A Comprehensive Understanding of Polyester Stereocomplexation

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A Comprehensive Understanding of Polyester Stereocomplexation Zhao-Qian Wan, Julie M. Longo, Li-Xin Liang, Hong-Yu Chen, Guang-Jin Hou, Shuai Yang, Wei-Ping Zhang, Geoffrey W. Coates, and Xiao-Bing Lu J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b07058 • Publication Date (Web): 28 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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Journal of the American Chemical Society

A Comprehensive Understanding of Polyester Stereocomplexation

Zhao-Qian Wan,† Julie M. Longo,‡ Li-Xin Liang,§ Hong-Yu Chen,§ Guang-Jin Hou,§ Shuai Yang,† Wei-Ping Zhang,† Geoffrey W. Coates*,‡ and Xiao-Bing Lu*,† †

State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024,

China ‡ Department

of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca,

New York 14853-1301, United States §State

Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of

Sciences, Dalian 116023, China

ABSTRACT: We report a comprehensive understanding of the stereoselective interaction between two

opposite

enantiomeric

polyesters

prepared

from

the

regioselective

copolymerization of chiral terminal epoxides and cyclic anhydrides. For many of the resultant polyesters, the interactions between polymer chains of opposite chirality are stronger than those of polymer chains with the same chirality, resulting in the formation of a stereocomplex with enhanced melting point (Tm) and crystallinity. The backbone, tacticity, steric hindrance of the pendant group, and molecular weight of the polyesters have significant effects on stereocomplex formation. Bulky substituent groups favor stereocomplexation, resulting in a greater rise in Tm in comparison with the component enantiomeric polymers. Stereocomplex assembly of discrete (R)- and 1

ACS Paragon Plus Environment

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(S)-poly(phenyl glycidyl ether-alt-phthalic anhydride)s oligomers revealed that the minimum degree of polymerization required for stereocomplex formation is five. Raman spectroscopy and solid-state NMR studies indicate that stereocomplex formation significantly restricts the local mobilities of C=O and C–H groups along the backbone of chains. The reduced mobility results in the enhanced spin-lattice relaxation time and both 1H and 13C downfield shifts due to the strong intermolecular interactions between (R)- and (S)-chains.

INTRODUCTION Chiral and steric recognitions by noncovalent interactions play critical roles in selectively spontaneous assembly of molecules into stable and well-defined aggregates with specific functions. For example, the intermolecular reading of genetic code widely observed in biological systems.1-3 In the realm of macromolecules, there are few enantiopure polymers that form stereocomplexes through an interlocked orderly assembly between two opposite enantiomeric chains when mixed in equivalent amounts.4-11 In comparison with the parent polymers, the stereocomplex usually exhibits improved physical properties, such as higher levels of crystallinity and melting points

(Tm). One of the most widely studied examples is poly(lactic acid)

(PLA).4, 12-13 Stereocomplexed PLA possesses a Tm of 230 °C, which is about 50 °C higher than that of its parent polymers, enantiopure poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA). Additionally, the stereocomplexation of PLLA and PDLA significantly increases its hydrolytic/thermal degradation-resistance and gas 2


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barrier properties.9 Epoxides have been widely studied in alternating copolymerizations with carbon dioxide to produce polycarbonates and with cyclic anhydrides to produce polyesters.14-19 The stereogenic centers in the substituted epoxides allow the possibility of forming tactic polymers by stereospecific copolymerization.20 In recent years, a variety of stereoregular polycarbonates and polyesters were prepared by means of regio- and/or stereoselective ring-opening epoxide copolymerization.21-24 Most isotactic polycarbonates from CO2 and meso-epoxides are semicrystalline materials, and their crystallinities depend on tacticity. Notably, blending equivalent amounts of enantiomeric polycarbonates derived from meso-epoxides readily formed crystalline stereocomplexes.25 On the contrary, cocrystallization was not observed in a 1:1 mixture of highly isotactic (R)- and (S)-polycarbonates from terminal epoxides, except for styrene oxide derivatives.26 Interestingly, (R)- and (S)-poly(propylene succinate), which each have a very low melting polymorph with a Tm around 70 °C, formed a stereocomplex with improved crystallinity and an increased Tm of 120 °C.27 The half-life of recrystallization for the stereocomplex is approximately 3 orders of magnitude faster than that of the parent enantiopure polymers. However, the critical factors affecting polymer stereocomplexation and the relationship between molecular weight and stereocomplex formation were not investigated. Herein, we present a comprehensive study regarding the effects of the backbone structure, tacticity, steric bulk of the pendant group, and molecular weight of the polyesters derived from terminal epoxides on stereocomplex formation. Furthermore, 3



