In Situ Small-Angle X-ray Scattering Studies During Reversible Addition-Fragmentation Chain Transfer Aqueous Emulsion Polymerization.

Emma E Brotherton,Amy A Cockram,Steven P Armes,Erik J Cornel,Paul D Topham,Fiona L Hatton,Matthew J Derry,Adam Czajka,Oleksandr O Mykhaylyk

doi:10.1021/jacs.9b06788

Abstract

Polymerization-induced self-assembly (PISA) is a powerful platform technology for the rational and efficient synthesis of a wide range of block copolymer nano-objects (e.g., spheres, worms or vesicles) in various media. In situ small-angle X-ray scattering (SAXS) studies of reversible addition–fragmentation chain transfer (RAFT) dispersion polymerization have previously provided detailed structural information during self-assembly (see M. J. Derry et al., Chem. Sci.2016, 7, 5078–509030155157). However, conducting the analogous in situ SAXS studies during RAFT aqueous emulsion polymerizations poses a formidable technical challenge because the inherently heterogeneous nature of such PISA formulations requires efficient stirring to generate sufficiently small monomer droplets. In the present study, the RAFT aqueous emulsion polymerization of 2-methoxyethyl methacrylate (MOEMA) has been explored for the first time. Chain extension of a relatively short non-ionic poly(glycerol monomethacrylate) (PGMA) precursor block leads to the formation of sterically-stabilized PGMA-PMOEMA spheres, worms or vesicles, depending on the precise reaction conditions. Construction of a suitable phase diagram enables each of these three morphologies to be reproducibly targeted at copolymer concentrations ranging from 10 to 30% w/w solids. High MOEMA conversions are achieved within 2 h at 70 °C, which makes this new PISA formulation well-suited for in situ SAXS studies using a new reaction cell. This bespoke cell enables efficient stirring and hence allows in situ monitoring during RAFT emulsion polymerization for the first time. For example, the onset of micellization and subsequent evolution in particle size can be studied when preparing PGMA29-PMOEMA30 spheres at 10% w/w solids. When targeting PGMA29-PMOEMA70 vesicles under the same conditions, both the micellar nucleation event and the subsequent evolution in the diblock copolymer morphology from spheres to worms to vesicles are observed. These new insights significantly enhance our understanding of the PISA mechanism during RAFT aqueous emulsion polymerization.

Highlights

Emulsion polymerization is a remarkably efficient and environmentally-friendly process that is applicable to many water-immiscible vinyl monomers, including styrene, methacrylates, acrylates, vinyl acetate, vinyl chloride etc.[1,2] It is employed on a global scale by many chemical companies to prepare tens of millions of tons of copolymer latexes every year
For well-stirred reaction mixtures, the poly(glycerol monomethacrylate) (PGMA) macro-CTA acts as an emulsifier: laser diffraction and optical microscopy studies indicated the formation of polydisperse methoxyethyl methacrylate (MOEMA) monomer droplets with a volume-average diameter of around 14 μm at 20 °C
PGMA29-PMOEMAy diblock copolymer nano-objects were analyzed by dynamic light scattering (DLS) and transmission electron microscopy (TEM) in order to construct a phase

Summary

■ INTRODUCTION

Emulsion polymerization is a remarkably efficient and environmentally-friendly process that is applicable to many water-immiscible vinyl monomers, including styrene, methacrylates, acrylates, vinyl acetate, vinyl chloride etc.[1,2] It is employed on a global scale by many chemical companies to prepare tens of millions of tons of copolymer latexes every year. Over the past decade or so, there has been considerable interest in conducting aqueous emulsion polymerizations using pseudo-living radical polymerization.[12−16] In particular, reversible addition−fragmentation chain transfer (RAFT) polymerization enables the controlled polymerization of a wide range of functional monomers to form well-defined amphiphilic diblock copolymers.[17−20] In principle, this surfactant-free approach should enable access to nanoparticles with various copolymer morphologies using a technique known as polymerization-induced self-assembly (PISA).[21] in practice there are many literature examples of RAFT aqueous emulsion polymerization formulations that only produce kineticallytrapped spheres.[22−34] Exceptions to this restrictive paradigm usually involve the use of statistical copolymers as the watersoluble precursor block to form spheres, worms/fibers or vesicles.[35−43] the use of ionizable monomers in such examples means that the copolymer morphology depends on parameters such as the stabilizer block composition,[35−38] solution pH35,37,39 and salt concentration.[35,36] It is well-known that RAFT aqueous dispersion polymerization of 2-hydroxypropyl methacrylate (HPMA), which has an aqueous solubility of 100 g dm−3 at 70 °C, invariably allows convenient access to spheres, worms or vesicles provided that a sufficiently short non-ionic steric stabilizer block is utilized.[44,45] In view of such observations, we hypothesized that the aqueous solubility of the vinyl monomer might be an important parameter when attempting to prepare worms or vesicles via RAFT aqueous emulsion polymerization. Such detailed studies enhance our understanding of the true nature of these heterogeneous polymerizations.[52]

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES