Biomacromolecules 2002, 3, 1392-1399
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Methacrylate Derivatives Incorporating Pyroglutamic Acid Tara J. Smith and Lon J. Mathias* School of Polymers and High Performance Materials, University of Southern Mississippi, Box 10076, Hattiesburg, Mississippi 39406-0076 Received September 3, 2002
Methacrylates containing pyroglutamic acid were synthesized in good yields. Methyl R-pyroglutamyl methylacrylate (PyMM) and methyl R-pyroglutamidoundecanoyl methylacrylate (PyUM) give very fast photopolymerization rates both in homopolymerizations and with widely used commercial monomers N-vinyl pyrrolidinone (NVP) and hydroxyethyl methacrylate (HEMA). Soluble or cross-linked homopolymers can be obtained depending upon polymerization temperature. Pyroglutamic methacrylates polymerize without added initiator in the melt. Solution cast, photocured, and thermally cured coatings gave good to excellent adhesion to poly(ethylene terephthalate) and glass surfaces. Introduction Acrylates and methacrylates are some of the most widely used commercial materials today. Methacrylate derivatives have found uses in medical and dental applications such as bone cements, dental fillings, bioadhesives, and hydrogels. One problem with many of these methacrylate derivatives is the leaching of unpolymerized monomer1 causing irritation of the surrounding tissues and even cell death.2 The amount of unpolymerized monomer is directly proportional to polymerization conditions, especially the polymerization rate. Thus, incorporation of pendent groups that do not alter the fast polymerization rates or inherent adhesion properties of methyl methacrylate (MMA)-based systems is preferred. Copolymers containing N-vinylpyrrolidone (NVP) or hydroxyethyl methacrylate (HEMA) have been investigated as UV curable bioadhesives, hydrogels, and biocoatings. For example, it was shown that NVP copolymers adhered to porcine intestine and created a hydrophilic surface for cell adhesion and cell growth.3 Copolymers of NVP and poly(ethylene glycol) diacrylate gave hydrogels capable of drug delivery.4 Both NVP and HEMA are used in soft contact lenses, with HEMA being the most widely used hydrophillic monomer in the soft contact lens field.5 Recently, poly(HEMA) hydrogels have been investigated as porous skirts for artificial corneas based on synthetic materials.6 Copolymerizing hydrophobic monomers with NVP, HEMA, acrylic acid, and acrylamide greatly expands their applicability in biomaterials. For example, copolymer coatings of 70% MMA with HEMA gave good adhesion to vascular stents and resulted in significantly reduced vessel wall response.7 Also, AB block copolymerization of acrylic acid with vinyl terminal oligo-(methyl methacrylate) (MMA) gave a micellar delivery system for controlled release of hydrophobic drugs.8 Incorporation of MMA into polyelectrolyte copolymers resulted in bioadhesive gels capable of specific drug delivery * To whom correspondence should be addressed. E-mail:
[email protected].
lon.
