Green Polymer Chemistry: Living Dithiol ... - American Chemical Society

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Green Polymer Chemistry: Living Dithiol Polymerization via Cyclic Intermediates Emily Q. Rosenthal,† Judit. E. Puskas,*,†,‡ and Chrys Wesdemiotis†,§ †

Departments of Polymer Science, ‡Chemical and Biomolecular Engineering, and §Chemistry, The University of Akron, Akron, Ohio 44325, United States ABSTRACT: This paper reports the synthesis and characterization of disulfide polymers obtained by oxidation of 2-[2-(2sulfanylethoxy)ethoxy]ethanethiol (DODT) using a benign, synergistic system comprised of air, dilute hydrogen peroxide and triethylamine as a catalyst that can be recycled. The dn/dc value of the polymer in THF was determined to obtain absolute molecular weight measurements. High molecular weight disulfide polymers (up to Mn = 250000 g/mol) with polydispersity indices as low as Mw/Mn = 1.15 were obtained. Thermal analysis by DSC and TGA demonstrated that the rubbery polymers had a Tg of −50 °C and began to degrade at 250 °C. Dithiothreitol reduced the polymers back to the original monomeric units in 33 h. MALDI-ToF showed the involvement of oligodisulfide rings (2−14 mers) in the polymerization that displayed the characteristics of a living/controlled polymerization; poly(DODT) was readily chain extended with 1,2ethanedithiol. The chain extension indicates a class of living polymerization which is governed by radical recombination.

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INTRODUCTION Disulfide bonds are found ubiquitously in biology and are an integral component of human cellular redox cycles. Disulfidebonds have attracted the biomaterials community because of the ease with which the covalent bond may be broken under specific biological conditions.1,2 The reducing environment found within the cell cytosol cleaves disulfide bonds to produce thiol groups, thereby releasing molecules previously conjugated to or trapped by sulfur−sulfur bonds.1,3 The oxidizing extracellular environment favors disulfide bond maintenance, allowing biomaterials to remain intact during circulation or in the extracellular milieu.1 Glutathione is the most abundant reducing agent found in biological systems, although free cysteine residues may also act as reducing agents Several review articles demonstrate the value of disulfide bonds to the biomedical field.,1,2,4,5 However, not all disulfides and thiols are suitable for biomaterial applications. The greatest concern is the production of “active oxygen” during thioldisulfide shuffling reactions that may lead to tissue damage and hemolysis.6,7 Potential toxicity is related to the reactivity of the thiol, so aminothiols and aromatic thiols are of particular concern. Intermediate thiyl radicals may react with protein disulfides and the double bonds in unsaturated fatty acids or vitamins, thereby causing damage.8 Despite these concerns, growing demand for new biomaterials has led to increased interest in new synthetic strategies for disulfide bondcontaining polymers and networks. Polysulfide and disulfide polymers were first developed in the late 1920s by J.C. Patrick and N. M. Mnookin by heating α,ωdihalogenated linear organic compounds with inorganic polysulfide salts in an emulsion (Scheme 1).9−11 © 2011 American Chemical Society

Scheme 1. Traditional Polysulfide Polymer Synthesis Developed at Thiokol Corp.9

Using this method, dozens of polysulfide polymers and copolymers were created. Polysulfide polymers may be converted to polymers with mostly disulfide bonds in postpolymerization processing such as reacting the polymers with iodine or dimethyl sulfoxide.12,13 Thiokol Corporation was founded based on this research and grew due to the unique adhesive and solvent resistant properties of the polymers. Over 30 patents detailing the synthesis and postsynthetic processing of polysulfides were awarded to Thiokol based on the work of Patrick, Mnookin, Fettes and many others. Today, the term “Thiokol” is often applied without ambiguity to indicate polysulfide or disulfide polymers.14 Despite their long history, only limited data on the structural characterization of these polymers is readily accessible. In addition, the process uses Received: October 7, 2011 Revised: November 15, 2011 Published: December 1, 2011 154

dx.doi.org/10.1021/bm201395t | Biomacromolecules 2012, 13, 154−164

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Table 1. Conditions and Results of Five Example Polymerization Reactions reaction time (min)

final concentrations (M)

sample

premixing

oxidative

max T (°C)

DODT

Et3N

H2O2

% conv.

