Synthesis of Biodegradable Polyisobutylene Disulfides by Living

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Synthesis of Biodegradable Polyisobutylene Disulfides by Living Reversible Recombination Radical Polymerization (R3P): Macrocycles? Judit E. Puskas*,§ and Sanghamitra Sen§ Chemical and Biomolecular Engineering, The University of Akron, Akron, Ohio 44325, United States ABSTRACT: This paper reports the synthesis and characterization of polyisobutylene disulfides by living reversible recombination radical polymerization (R3P). First telechelic thiol-functionalized PIB macromonomer (HS− PIB−SH) was synthesized using enzyme-catalyzed transesterification. Then the HS−PIB−SH was chain-extended using hydrogen peroxide, triethylamine catalyst and air to yield polyisobutylene disulfides up to Mn = 96 000 g/mol. 750 MHz HMBC correlation NMR spectroscopy along with Raman spectroscopy showed the absence of −SH end groups. Thermal analysis by DSC showed a Tg of −62.5 °C and TGA indicated that the polymer began to degrade at 340 °C. The increased Tg together with structural and SEC analysis indicate that this R3P likely yields macrocycles. Dithiothreitol (DTT) reduced the polyisobutylene disulfides to a mixture of pentamer-to-dimer PIB-dithiols.

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INTRODUCTION Polymers containing disulfide (−S−S−) repeat units in the main chain are known as polydisulfide polymers.1−5 Disulfide bonds can be cleaved in a reducing environment to generate thiols (−SH)6−9 and thiols can be transformed into disulfides in an oxidizing atmosphere. This dynamic disulfide/thiol redox system is a unique reversible chemical reaction that can frequently be observed in living organisms and plays a pivotal role in protein folding to generate the active conformation.1,10 The oxidizing extracellular and reducing intracellular environment generates a high redox potential difference.10 Disulfide bonds are stable in the extracellular atmosphere. However, they cleave into the corresponding thiols after entering the reducing intracellular environment. Hence disulfide/thiol systems have widely been exploited for drug delivery applications.10−13 We recently reported the living reversible radical recombination polymerization (R3P) of 2,2′-(ethylenedioxy)diethanethiol (DODT).14−17 A “green” air oxidation method was developed for this polymerization. The system showed living characteristics: poly(DODT) was readily chain-extended with ethylene dithiol (ED) and the molecular weight increased with conversion. NMR spectroscopy and MALDI−ToF demonstrated that rings are involved in the polymerization. We proposed a polymerization mechanism governed by active-dormant equilibria between disulfide bonds and sulfide radicals. The reduction of the disulfide bonds in poly(DODT) was also reported in the presence of dithiothreitol (DTT); the polymer completely degraded to monomer after 33 h. In this study, we will report the synthesis and characterization of polyisobutylene disulfides through R3P using thiol-functionalized PIB (HS−PIB−SH) as a macromonomer, 30 wt % aqueous hydrogen peroxide (H2O2), triethylamine (TEA) catalyst, and air. We will present evidence that this R3P likely © XXXX American Chemical Society

yields macrocycles. DTT degraded the polyisobutylene disulfides into a mixture of pentamer-to-dimer, making this the first biodegradable PIB.

