PLA–PHB–PLA Triblock Copolymers: Synthesis by Sequential

May 15, 2013 - The relationship between zero-shear viscosity can be described by the scaling law of ηo ∝ Mw3.4, which was determined previously for...
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PLA−PHB−PLA Triblock Copolymers: Synthesis by Sequential Addition and Investigation of Mechanical and Rheological Properties Dinesh C. Aluthge,† Cuiling Xu,† Norhayani Othman,‡,§ Nazbanoo Noroozi,‡ Savvas G. Hatzikiriakos,‡ and Parisa Mehrkhodavandi†,* †

Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia, Canada Department of Chemical and Biological Engineering, University of British Columbia, Vancouver, BC, Canada § Faculty of Chemical Engineering, Universiti Teknologi Malaysia, Skudai 81310, Johor Bahru, Malaysia ‡

ABSTRACT: The dinuclear indium catalyst [(NNO)InCl]2(μ-OEt)(μ-Cl), previously reported to be highly active for the living ring-opening polymerization of cyclic esters lactide (LA) and β-butyrolactone (BBL), was used to generate a series of triblock copolymers of poly(lactic acid) (PLA) and poly(hydroxybutyrate) (PHB). Copolymers PLLA−PDLLA−PLLA and PLLA−PDLLA−PDLA, synthesized via sequential monomer addition, showed low molecular weight distributions and excellent correlation between the calculated and experiment molecular weights. Significantly, triblock copolymers of the type PLA−PHB−PLA were also synthesized for the first time through a sequential addition technique. Analysis of polymers after each addition of monomer showed that although only 85% conversion was achieved after addition of BBL, the remaining chain ends were active and addition of more lactide yielded a triblock. Rheological studies of PLLA−PHB−PDLA indicated solid like behavior even well above the temperature at which stereocomplex formation was observed. These elastomeric triblocks exhibited elongations at break 5−10 times greater than those of corresponding PLLA−PDLLA−PLDA triblocks.

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ROP of BBL,2c examples of copolymers of LA and BBL are rare.8 These polymers are important because the incorporation of racemic BBL in the middle block of an A−B−A hard−soft− hard triblock produces a biodegradable elastomer whose mechanical properties can be controlled by the ratio of the hard and soft segments. In particular, in PLLA−PHB−PDLA triblocks, the PLLA and PDLA blocks can form stereocomplexes with a highly crystalline hard segment. It is very challenging to obtain PHB-containing A−B−C triblocks generated from simple consecutive addition of monomers, in part due to scrambling via transesterification.9 Kimura et al. have reported the formation of PLLA−PHB− PLLA (A−B−A) polymers through a multistep synthesis involving the initial tin-catalyzed synthesis of bishydroxyterminated PHB via ring-opening polymerization of BBL in the presence of 1,4-butanediol, followed by reaction with L-LA to obtain a triblock (PLLA−PHB−PLLA).10 This polymer exhibits increased Young modulus and crystallinity, with certain compositions, and decreased elongation at break compared to pure PLLA and PHB. These mechanical properties render such materials potentially suitable as biodegradable thermoplastic elastomers. To our knowledge, there are no examples of wellcharacterized stereocomplex forming PLLA−PHB−PDLA (A− B−C) triblocks.

INTRODUCTION In recent years, biodegradable polyesters such as poly(lactic acid) (PLA)1 and poly(3-hydroxybutyrate) (PHB)2 have been investigated in a variety of applications such as packaging and drug delivery. The ring-opening polymerization (ROP) of cyclic esters such as lactide (LA)3 and β-butyrolactone (BBL)2c catalyzed by discrete metal complexes has been explored in an attempt to control polymer properties and limit uncontrolled chain transfer/termination events. However, the brittleness and the relatively weak mechanical properties associated with these crystalline polymers often prohibit processing and wide ranging applications.4 Blending of various isomeric types of PLA generated from LLA (PLLA), D-LA (PDLA), and racemic LA (PDLLA) has been implemented to mitigate brittleness and improve other mechanical properties through control of crystallization and morphology. Blending of PLLA and PDLA frequently leads to stereocomplex formation with markedly higher melting points than conventional PLAs.5 Another strategy to improve and control mechanical properties is by synthesizing diblock copolymers such as PLLA−PDLA which form stereocomplex crystallites of high melting point (above 200 °C) that cause significant viscosity enhancement.6 These diblocks also exhibit improvement in elongation of break and tensile strength. In an attempt to improve further PLA properties such as melting point, elongation at break and tensile strength, copolymers of LA and other cyclic esters such as glycolide or caprolactone have been developed.7 However, due to the difficulties with the © XXXX American Chemical Society

Received: March 11, 2013 Revised: April 22, 2013

A

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Scheme 1. Polymerization of Cyclic Esters by [(NNO)InCl]2(μ-Cl)(μ-OEt) (1)

