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Letter pubs.acs.org/acscatalysis

Access to Biorenewable Polycarbonates with Unusual GlassTransition Temperature (Tg) Modulation Nicole Kindermann,† À lex Cristòfol,† and Arjan W. Kleij*,†,‡ †

Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science and Technology (BIST), Av. Països Catalans 16, 43007 − Tarragona, Spain ‡ Catalan Institute of Research and Advanced Studies (ICREA), Pg. Lluís Companys 23, 08010 − Barcelona, Spain S Supporting Information *

ABSTRACT: A sequential and mild approach toward the synthesis of poly(limonene)dicarbonate (PLDC) has been developed using readily available limonene oxide (LO) and CO2 as renewable reagents and an air-stable Al(III) complex as catalyst for the alkene-rich poly(limonene)carbonate (PLC). The developed sequence allows for the stepwise construction of different PLDC polymers, using PLC as a synthetic intermediate with molecular weights of up to 15.3 kg/mol and tunable glass-transition temperature (Tg) values of up to an unprecedented 180 °C using a commercially available mixture of cis/ trans (+)-LO. KEYWORDS: aluminum, carbon dioxide, limonene oxide, polycarbonates, renewables

T

he copolymerization of aliphatic epoxides and CO21 is a well-established approach toward the synthesis of thermoplastic polymers with properties that are mainly dependent on the nature of the epoxide monomer. Despite the fact that most reported polycarbonates are based on petroleum-derived epoxide monomers, such as cyclohexene oxide,2 propylene oxide,3 and others,4 recently, the attention has been shifting toward the use of (partially) bio-based epoxides as a way to produce polymers with an improved sustainability footprint. In this respect, the use of terpene oxides has yet offered new potential for bio-derived polycarbonates5 and polyesters5a,6 displaying similar or even improved properties, compared to fossil-fuel-based conventional polymers. In particular, commercially available and inexpensive limonene oxide (LO; Scheme 1)7 has been recognized as an attractive, readily available, bifunctional, and renewable epoxide monomer and its use in polycarbonate material development is steadily increasing.

Ever since the seminal work from Coates et al., who developed the first efficient copolymerization of LO and CO2 using a Zn(β-diiminate)-based catalyst,8 efforts have been made to prepare functional (crystalline) materials from poly(limonene carbonate) (PLC; see Scheme 1).9 More recently, Sablong and co-workers reported a related polycarbonate based on limonene dioxide (LDO) giving rise to an epoxide-rich polymer that showed potential for post-synthetic modifications, thereby creating new opportunities for thermoset coating resins.9e The pendent alkene groups in PLC are also useful for polymer curing, as recently demonstrated.5b Our continuous interest in the area of bio(poly)carbonates5b,c,10 and the importance to find suitable alternatives for existing commercial bis-phenol A (BPA)/CO2-based copolymers with a glasstransition temperature (Tg) of ∼150 °C, prompted us to consider PLC as a starting point toward biosourced polycarbonates with unusual thermal properties. Until now, only very few examples are known of polycarbonates with high glass transitions, including the poly(indene carbonate) reported by Darensbourg (Tg = 138 °C),4b and a PLDC (Scheme 1) reported recently by Sablong (Tg = 146 °C).9e Despite this notable progress, there is still a need to design new polymer formulations, which complement these former achievements, such that bioalternatives become available for a wider array of polycarbonate applications currently enabled through the use of BPA. Inspired by this challenge, here we report on the development of PLDC polymers that are built up in a sequential approach using PLC as a synthetic intermediate. The overall process takes advantage

Scheme 1. Sequential Synthetic Approach Used in this Work toward the Preparation and Tg Modulation of PLDC

Received: March 9, 2017 Revised: April 27, 2017 Published: April 28, 2017 © 2017 American Chemical Society

3860

DOI: 10.1021/acscatal.7b00770 ACS Catal. 2017, 7, 3860−3863

Letter

ACS Catalysis

PLCO (Mn = 4.40 kg/mol, Đ = 1.31) derived from trans-PLC at 4-fold scale in a pressure reactor at different reaction temperatures (53−113 °C), using only PPNBr or PPNCl as catalysts (Table 1, entries 7−13). At higher temperatures, shorter reaction times are required for full conversion, but we were pleased to find that, at relatively mild temperatures (73 °C, entry 8), using 10 mol % of PPNCl, quantitative conversion of PLCO to PLDC could already be observed within 48 h. Furthermore, the use of PPNBr under these conditions provided incomplete epoxy conversion (entry 11 in Table 1, 70%), demonstrating that sterically congested epoxides are best converted with less potent, though smaller nucleophiles, as recently also shown for bulky 1,2-disubstituted oxiranes.13 Interestingly, the isolated PLDC polymer from entry 8 was analyzed by differential scanning calorimetry (DSC) analysis and provided an unexpected but promising high glass-transition temperature of 165 °C (Mn = 6.19 kg/mol, Đ = 1.17). Subsequently, various PLCO grades (Table 2, P1−P5) were prepared having different molecular weights (see the SI) and

