Renewable Polymers from Itaconic Acid by Polycondensation and

Article pubs.acs.org/Macromolecules

Renewable Polymers from Itaconic Acid by Polycondensation and Ring-Opening-Metathesis Polymerization Matthias Winkler,† Talita M. Lacerda,†,‡ Felix Mack,† and Michael A. R. Meier*,† †

Laboratory of Applied Chemistry, Institute of Organic Chemistry, Karlsruhe Institute of Technology (KIT), Fritz-Haber-Weg 6, 76131 Karlsruhe, Germany ‡ São Carlos Institute of Chemistry and Department of Materials Engineering/São Carlos School of Engineering, University of São Paulo, Av. Trabalhador São-carlense 400, CEP 13566-590, São Carlos, SP, Brazil S Supporting Information *

ABSTRACT: Itaconic acid, a renewable feedstock obtained by fermentation of carbohydrates, is used as a key substrate to produce aliphatic unsaturated polyesters as well as polynorbornenes. Renewable unsaturated polyesters were prepared by direct polycondensation of dimethyl itaconate (DMI) with diols, whereas the ring-openingmetathesis polymerization (ROMP) of a DMI derived norbornene led to polynorbornenes. The direct polycondensation of DMI was performed with tin(II) ethylhexanoate as catalyst, 4-methoxyphenol as radical inhibitor, and different diols to yield linear unsaturated polyesters with a molecular weight up to 11 500 Da without isomerization or cross-linking of the vinylic double bond. Further modification of the unsaturated polyesters by thia-Michael addition yielded polysulfides, which were subsequently oxidized to polysulfones. Moreover, the solvent-free and straightforward Diels− Alder reaction of DMI and cyclopentadiene was used to prepare a partially renewable norbornene monomer, which was used in ROMP with different catalysts to prepare polymers with low dispersities and adjustable molecular weights. The thereof derived unsaturated and functionalized renewable polynorbornenes were further modified by hydrogenation, and their thermal properties were evaluated.

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with a high IA content.9 However, this method took advantage of the CC double bond of IA, as it is the case of most of the already published studies. The work of Mishra and Ray, for example, involved the synthesis of a pH-sensitive hydrogel system composed of IA and N-[3-(dimethylamino)propyl]methacrylamide, prepared by copolymerization with N,Nmethylenebis(acrylamide) as a cross-linker.10 Tomić et al. prepared two series of hydrogels based on 2-hydroxyethyl acrylate, IA, and two poly(ethylene glycol) dimethacrylates of different ethylene glycol chain lengths by free radical crosslinking copolymerization.11 Zhou and Chuai developed a high oil-absorbing resin by suspension copolymerization with ethylene propylene diene, α-methylstyrene, and IA as monomers, benzoyl peroxide as initiator, and divinylbenzene as cross-linking agent.12 Studies related to the direct polycondensation of IA are scarcer, mainly because the vinylic double bond of IA tends to isomerize at high temperatures or to promote radical crosslinking. Thus, among the few studies dealing with this polymerization reaction, the ones that were successful involved mild conditions, which can for instance be achieved by

INTRODUCTION The possible shortage of crude oil and unpredictable price increases have led to a constant expansion of initiatives dedicated to find alternative resources for chemicals and especially polymers. In particular, there is a great interest in monomers from renewable resources, since the polymer industry depends on fossil resources.1−4 Inter alia, itaconic acid (IA), easily obtained from plant biomass, has great potential as feedstock for the production of monomers and polymers.5,6 Currently, IA is produced in an industrial scale of about 80 000 tons per year by fermentation of carbohydrate biomass using strains of filamentous fungus Aspergillus, such as Aspergillus terreus and Aspergillus itaconicus.7,8 High yields of IA are obtained when using glucose or sucrose as substrates in the respective fermentation, but other sources such as starch, molasses, or wood can be alternatively used.6,7 IA is a very interesting “green” starting material for the production of polymers, since it is a potential renewable substitute of fossilderived acrylic and methacrylic acids. Additionally, its two carboxyl groups allow its use as AA-type monomer for polycondensation reactions. One of the pioneering studies on the application of IA to produce polymers was reported by Marvel and Shepherd, who described the radical copolymerization of IA with acrylic acid, using potassium persulfate as initiator, to produce copolymers © XXXX American Chemical Society

Received: January 9, 2015 Revised: February 12, 2015

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Scheme 1. Synthesis of Renewable Polyesters (P1−P9) by Direct Polycondensation of DMI 1 with Different Diols (2−4) and a Radical Inhibitor; Polyesters Were Subsequently Modified via Thia-Michael Addition Reactions with Different Thiols (5−7) and Oxidized To Yield Polysulfones (P10−P13)

enzymatic polymerization. The latter approach was followed by Loos and co-workers, who inter alia demonstrated the successful enzymatically copolymerization of dimethyl succinate, itaconate, and 1,4-butanediol.13,14 Kobayashi et al. employed itaconic anhydride for lipase-catalyzed ring-opening addition polycondensation to afford copolymers with molecular weights up to 3500 Da.15 However, the use of IA or itaconic anhydride as single comonomer with diols yielded only low molecular weight polymers (Mn < 1000 Da). Most promising are the results recently reported from Khan et al., who made use of the Candida antarctica lipase B in order to prepare unsaturated polyesters with molecular weights up to 7900 Da by enzymatic polymerization of dimethyl itaconate (DMI) and oligo(ethylene glycol).16 IA is without doubt an interesting renewable substrate for the preparation of functionalized polyesters. A very straightforward way to prepare IA-based polymers is, in principle, the polycondensation with an appropriate diol to obtain polyesters with vinylic double bonds along the aliphatic backbone. Such unsaturated polyesters are attractive materials due to the possibility of postpolymerization modification of the vinylic double bonds along the polymer chain. This could be achieved, for instance, by cross-linking or Michael addition reactions, which enable the preparation of materials with shape-memory or self-degradable properties. 16−18 However, the direct polycondensation of IA or DMI with diols is not as straightforward as for conventional diacids or diesters, such as adipic acid or dimethyl adipate, since the α,β-unsaturated carbonyl group is less reactive and the vinylic double bond tends to undergo side reactions. For this reason, IA is usually used in terpolymerizations with other diacids (diesters) to yield moderate molecular weight copolymers. So far, usually only low molecular weights polymers were obtained in direct polycondensations of IA (Mn of about 1200−1300 Da).17,19,20 During the review process of this manuscript, Ramakrishan et al. described direct polymerizations of IA in the presence of a radical scavenger at a polymerization temperature of 160 °C. This resulted in ill-defined polyesters with high molecular weights and very high dispersities of 3.2−26.2, indicating the cross-linking of the polymer chains by radical side reaction of the vinylic double bond.21 In the present work, we describe the successful synthesis of unsaturated renewable polyesters without any isomerization of IA with molecular weights up to 11 500 Da, produced from the ester counterpart of IA (DMI) and different diols. Further modification of those polyesters by

thia-Michael addition reactions with mercaptans yielded polysulfides, which were then oxidized to polysulfones. Recently, Li and co-workers reported the preparation of the diene monomer di(10-undecenyl) itaconate, easily prepared by esterification of IA and 10-undecenol.22 Fully renewable unsaturated polyesters with molecular weights of up to 44 kDa were synthesized via acyclic-diene-metathesis (ADMET) polymerization. Additionally, the authors presented the postpolymerization modification of the unsaturated polyesters by Michael addition reactions, which led to polyesters with interesting material properties, e.g., self-degradability. The Diels−Alder (DA) reaction corresponds to a very interesting reaction with regard to green chemistry and should also be useful to prepare monomers from IA. In the present work, we thus report the preparation of a norbornene derived from the DA reaction of DMI and cyclopentadiene (CPD), an interesting monomer to be further applied in ring-opening metathesis polymerization (ROMP) to prepare defined polynorbornenes with adjustable molecular weights.

