Kinetic study of TBD catalyzed δ-valerolactone polymerization using a gas-driven droplet flow reactor

Since their discovery, aliphatic polyesters have found numerous applications in medical fields owing to their excellent biocompatibility and biodegradability. Presently, the ring-opening based polymerization process is widely used in the synthesis of polyesters in the presence of catalysts. These catalysts are mostly organic and are categorized as monofunctional and bifunctional catalysts. 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) is a good example of bifunctional catalysts that can be used to speed up the ring-opening polymerization process. Additionally, it has a simple molecular structure hence highly preferred for industrial applications. However, the ring-opening polymerization reaction under TBD catalysts requires quenching to obtain high-quality polymers. As such, there is a high need for precious control of the reaction process which requires an in-depth understanding of the kinetics of TBD catalyzed ring-opening polymerizations.

To this end, Shiyao Lu and Kai Wang from Tsinghua University recently extended the use of droplet flow reactors in carrying out polymerization processes. In particular, they developed a gas-driven droplet flow reactor for investigating the kinetics of the TBD catalyzed δ-valerolactone ring-opening polymerization reaction with very little assumption of reactants. Their main objective was to achieve more controllable preparation of aliphatic polyester materials based on the provided kinetic parameters. The work is currently published in the research journal, Reaction Chemistry & Engineering.

In brief, the experimental platform was constructed from simple valves and low-cost but highly efficient tubing. The microreactor platform was validated based on systematic experiments aimed at showing the variation of the δ-valerolactone conversion and polyvalerolactone molecular weights. On the other hand, the droplet residence time was changed by varying the flow rate of high purity nitrogen which in turn varied the velocity flow in the droplet.

With ring-opening polymerization of δ-valerolactone as a model, it was possible to obtain the reaction orders of the TBD as well as the end hydroxyl of the polymer. Even though only a 2 ml reaction solution was used for each test, it significantly reduced the work involved in preparing the anhydrous reactants. Also, for the kinetic model, the orders of the catalyst and alcohol were observed to be close to 1 and 0 respectively. This indicated the activation reaction between the catalysts and the monomer otherwise represented by the activation energy and the rate-determining step which was concluded to be the reaction between the TBD and δ-VL.

The reaction of the activated monomer and the end hydroxyl in the polymer chain was enough to complete the reaction even though the activation energy of the rate-determining reaction was reported to be approximately 7.302 kJ mol^-1. In general, a more detailed mechanism of the TBD catalyzed ring-opening polymerization was presented based on the kinetic parameters. These insights were of great importance in achieving controllable preparation of aliphatic polyester material.  Lu -Wang study will, therefore, advance the use of droplet flow reactors in studying of the mechanisms involved in different polymerization reactions.