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Synthesis of miktoarm star-shaped and inverse star-block copolymers by a combination of ring-opening polymerization and click chemistry

  • Xiaoqi Yan , Jianbo Li ORCID logo EMAIL logo and Tianbin Ren
Published/Copyright: September 13, 2018
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e-Polymers
From the journal e-Polymers Volume 18 Issue 6

Abstract

Based on the combination of the “arm-first” and “core-first” strategies, the miktoarm star-shaped copolymer PLLA2PCL2 and the inverse star-block copolymer (PCL-b-PLLA)2-core-(PCL-b-PLLA)2 were designed and synthesized by the combination of ring-opening polymerization (ROP) and a click reaction. The miktoarm star-shaped copolymer PLLA2PCL2 was synthesized by a click reaction of an azido macroinitiator PLLA2(N3)2 and HC≡C-PCL. The inverse star-block copolymer (PCL-b-PLLA)2-core-(PCL-b-PLLA)2 was synthesized by a click reaction of an azido macroinitiator (PCL-b-PLLA)2(N3)2 and HC≡C-PCL-b-PLLA. The structures of these star polymers were confirmed by nuclear magnetic resonance (NMR), Fourier-transform infrared (FT-IR) spectroscopy and gel permeation chromatograph (GPC). The inverse star-block copolymer could be used to study the potential relationship between polymer structure and properties, which has a unique structure and good crystallization properties.

Keywords: click chemistry; inverse star-block copolymer; poly(ε-caprolactone); poly(L-lactic acid); ring-opening polymerization

1 Introduction

Star polymers have attracted widespread interest due to their unique three-dimensional structures and properties, such as a smaller hydrodynamic volume than linear polymers and a much lower solution viscosity. In particular, with the development of reactive/controlled radical polymerization technologies such as reversible addition-fragmentation chain transfer (RAFT) polymerization (1), (2), (3), atom transfer radical polymerization (ATRP) (4), (5), (6) and nitrogen-oxygen stable radical polymerization (7), the synthesis of a well-defined star polymer has become much easier recently. In addition, the ring-opening polymerization (ROP) has also been used to prepare star copolymers (8), (9), (10). Among these star polymers, the synthesis of a miktoarm star-shaped copolymer with different linear arms usually involves a variety of synthetic methods and steps, which is therefore challenging (11), (12), (13). The inverse star-block copolymer (AB)2-core-(AB)2 is a new class of miktoarm star-shaped copolymers with the same chemical composition of each arm, but a different sequence of chain segments. It consists of four A-b-B diblock copolymers, two of which are connected to the center point by the A ends and the other two are connected to the center point by the B ends. Yang et al. synthesized a well-defined inverse star-block copolymer (PCL-PS)2-core-(PCL-PS)2, which was found to have the special structure of the radial block copolymer that affects the phase separation of PS and PCL segments and the crystallization behavior of PCL (14).

Poly(L-lactic acid) (PLLA) and poly(ε-caprolactone) (PCL) are two environmental friendly polymers with great biodegradability, biocompatibility and non-toxicity. And they also have excellent crystallinity. The synthesis of many copolymers containing PLLA and PCL segments, such as block, star, star-block and miktoarm star-shaped copolymers has been studied to research the effect of structure on the physical properties (15), (16), (17), (18), (19).

In this paper, in order to provide a new research object for polymer physics’ research and make it easier to study the effect of structure on physical properties, the miktoarm star-shaped copolymer PLLA2PCL2 and the inverse star-block copolymer (PCL-b-PLLA)2-core-(PCL-b-PLLA)2 were designed. The synthetic strategy and route was as shown in Scheme 1. The structures of the miktoarm star-shaped copolymer and the inverse star-block copolymer are characterized by nuclear magnetic resonance (NMR), Fourier transform infrared (FT-IR) spectrometry and gel permeation chromatograph (GPC).

Scheme 1: (A) Synthetic strategy. (B) Synthetic route of miktoarm star-shaped copolymer A2B2. (C) Synthetic route of the inverse star-block copolymer (AB)2-core-(AB)2.
Scheme 1:

(A) Synthetic strategy. (B) Synthetic route of miktoarm star-shaped copolymer A2B2. (C) Synthetic route of the inverse star-block copolymer (AB)2-core-(AB)2.

