Enthalpy-Driven Polyisobutylene Depolymerization - Macromolecules

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Enthalpy-Driven Polyisobutylene Depolymerization Christopher B. Watson, Dustin Tan, and David E. Bergbreiter* Texas A&M University, Department of Chemistry, P.O. Box 30012, College Station, Texas 77842-3012, United States

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S Supporting Information *

ABSTRACT: Polyisobutylene (PIB) oligomers containing terminal alkene groups depolymerize rapidly at room temperature in the presence of trifluoromethanesulfonic acid and an arene solvent like benzene. This dramatically lower temperature depolymerization behavior is due to an enthalpically driven process wherein the isobutylene groups formed by a chain scission event after reacting a strong Brønsted acid with alkene groups at the polyisobutylene oligomer terminus are trapped by the solvent. In an arene solvent like benzene or toluene, tert-butyl carbocations formed from this isobutylene group rapidly react with benzene or toluene in a thermodynamically favorable process to form tert-butylarene products. This process is presumably favored in part because the Csp2−Csp3 bond that forms tertbutylarene is stronger than the Csp3−Csp3 bonds in the polyisobutylene polymer. These studies show that polyisobutylene oligomers depolymerize in only a few minutes at ambient or even sub-ambient temperature using this chemistry.

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INTRODUCTION While the chemistry involved in polymer formation is the most common focus of synthetic polymer chemistry, polymer depolymerization is also important.1 This is especially true for synthetic polymers and particularly true for polymers that are formed by reactions that generate carbon−carbon σ bonds from monomers containing reactive π bonds. The formation of a polyolefin like polyisobutylene (PIB) is a good example. Polyisobutylene is typically prepared by cationic polymerization.2 In this polymerization process, the favorable enthalpy (ΔH) of formation of a σ bond from a π bond overcomes an unfavorable entropy term (ΔS) to make the free energy term (ΔG) of the Gibbs equation negative. The result is a thermodynamically favorable polymerization process that yields a high-molecular-weight polymer provided that the temperature is not so high that the entropy term dominates. While that propitious state of affairs is advantageous in forming polyisobutylene, the alkane-like character of the bonds in the repeating units in the polyolefin chain also makes depolymerization unfavorable. The results of this study show how this situation can be reversed, and that a sufficiently strong acid, an appropriate solvent that can serve as a trapping agent for the monomer isobutylene, and a suitably reactive end group on polyisobutylene can lead to a thermodynamically favored depolymerization reaction. The studies described below detail the conditions under which this process can be made to work and show what types of end-functionalized polyisobutylene oligomers rapidly undergo an end group-dependent depolymerization in minutes even at 0 °C. Polyolefin formation including formation of polyisobutylene from isobutylene monomers is thermodynamically favored. However, polymers, in general, including polyisobutylene that have reactive end groups can lose monomer units and undergo depolymerization above a so-called ceiling temperature (Tc).1 At this temperature, the depolymerization process becomes © XXXX American Chemical Society

thermodynamically more favorable than polymerization. This occurs because the addition of a monomer to a reactive end group of a macromonomer is, to some extent, reversible. The value of Tc varies widely for different polymers and depends on the contrasting effects of ΔH and ΔS as discussed above. In the case of polyisobutylene, this Tc is relatively low because the carbon−carbon bonds linking one monomer to another are weaker than similar carbon−carbon bonds in other polyolefins like polyethylene. The estimates for the value of Tc for polyisobutylene vary from 88−120 °C.3,4 Other hydrocarbon polymers, like poly(α-methylstyrene), can have even lower Tc values. Low Tc values are not uncommon for other types of polymers if the bond energies for the π bond of the monomer and the monomer−monomer σ bonds in the product polymer are more comparable.5,6 In some cases, the inability to have an enthalpic driving force precludes polymerization. For example, the polymerization via ring-opening metathesis polymerization (ROMP) of a relatively unstrained alkene like cyclohexene is not possible because there is little difference in energy between the product polymer and the monomer cyclohexene. However, while polyisobutylene’s Tc is still much lower than the ca. 400 °C Tc reported for other polyolefins like polyethylene,2 its depolymerization, like that of other stable polymers, still requires an elevated temperature. Depolymerization of polymers, in general, or polyisobutylene specifically at milder temperatures would nonetheless be potentially useful as such a process could be used as a way to degrade polymers that do not biodegrade readily. While the ceiling temperature (Tc) of polymers is a thermodynamic parameter that cannot be reduced, facile depolymerization like that shown in this paper where an arene solvent captures a Received: February 12, 2019 Revised: March 17, 2019

