Synthetic and mechanistic aspects of anionic polymerization of methyl methacrylate using tetrabutyl ammonium thioimidate

The tetrabutylammonium salts of ionic organo-initiator containing N,N'-diisopropylthiourea (TUA-1) or N,N'-diethylthiourea (TUA-2) serve as inexpensive initiators for the anionic polymerization of methyl methacrylate (MMA) at room temperature. The molecular weights of obtained polymers are in the range of 1500–22,700 g mol⁻¹ and the molecular weight distributions are fairly broad (Đ = 1.9–2.5) in optimized cases. The molar ratio of monomer to initiator can be achieved up to 800. Side-reactions, for example, backbiting, transfer reactions result in the polymerization being a non-living manner, thus leading to broad molecular weight distributions of the resulting polymers. The effects of counterion nature were also studied from the polymerization of MMA using TUA-1 anion with sodium or potassium salts as counterions under identical conditions. Detailed investigation indicates that the polymerization proceeds via a sulfur anion initiated repeated 1,4-Machael addition. In general, thioimidate initiators induced MMA polymerization feature certain induction periods, which is ascribed to slow addition thioimidate to CC double bond of MMA as a result of low initiator efficiency.