Synthesis of Amphiphilic ABC 3-Miktoarm Star Terpolymer by

Synthesis of Amphiphilic ABC 3-Miktoarm Star Terpolymer by

(Parte 1 de 3)

Synthesis of Amphiphilic ABC 3-Miktoarm Star Terpolymer by Combination of Ring-Opening Polymerization and “Click” Chemistry

You-Yong Yuan,†,‡ Yu-Cai Wang,‡ Jin-Zhi Du,‡ and Jun Wang*,†,§

Hefei National Laboratory for Physical Sciences at Microscale, Department of Polymer Science and Engineering, and School of Life Sciences, UniVersity of Science and Technology of China, Hefei, Anhui 230027, People’s Republic of China

ReceiVed June 30, 2008; ReVised Manuscript ReceiVed September 28, 2008

ABSTRACT: Novel biodegradable amphiphilic ABC 3-miktoarm star terpolymers composed of poly(εcaprolactone)(PCL), monomethoxypoly(ethyleneglycol) (MPEG), and polyphosphoester(PPE) were synthesized by a combination of ring-opening polymerization and “click” chemistry. MPEG was first end-capped by epoxide ring, which was opened by sodium azide in the presence of ammonium chloride to give modified MPEG bearing reactive azide and hydroxyl groups (MPEG(-OH,-N3)). “Click” chemistry was then applied to conjugate

R-propargyl-ω-acetyl-poly(ε-caprolactone) and MPEG(-OH,-N3), resulting in a diblock copolymer of MPEG and PCL with reactive hydroxyl groups at the junction point (MPEG(-OH)-b-PCL), which further initiated ring- opening polymerization of 2-ethoxy-2-oxo-1,3,2-dioxaphospholane (EEP) under the catalytic action of stannous octoate to obtain the desired well-defined (MPEG)(PCL)(PEEP) 3-miktoarm star terpolymers. Such terpolymers and theirintermediateswere characterizedby 1H NMR, FT-IR,and gel permeationchromatography.Thesepolymers are expected to be promising vehicles for drug delivery applications.

Introduction

ABC 3-miktoarmstar terpolymersare star-shapedcopolymers composedof threechemicallyand molecularlydifferentpolymer chains emanating from a central junction point.1,2 Previous research showed that ABC 3-miktoarm star terpolymers exhibit quite different microdomain morphology in bulk3 and selfassembly behavior in solution4,5 as compared to linear ABC triblock copolymers. Interestingly, they could form multicompartmental micelles as reported by Lodge and co-workers.6,7 This kind of micelle forms segregated microdomains with different chemical environments that may be used to store various drug molecules in desired ratio.7 This kind of miktoarm star polymers may have potential application in drug delivery.8

A preferred polymeric drug delivery system should be biocompatible, biodegradable, and functionable for introducing chemicalreactivegroups or biologicallyrelevantand compatible molecules.9 Poly(ethylene glycol) (PEG) and poly(ε-caprolactone) (PCL) are biocompatible materials that have been widely investigated in biomedical applications. PEG can prevent recognition by the reticuloendothelialsystem, and thus prevents premature elimination of the micelles from the bloodstream and yieldsa prolongedsystemiccirculation.10 PCL is of great interest in biomedicalresearchbecauseof its low cost, slow degradation, high permeability to many drugs, and nontoxicity.1 Recently, polyphosphoesters (PPE) have received considerable attention in biomedical applications due to their biodegradability, biocompatibility, and pendant functional ability in addition to a variable backbone structure, thereby allowing introduction of bioactive molecules and modification of its chemical and physical properties.12,13 For example, amino groups have been introducedinto the side chainsfor condensationof plasmidDNA in gene delivery.14,15 Other functionalgroupssuch as hydroxyl,16 methacrylate,17 and bromoisobutylate18 have also been introduced as pendent groups of polyphosphoesters. In addition, Iwasaki et al. have demonstrated that poly(ethyl ethylene phosphate) and its copolymer with poly(isopropyl ethylene phosphate) can be thermo responsive.19 We hypothesize that incorporation of biocompatible PEG, PCL, and PEEP segments into a 3-miktoarm star terpolymer would render this polymeric structure attractive for building a versatile drug delivery system.

Miktoarm star polymers have been synthesized since the 1990s.20 Because of the synthesis and purification difficulties, preparation of ABC 3-miktoarm star terpolymers is still challenging. Until recently, limited strategies have been applied to synthesize miktoarm star terpolymers.1,2 Three synthetic methods have been mainly used to synthesize ABC 3-miktoarm star terpolymers. The first method uses multifunctional chlorosilane to react subsequently with active chain ends of different living linear polymers.21,2 The second method utilizes linear polymer with an end group of 1,1-diphenylethylene (DPE) or 1,4-bis(1- phenylethenyl)benzene (DDPE) to react with another living macroanion polymer chain, which generates an anionic species at the junction point of the diblock copolymer. Another monomer is then polymerized from this junction point to obtain the desired miktoarm star terpolymers.23-25 The third method is using a polymer chain capped at one end by two functional groups that are able to initiate independently the polymerization of two differentkinds of monomers.26,27 In addition,trifunctional initiator has also been utilized for miktoarm synthesis.28 Yet the first two methods are restricted to a few monomers and contain stringent polymerizationconditions. Regarding the third method, it is difficult to design and synthesize a capping molecule or an initiator with multifunctional groups for the different polymerization methods, because different polymerization methods generallyrequire differentpolymerizationconditions. Recently, “click” chemistry has been used extensively in polymer and material sciences to construct polymer architecture because of its high selectivity, near-perfect reliability, and high yield. Most importantly, it is exceptionally tolerant toward a wide range of functional groups and reaction conditions.29,30 Such advantages have been applied for the synthesis of miktoarm polymers. For example, Monteiro et al. used “click” chemistry and the ATRP technique to synthesize 3-miktoarm star terpolymers.31,32 In the present work, by a combination of ring-opening polymerization and “click” chemistry, we report

* Corresponding author. Fax: +86 551 360 0402. E-mail: jwang699@ ustc.edu.cn.

† Hefei National Laboratory for Physical Sciences at Microscale. ‡ Department of Polymer Science and Engineering. § School of Life Sciences.

10.1021/ma801452n C: $40.75 2008 American Chemical Society Published on Web 10/25/2008

a facile and useful method to synthesize ABC 3-miktoarm star terpolymers of MPEG, PCL, and PPE with well-defined structures.

Experimental Section

Materials. 2-Ethoxy-2-oxo-1,3,2-dioxaphospholane (EEP) was synthesized by a method described previously33 and distilled under reduced pressure just before use. Monomethoxy poly(ethylene

glycol)s (MPEG, Mn ) 750, 2000, 5000 g/mol, Acros Organics, Belgium) were dried by azeotropically distillation from anhydrous toluene. Epichlorohydrin (ECH) and ε-caprolactone (ε-CL) (Acros Organics, 9%) were dried over calcium hydride for 48 h at room temperature, followed by distillation under reduced pressure just before use. Stannous octoate (Sn(Oct)2) (Sinopharm Chemical Reagent Co., Ltd., China) was purified according to a method described in the literature.34 Acetyl chloride (AcCl) was distilled just before use. Triethylamine (TEA) was refluxed with phthalic anhydride, potassium hydroxide, and calcium hydride in turn and distilled just before use. Tetrahydrofuran (THF) and toluene were refluxed over potassium-sodium alloy under N2 atmosphere and distilled just before use. Other reagents were used as received.

Synthesisof MPEG(-OH,-N3) (Scheme1). First, three R-methoxy-ω-epoxypoly(ethyleneglycol)swith differentmolecularweights were synthesized according to the literature.35 MPEG(-OH,-N3) was synthesized by reaction of R-methoxy-ω-epoxypoly(ethylene

glycol) with sodium azide in the presence of ammonium chloride as shownin Scheme1. Typically,R-methoxy-ω-epoxypoly(ethylene glycol)(1.33mmol)was dissolvedin DMF (5 mL),and thensodium azide(3.99mmol)and ammoniumchloride(3.99mmol)wereadded to the mixture and stirred at 50 °C for 36 h. After removal of DMF under reduced pressure, the residue was dissolved in dichloromethane and filtered to remove insoluble impurities, followed by precipitation in cold diethyl ether. The product denoted as

MPEG750(-OH,-N3) (subscript represents the number average molecular weight of the raw MPEG) was collected and dried under vacuum overnight with a yield of ca. 85%. Other MPEG-

(-OH,-N3)s were prepared with a similar method.

