Janus Polymerization

Is it possible to combine cationic and (coordinated) anionic ring-opening polymerizations (CROP and AROP) into a unimolecular chain? In this work, we design polymerizations of ε-caprolactone (CL) and tetrahydrofuran (THF) using a single catalytic system of lutetium triflate/propylene oxide (Lu­(OTf)₃/PO) in which a cationic copolymerization of THF with CL and an AROP of CL perform at two chain ends of a growing chain. Kinetic and mechanistic studies demonstrate that the two chain-growth ROPs which have no mutual interference up to 90% of CL conversion keep a living manner producing diblock copolymers of PCL-<i>b</i>-P­(THF-<i>co</i>-CL) with narrow polydispersities below 1.2. Afterward, the depletion of CL triggers a final step-growth polymerization via intermolecular coupling reaction of cationic and anionic chain ends, developing multiblock polyester–polyether elastomers of [PCL-<i>b</i>-P­(THF-<i>co</i>-CL)]_<i>m</i>. A concept of “Janus polymerization” is defined as a combination of cationic and anionic polymerizations on the two ends of a single growing chain, involving an initially living chain-growth polymerization and a subsequently self-triggered step-growth polycondensation