Isospecific, Chain Shuttling Polymerization of Propylene Oxide Using

Article pubs.acs.org/JACS

Isospecific, Chain Shuttling Polymerization of Propylene Oxide Using a Bimetallic Chromium Catalyst: A New Route to Semicrystalline Polyols M. Ian Childers,† Andrew K. Vitek,‡ Lilliana S. Morris,† Peter C. B. Widger,† Syud M. Ahmed,† Paul M. Zimmerman,*,‡ and Geoffrey W. Coates*,† †

Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301, United States Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109-1055, United States

‡

S Supporting Information *

ABSTRACT: Hydroxy-telechelic poly(propylene oxide) (PPO) is widely used industrially as a midsegment in polyurethane synthesis. These atactic polymers are produced from racemic propylene oxide using chain shuttling agents and double-metal cyanide catalysts. Unlike atactic PPO, isotactic PPO is semicrystalline with a melting temperature of approximately 67 °C. Currently there is no practical route to hydroxy-telechelic isotactic PPO using racemic propylene oxide as the monomer. In this paper, hydroxy-telechelic isotactic PPO is synthesized from racemic propylene oxide with control of molecular weight using enantioselective and isoselective bimetallic catalysts in conjunction with chain shuttling agents. The discovery of an easily accessible bimetallic chromium catalyst is reported for this transformation. Diol, triol, and polymeric chain shuttling agents are used to give hydroxy-telechelic isotactic PPO of varying functionality and structure. Detailed quantum chemical studies are used to reveal the polymerization mechanism and origin of stereoselectivity.

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INTRODUCTION Polymers with reactive end-groups are useful materials as they can be readily integrated into more complicated macromolecular assemblies in a well-defined manner.1 For example, low molecular weight polymers with multiple terminal hydroxyl groups (hydroxy-telechelic polymers, also known as “polyols”) react with multifunctional isocyanates to form polyurethanes, a class of polymers with an estimated production of 14 million tons in 2011 (∼5% of worldwide polymer production).2 These materials are used in a wide variety of products including soft and rigid foams, adhesives, and elastomers. Atactic poly(propylene oxide) (aPPO) polyols are commonly employed due to their low cost and the desirable properties they impart on the final polyurethane. The functionality (i.e., the number of hydroxyl groups) of the polyol affects the structure and thus properties of the resulting polyurethane product. For example, diols produce linear polyurethanes when reacted with diisocyanates, while polyols with higher functionality produce cross-linked polyurethanes. In addition to functionality and molecular weight, the tacticity of PPO impacts its properties. Atactic PPO is amorphous (Tg = −70 °C), whereas isotactic PPO (iPPO) is a semicrystalline material (Tm = 67 °C). Although aPPO is typically used in polyurethanes, mixtures of iPPO and aPPO are said to be useful polyurethane midsegments for flexible foams © 2017 American Chemical Society

and elastomers, while pure iPPO imparts improved properties to rigid polyurethane foams and films.3 An early study reported that a 3000 Da 40% iPPO diol produced a polyurethane with higher peel strength and rigidity than its atactic counterpart.4 However, while iPPO imparts improved properties, it has proven difficult to synthesize and to the best of our knowledge, only a few reports of hydroxy-telechelic iPPO have been disclosed. Historically, iPPO diols were produced by the chainscission of high molecular weight iPPO produced by partially isoselective heterogeneous catalysts.3a−d Other groups have used isoselective catalysts to produce telechelic iPPO from racemic epoxides. Schäfer et al. used a bimetallic μ-oxoalkoxide zinc−aluminum catalyst to produce partially crystalline PPO with an mm-triad content of approximately 64% and hydroxy end-groups.5 A tin phosphate condensate catalyst has been reported for the production of telechelic iPPO, but the resulting polymers have isotacticity levels below 60% mm-triad content and broad molecular weight distributions (2.4−4.6). Furthermore, preparation of the catalyst requires fractionation, and isolation of the isotactic polymer product necessitates removal of atactic polymer and other side products.6 Tokunaga et al. reported that semicrystalline diol iPPOs can be formed by Received: January 6, 2017 Published: July 3, 2017 11048

