Ring-Opening Copolymerization of Epoxides and Cyclic Anhydrides

Review pubs.acs.org/CR

Ring-Opening Copolymerization of Epoxides and Cyclic Anhydrides with Discrete Metal Complexes: Structure−Property Relationships Julie M. Longo, Maria J. Sanford, and Geoffrey W. Coates* Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301, United States ABSTRACT: Polyesters synthesized through the alternating copolymerization of epoxides and cyclic anhydrides compose a growing class of polymers that exhibit an impressive array of chemical and physical properties. Because they are synthesized through the chain-growth polymerization of two variable monomers, their syntheses can be controlled by discrete metal complexes, and the resulting materials vary widely in their functionality and physical properties. This polymer-focused review gives a perspective on the current state of the field of epoxide/anhydride copolymerization mediated by discrete catalysts and the relationships between the structures and properties of these polyesters. 6.4. Polyesters with High Tg Values 6.4.1. Semiaromatic Polyesters 6.4.2. Aliphatic Polyesters 6.4.3. Terpolymers 7. Post-Polymerization Modification 7.1. Motivations for Post-Polymerization Modification 7.2. Functionalization via Thiol−Ene and Tetrazine Click Chemistry 7.3. Post-Polymerization Modification Outlook 8. Conclusions and Outlook Author Information Corresponding Author Notes Biographies Acknowledgments References

CONTENTS 1. Introduction 1.1. Background 1.2. Synthesis of Polyesters 1.3. Catalyst Development 1.4. Mechanism of Polymerization with (salen)MX-type Catalysts 2. Monomer Scope 2.1. Epoxide and Anhydride Scope 2.2. Renewable Monomers 2.3. Terpolymerizations with CO2 2.4. Terpolymerizations with Lactones and Epoxides 2.5. Block Copolymers via Sequential Addition of Anhydrides 3. Catalyst Activity 3.1. Quantification of Activity 3.2. Activity of β-Diiminate and Porphyrin Catalysts 3.3. Activity of Bimetallic Catalysts 3.4. Activity of (salen)MX Catalysts 4. Control of Molecular Weight 4.1. Challenges in Control of Molecular Weight 4.2. Advances in Molecular Weight Control 5. Regio- and Stereocontrol of Epoxide/Anhydride Copolymerization 5.1. Quantification and Analysis of Regio- and Stereocontrol 5.2. Advances in Regiocontrol 5.3. Advances in Stereocontrol 5.3.1. Control of Cis/Trans Isomerization 5.3.2. Control of Epimerization 6. Thermal Properties 6.1. Semicrystalline Polyesters and Stereocomplexes 6.2. Amorphous Polyesters: Introduction 6.3. Polyesters with Low Tg Values © XXXX American Chemical Society

A A B B D D D E H I I J J J L L M M M

Y Y Y Z AA AA AA AB AB AB AB AB AB AC AC

1. INTRODUCTION 1.1. Background

Polyesters are a ubiquitous class of polymers that can be broadly divided into semiaromatic and aliphatic polyesters. Semiaromatic polyesters such as poly(ethylene terephthalate) (PET) are used in a variety of bulk applications owing to their good mechanical strength, barrier properties, and other characteristics.1 For example, PET is used extensively in fibers, blow-molded bottles, and other packaging applications because of its excellent barrier properties and is produced annually on a multibillion-pound scale.1 In addition, semiaromatic polyesters have found use as liquid crystalline polymers.2,3 Aliphatic polyesters have received attention as appealing, potentially sustainable alternatives to petroleum-based polymers because of their numerous renewable sources,4−8 facile hydrolytic degradation to typically benign

P P Q R R S S S U W

Received: August 15, 2016

A

DOI: 10.1021/acs.chemrev.6b00553 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

products,6,9−11 and high biocompatibility.12 As a result, they have been used in applications ranging from specialized biomedical devices to bulk packaging.12−18

and DMAP can catalyze these copolymerizations without a catalyst.20,47 Williams and co-workers19 recently published an excellent review that focused on epoxide/anhydride copolymerization catalysts. This review focuses on the structure− property relationships of polyesters formed from epoxide/ anhydride copolymerization, and only select highlights of catalyst development are discussed. The earliest epoxide/anhydride copolymerizations were initiated with amines and metal alkoxides, yielding lowmolecular-weight polymers with broad dispersities and, in many cases, significant polyether contamination from epoxide homopolymerization.54−58 Heterogeneous catalysts that exhibit good control, such as double metal cyanides, 59 metal alkoxides,53,60 and zinc glutarate,23 continue to be used for these copolymerizations, although the materials they form are not the main focus of this review. The first well-controlled polymerization of propylene oxide (PO) with phthalic anhydride (9) was reported by Aida and Inoue49 in 1985 and used an aluminum porphyrin catalyst (cat1) and a tetralkyl ammonium halide cocatalyst (Figure 1).

1.2. Synthesis of Polyesters

The most common route to polyesters is the step-growth polymerization of diacids or diesters with diols.1 However, this method requires the removal of a small-molecule byproduct water or an alcoholthat necessitates high temperatures and makes the approach energy-intensive. Additionally, as with all step-growth polymerizations, this polymerization must be run to a high conversion of monomer to achieve products with high number-average molecular weight (Mn), and the dispersity (Đ or Mw/Mn, where Mw is the weight-average molecular weight) of the polymers is, in general, approximately 2.1 In contrast to stepgrowth polymerization, chain-growth polymerization produces no small-molecule byproducts and give high-molecular-weight polymers with controlled dispersity at lower conversion.1 Given the many uses of polyesters, there has been great interest in developing chain-growth methods for their synthesis. Aliphatic polyesters from AB-type monomers are commonly made through chain-growth ring-opening polymerization (ROP) of lactones.19 ROP of lactones has been studied extensively, and a wide variety of initiators, including organocatalysts, metal alkoxides, and various metal complexes, have been explored.12 However, particularly at high conversion, the ROP of lactones can be limited by detrimental side reactions such as transesterification. Additionally, the resulting polyesters have a limited range of properties owing to the minimal functional diversity of available lactones and the lack of post-polymerization functionalization.12,19 An alternative chain-growth route to both aliphatic and semiaromatic polyesters is the alternating copolymerization of epoxides and cyclic anhydrides (Scheme 1).19 The use of two distinct monomer sets allows for facile tuning of properties and post-polymerization modification.19,20 Scheme 1. Alternating Copolymerization of Epoxides with Cyclic Anhydrides

Figure 1. Porphyrin and corrole catalysts.

1.3. Catalyst Development

This system led to the discovery of several related catalysts, including other aluminum derivatives,37,50 as well as chromium,32,37,38 cobalt,37 iron,44 and manganese44 complexes (see Figure 1). In 2007, Coates and co-workers61,62 found that a βdiiminate zinc acetate [(BDI)ZnOAc] catalyst (cat13), which had previously been used for epoxide/CO2 copolymerization, is an effective catalyst for copolymerizations of a number of epoxides and cyclic anhydrides (Figure 2). This was followed in

A diverse array of metal complexes has been used to catalyze the copolymerization of epoxides and anhydrides, including zinc, 2 1 − 3 0 magnesium, 2 4 , 2 6 , 3 1 chromium, 2 6 , 3 2 − 4 2 cobalt,26,30,33,35−37,40,43 manganese,33,40,44−46 iron,47,48 aluminum,33,36,37,40,48−53 and nickel30 complexes, many of which show markedly higher activity with the addition of a nucleophilic cocatalyst. Cocatalysts including bis(triphenylphosphine)iminium salts ([PPN]X), 4-dimethylaminopyridine (DMAP), phosphines, and ammonium salts have been reported for these reactions, and [PPN]X and DMAP are reportedly the most effective. Intriguingly, at high enough temperatures, [PPN]Cl

Figure 2. β-Diiminate zinc acetate catalyst. B


Chemical Reviews

Review

Figure 3. N,N′-Bis(salicylidene)cyclohexanediamine (salcy) catalysts.

Figure 4. N,N′-Bis(salicylidene)phenylenediamine (salph) catalysts.

2011 by two reports of another epoxide/CO2 copolymerization catalyst, N,N′-bis(salicylidene)cyclohexanediamine chromium(III) chloride [(salcy)CrCl] (cat14), for epoxide/anhydride copolymerization (Figure 3).33,42 Since those initial reports, (salcy)MX complexes with a variety of backbones have become some of the most widely used complexes for epoxide/anhydride copolymerization and have included chromium,42,34,26,33,36,41 cobalt,26,33,36,42,43,63,64 aluminum,33,36 manganese,33,46 and iron47 complexes (Figure 3). Similar backbones, including N,N′-bis(salicylidene)phenylenediamine (salph)32,35,36,40,46−48,51,52(Figure 4) and N,N′-bis(salicylidene)ethylenediamine (salen)36,46 (Figure 5) structures, have also been widely reported. One report of similar (salan)CrX complexes (Figure 6) has also been published.39 Another recent report described the use of tetradentate sulfur (OSSO) ligands, which are structurally similar to (salen)CrCl complexes (Figure 7).65 Kleij and Coates and co-workers48 reported the use of a geometrically more flexible iron aminotriphenolate complex (cat64) as well (Figure 8).

Figure 5. N,N′-Bis(salicylidene)ethylenediamine (salen) catalysts.

Finally, multinuclear zinc and magnesium catalysts have been studied extensively by Williams and co-workers,24,26−29,66 Ko and co-workers,30 and Lü and co-workers21,22,25 and are a class of C


Chemical Reviews

Review

[PPN]Cl can be drawn on the basis of studies by Darensbourg et al.,34 Duchateau and co-workers,32 and Chisholm and coworkers38 (Scheme 2). There are three possible cycles involving different pentacoordinate and hexacoordinate species. Common to all three is the general sequence of events in which an epoxide is first activated by a metal center and then attacked by a carboxylate. This generates an alkoxide, which subsequently reacts with an anhydride to form a new ester linkage, enchaining one complete repeat unit and regenerating a carboxylate and an open coordination site on the metal center. In cycle 1, the polymerization proceeds through a hexacoordinate monoalkoxide monocarboxylate species (AC) and a hexacoordinate bis(alkoxide) species (AA; Scheme 2). In cycles 2 and 3, a bis(carboxylate) (CC) is formed instead of a bis(alkoxide). Proceeding from species AC, either an alkoxide (cycle 2) or a carboxylate (cycle 3) can dissociate. Notably, for all three cycles, many of the steps could be drawn as equilibria. Given that a better understanding of the mechanism of these copolymerizations, including a deeper knowledge of the role of the cocatalyst, could lead to further catalyst development, our group and others continue to study the mechanism of polymerizations with (salen)MX catalysts. Finally, when the mechanisms of epoxide/anhydride copolymerization are considered for all catalyst systems, it is important to note that, to avoid polyether formation, the metal alkoxide must selectively attack anhydrides and not epoxides. Most of the discrete catalyst systems for these polymerizations meet this requirement, but significant polyether formation is noted throughout the review.

Figure 6. Salan catalysts.

2. MONOMER SCOPE 2.1. Epoxide and Anhydride Scope

One appealing aspect of epoxide/anhydride copolymerization is that the use of two types of monomers greatly increases the diversity of the potential polymer structures. To date, more than 20 epoxides and 20 anhydrides have been reported as monomers, leading to more than 400 possible polymer structures. This high number of structures also yields a diverse set of polymer properties, which are discussed in greater detail throughout the remainder of this review. All reported epoxide/anhydride combinations are summarized in Figures 10−14. Although a large number of both epoxides and anhydrides have been reported, propylene oxide (PO) (Figure 11), cyclohexene oxide (CHO; Figure 14), and phthalic anhydride (9) have been the most widely studied, which leaves many opportunities to study combinations of less commonly used and more functionally diverse monomers. Both the epoxides and cyclic anhydrides in these copolymerizations can be classified by their side chains and by their number of rings. In several instances, different classes of catalysts polymerize different classes of monomers, and polymer properties can vary on the basis of whether the selected epoxide and anhydride are mono-, bi-, or tricyclic and whether the polyester is semiaromatic or aliphatic. For example, crystallinity and thermal properties are often affected by polymer backbone structure and the length and rigidity of epoxide side chains. Additionally, a number of reported epoxides and anhydrides contain unsaturated moieties that result in unsaturated polyesters that can be subjected to post-polymerization modifications, thereby increasing the range of potential uses, particularly in biomedical applications. A wide range of mono-, bi-, and tricyclic anhydrides has been studied (Figure 10). In contrast, to date, the majority of

Figure 7. Catalysts containing tetradentate sulfur (OSSO) ligands.

