Understanding the Limitations of the Solvent-Free Enzymatic Synthesis of Sorbitol-Containing Polyesters

Abstract

Enzymatic catalysis is an attractive approach toward the synthesis of sustainable polyesters, which also provides advantages in terms of selectivity compared to conventional methods. Furthermore, the use of immobilized enzymes allows for solvent-free, ecofriendly polycondensation routes, but also leads to some limitations in terms of applicability to certain systems. A systematic study has been performed on the synthesis of close to linear aliphatic polyesters from biobased, commercially available sorbitol, 1,10-decanediol, and dimethyl adipate. Polycondensation reactions were carried out in the melt using SPRIN liposorb CALB (trade name for the immobilized form of Candida antarctica lipase B) as catalyst, targeting a number-average molecular weight between 4 and 6 kg/mol, and an amount of pendant and terminal hydroxyl groups within the range commonly used for coating applications. The efficacy with which the increasing amounts of sorbitol were built into the polyester backbone was studied in detail via 13C NMR spectroscopy. In addition, the particular selectivity for primary vs secondary hydroxyl groups of the biocatalyst was confirmed via 31P NMR spectroscopy. Extensive structural characterization was carried out via MALDI-ToF-MS analysis, which also provided further insights into limitations of the system related to sorbitol incorporation. Differential scanning calorimetry and X-ray diffraction analysis revealed that the melting temperature and crystallinity of the materials are lower when increased amounts of sorbitol are incorporated into the polyesters.

Green and selective polycondensation methods toward linear sorbitol‐based polyesters: enzymatic versus organic and metal‐based catalysis

“…The possibility of distinguishing between unreacted primary and secondary hydroxyl groups owing to different chemical shifts allows to evaluatet he ratio between them. As shown in our previous study, [22] if this ratio is higher than 4, sorbitol is mostly implemented through its primary hydroxyl groups corroborating as elective esterification for the primary over the secondary hydroxyl groups of the sorbitol unit. As summarized in Ta ble 2, when CALB, TBD, and DBTO weree mployed (entries 1, 5, and 7), the ratio between secondary versus primary hydroxyl groups showedt ob ei nt he same range, that is, be-tween5 .7 and 7.7, with a slightly higherv alue for TBD.…”

“…[20] When SPRIN liposorb CALB (a different immobilizedf orm of CALB) is used as biocatalyst,t he same catalytic activity as found for 10 wt %N ovozyme 435 is obtained by using only 7wt%. [22] Thus 7wt% of SPRIN liposorb CALB with respect to the total amount of monomers was used in our study.T he polymerization reaction conditions are depicted in Scheme2.T hrough elemental analysisi tw as possible to estimate the mol %o fr eal enzyme catalyst that would correspondt o7wt %o fi mmobilized enzyme (detailed calculations can be found in the Supporting Information). The immobilized form of SPRIN liposorb CALB consists of enzyme covalently immobilized on ad ivinylbenzene( DVB) cross-linked hydrophobic macroporous polymerb ased on methacrylic esters.…”

“…For entry 1, when using CALB at 100 8C, an initial feed of 4.8 mol %o fs orbitol leads to an incorporated amount of 3.4 mol %s orbitoli nto the polyester.I no ur prior work, the incorporation of sorbitol in ar elatively apolar polyester system, consisting of dimethyl adipate and 1,10-decanediol, was observed to be quite low. [22] In fact, starting from 5mol %i nitial feed of sorbitol, owing to the non-homogeneous mixturea nd lower affinity shownb yC ALB for sorbitol with respect1 ,10-decanediol, only 1.4mol %o fs orbitol wasf ound to be incorporated into the polyester.T he use of am ore polar diol such as 1,3-PD results in ah omogeneous mixture which favorss orbitol incorporation into the polyester.…”

Synthesis of water-soluble surfactants using catalysed condensation polymerisation in green reaction media

“…Nonetheless, high costs associated with enzymes hinder their widespread industrial use, and this is an important factor to consider when synthesising surfactants for commercial applications. 27,28 Our aim in this work is to look at alternative catalysts that are already established and to compare performance directly with the high degree of control that would be expected from the use of Novozym 435.…”

Lipase-ultrasound assisted synthesis of polyesters

“…Previous studies have been evidencing the poor stability of the enzyme, the slow reaction rates and the low molecular weight of the products. These problems remain to limit large-scale commercial use of lipases [8]. To overcome these limitations, attempts have been made to improve the performance of lipases by the application of several new techniques.…”