Forced gradient copolymerisation: a simplified approach for polymerisation-induced self-assembly

In this work, a novel and versatile gradient copolymerisation approach to simplify polymeric nanoparticle synthesis through polymerisation-induced self-assembly (PISA) is reported. In contrast with the commonly performed two-step PISA process, which involves chain-extension of a pre-synthesised stabiliser (or solvophilic block), this work demonstrates a one pot PISA approach via the formation of gradient copolymers through a gradual injection of the core-forming monomer in the presence of a solvophilic monomer. To demonstrate this concept, two model PISA systems were tested using a methacrylate monomer pair and an acrylamide pair. PISA using dimethylacrylamide (DMA) and diacetone acrylamide (DAAm) was first established to form a range of nanoparticle morphologies (spheres and worms), and importantly, a pure worm phase was observed without the addition of a co-solvent or a second solvophilic monomer during the polymerisation of the core-forming monomer. To demonstrate the gradient approach can be applied to other PISA monomer pairs, this methodology was applied to a system using oligo(ethylene glycol)methyl ether methacrylate (OEGMA300) and 2-hydroxypropyl methacrylate (HPMA) as monomers. PISA of this monomer pair resulted in the formation of nanoparticles with various morphologies, including spheres, worms and vesicles. More interestingly, the nanoparticles formed using these gradient copolymers presented thermoresponsive behaviour, exhibiting a sphere-to-worm transition with an increase in temperature from 25 °C to 40 °C. Thus, this facile gradient copolymerisation approach was shown to simplify the PISA process into a single step approach with easily tuneable solvophilic block length and copolymer composition, and additionally provide nanoparticle structures that afford unique properties.