because of their pH responsiveness.9 Tailored release profiles of both hydrophilic and hydrophobic drugs could be obtained by varying the copolymer ratios.10,11 One method of increasing rates of polymerization is by incorporation of pendent heterocyclic groups, which have been shown to reduce termination and perhaps increase chain transfer.12 High rates of polymerization lead to less leachable monomer from these systems. Larger pendent groups can also increase glass transition temperatures (Tg’s) and reduce shrinkage of the copolymer systems usually used in biomaterial applications. The chemistry of R-hydroxymethacrylate (HMA) derivatives allows tailoring of both polymerizability and final polymer properties.13-15 Several ester monomers (RHMA where R is the ester group) are now available commercially,16 and this makes possible synthesis of many new derivatives with even more varied properties. For example, conversion of the hydroxymethyl group to esters was found to dramatically increase rates of polymerization.17 We are exploring families of several new monomers based on naturally occuring starting materials. One family includes pyroglutamyl methacrylate (PGM) derivatives that incorporate amino acid-based heterocyclic pendent groups capable of strong hydrogen-bonding interactions. These moieties also have the ability to chelate inorganic fillers and bind to inorganic surfaces in the body. In these systems, the pyroglutamyl unit is linked to the R-hydroxymethacrylate moieties through an ester linkage, which should greatly enhance the rates of polymerization and copolymerization of these monomers. PGM derivatives range from water-soluble to hydrophobic and easily copolymerize with each other and with commercially available monomers that allow tailoring of the physical properties. Experimental Section Materials. Methyl R-hydroxyl methylacrylate (MHMA) was donated by Nippon Shokubai Co., Tokyo, Japan. Pyroglutamic acid was purchased from ICN Pharmaceuticals, Inc.,
10.1021/bm025663i CCC: $22.00 © 2002 American Chemical Society Published on Web 10/19/2002
Methacrylate Derivatives
Costa Mesa, CA, and used without further purification. Methyl R-chloromethyl acrylate (MCMA) was synthesized from MHMA as previously reported.18 All other materials were purchased from Aldrich or Fischer/Acros and used as received unless otherwise specified. Synthesis of Methyl (r-pyroglutamyl methylacrylate) (PyMM). Pyroglutamic acid (40 mmol) was dissolved in ethanol. K2CO3 (120 mmol) was added, and the mixture was allowed to stir at room temperature for 4 h. The reaction mixture was filtered and concentrated, leaving the potassium carboxylate salt of pyroglutamic acid (PGC) as a white powder. MCMA (44 mmol) was added in 50 mL of chloroform. The biphasic mixture was heated to 60 °C and left for 48 h. The reaction mixture was filtered, concentrated, and precipitated into stirring hexanes to give PyMM in 40% yield. Thin layer chromatography (TLC) using chloroform as a mobile phase showed complete removal of MCMA and only one spot corresponding to PyMM; mp 74 °C. 1H NMR (CDCl3): δ ) 2.41 (m, 4H, CH2CH2), 3.80 (s, 3H, OCH3), 4.28 (q, 1H, CH2CH), 4.89 (s, 2H, CH2OCO), 5.89 (s, 1H, CH2dCH), 6.03 (s, 1H, NH), 6.41 (s, 1H, CH2dCH). 13C NMR (CDCl3): δ ) 25.0 (CHCH2CH2), 29.3 (CHCH2CH2), 52.3 (CH3O), 55.5 (CHNH), 63.7 (CH2OCO), 128.9 (CH2d C), 134.7 (CdCH2), 165.5 (COOCH3), 171.6 (COOCH2), 178.0 (CONH). FT-IR (KBr pellet): 3208, 2964, 1947, 1762, 1708, 1723, 1708, 1286, 1211, 1161 cm-1. Synthesis of Methyl (r-pyroglutamidoundecanoyl methylacrylate) (PyUM). Pyroglutamic diketopiperazine (PyDKP) was synthesized according to a previously published procedure,19 and then PyDKP was ring-opened using 11-aminoundecanoic acid (AUA). To a nitrogen-purged onenecked flask equipped with a magnetic stirbar, PyDKP (11.3 mmol), AUA (22.5 mmol), and trifluoroethanol (35 mL) were added and cooled to 0 °C. Triethylamine (33.8 mmol) was then added. The reaction was allowed to warm to room temperature and stirred 20 h. The reaction mixture was concentrated leaving the white pyroglutamidoundecanoic triethylamine salt. This salt was dissolved in methanol and filtered to remove bis-glutamidoundecanoic diketopiperazine. To the methanol-soluble salt, MCMA (37.8 mmol) was added, and the mixture was allowed to react at room temperature for 24 h and then at 35 °C for 2 h. Methanol was removed in vacuo, and the resulting white solid was washed with water (2 × 25 mL) and dilute HCl solution (1 × 25 mL) and dried. Any MCMA not removed by rotary evaporation can be removed by washing with hexanes giving PyUM in 68% yield; mp 83 °C. 1H NMR (CDCl3): δ ) 1.23-1.58, 3.20 (m, 20H, (CH2)10), 2.30 (m, 4H, COCH2CH2), 3.74 (s, 3H, OCH3), 4.10 (q, 1H, CH2CH), 4.76 (s, 2H, COOCH2CdCH2), 5.80 (s, 1H, CH2dC), 6.32 (s, 1H, CH2dC), 6.62 (t, 1H, NH). 