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DOI: 10.1021/acs.macromol.9b00313 Macromolecules XXXX, XXX, XXX−XXX

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benzene at room temperature or 80 °C afforded a viscous residue of alkene-terminated polyisobutylene, a similar workup of a room-temperature CF3SO3H-catalyzed reaction of PIB1000 with benzene as a solvent afforded virtually no polyisobutylene residue. Similar results were seen if toluene was used as the solvent. These unexpected results wherein the polyisobutylene appeared to have degraded suggested that the effective temperature required for polyisobutylene depolymerization in the CF3SO3H experiment had dropped to room temperature or below. To confirm this supposition, qualitative 1H NMR spectroscopy experiments were carried out, which compared the integral of the signals for the methylene proton of polyisobutylene at 1.4 δ with the integral of a signal at 1.3 δ that corresponded to the chemical shift for the methyl groups of tert-butylbenzene (or 4-tert-butyltoluene). In a typical procedure with these arene solvents, PIB1000 that contained an alkene terminal group was allowed to react with CF3SO3H (2.2 equiv/vinyl group of the polyisobutylene) in toluene or benzene at ambient temperature for 20 h. The reaction mixture was then quenched, and the arene solvent was removed at a reduced pressure. The residue was then analyzed by 1H NMR spectroscopy. These studies showed that the alkene proton signals for the polyisobutylene largely disappeared. This decrease in intensity for the signal at 1.4 δ that corresponds to the methylene group in the starting polyisobutylene was accompanied by an increase in the intensity of the methyl signals at 1.3 δ for tert-butyl methyl groups. This same experiment was then carried out with PIB1000 derivatives containing ketone, acid, iodide, alcohol, phenol, anisole, and alkene terminal groups in toluene. Depolymerization was only seen for arene-terminated and alkene-terminated polyisobutylene derivatives as shown in Figure 1 based on the lack of

monomer by a thermodynamically favorable Friedel−Crafts reaction is an alternative way to effect facile depolymerization. While the study below uses an acid to generate carbocations that in turn generate monomers that can be chemically captured, similar chemistry might also be effected by using other mechanical processes that generate appropriate functional end groups, chemically by end group modification or by other chain scission events.