PDI = 1.14) of the PLLA was measured. This step was repeated for DL-LA (Mn,theo = 115 kDa, Mn,expt = 140 kDa, PDI = 1.12) and the third monomer L-LA to yield the final triblock polymer (Mn,theo = 157 kDa, Mn,expt = 190 kDa, PDI = 1.24). Conversions, as determined by 1H NMR spectra of the samples, were >98% after each addition. Overlaid GPC traces of the different steps in this synthesis show that the peak corresponding to the MA block is not observed after addition of MB and that the MA−MB diblock is not observed after addition of the third monomer (Figure 1). The correlation of the theoretical and expected molecular weights as well as the narrow molecular distributions observed for these triblocks indicate a lack of chain termination events. Formation of stereoblocks causes a significant decrease in polymer solubility. It is difficult to dissolve copolymers containing both L and D blocks (Table 1, entries 3−6) in most common solvents, including THF at room temperature and trichlorobenzene at elevated temperature. Under heating and sonication, these triblock copolymers can be slowly dissolved in CH2Cl2 and CHCl3. Therefore, the GPC measurements of these triblocks were carried out on a facility equipped with a refractive index detector using CHCl3 as the solvent (notated as GPC−RI). To compare the differences between results from different GPC facilities, entry 2, which is soluble both in THF and CHCl3, was run with GPC−RI and its result is 175 kDa with a PDI of 1.19. Compared to the value in Table 1 (190 kDa, run with a GPC facility on THF, equipped with a laser light scattering detector, notated as GPC−LLS), the difference between these two GPC facilities is within the measurement error. Triblock copolymers consisting of PLLA and PDLA blocks on each end respectively and a racemic block in the center have not been previously reported. The ability to generate these triblock polymers can be attributed to the highly controlled nature of ROP with catalyst 1. PLA−PHB−PLA Triblock Polymers. We used the same strategy to synthesize triblock copolymers with different weight percentages of racemic PHB as the middle segment (MB). Our initial goal was to determine whether catalyst 1 was indeed capable of a sequential block polymerization of LA (MA), BBL (MB), and LA (MC). We set out to synthesize a PLLA−PHB− PLLA triblock with 28 wt % PHB. The reaction was carried out at 25 °C in dry THF under a nitrogen atmosphere with [1]o ≈ 1 mM under constant stirring. This appears to be the ideal catalyst concentration under these conditions as a higher concentration (i.e., ≥ 1.5 mM) would cause polymer

We are interested in developing catalysts for the controlled ROP of cyclic esters and have reported the first example of an indium complex used as an initiator for the living polymerization rac-LA11 to form PLAs with moderate isotactic enrichment and low polydispersity indices (PDIs).12 Our diaminophenoxy supported dinuclear indium catalyst [(NNO)InCl]2(μ-Cl)(μ-OEt) (1) can catalyze the living homopolymerization of both LA and rac-BBL (to generate atactic PHB), with very minimal chain transfer/chain termination events (Scheme 1). We have reported on some properties and applications of these catalysts and the resulting polymers.6,12,14 In particular, we have reported on the formation of PLLA− PDLA diblocks and have studied their rheological and mechanical properties.6 Following our contributions, other research groups have reported the synthesis of indium complexes that were used as lactide polymerization initiators.13 Herein we describe the synthesis and characterization of triblock copolymers containing PLA and PHB using consecutive monomer addition with catalyst 1.

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RESULTS AND DISCUSSION PLA-Only Triblock Polymers. In a previous publication, we reported the synthesis of nearly monodispersed diblock copolymers of DL-LA or D-LA with L-LA by sequential living ring-opening polymerization with catalyst 1 (Scheme 2).6 This technique can also be used to synthesize triblock copolymers of different LA isomers (Table 1). The triblock polymers (e.g., Table 1, entry 2) were synthesized in a three-step process. In the first step L-LA (294 equiv) was added to a solution of 1 in dichloromethane and the reaction was stirred at room temperature overnight. The molecular weight (Mn,theo = 42.3 kDa, Mn,expt = 44.4 kDa, Scheme 2. Formation of Triblock PLA by Sequential Addition of LA

B

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Table 1. Summary of Synthesized Lactide Triblock Copolymers entrya 1 2 3 4

wt % LA isomers (MA−MB−MC) 36L−28DL−36L 27L−46DL−27L 36L−28DL−36D 27L−46DL−27D

MA:1 423 294 426 294

MB:1 322 505 325 505

MC:1 423 294 426 294

Mn,theob (kDa) b

168 157b 170b 157b

Mn,exptc (kDa)

