Polyethylene glycol methacrylate stabilised diblock copolymers created by RAFT-PISA and their associated thermoresponsive behaviours

Reversible addition-fragmentation chain transfer (RAFT) polymerisation-induced self-assembly (PISA) has emerged as a powerful method for the synthesis of well-defined block copolymer nanoparticles in a single step. By exploiting the amphiphilicity of block copolymers, PISA enables the formation of self-assembled structures such as micelles, worms, and vesicles. In this work, RAFT-PISA in methanol was employed to synthesise thermoresponsive nanoparticles, with a focus on understanding the relationship between polymer composition, self-assembly behaviour, and temperature-dependent solvation transitions. The homopolymerisation of diethylene glycol methyl ether methacrylate (DEGMA) was successfully achieved using RAFT solution polymerisation in methanol, with 4-cyano-4-(phenylcarbonothioylthio)pentanoic acid (CPDT) identified as the most effective chain transfer agent (CTA). Using CPDT, macro-CTAs of poly(diethylene glycol methyl ether methacrylate) (PDEGMA), poly(triethylene glycol methyl ether methacrylate) (PTEGMA), and poly(oligoethylene glycol methyl ether methacrylate) (POEGMA) were synthesised and kinetically analysed. All homopolymerisations exhibited first-order kinetics with respect to monomer concentration, demonstrating controlled radical polymerisation behaviour. These macro-CTAs were subsequently chain-extended via RAFT methanolic dispersion polymerisation with methyl methacrylate (MMA), yielding diblock copolymers. While PDEGMA-based diblocks displayed colloidal instability, PTEGMA-b-PMMA and POEGMA-b-PMMA copolymers exhibited improved stability, forming well-defined nanoparticles. Increasing the PMMA block length induced a morphological transition from spherical micelles to vesicles. However, the polymerisation of POEGMA-b-PMMA showed poor RAFT control, resulting in broad molecular weight distributions and heterogeneous particle formation. Further investigations explored the chain extension of PTEGMA macro-CTAs with styrene (Sty) and methyl methacrylate derivatives (MeMBl). The choice of RAFT agent significantly influenced polymerisation control and nanoparticle morphology, with a CPADB-derived PTEGMA macro-CTA enabling improved RAFT control in PTEGMA-b-PSty diblock copolymers. Notably, PTEGMA-b-PMeMBl represented the first reported case of MeMBl incorporation into a diblock copolymer via PISA, yielding nanoparticles with well-defined size distributions. The thermoresponsive behaviour of PTEGMA-b-PMMA nanoparticles was investigated using turbidity measurements, dynamic light scattering (DLS), and nuclear magnetic resonance (NMR) spectroscopy. Results revealed a dual lower critical solution temperature (LCST)- and upper critical solution temperature (UCST)-type transition, dependent on solvent composition and PMMA core size. Crosslinked particles confirmed that solvation of the PMMA core was essential for thermoresponsive behaviour. Small-angle X-ray scattering (SAXS) and nano differential scanning calorimetry (nanoDSC) further validated particle solvation upon heating. The solvent-dependent nature of the phase transition was examined, with methanol identified as the optimal solvent for preserving thermoresponsive properties, whereas higher water content disrupted self-assembly. Overall, this study provides new insights into the RAFT polymerisation of thermoresponsive block copolymers, elucidating the key factors governing colloidal stability, nanoparticle morphology, and solvent-mediated phase transitions. These findings contribute to the broader development of stimuli-responsive polymeric materials for advanced applications.