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DOI: 10.1039/C6RA00289G (Paper) RSC Adv., 2016, 6, 17531-17538

Bimetallic Schiff base complexes for stereoselective polymerisation of racemic-lactide and copolymerisation of racemic-lactide with ε-caprolactone†

Zhiqiang Sun, Ranlong Duan, Jingwei Yang, Han Zhang, Shuai Li, Xuan Pang*, Wenqi Chen and Xuesi Chen
Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China. E-mail: xpang@ciac.ac.cn

Received 5th January 2016 , Accepted 4th February 2016

First published on 5th February 2016

Abstract

Three pairs of bimetallic Schiff base aluminum complexes (1a–6a) with different tetraamine backbone bridging parts and bulky silyl-substituted moieties on salicylidene were synthesized. Using these complexes as catalysts for ring opening polymerisation (ROP) of racemic-lactide (rac-LA), systematical stereoselectivity and kinetic studies on ROP of rac-LA were carried out. The Gibbs free energy difference between homo-propagation and cross-propagation (ΔG≠) calculated by activation entropy and enthalpy showed that bulky substituents at the 3-position enhanced the stereoselectivity. And the kinetic study results indicated that bulkier substituents with more steric hindrance reduced the ROP reactivity. Moreover, these complexes could also be used for copolymerisation of rac-LA and ε-caprolactone (ε-CL). Copolymers of poly(LA-co-CL) with various monomer ratios were synthesized. Kinetic research on copolymerisation as well as microstructure analysis demonstrated that poly (LA-co-CL) achieved by these complexes were tapered block copolymers.

Introduction

Due to the recent concerns about environmental pollution, investigation of biodegradable materials has attracted more and more attention.1–5 As leading biodegradable synthetic polyesters, polylactide (PLA) and poly(ε-caprolactone) (PCL) have been intensively studied.1,5–8 Generally, the method to obtain PLA and PCL is ring-opening polymerisation (ROP) of LA and ε-CL, respectively.9–16 As for the polymerisation of LA, the chiral centers in the monomer play a very important role in determining the properties of the resulting PLA materials. The chain microstructures of PLA are crucial for its physical, mechanical, rheological and degradation properties.17–19 In the past twenty years, much effort has been focused on the development of lactones polymerisation using Schiff base metal complexes as catalysts. In the year of 1996, Spassky and coworkers firstly reported that an enantiomeric pure Schiff base aluminum alkoxide catalyst could lead to an isotactic type PLA materials with gradient S- and R-LA units in the polymer chains.20 Since then, many Schiff base complex systems based on a five-coordinated aluminum atom were explored and intensively studied for stereoselective polymerisation of lactide (LA).21,22 Both chiral23,24 and achiral25,26 salen-type Schiff base catalysts were applied for preparation of highly stereo-regular PLA materials via the enantiomorphic site control mechanism and chain-end control mechanism, respectively.

In recent years, a number of Schiff base complexes with multimetallic active centers had been synthesized and applied for ROP of LA and ε-CL.27–31 Darensbourg's group had synthesized a series of aluminum half-salen complexes containing both chiral and achiral ligands, and applied them for ROP of rac-LA.27,28 Hillmyer and coworkers reported a dimeric Zn(II) alkoxide complex, which gave atactic PLA with good molecular weight control.32 Lin reported a series of dinuclear zinc half-salen complexes shown high reactivities towards the controlled polymerisation of LA monomers.33 However, comparing with their monometallic counterparts, fewer studies were reported on the influence of the phenolate substituents and bridging part on the stereoselectivity and activity of multimetallic Schiff base complexes.34–36 In recent years, our group has synthesized a number of bimetallic aluminum salen complexes (Scheme S1†), and used them as catalysts for polymerisation of LA and ε-CL.37–42 Based on our previous work, substituents at the ortho position might have great influence on stereoselectivity and activity for ROP of rac-LA. Specifically, complexes with bulky substituents are inclined to provide high stereoselectivity and low activity. On the other hand, salen complexes with same phenolate substituents and different bridging parts also vary in catalytic activities.37–40

Herein, a new series of bimetallic Schiff base ligands (L1–L6) with different tetraamine backbone bridging parts and bulky silyl-substituted moieties on 3-position of the salicylidene were synthesised and used for preparing aluminum complexes (1a–6a) (shown by Scheme 1). These complexes were applied as catalysts for ROP of rac-LA. All of them gave isotactic polylactides with controlled molecular weight and narrow molecular weight distributions. The kinetic studies indicated that polymerisations catalyzed by these complexes were first-ordered with respect to lactide monomers. A systematic research of polymerisation demonstrated that the steric substituents on the phenolate ring of the complexes had remarkable influence on the stereoselectivity and the polymerisation activity. The Gibbs free energy change difference between homo-propagation and cross-propagation showed that bulky substituents at the 3-position enhanced the stereoselectivity. And the kinetic study results indicated that bulkier substituents with more steric hindrance reduced the polymerisation rate. Moreover, these complexes had also been used for copolymerisation of rac-LA and ε-CL. Copolymers of poly(LA-co-CL) with various monomer ratios were obtained by changing the feed ratio of comonomers. Kinetic studies on copolymerisation and sequential distribution analysis demonstrated that the reactivity ratio of rac-LA was obviously higher than ε-CL. The resultant poly(LA-co-CL) were tapered block copolymers, with the average lengths of LA and CL sequences both slightly longer than 2.0 for copolymerisation of rac-LA[thin space (1/6-em)]:[thin space (1/6-em)]ε-CL (50[thin space (1/6-em)]:[thin space (1/6-em)]50).


image file: c6ra00289g-s1.tif
Scheme 1 General synthetic route for L1–L6 and 1a–6a.