DOI: 10.1021/jacs.7b00194 J. Am. Chem. Soc. 2017, 139, 11048−11054

Article

Journal of the American Chemical Society polymerizing PO with a cobalt(III) complex and related compounds in the presence of acetic acid and then hydrolyzing the resulting end-groups with KOH.7 The molecular weights of these polymers are controlled by the catalyst/acetic acid ratio, but it is not clear if the acetic acid controls the molecular weight by acting as a chain shuttling agent (CSA) or through other means, since the ratio of PO to acetic acid (∼6) is too low for the degree of polymerization observed (∼45) if all the acetic acid functions as a CSA. The solid-state nature of (salph)Co(III) complexes8 and the stereochemical challenges of chain shuttling are also not addressed. Because of the potential improvement in polymer properties for polyurethanes made with iPPO polyols, our group has been working toward their synthesis (Scheme 1).

Scheme 2. Kinetic Resolution of PO by (S)-1 for the Synthesis of iPPO

solely iPPO chains, including (salph)Co(III) complexes8,15 and homogeneous racemic bimetallic (salen)Co(III) (rac-1) complexes.12,16 In theory it is possible to add an alcohol CSA to any of these routes to produce hydroxy-telechelic iPPO, though the stereochemical possibilities are more complex for the isoselective catalyst route (vide infra). To better understand how these bimetallic catalysts previously synthesized in our group could be used to create hydroxy-telechelic iPPO, the proposed mechanisms of several of the steps must be discussed (Scheme 3). In the initiation

Scheme 1. Commercial Production of Atactic HydroxyTelechelic PPO and Our Goal of Isotactic HydroxyTelechelic PPO

Scheme 3. Proposed Basic Mechanistic Steps for Polymerization of PO by Bimetallic Catalyst

Industrially, chain shuttling polymerizations of PO using alkali hydroxide and double-metal cyanide (DMC) catalysts are commonly used for the synthesis of aPPO. Use of DMC catalysts in conjunction with protic CSAs give polyols with the same functionality as the parent CSA. Inoue and co-workers were among the first to use CSAs with discrete catalysts, finding that adding alcohols to the polymerization of PO with (tetraphenylporphinato)aluminum chloride catalysts led to lower molecular weight polymers with hydroxy end-groups, which they termed an “immortal polymerization.”9 While the synthesis of aPPO is relatively straightforward, the chirality of PO complicates the production of iPPO and has severely limited investigations into uses of this polymer. There are three potential routes to iPPO and hydroxy-telechelic iPPO. First, enantiopure PO can be polymerized in a regioregular fashion.10 Recently, Hawker and co-workers used a similar approach to synthesize iPPO-b-poly(ethylene oxide)-b-iPPO triblock copolymers using enantiopure epoxides.11 Unfortunately, the high cost of enantiopure PO renders this approach uneconomical. Second, racemic PO can be polymerized by a chiral, enantioselective catalyst to give enantiopure iPPO, leaving unreacted, enantiopure PO of the opposite stereoconfiguration. While our group has prepared enantiopure bimetallic (salen)Co(III) catalysts ((S)-1) (Scheme 2) for this transformation,12 there are few other enantioselective catalysts, and those that exist suffer from poor enantioselectivities.13 While this approach allows for the use of racemic monomer, efficient recycling of the epoxide product must be addressed. Finally, an isoselective catalyst can be used to form iPPO from both enantiomers of the epoxide. This route is especially appealing for the large-scale production of iPPO because all of the PO can be polymerized in one step and the stereoregularity of the polymer is maintained at high conversions. Numerous heterogeneous catalysts are known to produce a mixture of iPPO and aPPO chains,14 but only a few catalysts produce

step, a molecule of PO binds to an open coordination site in the catalytic cleft. Once bound, a chloride attacks to open the epoxide, yielding a bound alkoxy group. To propagate, another molecule of PO binds to an open coordination site, where it is opened by the alkoxy group bound to the adjacent metal center. This process continues to produce a polymer chain. In the presence of alcohols, chain transfer can also occur. If a dormant chain comes into the catalytic cleft, alcoholysis transfers the proton from the dormant chain to the active chain. This proton transfer makes the previously active chain dormant and the dormant chain active. These alcohols act as CSAs and allow for the production of multiple polymer chains per catalyst center and control of polymer molecular weight by variation of the monomer to CSA ratio.17 Seeking to address the limits of telechelic iPPO synthesis, this paper describes two direct routes to semicrystalline hydroxy-telechelic iPPO from rac-PO using alcohol CSAs with enantioselective and isoselective homogeneous catalysts.