Synthesisof r-Propargyl-ω-acetyl-poly(ε-caprolactone)(Scheme 2). R-Propargyl-ω-acetyl-PCL was synthesized in two steps. First, propargyl-terminated PCL was obtained by ring-opening polymerization of ε-CL under the co-initiation of propargyl alcohol and

Sn(Oct)2. The polymerization was performed in a glovebox with a water contentof less than 0.1 ppm. In a typicalprocedure,propargyl alcohol (0.070 g, 1.25 mmol) and ε-CL (10 g, 87.6 mmol) were added to freshly dried toluene (50 mL) in a flask, and then Sn(Oct)2 (0.254 g, 0.63 mmol) was added and the solution was stirred at 80

°C for 4 h. The mixture was concentrated under reduced pressure. After precipitation in cold diethyl ether, the polymer was obtained and dried under vacuum overnight with a yield of ca. 67%. Other propargyl-terminated PCLs were prepared with a similar method.

The hydroxyl end group of the propargyl-terminated PCL was then blocked by acetyl chloride. In a typical example, propargylterminated PCL2 (1.0 g, 0.18 mmol) was azeotropically distilled from50 mL of toluene(removingabout30 mL of toluene),followed by introduction of triethylamine (TEA) (0.036 g, 0.36 mmol) via a syringe. Acetyl chloride (AcCl) (0.028 g, 0.36 mmol) in 5 mL of toluene was added dropwise to the above solution at 0 °C, and the mixture was further stirred overnight. Thereafter, insoluble salt was filtered off, and the solution was concentrated under reduced pressure. The polymer was collected by precipitationin cold diethyl ether, filtration, and dried under vacuum overnight with a yield of ca. 81.5%. Other propargyl-terminatedPCLs were end-capped with a similar method.

SynthesisofMPEG(-OH)-b-PCLby“Click”Chemistry(Scheme 3). In a typical procedure,R-propargyl-ω-acetyl-PCL2(0.50 g, 8.80 was degassed by five freeze-pump-thaw cycles, and then CuI (0.005 g, 0.3 equiv) and 1,8-diazabicyclo[5.4.0]undec-7-ene(DBU) (0.270 g, 20 equiv) were added in a glovebox. The solution was stirred at 35 °C for 24 h. After the reaction, the solution was passed through a neutral alumina column to remove copper catalyst. The solution was concentrated and poured into excess water (20 mL) and was dialyzed extensively using Spectra/Por dialysis tubing (MWCO ) 15 0). The purified diblock copolymer was obtained by lyophilization. Synthesisof ABC 3-MiktoarmStar Terpolymers(MPEG)(P-

CL)(PEEP)(Scheme4). Typically, MPEG750(-OH)-b-PCL2 (0.10 g, 0.015 mmol) was azeotropically distilled from toluene (20 mL) and dried under vacuum overnight. EEP (0.233 g, 1.53 mmol) and

Sn(Oct)2 (0.003 g, 0.008 mmol) were then added, and the polymerization was carried out at 35 °Ci nT HF (2 mL) for3hi n a glovebox. The solution was concentrated, and the residue was precipitated into cold diethyl ether containing 10% methanol (v/ v). After filtration, miktoarm star terpolymer was obtained by vaccuum-drying overnight.

Characterization Methods. 1H NMR spectra were recorded on a Bruker AV300 NMR spectrometer (300 MHz) at room temper- ature with CDCl3 as solvent. FT-IR spectra were measured on a Bruker Vector 2 Fourier transform infrared spectrometer at

Number and weight average molecular weights (Mn and Mw) and molecular weight distributions (polydispersity index, PDI ) Mw/

Scheme 1. Synthesis Route of MPEG(-OH,-N3) Scheme 2. SynthesisRoute of r-Propargyl-ω-acetyl-PCL