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RESULTS AND DISCUSSION Our initial studies on polyisobutylene chemistry in strong acids were focused on the synthesis of aryl-terminated polyisobutylene ligands and reagents for applications in catalysis, synthesis, and separation chemistry.7−9 Others have shown that polyisobutylene with tertiary chloride end groups reacts with arenes in the presence of Lewis acids like aluminum chloride to produce phenyl- or tolyl-terminated polyisobutylenes. 10 However, our initial studies using polyisobutylene with alkene groups as substrates in H2SO4-catalyzed electrophilic aromatic substitution failed with arenes like benzene or toluene.11 No aryl-terminated products were seen at room temperature, and using H2SO4 and an alkene-terminated polyisobutylene substrate at elevated temperature did not facilitate the reaction. The only electrophilic aromatic substitution products produced were small amounts of tert-butylbenzene or 4-tertbutyltoluene. Presumably, these products formed from isobutylene formed in situ that produced electrophilic tertbutyl cations. This result contrasted with the results we and others have seen in electrophilic aromatic substitution of arenes with electron-donating groups using alkene-terminated polyisobutylene. Specifically, we and others have reported that alkene-terminated polyisobutylene oligomers readily undergo electrophilic aromatic substitution with electron-rich arenes like phenol, anisole, catechol at room temperature using sulfuric acid or at elevated temperature with 2,5-dialkylanilines using AlCl3.7,8,12−15 These same studies also showed that polyisobutylene terminated with an electron-rich aryl derivative like phenol is in equilibrium with a cationic polyisobutylene intermediate in the presence of sulfuric acid since phenol-terminated polyisobutylene exchanges its aryl group with anisole when a phenol-terminated polyisobutylene is allowed to react with H2SO4 catalyst in the presence of excess anisole. While these results suggest that Brønsted acids like H2SO4 or CH3SO3H can generate carbocations from suitably functionalized polyisobutylene oligomers, our results showed that stoichiometric amounts of less reactive arenes like benzene or toluene did not readily form phenyl- or tolyl-terminated polyisobutylene in useful yields from electrophilic polyisobutylene intermediates, at least, under our conditions. In the later work, we sought to address the failure of H2SO4 or CH3SO3H to promote acid-catalyzed electrophilic aromatic substitution of benzene by an alkene-terminated polyisobutylene by using a much stronger acid, CF3SO3H, at 25 °C in benzene or toluene as a solvent. Our hope was that this stronger Brønsted acid CF3SO3H would produce a higher concentration of carbocationic-terminated polyisobutylene and that the excess arene solvent would then react with these electrophilic oligomers to form phenyl- or tolyl-terminated polyisobutylene. Since room temperature would be well below the Tc, we would avoid the depolymerization that complicated elevated temperature reactions. However, while an earlier unsuccessful H2SO4-catalyzed reaction of polyisobutylene (PIB1000, 1000 Da alkene-terminated polyisobutylene) with

Figure 1. Qualitative 1H NMR spectroscopic studies showing the extent of depolymerization of polyisobutylene oligomers with various end groups in toluene in the presence of 2.2 equiv of CF3SO3H at 25 °C for 20 h.

disappearance of signals for the −CH2− signals for terminally functionalized polyisobutylene oligomers containing −CH2COCH3, −CH2CO2H, −CH2I, and −CH2OH terminal groups. Analysis of the product residue after solvent removal in 1H NMR spectroscopy experiments that used benzene or toluene showed that most of the initial polyisobutylene had depolymerized and the products were 4-tert-butyltoluene in toluene or a mixture of tert-butylbenzene and 1,4-di-tertbutylbenzene if benzene was the solvent. tert-Butylbenzene was B

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reduced temperature (Figure 3). In these cases, we used a relatively concentrated solution containing 1 g of PIB1000 in 6

the major product in benzene. Experiments carried out using PIB1000 or PIB450 (450 Da alkene-terminated polyisobutylene) that contained an alkene terminal group using CF3SO3H in dichloromethane in the absence of an arene showed that little or no polyisobutylene depolymerization occurredthe only change observed was an increase in the trisubstituted so-called β alkene-terminated end group relative to the disubstituted or α end group. While the 1H NMR spectroscopy results were consistent with the initial qualitative observation that the polyisobutylene depolymerized in the presence of CF3SO3H in toluene or benzene to form tert-butylated arene derivatives, these 1H NMR spectroscopic studies were, at best, qualitative measures of the extent of polyisobutylene depolymerization. Thus, further studies used GPC analyses in two sorts of experiments. In the first set of experiments using GPC analysis, a solution of PIB1000 was allowed to react with 2.2 equiv of CF3SO3H in benzene, toluene, or anisole at 25 °C for 20 h. Then, a sample of the reaction solution was passed through a short column of silica gel to remove CF3SO3H. The filtrate was then concentrated at a reduced pressure to remove the arene solvent, and the residue was dissolved in THF and analyzed by GPC. These studies shown in Figure 2 showed that PIB1000

