Volume 50 Number 38 18 May 2014 Pages 4863–4960
ChemComm Chemical Communications www.rsc.org/chemcomm
ISSN 1359-7345
COMMUNICATION Nicholas J. Robertson et al. Controlled hydrogenative depolymerization of polyesters and polycarbonates catalyzed by ruthenium(II) PNN pincer complexes
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Cite this: Chem. Commun., 2014, 50, 4884 Received 21st January 2014, Accepted 7th March 2014 DOI: 10.1039/c4cc00541d
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Controlled hydrogenative depolymerization of polyesters and polycarbonates catalyzed by ruthenium(II) PNN pincer complexes† Eric M. Krall,a Tyler W. Klein,a Ryan J. Andersen,a Alex J. Nett,b Ryley W. Glasgow,b Diana S. Reader,a Brian C. Dauphinais,a Sean P. Mc Ilrath,a Anne A. Fischer,b Michael J. Carney,b Dylan J. Hudsona and Nicholas J. Robertson*a
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Ruthenium(II) PNN complexes depolymerize many polyesters into diols and polycarbonates into glycols plus methanol via hydrogenation. Notably, polyesters with two methylene units between ester linkages depolymerize to carboxylic acids rather than diols. This methodology represents a new approach for producing useful chemicals from waste plastics.
Recent efforts to mitigate the environmental impact of plastics have focused on two main areas: reducing the use of petroleum-based feedstocks and decreasing the large volumes of plastics currently heading to landfills.1 For the former, new synthetic routes have been developed to produce biorenewable monomers used to prepare well-established commodity polymers. For example, Coca-Cola’s polyethylene terephthalate (PET) PlantBottlet uses ethylene glycol (EG) derived from sugarcane. In addition, new biorenewable and biodegradable polymers that can replace traditional plastics are under development, with polylactic acid (PLA, from NatureWorks LLC) being a noteworthy alternative to petroleum derived polymers.2 However, high costs associated with biorenewable pathways currently limit widespread use of these materials.2a For the latter, most polymer recycling is currently accomplished through melting and re-molding post-consumer plastic. Unfortunately this route often produces inferior polymer compared to virgin resin due to impurities (e.g., pigments, additives, other polymers, catalyst residues) in post-consumer material.3 A smaller but potentially more promising route involves recycling plastics through controlled depolymerization.1b Depolymerization yields the polymer’s monomers, which can be purified and repolymerized to form virgin resin or used for other synthetic purposes. Current depolymerization methods, such as ester hydrolysis, a
Northland College, 1411 Ellis Ave., Ashland, Wisconsin, 54806, USA. E-mail: [email protected]; Tel: +1 715 682 1321 b University of Wisconsin-Eau Claire, 105 Garfield Ave., Eau Claire, Wisconsin 54701, USA † Electronic supplementary information (ESI) available: Experimental details and 1 H and 13C NMR spectra for the depolymerizations of entries 1–8, Table 1. See DOI: 10.1039/c4cc00541d
4884 | Chem. Commun., 2014, 50, 4884--4887
transesterification, olefin metathesis and retro Diels–Alder reactions,3,4 are typically not cost competitive because the resulting monomers are currently more cheaply derived from petroleum than through depolymerization.3c Thus, alternative depolymerization technologies are needed, including those with more efficient catalysts or that can produce value-added chemicals from post-consumer polymeric materials, such as PET.3b Recently reported ruthenium-based catalysts offer this sort of potential. The Milstein catalyst (2, Scheme 1) is a remarkably versatile catalyst for the dehydrogenative coupling of alcohols to form esters, as well as the coupling of alcohols and amines to afford amides.5 More recently, we and others used 2 to prepare high molecular weight polymers, despite proceeding via a stepgrowth polymerization mechanism, by removal of the hydrogen
Scheme 1 Catalyst precursors (1 and 3) are activated in situ using KOtBu generating Ru(II) PNN catalysts (2 and 4). Hydrogenation of polyesters using Ru(II) PNN catalysts produces diols (A) and hydrogenation of polycarbonates with Ru(II) PNN catalysts yields glycols and methanol (B). Polyesters with two methylene units between esters furnish carboxylic acids. R includes aliphatic or aromatic segments for polyesters and aliphatic segments for polycarbonates.
