transfer constants to monomer (Cm's)for IpAc at three temperatures were determined by a Mayo plot to be 2 orders larger than that of VAc. The temperature dependence of Cm shows that the activation energy for the chain-transfer reaction is smaller than that for the propagation reaction, which was supported by ESR observation of allylic radical in the photoinitiated polymerization at -160 °C. The difficulty in preparing high-molecular-weight polymers from IpAc was concluded to be due to considerable chain-transfer reaction to monomer as compared with those of monomers which provide high-molecular-weight polymers. In addition, the temperature dependence of the ESR spectrum shows that the propagating radical has two stable conformations which rapidly interchange at room temperature.