Postpolymerization modification is a powerful strategy to change the chemical functionality of pre-made polymers, but only limited approaches exist to modify functionality as well as the shape and behaviour of nano-particles. Herein, poly[poly(ethylene glycol) methyl ether methacrylate]-poly(2,3,4,5,6-pentafluorobenzyl methacrylate) nano-objects (pPEGMA-pPFBMA) prepared via RAFT dispersion polymerization with concurrent polymerization-induced self-assembly (PISA) in ethanol with either spherical or worm-shaped morphology were modified, post-synthesis, with a selection of 15 different thiols through thiol–para-fluoro substitution reactions in the nano-object cores. Depending on the choice of thiol, spherical nano-objects underwent an order–disorder transition to form unimers, increased in size, or underwent an order–order transition to form worm-shaped nano-objects. The core solvophobicity was found to be more important in driving a morphological transition than the modification efficiency, mass increase of the core block, or the glass transition temperature of the (partially) modified cores. These findings are relevant to the development of a “universal nanoparticle precursor” approach that allows the tuning of functionality, behaviour, size, and shape of a pre-made nano-object sample on demand.
The use of 2,3,4,5,6-pentafluorobenzyl methacrylate (PFBMA) as a core-forming monomer in ethanolic RAFT dispersion polymerization formulations is presented. Poly[poly(ethylene glycol) methyl ether methacrylate] (pPEGMA) macromolecular chain transfer agents were chain extended with PFBMA leading to nanoparticle formation via polymerization-induced self-assembly (PISA). pPEGMA-pPFBMA particles exhibited the full range of morphologies (spheres, worms, and vesicles) including pure and mixed phases. Worm phases formed gels that underwent a thermo-reversible degelation and morphological transition to spheres (or spheres and vesicles) upon heating. Post-synthesis, the pPFBMA cores were modified through thiol–para-fluoro substitution reactions in ethanol using 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as the base. For monothiols, conversions were 64% (1-octanethiol) and 94% (benzyl mercaptan). Spherical and worm-shaped nano-objects were core cross-linked using 1,8-octanedithiol, which prevented their dissociation in non-selective solvents. For a temperature-responsive worm sample, cross-linking additionally resulted in the loss of the temperature-triggered morphological transition. The use of the reactive monomer PFBMA in PISA formulations presents a simple method to prepare well-defined nano-objects similar to those produced with non-reactive monomers (e.g. benzyl methacrylate) and to retain morphologies independent of solvent and temperature.
A methacrylic polymer undergoing highly efficient para-fluoro substitution reactions is presented. A series of well-defined poly(2,3,4,5,6-pentafluorobenzyl methacrylate) (pPFBMA) homopolymers with degrees of polymerization from 28 to 132 and Ð ≤ 1.29 was prepared by the RAFT process. pPFBMA samples were atactic (with triad tacticity apparent in 1H and 19F NMR spectra) and soluble in most organic solvents. pPFBMA reacted quantitatively through parafluoro substitution with a range of thiols (typically 1.1 equiv thiol, base, RT, < 1h) in the absence of any observed side reactions. Para-fluoro substitution with different (thio)carbonylthio reagents was possible and allowed for subsequent one-pot cleavage of dithioester pendent groups with concurrent thia-Michael side group modification. Reactions with aliphatic amines (typically 2.5 equiv amine, 50–60 °C, overnight) resulted in complete substitution of the para-fluorides without any observed ester cleavage reactions. However, for primary amines, H2NR, double substitution reactions yielding tertiary (–C6F4)2NR amine bridges were observed, which were absent with secondary amine reagents. No reactions were found for attempted modifications of pPFBMA with bromide, iodide, methanethiosulfonate, or thiourea, indicating a highly selective reactivity toward nucleophiles. The versatility of this reactive platform is demonstrated through the synthesis of a pH-responsive polymer and novel thermoresponsive polymers: an oligo(ethylene glycol)-functional species with an LCST in water and two zwitterionic polymers with UCSTs in water and aqueous salt solution (NaCl concentration up to 178 mM).