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Communication Table 1
Catalytic hydrogenation of polyesters and polycarbonates with ruthenium(II) PNN pincer complexesa
Catalyst
Loadingb
Solvent
p(H2)/atm
Temp. (1C)
1
2
100 : 1 : 1
Anisole
13.6
120
80e
2
4
50 : 1 : 1
THF
47.6
120
93
3
4
50 : 1 : 2
Anisole/THF
54.4
160
499
4
4
50 : 1 : 2
Anisole/THF
54.4
160
499
5f
2 or 4
100 : 1 : 2
Anisole/THF
54.4
160
499
6f
2 or 4
100 : 1 : 2
Anisole/THF
54.4
160
91
7
4
50 : 1 : 2
Anisole/THF
54.4
160
88
8
4
50 : 1 : 2
Anisole/THF
54.4
160
90
Entry
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Polymer
Observed product(s)c
% Conv.d
a Catalyst precursor activated with KOtBu for 5 min. under N2 before transferring to a pressure reactor containing polymer. The reaction mixture was pressured and heated to target temperature for 48 h. b Loading ratio is polymer repeat units : catalyst precursor : KOtBu. c Products were determined by 1H and 13C NMR and no starting material or other products were observed. d % Conversions were calculated by 1H NMR signal integrations referenced to an internal CH2Cl2 standard. e Isolated yield. f Reaction time of 24 h.
byproduct to drive the reaction to high conversion.6 Under appropriate conditions, the catalytic cycle can be reversed and has been used to effectively hydrogenate esters to their corresponding alcohols through the addition of hydrogen gas to the reaction.7 Recent results have also shown that the catalyst can efficiently hydrogenate organic carbonates to methanol and the corresponding diols.8 We were intrigued by this reversibility as a potential means to controllably depolymerize polyesters to diols and polycarbonates to glycols and methanol (Scheme 1, A and B, respectively). We first sought to depolymerize a series of linear aliphatic polyesters that we previously synthesized through in vacuo dehydrogenation polymerization of diols using 2 generated in situ by deprotonation of 1 via potassium tert-butoxide (Scheme 1).6b These polyesters were readily depolymerized back to their corresponding a,o-diols when exposed to 13.6 atm hydrogen gas for 48 h at 120 1C in the presence of 2. 1H and 13 C NMR analysis of the crude reaction mixtures showed complete depolymerization. Upon cooling and releasing the hydrogen gas,
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the diols crystallized from the reaction mixture and were isolated with simple filtration. Using this process, we obtained an 80% isolated yield of 1,10-decanediol (Table 1, entry 1). However, the same conditions proved ineffective for the hydrogenation of caprolactone to 1,6-hexanediol. Recently, Ru(II) PNN complex 4 (Scheme 1) in tetrahydrofuran (THF) was successfully used by Milstein and coworkers for the hydrogenation of lactide to propylene glycol;7a we therefore thought 4 may be a more effective catalyst for the hydrogenation of caprolactone. Indeed, 4 was able to completely hydrogenate caprolactone to 1,6-hexanediol with 47.6 atm H2 (Table 1, entry 2). We suspect that the hydrogenation mechanism is the same as that proposed by Milstein and coworkers for ester hydrogenation using 2.7c Given our initial successes, we turned our attention to using 2 and 4 to controllably depolymerize a variety of polyesters and polycarbonates. Most efforts to depolymerize PET have employed glycolysis or solvolysis to the corresponding terephthalates.3c,9 With less than 30% of PET materials being recycled in the United States,3c,10 enormous waste streams are heading to landfills after only a
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Fig. 1 13C NMR spectra of crude reaction mixtures in CDCl3 following depolymerization of PET (A) and PLA (B). K = Anisole, ’ = THF, BDM = 1,4-benzenedimethanol, EG = ethylene glycol, PG = propylene glycol.
