Expanding the Scope of RAFT Multiblock Copolymer Synthesis Using the Nanoreactor Concept: The Critical Importance of Initiator Hydrophobicity

Abstract

Precise multiblock copolymer synthesis coupled with self-assembly offers morphology control on length scales ranging from a few nanometers to micrometer scale, providing enormous opportunities for future development of advanced materials and applications. The scope of multiblock copolymer synthesis via RAFT polymerization has recently been expanded by application of the nanoreactor concept for emulsion polymerization. This enabled use of slow propagating monomers, such as styrenes and methacrylates, in multiblock synthesis. However, severe limitations attributed to the high polymer glass transition temperature (T g ) of some polymers have hitherto remained. The use of monomers that give such high-T g polymers effectively prevented penetration of aqueous-phase-generated radicals into the polymer particles wherein the RAFT functionality is located. We here demonstrate that these constraints can be relieved by judicious choice of the radical initiator. Multiblock homopolymers were synthesized by seeded RAFT emulsion polymerization using initiators that differ substantially in hydrophobicity. Ten sequential chain extensions using tert-butyl methacrylate (T g of PtBMA = 118 °C) with targeted block DP = 100 were conducted at 80 °C for each initiator. Markedly narrower molecular weight distributions were obtained when more hydrophobic initiators were used. The same polymerizations targeting low -T g polymers (PnBMA; T g = 20 °C) resulted in only minor differences in control when the different initiators were used, supporting our hypothesis on the role of radical penetration. The present results are anticipated to significantly expand the scope of RAFT polymerization in aqueous emulsion by allowing access to a wider range of low-dispersity multiblock copolymers.

Crosslinked p(MMA) particles by RAFT emulsion polymerisation: tuning size and stability

“…It has recently been shown that the use of hydrophilic initiators in emulsion polymerisation can reduce radical penetration of polymer particles, particularly when the particles possess glass transition temperatures higher than the temperature of the reaction ( pMMA T g ≈ 100 °C, 53 T R = 70 °C). 54 Previous studies have noted that whilst the plasticising effect of unreacted monomer may lower the T g inside the particles, this effect is not considered significant enough to have a large influence on radical penetration. 54,55 Reduced radical penetration may reduce access of radicals to RAFT functionality contained within the particles, and therefore decreases the ability of RAFT agents to control the polymerisation.…”

“…54 Previous studies have noted that whilst the plasticising effect of unreacted monomer may lower the T g inside the particles, this effect is not considered significant enough to have a large influence on radical penetration. 54,55 Reduced radical penetration may reduce access of radicals to RAFT functionality contained within the particles, and therefore decreases the ability of RAFT agents to control the polymerisation. This can result in polymer particles with higher levels of dispersity and may be the reason for the high molecular weight shoulder and broad dispersity seen for these samples.…”

Multiblock copolymer synthesis via aqueous RAFT polymerization-induced self-assembly (PISA)

“…52 Given that the multiblock is a homopolymer of PS, which has a high T g , 73 potential issues with slow radical penetration must be considered. In our recent work, 52 we demonstrated that the use of an oil-soluble initiator can overcome radical penetration issues resulting in well-defined multiblock copolymer via sequential RAFT emulsion polymerization. AIBN is able to overcome radical penetration due to radical formation occurring within the particles instead of in the aqueous phase as for KPS.…”

“…To implement RAFT in emulsion polymerisation, it has been demonstrated that a macroRAFTmediated approach avoids the poor control and colloidal stability often seen in ab initio RAFT emulsion polymerisations. [42][43][44][45][46] Amphiphilic macroRAFT approaches have been widely used to synthesize high molecular weight multiblock copolymers, 15,[36][37][38][39][47][48][49][50][51][52] including multiblock copolymers of low k p monomers such as styrene and methacrylates. Amphiphilic approaches rely on an amphiphilic diblock synthesized in a two-step solution polymerization, that self-assembles in water.…”

The effect of surface-active statistical copolymers in low-energy miniemulsion and RAFT polymerization

“…[4][5][6][7] The combination of miniemulsions with controlled radical polymerizations is particularly attractive due to the higher polymerization rates and minimized termination observed when compared to traditional solution polymerizations. [8][9][10][11][12][13] Another unique property of an ideal miniemulsion polymerization is the so-called 1 : 1 copy feature whereby each monomer nanodroplet is transformed to one polymeric nanoparticle of a similar size. [14][15][16][17] Miniemulsions require high energy input to trigger their formation.…”