PDI

169 190 152d 153d

1.06 1.24 1.15d 1.16d

Reactions were carried out in CH2Cl2, 25 °C, [1]o ≈ 1 mM; conversions >98% were observed by 1H NMR spectroscopy after each addition. Calculated from Mn,theo = (45 g mol−1 + 144 g mol−1 × (MA+MB+MC) × conversion: 1. cEntries 1−2 were soluble in THF (2 mg mL−1) and molecular weights were determined by GPC−LLS (flow rate = 0.5 mL min−1.) Universal calibration was carried out with polystyrene standards, using the Mark−Houwink parameters (K = 1.832 × 10−4 dL g−1, a = 0.69), laser light scattering detector data, and concentration detector.15 d Samples were run using a CHCl3 mobile phase (GPC-RI), calibrated with polystyrene standards (flow rate =1 mL min−1) with a correction factor of 0.58 for PLA.16 a b

To confirm the synthesis of a triblock copolymer GPC analysis was carried out after formation of each polymer block (Figure 3). The diblock and triblock polymers were not soluble in THF, thus GPC analysis was carried out in CHCl3. Correction factors for PLA (0.58) and PHB (0.54) were used to determine molecular weights obtained from GPC-RI in CHCl3 against polystyrene calibration standards.16 For the diblocks and triblocks, correction factors were calculated by considering the amounts of PLA and PHB in the polymers as indicated in the Table 2 footnote. The GPC data clearly show a stepwise increase in the Mn values with each monomer addition indicating the synthesis of a triblock. The molecular weight distributions show well-controlled living polymerization of L-LA in the first block as expected (PDI = 1.11). However, after the formation of the PLLA−PHB diblock, the PDI increases to 1.29. This slight increase in molecular weight distribution is apparent in the GPC trace of diblock (Figure 3). The experimental Mn (101 kDa) is 20% higher than the theoretical value (84.3 kDa) and is consistent with some catalyst decomposition as reported previously for the polymerization of BBL with catalyst 1.14g Addition of the third block of L-LA results in an increase in Mn, with the theoretical value (159 kDa) higher than the theoretical value (120 kDa), as expected with decomposition of ∼20% of the growing polymer chains after the second addition. However, it is clear that the [In]− PHB−PLLA chain ends are still active and after addition of the third block do form the triblock PLLA−PHB−PLLA in a living fashion and with low polydispersity. In order to study the thermal, mechanical, and rheological properties of these novel polymers, we synthesized a series of PLA−PHB−PLA triblocks with varying PHB content and LA stereoisomers (Table 3). It should be noted that the rac-BBL was rigorously dried over CaH2 and freshly distilled prior to each polymerization. Omission of this step resulted in the failure of the catalyst to polymerize the BBL. During the synthesis, the polymers with the highest PHB content (36L− 28BBL−36L) reached the maximum conversion of BBL (∼85%), while the 36L−5BBL−36L triblock only achieved ∼72% conversion. This can be attributed to the fact that at a low monomer concentration the catalyst ceases to polymerize. Thermal Study of Triblock Polymers. We began our studies with a comparison of the thermal properties of the PLAonly triblocks (Table 4) and those containing PHB in the center block (Table 5). Triblock copolymers of PLLA− PDLLA−PLLA with 36% PLLA at each end (Table 4, entry 1) shows a melting temperature of 168.9 °C. As the PLLA length decreases to 27% the polymer becomes amorphous as no melting peak detected during second heating (Table 4, entry 2). In contrast, triblocks incorporating PDLA possess

Figure 1. Overlap of GPC traces for the synthesis of 27L−46LD−27L. Right (- -) MA (Mn = 44.4 kDa, PDI = 1.14). Middle (− −) MA + MB (Mn = 140 kDa, PDI = 1.12). Left (red ) MA + MB + MC (Mn = 190 kDa, PDI = 1.24).

precipitation and lower concentrations (∼0.75 mM) would diminish BBL conversion. 1 H NMR spectroscopy is used to determine monomer conversion after each step in the synthesis (Figure 2). The first segment is generated with 426 equiv of LA as expected with over >95% conversion. The second block was generated by addition of 544 equiv of rac-BBL to the mixture. Sixteen hours after the addition, the 1H NMR spectrum of the mixture shows that the rac-BBL polymerization has reached ∼85% conversion. Further reaction time does not result in appreciable monomer conversion. When the concentration of rac-BBL approaches 80 mM the polymerization ceases under these conditions. The polymerization of the third and final segment of the triblock occurs by addition of 426 equiv L-LA to the reaction mixture. After 8 h, > 96% conversion is achieved. We expect very minimal, if any, random incorporation of monomers in each of these sequences. After the first addition of LA, the monomer has reached nearly full conversion (within the detection limit of 1H NMR spectroscopy, i.e. > 95%) prior to the addition of the second monomer (BBL). After addition and polymerization of BBL over the reaction period, a considerable amount of monomer (∼15%) remains in solution. However, we have shown previously that at 25 °C BBL below a certain concentration is not ring-opened by our catalyst.14g At this point the BBL concentration is below the level necessary for polymerization at room temperature. We do not observe an appreciable conversion of the unreacted BBL (within the 1H NMR detection limit) after addition of the second aliquot of LA. Importantly, unreacted BBL is observed after the completion of formation of the triblock. This indicates that the extent of monomer scrambling is minimal and that the unreacted BBL monomer is not polymerized after the addition of the final PLLA block. C

