Letter pubs.acs.org/macroletters
Unique Base-Initiated Depolymerization of Limonene-Derived Polycarbonates Chunliang Li,†,‡ Rafael̈ J. Sablong,*,†,§ Rolf A. T. M. van Benthem,†,⊥ and Cor E. Koning†,∥ †
Laboratory of Physical Chemistry, Eindhoven University of Technology P.O. Box 513, 5600 MB Eindhoven, The Netherlands Dutch Polymer Institute DPI, P.O. Box 902, 5600 AX Eindhoven, The Netherlands § Polymer Technology Group Eindhoven B.V. (PTG/e B.V.) P.O. Box 6284, 5600 HG Eindhoven, The Netherlands ∥ DSM Coating Resins Ceintuurbaan 5, 8022 AW Zwolle, The Netherlands ⊥ DSM Ahead BV Urmonderbaan 22, 6167 RG Geleen, The Netherlands ‡
S Supporting Information *
ABSTRACT: The depolymerization of poly(limonene carbonate) (PLC) initiated by 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) was investigated. The strong organic base TBD was capable of deprotonating the OH-terminated PLC, leading to fast degradation via backbiting reactions at high temperature. An interesting feature of the base-initiated breakdown of PLC lies in the quantitative depolymerization into the corresponding initial limonene oxide monomer. This result implies the complete back-to-monomer recyclability of the fully biobased PLC, which accordingly can be considered as a really sustainable material. Additionally, the stability of PLC when exposed to TBD was enhanced by an end-capping reaction, which further supported the proposed degradation pathway.
A
Scheme 1. Different Depolymerization Pathways of APCs10,12,15−17
liphatic polycarbonates (APCs) have received increasing attention because of their potential degradability and biocompatibility. Some of them have even been commercialized and used in various fields such as polyurethanes, biomedical materials, and foam materials.1−3 They can be prepared by the catalytic copolymerization of epoxides and CO2, in which the use of epoxides of different structures and functional groups can tune the properties of the resulting APCs like, for example, thermal behavior, crystallinity, hydrophilicity, and so on.4−6 Besides, nontoxic and cheap CO2 is considered as a green and economical building block for copolymer synthesis. Many efforts have been made to develop new epoxide/CO2 copolymerization catalysts to achieve high selectivity and activity.7−11 Attention has also been given to understand the depolymerization pathways of APCs via experimental studies and computer simulation, indicating the possibility of chemically recycling such polymers.12−16 In most cases, the depolymerization of aliphatic polycarbonates undergoes an endwise scission reaction, yielding selectively five-membered cyclic carbonates at moderate temperatures (see pathway a in Scheme 1).14,17 These five-membered cyclic carbonates with small ring strain are stable and their ring-opening polymerization is thermodynamically disfavored, typically accompanied by the elimination of CO2 to give poly(ether-carbonate)s with low recyclability.18−20 In comparison, epoxides, produced by the depolymerization of some APCs, can be recycled and reused to prepare the corresponding pure APCs.10,19,20 Darensbourg et al. reported a unique, either metal-free or metal-assisted depolymerization pathway for poly(trans-cyclopentene carbonate) (PCPC), in which the depolymerization of © XXXX American Chemical Society
Received: April 25, 2017 Accepted: June 12, 2017
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DOI: 10.1021/acsmacrolett.7b00310 ACS Macro Lett. 2017, 6, 684−688
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ACS Macro Letters
Figure 1. (a) Number-average molecular weight (Mn) and polydispersity index (Đ) and (b) PLC polymer and epoxide concentration change as a function of reaction time during the depolymerization of PLC in the presence of TBD in toluene at 110 °C.
