Application of Thiourea/Halogen Bond Donor Cocatalysis in Metal-Free Cationic Polymerization of Isobutyl Vinyl Ether and Styrene Derivatives
Koji Takagi, Hiroto Murakata, Tomoki Hasegawa
Abstract
Cationic polymerization of isobutyl vinyl ether (IBVE) was investigated using Schreiner’s thiourea (STU) combined with Im-TEPB bearing a 2-iodoimidazolium cation and a noncoordinating borate anion. Both the CF3CO2H adduct and the HCl adduct of IBVE had the ability to initiate polymerization, but the number-averaged molecular weight was rather low compared with the theoretical value and the molecular weight distribution was fairly broad (Mw/Mn > 2.6). On the other hand, fast cationic polymerization of p-methoxystyrene (pMOS, 50 equiv relative to pMOS·HCl) proceeded using STU/Im-TEPB cocatalysts, giving rise to poly(pMOS) with Mn = 4580 and Mw/Mn = 1.38. STU/Im-TfO exhibited decreased activity due to the coordination of a trifluoromethanesulfonate counteranion with the NH group of STU, which was supported by the NMR spectrum, DFT calculation, and UV–vis titration experiment. The molecular weight of poly(pMOS) could be roughly regulated by STU/Im-TEPB cocatalysis between Mn = 2400 and Mn = 7500 by the monomer feed ratio with keeping the molecular weight distribution below 1.41, although these values were lower than the theoretical ones probably due to the unignorable chain-transfer reaction. The polymerization kinetics revealed that the monomer consumption rate depends on the concentration of Im-TEPB rather than STU, which reveals the catalytic function of STU/Im-TEPB.