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EXPERIMENTAL DETAILS

For information regarding experimental procedures, materials, and computational details, see the Supporting Information. 11049


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Journal of the American Chemical Society Table 1. Polymerization of rac-PO Using (S)-1 with 1,6-Hexanediol as a CSAa

entry 1 2 3 4 5

PO/CSA h

n.a. 400 200 100 50

conv (%)b 50 52 54 50 46

Mn theo. (kDa)c

Mn NMR (kDa)d

120 12.4 6.4 3.0 1.4

h

n.a. 16.0 8.3 4.2 2.2

Mn GPC (kDa)e

Đe

[mm] (%)f

Tm (°C)g

230 21.0 12.5 5.4 2.5

2.6 2.5 2.3 2.8 2.9

96.4 94.4 92.1 94.5 96.4

66 67 66 65 61

Polymerization conditions: 1 mL of PO, [PO] = 2.0 M in toluene; [PO]:[1]:[(PPN)(OPiv)] = 4000:1:2; Trxn = 22 °C, trxn = 1 h. bDetermined gravimetrically. cCalculated using one polymer chain per (S)-1 and CSA (Supplementary eq 1). dDetermined by 1H NMR spectroscopy (Supplementary eq 2). eDetermined by GPC calibrated with polystyrene standards at 30 °C in THF. fDetermined by 13C NMR spectroscopy (Supplementary eq 6). gDetermined by DSC. Reported Tm values are from the second heat. hn.a. = not applicable. a

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RESULTS AND DISCUSSION Enantioselective Synthesis of Telechelic iPPO via a Kinetic Resolution Polymerization of rac-PO. We initially sought to produce hydroxy-telechelic iPPO using our enantioselective enantiopure bimetallic cobalt catalyst (S)-1 and the ionic cocatalyst bis(triphenylphosphine)iminium ([PPN + ] = [Ph 3 P−NPPh 3 + ]) pivalate ([OPiv − ] = [tBuCO2−]). While 1 exhibits unprecedented enantioselectivity and activity for epoxide polymerization in the absence of CSAs, it produces polymers with multiple types of functionalities (chloride, carboxylate, and hydroxy) given the reactive species present in the catalyst and cocatalyst, and it is not possible to control the molecular weight of the polymer by varying the epoxide to catalyst ratio.12,16 The lack of molecular weight control is attributed to slow or incomplete initiation and rapid propagation. Based on reports using (salen)Co(III) catalysts with alcohols as CSAs for epoxide/comonomer copolymerizations,18 we hypothesized that adding 1,6-hexanediol (HD) as a CSA would control the molecular weight of the polymer chains without lowering the enantioselectivity of the catalyst. As seen in Table 1, all samples produced in this polymerization were highly isotactic and semicrystalline (Tm = 61−67 °C). Analysis of polymer tacticity by 13C NMR spectroscopy showed a ratio of stereoerrors [mr] = [rm] ≈ [rr] (Supplementary Table 1), which is consistent with an enantiomorphic site control mechanism.12 Terminal methine end-groups were observed by 1H NMR (δ = 3.91 ppm) and 13C NMR (δ = 65.60 ppm) spectroscopies and integrated cleanly with respect to the internal methylenes of the HD midsections, showing that diol CSAs can be used with (S)-1 to enantioselectively synthesize iPPO diols from racemic PO. Isoselective Synthesis of Telechelic iPPO Using a Chiral, Racemic Catalyst. While catalyst (S)-1 allowed for the formation of iPPO diols, using an enantiopure chiral catalyst limits the maximum yield of polymer to 50%. Using a chiral, racemic catalyst would increase the theoretical yield to 100%. In the absence of CSAs, racemic catalyst and racemic monomer yield racemic, isotactic polymer (Scheme 4). Using a chiral, racemic catalyst with CSAs, however, presents several kinetic possibilities.19 If chain transfer is fast relative to propagation, an atactic polymer with uniform molecular weight will be produced. If there is a slow chain transfer rate, the resulting polymer will be isotactic, but will have a broad

