Biomacromolecules 2008, 9, 3051–3056
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Organocatalytic Approach to Amphiphilic Comb-Block Copolymers Capable of Stereocomplexation and Self-Assembly Kazuki Fukushima,†,‡ Russell C. Pratt,† Fredrik Nederberg,†,‡ Jeremy P. K. Tan,§ Yi Yan Yang,§ Robert M. Waymouth,‡ and James L. Hedrick*,† IBM Almaden Research Center, 650 Harry Road, San Jose, California 95120, Department of Chemistry, Stanford University, Stanford, California 94305, Institute of Bioengineering and Nanotechnology, 31 Biopolis Way, The Nanos, Singapore 138669 Received May 13, 2008; Revised Manuscript Received July 23, 2008
Biocompatible amphiphilic block copolymers comprised of poly(ethylene glycol) (PEG) as the hydrophilic component and a poly(methylcarboxytrimethylene carbonate) (PMTC) as a hydrophobic backbone having either poly(L-lactide) (L-PLA) or poly(D-lactide) (D-PLA) branches were prepared by organocatalytic ring-opening polymerization (ROP). The polycarbonate backbone was prepared by copolymerization of two different MTCtype monomers (MTCs) including a tetrahydropyranyloxy protected hydroxyl group, a masked initiator for a subsequent ROP step. Interestingly, the organic catalyst used in the ROP of MTCs was also effective for acetylation of the hydroxyl end-groups by the addition of acetic anhydride added after polymerization. Acidic deprotection of the tetrahydropyranyloxy (THP) protecting group on the carbonate chain generated hydroxyl functional groups that served as initiators for the ROP of either D- or L-lactide. Comb-shaped block copolymers of predictable molecular weights and narrow polydispersities (∼1.3) were prepared with up to 8-PLA branches. Mixtures of the D- and L-lactide based copolymers were studied to understand the effect of noncovalent interactions or stereocomplexation on the properties.
Introduction Application of amphiphilic block copolymers as drug carriers has been an active area of research, and among the materials surveyed, polylactide (PLA) based copolymers are the most widely studied. Amphiphilic block copolymers of poly(ethylene glycol) (PEG) and PLA form micelles in aqueous solution and have been investigated in a variety of delivery systems.1-3 Glycolide,4-7 -caprolactone,8,9 amino acids,10-12 and R-hydroxy acids13 have been incorporated into the copolymers either to control degradability, enhance solubility, or add functionality. Hydrophilic vinyl polymers such as poly(N-isopropylacrylamide) (PNIPAM),6 poly(2-hydroxyethyl methacrylate) (PHEMA),14 and poly(vinyl alcohol) (PVA)15 have also been investigated in combination with block PLAs. An interesting aspect of some of these efforts is the unique ability of stereoregular PLA-based homo- and copolymers to form stereocomplexes. Enantiomeric mixture of L-PLA and D-PLA form stable stereocomplexes16 with significantly enhanced thermal and mechanical properties relative to the homochiral L-PLA. In copolymer systems, this noncovalent interaction serves as a driving force to produce robust, but reversible, self-assembled structures.17-20 For example, mixed micelles of linear triblock copolymers (PLLAPEG-PLLA/PDLA-PEG-PDLA) dispersed in water form thermally responsive hydrogels.17,21 Other examples include the enantiomeric mixtures of PLA-grafted dextran,20 poly(NIPAMco-HEMA) with PLA grafts,19 as well as the 8-arm PEG-PLA star block copolymers18 that yield reversible hydrogels via stereocomplexation. We have recently reported new amphiphilic * To whom correspondence should be addressed. E-mail: hedrick@ almaden.ibm.com. † IBM Almaden Research Center. ‡ Stanford University. § Institute of Bioengineering and Nanotechnology.
block copolymer architectures such as H-shaped22 PEG-PLA copolymer using organocatalytic strategies. Here we describe a route to amphiphilic comb-shaped block copolymers having stereoregular polylactide side chains emanating from the hydrophobic block. As described above, grafting PLA from hydrophilic polymers has been reported, however, there are still no reports regarding amphiphilic block copolymers with such a complex architecture that employs PLA branches as complexation sites. Shown in Scheme 1 is the general synthetic strategy in which a monohydroxyl poly(ethyene oxide) oligomer is used as an initiator for the organocatalytic ringopening polymerization (ROP) of functional cyclic carbonate monomers (MTCs) having latent initiator functionalities for the subsequent ROP of stereoregular lactides.