Scheme 3. Synthesis Route of MPEG(-OH)-b-PCL

Scheme 4. Synthesis Route of (MPEG)(PCL)(PEEP) Miktoarm Terpolymers

Macromolecules, Vol. 41, No. 2, 2008 Synthesis of Miktoarm Star Terpolymer 8621

Mn) were determined by gel permeation chromatography (GPC) measurements on a Waters GPC system, which was equipped with

a Waters 1515 HPLC solvent pump, a Waters 2414 refractiveindex detector,and WatersStyragelHigh Resolutioncolumns(HR4,HR2, HR1, effectivemolecularweightrange50-5000, 500-20 0, and 100-5000, respectively).Chloroform(HPLC grade,J.T. Baker, stabilized with 0.75% ethanol) was used as eluent and delivered at a flow rate of 1.0 mL/min at 40 °C. Monodispersed polystyrene standards with a molecular weight range 1310-(5.51 × 104) were used to generate the calibration curve.

Results and Discussion

(MPEG750, MPEG2000, MPEG5000) were used in this work. The hydroxyl end group of MPEG was first converted to epoxide group according to the literature.35 Figure 1A shows the representative 1H NMR spectrum of R-methoxy-ω-epoxypoly-

(ethylene glycol)2000 (MPEG2000-epoxide, subscript represents the number average molecular weight of the raw MPEG). The signals at 2.62, 2.79, 3.15 ppm are assigned to the protons of the epoxide ring as labeled in Figure 1A, indicating that the reactive epoxide group has been introduced to the MPEG2000. By comparing the integral of proton signal of the epoxide ring at 2.79 ppm (d) to that of the methylene protons of MPEG2000 at 3.62 ppm (b), the end-capping efficiency was estimated to be about 90.9%. End-capping efficiencies of MPEG750 and

MPEG5000 were 90.3% and 95.0%, respectively. Subsequently, the epoxide ring was opened by sodium azide in the presence of ammonium chloride to generate

MPEG(-OH,-N3) with both azide and hydroxyl groups. This approach has recently been employed by Matyjaszewski et al.

to produce azide-functionalized poly(glycidyl methacrylate-comethyl methacrylate) polymers.36 Figure 1B shows the typical

1H NMR spectrum of MPEG2000(-OH,-N3) in CDCl3.I ti s noteworthy that the signals of the epoxide ring in Figure 1A

(c, d, e) have disappearedcompletely,indicatingthat the epoxide ring has been opened by sodium azide. The peak at 4.25 ppm (f) should be attributed to the proton of methine proton of

-CHOH, and the signal of methylene protons of -CH2N3 is overlapped with that of methylene protons of MPEG backbone according to the literature.35,37 FT-IR measurements were performed to further confirm the formation of MPEG2000(-OH,-N3). As shown in Figure 2, the appearance of an absorption peak at 2099 cm-1 is clearly observed, which corresponds to the vibration frequency of the azide group.36

Synthesis of r-Propargyl-ω-acetyl-PCL (Scheme 2). The propargyl-terminatedPCL was first synthesizedin solutionusing propargyl alcohol as the initiator and Sn(Oct)2 as the catalyst. The reaction was performed in toluene at 80 °C according to the literature.38 Three different monomer/initiator ratios were utilized to obtain propargyl-terminated PCL and achieve different molecular weights. Detailed information is listed in Table 1.

The existence of propargyl group is verified by a typical 1H

NMR spectrum of the PCL2 as shown in Figure 3A. Resonance at 4.68 ppm (a) is the characteristic signal of the methylene protons of propargyl group (HCtCCH2-), while resonance of the alkynyl proton (i) is overlapped with that of methylene protons of PCL at 2.35 ppm (b). The triplet at 3.65 ppm (g) is assigned to the PCL methylene protons conjoint with the hydroxyl end group. The degree of polymerization(DP) of PCL can be calculated by the integration ratio of signals at 4.07 ppm (f) of the PCL repeat units and that of signal at 3.65 ppm (g). The DP of PCL1, PCL2, and PCL3 are 15, 49, and 104, respectively, as summarized in Table 1. It should be noted that all of the propargyl-terminated PCL polymers possess a low

Figure 1. 1H NMR spectra of MPEG2000-epoxide (A) and MP-

Table 1. Characteristics of r-Propargyl-ω-acetyl-poly(ε-caprolactone) code [M]0/[I]0a Mw/Mnb DPc Mn c a [M]0 and [I]0 are the initial concentrations of ε-CL and propargyl alcohol, respectively. b Determined by GPC. c Calculated on the basis of

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