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Preparation of amphiphilic poly(ethylene glycol)-b-poly(#butyrolactone) diblock copolymer via ring opening polymerization catalyzed by a cyclic trimeric phosphazene base or alkali alkoxide Yong Shen, jinbo Zhang, Zhichao Zhao, Na Zhao, Fusheng Liu, and Zhibo Li Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.8b01239 • Publication Date (Web): 05 Nov 2018 Downloaded from http://pubs.acs.org on November 7, 2018

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Biomacromolecules

Preparation of amphiphilic poly(ethylene glycol)-bpoly(γ-butyrolactone) diblock copolymer via ring opening polymerization catalyzed by a cyclic trimeric phosphazene base or alkali alkoxide Yong Shen, [a],‡ Jinbo Zhang,[b],‡ Zhichao Zhao, [a]Na Zhao, [b]Fusheng Liu,[a] and Zhibo Li*, [b] [a] College

of Chemical Engineering, Qingdao University of Science and Technology, Qingdao 266042, China.

[b] Key

Laboratory of Biobased Polymer Materials, Shandong Provincial Education

Department; College of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China.

ABSTRACT Bio-based poly(γ-butyrolactone) (PγBL) as a fully biodegradable and bioabsorbable biomaterials has shown superior properties compared to other aliphatic polyesters. It is of great importance to prepare amphiphilic block copolymer containing PγBL block in order to make ordered nano-objects for biomedical applications such as drug delivery system. However, such amphiphilic copolymer containing PγBL segment was never successfully prepared mostly due to the synthetic challenges of ring-opening polymerization (ROP) of nonstrained -butyrolactone (BL) monomer. Here we reported the first preparation of amphiphilic 1 ACS Paragon Plus Environment

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poly(ethylene glycol)-b-poly(γ-butyrolactone) (PEG-b-PγBL) diblock copolymer by using PEG as a macroinitiator. We applied two types of bases to initiate the ROP of γBL. An organic cyclic trimeric phosphazene base (CTPB) was firstly applied to activate the terminal hydroxyl group of PEG as macroinitiator for ROP of γBL. On the other hand, sodium hydride was used to activate hydroxyl group of PEG to form sodium alkoxide as initiating system for ROP of γBL. Both catalytic/initiating system showed moderate control on ROP of γBL and successfully produced PEG-b-PγBL diblock copolymers with varied molecular weights and relatively narrow molecular weight distributions. The effects of catalytic systems, activation temperatures, monomer concentrations on γBL conversion and molecular weight of PEG-b-PγBL were carefully explored. The thermal properties and phase behaviors of obtained PEG-b-PγBL were also investigated.

Introduction Biodegradable amphiphilic polymers have drawn increased attention and got extensive applications in drug delivery, gene transfection, tissue repairing, etc, owing to their inherent abilities to self-assemble into micelles, vesicles, polymersomes and hydrogels.1-5 Aliphatic polyesters as hydrophobic segments are of particular importance because of their potential biocompatibility and biodegradability. In particular, poly(lactide) (PLA), poly(ε-caprolactone) (PCL), poly(lactic-co-glycolic acid) (PLGA) and poly(3-hydroxybutyrate) (P3HB) have been widely investigated as hydrophobic components in combination with poly(ethylene glycol) (PEG) as hydrophilic component.6-11 PEG-b-PLA and PEG-b-PCL are two most prominent amphiphilic copolymers for constructing drug delivery system, owing to their excellent physicochemical and biological properties.12-14 For example, Shi et al. synthesized a docetaxel (DTX)-conjugated PEG-b-PLA micelle with an 2 ACS Paragon Plus Environment

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average size of 58.2 ± 2.3 nm. This DTX-loaded PEG-b-PLA micelle significantly induced apoptosis of HSC-3 cancer cells and effectively suppressed the tumor progression in the HSC-3 xenograft model in vivo.15 Notably, the PEG-b-PLA-based anticancer drugs such as Genexol-PM (paclitaxel formulation) have been approved for clinical trials for the treatment of breast, lung, and ovarian cancers.16,