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Raman spectroscopy and solid state NMR were used to investigate the nature of the intermolecular interactions in a stereocomplexed polyester, poly(phenyl glycidyl ether-alt-phthalic anhydride) (poly(PGE-alt-PA)). RESULTS AND DISCUSSION Synthesis of Enantiomeric (R)- and (S)-polyesters for Screening Stereocomplex Formation. Various enantiopure polyesters (Scheme 1) were prepared by the regioselective copolymerization of a series of chiral terminal epoxides and cyclic anhydrides catalyzed by the binary catalyst consisting of [PPN][NO3] and fluorine-substituted salcyCo(ІІІ)NO3 complex, which is a highly active, regioselective Co(ІІІ) species with longer catalyst life time in the copolymerization of epoxide with anhydride

28

(salcy

=

=

N,N’-bis(salicylidene)cyclohexanediamine,

PPN

bis(triphenylphosphine)iminium). The number average molecular weights (Mn) of these isotactic polyesters ranged from 7.2 to 23.0 kDa, depending on the combination of epoxide and cyclic anhydride. All resultant polyesters exhibited high levels of enantioselectivity (≥98% (S)- or (R)-stereocenter) and regioregularity (≥98% head-to-tail linkages) due to selective ring-opening at the less sterically hindered methylene carbon of the epoxides. (see Supporting Information, Table S1) After precipitation in excess methanol, most of the isolated isotactic polyesters were semi-crystalline, although some were amorphous. However, upon soaking in methanol, these amorphous polyesters gradually transformed into semi-crystalline materials. This might be ascribed to their slow crystallization rates. Therefore, the 4


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isotactic polyesters used in this study were immersed in methanol overnight after precipitation to improve crystallization. Scheme 1. Regioselective Copolymerization of Chiral Terminal Epoxides and Cyclic Anhydride Using Chiral SalcyCoNO3 and [PPN][NO3]

SalcyCoNO3 [PPN][NO3]

O R2

O

+

R1

O

30 oC

R2

O O

O

TBO

O

Bn

TBGE

IGE

R2 n

Ph EPB

O

O

O

F

O

O

O

O

O

O MGE

F

O O

O

O

PA EGE

N Co

O NO3 t t Bu Bu SalcyCoNO3

O

BGE

O O

N R2

O O

PGE

O

O

R1

O O

Ph PO

O O O

MA

O SA

The copolymers of phthalic anhydride (PA) and enantiopure glycidyl ether derivatives are all semi-crystalline with Tms between 90 and 150 °C (see Figure 1 for named polymer structures). The copolymer (S)-poly(PO-alt-PA) shows a Tm of 142 °C. When (S)-EPB) or (S)-TBO) was copolymerized with PA, the corresponding (S)-poly(EPB-alt-PA) or (S)-poly(TBO-alt-PA) is amorphous with a glass transition temperature (Tg) of 59 °C or 94 °C, respectively. The copolymer of (S)-PGE and maleic anhydride (MA) ((R)-poly(PGE-alt-MA)) is a semi-crystalline polymer with a Tm of 123 °C. While both isotactic poly(PGE-alt-PA) and isotactic poly(PGE-alt-MA) are semicrystalline polymers, isotactic poly(PGE-alt-SA) is amorphous with a Tg of 23 °C, despite its high level (99%) of isotacticity. To test the stereocomplex formation, 50 mg (S)- and (R)-polyesters were dissolved 5



in 3 mL dichloromethane respectively, and the two solutions mixed. The mixed solution was added dropwise to an excess of methanol to precipitate the polymer. After drying, the resulting blend was analyzed by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). The melting point and crystalline diffraction peaks of each blend were compared with those of the parent polyesters to confirm stereocomplex formation.

