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Contributed by Jeffery A. Byers, Ph.D., Assistant Professor of Chemistry, Boston College
It is particularly challenging to find an application where synthetic plastics have not become important. From disposable bags, cutlery and cups, lightweight automobile parts, vibrant paints and robust coatings, sticky adhesives, flexible textiles, and even devices designed to deliver drugs, the usefulness of synthetic polymers have arguably made them the most important discovery of the twentieth century. However, as we get firmly entrenched in the twenty-first century, our society is faced with the challenge of continuing to produce synthetic polymers that have properties suitable for all of these applications (and more) but without the environmental disadvantages that have plagued many synthetic polymers derived from non-renewable resources. In order to achieve this goal, access to synthetic polymers that are degradable and derived from renewable resources is an ongoing effort for chemists and engineers. Unlike the synthetic polymers developed previously, these new materials must meet the materials properties requirements and have degradable properties that are in line with the intended application: plastic bags should last months and car bumpers should last years, not vice versa.
As part of this ongoing effort, our research group has targeted versatile catalyst systems that can incorporate multiple building blocks (i.e., different monomers), control three-dimensional structure (e.g., stereochemistry) and control polymer architecture (e.g., linear versus cross-linked polymer). A catalyst system that has been particularly versatile has been the iron-based complex shown below that, when combined with an alcohol initiator, is active for the ring opening polymerization of lactide (a 
An interesting and useful property of lactic acid is that it is a chiral molecule. Chiral molecules, like hands, are molecules whose mirror image is not superimposable. When these lactic acid units are assembled in a polymer, the two different possible chiral lactic acid units, or stereoisomers, can assemble in regular or irregular ways. Previously it has been shown that the physical properties[2] and degradation rates[3] of poly(lactic acid) can be profoundly affected by how the stereoisomers are distributed (e.g., alternating versus homogeneous distribution) and how regularly they are distributed. For example, polymers that have a random distribution of lactic acid stereoisomers are amorphous materials that degrade relatively quickly while those with uniform distribution are crystalline materials with high melting points and slower degradation rates. Thus, the ability to synthesize poly(lactic acid) with the stereoisomers distributed in different ways provides access to materials with a broad array of physical properties and degradation profiles.
Historically, carefully designed, and (often) chiral catalysts have been utilized to synthesize poly(lactic acid) in a stereoregular fashion. However, this approach can be synthetically laborious if the synthesis of poly(lactic acids) with different degrees of stereoregularity is desired. Alternatively, the iron-based catalysts that we have developed are capable of producing a range of stereoregular poly(lactic acids) from a single iron catalyst precursor.[4] The key to the success of this method was the discovery that stereoselectivity in the polymerization reaction could be affected by the identity of additives used as initiators. Whereas alcohol initiators led to stereoirregular poly(lactic acid), silanol initiators led to stereoregular poly(lactic acid). Not only did the different additives change the selectivity of the reaction, the regularity of this stereoselectivity (also known as tacticity) could be controlled by altering the identity of the silanol initiator and the stereoisomer of the lactide (see graph where higher Ps indicate more stereoregular polymer and Ps = 50 is stereorandom polymer). Since the silanol initiators were used as additives in the reaction, a variety of stereoregular poly(lactic acid) was obtainable without the need to synthesize a library of metal precursors. Mechanistic studies revealed that the origin of the stereoselectivity in these reactions was a consequence of the silanol additive converting the achiral iron complexes into chiral catalysts during the course of the reaction. Since the three dimensional structure of the catalyst was partially defined by the silanol additive, the degree of stereoselectivity could be altered by changing the identity of the silanol additive.
While control over stereochemistry is a powerful advantage for tailoring the properties of poly(lactic acid), the iron-based catalysts proved to be even more versatile. Unlike most catalysts used for lactide polymerization, the iron-based catalysts can be reversibly oxidized by sequential exposure to chemical oxidants and reductants. Inspired by previous reports that demonstrated that lactide polymerization rates could be altered by changing the oxidation state of a catalyst,[5],[6],[7] we were pleased to find that our iron-based catalyst system could be deactivated upon exposure to oxidants and then later reactivated upon exposure to reductants (see blue trace in graph below). The switching capabilities of the catalyst system rendered it useful for the diversification of poly(lactic acid) when we discovered that complementary reactivity was observed when epoxides were used as monomers.[8] In other words, the iron-based catalyst could be switched on and off for epoxide ring opening polymerization much like it could for lactide polymerization, but this time the oxidized form of the catalyst was active for polymerization whereas the reduced form of the catalyst was inactive for polymerization (see red trace in graph below).