17

PEG-b-PCL micelles were also successfully used for paclitaxel

encapsulation with high anticancer activity both in vitro and in vivo.18 However, the current amphiphilic polyester systems still suffer from unsuitable degradation rates in vivo and undesirable acidic degradation product release, which limit their applications in biomedical technology. It is therefore of great significance to develop new types of amphiphilic block copolymers containing biodegradable polyesters, which can meet a great combination of biodegradation rate, biocompatibility as well as bioabsorbability. As an important downstream chemical of succinic acid, γBL is a biomass derived monomer and has been considered as one promising replacement of petroleum derived chemicals for the preparation of aliphatic polyesters.19 Moreover, the bacterial fermentation product of γBL, which is generally named as poly(4-hydroxybutyrate) (P4HB), exhibits suitable degradation rate and will not lead to undesirable highly acidic degradation product accumulation in vivo.20 These properties make PγBL, the chemical equivalent of P4HB, a promising polymer as hydrophobic segment for biomedical applications.21 However, γBL as a monomer is ignored for a long time mainly owing to its non-strained five-membered ring and related unfavorable thermodynamics involved in its ROP process.22,

23

The synthetic challenge of high molecular weight PγBL at

ambient pressure was accomplished by Hong and Chen with using La[N(SiMe3)2]3 or yttrium amide as catalysts at low temperature in 2016.24 The commercial available organophosphazene superbase tert-Bu-P4 and a new cyclic trimeric phosphazene superbase (CTPB) developed by our

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group also showed high catalytic activity toward the ROP of γBL.25,

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26

Most recently, N-

heterocyclic olefins in combination with lithium salts were also investigated as catalysts for the ROP of γBL.27 These pioneered studies provide further opportunities to make amphiphilic polyesters from the non-strained γBL. Although the random copolymerization of γBL with other monomers has been achieved decades ago,28-32 the synthesis of block copolymer containing PγBL as one block still remains as a big challenge considering the necessary high γBL concentration, low polymerization temperature, inhomogeneous polymerization system and high propensity of transesterification in the sequential copolymerization of γBL with other monomers. The only reported example was the preparation of PγBL-b-PLLA via sequential ROP of γBL and L-LA.32 However, the previously reported hydrophobic (co)polyesters from γBL have poor water solubility and are not suitable as drug delivery carriers. Moreover, the reported strategy was not suitable to prepare PEG-b-PBL diblock copolymers. In this contribution, we reported the successful preparation of amphiphilic PEG-b-PγBL diblock copolymer from biorenewable γBL using PEG as a macroinitiator. Two initiation systems were developed, both of which allowed preparation of PEG-b-PγBL with tunable molecular weights and relatively narrow molecular weight distributions after simple precipitation procedure. Furthermore, the thermal properties and phase behaviors of obtained PEG-b-PγBL were also studied with thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Experimental Section Preparation of PEG sodium


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Poly(ethylene glycol) monomethyl ether (mPEG2k: average Mn~2000 Da, mPEG5k: average Mn~5000 Da) and poly(ethylene glycol) (PEG2k: average Mn~2000 Da) were first dried by azeotropic removal of water with toluene. The typical procedure for the preparation of mPEG2k sodium (mPEG2k-ONa) was described as followed. (25 mmol, 50 g) mPEG2k and (37.5 mmol, 0.9 g) NaH were dissolved in 250 mL anhydrous tetrahydrofuran (THF). The solution was refluxed under N2 for 3 days and then diluted with another 250 mL THF. The reaction mixtures were filtered and the filtrate was concentrated under reduced pressure to give mPEG2k-ONa as white powder. The obtained PEG sodium was used directly without further purification and characterization. Initiator precursors of mPEG5k sodium (mPEG5k-ONa) and PEG2k sodium (NaO-PEG2k-ONa) were prepared following the same procedure. General polymerization procedures A typical polymerization procedure was described as followed. A 25 mL flame-dried Schlenk tube was charged with (0.075 mmol, 150 mg) mPEG2k and (0.05 mmol, 60 mg) CTPB. The Schlenk tube was then immersed into a cooling bath setting at -40 C and 0.6 mL dichloromethane (DCM) was then added using a gastight syringe. The reaction mixtures kept stirring at -40 C for 30 min and then cooled to -50 C. (7.5 mmol, 0.57 mL) γBL was then injected into the Schlenk tube to start the polymerization. The polymerization was conducted at 50 C under N2 atmosphere for 4 hours before quenched by addition of a few drops of acetic anhydride. 3 mL DCM was then added into the Schlenk tube to dissolve the polymer. An aliquot of reaction solution was withdrawn and used for 1H NMR measurement to determine monomer conversion. The remaining mixtures were precipitated into cold methanol. The precipitate was then dissolved with minimum DCM and precipitated into methanol twice. The collected precipitate was then dried under vacuum at room temperature to give mPEG2k-b-PBL. 5 ACS Paragon Plus Environment


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When PEG sodium was used as catalyst/initiator, the procedure was similar with the description above except that no additional catalyst was added and a mixture of DCM and THF with VDCM/VTHF = 2/1 was used as solvent. mPEG2k-b-PBL: 1H-NMR (CDCl3, 500 MHz):  (ppm): 4.11 (t, J = 6.5 Hz, -C(=O)OCH2-), 3.64 (s, -OCH2CH2O-), 3.37 (s, -OCH3), 2.39 (t, J=7.5 Hz, -C(=O)CH2-), 1.96 (m, -CH2-).

