Article Cite This: Acc. Chem. Res. 2017, 50, 2861-2869
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Indium Catalysts for Ring Opening Polymerization: Exploring the Importance of Catalyst Aggregation Kimberly M. Osten and Parisa Mehrkhodavandi* Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia V6T1Z1, Canada CONSPECTUS: Inexorably, the environmental persistence and damage caused by polyolefins have become major drawbacks to their continued long-term use. Global shifts in thinking from fossil-fuel to renewable biobased resources have urged researchers to focus their attention on substituting fossil-fuel based polymers with renewable and biodegradable alternatives on an industrial scale. The recent development of biodegradable polyesters from ring opening polymerization (ROP) of bioderived cyclic ester monomers has emerged as a promising new avenue toward this goal. Ever increasing numbers of metal-based initiators have been reported in the literature for the controlled ROP of cyclic esters, in particular for the polymerization of lactide to produce poly(lactic acid) (PLA). PLA has several material weaknesses, which hinder its use as a replacement for commodity plastics. Despite many advances in developing highly active and controlled catalysts for lactide polymerization, no single catalyst system has emerged to replace industrially used catalysts and provide access to PLA materials with improved properties. We reported the first example of indium(III) for the ring opening polymerization of lactide. Since then, indium(III) has emerged as a useful Lewis acid in initiators for the controlled polymerization of lactide and other cyclic esters. In particular, we have developed a large family of chiral dinuclear indium complexes bearing tridentate diaminophenolate ligands and tetradentate salen and salan ligands. Complexes within our tridentate ligand family are highly active initiators for the moderately isoselective living and immortal polymerization of rac-lactide, as well as other cyclic esters. We have shown that subtle steric effects influence aggregation in these systems, with polymerization typically proceeding through a dinuclear propagating species. In addition, profound effects on polymerization activities have been observed for central tertiary versus secondary amine donors in these and other related systems. In contrast, our well-controlled and highly active chiral indium salen systems are more isoselective than the tridentate analogues and polymerize lactide via a mononuclear propagating species. Again, we have noticed that subtle steric and electronic changes to the ligand can influence both polymerization activity and stereoselectivity via aggregation phenomena. Recently, we have reported a promising new chiral indium catalyst supported by a tetradentate salan ligand. This catalyst is remarkably water and air stable and can be activated by linear and branched alcohols to provide controlled access to multiblock copolymers in air. This catalyst represents an important step forward toward generating new, commercially relevant catalysts for ROP of cyclic esters to produce novel biodegradable polymers, and highlights the unique value of indium-based catalysts in the field.
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INTRODUCTION
PLA is the most widely used and studied biodegradable polyester, due in part to its relatively facile production from renewable agricultural sources, its biodegradability and biocompatibility, and the ability to tailor its physical properties for a variety of applications by materials modifications.8−10 The ring-opening polymerization of lactide (LA), mediated by metal (vide inf ra) and organocatalysts,11,12 can produce high molecular weight PLA in a controlled manner from biomass feedstocks such as corn (Figure 2).4,13 In this process, the conversion of lactic acid to the monomer lactide produces three stereoisomers: D- and L-LA, and meso-LA. Racemic-lactide (racLA) is a 1:1 mixture of D- and L-LA (Figure 2).
Biodegradable Polyesters
The new paradigm of biorefineries has emerged from a widespread shift in focus within both the scientific community and the public at large1 to develop materials made from renewable resources that are recyclable or biodegradable.2 This rethinking has led to the investigation of renewable polymers, those derived mostly or entirely from biomass,3 and biodegradable polymers, polymers that are capable of being decomposed by bacteria or other living organisms.4 The ringopening polymerization (ROP) of cyclic esters is currently used in industry to generate biodegradable polyesters such as poly(caprolactone) (PCL) and poly(lactic acid) (PLA) (Figure 1).5 Poly(hydroxybutyrate) (PHB), another example of a biodegradable polyester, can be biosynthesized by bacteria,6 in addition to being generated via the ROP of β-butyrolactone (Figure 1).7 © 2017 American Chemical Society
Received: September 12, 2017 Published: October 31, 2017 2861
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difficult heat processing properties and thus limited applicability.30,31 Metal-mediated catalysis can overcome these limitations by controlling PLA tacticity, the relative stereochemistry of adjacent chiral centers along the polymer chain. By tailoring the microstructure of PLA (Figure 4), desirable properties may be imparted to the resulting polymers.32 Stereocontrol in metal-mediated ROP of LA can arise through two mechanisms: (1) enantiomorphic site control and (2) chain-end control.33−35 These two mechanisms are not necessarily independent and may both play an important role at different points in the polymerization process.36 Controlling the physical properties of PLA through metal catalysis, for example, through the stereoselective polymerization of racLA7,18,19,37−39 or the copolymerization of LA and other cyclic esters2,5,40−44 has become an important avenue toward the widespread replacement of traditional nondegradable plastics with biodegradable options (vide inf ra).
Figure 1. Examples of biodegradable polyesters generated via ROP of cyclic esters.
Indium Catalysts for the Controlled ROP of Cyclic Esters
We reported the first chiral indium catalyst (1) for the living ring-opening polymerization of lactide in 2008 as part of our efforts to develop active catalysts for the controlled ringopening polymerization of cyclic esters (Figure 5).45 At the time, we were interested in using indium(III) as a Lewis acid due to its unique combination of air and moisture stability, which has been demonstrated stoichiometrically and catalytically in a host of organic transformations.46,47 Prior to our work, the only indium catalyst in the literature for cyclic ester polymerization was reported by Huang et al. for the polymerization of ε-caprolactone (Figure 5).48 Subsequent to our initial report, a variety of indium-based initiators have been reported in the literature for the polymerization of lactide (Figure 5).49−61 In this Account, we present an overview of our efforts to develop indium-based catalyst systems for the synthesis of novel and commercially relevant biodegradable polymers.
Metal-Mediated Ring-Opening Polymerization of Cyclic Esters
Metal-mediated ring-opening polymerization of LA through a coordination−insertion mechanism14 is by far the most studied route for the controlled synthesis of PLA, with hundreds of catalysts reported to date.15−22 Catalysts that promote living ROP of LA and other cyclic esters, where the absence of termination reactions leads to a direct correlation between monomer concentration, catalyst loading, and molecular weight, are desirable.23 In addition, immortal polymerization, living polymerization in the presence of a chain transfer agent (CTA), has garnered increased research attention due to its potential to produce highly controlled and potentially functionalized polymers with lower catalyst loadings (Figure 3).24 The major drawback of immortal polymerization is the common instability of most metal-based initiators in the presence of large amounts of chain transfer agents, especially alcohols. The development of more tolerant initiators for the immortal polymerization of LA and other cyclic esters is a current area of focus for many groups, including our own (vide inf ra).24−29 Industrially produced PLA is predominantly poly(L-lactide) (PLLA), due to the natural occurrence of L-lactic acid.30 PLLA is a brittle and stiff semicrystalline, thermoplastic polymer with low Tg (50−60 °C) and high melting point (175 °C), with
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INDIUM COMPLEXES BEARING TRIDENTATE DIAMINOPHENOLATE LIGANDS Complex 1 is an excellent catalyst for the living polymerization of rac-LA, yielding isotactically enriched PLA (Pm ∼ 0.6) with high molecular weight and low dispersity (Figure 6).33,45 We have since used this catalyst for a number of diverse applications, including the living and immortal polymerization
Figure 2. Lifecycle and synthesis of LA and PLA. 2862
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Figure 3. Schematic representation of living and immortal polymerization.
Figure 4. Different microstructures of PLA arising from lactide polymerization.
of β-butyrolactone (BBL),25 and the controlled polymerization of meso-LA, giving syndiotactically enriched PLAs.67 We have also explored its reactivity and that of related zinc compounds, in the living and immortal copolymerizations of LA and BBL to form di- and triblock copolymers,68,69 as well as star-shaped copolymers.26,27,70 We subsequently developed a large family of indium catalysts using ligands of this type,71 focusing on modifications to ligand chirality,33 the bridging ligands (halogens, alkoxides, hydroxides),27,29,33,72 terminal amine,73 central amine,34 and phenolate substituents,35 and the ligand backbone (Figures 6 and 7).34,73 Indium mono- and bis-alkoxide complexes in this family are synthesized from dihalide precursors via salt metathesis with the appropriate alkoxide salts and are invariably dinuclear in solution and the solid state.34,35,73 The corresponding monoand bis-hydroxides can be isolated via reaction with water.33,45,72 Complexes with asymmetric bridging ligands form homochiral (RR/RR)/(SS/SS) dimers (or racemic mixtures thereof) and those with symmetric bridging ligands preferentially form heterochiral (RR/SS) dimers, unless enantiopure ligands are used in their formation. Mixing (RR/ RR) and (SS/SS) bis-alkoxy dimers will form the heterochiral dimer in solution.
We observe increased rates of LA polymerization in moving from chloride to iodide analogues (1−3), consistent with our previous work showing the higher electrophilicity of the indium center with chloride ligands.33,34,72 In addition, we have observed no influence on polymerization activity or selectivity upon changing from para-tert-butyl to methyl substituents on the phenolate rings of the ligand backbone (Figure 6).33,72 Our detailed experimental33 and computational studies74 of complexes 1−5 show that these catalysts remain dinuclear during polymerization. Key pieces of evidence that allowed us to confirm this mechanism are (1) complex 1 and its bis-alkoxy derivative 5 retain their dinuclear structures in solution, as evidenced by variable temperature (VT) NMR spectroscopy, 2D NOESY NMR spectroscopy, and diffusion coefficient measurements, (2) complexes (RR/RR)-1 and 5 have different stereoselectivities for rac-LA polymerization, with Pm values of 0.48 and 0.65, respectively, and display different kinetic profiles, (3) there is no evidence for dissociation of 1, while (4) for complexes with bulky terminal n-propyl amine (6)73 or orthotriphenylsilyl phenolate (7)35 substituents, we observe dissociation of the complexes during LA polymerization with a corresponding loss in isoselectivity. 2863
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Figure 5. Selected indium catalysts for cyclic ester polymerization.45,48,52,55,61−66
Figure 6. Family of indium alkoxide complexes bearing chiral diaminophenolate ligands (Ad = adamantyl, Cm = cumyl, or −C(CH3)2Ph).
long initiation periods leading to a lack of controlled polymerization. More electron rich phenoxides (-Ph-R3 = -Ph-OMe or -Ph-Me) do show improved performance.29 Phenol can be used as CTA in immortal ROP of LA by complex 1, and this was further expanded to include the use of aromatic diols, such as 1,8-naphthalenediol. For chelating diols, formation of stable and inactive species can be avoided by the use of coordinating solvents. Changing the central amine donor from a secondary (R = H) to a tertiary (R = Me) amine has a profound effect on the activity of these indium complexes, with a more than 2 orders of magnitude drop in activity for tertiary amine donors.34 This effect is independent of whether a chiral cyclohexyl or achiral ethyl backbone is utilized and has also been observed in our work on related zinc75,76 catalysts (Figure 8). In zinc complexes of this type, the central N-Me group causes a subtle steric change in the system that favors the formation of a mononuclear species, which is far less active than the dinuclear
Figure 7. Dinuclear indium complexes with hydroxide and phenoxide bridging ligands.
The nature of the bridging ligands affects initiation rates (Figure 7). Bridging hydroxy groups do not initiate polymerization; however, complexes with mixed hydroxy/ethoxy moieties (10) are active.33,45,72 The phenoxy-bridged analogues (12) are active for LA polymerization; however they suffer from 2864
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significant transesterification. Complexes with low steric bulk (14 and 15) result in lower Pm values as expected; however, increasing steric bulk (16 and 17) has a limited effect on isoselectivity (maximum Pm ∼ 0.75). There are also significant steric effects on the rates of propagation observed within this series; complexes with less steric bulk (R2 = Me, Br) having slower rates of initiation compared to those with more steric bulk (R2 = tBu, Cm, Ad). Unlike the catalysts with tridentate ligands, there is evidence for an equilibrium between the dimeric and monomeric forms with these salen-ligand supported catalysts, which can be perturbed with the addition of donors.86 We synthesized mononuclear analogues of these complexes by changing the alkoxide initiator to 2-pyridinemethoxide (Figure 9). The polymerization behaviors of the dinuclear complexes and their mononuclear analogues, such as 13 and 18, are identical, with the exception of a small initiation phase for the former; both have similar activity, isoselectivity (Pm ∼ 0.75), and kinetic profiles (kL‑LA/kD‑LA ∼ 5). In contrast, dinuclear analogues of less bulky catalysts (14 or 15) have longer initiation periods, which are eliminated entirely with their mononuclear analogues. These observations confirm that the propagating species for these dinuclear indium salen complexes is most likely mononuclear and that dissociation of the dimers is required for initiation of LA polymerization. This aggregation phenomenon is intensified when the salen backbone is changed to binam (1,1′-binaphthyl-2,2′-diamine), similar to aluminum analogues.87 Polymerization of rac-LA by indium salen complexes with binam backbones (21b) is slow, requiring elevated temperatures to reach full conversion,88 and produces atactic PLA with higher than expected Mn values (Figure 10). We show that in fact 21b converts to unreactive 21a under polymerization conditions.88
Figure 8. Dinuclear indium complexes bearing achiral and chiral diaminophenolate ligands with central tertiary and secondary amine donors.
