Introduction
Aliphatic polyesters prepared by ring-opening polymerization of ɛ-caprolactone (CL) and L-lactide (LA) have attracted considerable interest because they are biodegradable and biocompatible. However, it is difficult to produce aliphatic homopolyesters that fulfil practical requirements given the inherently poor properties of these polymers, such as low mechanical strength and poor heat resistance. To overcome these drawbacks, improvements have been made by preparing nanocomposites with organoclay.1, 2, 3, 4
Aromatic polyimides (PIs) are well known as plastics used in super-engineering applications because of their high mechanical strength and thermal resistance, as well as their good biocompatibility.5 We believe that an incorporation of aromatic PIs into the aliphatic polyesters could help improve the properties of aliphatic polyesters for use as biomaterials. Ding and Harris6 have demonstrated that the mechanical properties of nylon 6-block-PI-block-nylon 6 copolymers are superior to those of commercial nylon 6. Takeichi and coworkers have reported that the initial decomposition temperature of poly(urethane-imide) is higher than that of polyurethane.7 These reports prompted us to explore a method to prepare copolymers composed of aliphatic polyesters and aromatic PIs. The corresponding method is required to more readily introduce the PI segment into the target polyester backbone. In this note, we describe a facile synthesis of ABA-type triblock copolymers consisting of aliphatic polyesters and aromatic PIs by ring-opening polymerization of CL and LA initiated with hydroxy-terminated PI. It has been reported that PIs with hydroxy-terminated groups can be easily synthesized by reacting dianhydride, diamine and aminoalcohol.8, 9, 10, 11
Experimental Procedure
Materials
4,4’-(Hexafluoroisopropylidene)diphthalic anhydride (99%, 6FDA, Sigma Chemical Company, St Louis, MO, USA), 4,4’-diaminodiphenyl ether (99%, ODA, Wako Pure Chemical Industries, Osaka, Japan), p-aminophenethyl alcohol (85%, Wako Pure Chemical Industries) and CL (Wako Pure Chemical Industries) were used as received. LA was provided by Kyushu Institute of Technology. Dried N-methyl-2-pyrrolidone (NMP) was obtained from Wako Pure Chemical Industries. Other solvents were obtained from Wako Pure Chemical Industries and were used without further purification.
Measurements
The 1H nuclear magnetic resonance (1H-NMR; 400 MHz) spectra were recorded on a JEOL ECX-400II spectrometer (JEOL Resonance, Tokyo, Japan) in dimethyl sulfoxide (DMSO)-d6 using tetramethylsilane as an internal standard. Fourier transform infrared spectra of the polymers were recorded on a JASCO FT/IR-4100 spectrometer (JASCO, Tokyo, Japan). Thermal decomposition was investigated by thermogravimetry-differential thermal analysis (Thermo plus EVO2 TG8120, Rigaku, Japan) at a heating rate of 10 °C min−1 in air. Glass transition temperatures were determined by differential scanning calorimetry (DSC) using a Thermo plus EVO2 DSC8230 (Rigaku, Tokyo, Japan) for poly(LA)-b-PI-b-poly(LA) and a X-DSC 7000 (Hitachi, Tokyo, Japan) for poly(CL)-b-PI-b-poly(CL) at a heating rate of 10 °C min−1, under N2 gas. The number-average (Mn) and weight-average (Mw) molecular weights were estimated by size exclusion chromatography (SEC), which consisted of a Waters Alliance e2695 GPC system equipped with a 2414 refractive index detector (Waters, Milford, MA, USA) and poly(vinyl alcohol) gel columns (shodex Asahipak GF-310 HQ+GF−7M HQ with size exclusion limitations of 4 × 104 and 1 × 107 g mol−1, respectively) eluted with tetrahydrofuran (THF) at a flow rate of 0.6 ml min−1 at 35 °C using a calibration curve based on polystyrene standards.
Synthesis of hydroxy-terminated PI
Hydroxy-terminated PI was synthesized according to previous papers:9, 10, 11 6FDA (1.50 g, 3.38 mmol) in 7.0 ml of NMP was added to 9.5 ml of NMP containing ODA (0.565 g, 2.82 mmol) and p-aminophenethyl alcohol (0.155 g, 1.13 mmol). The reaction mixture was stirred under argon gas at room temperature for 24 h, followed by refluxing with the addition of m-xylene (3 ml) at 180 °C for 6 h. Then the solution was poured into excess ethanol to precipitate the PI with hydroxy groups at both chain ends (Yield=71% (1.57 g), Mn(NMR)=5700, Mn(SEC)=3200).
1H NMR (DMSO-d6, 400 MHz, δ, p.p.m.): 2.78 (HOCH2CH2–); 3.65 (HOCH2CH2–); 4.7 (HOCH2CH2–); 6.51–8.34 (aromatic ring).
