Synthesis of a model cyclic triblock terpolymer of styrene, isoprene, and methyl methacrylate

The synthesis of a model cyclic triblock terpolymer [cyclic(S-b-I-b-MMA] of styrene (S), isoprene (I), and methyl methacrylate (MMA) was achieved by the end-to-end intramolecular amidation reaction of the corresponding linear α,ω-amino acid precursor [S-b-I-b-MMA] under high-dilution conditions. The linear precursor was synthesized by the sequential anionic polymerization of S, I, and MMA with 2,2,5,5-tetramethyl-1-(3-lithiopropyl)-1-aza-2,5-disilacyclopentane as an initiator and amine generator and 4-bromo-1,1,1-trimethoxybutane as a terminator and carboxylic acid generator. The separation of the unreacted linear polymer from the cyclic terpolymer was facilitated by the transformation of the unreacted species into high molecular weight polymers by the evaporation of the reaction solvent and the continuation of the reaction under high-concentration conditions. The intermediate materials and the final cyclic terpolymer, characterized by size exclusion chromatography, vapor pressure osmometry, thin-layer chromatography, IR and NMR spectroscopy, exhibited high molecular weight and compositional homogeneity. Dilute-solution viscosity measurements were used as an additional proof of the cyclic structure.