Stable Free Radicals Produced in Acrylate and Methacrylate Free Radical Polymerization:  Comparative EPR Studies of Structure and the Effects of Cross-Linking

Abstract

Electron paramagnetic resonance (EPR) examinations of the stable free radicals from the polymerizations of a series of monofunctional, difunctional, and trifunctional acrylates and methacrylates have been made. The continuous-wave (CW) EPR of polymers from selectively deuterated monofunctional monomers and the pulsed EPR of the nondeuterated polymers were also examined. The aim of the experiments was to attempt to clarify several points of disagreement concerning the identity and assignment of polymer free radicals. The EPR results on the polymethacrylates are in general agreement with past results and interpretations, in which isotropic hyperfine interactions are assigned to the methyl and methylene protons to the central carbon of the propagating polymer radical. The rate of propagation of the observed radicals has been arrested by the high viscosity achieved in the polymer. The CW EPR spectra of the polyacrylates are well-simulated with two different models. In one model, the spectra are assigned to the propagating free radical with isotropic -methylene proton interactions and an anisotropic R-proton coupling. In the other model, it is assumed that the secondary propagating radical abstracts a hydrogen from the polymer chain to form a tertiary radical flanked by two isotropically coupled methylene groups. The latter model gives a slightly better fit to the data. One of the unresolved problems with the EPR spectra of acrylate and methacrylate polymers has been the absence or selective broadening of certain expected hyperfine lines. One hypothesis is a dynamic polymer motion that exchanges methylene proton positions, resulting in an alternating homogeneous line width [Sakai, Y.; Iwasaki, M. J. Polym. Sci. A-1 1969, 7, 1719. The spin-spin relaxation times, T2, measured with pulsed EPR fail to support this hypothesis. We show that a static orientation distribution of the methylene protons with respect to the axis of the orbital of the odd electron [Best, M. E.;Kasai, P. H. Macromolecules 1989, 22, 2622 successfully leads to an alternating heterogeneous line width.

Influence of Mid‐Chain Radicals on Acrylate Free Radical Polymerization: Effect of Ester Alkyl Group

“…[5] The hyperfine coupling constant evaluated from the line spacing (27 G) in Figure 2 is in good agreement with the literature value of 27.5 G. [29] This suggests that MCRs formed by intermolecular hydrogen abstraction also exhibit three-line EPR spectra. [28,30] The EPR spectrum observed at 27% conversion during the tBA polymerization at 60 8C is in agreement with the simulated spectrum as shown in Figure 3a and b: a a ¼ 18.1 G and a b ¼ 10.3 and 28.3 G for PRR (95%) and a b ¼ 28.0 G for MCR (5%). The small radical expelled from 1 was not detected.…”

“…[28][29][30] Gilbert et al reported that MCRs formed by intramolecular hydrogen abstraction (backbiting) of the tetramer or higher oligomer radicals of acrylic acid exhibit three-line EPR spectra. [29] Photolysis of TBP in the presence of poly(tBA) in benzene was carried out at 60 8C and a broad three-line spectrum assignable to 2 was observed ( Figure 2).…”

Kinetic model of block photopolymerization of glycidyl methacrylate to high conversion

“…The main general ones are (1) the S-like shape of the kinetic curves, indicating the presence of autoacceleration and autodecelation processes; (2) a great posteffect, that is, being dark after UV illumination, stopping the postpolymerization process observed from the autoacceleration stage; and (3) a high (to 10 Ϫ1 mol/m 3 ) concentration of radicals discovered by EPR spectroscopy "in situ" until the end of the polymerization process. [1][2][3][4][5][6][7][8] Two main conceptions have been formulated with the aim of explanation of such peculiarities for the polymerization process to high conversion. The first is based upon the diffusion-controlled character of elementary reactions assigned to the classic kinetic scheme.…”

Polymerization of monomers initiated by silyl centers in SiO deposits prepared by pulsed laser ablation

“…spectra. Whereas the EPR parameters of the radical for EGDM are available, 14 in case of styrene we calculated some EPR parameters of the initiator and propagator radicals for better identification and compared with the measured ones. As a model compound for the initiator radical in eq.…”