Despite their large strain energies and the diversity of synthetically accessible structures, cyclopropenes (CPEs) have rarely been used in ring-opening metathesis polymerization (ROMP). In our investigations of these monomers, we discovered that their polymerization rates and the livingness of polymerization are highly dependent on their substitution patterns and substituent sterics and electronics. Most intriguingly, we synthesized a class of 1,1-disubstituted CPEs that does not homopropagate but rather engages in single addition to the Grubbs third generation metathesis catalyst. With low-strain cyclic olefins, these monomers participate in alternating ROMP, producing polymers with controlled molecular weights, very low dispersities, and regular microstructures, as evidenced by NMR spectroscopy and MALDI-TOF MS. We expanded the system to copolymerize CPEs with dioxepins to produce polyacetals/ketals that degrade into small molecules under acidic conditions.<br/><br/>With their selective single addition, we believed CPEs could be used to add single equivalences of functional groups along a ROMP polymer backbone. While these monomers allowed for the facile and fast synthesis of mono- and heterotelechelic polymers, quantitative reinitiation off a terminal ring-opened CPE was initially challenging due to substituent chelation. However, by modulating the amount of added 3-bromopyridine ligand and reaction temperature, we were able to synthesize polymers with multiple single additions of CPEs. This allowed us to modify polymers at defined locations along the polymer chain and synthesis of block copolymers with complex architectures.

Ring-opening metathesis of cyclopropenes for polymer sequence control

2019 Fall National Meeting

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