43840-AC7
Copolymerization of Carbon Dioxide with Cyclic Ketene Acetals
With support from the PRF-AC grant, we investigated several approaches to examine the feasibility of alternating ring-opening copolymerizations of carbon dioxide with cyclic ketene acetals (CKA) for the synthesis of biomedically important aliphatic polyesters, poly(ester-amide)s and poly(ester-carbodiimide)s. Utilization of carbon dioxide as an inexpensive, nonflammable, naturally abundant and renewable monomer for polymer synthesis has attacked increasing interest due to its relevance to sustainability, and this PRF support enabled us to explore possible new coupling reactions of carbon dioxide with other copolymerizable organic monomers for the production of useful macromolecular materials.
We initially examined the direct copolymerization of carbon dioxide with a seven-membered O,O-CKA, 2-methylene-1,3-dioxepane, at varied carbon dioxide pressure, solvent polarity, and reaction temperature. Under all the conditions employed, we obtained only a small fraction of low molecular-weight oligomeric products. Next, we carried out these reactions in the presence of electrophilic-nucleophilic ion pairs such as tetrabutylammonium diethylmalonate which are proposed to function as both initiator for the ring-opening process and stabilizer for the reactive propagating center; our results showed such ion pairs actually shut down the coupling reaction. Furthermore, we investigated the copolymerization of carbon dioxide with 2-methylene-1,3-dioxepane using N-heterocyclic carbenes (NHCs) as catalyst, but we found that such NHCs actually inhibited this coupling reaction. Lastly, we varied the cyclic ketene acetal structures in terms of ring size and heteroatom substituents and did not observe the formation of high molecular weight polymers upon copolymerization with carbon dioxide under various conditions.
However, we discovered a new polymerization reaction while investigating other organic comonomers for their possible coupling reactions with carbon dioxide. Specifically, we discovered that silyl ketene acetals (SKA) can be oxidatively activated with a catalytic amount of Ph3CB(C6F5)4. The intriguing, “monomer-less” initiation involves oxidative activation of SKA by Ph3CB(C6F5)4, leading to the Me3Si+-activated MMA (methyl methacrylate) derived from vinylogous hydride abstraction of SKA with Ph3C+; subsequent Michael addition of MeSKA to the activated MMA affords the highly active, ambiphilic propagating species containing both nucleophilic SKA and electrophilic silylium ion sites. A propagation “catalysis” cycle consists of a fast step of recapturing the silylium catalyst from the ester group of the growing polymer chain by the incoming MMA, followed by a rate-determining step of the C–C bond formation via intermolecular Michael addition of the polymeric SKA to the silylated MMA. This polymerization is also a living/controlled at ambient temperature, readily producing PMMA of low to high Mn (> 105) with narrow MWDs evidenced by PDI values of 1.04–1.12, with a Ph3CB(C6F5)4 activator loading as low as 0.025 mol% relative to monomer. Most recently, we investigated structure–property relationships of this novel polymerization system. Of several valuable findings obtained from this study, a most interesting and significant result is that there exhibits remarkable selectivity of the silyl group structure of the acetal initiator (and thus the derived silylium ion catalyst) for monomer structure: initiators having small silyl groups promote highly active and efficient polymerization of methacrylates, but they are poor initiators for polymerization of less sterically hindered, active α-H bearing acrylate monomers. On the other hand, initiators incorporating bulky silyl groups such as the triisobutylsilyl derivative, exhibit low activity toward methacrylate polymerization but exceptionally high activity (completed reaction in < 1 min), efficiency (achieved quantitative initiator efficiency), and degree of control (regulated low to high Mn (> 105) with narrow MWDs), for acrylate polymerization at ambient temperature in polar noncoordinating or nonpolar hydrocarbon solvents. We published these finding in two Macromolecules papers, acknowledging the PRF-AC support.
