J Polym Res (2012) 19:9800 DOI 10.1007/s10965-011-9800-6

ORIGINAL PAPER

Novel ternary block copolymerization of carbon dioxide with cyclohexene oxide and propylene oxide using zinc complex catalyst Shaoyun Chen & Min Xiao & Shuanjin Wang & Dongmei Han & Yuezhong Meng

Received: 15 August 2011 / Accepted: 19 November 2011 / Published online: 22 February 2012 # Springer Science+Business Media B.V. 2012

Abstract Block copolymers of poly (propylene carbonate— cyclohexyl carbonate) (PPC-PCHC) were successfully synthesized by a one-pot method with the zinc complex catalyst (Zn2G). The IR and 1H-NMR and 13C-NMR spectra verified the introducing of PCHC segments in the copolymers. The GPC curves of the copolymers appeared only one peak and the DSC results showed three glass transition temperatures at 40 °C, 66 °C and 115 °C, indicating the three-block copolymer structure. TGA tests revealed that the thermal decomposition temperature of the synthesized block copolymers increased up to about 300 °C. The mechanical properties proved to be also enhanced greatly as evidenced by static and dynamic mechanical tests. The thermal and mechanical properties of the resultant block copolymers lay between those of PPC and PCHC, demonstrating the desired properties of a polymer can be achieved via block copolymerization. Keywords Block copolymerization . Carbon dioxide . Cyclohexene oxide . Propylene oxide Abbreviations PPC-PCHC poly (propylene carbonate cyclohexyl carbonate) PPC poly (propylene carbonate) Zn2G zinc complex catalyst GPC gel permeation chromtograghy Tg glass transition temperature DSC differential scanning calorimetry FT-IR Fourier transform infrared spectra NMR Nuclear Magnetic Resonance S. Chen : M. Xiao : S. Wang : D. Han : Y. Meng (*) School of Physics and Engineering, Sun Yat-Sen University, Guangzhou, Guangdong Province, China e-mail: [email protected]

TGA DMA

Thermogravimetric analysis dynamic mechanical analyser

Introduction Carbon dioxide is currently regarded as a major greenhouse gas which contributes about 66% of climate warming. On the other hand, it is the cheapest and most abundant raw material source of carbon [1–4].Thus, the efficient fixation of CO2 into useful chemicals became a topic of intensive study for the sake of environmental concerns and the circulation of carbon. A representative example is the copolymerization of carbon dioxide and epoxides to produce biodegradable aliphatic polycarbonate [5–8]. Since the pioneering work of Inoue in 1969 [5], the copolymerization of CO2 and epoxides have been widely investigated. It has been demonstrated that CO2 can copolymerize with epoxides such as ethylene oxide (EO) [6], propylene oxide (PO) [7], cyclohexene oxide (CHO) [8] et al. Among these, the copolymerization of CO2 with PO to synthesize poly (propylene carbonate) (PPC) has been extensively reported [9–11].Most of the researches are focused on how to promote the efficiency of polymerization and activity of catalysts. Recently, our laboratory [9] successfully synthesized PPC via the copolymerization of CO2 with propylene oxide using a supported zinc glutarate catalyst. By optimizing the reaction conditions, high-molecular-weight PPC with an alternating structure was obtained with high yield. However, the glass transition temperature (Tg) of PPC is rather low due to its flexible carbonate linkage in the backbone. The practical application of PPC has been limited by the poor thermal stability [7]. More recently, the work about the modification and applications of PPC has been paid