We next capitalized on the complementary reactivity of the catalyst towards polymerization of epoxides and lactide by carrying out redox switchable copolymerization reactions starting from a mixture of an epoxide and lactide (see top of Scheme below).[8] Polyester-polyether diblock copolymers could be synthesized by starting with the catalyst in its reduced state resulting in the selective polymerization of lactide followed by chemical oxidation resulting in the selective polymerization of the epoxide. The alternative diblock copolymer starting with epoxide polymerization first followed by lactide polymerization could also be synthesized by starting with the catalyst in its oxidized state followed by chemical reduction. These results were exciting because they provide access to copolymers that are difficult to synthesize in one step, and they also suggested that multi-block copolymers of various polyester/polyether composition could be synthesized from the same reaction feed by altering the oxidation of the catalyst and the time between redox events. Such copolymers are expected to exhibit physical properties and degradation rates that differ significantly compared to poly(lactic acid).
In addition to synthesizing block copolymers using the iron-based polymerization catalysts, we have applied the redox-switching capabilities of the catalyst to create a redox-triggered crosslinking reaction (see bottom of the Scheme below).[9] Cross-linked polymers are linear polymers that have been chemically bonded to one another. Cross-linked polymers are tough materials that often demonstrate superior properties compared to linear polymers, but they are commonly difficult to process. As a result, triggered crosslinking reactions are desirable because they allow one to process the linear polymer prior to its crosslinking. Although many external stimuli have been used to trigger crosslinking reactions (e.g., heat, light, acid, etc.), to the best of our knowledge the only example of redox-triggered crosslinking reactions that has been reported to date have been for the reversible formation of disulfide bonds (i.e., RS—SR).[10] The development of a redox-triggered crosslinking reaction would complement the established methods. Moreover, while cross-linked poly(lactic acid) has been achieved through high energy light[11],[12] or electron beam irradiation,[13] the effect that crosslinking has on the physical, mechanical, and degradation properties of the polymer have largely been unexplored.
To utilize the redox-switchable, iron-based complexes for crosslinking applications, we synthesized a monomer that incorporated both a lactide-like cyclic diester and an epoxide (see bottom of the Scheme below). Exposing this monomer to the reduced form of the complex resulted in the formation of a linear polyester that contained epoxide functional groups as side chains. Catalyst oxidation and concentration of the reaction mixture led to the rapid intermolecular reaction between the epoxide side chains, which resulted in crosslinking of the polymer. This material demonstrated significantly different thermal and swelling properties compared to linear poly(lactic acid). Furthermore, copolymerizing the epoxide functionalized cyclic diester with lactide using the reduced form of the catalyst followed by oxidation resulted in crosslinked poly(lactic acid) with variable crosslinking densities. This capability led to the production of a series of polymers derived from lactic acid that possessed different thermal properties. It is expected that future exploration of the polymer's mechanical properties and degradation rates will lead to polymers with significantly different properties compared to poly(lactic acid).
While investigation of the thermal, mechanical and degradation properties of the polymers that we have synthesized are still in its infancy, access to a wide variety of polymers with varying composition, stereoregularity, and three dimensional structure has been made possible by utilizing the iron-based catalysts developed in our lab. It is expected that the synergism between catalyst structure and polymer properties will continue to pervade with the ultimate goal of developing degradable polymers derived from renewable resources that can be used for a wider variety of applications.
REFERENCES
[1] Biernesser, Ashley B.; Li, Bo; Byers, Jeffery A.* “The redox controllable polymerization of lactide catalyzed by bis(imino)pyridine iron bis-alkoxide complexes” Journal of the American Chemical Society, 2013, 135(44), 16553-16560, DOI:10.1021/ja407920d.
[2] Auras, Rafael A.; Lim, Loong-Tak; Selke, Susan E. M.; Tsuji, Hideto Poly(lactic acid): Synthesis, Structures, Properties, Processing, and Applications; John Wiley & Sons, Inc.: Hoboken, NJ, 2011, ISBN:978-0-470-29366-9.
