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43840-AC7
Copolymerization of Carbon Dioxide with Cyclic Ketene Acetals

Eugene Y.-X. Chen, Colorado State University

Utilization of carbon dioxide as an inexpensive, nonflammable, naturally abundant and renewable monomer for polymer synthesis has been actively pursued by chemists and chemical engineers for more than three decades. Despite recent significant advances made in the carbon dioxide utilization for polymer synthesis, organic comonomers are currently limited to epoxides and aziridines. 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. Specifically, we have taken the following three 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.

First, we examined the direct copolymerization of carbon dioxide with a seven-membered O,O-CKA, 2-methylene-1,3-dioxepane, at varied carbon dioxide pressure (200 to 800 psig), solvent polarity (from toluene to o-dichlorobenzene), and reaction temperature (from 25 to 110 șC). Under these conditions, we obtained only a small fraction of low molecular-weight oligomeric products. Next, we carried these reactions in the presence of electrophilic-nucleophilic ion pairs such as tetrabutylammonium diethylmalonate; such bifunctional ion pairs were proposed to function as both initiator for the ring-opening process and stabilizer for the reactive propagating center (anionic chain end). The preliminary results showed such ion pairs actually shut down the coupling reaction. Third, we investigated the possibility of alternating copolymerization of carbon dioxide with 2-methylene-1,3-dioxepane using N-heterocyclic carbenes as catalyst. Our initial experiments showed that 1,3-bis(2,4,6-trimethyl-phenyl)imidazol-2-ylidene inhibited this copolymerization. Future work will include investigation of the possible copolymerization of carbon dioxide with other cyclic ketene acetal structures varying in ring size and heteroatom substituents.

While investigating other organic comonomers for their possible coupling reactions with carbon dioxide, we accidentally discovered that silyl ketene acetals (SKA), which are commonly employed as initiators for group-transfer polymerization via reductive activation, can be oxidatively activated with a catalytic amount of an olefin polymerization activator, trityl tetrakis(pentafluorophenyl)borate. This novel SKA activation mode directly affords the first methyl methacrylate addition product, namely the highly active propagating species that contains both the nucleophilic and electrophilic catalyst sites to promote the controlled methacrylate polymerization via cooperative catalysis. In addition to its mechanistic distinction in initiation and propagation, this new organic polymerization system readily produces polymethacrylates of low to high molecular weights with narrow molecular weight distributions at ambient temperature. The details of these findings were included in a manuscript currently under review.

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