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much attention and recognition by researchers. The most important work is to enhance the thermal and mechanical properties of PPC, and introducing a third monomer into the copolymerization of CO2 and PO is considered a profitable means [12–16]. Terpolymerization of CO2, propylene oxide (PO) with cyclohexene oxide (CHO), [(2-naphthyloxy)methyl] oxirane (NMO), N-(2,3-epoxylpropyl) carbazole or maleic anhydride (MA) has been reported [17–22]. The resultant products show improved thermal and mechanical properties as compared with PPC. However, one glass transition which is higher than that of PPC but not higher than 45 °C was observed in most of the terpolymers indicating they are random copolymers. Block copolymers have found widespread use in drug delivery [23], thermoplastic elastomers [24] and so on. These types of polymers are typically synthesized by sequential monomer addition or macroinitiation [25]. Inspired by the structure of styrenic block copolymers and the articles reported [26–28], in this work, a di-block copolymers poly (propylene carbonate— cyclohexyl carbonate) (PPC-PCHC) derived from CO2, PO and CHO were prepared through a one-pot procedure with multi-component zinc catalyst (Zn2G). The thermal and mechanical properties of these di-block copolymers were expected to be further enhanced as compared with the random copolymers.

S. Chen et al.

Block copolymerization The block copolymerization was carried out in a 500 mL stainless steel autoclave equipped with a mechanical stirrer. Multi-component catalyst (Zn2G) was introduced into the autoclave and the autoclave with catalyst inside was dried for 24 h under vacuum at 80 °C and cooled down to room temperature before the reaction mixture was injected into it. Then the purified PO was immediately added into the autoclave. The autoclave was pressurized to 5.2 MPa via a CO2 cylinder and heated at 70 °C for 24 h. Following the evacuation of CO2 and unreacted PO, CHO was introduced into the autoclave in an inert atmosphere. The autoclave was repressurized with 5.2 MPa of CO2 and the reaction was performed at 80 °C for another 24 h. Then the pressure in the autoclave was reduced to atmosphere to terminate the block copolymerization. The resulting block copolymer was dissolved in a proper volume of chloroform and 15 mL dilute HCl (5 wt.%) was added to extract the catalyst residual from the product solution. The organic layer was then washed with distilled water for three times. The viscous solution was concentrated to a proper concentration by using a rotary evaporator. Finally, it was precipitated by being poured into vigorously stirred ethanol. The as-made block copolymer was filtered and dried under vacuum at a temperature of 120 °C until a constant weight was obtained. Measurements

Experimental Materials Carbon dioxide of a purity of 99.99% was used without further treatment. PO of a purity of 99.5% and CHO of a purity of 95.0% were refluxed over CaH2 for 4 h and 24 h, respectively and then distilled under dry nitrogen gas. Prior to use they were stored over 4-Å molecular sieves. Other solvents and reagents such an ethanol, acetone, chloroform were of analytical grade and used without further purification. Supported multi-component zinc dicarboxylate catalyst was prepared according to previous work [29] The catalyst was white powder with Zn content of 11.6 wt.%. Scheme 1 Di-block copolymerization of carbon dioxide with cyclohexene oxide in based poly(propylene carbonate)

FT-IR spectra were obtained by a PerkinElmer Spectrum 100 FTIR spectrometer. 1H-NMR and 13 C-NMR spectra of the block copolymer at room temperature using tetramethylsilane as an internal standard and D-chloroform(CDCl3) as solvent were recorded on a Bruker DRX-400 NMR spectrometer. Molecular weight (Mw and Mn) of the resultant polymer product was measured using a gel permeation chromatography (GPC) system (Waters 515 HPLC Pump, Waters 2414 detector) with a set of three columns (Waters Styragel 500,10000,and 100000 Å) and chloroform (HPLC grade) as eluent. The GPC system was calibrated by a series of polystyrene standards with polydisperisties of 1.02. The glass transition temperature (Tg) of the copolymers was measured by a differential scanning calorimeter

Novel ternary block copolymerization of carbon dioxide Table 1 The results of CO2, PO, and CHO block copolymerizationa

a Polymerization conditions: CO2 pressure 5.2 MPa, stirred at 100 rpm in a 500-mL autoclave b The molar fraction of the loaded CHO with respect to the total moles of CHO and PO c

As g of polymer per g of catalyst

d

Molecular weight was determined by GPC

e

Determined by 1H NMR spectroscopy

f

Could not be tested by GPC

Copolymer

PO:CHOb (feed molar ratio)

