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Article

Immortal Ring-Opening Polymerization of rac-Lactide Using Polymeric Alcohol as Initiator to Prepare Graft Copolymer

1
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China
2
University of the Chinese Academy of Sciences, Changchun Branch, Changchun 130022, China
*
Author to whom correspondence should be addressed.
Submission received: 24 December 2015 / Revised: 8 January 2016 / Accepted: 11 January 2016 / Published: 15 January 2016
(This article belongs to the Special Issue Metal-Mediated Polymer Synthesis)

Abstract

:
In the presence of a small molecular protic initiator, immortal ring-opening polymerization (ROP) of lactide (LA) is a highly efficient strategy to synthesize polylactide in a controllable manner, while using polymeric alcohol as an initiator has been less investigated. A series of polymeric alcohols (PS–OH) composed of styrene and 4.3%–18% hydroxyl functional styrene (diethyl(hydroxy(4-vinylphenyl)methyl)phosphonate, St–OH) were synthesized through reversible addition-fragmentation transfer (RAFT) polymerization. Using PS–OH as an initiator, the immortal ROP of rac-LA was catalyzed by dibutylmagnesium (MgnBu2) under various ratios of monomer to hydroxyl group within PS–OH to generate polystyrene-g-polylactide (PS–g–PLA) copolymers with different graft lengths. After thermal annealing at 115 °C, the PLA domain aggregated to nanospheres among the PS continuum. The size of the nanospheres, varying from 130.1 to 224.2 nm, was related to the graft density and length of PS–g–PLA. Nanoporous films were afforded through chemical etching of the PLA component.

Graphical Abstract

1. Introduction

Graft copolymers are an important building blocks to form self-assembly aggregates because of convenient adjusting of the self-assembly behavior by changing the grafting side-chain density and length [1,2,3,4,5,6,7,8] as compared with the corresponding widely investigated block copolymers. Graft copolymers could be synthesized by three methods of grafting through [9,10,11,12,13], grafting from [14,15] and grafting on [16] by merit of the anionic/radical living polymerization techniques, of which the grafting-from is considered a particularly attractive procedure [17] and a large number of graft copolymers with various backbones and side-chains have been synthesized by combining the ‘‘grafting-from’’ strategy with living/controlled radical polymerization. In comparison, the combination of ring-opening coordination polymerization of cyclic ester and the grafting-from strategy to synthesize a graft copolymer has aroused less attention [18] and few successful examples, since coordination polymerization is usually non-living without functional group fidelity as the further building block for grafting joints.
Biodegradable and biocompatible polylactide (PLA) derives from the bio-resourced lactic acid and has become one of the most promising environmentally friendly materials. Up until now, the highly efficient strategy of preparing PLA has been no doubt attributed to the immortal ring-opening polymerization (ROP) of lactide (LA) coined by Inoue et al. [19,20,21] when they studied the ROP of epoxides using a metal-based catalyst under the presence of excess amounts of alcohol. Therefore, immortal polymerization allows us to generate hundreds of polymer chains per metal with precisely controlled molecular weight and molecular weight distribution, moreover with functional group ends. To date, the most suitable and widely investigated protic initiators are small molecular alcohols (ROH) to give hydroxyl end-functionalized PLA [22,23,24,25,26,27,28] or star PLA [29]. The behavior of using polymeric alcohols as the protic initiators has been explored less.
Herein we report firstly the synthesis of a series of polymeric alcohols (PS–OH) composed of styrene and 4.3%–18.0% St–OH through reversible addition-fragmentation transfer (RAFT) polymerization, and the immortal ROP of rac-LA catalyzed by MgnBu2 with PS–OH as the initiator to generate polystyrene–g–polylactide (PS–g–PLA) copolymers with different graft lengths. The relationship of the thermal aggregation behavior of the (PS–g–PLA) graft copolymers and the formation of the nanoporous microstructure membranes will also be discussed.

2. Experimental Section

2.1. General Procedures

All reactions were carried out under a dry nitrogen atmosphere using Schlenk techniques or in a glovebox filled with dry nitrogen. THF was dried by distillation over sodium with benzophenone as indicator under a nitrogen atmosphere and was stored over freshly cut sodium in a glovebox. MgnBu2, azodiisobutyronitrile (AIBN) and dibenzyl carbonotrithioate were purchased from Sigma-Aldrich (St. Louis, MO, USA). rac-LA was recrystallized with dry ethyl acetate three times. Styrene was dried over CaH2 under stirring for 48 h and distilled under reduced pressure before use. Glassware and flasks using in the polymerization were dried in an oven at 115 °C overnight and exposed to a vacuum-nitrogen cycle three times. The molecular weight and weight distribution of the polymers were measured using a TOSOH HLC 8220 GPC instrument at 40 °C with THF as eluent against polystyrene standards. Polymers for NMR spectroscopy measurements were prepared by use of NMR tube sealed by paraffin film. The 1H and 13C NMR spectra were recorded on a Bruker AV400 spectrometer (FT, 400 MHz for 1H, 100 MHz for 13C, Brucker BioSpin GmbH, Rheinstetten, Germany). AFM was in tapping mode under an air atmosphere with SPA380HV with an SPI3800N controller (Seiko Instruments Inc., Chiba, Japan). The piezo scanner was able to scan with a horizontal range of 150 μm and a vertical range of 150 μm. TEM was carried out on a JEM-1011 (JEOL Ltd, Tokyo, Japan) device. The electron gun was equipped with a field emission electron source and was operated at 100 keV.

2.2. Material and Methods

2.2.1. A Typical Procedure to Synthesize PS–OH

A typical polymerization procedure was described here. In a glovebox, AIBN (10 μmol, 1.6 mg), dibenzyl carbonotrithioate (100 μmol, 29.1 mg), St–OH [30,31] (1 mmol, 0.27 g) and styrene (10 mmol, 1.04 g) were added into a 30 mL ampule. Then, the ampule was quickly taken out of the glovebox and put into a pre-equilibrated 70 °C oil bath. After 24 h, the product was precipitated in excess ethanol and dried in vacuo at 40 °C for constant weight. Finally, a yellow powder PS–OH was obtained. The molecular weight and the molecular weight distribution of the resulting polymer were characterized by GPC analysis. The content of St–OH was determined by 1H NMR.

2.2.2. A Typical Procedure for Immortal ROP

A typical polymerization of rac-LA was performed in an ampule (30 mL) in a glovebox. The polymeric initiator PS8.6–OH (1.18 g) was completely resolved by THF (20 mL). Then, MgnBu2 (43 μL, 1 M) was injected into the system. After 10 min, rac-LA (4.3 mmol, 0.62 g) was quickly added. The polymerization mixture was stirred at room temperature for 1 h. Several drops of acidified methanol were injected to terminate the polymerization. Then the reaction solution was poured into excess ethanol. After filtration, the residue was dried in vacuum at 40 °C for constant weight. The molecular weight and the molecular weight distribution of the resulting polymer were characterized by GPC analysis. The graft length was determined by 1H NMR.

2.2.3. Substrate Preparation and Spin-Coating

The silicon wafers for AFM were pretreated in a 2:1 v/v solution of 98% H2SO4 and 30% H2O2 at 100 °C for 1 h to generate a SiOχ layer on the surface. Then, the silicon wafers were cleaned by successive sonication steps in acetone and deionized water for 15 min each three times and blow-drying by nitrogen thoroughly before spin-coating. Thin films of the PS–g–PLA were prepared by spin-coating a chlorobenzene solution of PS-g-PLA (5 mg·mL−1) on silicon substrates first at 500 rpm for 10 s and at 3000 rpm for 30 s. The thin films were dried at 40 °C under a vacuum pressure. The preparation of the glass sheets was similar to the procedure of the silicon wafers except for acid picking procedure.

2.2.4. Thermal Annealing

The thermal annealing process was performed at 115 °C for 4 h and then gradually cooled to room temperature.

2.2.5. Etching Procedure of the PS–g–PLA

PS–g–PLA film was placed 9 cm from the lamps and exposed to UV chamber (An-Ting Electronic Instrument Factory, Shanghai, China) (wavelength 365 nm, power 16 W) for 1 h under air, then dipped into a 0.5 M sodium hydroxide solution containing 40/60 (v/v) methanol/water. After 30 min, the PS–g–PLA film was washed with a 40/60 (v/v) methanol/water and collected on carbon-coated copper TEM grid.

