Alternating Copolymerization of Propylene Oxide and Cyclohexene Oxide with Tricyclic Anhydrides: Access to Partially Renewable Aliphatic Polyesters with High Glass Transition Temperatures

Renewable, biodegradable polymers, such as aliphatic polyesters, based on sustainable sources have attracted considerable interest as alternatives to petroleum-based polymers. One limiting factor in the development of aliphatic polyesters as replacements for these materials has been their relatively low glass transition temperatures (<i>T</i>_g). For example, commercially available poly­(lactic acid) has a <i>T</i>_g of approximately 60 °C. Epoxide/anhydride copolymerizations offer an alternative to the ring-opening polymerization of lactones for the synthesis of aliphatic polyesters and allow for tuning of polymer properties through two distinct monomer sets. We synthesized six partially or fully renewable tricyclic anhydrides and copolymerized them with propylene oxide (PO) and cyclohexene oxide (CHO). By varying both the epoxide and the anhydride, we were able to tune the <i>T</i>_g of the resulting polymers over a nearly 120 °C range from 66 °C to an exceptionally high 184 °C. Polymers produced with PO had a lower range of <i>T</i>_g values (66–108 °C) and higher molecular weights up to 32.2 kDa, while those produced with CHO had higher <i>T</i>_g values (124–184 °C) and lower molecular weights, showing the profound influence of both monomer sets. To the best of our knowledge, these are the highest <i>T</i>_g values reported for entirely aliphatic polyesters

Mechanistic Insights into the Alternating Copolymerization of Epoxides and Cyclic Anhydrides Using a (Salph)AlCl and Iminium Salt Catalytic System

Mechanistic studies involving synergistic experiment and theory were performed on the perfectly alternating copolymerization of 1-butene oxide and carbic anhydride using a (salph)­AlCl/[PPN]Cl catalytic pair. These studies showed a first-order dependence of the polymerization rate on the epoxide, a zero-order dependence on the cyclic anhydride, and a first-order dependence on the catalyst only if the two members of the catalytic pair are treated as a single unit. Studies of model complexes showed that a mixed alkoxide/carboxylate aluminum intermediate preferentially opens cyclic anhydride over epoxide. In addition, ring-opening of epoxide by an intermediate comprising multiple carboxylates was found to be rate-determining. On the basis of the experimental results and analysis by DFT calculations, a mechanism involving two catalytic cycles is proposed wherein the alternating copolymerization proceeds via intermediates that have carboxylate ligation in common, and a secondary cycle involving a bis-alkoxide species is avoided, thus explaining the lack of side reactions until the polymerization is complete