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Figure 2. 1H NMR spectra (400 MHz, 25 °C, CDCl3) showing monomer conversion after each monomer addition during the synthesis of 36PLLA28PHB-36PLLA triblock copolymer. Reactions were carried out in THF, 25 °C, [1]o ≈ 1 mM. (a) First PLLA block showing ∼95% conversion after 5 h. (b) PLLA−PHB diblock showing ∼85% rac-BBL conversion after 16 h. (c) Final PLLA−PHB−PLLA triblock showing an overall L-LA conversion of ∼96% after 8 h.

has a higher melting temperature than the triblock with shorter and D-blocks. Copolymers with atactic PHB as the middle block also display one melting temperature as an indication of stereocomplex crystallites. The melting temperature of triblock copolymer of PLLA−PHB−PLLA with 47% PLLA at each end (Table 5, entry 1) is 171.3 °C (typical for crystalline PLA), which is slightly lowered by decreasing the length of PLLA. Triblocks with PLLA and PDLA at each end (Table 5, entries 4, 5 and 6) also show one melting peak at much higher temperature compared to polymers with PLLA at each end. Longer L- and D- blocks result in higher melting temperature. These melting temperatures are well above 200 °C and corresponds to the formation of stereocomplex crystallites as also seen in the PLLA−PDLLA−PDLA triblocks, entries 3 and 4 of Table 4. Similar observations were made by Woo et al.17 who reported that the crystallization of stereocomplex PLA was significantly hindered in blends with PHB and less perfect crystals were found with increasing PHB content. L-

Figure 3. Overlap of GPC traces for the synthesis of 27L−46LD−27L. Right (blue ) MA (Mn = 57.1 kDa, PDI = 1.11). Middle (red −) MA + MB (Mn = 84.2 kDa, PDI = 1.29). Left (green ) MA + MB + MC (Mn = 119.5 kDa, PDI = 1.19).

exclusively stereocomplex crystallites with only one melting temperature (Table 4, entries 3,4). As seen above, the melting temperature and crystallinity of the stereocomplex decreases with decrease of the PLLA and PDLA length; 36L−28DL−36D D

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Table 2. GPC Data for the Synthesis of PLLA−PHB−PLLA Polymer

entrya

sample

MA:1

MB:1

MC:1

Mn,theob (kDa)

Mn,exptc (kDa)

PDIc

1 2 3

PLLA PLLA−PHB PLLA−PHB−PLLA

426 426 426

− 544 544

− − 426

59 101 159

57.5 84.3 120

1.11 1.29 1.19

a Reactions were carried out in THF, 25 °C, [1]o ≈ 1 mM. Conversions were determined by 1H NMR spectroscopy with each block. bCalculated from Mn,theo = 45 g mol−1 +144 g mol−1 × (MA + MC):1 × conversion +86 g mol−1 × (MB):1 × conversion. cGPC measurements in CHCl3 (GPC− RI), calibrated with polystyrene standards (flow rate =1 mL min−1). The results were multiplied by a correction factor (X), calculated by X = 0.58 × nPLA + 0.54 × nPHB (nPLA = mole fraction of PLA in sample, nPHB = mole fraction of PHB in sample).16

Table 3. Summary of Synthesized PLLA−PHB−PLA Copolymers entrya

mol % LA isomers (MA−MB−MC)

MA:1

MB:1

MC:1

Mn,theo (kDa)b

Mn,expt (kDa)c

PDI

PHB %

1 2 3 4 5 6

36L−28BBL−36L 45L−10BBL−45L 47L−5BBL−47L 36L−28BBL−36D 45L−10BBL−45D 47L−5BBL−47D

426 530 559 426 530 559

544 197 99 544 197 99

426 530 559 426 530 559

152 154 156 157 158 156

115 126 130 113 129 138

1.25 1.16 1.25 1.32 1.27 1.22

26 8.2 3.9 29 9.3 4.5

Reactions were carried out in THF, 25 °C, [1]o ≈ 1 mM at the beginning of the polymerization. Conversions were determined by 1H NMR spectroscopy with each block. bCalculated from Mn,theo = 45 g mol−1 +144 g mol−1 × (MA + MC):1 × conversion + 86 g mol−1 × (MB):1 × conversion. cGPC measurements were in CHCl3 (GPC−RI), calibrated with polystyrene standards (flow rate =1 mL min−1) The results were multiplied by a correcting factor (X), X = 0.58 × nPLA + 0.54 × nPHB (nPLA = mole fraction of PLA in sample, nPHB = mole fraction of PHB in sample).16 a

Table 4. Glass Transition Temperature, Tg, the Melting Peak Temperature, Tm, the Enthalpy of Crystallization, ΔHc, and the Enthalpy of Heating, ΔHm, of the PLA Triblock Copolymersa entry

wt % of monomers (MA−MB−MC)