Figure 8. Iron aminotriphenolate catalyst.

catalysts quite distinct from the (salen)MX complexes (Figure 9). It should be noted that cat76 also contains a bridging acetate, and cat77−cat79 contain two bridging acetates. Each of the catalysts has unique strengths and challenges, and this great diversity allows for the synthesis of a wide range of polyesters with a variety of properties. 1.4. Mechanism of Polymerization with (salen)MX-type Catalysts

Although the mechanism of epoxide/anhydride copolymerization has yet to be determined definitively, when the many similarities of these reactions to epoxide/CO2 copolymerization are considered, a proposed mechanism for the reaction of (salen)MX complexes with a nucleophilic cocatalyst such as D


Chemical Reviews

Review

Figure 9. Multinuclear and salen catalysts with pendant functionality.

was the first of a renewable monomer being used in a welldefined alternating epoxide/anhydride copolymerization (Scheme 3a).60 In 2005, Takasu et al.53 epoxidized sugar-based alkenes and copolymerized the resulting epoxides with 1 and glutaric anhydride (6) by using an aluminum alkoxide initiator (Scheme 3b). Although the molecular weights of the products were quite low, these polymers are interesting examples of potentially fully renewable materials. The initial report from Coates and co-workers62 of (BDI)ZnOAc (cat13)-catalyzed copolymerization included some renewable monomers. Most notably, limonene oxide (LO), the epoxidized form of the highly abundant terpene limonene, was copolymerized with diglycolic anhydride (7) to produce a material with a moderately high molecular weight (Mn = 36 kDa). With maleic anhydride (2), the molecular weight of the product was lower (Mn = 12 kDa) but still significantly higher than those of the renewable monomers reported earlier by Maeda et al.60 and Takasu et al.53 (Scheme 4a). Succinic anhydride (1) was also copolymerized with 4-vinylcyclohexene oxide and cyclohexene oxide to give products with molecular weights of 20 and 12 kDa, respectively (Scheme 4b).62 DiCiccio and Coates42 reported the use of glycidol-based epoxides with maleic anhydride (2) in 2011 (Scheme 5). Cis/

the epoxides in use are monosubstituted monocyclic epoxides (Figure 12), and only a few examples of disubstituted monocyclic epoxides (Figure 13) and bi- or tricyclic epoxides (Figure 14) have been reported. 2.2. Renewable Monomers

Biodegradable polymers, sourced from sustainable feedstocks including biomass, have recently attracted considerable interest as alternatives to fossil-fuel-based polymers.5,7,67,68 Aliphatic polyesters are particularly appealing alternatives to petroleumbased polymers because of their many potential renewable sources,4−8 facile degradation to typically benign products,6,9−11 and generally high biocompatibility.12 Therefore, aliphatic polyesters have been used in a diverse range of applications from specialized biomedical devices to bulk packaging.12−18 Many renewable monomers for epoxide/anhydride copolymerization have been explored, resulting in a range of renewably sourced aliphatic polyesters with desirable properties. One of the earliest reports of a well-defined epoxide/ anhydride copolymerization used succinic anhydride (1), which can be renewably sourced from succinic acid. Maeda et al.60 used magnesium ethoxide to copolymerize 1 and ethylene oxide. Although the molecular weights of the products were generally low and dispersity values were as high as 5.5, this report E


Chemical Reviews

Review

Scheme 2. Mechanistic Possibilities for Epoxide/Anhydride Copolymerization by Use of (salen)MX Complexesa

a

Counterions are not shown for clarity. Salen = N,N′-bis(salicylidene)ethylenediamine.

Figure 10. Anhydride scope.

trans isomerism of the alkene yielded eight partially renewable, unsaturated polyesters. Also in 2011, Thomas and co-workers33 reported an elegant tandem synthesis of anhydrides and subsequent polymerization with epoxides (Scheme 6a). The monomer scope included two renewable epoxidesLO and αpinene oxideand four renewably sourced anhydrides (Scheme 6b). Despite the bulk of many of the monomers, which can

inhibit access to higher molecular weights, the molecular weights of these polymers were as high as 27 kDa. In 2012, Duchateau and co-workers35 showed that succinic anhydride (1) and citraconic anhydride (4) could be copolymerized with styrene oxide (SO) to yield partially renewable semiaromatic polyesters. They followed this result with a report of poly(limonene phthalate), a partially renewable semiaromatic polyester.40 Meier and Williams and co-workers26 F


Chemical Reviews

Review

Figure 11. Anhydrides copolymerized with propylene oxide (PO).

biomass.74−76 This result was followed by a report48 of six tricyclic anhydrides based on terpenes, citraconic anhydride, and 2,5-dimethylfuran (18, 20, 21, and 23−25; see Scheme 30). These anhydrides yielded polymers with exceptionally high Tg values (up to 184 °C) when copolymerized with CHO, which will be discussed in detail below. An appealing aspect of renewable monomers is their relatively wide variety, which leads to a wide range of possible properties. This variability is highlighted by contrasting the high Tg values reported by Coates and co-workers48,51 with the results of recent work by Meier and co-workers,41 who used methyl 9,10epoxystearate with several anhydrides, including 1 and the Diels−Alder adduct of methyl α-eleostearate and maleic anhydride (15), to yield polymers with Tg values as low as −44 °C (see Scheme 26). This approach offers Tg values spanning more than 200 °C for polymers based on renewably sourced monomers. The wide array of renewable mono-, bi-, and tricyclic monomers that have been used in epoxide/anhydride copolymerization shows the diversity of functionality and structure that can be renewably incorporated into polyesters (Figure 15). A particularly appealing aspect of the use of renewable monomers is that many of the polyesters with the best thermal properties and highest molecular weights to date have been derived from partially renewably resources.33,43,48,51,62 Many additional renewable monomers and combinations of monomers remain to be investigated and will further expand the scope of renewable polymers that can be synthesized through these copolymerizations.

also showed that CHO and 1,4-cyclohexadiene oxide (CHDO) can be synthesized via metathesis of plant oils, establishing a renewable source of these epoxides. These reports effectively show that a variety of semiaromatic polyesters can be synthesized with a renewable component in either the epoxide or the anhydride (Scheme 7). Given that semiaromatic polyesters typically have thermal properties superior to those of aliphatic polyesters, this finding is important for efforts to improve the sustainability of epoxide/anhydride copolymerization. Additionally, although phthalic anhydride (9) is currently not renewably sourced, a recent report suggests that its bioderivation may be possible in the future.69 Although Coates and co-workers43 reported the use of 1 to synthesize partially renewable poly(propylene succinate), which formed a stereocomplex with improved thermal properties (see Figure 19), they have recently focused on incorporating renewable feedstocks through the alternating copolymerization of epoxides and tricyclic anhydrides.48,51,52 These anhydrides are easily synthesized via the Diels−Alder reaction of a wide range of commercially available, inexpensive, biosourced dienes and dienophiles, which offer ample opportunities to incorporate renewable feedstocks. In 2015, Van Zee and Coates51 reported the use of terpene-based α-terpinene and α-phellandrene containing tricyclic anhydrides (21−23) in copolymerizations with PO to give products with glass-transition temperature (Tg) values up to 109 °C (see Scheme 29).70−73 The following year, Coates and co-workers52 reported the use of a citraconic anhydride-based tricyclic anhydride (18; Figure 15). Citraconic anhydride can be derived from itaconic acid, which is one of the U.S. Department of Energy’s top 12 value-added chemicals from G


Chemical Reviews

Review

Figure 12. Anhydrides copolymerized with monosubstituted epoxides, ethylene oxide, and isobutylene oxide.

2.3. Terpolymerizations with CO2

to the metal center (steps C and D), although kd is faster than kc, which means that epoxide/CO2 copolymerization is faster than epoxide/anhydride copolymerization. However, the rate of insertion of the anhydride (step A) is much faster than the rate of CO2 insertion (step B). This difference means that, in the presence of anhydride, only the metal carboxylate intermediate is present, and in turn, only polyester is formed. Once all the anhydride has reacted, CO2 insertion occurs and polycarbonate is formed. In situ IR spectroscopic experiments verified these results, showing that the polyester formed at a moderate rate, followed by rapid formation of polycarbonate after all of the anhydride was consumed (Scheme 9b). Similarly, in 2010, Sun et al.59 showed that a heterogeneous double metal cyanide catalyst could terpolymerize CHO with maleic anhydride (MA) and CO2. In this case, a mostly random copolymer was formed, although polyester generally formed more quickly than polycarbonate, so the structure could be partially tapered. In 2011, Duchateau and co-workers32 widened

One approach to tuning polyester properties such as biodegradability is through the incorporation of a third monomer to alter polymer backbone functionality. One such monomer is CO2, which can be copolymerized with epoxides to give polycarbonates. As discussed above, many catalysts are active for both polyester and polycarbonate formation.62 In 2008, Jeske et al.61 were the first to use a discrete catalyst (cat13) in a terpolymerization of CHO and diglycolic anhydride (DGA) (7) with CO2 (Scheme 8). Surprisingly, they found that extremely well-defined polyester-b-polycarbonate was formed, with the polyester forming first despite the fact that the overall rate of polycarbonate formation was known to be much faster than the overall rate of polyester formation. Coates and co-workers61 proposed a mechanism for the formation of block copolymers in which two catalytic cycles compete for polyester and polycarbonate formation (Scheme 9a). In both cycles, the rate-determining step is epoxide insertion H


Chemical Reviews

Review

Figure 13. Anhydrides copolymerized with 1,2-disubstituted monocyclic epoxides.

poly(propylene phthalate) and to add PO to poly(butylene phthalate) to yield polyester-b-polyether structures. ABA triblock copolymers were also prepared via sequential addition of phthalic anhydride (9) after the formation of the ether block. In addition, Inoue and co-workers successfully added βbutyrolactone or ε-caprolactone in excess epoxide to poly(propylene phthalate) to form entirely polyester diblocks of various types. Building on Inoue’s results, Nozaki and coworkers44 formed one-pot polyester-b-polyether materials by polymerizing PO and glutaric anhydride (6) with a (corrole)Mn catalyst (cat10) in excess PO. In 2015, Williams and co-workers27 adopted a new one-pot approach with a binuclear zinc catalyst (cat70) to form multiblock copolymers from CHO, 9, and ε-decalactone (εDL). As in the report of Jeske et al.,61 insertion of 9 versus DL to the zinc alkoxide was the product-determining step, and insertion of 9 was much faster. Therefore, polyester formed exclusively, and in the presence of cyclohexane diol, the polymer contained only hydroxy end groups. Once all of 9 reacted, ε-DL polymerized off the hydroxy end groups to give ABA polyester triblock copolymers (Scheme 11). This strategy can be extended to synthesize pentablock and heptablock copolymers through further addition of the appropriate monomers. This result is the first and only example of higher-order block copolymers containing epoxide/anhydride polyesters. In 2016, Williams and co-workers66 showed that one-pot polyester-b-polyester AB diblocks could be formed from a similar terpolymerization of CHO, 9, and ε-caprolactone.

the scope of these terpolymerizations by polymerizing 1, 9, and 13 with CHO and CO2 by using chromium porphyrin (cat2) and salph (cat33) catalysts. Darensbourg et al.34 also reported on the terpolymerization of 9 with CHO and CO2, using a (salcy)CrCl catalyst (cat14) and [PPN][N3] to give formation of a polyesterb-polycarbonate diblock copolymer. Likewise, Williams and coworkers24,28,66 demonstrated the formation of polyester-bpolycarbonate via terpolymerization of CHO, 9, and CO2 with dimagnesium and dizinc catalysts. With the bicyclic epoxide CHO, the identity of the anhydride can be varied without greatly affecting the outcome of the terpolymerization, which in all of these examples favors polyester formation followed by polycarbonate formation (Scheme 10a). The monocyclic epoxide PO has also been used in terpolymerizations with anhydrides and CO2 to form mainly blocky structures, which are expected because the epoxide ring opening is not the product-determining step. In 2014, Duan et al.63 used a bifunctional (salen)CoNO3 catalyst (cat31) to form polyester-b-polycarbonate from PO with exo- or endo-norbornene anhydride (16) and CO2 (Scheme 10b). They discovered that, at high anhydride concentrations, exo-16 maintained higher catalyst activity but endo-16 incorporated a greater amount of anhydride in the terpolymer (12.1% versus 20.5%). Lee and coworkers64 also reported a terpolymerization of PO with 9 and CO2 in 2014. Although they used a discrete homogeneous (salcy)CoOAc catalyst (cat32) and achieved the highest molecular weights to date (381 kDa) for these polymerizations, they did not observe well-defined blocks but rather gradient block copolymers (Scheme 10b). 2.4. Terpolymerizations with Lactones and Epoxides

2.5. Block Copolymers via Sequential Addition of Anhydrides

Because of the distinct catalytic cycles that occur in these terpolymerizations, CO2 is unusual in its capacity to form welldefined blocks with polyesters in one-pot reactions. Both lactones and epoxides also form blocks with polyesters through sequential monomer addition and, in rare cases, through one-pot procedures. In one of the earliest examples of epoxide/anhydride copolymerization, Inoue and co-workers50 used a porphyrin AlCl catalyst (cat1) to add ethylene oxide and 1-butene oxide to

In 2016, Coates and co-workers52 demonstrated the sequential formation of diblock copolymers in the polymerization of two tricyclic anhydrides and PO with a fluorinated (salph)AlCl catalyst (cat39; Scheme 12). The first block was synthesized with norbornene anhydride (16) and excess PO. Once 16 was completely consumed, a second hydrogenated tricyclic anhydride (17) was added, forming the second block. The use of saturated and unsaturated monomers allows for the possibility of I


Chemical Reviews

Review

and the ability to synthesize a wider range of purely polyester multiblock copolymers.

