Synthesis and Characterization of Salicylic Acid-Based Poly(Anhydride-Ester) Copolymers

Abstract

A series of poly(anhydride-esters) based on poly(1,10-bis(o-car-boxyphenoxy)decanoate) (CPD) and poly(1,6-bis(p-carboxyphenoxy)hexane) (p-CPH) were synthesized by melt-condensation polymerization. Poly-(anhydride-esters) that contain CPD hydrolytically degraded into salicylic acid, however, these homopolymers have mechanical and thermal characteristics that limit their use in clinical applications. The synthesis and characterization of copolymers of CPD with p-CPH, a monomer known to generate mechanically stable homopolymers, was investigated. By changing the CPD to p-CPH monomer ratios, the salicylic acid loading and thermal/mechanical properties of the copolymers was a controlling factor; increasing the CPD concentration increased the salicylate loading but decreased the polymer stability; whereas increasing the p-CPH concentration increased the thermal and mechanical stability of the copolymers. Specifically, decreasing the CPD:p-CPH ratio resulted in lower salicylate loading and increased thermal decomposition temperatures. The glass transition temperatures (°C) varied from 27 to 38°C, a desirable range for elastomeric biomedical implants.

INTRODUCTION

In the early 1980s, polyanhydrides were examined for sustained drug release for controlled drug delivery applications [1–4]. Over the next several decades, extensive research was conducted on polyanhy-drides for various biomedical applications that yielded numerous new polymer systems [5–15]. Polyanhydrides have excellent controlled release characteristics and degrade in vitro and in vivo to their acid counterparts as non-cytotoxic products [4,6,16]. For example, polyan-hydrides containing sebacic acid and 1,3-bis(p-carboxyphenoxy)-propane are hydrolytically degradable copolymers (1) used as implanted devices to control chemotherapeutic drug delivery (Figure 1) [15,17].

Figure 1.

Polyanhydride copolymer matrix for drug delivery.

The degradation rate of biodegradable polyanhydrides, such as polymer 1, was controlled by the manipulation of the polymer composition to release admixed drug molecules [18–21]. The polyanhydride copolymer (1) of sebacic acid and 1,4-bis(p-carboxyphenoxy)propane (CPP) (Figure 1) has excellent biocompatibility characteristics and degrades by a surface-erosion process [4,7,11,22,23]. The controlled, non-enzymatic degradation of polyanhydrides make these polymers particularly desirable for a variety of drug delivery applications. In addition, these materials are potentially useful as anti-biofilm coatings [24–26], tissue adhesion [27–29], and non-inflammatory coatings for medical devices [30–32].

Previously, our laboratory reported the synthesis of polyanhydrides that hydrolytically degrade into drugs, that is, the drug molecules were chemically incorporated into the polymer backbone rather than being physically admixed [33]. These poly(anhydride-esters) (2) released salicylic acid (3) and sebacic acid (4) upon hydrolytic degradation of the Molar ratio m:n = 80:20; polyanhydride copolymer polymer backbone (Figure 2) [33–37]. Polymer 2 is unique in that the drug was chemically incorporated into the polymer backbone, not attached as a side group nor physically admixed by melting or co-dissolved with the polymer. The sebacic acid (4) is the “linker” in the polymer used to connect the salicylic acid units; it is also the major component of polymer 1 (Figure 1) [15].

Figure 2.

Hydrolysis of salicylic acid-based poly(anhydride-ester).

A unique feature of this polymeric prodrug (2) design is that the polymer itself is a controlled drug-release system with high drug-loading capabilities (62wt% of polymer 2 is salicylic acid). Therefore, drug release is directly dependent on the hydrolytic cleavage of the anhydride and ester bonds, providing an advantage as a drug-delivery system; drug release was directly controlled by the composition of the polymer backbone [36–38]. An additional feature is that the polymeric prodrug can be easily manipulated to yield fibers by extrusion, films by solvent-casting, or microspheres by emulsion-precipitation methods [34,39].