Experimental section

General

All experiments were carried out in a dry nitrogen atmosphere using Schlenk techniques. Starting materials for the production of ligands L1–L6, rac-LA, ε-CL were purchased from Aldrich Inc. Toluene was distilled from Na/benzophenone before use. Ethyl acetate and iso-propanol were distilled from CaH2 under the protection of argon. rac-LA were purified by recrystallisation from ethyl acetate and dried under vacuum at room temperature. NMR spectra were recorded on Bruker AV 300 M and Bruker AV 400 M in CDCl3 (δ = 7.27) at 25 °C. Chemical shifts were given in parts per million (ppm) from tetramethylsilane. Gel permeation chromatography (GPC) measurements were conducted with a Waters 515 GPC with CHCl3 as the eluent (flow rate: 1 mL min−1 at 35 °C). The molecular weights were calibrated against the polystyrene standards.

Synthesis of ligands and complexes

General procedure. To a stirred solution of substituent salicylaldehyde (0.4 mol L−1) in ethanol (40 mL), a solution of tetraamine (compound Y in Scheme 1) (0.1 mol L−1) in ethanol (40 mL) was added drop-wise. The reaction mixture was refluxed for 12 h. After removal of the solvent under vacuum, a solid was produced and purified by recrystallisation in C2H5OH/CH2Cl2. For the synthesis of Schiff base aluminum complexes 1a–6a, AlEt3 (0.2 mmol) in toluene (5 mL) was added to the stirred 1 mL toluene solution of ligand precursors L1–L6 (0.1 mmol) at room temperature. The reaction was maintained at 70 °C for 12 h. Despite of our efforts, single crystals suitable for X-ray crystal structure determination were not obtained for 1a–6a.
Synthesis of ligands L1–L6.
L1. 1H NMR (300 MHz, CDCl3): δ = 0.41(s, SiCH3 36H), 1.35(s, CCH3 36H), 3.82(s, CH2N 8H), 7.31(d, ArH 4H), 7.53(d, ArH 4H), 8.50(s, NCH 4H). 13C NMR (100 MHz, CDCl3): δ = −0.93(Si(CH3)3), 31.49(C(CH3)3), 34.02(ArC(CH3)3), 44.32((CH2)2C(CH2)2), 61.54(CCH2N), 116.90, 126.59, 129.54, 135.25, 140.80, 163.60(Ar), 167.75(NCH). HRMS m/z, [M + H]+: 1061.65596. Elem. anal.: calcd C 69.00, H 9.11, N 5.28; found C 68.93, H 9.09, N 5.25.
L2. 1H NMR (300 MHz, CDCl3): δ = 0.37(s, Si(CH3)2 24H), 0.96(s, SiC(CH3)3 36H), 3.76(s, CH2N 8H), 6.89, 7.24, 7.43(m, ArH 12H), 8.40(s, NCH 4H), 13.36(s, OH 4H). 13C NMR (100 MHz, CDCl3): δ = −4.65(Si(CH3)2), 17.74(SiC(CH3)3), 27.20(SiC(CH3)3), 44.05((CH2)2C(CH2)2), 61.67(CCH2N), 117.71, 118.37, 125.06, 133.15, 139.77, 166.00(Ar), 167.61(NCH). HRMS m/z, [M + H]+: 1005.59264. Elem. anal.: calcd C 68.07, H 8.82, N 5.57; found C 67.95, H 8.85, N 5.57.
