Enzymatic Synthesis of Biobased Polyesters with Aromatic Diols

Biocatalyzed synthesis of furan and pyridine diols-based polyesters. Tunable crystallinity based on the aliphatic diester used for their synthesis. Complete characterization and analysis of the obtained materials.

In the present work, the biocatalyzed synthesis of a series of aromatic-aliphatic polyesters based on the aliphatic diesters dimethyl succinate, dimethyl adipate and dimethyl sebacate and the aromatic diols 2,5-bis(hydroxymethyl)furan, 3,4-bis(hydroxymethyl)furan and 2,6-pyridinedimethanol were investigated.

A similar series of polyesters based on the petroleum-based 1,3-benzenedimethanol, 1,4-benzenedimethanol and 1,4-benzenediethanol were also synthesized for comparison.

Data show that the enzymatic syntheses were successful starting from all diols, with the obtained polymers having isolated yields between 67 and over 90%, number average molecular weights between 3000 Da and 5000 Da and degree of polymerization (DP) of 6-18 (based on the used aliphatic diesters and aromatic diols) when polymerized in diphenyl ether as solvent.

Only using 3,4-bis(hydroxymethyl)furan as the diol led to shorter oligomers with isolated yields around 50% and DPs of 3-5.

DSC and TGA thermal analyses show clear correlation between polymer crystallinity and aliphatic carbon chain length of the diester.

1. Introduction

With the society’s increasing awareness and concern towards the use and disposal of single use plastics, governments and private enterprises are combining efforts to reduce plastic-derived pollution [1].

Work toward increasing plastic recyclability, reduction of pollution and synthesizing alternative bio-materials from renewable sources is aimed to reduce our dependence from non-renewable petroleum-based raw materials.

Hence, the use of resources like plastic garbage as feedstock [2] would allow the plastic industry to close the carbon cycle [3].

Studies on the replacement of terephthalic acid (TA) with bio-based TA [4] or its furan-based counterpart, i.e. 2,5-furandicarboxylic acid, indicated the possibility to synthesize furan diacid-based polyesters and co-polyesters using both traditional chemocatalysis [5], [6] or enzymatic synthesis [7], [8].

Most previous studies indeed focused on synthesis of poly(ethyene-2,5-furandicarboxylate) (PEF) that is aimed to substitute poly(ethylene terephthalate) (PET) in the production of water and soft drinks bottles.

Furan-based alternative polyesters such as poly(1,4-butylene 2,5-furandicarboxylate) [9], poly(1,4-butylene 2,5-thiophenedicarboxylate) [10], [11], [12] and various copolymers containing a wide range of aliphatic diols and diesters [13] were also investigated and showed interesting barrier properties that could lead to their use as film packaging.Scheme 1.

All these materials, despite the advantage of being bio-based, are enzymatically better degradable than the petrol-based PET and it is therefore easier to recycle them using biotechnological methods.

In fact, while PET can be only partially degraded, with reported weight losses usually between 5% and 20% [14], [15], PEF can be completely degraded into its constituent monomers and soluble oligomers in three days [16].

More recently, lignin-derived diesters simply called pyridinedicarboxylic acids were also suggested for the substitution of the terephthalic unit.

These monomers might offer increased rigidity if incorporated into a polymer, yet retaining a potentially interesting functionality which may affect the stacking/crystallization behaviour of the polycondensation product [17].

Despite the fact that several studies tried to substitute TA with various aromatic diesters, only few attempts were made towards the utilization of aromatic diols for giving rigidity to the polyester chain.

A first attempt using 2,5-bis(hydroxymethyl)furan (2,5-BHMF) as the diol was made by Loos and co-workers who synthesized a series of aliphatic-aromatic polyesters using a three-stage method (obtained Mns≈ 2000 Da) [16] and further investigated copolymers containing both 2,5-BHMF and diethyl-2,5-furandicarboxylate that achieved polymers having higher molecular weights [18], [19].

Other isosorbide-like diols were also used to give rigidity to the polymer chain in various extends (despites it leads to a decrease in crystallinity), with all relative works on the topic employ traditional metal compounds as the catalysts [20], [21].

The focus of this paper is directed at the investigation of the biocatalyzed synthesis of furan- and pyridine-derived aromatic diols polyesters, along with characterization of their thermal properties.

A series of aromatic-aliphatic polyesters based on benzene-derived diols are also synthesized for comparison with the bio-based monomers.

2. Results & discussion

The synthesis of the aromatic diols-based polyesters was conducted in diphenyl ether (DPE) as reaction solvent.

This synthesis method was selected because some of the dimethanol diols used in the study have melting temperatures (see Supplementary Table 1) above the enzyme’s deactivation temperature.

Therefore, solventless synthesis protocols reported for other aliphatic and aromatic monomers was deemed inappropriate for this study.

The DPE-based synthesis employed immobilized Candida antarctica lipase B (iCaLB) as the catalyst and was successful for all selected aromatic diols.

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Enzymatic synthesis of biobased polyesters utilizing aromatic diols as the rigid component

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