Expanded Scope of Methodology and Polymer Functionality for Photoredox-Mediated Ring-Opening Metathesis Polymerization

Photoredox-mediated ring-opening metathesis polymerization (Photo-ROMP) is an emerging polymerization method. In contrast to traditional ROMP, which uses a well-defined metal initiator (typically Ru, W, or Mo) that covalently bonds in stoichiometric amounts with each polymer chain end, Photo-ROMP utilizes a photo-oxidant to oxidize an organic polymerization initiator. This obviates the need for purification processes to remove the metal initiator for use of the resulting polymer in biomedical, electronics, and optics applications. We have recently studied the efficacy of a variety of photo-oxidants and expanded the monomer scope to include polar functionality. In an evaluation of pyrylium and thiopyrylium photo-oxidants, more electron-rich photo-oxidants (weaker photo-oxidants) achieved higher conversion to polynorbornene. Additionally, thiopyrylium photo-oxidants displayed higher rates of conversion than similarly functionalized pyryliums. While our previous studies identified 2,4,6-tris(4-methoxyphenyl)pyrylium tetrafluoroborate as an effective photo-oxidant for Photo-ROMP, in our recent evaluation of additional pyryliums and thiopyryliums, the correspondingly functionalized thiopyrylium emerged as a successful alternative for achieving rapid polymerization. In addition to evaluating various pyrylium and thiopyrylium photo-oxidants, we recently identified a variety of monomers and co-monomers with polar functionality for Photo-ROMP. Our previous studies identified norbornene, dicyclopentadiene, and dihydrodicyclopentadiene as effective monomers for Photo-ROMP. Recently, we've identified several new monomers. Remarkably, a TBS-ether-funtioncalized norbornene was co-polymerized with norbornene and homopolymerized in good yields. The resulting polymer was successfully deprotected to give the corresponding alcohol-functionalized polynorbornene. Additionally, a Boc-functionalized copolymer with norbornene was successfully synthesized and the Boc groups subsequently thermally removed. Interestingly, while 2-chloro-2,3-dihydro- endo -dicylopentadiene was precluded from copolymerization with norbornene due to slow initiation kinetics, it did successfully homopolymerize via Photo-ROMP to give the resulting homopolymer in moderate yields. These functionalized monomers could facilitate post-polymerization modification. In addition to these recent developments in expanding the methods and scope of Photo-ROMP, ongoing work in evaluating alternative photo-oxidants and monomers will also be discussed in this presentation.