3. Results and Discussions

3.1. Synthesis of Polymeric Initiator (PS–OH)

The polymeric initiator composed of styrene and 4.3%–18% St-OH was synthesized in bulky via RAFT polymerization using AIBN and dibenzyl carbonotrithioate as the initiators at 70 °C (Scheme 1). Firstly, copolymerization of styrene and St–OH was performed under various St–OH-to-St ratios to low conversions. The reactivity ratios of rSt = 0.96 and rSt–OH = 0.11 were determined according to the Finemann-Ross equation (Figure 1) with rSt < 1, rSt–OH < 1, and rSt × rSt–OH < 1, suggesting the azeotropic point appeared at the St–OH-to-St feeding ratio of 0.04:1, where the composition of the copolymer was kept constant at various monomer conversions [32]. Based on this, we chose St–OH-to-St feeding ratios varying from 0.05:1 to 0.25:1 to synthesize the statistical copolymers PS–OH with different OH contents (Table 1). The resonance of the methine proton within St–OH shifts upfield (4.90 ppm) (Figure S1, Supplementary Material) compared to that (5.00 ppm) (Figure S2, Supplementary Material) in the monomer. The St–OH content within the copolymer was calculated on the basis of the relative integration intensity between the aromatic protons at 6.50 ppm and the methine proton of St–OH. Therefore, all polymerizations were carried out at 70 °C for 24 h to total monomer conversions around 70%. When the St–OH:St feed ratio (0.5:10 mmol) was near the azeotropic point, the St–OH content within the copolymer was 4.3%, which is close to that in the feeding [32]. Increasing the amount of St–OH from 1.0 to 2.5 mmol, copolymers with St–OH content increasing from 8.9% to 18.0% were obtained, although the St–OH contents were slightly lower than those loading amounts owing to the relatively low competitive reactivity of monomer St–OH (Table 1, entries 1–4).
Scheme 1. Synthesis of PS–OH by RAFT copolymerization of styrene and St–OH.
Scheme 1. Synthesis of PS–OH by RAFT copolymerization of styrene and St–OH.
Polymers 08 00017 g005
Figure 1. Fineman-Ross plot for copolymerization of styrene and St–OH at 70 °C (F = [St]/[St–OH] in feed, f = [St]/[St–OH] in copolymer, rSt = 0.96, rSt–OH = 0.11).
Figure 1. Fineman-Ross plot for copolymerization of styrene and St–OH at 70 °C (F = [St]/[St–OH] in feed, f = [St]/[St–OH] in copolymer, rSt = 0.96, rSt–OH = 0.11).
Polymers 08 00017 g001
Table 1. Syntheses of PS-OH by RAFT copolymerization of styrene and St–OH. a
Table 1. Syntheses of PS-OH by RAFT copolymerization of styrene and St–OH. a
EntrySt–OH (mmol)St (mmol)Yield b (%)Cont.theory c (%)Cont. d (%)Mn e (×104)PDI e
10.51068.94.84.30.821.17
21.01071.39.18.90.881.14
31.51069.313.012.50.901.16
42.51074.420.018.00.951.18
a Conditions: AIBN: 10 μmol, dibenzyl carbonotrithioate: 100 μmol, Tp = 70 °C, Time = 24 h; b Isolated yield after precipitation; c Cont.theory = nSt–OH/(nSt + nSt–OH); d Determined by 1H NMR spectroscopy; e Determined by size exclusion chromatography calibrated vs. polystyrene standards.