[3] Gorrasi, Giuliana; Pantani, Roberto* “Effect of PLA grades and morphologies on hydrolytic degradation at composting temperature: Assessment of structural modification and kinetic parameters” Polymer Degradation and Stability, 2013, 98(5), 1006-1014, DOI:10.1016/j.polymdegradstab.2013.02.005.
[4] Manna, Cesar M.; Kaur, Aman; Yablon, Lauren; Haeffner, Fredrick; Li, Bo; Byers, Jeffery A.* “Stereoselective catalysis achieved through in situ desymmetrization of an achiral iron catalyst precursor” Journal of the American Chemical Society, 2015, 137(45), 14232-14235, DOI:10.1021/jacs.5b09966.
[5] Gregson, Charlotte K. A.; Gibson, Vernon C.*; Long, Nicholas J.; Marshall, Edward L.; Oxford, Phillip J.; White, Andrew J. P. “Redox Control with Single-Site Polymerization Catalysts” Journal of the American Chemical Society, 2006, 128(23), 7410-7411, DOI:10.1021/ja061398n.
[6] Broderick, Erin M.; Guo, Neng; Vogel, Carola S.; Xu, Culling; Sutter, Jorg; Miller, Jeffrey T.; Meyer, Karsten; Mehrkhodavandi, Parisa; Diaconescu, Paula L.* “Redox Control of a Ring-Opening Polymerization Catalyst” Journal of the American Chemical Society, 2011, 133(24), 9278-9281, DOI:10.1021/ja2036089.
[7] Wang, Xinke; Thevenon, Arnaud; Brosmer, Jonathan L.; Yu, Insun; Khan, Sneed I.; Mehrkhodavandi, Parisa; Diaconescu, Paula L.* “Redox Control of Group 4 Metal Ring-Opening Polymerization Activity toward L-Lactide and ∈-Caprolactone” Journal of the American Chemical Society, 2014, 136(32), 11264-11267, DOI:10.1021/ja505883u.
[8] Biernesser, Ashley B.; Delle Chiaie, Kayla; Curley, Julia B.; Byers, Jeffery A.* “Block Copolymerization of Epoxides with Lactide Facilitated by Redox Switchable Iron Polymerization Catalysis” Angewandte Chemie, International Edition, 2016, 55, 5251-5254, DOI:10.1002/anie.201511793.
[9] Delle Chiaie, Kayla; Yablon, Lauren L.; Biernesser, Ashley B.; Michalowski, Gregory R.; Sudyn, Alexander W.; Byers, Jeffery A.* “Redox-Triggered Crosslinking Reactions” Polymer Chemistry, 2016, 7, 4675-4681, DOI:10.1039/C6PY00975A.
[10] Chan, Nicky; Yee, N.; An, So Y.; Oh, Jung K. “Tuning Amphiphilicity/Temperature-Induced Self-Assembly and In-Situ Disulfide Crosslinking of Reduction-Responsive Block Copolymers” Journal of Polymer Science Part A, Polymer Chemistry, 2014, 52, 2057-2067, DOI:10.1002/pola.27216.
[11] Yang, Sen-lin; Wu, Zhi-Hua; Yang, Wei; Yang, Ming-Bo “Thermal and Mechanical Properties of Chemical Crosslinked Polylactide (PLA)” Polymer Testing, 2008, 27, 957-963, DOI:10.1016/j.polymertesting.2008.08.009.
[12] Quynh, Tran M.; Mitomo, H.; Nagasawa, N.; Wada, Y.; Yoshii, F.; Tamada, M.* “Properties of Crosslinked Polylactides (PLLA & PDLA) by Radiation and its Biodegradability” European Polymer Journal, 2007, 43(5), 1779-1785, DOI:10.1016/j.eurpolymj.2007.03.007.
[13] Phong, Lester; Han, Ernest S. C.; Xiong, Sijing; Pan, Jie; Loo, Say C. J.*, “Properties and Hydrolysis of PLGA and PLLA Cross-Linked with Electron Beam Radiation” Polymer Degradation and Stability, 2010, 95(5), 771-777, DOI:10.1016/j.polymdegradstab.2010.02.012.
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