Ring-opening metathesis polymerization of cyclopropenes and ladderenes
Keywords: Chemistry.

The importance of polymer microstructure is manifested in nature, where the precisely regulated monomer sequence in biomacromolecules enables their controlled folding and assembly to afford vital biological functions. In the pursuit of man-made materials with sophisticated properties, the field of polymer chemistry has developed rapidly to enable the synthesis of polymers with controlled microstructures. Specifically, living polymerization techniques, such as ring-opening metathesis polymerization (ROMP), enable excellent control over polymer molecular weight, composition, and architecture. ROMP is driven primarily by the release of ring strain in cyclic olefin monomers, but surprisingly few studies have been performed on the ROMP of cyclopropenes (CPEs), the most strained monocyclic olefins. In 2015, our group discovered that a class of 1,1-disubstituted CPE undergoes selective single addition to Grubbs third generation catalyst. Monomers which undergo single addition are extremely rare for chain-growth polymerizations and are particularly useful for manipulating the monomer sequence in a polymer. As such, we focused our research efforts on discovering other CPEs that can undergo single addition and uncovering the mechanism and role of their substituents. We have synthesized a library of CPEs with various disubstitution patterns and substituents and systematically investigated their reactivity with Grubbs catalyst. We have found that 1,1-disubstitution of CPE is crucial for both preserving the stability of the propagating chain end and tuning the metathesis reactivity from living polymerization to single addition. The distinct reactivities stemmed from differences in sterics and/or chelation at the Ru alkylidene from C1 substituents after a single CPE ring-opening event, affecting the barrier to propagation. We have utilized the single addition reactivity of CPEs to synthesize alternating copolymers with diverse side chain and backbone functionalities from alternating ROMP of CPE with low-strain cyclic olefins. Recently, we have developed a strategy to precisely place discrete functionalities and side chains via ring-opened CPEs at pre-determined locations along a living ROMP polymer chain, with control over the location and number incorporated. This advance in polymer chemistry opens many exciting opportunities to manipulate the functionalities along well-controlled polymer chains for understanding the effects of their placement and sequence on polymer behaviors, controlling polymer folding/assembly, as well as synthesizing polymers with more complex nonlinear architectures with precision. Our group also recently reported a unique polymechanophore system, polyladderene, that undergoes force-triggered rearrangement into semi-conducting, insoluble polyacetylene. A notable feature in the design was the terminal strained cyclobutene on ladderene that allowed rapid ROMP. We have developed synthetic procedures to prepare triblock copolymers containing mechanically active polyladderene, since block copolymers can self-assemble in solution and bulk and facilitate incorporation of polyladderene with common polymers to impart the dramatic stress-response of polyladderene to diverse materials