10−9−70 10−120−70 10−120−55 10−25−ice 160−25−ice

10 10 10 10 160

9 120 120 25 25

70 70 55 (ice bath) (ice bath)

0.39 0.39 0.36 0.35 0.35

0.88 0.88 0.90 0.69 0.69

0.72 0.72 0.72 0.75 0.75

73 80 90 88 87

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halogenated starting compounds reacted at above ambient temperatures making it energy inefficient. The oxidation of thiols to disulfides has been applied to the synthesis of disulfide polymers with varying degrees of success since the 1940s. The oxidative polymerization by oxygen alone has not been shown to produce polymer with more than about eight repeat units.15−17 Patrick used oxygen to connect previously made oligomers through their terminal thiols, but the molecular weight of the resulting polymers was not reported.18 An early account by Goethals and Sillis used dimethylsulfoxide and heat to create linear disulfide polymers with molecular weights of up to Mn = 10500 g/mol.19 Hydrogen peroxide has also been used for the oxidation of thiols.20 Hanhela and Mazurek polymerized dithiol monomers using a hydrogen peroxide and sodium hydroxide oxidation system, and obtained liquid oligomers.21 Recently, Park et al. made disulfide polymers from α,ω-thioacetate-functionalized triethylene glycol and ethylene glycol oligomers.22,23 Their aim was to produce a polymer analogous to PEG but with a steeper biodegradation profile. Thioacetate-functionalized oligomers23 were dissolved in an ammonia/methanol solvent mixture to remove the protecting group in situ while exposing them to a pure oxygen atmosphere or to 20% aqueous solutions of DMSO. Polymerization of triethylene glycol dithioacetate yielded a polymer with Mn = 61000 g/mol (relative to PEG standards) and Mw/Mn = 2.3 in 48 h of oxygen exposure.22 Exposure of the same monomer to DMSO solution yielded polymers with Mn = 6500 g/mol and Mw/Mn = 2.8 in 48 h.23 Polymers with 12 ethylene glycol units between disulfide bonds (reported Mn ≈ 21000 g/mol) were degraded completely in 2 h when exposed to 5.0 mM glutathione in 150 mM phosphate buffered saline. Reducing energy consumption and the subsequent global impact of carbon dioxide emissions is among the greatest environmental concerns today. Additionally, in 12 principles of green chemistry, Anastas and Warner express the importance of eliminating wasteful derivatives such as protecting groups and using safer chemicals both from an environmental and accident prevention outlook.24 We report an efficient and “greener” method for the oxidative polymerization of 2-[2(2-sulfanylethoxy)ethoxy]ethanethiol (DODT) yielding high molecular weight degradable disulfide polymers with narrow molecular weight distribution. Our method produces the desired polymers at ambient temperatures and pressure, does not require protecting groups for the thiol groups and uses no chlorinated reagents or solvents. Additionally, the use of a dilute hydrogen peroxide solution as an oxidant eliminates the explosion and fire hazard associated with pure oxygen. The organic base activator can be recycled and reused under industrial conditions. MALDI-ToF showed the involvement of rings in the polymerization that also displayed living characteristics.