Figure 3. GPC analyses of alkene-terminated PIB1000 after reacting with 2.2 equiv of CF3SO3H in benzene at 0 °C for 0 min (orange line), 5 min (purple line), and 60 min (green line).

mL of benzene with 2.2 equiv of CF3SO3H. The reactions were carried out in an ice bath at 0 °C. Depolymerization of PIB1000 using 1 equiv of CF3SO3H also occurred with the same major products being formed. The depolymerization was rapid even under these sub-ambient temperature conditions. These GPC experiments showed that three products formed. The major products with a 10 and 10.5 min elution time were 1,4-di-tertbutylbenzene (“b”) and tert-butylbenzene (“c”), respectively. The identity of the major products was confirmed by 1H NMR, 13C NMR, and mass spectroscopy. There was also a minor product at a 9.6 min retention time labeled “a” in Figure 3, which can also be seen in Figure 2. Based on mass spectroscopy and 1H NMR spectroscopy, this product was identified as 1,1,2,3-tetramethylbutylbenzene and is presumably formed as a result of the rearrangement process shown in Scheme 1. In this process, the tertiary 2,4,4-trimethyl Scheme 1. Rearrangements That Could Convert the Tertiary 2,4,4-Trimethyl Carbocation 1 into A Structurally Isomeric 2,3,4-Trimethylpentyl Tertiary Carbocation 4 That Could Form The Minor Product 1,1,2,3Tetramethylbutylbenzene

Figure 2. GPC analysis of PIB1000 starting material and products from the reaction of alkene-terminated PIB1000 with CF3SO3H in benzene, toluene, and anisole showing the extent of depolymerization of polyisobutylene at 25 °C for 20 h.

oligomers were depolymerized in both benzene and toluene over 20 h at 25 °C. Little or no depolymerization occurred in anisole. Similar results were seen with toluene or with alkeneterminated PIB450 and PIB2300 oligomers. The lack of depolymerization in anisole as a solvent is presumably due to a solvent leveling effect wherein protonated anisole, a weaker acid than protonated benzene or toluene, forms. We also observed no depolymerization in nitrobenzene, a result we attribute to the known low-reactivity of this arene in Friedel−Crafts alkylation. We also examined the reaction of PIB1000 with weaker Brønsted acids like H2SO4 or CH3SO3H at 25 °C. While these acids could be used to functionalize PIB with reactive arenes like phenol, anisole, or catechol, no significant depolymerization at 25 °C was seen in benzene or toluene with these weaker Brønsted acids by GPC analysis. GPC analyses were also used to follow the time course of the depolymerization of PIB1000 in benzene with CF3SO3H at

carbocation 1 could isomerize to form a secondary carbocation (2) or a corner-protonated cyclopropane intermediate (3) that could in turn form a 2,3,4-trimethylpentyl tertiary carbocation 4 that is structurally isomeric with 1. The reaction of this cation 4 with benzene would lead to the small amount of product “a”. We only saw evidence for this rearrangement process on the eight-carbon cation 1. If it had occurred on C

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Macromolecules Scheme 2. Triflic Acid-Promoted Reaction of Alkene-Terminated PIB1000 in the Presence of Benzene To Form tertButylbenzenea

a

(a) Reaction via the equilibrium formation of some isobutylene by the depolymerization of the polyisobutyl cation and (b) reaction of this isobutylene with a Brønsted acid to form tert-butyl carbenium ions that, in a more enthalpically favorable step, form tert-butylbenzene by Friedel− Crafts chemistry.