single use. Through the use of new catalysts, there is potential to recycle a greater fraction of this waste stream by converting it into a new chemical feedstock. Given our aliphatic polyester depolymerization results catalyzed by 2 and 4, we were intrigued by the potential to depolymerize PET to furnish EG and 1,4-benzenedimethanol (BDM) rather than a phthalate. Preliminary depolymerizations of PET were difficult to characterize due to the insolubility of partially depolymerized products. Addition of an aliquot of the crude reaction mixture to methanol allowed us to quickly assess the extent of depolymerization, as remaining polymeric or oligomeric species would precipitate whereas complete depolymerization showed no precipitate. The conditions used for the hydrogenation of caprolactone proved ineffective for hydrogenation of PET, presumably due to insolubility of PET in THF. 1H NMR of the reaction mixture showed the presence of BDM and EG, suggesting that some degree of depolymerization was occurring; however, precipitate formation in methanol implied that polymeric or oligomeric materials remained. To increase polymer solubility, we raised the temperature and used a 50 : 50 mixture of anisole : THF. These conditions were more effective, but it was not until we activated catalyst precursor 3 with two equivalents of KOtBu that the depolymerization went to completion (Table 1, Entry 3). With these conditions, no precipitate was observed in methanol and both 1H and 13C NMR (Fig. 1A) showed quantitative conversion to EG and BDM. It should be noted that the PET flakes (ca. 300 mg) were cut from a used water bottle and were depolymerized without further preparation, indicating that the catalyst system is tolerant of impurities such as pigments and additives in commercial resins. Interestingly, 2 did not fully depolymerize PET under conditions in which 4 achieved complete depolymerization. Milstein and coworkers used 4 to hydrogenate lactide to propylene glycol (PG),7a which suggested this system could also depolymerize PLA to PG.11 Under the same conditions used by Milstein to hydrogenate lactide, 4 was unable to hydrogenate PLA, which we suspected was again due to the insolubility of the polymer in THF. Upon switching to an anisole : THF mixture, 4
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quantitatively converted PLA into PG (Table 1, entry 4; Fig. 1B). Complex 2 was again unable to depolymerize PLA under these same conditions. The inability of 2 to fully depolymerize PET or PLA under conditions in which 4 was effective is perhaps due to the less sterically bulky dipyridyl backbone of 4 compared to the diethylaminomethyl arm on 2. These performance differences suggest that further ligand modifications could yield even more efficient catalysts. Milstein and coworkers further demonstrated the ability of 2 and 4 to hydrogenate organic carbonates and formates to yield methanol.8 More recently, Ding and coworkers used a Ru(II) PNP pincer complex to hydrogenatively depolymerize polypropylene carbonate (PPC) into PG and methanol.12 We were intrigued by these results to see if 2 and 4 could also hydrogenatively depolymerize PPC and polyethylene carbonate (PEC) into PG/methanol and EG/methanol, respectively. Indeed, both 2 and 4 were effective catalysts for the controlled depolymerization of PPC and PEC into their respective glycols and methanol (Table 1, entries 5, 6). The activity of 4 for hydrogenation of PPC was also