Metal-Free Photoredox-Mediated Ring Opening Metathesis Polymerization

Abstract

Metal-free, photoredox-mediated ring-opening metathesis polymerization (MF-ROMP) is a novel approach to the synthesis of ROMP polymers. Whereas traditional, metal-mediated ROMP uses well-defined transition metal-alkylidene initiators in stoichiometric amounts, MF-ROMP uses a photo-oxidant to oxidize an organic vinyl ether initiator. This obviates the need for purification to remove residual metal species for certain applications, such as drug delivery and organic electronics. A variety of pyrylium and thiopyrylium photo-oxidants have been evaluated for efficacy in MF-ROMP. Generally, the more electron-rich (thio)pyrylium salts, which are milder oxidants, achieved higher conversions of norbornene in MF-ROMP, with thiopyrylium salts out-performing corresponding pyrylium salts in both rate and conversion. While 2,4,6-tris(4-methoxyphenyl)pyrylium tetrafluoroborate had been the workhorse for research to date, the corresponding thiopryrylium emerged as a candidate for applications requiring rapidly reaching critical conversion, such as applications requiring gelation or crosslinking. Inspired by the broad functional group tolerance of metal-mediated ROMP, we sought to expand the scope of MF-ROMP, with particular emphasis on alcohol functionality. We were initially encouraged by the results of additive screenings, which demonstrated that the polymerization was tolerant to sterically hindered alcohols. Comonomers with polar functionality incorporated in copolymerization to varying extents. Remarkably, the TBS-ether functionalized norbornene both homopolymerized and copolymerized with norbornene, with quantitative incorporation in the case of copolymerization. The resulting polymers was deprotected in solution with TBAF. Notably, Boc-carbonate-functionalized norbornene copolymerized with norbornene, and was subsequently deprotected in the solid state to yield an alcohol-functionalized polynorbornene. Interestingly, 2-chloro-2,3-dihydro-endo-dicyclopentadiene did not copolymerize. Monitoring the homopolymerization revealed a low rate of conversion and a sigmoidal shape to the conversion vs. time plot, which is consistent with slow initiation kinetics. Each of these polar monomers can facilitate post-polymerization functionalization. An investigation of the stereochemistry of MF-ROMP polymers revealed predominantly trans olefin geometry. Quantitative 13C NMR spectroscopy was used to determine that MF-ROMP polymers are atactic. Additionally, block copolymers were synthesized via sequential addition of monomers, consistent with living characteristics. While the unique vinyl ether chain end of MF-ROMP can offer synthetic advantages, minimizing initiator loading improves cost effectiveness. Additionally, low-molecular weight polymers are desirable for certain applications, such as thermosetting. We demonstrated that chain transfer with α-olefins cleaved the vinyl ether chain ends via metathesis and offered molecular weight control.