Biomacromolecules 2002, 3, 153-158
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Synthesis and Characterization of Triblock Copolymers of Methoxy Poly(ethylene glycol) and Poly(propylene fumarate) Esfandiar Behravesh, Albert K. Shung, Seongbong Jo, and Antonios G. Mikos* Department of Bioengineering, Rice University, MS-142, P.O. Box 1892, Houston, Texas 77251-1892 Received August 27, 2001; Revised Manuscript Received November 1, 2001
Amphiphilic block copolymers were synthesized by transesterification of hydrophilic methoxy poly(ethylene glycol) (mPEG) and hydrophobic poly(propylene fumarate) (PPF) and characterized. Four block copolymers were synthesized with a 2:1 mPEG:PPF molar ratio and mPEGs of molecular weights 570, 800, 1960, and 5190 and PPF of molecular weight 1570 as determined by NMR. The copolymers synthesized with mPEG of molecular weights 570 and 800 had 1.9 and 1.8 mPEG blocks per copolymer, respectively, as measured by NMR, representing an ABA-type block copolymer. The number of mPEG blocks of the copolymer decreased with increasing mPEG block length to as low as 1.5 mPEG blocks for copolymer synthesized with mPEG of molecular weight 5190. At a concentration range of 5-25 wt % in phosphate-buffered saline, copolymers synthesized with mPEG molecular weights of 570 and 800 possessed lower critical solution temperatures (LCST) between 40 and 45 °C and between 55 and 60 °C, respectively. Aqueous solutions of copolymer synthesized with mPEG 570 and 800 also experienced thermoreversible gelation. The sol-gel transition temperature was dependent on the sodium chloride concentration as well as the mPEG block length. The copolymer synthesized from mPEG 570 had a transition temperature between 40 and 20 °C with salt concentrations between 1 and 10 wt %, while the sol-gel transition temperatures of the copolymer synthesized from mPEG molecular weight 800 were higher in the range 75-30 °C with salt concentrations between 1 and 15 wt %. These novel thermoreversible copolymers are the first biodegradable copolymers with unsaturated double bonds along their macromolecular chain that can undergo both physical and chemical gelation and hold great promise for drug delivery and tissue engineering applications. Introduction Many developments have been made in creating improved biomaterials by careful modulation of the polymer or copolymer composition. One such development has been the copolymerization of poly(ethylene glycol) (PEG) with biodegradable polymers.1-4 The incorporation of PEG, the most common moiety used to promote biocompatibility, has consistently improved the hydrophilic properties of the resulting copolymer.1,5,6 For example, diblock copolymers of methoxy poly(ethylene glycol) (mPEG) and poly(lactic acid) (PLA) exhibited significantly less protein adsorption and marrow stromal cell attachment compared to PLA homopolymers.2 Block copolymers of poly(propylene fumarate) (PPF) and poly(ethylene glycol) (PEG) have been previously synthesized.7,8 Poly(propylene fumarate) is a biodegradable polyester which by itself has been thoroughly characterized and investigated for orthopedic applications.9,10 It consists of propylene fumarate units and can be cross-linked in situ via its fumarate double bonds. The copolymerization of PPF with PEG, resulted in a hydrophilic, biodegradable, biocompatible, and in situ cross-linkable copolymer.1,5,7,8,11,12 The poly(propylene fumarate-b-ethylene glycol) (P(PF-b-EG)) co* Corresponding author. Telephone: (713) 348-5355. Fax: (713) 3484244. E-mail:
[email protected].
polymer has been examined for use in cardiovascular applications. Suggs et al. reported a significant decrease in platelet adhesion measured under flow conditions on the copolymer when compared to PPF.13 Using an in vivo rat cage implant system, P(PF-b-EG) exhibited an exudate leukocyte concentration comparable to that of the negative control.5 Biodegradable copolymers incorporating PEG can possess unique thermoreversible properties, depending on the hydrophobic block structure and PEG block length. Copolymers consisting of PEG and poly(R-hydroxy esters) possess lower critical solution temperatures (LCST) and sol-gel transition temperatures dependent on the PEG block length.14,15 These copolymers undergo gelation in water upon reaching their LCST and have thermoreversible properties suitable for drug delivery systems. Another class of biodegradable copolymers, poly(organophosphazene) incorporating mPEG, also possess a LCST.4,16 Lee et al. showed decreased LCST of the poly(organophosphazene) based copolymers with increasing sodium chloride concentration. However, unlike copolymers synthesized with a PPF block, these thermoreversible copolymers do not offer further chemical cross-linking. In this study, we investigated the synthesis of ABA-type block copolymers of PPF and mPEG of variable mPEG block molecular weights. In addition, we examined the thermoreversible properties of the block copolymers as a function of the molecular weight of the PEG block.
10.1021/bm010137x CCC: $22.00 © 2002 American Chemical Society Published on Web 12/12/2001
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Biomacromolecules, Vol. 3, No. 1, 2002
Figure 1. Reaction scheme for the copolymerization of poly(propylene fumarate) and methoxy poly(ethylene glycol).
Experimental Section Materials. Diethyl fumarate (Acros, Pittsburgh, PA), propylene glycol (Fisher, Pittsburgh, PA), and methoxy poly(ethylene glycol) (mPEG) of four nominal molecular weights of 550, 750, 2000, and 5000 (Aldrich, Milwaukee, WI) were used as received. One molar anhydrous zinc chloride in diethyl ether and hydroquinone were purchased from Sigma and used as received. All solvents used for purification were acquired from Fisher as reagent grade. Sodium chloride (Fisher) and phosphate-buffered saline (PBS) (GibcoBRL, Grand Island, NY) were purchased and used as received. Trifluoroacetic anhydride was purchased from Sigma. Polymer Synthesis. Poly(propylene fumarate) (PPF), the hydrophobic block, was synthesized as previously described.10 Briefly, diethyl fumarate, propylene glycol, zinc chloride, and hydroquinone were combined in a three-neck round-bottom flask in a 1:3:0.01:0.003 molar ratio. Zinc chloride was added as a catalyst while the hydroquinone was added to prevent spontaneous cross-linking during the transesterification reaction. The contents of the reactor were then mixed using an overhead mechanical stirrer. This mixture was heated to a temperature of 150 °C under a nitrogen purge to form a diester intermediate, bis(2-hydroxypropyl) fumarate, and the byproduct, ethanol, which was removed by distillation. The diester intermediate was then transesterified under vacuum (
