Poly(glycerol monomethacrylate)–Poly(benzyl methacrylate) Diblock Copolymer Nanoparticles via RAFT Emulsion Polymerization: Synthesis, Characterization, and Interfacial Activity

Abstract

A poly(glycerol monomethacrylate) (PGMA) macromolecular chain transfer agent has been utilized to polymerize benzyl methacrylate (BzMA) via reversible addition−fragmentation chain transfer (RAFT)-mediated aqueous emulsion polymerization. This formulation leads to the efficient formation of spherical diblock copolymer nanoparticles at up to 50% solids. The degree of polymerization (DP) of the core-forming PBzMA block has been systematically varied to control the mean particle diameter from 20 to 193 nm. Conversions of more than 99% were achieved for PGMA 51 −PBzMA 250 within 6 h at 70 °C using macro-CTA/initiator molar ratios ranging from 3.0 to 10.0. DMF GPC analyses confirmed that relatively low polydispersities (M w /M n < 1.30) and high blocking efficiencies could be achieved. These spherical nanoparticles are stable to both freeze−thaw cycles and the presence of added salt (up to 0.25 M MgSO 4 ). Three sets of PGMA 51 −PBzMA x spherical nanoparticles have been used to prepare stable Pickering emulsions at various copolymer concentrations in four model oils: sunflower oil, n-dodecane, nhexane, and isopropyl myristate. A reduction in mean droplet diameter was observed via laser diffraction on increasing the nanoparticle concentration. Finally, the cis diol functionality on the PGMA stabilizer chains has been exploited to demonstrate the selective adsorption of PGMA 51 −PBzMA 100 nanoparticles onto a micropatterned phenylboronic acid-functionalized planar surface. Formation of a cyclic boronate ester at pH 10 causes strong selective binding of the nanoparticles via the cis-diol groups in the PGMA stabilizer chains, as judged by AFM studies. Control experiments confirmed that minimal selective nanoparticle binding occurred at pH 4, or if the PGMA 51 stabilizer block was replaced with a poly(ethylene glycol) PEG 113 stabilizer block.

A Critical Appraisal of RAFT-Mediated Polymerization-Induced Self-Assembly

“…Moreover, if sterically stabilized particles are required, then an optimized aqueous PISA formulation usually offers high blocking efficiencies (and hence effective steric stabilization) via surfactant-free formulations. 35,69,76,79,93−96 This is in striking contrast to the relatively low grafting efficiencies usually achieved when using either macromonomers, block copolymers, or graft copolymer stabilizers for aqueous emulsion polymerization. 97−100 …”

“…34,46,54,70,71,89,102,103 Most literature examples of RAFT aqueous emulsion polymerization syntheses only result in the formation of kinetically-trapped spheres, even when targeting highly asymmetric diblock compositions. 35,95,96,104,105 These observations are currently not properly understood and surely warrant further work. In contrast, most RAFT dispersion polymerization formulations typically exhibit the expected range of copolymer morphologies (spheres, worms, and vesicles) provided that such syntheses are conducted at relatively high solids (>20% w/w) while using a sufficiently short stabilizer block as a macromolecular chain transfer agent (macro-CTA).…”

“…This approach enables the design of a wide range of spheres, worms, or vesicles exhibiting nonionic, 34,89,111 zwitterionic, 81,103,112 anionic, 102 or cationic 56,71,113 character in the case of RAFT aqueous dispersion polymerization. Similarly, RAFT aqueous emulsion polymerization has been used to produce non-ionic, 35,114 anionic, 105,115−117 or cationic spheres. 118 As expected, the chemical nature of the stabilizer block directly influences the colloidal stability of the nanoparticles.…”

Polymerization-induced self-assembly of block copolymer nanoparticles via RAFT non-aqueous dispersion polymerization

“…In essence, hydrophilic particles typically form oil-in-water (o/w) emulsions, whereas hydrophobic particles usually stabilize water-in-oil (w/o) emulsions. The development of robust PISA formulations provided a timely opportunity to compare the performance of hydrophilic block copolymer spheres, worms or vesicles synthesized via RAFT aqueous dispersion polymerization for the preparation of o/w Pickering emulsions [20,40,108]. One interesting question in this context is whether worms offer any advantages over spheres.…”

“…Initial research focused on RAFT aqueous emulsion polymerization using water-immiscible monomers such as methyl methacrylate, n-butyl acrylate or styrene [15][16][17][18][19][20]. Such formulations can be very efficient [15][16][17], but in many cases this approach leads to the formation of kinetically-trapped spheres, rather than the full range of copolymer morphologies [15][16][17][18][19][20][21]. In contrast, there are many examples of RAFT dispersion polymerization formulations that yield spheres, worms and vesicles [17,[22][23][24][25][26][27][28][29].…”