Orthogonal Cationic and Radical RAFT Polymerizations to Prepare Bottlebrush Polymers

Abstract

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.

Mixed Polymer Brushes for “Smart” Surfaces

“…To eliminate this additional synthetic step and further mitigate the possibility of undesired side reactions and cross-reactivity, research has moved towards orthogonal polymerization techniques. As successful orthogonal polymerizations of block copolymers and bottlebrush copolymers via RDRPs have been reported [117][118][119][120][129][130][131][132][133][134], we can foresee growing opportunities for fabricating MPBs via SI-RDRPs with improved orthogonality.…”

tert‐Butyl Esters as Potential Reversible Chain Transfer Agents for Concurrent Cationic Vinyl‐Addition and Ring‐Opening Copolymerization of Vinyl Ethers and Oxiranes

“…The RAFT process was also applied to the living cationic polymerization of vinyl ethers (VEs) and styrene derivatives. [ 9–17 ]…”

Photoorganocatalyzed Divergent Reversible‐Deactivation Radical Polymerization towards Linear and Branched Fluoropolymers

“…Beyond branched polymers, Boyer [8] and You [9] have demonstrated the employment of photocatalysts to regulate the topology of polymers from linear to graft via photopolymerization with chain transfer agent (CTA) in a two‐step fashion. Although previous examples [10] indicate the importance of matching the electric potential of photoredox catalyst (PC) with initiator/CTA in photoinduced reversible deactivation radical polymerization (photo‐RDRP), [11] a PC controlled divergent pathway toward both linear and branched polymers has not been investigated to our knowledge.…”