13C NMR (CDCl3): δ ) 25.0 (CHCH2CH2), 26.1, 27.0, 29.6 (m), 34.3, 39.8 ((CH2)10), 29.6 (CHCH2CH2), 52.2 (CH3O), 57.3 (CHNH), 62.4 (CH2OCO), 127.6 (CH2dC), 135.4 (CdCH2), 165.8 (COOCH3), 172.2 (CONHCH2), 173.4 (CH2COOCH2), 179.7 (CH2CONH). Polymerizations. Solution and bulk polymerizations were done with AIBN initiator that had been recently recrystalized from methanol. Solution polymerization of PyMM was carried out in several solvent systems including a 3:1
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benzene-toluene mixture, propanol, water, or a 7:3 methyl ethyl ketone (MEK)-propanol mixture. The polymers were precipitated into methanol or ether. Aqueous polymerizations used VA-O44 initiator (Wako Chemicals) and precipitation into acetone. PyUM was polymerized in benzene or DMF using AIBN initiator, and the resulting polymer was precipitated into acetone. During all of these reactions, polymer precipitation occurred; the obtained products were partially soluble or insoluble after drying. Soluble polymers of PyUM and PyMM were obtained using DMF as a solvent and AIBN as an initiator. Monomers and 1 mol % of initiator were dissolved in DMF and degassed by bubbling nitrogen through the solutions for 20 min. The polymerization mixture was then placed in an oil bath that was preheated to 60 °C. Heat was immediately turned down, and the bath was allowed to cool to 35 °C. The polymerization proceeded at 35 °C for 40 h, during which time the solution remained homogeneous. Polymers were then precipitated into water or ether. Specific shifts for poly(PyUM) are as follows: 13C NMR (DMSO) δ ) 24.7 (CHCH2CH2), 26.0, 27.1, 29.7 (m), 34.0 ((CH2)10), 29.7 (CHCH2CH2), 47.6 (CH3O), 56.4 (CHNH), 52.6 (CH2-C), 39.7 (C-CH2), 172.8 (CH2COOCH2), 178.0 (CH2CONH). Analyses. Solution NMR spectra were collected on a Bruker AC-300 MHz spectrometer operating at 75.47 MHz spectral frequency. Solid-state spectra were collected on a 400 MHz Bruker MSL NMR operating at a spectral frequency of 100.613 MHz for carbon. FT-IR spectra were recorded on a Nicolet 5DX using KBr disks. Thermal analyses were performed on a TA Instruments 9900 analyzer equipped with 910 differential scanning calorimeter and 952 thermal gravimetric analyzer cells using heating rates of 10 °C/min under nitrogen purge. The equilibrium swelling values of cross-linked polymers were determined by swelling the polymers in various solvents at room temperature. Polymerizations were done neat above the melts of the respective monomers. Solubles were removed by repetitive soaking in methanol and decanting. Polymers were dried under vacuum for 48 h. The swollen weight of the polymer was determined by blotting off the sample with filter paper and weighing. Results are presented as weight percent increase with respect to the dried gels. Photopolymerizations were carried out using a TA Instruments 930 differential photocalorimeter (DPC) with Irgacure 651 initiator. Results of the DPC experiments were evaluated using Microcal Origin 4.1. Results and Discussion PyMM was synthesized in two steps (Figure 1) from pyroglutamic acid and MCMA with an overall yield of 40% and 99% purity as determined by proton NMR and TLC analysis. The monomer is soluble in water, CHCl3, methanol, ethyl acetate, CH2Cl2, and benzene. Figure 2 gives the 13C NMR of this monomer along with that of its polymer obtained by solution polymerization in benzene, or a 1:3 toluene/benzene solvent mixture which shows a Tg of 157 °C by DSC and a 5% weight loss at 260 °C by TGA. These polymerization conditions resulted in poly(PyMM), which
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Figure 1. Synthesis of methyl pyroglutamyl methacrylate (PyMM).