3. CATALYST ACTIVITY 3.1. Quantification of Activity

Low catalyst activity is a challenge in copolymerization of cyclic anhydrides and epoxides. For example, in one of the earliest examples of epoxide/anhydride copolymerization, Aida and Inoue49 report that full conversion of PO and phthalic anhydride (9) with tetraphenylporphyrin (TPP)AlCl (cat1) required 4−7 days at room temperature. Since then, catalyst development in this field has resulted in multiple reports of exceptional activity, although many catalyst activities are not reported in easily comparable ways. For example, Sun et al.59 were among the first to report high catalyst activity for the terpolymerization of CHO, maleic anhydride (2), and CO2 with an efficiency of 12.7 kg of polymer/g of zinc in their heterogeneous double metal cyanide catalyst, but only one other report used catalyst efficiency to quantify catalyst activity.64 In another example, Lu and coworkers39 compared the activities of mononuclear (cat55, cat56) and binuclear (cat57) (salan)CrCl catalysts and found that the activity of the binuclear catalyst was 4−7 times higher, but they did not report absolute numbers for activity that could be compared with those of other catalysts. One way to quantify catalyst activity is to calculate turnover number (TON) and turnover frequency (TOF). TON is a measure of the amount of monomer that a catalyst can convert to polymer and is calculated by dividing the moles of polymer formed by the moles of catalyst. TOF is calculated in turn by dividing TON by the reaction time (in hours) to quantify how quickly the catalyst converts monomer to polymer. For clarity, only studies in which catalytic activity is reported in terms of TOF (or in which TOF can be easily calculated from the data available) are discussed here. Additionally, it should be noted that multiple factors affect the TOF of various catalysts, including reaction temperature, cocatalyst concentration, and ratio of epoxide to anhydride, as the copolymerization is generally accepted to be first-order with respect to epoxide. As expected, TOF generally increases with temperature, as is discussed in the examples below, and also typically increases as cocatalyst concentration increases. Because the majority of the reports on epoxide/anhydride copolymerization use different reaction conditions, direct comparisons of catalysts should be made with care. 3.2. Activity of β-Diiminate and Porphyrin Catalysts

The earliest report of an epoxide/anhydride catalyst with high activity was published by Coates and co-workers61 in 2008 for the terpolymerization of CHO, DGA (7), and CO2 with (BDI)Zn (cat13) to form polyester-b-polycarbonate. Whereas polyester formation from CHO and 7 alone had a TOF of only 31 h−1,62 the TOF of terpolymerization ranged from 153 to 650 h−1. Duchateau and co-workers also reported several examples of high catalyst activity for various anhydrides with CHO,32,36 SO,35 and LO.40 In 2011, Huijser et al.32 reported copolymerization with TOF values up to 95 h−1 for CHO and succinic anhydride (1) with a (TPP)CrCl catalyst (cat2). In 2012, Hosseini Nejad et al.36 compared various catalysts and cocatalysts for the copolymerization of CHO and 9, reporting the highest activities for (salph)CrCl (cat33) with DMAP (228 h−1), PCy3 (228 h−1), and [PPN]Cl (245 h−1). In another 2012 report, Hosseini Nejad et al.35 reported an impressive TOF of 480 h−‑1 for the copolymerization of SO and 9 with cat33 using tris(2,4,6-

Figure 14. Anhydrides copolymerized with bi- or tricyclic epoxides.

selective post-polymerization modification of individual blocks. This example, along with the others previously described, highlights the progress of alternating ring-opening copolymerization of cyclic anhydrides and epoxides toward sequencecontrolled polymerization incorporating a wide range of functionalities. Although numerous examples of terpolymerizations with CHO, PO, and 9 have been reported, expanding the scope of the monomers used in terpolymerizations could lead to new desirable materials, further tuning of polymer properties, J


Chemical Reviews

Review

Scheme 3. Early Reports of Copolymerization of (a) Ethylene Oxide with Succinic Anhydride and (b) Sugar-based Epoxides with Succinic and Glutaric Anhydride

Scheme 4. (a) Limonene Oxide- and (b) Succinic Anhydridebased Polyesters

Scheme 6. (a) Tandem Anhydride Synthesis and Copolymerization of Renewable Monomers and (b) Monomer Scope for Tandem Reactions

Scheme 5. Glycidol-based Epoxides Copolymerized with Maleic Anhydride

trimethoxyphenyl)phosphine or triphenylphosphine as a cocatalyst. Finally, Duchateau and co-workers40 reported in 2013 that the use of cat33 and [PPN]Cl for the copolymerization of LO and 9 resulted in a TOF of 78 h−1. In 2013, Chisholm and co-workers37 compared the activities of various aluminum, chromium, and cobalt porphyrin catalysts (cat1−cat9)using 1 equivalent of [PPN]Clfor the copolymerization of PO with succinic anhydride (1) and methyl succinic anhydride (3; Table 1). Both sets of monocyclic monomers were polymerized in the melt, at 30 °C for 3 and 130 °C for 1. As mentioned above, activity increases with temperature, resulting in much higher TOF values for 1 than for 3. For both anhydrides, chromium showed higher activity than cobalt (and both of these showed higher activity than K


Chemical Reviews

Review

compared with the Mg2Br2 catalyst (cat67), the Mg2OAc2 catalyst (cat65) is more than 10 times faster for CHO/9 polymerization. Similarly, the Zn2Br2 catalyst (cat68; TOF = 17 h−1) is significantly slower than both the Zn2OAc2 (cat66; TOF = 24 h−1) and the Zn2Ph2 (cat70; TOF = 25 h−1) catalysts.27 Thevenon et al.28 were able to improve the activity of CHO/9 polymerizations further by using asymmetric Zn2OAc2 catalysts (cat71 and cat72). Use of a salcy backbone (cat72) resulted in a TOF of 70 h−1, and amine linkages in the ligand framework gave polyether linkages. The authors showed that use of a flexible salen backbone and imine linkages (cat71) resulted in a TOF of 198 h−1, the highest for this system to date. Williams and co-workers26 also investigated the activity of cat65 for cyclohexadiene oxide (CHDO) with 9, as well as the activity of (salcy)CoCl (cat18) and (salcy)CrCl (cat14) with 1 equivalent of [PPN]Cl (Table 3). For this set of monomers, activity with both cat18 (TOF = 60 h−1) and cat14 (TOF = 246 h−1) was much higher than that with cat65 (TOF = 6.25 h−1). In fact, the activity reported with cat14 is one of the highest across all catalyst and monomer sets for the copolymerization of cyclic anhydrides and epoxides. This report also demonstrates the extent to which a small change in the monomer can affect catalyst activity, as cat65 has a TOF of 97 h−1 for copolymerization of saturated monomer CHO with 9. Additionally, both Chisholm and Williams found that Cr was the most active metal for their epoxide/anhydride copolymerization systems.

Scheme 7. Partially Renewable Semiaromatic Polyesters with (a) Styrene Oxide, (b) Limonene Oxide, and (c) 1,4Cyclohexadiene Oxide

3.4. Activity of (salen)MX Catalysts

Coates and co-workers51,52 were the first to report activity for (salph)AlCl catalysts (cat36, cat39, and cat40 with [PPN]Cl at 60 °C) in the copolymerization of anhydrides and epoxides. For PO and anhydride 21, cat36 gave a TOF of 63 h−1, whereas the Cr analogue (cat33) had a TOF of 115 h−1. In 2016, Coates and co-workers52 reported the copolymerization of PO and various tricyclic anhydrides formed from Diels−Alder adducts of maleic anhydride or citraconic anhydride with cat36, which yielded TOF values ranging from 63 to 185 h−1 (Table 4). In addition, while investigating the epimerization of these polymers at long reaction times (vide infra), the Coates group demonstrated the effect of cocatalyst concentration and ligand electronics on TOF. For both factors, modifications that prevented epimerization lower [PPN]Cl concentration and electron-withdrawing fluoro substituents on the ligand (cat39)also decreased catalyst activity. Even though this class of tricyclic anhydrides is much more sterically hindered than any other class of anhydrides used in polyester synthesis, the TOF values are comparable to many of those reported for mono- and bicyclic anhydrides. Although these examples demonstrate great advances in catalyst activity for polyester synthesis, the four best TOF values in the epoxide/anhydride literature all employ (salcy)Co catalysts (cat26,77 cat31,63 and cat3264). One of the highest reported TOF values for a pure polyester comes from Coates and co-workers,77 who used an electron-withdrawing salcy ligand (cat26) and 1 equivalent of [PPN][NO3]. In this example, maleic anhydride (2) was copolymerized with various epoxides at 30 °C to give TOF values ranging from 25 to 308 h−1, in which the highest TOF was observed for the copolymerization of epichlorohydrin and 2 (Scheme 13a). Duan et al.63 reported the first example of a bifunctional catalyst (cat31) for epoxide/ anhydride copolymerization in which cationic species functioning as cocatalysts were tethered to the salcy ligand. In the terpolymerization of PO, CO2, and either exo- or endonorbornene anhydride (16), cat31 yields polyester-b-polycar-

aluminum). Additionally, the PO/3 copolymerization was optimized with a TPP ligand (cat2) to achieve an optimized TOF of 52 h−1. At elevated temperature, cat2 achieved an optimized TOF for PO/1 copolymerization of ∼230 h−1. In another 2013 report, Chisholm and co-workers38 used cat2 with 1 equivalent of [PPN]Cl to achieve a TOF of ∼200 h−1 with SO and 1 at 70 °C (Table 1). 3.3. Activity of Bimetallic Catalysts

Williams and co-workers24,27,28 have compared the activity of several bimetallic catalysts (cat65−cat72) for the copolymerization of CHO and phthalic anhydride (9) at 100 °C (Table 2). Unlike many of the porphyrin and salen catalysts reported for these copolymerizations, these catalysts require no cocatalyst. In addition, these authors provide the only reported TOF values for a bicyclic epoxide. In 2014, the first of these reports24 showed that a Zn2(OAc)2 catalyst (cat66) gives a TOF of 24 h−1, whereas the corresponding dimagnesium catalyst (cat65) gives a TOF of 97 h−1, almost 4 times as high. In 2015, Garden et al.29 showed that a MgZnBr2 version of this catalyst (cat69) has much higher activity (TOF = 188 h−1) than either the Mg2Br2 (cat67; TOF = 9 h−1) or Zn2Br2 (cat68; TOF = 17 h−1) version. Furthermore, to demonstrate that cat69 was in fact responsible for the increase in activity, they also performed the polymerization in a 1:1 mixture of cat67 and cat68, which resulted in a TOF of only 5 h−1. This is the first example of a heteronuclear bimetallic catalyst for the copolymerization of cyclic anhydrides, but it also shows how important the identity of the axial group is for catalyst activity: L


Chemical Reviews

Review

Figure 15. Summary of renewable epoxides and cyclic anhydrides.

cyclic anhydrides and epoxides, investigations of TOF for a wider range of catalysts and monomer sets would increase our understanding of the relationships between catalysts, polymer structure, and activity. In addition, kinetic studies of the effects of cocatalysts and temperature, similar to those reported by Darensbourg et al. in 2012,34 would expand our knowledge of catalyst activity in this field.

Scheme 8. Terpolymerization of CHO, DGA, and CO2 with a (BDI)ZnOAc Catalysta

4. CONTROL OF MOLECULAR WEIGHT 4.1. Challenges in Control of Molecular Weight

One of the many advantages of chain-growth polymerization over step-growth methods is the ability to control molecular weight and the dispersity of the resulting polymers. However, although chain-growth polymerizations generally allow wellcontrolled systems, many epoxide/anhydride copolymerizations have been plagued by high dispersity values owing to transesterification at high conversion. Additionally, many of the molecular weights reported for these polymers are lower than expected because adventitious water or hydrolyzed anhydride is present, leading to an increase in the number of initiators and to overall lower molecular weight.36 Small amounts of bifunctional initiators such as water also result in bimodal molecular weight distributions, as noted in several instances. Despite this challenge, there have been several reports of well-controlled polymerizations (Đ ≤ 1.30) and high-molecular-weight polymers, which are extremely important if these materials are to compete with commercially available polymers. In this review, narrow molecular weight distributions are defined as 1.01−1.29 and moderate distributions as 1.3−1.5.

a

CHO = cyclohexene oxide; DGA = diglycolic anhydride; (BDI)ZnOAc = β-diiminate zinc acetate.

bonate with optimized TOF values of 1108 or 1144 h−1, respectively (Scheme 13b). Lee and co-workers64 also used a tethered salcy ligand (cat32) for the copolymerization of PO and phthalic anhydride (9) and achieved a TOF of 1901 h−1 (Scheme 13c). Their terpolymerization of PO, 9, and CO2 yielded gradient polyester-b-polycarbonate with a remarkable TOF of 12 000 h−1 (Scheme 13c). These are by far the highest TOF values reported in the epoxide/anhydride literature to date, and these tethered cocatalysts also typically result in much higher molecular weights (see Figure 16d). Although there are a number of excellent examples of high-activity catalysts for the copolymerization of

4.2. Advances in Molecular Weight Control

The initial reports of Inoue and co-workers49,50 in 1985 showed that use of a porphyrin AlCl catalyst (cat1) resulted in extremely M


Chemical Reviews

Review

Scheme 9. (a) Proposed Mechanism for Terpolymerization of CHO, DGA, and CO2a and (b) In Situ IR Spectroscopy Showing Formation of Polyester and Polycarbonate Blocks

Scheme 10. Anhydrides Used for Terpolymerizations with (a) CHO and CO2 and (b) PO and CO2a

a

CHO = cyclohexene oxide; PO = propylene oxide.