3.2. Immortal ROP Using PS–OH as Initiator to Prepare PS–g–PLA

Recently, we reported that the ligand-free, simple, and readily available MgnBu2 and Ph2CHOH can mediate the rapid immortal ROP of L-LA with high catalyst activity and remarkable catalyst efficiency to generate up to 500 PLA polymer chains per metal center, in which the molecular weights were accurately controlled by the [Ph2CHOH]0 loading relative to [Mg]0, while the molecular weight distributions remained narrow and nearly constant [25]. Hence, the hydroxyl functionalized polystyrenes (PS–OH) obtained above were anticipated to play the role of Ph2CHOH as the chain transfer agent. The binary catalytic system composed of MgnBu2 and an excess amount of PS-OH was assayed in the activity toward the immortal ROP of rac-LA. The use of rac-LA to give atactic PLA was important to the degradation of PLA [33]. Firstly, the polymeric initiator PS4.3–OH (1.11 g contains 0.43 mmol St–OH and the footnote represented the content of St–OH within PS–OH) was dissolved in THF, then hexane solution of MgnBu2 (43 μL, 1 M) was injected. After 10 min, rac-LA (0.62 g, 4.3 mmol) was transferred into the above solution, which was kept stirring at r.t. for 1 h to achieve quantitative conversion (Table 2, entry 1). In the 1H NMR of the resultant polymer PS–g–PLA (Figure 2), the resonance of the methine proton within St–OH appears at 6.05 ppm (H5), shifting downfield compared to that within PS–OH (4.90 ppm), which is similar to that within the phosphonate ester attaching to polylactide [30] (6.14 ppm), indicating that the PLA chains indeed grafted from the hydroxyl groups of PS–OH. The multiplets at 4.36 ppm (H8) are assigned to the methine protons of LA connected with the hydroxyl group (H7, 2.74 ppm) [25]. The multiplets at 5.18 and 1.57 ppm are attributed to the methine and methyl protons of the PLA polymer, respectively. The graft length of the LA side-chain was 10.2, calculated according to the following formula: GN = (I5.18 + I4.36)/(2 × I6.05) (Table 2, entry 1), close to the theoretical graft value of 10. Fixing the ratio of [LA]0:[St–OH]0 to be 100:10, no matter how the content of St–OH within PS–OH (5% to 20%) was changed, the grafted PLA chain length in the obtained PS–g–PLA copolymers was maintained around the designed 10 (Table 2, entries 1–4). Hence, a series of PS–g–PLA samples with the same graft length and different graft densities were prepared. Similarly, keeping the content of St–OH constant (8.6%), the change of the ratio of [LA]0:[St–OH]0 in the range of 5:1 to 20:1 caused the grafted PLA chain length to increase from 4.8 to 19.8 (Table 2, entries 2, 5 and 6), indicating that the grafted side-chain length could be swiftly controlled.
Table 2. Polymerization of rac-LA catalyzed by MgnBu2/PS–OH. a
Table 2. Polymerization of rac-LA catalyzed by MgnBu2/PS–OH. a
EntryPS–OH/grac-LA (mmol)Graft length (GN) bMn c (×104)PDI c
1PS4.3–OH/1.114.310.21.471.42
2PS8.6–OH/1.188.69.81.611.45
3PS12.5–OH/1.2412.510.11.731.55
4PS18.0–OH/1.3418.09.71.811.48
5PS8.6–OH/1.184.34.81.061.44
6PS8.6–OH/1.1817.219.81.521.51
a [MgnBu2]0:[rac-LA]0 = 1:100, THF: 20 mL, Temperature: 25 °C, Time: 1 h; b Determined by 1H NMR spectroscopy; c Determined by size exclusion chromatography calibrated vs. polystyrene standards.
Figure 2. The 1H NMR spectrum (400 MHz, CDCl3, 298 K) of the resulting PS8.6g–PLA4.8 (Mn,SEC = 10,600 g/mol, PDI = 1.44, Table 2, entry 5).
Figure 2. The 1H NMR spectrum (400 MHz, CDCl3, 298 K) of the resulting PS8.6g–PLA4.8 (Mn,SEC = 10,600 g/mol, PDI = 1.44, Table 2, entry 5).
Polymers 08 00017 g002

3.3. Annealing-Mediated Assembly of the PS–g–PLA Films

The chemical incompatibility between the PS and PLA graft enabled the PS–g–PLA films to spontaneously form microphase-separation structures with the lowest free energy. However, it was difficult to self-assemble with reliance only on the inner superiority of mutual balance between the chemical incompatibility and covalent bond linkage of the constituent graft copolymer. The thermal field which provides energy for the thermodynamic movement of PS and PLA chains, and thus promotes the reorganization of their conformation and orientation [34], is thought to be a facile way to regulate the self-assembling of PS–b–PLA films [35]. The segment adjustment was achieved only when the system temperature was higher than the Tg of both PS (109.5 °C) and PLA (69.65 °C). Therefore, the thermal annealing of the PS–g–PLA film was carried out at 115 °C for 4 h. The phase images for PS–g–PLA showed a dispersive nanosphere phase for PLA and a continuous phase structure for PS. The average diameter (D) and the density of the nanosphere within PS4.3g–PLA10 (4.3 represents the graft density (4.3%), 10 represents the length of PLA), measured using Nano Measurer 1.2, were 130.1 nm and 179/(100 μm2), respectively (Figure 3A; Table 3, entry 1). Keeping the length of the PLA graft constant, increasing the graft density from 8.6% to 12.5%, the density of the nanospheres increased correspondingly from 239 to 293/(100 μm2), while the average diameters were nearly similar (158.2 and 152.4 nm) (Figure 3B,C; Table 3, entries 2–3). When the graft density was further increased to 18.0%, however, the nanosphere became larger (212.6 nm), accompanied by a drop of the density (Figure 3D; Table 3, entry 4), suggesting that more PLA side-chains aggregated into one nanosphere. Under the same graft density (8.6%), AFM phase images of copolymers with short grafts such as PS8.6g–PLA5 (228/(100 μm2)) and PS8.6g–PLA10 (239/(100 μm2)) showed a similar density of nanospheres (Figure 4A,B; Table 3, entries 2 and 5), while the long PLA repeat unit such as PS8.6–g–PLA20 led to a reduced nanosphere density of 169/(100 μm2) (Figure 4C; Table 3, entry 6), which might be attributed to the serious chain entanglement. After etching the PLA phase by NaOH/methanol solution, the corresponding nanoporous films were confirmed by TEM (Figure S3, Supplementary Material).
Table 3. The average particle size and number of the PS–g–PLA with different graft densities and lengths after thermal annealing. a
Table 3. The average particle size and number of the PS–g–PLA with different graft densities and lengths after thermal annealing. a
EntryPS–g–PLAAverage particle size b (nm)Average particle number b (100 μm2)
1PS4.3g–PLA10130.1179
2PS8.6g–PLA10158.2239
3PS12.5g–PLA10152.4293
4PS18.0g–PLA10212.6158
5PS8.6g–PLA5133.5228
6PS8.6g–PLA20224.2169
a Conditions: thermal annealing temperature: 115 °C, time: 4 h; b The average particle size and number of the PS–g–PLA were measured by Nano Measurer 1.2.
Figure 3. AFM phase images and the sizes of the nanospheres of the PS–g–PLA with different graft densities after thermal annealing at 115 °C for 4 h: (A) PS4.3g–PLA10; (B) PS8.6g–PLA10; (C) PS12.5g–PLA10; (D) PS18.0g–PLA10.
Figure 3. AFM phase images and the sizes of the nanospheres of the PS–g–PLA with different graft densities after thermal annealing at 115 °C for 4 h: (A) PS4.3g–PLA10; (B) PS8.6g–PLA10; (C) PS12.5g–PLA10; (D) PS18.0g–PLA10.
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Figure 4. AFM phase images of the PS–g–PLA with different graft lengths after thermal annealing at 115 °C for 4 h: (A) PS8.6g–PLA5; (B) PS8.6g–PLA10; (C) PS8.6g–PLA20.
Figure 4. AFM phase images of the PS–g–PLA with different graft lengths after thermal annealing at 115 °C for 4 h: (A) PS8.6g–PLA5; (B) PS8.6g–PLA10; (C) PS8.6g–PLA20.
Polymers 08 00017 g004

