Scalable production of mechanically tunable blockpolymers from sugarMingyong Xionga,1, Deborah K. Schneidermanb,1, Frank S. Batesa, Marc A. Hillmyerb,2, and Kechun Zhanga,2

Departments of aChemical Engineering and Materials Science and bChemistry, University of Minnesota, Minneapolis, MN 55455-0431

Edited by Malcolm H. Chisholm, The Ohio State University, Columbus, OH, and approved April 11, 2014 (received for review March 11, 2014)

Development of sustainable and biodegradable materials is essen-tial for future growth of the chemical industry. For a renewableproduct to be commercially competitive, it must be economicallyviable on an industrial scale and possess properties akin or superiorto existing petroleum-derived analogs. Few biobased polymershave 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 intoa rubbery (i.e., low glass transition temperature) polymer. We fur-ther demonstrate that block copolymerization of βMδVL and lac-tide leads to a new class of high-performance polyesters withtunable mechanical properties. Key features of this work includethe creation of a total biosynthetic route to produce βMδVL, anefficient semisynthetic approach that employs high-yielding chem-ical reactions to transform mevalonate to βMδVL, and the use ofcontrolled polymerization techniques to produce well-definedPLA–PβMδVL–PLA triblock polymers, where PLA stands for poly(lactide). This comprehensive strategy offers an economically via-ble approach to sustainable plastics and elastomers for a broadrange of applications.

rubbery polyester | block copolymer | biobased production |mevalonate pathway

Polymeric materials account for nearly $400 billion in eco-nomic activity annually and represent the third largest

manufacturing industry in the United States (1). Currently pe-troleum-derived polymers—for example polyethylene, polystyrene,and polyvinylchloride—dominate this market. Although thesematerials are widely useful, their manufacture and disposalpresent inescapable environmental challenges that are costly tocorrect and simply unsustainable in the long term. To ensure thecontinued vitality of the polymer enterprise, it is necessary toinvent high-performance polymers that are both sustainable andcost-competitive. In recent years, ingenious advances in syntheticbiology have enabled the economical production of fuels (2–7),chemicals (8–12), and complex natural products (13–16) fromrenewable sugars. This elegant manipulation of existing organ-isms to efficiently produce valuable metabolites from inexpensivefeedstocks represents a triumph of modern science and engi-neering and offers society the promise of renewable, environ-mentally compatible next-generation materials.Whereas there has been steady scientific progress toward the

production of synthetic polymers from renewable resources, thefraction of high-performing, biobased, degradable polymers onthe market is today minuscule (1). Arguably the most successfulexample to date is poly(lactide) (PLA), a compostable aliphaticpolyester derived from the fermentation product lactic acid.However, the brittle nature of PLA and other commercial ali-phatic polyesters such as poly(butylene succinate) and poly(hydroxyalkanoate)s has thwarted their broad-based utility. Ex-panded market penetration will pivot on the development ofproducts endowed with tunable combinations of properties, e.g.,soft, ductile, and tough (17, 18). Among myriad strategies thathave been used to this end, PLA-containing block polymersrepresent a particularly attractive approach. These hybridmaterials offer opportunities to easily tune a wide range of

physical and mechanical properties by controlling molecular ar-chitecture, composition, and molar mass.A specific example that illustrates the utility of this approach is

ABA triblock thermoplastic elastomers. Here, rigid (glassy orsemicrystalline) “A” endblocks microphase separate to form nano-scopic domains that tether together the soft, low glass transitiontemperature (Tg) regions comprised of the center block “B” com-ponent (19–22). This design results in outstanding mechanicalproperties and has been the basis for the commercially successfulpoly(styrene)–poly(diene) variants (23). Many PLA-containingblock polymers have been reported; several have been shown tobe flexible, tough, elastic, and hydrolytically degradable (24–27).Unfortunately the starting materials used to prepare these pol-ymers are either derived from fossil fuels or from natural prod-ucts that are prohibitively expensive. The development of aneconomically viable rubbery polymer that can be used in com-bination with PLA to prepare mechanically superior block pol-ymers therefore is the grand challenge that must be addressedfor these materials to be competitive with the analogous styrenicblock polymers.The need for an efficient, scalable, process to a methyl branched

