Letter pubs.acs.org/macroletters
Acrylic Platform from Renewable Resources via a Paradigm Shift in Lactide Polymerization Timothy C. Mauldin,† Jason T. Wertz,‡ and Dylan J. Boday*,† †
IBM Corporation, Tucson, Arizona 85744, United States IBM Corporation, Poughkeepsie, New York 12601, United States
‡
S Supporting Information *
ABSTRACT: A new polyacrylate, poly(methylidenelactide), with high thermal stability and derived from biobased resources is reported. This polymer is formed from the radical polymerization of a modified lactide derivative and represents one of the few examples of an acrylic from which the entire mass is bioderived and is made from a simplistic synthesis. Furthermore, poly(methylidenelactide) serves as a foundation for a platform of new acrylic structures, owing to pendant cyclic diesters that are susceptible to postpolymerization modification via simple transesterification chemistry. Several examples of unique acrylics made from poly(methylidenelactide) are synthesized and characterized. crylics are a class of polymers that find widespread use as adhesives, rubbers, coatings, structural applications, and so on. Acrylates and methacrylates are traditionally manufactured from petrochemicals, which conflicts with a growing desire to produce organic materials from agricultural feedstocks,1 owing to concerns over finite petroleum supplies, carbon emissions, and so on. However, examples of acrylics derived entirely from renewable resources are not particularly common, with notable examples being a variety of processes designed to convert sugars and starches to acrylic acid via genetically engineered E. coli strains and other conversions of renewable resources, such as itaconic acid and levulinic acid to assorted acrylics.2 But it would be desirable to obtain biobased acrylics of which (1) the entire mass is derived from biological resources, (2) the renewable feeds are already commodity chemicals produced on large volumes, and (3) are made from a simple and scalable synthetic pathway. We describe the synthesis of a series novel polyacrylates derived from postpolymerization alcoholysis of poly(methylidenelactide), which is derived from the cyclic diester lactide through a series of simple and high-yielding synthetic steps. Lactide is produced on commercial volumes and used almost exclusively for the preparation of the polyester poly(lactic acid) by a ring-opening polymerization motif.3 Poly(methylidenelactide) has been reported only sporadically in the literature,4a−c the paucity of sources describing the utility of this polymer is likely due to difficulties revolving around its poor solubility, sluggish dissolution, and that it is challenging to handle the polymer as a melt. Britner et al. in part found a way to overcome this challenge by functionalizing poly(methylidenelactide) with amines to form several unique polyacrylamides.4c Yet here, we demonstrate the ability to convert poly(methylidenelactide) to a wide-range of novel acrylic materials via ring-opening transesterification. Hence, we believe this work represents a platform by which a series of
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complex acrylic structures are achievable through straightforward and scalable chemistry. Methylidenelactide (1) was synthesized from a sequential bromination and dehydrohalogenation of lactide by modification of a procedure reported elsewhere (Figure 1).4 Poly-
Figure 1. Synthesis of methylidenelactide (1) and poly(methylidenelactide) (2) and a collection of physical and thermal properties for 2.
