Sara Orski, Santanu Kundu, Richard Gross, and Kathryn Beers

The ring–opening polymerization (ROP) of lactones by enzymatic catalysts, such as Candida antarctica Lipase B (CAL B), provides a greener alternative to synthesize biodegradable polyesters over traditional metal-catalyzed reactions.  Heterogeneous catalysts, where the enzyme is immobilized on a solid support resin, can allow the enzyme to be easily isolated from the reaction mixture and reused for subsequent reactions. ROP reaction conditions, such as temperature and trace water content, play a critical role in controlling polymerization kinetics and molecular weight, but also can influence the enzyme stability on the resin. Catalyst desorption occurs due to the weak hydrophobic interactions between CAL B and the crosslinked poly(methyl methacrylate) (PMMA) resin surface. Leaching of CAL B from the solid support decreases catalyst concentration and contaminates the polyester product. Catalyst stability is therefore dependent on the physiochemical interaction between CAL B and the PMMA surface.  The complex structure of the porous 3D PMMA resin makes systematic characterization of CAL B/PMMA interfacial stability convoluted. A direct and quantitative method is needed to evaluate stability of the catalyst/polymer solid support interface decoupled from the 3D porous resin and mechanical stresses sustained by the heterogeneous catalyst during polymerization.

Quantitative study of the catalyst/polymer surface microenvironment was conducted using a quartz crystal microbalance with dissipation monitoring (QCM-D). A simplified 2D experimental model of the CAL B/PMMA interface was fabricated using a photo crosslinked PMMA thin film immobilized on a quartz crystal sensor.  In situ changes to enzyme mass and viscoelastic properties were monitored as experimental conditions were varied. Enzyme surface stability was evaluated with increasing water content of toluene and polycaprolactone solutions, and with increasing reactor temperature, mimicking reaction environments where enzyme leaching and changing enzymatic activity has been previously demonstrated. The CAL B/PMMA interface demonstrated up to 20 % loss of enzyme catalyst with increasing trace water content.  Increased polycaprolactone (PCL) binding at the enzyme surface was also observed, indicating greater PCL affinity for a more hydrated enzyme active site.  The enzyme layer destabilized with increasing temperature, yielding almost complete reversible catalyst desorption in the model. Determination of optimal reaction conditions at the polymer/enzyme interface can further control of polymerization and improve catalyst retention throughout the reaction. Furthermore, understanding the enzyme/polymer interface is a fundamental step in developing next generation biocatalysts with improved stability and recyclability for commercialization.