RAFT polymerization of renewable monomers with dithiobenzoates: Effect of Z-group substituents and reaction conditions

Abstract

The use of renewable polymer feedstocks has attracted significant attention recently due to increased concerns about petrochemical-based materials and limited fossil-based resources. As a promising candidate, lignin-derived monomers have been polymerized via controlled radical polymerization to yield bio-based materials. Arguably, the most commonly employed polymerization methodology is reversible-addition-fragmentation chain-transfer (RAFT) polymerization which utilizes, almost exclusively, 2-cyano-2-propyl benzodithioate as a chain transfer agent (CTA). Despite the robustness of this approach, polymerization of lignin-derived methacrylates is far more challenging than of the commonly used monomers (e.g. methyl methacrylate), often resulting in polymers with moderate control over the molecular weight and dispersity (Đ). To improve control over the RAFT polymerization, we investigate here a series of 2-cyano-2-propyl benzodithioate derivatives, where the electronic properties of the Z group were systematically altered. The introduction of electron-withdrawing groups in the para-position of the phenyl ring of the Z group increased control over the polymerization by improving the chain transfer coefficient of the CTA, resulting in enhanced control over molecular weights and lower Đs. Conversely, the incorporation of electron-donating groups had the opposite effect. 2-cyano-2-propyl 4-cyanobenzodithioate was found to be the optimal CTA for the polymerization of a series of lignin-derived monomers resulting in Đs < 1.25. The robustness of the system was highlighted by the synthesis of higher molecular weights polymers and the efficient chain-extension of macroCTAs; the latter case yielding bio-based block copolymers with a Đ as low as 1.16. This work highlights that RAFT polymerization can, upon judicious optimization, be successfully employed for the synthesis of renewable polymers.