13C

NMR (CDCl3, 125.7MHz):  (ppm): 172.7, 70.6, 63.6, 30.7, 24.0. mPEG5k-b-PBL and PBLb-PEG2k-b-PBL gave similar 1H-NMR and 13C NMR spectra.

Results and discussion Although it was known for a long time that the ROP of BL was difficult due to its nonstrained five-membered ring, most recent research demonstrated that suitable catalysts such as rare earth metal, organophosphazene superbase and N-heterocyclic olefins in combination with high monomer concentration and low polymerization temperature can resolve the challenge to obtain PγBL with relatively high molecular weight.24-27 Moreover, the nature of catalysts and solvents played critical roles on the ROP of γBL. Most recently, we reported a novel cyclic trimeric organophosphazene superbase (CTPB), which has a pKa about 33.3 in acetonitrile determined by NMR. It has good solubility in organic solvent even at low temperature given its organic compound nature and showed high catalytic activity toward the ROP of γBL.26 Moreover, CTPB has relatively lower pKa than that of tert-Bu-P4 (pKa = 42.7), and shows better control toward the ROP of γBL. Hence, we firstly used CTPB as superbase to activate the hydroxyl group of mPEG to form a complex to initiate ROP of BL. To prepare PEG-b-PBL diblock copolymer, the first requirement is that the ROP of BL needs to be done at low temperature and high monomer concentration. Considering this issue, previously used toluene cannot be used here since PEG cannot be dissolved in toluene at low temperature. Other solvents 6 ACS Paragon Plus Environment

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such as THF, acetonitrile and N, N-dimethylformamide did not work well due to solubility problem for PEG, either. Accordingly, DCM was chosen as solvent. Another concern is that CTPB may react with DCM given the superbase nature of CTPB. Before polymerization, we dissolved CTPB in DCM and found that the solvent became yellow slowly. It was supposed that DCM reacted with CTPB to give carbene and CTPB-H+. The presence of CTPB-H+ was demonstrated by the collaborative analyses of

1H

NMR and matrix-assisted laser

desorption/ionization time-of-flight mass spectroscopy (MALDI-TOF MS) (Figure S1 and S2). In contrast, it was reported that tert-Bu-P4 decomposed rapidly in DCM probably due to higher pKa than CTPB.25 Meanwhile, if we mixed them at low temperature such as below -30 C, there is no noticeable change for a rather long time, which suggested that such side reaction can be suppressed at low temperature. On the other hand, CTPB would prefer to react with hydroxyl groups in the presence of PEG. Considering that DCM is large excess compared to hydroxyl groups, we decided to mix the CTPB and PEG with DCM at low temperature to suppress the undesired reaction between CTPB and DCM. Because PEG dissolves very slowly in DCM at low temperature, the activation of hydroxyl groups might not be complete. To balance these two aspects, different reaction conditions were thus investigated here to find suitable conditions.



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Scheme 1. Illustration of preparation of PEG-b-PγBL with (a) PEG as initiator and CTPB as catalyst and (b) mPEG sodium as initiator. As illustrated in Scheme 1a, mPEG2k was firstly mixed with CTPB in DCM at 10 C and stirred for 30 min to ensure the activation of terminal hydroxyl group of mPEG2k before addition of γBL. The reaction mixtures were cooled down and polymerization then conducted at -50 C for 4 h to give a 5% γBL conversion (Table 1, run 1). This value is much lower than that obtained in previous study, in which benzyl alcohol was used as initiator and toluene as solvent.26 This reason is probably ascribed to the possible side reaction between CTPB and DCM, which resulted in the low activation efficiency of mPEG2k. Then, mPEG2k was activated with CTPB at even lower temperature for 30 min to suppress the possible reaction between CTPB and DCM. The γBL conversion increased to 16 % and 52 % when mPEG2k was activated at -30 C and -40 C, respectively (Table 1, run 2 and 3). The molecular weight of obtained mPEG2k-b-PγBL also increased to 6.4 kDa and 12.9 kDa, respectively, as the γBL conversion increased. Note that mPEG2k cannot completely dissolve in DCM at -50 C even with an