Figure 1. Equivalent isotactic (R)- and (S)-polyester blends used for screening stereocomplex formation by DSC and WAXD analysis (ΔTm = Tm(blend)-Tm(iso)).

No stereocomplexation was observed when equal masses of (S)- and (R)-poly(PO-alt-PA) were blended (see Supporting Information, Figures S15 and S16) even though enantiomeric mixtures of structurally similar poly(PO-alt-SA) form stereocomplexes. In contrast, the blending of (S)- and (R)-poly(PGE-alt-PA) formed a 6


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stereocomplex with a Tm of 184 °C, which is 60 °C higher than that of its parent polymers (Figure 2a). While no Tg was detected in the stereocomplexed poly(PGE-alt-PA), a Tg of 68 °C was observed in (S)-poly(PGE-alt-PA) (Figure 2a). This implies that the stereocomplexed poly(PGE-alt-PA) has higher levels of crystallinity than the component enantiomeric polymers. WAXD further confirmed stereocomplex formation. The stereocomplexed poly(PGE-alt-PA) has four major crystalline diffraction peaks at 6.1°, 20.2°, 22.2°, and 25.2°. In contrast, (S)-poly(PGE-alt-PA) has three strong peaks at 11.5°, 17.2°, and 19.0° (Figure 2b).

stereocomplex

stereocomplex O O

O O O

O

O

n

O n O Ph

O Ph

Figure 2. a) DSC thermograms and b) WAXD profiles of (S)-poly(PGE-alt-PA) and stereocomplexed poly(PGE-alt-PA).

We next sought to further investigate the effect of epoxide side chain structure on stereocomplex formation. A series of (R)- and (S)- poly(glycidyl ether-alt-PA) polyesters was synthesized and the thermal properties of the component enantiomeric polyesters and corresponding blends investigated by DSC (Figure 1). Blending (R)and (S)-poly(BGE-alt-PA) in 1:1 ratio also resulted in stereocomplex formation (see Supporting Information, Figures S17 and S18). Stereocomplexed poly(BGE-alt-PA) 7



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shows a Tm of 150 °C, which is 56 °C higher than that of its parent polyesters. To verify whether the aromatic group in the pendant chains is essential for stereocomplexation, the blending of enantiomeric polyesters without aromatic groups in the side chains was performed. Blending (R)- and (S)-poly(TBGE-alt-PA) also resulted in stereocomplex formation, as confirmed by DSC and WAXD analysis (see Supporting Information, Figures S19 and S20). The DSC plot shows that the stereocomplexed poly(TBGE-alt-PA) has a Tm of 162 °C, 68 °C higher than that of the component enantiomeric polyesters. This result illustrates that the aromatic groups in the pendant chains of enantiomeric polyesters are not essential for stereocomplexation. To test the steric effect of the side chains on stereocomplex formation, enantiomeric polyesters with different substituent groups on the pendant chain were investigated. Blending equal masses of (R)- and (S)-poly(IGE-alt-PA) formed a stereocomplex with a Tm of 143 °C, 30 °C higher than that of its parent polyesters (see Supporting Information, Figures S21 and S22). Blending equal masses of (R)- and (S)-poly(EGE-alt-PA) also formed a stereocomplex with a Tm of 111 °C, which is only 2 °C higher than that of the component enantiomeric copolymers (Figure 3a). Although there is only a slight difference in Tm, the stereocomplexed poly(EGE-alt-PA) shows unique crystalline behavior, distinct from its parent polymers (Figure 3b). Five diffraction peaks at 8.1°, 12.7°, 19.5°, 21.3°, 24.3°, and 25.4° were found in the WAXD of (S)-poly(EGE-alt-PA), while stereocomplexed poly(EGE-alt-PA)

has

five

large

diffraction 8


peaks

at

7.0°,

19.1°,

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stereocomplex

stereocomplex O O

O O O

O

O

O

n

n O

O

Figure 3. a) DSC thermograms and b) WAXD profiles of (S)-poly(EGE-alt-PA) and the blend of (R)- and (S)-poly(EGE-alt-PA) in 1:1 ratio.

