Rapid production of block copolymer nano-objects via continuous-flow ultrafast RAFT dispersion polymerisation

Abstract

Ultrafast RAFT polymerisation is exploited under dispersion polymerisation conditions for the synthesis of poly(dimethylacrylamide)-b-poly(diacetoneacrylamide) (PDMAm_x-b-PDAAm_y) diblock copolymer nanoparticles. This process is conducted within continuous-flow reactors, which are well suited to fast reactions and can easily dissipate exotherms making the process potentially scalable. Transient kinetic profiles obtained in-line via low-field flow nuclear magnetic resonance spectroscopy (flow-NMR) confirmed the rapid rate of polymerisation whilst still maintaining pseudo first order kinetics. Gel permeation chromatography (GPC) reported molar mass dispersities, Đ < 1.3 for a series of PDMAm_x-b-PDAAm_y diblock copolymers (x = 46, or 113; y = 50, 75, 100, 150 and 200) confirming control over molecular weight was maintained. Particle characterisation by dynamic light scattering (DLS) and transmission electron microscopy (TEM) indicated successful preparation of spheres and a majority worm phase at 90 °C but the formation of vesicular morphologies was only possible at 70 °C. To maintain the rapid rate of reaction at this lower temperature, initiator concentration was increased which was also required to overcome the gradual ingress of oxygen into the PFA tubing which was quenching the reaction at low radical concentrations. Ill-defined morphologies observed at PDAAm DPs close to the worm-vesicle boundary, combined with a peak in molar mass dispersity suggested poor mixing prevented an efficient morphological transition for these samples. However, by targeting higher PDAAm DPs, the additional monomer present during the transition plasticises the chains to facilitate formation of vesicles at PDAAm DPs of ≥300.