A potential drawback to polymer 2 is that it has a low glass transition temperature (T_g) of 27°C, which means that the material softens at body temperature [39,40]. For medical applications that require load-bearing, such as orthopedic devices, it is necessary to enhance the thermal and mechanical stability of these polymers. In previous work, we investigated copolymers in which the polymer properties were modulated by modifying the ratio of comonomers such as sebacic acid (4) and p-CPH (5) [41–43]. In this study, we prepared a series of copolymers composed of CPD (2) and p-CPH (5). The monomer p-CPH (5) was chosen to improve the thermal and mechanical characteristics; it is also structurally related to the CPP compound in polymer 1 (Figure 1). Homopolymers of p-CPH have higher mechanical strength and improved stability over polymer 2 [34,35,40]. Homopolymer 2 is an amorphous polymer (no melting temperature) with a glass transition temperature near room temperature, which limits the fabrication of these polymers into biomedical devices. In contrast, homopolymer 5 is a crystalline polymer (T_m = 147°C) with a higher T_g (48°C) [42]. Therefore, copolymers of CPD (2) and p-CPH (5) were anticipated to enhance the handling properties of CPD homopolymers (2). For example, a copolymer of CPD (2) and p-CPH (5) (50:50) generated stable membranes that withstand surgical implantation procedures [40]. In this paper, the synthesis and characterization of a series of poly(anhydride-esters) composed of CPD and p-CPH are described. The polymers were characterized by FT-NMR and FT-IR spectroscopy, as well as by molecular weight, polydispersity, thermal transition temperatures and drug loading capacity.

EXPERIMENTAL

Materials

Proton nuclear magnetic resonance (¹H-NMR) spectra were recorded on either a Varian 200MHz or 300MHz spectrometer. The deuterated solvent used to dissolve the samples (5–10mg) was DMSO- d₆, or CDCl₃, with the solvent as the internal reference. Infrared (IR) spectra were measured on a Thermo Nicolet/Avatar 360 FT-IR spectrometer, by depositing samples onto NaCl plates (if liquid) or solvent-casting samples from methylene chloride onto NaCl plates (if solid). Elemental analyses were provided by QTI (Whitehouse, NJ). Melting points (T_m) were determined on a Thomas-Hoover apparatus.

Polymer Preparation

The CPD diacid (6) and p-CPH diacid (7) were synthesized according to methods previously reported [44]. In brief, the diacid (1.0g) was added to an excess of acetic anhydride (100mL), then stirred at room temperature (for CPH 6) or reflux temperature (for p-CPH 7) until a clear homogenous solution was observed (~120min). The monomer was isolated by removing excess acetic anhydride under vacuum, and then washed with diethyl ether (50mL); monomers (8 and 9) were isolated as white solids.

Different copolymers of CPD and p-CPH were synthesized by melt-condensation polymerization. The molar ratios of CPD to p-CPH ranged from 9:1, 7:3, 6:4, 5:5, 4:6, 3:7, and 1:9. A typical procedure was as follows: monomers (8 and 9) (1.5 to 2.0g total) were placed in a two-necked round-bottom flask equipped with an overhead stirrer (T-line Laboratory Stirrer, Talboys Engineering). The reaction mixture was heated to 180°C using a temperature controller (Cole Parmer) in a silicone oil bath under high vacuum (<2mmHg) for 3 to 6h. During this time, the melt was actively stirred at ~100rpm by the overhead stirrer. Polymerization was complete when the viscosity of the melt remained constant or solidified. The polymer (10) was cooled to room temperature, dissolved in a minimal volume of methylene chloride (5mL), and reprecipitated into a 20-fold excess of diethyl ether (100mL).

CPD:p-CPH Poly(anhydride-ester) (10)

The yield was quantitative (off-white to white solid). The ¹H-NMR and FT-IR data of all the copolymers were similar with only variations in peak intensities. ¹H-NMR (DMSO-d₆): d 8.20 (b, 2H, ArH), 8.07 (b, 4H, ArH), 7.95 (b, 2H, ArH), 7.75 (b, 2H, ArH), 7.40 (b, 2H, ArH), 6.93 (b, 4H, ArH), 4.10 (b, 4H, CH₂), 2.20 (b, 4H, CH₂), 1.87 (b, 4H, CH2), 1.55 (b, 8H, CH₂), 1.25 (b, 8H, CH₂). IR (NaCl, cm⁻¹): 1792 (C =O, anhydride), 1760 (C=O, ester), 1740 (C=O, anhydride), 1710 (C—O, ester), 1185 (C—O, ether).

Polymer Characterization

Weight-average molecular weights (M_w) were determined by gel permeation chromatography (GPC) on a Perkin-Elmer liquid chromatography system consisting of a Series 200 refractive index detector, a Series 200 LC pump, and an ISS 200 advanced sample processor. A Dell OptiPlex GX110 computer running Perkin-Elmer TurboChrom 4 software was used for data collection and processing, and to automate the analysis via Perkin-Elmer Nelson 900 Series Interface and 600 Series Link. Polymers were dissolved in methylene chloride (5mg/mL) and filtered through 0.45μm poly(tetrafluoroethylene) (PTFE) syringe filters (Whatman, Clifton, NJ) before elution. Samples were resolved on a Jordi divinylbenzene mixed-bed GPC column (7.8×300mm) (Alltech Associates, Deerfield, IL) at 25°C, with methylene chloride as eluent at a flow rate of 0.5mL/min. Molecular weights were calibrated relative to narrow molecular weight polystyrene standards (Poly-sciences, Dorval, Canada).

