Expanding the scope of organocatalysis for alternating copolymerization of dihydrocoumarin and styrene oxide

Abstract

Metal-free copolymerization of 3,4-dihydrocoumarin and styrene oxide catalyzed by several organobases has been investigated. The products were shown to have strict alternating sequence distributions regardless of catalyst used. Compared with previously employed t-BuP4 superbase, phosphazene bases (PBs) with lower basicity (i.e. t-BuP2 and t-BuP1) afforded higher monomer conversions, higher molar masses, and significantly lower proportions of macrocycles in the products. Proton exchange between the mildly basic PBs and the hydroxy species was considered responsible for such improvements, which reduced chain-end nucleophilicity and thus suppressed side reactions. The use of N-heterocyclic bases, i.e. TBD and DBU, also resulted in products comprising linear and cyclic alternating copolymers. However, the molar masses of the two populations were not distinctly different as observed in the cases of PBs. Alternating copolymerization catalyzed by TBD or DBU also occurred in the absence of a hydroxy compound, and the products were shown to be predominantly macrocycles. Unlike initiator/chain-end activation by non-nucleophilic PBs, a mechanism involving nucleophilic monomer activation and zwitterionic chain growth was therefore suggested for such bases.