Tg (°C)

Tm (°C)

ΔHc (J g−1)

ΔHm (J g−1)

1 2 3 4

36L−28DL−36L 27L−46DL−27L 36L−28DL−36D 27L−46DL−27D

53.3 57.3 62.8 62.8

168.9 − 212.8 208.4

− − 20.6 11.1

4.6 − 32.9 17.6

Table 5. Glass Transition Temperature, Tg, the Melting Point Transition Temperature, Tm, the Enthalpy of Crystallization, ΔHc, and the Enthalpy of Heating, ΔHm of Triblock Copolymers Having PHB in the Center Blocka entry

wt % of monomers (MA−MB−MC)

Tg (°C)

Tm (°C)

ΔHc (J g1−)

ΔHm (J g−1)

1 2 3 4 5 6

47L−5BBL−47L 45L−10BBL−45L 36L−28BBL−36L 47L−5BBL−47D 45L−10BBL−45D 36L−28BBL−36D

53.2 48.6 38.7 4.5 6.4 4.8

171.3 171.0 166.7 207.6 204.8 208.9

16.5 27.0 19.8 36.9 38.0 28.4

39.1 33.3 37.8 41.1 40.7 36.2

a

The thermal analysis of the samples was performed by using a differential scanning calorimeter (DSC), DSC-60 SHIMADZU calibrated by indium (± 0.5 °C). Calorimetry was performed in nitrogen atmosphere with approximately 2.3−2.5 mg of sample. Samples were heated to 200 °C to determine Tm and ΔHm and cooled down to room temperature to determine Tc and ΔHc with a heating/ cooling rate of 2 °C/min.

a

The thermal analysis of the samples was performed by using a differential scanning calorimeter (DSC), DSC-60 SHIMADZU calibrated by indium (± 0.5 °C). Calorimetry was performed in nitrogen atmosphere with approximately 2.3−2.5 mg of sample. Samples were heated to 200 °C to determine Tm and ΔHm and cooled down to room temperature to determine Tc and ΔHc with a heating/ cooling rate of 2 °C/min.

Rheological Study of Triblock (PLLA−PDLLA−PLLA and PLLA−PDLLA−PDLA) Copolymers. The linear viscoelastic moduli and the complex viscosity of the triblock copolymers PLLA−PDLLA−PLLA (Table 4 entries 1,2) with different PLLA and PDLLA ratios were determined (Figure 4). The viscoelastic behavior for these triblock copolymers was independent of PLLA and PDLLA ratios. Small differences in the viscoelastic properties of the triblock copolymers are due to small differences in the molecular weights of the blocks. The terminal zone at low frequencies has been reached as indicated by slope of 1 and 2 for G″ and G′, respectively. The vertical shift factors, bT are small, simply reflecting the density variations, while the horizontal shift factors, aT, are similar to

those reported for PLA homopolymers14d and various PLA block copolymers.6 In addition, the data in Figure 4 implies that the plateau modulus is about 1 MPa, similar to those reported by others.6,18 The zero-shear viscosities of the triblock copolymers PLLA− PDLLA−PLLA as well as those previously reported values for nearly monodispersed PLAs, L/LD blends, and diblock copolymers PLLA−PDLLA are plotted as a function of molecular weight in Figure 5. The relationship between zeroshear viscosity can be described by the scaling law of ηo ∝ Mw3.4, which was determined previously for a series of nearly E

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Figure 4. Master curve of the linear viscoelastic moduli, G′ and G″ and complex viscosity |η*(ω)| of 36L−28DL−36L (unfilled symbols) and 27L− 46DL−27L (filled symbols) copolymers at the reference temperature of 180 °C.

Figure 5. Scaling between the zero-shear viscosity and molecular weight of PLAs at the reference temperature of 180 °C. Figure 6. Thermal stability of representative triblock copolymers.

monodispersed PLAs.14d,18 The slope of 3.4 implies linear microstructure and it is also in agreement with the solution properties results shown previously.14d The zero-shear viscosities of blends, diblocks and triblocks also agree with this scaling law implying the similarities in the behavior between PLA homopolymers, diblocks and triblocks of linear architecture. Rheological Study of Triblocks PLLA−BBL−PLLA and PLLA−BBL−PDLA) copolymers. Figure 6 shows representative results of the thermal stability of triblock copolymers investigated by dynamic time sweep measurements at the constant frequency of 1 rad/s and the temperature of 190 and 230 °C. The triblock copolymers of type PLLA−BBL−PLLA were more thermally stable than PLLA−BBL−PDLA. It should be noted that 3.5 wt % of TNPP, typically used in PLA polymers as a thermal stabilizer were added to both types of copolymer. The presence of larger amounts of PHB as the middle block decreases the thermal stability of the copolymer. Figures 7a and 7b show the master curves of the linear viscoelastic moduli and the complex viscosity of the triblock copolymers 47L−5BBL−47L and 45L−10BBL−45L with different middle block length ratios in the temperature range of 180 to 200 °C. In the case of 47L−5BBL−47L, the terminal

