(PTT-b-PTMO-b-PTT)n Multiblock Copolymers - ACS Publications

Sep 11, 2013 - One Pot, One Feeding Step, Two-Stage Polymerization Synthesis and Characterization of (PTT-b-PTMO-b-PTT)n Multiblock Copolymers...
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One Pot, One Feeding Step, Two-Stage Polymerization Synthesis and Characterization of (PTT‑b‑PTMO‑b‑PTT)n Multiblock Copolymers Qiaozhen Xu, Jianying Chen, Weichun Huang, Taoguang Qu, Xiaohong Li, Yaowen Li, Xiaoming Yang, and Yingfeng Tu* Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of Polymer Science and Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China S Supporting Information *

ABSTRACT: We present here the one pot, one step synthesis of poly(trimethylene terephthalate)-block-poly(tetramethylene oxide) multiblock copolymers (PTT-b-PTMO-b-PTT)n by melt polymerization of cyclic oligo(trimethylene terephthalate)s (COTTs) with PTMO macroinitiator. An improved quantitative 1H NMR characterization technique was developed and applied to investigate the multiblock copolymers’ structure with different reaction time by the chain end and functional groups estimation. The polymerization kinetics was revealed, and the results indicated a two-stage polymerization mechanism: melt ring-opening polymerization of cyclic oligo(trimethylene terephthalate)s (COTTs) by PTMO macroinitiator to form triblock copolymers at first stage, followed by the in-situ condensation polymerization of block copolymers to produce multiblock copolymers at second stage. This was further confirmed by the gel permeation chromatography (GPC) and viscosity experiments. To our knowledge, this is the first reported poly(ether ester) multiblock copolymers synthesized by one step polymerization process and with controlled structures. These multiblock copolymers show good thermal stability and double crystalline properties.

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INTRODUCTION Poly(ether ester) copolymers are a class of important industrial thermal plastic polymers with polyethers as soft segments and crystalline polyesters as hard segments, and have many applications in molded shoe soles, ski boots, automotive parts, wires and cables, shock absorbers, and biomedical applications.1−5 The most frequently used soft segments are poly(tetramethylene oxide) (PTMO, also known as poly(tetrahydrofuran) or PTHF), while the hard segments are aromatic semicrystalline polyesters, due to their good mechanical properties, toughness, chemical resistance, excellent surface appearance, and stable electrical insulation properties.2,6,7 Normally, these copolymers are synthesized by condensation polymerization of low molecular weights dihydroxyl-terminated polyethers with polyester monomers at high temperatures.8−12 This process produces random multiblock copolymers with some drawbacks; e.g., the structure of the hard polyester segments is uncontrollable, the total molecular weights of the polymers are not too high, and it requires high vacuum to remove the produced alcohols.13 Compared to the well-studied poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT) polyesters, poly(trimethylene terephthalate) (PTT) has attracted great interest recently due to its excellent elastic recovery and moderate modulus properties, after the economically and ecologically production of 1,3-propanediol monomer from starch by using a fermentation process.14 However, research works focused on poly(ether ester)s based on PTT are only a © 2013 American Chemical Society

few. Szymczyk et al. reported the synthesis of a series of random segmented block copolymers of poly(trimethylene terephthalate) and polyethers by two-step condensation polymerization, with the rigid segments as well as the flexible poly(ethylene oxide) or poly(tetramethylene oxide) segments randomly distributed along the chain.9,10,15,16 These types of copolymers have good thermoplastic elastomer properties, with the phase structure as well as thermal and mechanical properties affected by copolymer composition. However, there is no detailed analysis of the structure of copolymers, since it can hardly obtain regular and orderly structure due to uncontrolled polymerization process as stated above, and the reproducibility from two-step condensation polymerization is not good. To resolve these problems, one needs to find other polymerization methods to produce poly(ether ester)s with well-defined structure. Besides condensation polymerization, ring-opening polymerization (ROP) of lactones and lactide can produce polyesters with living polymerization characters.4,17−21 This method works well with aliphatic polyesters but has problems on aromatic polyesters due to the difficulty in synthesis of their corresponding cyclic monomers, until Brunelle et al. reported the synthesis of corresponding cyclic oligomers.22−24 Compared to traditional condensation polymerization, ROP of cyclic Received: May 9, 2013 Revised: August 24, 2013 Published: September 11, 2013 7274

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Scheme 1. Synthetic Route to (PTT-b-PTMO-b-PTT)n Multiblock Copolymers: (1) Formation of PTT-b-PTMO-b-PTT Triblock Copolymers by Ring-Opening Polymerization of COTTs with PTMO; (2) Formation of Multiblock Copolymers by Condensation Polymerization of Triblock Copolymers; (3) Possible Transesterification Reactions during Melt Polymerization Process

were synthesized, and the thermal properties of these materials were studied by TGA and DSC.

