The Power of RAFT for Creating Polymers Having Imbedded Side-Chain Functionalities: Norbornenyl-Functionalized Polymers and their Transformations via ROMP and Thiol-ene Reactions
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abstract
Robust, efficient and orthogonal chemistries are becoming increasingly important tools for the construction of increasingly sophisticated materials. In this article, the selectivity of reversible additionfragmentation chain transfer (RAFT)-based radical polymerization of bifunctional monomers is exploited for the preparation of statistical and block copolymers that contain imbedded side-chain functionalities, which are then shown to exhibit two different orthogonal types of chemical reactivity to afford discrete nanoscale objects and functional derivative structures. Based on the radical reactivity ratios calculated from AlfreyPrice theory, a bifunctional monomer 4-(5-norbornene-2-methoxy)-2,3,5,6-tetrafluorostyrene (1) was designed and synthesized, for its highly reactive tetrafluorostyrenyl group relative to its norbornene (Nb) group. Selective RAFT copolymerization of 1 with styrene (St) afforded copolymers with over 50 mol-% structural units having a pendent norbornenyl functionality while maintaining narrow molecular weight distribution (polydispersity index (PDI) = 1.23). Diblock copolymers (PDI = 1.091.23) with Nb side-chain substituents regioselectively placed along one segment of the block copolymer structure were also prepared by RAFT copolymerizations of 1 with St or 2,3,4,5,6-pentafluorostyrene, using either polystyrene or poly(styrene-alt-maleic anhydride)-based macro chain-transfer agents. A well-defined star block copolymer (PDI = 1.23) having a poly(norbornene)-based core and polystyrene arms was obtained by ring-opening metathesis polymerization using the regioselective diblock copolymer PSt-b-P(1-co-St) as the multifunctional macromonomer and Grubbs catalyst (first generation) as the initiator. Photo-induced thiol-ene reactions of Nb-functionalized polymers with thiols were fast and efficient, yielding polymers with new side-chain structures.