poly(cyclohexene carbonate) (PCHC) under the same conditions.34 This unexpected result indicated the potential of recycling PLC into its parent epoxide monomer. The investigation was also extended to OH-terminated PLOC, with the same polymer backbone as PLC. We initially performed the base-initiated degradation of PLC and PLOC in toluene with TBD at 110 °C for 16 h ([M]:[TBD] = 25), following the same conditions as described by Darensbourg.21 Interestingly, the 1H NMR analysis of the product revealed the full conversion of the polymers into the initial LMO or LDO monomers (see Figure S1), without formation of any cyclic carbonate, as confirmed by IR (see Figure S4). GC-MS analysis of the PLC depolymerization material revealed only two neighboring peaks (m/z 152), corresponding to cis- and transLMO, respectively (see Figure S6). The intriguing results prompted us to study this reaction in more detail using PLC as the model polymer. The reaction rate was significantly reduced at 100 °C while no degradation was observed at all at 80 °C, not even after 24 h. The kinetic study of the TBD-catalyzed depolymerization was carried out by following the changes in the number-average molecular weight (Mn) and PLC concentration as a function of reaction time at 110 °C. As shown in Figure 1a, the Mn first dropped rapidly from 12.5 to 6.4 kDa within 15 min and then decreased much more slowly. The polydispersity index (Đ) became quite broad during the degradation process, presumably because of the deprotonation reaction equilibrium.10 Besides, the intensity of the peak assigned to the resulting small molecules in the SEC traces increased with prolonged reaction time as a result of the depolymerization (Figure S3). The development of the PLC and epoxide concentration was also followed in time with NMR (Figure 1b). The 1H NMR spectra showed that the intensities of several characteristic peaks, corresponding to the methane (Ha, Ha′) and alkene protons (Hb, Hb′) in the polymer and the epoxide respectively, changed with reaction time (see Figure 2). The Ha resonance decreased gradually to zero after 290 min, while the Ha′ signal reached its maximum intensity. The depolymerization of PLC without TBD led to a negligible change in PLC concentration and molecular weight. The depolymerization of PLC was also accompanied by the release of CO2, leading to an increase of the internal pressure in the reactor. However, no effect of CO2 pressure was observed on the final reaction products, as indicated by the similar products during PLC depolymerization under 40 bar CO2. This behavior is somewhat different from the previous observations during the depolymerizations of PICs and PCPCs, which tend to produce the corresponding cyclic carbonates under high
the hydroxyl-terminated polymers resulted in a mixture of cyclopentene oxide and cis-cyclopentene carbonate. The depolymerization was mainly initiated by the deprotonation of the hydroxyl polymer ends and underwent alkoxide backbiting. The ratio of these two products varied with the reaction conditions (catalyst, CO2 pressure, temperature, pathway b).12,21 The highest selectivity of the reaction toward cyclopentene oxide reached 92%. Computational simulations also strongly supported the observation of high selectivity toward epoxide formation over cyclic carbonate formation. To the best of our knowledge, this was the first example of recycling polycarbonates derived from CO2 and epoxides into the corresponding monomer. Similarly, the depolymerization of poly(indene carbonate) (PIC) could also occur along with the formation of indene oxide, which was totally suppressed under high pressure of CO2.22 Very recently, another degradable polycarbonate, synthesized by copolymerization of CO2 with 1benzyloxycarbonyl-3,4-epoxy pyrrolidine, proved to be fully recyclable (Scheme 1, pathway c).23 Nowadays, biobased monomers have been extensively developed, aiming at decreasing the use of petrochemical resources and reducing the carbon footprint. Several biobased epoxides have been studied for the preparation of functional APCs.24−33 The coupling of CO2 and limonene 1,2-monoxide (LMO) and dioxide (LDO), derived from limonene, the main component of orange oil, yielded poly(limonene carbonate) (PLC) and poly(limonene-8,9-oxide carbonate) (PLOC) with extra functionalities along the polymer chain (isopropenyl and methyloxiranyl groups, respectively). The functional limonenebased APCs were readily subjected to multiple postmodifications via thiol−ene, thiol-epoxy, or epoxy-carboxylic acid reactions, allowing the attachment of small molecules, grafting of polymers and cross-linking reactions for solvent-resistant film formation.25,26 Herein, we present a unique degradation pathway that almost quantitatively converts these polycarbonates into their corresponding monomers (Scheme 1d). It is particularly interesting for the recycling of limonene-based APC-based materials, which can be potentially regarded as fully sustainable materials. The motivation for the present investigation originates from an interesting observation during the preparation of α,ωdihydroxyl-terminated PLCs. Upon treatment of PLC with diols in the presence of 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD as transcarbonation catalyst, a mixture of limonene 1,2diol, oligomers and, surprisingly, trans-limonene oxide was produced at moderate temperature (60 °C). However, no cyclic carbonate was detected, which differed from the degradation of 685
DOI: 10.1021/acsmacrolett.7b00310 ACS Macro Lett. 2017, 6, 684−688
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ACS Macro Letters
TBD is an active catalyst for such reactions.21 The transcarbonation initially results in the formation of “dead” polymer chains (i.e., cyclic PLC, Figure 3b, and nonhydroxyl PLC species, Figure 3c) and new alkoxide chain ends, which again initiate the backbiting or participate in another transcarbonation reaction. The “dead” polymer chains may undergo new transcarbonation reactions and finally degrade into LMO. This hypothesis was confirmed by the faster depolymerization of acetate end-capped PLC with TBD in the presence of (R)limonene-1,2-diol. These side reactions also caused a broadening of the molecular weight distribution during the depolymerization. The unexpected backbiting pathway could originate from the stereochemical structure of the monomer unit. The preference for axial attack during the epoxide ring-opening results in a 1,2diaxial disposition of the zinc alkoxide moiety and the polymer chain (Scheme 2a). Meanwhile, the large isopropenyl
Figure 2. 1H NMR spectra of the crude depolymerization products.
