Metal-Free ROMP

Photoredox-mediated metal-free ring-opening metathesis polymerization (MF-ROMP) is an alternative to traditional metal-mediated ROMP that avoids the use of transition metal initiators while also enabling temporal control over the polymerization. Herein, we explore the effect of various additives on the success of the polymerization in order to optimize reaction protocols and identify new functionalized monomers that can be utilized in MF-ROMP. The use of protected alcohol monomers allows for homo- and copolymers to be prepared that contain functionality beyond simple alkyl groups. Several other functional groups are also tolerated to varying degrees and offer insight into future directions for expansion of monomer scope.

Systematically varied pyrylium and thiopyrylium photo-oxidants have been evaluated in the metal-free ring-opening-metathesis polymerization (MF-ROMP) of norbornene. Across the series, we observed higher conversion into polynorbornene from thiopyrylium species in comparison with pyrylium salts that were otherwise similarly functionalized. Additionally, more electron-rich photo-oxidants (i.e., weaker oxidants) correlated with higher conversions.

This account details externally regulated ring-opening metathesis polymerization (ROMP) methods. Various external stimuli are discussed which collectively span chemical, thermal, photochemical, electrochemical, and mechanical modes of catalyst activation. Specific attention is also given to the recent development of a metal-free approach to ROMP that includes electro-organic and photoredox-mediated systems.

Metal-free ring-opening metathesis polymerization (ROMP) utilizes organic photoredox mediators as alternatives to traditional metal-based ROMP initiators to allow the preparation of polymers without residual metal contamination. Herein we report studies exploring the use of endo-dicyclopentadiene (DCPD), a common ROMP monomer, to form linear polyDCPD and copolymers with norbornene. Subsequent cross-linking of the linear polyDCPD using thiol–ene chemistry allows for a completely metal-free preparation of cross-linked polyDCPD. Furthermore, the examination of a number of structurally related monomers offers insights into mechanistic details of this polymerization and demonstrates new monomers that can be utilized for metal-free ROMP.

We have developed a method to achieve ring-opening metathesis polymerization (ROMP) mediated by oxidation of organic initiators in the absence of any transition metals. Radical cations, generated via one-electron oxidation of vinyl ethers, were found to react with norbornene to give polymeric species with microstructures essentially identical to those traditionally obtained via metal-mediated ROMP. We found that vinyl ether oxidation could be accomplished under mild conditions using an organic photoredox mediator. This led to high yields of polymer and generally good correlation between Mn values and initial monomer to catalyst loadings. Moreover, temporal control over reinitiation of polymer growth was achieved during on/off cycles of light exposure. This method demonstrates the first metal-free method for controlled ROMP.