lactone suitable for ring-opening transesterification polymerization(ROTEP) led us to examine candidates that could be produced bybiosynthesis with both high titer and high yield. After surveyingthe appropriate metabolic pathways, we identified β-methyl-δ-valerolactone (βMδVL) as an attractive target. Based on ourexperience with simple aliphatic polyesters we anticipated thatthe elastomeric polymer PβΜδVL could be combined with hardsemicrystalline or glassy poly(lactide) (PLLA or PLA, respectively)to produce P(L)LA–PβMδVL–P(L)LA triblock polymers that

Significance

In recent years there has been extensive research toward thedevelopment of sustainable polymeric materials. However,environmentally benign, bioderived polymers still representa woefully small fraction of plastics and elastomers on themarket today. To displace the widely useful oil-based polymersthat currently dominate the industry, a bioderived syntheticpolymer must be both cost and performance competitive. Inthis paper we address this challenge by combining the efficientbioproduction of β-methyl-δ-valerolactone with controlled poly-merization techniques to produce economically viable blockpolymer materials with mechanical properties akin to com-mercially available thermoplastics and elastomers.

Author contributions: M.X., D.K.S., M.A.H., and K.Z. designed research; M.X. and D.K.S.performed research; M.X., D.K.S., F.S.B., M.A.H., and K.Z. analyzed data; and M.X., D.K.S.,F.S.B., M.A.H., and K.Z. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

Freely available online through the PNAS open access option.1M.X. and D.K.S. contributed equally to this work.2To whom correspondence may be addressed. E-mail: kzhang@umn.edu or hillmyer@umn.edu.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1404596111/-/DCSupplemental.

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offer exceptional mechanical properties (26, 27). βMδVL canbe generated by direct fermentation of glucose or through theproduction of the intermediate compound mevalonate. Thesetwo approaches to βMδVL, one entirely biochemical and theother semisynthetic, represent facile chemical transformations(Fig. 1). Because the theoretical yield of this lactone monomer is0.42 g/g glucose, we estimate that this product can be producedfor less than $2 a kilogram (Table S1), compatible with large-volume commodity applications.With the bioderived βMδVL in hand, we demonstrated that

bulk ROTEP using an organocatalyst proceeds at room temper-ature to high conversion, yielding an amorphous polyester withTg = –51 °C (26, 28). Simple chain extension through reaction withlactide (LLA or LA) leads to P(L)LA–PβMδVL–P(L)LA tri-block polymers that can be tuned to have mechanical propertiesranging from stiff tough plastics to soft and highly extendibleelastomers (Fig. 1C).

ResultsConstruction of a Nonnatural Metabolic Pathway for Biosynthesis ofβMδVL. Because βMδVL is not a natural metabolite, we con-structed an artificial biosynthetic pathway to create an engineeredEscherichia coli strain (Fig. 1A). Our biosynthetic strategy buildson the mevalonate pathway that previously has been engineeredfor the synthesis of artemisinin and terpenoids in Saccharomycescerevisiae and E. coli, respectively (29, 30). In this work we im-proved the production of mevalonate in E. coli and also ex-panded the pathway to synthesize βMδVL from this precursor.The overall nonnatural pathway has three components: (i)overexpression of the mevalonate-producing enzymes; (ii) in-troduction of the fungal siderophore proteins to synthesizeanhydromevalonolactone (AML); (iii) reduction of AML to βMδVLby enoate reductases.To generate an acetoaceyl-CoA pool we used the E. coli