in the liquid state and found that the overall ΔH for the formation of tert-butylbenzene from benzene and polyisobutylene is −7.3 kcal/mol.16−18 Other calculations using the literature values for ΔHfo for isobutylene, benzene, tertbutylbenzene and the heat of polymerization of polyisobutylene afforded similar overall ΔH values for tert-butylbenzene formation from polyisobutylene and benzene (see Supporting Information for the details of these calculations). We made no adjustments for pressure in these calculations. Direct GPC analyses of reaction mixtures without solvent removal were also carried out. In these cases, a solution of 1.0 g of PIB1000 was allowed to react with 11 equiv of CF3SO3H in benzene or toluene for 20 h. The only workup before GPC analysis involved passing the solution of the reaction mixture through a short silica plug in a disposable pipette to remove triflic acid prior to GPC analysis. GPC analyses of these reaction mixtures (Figure 4) showed that vinyl-terminated

larger oligomers, then we would have expected to see other products of intermediate size in the GPC experiments. While we have not investigated this chemistry in detail, we speculate that the absence of rearrangements like this in larger cationic oligomers may be due to this isomerization proceeding via a corner-protonated cyclopropane intermediate instead of a secondary carbocation. Such corner-protonated cyclopropane intermediates that might have formed from cationic oligomers with three or more isobutyl groups would be expected to be more sterically strained because the isopropyl group cis to the methyl group in 4 would have other alkyl groups larger than the methyl or isopropyl in a similar cis arrangement. The relative amounts of 1,4-di-tert-butylbenzene and tertbutylbenzene that appeared to form varied. We believe this is mostly due to the workup and that the predominance of the 1,4-di-tert-butylbenzene product seen in some GPC analyses likely reflects losses of the more volatile tert-butylbenzene in the evaporative workup. However, the formation of significant amounts of 1,4-di-tert-butylbenzene was seen even where there was no evaporative workup (vide infra), a result that is consistent with the relative reactivity of tert-butylbenzene and benzene in Friedel−Crafts chemistry. The chromatograms also always contained a small amount of residue due to the ca. 5% hydrogenated PIB1000 present in the starting polyisobutylene oligomers as received from TPC. In separate experiments, we showed that hydrogenated PIB1000 prepared from alkene-terminated PIB1000 using a Pd/C catalyst and hydrogen does not significantly degrade on the treatment with 2.2 equiv of CF3SO3H in benzene for 20 h at 25 °C based on either GPC or 1H NMR spectroscopy. Depolymerization at a Tc is most commonly rationalized in terms of the counteracting effects of unfavorable entropy (ΔS) and a favorable enthalpy (ΔH) on the free energy (ΔG). While we make analogies to this in our introduction, the depolymerization of PIB in benzene or toluene promoted by triflic acid is actually not an entropy driven process. The depolymerization process described above is instead driven by a favorable enthalpy change for the overall reaction shown in Scheme 2; where the change in entropy going from a collection of solvent molecules and a single polymer molecule to a collection of an essentially equivalent number of tert-butylated solvent molecules is unlikely to be large in either a positive or negative sense. However, this overall reaction can be favorable if the ΔH is favorable. While the observed results above are empirical evidence for this hypothesis, we also used the existing literature thermodynamic data to confirm if the ΔH is indeed likely to be favorable. To do this, we calculated the ΔH for the reaction shown in eq 1 using literature values for the heat of formation of tert-butylbenzene, benzene, and polyisobutylene

Figure 4. GPC analyses carried out without an evaporative removal of solvent showing the effect of the terminal group in attempted depolymerization of PIB1000 derivatives after 20 h treatment with 11 equiv of CF3SO3H in toluene (solid lines) or benzene (dotted lines) at 25 °C.

polyisobutylene depolymerized in benzene at 25 °C after 20 h using 11 equiv of CF3SO3H. The major product was tertbutylbenzene or 4-tert-butyltoluene. The effect of end group on this depolymerization was also analyzed by GPC without an evaporative workup (Figure 4). As was seen in the initial 1H NMR spectroscopy experiments, polyisobutylene oligomers with carboxylic acid, primary iodide, or methyl ketone end groups did not depolymerize over 20 h with 2 equiv of CF3SO3H. However, polyisobutylene with D