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extended period of 3 h. As a consequence, -40 C was chosen as the activation temperature for mPEG2k in the remaining experiments. The ROP of γBL with mPEG2k as macroinitiator was further investigated under different reaction conditions. The γBL conversion remained unchanged and the obtained mPEG2k-bPγBL had comparable molecular weight even with an extended polymerization time of 12 h (Table 1, run 4). The γBL conversion dramatically decreased when the polymerization was preformed either with a lower monomer concentration (4 mol L-1, Table 1, run 5) or at higher temperature (-40 C, Table 1, run 6). Increasing the molar ratio of γBL/mPEG2k from 90/3 to 300/3 led to a gradual increase of molecular weight of obtained mPEG2k-b-PγBL from 5.9 kDa to 12.9 kDa (Table 1, run 3, 7, 8). However, further increasing the molar ratio of γBL/mPEG2k resulted in both decrease of γBL conversion and diblock copolymer molecular weight (Table 1, run 9-11). The trace impurities existed in the excess γBL may react with CTPB and resulted in the decreased catalytic activity and decreased γBL conversion. In addition to organic superbase, some common inorganic bases, including KOMe and NaH, were also investigated in this study. Overall, the ROP of γBL with KOMe and NaH as catalysts gave lower monomer conversions and broader molecular weight distribution compared to that with CTPB (Table 1, run 12 and 13). Besides, KOMe and NaH may be not suitable as catalysts to prepared block copolymers due to the possible occurrence of initiation pathways through methanolate or monomer deprotonation.25 Note that only low to moderate γBL conversions were obtained at different reaction conditions. We calculate the equilibrium monomer concentration provided that the polymerization system reaches thermodynamic equilibrium to give the readers an idea. According to the equation ln[γBL]eq = ∆Hp/RT - ∆Sp/R with using the reported thermodynamic parameters by Hong and 9 ACS Paragon Plus Environment


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Table 1. Results of ROP of γBL with PEG/base systems. a

Run

Base

Activation Temp (C) b

1

CTPB

10

300/2/3

2

CTPB

-30

3

CTPB

4e

γBL/C/

[γBL]

C(γBL)

PEG2k

(mol L-1)

(%) c

Mn (kDa) d

Ðd

Yield (%)

6

5

n.d.

n.d.

n.d.

300/2/3

6

16

6.4

1.66 7

-40

300/2/3

6

52

12.9

1.89 16

CTPB

-40

300/2/3

6

48

13.1

1.50 19

5

CTPB

-40

300/2/3

4

9

n.d.

n.d.

n.d.

6f

CTPB

-40

300/2/3

6

8

n.d.

n.d.

n.d.

7

CTPB

-40

90/2/3

6

41

5.9

2.41 11

8

CTPB

-40

150/2/3

6

60

8.6

1.87 19

9

CTPB

-40

450/2/3

6

33

7.0

2.29 14

10

CTPB

-40

600/2/3

6

27

7.0

1.69 9

11e

CTPB

-40

600/2/3

6

32

9.3

1.59 21

12g

KOMe

-40

300/2/3

6

57

5.5

2.66 22

13g

NaH

-40

300/2/3

6

38

6.1

1.88 h

17

a

Conditions: initiator: mPEG2k (0.075mmol, 150 mg), solvent: DCM, polymerization temperature: -50 C, time: 4h. b mPEG2k, CTPB and DCM were mixed and stirred at this temperature; c Determined by 1H NMR spectroscopy. d Determined from the isolated samples by SEC in THF with polystyrene as references. e The polymerization time was 12 h. f The polymerization temperature was -40 C. g VDCM/VTHF = 2/1 was used as solvent. h Bimodal distribution.

Chen (∆Hp = -5.4 kJ mol−1 and ∆Sp = -39.6 J mol−1 K−1),24 the equilibrium monomer concentration at -40 C and -50 C is calculated to be 7.22 mol L-1 and 6.38 mol L-1, respectively. Considering the initial monomer concentration we used is 6 mol L-1, it seems that


