Discriminatory Photoactivation of Diastereomeric RAFT Agents

Abstract

Reversible deactivation radical polymerization (RDRP) techniques regulate free radical polymerization through a process of reversible activation/deactivation of propagating radicals, which thus provides dormant polymer chains with unique capping moieties (alkoxyamine, halogen, thiocarbonylthio, etc.) as chain ends. The dormant chain ends are the reactive sites of initiation, propagation, and termination, and therefore their chemical structures are crucial for the mechanism and kinetics of RDRP. The investigation of stereochemical effects of dormant chain ends is challenging due to limited models that are highly accessible and able to represent long polymer chains. Herein, we employ a single unit monomer insertion (SUMI) technology to prepare ideal models (oligomeric macro-RAFT agents) for the investigation of the stereochemical effect on their photoactivation. Two diastereomeric macro-RAFT agents were prepared in high yield by SUMI, which were then photoactivated for photoinduced electron/energy transfer-reversible addition- fragmentation chain-transfer (PET-RAFT) polymerization under visible light. Discriminatory activation kinetics was observed with one diastereomer much faster than the other. The mechanistic explanation of such interesting behavior was suggested with the aid of quantum chemical calculations. Higher C–S bond dissociation energy and greater stability contributed to slower photoactivation of one of diastereomeric RAFT agents as compared to the other.

Single Unit Monomer Insertion: A Versatile Platform for Molecular Engineering through Radical Addition Reactions and Polymerization

“…It is surprising to find that these diastereomeric RAFT agents presented distinct activation behavior in the presence of visible light (red LED) and a photocatalyst. As a result, the conversion of Dia2 with irradiation time (Figure b) was much faster than that of Dia1 due to distinct C–S bond dissociation energies for Dia1 and Dia2, as confirmed by quantum chemical calculations . These findings demonstrated the significant stereochemical effect of RAFT agents on the initiation and mediation of photo-RAFT polymerization.…”

“…The integral of the peak signal at δ 1.0 ppm is defined as 3.0, which is assigned to the methyl group on the hydrocarbon tail of −C 12 H 25 . Reproduced with permission from ref .…”

“…For clarity, the stereoconfigurations of Dia1 ( R , R ) and Dia2 ( R , S ) were excluded due to the same reactivity as their enantiomeric counterparts of Dia1 ( S , S ) and Dia2 ( S , R ). Reproduced with permission from ref .…”

Sequential and alternating RAFT single unit monomer insertion: model trimers as the guide for discrete oligomer synthesis

“…SUMI has been recognized as a powerful tool in engineering radical addition reactions and polymerization, not only in sequence-defined polymer synthesis, but also for organic transformations, stereochemistry regulation and the kinetic investigation of free radical polymerization. 16,23 The history of SUMI can be dated back to the 1980s to the discovery of unimer formation from reactions between alkoxyamine and vinyl monomers, 24,25 and facile radical generation from xanthates and subsequent radical addition to various alkenes, 26 although it was initially not termed "SUMI". With the advances of RDRP, RDRP SUMI approaches have attracted increasing attentions to prepare functional RDRP initiators, 17,[27][28][29] polymer end-group or mid-chain functionalization [30][31][32][33][34] as well as sequenced-defined polymers.…”

“…[37][38][39][40][41][42] Our group has been focussing on photochemical approaches to activate and mediate RAFT SUMI ( photo-RAFT SUMI) including photoiniferter and photoinduced electron/ energy transfer (PET)-RAFT processes by visible light. 16,18,19,23,[43][44][45] The photo-activation of RAFT agents to generate initiating radicals is able to effectively suppress exogenous initiator-derived by-products in comparison to conventional RAFT polymerization. Meanwhile, the use of visible light allows the minimization of RAFT decomposition commonly observed under UV light irradiation in photoiniferter polymerization involving xanthates and dithioesters.…”