recently disclosed by Milstein and coworkers.13 Finally, poly(R-3-hydroxybutyric acid) (PHB, Table 1, entry 7) is an enantiomerically pure naturally occurring polyester that has one additional methylene unit compared to PLA. We were intrigued by this polymer because successful depolymerization would presumably furnish enantiomerically pure bio-derived R-1,3-butanediol, which could serve as a valuable starting material for chemical synthesis.14 Surprisingly, however, depolymerizing PHB with 4, using the same conditions that were effective for PLA and PET, yielded butyric acid as the sole product (Table 1, entry 7). We also depolymerized poly(3-hydroxypropionic acid) (P3HP, Table 1, entry 8) using 4 and obtained propionic acid in 90% conversion with no starting material or other products observed. Given that polyesters with five or more methylene units between ester linkages and PLA are depolymerized to yield diols (Table 1, entries 1, 2, and 4), we are currently investigating the depolymerization of polyesters with three and four methylene units to determine when the mechanism transitions between diol and carboxylic acid formation. Additionally, we are performing deuteration studies to elucidate the hydrogenation sequence of this unique reaction. This work demonstrates that hydrogenative depolymerization of polymers is an effective method for accessing useful small molecules and offers the promise of converting waste streams of polyesters and polycarbonates into chemical feedstocks. With the unexpected exception of PHB and P3HP, polyesters are depolymerized to diols and polycarbonates are depolymerized to the corresponding glycol and methanol. Future work will focus on better understanding the mechanism of this system and expanding the scope of depolymerizable polymers to include engineering polyesters and polycarbonates, as well as polyamides. This work was supported by a Cottrell College Science Award from the Research Corporation for Science Advancement to NJR, and by the UW-Eau Claire Office of Research and Sponsored Programs. We gratefully acknowledge the Coates lab at Cornell University, Novomer Inc., and Empower Materials Inc. for their generous donations of P3HP, PPC and PEC, respectively. Finally, we thank Dr John Kramer for useful discussions.
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Notes and references 1 (a) K. Yao and C. Tang, Macromolecules, 2013, 46, 1689; (b) H. Nishida, Polym. J., 2011, 43, 435; (c) C. K. Ober, S. Z. D. Cheng, P. T. Hammond, M. Muthukumar, E. Reichmanis, K. L. Wooley and T. P. Lodge, Macromolecules, 2009, 42, 465. 2 (a) A. H. Tullo, Chem. Eng. News, 2013, 91(28), 18; (b) G. Q. Chen and M. K. Patel, Chem. Rev., 2012, 112, 2082; (c) C. K. Williams and M. A. Hillmyer, Polym. Rev., 2008, 48, 1. 3 (a) H. W. Horn, G. O. Jones, D. S. Wei, K. Fukushima, J. M. Lecuyer, D. J. Coady, J. L. Hedrick and J. E. Rice, J. Phys. Chem. A, 2012, 116, 12389; (b) F. A. Leibfarth, N. Moreno, A. P. Hawker and J. D. Shand, J. Polym. Sci., Part A: Polym. Chem., 2012, 50, 4814; (c) K. Fukushima, O. Coulembier, J. M. Lecuyer, H. A. Almegren, A. M. Alabdulrahman, F. D. Alsewailem, M. A. McNiel, P. Dubois, R. M. Waymouth, H. W. Horn, J. E. Rice and J. L. Hedrick, J. Polym. Sci. Part A: Polym Chem., 2011, 49, 1273. 4 A. W. Kawaguchi, A. Sudo and T. Endo, ACS Macro Lett., 2013, 2, 1. 