(methylidenelactide) (2) was formed through radical polymerization with the thermal initiator 2,2-azobis(isobutyronitrile) (AIBN) in tetrahydrofuran (THF) at 60 °C for 30 h. The resulting polymer (Mn = 20.5 kg/mol, Đ = 1.82) is amorphous and a thermally robust material with a glass transition of 246 °C5 and a decomposition temperature (i.e., temperature at 5% mass loss) of 302 °C. Received: January 11, 2016 Accepted: March 30, 2016
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DOI: 10.1021/acsmacrolett.6b00023 ACS Macro Lett. 2016, 5, 544−546
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ACS Macro Letters Our goal was to use 2 as a precursor to form a platform of acrylic structures via chemical modification of the pendant diester through, for example, ring-opening transesterification with alcohols. However, the search for an adequate solvent for such reactions was initially discouraging, as 2 possesses only sparing solubility and markedly slow dissolution kinetics in tetrahydrofuran and dimethyl sulfoxide, and it is completely insoluble in a wide range of other common organic solvents. However, serendipitously, it was found that 2 is highly soluble and dissolves rapidly in a variety of THF/alcohol cosolvent blends. For example, in THF and methanol, 2 exhibited optimal solubility in a 2:1 (v/v) THF/methanol solution, substantially higher solubility than in either THF or methanol alone, the latter in which 2 is completely insoluble. Other cosolvent blends (THF/ethanol, THF/n-butanol, etc.) were similarly found to dissolve 2 to a higher extent than the individual components of each blend. Furthermore, a crude set of Hansen solubility parameters for 2 was derived from cosolvent solubility data (see Supporting Information) as δD = 16.2 MPa1/2, δP = 7.9 MPa1/2, and δH = 12.8 MPa1/2, the predictive power of which allowed for the selection of less orthodox, yet singlecomponent, solvents for 2. Most relevant to the goals in this work to functionalize 2 with alcohols, benzyl alcohol (Hansen solubility parameters: δD = 18.4 MPa1/2, δP = 6.3 MPa1/2, and δH = 13.7 MPa1/2, a satisfactory match to that of 2) was found to be a suitable solvent for 2, even without the inclusion of THF as a cosolvent. Triazabicyclodecene (TBD) was used as a catalyst to modify the pendant diesters on 2 via ring-opening transesterification with methanol. Excess methanol (300 equiv relative to diester) was used to drive the functionalization. Owing to two inequivalent esters, an initial transesterification of each pendant diester ring can form two possible products, denoted as II and III in Figure 2. A subsequent transesterification of either intermediate yields the product IV. All three transesterified
products exist in equilibrium with the initial cyclic diester I. Upon reaching steady-state concentrations at various temperatures, the catalyst was quenched with benzoic acid, and the molecular weight was measured by GPC and product distributions determined by 1H NMR. When equilibrium is reached at −25 °C, the resulting polymer moderately increases in molecular weight from 20.5 kg/mol (for neat 2) to 23.6 kg/mol (Figure 3). Such a
Figure 3. GPC chromatograms of poly(methylidenelactide) (2) compared with the result of 2 subject to transesterification conditions (methanol and a catalytic amount of TBD) at various temperatures. An additional transesterification at 60 °C is omitted due to the resulting polymer’s insolubility in common GPC mobile phases.
molecular weight increase is expected for a polymer rich in intermediate structures II and III, in which a molecule of methanol is formally added to each pendant diester. Conversely, the doubly transesterified diester IV is expected to yield a net decrease in molecular weight, relative to the cyclic diesters present in neat 2, owing to the loss of a lactyl substructure from each diester as either intermediate II or III is converted to the final product IV. But in the case of the equilibrium reached at −25 °C, the aggregate molar concentrations of intermediate structures II and III (7.1% + 14.5% = 21.6% of all diesters exist as II or III, shown in Figure 2) exceeds that of the doubly transesterified product IV (15.0%), and hence, the increase in molecular weight is consistent with expectations. Oddly, the chromatogram corresponding to the polymer formed at −25 °C is bimodal, which is not the case for either neat 2 or any of the equilibrium products formed at other temperatures. The nature of this bimodality is not immediately obvious and is the subject of future investigations. Upon allowing 2 to reach steady-state equilibrium concentrations in separate reactions at higher temperatures, the equilibrium shifts toward the doubly transesterified product IV. The molecular weight of 2 subject to transesterification at 5 and 23 °C was 14.6 kg/mol and 11.5 kg/mol, respectively, both marked decreases from neat 2’s molecular weight of 20.5 kg/ mol. This drop in molecular weight is consistent with an increasing concentration of equilibrium product IV (which is of a leaner mass than the cyclic diester I and intermediates II/III) from 0 mol % in neat 2 to 34.5 and 67.3 mol % at 5 and 23 °C, respectively. Equilibrium under transesterification conditions was also conducted at 60 °C; however, the structure of this product changed substantially enough to render it insoluble in all single-component solvents (including THF), and thus, the molecular weight of this polymer was not determinable by GPC. 1H NMR, however, indicated that 80.5 mol % doubly