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EXPERIMENTAL SECTION

Materials. Asymmetric hydroxyl-functionalized PIB (HO−PIB− (S)OH for short) was synthesized as described earlier.18 Triethylamine (TEA), aqueous hydrogen peroxide (H2O2, 3 and 30 wt %), Dithiothreitol (DTT), methyl-3-mercaptopropionate, Candida antarctica lipase B (CALB, 33273 Da, 20 wt % immobilized on a macroporous acrylic resin Novozyme 435), magnesium sulfate, toluene, methanol, hexane and THF were purchased from Sigma-Aldrich. Deuterated chloroform was purchased from Cambridge Isotope Laboratories. All reagents and solvents were used as received. Procedures. Synthesis of HS−PIB−SH. HO−PIB−(S)OH (Mn = 1700 g/mol, Mw/Mn = 1.3, 10.00 g, 0.0043 mol) was heated under vacuum (Schlenk line) at 50 °C in a 50 mL round bottomed flask, until bubble formation ceased, to completely dry the sample. Then toluene (8.00 mL) and methyl-3-mercaptopropionate (13.35 g, 0.11 mol, 10.00 equiv) were added to the flask and stirred until it formed a homogeneous solution. Then CALB (0.95 g resin, 0.0003 mol/L CALB) was added to the reaction mixture. The reaction was allowed to proceed for 6 h at 50 °C and 520 Torr vacuum to remove the methanol forming during the reaction. The product was recovered by precipitation from cold methanol (5 °C), redissolved in hexane and reprecipitated three more times from cold methanol and dried in the vacuum oven for 72 h. Synthesis of Polyisobutylene Disulfides. HS−PIB−SH (Mn = 2300 g/mol, Mw/Mn = 1.3, 5.00 g, 0.0217 mol) was dissolved in hexane (2.00 mL) in a 10 mL round bottomed flask. To this solution was added TEA (0.66 g, 0.07 mol), and the mixture was stirred at room temperature for 20 min. Then aqueous H2O2 (3 and 30 wt % as stated in Table 1) Received: November 4, 2016 Revised: March 9, 2017

A

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Macromolecules Table 1. Scouting Experiments SEC data

a

sample IDa

wt % of aqueous H2O2

equiv of H2O2 used per mol of −SH

temperature (°C)

time (min)

Mn (g/mol)

Mw/Mn

HS−PIB−SH 2−3−0−15 2−30−0−15 2−30−25−15 2−30−50−15 2−30−25−60 4−30−25−15 6−30−25−15

− 3 30 30 30 30 30 30

− 2 2 2 2 2 4 6

− 0 0 25 50 25 25 25

− 15 15 15 15 60 15 15

2300 2600 11000 11000 9100 10000 42000 96000

1.3 1.3 1.4 1.4 1.7 1.4 1.6 1.5

The ID 2-3-0-15 denotes a sample in which the polymerization was conducted with 2 equiv. of 3 wt % H2O2 at 0 °C for 15 min

