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Supplemental Information

for

Polylactide Vitrimers

Jacob P. Brutman, Paula A. Delgado, Marc A. Hillmyer*

Department of Chemistry, University of Minnesota, 207 Pleasant St. SE, Minneapolis, MN

55455-0431, USA

Experimental Details:

Materials:

Pentaerythritol (PERYT), methylenediphenyl diisocyanate (MDI) and stannous(II)

octoate [Sn(Oct)2] were purchased from Sigma-Aldrich and were used as received. (±)-lactide

was provided Altasorb, stored under a N2 atmosphere and was also used as received.

Synthesis of hydroxyl-terminated star-shaped poly((±)-lactide), HTSPLA:1,2

Sn(Oct)2 (0.005 eq., 0.028 g, 0.7 mmol) was dissolved in a minimal amount of toluene

(ca. 0.5 mL) and charged in a pressure vessel, along with (±)-lactide (25 eq., 50. g, 350 mmol)

and PERYT (1 eq., 1.9 g, 14 mmol) (Scheme 1). The reaction mixture was heated to 160 °C for 3

h, then allowed to cool to room temperature and dissolved in an approximately equal volume of

dichloromethane (DCM). The subsequent solution was precipitated in ethanol (ca. 10 times

volume of product solution), redissolved in an approximately equal volume of DCM and

reprecipitated in hexanes (10 times volume of DCM solution). The resulting polymer was then

dissolved in DCM, transferred to a polypropylene container and dried with N2 flow for

approximately 24 h. The polypropylene container was then put in a vacuum oven at

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approximately 60 °C and 20 mTorr for 72 h, and subsequently cooled to room temperature. The

resulting polymer was obtained in an 80% isolated yield. 1H NMR (500 MHz, CDCl3; 25 °C): δ

(ppm) = 5.2 [bm, -COCH(CH3)O-], 4.35 [m, -COCH(CH3)OH end group], 4.16 [bs, C(CH2)4-

PERYT], 1.56 [bm, -COCH(CH3)O-], Mn = 4000 g mol-1. LS-SEC (THF): (dn/dc) = 0.049 mL g-

1, Mm = 5000 g mol-1, Ð = 1.20. DSC: Tg = 35 °C.

Synthesis of isocyanate-based crosslinked star-shaped poly((±)-lactide), ICSPLA:

HTSPLA (1 eq.), MDI (1 to 2.2 eq.), and tris(nonylphenyl)phosphite (0.35 % by weight)

were dissolved in an approximately equal volume of DCM. A stock solution of Sn(Oct)2 in DCM

was prepared (200 mg catalyst per mL solvent), at which point an aliquot of the catalyst solution,

varying from 0.0050 to 0.10 initial mol ratio of Sn(Oct)2:OH, was added to the HTSPLA and

MDI solution. After complete mixing, the resulting solution was poured into a polypropylene

mold (64 mm (D) x 74 mm (H)). The molds were covered with aluminum foil and left

unperturbed for approximately 48 h. Tensile bars were then cut from the resulting films and

placed under reduced pressure at approximately 80 °C and 20 mTorr for 48 h to ensure all

solvent was removed. A typical tensile bar was ca. 0.5 mm (T) × 3 mm (W) × 25 mm (L) and

had a gauge length of 14 mm. Samples were named according to their isocyanate:hydroxyl

(IC:OH) ratio and catalyst loading, e.g., a sample with 0.75:1 IC:OH ratio and a 0.025 initial mol

ratio of Sn(Oct)2:OH was named ICSPLA-0.75-0.025.

Synthesis of ICSPLA-0.75-0

ICSPLA-0.75-0 was synthesized as a control sample; no Sn(Oct)2 was added to facilitate

transesterification-based healing. HTSPLA (1 eq.) and MDI (1.5 eq.) were dissolved in an

approximately equal volume of DCM and subsequently poured into a polypropylene mold (64

mm (D) x 74 mm (H)). The mold was then covered with aluminum foil and placed under an

active nitrogen atmosphere for 48 h. The molds were then placed in a vacuum oven at 60 °C and

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20 mTorr for 24 h. The resulting film was then compression molded performed at 120 °C and 4

MPa for 3 h, after which the samples were removed and placed under reduced pressure at 80 °C

and 20 mTorr for 12 h in order to ensure full conversion of the crosslinking reaction. A typical

tensile bar was ca. 0.5 mm (T) × 3 mm (W) × 25 mm (L) and a gauge length of 14 mm.