over the use of an air-stable Al(III) catalyst and user-friendly and efficient transformations under mild reaction conditions. This allowed modulating the Tg values of the PLDC polymers up to an unprecedentedly high 180 °C. Contrary to the use of limonene dioxide (LDO)9f,11 by Sablong et al., we preferred to take the less expensive and more readily available cis/trans (+)-LO as starting material toward the synthesis of PLDC through the functionalization of PLC by a sequential epoxidation/carboxylation approach (Scheme 1). This approach also has the advantage that both isomers of LO can be used for polycarbonate construction and the commercially available LO therefore does not need to be upgraded to a single diastereoisomer. As a catalyst, an aminotriphenolate Al(III) complex ([AlMe], 1) combined with PPNCl (bis-triphenylphosphine iminium chloride) proved to be a suitable binary catalyst to access different grades of PLC with molecular weights ranging from 1.3 to 15.1 kg/mol (see Table S1 in the Supporting Information (SI)). The epoxidation of the trans-PLC polymer12 to PLCO was first examined using an oligomeric carbonate (number-average molecular weight Mn = 1.29 kg/mol, Đ = 1.74) and using a standard oxidant (m-CPBA) in CH2Cl2 at 0 °C for 12 h. Nuclear magnetic resonance (NMR) analysis of the epoxidized polymer showed full conversion of the alkene into epoxide groups and, importantly, the polycarbonate backbone was not affected by this simple and mild oxidation procedure (details in the SI, Table S2 and Figures S12−S15). Having established that PLCO can be easily attained under mild reaction conditions, next, we turned to the use of the epoxy groups in generating PLDC (Table 1, entries 1−6) using

Table 2. Formation of PLDC Using Different Grades (P1− P5) of PLCO and Thermal Analysis Data of the PLDC Polymersa

Table 1. Screening of Catalysts for the Conversion of PLCO into PLDC Using [AlMe]/PPN-X (X = Cl, Br, I) as a Catalysta entry

catalyst

amount (mol %)

reaction time, t (h)

temperature, Tb (°C)

conversionc (%)

1 2 3 4 5 6 7 8 9 10d 11 12 13d

1 + [Cl] [Cl] 1 + [Br] [Br] 1 + [I] [I] [Cl] [Cl] [Cl] [Cl] [Br] [Br] [Br]

1.0, 5.0 5.0 1.0, 5.0 5.0 1.0, 5.0 5.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0

24 24 24 24 24 24 72 48 36 16 72 36 16

90 90 90 90 90 90 53 73 93 113 73 93 113

99 80 82 63 44 24 50 >99 >99 >99 70 >99 >99

entry

PLCO

Mn (kg/mol)b

Đb

Tg (°C)c

T5d (°C)d

1 2 3 4 5 6f

P1 P2 P3 P4 P5 P5

1.48 3.87e 10.6 6.19 15.3 15.0

1.38 1.18 1.21 1.17 1.51 1.31

152 154 178 165 178 180

233 221 227 199 224 229

a

General conditions: 200 mg PLCO (P1−P5), 10.0 mol % PPNCl, p(CO2)° = 20 bar, 73 °C (inside), MEK (2.0 mL); bMeasured by GPC (THF, 25 °C) using polystyrene standards, data for the PLDC product after isolation. cDetermined by DSC; data are taken from the second heating. dDecomposition temperature (Td) determined by TGA at 5% weight loss. eNote that, for this PLDC, the conversion of epoxy groups was 90% based on 1H NMR analysis. fThe PLDC was prepared at 98 °C using NaBr (10 mol %) as a catalyst at 25 bar CO2 for 16 h; see ref 9e.

a For entries 1−6: 25 mg of PLCO (Mn = 1.38 kg/mol, Đ = 1.75), p(CO2)° = 20 bar, MEK (0.50 mL). For entries 7−13: 100 mg of PLCO (Mn = 4.40 kg/mol, Đ = 1.31), p(CO2)° = 20 bar, MEK (2.0 mL). bFor entries 1−6, a set temperature was used; for entries 7−13, the temperature inside the reactor. cConversion of epoxy groups determined by 1H NMR. dToluene as solvent.