5 (a) C. Gunanathan and D. Milstein, Science, 2013, 341, 1229712; (b) C. Gunanathan and D. Milstein, Acc. Chem. Res., 2011, 44, 588; (c) D. Milstein, Top. Catal., 2010, 53, 915; (d) C. Gunanathan, Y. BenDavid and D. Milstein, Science, 2007, 317, 790; (e) J. Zhang, G. Leitus, Y. Ben-David and D. Milstein, J. Am. Chem. Soc., 2005, 127, 10840. 6 (a) B. Gnanaprakasam, E. Balaraman, C. Gunanathan and D. Milstein, J. Polym. Sci., Part A: Polym. Chem., 2012, 50, 1755; (b) D. M. Hunsicker, B. C. Dauphinais, S. P. Mc Ilrath and N. J. Robertson, Macromol. Rapid Commun., 2012, 33, 232; (c) H. Zeng and Z. Guan, J. Am. Chem. Soc., 2011, 133, 1159; (d) A. Staubitz, M. E. Sloan, A. P. M. Robertson, A. Friedrich,
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7
8 9
10 11
12 13 14
¨nne and I. Manners, S. Schneider, P. J. Gates, J. S. auf der Gu J. Am. Chem. Soc., 2010, 132, 13332. (a) E. Balaraman, E. Fogler and D. Milstein, Chem. Commun., 2012, 48, 1111; (b) E. Balaraman, B. Gnanaprakasam, L. J. W. Shimon and D. Milstein, J. Am. Chem. Soc., 2010, 132, 16756; (c) J. Zhang, G. Leitus, Y. Ben-David and D. Milstein, Angew. Chem., Int. Ed., 2006, 45, 1113. E. Balaraman, C. Gunanathan, J. Zhang, L. J. W. Shimon and D. Milstein, Nat. Chem., 2011, 3, 609. (a) S. Chaudhary, P. Surekha, D. Kumar, C. Rajagopal and P. K. Roy, J. Appl. Polym. Sci., 2013, 129, 2779; (b) F. Chen, G. Wang, C. Shi, Y. Zhang, L. Zhang, W. Li and F. Yang, J. Appl. Polym. Sci., 2013, 127, 2809; (c) V. Sinha, M. R. Patel and J. V. Patel, J. Polym. Environ., 2010, 18, 8; (d) N. D. Pingale and S. R. Shukla, Eur. Polym. J., 2008, 44, 4151; (e) G. P. Karayannidis and D. S. Achilias, Macromol. Mater. Eng., 2007, 292, 128; ( f ) D. E. Nikles and M. S. Farahat, Macromol. Mater. Eng., 2005, 290, 13. S. Miller, ACS Macro Lett., 2013, 2, 550. Very recently, a barium-promoted copper chromite heterogeneous catalyst (130 wt%) was used to depolymerize PLA to PG, presumably through methanolysis followed by hydrogenation. I. A. Shuklov, ¨rner, ¨hlein and A. Bo N. V. Dubrovina, J. Schulze, W. Tietz, K. Ku Chem. – Eur. J., 2014, 20, 957. Z. Han, L. Rong, J. Wu, L. Zhang, Z. Wang and K. Ding, Angew. Chem., Int. Ed., 2012, 51, 13041. D. Milstein, E. Balaraman, C. Gunanathan, B. Gnanaprakasam and J. Zhang, US Pat. Ap., US20130281664A1, 2013. N. Kataoka, A. S. Vangnai, T. Tajima, Y. Nakashimada and J. Kato, J. Biosci. Bioeng., 2013, 115, 475.
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COMMUNICATION Nicholas J. Robertson et al. Controlled hydrogenative depolymerization of polyesters and polycarbonates catalyzed by ruthenium(II) PNN pincer complexes
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Cite this: Chem. Commun., 2014, 50, 4884 Received 21st January 2014, Accepted 7th March 2014 DOI: 10.1039/c4cc00541d
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Controlled hydrogenative depolymerization of polyesters and polycarbonates catalyzed by ruthenium(II) PNN pincer complexes† Eric M. Krall,a Tyler W. Klein,a Ryan J. Andersen,a Alex J. Nett,b Ryley W. Glasgow,b Diana S. Reader,a Brian C. Dauphinais,a Sean P. Mc Ilrath,a Anne A. Fischer,b Michael J. Carney,b Dylan J. Hudsona and Nicholas J. Robertson*a
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Ruthenium(II) PNN complexes depolymerize many polyesters into diols and polycarbonates into glycols plus methanol via hydrogenation. Notably, polyesters with two methylene units between ester linkages depolymerize to carboxylic acids rather than diols. This methodology represents a new approach for producing useful chemicals from waste plastics.