solution was added dropwise under constant stirring while air was bubbled through the solution. After the completion of the addition (5 min), the mixture was stirred for an additional time period (see Table 1) under constant air bubbling. The colorless solutions turned white with time as they formed a two-phase system and the polymer was present in the hexane phase. Specific conditions are listed in the figure captions and Table 1. The product was recovered by precipitation from cold methanol (5 °C), redissolved in hexane and reprecipitated from cold methanol three times. For 2D NMR and Raman spectroscopy studies a lower molecular weight polyisobutylene disulfide (Mn = 25 000 g/mol) was synthesized by running the oxidative polymerization reaction for 1 min while keeping all other parameters unchanged. Kinetic Study. A stock solution was prepared by dissolving HS−PIB− SH (Mn = 3200 g/mol Mw/Mn = 1.3, 7.00 g, 0.0304 mol) in hexane (3.00 mL). To this solution was added TEA (0.92 g, 0.0900 mol), and the mixture was stirred at room temperature for 20 min. Five glass vials were filled with 6.10 μL of 30 wt % hydrogen peroxide. To each vial was added 0.30 mL of HS−PIB−SH stock solution using a syringe while stirring under constant air bubbling. Each vial was allowed to react for a specified time (1, 3, 5, 10, and 15 min) under constant bubbling of air before the reaction was stopped by pouring the reaction mixture into cold methanol (5 °C). The products were redissolved in hexane and reprecipitated from cold methanol. Finally, the products were dried in the vacuum oven for 72 h and analyzed by SEC and 1H NMR spectroscopy. Degradation of Polyisobutylene Disulfides. In a 50 mL roundbottomed flask, a polyisobutylene disulfide sample (Mn = 96 000 g/mol Mw/Mn = 1.5, 1.900 g, 0.02 mmol) polymer was dissolved in THF (6.00 mL). Separately DTT (2.5 g, 16.20 mmol) was also dissolved in THF (4.00 mL) and added to the polymer solution. The mixture was stirred vigorously with a magnetic stir bar. Aliquots (1.00 mL) were collected at regular time intervals (24, 48, 72, 96, and 120 h, Table 3). Then 5 mL of hexane was added to the reaction mixture to extract the polymer. The solution was then washed with water for five times to remove the unreacted DTT and remaining THF. The organic layer was separated and dried with magnesium sulfate. The product was recovered by evaporation of the solvent under reduced pressure and further dried in the vacuum oven for 72 h. The progress of the reaction was monitored by 1H NMR spectroscopy and SEC. Polymer Characterization. Nuclear Magnetic Resonance (NMR) Spectroscopy. For 1D NMR, 1H and 13C NMR spectra were recorded on a Varian 300 MHz NMR. The resonances of nondeuterated chloroform residues at 7.27 and 77.23 ppm were used as internal references. 1H NMR samples were prepared in 5 mm NMR tubes with 25−50 mg of polymer dissolved in 0.7 mL of NMR solvent. 13C NMR samples were prepared in 5 mm NMR tubes with 100 mg of polymer dissolved in 0.7 mL of NMR solvent and 10,000 scans with 5 s relaxation time were collected. Two-dimensional (2D) gradient heteronuclear multiple bond correlation (gHMBC) NMR experiments were performed at 25 °C, with 90° pulse widths of 5.625 and 15.0 μs for 1H and 13C respectively, on a Varian 750 MHz NMR instrument using triple resonance 1 H(13C/15N) pulse field gradient cryoprobe. The relaxation delay was 1 s and data was acquired without 13C decoupling using an acquisition

time of 0.216 s and a spectral width of 9470 Hz in the f2 dimension, and 16 transients were averaged for 256 t1 increments to provide a spectral window of 45223.3 Hz in the f1 dimension. The total experiment time was 3 h. The NMR data were processed with the ACD/Laboratories software. Raman Spectroscopy. The Raman spectra were accumulated over 50 s with a resolution of 4 cm−1 and excitation source with 534 nm radiation from a Bruker Optics GmbH laser with 50 μm aperture. Size Exclusion Chromatography (SEC). SEC measurements were performed using a system consisting of an Agilent 1260 Infinity Isocratic Pump, an Agilent 1260 Infinity variable wavelength detector (UV), a Wyatt OPTILAB T-rEX interferometric refractometer (RI), a Wyatt ViscoStar-II viscometer (VIS), a Wyatt DAWN HELIOS-II multiangle static light scattering detector (LS) with built-in WyattQELS dynamic light scattering module (QELS), an Agilent 1260 Infinity Standard Autosampler, and six StyragelVR columns (HR6, HR5, HR4, HR3, HR1, H0.5) thermostated at 35 °C. Tetrahydrofuran (THF) continuously distilled from CaH2 was used as the mobile phase at a flow rate of 1 mL/min. The results were analyzed by using the ASTRA 6 software (Wyatt Technology). Thermal Analysis. Differential scanning calorimetry (DSC) experiments were carried out on a TA Q2000 DSC using a heat−cool-heat thermal cycle with 10 °C/min in the temperature range from −150 to +130 °C under nitrogen. Glass transition temperatures (Tg) were calculated from the second heating cycle as the mean value between the onset and end point temperatures. Thermal gravimetric analysis (TGA) experiments were performed on a TA 5000 TGA instrument using a heating rate of 10 °C/min from room temperature to 600 °C under nitrogen. Analysis of the thermal data was performed using the Universal Analysis 2000 software.