was soluble in DMSO, TFE/CHCl3 (2:8), and DMF and slightly soluble in CHCl3. It was insoluble in water, acetonitrile, benzene, and ethyl acetate. Although initial PyMM polymerizations in benzene gave soluble polymer, the yields were low, so higher boiling solvents were used for polymerizations above 80 °C. For example, solution polymerizations in MEK-propanol (7:3) or water or bulk polymerization at 80 °C gave polymers that swelled in many organic solvents but that were insoluble in all common solvents tried. However 13C solid-state NMR spectral comparison (Figure 3) confirmed overall polymer composition. The Tg of these insoluble polymers was 156 °C, while 5% weight loss in nitrogen occurred from 240 to 250 °C. These properties indicate only a slight degree of branching and cross-linking, possibly via chain transfer involving the pyroglutamate ring hydrogens. The methine hydrogen loss, for example, would generate a captodative radical capable of cross-linking by radical-radical coupling (see below for further discussion). Synthesis of PyUM is outlined in Figure 4. Key to this process is the PyDKP, a reactive intermediate, which displays five-membered ring opening under basic conditions and sixmembered ring opening under acidic conditions.19 When polymerized at high temperatures, PyUM was also crosslinked and insoluble in common organic solvents. During solution polymerization in benzene at 70 °C, poly(PyUM) precipitated from the solution. The resulting polymer was soaked in methanol and dried. No Tg was observed between 30 and 250 °C, and the 5% weight loss value was observed at 288 °C. Polymerizations of PyMM and PyUM in DMF at lower temperatures resulted in soluble polymer in ca. 80% yields. For example, the polymerization mixture was first placed in a preheated oil bath at 55 °C for 2 min and then allowed to react at room temperature for 48 h. Because of the high affinity of DMF for these polymers, poly(PyUM) did not precipitate in methanol, acetone, or ether. However, after several washes with ether, the polymer did precipitate into acetone. Poly(PyMM) was precipitated into methanol. Poly(PyUM) was soluble in DMSO and DMF and insoluble in THF, CHCl3, and benzene, similar to the behavior of poly(PyMM). Thermal properties for both the cross-linked and soluble polymers of PyMM and PyUM were similar and are summarized in Table 1. 13C NMR spectra of soluble polymers showed peaks that correlated well with those seen in the solid-state NMR of the cross-linked poly(PyMM) samples (Figures 2 and 4). Figure 5 shows the carbon NMR and peak assignments for
Smith and Mathias
PyUM and poly(PyUM). For the polymer, as for poly(PyMM), a large broadening of the peaks is observed for the backbone group and the quaternary backbone carbon is relatively sharp. This correlates with results seen in other RHMA polymerizations. Swelling studies of the cross-linked polymers were carried out in CHCl3, ethanol, DMF, and water (Figure 6). Surprisingly, poly(PyMM) picked up more water (65 wt %) than ethanol (25 wt %). In chloroform, poly(PyMM) reached equilibrium swelling with about 83 wt % solvent, while in DMF, the polymer contained 260 wt % before the sample lost structural integrity. Poly(PyUM) swelled to 140 wt % increase with chloroform before the sample lost structural integrity (Figure 7). In DMF, swelling was very rapid, but the sample broke apart at 77 wt % solvent uptake. Poly(PyUM) swelled more in ethanol (102 wt %) than in water (24 wt %). As expected, poly(PyMM) swelled more in water than poly(PyUM), which is more hydrophobic. Poly(PyUM) had a higher affinity for chloroform and ethanol than poly(PyMM). These results indicate very lightly cross-linked systems with high affinities for polar solvents. Because the dried gels were brittle and could not be pressed into disks, sample dimensions were not constant. This may affect rates of swelling; however, equilibrium swelling values should not depend on sample dimensions. Clearly, under certain polymerization conditions, self-cross-linking occurs to give products with high swelling ratios, making