20.9°, 22.5°, and 24.4° (Figure 3b). Interestingly, further decreasing the steric hindrance of the substituent groups in the pendant chain resulted in no stereocomplexation. The 1:1 blend of (R)- and (S)-poly(MGE-alt-PA) showed the same diffraction peaks at 2θ angles as (S)-poly(MGE-alt-PA) (see Supporting Information, Figures S23 and S24). These results suggest that steric hindrance in the pendant chains of enantiomeric polyesters derived from PA and epoxides plays an important role in stereocomplex formation. It should be noted that all the stereocomplexes discussed above are based on glycidyl ether derivatives. Blending two opposite enantiomeric polyesters from EPB or TBO also formed stereocomplexes (see Supporting Information, Figures S25, S26, S27, and S28), confirmed by DSC and WAXD. This result suggests that an oxygen atom in the pendant chains of enantiomeric polyesters is not essential to stereocomplexation. To investigate the effect of the backbone structure of enantiomeric polyesters on 9



stereocomplex formation, enantiomeric poly(PGE-alt-MA) and poly(PGE-alt-SA) were prepared. No stereocomplexation occurs

between the two opposite

enantiomeric poly(PGE-alt-SA)s, (see Supporting Information, Figures S29 and S30) while the

1:1 blend of (R)- and (S)-poly(PGE-alt-MA) formed a stereocomplex with

a Tm of 137 °C, which is

14 °C higher than that of either isotactic polymer. (see

Supporting Information, Figures S31 and S32). As noted above, stereocomplexed poly(PGE-alt-PA) showed a Tm 60 °C higher than that of isotactic poly(PGE-alt-PA). This demonstrates that the backbone rigidty of enantiomeric polyesters also has a critical influence on stereocomplex formation, and that enantiomeric polyesters based on PA and epoxides with bulky substituent groups were more likely to form stereocomplexes. The Effect of Tacticity and Mn on Stereocomplexation Having discovered a series of crystalline polyesters and their stereocomplexes, we probed the effect of tacticity on crystallinity and stereocomplex formation, using poly(PGE-alt-PA) as a model polyester. Various poly(PGE-alt-PA)s with 60, 74, 82, and 99% enantiomeric excess (ee) were prepared with Mn values between 12.0 and 14.0 kDa. Isotactically-enriched poly(PGE-alt-PA)s with 60% and 74% ee are amorphous materials with Tg values of 63 °C and 70 °C, respectively (Figure 4a). In contrast, an enhanced Tg of about 80 °C and a very small melting endothermic peak at 110 °C with a melting enthalpy (ΔH) of 7 J/g was observed in the poly(PGE-alt-PA) sample with 82% ee, indicating a very low degree of crystallinity (Figure 4a). The highly isotactic poly(PGE-alt-PA) with 99% ee exhibited a sharp and high 10


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Figure 4. DSC thermograms of a) isotactically-enriched poly(PGE-alt-PA) with different ee values; and b) the 1:1 blend of two opposite enantiomeric poly(PGE-alt-PA)s with different ee values.

crystallization endothermic peak at about 120 °C (Figure 4a). The influence of the tacticity of the parent poly(PGE-alt-PA)s on stereocomplexation is shown in Figure 4b. No stereocomplexation is observed for the 1:1 mixture of the two opposite configuration poly(PGE-alt-PA)s with 60% ee, while the blending of parent copolymers with 74%, 82%, and 99% ee resulted in stereocomplex formation (Figure 4b). The Tm and ΔH values increase with the tacticity of the parent poly(PGE-alt-PA)s. Subsequently, the stereocomplexation of enantiomeric poly(PGE-alt-PA) with different molecular weights was investigated. To control the molecular weight, benzyl alcohol was used as a chain transfer agent. The blending of (R)- and (S)-poly(PGE-alt-PA) with Mns of 3.6 kDa and dispersities (Ð) of 1.20 resulted in the formation of a stereocomplex with a