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RESULTS AND DISCUSSION Synthesis of HS−PIB−SH. Asymmetric telechelic HO− PIB−(S)OH synthesized as reported earlier18 was subjected to CALB-catalyzed transesterification reaction with methyl-3mercaptopropionate to generate the dithiol macromonomer (HS−PIB−SH) as shown in Scheme 1. The progress of the reaction was monitored by 1H NMR spectroscopy. Figure 1 shows the 1H NMR spectra of HO−PIB− (S)OH (A) and HS−PIB−SH (B). The peaks for the methylene protons next to the hydroxyl groups in HO−PIB−(S)OH (n = 3.75, a1 and a2 = 3.50 and 3.35 ppm, Figure 1A) shifted downfield (n′ = 4.26 and a1′ and a2′ = 3.95 and 3.82 ppm, Figure 1B) after the transesterification reaction as the hydroxyl groups were replaced by more electronegative ester groups. Two new peaks were generated at r = 2.75 and s = 2.65 ppm, corresponding to the methylene protons next to the thiol and carboxyl groups, respectively. The (n′:a1′:a2′:(m + r):s = 2:1:1:6:4) ratio indicates quantitative functionalization. 1H NMR (Figure 2A) studies further disclosed that the two hydroxyl groups of the asymmetric HO−PIB−(S)OH displayed different reactivity toward esterification. One of the hydroxyl groups was introduced through the B

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Macromolecules Scheme 1. Synthesis of HS−PIB−SH

Figure 2. (A) Progress of the transesterification reaction followed by 1H NMR. (B) Thiol end group structures from HO-1 (i) and HO-2 (ii).

2).18 The hydroxyl group coming from the PE (HO-1) converted to the ester within 3 h, yielding structure i as shown in Figure 2B. However, the hydroxyl group introduced through the thiol− ene click reaction (HO-2) took an additional 3 h for completion, yielding structure ii (Figure 2B). This difference of reactivity is surprising since HO-1 appears to be more sterically hindered

propylene epoxide (PE) initiator during the synthesis of allyl− PIB−OH by carbocationic polymerization (HO-1), while the other was introduced through the UV assisted thiol−ene click reaction of mercaptoethanol to allyl−PIB−OH ((S)HO or HO-

Figure 1. 1H NMR spectra of (A) HO−PIB−(S)OH (n:a1:a2 = 2:1:1) and (B) HS−PIB−SH (Mn = 2300 g/mol, Mw/Mn= 1.3) (n′:a1′:a2′:(m + r):s = 2:1:1:6:4) (300 MHz; 5 s relaxation; 64 scans). C

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Figure 3. 13C NMR spectrum of (A) HO−PIB−(S)OH and (B) HS−PIB−SH (Mn = 2300 g/mol, Mw/Mn = 1.3, 5 s relaxation; 10 000 scans).

than HO-2. It should be noted here that the signals of the −SH protons, expected to appear around 1.5 ppm, are not visible due to overlapping with the methyl proton signals of the PIB chain at 1.45 ppm. Figure 3 shows the 13C NMR spectra of HO−PIB− (S)OH and HS−PIB−SH. A new peak at 171 ppm (T) in the NMR spectrum of HS−PIB−SH (Figure 3B) clearly shows the introduction of the carboxyl group from the transesterification reaction. Additionally, two new peaks were generated at 38.50 (S) and 19.80 (R) ppm corresponding to the methylene carbon next to the carboxyl and thiol groups, respectively. Thus, 13C NMR verified the presence of −SH end groups. R3P of HS−PIB−SH. HS−PIB−SH was polymerized exploiting the new reversible radical recombination polymerization (R3P) living system discovered in our laboratory (see Scheme 2).14−17 Rosenthal et al. had shown that the aliphatic dithiol, 2,2′-(ethylenedioxy)ethanedithiol (DODT) was successfully polymerized using triethylamine (TEA) catalyst and 3 wt % aqueous solution of H2O2 to high molecular weight disulfides.