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Yield

c

Mn/Mw/PId

Composition (molar fraction%)e fCO2

fPC

fCHC

fPE

PPC

1:0

83.2

217 K/378 K/1.74

47.3

47.3

–

5.4

PPC-PCHC1 PPC-PCHC2

1:1 2:1

67.9 72.8

272 K/614 K/2.25 309 K/700 K/2.27

48.0 48.3

31.0 34.7

17.0 13.6

4.0 3.4

PPC-PCHC3

3:1

71.2

242 K/581 K/2.40

48.6

38.1

10.5

2.8

PPC-PCHC4 PPC-PCHC5

4:1 5:1

75.8 65.5

297 K/735 K/2.47 268 K/568 K/2.12

48.8 48.5

39.4 39.6

9.4 8.9

2.4 3.0

PPC-PCHC6

6:1

78.7

248 K/688 K/2.63

47.9

40.0

7.9

4.2

PPC-PCHC7 PPC-PCHC8

7:1 8:1

80.7 73.1

253 K/461 K/1.82 274 K/788 K/2.87

PCHC

0:1

56.0

–f

48.8 48.3 20.6

43.0 43.2 –

5.8 5.1 20.6

2.4 3.4 79.4

(Netzsch Model 204) and the measurements were carried out under nitrogen flow from −20 °C to 150 °C at a heating rate of 10 °C/min. Tg of the samples was determined from the second run. Thermogravimetric analysis (TGA) measurements were performed in a PerkinElmer Pyris Diamond TG/DTA analyzer under a protective nitrogen atmosphere. The temperature ranged from 50 °C to 500 °C with a heating rate of 10 °C/min. The static mechanical properties including tensile strength and elongation at break of the block copolymer were measured at 25 °C using a computer-controlled tensile tester (SANS-CMT4014) according to the ASTME-104 standard. The crosshead speed was kept at 50 mm/min. Five specimens of each sample were tested, and the average results were reported. Dynamic mechanical properties of

Fig. 1 The GPC results of the resultant polymers

the samples (3.14×(5 mm) 2 ×1 mm) were measured by a dynamic mechanical analyser (DMA) SDT861e (Mettler. ToleDo company) under nitrogen atmosphere at a heating rate of 2 °C/min from −30 °C to 150 °C. The tests were done under the frequency of 0.3, 0.5, 1.0, 3.0, 5.0, 10.0 Hz.

Results and discussion Proposed reaction mechanism of the block copolymerization The copolymerization of CO2 and epoxides to produce aliphatic polycarbonate using ZnGA as catalyst has been extensively investigated [10, 30, 31]. The mechanism of this reaction is considered to be anionic coordination mechanism. CO2 was activated by coordinating with Zn metal active centers of the catalyst, and then inserted into Zn-O bond leading to produce carbonate anion. Epoxide could

Fig. 2 FTIR spectra of the resultant polymers

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Fig. 3 a 1H NMR spectrum of PPC, b 1H NMR spectrum of PPC-PCHC

S. Chen et al.

Novel ternary block copolymerization of carbon dioxide

Fig. 4 a

13

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C-NMR spectrum of PPC b 13 C- NMR spectrum of PPC-PCHC c COSY spectrum of PPC-PCHC

also be activated by carbonate anion following the ring-open reaction and insertion into growing main chain. In this work,

we use a one-pot method to synthesize di-block copolymers, as shown in Scheme 1.

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S. Chen et al.

Fig. 4 (continued)

The one-pot method involved two steps. Firstly, PPC was synthesized via the copolymerization of CO2 and PO. The reaction was stopped by evacuating CO2 and PO but the chain termination reaction was supposed to not happen. The PPC with active end was named ‘Living Poly (propylene carbonate) (PPC) Oligomer’. Secondly, when CHO and CO2 were introduced into the autoclave, the ‘living PPC oligomer’ acted as a macroinitiator for the copolymerization of CHO and CO2, thus obtain the di-block copolymer. It is supposed that PO and CHO could homopolymerize to give polyether linkage, which existed in the resulted polymer in a small amount.