analogue;28,76 however, such a clear distinction cannot be made for the indium systems. We do observe evidence for hydrogen bonding across the indium dimers in the solid state between the secondary amino N−H and the terminal halide ligands; however, there is no clear connection between disruption of hydrogen bonding (e.g., replacement of N−H with N-Me) and the loss of activity. We also observe no clear electronic differences between N-Me and N−H systems theoretically34 or experimentally,77 and we conclude that a subtle combination of steric and electronic factors may be at play within these systems to affect their polymerization activity. Regardless, we have used these results to inform the design of more active indium catalysts supported by pentadentate dinucleating ligand systems78 and salan-type ligand systems (vide inf ra).79
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INDIUM COMPLEXES BEARING TETRADENTATE SALEN LIGANDS We reported the first example of a highly active, isoselective indium complex (13)66 bearing a chiral diimino-bisphenolate (salen) ligand for the polymerization of rac-LA (Figure 9).
Figure 10. Chiral indium salen complexes with binam backbones.
Figure 9. Family of dinuclear and mononuclear indium salen complexes based on a chiral cyclohexyl ligand backbone.
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MEASURING AND REPORTING POLYMER TACTICITY Although the Pm value of PLA synthesized using catalyst 13 was comparable to other systems reported in the literature at the time,89 we noticed a dramatic difference in the physical properties of our polymers (amorphous, Tg ∼ 55 °C) compared to those reported in the literature (crystalline, Tm > 170 °C).61,89,90 We attribute these differences to the different ways in which tacticity is calculated and reported in the literature. To our knowledge, two distinct methods, both relying on Bernoullian statistical equations and 1H{1H} NMR spectroscopic data (Figure 11),91−93 are used in the literature for the calculation of the tacticity of PLA synthesized from rac-LA: (1) analysis of stereoerror tetrad integrations and calculation of tacticity based on two parameters, Pm and Pr (method A) and
These indium complexes are orders of magnitude more active than analogous aluminum catalysts.80−83 Although achiral indium salen complexes had been reported previously,84 prior to our work there was only one example of a chiral indiumsalen complex, which was not used as a catalyst for polymerization.85 Complexes (±)- and (R,R)-13 are highly active and controlled for the living polymerization of rac-LA and yield isotactic PLA (Pm ∼ 0.75) through a predominantly site-selective mechanism (Figure 9).66 They are also active for the polymerization of meso-LA to form syndiotactically enriched PLA.67 We subsequently investigated a series of these complexes bearing different phenoxide substituents (Figure 9).86 All enantiopure (RR/RR) complexes in the series give good control of molecular weights and dispersities, albeit with 2865
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Figure 12. Air and moisture stable dinuclear indium complex bearing a chiral salan ligand.
4 h, 90−99% conversion) and in the melt (120 °C, 2 h, 70− 80% conversion) with good control over molecular weights and low dispersities. The complex can also be activated by the aromatic triol 1,3,5-tris(hydroxymethyl)benzene (THMB), similar to our tridentate systems, allowing for the synthesis of star-shaped polymers of L- and D-LA, including di- and triblock copolymers via sequential monomer addition in air, again with excellent control of molecular weights and low dispersities. In addition, the catalyst is active for the controlled polymerization of BBL and its copolymerization with rac-LA, albeit under airfree conditions with purified monomers. This catalyst is unique in the literature for its stability and its control over polymerizations carried out in the melt without protection from air or moisture and with unpurified lactide. It represents an important step forward in developing industrially relevant catalysts for the controlled polymerization of cyclic esters. Indeed, high molecular weight, low dispersity PLLA− PDLA-PLLA star-shaped triblock copolymers made with this catalyst exhibit high melting temperatures (>200 °C) and can be made in one-pot in only 5 h. This represents a marked improvement over the industrially used catalyst tin(II) octanoate, which requires over 7 days and two steps to catalyze the synthesis of similar high molecular weight PLLA−PDLA star-shaped diblock copolymers.95
Figure 11. 1H{1H} NMR spectrum of isotactically enriched PLA synthesized from rac-LA, showing manual integrations (below peaks) and peak deconvolution traces (black lines) and integrations obtained for each individual tetrad (above peaks), with inset showing an example tetrad along a PLA chain (mrm).
