Multiblock Copolymer Synthesis via Reversible Addition–Fragmentation Chain Transfer Emulsion Polymerization: Effects of Chain Mobility within Particles on Control over Molecular Weight Distribution

Abstract

Synthesis of multiblock copolymers using seeded reversible addition−fragmentation chain transfer (RAFT) emulsion polymerization has been explored with a view to elucidate how certain experimental conditions influence the control over molecular weight distribution (MWD). Two separate parameters have been explored in detail: (i) the ratio of monomer concentration to the RAFT end group concentration within particles and (ii) the glass transition temperature (T g ) of the particles. The parameters (i) and (ii) are interrelated as an increase in the ratio [monomer]:[RAFT] leads to a lower T g because of the increased plasticization of the polymer particle by the monomer. Three different monomers were employed, each giving a polymer of different T g values: n-butyl methacrylate (T g = 20 °C), iso-butyl methacrylate (T g = 57 °C), and tert-butyl methacrylate (T g = 118 °C). The results show that the level of control over the MWDs via the RAFT mechanism is markedly reduced under conditions where T g of the polymer particles is high. This is attributed to the high T g value, leading to low radical penetration rates (low diffusion rates) of radicals generated via initiation in the aqueous phase, preventing propagating radicals from reaching the core region of the particles before bimolecular termination occurs. In the present system, the RAFT end groups are predominantly (but not at all exclusively) located in the core region of the particles.

Synthesis of Multicompositional Onion‐like Nanoparticles via RAFT Emulsion Polymerization

“…These unsuccessful chain extensions may be related to poor radical penetration into the polymer particles due to the high T g of PtBMA (118 8 8C) and PS (100 8 8C) as these blocks are forming. [15] This radical penetration issue was not significant for MB1 (Table 1, Figure 2a)a st he cumulative DP of the first two blocks was relatively low (475 (MB1) vs.700 (MB4)). In order to circumvent this issue,t he polymerization was repeated in the presence of toluene for the 3 rd and 4 th block.…”

“…The syntheses of the first block and the second block were successful, but the subsequent chain extensions with t BMA and S did not show the expected shifts in the MWD (Figure S8). These unsuccessful chain extensions may be related to poor radical penetration into the polymer particles due to the high T g of P t BMA (118 °C) and PS (100 °C) as these blocks are forming [15] . This radical penetration issue was not significant for MB1 (Table 1, Figure 2 a) as the cumulative DP of the first two blocks was relatively low (475 (MB1) vs. 700 (MB4)).…”

“…[14] If the glass transition temperature (T g )is greater than the polymerization temperature,l ow diffusion rates of oligomeric z-mer radicals entering the particles (and subsequently propagating within the particles) may restrict access to the RAFT end groups located primarily in the core region, thus negatively impacting RAFT control. [11,15] The T g values for the seed polymers are 20 8 8C(PnBMA;MB1), 54 8 8C (PBzMA;M B2) and 118 8 8C( P tBMA;M B3). [16] Thus,t he T g values of the seeds in MB1 and MB2 are lower than the polymerization temperature (80 8 8C), thus providing ar ationale for the lower values for MB1 and MB2 (1.12 (MB1), 1.22 (MB2) and 1.46 (MB3);Figure S4).…”

“…The aqueous phase initiator KPS generates hydrophilic radicals in the aqueous phase that enter the particles after a few propagation steps in the aqueous phase (forming z ‐mers) [14] . If the glass transition temperature ( T g ) is greater than the polymerization temperature, low diffusion rates of oligomeric z ‐mer radicals entering the particles (and subsequently propagating within the particles) may restrict access to the RAFT end groups located primarily in the core region, thus negatively impacting RAFT control [11, 15] . The T g values for the seed polymers are 20 °C (P n BMA; MB1), 54 °C (PBzMA; MB2) and 118 °C (P t BMA; MB3) [16] .…”

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

“…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.…”