CO2, PO and CHO. In order to verify the reaction mechanism of block copolymerization, we need to know whether the resultant polymers are block copolymers PPC-PCHC or just the mixture of PPC and PCHC. From the GPC results, the number-averaged molecular weight of PPC-PCHC is slightly

Structure characterization of Di-block copolymers In this work, the feed ratios of PO to CHO were changed to synthesize the di-block copolymers with varying compositions. The results of the block copolymerization are summarized in Table 1. The copolymers with high molecular were synthesized in good yields ranged from 56.0 g to 83.2 g per gram of catalyst depending on the molar feed ration of PO to CHO. This indicates the high catalytic activity of Zn2G catalyst in the block copolymerization of

Fig. 5 Effect of the molar feed ratio on the composition of PPC-PCHC

Novel ternary block copolymerization of carbon dioxide

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Table 2 The thermal properties of the resultant polymers Sample

Tg (°C) (N2)

T-5% (°C)

Tmax (°C)

PPC PPC-PCHC1

44 41/67/120

253.7 254.3

278.5 307.1/340.2

PPC-PCHC2

40/68/117

261.5

303.6/345.1

PPC-PCHC3 PPC-PCHC4

42/66/110 41/67/119

263.1 261.6

305.9/350.0 304.2/342.7

PPC-PCHC5 PPC-PCHC6

39/67/119 37/68/118

264.2 258.8

302.2/342.7 300.0/340.0

PPC-PCHC7

41/66/113

259.6

298.2/339.1

PPC-PCHC8 PCHC

40/67/112 66/121

256.3 284.0

297.1/342.3 350.7/393.7

larger than PPC, while the weight-averaged molecular weight is much larger, which contributes a wider polydistribution index. This means that the proportion of high molecular weight of PPC-PCHC is greater than PPC, demonstrating that most of CHO polymerized with CO2 at the living end of PPC oligomer main chain to get the block copolymer. This contributed only one peak in GPC curve. However, we can not exclude that a small proportion of PPC and PCHC exited, i.e., there is a small tail in the GPC curve as shown in Fig. 1. As described in experimental section, the structure of resulting copolymers was characterized by FTIR, 1H-NMR, 13 CNMR techniques. The representative absorptions of PPC’s FTIR spectra were at around 1460 cm−1,1380 cm−1,2990 cm−1 (CH3);790 cm−1,1460 cm−1,2890 cm−1 (CH2);1340 cm−1 (CH);1750 cm−1 (s, C0O);1250 cm−1 (s, C-O);1074 and

Fig. 6 DSC curves of the resultant polymers

790 cm−1 (s, C-O-C). The existence of two adsorption peaks of carbonyl group (C 0O) and ether group at 1750 and 1250 cm−1 provided an evidence for the presence of carbonate unit in the resultant block copolymers. It can also be seen from Fig. 2 that there exists a new absorption peak at 1020 cm−1 which represents cycloalkane ring deformation vibration, indicating the presence of cyclohexene group in the resultant block copolymers. Figure 3 shows the typical 1H NMR spectra for the PPC, PPC-PCHC block copolymer. The 1H NMR spectrum of PPC (Fig. 3 (a)) indicates the existence of carbonate linkages with small amount of ether linkages on PPC backbone, the main signals are assigned as follows: 1H-NMR (CDCl3), δ (ppm) 1.3 [3H, -CH3], 3.5 [3H, -CH2CH-], 4.2 [2H, CH2CH-] and 5.0 [1H,-CH2CH-]. The signals for PPCPCHC1 are assigned as follows: 1.3 [3H, CH3], 4.2 [2H, CH2CH], 5.0 [1H, CH2CH]. Comparing to PPC, the peaks of 1.7 ppm, 2.1 ppm, 4.6 ppm representing CH2 of M-, O-cycloalkane ring and CH link to carbonate indicated the cyclohexene carbonate unit in the block copolymer. The 13 C-NMR spectra of PPC and PPC-PCHC1 are shown in Fig. 4 (a) and (b) respectively. The main signals for PPC are assigned as follows: 13 C-NMR (CDCl3), δ (ppm) 16.2 (-CH3), 69.0 (-CH2CH-), 72.3 (-CH2CH-), 153–155 (-OCOO-). The signals for PPC-PCHC are assigned as follows: 13 C-NMR (CDCl 3 ), δ(ppm) 16.1 (-CH 3 ), 69.0 (-CH 2 CH-), 72.3 (-CH2CH-), 77.3 (-OCHCHO-) (overlapped with the peak CDCl3, which could be investigated in the COSY spectrum of PPC-PCHC),153–155 (-OCOO-). Compared with PPC, the 13 C NMR signals at 22.9, 29.6 which represent the carbon of O-, M-cycloalkane ring indicates that CHO copolymerized with CO2 in based ‘living PPC oligomer’.