L3. 1H NMR (300 MHz, CDCl3): δ = 0.38(s, SiCH3 36H), 1.35(s, CCH3 36H), 2.08(s, CCH2C 4H), 3.79(s, CH2N 8H), 7.30(d, ArH 4H), 7.50(d, ArH 4H), 8.47(s, NCH 4H), 13.29(s, OH 4H). 13C NMR (100 MHz, CDCl3): δ = −0.91(Si(CH3)3), 31.55(C(CH3)3), 34.06(CCH2C), 36.66((CH2)2C(CH2)2), 67.02(CCH2N), 116.92, 126.54, 129.56, 135.03, 140.68, 163.56(Ar), 167.12(NCH). HRMS m/z, [M + H]+: 1101.68724 elem. anal.: calcd C 69.76, H 9.15, N 5.08; found C 69.69, H 9.14, N 5.01.
L4. 1H NMR (300 MHz, CDCl3): δ = 0.35(s, Si(CH3)2 24H), 0.94(s, SiC(CH3)3 36H), 2.04(s, CCH2C 4H), 3.73(s, CH2N 8H), 6.91, 7.21, 7.44(m, ArH 12H), 8.31(s, NCH 4H), 13.43(s, OH 4H). 13C NMR (100 MHz, CDCl3): δ = −4.67(Si(CH3)2), 17.67(SiC(CH3)3), 27.15(SiC(CH3)3), 33.65(CCH2C), 36.54((CH2)2C(CH2)2), 66.98(CCH2N), 117.76, 118.22, 125.08, 133.00, 139.56, 165.97(Ar), 166.89(NCH). HRMS m/z, [M + H]+: 1045.62383 elem. anal.: calcd C 68.91, H 8.87, N 5.36; found C 69.93, H 8.90, N 5.33.
L5. 1H NMR (300 MHz, CDCl3): δ = 0.35(s, SiCH3 36H), 1.31(s, CCH3 36H), 3.51(s, CCH2N 4H), 3.987(d, CCH2O 4H), 4.03(s, CCH2N 4H), 4.21(d, CCH2O 4H), 5.54(s, ArCH 2H), 7.26, 7.46, 7.56(m, ArH 12H), 8.37, 8.44(s, NCH 4H). 13C NMR (100 MHz, CDCl3): δ = −1.63(Si(CH3)3), 30.82(C(CH3)3), 33.35(ArC(CH3)3), 37.61(OCH2CCH2N), 58.98, 60.60(CCH2N), 71.32(CCH2O), 101.01(ArCO), 116.03, 116.23, 125.44, 125.81, 126.08, 128.91, 134.42, 134.73, 138.17, 140.04, 140.19, 162.83(Ar), 167.17(NCH). HRMS m/z, [M + H]+: 1296.74435. Elem. anal.: calcd C 68.58, H 8.55, N 4.32; found C 68.55, H 8.86, N 4.29.
L6. 1H NMR (300 MHz, CDCl3): δ = 0.34(s, Si(CH3)2 24H), 0.93(s, SiC(CH3)3 36H), 3.49(s, CCH2N 4H), 3.98(d, CCH2O 4H), 4.02(s, CCH2N 4H), 4.20(d, CCH2O 4H), 5.54(s, ArCH 2H), 6.86, 7.23, 7.41, 7.56(m, ArH 16H), 8.32, 8.41(s, NCH 4H), 13.06, 13.36(s, OH 4H). 13C NMR (100 MHz, CDCl3): δ = −4.75(Si(CH3)2), 17.63(SiC(CH3)3), 27.08(SiC(CH3)3), 38.18(OCH2CCH2N), 59.64, 61.20(CCH2N), 72.02(CCH2O), 101.67(ArCO), 117.72, 118.31, 125.48, 126.05, 133.05, 138.77, 139.54, 165.96(Ar), 167.58(NCH). HRMS m/z, [M + H]+: 1239.68251. Elem. anal.: calcd C 67.81, H 8.29, N 4.52; found C 67.84, H 8.25, N 4.49.
Synthesis of complexes 1a–6a.
1a. 1H NMR (400 MHz, CDCl3): δ = −0.12(m Al–CH2 4H), 0.31(s SiCH3 36H), 0.96(m (Al–CH2)CH3 6H), 1.30(s CCH3 36H), 3.74(d CH2N 8H), 7.05(d ArH 4H), 7.68(m ArH 4H), 8.22(d NCH 4H). 13C NMR (100 MHz, CDCl3): δ = −0.97(Si(CH3)3), 1.61(Al–CH2CH3), 9.16(Al–CH2CH3), 31.46(C(CH3)3), 33.93(ArC(CH3)3), 44.02((CH2)2C(CH2)2), 61.18(CCH2N), 116.90, 132.15, 138.61, 140.72, 142.50, 170.84(Ar), 177.17(NCH). Elem. anal.: calcd C 66.74, H 8.79, N 4.61; found C 66.84, H 8.75, N 4.59.