these excellent candidates for coatings applications in which cross-linking is desirable. PyUM (10-50 wt %) was copolymerized with HEMA using AIBN at 80 °C, above the melting point of PyUM. The cross-linked products were soaked in methanol (6 × 5 mL over 5 days) and dried under vacuum. The swelling ratios of the gels in water were determined gravimetrically and are shown in Figure 8. In general, increasing the amount of PyUM decreased the swelling in water with the exception of the 50 wt % PyUM copolymer. At PyUM feeds greater than 25 wt %, Tromsdorff autoacceleration was observed before complete melting of the PyUM monomer. The unusual swelling behavior may result from this nonhomogeneity. Also, at feeds of PyUM greater than 35 wt %, the soluble percentage increases indicating incomplete reaction of the PyUM. Table 2 summarizes the weight percent soluble fraction in these gel systems. Both PyMM and PyUM are very reactive monomers toward polymerizaton. TGA data indicated that PyMM was as thermally stable as its homopolymer, showing a 5 wt % loss at 259 °C. DSC analysis of the monomer revealed that PyMM spontaneously polymerized upon melting. In fact, the Tg for poly(PyMM) is seen in the first DSC scan after the monomer melting endotherm and the polymerization exotherm (Figure 9). The second scan showed only the Tg of the homopolymer, indicating that little or no monomer was left unpolymerized. The upper part of Figure 9 shows the DSC traces of the monomer and soluble poly(PyMM). The slightly lowered Tg in the second scan of the monomer could be the result of trapped impurities. The lower part of Figure 9 shows the DSC traces of PyUM and the soluble poly(PyUM). Again, after melting, a polymerization exotherm
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Methacrylate Derivatives
Figure 2.
13C
NMR spectra of PyMM and poly(PyMM).
Figure 3. Solid-state
13C
NMR spectra of various samples of cross-linked poly(PyMM).
Figure 4. Synthesis of PyUM.
is observed. In the second scan, no monomer melting or polymer thermal transitions are seen, as is also the case for poly(PyUM) obtained by deliberate polymerization.
Various pyroglutamate derivatives have shown liquid crystalline character in the melts indicating strong intermolecular association through hydrogen bonding.20 Hyperchem
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Table 1. Thermal Properties of Poly(PyMM) and Poly(PyUM) polymer poly(PyMM)
poly(PyMM-co-NVP) poly(PyUM) a
polymerization solvent
Tg (°C) (DSC)
5% wt loss (°C) (TGA)
benzene/toluenea MEK/propanol bulk water watera benzene DMFa
165 156 147 150 154 ND ND
260 249 244 240 246 288 287
Soluble polymer.
molecular models of these hydrogen-bonded dimers show that the carboxylate groups point in the same direction with respect to the average plane of the pyrrolidone moiety. This indicates that the methacrylate groups are aligned because of hydrogen-bonding interactions, giving rise to enhanced reactivities of the double bonds for polymerization (Figure 10). Strong hydrogen bonding may also account for disparities between inherent viscosity and solubility. Polymers are completely insoluble in many organic solvents seeming to indicate high molecular weights. However, they are readily solvated in very polar solvents with groups capable of hydrogen bonding. Viscosity measurements for poly(PyUM) done in DMF at 35 °C showed a relatively low inherent viscosity of 0.30 dL/g.
Figure 5.
13C
Photopolymerizations. Thermal and photopolymerizations were carried out on the PyMM by itself and in copolymerizations with NVP. Initiated bulk monomer studies in the melt indicated an increase in polymerization rate over typical methacrylates. Also, self-initiated polymerization was observed during the isotherm prior to irradiation with UV light. A blank sample was run above the melt with no photoinitiator present, and a broad polymerization exotherm was observed, confirming uninitiated polymerization occurs spontaneously in the melt. PyMM was copolymerized at room temperature with NVP, in which it was soluble (Figure 11). At low NVP amounts (
