In Situ Spectroscopic Studies of Highly Transparent Nanoparticle Dispersions Enable Assessment of Trithiocarbonate Chain-End Fidelity during RAFT Dispersion Polymerization in Nonpolar Media.

Erik J. Cornel,Timothy Smith,Sandra van Meurs,Steven P. Armes,Paul S. O’Hora

doi:10.1021/jacs.8b07953

Abstract

We report the synthesis of highly transparent poly(stearyl methacrylate)-poly(2,2,2-trifluoroethyl methacrylate) (PSMA–PTFEMA) diblock copolymer nanoparticles via polymerization-induced self-assembly (PISA) in nonpolar media at 70 °C. This was achieved by chain-extending a PSMA precursor block via reversible addition–fragmentation chain transfer (RAFT) dispersion polymerization of TFEMA in n-tetradecane. This n-alkane has the same refractive index as the PTFEMA core-forming block at 70 °C, which ensures high light transmittance when targeting 33 nm spherical nanoparticles. Such isorefractivity enables visible absorption spectra to be recorded with minimal light scattering even at 30% w/w solids. However, in situ monitoring of the trithiocarbonate RAFT end-groups during PISA requires selection of a weak n → π* band at 446 nm. Conversion of TFEMA into PTFEMA causes a contraction in the reaction solution volume, leading to an initial increase in absorbance that enables the kinetics of polymerization to be monitored via dilatometry. At ∼98% TFEMA conversion, this 446 nm band remains constant for 2 h at 70 °C, indicating surprisingly high RAFT chain-end fidelity (and hence pseudoliving character) under monomer-starved conditions. In situ 19F NMR spectroscopy studies provide evidence for (i) the onset of micellar nucleation, (ii) solvation of the nanoparticle cores by TFEMA monomer, and (iii) surface plasticization of the nanoparticle cores by n-tetradecane at 70 °C. Finally, the kinetics of RAFT chain-end removal can be conveniently monitored by in situ visible absorption spectroscopy: addition of excess initiator at 70 °C causes complete discoloration of the dispersion, with small-angle X-ray scattering studies confirming no change in nanoparticle morphology under these conditions.

Highlights

Poly(stearyl methacrylate)− poly(2,2,2-trifluoroethyl methacrylate) (PSMA−PTFEMA) diblock copolymer nanoparticles were prepared via reversible addition−fragmentation chain transfer (RAFT) dispersion polymerization in either n-tetradecane or ndodecane
The PTFEMA block became insoluble at a certain critical degree of polymerization (DP) as it grows from the soluble PSMA block; this leads to micellar nucleation and eventually the formation of sterically stabilized nanoparticles
Because RAFT polymerizations are typically performed at 60−90 °C, such turbidity prevents in situ visible absorption spectroscopy studies from being performed during polymerization-induced self-assembly (PISA) syntheses conducted in this solvent

Summary

■ INTRODUCTION

Block copolymer self-assembly in solution has become a wellestablished route for accessing a wide range of organic nanoparticles of varying size, morphology, and surface chemistry.[1,2] One of the most powerful and versatile means of preparing functional block copolymers from various vinyl monomers is reversible addition−fragmentation chain transfer (RAFT) polymerization.[3−6] This controlled radical polymerization technique provides good control over the molecular weight distribution and offers sufficient pseudoliving character to enable the synthesis of well-defined diblock copolymers.[7−20] Self-assembly is traditionally achieved via post-polymerization processing, but over the past decade many research groups have demonstrated that polymerization-induced self-assembly (PISA) offers decisive advantages for the efficient synthesis of diblock copolymer nanoparticles directly in a wide range of solvents (e.g., water, polar solvents, nonpolar solvents, ionic liquids, etc.).[21−25] When PISA is conducted via RAFT dispersion polymerization a soluble homopolymer precursor is chain-extended using a second miscible monomer, which forms an insoluble block when polymerized. We report the RAFT dispersion polymerization of TFEMA in n-tetradecane to afford poly(stearyl methacrylate)poly(2,2,2-trifluoroethyl methacrylate) (PSMA−PTFEMA) spherical nanoparticles of 33 nm diameter These nanoparticles are almost perfectly isorefractive with the solvent at the reaction temperature of 70 °C, which enables high-quality visible absorption spectra to be recorded in situ without any interference from particle scattering. This allows the RAFT chain-end fidelity to be conveniently monitored throughout the polymerization, even when preparing such nanoparticles at 30% w/w solids. Such highly transparent dispersions may offer new opportunities for further scientific studies in the field of colloid science.[54−56]

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES