urea catalyzed ring-opening polymerization of cyclic esters

Journal Pre-proof Bifunctional Phosphazene-Thiourea/Urea Catalyzed Ring-opening Polymerization of Cyclic Esters Ruiting Yuan, Guangqiang Xu, Chengdong Lv, Li Zhou, Rulin Yang, Qinggang Wang

PII:

S2352-4928(19)31138-9

DOI:

https://doi.org/10.1016/j.mtcomm.2019.100747

Reference:

MTCOMM 100747

To appear in:

Materials Today Communications

Received Date:

20 September 2019

Accepted Date:

4 November 2019

Please cite this article as: Yuan R, Xu G, Lv C, Zhou L, Yang R, Wang Q, Bifunctional Phosphazene-Thiourea/Urea Catalyzed Ring-opening Polymerization of Cyclic Esters, Materials Today Communications (2019), doi: https://doi.org/10.1016/j.mtcomm.2019.100747

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Bifunctional Phosphazene-Thiourea/Urea Catalyzed Ring-opening Polymerization of Cyclic Esters

Ruiting Yuan,‡a Guangqiang Xu,‡a Chengdong Lv,a,b Li Zhou,a,b Rulin Yang,a,b and Qinggang Wang * a,b

a

Key Laboratory of Biobased Materials, Qingdao Institute of Bioenergy and Bioprocess

b

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Technology, Chinese Academy of Sciences, Qingdao, 266101, China. Center of Materials Science and Optoelectronics Engineering, University of Chinese

Academy of Sciences, Beijing, 100049, China. * Corresponding author E-mail: [email protected]

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‡ These authors contributed equally to this work.

New bifunctional phosphazene-thiourea/urea organocatalysts for ROP.

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

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Highlights

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Graphical abstract

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A broad monomer scope of lactones (rac-lactide, δ-valerolactone and ε-caprolactone )

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Isoselective polymerization of rac-LA with Pm value of 0.79.

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Corresponding polyesters with controlled MWs, narrow MWDs, well-defined end groups.

ABSTRACT

A series of new bifunctional phosphazene-thiourea/urea catalysts have been designed and applied to ring-opening polymerization (ROP) of rac-lactide, δ-valerolactone and ε-caprolactone. The ROPs were promoted efficiently, delivering the corresponding polyesters with controlled molecular weights, narrow molecular weight distributions, well-defined end group fidelities and without undesired epimerization of rac-lactide. Kinetic experiment of rac-LA confirmed the controlled/living nature of the ROP process. The isoselective polymerization of rac-LA was achieved with a highest Pm value of 0.79 (based on Bernouillan statics) under mild conditions. In addition, the

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ROPs of δ-VL and ε-CL were also smoothly achieved with high conversions and narrow molecular weight distributions. These catalysts exhibit a broad monomer

scope of lactones, providing a facile synthesis methodology of metal-free and applicable polyester materials.

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Keywords: Phosphazene-Thiourea/Urea bifunctional organocatalysts, Ring-opening polymerization, Cyclic esters

INTRODUCTION Aliphatic polyesters have been investigated for a wide array of biomedical, agricultural, pharmaceutical and biodegradable plastic applications due to their outstanding materials properties and facile biodegradation. 1-4 In recent years, the use of organocatalysts for the ring-opening polymerization (ROP) of cyclic esters opened a new avenue for the synthesis of metal-free polymers, owing to increasing concerns in biomedical and microelectronic applications. 5-8 Furthermore, organocatalysts display huge potential for the design and

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synthesis of new type of polymers with varying chain structures, topologies and

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functional groups.9-11

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Figure 1. Organocatalysts for stereoselective ROP of rac-LA

Among highly efficient organocatalytic polymerizations, phosphazene bases

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developed by Schwesinger12,13 have been received significant attention, due to the strongest basicity and highest reactivity.14,15 Different types of monomers, such as cyclic esters,16,17 epoxides,18,19 cyclosiloxanes,20,21 lactam,22 acrylates23 and cyclic carbonates24,25 have been well investigated for the polymerization and copolymerization in the past two decades.11 The catalytic activities and stereoselectivities of phosphazene bases are heavily dependent on the molecular structure and basicity. For example, using tBu-P1 or BEMP as catalyst for ROP

of rac-LA, low activities (> 60 h) and moderate Pm = 0.70 (the probability of forming a meso dyad) were obtained.16,26 When using a stronger phosphazene bases

(tBu-P2,

CTPB),

excellent

activities

were

observed

at

room

temperature.10,17 However, in order to achieve high stereoselectivity, these catalysts required low temperature (-78 oC) conditions to reduce the occurrence of undesirable side reactions (epimerization, transesterification etc.). And for ε-caprolactone (ε-CL), only 14% conversion of monomer was observed after 240 h at 80

o

C utilizing BEMP as catalyst,16 while CTPB provided

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polycaprolactone with relatively broad molecular weight distributions (Ð = 1.35-2.20) at room temperature. 26 Therefore, it is of challenge and demanding to develop new phosphazene catalyst for the ROP of lactones with high reactivity as well as high stereoselectivity under mild conditions.

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In this context, a variety of bifunctional phosphazene-thiourea/urea catalysts were designed and synthesized for the ROP of lactones, which were composed

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of ethyl/cyclohexyl linking thiourea/urea and stronger H-bond accepting phosphazene base (Figure 1). In these catalysts, the presence of thioureas/ureas might facilitate controllable polyesters synthesis with well-defined

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27-30

structures and narrow polydispersities, as well as stronger phosphazene bases

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might provide high reactivity and selectivity. 31-33 Three common cyclic esters, rac-lactide (rac-LA), δ-valerolactone (δ-VL) and ε-caprolactone (ε-CL) were selected to test the catalytic performance of three phosphazene-thioureas

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(PTU-1, 2, 3) and two phosphazene-ureas (PU-1, 2) catalysts. What’s more,

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tacticity of polylactide was also explored during the ROP process of rac-LA.

RESULTS AND DISCUSSION Table 1. ROP of cyclic esters using PTUs/PUs a)

Conv.b)

Mnc)

(%)

(g/mol)

12 h

99

50/2.5/1

10 h

PTU-3

50/2.5/1

rac-LA

PU-1

rac-LA

PU-2

6

rac-LA

7e)

Entry

Pm(CEC)d)

Pm(ESC)d)

7300

1.13

0.68

0.76

94

3900

1.12

0.70

0.78

12 h

99

1400

1.90

0.66

0.74

50/2.5/1

12 h

95

2900

1.14

0.72

0.80

50/2.5/1

5h

99

5700

1.12

0.74

0.82

PU-2

50/5/1

1.5 h

90

2700

1.15

0.69

0.79

D-LA

PU-2

50/2.5/1

4h

91

7700

1.10

0.99

0.99

8

rac-LA

PU-2

100/2.5/1

14 h

94

9400

1.12

0.72

0.80

9

rac-LA

PU-2

200/2.5/1

14 h

83

11500

1.11

0.79

0.86

10

rac-LA

PU-2

200/2.5/1

24 h

97

16100

1.25

0.73

0.81

11

rac-LA

PU-2

500/2.5/1

23 h

95

26700

1.14

0.76

0.83

100/2.5/2.5/1

2 min

99

14100

1.51

0.50

0.50

Time

1

rac-LA

PTU-1

50/2.5/1

2

rac-LA

PTU-2

3

rac-LA

4 5

tBu-P

12f)

rac-LA

13f)

rac-LA

14g)

δ-VL

PTU-2

15g)

δ-VL

PU-2

16g)

δ-VL

17g) 18g)

1+U tBu-P

1+TU

100/2.5/2.5/1

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[M]/[Cat.]/[I]

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Cat.

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Ðc)

Monomer

10

min

99

16700

1.57

0.68

0.76

18 h

98

2700

1.14

--

--

50/1/1

72 h

97

2600

1.18

--

--

PTU-2

100/1/1

96 h

91

3100

1.14

--

--

ε-CL

PTU-2

50/1/1

87 h

99

2800

1.16

--

--

ε-CL

PU-2

50/1/1

87 h

48

2400

1.11

--

--

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a)Unless

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50/1/1

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otherwise specified, reactions were initiated by benzyl alcohol and run in CH2Cl2 at room temperature with 2.5 mol % catalyst loading relative to monomer. [monomer]0 = 1 M. b)Measured by 1H NMR. c)Measured by GPC in THF using polystyrene standards for calibration, and corrected using different Mark-Houwink factors: 0.58 for polylactide, 0.57 for polyvalerolactone and 0.56 for polycaprolactone.28 d) Probability of finding meso dyads calculated from homonuclear decoupled 1H NMR spectrum after deconvolution; calculations were based on CEC/ESC mechanism. e)The reaction monomer was enantiomerically pure D-LA. f)Reaction was catalysted by a mixture of t-Bu-P1 and TU/U in the molar ratio 1:1. g)Reactions were initiated by benzyl alcohol and run in toluene at 50 oC with 2 mol % catalyst loading relative to monomer.

The catalytic performance of ROP of rac-LA was first investigated in CH2Cl2 at room temperature with a ratio of 50:2.5:1 for [LA] 0/[Cat.]0/[BnOH]0. The microstructure of the prepared PLAs were determined by the analysis of the methine region of the homonuclear decoupled 1H NMR spectra (Figure 3,

SI, Figures S28-31).34,35 With PTU-1, the monomer conversion reached 99% in 12 h with a narrow molecular weight distribution (MWD) of 1.13 and controlled Mn of 7300 g/mol (Table1, entry 1). Compared with Dixon’s bifunctional iminophosphorane catalyst (Figure 1) 38, a higher isoselectivity of Pm = 0.68 (based on Bernouillan statics) was obtained, which suggested phosphazene structure in bifunctional catalysts indeed has a positive influence on the isoselectivity of polylactides. Furthermore, compared with tBu-P1 or BEMP, the reaction time of PTU-1 catalyzed polymerization was much

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shorter.16 Encouraging by the preliminary result, a more rigid PTU-2 was synthesized and tested. Similar activity while higher selectivity (Pm = 0.70)

were obtained, which indicated that the linker (cyclohexyl vs ethyl) also has notable effect on the tacticity of polymers (entry 2). Compared with

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Takemoto’s organocatalyst (Figure 1)36,37, the high activity and selectivity of PTU-2 once again illustrated the importance of phosphazene in this bifunctional

polymerization,

catalyst

PTU-3

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catalyst. In order to investigate the effect of the catalyst configuration on the was

prepared

by

switching

the

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trans-diaminocyclohexane to cis structure, but the tacticity of the corresponding polymer was not improved. Meanwhile broad MWD (1.90) and low Mn (1400

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g/mol) were obtained, which indicated that serious chain transfer occurred during the ROP process (entry 3).16,39 To further increase the possibility of stereoselectivity, bifunctional

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phosphazene-urea catalysts PU-1, 2 were synthesized for ring-opening polymerization of rac-lactide under standard conditions. Using PU-1 as

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catalyst, higher Pm (0.72) and narrow MWD (1.14) was achieved (entry 4). Furthermore, when utilizing PU-2 as catalyst, the conversion reached 99% within 5 h, affording the PLAs with even higher Pm of 0.74 and narrow MWD of 1.12 (Table 1, entry 5). Doubling of the amount of catalyst, the reaction time was shorted to 1.5 h, and the MWD (1.15) was still narrow (entry 6). It is clear that the urea functional group in the bifunctional catalyst is critical to achieve

both high catalytic activity and selectivity. Exploring the influence of

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epimerization

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Figure 2. (a) Homonuclear decoupled 1H NMR spectrum of PDLA. (b) Plot of PLA Mn and PDI versus the molar ratio of monomer to initiator ([LA] 0/[BnOH]0) for the polymerization of rac-LA using PU-2. (c) Representative GPC traces of the polylactide prepared by PU-2. (d) Plot of monomer conversion versus reaction time for the polymerization of rac-LA using PU-2. (e) Semilogarithmic plot of ln([LA] 0/[LA]t) versus time for rac-LA polymerization. (f) Mn and PDI versus monomer conversion for PLA synthesized from PU-2 at room temperature. (d), (e), (f) are all for Table 1, entry 8.

side reaction on the tacticity of polymer, optically pure D-LA was used for PU-2 catalyzed polymerization (entry 7).40 A high Pm of 0.99 in 1

H

NMR

spectrum showed

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homodecoupled

that

epimerizations

were

well-suppressed (Figure 2a). What’s more, the low ratio (< 1 mol %) of

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[meso-LA]/([meso-LA] + [D-LA] + [PLAs]) was also calculated by the methyl peak at 1.72 ppm in 1H NMR spectrum (entry 7, Figure S32). Hence, when

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calculating the Pm value of the polymerization, the effect of epimerization was ignored.

Next, the ROP of rac-LA catalyzed by PU-2 at varied feeding

monomer/initiator ([LA]0/[BnOH]0) ratios (Table 1, entries 8-11) were investigated. A highest Pm value of 0.79 (based on Bernouillan statics,41 entry 9) was achieved with a ratio of 200:2.5:1 for [LA]0/[Cat.]0/[BnOH]0. The number-average molecular weights (Mns) of the PLAs showed a linearly trend

with the increasing monomer-to-initiator ratio ([LA]0/[BnOH]0) and the polydispersities (PDIs) were maintained in the range of 1.10–1.12 (Figure 2b), implying the highly controlled and “living” character of the ROP process.42 All the produced polymers showed sharp and unimodal GPC peaks (Figure 2c). Furthermore, a distinct first-order kinetic characteristic (kapp = 0.182 h-1) between the ln ([LA]0/[LA]t) and reaction time was attained (Figure 2d, e, SI, Table S2), also suggesting the high controlled nature of the phosphazene-urea catalytic system.43,44 Besides, a linear relationship [y = 242.1x-1102 (R2 =

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0.998)] between Mn and the conversion in conjunction with narrow PDIs indicative of a single site of controllable reaction (Figure 2f, Table S3).45 The MALDI-TOF of the produced PLAs showed the main series of peaks at m/z = 108 + 23 + 144*n, corresponding to the primary sequences of BnOH + Na +

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n*LA, which suggested a linear feature of the obtained polymers and

end-capped with benzyloxy group (SI, Figure S33). The well-defined end group

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fidelity was also proved by the 1H NMR spectrum of the resultant PLA (SI, Figure S24).

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Waymouth et al exploited the alkoxides and thioureas/ureas catalytic system for the living ring-opening polymerization of rac-LA.33,46,47 Mechanistic studies

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reveal that thiourea/urea anions are responsible for the high activity. In this paper, the co-catalytic systems consisting of tBu-P1 and TU/U were also investigated for isoselective ring-opening polymerization of rac-LA (Table 1,

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entries 12-13). For tBu-P1/U system, completely random polymer was obtained, while for tBu-P1 /TU system, isotactic polylactide with a Pm value of 0.68 was

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obtained (Figures S30, 31). These results highlighted that the stereoselectivity of bifunctional catalysts are much higher than co-catalyst systems (PTU/U-2 vs t

Bu-P1 + TU/U). In order to understand the isoselective mechanism for ring-opening

polymerization of rac-LA in this system, both Bernouillan statics41 (chain-end control mechanism) and non-Bernouillan statics48 (enantiomorphic site control mechanism) were used to calculate Pm values. In the homodecoupled 1H NMR

spectrum after deconvolution (Figure 3) of the sample (Table 1, entry 11), where Pm = 0.76 was calculated by the CEC mechanism and 0.83 by the ESC ppm

peak

ratio

5.22

rmr

0.0335

0.74

0.93

5.21

rmm

0.1053

0.70

0.70

5.18

mmr

0.0923

0.76

0.76

5.17

mmm 0.6897

0.78

0.85

5.16

mrm

0.84

0.91

0.76

0.83

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0.0792

Pm(CEC) Pm(ESC)

Figure 3. Homodecoupled 1H NMR spectrum after deconvolution of polylactide (Table 1, entry 11)

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mechanism. According to the CEC mechanism, the calculation results of [mmr] = [rmm] ≠ [rmr] should be obtained. While in the ESC mechanism, the tetrad

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ratios should be [mmr] = [rmm] = [rmr] = [mrm]/2. The experimental tetrad ratios were calculated: [mmr] = 0.092, [rmm] = 0.105 and [rmr] = 0.034 by

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MestreNova software after deconvolution of the peaks, which was close to 1/1/0.3. The experimental data suggested chain-end control might be the most likely stereo-control mechanism. This conclusion was further confirmed by the

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fact that the Pm values keep constant at different conversions during the polymerization process (Pm = 0.72-0.77, SI, Table S3).41

All

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In addition, more challenging monomers (δ-VL, ε-CL) were also explored. the

polymerizations

were

performed

in

1

M

toluene

with

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[M]0/[Cat.]0/[BnOH]0 = 50/1/1 at 50 oC. Delightedly, both the PTU-2 and PU-2 effectively promoted the ROP of δ-VL and ε-CL with high conversion (Table 1, entries 14-18). Using PTU-2 as catalyst, ROP of δ-VL smoothly proceeded and reached a 98% monomer conversion in 18 h to give PVLs with a molecular weight of 2700 g/mol and PDI of 1.14 (Table 1, entry 14). Under the same reaction conditions, PU-2 required longer time to achieve high conversion (entry 15). Higher catalytic activity of PTU-2, comparing with PU-2

counterpart, might be associated with the stronger hydrogen binding of thiourea to the carbonyl of δ-VL.31,32,46 Increasing the amount of monomer, a high conversion was still obtained (entry 16). Employ of PTU-2 as catalyst, ROP of ε-CL was achieved within 87 h, yielding the PCLs with narrow MWD of 1.16 and controlled Mn of 2800 g/mol (entry 17), which suggested that the polymerization efficiency of ε-CL was considerably lower than δ-VL in this catalytic system.38 All the results strongly outlined the PTU/PU catalysts

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exhibited broad monomer scopes.

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Figure 4. 1H NMR spectra of (1) BnOH, (2) PU-2, (3) PU-2/BnOH = 1:1 and (4) t Bu-P1/BnOH = 1:1 at room temperature in THF-d8.

To explore the polymerization mechanism, a comparative

1

H NMR

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measurement was performed (Figure 4). The 1H NMR spectrum of PU-2 (2, Figure 4) revealed that the N-H proton of urea formed strong hydrogen bond

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with the phosphazene, which was verified by the high chemical shift (δ = 10.6) of peak (f). On addition of BnOH to the PU-2 (3, Figure 4), the chemical shifts of PU-1 and BnOH were not significantly changed. While the shape of CH2 peak (b) of PhCH2OH changed from doublet to singlet, which indicated that hydrogen bond interactions between the PU-1 and BnOH might be formed. In contrast, when BnOH was mixed with superbase tBu-P4, the CH2 peak (b) of

PhCH2OH shifted to downfield region, due to the complete deprotonation of BnOH. Based on the above data, a bifunctional ROP mechanism of lactones has been proposed as shown in Scheme 1. The bifunctional organocatalysts promote the polymerization of lactones, through hydrogen bonds of both the monomer and initiator. Further chain propagations proceed as similar transition

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state, affording the polyesters with good reactivity and selectivity.

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Scheme 1. Proposed mechanism for ROP of cyclic esters using bifunctional phosphazene-thiourea/urea catalysts.

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CONCLUSIONS

In summary, phosphazene-thiourea/urea catalysts were proved to be efficient bifunctional organocatalysts for ROP of cyclic esters including rac-lactide,

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δ-valerolactone and ε-caprolactone. In these catalysts, the presence of thioureas/ureas and stronger phosphazene bases facilitates controllable

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polyesters synthesis with well-defined structures, narrow polydispersities, high reactivities and selectivities. Using phosphazene-urea PU-2 as catalyst, isotactic polylactides were obtained with high Pm value of 0.79 and fast polymerization under mild conditions. Compared with other phosphazene bases (BEMP, tBu-P2 or CTPB), PU-2 has the highest stereoselectivity at room temperature and much higher catalytic activity than tBu-P1 or BEMP. Importantly, these catalysts show unique advantages such as versatile, broad applicability and easy

handling, low toxicity, relying on simple and inexpensive starting materials. Further investigations of different types of monomers are in progress in our laboratory. APPENDIX A. SUPPLEMENTARY DATA Supplementary data to this article can be found online from Elsevier. Details of bifunctional phosphazene-thiourea/urea catalysts preparation steps and spectral characterizations; polymerization experiments; additional polymerization data; polymer NMR

Conflicts of interest The authors declare no competing financial interest.

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Acknowledgements

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spectrum, MALDI-TOF mass spectrum and kinetic studies (PDF).

We gratefully acknowledge the generous support by the National Key R&D Plan

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(2017YFC1104800), Young Taishan Scholars Program of Shandong Province (tsqn201812112), “135” Projects Fund of CAS-QIBEBT Director Innovation

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Foundation and DICP& QIBEBT United Foundation (UN201701).

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