2 Experimental

2.1 Materials

Propargyl alcohol (99%; Shanghai Jing-Chun Co., Ltd., Shanghai, China) was dried over anhydrous magnesium sulfate, then was distilled under reduced pressure. Methylbenzene was refluxed by a sodium metal and distilled under ordinary pressure before use. 2,2-Bis(bromomethyl)-1,3-propanediol (BPOH, 98%, Aldrich, St. Louis, MO, USA) was purified by recrystallization from methylbenzene and dried in a vacuum at 30°C. ε-Caprolactone (CL, 99%; Acros Organic, Gael, Belgium) was distilled over CaH2 and stored under N2 before use. L-lactide (LLA; Shanghai Tong-Jie-Liang Biomaterials Co., Ltd., Shanghai, China) was purified twice by recrystallization from ethyl acetate and dried in a vacuum at room temperature. Tin 2-ethylhexanoate [Sn(Oct)2; Aldrich, St. Louis, MO, USA] was distilled under reduced pressure before use. Cuprous bromide (CuBr, ≥98.5%) was washed to colorless with acetic acid, washed 3 times with anhydrous ethanol and anhydrous ether and dried in a vacuum. Sodium nitride (NaN3, 99%; Shanghai Jing-Chun Co., Ltd., Shanghai, China), N,N,N′,N,N″-pentamethyl diethylene triamine (PMDETA, 98%; Alfa Aesar, Heysham, UK) and Merrifield resin (BR; Shanghai Ju-Yuan Biomaterials Co., Ltd., Shanghai, China) were used as received without further purification. All other chemicals were of analytical grade and purchased from Sinopharm Chemical Regent Company (Shanghai, China).

2.2 Synthesis of the miktoarm star-shaped copolymer PLLA2PCL2

2.2.1 Preparation of Merrifield’s resin-azide

The brief procedure for the preparation of Merrifield’s resin-azide was as follows (20). The Merrifield’s resin (5.0 g, 1.6 mmol/g, 8 mmol Cl) and NaN3 (5.2 g, 80 mmol) were added to DMF (200 ml) and the resulting mixture was stirred at 50°C for 48 h. The obtained resin was washed with water and THF 3 times and dried at 40°C in a vacuum oven to constant weight.

2.2.2 Synthesis of the alkynyl-terminated poly(ε-caprolactone) (HC≡C-PCL)

The HC≡C-PCL was prepared by the ROP of ε-caprolactone with propargyl alcohol as a small molecule initiator. The synthesis of HC≡C-PCL was as follows: ε-caprolactone (2.854 g, 25 mmol), propynyl alcohol (0.070 g, 1.25 mmol), catalytic amount of Sn(Oct)2 and thoroughly dried toluene (20 ml) were successively added into a polymerization tube. After three cycles of vacuum-nitrogen filling, the polymerization tube was placed in a 110°C oil bath for 24 h under nitrogen protection. The obtained product was dissolved with chloroform, then precipitated twice in methanol. The product after suction filtration was dried in a vacuum at room temperature to constant weight.

Mn,NMR=2290, Mn,GPC=2350, Mw/Mn=1.17.

2.2.3 Synthesis of the two-arm poly(L-lactic Acid) (PLLA2Br2)

The PLLA2Br2 was prepared by the ROP of L-lactide with 2,2-bis(bromomethyl)-1,3-propanediol (BPOH) as a small molecule initiator. The synthesis of PLLA2Br2 was as follows: L-lactide (1.441 g, 10 mmol), BPOH (0.065 g, 0.25 mmol) and catalytic amount of Sn(Oct)2 were successively added into a polymerization tube. After three cycles of vacuum-nitrogen filling, the polymerization tube was placed in a 115°C oil bath for 24 h under nitrogen protection. Postprocessing operations are the same as before.

Mn,NMR=5900, Mn,GPC=5630, Mw/Mn=1.19.