Experimental Section Materials and Methods. All reagents were purchased from Aldrich and used as received unless otherwise noted. N-(3,5-Trifuluoromethyl)phenyl-N′-cyclohexylthiourea (TU) was prepared as previously reported23 and dried by stirring in dry THF with CaH2, filtering, and removing solvent in vacuo. 1,8-Diazabicyclo[5,4,0]undec-7-ene (DBU) was stirred over CaH2, vacuum distilled, then stored over molecular sieves (3 Å). (-)-Sparteine was twice distilled from CaH2 under dry N2 and stored over molecular sieves (3 Å). Dry CH2Cl2, THF, and toluene were obtained by using a solvents drying system from Innovative. Ion exchange resins, Amberlyst-15 and Dowex 50W-X2, were rinsed with the reaction solvent prior to use. L- and D-Lactides (purac 99%) were azeotropically distilled from toluene and recrystallized from dry toluene three times prior to use. Monomethoxy-PEG-OH (Mn 5000 g/mol, PDI 1.04) was azeotropically distilled from toluene and dried in vacuo. 1H and 13C NMR spectra were recorded on a Bruker Avance 400 instrument operated at 400 and 100 MHz, respectively, using CDCl3 solutions unless noted otherwise. Mass spectrometry service (high resolution electrospray ionization, HR-ESI-MS) was
10.1021/bm800526k CCC: $40.75 2008 American Chemical Society Published on Web 10/10/2008
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Scheme 1. Synthetic Route to PEG-b-PMTC(Et-co-HE)-g-PLAa
a Reagents and conditions: (i) MeO-PEG-OH (5k), TU, DBU, CH2Cl2, rt, 1 h; (ii) Ac2O, overnight; (iii) Dowex 50W-X2, MeOH, 50 °C, 2 h; (iv) L(D)lactide, TU, (-)-sparteine, CH2Cl2, rt, 1 h; (v) DBU, Ac2O, CH2Cl2, rt, overnight.
provided by Stanford University Mass Spectrometry. Gel permeation chromatography (GPC) was performed in THF at 30 °C using a Waters chromatograph equipped with four 5 µm Waters columns (300 mm × 7.7 mm) connected in series with increasing pore size (10, 100, 1000, 105, 106 Å), a Waters 410 differential refractometer for refractive index (RI) detection, and calibrated with polystyrene standards (750 - (2 × 106) g/mol). Differential scanning calorimetry (DSC) was performed using a TA differential scanning calorimeter Q1000 that was calibrated using high purity indium at a heating rate of 10 °C/min. The second heating scan was employed to show the thermal behavior free from the influence of the preparation condition of samples. Synthesis of 1: MTC-OEt. (i) 2,2-Bis(methylol)propionic acid (bisMPA; 22.1 g, 0.165 mol) was added in ethanol (150 mL) with Amberlyst-15 (6.8 g) and refluxed overnight. The resins were then filtered out and the filtrate was evaporated. Methylene chloride (200 mL) was added to the resulting viscous liquid to filtrate the unreacted reagent and byproduct. After the solution was dried over MgSO4 and evaporated, ethyl 2,2-bis(methylol)propionate was obtained as a clear and colorless liquid (21.1 g, 86%). (ii) A solution of triphosgene (19.5 g, 0.065 mol) in CH2Cl2 (200 mL) was added stepwise to a CH2Cl2 solution (150 mL) of ethyl 2,2bis(methylol)propionate (21.1 g, 0.131 mol) and pyridine (64 mL, 0.786 mol) over 30 min at -75 °C with dry ice/acetone. The reaction mixture was kept stirring for another 2 h under chilled condition and then allowed to heat to room temperature. Saturated NH4Cl aqueous solution (200 mL) was added to the reaction mixture to decompose excess triphosgene. The organic phase was then treated with 1 N HCl aq (200 mL), followed by saturated NaHCO3 (200 mL), brine (200 mL), and water (200 mL). After the CH2Cl2 solution was dried over MgSO4 and evaporated, the residue was recrystallized from ethylacetate to give white crystals (13.8 g, 56%). 