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S. Chen et al.

According to the area integration of the 1H NMR spectra of the block copolymers, the composition of resultant copolymers can be calculated based on the following equation: fCHC ¼

fCO2 ¼

fPE ¼

A4:6 ½ðA4:6 þ A4:2 Þ  2 þ 0:8  A3:5 100% A4:6 þ A4:2 ½ðA4:6 þ A4:2 Þ  2 þ 0:8  A3:5 100% 0:8  A3:5

½ðA4:6 þ A4:2 Þ  2 þ 0:8  A3:5 

100%

ð1Þ

ð2Þ

ð3Þ

The compositions of resultant copolymers including the molar fractions of PC, CHC, CO2 and PE are listed in Table 1. It can be seen that the existence of small fraction of random incorporated ether units in PPC and PPC-PCHC backbone, indicating the almost alternative structure of PPC and PPC-PCHC. It can be also seen that the content of polyether linkage in case of PCHC synthesis is quite high, nearly 80%. This means that large amount of CHO homopolymerized instead of copolymerized with CO2. This is similar with the case that the homoolymerization of CHO easily takes place using the catalyst systems of zinc complexes [32–34]. Figure 5 shows the relationship of the ratio of propylene carbonate unit to cyclohexene carbonate unit and the feed molar ratio of epoxide monomers. From this figure, we can see that the content of cyclohexene carbonate unit increases with increasing the feed ratio of CHO to PO. However, the content of cyclohexene carbonate unit in most of the copolymers is less than the content of real feeding. This is believed to be resulted from the lower activity of CHO compared to Fig. 7 TGA curves of the resultant polymers

Table 3 The mechanical properties of the resultant polymers Sample

Tensile strength(MPa)

Elongation(%)

PPC PPC-PCHC1

27.6 41.0

23.0 5.6

PPC-PCHC2

43.0

9.3

PPC-PCHC3 PPC-PCHC4

38.1 40.2

9.0 9.2

PPC-PCHC5

42.0

7.1

PPC-PCHC6 PPC-PCHC7

36.9 37.1

9.8 10.2

PPC-PCHC8

36.8

PCHC

–

8.9 –

PO that may arise from the difficulty of coordination between catalyst and CHO. Moreover, CHO monomer diffuses much slowly to active end because of its high viscosity in the second reaction stage. This subsequently favors the formation of polyether linkages. Thermal properties of Di-block copolymers As well known, glass transition temperature is a very important parameter for the application of a plastic. It is the macro reflection of polymer chain’s state from freezing to moving, while the chain movement is achieved through the rotation of bonds. Therefore, the stiffness of a polymer chain is one of the most important factors for increasing Tg. The Tg of PCHC is much higher than PPC due to the rigid sixmembered ring existed in CHO. The thermal properties of the resultant copolymers and block copolymers based on PPC are shown in Table 2. It is apparent that the resultant

Novel ternary block copolymerization of carbon dioxide

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Fig. 8 DMA curves of a PPC, b PPC-PCHC4, c Comparison at 10 Hz

PPC-PCHC copolymers exhibited three Tg (nearly 40 °C, 66 °C, 115 °C) (Fig. 6), representing the Tg of PPC segment, PCHO segment (the polyether of homopolymerization of CHO), and PCHC segment in its turns. This demonstrates that the block copolymers were successfully synthesized. The Vica soft point of (VST) of PPC, PPC-PCHC1 and the random copolymer PPCCHC were measured. However,

Fig. 9 Energy minimized space filling structure of a PPC; b PPC-PCHC

the VST value of PPC is too low to be detected, while the VST values of PPC-PCHC1 and PPCCHC are 55.6 °C and 46.4 °C, respectively. The thermal deformation temperature of the copolymers is greatly improved by introducing the third monomer CHO, and the improvement is more significant for the block copolymer due to the stiff segment PCHC in pure PPC.