Scheme 4. Kinetic Possibilities of Polymerizing PO with a Chiral, Racemic Catalyst in the Absence and Presence of CSAs

molecular weight distribution. Finally, with a moderate chain transfer rate, racemic monomer and catalyst will give polymer with uniform molecular weight and isotactic stereoblocks, which is the type of material we sought to produce. The synthesis of rac-1, however, requires individual synthesis of each enantiomer followed by mixing to avoid the formation of inseparable diastereomers due to the reaction of the chiral diamine with the chiral binaphthol linker. Our group has previously reported the one-pot synthesis of racemic bimetallic cobalt catalysts with achiral ethylene diamine units,16 but their 11050


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polymerize PO using a monometallic (salalen)CrCl analogue of 2 yielded less than 1% conversion (Supplementary Figure 29), suggesting that the bimetallic geometry of 2 is important for catalytic activity and that the mechanism of polymerization may be related to that of 1 (Scheme 3). We sought to produce iPPO diols using rac-2 and [PPN]Cl with rac-PO and 1,6-hexanediol (HD) as a CSA. As the ratio of PO to HD decreased, the molecular weight of the polymers produced also decreased, while the mm-content of the polymers remained above 87% (Table 2, entries 2−4). Integration of the terminal polymer methine proton (δ = 3.91 ppm) with the internal methylenes of the HD CSA (δ = 1.54 and 1.32 ppm) indicated that these groups were present in a 1:1 molar ratio, consistent with one polymer chain per CSA. Analysis of the methine region of the 13C NMR spectra of these polymers revealed stereochemical errors of the type [mr] = [rm] > [rr],19 consistent with the formation of stereoblocks of iPPO due to stereoerrors resulting from both the insertion of the opposite enantiomer of PO and propagation from stereochemically mismatched polymer chain-end and catalyst combinations. Even at catalyst loadings of less than 0.01 mol % (Table 2, entry 3), the polymerization proceeded to high conversion and the molecular weight of the polymer was controlled by the ratio of PO to HD. Producing a large batch of polymer allowed us to determine the hydroxyl value of the polymer to be 19.3 mg KOH/g, compared to a theoretical value of 19.4 mg KOH/g, confirming the presence of hydroxyl end-groups. All of the polyether diols were isolated as semicrystalline solids with melting points of 57−65 °C, demonstrating that it is possible to produce semicrystalline iPPO diols from rac-PO and racemic catalysts under chain shuttling reaction conditions. For polymerizations at 22 °C, we observed relatively low dispersities and high isotacticities, consistent with a moderate rate of chain shuttling. Because temperature can have a marked influence on chain shuttling systems, we next examined the effect of increased reaction temperature on the rates of chain shuttling and propagation (Table 2). At 120 °C in the absence of CSA, rac-2 produced iPPO with only a small decrease in mm-content and Tm relative to the polymerization at 22 °C. However, upon the addition of CSA, the level of isotacticity dropped dramatically and the resulting polyether was predominantly atactic and amorphous. We propose that as the temperature of the polymerization increases, the chain transfer process is favored relative to propagation, resulting in a change from isotactic stereoblock polymers to largely atactic polymers as outlined earlier in Scheme 4. Having produced iPPO diols using a small molecule CSA, we explored oligomeric and polymeric CSAs.18,26 Using trifunctional glycerol propoxylate (gPO) as a CSA (Table 3, entries 1 and 2) led to trifunctional semicrystalline iPPO polyols that may be useful as cross-linkers in polyurethane synthesis. Atactic PPO diols of various molecular weights worked well as CSAs (Table 3, entries 3−6), leading to iPPO-b-aPPO-b-iPPO stereoblock polymers with varying levels of isotacticity and semicrystallinity, which were tunable by simply changing the CSA molecular weight and/or the ratio of CSA to PO. The polymerization was also shown to be stable to the addition of ester functionality, as poly(caprolactone) (PCL) diols lead to iPPO-b-PCL-b-iPPO block copolymers with no transesterification observed by 1H and 13C NMR spectroscopy (Table 3, entries 7 and 8). Quantum Chemical Reaction Mechanism. To better understand the mechanism of polymer formation using rac-2,