EXPERIMENTAL SECTION

Materials. 2-[2-(2-Sulfanylethoxy)ethoxy]ethanethiol (DODT) was purchased from Sigma-Aldrich. Triethylamine and dithiothreitol (DTT) were also purchased from Sigma-Aldrich and were kept under an inert atmosphere until use. Hydrogen peroxide solution (3% by weight) with 0.05% phenacetin stabilizer was purchased from J.T. Baker. Hydrogen peroxide (3% by weight) without a stabilizer was also purchased from Fisher Scientific. Acetone and THF were purchased from Fischer Scientific. Ethyl acetate was purchased from VWR. Deuterated NMR solvents were purchased from Cambridge Isotope Laboratories. All reagents and solvents were used as received. Synthesis of Disulfide Polymers. In a typical polymerization reaction, the dithiol monomer and triethylamine (1:1.25 equiv ratio) were mixed first and reacted (specific times and conditions are given in Table 1). To the bulk mixture, 2.0 equiv of hydrogen peroxide (3% aqueous solution by weight) were added in 10−15 aliquots of equal volume during a time period of 5−20 min to keep the reaction temperature below the values specified in Table 1. The final concentrations are listed in Table 1, although it has to be noted that polymer started to precipitate after the addition of one equivalent of hydrogen peroxide. Air was then bubbled into the reaction flask while maintaining vigorous stirring using a magnetic stir bar in an open environment. The air oxidation step was allowed to proceed for specified time periods given in Table 1. The precipitated polymer was then removed from the reaction flask, rinsed with water and extracted with acetone for 72 h (refreshed every 24 h) to remove residual triethylamine and monomer. The excess acetone was decanted and the polymer was dried in a vacuum oven until constant weight was achieved. Kinetic Studies. A 2.09 M stock solution of DODT in triethylamine was prepared and stirred for approximately 20 min. Four glass vials were filled with 6.92 mL of hydrogen peroxide. To each vial, 1.54 mL of stock was added at once using a syringe while stirring. Each vial was allowed to react for a specified amount of time (1, 5, 30, and 120 min) before the reaction was stopped by pouring the reaction mixture into methanol. The precipitated polymers were transferred from the solvent mixture to massed aluminum pans. Additional solvent (chloroform) was added to aid in the extraction of water. The polymers were dried until a constant mass was reached and analyzed by SEC. Chain Extension Studies. Previously synthesized DODT polymer was dissolved in THF (0.5008 g in 10 mL THF). The reaction flask was then placed in an oil bath thermostatted at 25.0 °C, and allowed to equilibrate. Based on the mass of polymer added, the solution had a poly(DODT) repeat unit concentration of 0.28 M. A 2.31 M solution of ED in triethylamine was prepared in a volumetric flask and allowed to stir for 1 h. Using a syringe, 2.72 mL of ED solution (0.5915 g ED, 6.28 mmol ED) was added to the DODT solution and stirred for 10 min. The concentration of DODT repeat units and ED monomer in the solution was 0.22 and 0.49 M, respectively. Hydrogen peroxide (2.68 mL; 3.1 mmol) was added over 1.5 min while stirring vigorously. The temperature of the reaction flask peaked at 30.2 °C following the addition of hydrogen peroxide. Aliquots were taken at 4 and 30 min. The oil bath temperature was then raised to 55 °C and kept at this temperature for 40 min before bringing the reaction to reflux at 70 °C. The flask was removed from heat at the onset of reflux and allowed to stir for 16 h.

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Polymer Characterization. NMR. 1H and 13C spectroscopy was performed using a Mercury 300 MHz instrument. 1H NMR was also performed using a 500 MHz Varian INOVA 500 instrument. Analysis of the spectra was performed using a 1 D NMR processor software produced by Advanced Chemistry Development. FTIR. Infrared analysis was carried out using a Digilab Excalibur Series FTS 3000 instrument. Samples were dissolved in THF and cast onto a KBr crystal for analysis. Size Exclusion Chromatography (SEC). Molecular weights (MW) and molecular weight distributions (MWD) were determined by SEC on a system equipped with six Waters Styragel columns, a Waters 2487 dual absorbance UV detector, a Wyatt Optilab DSP interferometric refractometer, a Wyatt DAWN EOS multiangle laser light scattering detector, and a Wyatt Viscostar viscometer. The data from the SEC was processed using Astra Version 5.3.4.14. The dn/dc of the polymer (0.132 mL/g) was determined with the Optilab DSP RI detector using a increasing series of dilute polymer solutions. Mass Spectrometry (MALDI-ToF). Matrix assisted laser desorption/ionization time-of-flight mass spectrometry was performed on a Bruker UltraFlex-III time-of-flight mass spectrometer equipped with a Nd:YAG laser emitting at 355 nm. Mass spectra were measured in positive reflectron mode, using T-2-(3-(4-t-butyl-phenyl)-2-methyl-2propenylidene) malononitrile (DCTB) as matrix and sodium trifluoroacetate (NaTFA) as cationizing salt. Polymer, matrix, and cationizing salt were dissolved in anhydrous THF at concentrations of 1, 20, and 10 mg/mL, respectively. The DCTB and NaTFA solutions were mixed in the ratio 10:1 (v/v). Sample preparation involved depositing 0.5 μL of matrix/salt mixture on the wells of a 384-well ground-steel plate, allowing the spots to dry, depositing 0.5 μL of polymer on top of the dry matrix spot, and adding another 0.5 μL of matrix/salt on top of the dry sample (sandwich method). This protocol produced [M + Na]+ ions from the polymer’s n-mers. Thermal Analysis. Differential scanning calorimetry (DSC) was carried out on a TA Q2000 DSC using a heat−cool−heat thermal cycle. A typical thermal cycle started by heating the sample from 40 to 175 °C at 10 °C/min to remove any previous thermal history. In the second step of the cycle, the sample was cooled at 10 °C/min to reach −80 °C. The third step of the cycle heated the sample from −80° to 160 at 10 °C/min. To be sure of the absence of a crystalline region, some samples were cooled to −150 °C. Thermal gravimetric analysis (TGA) was performed on a TA 5000 TGA using a heating rate of 10 °C/min from room temperature to 500 °C under N2 atmosphere. Analysis of the thermal data was performed using the Universal Analysis 2000 software. Degradation of the Polymer by Dithiothreitol. In a 500 mL round-bottomed flask, 0.3123 g polymer was dissolved in 100 mL of THF. To the flask, 40 mL of a 50.05 mM aqueous dithiothreitol (DTT) solution was added. The mixture was stirred vigorously with a magnetic stir bar. Aliquots (10 mL) were taken at timed intervals for NMR analysis. The reduction reaction was stopped by adding 5 mL of chloroform and 5 mL of saturated NaCl solution to the aliquot which separated the polymer and monomer from the DTT. The organic phase was then dried and analyzed by NMR.

