Article pubs.acs.org/Macromolecules
Homo- and Block Copolymerizations of ε‑Decalactone with L‑Lactide Catalyzed by Lanthanum Compounds Jin-Ou Lin,† Wanli Chen,‡ Zhiquan Shen,† and Jun Ling†,* †
MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China ‡ Center of Analysis & Measurement, Zhejiang University of Technology, Hangzhou 310014, China ABSTRACT: Biobased and environmental-friendly polylactones provide a solution to the increasing crisis of fossilsourced products. Ring-opening polymerization (ROP) of εdecalactone (DL) catalyzed by lanthanum tris(2,6-di-tert-butyl4-methylphenolate) [La(OAr) 3 ] and lanthanum tris(borohydride) [La(BH4)3(THF)3] is reported for the first time in this paper. Both of them exhibit good activities producing poly(ε-decalactone) (PDL) with molecular weight (MW) up to 26.4 kg/mol and polydispersity index (PDI) as low as 1.10. PLLA-b-PDL-b-PEG-b-PDL-b-PLLA pentablock copolymers with predictable MWs and relatively narrow PDIs (1.19−1.28) are synthesized by sequential ROP of DL and Llactide (LLA) catalyzed by La(OAr)3 in the presence of poly(ethylene glycol) (PEG). Chain extension reactions of the obtained pentablock copolymers are carried out using L-lysine diisocyanate (LDI) to produce multiblock copolymers with relatively high MW. The thermal behaviors studied by DSC and DMA measurements indicate that PDL is completely amorphous under ambient temperature and the copolymers with two Tgs suggest microphase separation of hard and soft domains. We employ tensile tests to assess mechanical properties and find excellent elongation up to 723% of the chain-extended samples. Considering the biorenewable resource of DL and LLA, a novel, biobased, biodegradable, and biocompatible elastomer is successfully synthesized.
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INTRODUCTION In the past decades, numerous reports have been focusing on the study of aliphatic polyesters. They are widely applied in biomedical fields with many desirable properties, for instance, biocompatibility, biodegradation and nontoxicity.1−4 Polymers from biobased renewable lactone and lactide provide a solution to the increasing crisis of fossil-based polymeric materials nowadays. Ring-opening polymerization (ROP) of cyclic esters such as ε-caprolactone (CL),5 L-lactide (LLA),6 and trimethylene carbonate (TMC)7 or copolymerization of epoxides and cyclic anhydrides8,9 is an effective way to synthesize aliphatic polyesters. Among the catalysts used, rare earth metal compounds have been extensively studied due to their advantages of high activity, low toxicity and low remnants.10−13 Being a kind of flavoring agent derived from caster oil, εdecalactone (DL), a lactone with seven membered ring and a nbutyl group on the 5-carbon, has only been reported once.14,15 Lin and his co-workers found that zinc benzylalkoxide complexes were efficient catalysts for ROP of DL at 50 °C in toluene. The molecular weights (MWs) of homo-PDL could be controlled ranging from 5550 to 15700 with polydispersity indices (PDIs) between 1.09 and 1.12.14 Limited investigations were involved toward either DL copolymerization or its physical properties. Because the pendent n-butyl group destroys the crystallization of the backbone, PDL is expected to be an © XXXX American Chemical Society
amorphous polymer differing from PCL, its semicrystalline analogue (Scheme 1). Scheme 1. Structures of CL, DL, and the Corresponding Polymers
Poly(L-lactide) (PLLA), the most attractive and widely used biosourced polyester is a semicrystalline, hard and rather brittle material.16,17 A common strategy to avoid these disadvantages is to copolymerize LLA with other monomers to meet the demands to be an applicable material whose architecture and composition can be modulated.18−21 In copolymers, PLLA Received: June 13, 2013 Revised: September 5, 2013
A
dx.doi.org/10.1021/ma401218p | Macromolecules XXXX, XXX, XXX−XXX
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Table 1. Homopolymerization of ε-Decalactone Catalyzed by Different Catalystsa
a
sample
catalyst
[DL]/[La]
solvent
T (°C)
t (h)
yield (%)
Mnb
PDIb
1 2 3 4 5
La(BH4)3(THF)3 La(BH4)3(THF)3 La(OAr)3 La(OAr)3 Sn(Oct)2
60 60 92 192 18c
THF toluene toluene toluene −
30 100 30 30 150
2 2 1.5 1.5 20
78 82 95 84 88
4500 5000 23400 26400 5000
1.18 1.16 1.10 1.16 1.30
The concentration of DL was 1.5 mol/L except sample 5 which was in bulk. bMeasured by SEC. cThe ratio was [DL]/[PEG]. finish at 80 °C for 13 h. The product was isolated by precipitation from hexane and dried under vacuum. Measurements. Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker Avance DMX 500 spectrometer (1H: 500 MHz) using CDCl3 as solvent and tetramethylsilane as internal reference. MWs and PDIs were determined by size-exclusion chromatography (SEC) on a Waters-150C apparatus equipped with Waters Styragel HR3 and HR4 columns and a Waters 2414 refractive index detector. THF was used as the eluent with a flow rate of 1.0 mL/min at 40 °C, and narrow PDI polystyrene was used as the calibration standard. The differential scanning calorimetry (DSC) analyses were performed on a Q200 TA Instruments. Samples were heated from room temperature to 100 °C at a rate of 10 °C/min under a nitrogen purge and held for 2 min to erase the thermal history. They were then cooled to −90 °C with a rate of 10 °C/min and held for another 2 min. The second heating scan from −90 to +100 °C was then recorded. Mechanical Properties. Solution-cast samples were prepared from THF solutions in Teflon molds. The solvent was slowly evaporated at room temperature for 2 days. Samples were cut into dog-bone shape (35 mm in length, 2 mm in width and 0.2 mm in thickness) using a GB/T-528 standard dumbbell cutter. Mechanical properties including Young’s modulus, strength, and elongation were measured at ambient temperature on a Reger RWT 10 instrument at 10 mm/min extension rate. Dynamic Mechanical Analysis (DMA). Rectangular samples (30 mm in length, 6 mm in width and 0.2 mm in thickness) were cut from the solution-cast films and tested on a dynamic mechanical analyzer (TA Instruments Q800). The temperature rate was set at 3 °C/min from −110 to +80 °C and frequency of 1 Hz.
serves as hard segment while the other amorphous component acts as soft segment with a low Tg, such as petroleum-based polyisoprene,22 polyisobutylene20 and polyTMC23 to form thermoplastic elastomers with microphase separation. Moreover, compared with triblock copolymers, multiblock copolymers have the advantage of better mechanical properties and better control over the degradation time.20 Herein, we report a novel fully biobased and environmental-friendly thermoplastic elastomer consisting of multiblock copolymers of DL and LLA. Two rare earth metal compounds, lanthanum tris(2,6-di-tertbutyl-4-methylphenolate) [La(OAr)3] and lanthanum tris(borohydride) [La(BH4)3(THF)3] are chosen as catalysts, and a “one-pot” block copolymerization strategy without PDL purification is applied.
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EXPERIMENTAL SECTION
Materials. ε-Decalactone (DL, Sigma-Aldrich) was dried over CaH2 followed by distillation under reduced pressure prior to use. LLactide (LLA, Changchun SinoBiomaterials Co., Ltd.) was recrystallized from a mixture of dry ethyl acetate with ethanol twice, dried at 40 °C under vacuum for 10 h and stored under argon atmosphere. Poly(ethylene glycol) (PEG, Alfa Aesar, Mn = 600) with dihydroxyl end groups was vacuum-dried at 70 °C overnight. L-Lysine diisocyanate (LDI, Nantong Dahong Chemical Co.) was distilled prior to use. Tetrahydrofuran (THF) and toluene were refluxed over potassium/benzophenone ketyl before use. La(BH4)3(THF)3 and La(OAr)3 were synthesized according to the methods described in the literatures.24,25 All Other chemicals were used as received. Homopolymerization of ε-Decalactone (DL). All polymerizations were carried out via Schlenk techniques in 20 mL ampules that previously flamed and purged with dry argon. In a typical procedure, 0.5 mL of DL (0.479 g, 2.81 mmol) was first dissolved in 1.6 mL of toluene at a given temperature and 0.5 mL of La(BH4)3(THF)3 (0.0891 mol/L in THF) was then added into the ampule to start the polymerization. After the desired reaction time, the product was quenched by