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the polymerization cannot happen. Actually, the generated mPEG-b-PBL with relative high molecular weights precipitated out of solution with proceeding of polymerization and the system became inhomogeneous. As a consequence, the propagation/depropagation equilibrium is broken and shifts towards polymerization. Though the achievable conversion cannot be calculated base on thermodynamic equilibrium herein, the calculation clarifies why γBL conversion is still moderate with an extended time of 12 h. The obtained mPEG2k-b-PγBL copolymers were further analyzed with the collaboration of SEC, NMR and MALDI-TOF MS. Right after ROP of BL, an aliquot of sample solution was withdrawn from the reaction mixtures for SEC characterization (Table 1, run 3). It was found that the crude products displayed rather complicated SEC traces with an apparent trimodal distribution, indicating mixed products containing three polymer species. Furthermore, the SEC trace can be deconvoluted into four peaks (Figure S3). These four peaks were tentatively assigned as high molecular weight mPEG2k-b-PγBL, low molecular weight mPEG2k-b-PγBL, mPEG2k and cyclic PγBL homopolymer in the order of retention time. The percentage of mPEG2k and cyclic PγBL homopolymer was calculated to be about 13.4 % and 6.2 % from their integral areas, respectively. As mentioned above, with proceeding of the polymerization, the formed mPEG2k-b-PγBL diblock copolymers with relative high molecular weights precipitated out of solution and the reaction mixtures became a jellylike system. The inhomogeneous reaction system may result in the insufficient proton transfer of activated terminal hydroxyl group of mPEG and chain termination. The occurrence of intramolecular transesterification (vide infra) led to the formation of cyclic PγBL homopolymers. Besides, intermolecular transesterification also accounted for the broad distribution of obtained products. Accordingly, the complex compositions in the reaction mixtures were attributed to the insufficient proton transfer, 11 ACS Paragon Plus Environment


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inhomogeneous reaction system (jellylike system) and concomitant occurrence of intermolecular and intramolecular transesterification. mPEG2k can be easily removed by precipitating the reaction mixtures into methanol owing to its distinct solubility difference with mPEG2k-b-PγBL diblock copolymer. After two cycles of precipitation, the sample displays a unimodal SEC trace with a little tailing, indicating that the remaining components are mainly mPEG2k-b-PγBL with a small amount of cyclic PγBL homopolymers (Figure 1c).

Figure 1. SEC traces of (a) mPEG2k, (b) product obtained before precipitation and (c) mPEG2kb-PγBL (Table 1, run 3) obtained after precipitation. Figure S4 gives a representative 1H NMR spectrum of crude product before precipitation, suggesting the coexistence of unreacted mPEG, unreacted γBL and mPEG2k-b-PγBL diblock copolymer. Figure 2 gives the corresponding 1H NMR spectrum of purified mPEG2k-b-PγBL diblock copolymer, which shows all characteristic peaks of mPEG and PγBL with corresponding integral area ratios. This 1H-NMR result demonstrates that the unreacted mPEG, unreacted γBL and other byproducts can be effectively removed via simple precipitation, which is consistent with above SEC result. The

13C

NMR spectrum given in Figure 3 also agrees well with the

chemical structure of mPEG2k-b-PγBL. The MALDI-TOF mass spectrum of mPEG2k-b-PγBL 12 ACS Paragon Plus Environment

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in Figure S5 displays two groups of polymer signals. End group analyses show a good agreement with the cyclic PγBL structure for the polymer population from 2000 Da to 4500 Da. Although the resolution in the high molecular weight region is not high enough to show the distinct isotopic signals, the polymer population from 4500 Da to 8000 Da is assigned as linear mPEG2k-b-PγBL based on the presence of 44 Da and 86 Da intervals between neighboring peaks. The analysis of end group of linear mPEG2k-b-PγBL is thus not given due to the unsatisfied signal resolution. Note that the higher intensity of cyclic PγBL homopolymer is probably due to their smaller molecular weight and higher ionization propensity rather than indicating their actual percentage. No signals of mPEG2k were observed from the MALDI-TOF mass spectrum, indicating the almost complete removal of mPEG2k from the isolated product. The coexistence of cyclic PγBL homopolymer and linear mPEG2k-b-PγBL agrees well with the above SEC results.

Figure 2. Representative 1H NMR spectrum of purified mPEG2k-b-PγBL measured in CDCl3 (Table 1, run 3).



