Scalable production of mechanically tunable block polymers from sugar

Abstract

Development of sustainable and biodegradable materials is essential for future growth of the chemical industry. For a renewable product to be commercially competitive, it must be economically viable on an industrial scale and possess properties akin or superior to existing petroleum-derived analogs. Few biobased polymers have met this formidable challenge. To address this challenge, we describe an efficient biobased route to the branched lactone, β-methyl-δ-valerolactone (βMδVL), which can be transformed into a rubbery (i.e., low glass transition temperature) polymer. We further demonstrate that block copolymerization of βMδVL and lactide leads to a new class of high-performance polyesters with tunable mechanical properties. Key features of this work include the creation of a total biosynthetic route to produce βMδVL, an efficient semisynthetic approach that employs high-yielding chemical reactions to transform mevalonate to βMδVL, and the use of controlled polymerization techniques to produce well-defined PLA-PβMδVL-PLA triblock polymers, where PLA stands for poly(lactide). This comprehensive strategy offers an economically viable approach to sustainable plastics and elastomers for a broad range of applications.

Keywords: biobased production; block copolymer; mevalonate pathway; rubbery polyester.

Fig. 1.

(A) Total biosynthetic pathway for the production of βMδVL. (B) A semisynthetic route to produce βMδVL from mevalonate. (C) Conversion of βMδVL to an elastomeric triblock polymer that can be repeatedly stretched to 18 times its original length without breaking.

Fig. 2.

Total biobased production of βMδVL and semisynthetic route to this monomer. (A) Fermentation of mevalonate from different combinations of MvaS and MvaE. MvaE from E. faecalis plus MvaS from: I, E. faecalis; II, Staphylococcus aureus; III, L. casei. MvaS from L. casei plus MvaE from: IV, S. aureus; V, L. casei; VI, M. maripaludis; VII, M. voltae. (B) Anhydromevalonolactone fermentation with siderophore enzymes SidI and SidH from: A, A. fumigatus; B, N. crassa; C, P. nodorum; D, S. sclerotiorum. (C) Production of βMδVL through fermentation with enoate-reductase: 1, Oye2 from S. cerevisiae; 2, Oye3 from S. cerevisiae; 3, wild-type YqjM from B. subtilis; 4, Mutant YqjM (C26D and I69T) from B. subtilis. (D) Production of mevalonate by fermentation of glucose in a 1.3-L bioreactor. (E) Acid catalyzed dehydration of mevalonate to anhydromevalonolactone monitored by refractive index (RI). (F) NMR spectrum of purified βMδVL prepared via hydrogenation of anhydromevalonolactone. Data shown in A–C include error bars that identify the range of the results obtained from three experiments (n = 3).

Fig. 3.

Polymerization of βMδVL leading to PβMδVL and chain extension with (±)- or (-)-lactide (LA or LLA) yielding P(L)LA–PβMδVL–P(L)LA triblock polymer. TBD and HO-R-OH represent triazabicyclodecene and 1,4-phenylenedimethanol, respectively.

Fig. 4.

(A) Overlay of size exclusion chromatography traces obtained from PβMδVL (20.0 kg mol⁻¹) and a corresponding PLLA–PβMδVL–PLLA (9.1–20.0–9.1 kg mol⁻¹) triblock polymer. (B) ¹³C NMR spectra obtained from (Bottom) PβMδVL, (Middle) PLLA (10.0 kg mol⁻¹), and (Top) PLLA–PβMδVL–PLLA (9.1–20.0–9.1 kg mol⁻¹). (C) DSC thermograms recorded for (Bottom) PβMδVL and (Middle) PLA–PβMδVL–PLA (16.2–20.0–16.2 kg mol⁻¹), and (Top) PLLA–PβMδVL–PLLA (9.1–20.0–9.1 kg mol⁻¹). Data were taken while heating at a rate of 5 °C min⁻¹ after cooling from 200 °C at the same rate. (D) SAXS pattern recorded at room temperature from PLA–PβMδVL–PLA (16.2–20.0–16.2 kg mol⁻¹). Diffraction peaks at q*=0.185 nm⁻¹, 2q*, 3q*, and 4q* are consistent with a periodic (d = 33 nm) lamellar morphology.

Fig. 5.

(A) Representative stress (σ) versus strain (ε) results obtained in uniaxial extension for triblock polymers containing different volume fractions of semicrystalline (f_LLA) and glassy (f_LA) blocks. Incorporation of relatively small amounts of the hard block (f_LA = 0.29 and f_LLA = 0.32) results in a soft (elastic modulus E = 1.9 and 5.9 MPa, respectively), highly extendable elastic material. Increasing the hard block content (f_LA = 0.59) leads to a stiff (E = 229 MPA) and ductile plastic. (B) Stress versus strain response of PLLA–PβMδVL–PLLA (18.6–70.0–18.6 kg mol⁻¹) (f_LLA = 0.32) during cyclic loading (1–20 cycles) to 67% strain at a rate of 5 mm min⁻¹. These results demonstrate nearly ideal elastic behavior with nearly complete recovery of the applied strain.