Tm of 165 °C (Figure 5a). Interestingly, the

blend of 1.8 kDa enantiomeric poly(PGE-alt-PA) also formed the stereocomplex, 11



Figure 5. a) DSC thermograms and b) WAXD profiles of (S)-poly(PGE-alt-PA) oligomers with molecular weight of 1.8 kDa and 3.6 kDa, and the 1:1 blend of two opposite enantiomeric poly(PGE-alt-PA)s with molecular weight of 1.8 kDa and 3.6 kDa.

confirmed by WAXD (Figure 5b). However, its Tm is close to that of its parent polyesters (Figure 5a). This result provides evidence for the critical number of ordered repeat units for forming this stereocomplex. We suspect the minimum molecular weight required for poly(PGE-alt-PA) stereocomplexation is about 1.8 kDa. Since the molecular weight of the repeat unit of poly(PGE-alt-PA) is 298.1 g/mol, the minimum degree of polymerization (DP) for stereocomplex formation must be less than 6. Inspired by the excellent researches on length-dependent structure property relationship,29-32 thin-layer chromatography (150:1, chloroform:methanol) was used to isolate discrete stereoregular oligomers from the enantiomeric poly(PGE-alt-PA) with an original Mn of 1.8 kDa and a Ð of 1.21. Enantiomeric oligomers with DP’s of 4 and 5 were obtained, and confirmed by MALDI-TOF MS (Figure 6), GPC (see Supporting Information, Figure S40) and 1H NMR (see Supporting Information, 12


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a)

O O Bn O

O

O H n O

Oligmer mixture Mn = 1.8 kDa DGPC = 1.21

b)

[M + Na]+Found = 1323

298.1 a.m.u

Ph

[M + K]+Found = 1339

DP = 4

chromatography separation

[M + Na]+Found = 1621

O O

O O Bn O

O

Bn O

O H 4

298.1 a.m.u O

Ph

Ph

DP = 5

DP = 4

Figure

DP = 5

O

O

[M + K]+Found = 1637

O H 5

6.

a)

Schematic

(R)-poly(PGE-alt-PA)

;

b)

illustration

of

MALDI-TOF

the mass

separation spectra

of

of the

1.8

kDa

isolated

(R)-poly(PGE-alt-PA) samples (DP = 4, and 5).

Figure 7. a) DSC thermograms and b) WAXD profiles of (S)-poly(PGE-alt-PA) with a DP of 4 and 5, the 1:1 blend of two opposite enantiomeric poly(PGE-alt-PA) oligomers with a DP of 4 and 5.

Figures S13 and S14) characterization.

No stereocomplexation was observed when

(R)- and (S)-poly(PGE-alt-PA) with a DP of 4 were mixed (Figures 7a and 7b). However, when (R)- and (S)-poly(PGE-alt-PA) oligomers with DP of 5 were blended, 13



Page 14 of 25

stereocomplexation was observed by both DSC and WAXD (Figures 7a and 7b). While the parent polyesters with DP of 5 were amorphous, the blend was semicrystalline with a melting point of 81 °C and a ΔH value of 56 J/g. Therefore, the minimum DP required for stereocomplexation between (R)- and (S)-poly(PGE-alt-PA) is five.

Raman and Solid State NMR Investigation of Polyester Stereocomplexation

Figure 8. Raman spectra (1650-1830 cm-1) of (A) isotactic

and (B) stereocomplexed

poly(PGE-alt-PA).