zone at low frequencies has been reached as indicated by the slopes of 1 and 2 for G″ and G′, respectively. The linear viscoelastic properties of 36L−28BBL−36L were measured at 180 °C (Figure 7c). Above 180 °C, due to the low viscosity of the copolymer and thermal degradation, the measurement could not be carried out. As discussed above, triblocks with PLLA and PDLA at each end (Table 5, entries 4, 5, and 6) form stereocomplex crystallites at temperatures above 200 °C. These polymers barely flow and exhibit solid-like behavior even at temperatures well above the stereocomplex formation (Figure 8). In particular, for 47L−5BBL−47D, it can be observed from Figure 8 that G′ is well above G″ over the whole range of frequencies investigated. Tensile Properties of Triblock Copolymers. The tensile properties of the triblock copolymers were measured (Table 6). For the PLA-only copolymers, the elastic modulus of triblocks remains unchanged regardless of the outer block and block length ratio of PLA (Table 5, entries 1−4). The tensile strength of copolymers PLLA−PDLLA−PLLA and PLLA−PDLLA− PDLA with 28% PDLLA are comparable to that of PLLA,19 whereas the tensile strength of copolymers with 46% PDLLA F

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Figure 7. Master curves of the linear viscoelastic moduli, G′ and G″ and complex viscosity |η*| of copolymers of type L−BBL−L at the reference temperature of 180 °C: (a) top right, 47L−5BBL−47L; (b) top left, 45L−10BBL−45L; (c) bottom, 36L−28BBL−36L.

Table 6. Tensile Properties of Triblock Copolymers entry

samples

1 2 3 4 5 6 7 8 10

36L−28DL−36L 27L−46DL−27L 36L−28DL−36D 27L−46DL−27D 47L−5BBL−47L 45L−10BBL−45L 36L−28BBL−36L 47L−5BBL−47D 36L−28BBL−36D

tensile strength (MPa) 56 40 58 45 34 40 20 33 15

± ± ± ± ± ± ± ± ±

2 4 4 6 1.5 7.6 1.7 2.1 1.7

elastic modulus (MPa) 2548 2674 2637 2654 847 729 338 548 431

± ± ± ± ± ± ± ± ±

124 118 110 102 149 116 2 32 85

elongation at break (%) 4.8 4.6 3.5 2.8 9.7 14.2 21.0 10.6 7.3

± ± ± ± ± ± ± ± ±

0.1 0.3 0.1 0.1 0.1 0.6 1.1 1.7 0.9

having only PLA blocks. However, the elongation at break of these copolymers improved significantly compared to the PLA only triblocks due to the elastomeric nature of atactic PHB. We observed that shorter block of PLLA and PDLA and longer block of PHB decrease the tensile strength of triblocks. These results agree with literature reports and highlight the elastomeric nature of PLLA−PHB−PLLA triblocks.10,21 The elongation at break of PLLA−PHB−PLLA copolymers improved about 10 times compared to pure PLLA. It should be noted that Kimura et al. studied lower molecular weight PHB (Mn 31 kDa), which is miscible with PLLA. In contrast, high molecular weight PHB blocks used in our study can be phase-separated from PLA.10 This could be the reason for the poor tensile properties observed in the present study for the triblock with 28% PHB.

Figure 8. Linear viscoelastic moduli, G′ and G″, of copolymer 47L− 5BBL−47D at various temperatures (180−220 °C). This behavior is solid-like as G′ is always above G″ over the whole range of frequency investigated.

decreases slightly with the value comparable to the tensile strength of PDLLA.20 Improvements of 85% and 77% were found in the elongation at break for copolymers PLLA− PDLLA−PLLA as compared to PLLA. The elongation at break of triblocks PLLA−PDLLA−PDLA was higher compared to the copolymers without stereocomplex formation.19,20 In comparison, triblocks incorporating PHB have shown a significant drop in their tensile properties compared to those G