Thermal analyses were performed on a Perkin-Elmer system consisting of Pyris 1 DSC and TGA 7 analyzers with TAC 7/DX instrument controllers. Perkin-Elmer Pyris software was used for data collection on a Dell OptiPlex GX110 computer. For DSC, samples (5mg) were heated under dry nitrogen gas. Data were collected at heating and cooling rates of 10°C/min with a two-cycle minimum. For TGA, samples (10mg) were heated under dry nitrogen gas. Data were collected at a heating rate of 10°C/min. Decomposition temperatures were defined as the onset of decomposition.

RESULTS AND DISCUSSION

A series of copolymers based on CPD and p-CPH were synthesized and the physicochemical properties compared as a function of polymer composition. The polymers described herein were prepared via melt-condensation polymerization methods as previously detailed [44]. An outline of the CPD:p-CPH poly(anhydride-ester) (10) synthesis, starting from the relevant diacids (6 and 7) is outlined in Figure 3.

Figure 3.

Synthesis of CPD:p-CPH copolymers.

Diacids 6 and 7 were synthesized as previously reported [33,44]. CPD (6) and p-CPH (7) were separately stirred in excess acetic anhydride at room temperature and reflux temperature, respectively. Monomers (8 and 9) were obtained as white solids. Various molar ratios of the two solids, 8 and 9, (9:1, 7:3, 6:4, 5:5, 4:6, 3:7, 1:9) were melt-polymerized to yield the corresponding CPD:p-CPH copolymers (10). Melt-condensation polymerization was performed using a modular apparatus in conjunction with an overhead mechanical stirrer, the polymerization was deemed complete upon solidification.

The molecular weights obtained for these copolymers were typically above 25,000, corresponding to >50 repeat units. The molecular weights for these poly(anhydride-esters) characteristic for aromatic polyanhydrides synthesized by melt condensation [4,41,45]. The glass transition temperatures (T_g) ranged from 27 to 38°C, with no indication of a melting temperature (T_m) for five of the seven polymers, indicating that these are highly amorphous copolymers. CPD:p-CPH (9:1) copolymers gave the lowest T_g (27°C) and the CPD:p-CPH (1:9) copolymer yielded the highest T_g (38°C). Poly(p-CPH) is a biodegradable, highly crystalline polymer with a T_g = 48°C and T_m = 142°C [46]; whereas, poly(CPD) is a biodegradable, amorphous polymer with a T_g = 27°C [33,44]. By melt-polymerizing monomers of CPD and p-CPH to form a copolymer, a series of polymers with T_g values in a narrow range were obtained. Only copolymers with high p-CPH ratios (>70%) were crystalline.

This work expands upon our preliminary in vivo studies using a 50:50 copolymer, in which the copolymer displayed superior handling properties over the CPD homopolymer [39,40]. In addition to enhancing the mechanical and thermal properties, copolymerizing poly(CPD) and poly(p-CPH) at different ratios produced a series of salicylic acid-based poly(anhydride-esters) with drug loads of salicylic acid ranging from 6 to 50wt%.

CONCLUSIONS

A series of biodegradable polymers derived from CPD and p-CPH monomers were synthesized. By copolymerizing monomers of CPD and p-CPH at various ratios, a series of poly(anhydride-esters) that contain varying levels of salicylic acid (6 to 50wt%) were obtained. Overall, the addition of p-CPH as a monomer did not significantly influence the molecular weights or T_g values. Notably, most copolymer compositions were amorphous; T_m values were only observed for copolymers with 70 or 90% p-CPH composition and were very similar (27 to 38°C).

Table 1.

Comparison of CPD:p-CPH copolymer properties.

CPD:p-CPH ratio		Mw	PDI	Tg (°C)	Tm (°C)	Td (°C)	Salicylic acid (wt%)
Theor.	Observ.a						Theor.	Observ.a
9:1	8:1	29,500	1.2	27	b	390	56	50
7:3	6:3	76,300	1.3	32	b	392	44	38
6:4	5:4	37,400	1.6	28	b	393	37	32
5:5	5:5	47,500	1.4	33	b	407	31	31
4:6	4:6	27,800	2.0	28	b	416	25	25
3:7	3:7	43,000	3.1	34	51	428	19	19
1:9	1:9	28,500	1.2	38	128	431	6	6

^aCalculated from ¹H-NMR spectral data.

^bMelting temperature (T_m) not observed.