Study of BDE by molecular modeling of a series of indolinic aminoxyl radicals

The control of radical polymerizations depends on the reversible dissociation of a growing chain capped by a nitroxide moiety. When either kd increases or kc decreases, the polymerization time decreases. K is a thermodynamic parameter and is related to the bond dissociation enthalpy, BDE, of the C–O bond (see Scheme 6.36).

Scheme. 6.36. Pathway of the homolytic dissociation of an alkoxyamine.

The reversible homolysis of an alkoxyamine is influenced by steric and/or electronic factors of the nitroxyl and polymer moieties. The steric congestion is reduced by the dissociation, so that kd and K increase with the steric hindrance around the C–O bond. If the nitroxide or the polymer radical is stabilized by delocalization of the unpaired electron, the BDE decreases.

It has been established that the BDE of a nitroxide capped polymer alkoxyamine is close to the BDE in a model alkoxyamine. Thus molecular modeling of the BDE of alkoxyamines can provide thermodynamic information to estimate the importance of different factors on the dissociation equilibrium. These estimations may be very helpful to design new nitroxides for control of free–radical polymerization of a given monomer.

Quantitative information about radical combination reactions may be provided by ab–

initio molecular orbital calculations ⁶⁵. The ab–initio calculations allow to obtain information on reaction energy barriers (see Fig. 6.36). The figure shows a schematic potential energy profile corresponding to the dissociation of an alkoxyamine, proceeding via a transition structure (TS) to produce a radical R^• and a nitroxide. The calculations allow to obtain complete geometries, bond and torsional angles, of all species. It is also possible to determine thermochemical quantities such as reaction barrier and exothermicity. Calculations may be carried out using either a restricted (RHF) or unrestricted (UHF) Hartree-Fock methods ⁶⁶.

Considering the size of the studied molecules, Tordo et al ⁶⁷ used semi–empirical methods (AMPAC software) for these calculations. It was already shown that PM3 parametrization offers the best description of alkoxyamines and nitroxides (compared to AM1 and SAM1 parametrizations). At first, the BDE calculations were performed at the RHF level in order to avoid spin contamination in the conjugated nitroxides. The UHF BDEs are known to be underestimated but this method offers realistic relative BDEs for this kind of compounds. The polyethylene chain was simulated with an n–hexyl chain and the preferred conformer of each alkoxyamine was found after a simulated annealing. Because of the large number of variables, some negligible internal coordinates (like C–H stretchings or the deformations of the phenyl rings) were frozen during the simulated annealing.

Previous calculated BDE of alkoxyamines with conjugated leaving radicals have shown a good correlation with the experimental temperatures reported in Fig. 6.37. The cleavage temperatures are determined by ESR.

Fig. 6.37. Correlation between the calculated BDE and the cleavage temperature of the C–O bond in alkoxyamines with conjugated leaving groups.

The BDE of the O–C bond in hexyl–DPAIO was calculated to be around 185 ± 10 kJ⋅mol^–1. The validity of the BDE calculations have been verified on a small series of nitroxide–alkyl alkoxyamines. This correlation which is reported in Fig. 6.38 allows the estimation of the cleavage temperature of the C–O bond in hexyl–DPAIO at around 210°C.

Fig. 6.38. Estimation of the cleavage temperature of the C–O bond in hexyl–DPAIO.

The experimental study of thermal degradation shows that it is the N–O bond of the alkoxyamine which breaks instead of the C–O bond which should break in order to allow for control of the radical polymerization. Further BDE calculations were performed in order to compare the strength of the N–O and C–O bonds. BDE calculations for the two bonds of different alkoxyamines are reported in Table 6.39. Firstly, as experimentally observed, the N–O bond was found to be weaker than the C–O bond in the case of hexyl–DPAIO. The same conclusion is reached for alkoxyamines hexyl–4a and hexyl–4b (see structures in Scheme 6.40) . The weakness of the N–O bond is strongly dependent on steric hindrance as can be seen from a comparison of 4a and 4b. The presence of bulky substituents on the nitroxide induces a strong decrease of the N–O BDE. The C–O bond in hexyl–nitroxide alkoxyamines appears to be extremely strong as compared with that in styryl–TEMPO and other TEMPO–derived alkoxyamines (Table 6.39). These calculations also show that the C–O bond is weaker than the N–O bond in S–TEMPO, allowing a satisfactory controlled polymerization.

Table 6.39. Estimation of BDE of the C–O and N–O bonds in different alkoxyamines (see text for the method used)

BDE calculations have been performed on a series of indolinic aminoxyl radicals with various substituents. For the nitroxides 2a–c, the BDE of the C–O bond should be reduced by their greater stabilization due to the extended delocalization of the unpaired electron.

Moreover, a favorable steric factor is introduced in 2c. In the 3a–c series, the steric hindrance around the aminoxyl moiety is varied and the BDE should decrease due to destabilization of the alkoxyamine. In the case of nitroxide 4, the aminoxyl function is a six–membered ring and the BDE of such alkoxyamine is known to be lower than those involving five–membered rings.

The calculations using UHF method show a decrease of the BDE of the C–O bond in hexyl–2a, hexyl–2b, hexyl–2c relative to hexyl–DPAIO. As expected, the increase of the steric hindrance in 3b and 3c results in a slightly decrease of the BDE relative to the BDE of hexyl–DPAIO. The BDE of hexyl–4 is also lowered at the RHF and UHF levels but not as much as it was expected considering the stabilization of the nitroxide.

The calculations however show that the N–O bond in the different alkoxyamines hexyl–2a–c, hexyl–3a–c, and hexyl–4a–b was also shown to be much weaker than the C–O bond (Tordo et al. ⁶⁷). No substituents were found to be able to strongly increase the strength of the N–O bond.

Scheme 6.40. Series of studied indolinic aminoxyl radicals.