Scheme 2. Effective Method for the Synthesis of Disulfide Polymers by Thiol Oxidation

hydrogen peroxide was added over 10 to 15 min. For example, the sample identification number 10−9−70 in Table 1 stands for 10 min of premixing, 9 min of oxidation reaching a maximum temperature of 70 °C. The polymerizations produced transparent, rubbery polymers that were soluble in THF, DMSO and chloroform but insoluble in hexanes, methanol, acetone, or water. While the monomer is soluble in triethylamine and triethylamine is miscible with water below 18 °C,25 the polymers were not soluble in the reaction mixture and precipitated from the solution. The polymers were opaque white during polymerization but became clear and colorless upon drying. In all cases, high conversions were reached, with sample 10−120−55 yielding 90% conversion. This sample was fully characterized. NMR. The 1H NMR spectrum of the monomer (Figure 1A) shows four peaks. The sulfhydryl proton (a) is split by the two neighboring protons and appears as a triplet (1.58 ppm). This peak is absent in Figure 1B, supporting the conversion of monomer to polymer. The methylene protons (b) adjacent to the thiol group display a quadruplet (2.71 ppm) as they are split by both the two neighboring methylene protons and the sulfhydryl proton. In the polymer spectrum, the thiol-adjacent methylene protons are only split by the neighboring methylene protons and therefore appear as a triplet (2.89 ppm). In the monomer, the methylene protons adjacent to the oxygen (c) are split by the neighboring methylene group and they appear as a triplet (3.62 ppm). However, the strong singlet peak (3.61 ppm) from the four equivalent hydrogens (d) found on the two center carbon atoms overlaps the triplet. In the polymer spectrum (sample 10−120−55), the two peaks no longer overlap and may be integrated separately. The integration values match the expected values since each monomer contains four of each type of methylene protons. The triplet peak from the terminal thiol protons (1.58 ppm) was not detected in 10− 120−55 which turned out to have the highest molecular weight (see SEC data). However, the other samples displayed the SH proton signal. Clear shifts in the carbon NMR spectra of the starting material versus the polymeric product (Figure 2) are visible. The greatest shift is seen in the sulfur-adjacent carbon which appears at 24.39 ppm in the monomer and 38.39 ppm in the polymer, due to the weaker polarization of the carbon−sulfur bonds in the disulfide state. No signals corresponding to carbons adjacent to SH end groups were detected. The oxygen adjacent carbon (C) and the central carbon atoms (D) both shifted upfield. The central carbon peak shifted less than 1.0 ppm upfield from 70.13 to 69.95 ppm. The oxygen adjacent atoms showed a larger shift from 72.82 to 70.33 ppm. FTIR. The infrared spectra of the DODT monomer and the polymer from Sample 10−120−55 are shown in Figure 3. The