endogenous enzyme acetyl-CoA acetyltransferase AtoB. FirstHMG-CoA synthase (MvaS) and HMG-CoA reductase (MvaE)were cloned to provide a route for the production of mevalonatefrom this pool. To maximize mevalonate flux, the Protein-Protein BasicLocal Alignment Search Tool (BlastP) was used to identify MvaSand MvaE from various organisms: Enterococcus faecalis, Staph-ylococcus aureus, Lactobacillus casei, Methanococcus maripaludis,and Methanococcus voltae. Combinatorial tests were used to

identify the optimum set of MvaS and MvaE for mevalonate pro-duction (Fig. 2A and Table S2). high-performance liquid chro-matography (HPLC) analyses indicated that, with the exceptionof MvaS from M. maripaludis and M. voltae, all heterologousenzymes were active in E. coli. The strain expressing both mvaSand mvaE genes from L. casei produced 14.6 g L–1 mevalonatefrom 40 g L–1 glucose, better than any other combination (V inFig. 2A). The strain expressing the L. casei enzymes produced 50%more mevalonate than other engineered E. coli strains described inprevious work (30–32).To biosynthesize anhydromevalonolactone from mevalonate,

we explored the potential of siderophore pathway enzymes. Infungi acyl-CoA ligase SidI converts mevalonate to mevalonyl-CoA, and enoyl-CoA hydratase SidH transforms mevalonyl-CoAto anhydromevalonyl-CoA (33). We speculated that anhy-dromevalonyl-CoA could spontaneously cyclize into anhy-dromevalonolactone intracellularly. To test this hypothesis, wesynthesized sidH and sidI genes based on E.-coli-optimized codonsfor protein expression. Using BlastP analysis, homologous SidIand SidH from Aspergillus fumigatus, Neurospora crassa, Phaeos-phaeria nodorum, and Sclerotinia sclerotiorum were chosen forfermentation experiments (34). Results revealed that the strainexpressing SidI and SidH from A. fumigatus produced 730 mg L–1

anhydromevalonolactone; the strain carrying enzymes fromN. crassa also generated 540 mg L–1 anhydromevalonolactone(Fig. 2B and Table S3). The other two enzyme pairs did notfunction properly in E. coli.To explore the reduction of anhydromevalonolactone to βMδVL

by enoate reductases, we cloned Oye2 and Oye3 from S. cerevisiae(35), wild-type and mutant YqjM (C26D and I69T) from Bacillussubtilis (36). The strains carrying the anhydromevalonolactonepathway and Oye2 or mutant YqjM successfully produced 180 or270 mg L–1 βMδVL, respectively, directly from glucose (Fig. 2Cand Table S4). Thus, we successfully constructed an artificialbiosynthetic pathway to βMδVL from glucose.

Semisynthesis of βMδVL from Mevalonate. Whereas directed evo-lution approaches undoubtedly will improve the aforementionedβMδVL biosynthetic pathway (5), we also developed a semi-synthetic approach for the immediate large-scale production ofβMδVL. In this route the fermented mevalonate is first dehy-drated to anhydromevalonolactone, then reduced to βMδVL(Fig. 1B).To scale up the production of mevalonate, the E. coli strain

carrying genes from L. casei was tested for fermentation in a 1.3-Lbioreactor. During the fermentation, the strain achieved a produc-tivity of 2 g L–1h–1 mevalonate with the final titer reaching 88 g L–1

(Fig. 2D and Table S5). The yield for this semibatch fermentationwas 0.26 g/g glucose. To prepare anhydromevalonolactone weadded sulfuric acid directly to the fermentation broth and heatedto reflux to catalyze the dehydration of mevalonate (Fig. 2E andFig. S1). At a catalyst loading of 10% by volume, 98% of themevalonate was converted to anhydromevalonolactone with a se-lectivity of 89% (Table S6). The resulting anhydromevalonolactonewas isolated by solvent extraction using chloroform and hydroge-nated to βMδVL using Pd/C as the catalyst (bulk, room tempera-ture, 350 psi H2, 12 h) to >99% conversion. The crude product wassubsequently purified by distillation to obtain polymerization-grademonomer (Fig. 2F).