Characterization Methods:

1H-NMR spectroscopy was performed on a Varian Inova 500 MHz spectrometer (Santa

Clara, CA). Solutions were prepared in CDCl3 (Cambridge Isotope Laboratories). All spectra

were acquired at 20 °C with 64 scans. Chemical shifts are reported in ppm with respect to the

residual chloroform signal (7.26 ppm).

Uniaxial tensile testing was conducted using dog bone shaped tensile bars (ca. 0.5 mm

(T) × 3 mm (W) × 25 mm (L) and a gauge length of 14 mm). The samples were aged for 48

hours at 25 °C in a desiccator prior to testing. Tensile measurements were performed on a

Shimadzu Autograph AGS-X Series tensile tester (Columbia, MD) at 25 °C with a uniaxial

extension rate of 5 mm min-1. Young’s modulus (E) values were calculated using the Trapezium

software by taking the slope of the stress-strain curve from 0 to 1% strain. Reported values are

the average and standard deviations of at least five samples.

Dynamic mechanical thermal analysis (DMTA) and stress relaxation analysis (SRA)

were performed on a TA Instruments RSA-G2 analyzer (New Castle, DE) using dog bone shape

films (ca. 0.5 mm (T) × 3 mm (W) × 25 mm (L) and a gauge length of 14 mm).

DMTA experiments were conducted in tension film mode, where the axial force was first

adjusted to 0 N with a sensitivity of 0.05 N. The strain adjust was then set to 30%, with a

minimum strain of 0.05%, a maximum strain of 5% and a maximum force of 1 N in order to

prevent the sample from going out of the specified strain range. A temperature ramp was then

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performed from 30 °C to 120 °C at a rate of 5 °C min-1, with an oscillating strain of 0.05% and

an angular frequency of 6.28 rad s-1. The Tg was calculated from the maximum value of the loss

modulus. The crosslinking density (νe) and the molar mass between crosslinks (Mx) were

calculated using the storage modulus (E’) at 100 °C and equation 1.

𝐸! 𝑇 = 3𝐺′(𝑇) = 3𝑅𝑇𝜈! =!"#!!

(S1)

Where E’ and G’ are the storage and shear modulus respectively, R is the universal gas constant,

T refers to the absolute temperature in the rubbery region (ca. 373 K) and ρ is the density of the

poly((±)-lactide (ca. 1.25 g cm-3).

Figure S1: a) Storage and b) loss modulus of ICSPLA materials while varying catalyst loading as well as c) loss modulus of ICSPLA materials while varying IC:OH ratio. Data acquired from uniaxial tension deformation (30 to 120 °C at 5 °C min-1, ω=6.28 rad s-1 and γ=0.05%) with a rectangular sample geometry.

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Table S1: Thermal and thermomechanical characterization of ICSPLA materials with respect to IC:OH and Sn(Oct)2 to initial OH ratios.

Sample IC:OH ratio

Sn(Oct)2 to initial OH ratio

Tg, DMTA (°C)a

Tg, DSC (°C)

E' at 100 °C (MPa)

νe (10-4 mol

cm-3)b,c Mx (kg mol-1)b,c

ICSPLA-0.60-0.025 0.60:1 0.025 55 50. 1.1 1.2 10.

ICSPLA-0.70-0.025 0.70:1 0.025 57 51 1.6 1.7 7.3

ICSPLA-0.80-0.025 0.80:1 0.025 60 55 2.3 2.5 5.1

ICSPLA-0.90-0.025 0.90:1 0.025 62 57 3.3 3.6 3.5

ICSPLA-1.00-0.025 1.0:1 0.025 64 57 3.7 4.0 3.1

ICSPLA-1.10-0.025 1.1:1 0.025 65 57 4.5 4.8 2.6

ICSPLA-0.75-0.013 0.75:1 0.013 59 56 1.1 1.2 11

ICSPLA-0.75-0.025 0.75:1 0.025 58 52 1.3 1.4 9.0

ICSPLA-0.75-0.050 0.75:1 0.050 58 56 1.3 1.4 9.0

aDetermined by the peak temperature of the loss modulus after uniaxial tension deformation (30 to 120 °C at 5 °C min-1, ω=6.28 rad s-1 and γ=0.05%) with a rectangular sample geometry; bcalculated using equation 1 at 100 °C; cbased on density of 1.25 g cm-3.

