No abstract

Benzyl (diethoxyphosphoryl)dithioformate (1) and benzyl (diethoxythiophosphoryl)dithioformate (2) were studied as RAFT agents in the polymerization of styrene. The reactive intermediates involved in the process were identified by electron spin resonance spectroscopy as the species deriving from the addition of the polymer propagating radical to the chain transfer agent, in agreement with the RAFT mechanism. Both thermally and AIBN-initiated RAFT styrene polymerizations were performed at different temperatures. In general, the molar mass increased with time as expected for a controlled polymerization process. The molar mass distribution also progressively increased. Inefficient control of the molar mass distribution appears to be related to the rate of radical formation, which is not sufficiently fast compared to the overall monomer conversion, as well as to the rate of bond dissociation and reformation with respect to the propagation rate. As a result, both 1 and 2 behave as RAFT agents but, at relatively high conversion, afford polymers endowed with polydispersities as large as in conventional radical polymerizations.

Absolute rate constants for the reactions of rert-butoxyl radicals with some organic aldehydes have been measured in solution by using laser flash photolysis techniques. At 297 K, the rate constants are 8.9, 6.8, 1.0, and 0.05 X 107 M-1 s-1 for EtCHO, C6H5CHO, Me2NCHO, and EtOCHO, respectively. Arrhenius parameters were determined for benzaldehyde. Polar effects are very important in determining the reactivity of organic aldehydes toward tert-butoxyl radicals. The EPR spectra are described for a number of furoyl, thenoyl, and pyrroyl radicals (ArCO) which were generated photolytically over a wide range of temperatures. Conformational isomers were detected and their structural assignment obtained from the magnitude of the hyperfme splittings of the ring protons, by comparison with the conformers of the parent aldehydes, and from the solvent dependence.

Benzyl (diethoxyphosphoryl)dithioformate (1) has been used to control the AIBN initiated radical polymerization of styrene. In a comparative study with the more popular RAFT transfer agent benzyl dithiobenzoate (2), compound 1 led to higher molecular weight polymers, although with a slightly worse polydispersity. During the polymerization process mediated by 1 a few radicals have been intercepted and characterized by means of ESR spectroscopy. The nature and relative amount of the various radical species varied as the polymerization proceeded, matching expectations based on the generally accepted RAFT mechanism. The present ESR results induce a reassessment of the identity of a radical species observed in a previous preliminary study.

The 3,3''',3'''''-tris(butylsulfanyl)- 2,2':5',2'':5'',2''',5''',2'''':5'''',2'''''-sexithiophene 1 was investigated through spectroscopic (NMR, EPR, UV/Vis-NIR), electrochemical, spectroelectrochemical and theoretical (DFT) studies. The charged species obtained upon its oxidation and reduction were characterised, showing that 1 can exist in at least five different oxidation states, that is, a neutral species, a radical cation, a dication, a radical anion, and a dianion. The long term stability of the radical cation 1+. was evidenced by the 1H NMR study in the presence of small quantities of trifluoroacetic acid (TFA). This approach allowed a comparison of the relative broadening of proton signals of 1, induced by the electron exchange process with traces of radical cation 1+., and the hfc (hyperfine coupling) constants obtained from the EPR study and DFT calculations. In the radical cation, all of the heterocyclic sulphur atoms are not significantly involved in the delocalisation of the unpaired electron, whereas the opposite holds for the radical anion. Time-dependent DFT calculations reproduced well the wavelengths of the optical transitions observed in the spectroelectrochemical experiments for all the five oxidation states and support the formation of the dianion 1(2-).