converted to their PLDC polymers using the optimized conditions. Precursor polymer P1, derived from a commercial 40:60 cis/trans mixture, was converted to a PLDC polymer (Table 2, entry 1) exhibiting a Tg value of 152 °C. Highermolecular-weight versions of PLCO derived from cis/trans (+)-LO (Table 2, entries 2 and 3) were then also investigated as substrates. The thermal properties of the resultant PLDCs were examined (see Figures S27−S35 in the Supporting Information), and an increase in Tg to a remarkable 178 °C was noted. Importantly, the use of P1−P314 was compared with

a parallel pressure reactor system (details in the SI (Tables S3 and S4)). Despite the fact that the presence of Al-complex 1 was beneficial for the conversion of the epoxy into cyclic carbonate groups at 90 °C (entries 1, 3, and 5 in Table 1), the use of a nucleophilic additive (Cl, Br, or I) also gave appreciable conversion (entries 2, 4, and 6 in Table 1). Therefore, next, we examined the conversion of a higher-molecular-weight-grade 3861


Letter

ACS Catalysis PLCO samples derived from pure cis-LO (P4 and P5; entries 4 and 5 in Table 2) and the thermal properties of the highestmolecular-weight PLDC (entry in Table 2) were of a similar order of magnitude, compared with the highest Tg value measured for PLDC obtained from a mixture of cis/trans (+)-LO (entry 3). These results suggest that, for the preparation of PLDC grades with high glass transitions, the use of the commercially available and less-expensive cis/trans mixture of LO is more attractive, compared to the upgraded, pure cis-LO. Intrigued by the results provided in Table 2 and the much lower glass transition reported for trans-PLDC by Sablong and co-workers (Mn = 11.2 kg/mol, Đ = 1.35, Tg = 146 °C), we decided to use P5 and converted it in a similar way (NaBr, 98 °C, 25 bar CO2; see Figure S25 in the SI), as described previously.9e The thermal properties of the PLDC produced were evaluated (Table 2, entry 6), and, surprisingly, we found a rather similar Tg of 180 °C, compared to the PLDC polymers of entries 3 and 5 prepared using our sequential approach, starting from LO instead of LDO. NMR comparison between the different stages of polymer construction was carried out using the highest molecular weight PLDC materials obtained through initial conversion of either commercial cis/trans-LO or upgraded cis-LO (Figure 1; analyses were related to the polymer precursors and PLDCs of entries 3 and 5 in Table 2). In each individual step, >98% conversion of the functional groups could be ascertained by NMR analyses. The PLDC polymers derived from cis/trans-(+)-LO show clear additional spectroscopic fingerprints best recognized in

the regions where the polycarbonate backbone CH fragment (in blue) and epoxide H atoms (yellow) resonate (5.1−5.3 and 2.5−2.7 ppm, respectively). Assuming that the epoxidation of the PLC precursors does not occur stereoselectively, distinct patterns for the resultant PLCO and PLDC polymers should be observed with several stereoisomeric repeat units. Apparently, the presence of a manifold of diastereoisomeric limonene− dicarbonate fragments in the PLDC polymers has little to no effect on the glass-transition behavior. Also, from the data in Table 2, it can be inferred that low-molecular-weight PLDC (entry 1) already gives rise to relatively high Tg values, which are potentially useful rigid building blocks in sequential copolymerization and chemical transformation strategies.15 These very high glass transitions for the PLDC polymers are ascribed to a reduced degree of rotational freedom for the substituted cyclohexyl fragments with the pendent cyclic carbonate group limiting the axial−equatorial conformational interconversion. In addition, the increased dipole moment in the pendent groups in PLDC may affect the overall rigidity of the polymer. In summary, here, we have reported a simple, although efficient, sequential approach for the synthesis of PLDC from commercial and inexpensive cis/trans (+)-LO with an unprecedented high glass transition of up to 180 °C. Efficient Al-catalysis can be used to prepare PLC polymers with preselected molecular weights between 1.3 and 15.1 kg/mol, and the conversion of the pendent alkene groups is simply and quantitatively achieved by standard epoxidation conditions, followed by chloride-assisted carboxylation, affording the PLDC products. This sequential procedure thus allows the preparation of small oligomeric, highly rigid polycarbonates with hydroxyl end-groups16 that can be used as precursors toward new coating applications. The higher-molecular-weight PLDCs could be used to create new polymer blends with tunable properties. Apart from this potential, the epoxy- and cyclic carbonate-functionalized PLCO/PLDC polymers offer great potential as “platform” molecules, since the conversion of the oxirane/carbonate units may allow for the introduction of new functional groups: such functionality includes oxazolidinones,17a cyclic sulfite/sulfate groups,17b and carbamates17c,d that are useful for post-modification and/or curing, allowing for the design of precision polymers. Studies along these lines are now ongoing in our laboratory.