Ring-opening polymerization of CL initiated by PI
The typical procedure for the ring-opening polymerization of CL initiated by the reactive PI, shown in Scheme 1, is described as follows. Tin(II) 2-ethylhexanoate (Sn(Oct)2, 6 mg, 14.8 μmol) was added to a test tube containing CL (0.699 g, 6.12 mmol) and PI (0.352 g, 0.0617 mmol) in THF (1.0 ml). The reaction mixture was heated at 130 °C and stirred for 24 h under an argon atmosphere. The resultant polymer was purified by reprecipitation with an excess of methanol (0.64 g, Yield=61%).
1H NMR (DMSO-d6, 400 MHz, δ, p.p.m.): 1.00–1.71 (–C(=O)CH2CH2CH2CH2CH2O–); 2.08–2.35 (–C(=O)CH2CH2CH2CH2CH2O–); 2.89–2.97 (–C6H4CH2CH2O–); 3.82–4.05 (–C(=O)CH2CH2CH2CH2CH2O–); 4.22–4.29 (–C6H4CH2CH2O–); 6.98–8.33 (aromatic ring). 13C NMR (DMSO-d6, 100 MHz, δ, p.p.m.): 24.5(–C(=O)CH2CH2CH2CH2CH2O–); 25.3 and 28.2 (–C(=O)CH2CH2CH2CH2CH2O–); 32.6 (–C6H4CH2CH2O–); 33.8 (–C(=O)CH2CH2CH2CH2CH2O–); 61.0 (–C(CF3)6–); 63.9 (–C(=O)CH2CH2CH2CH2CH2O–) and (–C6H4CH2CH2O–); 99.9 (–C(CF3)6–); 119.5, 127.6, 129.8, 133.1, 137.7, and 156.5 (aromatic ring); 166.5 (carbonyl group of imide ring); 173.2 (–C(=O)CH2CH2CH2CH2CH2O–).
Ring-opening polymerization of LA initiated by PI
The typical procedure for the ring-opening polymerization of LA initiated by PI, as shown in Scheme 1, is described as follows. Sn(Oct)2 (6 mg, 14.8 μmol) was added to a test tube containing LA (0.882 g, 6.12 mmol) and PI (0.352 g, 0.0617 mmol) in THF (1.0 ml). The reaction mixture was heated at 130 °C and stirred for 12 h under an argon atmosphere. The resultant polymer was purified by reprecipitation with excess methanol (0.63 g, Yield=51%).
1H NMR (DMSO-d6, 400 MHz, δ, p.p.m.): 1.17–1.54 (–C(=O)CH(CH3)OC(=O)CH(CH3)O–); 2.91–3.01 (–C6H4CH2CH2O–); 4.13–4.23 (–C6H4CH2CH2O–); 4.29–4.40 and 5.02–5.27 (–C(=O)CH(CH3)OC(=O)CH(CH3)O–); 6.72–8.33 (aromatic ring). 13C NMR (DMSO-d6, 100 MHz, δ, p.p.m.): 17.0 (–C(=O)CH(CH3)OC(=O)CH(CH3)O–); 35.2 (–C6H4CH2CH2O–); 66.0 (–C(CF3)6–); 69.2 (–C(=O)CH(CH3)OC(=O)CH(CH3)O–) and (–C6H4CH2CH2O–); 119.6, 124.9, 127.7, 129.9, 133.2, 133.6, 137.8, and 156.6 (aromatic ring); 166.6 (carbonyl group of imide ring); 169.7 (–C(=O)CH(CH3)OC(=O)CH(CH3)O–).
Results and discussion
Synthesis of hydroxy-terminated PI
Prior to the synthesis of ABA triblock copolymers, hydroxy-terminated PI was synthesized by a polyaddition reaction. The 1H NMR spectrum of PI is shown in Figure 1. The peaks at 2.78, 3.65 and 4.7 p.p.m. assignable to the protons of the –CH2CH2OH groups of PI are clearly observed. These results are consistent with those of previous studies.9, 10, 11 The 1H NMR spectroscopic analysis reveals that the Mn(NMR) value is 5700. The Mn and Mw/Mn values estimated by SEC analysis were 3200 and 2.03, respectively.
1H NMR spectrum of PI measured in DMSO-d6.