2a. 1H NMR (400 MHz, CDCl3): δ = 0.01(m Al–CH2 4H), 0.31(s Si(CH3)2 24H), 0.93(s SiC(CH3)3 36H), 1.01(m (Al–CH2)CH3 6H), 4.01(s CH2N 8H), 6.71, 6.93, 7.58(m ArH 12H), 8.21(s NCH 4H). 13C NMR (100 MHz, CDCl3): δ = −4.55(Si(CH3)2), 1.53(Al–CH2CH3), 9.13(Al–CH2CH3), 17.87(SiC(CH3)3), 27.28(SiC(CH3)3), 45.09((CH2)2C(CH2)2), 62.87(CCH2N), 118.11, 130.26, 135.32, 137.19, 144.37, 146.59, 170.14(Ar), 177.37(NCH). Elem. anal.: calcd. C 65.78, H 8.51, N 5.03; found C 65.74, H 8.55, N 4.99.
3a. 1H NMR (400 MHz, CDCl3): δ = −0.03(m Al–CH2 4H), 0.32(s SiCH3 36H), 1.05(m (Al–CH2)CH3 6H), 1.25(s, CCH3 36H), 2.58(s CCH2C 4H), 3.82(s CH2N 8H), 6.97(d ArH 4H), 7.64(d ArH 4H), 8.07(s NCH 4H). 13C NMR (100 MHz, CDCl3): δ = −0.94(Si(CH3)3), 1.25(Al–CH2CH3), 9.09(Al–CH2CH3), 31.39(C(CH3)3), 33.98(CCH2C), 38.76((CH2)2C(CH2)2), 65.39(CCH2N), 116.62, 131.75, 132.23, 140.12, 142.16, 168.08(Ar), 174.91(NCH). Elem. anal.: calcd C 67.50, H 8.83, N 4.63; found C 67.54, H 8.85, N 4.59.
4a. 1H NMR (400 MHz, CDCl3): δ = 0.02(m Al–CH2 4H), 0.34(s Si(CH3)2 24H), 0.94(s SiC(CH3)3 36H), 1.06(m (Al–CH2)CH3 6H), 2.62(s CCH2C 4H), 3.87(s CH2N 8H), 6.72, 6.89, 7.62(m ArH 12H), 7.98(s NCH 4H). 13C NMR (100 MHz, CDCl3): δ = −4.51(Si(CH3)2), 1.28(Al–CH2CH3), 9.11(Al–CH2CH3), 17.78(SiC(CH3)3), 27.31(SiC(CH3)3), 34.95(CCH2C), 40.03((CH2)2C(CH2)2), 67.45(CCH2N), 117.65, 130.09, 133.14, 137.09, 145.96, 167.02, 170.13(Ar), 175.19(NCH). Elem. anal.: calcd C 66.62, H 8.56, N 4.86; found C 66.54, H 8.55, N 4.89.
5a. 1H NMR (400 MHz, CDCl3): δ = −0.02(m Al–CH2 4H), 0.32(s, SiCH3 36H), 1.04(m (Al–CH2)CH3 6H), 1.31(s CCH3 36H), 3.62(s CCH2N 4H), 3.84(d CCH2O 4H), 3.98(s CCH2N 4H), 4.12(d CCH2O 4H), 5.55(s ArCH 2H), 7.14, 7.19, 7.60(m ArH 12H), 8.07, 8.21(s NCH 4H). 13C NMR (100 MHz, CDCl3): δ = −0.98(Si(CH3)3), 1.20(Al–CH2CH3), 9.10(Al–CH2CH3), 31.52(C(CH3)3), 34.06(ArC(CH3)3), 39.80(OCH2CCH2N), 59.03, 60.62(CCH2N), 71.06(CCH2O), 101.24(ArCO), 117.14, 126.26, 132.25, 140.17, 142.18, 168.06, 169.88(Ar), 177.58(NCH). Elem. anal.: calcd C 66.72, H 8.33, N3.99; found C 66.74, H 8.35, N 3.89.
6a. 1H NMR (400 MHz, CDCl3): δ = −0.02(m Al–CH2 4H), 0.32(s Si(CH3)2 24H), 0.94(s SiC(CH3)3 36H), 1.03(m (Al–CH2)CH3 6H), 3.65(s CCH2N 4H), 3.99(d CCH2O 4H), 4.07(m CCH2N 4H), 4.22(d CCH2O 4H), 5.55(s ArCH 2H), 6.80, 7.19, 7.44, 7.57(m ArH 16H), 8.07, 8.39(s NCH 4H). 13C NMR (100 MHz, CDCl3): δ = −4.56(Si(CH3)2), 1.02(Al–CH2CH3), 9.07(Al–CH2CH3), 17.79(SiC(CH3)3), 27.34(SiC(CH3)3), 39.83(OCH2CCH2N), 58.99, 61.39(CCH2N), 70.00(CCH2O), 101.27(ArCO), 117.76, 126.20, 130.43, 133.25, 136.73, 139.74, 145.62, 169.86(Ar), 177.59(NCH). Elem. anal.: calcd C 65.94, H 8.08, N 4.16; found C 65.84, H 8.05, N 4.19.

Polymerisation of rac-LA

In a typical polymerisation experiment, the rac-LA (20.0 mmol), complexes 1a–6a (0.10 mmol in 5.0 mL of toluene), iso-propanol (0.20 mmol in 2.0 mL of toluene), and toluene (33.0 mL) were added into a flame-dried vessel containing a magnetic stirring bar. The mixtures were stirred at 70 °C, 90 °C or 110 °C in an oil bath. Conversion of the monomer was determined by 1H NMR. The polymers were isolated by precipitation and centrifugation in cold ethanol, and dried under vacuum at room temperature for 24 h.

Copolymerisation of rac-LA with ε-CL

A typical example of the copolymerisation by using 2a was as follows. The rac-LA (10.0 mmol), ε-CL (10.0 mmol), complexes 2a (0.10 mmol in 5.0 mL of toluene), iso-propanol (0.20 mmol in 2.0 mL of toluene), and toluene (33.0 mL) were added into a flame-dried vessel. The mixture was stirred at 90 °C in an oil bath. The copolymers were isolated by precipitation and centrifugation in cold ethanol, and dried under vacuum at room temperature for 24 h.

Result and discussion

Ligand and complex synthesis

As shown in Scheme 1, Schiff base ligands L1–L6 were synthesized by reaction of tetraamine (named as compound Y) with substituented salicylaldehyde. Ligands L1–L6 had different bridging parts (X) and bulky phenolate substituents. L1 & L2, L3 & L4, L5 & L6 had the identical tetraaimine backbones but different silyl-substituted moieties on 3-position of the salicylaldehyde, trimethylsilyl (TMS) for L1, L3, L5 and tert-butyldimethylsilyl (TBDMS) for L2, L4, L6. The 1H NMR spectra of L1 showed signals at δ = 8.50 and 3.82 ppm, which were attributed to the NCH and NCH2 protons of the pentaerythrityl tetramine, respectively. The intensity ratio of the two signals was 1[thin space (1/6-em)]:[thin space (1/6-em)]2, which confirmed the structure of L1. And then reaction of ligands L1–L6 with AlEt3 in toluene produced bimetallic Schiff base aluminum complexes 1a–6a, accordingly. The 1H NMR spectrum of complex 1a showed signals at δ = 0.96 and −0.12 ppm, which were attributed to the methyl and methylene protons of the aluminum ethyl group, respectively. Comparing with L1, the corresponding signals of NCH and NCH2 protons in complex 1a moved to δ = 8.22 and 3.74, respectively. The intensity ratio of the signals at δ = 8.22, 3.74, 0.96 and −0.12 ppm was 2[thin space (1/6-em)]:[thin space (1/6-em)]4[thin space (1/6-em)]:[thin space (1/6-em)]3[thin space (1/6-em)]:[thin space (1/6-em)]2, which confirmed the structure of 1a.

Stereoselective polymerisation of rac-LA

To examine the influence of bulky substituents on their catalytic performance, ROP of rac-LA in toluene using 1a–6a in presence of iso-propanol were investigated (Table 1). The 1H NMR spectrum of the PLA oligomers (Fig. S1†) showed that: the integral ratio of the peaks at δ = 4.35 ppm (the methine proton neighboring –OH end group) and δ = 1.24 ppm (the methyl protons of the isopropoxycarbonyl end group) was close to 1[thin space (1/6-em)]:[thin space (1/6-em)]6. And the MALDI-TOF analysis (Fig. S2†) exhibited oligomers of the formula H(OCHMeCO)2nOiPr·Na+. All these results indicated that the polymer chains were end-capped with a –OH group and an isopropyl ester group, and the ring-opening occurred through a coordination-insertion mechanism.43,44 Furthermore, the relation between the number-average molecular weight (Mn) and the monomer conversion also had been investigated (Fig. S3†), the linear relationship and the low polydispersity (PDI) elucidated that all polymerisations catalysed by 1a–6a showed a characteristic of controlled propagation.
Table 1 Polymerisation data of rac-LA using complexes 1a–6aa
Entry Complex T/°C T [h] Conv.b% Mn(c)c [103] Mnd [103] PDId Pme
a The polymerisations were carried out in toluene solution. [LA]0 = 0.5 mol L−1, with [M]0/[Cat]/[iso-propanol] = 200[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]2.b Measured by 1H NMR spectra.c Calculated from the molecular weight of LA × [M/2]0/[Cat] × conversion + Mw(iPrOH).d Obtained from GPC analysis and calibrated against polystyrene standard, calculated according to formula Mn = 0.58Mn(GPC).e The parameter Pm is the probability of meso linkages, according to [mmm] = Pm2 + (1 − Pm)Pm/2, [mmr] = [rmm] = (1 − Pm)Pm/2.
1 1a 70 4 52 7.5 10.1 1.10 0.94
2 1a 90 3 81 11.7 11.2 1.13 0.83
3 1a 110 2 78 11.2 9.2 1.13 0.75
4 2a 70 7.5 36 5.2 3.5 1.11 0.95
5 2a 90 6 59 8.5 7.5 1.20 0.87
6 2a 110 2.7 73 10.6 10.2 1.20 0.80
7 3a 70 5 81 11.7 9.1 1.08 0.89
8 3a 90 3 84 12.2 12.5 1.19 0.77
9 3a 110 1 76 11.0 8.6 1.28 0.68
10 4a 70 7.5 42 6.1 4.6 1.10 0.91
11 4a 90 6 73 10.6 10.0 1.13 0.83
12 4a 110 2 65 9.4 5.9 1.21 0.78
13 5a 70 5 69 10.0 11.0 1.14 0.88
14 5a 90 3 81 11.7 13.2 1.20 0.82
15 5a 110 1.8 83 12.0 15.0 1.47 0.73
16 6a 70 8 47 6.8 6.0 1.11 0.90
17 6a 90 5.7 45 6.5 4.7 1.40 0.85
18 6a 110 2.5 33 4.8 4.5 1.59 0.77

The stereochemical microstructures of the resultant PLAs were determined from the methine region of the homonuclear decoupled 1H NMR (shown by Fig. S4†). From the Pm values listed in Table 1, we could see that all of these six complexes showed high stereoselectivity in the polymerisation of rac-LA. Moreover, the silyl-substituent (R) on 3-position of the salicylidene showed regular influence on this stereoselectivity. The Pm values of PLA materials produced by 1a/iso-propanol at 70 °C, 90 °C and 110 °C were 0.94, 0.83 and 0.75, respectively. And complex 2a with same bridging part produced PLAs with Pm values of 0.95, 0.87 and 0.80, accordingly. The improvement in stereoselectivity was attributed to the bulkier TBDMS (R) group in complex 2a compared to TMS in 1a. Similar Pm value increases were found in PLA materials produced by complexes 3a & 4a, 5a & 6a at different temperature. All these stereochemical microstructure results gave experimental evidences for previous studies that the enhancement of stereoselectivity requires bulky substituents.25,26

As all these six ligands were achiral, it was supposed that the ROP of rac-LA by these catalysts took place via chain-end control mechanism. The reactions showed to be in accordance with a first-order Markovian statistics.45,46 According to the Markovian statistics and absolute reaction rate theory, the activation entropy difference and activation enthalpy difference between homo-propagation (km) and cross-propagation (kr) would be determined by eqn (i):47,48

 
Pm/(1 − Pm) = km/kr = exp[(ΔS≠m − ΔS≠r)/R − (ΔH≠m − ΔH≠r)/RT] (i)
where R is the universal gas constant (1.987 cal mol−1 K−1) and T is the Kelvin temperature. The (ΔS≠m − ΔS≠r) was the entropy difference between homo-propagation and cross-propagation, and the (ΔH≠m − ΔH≠r) was the enthalpy difference between homopropagation and cross-propagation. If km > kr, formation of isotactic sequences was favored. On the contrary, if km < kr, heterotactic sequences were produced. ln[Pm/(1 − Pm)] was plotted against 1/T to calculate the values of (ΔS≠m − ΔS≠r) and (ΔH≠m − ΔH≠r). As shown by Fig. 1, the entropy and enthalpy differences of 3a/iso-propanol were calculated as −22.07 cal K−1 mol−1 and −8.94 kcal K−1 mol−1, respectively.


image file: c6ra00289g-f1.tif
Fig. 1 Relationship between polymerisation temperature and stereochemistry of the resulting poly(rac-LA)s by using 3a/iso-propanol.

As a convenient criterion for the spontaneity of processes at constant pressure and temperature, Gibbs free energy is the chemical potential that is minimized when a system reaches equilibrium. And the relative Gibbs-free energy difference could be calculated for predicting the stereo-microstructure of polymers produced from ROP of enantiomers.49 From the values of (ΔS≠m − ΔS≠r) and (ΔH≠m − ΔH≠r) shown in Table 2, we can calculate the relative Gibbs free energy change difference between homo-propagation and cross-propagation (ΔG≠) by:

 
ΔG≠ = ΔG≠m − ΔG≠r = (ΔH≠m − TΔS≠m) − (ΔH≠r − TΔS≠r) = ΔH≠m − ΔH≠r − T(ΔS≠m − ΔS≠r) (ii)

Table 2 The entropy and enthalpy difference of 1a–6a/iso-propanol
Complex (ΔS≠m − ΔS≠r) cal K−1 mol−1 (ΔH≠m − ΔH≠r) kcal K−1 mol−1 ΔG≠ kcal K−1 mol−1
70 °C 90 °C 110 °C
1a −29.72 −12.07 −1.88 −1.28 −0.68
2a −26.22 −11.14 −2.16 −1.62 −1.10
3a −22.07 −8.94 −1.37 −0.93 −0.49
4a −15.54 −6.85 −1.52 −1.21 −0.90
5a −15.05 −6.52 −1.36 −1.06 −0.76
6a −14.48 −6.47 −1.50 −1.21 −0.92

The ΔG≠ values between homo-propagation and cross-propagation for 1a–6a at different temperature (Table 2) were all negative, which elucidated the preference of isotactic stereosequence for all these 6 complexes. Moreover, Fig. 2 showed the plots of ΔG≠ values between homo-propagation and cross-propagation for 1a–2a at different temperature. It had been found that for each of the two complexes, absolute values of ΔG≠ between homo-propagation and cross-propagation decreased gradually with the temperature rise, and the Pm values decreased accordingly.


image file: c6ra00289g-f2.tif
Fig. 2 Plots of ΔG≠ values between homo-propagation and cross-propagation for 1a–2a vs. temperature.

These results gave a theoretical explanation for decrease of isotactic stereosequence in polymer chains fabricated with increasing temperature. What's more, 2a with bulkier substituents showed more negative ΔG≠ values at different temperature, comparing with its counterpart 1a with same bridging part but smaller substituent. Similar results were observed between the other two pairs of complexes (3a & 4a, 5a & 6a shown by Fig. S5†), which systematically indicated that bulky substituents at the 3-position enhanced the stereoselectivity for ROP of rac-LA.

Kinetics of rac-LA polymerisation

The ROP processes of rac-LA were investigated by kinetic studies using complexes 1a–6a. Fig. 3 showed the data of monomer conversion versus time collected for complexes 5a & 6a in presence of iso-propanol at 70 °C, 90 °C and 110 °C. In all cases, first-order kinetics in monomer was observed. Similar results were obtained for other four complexes (shown by Fig. S6†). The polymerisation of rac-LA by using 1a–6a proceeded according to the eqn (iii):
 
−d[LA]/dt = kapp[LA] (iii)
image file: c6ra00289g-f3.tif
Fig. 3 Kinetic plots of the rac-LA conversion ([M]0/[Cat] = 200[thin space (1/6-em)]:[thin space (1/6-em)]1) vs. the reaction time at different temperature: (A) complex 5a, (B) complex 6a.