The SRA experiments were performed in a strain control at specified temperature (100-

140 °C). The samples were allowed to equilibrate at this temperature for approximately 15

minutes, after which the axial force was then adjusted to 0 N with a sensitivity of 0.05 N. Each

sample was subjected to an instantaneous 9.3% strain. The stress decay was monitored, while

maintaining a constant strain (9.3%), until the stress relaxation modulus had relaxed to at least

37% (1/e) of its initial value. This was performed three times for each sample.

Freezing transition temperature (Tv) determination: The characteristic relaxation time (τ*) was

defined as the time required for the stress relaxation modulus to reach 37% (1/e) of its initial

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value and determined via SRA at varying temperatures from 100 to 140 °C. These points were

then plotted versus 1000/T and fit to the Arrhenius relationship in equation 2.3

𝜏∗ 𝑇 =   𝜏!𝑒!!/!" (S2)

Where τ0 is the characteristic relaxation time at infinite T, Ea is the activation energy of the

transesterification reaction (kJ mol-1), R is the universal gas constant and T is the temperature at

the SRA was performed (373-413 K).

Tv is defined as the point at which a vitrimer exhibits a viscosity of 1012 Pa s, also known

as the liquid to solid transition viscosity (η).3,4 Using Maxwell’s relation (equation S3) and E’

determined from DMTA, τ* was determined to be ca. 1.4 x 106 s at Tv (i.e., for these samples the

relaxation time for a sample with modulus E’ at the defined liquid to solid transition viscosity of

1012 Pa s is estimated as 1.4 x 106 s for all temperatures since E’ is relatively invariant in the

rubbery state). The Arrhenius relationship from equation 2 was then extrapolated to τ* = 1.4 x

106 to determine Tv for each sample.

𝜂 = !!𝐸′ ∗ 𝜏∗ (S3)

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Figure S2: Arrhenius analysis of τ* versus 1000/T with respect to IC:OH ratio (left) and Sn(Oct)2 to initial OH ratio (right). Samples ICSPLA-1.0 and ICSPLA-1.1 not shown as their lines overlap with ICSPLA-0.9. Table S2: Tensile testing, gel fraction and stress relaxation characterization of ICSPLA materials.

Sample εb (%) σTS (MPa) E (GPa) Gel

fractiona

Tv (°C)b

ICSPLA-0.60-0.025 5.5 ± 1 50 ± 4 1.5 ± 0.2 0.94 57 ICSPLA-0.70-0.025 4.8 ± 0.5 59 ± 1 1.8 ± 0.1 0.96 59 ICSPLA-0.80-0.025 5.1 ± 2 60. ± 4 1.8 ± 0.2 0.97 60. ICSPLA-0.90-0.025 5.1 ± 0.8 60. ± 2 1.7 ± 0.1 0.98 61 ICSPLA-1.00-0.025 5.0 ± 0.4 56 ± 3 1.8 ± 0.2 0.98 61 ICSPLA-1.10-0.025 5.5 ± 1 52 ± 2 1.7 ± 0.1 0.97 61 ICSPLA-0.75-0.013 4.5 ± 0.7 54 ± 2 1.6 ± 0.1 0.99 63 ICSPLA-0.75-0.025 4.5 ± 1 51 ± 4 1.6 ± 0.1 0.99 61 ICSPLA-0.75-0.050 5.4 ± 1 49 ± 4 1.5 ± 0.1 0.99 59

Tensile testing of the materials was performed after aging the samples at 25 °C for 48 hours. aperformed via solvent extraction after 48 h in methylene chloride. bCalculated at τ* = 1.4 x 106 s on Figure 2.

Differential scanning calorimetry (DSC) was conducted on a TA Instruments Discovery

DSC (New Castle, DE). The instrument was calibrated using an indium standard. All samples

were prepared using T-Zero hermetic pans (ca. 5 mg) under a N2 purge of 50 mL min-1. The

thermal history of the samples was first erased by heating to 150 °C at a rate of 10 °C min-1 and

isothermally annealed for 1 min. The samples were then cooled at 10 °C min-1 to -80 °C

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followed by a second heating cycle to 100 °C at 10 °C min-1. Glass transition temperatures (Tg)

were acquired at the mid-point of each transition using the Trios software and reported values are

the average of at least three samples.