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RESULTS AND DISCUSSION Renewable Polyesters by Polycondensation. The polycondensation of DMI with different diols was first studied under different conditions to yield polyesters with optimized molecular weights. Polymerization reactions using 1,5,7triazabicyclo[4.4.0]dec-5-ene, Ti(OiPr)4 or p-toluenesulfonic acid as catalysts were not successful. In a temperature range of 120−140 °C and under high vacuum (10−2 mbar), either only low molecular weight (Mn < 1300 Da) or completely insoluble materials were obtained. In polymerization reactions yielding insoluble materials, the vinylic double bonds led to crosslinking of the polymer chains. In order to avoid such radicalmediated side reactions, 4-methoxyphenol, a radical inhibitor for acrylate monomers, was thus used for all further polycondensation reactions. The best results for the polycondensation of 1 were obtained with tin(II) ethylhexanoate (1 mol %) as catalyst and 4-methoxyphenol (0.5 wt % related to all reactants) at 130 °C. The reactions were first stirred under an argon atmosphere at 130 °C, and the pressure was gradually reduced to 10−2 mbar (more details can be found in the Supporting Information). In this way, linear polymers without cross-linking were obtained. Size-exclusion chromatography (SEC) and differential scanning calorimetry (DSC) of the polycondensates obtained from DMI with 1,6-hexanediol (2), 1,10-decanediol (3), or 1,12-dodecanediol (4) revealed the successful synthesis of unsaturated polyesters with a molecular B

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di(10-undecenyl) itaconate, easily prepared by esterification of IA and 10-undecenol.22 Compared to P1−P3, the synthesized polyesters of a molecular weight of about 44 kDa have a different structure. The ADMET polymers have additional aliphatic double bonds along their backbone, and although these polyesters have long-aliphatic chain segments, no glass or melt transitions were observed. To modify polymers or also peptides functionalized with α,βunsaturated double bonds, the thia-Michael addition using readily available thiols proved to be a powerful tool for very efficient postpolymerization modification.24−26 Thus, thiaMichael addition reactions were further used as tool to functionalize the vinylic double bonds of the synthesized polyesters. Complete functionalization of the vinylic double bonds was achieved by using different thiols with hexylamine (10 mol %) as catalyst at 40 °C (Scheme 1).27 1H NMR analysis of the modified polyesters revealed full conversion of the double bonds as evidenced by the disappearance of the vinylic proton signals at 6.31 and 5.70 ppm and the newly emerging signals resulting from the introduced thiol (Figure 1 and Figures S5−S10). Moreover, SEC analysis of the crude reaction mixture showed homogeneous molecular weight distributions and molecular weights similar to the nonfunctionalized polyesters, which, in combination with the 1H NMR analysis, confirms the successful modification via thia-Michael addition without degradation of the polymers or side reactions (Table 2 and

weight of about 9900−11 500 Da and melting points (Tm) ranging from −40 to 44 °C (Scheme 1 and Table 1), depending Table 1. Results of SEC and DSC Analysis of P1−P3

a

polymer

[diol]

Mna [Da]

Đ

Tm [°C]

P1 P2 P3

2 3 4

11500 10200 9950

1.64 1.85 1.75

−40 35 44

Determined by SEC relative to narrow PMMA standards.

