CALB- Catalyzed Polymer Synthesis

The activity of both Novozym 435 and crosslinked Novozym 435 was measured using the standard PLU-assay. The PLU (propyl laurate units) assay is a method to determine the activity of immobilized lipases by ester synthesis. One PLU corresponds to 1 pmol of propyl laurate formed from condensation of lauric acid and 1-propanol per gram of enzyme per minute under specified conditions. The PLU value is determined by measuring the area under the peak of formed propyl laurate versus a standard sample of propyl laurate with known concentration using a gas chromatograph with an FID detector. Crosslinked Novozym 435 showed an activity of 4800 PLU g 1, whereas the activity of Novozym 435 before crosslinking was measured to be 12200 Pl.U g 1. 

Polycaprolactone synthesis was performed in two scales preparative, and screening scale. In preparative scale, synthesis was performed in a three-neck flask with an overhead mechanical stirrer and a temperature controller in presence of an anhydrous solvent. Toluene was selected as the best solvent for PCL synthesis [24], In screening scale, synthesis was performed in smaller scale inside glass vials in a J-Kem parallel synthesizer with either 6-vial or 12-vial reactor. 

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Immobilized CALB has frequently been applied in the literature as a catalyst for polymerization of aliphatic polyesters, polycarbonates, polyurethanes and their copolymers. In the present work on CALB catalyzed polymerization, the ring opening of e-caprolactone to polycaprolactone was selected as the model polymerization reaction (Figure 3.3). This model reaction has been well established in the literature [24-27] as an example of a polymerization reaction that can be successfully catalyzed by immobilized lipases (see also Chapter 4). Polymer synthesis and characterization was performed in four steps (i) polymerization (ii) separation (iii) purification and (iv) characterization. 

Lipase-catalyzed polyester synthesis has received considerable interest [31] due to the harsh conditions used in traditional chemical polymerization (> 200 C), and has also been subjected to reaction in ILs. Both CaLB and PcL lipases have been found to catalyze polyester synthesis in [BMIM][BF4] and [BMIM][PF6], but this approach does not seem to offer any advantages over state-of-the-art lipase-mediated polyester synthesis [32, 33] [Eq. (3)]. Still, the polydispersity index of the polymers formed in ILs were remarkably affected, giving values close to 1, which indicate a very narrow molecular weight range compared to material prepared by conventional polymerization processes [33]. 

For instance, in the last decade synthesis of poly(ester-alt-ether) was intensively studied. A common enzyme used in these syntheses is CALB. Polymerization of l,5-dioxepan-2-one (DXO) was performed by enzyme-catalyzed ROP in order to avoid contamination of product polymers by toxic organometallic catalysts [92], High molecular weight of poly(DXO) was obtained (Mn = 56000 Mw = 112000, 97% yield) at 60 °C for 4h. The polymerization had the characteristics of a living polymerization, as indicated by the linearity of plots between M and monomer conversion, meaning that the product molecular weight could be controlled by the stoichiometry of the reactants. Similarly, Nishida et al. [91] carried out enzymatic ROP of l,4-dioxan-2-one at 60 °C catalyzed by Novozym 435 that resulted in a polymer with Mw = 41000 in 77% yield. 

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