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alcohol, phenol, catechol, or anisole end groups did depolymerize when benzene or toluene was used as the solvent in the presence of 11 equiv of CF3SO3H. The experiments above used a large excess of benzene as a solvent to trap the tert-butyl cations formed from the isobutylene monomer generated by an end group chain scission event. We also examined depolymerization using lower amounts of benzene. Depolymerization in these cases was similarly effective as shown in Figure 5. In these cases, the

Article

EXPERIMENTAL SECTION

1

H and 13C NMR spectra were recorded on an INOVA spectrometer at 300 and 75 MHz in CDCl3. All GPC analyses were carried out on reaction samples that were first passed through a flash column of SiO2 to remove triflic acid. After removal of triflic acid, the product solutions were either concentrated on a rotary evaporator at a reduced pressure or were used directly. In all cases, the samples were dissolved in THF (25 mg per mL) for gel permeation chromatography (GPC) analysis. GPC data were obtained on a Viscotek VE GPC system with an Agilent Resipore column using an elution rate of 1.0 mL per min. Ozonolysis was carried out using a Welbach ozonolysis apparatus. PIB450, PIB1000, and PIB2300 alkene was generously donated by Texas Polymer Corporation.19 All other chemicals were purchased from commercial sources and used as received. NMR Spectroscopic Studies of Depolymerization of Terminally Functionalized Polyisobutylene Oligomers. In the general procedure for initial depolymerization studies in toluene using 1 H NMR spectroscopy, the PIB1000 substrate (1 g, 1 mmol) was dissolved in 6 mL of toluene. Then (0.2 mL, 2.2 mmol) of triflic acid was added, and the resulting mixture was allowed to stir for 20 h at 25 °C. A 1 mL aliquot of the solution was then removed and passed through a silica-packed glass pipet to remove triflic acid. The eluent was then concentrated using a rotary evaporator at a reduced pressure, and the residue was dissolved in CDCl3 for 1H spectroscopic analysis. The extent of depolymerization was analyzed using the integral of the signal for the tert-butyl group of 4-tert-butyltoluene at 1.3 ppm that was seen when depolymerization occurred. When the integral of the residual poly(isobutylene methylene) protons in the isobutylene repeating units due to the unreacted PIB1000 derivative at 1.42 ppm was calibrated to 2, the percent conversion to 4-tert-butyltoluene that is shown in Figure 1 could be calculated using eq 1.

Figure 5. GPC analyses of alkene-terminated PIB1000 after reacting with 2.2 equiv of CF3SO3H and 0.1 or 0.2 equiv of benzene at 25 °C.

reaction of PIB1000 with a 1/10 or 1/20 molar ratio of PIB1000/ benzene still led to PIB depolymerization with the only difference being the formation of more di-tert-butylbenzene in the experiment that used a 1/10 molar ratio of PIB1000/ benzene versus experiments that used larger amounts of benzene.

%conversion =

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CONCLUSIONS The results above show that polyisobutylene depolymerization can be driven to completion enthalpically using arenes like benzene or toluene as solvents at temperatures substantially below polyisobutylene’s ceiling temperature using CF3SO3H as an acid. This depolymerization can be complete in as little as 5 min at 0 °C based on GPC analysis. The major products in benzene are tert-butylbenzene and 1,4-di-tert-butylbenzene in benzene and 4-tert-butyltoluene in toluene. A small amount of 1,1,2,3-tetramethylbutylbenzene was also formed in depolymerizations conducted in benzene. The extent of depolymerization and whether depolymerization even occurs depends on the end group of polyisobutylene and the nature of arene solvent. No depolymerization occurs in nitrobenzene. While anisole can be used to form 4-polyisobutyl-anisole, polyisobutylene depolymerization was not seen using anisole. Polyisobutylene-containing end groups that do not readily form a stable tertiary carbocation either did not readily depolymerize under these conditions or only underwent depolymerization with larger excesses of CF3SO3H. Polyisobutylene does not depolymerize to any significant extent using weaker Brønsted acids like H2SO4 or CH3SO3H at room temperature in benzene or toluene. These results show that using a solvent that traps a reactive intermediate that is derived from monomer by forming more stable bonds, in this case a σ bond that is stronger than the σ bonds in the polymer, provides an alternative way to depolymerize a polymer and are, in effect, a way to lower a polymer’s Tc. This same effect could presumably be extended to other materials if reactive groups at a polymer’s terminus produce monomers that can be trapped by a solvent or other suitable reagent.