on the applied diol. Running the polycondensations for longer reaction times or at higher temperatures did not lead to defined and/or higher molecular weight polyesters since cross-linking of the polymer chains was observed yielding polyesters with very high dispersities. The highest melting point was observed for P3, since the long aliphatic chain segments contribute to enhanced chain− chain interactions. Compared to well-established polyesters, for instance polycaprolactones (Tm values around 60 °C), the melting points of P1−P3 are lower, since van der Waals interactions at the aliphatic polymer chain segments are hindered by the vinylic double bonds.23 SEC analysis revealed homogeneous molecular weight distributions with dispersities close to 2. Additionally, NMR analysis confirmed the successful preparation of the desired linear unsaturated polyesters without isomerization or cross-linking of the vinylic double bonds (see Figure 1 or Figures S1 and S2 in the Supporting Information). Using optimized conditions, we demonstrate here that a direct polycondensation of DMI is a very straightforward way to prepare functionalized renewable polyesters, with the additional advantage that no preliminary protecting steps of the vinylic double bond was necessary, and the obtained α,β-unsaturated polyesters could be further modified to adjust their material properties. The key to the successful synthesis of the itaconic acid derived polycondensation products is obviously the use of a radical inhibitor and high vacuum, which prevented radical side reactions and enabled the synthesis of polyesters with molecular weights up to 11 500 Da. Li et al. prepared similar polyesters by ADMET polymerization of a diene monomer

Table 2. Results of SEC and DSC Analysis of P4−P9 polymer P4 P5 P6 P7 P8 P9

[P] + [thiol] P2 P2 P2 P3 P3 P3

+ + + + + +

5 6 7 5 6 7

Mn [kDa]

Đ

Tm [°C]

8.8 8.4 14.2 10.5 11.8 14.0

1.76 1.57 1.50 1.70 1.44 1.42

−41 (Tg) −33 −44 (Tg) −20 −7 0

Figure S11). The results obtained by DSC analysis showed that all modified polyesters (P4−P9) exhibited lower melting points

Figure 1. Comparison of 1H NMR (300 MHz/CDCl3) spectra of P2 (bottom) and P4 (top). C

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Scheme 2. Synthesis and Modification of Polynorbornenes Prepared by ROMP of the Itaconic Acid-Derived Norbornene 8 with Alkylidene Catalystsa

a

C1: umicore M31 ([1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene]dichloro-(3-phenyl-1H-inden-1-ylidene)(pyridyl)ruthenium(II)); C2: Grubbs third-generation dichloro[1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene](benzylidene)bis(3-bromopyridine)ruthenium(II).

The direct polycondensation of DMI with different diols is, therefore, a powerful approach that enabled the synthesis of renewable unsaturated polyesters, which can be modified by subsequent thia-Michael addition and oxidation of the respective polysulfides to prepare materials according to the desired properties. Renewable Polynorbornenes by ROMP. The Diels− Alder (DA) reaction of DMI and cyclopentadiene (CPD) leads to the formation of norbornene 8, an interesting monomer for the synthesis of polyesters or ROMP (Scheme 2). It has to be noted that the performed DA reaction of DMI and CPD yielded two diastereomers: the endo (75%) and exo (25%) isomer. The DA reaction was performed without any solvent, and only a small excess of CPD to obtain the Diels−Alder product as a mixture of the two diastereomers in good yields and high purity by very simple work-up (distillation). ROMP is a very mild and catalytic method to prepare welldefined polymers with adjustable molecular weights and low dispersities. In order to study the ROMP of 8 (endo isomer), two different catalysts C1 and C2 were used and the monomer to initiator (catalyst) ratio was varied (Scheme 2 and Table 3).

compared to the nonfunctionalized polyesters due to the increased steric hindrance of the introduced side-chain moieties, which hamper the chain−chain interaction of the long aliphatic chain segments. Polyester P9 (functionalized with mercaptoethanol 7) showed the highest melting point (0 °C), since the introduced hydroxyl groups introduce additional chain−chain interactions. The thia-Michael modified polyesters P4 and P6 exhibited no crystallinity with glass transition temperatures of −41 to −44 °C. In order to prepare polyesters with a similar structure to P4− P9, DMI can, in principle, be first modified via thia-Michael addition and then be used in polycondensation reactions with the respective diols.28 In this way, the vinylic double bond would be modified before the polycondensation, which should avoid any radical side reaction and increase the activity of the ester group next to the tertiary carbon. However, polycondensation reactions with DMI derived thia-Michael products were not successful as the thio-ethers underwent elimination reactions during the polycondensation, leading to undefined low molecular weight polymers and broadening of the molecular weight distribution. Polysulfides can be oxidized to polysulfones to obtain polymers with a significantly different solubility and modified thermal properties.29,30 In order to further modify the material properties of the IA-based polymers, the polysulfides P4, P5, P7, and P8 were oxidized to the respective polysulfones P10− P13. The complete oxidation of the polysulfides to the desired polysulfones was confirmed by NMR, SEC, DSC, and IR analysis (see Supporting Information). The oxidation reactions to the corresponding polysulfones led to increased melting points or glass transition temperatures, which can be explained by the increased chain−chain interactions due to the polar sulfone groups (Table S1). For instance, polysulfone P10 showed a 40 °C higher glass transition temperature than the respective polysulfide P4. Full oxidation of the thioethers was also evidenced by NMR analysis. A shift of the set of proton signals next to the oxidized sulfur confirmed the transformation to the respective polysulfones (Figures S13−S16), while for the soluble polysulfones SEC analysis revealed homogeneous molecular weight distributions and polymers with slightly increased molecular weights due to the altered hydrodynamic radius.

Table 3. Results of SEC and DSC Analysis of P14−P20, Using Different Monomer to Initiator Ratios ([M]:[I]) polymer

[M]:[I]

[cat.]

Mn [kDa]

Đ

Tg [°C]

P14 P15 P16 P17 P18 P19 P20

50:1 80:1 120:1 150:1 200:1 50:1 150:1

C1 C1 C1 C1 C1 C2 C2

16.0 28.4 48.6 65.2 87.5 19.7 51.4

1.07 1.09 1.14 1.20 1.26 1.14 1.42

57 68 58 60 76 69 82

The best performance was observed for catalyst C1, and a good control over molecular weight and low dispersities were obtained at room temperature within 30 min (Table 3). On the other hand, C2 led to polymers with a good control over molecular weight as well, though the dispersities of the obtained polynorbornenes were higher. ROMP using the Hoveyda−Grubbs second-generation catalyst were successful as well, although the polymerization resulted in higher dispersities of 1.5−1.9 due to slow initiation of this catalyst.31−33 D

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Figure 2. SEC analysis of P14−P17 (a) and plot of the molecular weight versus the monomer to initiator ratio (b).

of the double bond proton signals at 5.42−5.02 ppm (Figure S21). The molecular weights determined by SEC slightly increased after hydrogenation, whereas the molecular weight distributions remained homogeneous, while retaining low dispersities. DSC analysis showed lower glass transitions temperatures of the hydrogenated polyesters (Table S2), which can be explained by the increased flexibility of the polymer backbone.

All polynorbornenes prepared in this study were characterized by NMR, SEC, and DSC analysis. 1H NMR analysis confirmed the structure of the desired polymers and revealed a cis/trans ratio of the double bonds for all polymers of 43/57 (Figure S21). SEC analysis revealed narrow and homogeneous molecular weight distributions (Figure 2a). It is important to mention that the values of molecular weights determined by SEC are higher than the theoretical ones, since they were determined relative to narrow PMMA standards. Figure 2b illustrates the linear correlation of the molecular weight versus the monomer to initiator ratio, as expected for well-controlled ROMP reactions. The thermal analysis of the polymers revealed that all polynorbornenes are amorphous, with glass transition temperatures ranging from 57 to 82 °C. Interestingly, directly employing the mixture of the two diastereomers (endo/exo), obtained as product from the Diels−Alder reaction of DMI and CPD, yielded polymers with similar cis/trans ratios of the double bonds and the same thermal properties as for polynorbornenes derived from the single endo isomer. Thus, the work up of the Diels−Alder reaction becomes very straightforward, since the crude reaction mixture of the Diels−Alder reaction was purified by simple distillation. The polymers synthesized by ROMP are interesting materials since they have double bonds along the aliphatic backbone and methyl esters that are suitable for postpolymerization modification, which is very interesting to tailor the material properties. In further reactions, the cis- and transconfigured double bonds along the polymer backbone were hydrogenated in order to modify the ROMP polymers and study their thermal properties. Hydrogenation of ROMP polymers from norbornene monomers are usually performed with palladium- or ruthenium-based catalyst and diverse additives or promoters.34 Fogg and co-workers introduced an elegant one-pot ROMP and hydrogenation procedure by the addition of methanol as a polar co-solvent and triethylamine as additive.35,36 In order to compare the thermal properties of the ROMP polymers before and after hydrogenation, we used a two-step procedure; therefore, the polynorbornenes were first isolated and subsequently hydrogenated. The hydrogenation of P15 and P16 was performed in THF at 60 °C with Pd(OH)2/ C as catalyst and a hydrogen pressure of 40 bar to yield P21 and P22. Full hydrogenation of the unsaturated polymers was evidenced by 1H NMR analysis by means of the disappearance

