Polystyrene-block-polyethylene-block-polystyrene triblock copolymers: Synthesis and crystallization-driven self-assembly behavior

Abstract

In this work, we reported the synthesis of a series of novel semicrystalline ABA triblock copolymers with polyethylene midblock and polystyrene endblocks via the combination of ring-opening metathesis polymerization (ROMP), hydrogenation and reversible addition-fragmentation chain transfer (RAFT) polymerization. First, an α,ω-dihydroxyl-terminated polycyclooctadiene (PCOD) was synthesized via the ROMP of cyclooctadiene by the use of the Grubbs second generation catalyst and with cis-2-butene-1,4-diol as the chain transfer agent. The α,ω-dihydroxyl-terminated PCOD was then hydrogenated into an α,ω-dihydroxyl-terminated polyethylene (PE); the latter was allowed to react with 2-methyl-2-[(dodecylsulfanylthiocarbonyl) sulfanyl]propanoic acid to afford a macromolecular chain transfer agent (i.e., Macro-CTA) for the RAFT polymerization of styrene. By controlling the lengths of PS endblocks, a series of polystyrene-block-polyethylene-block-polystyrene (PS-b-PE-b-PS) triblock copolymers were obtained. The successful synthesis of the ABA triblock copolymers were demonstrated by means of nuclear magnetic resonance (NMR) spectroscopy and gel permeation chromatography (GPC). The results of differential scanning calorimetry (DSC) and atomic force microscopy (AFM) indicate that the PS-b-PE-b-PS triblock copolymers were microphase-separated in bulks and their morphologies were quite dependent on the mass ratios of PE to PS blocks. Transmission electron microscopy (TEM), atomic force microscopy (AFM) and dynamic light scattering (DLS) showed that the triblock copolymers displayed crystallization-driven self-assembly (CDSA) behavior in selective solvent (viz. 1,4-dioxane). Depending on the lengths of PS blocks, the CDSA behavior of all the triblock copolymers generated spherical and cylindrical micelles. It was found that the quantity of the cylindrical micelles increased with decreasing the lengths of PS blocks.