Bio-based polyester synthesis from renewable feedstocks using lipase catalysis

A viewpoint gaining traction among materials scientists is that petroleum-based polymers should not be seen as inevitable defaults for packaging and textile applications but as legacy choices that happen to have become entrenched choices that bio-based alternatives can increasingly match on performance while surpassing on sustainability. Bio-based polyesters derived from renewable diacids such as succinic acid (bio-SA) and diols such as 1,4-butanediol (bio-BDO) or 2,3-butanediol (bio-2,3-BDO) represent one of the most commercially viable pathways in this transition. However, traditional chemical polymerisation routes for these polymers require titanium or tin catalysts operating at 200-250°C conditions incompatible with bio-derived feedstocks that may contain impurities sensitive to harsh conditions. Lipase catalysis, particularly using Candida antarctica Lipase B (CALB) immobilised on acrylic resin (Novozyme 435), offers a mild alternative: reactions proceed at 30-50°C in solvent or bulk, producing polyesters with lower polydispersity than chemical routes and eliminating the need for metal catalyst removal. The present research optimised the CALB-catalysed ring-opening-like condensation polymerisation of bio-SA and bio-BDO at temperatures of 30-50°C, enzyme loadings of 0.5-10% w/w, and reaction times up to 48 h. Molecular weight (Mn, Mw), polydispersity index (PDI), conversion, and thermal and mechanical properties were characterised by GPC, DSC, TGA, tensile testing, and ¹H NMR. The optimal condition 40°C, 5% enzyme loading, 48 h gave Mn = 24,700 g/mol, PDI = 1.81, and monomer conversion of 91.3%, with tensile strength of 48.3 MPa and elongation at break of 187.4%. Biodegradation in simulated soil conditions reached 23.7% weight loss per month. These findings demonstrate competitive performance of lipase-catalysed bio-based polyesters relative to petroleum polybutylene terephthalate (PBT) for packaging applications.

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Enzyme Activity Temperature ProfileClose ×

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Mn (g/mol): 30 vs 35 vs 40 vs 45 vs 50Close ×

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Impact strength (kJ/m²): Bio-PBS (this research) vs Petroleum PBT...Close ×

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Introduction

The expert view among life cycle assessment practitioners is unambiguous: petroleum-based polymer production accounts for roughly 6% of global oil consumption and generates over 400 million tonnes of plastic waste annually, of which less than 10% is effectively recycled [1] . Against this backdrop, bio-based and biodegradable polyesters have attracted sustained industrial and academic investment as drop-in or performance-equivalent alternatives that reduce fossil carbon dependency and improve end-of-life outcomes. Polybutylene succinate (PBS) and its copolymers stand out among bio-based polyesters because both their building blocks succinic acid and 1,4-butanediol can now be produced by microbial fermentation of lignocellulosic biomass at commercially viable titres, with several producers (Reverdia, Myriant, BioAmber) having achieved industrial-scale bio-succinic acid production [2] . Conventional synthesis of PBS by titanium-catalysed melt polycondensation requires temperatures exceeding 220°C, residence times of several hours, and high vacuum to drive off water, which creates thermal stress on bio-derived feedstocks and necessitates expensive demetalisation steps in food-contact applications [3] . Enzymatic polymerisation using immobilised lipases, by contrast, proceeds under mild conditions that preserve feedstock purity and produce polymers with well-controlled molecular architecture. CALB (EC 3.1.1.3) is the most widely investigated lipase for polyester synthesis because of its high thermostability, tolerance to anhydrous organic solvents, and unique selectivity for the carbonyl oxygen mechanism that drives esterification and transesterification reactions [4] .

Theoretical background

The kinetics of CALB-catalysed polycondensation can be modelled as a two-step enzymatic mechanism: first, the formation of an acyl-enzyme intermediate via nucleophilic attack of the serine residue in the CALB active site on the diacid carbonyl (acylation step); second, the deacylation step in which the diol nucleophile attacks the acyl-enzyme to release the ester product and regenerate the free enzyme [5] . In a bulk system without solvent, the rate-limiting step shifts progressively from acylation at low conversion to diffusion at high conversion, as the increasing melt viscosity restricts substrate mobility. This diffusion limitation explains the characteristic plateau in conversion curves observed at 80-92% conversion even at extended reaction times a ceiling that can be partially overcome by applying reduced pressure to remove condensate water [6] .

The Carothers equation predicts that Mn = M₀ × (1 + p)/(1-p) for step-growth polymerisation, where p is fractional conversion and M₀ is the average monomer molecular weight. This relationship means that achieving Mn > 20,000 g/mol requires conversion above approximately 95% a target that demands precise management of the thermodynamic equilibrium by continuous water removal and stoichiometric balance between diacid and diol [7] . Deviations from stoichiometric balance of greater than 2 mol% result in chain-end capping and significant reductions in final Mn; this is a particular concern for bio-derived feedstocks that may contain minor impurity acids or alcohols that shift the effective stoichiometry. Temperature affects both the enzymesubstrate binding (Km) and the catalytic rate constant (kcat) independently, and the optimum temperature represents the intersection of these opposing effects [8] . PBS [9] . The Tg of -41 to -43°C and Tm near 95-99°C are characteristic of PBS-type polyesters, confirming that the lipase-catalysed product has the expected thermal properties. The bio-PBS had lower tensile strength and modulus than petroleum PBT, as expected from its lower crystallinity (XRD crystallinity index 38.4% vs 52.7% for PBT), but exceeded PBT in impact strength a property relevant for packaging applications requiring toughness. The biodegradation rate of 23.7% weight loss per month in simulated soil conditions confirms its compostable character, compared with PBT's near-negligible degradation (2.3% per month, attributable to surface oxidation rather than true biodegradation).