(integral at 1.3 ppm)*100 9 + (integral at 1.3 ppm)

(1)

GPC Studies of Low-Temperature Depolymerization Reactions. PIB1000 alkene (1.0 g, 1 mmol) was dissolved in benzene (6 mL) and placed in an ice bath. After being allowed to cool (some benzene freezing occurred), triflic acid (0.2 mL, 2.2 mmol) was added, and aliquots were removed at noted times and passed through silica as described above. The aliquots were then reduced via a reduced pressure and diluted with THF to 25 mg/mL, and GPC data were acquired. GPC Studies of Depolymerization of Polyisobutylene Oligomers with Different End Groups. Functionalized PIB1000 (1.0 g, 1 mmol) was dissolved in benzene or toluene (6 mL), and triflic acid (1.0 mL, 11 mmol) was added. This mixture was allowed to stir for 20 h at room temperature. A 1 mL aliquot was then taken and passed through silica as described above. This sample was then diluted with THF to 25 mg/mL, and GPC data shown in Figure 4 were acquired. GPC Studies of Depolymerization of PIB1000 Alkene in Concentrated Benzene Solutions. PIB1000 alkene (1.0 g, 1 mmol) was added with benzene (either 0.8 mL, 9 mmol or 1.6 mL, 18 mmol) and triflic acid (0.2 mL, 2.2 mmol). This was then stirred for 18 h at 25 °C, passed through a silica plug, and diluted in THF to 25 mg/mL, and GPC data shown in Figure 5 were acquired. Synthetic Procedures. These procedures are analogous to the procedures described in a prior work and the afforded products whose spectra were in agreement with those obtained in prior reports.12,20 PIB1000 Alcohol. PIB1000 alkene (50 g, 50 mmol) was dissolved in 100 mL of hexane. Borane dimethyl sulfide solution (8.5 mL, 85 mmol) was added, and the resulting solution was stirred for 24 h. The reaction flask was then cooled to 0 °C, and 40 mL of ethanol was added. Twelve milliliters of 4 N NaOH (aq) was then added followed by dropwise addition of 8 mL of 35% hydrogen peroxide. The reaction mixture was stirred for 2 h, and then 100 mL of water was added. The organic phase was then separated and washed with water and brine. The organic phase was then dried with sodium sulfate and concentrated under a reduced pressure. Forty-five grams (90%) of E