Scheme 11. Terpolymerization of CHO, Phthalic Anhydride, and ε-DL in the Presence of Cyclohexane Diol To Form ABA Triblock Copolymersa

a

CHO = cyclohexene oxide; DGA = diglycolic anhydride; R = alkyl or polymer.

well-controlled dispersity for the copolymerization of monocyclic epoxides and 9, with values ranging from 1.08 to 1.16 depending on the epoxide. The molecular weights of the materials were below 4 kDa, however (Figure 16a). This approach was the first major improvement over previous uses of heterogeneous metal complexes, which gave much broader molecular weight distributions and often resulted in the homopolymerization of epoxides to give polyether. The next significant advance in molecular weight was in 2007, when Coates and co-workers62 found that using a (BDI)ZnOAc catalyst (cat13) significantly expanded the monomer scope of epoxide/anhydride copolymerization and yielded polymers with significantly higher molecular weights (10−55 kDa) and dispersity values of 1.1−1.5 (Figure 16b). In 2011, Thomas and co-workers33 reported another increase in monomer scope while maintaining excellent control over molecular weight and dispersity (Đ = 1.1−1.3) through an elegant tandem synthesis and copolymerization. Their approach achieved molecular weights as high as 27 kDa for the copolymerization of LO and camphoric anhydride (14; Figure 16c). Also in 2011, DiCiccio and Coates42 reported molecular weights ranging from 17 to 33 kDa for polymers from the copolymerization of maleic anhydride (2) and various epoxides, although the dispersities were, in

a

CHO = cyclohexene oxide; ε-DL = ε-decalactone.

general, only moderately narrow (see Scheme 5). This was the first instance of access to unsaturated, aliphatic polyesters with molecular weights greater than 15 kDa, and these materials are of significant interest for biomedical applications. The highest molecular weights to date for polyesters from epoxide/anhydride copolymerization come from Lee and coN


Chemical Reviews

Review

Scheme 12. Diblock Copolymer Synthesis via a Sequential Addition Strategy

workers,64 who reported an Mn of 167 kDa for poly(propylene phthalate) and a narrow dispersity at full conversion (Đ = 1.21; Figure 16d). This report is especially impressive because many systems cannot maintain narrow dispersity at high conversion owing to transesterification. Additionally, by incorporating CO2 as a comonomer, the authors pushed molecular weights as high as 381 kDa. Tricyclic anhydrides are a growing class of monomers in this field and display impressive thermal properties.48,51,52 In 2015, Van Zee and Coates51 noted that the choice of catalyst used to promote the copolymerization of PO with tricyclic anhydride 21 had a pronounced effect on the dispersity of the polymer and the ability to achieve high-molecular-weight materials. Use of a chromium or cobalt catalyst (cat33 or cat34) in excess epoxide led to substantial transesterification and thus significant broadening of dispersity, whereas use of an aluminum catalyst (cat36) suppressed transesterification, retaining narrow dispersity even at high conversion. This approach provides access to materials with molecular weights as high as 55 kDa (see Scheme 20a). Further catalyst optimization to expand access to less bulky tricyclic anhydrides has recently been reported.52 Although cat36 effectively suppresses transesterification with bulky monomers, it is ineffective at controlling dispersity with less bulky monomers. By substituting fluoro groups (cat39) for the tert-butyl groups in the para position of the phenoxide moiety of the ligand, Coates and co-workers52 were able to quell

transesterification even with less bulky anhydrides, which allowed control over molecular weight and dispersity (see Scheme 20b and c). In 2016, Kleij and Coates48 copolymerized six partially or fully renewable anhydrides with PO and CHO. With PO, polymer molecular weights were as high as 32 kDa, and dispersity values were generally less than 1.15. Although molecular weight and dispersity can be controlled solely through catalyst choice, other modifications to the reaction mixture can also play roles in determining molecular weight and dispersity. Duchateau and co-workers40 found that the addition of protic chain transfer agents (CTAs) resulted in narrower dispersity, although it also lowered the overall molecular weights of the materials. The authors reported that the copolymerization of LO and phthalic anhydride (9) in the presence of CTAs gave poly(limonene phthalate) with dispersity values ranging from 1.06 to 1.3, an improvement over polymerizations without CTAs (Đ ≥ 1.3), similar to the observations of DiCiccio and Coates42 with a similar strategy. Although these reports show that achieving higher-molecularweight materials and gaining excellent control over molecular weight distribution is feasible, it should be emphasized that reports of low-molecular-weight materials or very broad dispersities abound. Significant room for improvement remains, and improved molecular weights and narrow dispersities should be a continuing focus to make progress in this field. In particular, bicyclic epoxides tend to be difficult to polymerize to highmolecular-weights with currently available catalysts. Additionally, O


Chemical Reviews

Review

Table 1. Activity of Porphyrin Catalysts with [PPN]Cl for the Copolymerization of Epoxides and Cyclic Anhydridesa

entry

R1

R2

catalyst

T (°C)

TOFb (h−1)

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

Me Me Me Me Me Me Me Me Me Me Me Me Ph

Me Me Me Me Me Me Me Me Me H H H H

cat4 cat1 cat7 cat5 cat2 cat8 cat6 cat3 cat9 cat1 cat2 cat3 cat2

30 30 30 30 30 30 30 30 30 130 130 130 70

8 19 9 50 52 25 17 25 19 ∼216 ∼230 ∼200 ∼200

Table 2. Activity of Bimetallic Catalysts for the Copolymerization of Cyclohexene Oxide and Phthalic Anhydride at 100 °Ca

entry

catalyst

M1

M2

X

TOFb (h−1)

1 2 3 4 5 6 7 8

cat66 cat65 cat69 cat68 cat67 cat70 cat71 cat72

Zn Mg Mg Zn Mg Zn Zn Zn

Zn Mg Zn Zn Mg Zn Zn Zn

OAc OAc Br Br Br Ph OAc OAc

24 97 188 17 9 25 198 70

a

General conditions: [epoxide]/[anhydride]/[catalyst] = 800:100:1. Turnover frequency = moles of monomer consumed per mole of catalyst per hour.

b

a

General conditions: [epoxide]/[anhydride]/[catalyst]/[(PPN)Cl] = 200:220:1:1, >99% conversion. bTurnover frequency = moles of monomer consumed per mole of catalyst per hour.

Table 3. Activity of Catalysts for the Copolymerization of Cyclohexadiene Oxide and Phthalic Anhydridea

most higher-molecular-weight examples are only moderately high, with only one example of molecular weights greater than 100 kDa, compared with those of other classes of polymers. Although some applications of polyesters such as coatings and resins benefit from low molecular weights, access to truly highmolecular-weight materials continues to be a key challenge to address, particularly because molecular weights above the chain entanglement molecular weight and the accompanying improvement in properties are necessary for many potential applications.

5. REGIO- AND STEREOCONTROL OF EPOXIDE/ANHYDRIDE COPOLYMERIZATION 5.1. Quantification and Analysis of Regio- and Stereocontrol

Yet another advantage of chain-growth polymerizations is the potential to control regiochemistry via catalyst design or monomer selection. Ring opening of an asymmetric terminal epoxide can occur either at the less sterically hindered methylene carbon or at the methine carbon, with a preference for inversion of the stereocenter (Scheme 14). If ring opening occurs exclusively at one of these positions, the resulting polymer will be regioregular, containing only head-to-tail (HT) linkages. If ring opening occurs randomly at either carbon, the polymer will be regioirregular, containing tail-to-tail (TT), HT, and head-tohead (HH) linkages. Furthermore, if the epoxide in the copolymerization is enantiopure, as shown in Scheme 14, regioregular ring opening at either position will also result in a stereoregular, isotactic polymer. By contrast, regioerrors in the ring opening of enantiopure epoxides also cause stereoerrors.

entry

catalyst

cocatalyst

T (°C)

TOFb (h−1)

1 2 3

cat65 cat18 cat14

[PPN]Cl [PPN]Cl

120 110 110

6 60 246

a

General conditions: [epoxide]/[anhydride]/[catalyst]/[(PPN)Cl] = 250:250:1:1. bTurnover frequency = moles of monomer consumed per mole of catalyst per hour.

The relationship between regio- and stereochemistry in the copolymerization of epoxides and cyclic anhydrides allows the use of careful analysis of stereoerrors to determine regioselectivity. If enantiopure epoxide is used in the copolymerization, the resulting polyester can be degraded to give diols with P


Chemical Reviews

Review

the 13C NMR spectra of poly(phenylethylene succinate)38 and poly(propylene succinate).37 In the former, signals from HT linkages were only slightly more prominent than those from HH and TT linkages, whereas in the latter, HT linkages were the major signals, a result that suggested regioregularity. For poly(propylene succinate), Chisholm and co-workers37 quantified the regioregularity, showing that cat1 yielded 86% HT linkages at room temperature. In 2013, Lu and co-workers39 used a bimetallic (salan)CrCl complex in the regioselective copolymerization of phenyl glycidyl ether (PGE) with maleic anhydride (2). The regioselectivity was assessed by using enantiopure (S)-PGE, which was degraded to give the corresponding (S)-diol with 98.5% enantiomeric excess and, thus, 98.5% HT linkages (Scheme 16a). Retention of the stereochemistry again suggests that the epoxide is predominantly opened at the methylene carbon to give regio- and stereoregular polymer. Coates and coworkers43,77,51 also quantified the regioregularity of polyesters by degrading polyesters formed from enantiopure PO. In 2014, Coates and co-workers43 found that enantiopure (salcy)CoNO3 with electron-withdrawing chloro groups on the ligand (cat23) yielded isotactic poly(propylene succinate) with 96−97% HT linkages (Scheme 16b). More surprisingly, Van Zee and Coates51 also demonstrated that an achiral (salph)CrCl catalyst (cat33) regioregularly inserted (S)-PO in copolymerization with the tricyclic anhydride 21 (Scheme 16c). A possible explanation for this selectivity is that the bulky carboxylate formed by the ring opening of the anhydride preferentially attacks the less sterically hindered methylene carbon of the epoxide. Coates and co-workers77 continued investigating the effects of catalyst choice on the regioregularity of epoxide/anhydride copolymerization. In agreement with an earlier report42 on the synthesis of poly(propylene maleate), (salcy)Co catalysts (cat19 and cat23−cat27) were found to produce polyesters that were much more regioregular than those produced by (salcy)CrCl (cat14), as shown by two-dimensional heteronuclear single quantum coherence (2D HSQC) NMR spectroscopy (Figure 17). Additionally, the identity of the axial ligand and the para substituents on the salcy ligand affected both the regioregularity and the redox stability of the catalysts. The combination of a nitro axial group and electron-withdrawing fluoro groups on the ligand yielded the most regioregular polyesters for a wide range of epoxides with 2 and for a range of anhydrides with PO (Scheme 17).77 To date, regioregularity in the ring-opening copolymerization of epoxides and anhydrides has been controlled only for systems containing terminal monocyclic epoxides. Van Zee and Coates51 investigated the regiochemistry of asymmetric anhydride insertion with (salph)CrCl catalyst cat33 but observed only regioirregular insertion. In all these examples, ring opening of the epoxide occurred mainly at the less sterically hindered methylene carbon, allowing for the retention of stereochemistry when enantiopure epoxides were used in place of racemic epoxides. We predict that multiple factors affect the capability of a catalyst to regioselectively polymerize terminal epoxides with cyclic anhydrides. First, bulky epoxides and bulky carboxylates both likely favor ring opening at the methylene carbon of the epoxide, owing to steric hindrance at the methine carbon.53,51 In the case described by Lu and co-workers,39 the bimetallic catalyst cat57 may have an asymmetric active site that highly favors methylene attack. Finally, as Coates and co-workers77 observed, changes in metal and ligand identity can increase the Lewis acidity of a catalyst, which results in tighter binding and greater epoxide

Table 4. Turnover Frequencies for Polyesters Formed from Propylene Oxide and Tricyclic Anhydrides with (salph)AlCl and [PPN]Cl at 60 °Ca

entry

anhydride

catalyst

[PPN]Cl (equiv)

TOFb (h−1)

1 2 3 4 5 6 7 8 9 10 11

21 19 16 17 18 16 16 16 19 19 19

cat36 cat36 cat36 cat36 cat36 cat36 cat36 cat36 cat36 cat40 cat39

1 1 1 1 1 1.1 0.9 0.5 0.9 0.9 0.9

63 122 95 152 185 106 92 36 88 80 49

a

General conditions: [epoxide]/[anhydride]/[catalyst] = 500:100:1. Salph = N,N′-bis(salicylidene)phenylenediamine. bTurnover frequency = moles of monomer consumed per mole of catalyst per hour.

stereocenters that are unaffected by the degradation process and can be easily quantified by gas chromatography (Scheme 15). Thus, any stereoerrors can be attributed to regioerrors from the ring opening of the epoxide. If the diol resulting from polymer degradation is of the same absolute stereochemistry as the epoxide starting material, ring opening occurred at the methylene carbon. Conversely, if the diol is of the opposite absolute stereochemistry as the epoxide, ring opening occurred at the methine carbon with an inversion of stereochemistry. The ratio of these diols can then be used to determine the regioregularity of epoxide ring opening. 5.2. Advances in Regiocontrol

The first example of control over regiochemistry in epoxide/ anhydride copolymerization was provided by Takasu et al.53 in 2005 in the regioregular ring opening of sugar-based epoxides with methylaluminum bis(2,6-di-tert-butyl-4-bromophenoxide), Mg(OEt)2, or Al(OiPr)3. These epoxides likely show preference for ring opening at the methylene carbon because of the extreme steric bulk at the methine carbon. Additionally, ring opening of enantiopure versions of these epoxides results in isotactic polymers. In 2013, Chisholm and co-workers38,37 investigated the regioregularity of polyesters formed from succinic anhydride (1) and SO or PO with (TPP)CrCl (cat2) or (TPP)AlCl (cat1), respectively. They found that, in the initial ring opening of SO by cat2, attack at the methylene versus the methine carbon occurred in a 2:1 ratio. By using enantiopure (R)-SO, they also showed that an SN2-type attack at the methine carbon resulted in a stereochemical inversion-to-retention ratio of 2:1.38 Furthermore, they observed HT linkages formed from regioregular enchainment as well as HH and TT linkages from regioerrors in Q