methanol containing 5% hydrochloric acid, precipitated by methanol, and dried in vacuo to constant weight. The polymerizations catalyzed by La(OAr)3 followed the same procedure. When catalyzed by stannous 2-ethylhexanoate [Sn(Oct)2], DL was first allowed to mix with PEG and then Sn(Oct)2 was added by syringe. The polymerization was reacted at 150 °C for 20 h and the product was obtained using the same work-up. Synthesis of PLLA-b-PDL-b-PEG-b-PDL-b-PLLA Pentablock Copolymers. As a typical pentablock copolymerization, 0.799 g of (0.221 mmol/L) PEG dissolved in THF was mixed with 8.5 mL of La(OAr)3 (0.0312 mol/L in toluene) in an ampule. The mixture was aged for 15 min and then DL (4.558 g, 26.8 mmol) was added to start the reaction at 60 °C. After 5 h polymerization, a THF solution of LLA (6.3 g, 43.8 mmol) was introduced and the reaction was performed at 90 °C for another 7 h. The polymer was precipitated by methanol and dried under vacuum to constant weight with a yield of 87%. Chain Extension of PLLA-b-PDL-b-PEG-b-PDL-b-PLLA. Pentablock copolymer (1.022 g, 0.110 mmol) was vacuum-dried at 70 °C for 10 h and then dissolved in 7 mL of dry toluene as a homogeneous solution. A mixture of LDI (27.5 mg, 0.122 mmol) and Sn(Oct)2 (0.1 wt %) in toluene solution was injected, and the reaction was allowed to
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RESULTS AND DISCUSSION ROPs of DL were carried out using La(BH4)3(THF)3, La(OAr)3 , and Sn(Oct) 2 as catalysts with the results
Figure 1. 1H NMR spectrum of PDL (sample 2) (∗, H2O). B
dx.doi.org/10.1021/ma401218p | Macromolecules XXXX, XXX, XXX−XXX
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3). Two methylene protons Hg and Hh on n-butyl chain are found overlapped with Hc by their coupling signals with Hf and Hi on the 1H−1H COSY spectrum (insert B in Figure 3). Hj (t, 4.03 ppm) and Ha′ (m, 3.58 ppm) are assigned to the hydroxymethylene and hydroxymethyne protons on two chain ends, respectively. According to the coupling signals of Hj and He′ (insert C in Figure 3), He′ is overlapping with Hd. Therefore, the La(BH4)3(THF)3-mediated homo-PDL is an α,ω-dihydroxyltelechelic polymer which can be employed as a macroinitiator and used for further functionalization. A similar α,ω-dihydroxyltelechelic polymer was observed in CL polymerizations catalyzed by rare earth tris(borohydride) complexes.26 It is noteworthy that all homo-PDL samples are presented in the form of a transparent viscous liquid under ambient temperature rather than a powder. DSC curve of the homoPDL is shown in Figure 5A. A glass-transition temperature (Tg) for homo-PDL is observed at −58.5 °C but no melting peak is detected, which indicates that PDL is completely amorphous because n-butyl pendant group destroys the crystallization of backbone. Homo-PCL, an analogue for comparison, has a Tg of about −60 °C and a melting temperature (Tm) in the range of 50 and 66 °C depending on its MW and crystallinity, known as a semicrystalline polymer.24,27 Derived from castor oil, a biobased renewable resource, DL is totally biocompatible. Taking the above into consideration, PDL is a promising material as soft segment for synthetic thermoplastic elastomer, together with hard segment such as PLLA. PEG is a widely used, biocompatible, and environmentalfriendly material. Various triblock and pentablock polyesters based on PEGs have been reported by ROP using Sn(Oct)2, Zn complex, CaH 2 , and rare earth metal compounds as catalysts,28−34 in which separation and purification of a triblock copolymer precursor are necessary to prepare a neat pentablock copolymer with PEG. In this study, pentablock PLLA-b-PDL-bPEG-b-PDL-b-PLLA polymers are prepared by a “one-pot” procedure without any precursor separation or purification,
Figure 2. 13C NMR spectrum of PDL (sample 2).
summarized in Table 1. Both the lanthanum catalysts exhibited good activities to polymerize DL under mild condition, i.e. 30 °C within 2 h, while Sn(Oct)2 required much higher temperature (150 °C) and much longer reaction time (20 h). PDL obtained by Sn(Oct)2 had broader PDI (1.30) than those by the two lanthanum complexes (