Ring-opening polymerization of CL initiated by PI
The ring-opening polymerization of CL was performed with PI in the presence of Sn(Oct)2 at 130 °C for 24 h in a feed molar ratio of [–OH in PI]0:[CL]0 of 2:100 to afford the corresponding polymer in 61% yield. The 1H and 13C NMR spectra of the resultant polymer are shown in Figure 2. The peaks at 1.00–1.71, 2.08–2.35 and 3.82–4.05 p.p.m. assignable to the methylene protons of poly(CL), as well as the aromatic protons of PI, were clearly observed in the 1H NMR spectrum of the resultant polymer. In addition, there was no peak at 4.7 p.p.m. attributed to the terminal hydroxy groups of PI, which implies that the ring-opening polymerization proceeded from both ends of PI. The 13C NMR spectrum of the resultant polymer shows peaks assignable to the methylene carbons and ester carbonyl carbons of poly(CL), as well as to the aromatic carbons of PI. These results indicate that the ring-opening polymerization of CL successfully proceeded, affording the corresponding poly(CL). This NMR spectroscopic analysis allowed us to determine the composition ratio of the resultant polymer, PI:poly(CL)=48:52 (wt:wt), by comparing the molecular weights between the PI segment (Mn(NMR)=5700) and the poly(CL) segment (Mn(NMR)=6300). The molecular weight of the poly(CL) segment was determined by comparing the integral ratio of the peaks attributed to the methylene protons in the PI segment and the poly(CL) protons in the 1H NMR spectrum. The polymerization degree of CL at one hydroxy group of PI was 27.6.
(a) 1H and (b) 13C NMR spectra of poly(CL)-b-PI-b-poly(CL) measured in DMSO-d6. Comp. (PI:poly(CL))=48:52 (wt:wt) determined by 1H NMR spectroscopy.
The SEC profiles of the resultant polymer show a unimodal elution peak that shifted to a higher molecular weight region from that of PI (Figure 3). The SEC and NMR results reveal that the ring-opening polymerization of CL was successfully initiated from both of the hydroxy-terminated groups of PI to give the corresponding ABA triblock copolymer. The corresponding Mn and Mw/Mn values were estimated to be 10 000 and 1.67, respectively.
SEC profile of poly(CL)-b-PI-b-poly(CL). Comp. (PI:poly(CL))=48:52 (wt:wt) determined by 1H NMR spectroscopy. A full color version of this figure is available at the Polymer Journal online.
When the ring-opening polymerization of CL initiated by PI in a feed molar ratio of [–OH in PI]0:[CL]0=2:200 was performed in a similar manner, the corresponding ABA triblock copolymer with a composition of PI:poly(CL)=30:70 (wt:wt), as determined by 1H NMR spectroscopy, was obtained in 69% yield. The polymerization degree of CL at one hydroxy group of PI was 58.0. The SEC result showed a unimodal peak with Mn=12 000 (Mw/Mn=1.74).
We attempted to prepare films of poly(CL)-block-PI-block-poly(CL) by using the solvent casting method; however, the films were too brittle to investigate their mechanical properties, likely because of the low molecular weight of the PI segment. Synthesis of triblock copolymers derived from PI with higher molecular weights is required to investigate the mechanical properties compared with pure poly(CL). We expect introducing the PI segment into poly(CL) may enhance its elastic modulus, and detailed results of these investigations will be reported elsewhere.
Ring-opening polymerization of LA initiated by PI
In a similar procedure to that used with CL, the ring-opening polymerization of LA by PI was performed in a feed molar ratio of [–OH in PI]0:[LA]0=2:100 to synthesize poly(LA)-block-PI-block-poly(LA). The resultant polymer was obtained in 51% yield (Table 1). Figure 4 shows the 1H and 13C NMR spectra of the resultant polymer. In addition to the peaks ascribed to the aromatic protons of PI, peaks assignable to the methyl and methine protons of poly(LA) were clearly observed at 1.17–1.54 and 5.02–5.27 p.p.m., respectively, in the 1H NMR spectrum. There was no peak at 4.7 p.p.m. attributed to the hydroxy groups of PI, which suggests that the ring-opening polymerization proceeded from both hydroxy groups. The 13C NMR spectrum shows peaks at 17, 69.2 and 169.7 p.p.m. ascribed to the methyl, methine and ester carbonyl carbons of poly(LA), respectively. The composition of the polymer (PI:poly(LA)) was calculated to be 48:52 (wt:wt) based on the molecular weights of the PI segment (Mn(NMR)=5700) and the poly(LA) segment (Mn(NMR)=6300). The polymerization degree of LA at one hydroxy group of PI was 21.7. The SEC profile, shown in Figure 5, shows a unimodal peak of the resultant polymer. The corresponding Mn and Mw/Mn values were 7700 and 1.56, respectively. These results indicate that poly(LA)-block-PI-block-poly(LA) was successfully synthesized by the ring-opening polymerization of LA from both of the hydroxy-terminal groups of PI.
(a) 1H and (b) 13C NMR spectra of poly(LA)-b-PI-b-poly(LA) measured in DMSO-d6. Comp. (PI:poly(LA))=48:52 (wt:wt) determined by 1H NMR spectroscopy.
