An orthogonal combination of cationic and radical RAFT polymerizations is used to synthesize bottlebrush polymers using two distinct RAFT agents. Selective consumption of the first RAFT agent is used to control the cationic RAFT polymerization of a vinyl ether monomer bearing a secondary dormant RAFT agent, which subsequently allows side-chain polymers to be grafted from the pendant RAFT agent by a radical-mediated RAFT polymerization of a different monomer, thus completing the synthesis of bottlebrush polymers. The high efficiency and selectivity of the cationic and radical RAFT polymerizations allow both polymerizations to be conducted in one-pot tandem without intermediate purification. Reversible addition-fragmentation chain-transfer (RAFT)has grown in the last 20 years as one of the most versatile polymerization techniques, enabling the control of molecular weight distribution, branched architecture, and chemical functionality for a wide range of polymeric systems. [1] Based on the RAFT versatility, recent years have seen an expansion of using various RAFT agents beyond traditional thermally initiated radical polymerizations. [2] One emerging direction is the use of RAFT agents that facilitate other classes of polymerization such as anionic ring-opening polymerization (AROP) [3] and cationic RAFT polymerization, [4] in synergy with radical mediated RAFT polymerization, to enable dual copolymerization strategies [4b,d, 5] and access unique copolymer compositions. [3c, 4b] Another emerging research area is the exploitation of the versatile photochromic behavior of RAFT agents for photo-controlled polymerization upon direct photo-fragmentation of the R-group of the RAFT agent by UV/Vis irradiation (following the iniferter mechanism) [6] or through a photocatalyst (photoinduced-electron/energy transfer). [7] In particular, Xu and Boyer demonstrated the wavelength dependency of selectively photo-activated RAFT agents [6b, 8] to allow selective RAFT control for orthogonal polymerizations, where an unselected yet activatable RAFT agent remains dormant during photo-controlled radical polymerization with another RAFT agent. [9] Such selective RAFT processes had been previously untapped owing to inherent difficulties in suppressing chain transfer activity of reactive RAFT agents. [10] Through selective photo-fragmentation, Matyjaszewski, Boyer, and co-workers elegantly demonstrated an orthogonal iniferter-RAFT polymerization. [9b] In their case (Figure 1 a), a RAFT-agent-bearing methacrylate monomer was first polymerized orthogonally to produce a linear polymeric chain with pendant RAFT agents remaining intact during the polymerization of the methacrylate unit. The pendant RAFT Figure 1. Generalized structure of a RAFT agent and comparison of two-step syntheses of bottlebrush macromolecules via orthogonal RAFT in a) previous work [9b] and b) this work.

Here, commercially available N-aromatic substituted bismaleimides were used in RAFT step-growth polymerization for the first time. In our initial report (J. Am. Chem. Soc. 2021, 143 (39), 15918-15923), maleimide precursors...

RAFT step-growth polymerization was previously demonstrated with monomers that bear low rate of homopropagation to favor the chain transfer process; by contrast, acrylates are known to be fast homopropagating monomers, thereby posing serious challenges for RAFT step-growth. Here, we identified a chain transfer agent (CTA) that rapidly yields single unit monomer inserted (SUMI) CTA adducts with a model acrylate monomer. Using a bifunctional reagent of this CTA, we successfully demonstrated RAFT step-growth polymerization with diacrylates, yielding linear polymer backbones. Furthermore, we achieved inclusion of functionality (i.e., disulfide) into RAFT step-growth polymer via a disulfide incorporated bifunctional CTA. Grafting from this backbone resulted in molecular brush polymers with cleavable functionality in each repeat unit of the backbone, allowing selective degradation to afford well-defined unimolecular species of two polymeric side chains. Given the wide selection of commercially available diacrylates, RAFT step-growth polymerization of diacrylates will further enable facile synthesis of complex architectures with modular backbones.

Research on controlling cationic polymerization through a degenerative chain-transfer (DCT) process has primarily focused on the family of vinyl ether monomers. To expand the monomer scope, a better understanding on what properties of chain-transfer agents (CTA)s to achieve satisfactory cationic DCT is necessary. In this work, we focused on para-methoxystyrene (pMOS) as the model monomer for electron-rich styrenics and screened a library of CTAs varying in acidity and nucleophilicity. Our results showed that increasing the nucleophilicity of the CTAs significantly improved the control over the cationic DCT polymerization of pMOS. In contrast, acidic CTAs, which provide good control over the cationic DCT polymerization of vinyl ethers, do not exert appreciable control of the cationic DCT polymerization of pMOS. Furthermore, we discovered two new CTAs for controlling the cationic DCT polymerization of pMOS, 1-butaneselenol and benzeneselenol. Lastly, this systematic study allowed us to develop a hypothesis about how the electronic structure of the propagating carbocation dictates the characteristics of a CTA necessary to control the cationic DCT polymerization of a given monomer.

An orthogonal combination of cationic and radical RAFT polymerizations is used to synthesize bottlebrush polymers using two distinct RAFT agents. Selective consumption of the first RAFT agent is used to control the cationic RAFT polymerization of a vinyl ether monomer bearing a secondary dormant RAFT agent, which subsequently allows side‐chain polymers to be grafted from the pendant RAFT agent by a radical‐mediated RAFT polymerization of a different monomer, thus completing the synthesis of bottlebrush polymers. The high efficiency and selectivity of the cationic and radical RAFT polymerizations allow both polymerizations to be conducted in one‐pot tandem without intermediate purification.