2.2.4 Synthesis of the two-arm azido-poly(L-lactic acid) [PLLA2(N3)2]

NaN3 (0.132 g, 4.8 mmol) was added to a solution of the resultant PLLA2Br2 (0.71 g, 0.24 mmol Br) in DMF (20 ml). After stirring the reaction mixture at 40°C for 72 h, excess NaN3 was removed by filtration, and the solvent was distilled off under reduced pressure. Then, chloroform was added to dissolve the polymer, extracted 3 times with deionized water to remove the remaining NaN3, and the organic layer was dried by anhydrous magnesium sulfate. After the polymer solution was concentrated, it was precipitated in ether. Post-processing operations are the same as before.

Mn,NMR=5800, Mn,GPC=5870, Mw/Mn=1.18.

2.2.5 Synthesis of the miktoarm star-shaped copolymer (PLLA2PCL2)

The synthesis of PLLA2PCL2 was as follows: PLLA2(N3)2 (0.58 g, 0.2 mmol -N3 group) and HC≡C-PCL (0.57 g, 0.25 mmol) were added into DMF (20 ml). After dissolution was complete, the solution was sparged with nitrogen for 15 min, then CuBr (36 mg, 0.25 mmol) and PMDETA (43 mg, 0.25 mmol) were added. The mixture in the flask was degassed by three freeze-vacuum-thaw cycles. Then, nitrogen gas was introduced into the flask and the flask was immersed into an oil bath at 60°C for 48 h. After the reaction was carried out for the prescribed time, the flask was rapidly cooled to room temperature by ice water. The Merrifield’s resin-azide (0.2 g, 0.32 mmol -N3 group) was added. The mixture was degassed by three freeze-vacuum-thaw cycles. Nitrogen gas was introduced into the flask and the flask was immersed into an oil bath at 60°C for 24 h. The polymer solution was passed through a short column of neutral alumina to remove the metal salt and Merrifield’s resin-azide. Postprocessing operations are the same as before.

Mn,NMR=11,060, Mn,GPC=7580, Mw/Mn=1.18.

2.3 Synthesis of the inverse star-block copolymer [(PCL-b-PLLA)2-core-(PCL-b-PLLA)2]

2.3.1 Synthesis of the alkynyl-terminated block copolymer (HC≡C-PCL-b-PLLA)

The HC≡C-PCL-b-PLLA was prepared by the ROP of L-lactide with HC≡C-PCL as a macroinitiator. The synthesis of HC≡C-PCL-b-PLLA was as follows: L-lactide (0.865 g, 6 mmol), HC≡C-PCL (0.69 g, 0.3 mmol) and catalytic amount of Sn(Oct)2 were successively added into a polymerization tube. The temperature of the oil bath was 115°C. The other procedure is as same as the synthesis of HC≡C-PCL

Mn,NMR=5100, Mn,GPC=5160, Mw/Mn=1.20.

2.3.2 Synthesis of the two-arm block copolymer [(PCL-b-PLLA)2Br2]

The (PCL-b-PLLA)2Br2 was prepared by the ROP of ε-caprolactone with PLLA2Br2 as macroinitiator. The synthesis of (PCL-b-PLLA)2Br2 was as follows: ε-caprolactone (0.548 g, 4.8 mmol), PLLA2Br2 (0.71 g, 0.24 mmol -OH), catalytic amount of Sn(Oct)2 and thoroughly dried toluene (10 ml) were successively added into a polymerization tube. The temperature of oil bath is 115°C. The other procedure is the same as the synthesis of PLLA2Br2.

Mn,NMR=10,300, Mn,GPC=9200, Mw/Mn=1.23.

2.3.3 Synthesis of the two-arm block azido-copolymer [(PCL-b-PLLA)2(N3)2]

NaN3 (0.182 g, 2.8 mmol) was added to a solution of the resultant (PCL-b-PLLA)2Br2 (0.72 g, 0.14 mmol Br) in 20 ml of DMF. The other procedure is the same as the synthesis of PLLA2(N3)2.

Mn,NMR=10,190, Mn,GPC=9600, Mw/Mn=1.21.

2.3.4 Synthesis of the inverse star-block copolymer [(PCL-b-PLLA)2-core-(PCL-b-PLLA)2]

The synthesis of (PCL-b-PLLA)2-core-(PCL-b-PLLA)2 was as follows: (PCL-b-PLLA)2(N3)2 (0.51 g, 0.1 mmol -N3 group) and HC≡C-PCL-b-PLLA (0.66 g, 0.13 mmol) were added into DMF (20 ml). After dissolution was complete, the solution was sparged with nitrogen for 15 min, then CuBr (37 mg, 0.26 mmol) and PMDETA (45 mg, 0.26 mmol) were added. The other procedure is the same as the synthesis of PLLA2PCL2.

Mn,NMR=20,920, Mn,GPC=14,900, Mw/Mn=1.22.

2.4 Characterization

1H NMR spectra were determined on a Bruker DMX-500 NMR instrument using CDCl3 as a solvent and tetramethyl silane as an internal standard. FT-IR spectra were recorded on an AVATAR 360 ESP FT-IR instrument. The measurement of molecular weight and molecular weight distribution of the polymers was carried out on a Waters 150C GPC. Polystyrene was used as a calibration standard. CHCl3 was used as the eluent at a flow rate of 1.0 ml/min.

3 Results and discussion

3.1 Synthesis of HC≡C-PCL

The FT-IR spectrum of HC≡C-PCL is shown in Figure 1A. The strong absorption peak at 1729 cm−1 could be attributed to the stretching vibration peak of C=O in the PCL molecular chain, and the absorption peak at 3250 cm−1 belongs to the characteristic peak of alkyne, which can prove that the synthesized PCL contains alkyne groups. For confirmation of the polymer structure, the 1H NMR spectra were measured, as shown in Figure 1C. Except for the characteristic signals of PCL at 4.06 ppm (d), 3.65 ppm (d’), 2.31 ppm (a), 1.65 ppm (b) and 1.39 ppm (c), we can find the signals at 2.47 ppm (h) and 4.68 ppm (g) ascribed to alkyne protons and alkynylparamethylenes, respectively. In addition, the molecular weight of HC≡C-PCL can also be calculated by the integrated areas Ig and Id of the two signal peaks at the 4.68 ppm and 4.06 ppm displacements according to equation [1].

Figure 1: FT-IR and 1H NMR spectra of HC≡C-PCL [(A), (C)] and HC≡C-PCL-b-PLLA [(B), (D)] .
Figure 1:

FT-IR and 1H NMR spectra of HC≡C-PCL [(A), (C)] and HC≡C-PCL-b-PLLA [(B), (D)] .

[1]Mn,NMR[HC​≡​C-PCL]=Id/Ig×114+56

Here, 114 and 56 are the molecular weights of the ε-caprolactone monomer and the terminal alkynyl group, respectively. The calculated results are listed in Table 1. The agreement of Mn,NMR, Mn,th and Mn,GPC proves that HC≡C-PCL was successfully prepared.

Table 1:

Molecular weight and conversion rate data for the synthetic polymers.

SampleMn,thaMn,NMRbMn,GPCcMw/MncConv, %d
HC≡C-PCL2300229023501.1798.3
HC≡C-PCL-b-PLLA5080510051601.2096.9
PLLA2Br25880590056301.1997.5
PLLA2(N3)25810580058701.1892.7
(PCL-b-PLLA)2Br210,36010,30092001.2397.8
(PCL-b-PLLA)2(N3)210,29010,19096001.2191.6
PLLA2PCL210,41011,06075801.18−
(PCL-b-PLLA)2-core-(PCL-b-PLLA)220,45020,92014,9001.22−

  1. aMn,th denotes the theoretical number-average molecular weight of polymers. Mn,th of HC≡C-PCL, HC≡C-PCL-b-PLLA, PLLA2Br2, and (PCL-b-PLLA)2Br2 can be calculated according to the equation: Mn,th=[monomer]/[initiator]×Mmonomer×Conv (%)+Minitiator, where [monomer] and [ initiator] are initial concentrations of monomers and initiators, Minitiator is the number-average molecular weight of initiator; Mn,th of PLLA2(N3)2 and (PCL-b-PLLA)2(N3)2 can be calculated according to the equation: Mn,th=Mn,th[prepolymer]+2×(80–42), where Mn,th[prepolymer] is the number-average molecular weight of PLLA2Br2 or (PCL-b-PLLA)2Br2, 42 and 80 are the molecular weight of azido group and bromine group, respectively; Mn,th[PLLA2PCL2]=Mn,th[PLLA2(N3)2]+2×Mn,th[HC≡C-PCL]; Mn,th[(PCL-b-PLLA)2-core-(PCL-b-PLLA)2]=Mn,th[(PCL-b-PLLA)2(N3)2]+2×Mn,th[HC≡C-PCL-b-PLLA].

  2. bDetermined by 1H NMR spectroscopy of polymers.

  3. cDetermined by GPC analysis with polystyrene standards. CHCl3 was used as eluent.

  4. dConversion of monomer obtained from gravimetry.

3.2 Synthesis of HC≡C-PCL-b-PLLA

The FT-IR spectrum of HC≡C-PCL-b-PLLA is shown in Figure 1B. The difference between Figure 1B and A is mainly the position of the C=O absorption peak. As shown in Figure 1B, the C=O absorption peak broadens and splits into two peaks, where the strong absorption peak at 1756 cm−1 can be attributed to C=O on the PLLA segment in the copolymer. This result can confirm that the PLLA chain exists on the synthesized copolymer. For confirmation of the polymer structure, the 1H NMR spectra were measured, as shown in Figure 1D. Compared with the 1H NMR spectrum of HC≡C-PCL, two strong signal peaks appear at 5.17 (e) and 1.58 ppm (f), which can be attributed to the proton of -CH-CH3 and -CH3 on repeat units in the PLLA molecule chain, respectively. It can be seen the signal peak at 3.65 ppm disappeared completely, a new signal appeared at 4.37 ppm (e’), which belongs to -CH-CH3 in the PLLA molecule chain. In addition, the molecular weight of HC≡C-PCL-b-PLLA can also be calculated by the integrated areas Ie, Ig and Id of the three signal peaks at the 5.17, 4.68 and 4.06 ppm displacements according to equation [2].

[2]Mn,NMR[HC​≡​C−PCL−b−PLLA]=Id/Ig×114+Ie/Ig×144+56

Here, 114, 144 and 56 are the molecular weights of the ε-caprolactone monomer, lactide monomer and terminal alkynyl group, respectively. The calculated results are listed in Table 1. The agreement of Mn,NMR, Mn,th and Mn,GPC proves the formation of HC≡C-PCL-b-PLLA copolymers.

3.3 Synthesis of PLLA2Br2

The FT-IR spectrum of PLLA2Br2 is shown in Figure 2A. The strong absorption peak at 1756 cm−1 can be attributed to the stretching vibrational peak of C=O on the PLLA chain, which can prove that the synthesized polymer is PLLA. The chemical structure of PLLA2Br2 was also characterized by 1H NMR, as shown in Figure 2D. In addition to the characteristic signal peaks of PLLA, the para-methylene protons near Br and O in the BPOH molecule are present at 3.48 ppm (j) and 4.23 ppm (i), respectively. In addition, the molecular weight of PLLA2Br2 can also be calculated by the integrated areas Ie and Ii of the two signal peaks at the 5.18 ppm and 4.23 ppm displacements according to equation [3].

Figure 2: FT-IR and 1H NMR spectra of PLLA2Br2 [(A), (D)], PLLA2(N3)2 [(B), (E)] and PLLA2PCL2 [(C), (F)] .
Figure 2:

FT-IR and 1H NMR spectra of PLLA2Br2 [(A), (D)], PLLA2(N3)2 [(B), (E)] and PLLA2PCL2 [(C), (F)] .

[3]Mn,NMR[PLLA2Br2]=Ie/Ii×144×2+262

Here, 144 and 262 are the molecular weights of the lactide monomer and BPOH, respectively. The calculated results are listed in Table 1. The Mn,NMR and the Mn,th are in agreement, but it is slightly higher than the molecular weight Mn,GPC.