SEC profile of poly(LA)-b-PI-b-poly(LA). Comp. (PI:poly(LA))=48:52 (wt:wt) determined by 1H NMR spectroscopy. A full color version of this figure is available at the Polymer Journal online.
When the ring-opening polymerization of LA was performed at a feed molar ratio of [–OH in PI]0:[LA]0=2:200, the corresponding ABA triblock copolymer with a composition ratio of PI:poly(LA)=32:68 (wt:wt) was obtained in 43% yield (Table 1). The polymerization degree of LA at one hydroxy group of PI was 41.2. The Mn and Mw/Mn values estimated by SEC analysis were 10 100 and 1.42, respectively.
The films prepared from poly(LA)-block-PI-block-poly(LA) by the solvent casting method were also too brittle to allow investigation of their mechanical properties.
Thermal properties
Thermogravimetric analysis was performed to investigate the thermal degradation behavior of ABA triblock copolymers composed of aromatic PIs and aliphatic polyesters. No significant difference in the onset degradation temperature was observed between the two poly(CL)-block-PI-block-poly(CL) copolymers (Figure 6). Thermogravimetric analysis curve showed a two-step degradation. In the low temperature region, below approximately 300 °C, weight loss of a few percentage points (<5%) was observed for poly(CL)-block-PI-block-poly(CL). The weight decreased abruptly upon heating at >300 °C because of thermal decomposition of the poly(CL) segment. Then a plateau was reached between approximately 420 °C and 500 °C. Weight loss was observed again at approximately 510 °C, and this was attributed to the decomposition of the PI segment. On the basis of these results, the compositions were calculated to be PI:poly(CL)=48:52 and 27:73 (wt:wt) for feed molar ratios of [–OH in PI]0:[CL]0=2:100 and 2:200, respectively. These values were close to the compositions determined by 1H NMR spectroscopy. In the case of poly(LA)-block-PI-block-poly(LA), as shown in Figure 7, the onset degradation temperature of the copolymer prepared at [–OH in PI]0:[LA]0=2:100 was approximately 6 °C higher than that prepared at [–OH in PI]0:[LA]0=2:200, indicating that the ratio of the PI segment to the poly(LA) segment affected the thermal resistance. The results of thermogravimetric analysis also showed a two-step decomposition behavior (that is, the weight losses at approximately 220–310 °C and >510 °C were assigned to the thermal decomposition of poly(LA) and PI, respectively). The corresponding compositions based on the thermal decomposition behavior were calculated to be PI:poly(LA)=49:51 and 30:70 (wt:wt) for [–OH in PI]0:[LA]0=2:100 and 2:200, respectively. These values were close to the compositions determined by 1H NMR spectroscopy.
Thermal degradation of poly(CL)-b-PI-b-poly(CL).
Thermal degradation of poly(LA)-b-PI-b-poly(LA).
DSC measurement of poly(CL)-block-PI-block-poly(CL) was performed at −100 °C to 100 °C. Glass transition temperatures (Tg) were detected at −63.9 and −64.4 for the triblock copolymers prepared with [–OH in PI]0:[CL]0=2:100 and 2:200, respectively. These Tg values were attributable to the poly(CL) segment.12 Similarly, DSC measurement of poly(LA)-block-PI-block-poly(LA) was performed at 0–200 °C. Tg values of 41.7 °C and 39.7 °C were determined for the copolymers prepared with [–OH in PI]0:[LA]0=2:100 and 2:200, respectively, and these values were attributable to the poly(LA) segment.13 These results indicate that a small improvement in heat resistance is achieved by incorporating a PI segment into an aliphatic polyester segment.
Conclusions
ABA-type triblock copolymers composed of aliphatic polyesters and aromatic PIs were synthesized in a facile manner by the ring-opening polymerization of CL and LA initiated by hydroxy-terminated aromatic PI. The aliphatic polyester segments of the resultant copolymers showed a small improvement in thermal stability as a result of the introduced aromatic PI.
Ring-opening polymerization of CL and LA initiated by PI.
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Acknowledgements
This research was supported by a Joint Project of Toyohashi University of Technology and Kosen for Education and Research. We thank Professor Dr Tsutomu Takeichi (Toyohashi University of Technology) for helpful discussions. We also thank Professor Dr Haruo Nishida (Kyushu Institute of Technology) for providing L-lactide.
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Morinaga, H., Tsuneishi, F. & Inada, S. Ring-opening polymerization of ɛ-caprolactone and L-lactide initiated by a hydroxy-terminated aromatic polyimide. Polym J 48, 311–315 (2016). https://doi.org/10.1038/pj.2015.107
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DOI: https://doi.org/10.1038/pj.2015.107