Since PIB is not soluble in TEA hexane (Hx) was used as cosolvent. Table 1 shows the results of the scouting experiments. Since 3 wt % H2O2 did not result in molecular weight increase, the oxidative polymerizations were conducted with 30-wt % aqueous solution of H2O2, keeping other parameters constant. The temperature increase during polymerization was not significant, in contrast to that reported for DODT. Increasing the temperature and reaction time did not have an effect on the molecular weight. However, increasing the H2O2 excess to 6/1 relative to the −SH concentration led to substantial molecular weight increase (Mn = 96 000 g/mol, Table 1). Further increase of the H2O2 concentration did not increase the molecular weight. Figure 4 shows SEC traces of the HS−PIB−SH macromonomer and the last polyisobutylene disulfide (sample 6−30−25−15), demonstrating the molecular weight increase. The Mw/Mn values

Scheme 2. Synthesis of Polyisobutylene Disulfide

Figure 4. SEC traces of HS−PIB−SH and sample 6−30−25−15. D

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Figure 5. (A) 1H NMR spectrum of sample 6−30−25−15 in Table 1 (300 MHz; 5 s relaxation; 64 scans) (n′:r′:(m + s′) = 2:4:6) (B) 13C NMR spectrum of sample 6−30−25−15 in Table 1 (300 MHz; 5 s relaxation; 10,000 scans) (Mn = 96 000, Mw/Mn = 1.5).

Figure 6. 1H - 13C correlation HMBC spectra for (A) HS−PIB−SH (Mn = 3200 g/mol, Mw/Mn = 1.3) and (B) a polyisobutylene disulfide (Mn = 25 000 g/mol, Mw/Mn = 1.3).

methylene protons next to the thiol and carboxyl groups in the HS−PIB−SH macromonomer (Figure 1B) shifted downfield to 2.95 ppm (r′) and 2.75 ppm (s′) as disulfide groups are more electronegative than −SH. Further characterization of the polydisulfide was conducted through 13C NMR spectroscopy

are relatively low, similarly to that reported for the DODT system.14 Structural Characterization. Figure 5A shows the 1H NMR of the last polydisulfide sample in Table 1 (6−30−25−15). Two peaks at 2.75 (r) and 2.65 (s) ppm corresponding to the E

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Kinetic Study. A summary of the molecular weights and conversion data from a kinetic experiment is presented in Table 2. The conversion rate is very fast -65% conversion is reached

as shown in Figure 5B. The peak corresponding to the methylene carbon next to the thiol group (R) in the starting SH−PIB−SH macromonomer (19.80 ppm as shown in Figure 3B) shifted to 34.02 ppm (R′) (Figure 5B) corresponding to the methylene carbon next to the disulfide bond. The higher electronegativity of the disulfide group compared to that of the thiol is responsible for this downfield shift. However, the methylene carbon next to the carboxyl group (S) showed an upfield shift from 38.50 to 34.80 ppm (S′). Importantly, no carbon signals next to −SH were detected. However, the Mn = 96 000 g/mol may be too high for end group detection and the low molecular weight tailing in Figure 4 may point to the presence of some leftover dithiol not detected by NMR. In order to gain more insight into the structure of this polymer, 750 MHz 2D HMBC (1H−13C correlation spectroscopy) experiments were conducted on both the macromonomer (HS−PIB−SH) and a lower molecular weight polyisobutylene disulfide (Mn = 25 000 g/mol) as shown in Figure 6, parts A and B. The HMBC NMR technique is suitable for determining longrange 1H−13C connectivity (two or three bonds away).19−21 The HMBC spectrum of HS−PIB−SH shows a correlation signal at 1.48/38.22 ppm. This signal corresponds to the interaction between the thiol proton and the methylene carbon (S) next to the carboxyl group (three bonds away). However, no correlation signal was observed at 1.48/34.80 ppm in the HMBC NMR spectrum of the polyisobutylene disulfide (the S carbon signal shifted to 34.80 (S′) from 38.22 (S) ppm after the disulfide bond formation, see Figure 5B), indicating the absence of −SH end groups. The correlation signal at 1.48/30.96 ppm corresponding to the interaction between the methylene proton j (from the backbone) and methyl carbon I (from the side chain) can be observed in the HMBC spectra of both HS−PIB−SH and the polyisobutylene disulfide. The other correlation peaks observed in both spectra between (1.46/26.00−1.46/54.00) ppm and (1.29/26.00−1.29/54.00) ppm correspond to the interaction between the PIB backbone methylene protons and the side chain methyl carbons and PIB backbone carbons and methyl side group protons, respectively. Raman spectroscopic studies were also performed on HS− PIB−SH and polyisobutylene disulfide to further elucidate the structures of the macromonomer and the polymer (Figure 7).