1H NMR: δ 4.68 (d, 2H, CH2OCOO), 4.25 (q, 1H, OCH2CH3), 4.19 (d, 2H, CH2OCOO), 1.32 (s, 3H, CH3), 1.29 (t, 3H, CH3CH2O). 13C NMR: δ 171.0, 147.5, 72.9, 62.1, 39.9, 17.3, 13.8. HR-ESI-MS: m/z calcd for C8H12O5 · Na, 211.0582; found, 221.0578. Synthesis of 2: MTC-OCH2CH2OTHP. MTC-OH 9 was prepared according to a procedure previously reported.24 To a dry THF solution of MTC-OH (9.3 g, 58 mmol) was added 3 drops of DMF followed oxalyl chloride (5.6 mL, 67 mmol), and the solution was stirred under a flow of N2 for 1 h, bubbled with N2 flow, evaporated under vacuum. The residue was redissolved in dry THF (50 mL) and gently dropped to a solution of 2-(tetrahydro-2H-pyran-2-yloxy)ethanol (8.4 mL, 61
mmol) and triethylamine (7.7 mL, 70 mmol) in dry THF (100 mL) over 40 min. The mixture was stirred for 3 h before a white precipitate was filtered out and the filtrate was concentrated. Afterward, ethyl acetate (100 mL) was added into the concentrated filtrate to wash with water (100 mL × 3). Purification by column chromatography (silica, 1:1 ethyl acetate/hexanes) provided the product as a colorless oil (8.0 g, 51%). 1H NMR: δ 4.70 (d, 2H, CH2OCOO), 4.61 (t, 1H, OCHO), 4.38 (m, 2H, OCOCH2CH2), 4.20 (d, 2H, CH2OCOO), 3.92 (m, 1H, CHaHbOCH), 3.82 (m, 1H, OCHaHbCH2CH2), 3.65 (m, 1H, CHaHbOCH), 3.51 (m, 1H, OCHaHbCH2CH2), 1.85-1.65 (m, 2H, CHCH2), 1.61-1.47 (m, 4H, CH2CH2CH2CH2), 1.35 (s, 3H, CH3). 13C NMR: d 170.9, 147.4, 98.7, 72.9, 65.0, 64.7, 62.1, 40.1, 30.3, 25.2, 19.2, 17.5. HR-ESI-MS: m/z calcd for C13H20O7 · Na, 311.1107; found, 311.1108. General Procedure for Ring-Opening Copolymerization of 1 and 2 with Monomethoxy-PEG-OH: PEG-b-PMTC(Et-co-THP) (Polymer 3). Monomethoxy-PEG-OH (Mn 5000 g/mol, 200 mg), 1 (115 mg, 0.61 mmol), 2 (116 mg, 0.40 mmol), and TU (17 mg, 0.05 mmol) were dissolved in CH2Cl2 (1 mL), and this solution was transferred to a vial containing DBU (7 mg, 0.05 mmol) to initiate polymerization at room temperature ([M]0/[I]0 ) 25). After 1 h, the mixture was transferred to another vial, placed in acetic anhydride (51 mg, 0.50 mmol), and stirred overnight to acetylate the hydroxyl end of the polymer. The solution was then precipitated into diethylether (30 mL) and the precipitate was centrifuged and dried in vacuo. 1H NMR: δ 4.62 (t, ∼9H, OCHO), 4.36-4.22 (m, ∼104H, CH2 PMTC backbone and OCOCH2CH2O), 4.17 (q, ∼25H, CH2CH3), 3.91-3.78 (m, ∼21H, OCOCH2CHaHbOCH, OCHaHbCH2CH2, and PEG satellite), 3.63 (m, ∼497H, CH2 PEG and OCOCH2CHaHbOCH), 3.55-3.39 (m, ∼23H, OCHaHbCH2CH2 and PEG satellite), 3.37 (s, 3H, CH3O-PEG), 2.04 (s, 3H, CH3OCO), 1.85-1.46 (m, ∼65H, CH2 THP), 1.28-1.20 (m, ∼100H, CH2CH3 and CH3). General Procedure for Deprotection of Tetrahydropyranyl Ether on Side Chains of Polymer 3: PEG-b-PMTC(Et-co-HE) (Polymer 4). Dowex 50W-X2 (120 mg) was added into a solution of polymer 3 (273 mg, [THP] ∼ 0.24 mmol) in methanol (5 mL), and the mixture was stirred at 50 °C for 2 h. The resin was removed by filtration before the solvent condensed under vacuum to precipitate into diethylether. The precipitate was redissolved in CH2Cl2 (15 mL) and dried by stirring with CaH2, filtering, and removing solvent in vacuo for the following lactide polymerization. 1H NMR: δ 4.38-4.21 (m, ∼95H, CH2 PMTC backbone and OCOCH2CH2O), 4.17 (q, ∼23H, CH2CH3), 3.77 (b,
Amphiphilic Comb-Block Copolymers
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Scheme 2. Synthesis of MTCsa
a Reagents and conditions: (a) BnBr, KOH, DMF, 100 °C, overnight; (b) triphosgene, pyridine, CH2Cl2,