low synthetic yields (∼14% for the ligand) and instability with alcohol CSAs in initial experiments deterred further investigation. We chose to employ chromium salalen complexes because monometallic (salalen)CrCl complexes have been used by Katsuki and co-workers for asymmetric hetero-Diels−Alder reactions20 and by Nozaki and co-workers for the copolymerization of cyclohexene oxide and CO2.21 Previous work on chromium salens by Chen and co-workers22 and porphyrins by Chatterjee and Chisholm23 suggested that bimetallic chromium complexes should be capable of homopolymerizing PO. Furthermore, reports by both Duchateau 24 and our laboratory25 showed that (salen)CrCl catalysts can chain shuttle with alcohols in the copolymerization of epoxides with cyclic anhydrides. Thus, we synthesized the bimetallic (salalen)CrCl complex, rac-2 (Scheme 5). Ligand-2 can be Scheme 5. Synthesis of Bimetallic Complex rac-2

prepared in a one-pot synthesis in either the enantiopure or racemic form via the condensation of N-methylethylenediamine with 3,3′-diformyl-2,2′-dihydroxy-1,1′-binaphthyl, followed by the SN2 reaction of the product with 3,5-di-tert-butyl-2hydroxybenzyl bromide. Notably, ligand-2 is obtained in 53% overall yield with no chromatography steps after purification by crystallization. Metalation of the ligand with anhydrous CrCl2 followed by air oxidation afforded bimetallic complex rac-2 in 94% yield. We screened rac-2 with commercially available [PPN]Cl and it was active for the isospecific polymerization of rac-PO (Table 2, entry 1), giving stereoerrors of the type [mr] = [rm] = [rr], indicative of an enantiomorphic site control mechanism.12 Preliminary experiments with (S)-2 showed that (S)-PO was preferentially consumed (krel ≈ 60) and (S)-iPPO was produced. Unfortunately, all attempts to date at crystallizing both (S)-2 and rac-2 to determine the solid-state structure have failed, possibly due to the multiple stereochemical possibilities for the N-methyl groups of the salalen ligand. The only reported crystal structure of a (salalen)CrCl complex places the Cr atom in a cis-β geometry.20 However, attempts to 11051


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Journal of the American Chemical Society Table 2. Polymerization of rac-PO Using rac-2 with 1,6-Hexanediola

entry

CSA

PO/CSA

Trxn (°C)

trxn (h)

conv (%)b

TONc

Mn theo. (kDa)d

MnNMR (kDa)e

Mn GPC (kDa)f

Đf

[mm] (%)g

[mr] + [rm] (%)g

[rr] (%)g

Tm (°C)h

1 2 3j 4 5 6

none HD HD HD none HD

n.a.i 100 100 50 n.a.i 30

22 22 22 22 120 120

1.5 2.3 21 3.0 1.0 24

90 99 99 99 79 96

4100 3700 11 500 4000 3400 3600

240 5.5 5.8 3.0 195 1.7

n.a.i 5.5 6.0 3.0 n.a.i 1.8

170 7.6 8.9 3.8 55.0 2.2

2.8 1.7 1.7 1.7 2.7 1.2

94.4 90.0 89.7 87.5 87.7 61.4

3.7 8.0 8.6 10.8 8.1 31.5

1.9 2.0 1.7 1.8 4.2 7.1

67 63 62 57 66 −k

a

Polymerization conditions: 1 mL of PO; [PO] = 4.8 M in dimethoxyethane (DME); [PO]:[2]:[(PPN)Cl] = 4000:1:2. bDetermined gravimetrically. cmmol PO consumed/mmol rac-2. dCalculated using one polymer chain per rac-2 and CSA (Supplementary eq 1). eDetermined by 1 H NMR spectroscopy (Supplementary eq 2). fDetermined by GPC calibrated with polystyrene standards at 30 °C in THF. gDetermined by 13C NMR spectroscopy. hDetermined by DSC. Reported Tm values are from the second heat. in.a. = not applicable. j25 mL of PO; [PO]:[3]:[(PPN)Cl] = 12 000:1:2. kNo Tm detected for this sample.