Chemical Reviews

Review

Scheme 13. Turnover Frequencies of Polyester and Polyester-b-Polycarbonate from (salcy)Co Catalystsa

a

(salcy)Co = (N,N′-bis[salicylidene]cyclohexanediamine)cobalt.

published examples of stereocontrol via enantioselective or isoselective catalysts for the copolymerization of racemic epoxides and anhydrides have been published, although Chisholm and co-workers,38 Lu and co-workers,39 and Coates and co-workers43,77,51 have produced isotactic polyesters from enantiopure epoxides to study regiocontrol, as discussed above. Indeed, only one study to date has investigated this type of stereocontrol. In 2013, Chisholm and co-workers38 determined that the ring opening of racemic SO with (TPP)CrCl (cat2) at the methine carbon results in a 2:1 ratio of stereochemical inversion to retention. Using these results, they compared the 13 C NMR spectra of poly[(R)-phenylethylene succinate] to (rac)-poly(phenylethylene succinate) and found that the polymer was stereoirregular. 5.3.1. Control of Cis/Trans Isomerization. One of the few examples of stereocontrol in polyester formation comes from the controlled cis/trans isomerization of poly(propylene maleate) to poly(propylene fumarate) described by DiCiccio and Coates42 in 2011 (Scheme 18). Catalytic diethylamine was added to poly(propylene maleate) to give poly(propylene fumarate), and the reaction was confirmed by 1H NMR spectroscopy. Over the course of the isomerization, the molecular weight remained constant (17 kDa) and Tg increased from 18 to 35 °C. To our knowledge, this result is the only example of controlled cis/trans isomerization of an alkene in polyesters synthesized by epoxide/ anhydride copolymerization. The vinyl protons in poly(propylene maleate) are likely more acidic than typical vinylic

Figure 16. Advances in molecular weight (Mn) and dispersity (Đ).

activation. These changes in turn seem to greatly increase the regioselectivity of the epoxide ring-opening step. 5.3. Advances in Stereocontrol

In olefin polymerizations, stereocontrol played a seminal role in synthesizing semicrystalline materials with properties so robust that they are now produced on the megaton scale every year. However, unlike studies of epoxide homopolymerization,78 no R


Chemical Reviews

Review

Scheme 14. Regio- and Stereochemistry of Ring Opening of an Enantiopure Epoxidea

a

O2CP = growing polymer chain.

20a). This report was followed in 2016 by further catalyst optimization because although cat36 prevented epimerization with bulky monomers, it allowed rapid epimerization with less bulky monomers.52 By replacing the tert-butyl groups in the para position of the ligand with fluoro groups (cat39), Coates and coworkers52 effectively suppressed epimerization and maintained high cis-diester content even when much less bulky anhydrides were used (Scheme 20d). As with transesterification, the amount of epimerization was correlated with the electron-donating capability of the ligand (Scheme 20b,d). Whereas Coates and co-workers51,52 focused on ways to decrease epimerization, Liu and Kim and co-workers20 exploited epimerization to synthesize both cis and trans stereoregular polymers and reported that the stereochemistry of the polymer could be controlled by changing the ratio of epoxide to anhydride and the exo/endo stereochemistry of the anhydride. When excess CHO was used, the resulting polymers were >99% trans, whereas with excess anhydride or a 1:1 ratio of CHO/exoanhydride, the polymers were 92% to >99% cis (Scheme 21). Similar to earlier findings,51 epimerization to the trans configuration decreased the Tg; samples with predominantly cis-diester linkages had Tg values of up to 130 °C, whereas the Tg values of materials with >99% trans linkages ranged from 112 to 116 °C.

Scheme 15. Regioselective Copolymerization of an Enantiopure Epoxide and Subsequent Degradation

protons because they are also α-protons, which increases their susceptibility to isomerization in the presence of an amine. 5.3.2. Control of Epimerization. Another type of stereocontrol in epoxide/anhydride copolymerization is control over polymer chain epimerization. Certain monomers, such as tricyclic anhydrides made via the Diels−Alder reaction, have inherent cis stereochemistry that is retained in the ring opening and leads to cis-diester linkages in the polymer (Scheme 19). However, epimerization to the thermodynamically more favorable trans configuration can occur (Scheme 19). Presumably, epimerization can occur in all polyesters with α-protons, but it is a measurable side reaction only in polymers with asymmetric carbons at the α-position. The control of epimerization has been studied recently due to its effects on polymer properties. In 2015, Van Zee and Coates51 noted that catalyst choice in the copolymerization of PO with tricyclic anhydride 21 had a dramatic effect on the Tg of the polymer. Use of a chromium (cat33) or cobalt catalyst (cat34) in excess epoxide led to significant epimerization, reducing the Tg as much as 31 °C from a high of 109 °C to a low of 78 °C. An aluminum catalyst (cat36), however, greatly reduced epimerization, allowing retention of 98% of the cis-diester linkages even at high conversion (Scheme

6. THERMAL PROPERTIES 6.1. Semicrystalline Polyesters and Stereocomplexes

Semicrystalline polymers such as polyethylene and polypropylene are the most widely produced polymers in the world, in large part because of their exceptional thermal properties. Polyesters like PET and polylactide are also semicrystalline, which makes them suitable for a large number of applications. Unfortunately, there are few reports of semicrystalline polyesters synthesized from epoxides and cyclic anhydrides, and these rare examples occur only in cases in which there is no stereochemistry to consider or isotactic polyesters are formed (Figure 18). Notably, however, not all examples of isotactic polyesters have melting temperatures (Tm) that can be determined by differential scanning calorimetry (DSC). For example, Chisholm and coworkers38 synthesized isotactic-enriched poly(phenylethylene S


Chemical Reviews

Review

Scheme 16. Regioregular Ring Opening of Terminal Epoxides with Salen-type Catalysts

Figure 17. HSQC NMR spectroscopy showing (a) regioirregular poly(propylene maleate), produced with (salcy)CrCl, and (b) regioregular poly(propylene maleate), produced with (salcy)CoOBzF5.

succinate) and Lu and co-workers39 synthesized isotactic poly(PGE-alt-MA), but these materials were not reported to be semicrystalline. In 1997, Maeda et al.60 showed that the mostly alternating copolymer of achiral monomers EO and succinic anhydride (1), with no stereochemistry to consider, has a Tm ranging from 85 to 99 °C depending on the anhydride content in the polymer (see Figure 18). Coates and co-workers,43,77 on the

contrary, have reported three examples of semicrystalline isotactic polyesters synthesized with enantiopure PO (Figure 18). Copolymerizations of (S)-PO or (R)-PO with 1, maleic anhydride (2), and phthalic anhydride (9) give polymers with Tm values of 79, 117, and 150 °C, respectively. Notably, in all three of these cases, Tm values are reported from the first DSC heat only, as these materials do not readily recrystallize from the melt. T


Chemical Reviews

Review

Scheme 17. Regioregular Polymerization of Cyclic Anhydrides and Epoxides with a p-Fluoro (salcy)Co Catalysta

a

(salcy)Co = (N,N′-bis[salicylidene]cyclohexanediamine)cobalt.

(propylene succinate) crystallize from solution and have a Tm of 79 °C (Figure 19a,b). However, these materials recrystallize very slowly from the melt, with crystallinity first reappearing after 7 days of annealing at 25 °C and comparable enthalpy (proportional to the percent crystallinity of the material) reappearing after approximately 3 weeks. Also, the isotactic polymers crystallize as multiple polymorphs with lower melting temperatures than the solution-crystallized samples (Figure 19c). A stereocomplex with improved thermal properties is formed from the stereoselective interaction of stereoregular polymers when (R)- and (S)-poly(propylene succinate) are mixed in a 1:1 ratio (Figure 19d). Not only does the stereocomplex have a Tm 40 °C higher than the parent polymers at 120 °C but it also recrystallizes immediately upon cooling to 90 °C. In fact, the stereocomplex recrystallizes approximately 3 orders of magnitude faster than the parent polymers. Powder X-ray diffraction shows that the stereocomplex has a crystal structure completely different from those of (R)- and (S)-poly(propylene succinate), which likely contributes to the difference in melting temperatures.43 Although stereochemistry certainly plays a role in the crystallization of these polyesters, no definite conclusions can be drawn about the influence of the polymer backbone structure given the very limited number of examples. If anything, small rigid epoxide substituents seem to enhance polyester crystallization. Additionally, Coates and co-workers43 have shown that stereocomplex formation can lead to increased melting temperatures and crystallinity in polyesters. Clearly, isotactic stereoregular polyesters could have more interesting physical and thermal properties than those of their atactic versions, although further research is necessary to confirm the relationships between polyester structure and crystallinity.

Scheme 18. Synthesis of Poly(propylene maleate) and Isomerization to Poly(propylene fumarate)

Scheme 19. Stereochemistry of Tricyclic Anhydridecontaining Polyesters before and after Epimerization and Mechanism of Epimerization

6.2. Amorphous Polyesters: Introduction

In epoxide/anhydride copolymerization, polymer properties can be widely tuned owing to the structural diversity of both monomer sets. This flexibility is particularly important for controlling the thermal properties of these materials. To be practically useful in many applications, materials need to have thermal transitions far removed from room temperature. Materials with Tg values below 0 °C have potential for use as

Crystallinity in these materials returns after extended annealing (more than 7 days) or precipitation into a nonsolvent such as methanol. In addition, interesting melting behavior is observed with stereoregular isotactic poly(propylene succinate).43 Unlike atactic poly(propylene succinate), both (R)- and (S)-polyU


Chemical Reviews

Review

Scheme 20. Control of Epimerization with (salph)AlCl Complexes:a (a) Metal Screening, (b) Ligand Electronics, (c) Control of Dispersity over Time, and (d) Epimerization over Time

a

salph = N,N′-bis(salicylidene)phenylenediamine.

Scheme 21. Control of Epimerization for Stereoregular Polyesters

Figure 18. Semicrystalline polyesters from the copolymerization of cyclic anhydrides and epoxides. V


Chemical Reviews

Review

Figure 19. Stereocomplexation of poly(propylene succinate), shown by differential scanning calorimetry, of (a) (S)-poly(propylene succinate), (b) (R)poly(propylene succinate), (c) (R)-poly(propylene succinate) annealed at 25 °C for up to 19 days, and (d) poly(propylene succinate) stereocomplex formed from a 1:1 mixture of (R)- and (S)-poly(propylene succinate).

midblocks in thermoplastic elastomers. On the other end of the spectrum, materials with Tg values above approximately 90 °C are useful for applications in which a rigid structure must be maintained at elevated temperatures. For certain applications of amorphous polymers, a Tg above 100 °C is particularly desirable to prevent polymer deformation in the presence of boiling water. The wide range of monomer combinations in epoxide/anhydride copolymerization has led to materials with a diverse range of useful, accessible Tg values, in turn allowing for exceptional control through the targeting of specific thermal properties in a desired polymer.

Scheme 22. Initial Reports of Diglycolic Anhydride Polymers with Low Tg with (a) Propylene Oxide and (b) Isobutylene Oxide

6.3. Polyesters with Low Tg Values

Several materials with low Tg values can be synthesized via epoxide/anhydride copolymerization. Some of the first reported epoxide/anhydride copolymerizations by Coates and co-workers62 yielded materials with low Tg values. The use of PO with diglycolic anhydride (7) yielded a material with a Tg of −2 °C, whereas 7 and isobutylene oxide (IBO) produced a material with a similar Tg of −1 °C (Scheme 22). In 2011, DiCiccio and Coates42 reported that the copolymerization of maleic anhydride (2) with allyl glycidyl ether (AGE) or methoxyethoxymethoxy (MEMO), an epoxide with a short chain of poly(ethylene oxide), yielded products with even lower Tg values. With AGE, the Tg values were −10 °C for the maleate analogue and −6 °C for the fumarate analogue. MEMO gave Tg values of −26 °C for the maleate isomer and −29 °C for the fumarate isomer (Scheme 23). Interestingly, although the cis/trans isomerism of the double bond in the polymer backbone affected Tg, no clear trend was apparent for the configuration that yielded materials with a lower

Tg. Although the effect of cis/trans isomerism in the above examples is small, the effect was quite pronounced in the copolymerization of 2 with 1-butene oxide. Poly(butene W