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S. Chen et al.

Figure 7 is the TG curves of resultant block copolymers. It can be seen that the thermal decomposition process of the block copolymer PPC-PCHC4 has two stages. The first stage is the decomposition of the PPC segment, and the second stage corresponds to the decomposition of the PCHC segment. The 5% weight loss temperature (T-5%), maximum weight loss temperature (Tmax) of PPC and PCHC were 253.7 °C, 278.5 °C and 284.0 °C, 350.7 °C in its turns as shown in Table 2. The thermal decomposition temperatures of block copolymers are between that of PPC and PCHC. The first Tmax (the Tmax of the PPC-PCHC segment) of the block copolymers is 20 °C higher than that of PPC. According to previous work [35], the thermal decomposition of PPC obeys two kinds of mechanism, the main chain scission reaction and the unzipping reaction. The unzipping reaction involves the backbiting of the terminal hydroxyl groups at the carbon of carbonate linkage leading to the formation of cyclic propylene carbonate. The introducing of PCHC segment at the end of the PPC segment restrains the backbiting process and depresses the unzipping reaction, leading to highly thermal stable block copolymers.

the rotation of the molecular chain, leading to an improvement of mechanical and thermal properties.

Mechanical properties of block copolymers

Acknowledgements The authors would like to thank the China HighTech Development 863 Program (2009AA034900,2009AA03Z340), Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2010), Guangdong Province Sci & Tech Bureau (Key Strategic Project Grant No. 2008A080800024, 10151027501000096), and Chinese Universities Basic Research Founding for financial support of this work.

The static mechanical properties of the resultant copolymers were determined in terms of tensile strength and elongation at break. The results are listed in Table 3. It is evident that the block copolymers exhibit a superior tensile strength to PPC. This is because CHO possess a rigid six-membered ring. The tensile strengths of PPC could be enhanced via a block copolymerization of CO2 with CHO. On the other hand, the elongation at break of the block copolymers is found to be lower than that of PPC, but higher than that of PCHC. The PCHC homopolymer was too brittle to be measured under the same conditions. The dynamic mechanical properties of PPC and the block copolymer PPC-PCHC4 are shown in Fig. 8. From this figure, we can see that with increasing frequency, both storage modulus and tanδ are right-shifting. The storage modulus of PPC-PCHC4 is much higher than that of PPC, indicating the stiffness of block copolymer is greatly enhanced compared to PPC. The tanδ spectrum of PPC-PCHC4 appeared two peaks at around 60 °C and 80 °C, which are related to the first two Tgs in DSC curves, but higher than DSC results, which is general rule for the Tg measurement using DMA technique. Third peak was not observed for homopolymerization CHO in the tanδ spectrum corresponding to the peaks in DSC curves because the quantity of PCHC homopomer was too little. Figure 9 shows the energy minimized PPC and PPCPCHC molecular chains. From the relative long molecular chain, it can be imaged that the cyclohexeyl groups restrict

Conclusions The ternary block copolymerization of CO2, CHO and living PPC can be effectively carried out using the same catalyst for the copolymerization of CO2 and PO through a one pot method. The resultant ternary block copolymers exhibited three glass transition temperatures at 40 °C, 66 °C and 115 °C. The Vica soft point of the block copolymers increased up to 57 °C, and the maximum weight loss temperatures of the block copolymers were 20 °C higher than that of PPC. The thermal stability of PPC can be greatly improved by the introduction of rigid CHO moiety. Moreover, the tensile strength of the block copolymers was enhanced to a very high value of 40 MPa. These results demonstrated that the novel block copolymerization method provides an effective way to synthesize highly thermal stable PPC based copolymers, and extends the potential application of PPC as a new biodegradable thermoplastic.

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