Recent efforts to mitigate the environmental impact of plastics have focused on two main areas: reducing the use of petroleum-based feedstocks and decreasing the large volumes of plastics currently heading to landfills.1 For the former, new synthetic routes have been developed to produce biorenewable monomers used to prepare well-established commodity polymers. For example, Coca-Cola’s polyethylene terephthalate (PET) PlantBottlet uses ethylene glycol (EG) derived from sugarcane. In addition, new biorenewable and biodegradable polymers that can replace traditional plastics are under development, with polylactic acid (PLA, from NatureWorks LLC) being a noteworthy alternative to petroleum derived polymers.2 However, high costs associated with biorenewable pathways currently limit widespread use of these materials.2a For the latter, most polymer recycling is currently accomplished through melting and re-molding post-consumer plastic. Unfortunately this route often produces inferior polymer compared to virgin resin due to impurities (e.g., pigments, additives, other polymers, catalyst residues) in post-consumer material.3 A smaller but potentially more promising route involves recycling plastics through controlled depolymerization.1b Depolymerization yields the polymer’s monomers, which can be purified and repolymerized to form virgin resin or used for other synthetic purposes. Current depolymerization methods, such as ester hydrolysis, a
Northland College, 1411 Ellis Ave., Ashland, Wisconsin, 54806, USA. E-mail: [email protected]; Tel: +1 715 682 1321 b University of Wisconsin-Eau Claire, 105 Garfield Ave., Eau Claire, Wisconsin 54701, USA † Electronic supplementary information (ESI) available: Experimental details and 1 H and 13C NMR spectra for the depolymerizations of entries 1–8, Table 1. See DOI: 10.1039/c4cc00541d
4884 | Chem. Commun., 2014, 50, 4884--4887
transesterification, olefin metathesis and retro Diels–Alder reactions,3,4 are typically not cost competitive because the resulting monomers are currently more cheaply derived from petroleum than through depolymerization.3c Thus, alternative depolymerization technologies are needed, including those with more efficient catalysts or that can produce value-added chemicals from post-consumer polymeric materials, such as PET.3b Recently reported ruthenium-based catalysts offer this sort of potential. The Milstein catalyst (2, Scheme 1) is a remarkably versatile catalyst for the dehydrogenative coupling of alcohols to form esters, as well as the coupling of alcohols and amines to afford amides.5 More recently, we and others used 2 to prepare high molecular weight polymers, despite proceeding via a stepgrowth polymerization mechanism, by removal of the hydrogen
Scheme 1 Catalyst precursors (1 and 3) are activated in situ using KOtBu generating Ru(II) PNN catalysts (2 and 4). Hydrogenation of polyesters using Ru(II) PNN catalysts produces diols (A) and hydrogenation of polycarbonates with Ru(II) PNN catalysts yields glycols and methanol (B). Polyesters with two methylene units between esters furnish carboxylic acids. R includes aliphatic or aromatic segments for polyesters and aliphatic segments for polycarbonates.
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Catalytic hydrogenation of polyesters and polycarbonates with ruthenium(II) PNN pincer complexesa
Catalyst
Loadingb
Solvent
p(H2)/atm
Temp. (1C)
1
2
100 : 1 : 1
Anisole
13.6
120
80e
2
4
50 : 1 : 1
THF
47.6
120
93
3
4
50 : 1 : 2
Anisole/THF
54.4
160
499
4
4
50 : 1 : 2
Anisole/THF
54.4
160
499
5f
2 or 4
100 : 1 : 2
Anisole/THF
54.4
160
499
6f
2 or 4
100 : 1 : 2
Anisole/THF
54.4
160
91
7
4
50 : 1 : 2
Anisole/THF
54.4
160
88
8
4
50 : 1 : 2
Anisole/THF
54.4
160
90
Entry
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Polymer
Observed product(s)c
% Conv.d
a Catalyst precursor activated with KOtBu for 5 min. under N2 before transferring to a pressure reactor containing polymer. The reaction mixture was pressured and heated to target temperature for 48 h. b Loading ratio is polymer repeat units : catalyst precursor : KOtBu. c Products were determined by 1H and 13C NMR and no starting material or other products were observed. d % Conversions were calculated by 1H NMR signal integrations referenced to an internal CH2Cl2 standard. e Isolated yield. f Reaction time of 24 h.
