Highly Thermally Stable and Miscible CO₂‑Based Block Copolymers by the Combination of Ring-Opening and RAFT...

In this study, the chain end of a reversible addition-fragmentation chain transfer (RAFT) polymerization agent of poly-(cyclohexene carbonate) (PCHC) was synthesized via the ring-opening copolymerization of CO₂ and cyclohexene oxide (CHO) by using s-dodecyl-s'-(α,α'-dimethyl-α″-acetic acid) trithiocarbonate (DDMAT) as a chain transfer agent. Various block copolymers of poly-(cyclohexene carbonate)-b-poly-(styrene-alt-N-(hydroxyphenyl)-maleimide) (PCHC-b-PSHPMI) were subsequently synthesized by the RAFT copolymerization of styrene and N-(hydroxyphenyl)-maleimide (HPMI) in the presence of azobis-(isobutyronitrile) (AIBN), which were characterized by using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR), and gel permeation chromatography (GPC). DSC thermal analyses indicated that the single T _g values were observed for all PCHC-b-PSHPMI copolymers, indicating miscible behavior, and the T _g value was 194 °C for the PCHC-b-PSHPMI78 copolymer. One- and two-dimensional (2D) FTIR spectroscopy revealed that these PCHC-b-PSHPMI copolymers actually provide relatively weak intermolecular O-H···OC hydrogen bonding, which was attenuated by the self-association of hydrogen bonding within the pure PCHC and pure PSHPMI segments. In the solid-state ¹³C NMR spectra, a pronounced chemical shift variation of the C-OH and CO units of the PSHPMI segment and CO units of the PCHC segment was also observed, which is attributable to the intermolecular hydrogen interactions in these PCHC-b-PSHPMI copolymers. Rotating-frame ¹H spin-lattice relaxation [T _1ρ(H)] analyses also indicated the complete miscible behavior of these block copolymers within the 2-3 nm length scale, and the relaxation times exhibited positive deviations from the linear predicted rule. These results suggest that the loose chain structure was formed because of the weaker intermolecular hydrogen bonding between the PCHC and PSHPMI segments in the block copolymers.