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

Abstract

Summary: The benzene solution polymerizations of tert‐butyl acrylate (tBA) and 2‐ethylhexyl acrylate (EHA) have been investigated with respect to the effects of mid‐chain radicals (MCRs) on the rate of polymerization, the total radical concentration, the apparent rate coefficients for propagation, termination, and β‐fragmentation, the contents of unsaturated end groups and branching, and the molecular weight distribution. The EHA polymerization involves a considerably higher concentration of MCR and is more significantly affected by MCR than the tBA polymerization. The MCR content as estimated by electron paramagnetic resonance (EPR) spectroscopy during photosensitized EHA polymerization was as high as 70% of the total radical concentration at 25 °C. However, no 1H and 13C NMR resonances due to unsaturated ends and branching, respectively, were detected for the poly(EHA) obtained at 25 °C. These findings indicate that a high MCR content does not directly correspond to significant MCR effects on the polymer structure. The rate constants for mutual reaction ($k'_{\rm t}$) and β‐fragmentation (kf) of MCR were estimated for the two assumed cases where MCR was consumed solely by (i) mutual reaction, and (ii) β‐fragmentation, based on the after‐effect of the photosensitized EHA polymerization monitored by EPR spectroscopy at 25 °C; $k'_{\rm t}$ = 3.5 × 103 L · mol · s−1 and kf = 4.2 × 10−2 s−1. These rate coefficients were compared with those for reactions of structurally similar radicals.

The role of mid‐chain radicals in acrylate free radical polymerization: Branching and scission

“…46 Another interesting implication from the scission is that the unsaturated polymer chain constitutes a macromonomer that can react with any growing radical forming a MCR. 47 Hence, effectively, the MCR can equilibrate with the SPR species when scission becomes fast enough and the concentration of macromonomers is high enough. Such formation of an equilibrium may play an important part in high-temperature acrylate polymerization, and this effect can indeed be ''isolated'' from the other MCR follow-up reactions resulting in the formation of pure macromonomers.…”

“…The same observation was made for 2-ethylhexyl acrylate and tert-butyl acrylate. 47 They assigned the spectra to an overlap of the MCR and SPR spectra whereby they assumed a three-line spectrum for the MCR (coupling of the radical center with two hydrogen atoms with the coupling constant a Hb ¼ 27.5 G) and eight lines for the SPR where two lines coincide at the center field (with two pairs of b-hydrogen atoms coupling with a Hb1 ¼ 10.7 G and a Hb2 ¼ 29.5 G and one hydrogen atom in a-position with a Ha ¼ 17.7 G). For the MCR, it was assumed that only two out of the four available hydrogens in b-position would couple with the radical center.…”

“…However, it is important to note that MCR b-scission already takes place at significant levels at temperatures as low as 60 C (Scheme 7). 47 Macromolecular construction often proceeds, however, via more elaborate control protocols, mostly with the aim of generating complex macromolecular architectures such as star polymers. As one of the most versatile living free radical processes, RAFT polymerization 1,75 is readily employed with multifunctional cores to generate star polyacrylates.…”

EPR Analysis of n‐Butyl Acrylate Radical Polymerization

“…Electron paramagnetic resonance (EPR) spectroscopy is a unique tool for studying radical polymerizations, as the type and concentration of radicals may be determined. [11] EPR studies on acrylate polymerization have been carried out mostly focusing on the type of radical, [5,8,9,[12][13][14] [3,4] The backbiting rate coefficient, k bb , has been deduced from PLP-SEC experiments under extended variation (tuning) of laser pulse repetition rate. [15] The resulting k bb data are close to the k bb values from an earlier 13 C NMR study.…”

Reassessment of Free-Radical Polymerization Mechanism of Allyl Acetate Based on End-Group Determination of Resulting Oligomers by MALDI-TOF-MS Spectrometry

“…In this connection, it is notably recalled that the free-radical polymerizations of ethylene, [32][33][34][35][36][37] vinyl chloride, 38,39 vinyl acetate, [40][41][42][43] and acrylate esters [44][45][46][47][48][49] are governed by the polymerization mechanism involving mid-chain radical formed through backbiting hydrogen abstraction of growing polymer radical. …”