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RESULTS AND DISCUSSION Synthesis of Disulfide Polymers. The recently developed efficient method for thiol oxidation is shown in Scheme 2. Table 1 shows experiments carried out under various conditions. The premixing time refers to time allowed for the monomer and triethylamine to react before the addition of hydrogen peroxide. Oxidative reaction time refers to the time the reaction was carried out after the final addition of hydrogen peroxide. Polymerization reactions used to make samples 3 and 4 were carried out in an ice bath using ice-chilled hydrogen peroxide. The maximum temperature reached in the reaction flask was controlled by the pace of hydrogen peroxide addition. For reactions reaching 70 °C, hydrogen peroxide was added over about 5 to 10 min, while in reactions reaching 55 °C, 156


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Figure 1. 1H NMR spectra of the starting material, DODT (A) and poly(DODT) sample 10−120−55 (B). Integration values are labeled below each peak (500 MHz; 12 s relaxation; 128 transients).

Figure 2. (A) 13C NMR spectrum of the starting material, DODT. (B) (500 MHz; 10 s relaxation; 1024 transients).

13

C NMR spectrum of poly(DODT) sample 10−120−55 in CDCl3

Figure 3. FTIR spectrum of DODT monomer followed by spectrum of poly(DODT) (sample 10−120−55): (a) 2557 cm−1 −S−H; (b) 667 cm−1 −C−S in thiols; (c) 1042 cm−1 −C−O stretching from ether bonds; (d) 644 cm−1; and (e) 467 cm−1 −C−S and −S−S− vibrations of the disulfide polymer.

strong peak at 2557 cm−1 (peak a) corresponding to S−H stretching of the terminal thiol groups of DODT26 is not seen in the spectrum of the product. The signal at 477 cm−1 (peak e), not present in the monomer, indicates the formation of S−S

bonds in the polymer. The C−H stretching signals between 2800 and 3000 cm−1 are present in both spectra. In the fingerprint region of the spectra, similar signals from C−C and C−H vibrational modes are seen with only minimal lateral 157


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shifting after polymerization. For example the C−S signal seen at 667 cm−1 (peak b) shifts to 644 cm−1 (peak d) after the reaction, now being affected by another sulfur atom rather than a hydrogen atom. Also, the signal at 1113 cm−1 that represents the C−O stretching is strong in both spectra, but the shoulder evolving from this band in the monomer grows into a peak after the polymerization and is typical of compounds containing multiple ether bonds (peak c). SEC. As mentioned in the Introduction, Park et al. used poly(ethylene glycol) PEG standards to obtain the molecular weights of their polymer.23 We found that although the polymer structure is visually reminiscent of PEG, the dn/dc = 0.132 mL/g measured in this work (see Experimental Section) is very different from the dn/dc of 0.068 mL/g measured for PEG (Mw = 6700) in THF.27 Therefore, the molecular weight data reported for poly(DODT) earlier is questionable. The Mn values shown in Table 2 were obtained using the

Scheme 3. Mechanism of Base-Catalyzed Thiol Oxidation by Oxygen

Table 2. SEC Data for the First Three Reactions Listed in Table 1, with dn/dc = 0.1320 mL/g

Table 3 summarizes the data for these reactions. The high MW peaks were integrated separately, while retention times and peak MWs are provided for the peaks in the low MW region. Comparison of 10−25−ice and 160−25−ice reveals that the premixing time did not have an effect on the molecular weight of the high MW fraction. Interestingly, the high MW fraction has very narrow polydispersity (Mw/Mn = 1.15). The low molecular weight peaks comprise 41.5 and 59.5% of the total injected polymer in the two samples, respectively. These samples were also analyzed by MALDI-ToF. MALDI-ToF Mass Spectrometry. Figure 6 shows the MALDI-ToF mass spectrum of the low molecular weight fraction of sample 10−120−55 and is representative of the MALDI-ToF spectra for the low molecular weight fractions of all of the DODT polymers. The mass spectrum shows repeating peaks which are spaced at 180 m/z units. This mass to charge ratio corresponds directly to the predicted mass of the repeat unit structure of poly(DODT). The mass of the monoisotopic peaks (i.e., those containing the lowest-mass isotopes) in each repeat unit also indicates the absence of end groups; for example, the peak at m/z 1823.7 arises from a 10mer without end groups; the corresponding calculated m/z, 10 × 180.03 (mass of DODT repeat unit) + 22.99 (mass of Na+) = 1823.3, agrees very well with the measured value. Thus, we conclude that the low molecular weight polymers are cyclic disulfides. The 500 MHz NMR did not detect the presence of thiol end groups in this polymer. Thermal Analysis. In the DSC trace of sample 10−120− 55, a strong glass transition is seen between −53 and −49 °C (Figure 7). No other transitions were observed in the −150 to 150 °C range. The TGA decomposition trace of sample 10− 120−55 shows 2% mass loss at 236.3 °C, 50% mass loss at 297.9 °C and the decomposition profile plateaus at 356.8 °C (Figure 8). The thermal degradation temperature as calculated by the Universal Analysis software was 283.7 °C. Polymer Degradation. The reduction of disulfide polymers by dithiothreitol is shown in Scheme 4. The reduction is best visualized by following the proton signals in the NMR spectra of the methylene group neighboring the sulfur atoms (Figure 9). As described previously, in the polymer, the methylene group appears as triplet, while in the monomer it appears as a quadruplet. As the polymer is reduced, the methylene triplet disappears and the quadruplet corresponding to the monomer appears. Degradation is seen in the