Chemical Reviews

Review

Scheme 23. Maleate- and Fumarate-based Polyesters

Scheme 25. Functionalizable Phthalic Anhydride-based Polymer

polymerization modification, which could be used to modulate Tg further or to introduce new functionalities. Recently, Meier and co-workers41 reported the use of fatty acid-based monomers for epoxide/anhydride copolymerization. The resulting materials had some of the lowest reported Tg values for this class of polyesters (Scheme 26). The use of methyl-9,10epoxystearate (MES) with 1 gives a perfectly alternating copolymer with a Tg of −44 °C, whereas the Tg values of products obtained with 9 ranged from −23 to −30 °C depending on molecular weight. The Diels−Alder adduct of methyl αeleostearate and maleic anhydride (15) was also tested with this epoxide, but significant polyether formation was observed. Although molecular weights were generally low and the copolymerization required extended reaction times at high temperatures, this result is a unique and interesting example of the versatility of epoxide/anhydride copolymerization, and it allows access to some extremely low Tg values. Low-Tg polymers synthesized via epoxide/anhydride copolymerization can be roughly divided into two classes: those with long side chains and those with extremely flexible backbones. DGA-based polymers and poly(propylene succinate) are examples of the latter, whereas the use of monocyclic epoxides and anhydrides results in high backbone flexibility. The other

maleate) had a Tg of 11 °C, whereas poly(butene fumarate) had a much lower Tg of −14 °C. In 2012, Darensbourg et al.34 reported what was, at the time, the lowest Tg for a polyester from epoxide/anhydride copolymerization. Copolymerization of racemic PO with succinic anhydride (1) yielded atactic poly(propylene succinate) with a Tg of −39 °C (Scheme 24). By contrast, Coates and coworkers43 reported that isotactic poly(propylene succinate) from enantiopure PO had a Tg of −4 °C and a Tm up to 120 °C (see Scheme 24). These results show the profound impact stereochemistry has on the thermal properties of polyesters. In 2015, Liu and co-workers65 reported that the copolymerization of phthalic anhydride (9) and AGE gave a polymer with a Tg of −1 °C (Scheme 25). This example is particularly notable not only because the use of 9 typically gives materials with Tg values slightly above ambient temperature but also because the alkene in the polymer opens the possibility for post-

Scheme 24. Effect of Tacticity on Tg of (a) Atactic Poly(propylene succinate) and (b) Isotactic Poly(propylene succinate)

X


Chemical Reviews

Review

typically yields materials with Tg values of 40−70 °C. A few examples of semiaromatic polyesters synthesized with 9 have Tg values above 100 °C. In 2015, Li and co-workers65 reported poly(cyclohexene phthalate) with a Tg of 146 °C and a molecular weight of 13.2 kDa and poly(4-vinylcyclohexene phthalate) with a Tg of 128 °C (Mn = 22.0 kDa; Scheme 27a). Although the

Scheme 26. Low-Tg Polymers from Methyl-9,10epoxystearate

Scheme 27. High-Tg Semiaromatic Polymers

molecular weight of poly(4-vinylcyclohexene phthalate) was higher than that of poly(cyclohexene phthalate), the pendent vinyl group of the former may have reduced the Tg. Meier and co-workers26 used (salcy)CrCl (cat14) with 9 and CHDO to synthesize a 7.5 kDa polymer with a Tg of 128 °C (Scheme 27b). This high Tg is attributable to the combination of two bicyclic monomers and the rigidity of the aromatic phthalate unit, and although it is quite high already, the molecular weight of the sample was relatively low compared with that of poly(cyclohexene phthalate), reported by Li and Liu and coworkers,65 which has a much higher Tg. This result suggests that the intrinsic maximum Tg of poly(cyclohexadiene phthalate) might even be higher. 6.4.2. Aliphatic Polyesters. After the report by Coates and co-workers in 2007,62 the next improvements in thermal properties of aliphatic polyesters occurred in 2012, when Darensbourg et al.34 reported that the alternating copolymerization of CHO and cyclohexene anhydride (10) gave a polymer with a Tg of 95 °C (Scheme 28). This report was followed by an early 2015 study by Van Zee and Coates,51 who used a terpene-based tricyclic anhydride (21) with PO to obtain a polymer with Tg up to 109 °C and molecular weight up to 55 kDa (Scheme 29). This example is especially

examples described above rely primarily on long side chains to achieve lower Tg values, and in the case of methyl 9,10epoxystearate, the effect is quite pronounced. Given the wide range of monomers that either have long side chains or incorporate flexible backbones, it should be relatively easy to expand the scope of low-Tg polyesters and tune their Tg values to precise values for a variety of applications. 6.4. Polyesters with High Tg Values

A study by Coates and co-workers62 in 2007 reported a moderate Tg of 62 °C for the products of the copolymerization of LO and maleic anhydride (2). Copolymerization of the structurally similar monomers CHO and succinic anhydride (1) resulted in a polyester with a similar Tg of 57 °C. Although these values are significantly above room temperature, a Tg significantly below the boiling point of water is impractical for many applications. Thus, there has been significant interest in synthesizing materials with Tg values higher than 100 °C. Although low Tg values are favored by long side chains and flexible backbones, high Tg values are favored by rigid backbones. Notably, the highest Tg values reported by Coates and co-workers62 in 2007 were for products synthesized with rigid bicyclic epoxides. Another way to introduce backbone rigidity is to include an aromatic group. Semiaromatic polyesters and aliphatic polyesters have both benefits and drawbacks; therefore, they are discussed separately herein. As mentioned earlier, one factor limiting these polymerizations is molecular weight, and because Tg depends on molecular weight, some of the reported Tg values may be lower than the upper limit intrinsic to a given polymer.1 6.4.1. Semiaromatic Polyesters. In general, it is easier to achieve a high Tg with semiaromatic polyesters than with aliphatic polyesters because of the inherent rigidity of aromatic moieties. One of the most commonly used semiaromatic anhydrides in epoxide/anhydride copolymerization is phthalic anhydride (9). With PO and other monocyclic epoxides, 9

Scheme 28. Copolymerization of Cyclohexene Oxide and Cyclohexene Anhydride To Yield High-Tg Polyester

Y


Chemical Reviews

Review

renewable anhydrides and PO or CHO. Their study focused on effects of the anhydride and epoxide on thermal properties. Although there were small variations among the polymers synthesized with the six anhydrides studied, a more dramatic effect was seen when the epoxide was changed. With PO, Tg values ranged from 66 to 108 °C, whereas with CHO, the range was 124−184 °C (Scheme 30). These examples suggest that the presence of at least one multicyclic monomer adds bulk and rigidity to the backbone and results in a high Tg. Thermal properties might be further improved by exploring new combinations of multicyclic monomers, as they have yielded the highest Tg values to date. 6.4.3. Terpolymers. As discussed above, terpolymerizations of epoxides, anhydrides, and CO2 have garnered considerable interest, in part because many polycarbonates have fairly high Tg values. Therefore, terpolymers with Tg values higher than those of pure polyesters can be synthesized if the terpolymerization is random or if the polyester and polycarbonate blocks are miscible.80 For example, in 2010, Zhang and co-workers59 used a Zn−Co(III) double metal cyanide catalyst to terpolymerize maleic anhydride (2), CHO, and CO2. The resulting polymer was essentially random, although the polyester generally formed earlier than the polycarbonate. The Tg values of the resulting materials were quite high, ranging from 88 to 109 °C (Scheme 31). In 2014, Williams and co-workers24 used bimetallic zinc (cat66) and magnesium catalysts (cat65) to copolymerize phthalic anhydride (9) and CHO, which yielded a polyester with a Tg up to 83 °C. When 9 and CHO were terpolymerized with CO2, however, the Tg of the product increased to 97−104 °C (Scheme 32). Although the polymer had block architecture, the blocks were miscible and only one Tg was observed. These results suggest that terpolymerizations with CO2 are attractive avenues to materials with improved thermal properties for applications that do not require a pure polyester.

Scheme 29. Terpene-based High-Tg Polyester

noteworthy because the material has a Tg nearly 10 °C higher than that of commercially used polystyrene. Later in 2015, Theato and co-workers20 reported that the copolymerization of norbornene anhydride (16) with CHO yielded polymers with Tg values ranging from 111 to 130 °C depending on molecular weight and whether the diester linkages had cis or trans relative stereochemistry (see Scheme 21). However, although the Tg values of these polymers were some of the highest among aliphatic polyesters synthesized through epoxide/anhydride copolymerization, the molecular weights were less than 10 kDa, and the polymerizations were not well-controlled, with dispersity generally ranging from 1.5 to 3.0. Notably, the highest Tg reported by Theato and co-workers20 was for a sample with a dispersity of 1.2 and a high cis-diester content. Cheng and coworkers79 also copolymerized CHO and 16 and reported similar results in 2015. In 2016, Kleij and Coates and co-workers48 reported some of the highest Tg values to date for aliphatic polyesters, with values as high as 184 °C for polymers synthesized with partially or fully

Scheme 30. Copolymerization of Tricyclic Anhydrides with PO and CHO To Yield Exceptionally High-Tg Materialsa

a

PO = propylene oxide; CHO = cyclohexene oxide. Z


Chemical Reviews

Review

Scheme 31. Random Terpolymer Polyester-co-polycarbonate from Maleic Anhydride, Cyclohexene Oxide, and CO2

Scheme 33. Initial Report of Postpolymerization Functionalization through Thiol−ene Click Chemistry

Scheme 32. Polyester-b-Polycarbonate from Phthalic Anhydride, Cyclohexene Oxide, and CO2

thiol−ene reaction with azobis(isobutyronitrile) (AIBN) as the initiator (Scheme 33). However, the authors did not confirm whether the thiol−ene reaction proceeded to full conversion or whether narrow dispersity was preserved. This report was followed by a study in 2015 by Liu and coworkers,20 who reported the synthesis of polyesters containing exo- and endo-16 (Scheme 34). These polymers were initially Scheme 34. Functionalization with Mercaptoacetic Acid, 2Mercaptoethanol, and Cysteamine

7. POST-POLYMERIZATION MODIFICATION 7.1. Motivations for Post-Polymerization Modification

Aliphatic polyesters have garnered interest for use in biomedical applications because of their biodegradability and biocompatibility.81 However, these typically hydrophobic materials often need to be functionalized to improve their properties for these applications. Few options for polymer functionalization are available for aliphatic polyesters synthesized through the ROP of lactones. Functionalized lactones can introduce additional functionality into the polymer, but the synthesis of these monomers is typically cumbersome and the scope is often quite narrow.81−88 Post-polymerization modification of these polyesters is also limited for similar reasons, as incorporating functionalizable substituents into the monomers is challenging. The wide range of monomers available for epoxide/anhydride copolymerization makes it easy to incorporate functional monomers and groups that can readily undergo post-polymerization modification. A common challenge for post-polymerization modification for all aliphatic polyesters, regardless of polymerization method, is the fragility of the polyester backbone, which is prone to hydrolytic degradation by both acids and bases. Light- and radical-induced thiol−ene and tetrazine click chemistry have been used for post-polymerization modification of various polyesters, both aliphatic and semiaromatic, to modulate the properties of polymers and introduce new functionalities.

insoluble in ethanol, as is common for many polyesters, but became soluble after functionalization with mercaptoacetic acid via the thiol−ene reaction and clearly demonstrated the potential use of post-polymerization modification to modulate polymer properties. The thiol−ene reaction can be used to introduce a variety of functionalities: the authors also reported introduction of hydroxyl and amine groups through use of 2-mercaptoethanol and cysteamine, respectively (Scheme 34). In 2015, Cheng and co-workers79 reported the use of lightpromoted thiol−ene click chemistry and tetrazine click chemistry with copolymers of SO and 16 (Scheme 35). These reactions reached full conversion, and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) analysis confirmed that the polymer backbone remained intact. Thermoresponsive behavior and post-polymerization cross-linking via olefin metathesis of the

7.2. Functionalization via Thiol−Ene and Tetrazine Click Chemistry

In 2014, Liu and co-workers63 used a bifunctional (salen)CoNO3 complex (cat31) for terpolymerization of PO, norbornene anhydride (16), and CO2 (Scheme 33). Although the TOF values for the formation of polyester-b-polycarbonate were extremely high (576−1144 h−1), the materials showed low incorporation of ester, low molecular weights, and broad dispersity. Functionalization of the polyester block of the copolymer with 2-mercaptoethanol was reported via a facile AA


Chemical Reviews

Review

8. CONCLUSIONS AND OUTLOOK The alternating copolymerization of epoxides and cyclic anhydrides is a field that has expanded greatly during the past 10 years. Whereas early examples of these polymerizations used heterogeneous catalysts and the first discrete catalysts had low activity and yielded products with low molecular weights and limited monomer scopes, recent developments have led to an extraordinarily wide range of functional polyesters with an everwidening range of properties. For example, Tg can be tuned to almost any desired value from −44 to 184 °C by combining various epoxides and cyclic anhydrides. Additionally, terpolymerizations with CO2 and other monomers have allowed even greater control over polymer properties. Although it has been shown that one polyester formed from epoxide/anhydride copolymerization can form a stereocomplex that results in a material with improved thermal properties, it is likely that the wide array of monomers available will yield additional combinations with similar crystallization behavior. Unsaturated polyesters synthesized from epoxide/anhydride copolymerization also have a great deal of unexplored potential for post-polymerization modification to form materials suitable for a wide range of possible uses, including biomedical applications. Improvements in molecular weights through catalyst design or post-polymerization modification will be essential for creating commercially viable materials that are also potentially biodegradable and biocompatible. Additionally, stereoselective copolymerization of racemic starting materials will likely be a target of future efforts to form semicrystalline polyesters with improved thermal properties. Although further research in this area is necessary to create materials with properties comparable to today’s commercial polymers, it is vitally important to continue to develop biodegradable, biocompatible polymers from renewable sources to address the ever-growing problems of fossil fuel reliance and plastic waste. The firm foundation already established for epoxide/anhydride copolymerization makes these reactions crucial routes to such materials.

