Effect of the Carboxylate Salt on the CON Bond Homolysis of SG1‐Based Alkoxyamines

Abstract

Alkoxyamines and persistent nitroxides are important regulators of the nitroxide mediated radical polymerization (NMP). Since the polymerization time decreases with increasing values of the homolysis rate constant (kd) for the C--ON bond between the polymer chain and the nitroxide moiety, the factors influencing kd are of considerable interest. Environmentally friendly polymerization methods, such as NMP in emulsion medium, have now been developed. The success of the polymerization depends not only on the strength of the C--ON bond but also on the solubility of the initiator/controller alkoxyamines, which control the nucleation. Various salts of SG1-based alkoxyamines (S+-OOCCMeH-SG1) labelled 1 a-f, where S+=Li+ (a), Na+ (b), K+ (c), nBu3HN+ (d), NH4+ (e), and nBu4N+ (f) are prepared, all of which are soluble in most organic solvents (e.g. pentane, dichlomethane, benzene, ethanol, etc.) as well as in water. Their kd values, which are measured in tert-butylbenzene, reveal unexpected behaviors depending on the diastereoisomers. For the RR/SS diatereoisomers (i.e. slowly homolyzing isomers), kd was not found to be sensitive to any of the salts, whereas for the RS/SR diastereoisomers (i.e. quickly homolyzing isomers), kd is observed to decrease to a greater or lesser extent depending on the type of salt. The conformations of the diastereoisomers of several salts are determined by means of DFT calculations, and the orbital interactions are studied by natural bond orbital (NBO) analysis.

Chemically Triggered C–ON Bond Homolysis of Alkoxyamines. Quaternization of the Alkyl Fragment

“…Unshielding (Δδ = 9À12 ppm, Table 1SI) was observed for 2b, 2dÀf, pointing to a positive charge at C4, hence, supporting an increase in χ at C5, leading to the observed increase in k d . As already reported, 16,19 no correlation was found between the geometric parameters of the reactive center (l C5;O7 , l N8;O7 , d C5;N8 , and R N8O7C5 in Table 2SI) and the k d of the alkoxyamine. In fact, the mesomeric forms of c• (Scheme 3) show the possibility of a nitroxide as a mesomeric form (IVc•), which would involve extra stabilization of the released radical, which in turn would overmatch the polar effect (decrease in χ) and would lead to the observed increase in k d .…”

“…This difference in reactivity between diastereoisomers has already been reported 12À15 and ascribed to either a hyperconjugation effect 16 or a remote steric effect. Indeed, in t-BuPh as solvent, intimate ion pairs are expected, 19 and hence, a part of the positive charge is used to neutralize the negative charge maintained in close vicinity, weakening the polar effect, 19 whereas in a more polar solvent (t-BuPh/CH 2 Cl 2 ) the charge dissociation is more effective, involving a stronger effect of the positive charge on the increase in the electronegativity χ at C5 (vide infra) and consequently an increase in k d . It prompted us to investigate the solvent effect on 2a, 2e, and 2f.…”

Diasteromeric Effect on the Homolysis of the C–ON Bond in Alkoxyamines: A DFT Investigation of 1,3-Diphenylbutyl-TEMPO

“…In recent articles [19,20], we investigated the hyperconjugation effect as well as the steric strain [21] using Density Functional Theory (DFT) calculations at the B3LYP/6-31G(d) level of theory. All calculations were performed using the Gaussian 03 molecular orbital package [22].…”

Novel Source of Trifluoromethyl Radical As Efficient Initiator for the Polymerization of Vinylidene Fluoride

“…These radicals are too stable to initiate any polymerization and have long half‐life but enable to trap any growing macroradical to favor a fast equilibrium between the living and the dormant chain. Non‐exhaustive possible radicals are nitroxides,10–20 cyanooxyl,21, 22 benzyl,23 those arising from tetraphenyl ethane,24 fullerenes radicals (in the polymerization of methyl methacrylate for example25). Furthermore, the structure of persistent radicals formed by UV irradiation of poly(dialkylsilylene)s was assessed by ESR and ENDOR spectroscopy.…”