sample

Mn (g/mol)

Mw (g/mol)

Mw/Mn

10−9−70 10−120−70 10−120−55

14000 47000 230000

19000 84000 345000

1.35 1.79 1.50

measured dn/dc. The SEC trace of sample 10−120−55 is shown in Figure 4 and is representative of the first three reactions listed in Table 1.

Figure 4. Light scattering (LS), refractive index (RI), and UV chromatograms from SEC analysis of sample 10−120−55 is representative of most polymerization reactions.

The first three reactions each show one large, predominant peak in their light scattering and RI traces. The SEC data for these are listed in Table 2. The UV trace tracks the RI trace in the high molecular weight region and then displays a small peak in the low molecular weight range of the plot which is too small to be detected by light scattering. Interestingly, there is no signal in between the low and high molecular weight fractions. The MW increased with increasing oxidation time. The highest molecular weight was reached when the temperature was kept below 55 °C. It is also interesting that the Mw/Mn values are ∼1.5, indicating recombination effects. Indeed, the base-catalyzed oxidation of thiols reportedly involves radical recombination,28 as shown in Scheme 3. The polymerization reactions which were carried out in an ice bath show a different pattern. The RI traces display a high MW peak, followed by a series of low MW peaks (see Figure 5). 158


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Figure 5. SEC RI trace of 10−25−ice (A) and 160−25−ice (B).

Table 3. SEC Data for the Polymerizations in Performed in an Ice Bath molecular weight sample 10−25−ice peak 4 3 2 1 sample 160−25−ice

Mn (g/mol)

Mw(g/mol)

113000 130000 retention time (min) peak MW (g/mol) 57.1 1100 58.3 800 60.5 400 molecular weight Mn (g/mol)

Mw(g/mol)

4 116000 133000 retention time (min) peak M.W. (g/mol) 3 57.2 1000 2 58.3 800 1 60.4 400

PDI (Mw/Mn) 1.15 approx. DP 6 4 2

Figure 7. DSC trace of poly(DODT) for sample 10−120−55. Heating rate: 10 °C/min.

PDI (Mw/Mn) 1.15 approx. DP 6 4 2

Figure 8. TGA decomposition profile of sample 10−120−55. Heating rate: 10 °C/min. Figure 6. MALDI-ToF mass frequency spectrum from poly(DODT) sample 10−120−55.

This peak is also present in the monomer spectrum but disappears after polymerization. Upon exposure to dithiothreitol this peak reappeared indicating the evolution of thiol functional groups. Polymerization Kinetics and Mechanism. A summary of the SEC and conversion data from the kinetic experiment is presented in Table 4. The kinetic investigation showed that the polymerization proceeded in two stages. The first stage was

first sample aliquot taken after 3 h of exposure to DTT. With the progression of time, the triplet becomes weaker while the quartet becomes stronger. By 33 h of exposure, the polymer peaks have almost disappeared entirely. Further evidence of degradation is provided by FTIR analysis of the degraded product which shows an S−H stretching peak at 2557 cm.−1 159


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Scheme 4. Disulfide Reduction by Dithiothreitol

Figure 10. Mn conversion plot for the polymerization of DODT.

