Radical Cation, Dimer Radical Cation and Neutral Radical of 2,3‐Dihydropyran – Possible Initiators of its Polymerisation?

Abstract

The radical cation, distonic dimer radical cation, and radical of 2,3‐dihydropyran (DHP), which may play an important role in mechanism of radical polymerisations of DHP, were radiolytically generated at low temperatures in freon matrices and investigated by electron paramagnetic resonance (EPR) spectroscopy, as well as by quantum chemical methods. DHP radical cation with a half‐chair conformation and six coupling protons (0.88, 0.18, 4.60, 1.88, 1.27, 0.56 mT) was found to be stable in CF3CCl3 between 77 and 145 K. The same radical cation was observed at 77 K also in CF2ClCFCl2, but on increasing temperature, transformation into the neutral 4‐dihydropyryl radical occurred; the latter also shows the half‐chair conformation at 140 K and is characterised by six coupling protons (0.14, 2.31, 0.92, 1.42, 0.28, 1.35 mT). At a high DHP concentration in CF2ClCFCl2 a distonic DHP dimer radical cation, with strongly separated spin and charge and only two coupling protons (1.8 and 3.2 mT) was identified at 100 K, also yielding, at higher temperatures, the 4‐dihydropyryl radical.The radiolytically generated species from DHP and their observed EPR spectra.magnified imageThe radiolytically generated species from DHP and their observed EPR spectra.

On the Radiation‐Induced Polymerisation of 2,3‐Dihydropyran

“…[9] When subtracting the spectrum belonging to the monomer radical cation (dotted line), as measured before at a low (0.2 mol-%) DHP concentration, [9] the spectrum of the dimer radical shown in Figure 1(b) is obtained. The latter was simulated as a doublet of doublets with the hfs splitting constants of 3.2 and 1.8 mT, respectively.…”

“…The latter was simulated as a doublet of doublets with the hfs splitting constants of 3.2 and 1.8 mT, respectively. [9] By quantum chemical calculations it was show that the dimer radical cation has a distonic character: spin is mostly localised on one part (%90%), while positive charge on the other part of dimer (%80%). The calculated hfs splitting constants of the a and the b-proton (structure 4) agree well with the experimental values (cf.…”

“…Figure 2(a) and (b)) and then a further slower transformation proceeds, yielding the dihydropyryl radical. [9] The spectra shown in Figure 2(b) and (c) can be well simulated as a mixture of the dimer radical cation and dihydropyryl radical; for the spectra simulations exactly the same sets of the hfs splitting constants as determined before and which are summarized in Table 1 were used. [9] At 130 K, only the spectrum of the dihydropyryl radical, [9] decaying by a second-order kinetics without further changes, can be seen (see Figure 2(d) and (e) and insert in Figure 2).…”

“…Calculated and experimental values of isotropic hfs splitting constants a(H)/mT for different transients from DHP (see also ref. [9] observed in the EPR spectra during the decay (see Figure 2). This reaction is most likely a bimolecular recombination (Reaction (7)) yielding dihydropyran dehydrodimer 8.…”

“…[8] In our previous low-temperature EPR study with frozen solutions of DHP in Freon, several radiolytically generated species -radical cation, dimer radical cation and radical of DHP -were identified. [9] Since the formation of some polymer was observed in concentrated solutions after irradiation, further investigation was focused on the role of the identified radical species in the polymerisation process. A similar study carried out with DHF has already been completed [10] and it was shown, that the polydisperse polymer is formed here mainly by a cationic process initiated by a dimer carbocation, while the free-radical mechanism via the dihydrofuryl radical leads to low molecular weight oligomers only.…”