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

A xanthate derived photoinitiator, S-methacryloyl O-ethyl xanthate (MAX) be aring an electron-deficient polymerizable double bond has been found to be capable of distinguishing MMA and styrene, thereby dictating their polymeri zation pathways in distinctly different and controlled fashion. The structu re, molecular weight, and the polydispersity of each polymer have been dete rmined by spectral analysis and size-exclusion chromatography (SEC). Photop olymerization of MMA using MAX under 350-nm irradiation led to the formatio n of narrow dispersed (M-w/M-n < 1.5) linear "macroinitiators" with methacr yloyl and thiocarbonyl thiyl end functional groups by a controlled free rad ical mechanism where the molecular weights remained nearly the same, indepe ndent of irradiation time. The presence of the thiocarbonyl thiyl group was further confirmed by the block copolymerization of methyl acrylate using t he macroinitiator. On the other hand, photopolymerization of styrene with M AX showed considerable increase in molecular weights and polydispersities w ith irradiation time, as in the case of a pseudo-"living" free radical poly merization. Nevertheless, in the present case, the molecular weight increas e 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 spect ral analysis. This unusual behavior of MAX is attributed to its "tricky" ap proach toward MMA and styrene, making use of the electron availability arou nd their double bonds. MAX behaves only as a photoinitiator toward the elec tron-deficient MMA, whereas it plays the dual role of a photoinitiator as w ell as a co-monomer toward electron-rich styrene due to a weak donor-accept or interaction, leading to the initial formation Of a macro-photoinitiator and the subsequent formation of branched and cross-linked polymers. Interes tingly, S-benzoyl O-ethyl xanthate, an analogous photoinitiator without a p olymerizable double bond, did not show any differences in the polymerizatio n of MMA and styrene, thereby emphasizing the role of the methacryloyl moie ty of MAX in controlling their polymerization pathways.