A xanthate-derived photoinitiator that recognizes and controls the free radical polymerization pathways of methyl methacrylate and styrene

Abstract

A xanthate derived photoinitiator, S-methacryloyl O-ethyl xanthate (MAX) bearing an electron-deficient polymerizable double bond has been found to be capable of distinguishing MMA and styrene, thereby dictating their polymerization pathways in distinctly different and controlled fashion. The structure, molecular weight, and the polydispersity of each polymer have been determined by spectral analysis and size-exclusion chromatography (SEC). Photopolymerization of MMA using MAX under 350-nm irradiation led to the formation of narrow dispersed (M_w/M_n < 1.5) linear "macroinitiators" with methacryloyl and thiocarbonyl thiyl end functional groups by a controlled free radical mechanism where the molecular weights remained nearly the same, independent of irradiation time. The presence of the thiocarbonyl thiyl group was further confirmed by the block copolymerization of methyl acrylate using the macroinitiator. On the other hand, photopolymerization of styrene with MAX showed considerable increase in molecular weights and polydispersities with irradiation time, as in the case of a pseudo-"living" free radical polymerization. Nevertheless, in the present case, the molecular weight increase and the broad polydispersity of polystyrene are explained on the basis of the branching of the polymer chain, which is supported by IR and NMR spectral analysis. This unusual behavior of MAX is attributed to its "tricky" approach toward MMA and styrene, making use of the electron availability around their double bonds. MAX behaves only as a photoinitiator toward the electron-deficient MMA, whereas it plays the dual role of a photoinitiator as well as a co-monomer toward electron-rich styrene due to a weak donor-acceptor interaction, leading to the initial formation of a macro-photoinitiator and the subsequent formation of branched and cross-linked polymers. Interestingly, S-benzoyl O-ethyl xanthate, an analogous photoinitiator without a polymerizable double bond, did not show any differences in the polymerization of MMA and styrene, thereby emphasizing the role of the methacryloyl moiety of MAX in controlling their polymerization pathways.