Table 2. Kinetics of the R3P of HS−PIB−SH time (min)

Mn (g/mol)

Mw/Mn

% conversion

0 1 3 5 10 15

3200 25000 39000 40000 88000 95000

1.3 1.3 1.7 1.6 1.5 1.5

− 65.00 77.00 88.75 98.00 98.86

within the first 1 min and 99% conversion was obtained after 15 min. The conversion was determined by 1H NMR from the ratio of the methylene proton peaks next to the disulfide bond at 2.95 ppm (r′) (Figure 6A) to the methylene proton peak next to the ester group at 4.26 ppm (n′) which remains unchanged during the oxidative polymerization reaction. Figure 8 displays the semilogarithmic rate plot (A) and the Mn−conversion plot (B). The rate plot is very close to linear. On the basis of the proposed mechanism, shown in Figure 9 for HS−PIB−SH, this polymerization is governed by a dormant-active equilibrium: K

Pn ⇄ *Pn*

(1)

Assuming that under highly oxidizing conditions only rings are involved (Pn stands for rings and *Pn* are diradicals), the rate of the reaction can be written as follows: R p = −d[SH]/dt = d[S−S]/dt = kp[*P*]

(2)

Since [*P*] = K[S−S]

(3)

d[S−S]/dt = kpK[S−S]

(4)

After separation of the variables and integration we get −ln(1 − p) = kpKt

where p is the fractional conversion. With increasing conversion the Mn increases exponentially (Figure 8B), similarly to that reported for DODT.14,15 This indicates that as the reaction progresses smaller diradical chains can combine. Thermal Properties. The thermal properties of sample 6− 30−25−15 in Table 1 were determined and compared to that of HS−PIB−SH (Figure 10). The TGA decomposition traces are nearly identical (Figure 9A). The glass transition temperatures (Tg) of both HS−PIB−SH and polyisobutylene disulfide were determined exploiting DSC (Figure 10B). HS−PIB−SH showed a Tg at −69.91 °C while the polyisobutylene disulfide showed a Tg at −62.55 °C. SEC Analysis. Detailed SEC analysis was carried out to study the molecular conformation. Puskas et al. reported correlations between radii of gyration and hydrodynamic radii and moleculer weight for linear PIBs eq 5 and 6:25

Figure 7. Raman spectra of HS−PIB−SH (Mn = 3200 g/mol, Mw/Mn = 1.3) and polyisobutylene disulfide (Mn = 25 000 g/mol, Mw/Mn = 1.3).

A strong peak at 2580 cm−1 corresponding to −SH stretching of the terminal thiol group of SH−PIB−SH can be observed in the Raman spectrum of SH−PIB−SH.22−24 However, this peak is absent in the Raman spectrum of the polyisobutylene disulfide. The new peak at 500 cm−1 corresponds to the −S−S− stretching signal, confirming the formation of disulfide bonds. F

R g,i = 1.12x10−2xM i 0.598

(5)

R h,i = 1.64x10−2xM i 0.550

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Figure 8. (A) Semilogarithmic rate plot and (B) Mn vs conversion plots for the polymerization of HS−PIB−SH.

Figure 9. Proposed polymerization mechanism.

