Surfactant-Free RAFT Emulsion Polymerization of Styrene Using Thermoresponsive macroRAFT Agents: Towards Smart Well-Defined Block Copolymers with High Molecular Weights.

Steffen Eggers,Volker Abetz

doi:10.3390/polym9120668

Steffen Eggers, Volker Abetz

Open Access

https://doi.org/10.3390/polym9120668

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Journal: Polymers	Publication Date: Dec 3, 2017
Citations: 16	License type: CC BY 4.0

Affiliation: Universität Hamburg, Helmholtz-Zentrum Hereon

Abstract

The combination of reversible addition–fragmentation chain transfer (RAFT) and emulsion polymerization has recently attracted much attention as a synthetic tool for high-molecular-weight block copolymers and their micellar nano-objects. Up to recently, though, the use of thermoresponsive polymers as both macroRAFT agents and latex stabilizers was impossible in aqueous media due to their hydrophobicity at the usually high polymerization temperatures. In this work, we present a straightforward surfactant-free RAFT emulsion polymerization to obtain thermoresponsive styrenic block copolymers with molecular weights of around 100 kDa and their well-defined latexes. The stability of the aqueous latexes is achieved by adding 20 vol % of the cosolvent 1,4-dioxane (DOX), increasing the phase transition temperature (PTT) of the used thermoresponsive poly(N-acryloylpyrrolidine) (PAPy) macroRAFT agents above the polymerization temperature. Furthermore, this cosolvent approach is combined with the use of poly(N,N-dimethylacrylamide)-block-poly(N-acryloylpiperidine-co-N-acryloylpyrrolidine) (PDMA-b-P(APi-co-APy)) as the macroRAFT agent owning a short stabilizing PDMA end block and a widely adjustable PTT of the P(APi-co-APy) block in between 4 and 47 °C. The temperature-induced collapse of the latter under emulsion polymerization conditions leads to the formation of RAFT nanoreactors, which allows for a very fast chain growth of the polystyrene (PS) block. In dynamic light scattering (DLS), as well as cryo-transmission electron microscopy (cryoTEM), moreover, all created latexes indeed reveal a high (temperature) stability and a reversible collapse of the thermoresponsive coronal block upon heating. Hence, this paper pioneers a versatile way towards amphiphilic thermoresponsive high-molecular-weight block copolymers and their nano-objects with tailored corona switchability.

Highlights

While reversible addition–fragmentation chain transfer (RAFT) polymerizations have nowadays entered various fields of chemistry [1], industry [2] and medicine [3,4], it still appears challenging to synthesize well-controlled high-molecular-weight polymers from slowly-propagating monomers; so-called low-kp monomers [5]
To still be able to use the tremendous advantages of RAFT emulsion techniques for synthesizing high-molecular-weight thermoresponsive styrenic block copolymers, we make use of adding slight amounts of a cosolvent pushing Polymers the phase transition temperature (PTT)
It has been shown that amphiphilic thermoresponsive block copolymers can be synthesized by surfactant-free RAFT emulsion polymerization in short reaction times

Summary

Introduction

While reversible addition–fragmentation chain transfer (RAFT) polymerizations have nowadays entered various fields of chemistry [1], industry [2] and medicine [3,4], it still appears challenging to synthesize well-controlled high-molecular-weight polymers from slowly-propagating monomers; so-called low-kp monomers [5]. Besides its potential for synthesizing high-molecular-weight block copolymers and allowing for high polymer loadings in the latexes up to 50 wt % [11,12,13], RAFT emulsion polymerization (both growing block and monomer are insoluble in the polymerization medium) and its relative RAFT dispersion polymerization (the monomer in contrast to the growing solvophobic polymer block is well-soluble) have attracted much attention in the past eight years or so because of a related process named ‘polymerization-induced self-assembly’ (PISA) Achievements in this field have recently been reviewed for instance by Armes et al [14], Truong et al [15] and Boyer et al [16]. PISA in emulsion polymerizations of styrene almost exclusively ends up in the formation of spherical micelles even if thermodynamics favor other morphologies, i.e., cylindrical structures (worms, rods) or vesicles, depending on the volume fractions of the different blocks and packing parameter [14,22,23]

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