Figure 2. Equilibrium between pendant cyclic diester I exclusively present in neat 2, singly transesterified products II and III, and doubly transesterified product IV, along with the steady-state concentrations of each product at various temperatures, determined by NMR (Supporting Information). 545
DOI: 10.1021/acsmacrolett.6b00023 ACS Macro Lett. 2016, 5, 544−546
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ACS Macro Letters
Patent WO 2013185009 A1, Metabolix, Inc., U.S.A., December 12, 2013. (d) Rosenberg, M. J. The Commercialization of BioAcrylic Acid; http://www.opxbio.com/2012/09/the-commercialization-ofbioacrylic-acid (accessed April 12, 2015). (e) Manzer, L. E. Appl. Catal., A 2004, 272, 249−256. (f) Braden, D. J.; Henao, C. A.; Heltzel, J.; Maravelias, C. C.; Dumesic, J. A. Green Chem. 2011, 13, 1755− 1765. (g) Gowda, R. R.; Chen, E.Y.-X. Org. Chem. Front. 2014, 1, 230−234. (3) (a) Inkinen, S.; Hakkarainen, M.; Albertsson, A.-C.; Södergård, A. Biomacromolecules 2011, 12, 523−532. (b) Gupta, G.; Revagade, N.; Hilborn, J. Prog. Polym. Sci. 2007, 32, 455−482. (c) Mehta, R.; Kumar, V.; Bhunia, H.; Upadhyay, S. N. J. Macromol. Sci., Polym. Rev. 2005, 45, 325−349. (4) (a) Scheibelhoffer, A. S.; Blose, W. A.; Harwood, H. J. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1969, 10, 1375−1380. (b) Miyake, G. M.; Zhang, Y.; Chen, E.Y.-X. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 1523−1532. (c) Britner, J.; Ritter, H. Macromolecules 2015, 48, 3516−3522. (d) Jing, F.; Hillmyer, M. A. J. Am. Chem. Soc. 2008, 130, 13826−13827. (5) Tg was measured by a second heating scan via Differential Scanning Calorimetry (DSC). However, two thermal transitions were observed, at 39 and 246 °C, both repeatable upon heating and cooling. Hence, Dynamic Mechanical Analysis (DMA) was performed to assign the thermal transitions, with the latter identified as the Tg (Supporting Information). Unfortunately, neither DSC nor DMA revealed thermal transitions for equilibrium products I, II, III, or IV before these products decomposed.
transesterified product IV was formed, further confirming the trend of an equilibrium shift toward product IV at elevated temperatures. The extent to which the equilibrium lies with product IV varies significantly with the alcohol used in the transesterification. In the example mentioned above, transesterification with methanol at 60 °C yielded 80.5 mol % of product IV. However, when the transesterifications were conducted with n-butanol and benzyl alcohol at 60 °C, the amount of doubly transesterified product (i.e., the structures analogous to equilibrium product IV for each alcohol) was 62.3 and 16.6 mol %, respectively, with corresponding drops in molecular weight (see Supporting Information). In summary, radical polymerization of a modified lactide monomer 1 yields a unique and thermally robust acrylic polymer, 2, that is derived entirely from biological resources. Compound 2 possesses pendant cyclic diesters susceptible to postpolymerization functionalization through, for example, ring-opening transesterification with a variety of alcohols. Furthermore, the extent by which singly (equilibrium products II and III) and doubly (equilibrium product IV) transesterified products are formed can be tuned with relative ease by adjusting the temperature at which the transesterification equilibrium is reached. This, combined with the fact that the ring-opening liberates a free hydroxyl group that is easily susceptible to further structural modification that is not elaborated on here, allows for a rich and varied class of achievable polymeric structures. Hence, the work reported herein not only introduces, perhaps, the first easily achievable, biobased acrylic, but we also anticipate a plethora of other novel biobased acrylics to be developed there from.
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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.6b00023. Details of syntheses, characterization, and NMR spectra (PDF).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We kindly thank Sue A. Roberts at the University of Arizona for assistance with X-ray diffraction analyses and Sarah Czaplewski at IBM for assistance with dynamic mechanical analysis.
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REFERENCES
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