Figure 3. Representative

13C

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NMR spectrum of purified mPEG2k-b-PγBL measured in CDCl3

(Table 1, run 3). A series of mPEG2k-b-PγBL diblock copolymer was successfully prepared with CTPB as catalyst at low temperature. By varying the monomer/PEG ratio, the molecular weight of PBL block can be changed albeit without great control. However, the possible incomplete activation of terminal hydroxyl group of PEG and side reaction of CTPB with DCM led to low initiation efficiency and complicate products. On the other hand, it was known that sodium or potassium alkoxide can also be used as initiation system for ROP of BL albeit without good control.25 Hence, we considered using alkali metal or alkali hydride to deprotonate the hydroxyl group of PEG to form alkali alkoxide as initiation system to initiate ROP of BL at low temperature. Again, the challenge here is to find a suitable solvent with necessary solubility for activated PEG at appropriate temperature. As a proof of concept, we used NaH to react with PEG to produce PEG sodium, which was used directly as initiator/catalyst without the presence of additional catalyst in the following polymerization. Firstly, mPEG2k-ONa powder was dissolved in a mixture of DCM and THF at different temperatures to investigate whether PEG sodium will


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react with DCM. The polymerization achieved a γBL conversion of 50 %, producing mPEG2kb-PγBL with molecular weight of 12.3 kDa when mPEG2k-ONa was dissolved in DCM/THF at -50 C (Table 2, run 1). Comparable γBL conversion and molecular weight of mPEG2k-b-PγBL were obtained even if mPEG2k-ONa was stirred in DCM/THF at 25 C for 30 min before addition of γBL (Table 2, run 2, Figure S6). It seems PEG sodium does not react with DCM or at least at a negligible reaction rate based on the above results. An aliquot of reaction mixtures was withdrawn and analyzed with SEC. The SEC trace shown in Figure 4b suggests a lower percentage of residual mPEG2k in the products compared to that with CTPB as catalyst. The percentage of mPEG2k-b-PγBL, mPEG2k and cyclic PγBL homopolymer was calculated to be 82.5%, 8.2% and 9.3% through deconvolution, respectively (Figure S7). By simply precipitating the reaction mixtures into methanol, mPEG2k-b-PγBL was obtained and displayed a unimodal distribution with a small shoulder in the low molecular weight region (Figure 4c). MALDI-TOF mass spectrum shown in Figure S8 indicates the coexistence of cyclic PγBL homopolymer and linear mPEG2k-b-PγBL. Relative lower cyclic PγBL homopolymer signals suggests the less transesterification and better control during polymerization with using PEG sodium as initiator/catalyst compared to that with using CTPB as catalyst. It was reported that base/urea system exhibited high catalytic activity and good control toward the ROP of cyclic lactones or lactides, such as L-LA, ε-CL and -valerolactone.33,

34

Two

different ureas, i.e., 1-(4-chlorophenyl)-3-phenylurea and 1-cyclohexyl-3-phenylurea were thus used as co-catalysts in combination with PEG sodium for the ROP of γBL. The results were summarized in Table S1. The ROP of γBL with ureas as co-catalysts achieved better control (lower dispersity, Ð = 1.19 ~ 1.27) as expected, however, accompanying with lower γBL conversion and lower molecular weight of mPEG2k-b-PγBL (Table 2, run 2 vs Table S1, run 2). 15 ACS Paragon Plus Environment


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The molecular weight of mPEG2k-b-PγBL can be varied by changing the feeding molar ratio of γBL/mPEG2k although not in a linear mode (Table 2, run 2 to 4). mPEG5k-ONa and NaOPEG2k-ONa was also used as initiator to prepared mPEG5k-b-PγBL with varied hydrophilic block length (Table 2, run 5) and PγBL-b-PEG2k-b-PγBL triblock copolymer (Table 2, run 6), respectively. Overall, the ROP using PEG sodium as initiator/catalyst shows better control compared to that with CTPB as catalyst, which is presumably attributed to the improved stability of PEG sodium in DCM and solvating effect of PEG towards sodium cation.

Table 2. Results of ROP of γBL with PEG sodium as initiator. a Dissolving C(γBL) Run Initiator γBL/I b (%) c Temp( C)

Mn (kDa) d

Ðd

Yield (%)

1

mPEG2k-ONa

-50

100/1

50

12.3

1.48

19.6

2

mPEG2k-ONa

25

100/1

56

12.8

1.44 21.4

3

mPEG2k-ONa

25

50/1

46

8.2

1.78 21.2

4

mPEG2k-ONa

25

200/1

34

6.5

2.07 18.2

5

mPEG5k-ONa

25

100/1

44

5.8

2.25 12.4

6

NaO-PEG2kONa

25

100/1

38

7.1

1.30 28.2

a

Conditions: solvent: VDCM/VTHF = 2/1, [γBL] = 6 mol L-1, polymerization temperature: -50 C, time: 4h. b PEG sodium was dissolved in DCM/THF at this temperature; c Determined by 1H NMR spectroscopy. d Determined from the isolated samples by SEC in THF with polystyrene as references. A series of mPEG2k-b-PγBL samples with varied molecular weight was then subjected to TGA and DSC analyses. Table S2 summarizes the temperatures at 5% weight loss (Td, maximum rates of decomposition (Td,

max).

