Abstract
An innovative enzymatic polycondensation of dicarboxylic acids and dialcohols in aqueous polymerization media using free and immobilized lipases was developed. Various parameters (type of lipases, temperature, pH, stirring type and rate, and monomer carbon chain length) of the polycondensation in an oil-in-water (o/w) miniemulsion (>80% in water) were evaluated. The best results for polycondensation were achieved with an equimolar monomer concentration (0.5 M) of octanedioic acid and 1,8-octanediol in the miniemulsion and water, both at initial pH 5.0 with immobilized Pseudozyma antarctica lipase B (PBLI). The synthesized poly(octamethylene suberate) (POS) in the miniemulsion is characterized by a molecular weight of 7800 g mol−1 and a conversion of 98% at 45 °C after 48 h of polycondensation in batch operation mode. A comparative study of polycondensation using different operation modes (batch and fed-batch), stirring type, and biocatalyst reutilization in the miniemulsion, water, and an organic solvent (cyclohexane:tetrahydrofuran 5:1 v/v) was performed. Regarding the polymer molecular weight and conversion (%), batch operation mode was more appropriate for the synthesis of POS in the miniemulsion and water, and fed-batch operation mode showed better results for polycondensation in the organic solvent. The miniemulsion and water used as polymerization media showed promising potential for enzymatic polycondensation since they presented no enzyme inhibition for high monomer concentrations and excellent POS synthesis reproducibility. The PBLI biocatalyst presented high reutilization capability over seven cycles (conversion > 90%) and high stability equivalent to 72 h at 60 °C on polycondensation in the miniemulsion and water. The benefits of polycondensation in aqueous media using an o/w miniemulsion or water are the origin of the new concept strategy of the green process with a green product that constitutes the core of the new greener polymer-5B technology.
Highlights
The polyester synthesis was carried out in 20 mL capped flasks used as a reactor at 25, 35, 45, or 65 ◦ C under direct magnetic stirring (250 rpm) (reactor with magnetic stirring (RMS)) for 48 h, and 10 mL of the miniemulsion was added to the reaction vessel containing the appropriate amount of free enzymes or immobilized enzymes (5 or 8 mg mL−1, respectively)
Poly(octamethylene suberate) (POS) synthesis was performed between octanedioic acid and 1,8-octanediol at an equimolar concentration (0.5 M) in three different polycondensation media: miniemulsion, water, and an organic solvent system comprising a mixture of cyclohexane and THF (5:1 v/v)
The influence of three different lipases, in free (CRL, Burkholderia cepacia lipase (BCL), PBL) and immobilized (Figure 2B). These results were due to the loss of the enzyme secondary structure with a (CRLI, Burkholderia cepacia lipase (BCLI), and PBLI) forms, on the polyester synthesis from different diacids and diols temperature increase and a parallel decrease in activity in the polycondensation media in a miniemulsion was evaluated
Summary
By direct polycondensation of dicarboxylic acids or their derivatives (hydroxyacids, esters) or diols (mainly dialcohols), more complex and structurally diverse monomers can be employed in comparison to enzymatic ROP [33] Those enzymatic polyester syntheses continue to operate under unfavorable polymerization conditions, such as the ones resulting from using organic solvents and high temperatures, or even when a low vacuum is used to remove the generated by-products, which may denature or inhibit the enzyme [32]. Aqueous miniemulsions have been reported as environmentally benign enzymatic reaction media for polymerization [37,44] In these polymerization media, there is no need to dry reagents before the polymerization, the water produced in dehydrative polyesterification is expelled to the continuous (aqueous) phase, and the biocatalyst stability is improved in comparison to an organic solvent. Lipase-catalyzed polycondensation in a renewed aqueous polycondensation medium, as part of the polymer-5B technology, was applied in this work to synthesize biodegradable and biocompatible polyesters with desirable polymer characteristics and good monomer conversion [48]
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