Polymerization of βMδVL. Based on previous work with alkyl-substituted δ-valerolactones, we suspected the ceiling temper-ature for the polymerization of βMδVL might be low (28, 37, 38).To favor high conversion we therefore conducted the polymer-izations in bulk monomer at room temperature using the highlyactive organocatalyst triazabicyclodecene (TBD) (Fig. 3) (39). Theaddition of TBD to neat βMδVL in the presence of benzyl alcohol(BnOH) as the initiator ([βMδVL]0/[TBD]0/[BnOH]0 = 492/1/1.7)

Fig. 1. (A) Total biosynthetic pathway for the production of βMδVL. (B) Asemisynthetic route to produce βMδVL from mevalonate. (C) Conversion ofβMδVL to an elastomeric triblock polymer that can be repeatedly stretchedto 18 times its original length without breaking.

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resulted in the rapid production of poly(βMδVL)—within 1 h atroom temperature (T = 18 °C) 75% of the monomer was con-sumed, within 3 h the reaction approached equilibrium (Fig. S2).Determination of the residual monomer concentration over a rangeof temperatures allowed us to calculate the thermodynamic param-eters for this polymerization (ΔHo

p =−13:8± 0:3 kJ mol�1 andΔSop =−46± 1 J mol−1K−1); these values correspond to a ceilingtemperature (Tc) of 227 °C for the polymerization of neat βMδVL.Practically, the limiting conversion for the bulk polymerizationis about 91% and 60% at 18 °C and 120 °C, respectively (Fig.S3). At 18 °C with as little as 0.05 wt% TBD, the bulk polymer-ization is well controlled with the conversion and ratio of mono-mer to added initiator dictating the molar mass of the polymer(Table S7). Using the diol initiator 1,4-phenylenedimethanol weobtained dihydroxy terminated poly(βMδVL).

Synthesis and Mechanical Properties of Block Copolymers. Poly(βMδVL) is an amorphous aliphatic polyester with a low glasstransition temperature (Tg = –51 °C). To explore the potential ofthis rubbery polymer as the soft segment in thermoplastic elas-tomers we used dihydroxy terminated poly(βMδVL) to preparetriblock polymers with PLA endblocks. This was easily accom-plished by adding a solution of (±) or (–)-lactide (LA or LLA)directly to a polymerization of βMδVL that was near equilibrium(Fig. 3). [Alternatively, purfied telechelic poly(βMδVL) (free ofresidual monomer and catalyst) could be dissolved in a solution

of lactide, and the polymerization initiated by addition of TBD.]Although depolymerization can be problematic for poly(βMδVL)when diluted in the presence of TBD, the conversion of βMδVLwas virtually unchanged before and after the chain extension withlactide, suggesting that the addition of lactide to the end of the poly(βMδVL) prevents significant depolymerization. Compared withthe poly(βMδVL) macroinitiators, polymers prepared using eitherextension strategy exhibited expected increases in mass averagemolar mass (Mm) as determined by multiangle laser light scattering-size exclusion chromatography (Fig. 4A). In addition, the 13C NMR

Fig. 2. Total biobased production of βMδVL and semisynthetic route to this monomer. (A) Fermentation of mevalonate from different combinations of MvaSand 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 bio-reactor. (E) Acid catalyzed dehydration of mevalonate to anhydromevalonolactone monitored by refractive index (RI). (F) NMR spectrum of purified βMδVLprepared via hydrogenation of anhydromevalonolactone. Data shown in A–C include error bars that identify the range of the results obtained from threeexperiments (n = 3).

Fig. 3. Polymerization of βMδVL leading to PβMδVL and chain extensionwith (±)- or (-)-lactide (LA or LLA) yielding P(L)LA–PβMδVL–P(L)LA triblockpolymer. TBD and HO-R-OH represent triazabicyclodecene and 1,4-phenyl-enedimethanol, respectively.