oligo(ethylene terephthalate)s (COETs) or cyclic oligo(butylene terephthalate)s (COBTs) produces their corresponding aromatic polyesters with rapid polymerization rate,25−28 much higher molecular weights,13,25 and little small molecular byproduct. For example, high molecular weights PBT and its copolymers with PET was prepared via ROP using several catalysts within 10−20 min, with the molecular weights about 95 000−115 000 Da.23 However, the reported research works on ROP of cyclic oligo(trimethylene terephthalate)s (COTTs) are few. Kyeong Pang et al. reported the synthesis of PTT by the ROP of cyclic trimethylene terephthalate dimer29 and found that titanium(IV) butoxide was the most effective catalyst. Wan et al. studied the catalyst’s efficiency in ROP of COTTs to PTT, and Sb2O3 and titanium(IV) butoxide were found to be effective.30 Since ring-opening polymerization of lactones with dihydroxyl-terminated polyethers in solution at mild conditions can produce polyester-block-polyether-block-polyester triblock copolymers,17−21,31 we propose the melt ROP of cyclic oligo(aromatic ester)s by dihydroxyl-terminated polyethers will produce triblock copolymers. Specially, PTT is chosen as the polyester segment while PTMO as polyether segment in this work. Since the polymerization must be carried above the melting temperature of the COTTs (210 °C), it is possible that condensation polymerization and transesterification reactions occur at this high temperature. As a result, multiblock copolymers can be obtained by this process. With the calling of a good calibration method for the analysis of absolute molecular weights and detailed structures of block copolymers, we developed an improved quantitative 1H NMR characterization technique in this work to reveal the block copolymers’ composition by chain end and functional groups analysis and the polymerization kinetics. As a series of thermal plastic polymers, block copolymers with different soft segment content

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EXPERIMENTAL SECTION

Materials. Terephthaloyl chloride (TPC) (Alfa Aesar, 99%) was recrystallized from petroleum ether three times. 1,3-Propanediol (PDO) (J&K Chemical, 98%) was purified by vacuum distillation after stirring with calcium hydride overnight. 1,4-Diazabicyclo[2.2.2]octane (DABCO) (Acros, 99%) was purified by sublimation in vacuum. Triethylamine (Et3N) (Xilong Chemical of Guangzhou, AR), dichloromethane (CH2Cl2) (Qiangsheng chemical of Suzhou, AR), and tetrahydrofuran (THF) (Qiangsheng chemical of Suzhou, AR) were purified by stirring with calcium hydride overnight and then distilled in the protection of nitrogen. Dihydroxyl-terminated PTMO (PTHF-2900, Sigma-Aldrich, numbers stand for average molecular weights) and titanium tetrabutoxide (Ti(n-C4H9O)4) (Alfa Aesar, 98%) were used as received. Synthesis of COTTs. COTTs were synthesized by using a similar procedure as references by terephthaloyl chloride (TPC) and 1,3-PDO under pseudo-high-dilution conditions in dichloromethane using 1,4diazabicyclo[2.2.2]octane (DABCO) as catalyst.23,32 Yield: 36%. 1H NMR (CHCl3-d, δ): 7.04−8.06 (m, 4H, Ar H), 4.52−4.62 (m, 4H, CH2O), 2.28 (m, 2H, OCH2CH2CH2O). Synthesis of (PTT-b-PTMO-b-PTT)n Block Copolymers. The synthesis of the poly(ether ester) multiblock copolymers is represented in Scheme 1. In a typical polymerization procedure, predetermined amounts of COTTs and PTMO-2900 were charged in a three-necked flask with mechanical stirring and nitrogen inlet. The mixture was heated to 240 °C for 5 min under a nitrogen atmosphere, and then Ti(n-C4H9O)4 was added to start the polymerization. For polymerization kinetic studies, a small portion of mixture was taken out at different reaction time for 1H NMR, GPC, and viscosity measurements. Instrumentation. The 1H NMR spectrum was recorded on an INOVA 400 MHz nuclear magnetic resonance instrument using CDCl3 or CF3COOD/CDCl3 (1:10 in volume, for all block copolymers) as the solvent and tetramethylsilane (TMS) as the 7275