(2) analysis of all five individual tetrad integrations by peak deconvolution and calculation of tacticity based on one unique parameter, Pm (method B). The use of Bernoullian equations to calculate tacticity comes with some important assumptions:94 (1) they are only valid for ideal catalytic systems; (2) they are only valid when no other reactions are occurring (i.e., transesterification or epimerization); (3) they do not account for any level of chain end control in the system (i.e., when both site control and chain end control are operative); and (4) the concentration of each monomer in solution is assumed to be constant over time. It is clear that systems with “intermediate tacticity values” or those resulting from chain end control mechanisms will likely not fulfill all these criteria. In method A, the calculation of Pm and Pr as separate variables allows for the level of agreement with the Bernoullian model to be assessed, by determining the deviation of the sum of these probabilities from its ideal value of 1;94 however, there are also increased levels of uncertainty introduced by this method due to calculating the tacticity using fewer integration values (method B overcomes this uncertainty somewhat by averaging values obtained from five peaks). When comparing tacticity values between different catalyst systems, it is important to report the methodology used in the calculation and compare numbers determined using the same methodology and under similar conditions. As noted above, the physical properties of PLA with the same tacticity reported by different research groups are not always similar.67 Therefore, we recommend using additional benchmarks, such as Tg and Tm values or rheological properties, to compare polymers with similar, “midrange” tacticity values.
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CONCLUDING REMARKS Through this Account, we hoped to illustrate the versatility and potential of indium complexes in ring opening polymerization of cyclic esters to form biodegradable polymers. Prior to our work, these complexes had not been used for the polymerization of lactide, but our work, and the subsequent work of many others in the field, has shown that with the judicial choice of supporting ligand, indium complexes can be excellent catalysts for ring opening polymerization. Indeed, as we have shown with our salan systems, in some cases indium catalysts can be superior to the industrial standard tin octanoate in every way. One of the most important factors we have noted with indium systems is the exquisite impact of subtle changes to ligand design on catalyst activity. Ligand design plays a crucial role in controlling catalyst aggregation in our systems. Flexible ligand designs can promote ligand disproportionation and form inactive catalysts, while ligands with high steric hindrance can prevent the formation of stable dinuclear species and impede complex formation. Secondary amine donors are preferable to imine or tertiary amine donors due to their ability to promote the formation of stable dinuclear species. Although neutral indium catalysts have been explored in the past decade, cationic indium complexes have received far less attention and, we
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INDIUM COMPLEXES BEARING TETRADENTATE SALAN LIGANDS Recently, we have reported the use of an air and moisture stable indium salan complex (22) for the controlled polymerization of cyclic esters in air (Figure 12).79 Activation of complex 22 with ethanol (2−100 equiv) allows for the immortal polymerization of unpurified, wet rac-LA in air both in solvent (toluene, 80 °C, 2866
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believe, have great potential in other areas of catalysis. This study of ligand design will allow us to tune the reactivity of cationic indium species accordingly.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Parisa Mehrkhodavandi: 0000-0002-3879-5131 Notes
The authors declare no competing financial interest. Biographies Kimberly M. Osten completed her Ph.D. at UBC, where she gained a love for inorganic chemistry and catalysis. After completing a postdoctoral fellowship at the University of Toronto in 2017, she will be pursuing a further postdoctoral position at Nagoya University, Japan. Parisa Mehrkhodavandi is an Associate Professor at the University of British Columbia. She received her Ph.D. at MIT (R. R. Schrock) and completed her postdoctoral training at Caltech (J. E. Bercaw). Her areas of interest are organometallic synthesis, catalyst design and development, and polymer science.
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ACKNOWLEDGMENTS P.M. thanks the many co-workers who have contributed to this research over the years. In addition, P.M. and K.M.O. thank Dr. Insun Yu, Dr. Dinesh Aluthge, Dr. Ese Chile, Alexandre Kremer, and Tannaz Ebrahimi for providing feedback on this manuscript. P.M. thanks Natural Sciences and Engineering Council of Canada for generously funding this work.
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REFERENCES
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