Synthesis of ABCBA-Type Pentablock Copolymers via Sequential Ring-Opening, Free Radical, and Photoinduced Free-Radical-Promoted Cationic Polymerization Methods

This study reports the synthesis and characterization of ABCBA-type pentablock copolymers comprising poly(cyclohexene oxide) (PCHO) as the A block, poly(epsilon-caprolactone) (PCL) or poly(L-lactide) (PLLA) as the B block, and polystyrene (PSt) as the C block. The target pentablock copolymers, PCHO-PCL-PSt-PCL-PCHO and PCHO-PLLA-PSt-PLLA-PCHO, were synthesized through a sequential combination of ring-opening polymerization (ROP), free radical polymerization (FRP), and, for the first time to the best of our knowledge, photoinduced free-radical-promoted cationic polymerization (FRPCP) in pentablock copolymer synthesis. In the initial ROP step, well-defined Bz-PCL and Bz-PLLA macrophotoinitiators were synthesized with photoactive benzoin (Bz) end groups. Next, Bz-PCL and Bz-PLLA were converted to the macro-azo initiators, Bz-PCL-N=N-PCL-Bz and Bz-PLLA-N=N-PLLA-Bz, by esterification with 4,4 '-azobis(4-cyanovaleric acid) (ACVA). These macro-azo initiators were employed as initiators in the FRP of styrene to form triblock copolymers, which were then used as prepolymers in the photoinduced FRPCP of cyclohexene oxide (CHO) to produce the pentablock structures. Structural characterization conducted via FT-IR and H-1 NMR spectroscopy confirmed the presence of each polymer segment within the pentablock architecture, and gel permeation chromatography (GPC) analysis further validated pentablock formation, showing unimodal distributions. UV-Vis and fluorescence measurements also verified the successful incorporation of the benzoin groups. This multistep synthetic strategy introduces an effective approach for constructing ABCBA-type pentablock copolymers, which might find the potential uses in advanced material design.