byproduct to drive the reaction to high conversion.6 Under appropriate conditions, the catalytic cycle can be reversed and has been used to effectively hydrogenate esters to their corresponding alcohols through the addition of hydrogen gas to the reaction.7 Recent results have also shown that the catalyst can efficiently hydrogenate organic carbonates to methanol and the corresponding diols.8 We were intrigued by this reversibility as a potential means to controllably depolymerize polyesters to diols and polycarbonates to glycols and methanol (Scheme 1, A and B, respectively). We first sought to depolymerize a series of linear aliphatic polyesters that we previously synthesized through in vacuo dehydrogenation polymerization of diols using 2 generated in situ by deprotonation of 1 via potassium tert-butoxide (Scheme 1).6b These polyesters were readily depolymerized back to their corresponding a,o-diols when exposed to 13.6 atm hydrogen gas for 48 h at 120 1C in the presence of 2. 1H and 13 C NMR analysis of the crude reaction mixtures showed complete depolymerization. Upon cooling and releasing the hydrogen gas,
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the diols crystallized from the reaction mixture and were isolated with simple filtration. Using this process, we obtained an 80% isolated yield of 1,10-decanediol (Table 1, entry 1). However, the same conditions proved ineffective for the hydrogenation of caprolactone to 1,6-hexanediol. Recently, Ru(II) PNN complex 4 (Scheme 1) in tetrahydrofuran (THF) was successfully used by Milstein and coworkers for the hydrogenation of lactide to propylene glycol;7a we therefore thought 4 may be a more effective catalyst for the hydrogenation of caprolactone. Indeed, 4 was able to completely hydrogenate caprolactone to 1,6-hexanediol with 47.6 atm H2 (Table 1, entry 2). We suspect that the hydrogenation mechanism is the same as that proposed by Milstein and coworkers for ester hydrogenation using 2.7c Given our initial successes, we turned our attention to using 2 and 4 to controllably depolymerize a variety of polyesters and polycarbonates. Most efforts to depolymerize PET have employed glycolysis or solvolysis to the corresponding terephthalates.3c,9 With less than 30% of PET materials being recycled in the United States,3c,10 enormous waste streams are heading to landfills after only a
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Fig. 1 13C NMR spectra of crude reaction mixtures in CDCl3 following depolymerization of PET (A) and PLA (B). K = Anisole, ’ = THF, BDM = 1,4-benzenedimethanol, EG = ethylene glycol, PG = propylene glycol.