bonds of other dimers or oligomeric rings. At higher monomer conversion the oligomer diradicals recombine, causing the exponential molecular weight growth. As the rings grow, cycle conformation may become unfavorable and the rings open to form linear polymers (Scheme 5). However, larger rings may exist as no end groups were detected in 10−120−55. Endo and co-workers reported that certain cyclic disulfide monomers undergo ring-opening polymerization and propagate with the involvement of cyclic structures. However, they proposed the formation of interlocking polymer cycles (catenanes) under the conditions they used.29,30 The combination of high molecular weight polymers and cyclic oligomers has precedent in the polycondensation of silicones in which about 18% of the total polymer is cyclic.14 High molecular weight linear silicones exist in a thermodynamically controlled equilibrium with oligomeric cyclic silicones. The equilibrium is highly dependent on polymer dilution and the structure of the side groups.31 In the case of poly(DODT), samples 10−25−ice and 160−25−ice show that cyclic oligomers (up to 14-mers) coexist with high polymers (DPn ∼ 600) having very narrow distribution (Mw/Mn = 1.15). This is the first example of disulfide polymers with such a low polydispersity synthesized by the oxidative polymerization of dithiols. Chain Extension. To further test our hypothesis we conducted a chain extension experiment from poly(DODT) 10−9−70 (see Table 2) with 1,2-ethanedithiol (ED). White flecks appeared in the solution immediately upon the addition of hydrogen peroxide to the poly(DODT)-ED solution. Aliquots of the reaction were taken at 4 min (0.5 mL) and 30 min (1.0 mL) and immediately diluted with water. The white flecks (insoluble) were filtered out and the remaining sample was poured into methanol. No precipitate was seen in the methanol portion. Similarly, the insoluble fraction of the final polymer product was filtered out and rinsed with methanol. About half of the solvent was removed by rotary evaporation before precipitation in methanol. All fractions, insoluble and soluble, were dried until a constant mass was reached. Of the material recovered from the 4 min aliquot, 18.9% was insoluble and 81.1% was soluble. Of the material recovered from the 30 min aliquot, 27.8% was insoluble and 72.2% was soluble. The total conversion for each aliquot was 92.5 and 82.3%, respectively. In the final product, 65.5% was insoluble and 34.5% was soluble, with 92.6% total conversion. The total yield was 1.00 g, thus, the soluble fraction (0.345 g) was less than the mass of the starting material. Poly(ethane disulfide) (poly(ED)) is known to be insoluble in most solvents.19 The homopolymerization of ED also yielded an

Figure 9. 1H NMR spectra taken at timed intervals during degradation of polymer sample 10−120−55 using dithiothreitol.

Table 4. SEC and Conversion Data from Kinetic Studies time (min)

Mn (g/mol)

Mw (g/mol)

Mw/Mn

% conv.

1 5 30 120

37000 197000 187000 253000

56000 287000 312000 367000

1.51 1.46 1.67 1.45

39.7 88.4 88.9 93.7

very fast: over 75% monomer conversion was achieved in 5 min. The second stage was slower, but the Mn increased exponentially in this stage, as shown in Figure 10. In Figure 11, the light scattering and refractive index traces for the 1 and 120 min samples are compared. Both traces move toward higher molecular weights while the molecular weight distribution remains around 1.5. Based on the data presented here, we propose the following mechanism. The thiol protons of the monomer are removed by triethylamine to form thiolate anions. Upon oxidation, the diradicals recombine to form cyclic dimers. The dimer rings are the dormant species that exist in equilibrium with their active diradical form. The first stages of propagation most likely involve the rapid insertion of these diradicals into disulfide 160


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Figure 11. Light scattering (LS) and refractive index (RI) traces for 1 min polymer sample and 120 min polymer sample.

Scheme 5. Proposed Mechanism of Dithiol Oxidation Polymerization

insoluble polymer, thus, we concluded that the insoluble fraction contained poly(ED). The 1H NMR spectrum of the 30 min aliquot and the final polymer are compared in Figure 12. Peaks from in the starting material at 3.75 (c), 3.65 (d), and 2.90 ppm (b) are retained in the product spectrum except they are dramatically broadened and show more irregular splitting patterns. A new peak also appears in the two product spectra at 3.02 ppm (e). Meng and co-workers found one singlet peak at 2.99 ppm corresponding to the 4 equiv methylene protons of each repeat unit of soluble cyclic oligomers of ED.32 Based on Meng’s data the peak at 3.02 ppm in our two product spectra is assigned to the protons of ED units in the polymer, however, the broad peak and multiplet splitting pattern