Scheme 35. Tetrazine and Thiol−ene Click Chemistry Modification

norbornene units were also investigated in this impressive study of post-polymerization functionalization. Whereas Liu and Kim and co-workers20 focused on modification through the anhydride-derived portion of their polymers, Liu and Li and co-workers65 exploited the epoxide moiety of the polymer for functionalization. They synthesized polyesters from phthalic anhydride (9) and 4-vinyl cyclohexene oxide and used mercaptoacetic acid and mercaptoethanol in the thiol−ene reaction to introduce carboxylic acid and ethanol functionality (Scheme 36). However, these thiol−ene reactions Scheme 36. Thiol−ene Functionalization of Semiaromatic Polyesters via the Epoxide Moiety

AUTHOR INFORMATION Corresponding Author

*E-mail [email protected]. Notes

The authors declare no competing financial interest. Biographies

reached only 86% and 65% conversion, respectively, likely due to the lower reactivity of the pendent alkene compared with the more reactive norbornene unit in the earlier examples by Liu and Kim and co-workers.20

Julie M. Longo was born in Atlanta, GA. She attended Emory University and received her B.S. in chemistry in 2011. She entered the Ph.D. program at Cornell University in 2011 and joined the lab of Geoff Coates, where she was a National Science Foundation graduate research fellow. Her research interests included developing biodegradable polymers from cyclic anhydrides and epoxides. She received her Ph.D. in August 2016.

7.3. Post-Polymerization Modification Outlook

Although examples of post-polymerization modification of polyesters are limited, they suggest that these polymers are well-suited to post-polymerization functionalization. By virtue of the use of two monomer sets, the possibilities for various combinations of functionalizable moieties are immense. Although thus far only the thiol−ene and tetrazine click reactions have been used for post-polymerization functionalization, other orthogonal functionalization reactions have potential as well. If coupled with well-defined block structures, epoxide/anhydride copolymerizations could open the door to a wide array of diverse, functionalized polyesters for a variety of applications.

Maria J. Sanford was born in 1991 and raised in Kennett Square, PA. She received her B.S. degrees in chemistry and pharmaceutical product development from West Chester University of Pennsylvania in 2013. She is currently a Ph.D. student in the lab of Geoff Coates, where her research interests include catalyst development for epoxide/anhydride copolymerization. Geoffrey W. Coates was born in 1966 in Evansville, IN. He received his B.A. in chemistry from Wabash College in 1989, working with Roy G. Miller, and Ph.D. in organic chemistry from Stanford University in 1994 AB


Chemical Reviews

Review

(15) Datta, R.; Henry, M. Lactic Acid: Recent Advances in Products, Processes and Technologies  a Review. J. Chem. Technol. Biotechnol. 2006, 81, 1119−1129. (16) Auras, R.; Harte, B.; Selke, S. An Overview of Polylactides as Packaging Materials. Macromol. Biosci. 2004, 4, 835−864. (17) Vink, E. T. H.; Rábago, K. R.; Glassner, D. A.; Gruber, P. R. Applications of Life Cycle Assessment to Natureworks Polylactide (PLA) Production. Polym. Degrad. Stab. 2003, 80, 403−419. (18) Ikada, Y.; Tsuji, H. Biodegradable Polyesters for Medical and Ecological Applications. Macromol. Rapid Commun. 2000, 21, 117−132. (19) Paul, S.; Zhu, Y.; Romain, C.; Brooks, R.; Saini, P. K.; Williams, C. K. Ring-Opening Copolymerization (ROCOP): Synthesis and Properties of Polyesters and Polycarbonates. Chem. Commun. 2015, 51, 6459− 6479. (20) Han, B.; Zhang, L.; Liu, B.; Dong, X.; Kim, I.; Duan, Z.; Theato, P. Controllable Synthesis of Stereoregular Polyesters by Organocatalytic Alternating Copolymerizations of Cyclohexene Oxide and Norbornene Anhydrides. Macromolecules 2015, 48, 3431−3437. (21) Zhu, L.; Liu, D.; Wu, L.; Feng, W.; Zhang, X.; Wu, J.; Fan, D.; Lü, X.; Lu, R.; Shi, Q. A Trinuclear [Zn3(L)2(OAc)2] Complex Based on the Asymmetrical Bis-Schiff-Base Ligand H2L for Ring-Opening Copolymerization of CHO and MA. Inorg. Chem. Commun. 2013, 37, 182−185. (22) Liu, D.-F.; Wu, L.-Y.; Feng, W.-X.; Zhang, X.-M.; Wu, J.; Zhu, L.Q.; Fan, D.-D.; Lü, X.-Q.; Shi, Q. Ring-Opening Copolymerization of CHO and MA Catalyzed by Mononuclear [Zn(L2)(H2O)] or Trinuclear [Zn3(L2)2(OAc)2] Complex Based on the Asymmetrical Bis-Schiff-Base Ligand Precursor. J. Mol. Catal. A: Chem. 2014, 382, 136−145. (23) Liu, Y.; Xiao, M.; Wang, S.; Xia, L.; Hang, D.; Cui, G.; Meng, Y. Mechanism Studies of Terpolymerization of Phthalic Anhydride, Propylene Epoxide, and Carbon Dioxide Catalyzed by ZnGA. RSC Adv. 2014, 4, 9503−9508. (24) Saini, P. K.; Romain, C.; Zhu, Y.; Williams, C. K. Di-Magnesium and Zinc Catalysts for the Copolymerization of Phthalic Anhydride and Cyclohexene Oxide. Polym. Chem. 2014, 5, 6068−6075. (25) Wu, L.-y.; Fan, D.-d.; Lü , X.-q.; Lu, R. Ring-Opening Copolymerization of Cyclohexene Oxide and Maleic Anhydride Catalyzed by Mononuclear [Zn(L)(H2O)] or Binuclear [Zn2(L) (OAc)2(H2O)] Complex Based on the Salen-Type Schiff-Base Ligand. Chin. J. Polym. Sci. 2014, 32, 768−777. (26) Winkler, M.; Romain, C.; Meier, M. A. R.; Williams, C. K. Renewable Polycarbonates and Polyesters from 1,4-Cyclohexadiene. Green Chem. 2015, 17, 300−306. (27) Zhu, Y.; Romain, C.; Williams, C. K. Selective Polymerization Catalysis: Controlling the Metal Chain End Group to Prepare Block Copolyesters. J. Am. Chem. Soc. 2015, 137, 12179−12182. (28) Thevenon, A.; Garden, J. A.; White, A. J. P.; Williams, C. K. Dinuclear Zinc Salen Catalysts for the Ring Opening Copolymerization of Epoxides and Carbon Dioxide or Anhydrides. Inorg. Chem. 2015, 54, 11906−11915. (29) Garden, J. A.; Saini, P. K.; Williams, C. K. Greater Than the Sum of Its Parts: A Heterodinuclear Polymerization Catalyst. J. Am. Chem. Soc. 2015, 137, 15078−15081. (30) Yu, C.-Y.; Chuang, H.-J.; Ko, B.-T. Bimetallic Bis(Benzotriazole Iminophenolate) Cobalt, Nickel and Zinc Complexes as Versatile Catalysts for Coupling of Carbon Dioxide with Epoxides and Copolymerization of Phthalic Anhydride with Cyclohexene Oxide. Catal. Sci. Technol. 2016, 6, 1779−1791. (31) Takasu, A.; Ito, M.; Inai, Y.; Hirabayashi, T.; Nishimura, Y. Syntheis of Biodegradable Polyesters by Ring-Opening Copolymerization of Cyclic Anhydrides Containing a Double Bond with 1,2Epoxybutane and One-Pot Preparation of the Itaconic Acid-Based Polymeric Network. Polym. J. 1999, 31, 961−969. (32) Huijser, S.; HosseiniNejad, E.; Sablong, R.; de Jong, C.; Koning, C. E.; Duchateau, R. Ring-Opening Co- and Terpolymerization of an Alicyclic Oxirane with Carboxylic Acid Anhydrides and CO2 in the Presence of Chromium Porphyrinato and Salen Catalysts. Macromolecules 2011, 44, 1132−1139.

with Robert M. Waymouth. After being an NSF postdoctoral fellow with Robert Grubbs at the California Institute of Technology, he joined the faculty at Cornell University in 1997 and was appointed to the first Tisch University Professorship in 2008. Research in the Coates group focuses on development of single-site catalysts for small molecule and polymer synthesis as well as preparation and characterization of well-defined polymer architectures.

ACKNOWLEDGMENTS We thank the NSF for generous financial support of our work in the field of epoxide/anhydride copolymerization through the Center for Sustainable Polymers, an NSF Center for Chemical Innovation (CHE-1413862). J.M.L. thanks the NSF for a graduate research fellowship. We gratefully acknowledge Kyle O’Connor, Anne LaPointe, and Nancy Jacoby for editorial assistance. REFERENCES (1) Odian, G. Principles of Polymerization, 4th ed.; John Wiley & Sons: Hoboken, NJ, 2004. (2) Tokita, M.; Watanabe, J. Several Interesting Fields Exploited through Understanding of Polymeric Effects on Liquid Crystals of MainChain Polyesters. Polym. J. 2006, 38, 611−638. (3) Tylkowski, B.; Bogdanowicz, K. A.; Ambrogi, V.; Lederer, A.; Patroniak, V.; Giamberini, M. Synthesis and Characterization of a New Family of Photoactive Liquid Crystalline Polyesters Based on αMethylstilbene. Polym. Int. 2014, 63, 315−326. (4) Hillmyer, M. A.; Tolman, W. B. Aliphatic Polyester Block Polymers: Renewable, Degradable, and Sustainable. Acc. Chem. Res. 2014, 47, 2390−2396. (5) Ragauskas, A. J.; Williams, C. K.; Davison, B. H.; Britovsek, G.; Cairney, J.; Eckert, C. A.; Frederick, W. J., Jr.; Hallett, J. P.; Leak, D. J.; Liotta, C. L.; et al. The Path Forward for Biofuels and Biomaterials. Science 2006, 311, 484−489. (6) Vert, M. Aliphatic Polyesters: Great Degradable Polymers That Cannot Do Everything. Biomacromolecules 2005, 6, 538−546. (7) Gross, R. A.; Kalra, B. Biodegradable Polymers for the Environment. Science 2002, 297, 803−807. (8) Amass, W.; Amass, A.; Tighe, B. A Review of Biodegradable Polymers: Uses, Current Developments in the Synthesis and Characterization of Biodegradable Polyesters, Blends of Biodegradable Polymers and Recent Advances in Biodegradation Studies. Polym. Int. 1998, 47, 89−144. (9) Olson, D. A.; Gratton, S. E. A.; DeSimone, J. M.; Sheares, V. V. Amorphous Linear Aliphatic Polyesters for the Facile Preparation of Tunable Rapidly Degrading Elastomeric Devices and Delivery Vectors. J. Am. Chem. Soc. 2006, 128, 13625−13633. (10) Müller, R.-J.; Kleeberg, I.; Deckwer, W.-D. Biodegradation of Polyesters Containing Aromatic Constituents. J. Biotechnol. 2001, 86, 87−95. (11) Brown, A. H.; Sheares, V. V. Amorphous Unsaturated Aliphatic Polyesters Derived from Dicarboxylic Monomers Synthesized by Diels− Alder Chemistry. Macromolecules 2007, 40, 4848−4853. (12) Lecomte, P.; Jérôme, C. Recent Developments in Ring-Opening Polymerization of Lactones. In Synthetic Biodegradable Polymers; Rieger, B., Künkel, A., Coates, G. W., Reichardt, R., Dinjus, E., Zevaco, A. T., Eds.; Advances in Polymer Science, Vol. 245; Springer-Verlag: Berlin and Heidelberg, Germany, 2012; pp 173−217; DOI: 10.1007/978-3642-27154-0. (13) Engels, H.-W.; Pirkl, H.-G.; Albers, R.; Albach, R. W.; Krause, J.; Hoffmann, A.; Casselmann, H.; Dormish, J. Polyurethanes: Versatile Materials and Sustainable Problem Solvers for Today’s Challenges. Angew. Chem., Int. Ed. 2013, 52, 9422−9441. (14) Gupta, A. P.; Kumar, V. New Emerging Trends in Synthetic Biodegradable Polymers − Polylactide: A Critique. Eur. Polym. J. 2007, 43, 4053−4074. AC


