RAFT Dispersion Polymerization of Methyl Methacrylate in Mineral Oil: High Glass Transition Temperature of the Core-Forming Block Constrains the Evolution of Copolymer Morphology

Abstract

RAFT dispersion polymerization of a prototypical methacrylic monomer, methyl methacrylate (MMA), is performed in mineral oil using various poly(lauryl methacrylate) (PLMA) precursors prepared with a trithiocarbonate-based RAFT agent. GPC analysis indicated reasonably narrow molecular weight distributions (M w /M n ≤ 1.39) for all diblock copolymers, with 1 H NMR studies indicating high MMA conversions (≥95%) for all syntheses. An efficient one-pot synthesis protocol enabled high blocking efficiencies to be achieved when targeting higher PMMA DPs. However, the relatively high glass transition temperature (T g ) of the corresponding core-forming PMMA block unexpectedly constrains the evolution in copolymer morphology during polymerization-induced self-assembly (PISA). More specifically, well-defined PLMA 22 −PMMA x spheres (x = 19−39) and relatively short worms (x = 69−97) can be obtained at 90 °C when using a PLMA 22 precursor but targeting higher x values (x ≥ 108) invariably leads to colloidally unstable aggregates of spheres, rather than long worms or vesicles. Interestingly, similar constraints were observed when targeting higher solids, when using n-dodecane instead of mineral oil, or when employing an alternative steric stabilizer block. Raising the PISA synthesis temperature from 90 to 115 °C (i.e., from below to above the T g of the final PMMA block) does not alleviate this unexpected problem. Moreover, only spherical nanoparticles can be obtained at 115 °C when targeting PMMA DPs between 50 and 400 with the same PLMA 22 precursor. This suggests that nanoparticle formation may occur by a chain expulsion/insertion mechanism at this relatively high reaction temperature. PLMA 22 −PMMA x nanoparticles were characterized in terms of their particle size and morphology using dynamic light scattering (DLS), transmission electron microscopy (TEM), and small-angle X-ray scattering (SAXS). DLS and TEM studies of a 0.1% w/w dispersion of PLMA 22 −PMMA 69 short worms indicated an irreversible worm-to-sphere transition on heating from 20 to 150 °C. Oscillatory rheology and TEM studies indicated that this thermal transition was only partially reversible for a 20% w/w dispersion of PLMA 22 −PMMA 69 short worms.

Tuning the Glass Transition Temperature of a Core-Forming Block during Polymerization-Induced Self-Assembly: Statistical Copolymerization of Lauryl Methacrylate with Methyl Methacrylate Provides Access to Spheres, Worms, and Vesicles

“… 45 , 47 However, only spherical nanoparticles were obtained when targeting PMMA DPs between 50 and 400 at 115 °C, rather than 90 °C (see Figure 1 b). 15 …”

“…Targeting PLMA 22 –PMMA 200 at 115 °C produced PLMA 22 –PMMA 192 spheres at 96% conversion (see Figure 2 a). 15 Introducing 5 mol % LMA into the core-forming block was insufficient to produce a higher order morphology, merely resulting in PLMA 22 –P(0.95MMA- stat -0.05LMA) 188 spheres (see Figure 2 b ) . However, employing 10 mol % LMA led to a mixed phase comprising mainly vesicles and worms with a minor population of spheres (see Figure 2 c).…”

“…In this context, it is notable that targeting PLMA 22 –PMMA y ( y = 50–400) nano-objects at 20% w/w solids in mineral oil invariably resulted in the formation of kinetically trapped spherical nanoparticles despite MMA conversions remaining incomplete (≥95%) after 17 h at 115 °C. 15 Herein, a 20% w/w dispersion of PLMA 22 –PMMA 291 spherical nanoparticles (previously prepared at 115 °C in mineral oil; final MMA conversion = 97%) was heated at 115 °C for 17 h in the presence of a large excess amount of LMA monomer ([LMA]/[PLMA 22 –PMMA 291 ] molar ratio = 50). The reaction mixture was sealed to prevent evaporative loss of LMA (plus residual MMA comonomer) and was not degassed prior to heating to prevent further copolymerization.…”

“…Unreacted monomer diffuses into the nanoparticle cores, which leads to a relatively high local concentration and hence a faster rate of polymerization. 10 − 15 This enables very high conversions to be achieved within relatively short time scales, which makes PISA much more efficient than the equivalent solution polymerization. 13 , 16 Depending on the monomer solubility within the continuous phase, PISA formulations can be classified as examples of either dispersion (miscible monomer) 4 , 5 , 13 − 15 , 17 − 24 or emulsion (immiscible monomer) 8 , 25 − 31 polymerization.…”

“…Recently, we reported the synthesis of poly(lauryl methacrylate)–poly(methyl methacrylate) [PLMA–PMMA] diblock copolymer nano-objects via reversible addition–fragmentation chain transfer (RAFT) dispersion polymerization in mineral oil. 15 For this PISA formulation, only kinetically trapped spheres or relatively short worms could be obtained. This was attributed to the high T g of the structure-directing PMMA block (126 °C) relative to the synthesis temperature (70–115 °C).…”