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CONCLUSIONS In summary, we have successfully prepared renewable unsaturated polymers based on itaconic acid by either polycondensation of dimethyl itaconate and different diols or ROMP of a dimethyl itaconate derived norbornene. Polycondensation reactions with different diols were optimized regarding the polymerization procedure in order to prepare linear dimethyl itaconate-based polyesters with a molecular weight up to 11 500 Da and without cross-linking or isomerization of the vinylic double bond. The prepared unsaturated polyesters were further modified by thia-Michael addition using three different thiols, whereas subsequent oxidation led to polysulfones with modified thermal properties, as evidenced by DSC analysis. The solvent-free, efficient, and very straightforward Diels−Alder reaction of dimethyl itaconate and cyclopentadiene was performed to synthesize a renewable norbornene monomer used in ROMP reactions to prepare functionalized polynorbornenes. Herein, C1 showed the best performance yielding polymers with low dispersities and adjustable molecular weights. Analysis of the ROMP polymers revealed an excellent control over the polymerization by means of a linear correlation of the Mn and the monomer to initiator ratio. Finally, the polynorbornenes prepared by ROMP were hydrogenated to study the thermal properties of the modified materials, which presented lower glass transitions as consequence of the increased flexibility of the polymer backbone.

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ASSOCIATED CONTENT

* Supporting Information S

Materials, synthesis details of monomers and polymers, the properties of monomers and polymers including 1H and 13C NMR spectra, IR, HRMS, molecular weight, and thermal information. This material is available free of charge via the Internet at http://pubs.acs.org. E

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(30) van den Berg, O.; Dispinar, T.; Hommez, B.; Du Prez, F. E. Eur. Polym. J. 2013, 49, 804−812. (31) Kim, K. O.; Shin, S.; Kim, J.; Choi, T.-L. Macromolecules 2014, 47, 1351−1359. (32) Thiel, V.; Hendann, M.; Wannowius, K.-J.; Plenio, H. J. Am. Chem. Soc. 2011, 134, 1104−1114. (33) Vougioukalakis, G. C.; Grubbs, R. H. Chem. Rev. 2009, 110, 1746−1787. (34) Sohn, B. H.; Gratt, J. A.; Lee, J. K.; Cohen, R. E. J. Appl. Polym. Sci. 1995, 58, 1041−1046. (35) Drouin, S. D.; Zamanian, F.; Fogg, D. E. Organometallics 2001, 20, 5495−5497. (36) Camm, K. D.; Martinez Castro, N.; Liu, Y.; Czechura, P.; Snelgrove, J. L.; Fogg, D. E. J. Am. Chem. Soc. 2007, 129, 4168−4169.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.A.R.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M. Winkler is gratefully thankful to Volker Winkler (KIT, IAMWPT) for inspiring discussions. T. M. Lacerda acknowledges FAPESP for her postdoctoral fellowship (2013/02663-7).

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