3.4 Synthesis of PLLA2(N3)2

The FT-IR spectrum is shown in Figure 2B. Its main difference from Figure 2A is that a new absorption peak appears at 2097 cm−1, which belongs to the azido group. 1H NMR of PLLA2(N3)2 is shown in Figure 2E. It can be seen that there are almost no changes in the chemical shifts attributed to the PLLA molecular chain. The signal peak at the 3.48 ppm (j) of methylene proton by Br completely disappears. At 3.40 ppm, a new signal peak appears that belongs to the methylene proton adjacent to the azido group. Moreover, the signal peak of the methylene proton of the O in BPOH molecule is also shifted from δ=4.23 to 4.11 ppm (i), which is also caused by the formation of an azido group on the central molecule. These results indicate that the Br atom on the central molecule of PLLA2Br2 has been converted to an azido group. Similarly, the molecular weight of PLLA2(N3)2 can also be calculated by equation [4].

[4]Mn,NMR[PLLA2(N3)2]=Ie/Ii×144×2+186

Here, 144 and 186 are the molecular weights of the lactide monomer and BPOH, respectively. The calculation results are listed in Table 1, which is similar to the molecular weight result of PLLA2Br2.

3.5 Synthesis of PLLA2PCL2

Figure 2C shows the FT-IR spectrum of PLLA2PCL2, which differs from Figure 2B mainly in the position of the C=O absorption peak. As shown in Figure 2C, the C=O absorption peak broadens and splits into two peaks, wherein the strong absorption peak at 1756 cm−1 and 1729 cm−1 can be attributed to C=O on the PLLA chain and the PCL chain, respectively. This result proves that the PCL and PLLA chains are present simultaneously. In addition, the characteristic peak belonging to the alkynyl group at 3250 cm−1 in Figure 2A and the azido group at 2097 cm−1 in Figure 2B disappear completely, indicating that the alkynyl group and the azido group were consumed in the click reaction. The 1H NMR spectrum of PLLA2PCL2 is shown in Figure 2F. The characteristic peaks at 4.68, 2.47 ppm in Figure 2D and 4.11, 3.40 ppm in Figure 2E disappear completely, which also indicated that the alkynyl group and the azido group were consumed. Similarly, the molecular weight of PLLA2PCL2 can also be calculated by equation [5].

[5]Mn,NMR[PLLA2PCL2]=(Id/Ii×114+Ie/Ii×144)×2+56×2+186

Here, 114, 144, 56 and 186 are the molecular weights of the ε-caprolactone monomer, lactide monomer, terminal alkynyl group and BPOH, respectively. The calculation results are listed in Table 1, which is similar to the molecular weight result of PLLA2Br2. The Mn,NMR and the Mn,th are in agreement, but they are also significantly different from the Mn,GPC. The Mn,GPC is significantly lower than Mn,NMR and Mn,th, which is due to the hydrodynamic volume difference between the star polymer and the linear polymer. The most important feature of star polymers is that they have more segment density than linear polymers of the same molecular weight, which makes star polymers generally have smaller hydrodynamic volumes. In a size exclusion chromatography device, the smaller hydrodynamic volume allows the polymer to have a higher outflow volume for a specific time. This means that when linear polymer standards are used in GPC testing, the apparent molecular weight of highly branched polymers such as star polymers is often far below their actual molecular weight (21). However, it can be referred to as a result as the Mn,GPC is higher than that of the linear raw material polymer. The narrow molecular weight distribution can prove that the star copolymer is successfully synthesized by the click reaction.

According to changes of GPC curves before and after the click reaction in Figure 3A, the GPC curves of the raw materials HC≡C-PCL and PLLA2(N3)2 are single peaks, and on the lower molecular weight side. The GPC curve of the product PLLA2PCL2 is also a single peak and has a significant shift to the high molecular weight direction. It indicates that the click reaction between HC≡C-PCL and PLLA2(N3)2 link the two types of molecular chains together successfully and form the miktoarm star-shaped copolymer PLLA2PCL2 with a higher molecular weight.

Figure 3: A. GPC traces of (a) HC≡C-PCL, (b) PLLA2(N3)2 and (c) PLLA2PCL2. B. GPC traces of (a) HC≡C-PCL-b-PLLA, (b) (PCL-b-PLLA)2(N3)2 and (c) (PCL-b-PLLA)2-core-(PCL-b-PLLA)2.
Figure 3:

A. GPC traces of (a) HC≡C-PCL, (b) PLLA2(N3)2 and (c) PLLA2PCL2. B. GPC traces of (a) HC≡C-PCL-b-PLLA, (b) (PCL-b-PLLA)2(N3)2 and (c) (PCL-b-PLLA)2-core-(PCL-b-PLLA)2.

