Abstract
The successful synthesis of multiblock copolymers based on high glass transition temperature (Tg) polymers via RAFT emulsion polymerization highly depends on overcoming the radical penetration issue, a significant barrier when radicals cannot effectively enter or diffuse through a glassy particle.1-3 High Tg values result in poor radical diffusion from the aqueous phase, preventing the propagating radicals from reaching the hydrophobic RAFT moieties at the particle core before undergoing bimolecular termination or undergoing significant uncontrolled propagation.1, 2 To provide a comprehensive understanding of this challenge, this study evaluates the interplay of critical chemical and physical parameters in RAFT emulsion systems. First, we analysed the influence of different initiators, such as potassium persulfate (KPS), 2,2′azobis(2-methylpropionitrile) (AIBN), and benzyl peroxide (BPO), as well as different [RAFT]/[initiator] ratios. Next, we evaluated the effect of monomer type by comparing the compatibilities of different seed latexes and chain extension pairings utilizing tert-butyl methacrylate (tBMA), butyl methacrylate (BMA), and tert-butyl acrylate (tBA). Finally, we studied the critical impact of different physical agitation methods (shaking, stirring, and both), reaction scale (small and large), and headspace (low and normal).2, 4 Experimental data indicate that successful chain extension mainly depends on initiator hydrophilicity and mixing dynamics. The water-soluble KPS achieves high conversion without RAFT control, whereas the use of hydrophobic BPO and AIBN leads to low conversions. Optimal results were achieved by combining a high [RAFT]/[initiator] ratio of 20 with moderate stirring speed of 250 rpm, shaking, or utilizing a low headspace to mitigate radical penetration issues.
Keywords: Multiblock copolymers, RAFT, Emulsion polymerization, Radical penetration