Solvent extraction experiments were performed by placing a small amount of crosslinked

polymer (ca. 20 mg) into a 20 mL vial filled with dichloromethane (DCM). The vial was then

closed and stirred for 48 h before removing the solvent by gravity filtration. The recovered

sample was dried under reduced pressure for 48 h at 60 °C and 20 mTorr, after which the sample

was weighed and the gel percent was determined. A high temperature swell test was also

performed with ICSPLA-0.025-0.75 submerged in 1,2,4-trichlorobenzene at 140 °C for 120 h.

Some yellowing, likely from oxidative degradation, was noticed about 9 h into the test, but the

sample did not exhibit full dissolution during the entirety of the test. More yellowing was

observed over this extended time period.

Figure S3: High temperature swell test with ICSPLA-0.025-0.75 for 48 h at 140 °C in 1,2,4-trichlorobenzene.

Light scattering size exclusion chromatography (LS-SEC) was performed at 35 °C using

three successive Phenomenex Phenogel-5 columns (Torrance, CA), which was equipped with a

Wyatt Technology DAWN DSP multiangle laser light scattering detector as well as a Wyatt

Optilab EX RI detector (Santa Barbara, CA); tetrahydrofuran was used as the mobile phase with

an elution rate of 1 mL min-1. Mm and Đ was determined by calculating the dn/dc value of the

polymer from the refractive index and the Agilent 1260LC data analysis software. Mn and Đ

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were determined based on a 10-point calibration curve using polystyrene standards purchased

from Polymer Laboratories.

Healing Strategy: Tensile bars broken via uniaxial tensile testing were placed in a dog bone

shaped mold between two Teflon® sheets. This was then placed in a press mold at 140 °C and

allowed to equilibrate for 2 minutes. Approximately 4 MPa of pressure was then applied for 30

minutes, at which point the mold was removed from the press mold and allowed to cool to room

temperature. The resulting tensile bars were ca. 0.5 mm (T) × 3 mm (W) × 25 mm (L) and a

gauge length of 14 mm. The bars were then allowed to age at room temperature for 48 h in a

desiccator, followed by uniaxial tensile testing to determine the recovery in their material

properties.

Figure S4: ICSPLA materials before (top) and after (bottom) healing via compression molding.

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Table S3: Uniaxial tensile testing of ICSPLA materials after healing

Sample εb (%)

Recovery (%)

σTS (MPa)

Recovery (%)

E (GPa)

Recovery (%)

ICSPLA-0.6-0.025 3.2 ± 0.9 58 ± 19 46 ± 6 92 ± 14 1.8 ± 0.2 120 ± 21 ICSPLA-0.7-0.025 2.4 ± 0.4 50 ± 10 51 ± 8 86 ± 14 2.2 ± 0.2 122 ± 13 ICSPLA-0.8-0.025 2.1 ± 0.8 41 ± 20 42 ± 17 70 ± 29 2.2 ± 0.1 122 ± 15 ICSPLA-0.9-0.025 2.6 ± 0.5 51 ± 13 50. ± 8 83 ± 14 2.1 ± 0.2 124 ± 14 ICSPLA-1.0-0.025 1.9 ± 0.9 38 ± 18 35 ± 15 63 ± 27 1.9 ± 0.1 106 ± 13 ICSPLA-1.1-0.025 1.4 ± 0.5 25 ± 10 27 ± 9 52 ± 17 1.9 ± 0.1 106 ± 8 ICSPLA-0.75-0.013 1.5 ± 0.5 33 ± 12 24 ± 7 44 ± 13 1.7 ± 0.1 106 ± 9 ICSPLA-0.75-0.025 3.0 ± 0.7 67 ± 24 44 ± 7 86 ± 15 1.8 ± 0.2 113 ± 14 ICSPLA-0.75-0.050 3.3 ± 0.8 61 ± 21 50. ± 6 102 ± 15 2.0 ± 0.5 133 ± 34

References

1 Korhonen, H.; Helminen, A.; Seppälä, J. V. Polymer 2001, 42, 7541-7549.

2 Karikari, A. S.; Mather, B. D.; Long, T. E. Biomacromolecules 2007, 8, 302-308.

3 Capelot, M.; Unterlass, M. M.; Tournilhac, F.; Leibler, L. ACS Macro Lett. 2012, 1, 789-792.

4 Montarnal, D.; Capelot, M.; Tournilhac, F.; Leibler, L. Science 2011, 334, 965-968.