Abstract
Among the various catalysts that can be used for polycondensation reactions, enzymes have been gaining interest for three decades, offering a green and eco-friendly platform towards the sustainable design of renewable polyesters. However, limitations imposed by their delicate nature, render them less addressed. As a case study, we compare herein bulk and solution polycondensation of 1,6-hexanediol and diethyl adipate catalyzed by an immobilized lipase from Candida antarctica. The influence of various parameters including time, temperature, enzyme loading, and vacuum was assessed in the frame of a two-step polymerization with the help of response surface methodology, a statistical technique that investigates relations between input and output variables. Results in solution (diphenyl ether) and bulk conditions showed that a two-hour reaction time was enough to allow adequate oligomer growth for the first step conducted under atmospheric pressure at 100 °C. The number-average molecular weight (Mn) achieved varied between 5000 and 12,000 g·mol−1 after a 24 h reaction and up to 18,500 g∙mol−1 after 48 h. The statistical analysis showed that vacuum was the most influential factor affecting the Mn in diphenyl ether. In sharp contrast, enzyme loading was found to be the most influential parameter in bulk conditions. Recyclability in bulk conditions showed a constant Mn of the polyester over three cycles, while a 17% decrease was noticed in solution. The following work finally introduced a statistical approach that can adequately predict the Mn of poly(hexylene adipate) based on the choice of parameter levels, providing a handy tool in the synthesis of polyesters where the control of molecular weight is of importance.
Highlights
Aliphatic and aromatic polyesters are more and more receiving special attention as biobased alternatives to petropolymers due to their vast availability, large range of applications, and to some of them, their biodegradability [1,2,3,4]
The most popular biocatalyst used is Novozym 435 (N435—lipase B from Candida Antarctica immobilized on an acrylic resin), where it showed improved properties in terms of specificity, thermal stability and selectivity, and was proposed in many research works as a versatile catalyst that can be beneficial in different synthetic routes, in the case of aliphatic polyesters [17,18]
N435 was reported as a catalyst for the combination of ring-opening polymerization and polycondensation reactions using glycidol, ω-pentadecalactone and adipic acid as starting materials [21]
Summary
Aliphatic and aromatic polyesters are more and more receiving special attention as biobased alternatives to petropolymers due to their vast availability, large range of applications, and to some of them, their biodegradability [1,2,3,4]. Zeng et al showed that in the polycondensation of glycerol, 1,8-octanediol and adipic acid, N435 produced a close to linear polyester, where the selectivity of acylation at the primary hydroxyl sites of glycerol was 74.9%, and the number average molecular weight (Mn ) achieved was 22,700 g·mol−1. Those results were superior to other catalysts such as scandium triflate and organocatalysts such as 1,5,7-triazabicyclo [4,4,0]dec-5-ene (TBD), diphenyl hydrogen phosphate and bis(1,1,2,2,3,3,4,4,4-nonafluoro-1-butanesulfonyl)imide, which did not exceed a selectivity of 65.9% or yield molecular weights higher than 6000 g·mol−1 [24]
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