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Macromolecules PIB1000 alcohol was obtained (residual hexanes were present): 1H NMR: 0.87−1.5 (m, 95H), 3.33 (dd, J = 6.9, 10 Hz, 1H), 3.50 (dd, J = 6, 10 Hz, 1H). PIB1000 Iodide. PIB1000 alcohol (10 g, 10 mmol), imidazole (1 g, 15 mmol), iodine (3.5 g, 13 mmol), and triphenylphosphine (3.5 g, 13 mmol) were dissolved in 100 mL of dichloromethane and stirred overnight. The reaction solution was concentrated under a reduced pressure, redissolved in hexanes, and washed with DMF and aqueous ethanol. The organic phase is then dried with sodium sulfate and concentrated under a reduced pressure to give 9 g (90%) of PIB1000 terminated with an iodo group (residual hexanes were present): 1H NMR: 0.87−1.5 (m, 240H), 3.15 (dd, J = 6.9, 9.7 Hz, 1H), 3.28 (dd, J = 3.6, 9.7 Hz, 1H). PIB1000 Methyl Ketone. PIB1000 alkene (11 g, 11 mmol) was dissolved in 50 mL of pentane and cooled to −78 °C. Ozone was then bubbled through the solution until it turned blue. Then, excess tributylphosphine (4.5 mL) was added, and the mixture was stirred for 3 h. The solution was then washed with aqueous ethanol, dried with sodium sulfate, and concentrated under a reduced pressure to give 9.8 g (89%) of PIB1000 terminated with a methyl ketone group (residual pentane was present): 1H NMR: 0.87−1.5 (m, 205H), 2.16 (s, 1H), 2.48 (s, 1H). PIB1000 Carboxylic Acid. PIB1000 methyl ketone (6.9 g, 6.9 mmol) was dissolved in 200 mL of THF. Iodine (6.9 g, 54 mmol) was added. Potassium hydroxide (25 g, 440 mmol) was dissolved in 200 mL of water. Tetrabutylammonium bromide (1.6 g, 5 mmol) was then added, and the resulting mixture was allowed to stir overnight. At that point, the reaction solution was concentrated under a reduced pressure, dissolved in hexanes, and washed with DMF, aqueous ethanol, and water. The organic phase was then dried with sodium sulfate and concentrated under a reduced pressure to give 5.7 g (83%) of the PIB1000 carboxylic acid-terminated oligomer product (residual hexanes were present): 1H NMR: 0.86−1.5 (m, 334H), 2.34 (s, 2H). PIB1000 Phenol. PIB1000 alkene (10 g, 10 mmol) and phenol (19 g, 220 mmol) were dissolved in 100 mL of dichloromethane. The solution was cooled to 0 °C, and 6 mL of concentrated sulfuric acid was added slowly. The solution was then stirred overnight and concentrated under a reduced pressure, and the residue was redissolved in 300 mL of hexanes. The organic solution was then washed with aqueous ethanol and dried with sodium sulfate to give 9 g (90%) of a PIB1000 oligomer product terminated with a 4hydroxyphenyl (phenol) group (residual hexanes were present): 1H NMR: 0.86−1.5 (m, 334H), 6.77 (d, J = 8.4 Hz, 2H), 7.24 (d, J = 8.4 Hz, 2H). PIB1000 Anisole. PIB1000 alkene (15 g, 15 mmol) was dissolved in 32 g of anisole. Nine milliliters of concentrated sulfuric acid was added, and the reaction solution was stirred overnight. The solution was then washed with aqueous 1 N NaOH, extracted with hexanes, and washed three times with acetonitrile. The organic phase was then dried with sodium sulfate and concentrated under a reduced pressure to give 16 g (>100%) of PIB1000 oligomer terminated with a 4-methoxyphenyl group (residual hexanes were present): 1H NMR: 0.86−1.5 (m, 209H), 1.82 (s, 2H), 1.86 (s, 6H), 3.82 (s, 3H), 6.85 (d, J = 9.0 Hz, 2H), 7.29 (d, J = 9.0 Hz, 2H).

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ORCID

David E. Bergbreiter: 0000-0002-1657-0003 Author Contributions

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support of this research from the National Science Foundation (CHE-1362735) and Robert A. Welch Foundation (Grant A0639) is gratefully acknowledged. We are also grateful to TPC Inc. (Houston, TX) for donating the PIB450, PIB1000, and PIB2300 materials used in these studies.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00313.

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REFERENCES

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Additional GPC data, NMR spectra, and other calculations of the enthalpy of the depolymerization process (PDF)

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DOI: 10.1021/acs.macromol.9b00313 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.9b00313 Macromolecules XXXX, XXX, XXX−XXX