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spectra of the resulting triblocks revealed no evidence of significanttranesterification between the poly(βMδVL) and poly(lactide) blocks,consistent with clean formation of the desired ABA triblock archi-tecture (Fig. 4B).Despite the structural similarity of poly(lactide) and poly

(βMδVL), P(L)LA–P(βMδVL)–P(L)LA triblock polymers readilymicrophase separate at only moderate molar masses as evidencedby differential scanning calorimetry (DSC) and small-angle X-rayscattering (SAXS). Both PLA–P(βMδVL)–PLA [from (±)-lactide,LA] and PLLA–P(βMδVL)–PLLA [from (-)-lactide, LLA] exhibitseparate glass transitions for the midblock and endblock segmentsby DSC (Fig. 4C). In addition, the SAXS data from these tri-blocks showed well-defined scattering peaks that correspond toself-assembled nanostructures with principal spacings rangingfrom 20 to 50 nm (Fig. 4D and Table S8).Predictably, the mechanical and thermal properties of these

ordered block polymers are influenced by molar mass, tacticityof the poly(lactide) segments, and composition. By changing theendblock from the minority component (fLA = 0.29) to the majoritycomponent (fLA = 0.59) it is possible to access either elastomers ortough plastics. Moreover, at a fixed composition and molar mass,the use of semicrystalline PLLA endblocks leads to remarkablystrong elastomers that rival commercially available petroleum basedblock copolymers in terms of recoverability, tensile strength, andultimate elongation (Fig. 5 and Table S9) (23).

ConclusionWe have developed a semisynthetic approach to βMδVL fromglucose that relies upon the fermentation of mevalonate andsubsequent transformation of mevalonate to βMδVL. The hightiter of the fermentation (88 g L−1) and the efficiency of thechemical reactions used to produce the final product make theoverall process scalable and commercially promising. We alsohave described a nonnatural total biosynthetic pathway for theproduction of βMδVL , which obviates the need for additionalchemical transformations. Optimization of this all-biosyntheticprocess (i.e., titer and yield comparable to the semisynthetic route)would further reduce the production cost of βMδVL. This bio-derived monomer can be readily converted from the neat state toa rubbery hydroxytelechelic polymer using controlled polymeri-zation techniques at ambient temperature; addition of either (±)or (-)-lactide to a poly(βMδVL) midblocks leads to well-definedABA triblock polymers. We have shown that the thermal andmechanical properties of these materials can be tuned by con-trolling molar mass, architecture, and endblock tacticity and havespecifically demonstrated thermoplastic elastomers with proper-ties similar to commercially available styrenic block polymers.This work lays the foundation for the production of new biobasedpolymeric materials with a wide range of potential propertiesand applications.

Fig. 4. (A) Overlay of size exclusion chromatography traces obtained from PβMδVL (20.0 kg mol−1) and a corresponding PLLA–PβMδVL–PLLA (9.1–20.0–9.1kg mol−1) triblock polymer. (B) 13C NMR spectra obtained from (Bottom) PβMδVL, (Middle) PLLA (10.0 kg mol−1), and (Top) PLLA–PβMδVL–PLLA (9.1–20.0–9.1kg 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 roomtemperature 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.

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Materials and MethodsFor details, see SI Appendix.

Plasmids and Strains. All cloning procedures were carried out in the E. colistrain XL10-gold (Stratagene). Genes for mevalonate and βMδVL pathwaywere either PCR amplified from genomic DNA templates or codon-opti-mized and synthesized by GenScript. Three kinds of plasmids have beenconstructed for the biosynthesis of βMδVL (Fig. S4). BW25113 (rrnBT14

ΔlacZWJ16 hsdR514 ΔaraBADAH33 ΔrhaBADLD78) transformed with the rele-vant plasmids was used to produce mevalonate or βMδVL.