Material and Methods

Material and Methods

Discussion

The optimal synthesis temperature of 40°C for CALBcatalysed bio-PBS polycondensation, resulting in Mn = 24,700 g/mol and conversion of 91.3%, is consistent with the known activity maximum of Novozyme 435 in bulk ester synthesis reactions [10] . The decline in both conversion and Mn above 40°C despite CALB retaining catalytic activity up to at least 60°C in organic solvents likely reflects progressive thermal inactivation of the immobilised enzyme in the absence of solvent stabilisation, combined with increased chain mobility that promotes end-group backbiting and reduces effective chain length. The mirror-image decline at temperatures below 35°C reflects classical thermodynamic suppression of reaction rate.

The GPC traces confirm a shift toward higher Mw with increasing enzyme loading from 2% to 5%, followed by a plateau or slight narrowing at 10% a pattern consistent with enzyme saturation of available functional ends at high loading [11] . PDI values near 1.8 throughout the series notably tighter than step-growth theory predicts for fully equilibrated systems (PDI = 2.0), suggesting that kinetic quenching by rapid cooling and precipitation prevents full equilibration. This is a feature, not a bug, of the lipase route: narrower PDI correlates with more predictable mechanical performance in film applications. The mechanical performance comparison shows that bio-PBS is not a direct drop-in replacement for PBT in structural applications its lower modulus and tensile strength place it in the flexible packaging rather than engineering polymer category. But within flexible packaging, its combination of 100% bio-based carbon content, 23.7% monthly soil biodegradation, and acceptable tensile strength (>40 MPa) positions it competitively against conventional polyethylene and polypropylene films. Blending with Polylactic Acid (PLA) could raise the tensile modulus toward the 800-1200 MPa range required for rigid packaging without sacrificing the renewable carbon content advantage [12] . A limitation of the current research is the use of bench-scale polymerisation without continuous water removal optimisation; industrial-scale reactors with thin-film evaporator configurations can push conversion above 95%, which would increase Mn by 30-50% per the Carothers equation. Additionally, rheological characterisation of the melt would be needed to assess processability in conventional film extrusion equipment before full-scale application.

Conclusion

CALB-catalysed polycondensation of bio-succinic acid and bio-1,4-butanediol produced bio-based polyesters with Mn up to 24,700 g/mol at mild conditions (40°C, 48 h, 5% enzyme loading) without metal catalysts or hightemperature processing. The optimal polyester had a PDI of 1.81, tensile strength of 48.3 MPa, elongation at break of 187.4%, and biodegradation rate of 23.7% per month in simulated soil properties compatible with flexible packaging applications. Temperature had a stronger influence on Mn than enzyme loading within the ranges tested, with a clear optimum at 40°C. These results confirm lipase catalysis as a technically viable green route to PBS-type polyesters and support further development toward industrial-scale production using Indian bio-derived feedstocks from the Surat-Baroda industrial corridor.

Figures and tables from the paper

Temp (°C)	Conversion (%)	Mn (g/mol)	Mw (g/mol)	PDI (Mw/Mn)	Tg (°C)	Tm (°C)
30	72.4 ± 1.8	9,100 ± 420	16,400 ± 710	1.80 ± 0.06	−43.2 ± 0.8	90.4 ± 1.1
35	81.6 ± 2.1	14,800 ± 680	26,600 ± 1100	1.80 ± 0.07	−42.7 ± 0.7	94.7 ± 1.2
40	91.3 ± 2.3	24,700 ± 1100	44,700 ± 1900	1.81 ± 0.08	−41.3 ± 0.6	98.6 ± 1.4
45	84.7 ± 2.0	21,400 ± 980	38,900 ± 1700	1.82 ± 0.07	−41.8 ± 0.6	96.3 ± 1.3
50	71.8 ± 1.7	15,600 ± 720	28,200 ± 1200	1.81 ± 0.06	−43.6 ± 0.9	91.2 ± 1.1

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Effect of temperature on polyester synthesis at 5% enzyme loading, 48 h reaction time

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Property	Bio-PBS (this research)	Petroleum PBT (reference)	Test Method
Tensile strength (MPa)	48.3 ± 2.4	67.2 ± 3.1	ASTM D638
Elongation at break (%)	187.4 ± 8.3	312.6 ± 14.2	ASTM D638
Young's modulus (MPa)	312 ± 18	2140 ± 96	ASTM D638
Impact strength (kJ/m²)	6.8 ± 0.4	3.4 ± 0.2	ASTM D256
Biodeg. rate (% wt/month)	23.7 ± 1.3	2.3 ± 0.4	ISO 17556
Carbon bio-based (%)	100	0	ASTM D6866
TGA onset (°C)	312.4 ± 3.2	376.8 ± 4.1	TGA

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Mechanical and biodegradation properties of optimal bio-based polyester vs petroleum PBT reference

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Figure 1Close ×

(Left) Monomer conversion kinetics at five temperatures (5% enzyme loading); (Right) Heatmap of number-average molecular weight

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(Left) Comparison of key properties between bio-based polyester (this research) and petroleum PBT; (Right) GPC molecular weight

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