Application of Thiourea/Halogen Bond Donor Cocatalysis in Metal-Free Cationic Polymerization of Isobutyl Vinyl Ether and Styrene Derivatives

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 pMOSmiddotHCl) 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.