The linear relationship illustrated a first-order in monomer (kapp = kp[Cat]). As there were two aluminum centers in these bimetallic complexes, the polymerisation of rac-LA initiated and catalyzed by 1a–6a followed a kinetic law as shown in the eqn (iv):

 
−d[LA]/dt = kp[Al][LA] (iv)

All of the kapp and kp were calculated for 1a–6a/iso-propanol in toluene. The results listed in Table 3 indicated that the substituent groups on the phenolate ring significantly affected the ROP activity. Complexes 1a and 2a had the same bridging part, but different phenolate substituents. Polymerisation data revealed that 1a had higher activity than that of 2a. For example, kp value of 1a at 70 °C (0.60 L mol−1 min−1), 90 °C (1.83 L mol−1 min−1) were more than three folds higher than that of 2a at 70 °C (0.19 L mol−1 min−1), 90 °C (0.47 L mol−1 min−1). Even at 110 °C, kp values of 1a (2.83 L mol−1 min−1) was much higher than 2a (1.67 L mol−1 min−1). The decrease of ROP activity was attributed to the bulkier TBDMS group of 2a compared with TMS group of 1a. Similar activity decreases were observed for complex pairs of 3a & 4a and 5a & 6a at different temperature. All these results indicated that bulky substituents on 3-position of phenolic ring resulted in a reduction of polymerisation rate remarkably. More precisely, the bulkier substituents with more steric hindrance may keep active species from being approached by rac-LA monomer, as a result, slowing down the polymerisation rate.26,38,48

Table 3 Kinetic data of complex 1a–6a for the ROP of rac-LA in toluene with [LA]0 = 0.5 mol L−1
Complex T/°C kapp/h−1 kp/L mol−1 min−1
1a 70 0.18 0.60
1a 90 0.55 1.83
1a 110 0.85 2.83
2a 70 0.057 0.19
2a 90 0.14 0.47
2a 110 0.50 1.67
3a 70 0.32 1.07
3a 90 0.65 2.17
3a 110 1.27 4.23
4a 70 0.07 0.24
4a 90 0.24 0.80
4a 110 0.59 1.97
5a 70 0.24 0.80
5a 90 0.56 1.87
5a 110 0.99 3.30
6a 70 0.073 0.25
6a 90 0.39 1.30
6a 110 0.72 2.40

Copolymerisation of rac-LA and ε-CL

In order to expand the application of these bulky silyl-substituted Schiff base complexes, we have used them as catalysts for ring opening copolymerisation of rac-LA and ε-CL. Copolymerisation of rac-LA and ε-CL mixture with different feed ratios were carried out using complex 2a as catalyst, and the copolymerisation data were collected in Table S1.† 1H NMR spectra were applied for composition analysis of the copolymer materials.

As seen from Fig. 4, the spectra change regularly according to the feed ratio of rac-LA and ε-CL comonomers. The multi-peaks between 4.95 and 5.35 ppm were assigned to the methine protons in LA units. The multi-peaks between 2.15 and 2.55 ppm were assigned to the methylene protons neighboring the carbonyl in CL units. The integral ratio of these two groups of peaks led to the LA/CL mole ratio in the copolymer materials. As listed in the 3rd column of Table S1,† LA/CL in copolymers changed closely to the feed ratio of rac-LA[thin space (1/6-em)]:[thin space (1/6-em)]ε-CL. It indicated the composition of the copolymers could be adjusted by controlling the feed ratio of monomer mixture.


image file: c6ra00289g-f4.tif
Fig. 4 1H NMR spectra of the copolymers fabricated by different rac-LA[thin space (1/6-em)]:[thin space (1/6-em)]ε-CL feed ratios with complex 2a.

The enlarged spectrum in Fig. 5 showed that there were two triplets at 4.07 and 2.32 ppm, which were assigned to (c) and (d) methylene protons in CL–CL connected units, respectively. And the multi-peak groups at 4.11–4.24 and 2.35–2.50 ppm were assigned to (a) and (b) methylene protons in CL-LA connected units. The two triplets could be found in all the spectra shown by Fig. 4, which indicated that CL–CL blocks did exist in all the copolymer materials produced by complex 2a. Moreover, an interesting phenomenon was found that integral ratios of (a)/(c) and (b)/(d) were obviously lower than the feed ratio of rac-LA[thin space (1/6-em)]:[thin space (1/6-em)]ε-CL for copolymerisation. We supposed that this phenomenon was attributed to the higher reactivity ratio of rac-LA than ε-CL50,51 in this catalysed copolymerisation system. And the hypothesis was further confirmed by the kinetic study on copolymerisation of rac-LA[thin space (1/6-em)]:[thin space (1/6-em)]ε-CL (50[thin space (1/6-em)]:[thin space (1/6-em)]50) with complex 2a.


image file: c6ra00289g-f5.tif
Fig. 5 1H NMR spectrum of poly(LA-co-CL) obtained by 2a for copolymerisation of rac-LA[thin space (1/6-em)]:[thin space (1/6-em)]ε-CL = 80[thin space (1/6-em)]:[thin space (1/6-em)]20.