Figure 10. (A) TGA and (B) DSC traces of HS−PIB−SH (Mn = 3200 and Mw/Mn = 1.3) and polyisobutylene disulfide (sample 6−30−25−15 in Table 1, Mn = 96 000, Mw/Mn = 1.5). Heating rate =10 °C/min. Tg calculated from second cycle.

Rg, Rh, scaling factors (νg and νh), MHS a and intrinsic viscosity

The z-average radii versus measured Mw plots of linear PIB standards showed excellent agreement with the Ri − Mi plots obtained for individual SEC slices. The Rg data indicated very good solution behavior close to the exponent of νg = 0.588 predicted by renormalization theory. The Mark−Houwink− Sakurada (MHS) equation yielded K = 2.2 × 10−2 (mL/g) and a = 0.667 (scaling prediction gives a = 0.794).26 Table 3 compares

(ηw) with those measured for 6−30−25−15 in Table 1 (Mw = 140 000 g/mol, Mw/Mn = 1.5). It can be seen that both Rg and Rh are larger than the values measured for linear PIB. However, both scaling factors (νg and νh) are lower than those for linear PIBs. The ηw is also lower than that measured for a linear PIB. G

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Macromolecules Table 3. Selected Conformational Parameters in Comparison with Linear PIBs sample linear PIB −(PIB−S− S)n−

Rg (nm)

Rh (nm)

νg

νh

MHS α

ηw (mL/g)

13.4 18.0

11.1 14.3b

0.598 0.520

0.550 0.500b

0.667 0.525

59.6a 48.83

15.3c a

Scheme 3. Degradation of the Disulfide Bonds in the Presence of DTT

0.500c b

Mw = 140 000 g/mol, Mw/Mn = 1.1. Viscometry. cQELS.

Macrocycles? Earlier it has been reported that cyclic disulfide monomers can undergo ring-opening polymerization with the involvement of cyclic intermediates.2,27 Formation of interlocked polymer cycles (catenanes) were reported as well.27 Recently Rosenthal et al. demonstrated by NMR and MALDI that rings were involved in the R3P of DODT.14 Our data indicate that R3P of HS−PIB−SH yields macrocycles. Both HMBC and Raman spectroscopy confirmed the absence of −SH end groups in the polyisobutylene disulfides. DSC analysis corroborates the ring structure: PIB with Mn close to 100 000 g/mol reportedly has a Tg of −73 °C.,28,29 much lower than the Tg = −62.55 °C measured for sample 6−30−25−15 in Table 1. Cyclic polymers are reported to have higher Tg compared to their linear counterparts, due to lack of intermolecular interaction and reptation.30−34 SEC analysis also supports the formation of macrocycles. The decrease in νg relative to linear PIB is consistent with that reported for linear and ring polyethylenes (νg,L = 0.53 and νg,R = 0.45)35 and by computer simulation for polymer rings (νg,L = 0.5 and νg,R = 0.40).36,37 PIB is known to be a most unusual polymer38 so the anomaly of larger radii measured for the PIB−disulfide could be due to the stiffness of the PIB chains. On the basis of our data it is plausible to state that the R3P of HS−PIB−SH yields macrocycles following the mechanism shown for DODT by Rosenthal-Kim.14,15 The proposed mechanism is shown in Figure 9. The oxidative environment favors ring closure via radical recombination. We plan to study the melt rheology of the polymers to get more evidence for the macrocycle structure. Polymer Degradation. Polyisobutylene disulfide samples were treated with the reducing agent DTT to study the degradation reaction (Scheme 3). The initial degradation experiment was conducted exploiting a 4-fold excess of DTT per each disulfide bond present in sample 6−30−25−15 as seen in Table 1. No substantial reduction in the molecular weight was observed even after 72 h. DTT excess was increased to 5-fold and the degradation reaction was conducted keeping the other parameters constant. Samples were collected at regular time intervals and the progress of the degradation reaction was visualized by 1H NMR spectroscopy as shown in Figure 11. The peak at 2.95 ppm (peak corresponding to the methylene protons next to the disulfide bond) started disappearing while peaks at 2.75 ppm (peak corresponding to the methylene protons next to the thiol group) and 2.65 ppm (peak corresponding to the methylene protons next to the carboxyl group in HS−PIB−SH) started reappearing after 24 h (Figure 11), which confirmed the cleavage of the disulfide bonds and formation of the corresponding −SH. However, it was found that the reduction of sample 6−30− 25−15 in Table 1 was much slower than that of poly DODT under similar conditions.14 Figure 12 shows the SEC chromatograms of the degraded polymers and Table 4 summarizes the