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techniques. A Bruker Avance 300 or 400 MHz spectrometer was used to record 1H spectra. A Bruker Avance 600 MHz spectrometer was used to acquire homonuclear decoupled 1H{1H} spectra of PLA. 1H NMR chemical shifts are given in ppm versus residual protons in deuterated solvents as follows: δ 7.27 CDCl3. Molecular weights and polydispersity indices were determined by triple detection gel permeation chromatography using a Waters liquid chromatograph equipped with a Waters 515 HPLC pump, a Waters 717 plus autosampler, Waters Styragel columns (4.6 × 300 mm) HR5E, HR4 and HR2, a Waters 2410 differential refractometer, a Wyatt tristar miniDAWN laser light scattering detector and a Wyatt ViscoStar viscometer. A flow rate of 0.5 mL/min was used and samples were dissolved in tetrahydrofuran (THF) (ca. 2 mg/mL). For samples insoluble in THF, GPC measurements were done using a CHCl3 mobile phase (GPC-RI), calibrated with monodispersed polystyrene standards (Polymer Laboratories) the range of 9000−233000 g/mol. A flow rate of 1 mL/min with a Waters Styragel column (HR4) was used, and the detection was through a Waters model 2410 refractive index detector. The results were multiplied by a correction factor of 0.58 for PLA.16 The measurements were carried out at laser wavelength of 690 nm, at 25 °C. The data were processed using the Astra software provided by Wyatt Technology Corp. A differential scanning calorimeter (DSC) Q1000 (TA Instruments) was employed to measure the glass transition (Tg) and melting (Tm) temperatures (±0.5 °C). Materials. THF were taken from an IT Inc. solvent purification system with activated alumina columns and degassed before use. CH2Cl2 was refluxed with CaH2, distilled and degassed before use. ethanol, CDCl3 were dried over CaH2, transferred under vacuum and degassed through freeze−pump−thaw cycles before use. DL-, L-, and Dlactide were donated by Purac Biomaterials and recrystallized twice in toluene. β-Butyrolactone, purchased from Aldrich, was stirred with CaH2 for 48 h, distilled under vacuum, degassed through freeze− pump−thaw cycles and kept in the freezer at −30 °C. The catalyst [(NNO)InCl] 2(μ-Cl)(μ-OEt) (1) was prepared according to previously published procedures.12 Representative Synthesis of Lactide Triblock Copolymers. Complex 1 (23 mg, 0.021 mmol) was dissolved in CH2Cl2 and transferred to a round-bottom flask. While stirring, a solution of L-LA in CH2Cl2 (1.267 g, 8.799 mmol) was added. Approximately 20 mL of CH2Cl2 was used. The mixture was allowed to stir for a few hours and then a solution of the second monomer rac-LA (0.966 g, 6.71 mmol) in CH2Cl2 was added to the reaction. The polymerization was allowed to stir overnight and a solution of the third monomer D-LA (1.267 g, 8.799 mmol) in CH2Cl2 (5 mL) was added to the reaction mixture. The reaction was allowed to stir overnight and then quenched with acidic Et2O (0.5 mL of 1.5 M HCl in Et2O). A few drops of the mixture were removed to check conversion and the remaining mixture was concentrated under vacuum and the polymer was precipitated with cold MeOH. The resulting polymer was washed with cold MeOH (3 × 3 mL) and dried under vacuum. The polymer was then redissolved in CH2Cl2, a thermal stabilizer tris(nonylphenyl)phosphite (TNPP) (0.35 wt % of polymer) was added, and the solvent was removed under vacuum overnight. Parallel experiments of the synthesis of only the first block and the first two block polymers were carried out under the same condition to get information on the molecular weight of each block. Representative Synthesis of Lactide/Butyrolactone Triblock Copolymers (Table 3). Complex 1 (22 mg, 0.020 mmol) was dissolved in THF and transferred to a round-bottom flask. While stirring, a solution of L-LA in THF (1.226 g, 8.514 mmol) was added. Approximately 20 mL of THF was used. The polymerization was allowed to stir for 4 h and a 1H NMR spectrum was obtained to establish monomer conversion. The 0.5 mL of the reaction mixture was withdrawn for GPC analysis. Then a solution of the second monomer rac-BBL (0.840 mL, 10.3 mmol) in THF (1 mL) was added to the reaction. A 1H NMR spectrum was obtained after 16 h to establish monomer conversion and 0.5 mL of the reaction mixture was withdrawn for GPC analysis. The reaction was allowed to stir overnight and a solution of the third monomer L-LA (1.222 g, 8.486