Kinetic plots in Fig. 6 showed that during the early stage of copolymerisation, rac-LA is preferentially polymerized. The kp value of rac-LA (1.25 L mol−1 min−1) was over twice of ε-CL (0.56 L mol−1 min−1) in the copolymerisation. Similar results were achieved in the kinetic study of rac-LA[thin space (1/6-em)]:[thin space (1/6-em)]ε-CL (50[thin space (1/6-em)]:[thin space (1/6-em)]50) with complex 1a (Fig. S7†). The kinetic studies inferred that tapered block copolymers were expected to be formed using complexes 1a and 2a. In order to assess the basic molecular feature of copolymers, the sequential distribution of the comonomers within the chains were analyzed by 13C NMR spectroscopy.


image file: c6ra00289g-f6.tif
Fig. 6 Kinetics of the rac-LA[thin space (1/6-em)]:[thin space (1/6-em)]ε-CL (50[thin space (1/6-em)]:[thin space (1/6-em)]50) copolymerisation using 2a at the reaction temperatures of 90 °C, [M]0/[Cat] = 200[thin space (1/6-em)]:[thin space (1/6-em)]1.

As a method very sensitive to the chemical environment of the studied nucleus, 13C NMR spectroscopy is a powerful tool for analyzing the actual average length of each type of monomer sequence. As for copolymers of LA and ε-CL, “carbonyl” signals between 175 and 165 ppm have been widely used in calculation of average length of LA sequence (LeLL) and CL sequence (LeCap).52,53 Fig. 7 showed the “carbonyl” signals of 13C NMR spectra of poly[(rac-LA)-co-(ε-CL)] obtained by copolymerisation of rac-LA[thin space (1/6-em)]:[thin space (1/6-em)]ε-CL (50[thin space (1/6-em)]:[thin space (1/6-em)]50). The signal assignments were labeled on the image according to the literature.53,54


image file: c6ra00289g-f7.tif
Fig. 7 13C NMR spectra (150 MHz) of copolymer obtained by 2a for copolymerisation of [rac-LA][thin space (1/6-em)]:[thin space (1/6-em)][ε-CL] = 50[thin space (1/6-em)]:[thin space (1/6-em)]50.

The experimental average lengths LeLL and LeCap could be calculated from eqn (v) and (vi):45,54

 
image file: c6ra00289g-t1.tif(v)
 
image file: c6ra00289g-t2.tif(vi)
where [sequence] indicated the contents of appropriate sequences in the copolymer chains represented by the intensities of assigned signals in 13C NMR spectrum. It had been calculated that the average lengths of LA and CL sequences of copolymer shown by Fig. 7 were LeLL ≈ 2.2 and LeCap ≈ 2.1, respectively.

Conclusions

In conclusion, we have synthesized a series of dinuclear Salen aluminum complexes, with 3-position of the salicylidene moieties have bulk silyl-substituents. Their catalytic behaviour in ROP of rac-LA had been investigated detailedly. All six complexes showed high stereoselectivity. The ΔG≠ values between homo-propagation and cross-propagation calculated by (ΔS≠m − ΔS≠r) and (ΔH≠m − ΔH≠r) were all negative for ROP at 70 °C, 90 °C and 110 °C, which indicated the preference of isotactic stereosequence for all the complexes. It had also been found that for each pair of the two complexes with the same bridging parts, complexes (2a, 4a and 6a) with bulkier substituents showed more negative ΔG≠ values at different temperature, comparing with their counterparts (1a, 3a and 5a) with smaller substituents. These results systematically indicated that bulky substituents at the 3-position enhanced the stereoselectivity for ROP of rac-LA. And the further kinetic studies showed that bulkier substituents with stronger steric hindrance reduced the ROP reactivity remarkably. Moreover, these complexes were also effective as catalysts for copolymerisation of rac-LA and ε-CL. By changing the feed ratio of rac-LA and ε-CL, copolymer materials with various monomer ratios were prepared intensively. Kinetic studies on copolymerisation and microstructure analysis results showed that reactivity of rac-LA was higher than ε-CL, and tapered block copolymers with gradient comonomer sequences were achieved by these complexes.

Acknowledgements

The financial support for this work was provided by the National Natural Science Foundation of China (No. 51203155, 51173183, 21574124, 51503203 and 51321062), the Ministry of Science and Technology of China (No. 2011AA02A202), and the Youth Foundation of Jilin Provincial Research Foundation for Basic Research (No. 20160520122JH).

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Footnote

† Electronic supplementary information (ESI) available: 1H NMR spectrum of oligomers of rac-LA, plots of PLA Mn and PDI (Mw/Mn) as a function of rac-LA conversion using complexes 1a–6a, NMR and HRMS spectra, kinetic plots for ROP and copolymerisations. See DOI: 10.1039/c6ra00289g
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