Culture Condition and Analysis. E. coli strains were growth at 37 °C in 2XYTrich medium (16 g/L Bacto-tryptone, 10 g/L yeast extract, and 5 g/L NaCl) sup-plemented with appropriate antibiotics (ampicillin 100 μg/mL and kanamycin50 μg/mL). Shake flask fermentations were performed with 125-mL conical flaskscontaining M9 medium. After adding 0.1 mM isopropyl-β-D-thiogalactoside, thefermentation was performed for 48 h at 30 °C. Scale-up fermentation wasperformed in 1.3 L Bioflo 115 Fermentor (NBS). The culturing condition wasset at 34 °C, dissolved oxygen level 20%, and pH 7.0. The concentrations ofmetabolites were measured by HPLC.

Dehydration, Hydrogenation, and Purification. The fermentation supernatantwas treated with concentrated H2SO4 to dehydrate mevalonate into anhy-dromevalonolactone. The reaction temperature was 121 °C. Chloroform wasused to extract anhydromevalonolactone from the reaction mixture. Theresults of dehydration are shown in Fig. S1 and Table S6. For hydrogenation,unreduced palladium on activated carbon [10% (wt/wt) Acros Organics] wasused as the catalyst. In a typical hydrogenation procedure, 10 g of catalyst wereadded to a 300 mL high-pressure reactor with 50 mL anhydromevalonolactone.The reaction was then allowed to stir under H2 (∼350 pounds per square inchgauge) at room temperature overnight to ensure quantitative conversion.The crude βMδVL was dried over calcium hydride for 12 h and then distilledunder vacuum (50 mTorr, 30 °C); the distilled product was passed through dry

basic alumina using cyclohexane as an eluent. The cyclohexane was removedunder vacuum to obtain purified βMδVL.

Polymer Synthesis. To synthesize poly(βMδVL) a monofunctional (benzyl al-cohol) or difunctional (1,4 benzene dimethanol) alcohol was added tomonomer in a pressure vessel or glass vial and stirred with a magnetic stirbar until completely dissolved. The ratio of monomer to alcohol was variedto target polymers of different molecular weights. When the initiator wasdissolved, an appropriate amount of catalyst (∼0.05–0.2 mol% TBD or ∼0.5mol% diphenyl phosphate) relative to monomer was added to initiate thepolymerization. The reaction was stirred and the conversion monitored us-ing 1H NMR of crude quenched aliquots. Poly((L)LA)-b-poly(βMδVL)-b-poly((L)LA) was prepared from a previously isolated and purified poly(βMδVL).The poly(βMδVL) prepolymer was first dissolved in a 1 M solution of (±)-lactide or (–)-lactide in dichloromethane. After dissolution of the prepol-ymer, 0.1 mol% TBD (relative to lactide) was added. The solution was stirredfor 10 min and then quenched by adding 5 equivalents of benzoic acidrelative to TBD. The crude polymer samples were purified by precipitation inmethanol from dichloromethane/chloroform and subsequently dried.

ACKNOWLEDGMENTS. D.K.S. thanks Byeongdu Lee for assistance withacquisition of SAXS data, and Justin Bolton for photography assistance.Funding for this work was provided by the Center for Sustainable Polymersat the University of Minnesota, a National Science Foundation (NSF)-supportedCenter for Chemical Innovation (CHE-1136607). The BioTechnology Instituteat the University of Minnesota is also acknowledged for support. Portionsof this work were performed at Sector 12-IDB of the Advanced PhotonSource (APS). Use of the APS, an Office of Science User Facility operatedfor the US Department of Energy (DOE) Office of Science by ArgonneNational Laboratory, was supported by the US DOE under Contract DE-AC02-06CH11357. Additional parts of this work were carried out in theUniversity of Minnesota College of Science and Engineering Characteriza-tion Facility, which receives partial support from NSF through the NationalNanotechnology Infrastructure Network program.

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

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