Abstract
This study investigated the effect of enzyme immobilization on its activity for the ring-opening polymerizations of lactones with differing ring sizes. Candida antarctica lipase B (CALB) was immobilized on a wide array of support materials that varied in polymer composition, relative hydrophobicity, pore diameter, and surface area. The reaction rates and polymer molecular weights were monitored by in situ NMR measurements. Synchrotron infrared microspectroscopy(SIRMS) was used to study the enzyme distribution and secondary structure within the support matrix. The fastest reaction rates resulted by immobilization of CALB on the macroporous matrices QDM 2-3-4 and Accurel. Both of these are constructed from polypropylene and had CALB distributed throughout the matrix particles. Immobilized CALB on QDM 2-3-4 gave ϵ-caprolactone (ϵ-CL) conversion that reached 50 and 95% in 12 and 60 min, respectively. In comparison, by using CALB immobilized on Deloxan HAP, a 50% CL conversion required 120 h. Other changes that were found by immobilization of CALB on different matrices are shifts in CALB selectivity and the frequency of step-condensation reactions at monomer conversions>50%. By changing the immobilization matrix, PCL molecular weight was increased from 5,000 to 15,000. All of the CALB-matrix systems investigated in this study gave polymerizations with ϵ-CL conversions that followed a first-order rate law. Changes in CALB catalytic activity and specificity imply variations in CALB conformation induced by the different surfaces. However, these conformational changes must be subtle since they were not found by synchrotron IR Microspectroscopy. Two common features observed that led to CALB-matrix systems with higher activity for polyester synthesis are: i) increased density of CALB molecules within the pores of matrices, and ii) distribution of CALB throughout the matrix.
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