3.6 Synthesis of (PCL-b-PLLA)2Br2

The FT-IR spectrum of (PCL-b-PLLA)2Br2 is shown in Figure 4A. Compared with Figure 4A, in addition to the strong absorption peak of PLLA at 1756 cm−1, a new absorption peak at 1729 cm−1 has also been split, which is attributed to the stretching vibration peak of C=O on the PCL. The chemical structure of (PCL-b-PLLA)2Br2 was also characterized by 1H NMR, as shown in Figure 4D. In addition to the characteristic signal peaks of PLLA and BPOH, the characteristic signal peaks of PCL at 4.07(d), 3.65(d′), 2.30(a), 1.65(b) and 1.38(c) appeared. In addition, the molecular weight of (PCL-b-PLLA)2Br2 can also be calculated by the integrated areas Ie, Id and Ij of the three signal peaks at the 5.18, 4.07 and 3.48 ppm displacements according to equation [6].

Figure 4: FT-IR and 1H NMR spectra of (PCL-b-PLLA)2Br2 [(A), (D)], (PCL-b-PLLA)2(N3)2 [(B), (E)] and (PCL-b-PLLA)2-core-(PCL-b-PLLA)2 [(C), (F)] .
Figure 4:

FT-IR and 1H NMR spectra of (PCL-b-PLLA)2Br2 [(A), (D)], (PCL-b-PLLA)2(N3)2 [(B), (E)] and (PCL-b-PLLA)2-core-(PCL-b-PLLA)2 [(C), (F)] .

[6]Mn,NMR[(PCL−b−PLLA)2Br2]=(Id/Ij×114+Ie/Ij×144)×2+262

Here, 114, 144 and 262 are the molecular weight of the ε-caprolactone monomer, lactide monomer and BPOH, respectively. The calculated results are listed in Table 1. The Mn,NMR and the Mn,th are in agreement, but it is slightly higher than the molecular weight Mn,GPC.

3.7 Synthesis of (PCL-b-PLLA)2(N3)2

The FT-IR spectrum of (PCL-b-PLLA)2(N3)2 is shown in Figure 4B. The difference of Figure 4B and A is mainly from a new absorption peak at 2097 cm−1, which is a characteristic peak belonging to the azido group. For confirmation of the polymer structure, the 1H NMR spectra were measured and a typical spectrum is shown in Figure 4E. It can be seen that the signal peak of the para-methylene proton at 3.48 ppm (j) by the Br completely disappears. At 3.40 ppm, there is a new signal peak attributed to the methylene proton next to the azide. Moreover, the signal peak of the methylene proton next to the O is also shifted from 4.23 to 4.04 ppm(i), which is due to the reduction of the chemical shift caused by the initiation of the azido group on the central molecule. These results indicated that (PCL-b-PLLA)2Br2 initiates the conversion of the bromine atom on the central molecule to an azido group. Similarly, the molecular weight of (PCL-b-PLLA)2(N3)2 can also be calculated in the same way according to equation [7].

[7]Mn,NMR[(PCL−b−PLLA)2(N3)2]=(Id/Ij×114+Ie/Ij×144)×2+186

Here, 114, 144 and 186 are the molecular weight of the ε-caprolactone monomer, lactide monomer and BPOH azide, respectively. The calculated results are listed in Table 1. The result is similar to (PCL-b-PLLA)2Br2.