suggest that neighboring repeat units may be either ED or DODT units. The solution NMR data demonstrate that ED incorporated into the poly(DODT). Interestingly, the ratio of the 3.75, 3.65, and 2.90 ppm peaks (DODT) to the 3.02 ppm peak (ED) changes from about 1:1 in the 30 min spectrum to almost 2:1 in the final polymer spectrum. At first, this may appear to indicate that ED units are leaving the polymer chain. However, taking into consideration the increase in both molecular weight and insoluble fraction, the data suggests that the increased incorporation of ED into the poly(DODT) chains causes them to become insoluble, leaving the soluble fraction with only copolymer containing less ED units. The mass decrease of the soluble portion of the polymer relative to the starting poly(DODT) supports this conclusion. 161


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Figure 12. 1H NMR spectra of the chain extension experiment (12 s relax, 64 transients in CDCl3).

Figure 13. 13C NMR spectrum of the final polymer from the chain extension experiment.

The carbon NMR spectrum (Figure 13) of the soluble portion of the final product shows the three peaks typical of poly(DODT) homopolymer with the oxygen-adjacent carbons at 70.32 ppm (C) and 69.66 ppm (D) and the sulfur-adjacent carbon (B) at 38.45 ppm. The new peak at 37.39 ppm corresponds to the 2 equiv carbon atoms of ED units (E). The 13C spectrum also demonstrates that ED incorporated into the poly(DODT). The solid-state carbon NMR (Figure 14) of the insoluble fraction of the final polymer shows two broad peaks, one at

37.06 ppm (sulfur-adjacent carbons) and another at 71.67 ppm (oxygen-adjacent carbons). Because the larger peak is centered at 37 ppm rather than 38 ppm, it appears the majority of the sulfur adjacent carbons come from ED units. If the insoluble fraction were only poly(ED), the spectrum would show a single peak at 37.06 ppm. Thus, the solid state NMR verified that the insoluble fraction is also a copolymer. Figure 15 compares the SEC RI traces of the starting material and the final polymer product. The trace of the final product 162


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Figure 14. Solid state 13C NMR spectrum of insoluble fraction of final product from chain extension study.

(sulfones, etc) is critical when developing new biomaterials designed to degrade under physiological conditions. Second, the described method also offers a mild oxidation system without the use of heavy metal catalysts, minimal energy use since no heating is required and a simple of purification step that does run the risk of leaving halogenated residues. The method is fast and efficient, reaching number average molecular weights of over 200000 g/mol in less than 3 h. Additionally, we have shown that cyclic oligomers are involved in the polymerization mechanism, and that the living character of the mechanism allows for additional monomer incorporation after initial polymerization. When using this synergistic oxidative system, lower reaction temperatures lead to narrower molecular weight distributions, and we have obtained distributions as low as 1.15.

Figure 15. RI SEC traces from the chain extension study.

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shifted toward higher molecular weight. Because no dn/dc value is available for the copolymer, the SEC data is based on 100% mass recovery of the injected sample. With this, the final product had Mn = 120000 g/mol. The SEC data for the chain extension study is summarized in Table 5.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ACKNOWLEDGMENTS The authors would like to thank Kurt Cheng Ching Chiang for help in measuring the dn/dc value of the polymer and Bimala Lama for running the solid state NMR. This material is based upon work supported by the National Science Foundation under Grant No. DMR-0804878. We also wish to thank the Goodyear Corporation for the donation of the 300 MHz NMR instrument used in this work and the Ohio Board of Regents and the National Science Foundation for funds used to purchase the 500 MHz NMR instrument used in this work.

Table 5. SEC from Chain Extension Study sample

Mn (g/mol)

Mw (g/mol)

Mw/Mn

poly(DODT) final product

20000 120000

35000 214000

1.79 1.76

The incorporation of ED into existing poly(DODT) chains supports our proposal that the disulfide bonds can be reactivated. More detailed investigations of the polymerization mechanism are in progress.

■

■

CONCLUSIONS The described method for the synthesis of high molecular weight disulfide polymers is a useful alternative to traditional polymerization methods for several reasons. First, the method has the advantage of ensuring that specifically disulfide bonds are formed to connect the monomer units. The absence of higher ranking sulfur bonds and sulfur atoms in higher oxidations states

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