5%)

and

All samples display a two-step decomposition

profile, where the PγBL block decomposes in the first stage followed by the decomposition of PEG block (Figure 5). The assignment is based on the decomposition temperature of mPEG2k 16 ACS Paragon Plus Environment

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and PBL homopolymer24, 25, 32 as well as the relative content of each block in the mPEG2k-bPBL block copolymers. The molecular weight plays a key role on the thermal stability of mPEG2k-b-PγBL, especially in the low molecular weight region. For example, as the molecular weight of mPEG2k-b-PγBL increases from 6.5 kDa to 8.6 kDa to 13.7 kDa, an increase of Td, 5% from 234 C to 253 C to 268 C is observed.

Figure 4. SEC traces of (a) mPEG2k, (b) products obtained before precipitation and (c) mPEG2k-b-PγBL (Table 2, run 2) obtained after precipitation. Table S3 summarizes the DSC results of mPEG2k-b-PγBL samples, which display glass transition temperatures (Tg) in the range from -56.4 C to -49.3 C. These values are slightly lower than the Tg of PγBL homopolymer reported before.24, 32 As seen in Figure S9, all three samples give a minor melting transition in a range from 13.8 C to 20.6 C and a major melting transition at around 56 C. The former is possibly due to crystallization of PEG component while the latter is attributed to PγBL component. The small shoulder peak around 44 C is assigned as the melting transition of cyclic PγBL homopolymer.24 The Tm of PγBL block is comparable with that of PγBL homopolymer, indicating that the existence of mPEG component has little effect on 17 ACS Paragon Plus Environment


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the crystallization behavior of PγBL block. On the other hand, the Tm of possible mPEG block is much lower (> 30 C) than that of mPEG homopolymer, but the underlying reaction is not clear. Moreover, the fusion enthalpy (∆Hm) of mPEG block, normalized by mPEG mass fraction (F), shows a significant decrease compared to that of mPEG homopolymer. For example, S1 with FmPEG = 14.6 % has a ∆Hm, mPEG = 5.3 J/g. After normalization, the value is 36.3 J/g and shows 81 % reduction compared to that of mPEG homopolymer. Both results suggest that the presence of PγBL crystals imposes spatial confinement on the mPEG crystallization and results in imperfect and smaller size of the crystal stacks of the mPEG block. Two crystallization peaks were observed from the cooling scan for S3, which has the highest percentage of mPEG component. The minor one around -24 C and major one around 24 C is attributed to the crystallization of mPEG block and PγBL block, respectively. Akin to the melting transition temperature, the crystallization temperature (Tc) of PγBL block is comparable with that of PγBL homopolymer whereas Tc of mPEG block is much lower than that of mPEG homopolymer. The crystallization enthalpy (∆Hc1) of mPEG block displays significant reduction compared to that of mPEG homopolymer, indicating PγBL block suppresses the crystallization of mPEG block. A control DSC experiment in which mPEG2k and PγBL homopolymers were mixing at similar composition to each block of S3 was further performed to validate the phase transition behavior of mPEG2k-b-PγBL. As seen in Figure S9, the blends of mPEG2k and PγBL homopolymers display distinct different DSC curve with that of mPEG2k-b-PγBL, further suggesting the successful preparation of block copolymers. The self-assemble behaviors of obtained amphiphilic mPEG2k-b-PγBL diblock copolymers were preliminarily investigated with dynamic light scattering (DLS) and transmission electron microscope (TEM). Figure S10 gives the DLS results of mPEG2k-b-PγBL with a molecular 18 ACS Paragon Plus Environment

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weight of 4.5 kDa (Table S1, run 1) at a concentration of 1 mg/mL and 5 mg/mL in water. The average size of assembles increases from 74 nm to 135 nm as the concentration increases from 1 mg/mL to 5 mg/mL. TEM images shown in Figure S11 suggest mPEG2k-b-PγBL can selfassemble to micelles and vesicles in water. The effects of compositions and block lengths of mPEG2k-b-PγBL diblock copolymers on their self-assemble behaviors are still in progress.