4. Conclusions

This work, for the first time, demonstrated that even in the presence of PS–OH as the protic alcohol, the immortal ROP of LA could also proceed, in a controllable manner, to generate a series of PS–g–PLA copolymers with different graft densities and graft lengths through controlling the feeding of LA and the content of the hydroxyl group within PS–OH. After thermal annealing, the PLA domain aggregated to nanospheres among the PS continuum. Under the same PLA graft length, the density of the nanosphere increased from 179 to 293/(100 μm2) with the graft density of PLA increasing in the range of 4.3% and 12.5%, which dropped at further increasing of the graft density due to more PLA participating in the aggregation to form a large nanosphere. When the graft density was kept constant, the short graft chain length seemed affected the diameter of the nanosphere more obviously, while the serious chain entanglement caused by long side-chains reduced the nanosphere density. Furthermore, the chemical etching of the PLA component resulted in a large number of hydroxyl functionalizations within the porous membrane. This functionality could be the “reactive handles” to achieve membranes of various applications.

Supplementary Materials

The following are available online at www.mdpi.com/2073-4360/8/1/17/s1.

Acknowledgments

The authors are grateful financial supports from the National Natural Science of China for project Nos. 21361140371, 51321062, 21274143.

Author Contributions

Dongmei Cui conceived and designed the experiments; Na Liu and Changguang Yao performed the experiments; Na Liu and Bo Liu analyzed the data; Na Liu and Bo Liu wrote the paper.

Conflicts of Interest

The authors declare no conflict of interest.

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MDPI and ACS Style

Liu, N.; Liu, B.; Yao, C.; Cui, D. Immortal Ring-Opening Polymerization of rac-Lactide Using Polymeric Alcohol as Initiator to Prepare Graft Copolymer. Polymers 2016, 8, 17. https://0-doi-org.brum.beds.ac.uk/10.3390/polym8010017

AMA Style

Liu N, Liu B, Yao C, Cui D. Immortal Ring-Opening Polymerization of rac-Lactide Using Polymeric Alcohol as Initiator to Prepare Graft Copolymer. Polymers. 2016; 8(1):17. https://0-doi-org.brum.beds.ac.uk/10.3390/polym8010017

Chicago/Turabian Style

Liu, Na, Bo Liu, Changguang Yao, and Dongmei Cui. 2016. "Immortal Ring-Opening Polymerization of rac-Lactide Using Polymeric Alcohol as Initiator to Prepare Graft Copolymer" Polymers 8, no. 1: 17. https://0-doi-org.brum.beds.ac.uk/10.3390/polym8010017

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