Figure 11. Progress of the reduction of polyisobutylene disulfide on treatment with DTT.

Figure 12. SEC analysis of the degradation of polyisobutylene disulfide.

molecular weight data. A decrease in the molecular weight was obtained with increasing reaction time. After 48 h the SEC trace became bimodal, showing a shoulder at about 4000 g/mol. The starting HS−PIB−SH had Mn = 2300 g/mol, so this shoulder corresponds to a dimer. After 120 h of reaction Mn = 7900 g/mol was achieved, with peak molecular weights of 11 000 and 3600 g/ mol (pentamer-to-dimer mixture). To study if the degradation can progress further, DTT was added in multiple go to the reaction mixture in equal parts. In this experiment a total of 2.5 g of DTT was added in four equal parts H

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Table 4. SEC Data of the Degradation Study Mn (g/mol)

Mw/Mn

high

low

0 24 48 72 96 120

96 000 42 000 16 000 9300 (8800) 8000 (8300) 7900

1.5 1.6 1.6 1.2 1.2 1.2

220 000 88 000 28 000 13 000 (16 000) 13 000 (12 000) 11 000

− − 4000 4000 (4000) 4500 (4000) 3600

(0.6250 g each time) in 24 h time intervals keeping the other parameters constant. Aliquots were collected after 72 and 96 h and the molecular weights of the degraded polymer were determined by SEC (in parentheses in Table 4). This experiment also showed bimodal distributions with a lowest Mn = 8300 g/ mol with peak molecular weights of 12 000 and 4000 g/mol. Further characterization of this interesting polymerization system is ongoing in our laboratories.

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CONCLUSIONS This study reported the successful synthesis of thiol-functionalized PIB macromonomers HS−PIB−SH exploiting enzymecatalyzed transesterification. These thiol functionalized PIB macromonomers were further subjected to a green oxidative polymerization method (R3P). The oxidative polymerization method reported here is energy efficient as the reaction is conducted at room temperature. The optimization of the oxidative polymerization reaction shows that highest molecular weight can be achieved when the macromonomer is treated with 30 wt % H2O2 solution under constant bubbling of air at room temperature for 15 min. Increasing the temperature to 50 °C shows no additional improvement in the molecular weight. The polydisulfides were characterized by 1H, 13C, and 2D HMBC NMR along with Raman spectroscopy. The absence of thiol end groups indicates that the macromonomer likely forms a cyclic polymer. The Tg increase from the macromonomer to the polydisulfide, and SEC analysis also support this conclusion. In this study it is shown that the disulfide bonds can be cleaved by DTT.

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peak MWs (g/mol) time (h)

Article

AUTHOR INFORMATION

Corresponding Author

*(J.E.P.) E-mail: [email protected]. Telephone: 330-9726203. ORCID

Judit E. Puskas: 0000-0001-5282-5256 Author Contributions §

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. All authors contributed equally. Funding

This material is based upon work supported by the National Science Foundation, under Grant No. DMR-0804878, and the Rubber Division of the American Chemical Society. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Dr. Barbara Fowler and Dr. Venkat Dudipala for helping with the Raman and 2D HMBC spectroscopic studies. I

DOI: 10.1021/acs.macromol.6b02397 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.6b02397 Macromolecules XXXX, XXX, XXX−XXX