Figure 5. (a) TGA and (b) DTG curves of mPEG2k-b-PγBL and mPEG2k. S1: 13.7 kDa, Ð = 1.34, S2: 8.6 kDa, Ð = 1.87, S3: 6.5 kDa, Ð = 1.19. Conclusion 19 ACS Paragon Plus Environment


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Although there is still a long way to achieve control ROP of BL due to the unfavorable thermodynamics, inhomogenous polymerization system and undesirable transesterification reaction, a series of amphiphilic PEG-b-PγBL block copolymers was prepared by choosing suitable polymerization conditions via a macroinitiator strategy. A suitable activation temperature was crucial to obtain moderate to high γBL conversion and satisfactory molecular weight of PEG-b-PγBL with CTPB as catalyst. The polymerization with PEG sodium as initiator/catalyst showed better control compared to that with CTPB as catalyst. Although the low initiation efficiency and possible reaction between catalysts and DCM led to complicate polymerization products, linear PEG-b-PγBL as main product can be obtained through simple precipitation. TGA results indicated that PEG-b-PγBL displayed improved stability with increasing the molecular weight of PγBL block. The presence of PγBL block significantly suppressed the crystallization of PEG block as suggested by DSC studies. The preliminary study indicated that PEG-b-PγBL can self-assemble to micelles and vesicles in water, which make them promising candidates as drug delivery carriers. Given the advantages of PγBL, such as suitable degradation rate and biocompatible degradation products, the amphiphilic PEG-b-PγBL is expected to exhibit superior performances compared to the existed amphiphilic aliphatic polyesters in biomedical applications. Supporting Information. The following files are available free of charge. Experimental details; TGA and DSC data; 1HNMR, 13C NMR and MALDI-TOF spectra; GPC curves; DLS results and TEM images. AUTHOR INFORMATION Corresponding Author 20 ACS Paragon Plus Environment

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* E-mail:[email protected] Author Contributions ‡These authors contributed equally. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors appreciate financial support from Major Program of Shandong Province Natural Science Foundation (ZR2017ZB0105), the Department of Science and Technology of Shandong Province (No. 2015GGX103037), and NSFC (No. 21434008 ， No. 21801151). Shen Y. appreciates financial support by Shandong Provincial Natural Science Foundation, China (ZR2018BEM029), China Postdoctoral Science Foundation (2016M600526) and the support from the Eco-chemical Engineering Cooperative Innovation Center of Shandong, Qingdao University of Science and Technology. REFERENCES 1. Motala-Timol, S.; Jhurry, D.; Zhou, J. W.; Bhaw-Luximon, A.; Mohun, G.; Ritter, H. Amphiphilic poly(L-lysine-b-caprolactone) block copolymers: Synthesis, characterization, and solution properties. Macromolecules 2008, 41, 5571-5576. 2. Zhu, Z. X. Effects of amphiphilic diblock copolymer on drug nanoparticle formation and stability. Biomaterials 2013, 34, 10238-10248. 3. Chang, L. L.; Liu, J. J.; Zhang, J. H.; Deng, L. D.; Dong, A. J. pH-sensitive nanoparticles prepared from amphiphilic and biodegradable methoxy poly(ethylene glycol)-block(polycaprolactone-graft-poly(methacrylic acid)) for oral drug delivery. Polym. Chem. 2013, 4, 1430-1438. 4. Guo, S. T.; Huang, Y. Y.; Zhang, W. D.; Wang, W. W.; Wei, T.; Lin, D. S.; Xing, J. F.; Deng, L. D.; Du, Q.; Liang, Z. C.; Liang, X. J.; Dong, A. J. Ternary complexes of amphiphilic polycaprolactone-graft-poly (N,N-dimethylaminoethyl methacrylate), DNA and polyglutamic acid-graft-poly (ethylene glycol) for gene delivery. Biomaterials 2011, 32, 4283-4292. 5. Jeong, B.; Bae, Y. H.; Lee, D. S.; Kim, S. W. Biodegradable block copolymers as injectable drug-delivery systems. Nature 1997, 388, 860. 6. Barouti, G.; Jaffredo, C. G.; Guillaume, S. M. Advances in drug delivery systems based on synthetic poly(hydroxybutyrate) (co)polymers. Prog. Polym. Sci. 2017, 73, 1-31.



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For Table of Contents use only

Preparation of amphiphilic poly(ethylene glycol)-b-poly(γ-butyrolactone) diblock copolymer via ring opening polymerization catalyzed by a cyclic trimeric phosphazene base or alkali alkoxide Yong Shen, [a],‡ Jinbo Zhang, [b],‡ Zhichao Zhao, [a] Na Zhao, [b] Fusheng Liu, [a] and Zhibo Li*, [b]


Preparation of Amphiphilic Poly(ethylene glycol)-b ... - ACS Publications

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