CONCLUSIONS In this work, we report the formation of stereocontrolled triblocks using sequential monomer addition with the living ring-opening polymerization catalyst [(NNO)InCl]2(μ-Cl)(μOEt) (1) and study the mechanical and rheological properties of the polymers. To our knowledge this is the first time sequential addition has been used to synthesize wellcharacterized triblock copolymers of the type PLLA−PHB− PDLA where the outer blocks can form stereocomplex crystallites. We first investigated the formation of two types of block copolymers including only PLA: PLLA−PDLLA−PLLA triblocks generated from L- and rac-LA, and PLLA−PDLLA− PDLA triblocks generated from L-, rac-, and D-LA. The polymerization reactions are highly controlled and the triblock copolymers show low molecular weight distributions and excellent correlation between the calculated and experiment molecular weights. Incorporation of PHB in the polymer was more challenging. Sequential addition of L-LA followed by BBL resulted in a PLLA−PHB diblocks with broader molecular weight distributions and higher than calculated molecular weights. In addition, only ∼85% of the BBL was consumed in this reaction. However, addition of the third block of L-LA resulted in higher molecular weight polymers consistent with the formation of a triblock PLLA−PHB−PLLA. It appears that although a small fraction of the catalyst is deactivated after the addition of PHB, the majority of chain ends remain active. We have previously reported that for PLLA−PDLA diblocks, formation of stereocomplex crystallites leads to a high melting point (>200 °C).6 In this work we show that while triblock polymers PLLA−PDLLA−PLLA crystallize at temperatures comparable to that of pure PLLA or PLDA, triblock polymers PLLA−PDLLA−PDLA with amorphous PLA in the middle block show crystalline stereocomplex behavior. Similarly, triblock copolymers PLLA−PHB−PDLA with amorphous PHB in the middle block exhibit exclusively stereocomplex crystallinity (T > 200 °C) while their PLLA−PHB−PLLA counterpart crystallize at temperatures comparable to those of pure PLLA or PLDA. Stereocomplex formation has an important impact the rheological and mechanical properties of these copolymers. The rheological properties of PLLA−PDLLA−PDLA and PLLA−PDLLA−PLLA are quite similar to those of pure PLLA and PLDA and their blends. However, the inclusion of PHB as the middle block changes the rheological properties of the triblocks and renders them more elastomeric in nature (solid-like behavior), particularly when stereocomplexes form and crystallization temperatures exceed 200 °C. In addition, an increase of the length of the PHB block renders these polymers thermally unstable. Tensile testing shows that diblock and triblock copolymers of PLLA and PDLLA which form stereocomplexes show promising improvements in tensile strength and elongation at break compared to the corresponding homopolymers and blends. On the other hand, the triblock copolymers of PLLA−PHB−PDLA with atactic PHB in the middle block showed poor tensile strength and significant improvement to the elongation at break since they have found to be elastomeric in nature (increase by a factor of 5−10 compared to PLLA/PDLLA/PLDA triblock).

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EXPERIMENTAL SECTION

General Methods. All the air and moisture sensitive manipulations were carried out in an MBraun glovebox or using standard Schlenk line H

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mmol) in THF (5 mL) was added to the reaction mixture. The reaction was allowed to stir for 8 h and then quenched with 0.5 mL of 1.5 M HCl in Et2O. A few drops of the mixture were removed to check monomer conversion by 1H NMR spectroscopy and the remaining mixture was concentrated under vacuum and the polymer was precipitated with cold MeOH. The resulting polymer was washed with cold MeOH (3 × 3 mL) and dried under vacuum. The polymer was then redissolved in CH2Cl2, a thermal stabilizer tris-nonylphenylphosphite (TNPP) (0.35 wt % of polymer) was added and the solvent was removed under vacuum overnight. DSC Measurement of PLA Homopolymers and Copolymers. Approximately 2−3 mg of the samples were weighed and sealed in an aluminum pan. The experiments were carried out under a nitrogen atmosphere. The samples were heated at a rate of 10 °C/min from 40 to 200 °C and held isothermally for 5 min to destroy any residual nuclei before cooling at 5 °C/min. The transition and melting temperatures were obtained from a second heating sequence, performed at 10 °C/min. Linear Viscosity Measurement. Shear measurements were performed using a MCR 501 rheometer (Anton Paar), equipped with 8 mm parallel plates. The dynamic linear viscoelastic measurements were carried out within the linear viscoelastic regime at temperatures in the range from 70 to 210 °C under nitrogen. The limit of linear viscoelasticity was first determined by a set of strain amplitude sweep tests, performed at frequencies equal to 0.1, 1, and 10 Hz at 180 and 190 °C. The dynamic measurements were conducted in the range of 0.01−100 rad/s at a strain of 2%. Gaps of 0.3−0.6 mm were used to minimize edge effects and ensure a reasonable aspect ratio of plate radius and gap. Dynamic time sweep measurements were carried out at an angular frequency of 1 rad/s and 180 °C to examine the thermal stability of the samples. Step shear strain measurements were performed at 150 °C to determine the shear damping function for the constitutive modeling. Mechanical Property Measurements. Tensile tests were performed using COM-TEN 95 series tensile testing equipment (COM-TEN Industries) at ambient conditions. Specimen of 36.6 mm in width and 90 mm in length were cut from the middle portion of the compressed films to avoid edge effects and edge imperfections. The films were compressed in a hot press at high temperature before being slowly cooled. A gage length of 40 mm, crosshead speed of 25 mm/min and a 40 pound (178 N) capacity of load cell was used for testing all samples. To eliminate specimen slippage from the grips, double adhesive masking tape was used to wrap around the top and bottom portions of the sample. For each sample five tests were run. The average modulus, tensile stress and elongation at break were calculated from the resultant stress−strain measurements and these are reported below along with standard deviations shown by the plotted error bars.

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AUTHOR INFORMATION

Corresponding Author

*[email protected] (P.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the NSERC for the Engage Grant (EGP 434440-12) and the in-kind contribution and collaboration with Ducan Industries.

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REFERENCES

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