3.8 Synthesis of (PCL-b-PLLA)2-core- (PCL-b-PLLA)2

The FT-IR spectrum of (PCL-b-PLLA)2(N3)2 was shown in Figure 4C. It can be seen that characteristic peaks belonging to the alkyne at 3250 cm−1 in Figure 1B and characteristic peaks of the azido group at 2097 cm−1 in Figure 4B disappeared completely, indicating that the alkyne and the azido group were reacted in the click reaction. Figure 4F is the 1H NMR spectrum of (PCL-b-PLLA)2-core-(PCL-b-PLLA)2. Compared with Figure 4E, the signal peaks for methylene protons and alkyne protons at 4.68 and 2.47 ppm have disappeared completely. Instead, the peak at 5.24(g) and 7.89 ppm(h) appear, which are assigned to protons of the 1,2,3-triazole ring para-methylene and the methylene proton. In addition, the signal peaks for protons of the methylene groups on the side of the O and -N3 at 4.04 and 3.40 ppm in Figure 4E are disappeared completely. Instead, the peak at 4.49(i) and 3.98 ppm(j) appear, which are assigned -CH2- next to the O and the 1,2,3-triazole ring. It is also illustrated that the alkyne and azido groups are consumed completely by the click reaction and 1,2,3-triazole ring groups are formed. In addition, the molecular weight of (PCL-b-PLLA)2-core-(PCL-b-PLLA)2 can also be calculated by the integrated areas Ie, Ii, and Id of the three signal peaks at the 5.16, 4.49 and 4.06 ppm displacements according to equation [5]. The calculation results are listed in Table 1. The Mn,NMR and the Mn,th are in agreement, but they are also significantly different from the Mn,GPC. The Mn,GPC is significantly lower than Mn,NMR and Mn,th, which is due to the hydrodynamic volume difference between the star polymer and the linear polymer.

Combining the changes of the GPC curves before and after the click reaction in Figure 3B, the GPC curves of the raw materials HC≡C-PCL-b-PLLA and (PCL-b-PLLA)2(N3)2 are both a single-peak and are on the lower molecular weight side. The GPC curve of the product (PCL-b-PLLA)2-core-(PCL-b-PLLA)2 is also a single-peak and has a significant shift to the high molecular weight direction. It is shown that the formation of a higher molecular weight reversal star-block copolymer.

4 Conclusions

The well-defined miktoarm star-shaped copolymer PLLA2PCL2 and inverse star-block copolymer (PCL-b-PLLA)2-core-(PCL-b-PLLA)2 were successfully prepared through a combination of ROP and click chemistry. The feasible synthetic strategy of the inverse star-block copolymer is a combination of the “core-first” and “arm-first” methods. Starting from the non-equivalent functional group initiator BPOH, PLLA2Br2 was produced by ROP with L-lactide, and (PCL-b-PLLA)2Br2 was synthesized by ROP with ε-caprolactone using PLLA2Br2 as a macroinitiator. Then, after the ROP of ε-caprolactone to prepare HC≡C-PCL using propylene alcohol as initiator, HC≡C-PCL-b-PLLA was further produced by ROP with L-lactide. Following click reaction was used to produce PLLA2PCL2 with PLLA2(N3)2 and HC≡C-PCL. The following click reaction was used to produce (PCL-b-PLLA)2-core-(PCL-b-PLLA)2 with (PCL-b-PLLA)2(N3)2 and HC≡C-PCL-b-PLLA. The FT-IR, 1H NMR and GPC demonstrate that star copolymers have been successfully synthesized. In conclusion, the above experiment proves that the miktoarm star-shaped and inverse star-block copolymers can be successfully synthesized by a combination of ROP and click reactions. Furthermore, we can infer that the miktoarm or inverse star-block copolymers with more abundant molecular composition can be synthesized by using the similar synthesis strategy and other living polymerization methods (such as ATRP, RAFT, etc.). The synthesized copolymers could be used to study the potential relationship between polymer structure and properties.

Funding source: National Natural Science Foundation of China

Award Identifier / Grant number: 51203118

Funding statement: This work was financially supported by the National High-Tech R&D Program of China (No. 2013AA032202), the National Natural Science Foundation of China (Funder Id: 10.13039/501100001809, No. 51203118), the Fundamental Research Funds for the Central Universities and the Open Funds for Characterization of Tongji University.

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Received: 2018-08-05
Accepted: 2018-08-19
Published Online: 2018-09-13
Published in Print: 2018-10-25

©2018 Walter de Gruyter GmbH, Berlin/Boston

This article is distributed under the terms of the Creative Commons Attribution Non-Commercial License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

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https://doi.org/10.1515/epoly-2018-0180
Keywords for this article
click chemistry; inverse star-block copolymer; poly(ε-caprolactone); poly(L-lactic acid); ring-opening polymerization
Creative Commons
BY-NC-ND 3.0

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