Scalable Routes to Block Copolymers via Cu-Mediated Reversible Deactivation Radical Polymerization

Abstract

Cu-mediated Reversible Deactivation Radical Polymerization (RDRP) has been investigated as a method to produce (meth)acrylic polymers of high chain-end functionality and well-defined structure, enabling the production of uniform block copolymer materials. The use of relatively inexpensive reagents with common, simple, and scale-appropriate reactor configurations are key features in overcoming the hurdles to commercialization. To that end, a previously-developed two-step process that produced a poly(methyl acrylate) (PMA) macroinitiator species continuously in a copper tubular reactor that was subsequently chain-extended in a semi-batch reactor was investigated. The complexing ligand N,N,N’,N’’,N’’-pentamethyldiethylenetriamine (PMDETA) utilized in the chain-extension step is significantly less costly than other commonly used ligands such as tris[2-(dimethylamino)ethyl]amine (Me6TREN), while conducting the chain-extension in semi-batch offers an effective means to manage polymerization exotherms at larger scales. Previous studies have demonstrated success in chain-extending with acrylates, however slow polymerization rates and poor initiation efficiency is observed with methacrylates. Using methyl acrylate (MA) as a model system, reaction conditions and feeding strategies were optimized to accelerate polymerization in the semi-batch system, leading to a 2-fold reduction in reaction time with no loss in control compared to previous studies of this system. It was concurrently demonstrated that pre-polymerization in copper tube reactor could be eliminated while still providing a chain-extendible species, a result that simplifies reactor operation, offers greater flexibility in initiator choice, and improves compositional control of the final product. Learnings from the acrylate system were applied to the polymerization of methyl methacrylate (MMA) and di(ethylene glycol) methyl ether methacrylate (DEGMEMA), with the elimination of the PMA macroinitiator and substitution of the initiator methyl 2-bromopropionate (MBP) with ethyl α-bromoisobutyrate (EBiB) providing significant improvement in the initiation efficiency over those observed in previous studies. The conditions developed for the homopolymerization systems were then applied to produce acrylate-acrylate and acrylate-methacrylate block copolymers, also exploring the influence of block order. Reactions with high inter-block conversion were completed in 4 h or less with no intermediate purification or additional catalyst, thus yielding a scalable method of producing block copolymer materials.