single use. Through the use of new catalysts, there is potential to recycle a greater fraction of this waste stream by converting it into a new chemical feedstock. Given our aliphatic polyester depolymerization results catalyzed by 2 and 4, we were intrigued by the potential to depolymerize PET to furnish EG and 1,4-benzenedimethanol (BDM) rather than a phthalate. Preliminary depolymerizations of PET were difficult to characterize due to the insolubility of partially depolymerized products. Addition of an aliquot of the crude reaction mixture to methanol allowed us to quickly assess the extent of depolymerization, as remaining polymeric or oligomeric species would precipitate whereas complete depolymerization showed no precipitate. The conditions used for the hydrogenation of caprolactone proved ineffective for hydrogenation of PET, presumably due to insolubility of PET in THF. 1H NMR of the reaction mixture showed the presence of BDM and EG, suggesting that some degree of depolymerization was occurring; however, precipitate formation in methanol implied that polymeric or oligomeric materials remained. To increase polymer solubility, we raised the temperature and used a 50 : 50 mixture of anisole : THF. These conditions were more effective, but it was not until we activated catalyst precursor 3 with two equivalents of KOtBu that the depolymerization went to completion (Table 1, Entry 3). With these conditions, no precipitate was observed in methanol and both 1H and 13C NMR (Fig. 1A) showed quantitative conversion to EG and BDM. It should be noted that the PET flakes (ca. 300 mg) were cut from a used water bottle and were depolymerized without further preparation, indicating that the catalyst system is tolerant of impurities such as pigments and additives in commercial resins. Interestingly, 2 did not fully depolymerize PET under conditions in which 4 achieved complete depolymerization. Milstein and coworkers used 4 to hydrogenate lactide to propylene glycol (PG),7a which suggested this system could also depolymerize PLA to PG.11 Under the same conditions used by Milstein to hydrogenate lactide, 4 was unable to hydrogenate PLA, which we suspected was again due to the insolubility of the polymer in THF. Upon switching to an anisole : THF mixture, 4
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quantitatively converted PLA into PG (Table 1, entry 4; Fig. 1B). Complex 2 was again unable to depolymerize PLA under these same conditions. The inability of 2 to fully depolymerize PET or PLA under conditions in which 4 was effective is perhaps due to the less sterically bulky dipyridyl backbone of 4 compared to the diethylaminomethyl arm on 2. These performance differences suggest that further ligand modifications could yield even more efficient catalysts. Milstein and coworkers further demonstrated the ability of 2 and 4 to hydrogenate organic carbonates and formates to yield methanol.8 More recently, Ding and coworkers used a Ru(II) PNP pincer complex to hydrogenatively depolymerize polypropylene carbonate (PPC) into PG and methanol.12 We were intrigued by these results to see if 2 and 4 could also hydrogenatively depolymerize PPC and polyethylene carbonate (PEC) into PG/methanol and EG/methanol, respectively. Indeed, both 2 and 4 were effective catalysts for the controlled depolymerization of PPC and PEC into their respective glycols and methanol (Table 1, entries 5, 6). The activity of 4 for hydrogenation of PPC was also recently disclosed by Milstein and coworkers.13 Finally, poly(R-3-hydroxybutyric acid) (PHB, Table 1, entry 7) is an enantiomerically pure naturally occurring polyester that has one additional methylene unit compared to PLA. We were intrigued by this polymer because successful depolymerization would presumably furnish enantiomerically pure bio-derived R-1,3-butanediol, which could serve as a valuable starting material for chemical synthesis.14 Surprisingly, however, depolymerizing PHB with 4, using the same conditions that were effective for PLA and PET, yielded butyric acid as the sole product (Table 1, entry 7). We also depolymerized poly(3-hydroxypropionic acid) (P3HP, Table 1, entry 8) using 4 and obtained propionic acid in 90% conversion with no starting material or other products observed. Given that polyesters with five or more methylene units between ester linkages and PLA are depolymerized to yield diols (Table 1, entries 1, 2, and 4), we are currently investigating the depolymerization of polyesters with three and four methylene units to determine when the mechanism transitions between diol and carboxylic acid formation. Additionally, we are performing deuteration studies to elucidate the hydrogenation sequence of this unique reaction. This work demonstrates that hydrogenative depolymerization of polymers is an effective method for accessing useful small molecules and offers the promise of converting waste streams of polyesters and polycarbonates into chemical feedstocks. With the unexpected exception of PHB and P3HP, polyesters are depolymerized to diols and polycarbonates are depolymerized to the corresponding glycol and methanol. Future work will focus on better understanding the mechanism of this system and expanding the scope of depolymerizable polymers to include engineering polyesters and polycarbonates, as well as polyamides. This work was supported by a Cottrell College Science Award from the Research Corporation for Science Advancement to NJR, and by the UW-Eau Claire Office of Research and Sponsored Programs. We gratefully acknowledge the Coates lab at Cornell University, Novomer Inc., and Empower Materials Inc. for their generous donations of P3HP, PPC and PEC, respectively. Finally, we thank Dr John Kramer for useful discussions.
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