To better understand the interaction between the two opposite enantiomeric polymers, Raman spectroscopy was used to investigate the differences in vibrational modes between isotactic and stereocomplexed poly(PGE-alt-PA) in the solid state.33 A single peak at 1729.0 cm-1 assigned to the ν(C=O) appears in the isotactic poly(PGE-alt-PA)

(Figure

8,

stereocomplexed-poly(PGE-alt-PA), the

plot

A),

while

in

the

ν(C=O) signal was split into two major

peaks at 1706.8 cm-1 and 1735.0 cm-1 (Figure 8, plot B). The splitting of the ν(C=O) 14


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signal implies that two different carbonyl environments exist after stereocomplexation. The new peak at 1706.8 cm-1 can be ascribed to the ν(C=O) signal generated by the stereocomplexation. The 22 cm-1 frequency shift of ν(C=O) after stereocomplexation indicates that some carbonyl groups may participate in hydrogen bonding as proton acceptors.34-35 The Raman shift of the vibration of the possible proton donor, ν(C-H), is also observed (see Supporting Information, Figure S35). Furthermore, the interactions between (R)- and (S)- chains in stereocomplexed poly(PGE-alt-PA), were investigated using solid state

13 C

cross polarization

(CP)/magic angle spinning (MAS) and 1 H- 13 C HETCOR NMR methods. 36-38 In comparison to the component enantiomeric polymers, the stereocomplexed

Figure 9. 13C CP/MAS spectra of (A) isotactic and (B) stereocomplexed

poly(PGE-alt-PA) has a different solid state

13C

poly(PGE-alt-PA).

CP/MAS spectrum: the carbonyl

carbons resonance signals in isotactic poly(PGE-alt-PA) are at 169.7 and 166.8 ppm whereas the same signals in stereocomplexed poly(PGE-alt-PA) shift to 171.1 and 165.9 ppm, respectively (Figure 9). The larger splitting of these resonance signals in 15



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the spectrum of the stereocomplex indicates a bigger difference in the chemical environment of the carbonyl carbons. Additionally, in the spectrum of isotactic poly(PGE-alt-PA), only two peaks at 130.0 and 132.9 ppm can be observed between 124 and 140 ppm. As for the spectrum of stereocomplexed poly(PGE-alt-PA), five peaks are observed in this range. In CP/MAS solid state NMR, the group or molecular motions decrease the cross polarization efficiency, which reduces resolution of the spectrum. The peaks from 124 to 140 ppm are mainly ascribed to the resonance of aromatic carbons in the backbones of polymers. Therefore, the increased spectrum resolution in the area implies that the motions of aromatic rings in the backbones of stereocomplexed poly(PGE-alt-PA) are restricted.

Table 1. 13C NMR T1 Relaxation Times for Solid (S)- and Stereocomplexed- Poly(PGE-alt-PA) at Ambient Temperature. O

j

c

O

O

n e

g

Entry

T1 (s) C=O

1a 2b

a

d

b

h f

O

k a

i

O

CH

CH2

a1

a2

b

c–f

g

h

i

340 >500

120 >500

75 77

52 52

72 72

19 17

76 11 0

j

k 84 55

a(S)-poly(PGE-alt-PA), bstereocomplexed-poly(PGE-alt-PA).

Spin-lattice relaxation time (T1) was measured for each carbon atom in isotactic

16


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and stereocomplexed poly(PGE-alt-PA) samples under the cross polarization condition by the application of the saturation recovery-based sequence. The T1 values of the carbonyl peaks for stereocomplexed poly(PGE-alt-PA) are longer than those of isotactic poly(PGE-alt-PA). For the carbonyl peak with lower chemical shift (165.9 ppm, Ca2), its T1 is over 500 s, while the T1 value of the corresponding carbonyl for isotactic poly(PGE-alt-PA) is 120 s. (Table 1) The much larger T1 value of the carbonyl peak at 165.9 ppm for stereocomplexed poly(PGE-alt-PA) indicates that the motion of the carbonyl is restricted after stereocomplexation. Similar T1 values for carbon atoms of the phenyl group (Cb, Cg, Ch) in the side chain were measured for isotactic and stereocomplexed poly(PGE-alt-PA)s. The T1 value of the methylene carbons (Cj, Ck) for stereocomplexed poly(PGE-alt-PA) (55 s) is shorter than that of isotactic poly(PGE-alt-PA) (84 s). (Table 1) The T1 value of the methine carbon (Ci) for stereocomplexed poly(PGE-alt-PA) (110 s) is longer than that of isotactic poly(PGE-alt-PA) (76 s), (Table 1) indicating that the motion of the methine is also restricted after stereocomplexation.