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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/acscatal.7b00770.

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Experimental procedures and polymer characterization data (PDF)

AUTHOR INFORMATION

Corresponding Author

Figure 1. Selected regions in the 1H NMR spectra (CDCl3, 4.0−5.5 and 2.0−2.7 ppm) of the PLDC polymers obtained through the use of cis/trans-(+)-LO (left panel; Table 2, entry 3), and pure cis-LO (right panel; Table 2, entry 5): (a) prepared PLC precursor, (b) PLCO intermediate, and (c) final PLDC product. Note that the gray spheres represent the polymer chain ends, and the asterisks denote solvent impurities.

*E-mail: [email protected]. ORCID

Arjan W. Kleij: 0000-0002-7402-4764 Notes

The authors declare no competing financial interest. 3862


Letter

ACS Catalysis

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(9) Recent examples include: (a) Auriemma, F.; De Rosa, C.; Di Caprio, M. R.; Di Girolamo, R.; Ellis, W. C.; Coates, G. W. Angew. Chem., Int. Ed. 2015, 54, 1215−1218. (b) Auriemma, F.; De Rosa, C.; Di Caprio, M. R.; Di Girolamo, R.; Coates, G. W. Macromolecules 2015, 48, 2534−2550. (c) Hauenstein, O.; Agarwal, S.; Greiner, A. Nat. Commun. 2016, 7, 11862. (d) Hauenstein, O.; Reiter, M.; Agarwal, S.; Rieger, B.; Greiner, A. Green Chem. 2016, 18, 760−770. (e) Li, C.; Sablong, R. J.; Koning, C. E. Angew. Chem., Int. Ed. 2016, 55, 11572−11576. (f) Li, C.; Sablong, R. J.; Koning, C. E. Eur. Polym. J. 2015, 67, 449−458. (g) Reiter, M.; Vagin, S.; Kronast, A.; Jandl, C.; Rieger, B. Chem. Sci. 2017, 8, 1876−1882. (10) Fiorani, G.; Stuck, M.; Martín, C.; Martínez Belmonte, M.; Martin, E.; Escudero-Adán, E. C.; Kleij, A. W. ChemSusChem 2016, 9, 1304−1311. (11) LDO has recently been demonstrated as a useful monomer for polyurethane synthesis, see: (a) Schimpf, V.; Ritter, B. S.; Weis, P.; Parison, K.; Mülhaupt, R. Macromolecules 2017, 50, 944−955. (b) Bähr, M.; Bitto, A.; Mülhaupt, R. Green Chem. 2012, 14, 1447− 1454. (12) This trans-PLC was derived from the pure cis-LO. (13) Rintjema, J.; Kleij, A. W. ChemSusChem 2017, 10, 1274−1282. (14) Interestingly, when P3 was converted to PLDC under similar conditions as reported in Table 2, using only a reaction time of 18 h, a polymer with 61% converted epoxy-groups was attained that already exhibited a high Tg of 167 °C (T5d = 231 °C), see the SI (Figures S36 and S37). (15) Wang, Y.; Fan, J.; Darensbourg, D. J. Angew. Chem., Int. Ed. 2015, 54, 10206−10210. (16) Recently MALDI-TOF mass spectrometric analysis of various PLCs showed that the polymer contains primarily OH end-groups (ref 5c). Upon combining with suitable diisocyanate precursors, various polyurethane formulations can be prepared. (17) (a) Laserna, V.; Guo, W.; Kleij, A. W. Adv. Synth. Catal. 2015, 357, 2849−2854. (b) Laserna, V.; Martin, E.; Escudero-Adán, E. C.; Kleij, A. W. Adv. Synth. Catal. 2016, 358, 3832−3839. (c) Guo, W.; González-Fabra, J.; Bandeira, N. A. G.; Bo, C.; Kleij, A. W. Angew. Chem., Int. Ed. 2015, 54, 11686−11690. (d) Sopeña, S.; Laserna, V.; Guo, W.; Martin, E.; Escudero-Adán, E. C.; Kleij, A. W. Adv. Synth. Catal. 2016, 358, 2172−2178.

ACKNOWLEDGMENTS We thank ICREA, the CERCA Program/Generalitat de Catalunya and the Spanish Ministerio de Economıá y Competitividad (MINECO: CTQ-2014-60419-R and Severo Ochoa Excellence Accreditation 2014−2018, SEV-2013-0319) for support. N.K. wishes to thank the COFUND postdoctoral program of the E.U.

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REFERENCES

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