Chemical Reviews

Review

(33) Robert, C.; de Montigny, F.; Thomas, C. M. Tandem Synthesis of Alternating Polyesters from Renewable Resources. Nat. Commun. 2011, 2, 586. (34) Darensbourg, D. J.; Poland, R. R.; Escobedo, C. Kinetic Studies of the Alternating Copolymerization of Cyclic Acid Anhydrides and Epoxides, and the Terpolymerization of Cyclic Acid Anhydrides, Epoxides, and CO2 Catalyzed by (Salen)CrIIICl. Macromolecules 2012, 45, 2242−2248. (35) Hosseini Nejad, E.; Paoniasari, A.; Koning, C. E.; Duchateau, R. Semi-Aromatic Polyesters by Alternating Ring-Opening Copolymerisation of Styrene Oxide and Anhydrides. Polym. Chem. 2012, 3, 1308− 1313. (36) Hosseini Nejad, E.; van Melis, C. G. W.; Vermeer, T. J.; Koning, C. E.; Duchateau, R. Alternating Ring-Opening Polymerization of Cyclohexene Oxide and Anhydrides: Effect of Catalyst, Cocatalyst, and Anhydride Structure. Macromolecules 2012, 45, 1770−1776. (37) Bernard, A.; Chatterjee, C.; Chisholm, M. H. The Influence of the Metal (Al, Cr and Co) and the Substituents of the Porphyrin in Controlling the Reactions Involved in the Copolymerization of Propylene Oxide and Cyclic Anhydrides by Porphyrin Metal(III) Complexes. Polymer 2013, 54, 2639−2646. (38) Harrold, N. D.; Li, Y.; Chisholm, M. H. Studies of Ring-Opening Reactions of Styrene Oxide by Chromium Tetraphenylporphyrin Initiators. Mechanistic and Stereochemical Considerations. Macromolecules 2013, 46, 692−698. (39) Liu, J.; Bao, Y.-Y.; Liu, Y.; Ren, W.-M.; Lu, X.-B. Binuclear Chromium-Salan Complex Catalyzed Alternating Copolymerization of Epoxides and Cyclic Anhydrides. Polym. Chem. 2013, 4, 1439−1444. (40) Nejad, E. H.; Paoniasari, A.; van Melis, C. G. W.; Koning, C. E.; Duchateau, R. Catalytic Ring-Opening Copolymerization of Limonene Oxide and Phthalic Anhydride: Toward Partially Renewable Polyesters. Macromolecules 2013, 46, 631−637. (41) Biermann, U.; Sehlinger, A.; Meier, M. A. R.; Metzger, J. O. Catalytic Copolymerization of Methyl 9,10-Epoxystearate and Cyclic Anhydrides under Neat Conditions. Eur. J. Lipid Sci. Technol. 2016, 118, 104−110. (42) DiCiccio, A. M.; Coates, G. W. Ring-Opening Copolymerization of Maleic Anhydride with Epoxides: A Chain-Growth Approach to Unsaturated Polyesters. J. Am. Chem. Soc. 2011, 133, 10724−10727. (43) Longo, J. M.; DiCiccio, A. M.; Coates, G. W. Poly(propylene succinate): A New Polymer Stereocomplex. J. Am. Chem. Soc. 2014, 136, 15897−15900. (44) Robert, C.; Ohkawara, T.; Nozaki, K. Manganese-Corrole Complexes as Versatile Catalysts for the Ring-Opening Homo- and Co-Polymerization of Epoxide. Chem. - Eur. J. 2014, 20, 4789−4795. (45) Liu, D.-F.; Zhu, L.-Q.; Wu, J.; Wu, L.-Y.; Lu, X.-Q. Ring-Opening Copolymerization of Epoxides and Anhydrides Using Manganese(III) Asymmetrical Schiff Base Complexes as Catalysts. RSC Adv. 2015, 5, 3854−3859. (46) Liu, D.; Zhang, Z.; Zhang, X.; Lü, X. Alternating Ring-Opening Copolymerization of Cyclohexene Oxide and Maleic Anhydride with Diallyl-Modified Manganese(III)−Salen Catalysts. Aust. J. Chem. 2015, 69, 47−55. (47) Mundil, R.; Hošt’álek, Z.; Šeděnková, I.; Merna, J. Alternating Ring-Opening Copolymerization of Cyclohexene Oxide with Phthalic Anhydride Catalyzed by Iron(III) Salen Complexes. Macromol. Res. 2015, 23, 161−166. (48) Sanford, M. J.; Peña Carrodeguas, L.; Van Zee, N. J.; Kleij, A. W.; Coates, G. W. Alternating Copolymerization of Propylene Oxide and Cyclohexene Oxide with Tricyclic Anhydrides: Access to Partially Renewable Aliphatic Polyesters with High Glass Transition Temperatures. Macromolecules 2016, 49, 6394−6400. (49) Aida, T.; Inoue, S. Catalytic Reaction on Both Sides of a Metalloporphyrin Plane. Alternating Copolymerization of Phthalic Anhydride and Epoxypropane with an Aluminum Porphyrin-Quaternary Salt System. J. Am. Chem. Soc. 1985, 107, 1358−1364. (50) Aida, T.; Sanuki, K.; Inoue, S. Well-Controlled Polymerization by Metalloporphyrin. Synthesis of Copolymer with Alternating Sequence and Regulated Molecular Weight from Cyclic Acid Anhydride and

Epoxide Catalyzed by the System of Aluminum Porphyrin Coupled with Quaternary Organic Salt. Macromolecules 1985, 18, 1049−1055. (51) Van Zee, N. J.; Coates, G. W. Alternating Copolymerization of Propylene Oxide with Biorenewable Terpene-Based Cyclic Anhydrides: A Sustainable Route to Aliphatic Polyesters with High Glass Transition Temperatures. Angew. Chem., Int. Ed. 2015, 54, 2665−2668. (52) Van Zee, N. J.; Sanford, M. J.; Coates, G. W. Electronic Effects of Aluminum Complexes in the Copolymerization of Propylene Oxide with Tricyclic Anhydrides: Access to Well-Defined, Functionalizable Aliphatic Polyesters. J. Am. Chem. Soc. 2016, 138, 2755−2761. (53) Takasu, A.; Bando, T.; Morimoto, Y.; Shibata, Y.; Hirabayashi, T. Asymmetric Epoxidation of α-Olefins Having Neighboring Sugar Chiral Templates and Alternating Copolymerization with Dicarboxylic Anhydrides. Biomacromolecules 2005, 6, 1707−1712. (54) Luston, J.; Manasek, Z. Copolymerization of Epoxides with Cyclic Anhydrides Catalyzed by Tertiary Amines in the Presence of ProtonDonating Compounds. J. Macromol. Sci., Chem. 1979, 13, 853−867. (55) Luston, J.; Vašs,̌ F. Anionic Copolymerization of Cyclic Ethers with Cyclic Anhydrides. Adv. Polym. Sci. 1984, 56, 91−133. (56) Masters, J. E. Epoxide-Anhydride-Hydroxy Polymer Compositions and Method of Making Same. U.S. Patent US 2908663, October 13, 1959. (57) Nagata, N.; Taniguchi, S. Copolymers of epoxy resins, unsaturated carboxylic acids or anhydrides, and other copolymerizable monomers for use as coatings and sizing agents. Patent GB 1081295 19670831, 1967. (58) Sakai, S.; Ito, H.; Ishii, Y. Reactivity and Polymerizability of Cyclic Compounds. IX. Copolymerization of Epoxides with Succinic Anhydride Catalyzed by Organotin Compounds. Kogyo Kagaku Zasshi 1968, 71, 186−187. (59) Sun, X.-K.; Zhang, X.-H.; Chen, S.; Du, B.-Y.; Wang, Q.; Fan, Z.Q.; Qi, G.-R. One-Pot Terpolymerization of CO2, Cyclohexene Oxide and Maleic Anhydride Using a Highly Active Heterogeneous Double Metal Cyanide Complex Catalyst. Polymer 2010, 51, 5719−5725. (60) Maeda, Y.; Nakayama, A.; Kawasaki, N.; Hayashi, K.; Aiba, S.; Yamamoto, N. Ring-Opening Copolymerization of Succinic Anhydride with Ethylene Oxide Initiated by Magnesium Diethoxide. Polymer 1997, 38, 4719−4725. (61) Jeske, R. C.; Rowley, J. M.; Coates, G. W. Pre-Rate-Determining Selectivity in the Terpolymerization of Epoxides, Cyclic Anhydrides, and CO2: A One-Step Route to Diblock Copolymers. Angew. Chem., Int. Ed. 2008, 47, 6041−6044. (62) Jeske, R. C.; DiCiccio, A. M.; Coates, G. W. Alternating Copolymerization of Epoxides and Cyclic Anhydrides: An Improved Route to Aliphatic Polyesters. J. Am. Chem. Soc. 2007, 129, 11330− 11331. (63) Duan, Z.; Wang, X.; Gao, Q.; Zhang, L.; Liu, B.; Kim, I. Highly Active Bifunctional Cobalt-Salen Complexes for the Synthesis of Poly(Ester-Block-Carbonate) Copolymer Via Terpolymerization of Carbon Dioxide, Propylene Oxide, and Norbornene Anhydride Isomer: Roles of Anhydride Conformation Consideration. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 789−795. (64) Jeon, J. Y.; Eo, S. C.; Varghese, J. K.; Lee, B. Y. Copolymerization and Terpolymerization of Carbon Dioxide/Propylene Oxide/Phthalic Anhydride Using a (Salen)Co(III) Complex Tethering Four Quaternary Ammonium Salts. Beilstein J. Org. Chem. 2014, 10, 1787−1795. (65) Si, G.; Zhang, L.; Han, B.; Duan, Z.; Li, B.; Dong, J.; Li, X.; Liu, B. Novel Chromium Complexes with a [OSSO]-Type Bis(Phenolato) Dianionic Ligand Mediate the Alternating Ring-Opening Copolymerization of Epoxides and Phthalic Anhydride. Polym. Chem. 2015, 6, 6372−6377. (66) Romain, C.; Zhu, Y.; Dingwall, P.; Paul, S.; Rzepa, H. S.; Buchard, A.; Williams, C. K. Chemoselective Polymerizations from Mixtures of Epoxide, Lactone, Anhydride, and Carbon Dioxide. J. Am. Chem. Soc. 2016, 138, 4120−4131. (67) Gandini, A. The Irruption of Polymers from Renewable Resources on the Scene of Macromolecular Science and Technology. Green Chem. 2011, 13, 1061−1083. AD


Chemical Reviews

Review

(68) Mecking, S. Nature or Petrochemistry?Biologically Degradable Materials. Angew. Chem., Int. Ed. 2004, 43, 1078−1085. (69) Mahmoud, E.; Watson, D. A.; Lobo, R. F. Renewable Production of Phthalic Anhydride from Biomass-Derived Furan and Maleic Anhydride. Green Chem. 2014, 16, 167−175. (70) Swift, K. A. D. Catalytic Transformations of the Major Terpene Feedstocks. Top. Catal. 2004, 27, 143−155. (71) Behr, A.; Johnen, L. Myrcene as a Natural Base Chemical in Sustainable Chemistry: A Critical Review. ChemSusChem 2009, 2, 1072−1095. (72) Wilbon, P. A.; Chu, F.; Tang, C. Progress in Renewable Polymers from Natural Terpenes, Terpenoids, and Rosin. Macromol. Rapid Commun. 2013, 34, 8−37. (73) Dakshinamoorthy, D.; Weinstock, A. K.; Damodaran, K.; Iwig, D. F.; Mathers, R. T. Diglycerol-Based Polyesters: Melt Polymerization with Hydrophobic Anhydrides. ChemSusChem 2014, 7, 2923−2929. (74) Top Value Added Chemicals from Biomass, U.S. Department of Energy, 2004; http://www.nrel.gov/docs/fy04osti/35523.pdf; Accessed July 3, 2016. (75) The Merck Index; 14th ed.; Merck: Whitehouse Station, NJ, 2006. (76) Hajian, H.; Yusoff, W. M. W. Itaconic Acid Production by Microorganisms: A Review. Curr. Res. J. Biol. Sci. 2015, 7, 37−42. (77) DiCiccio, A. M.; Longo, J. M.; Rodriguez-Calero, G. G.; Coates, G. W. Development of Highly Active and Regioselective Catalysts for the Copolymerization of Epoxides with Cyclic Anhydrides: An Unanticipated Effect of Electronic Variation. J. Am. Chem. Soc. 2016, 138, 7107−7113. (78) Childers, M. I.; Longo, J. M.; Van Zee, N. J.; LaPointe, A. M.; Coates, G. W. Stereoselective Epoxide Polymerization and Copolymerization. Chem. Rev. 2014, 114, 8129−8152. (79) Baumgartner, R.; Song, Z.; Zhang, Y.; Cheng, J. Functional Polyesters Derived from Alternating Copolymerization of Norbornene Anhydride and Epoxides. Polym. Chem. 2015, 6, 3586−3590. (80) Kember, M. R.; Buchard, A.; Williams, C. K. Catalysts for CO2/ Epoxide Copolymerisation. Chem. Commun. 2011, 47, 141−163. (81) Williams, C. K. Synthesis of Functionalized Biodegradable Polyesters. Chem. Soc. Rev. 2007, 36, 1573−1580. (82) du Boullay, O. T.; Saffon, N.; Diehl, J.-P.; Martin-Vaca, B.; Bourissou, D. Organo-Catalyzed Ring Opening Polymerization of a 1,4Dioxane-2,5-Dione Deriving from Glutamic Acid. Biomacromolecules 2010, 11, 1921−1929. (83) Jing, F.; Hillmyer, M. A. A Bifunctional Monomer Derived from Lactide for Toughening Polylactide. J. Am. Chem. Soc. 2008, 130, 13826−13827. (84) Trimaille, T.; Möller, M.; Gurny, R. Synthesis and Ring-Opening Polymerization of New Monoalkyl-Substituted Lactides. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 4379−4391. (85) Wright, C.; Banerjee, A.; Yan, X.; Storms-Miller, W. K.; Pugh, C. Synthesis of Functionalized Poly(Lactic Acid) Using 2-Bromo-3Hydroxypropionic Acid. Macromolecules 2016, 49, 2028−2038. (86) Pounder, R. J.; Dove, A. P. Towards Poly(Ester) Nanoparticles: Recent Advances in the Synthesis of Functional Poly(Ester)s by RingOpening Polymerization. Polym. Chem. 2010, 1, 260−271. (87) Fiore, G. L.; Jing, F.; Young, V. G., Jr.; Cramer, C. J.; Hillmyer, M. A. High Tg Aliphatic Polyesters by the Polymerization of Spirolactide Derivatives. Polym. Chem. 2010, 1, 870−877. (88) Dubois, P.; Jerome, R.; Teyssie, P. Macromolecular Engineering of Polylactones and Polylactides. 3. Synthesis, Characterization, and Applications of Poly(iε-Caprolactone) Macromonomers. Macromolecules 1991, 24, 977−981.

AE


Ring-Opening Copolymerization of Epoxides and Cyclic Anhydrides

Recommend Documents