Figure 10. 1H–13C HETCOR NMR spectra at the methine region of (A) isotactic and (B) stereocomplexed poly(PGE-alt-PA) at a contact time of 1000 µs. 17



1H–13C

HETCOR NMR experiments with a contact time of 1000 μs were carried

out for both isotactic and stereocomplexed poly(PGE-alt-PA) samples, and the spectra of the methine region are shown in Figure 10. While the chemical shifts of the methine hydrogen and carbon in isotactic poly(PGE-alt-PA) are 4.03 and 69.34 ppm, respectively, the corresponding signals in the stereocomplex are 5.82 and 71.89 ppm, respectively. The 1H downfield shift of 1.79 ppm and 13C downfield shift of 2.55 ppm indicate that the electronic cloud density of the methine hydrogen was weakened after stereocomplexation. This may be attributed to the electron-withdrawing inductive effect of the carbonyl oxygen atom in the polymer chain with opposite configuration. Taking the Raman spectroscopy and solid state NMR studies into consideration, we attribute the driving force of the stereocomplexation to the weak hydrogen bond interaction of C-H•••O=C between the oxygen atom of a carbonyl group of one enantiomer and the hydrogen atom on the methine C-H group at the chiral carbon of the other stereoisomer. CONCLUSIONS We report the synthesis of a variety of highly isotactic polyesters via the regioselective ring-opening copolymerization of enantiopure terminal epoxides with cyclic anhydrides mediated by the fluorine-substituted salcyCoNO3 complex in combination with [PPN][NO3]. Most isotactic polyesters are semicrystalline materials, possessing Tms between 92 and 142 °C. Notably, stereocomplexation was generally observed in the 1:1 mixture of (R)- and (S)-polyesters, affording the corresponding 18


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stereocomplexes with enhanced Tm and higher levels of crystallinity, as well as distinct crystallization behavior from its parent polymers. The steric hindrance of the pendant chain, backbone rigidity, tacticity, and molecular weight have significant effects on stereocomplex formation. Bulky epoxide substituent groups appear to benefit stereocomplexation with higher increases in Tm in comparison with the component enantiomeric polymers. The study on the stereoselective interaction between two opposite enantiomeric, discrete oligomers suggests that the minimum degree of polymerization for the stereocomplex formation between enantiomeric poly(PGE-alt-PA)s is 5. Raman spectroscopy study confirmed the obvious difference at the ν(C=O) signal. A single peak at 1729.0 cm-1 appears in the isotactic poly(PGE-alt-PA), while in the stereocomplex, the ν(C=O) signal was split into two major peaks at 1706.8 cm-1 and 1735.0 cm-1. Solid state

13C

cross polarization/magic angle spinning and 1H-13C

HETCOR NMR analysis demonstrated that strong intermolecular interactions between stereocomplexed R- and S-chains significantly restricts the local mobilities of C=O and C–H groups along the backbone of chains and results in the enhanced spin-lattice relaxation time and both 1H and

13C

downfield shifts. These studies

suggest the driving force of stereocomplexation is the weak hydrogen-bonding interaction between carbonyl and methine of the opposite enantiomeric polyesters.

ASSOCIATED CONTENT

Supplementary characterization data including 1H NMR and 13C NMR spectra, DSC 19



and XRD data, as well as GPC of various polyesters.

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected]; [email protected] Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT

This work is supported by the National Natural Science Foundation of China (NSFC, Grant 21690073), and Program for Changjiang Scholars and Innovative Research Team in University (IRT-17R14), and the Center for Sustainable Polymers, an NSF Center for Chemical Innovation (CHE-1413862).

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SYNOPSIS TOC:

25


A Comprehensive Understanding of Polyester Stereocomplexation

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