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In this article we review the current state of knowledge in the field of responsive nanoparticles for aqueous-based systems. The review focuses on the use of stimuli-responsive copolymers that can either self-assemble into 'soft' nanoparticles or that are attached to the surface of solid nanoparticles. We first describe the most common methods for synthesizing the different responsive polymers used in the design of such nanoparticles, highlighting living radical polymerization techniques in particular. Subsequently, we give examples of how copolymers containing such responsive blocks can selfassemble into a wide range of 'soft' structures. We also depict the main techniques for attaching responsive polymers to the surface of solid nanoparticles and list advantages and drawbacks of each. Finally, for both soft self-assembled systems and solid core-polymer shell systems, we show specific examples of these systems used in a varied range of applications including drug delivery, smart emulsifiers, transport across membranes, sensors and coatings.

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A hydroxyl-functionalized alkoxyamine derived from the SG1 nitroxide was used as a dual initiator for ring-opening polymerization (ROP) of ε-caprolactone (CL) and nitroxide-mediated polymerization (NMP) of n-butyl acrylate (BA) to obtain the corresponding block copolymer. Both sequential and one-step strategies were investigated, using the tin(II) 2-ethylhexanoate (Sn(Oct)2) ROP catalyst. The NMP first sequential approach (consisting of performing the NMP before the ROP) successfully provided the diblock copolymer through well-controlled NMP and ROP processes. This copolymer was fully characterized by size exclusion chromatography (SEC), liquid chromatography at the critical conditions (LC-CC) of PBA and PCL, and gradient polymer elution chromatography (GPEC). Conversely, the ROP first strategy led to badly defined PCL (bimodal distribution) in the first ROP step. This was attributed to a side reaction of Sn(Oct)2 with the SG1 nitroxide arising from the alkoxyamine dissociation at the ROP temperature. This strategy was consequently not suitable for obtaining the diblock copolymers. Finally, PBA-b-PCL copolymers were successfully prepared from the dual initiator through a NMP/ROP one-step process in toluene, as shown by 1H NMR spectroscopy, SEC, and GPEC. The success of this one-step approach was explained by a much faster consumption of the BA by NMP than that of the CL by ROP, making the process looking like a NMP first in a one pot experiment.

Glycidyl methacrylate (GMA) is an example of a heterobifunctional monomer, a so-called "Jekyll and Hyde" monomer, in that one functional group can be polymerized via conventional free radical polymerization and the other via cationic ring-opening polymerization. Laterally linked diblock copolymers differ from conventional linear diblock copolymers in that the blocks are not linked via their termini but at a point some way along each chain. This architecture has therefore some of the characteristics of a graft copolymer. Free radical copolymerization of a low level of GMA with methyl methacrylate (MMA) yields MMA/GMA copolymers with a few pendent epoxide groups. Likewise cationic ring-opening copolymerization of low levels of GMA with tetrahydrofuran (THF) yields THF/GMA copolymers with a few pendent methacrylate groups. Subsequent cationic polymerization of the first copolymer type with THF, and subsequent free radical polymerization of the second copolymer type with MMA, yields laterally linked block copolymers of MMA and THF. The two strategies are complementary but the polymerization of the THF block first followed by the MMA block is more efficient and leads to good incorporation of both of these monomer segments. Typically mole ratios of THF/MMA segments approaching 1/1 are achievable with copolymer recoveries up to ∼60% and overall M w values up to 125 kDa. While the products are significantly more heterogeneous in terms of the molar mass distributions of the blocks and the backbone architecture than are conventionally produced diblock copolymers, we believe that they are complementary to, rather than competitive with, conventional diblocks and that this new synthetic strategy and ease of synthesis offers potential for the exploitation of these materials.