Table 3. Polymerization of rac-PO Using rac-2 with Various CSAsa

entry 1 2 3 4 5 6 7 8

gPO gPO aPPO aPPO aPPO aPPO PCL PCL

CSA

PO/CSA

Trxn (°C)

trxn (h)

conv (%)b

TONc

Mn theo. (kDa)d

Mn NMR (kDa)e

Mn GPC (kDa)f

Đf

[mm] (%)g

[mr] + [rm] (%)g

[rr] (%)g

Tm (°C)h

(1.2 (2.0 (3.0 (3.0

130 90 40 40 80 40 160 80

22 22 22 22 22 22 22 22

19 22 22 18 17 17 18 18

99 100 98 97 94 99 98 98

3400 3400 4200 3200 3600 3800 4100 3600

8.3 5.2 3.2 4.0 7.5 5.5 9.0 4.5

8.0 5.0 3.1 3.9 7.4 5.6 9.4 4.4

10.2 6.6 4.8 6.0 10.9 7.9 17.3 10.7

1.6 1.5 1.8 1.5 1.7 1.4 1.7 1.5

87.8 84.7 65.4 58.4 66.4 52.7 92.4 89.8

10.2 12.3 24.6 29.0 23.3 33.7 5.5 7.9

2.1 3.0 10.1 12.6 10.3 13.6 2.1 2.3

62 59 59 59 64 58 66 62

kDa) kDa) kDa) kDa)

a Polymerization conditions: 1 mL PO; [PO] = 4.8 M in DME; [PO]:[3]:[(PPN)Cl] = 4000:1:2. bDetermined gravimetrically. cmmol PO consumed/mmol rac-2. dCalculated using one polymer chain per rac-2 and CSA (Supplementary eq 1). eDetermined by 1H NMR spectroscopy (Supplementary eq 2). fDetermined by GPC calibrated with polystyrene standards at 30 °C in THF. gDetermined by 13C NMR spectroscopy. h Determined by DSC.

ωB97X-D density functional28 with heptet spin and with the SMD solvation model29 using diethyl ether as the implicit solvent (see Computational Details in the Supporting Information). The full initiation and propagation mechanisms can be found in the Supporting Information.

density functional theory (DFT) simulations were employed. These techniques utilized the single-ended Growing String Method27 to determine structures, reaction paths and transition states for the initiation and chain-growth elementary steps. Free energies for all reported structures were obtained using the 11052


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These atomistic simulations provide a 3D structural explanation for the observed stereoselectivity of catalysis. As shown in Figure 1, the (R)- and (S)-monomers take different

Article

AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Geoffrey W. Coates: 0000-0002-3400-2552 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Office of Naval Research (N00014-14-1-0551) for financial support of this work. We thank Dr. Nathan Van Zee for helpful discussions and Mr. Anthony Condo for HRMS analysis.

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Figure 1. Stereochemical model for propagation transition states for (a) (R)-PO and (b) (S)-PO after initiation using (S)-PO.

orientations for their methyl groups at the chain growth steps, TSRS3 and TSSS3. Via Newman projections, it can be seen that the methyl group of (R)-PO sterically clashes with the proximate naphthyl group of the model. This interaction causes a shift to a higher energy, less preferred transition state geometry compared to (S)-PO insertion. In (S)-PO insertion TSSS3, the same methyl position is replaced by hydrogen, minimizing this unfavorable steric interaction and resulting in a barrier that is 4.0 kcal/mol lower than that of TSRS3.

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CONCLUSION In summary, we have shown that alcohols are effective chain shuttling agents for the enantioselective polymerization of PO by (S)-1, producing iPPO with controlled molecular weights and alcohol end-groups. To address the practical difficulties posed by using (S)-1, we then synthesized a racemic bimetallic (salalen)CrCl catalyst (rac-2) and used alkyl diol, PO-oligomer triols, and aPPO and PCL diols as CSAs to produce hydroxytelechelic iPPO. These telechelic polymers have controlled molecular weights and are semicrystalline. Amorphous hydroxytelechelic PPO was also produced by increasing the reaction temperature in conjunction with CSAs. Quantum chemical simulations reveal that the stereoselective step of the polymerization can be explained by steric interactions between the chiral PO monomer and the naphthyl region of the catalyst. We are continuing to explore the use of hydroxy-telechelic iPPO in polyurethane synthesis, as well as the polymerization of other epoxides with this catalyst system.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b00194. Synthetic procedures and characterization data for ligand-2 and complex rac-2, polymerization procedures, polymer characterization, and calculation details (PDF) 11053


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Isospecific, Chain Shuttling Polymerization of Propylene Oxide Using

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