pressure of CO2.21,22 These experiments confirmed the base initiation as the origin of the chain scission and ruled out the significance of the thermal activation. Another interesting phenomenon was observed during the NMR monitoring of the degradation of PLC. So, the signal corresponding to Ha′ of cis-LMO appeared at the beginning of the reaction and remained unchanged after a certain reaction time (Figure 2). It is known that trans-LMO shows a much higher reactivity than cis-LMO in the LMO/CO2 copolymerization catalyzed by diiminate zinc complexes.20 As a result, only a minor amount of cis-LMO was incorporated into the polymer, even after complete trans-LMO consumption. The polymerization sequence obviously leads to the formation of a polymer chain, ending with a segment rich in cis-moieties carrying a hydroxyl end-group (Figure 3). Hence, the depolymerization should start from this chain end, undergoing endwise scission reactions. It is worth noting that the other polymer chain end may not contain a hydroxyl group due to the epoxide rearrangement in the presence of zinc catalyst, generating alcohols as chain transfer agents.35 In order to ascertain the depolymerization pathway of PLC, the end-capping of the copolymer with acetate groups was performed, which retarded the depolymerization. Only traces of LMO were observed after 16 h under the same experimental conditions, pointing to a dominating endwise degradation. Meanwhile, intra- and intermolecular transcarbonation reactions may also occur during the degradation (Figure 3b,c) since
Scheme 2. (a) Ring opening of trans- and cis-LMO During Copolymerization; (b) Backbiting Reaction Resulting in the Formation of trans- and cis-LMO, Respectively
substituent of the monomer unit, preferentially occupying the equatorial position with respect to the tetra-substituted cyclohexane ring, leads to a conformationally biased (also called “anancomeric”) system.36,37 Thus, the alkoxide issued
Figure 3. Possible reactions during the depolymerization of PLC: (a) backbiting as the main process; (b, c) intra- and intermolecular transcarbonations. 686
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from the deprotonation of the hydroxyl polymer chain end is unable to attack the vicinal carbonyl group due to the “locked” ring structure, which would afford a five-membered cyclic carbonate. Instead, the attack of the secondary and tertiary alkoxide chain ends takes place on the adjacent carbon connected to the carbonate linkage, leading to the formation of cis- and trans-LMO, respectively (Scheme 2b). This mechanism was supported by the depolymerization of poly(1methycyclohexene carbonate) into mainly (93%) 1-methylcyclohexene carbonate under the same conditions (see Figures S2 and S5). In summary, we have investigated the TBD-catalyzed depolymerization of OH-terminated PLC and PLOC. The depolymerization was first initiated by TBD via deprotonation of the hydroxyl polymer chain ends to alkoxides and was subsequently followed by backbiting, either from the original chain end or from new chain ends generated by inter-intrachain transcarbonations. This mechanism selectively converted the polymer into the initial epoxide monomers. The stability toward backbiting of the acetate end-capped PLC strongly supported this reaction pathway. This depolymerization of limonene-based polycarbonates suggests that these fully biobased polycarbonates can be fully recycled back-tomonomer and therefore are truly sustainable materials. Studies on the metal-assisted depolymerization of PLC are in progress.
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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/acsmacrolett.7b00310. Experimental procedures, NMR spectra, SEC traces, and FTIR spectra (PDF).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Rafaël J. Sablong: 0000-0002-5718-124X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work forms part of the research programme of the Dutch Polymer Institute (DPI), Project #796p (C.L.). This project has also received funding from the European Union’s Seventh Framework Programme for research, technological development and demonstration under Grant Agreement No. 289253 (C.L.).
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