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Figure 1. 1H NMR spectra of COTTs, PTMO, and (PTT-b-PTMO-b-PTT)n copolymers with different reaction times. Solvent: CF3COOD/CDCl3 (50:500 in volume); concentration: 0.02 g/mL. Polymerization conditions: mPTMO/mCOTT = 2/1 with 0.05 wt % Ti(n-C4H9O)4 at 240 °C. internal standard, with the solution concentration of 0.02 g/mL. GPC experiments were performed on a modular system comprising a Waters 1515 pump, a 717 plus autosampler, and a 2487 UV detector with three 300 mm (length) × 7.5 mm (inner diameter) columns including particle size of 5 μm (PL gel Mixed-C, Polymer Laboratories). The polymers were dissolved (2.5 mg/mL) in a mixture solvent (chloroform and 1,1,1,3,3,3-hexafluoro-2-propanol, 95:5, v/v) and subjected for GPC characterization with the mobile phase flow rate of 0.60 mL/min at 35 °C. The molecular weights were calculated using nine narrow distribution polystyrene standards from 8 710 000 to 474 g/mol. The apparent viscosity test was taken on SNB-3 digital viscometer from Shanghai Nirun Intelligent Technology Co. Ltd., with the polymer concentration of 5 mg/mL in phenol/ tetrachloroethane (1:1, in mass). Thermogravimetric analysis (TGA) was performed at a heating rate of 10 °C min−1 from room temperature to 800 °C under a continuous nitrogen flow of 50 mL min−1 with a TA Instruments SDT-2960TG/DTA. The temperature of thermal degradation (Td) was measured at the point of 5% weight loss relative to the weight at room temperature. The differential scanning calorimetry (DSC) was carried out on the TA Q100 instrument under a nitrogen atmosphere in the temperature range from −20 to 250 °C at a heating and cooling rate of 10 °C min−1. The first cooling and second heating scans were used to determine the melting and crystallization peaks. For glass transition temperature measurements, the temperature range is from −160 to 270 °C with a heating rate of 20 °C min−1 to enhance the sensitivity.

the backbone, which makes their peak integration value smaller due to the shortage of NMR relaxation delay and causes bigger system errors. For the better structural determination of our multiblock copolymers, we used an improved quantitative 1H NMR method where the decoupling of 13C to 1H was applied only during the acquisition, with the delay time (20 s) set as 10 times of relaxation time (T1) and the experiments conducted at 90° pulse for the maximum signal acquisition.40 Figure 1 is the 1H NMR spectra of PTMO, COTTs, and (PTT-b-PTMO-b-PTT)n copolymers polymerized with different reaction time with the feeding ratio of 2/1 (mPTMO/mCOTT) and 0.05 wt % Ti(n-C4H9O)4. The corresponding assignment of peaks to the polymer chemical structure is shown above the figure. A detailed list of integration values for all peaks can be found in the Supporting Information (Table S1). It clearly shows that after reacted for 30 min the peak at chemical shift of 4.38 ppm corresponding to the functional end group (CH2OH) of PTMO disappeared, indicating all the end groups of PTMO were reacted with COTTs. On the other hand, peaks at position of 4.43, 3.97, and 2.14 ppm appeared. The peak at 4.43 ppm corresponds to the PTMO’s methylene protons at chemical position d, which is bonded with PTT segments by ester groups, while the other two correspond to the PTT’s methylene protons at polymer chain end at position g and i, respectively, as listed in Figure 1. For PTMO homopolymers, the average repeating units can be calculated from the ratio of integration value of methylene group at chain end (CH2OH, peak d*) to the total methylene oxide groups (CH2O, peaks d* and f). The calculated value is 43.6, which corresponds to the number-average molecular weights of 3100 g/mol. This value is very close to the molecular weights provided by the producer (2900 g/mol), indicating the accuracy of our NMR acquisition method. The diminished peak of g and i (δ = 3.97 and 2.14 ppm) with prolonged reaction time, which belongs to the terminal groups at chain end, clearly indicates the increment of polymer molecular weight. The total repeating units of TT (NTT) and TMO (NTMO) in copolymers can be calculated by integration

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RESULTS AND DISCUSSION The chain-end estimation method by 1H NMR to determine the number-average molecular weights of polymers is wellestablished.33−37 However, this method usually works well for polymers with small molecular weights of several thousand daltons. For polymers with molecular weights higher than 104 Da, the error becomes greater. This is due to the couplings of 13 C to 1H take at least 1% integrated peak area from those major peaks of 1H connected with 12C.38,39 Another fact people often overlooked is the different relaxation time of protons at the chain end and on the backbone. If the relaxation delay provided at the front of each acquisition scan was not long enough, NMR signals would be suppressed. Normally, the chain-end protons have longer relaxation time than those on 7276

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Table 1. Integration Value of Peaks by 1H NMR for PTMO Macroinitiator and (PTT-b-PTMO-b-PTT)n Multiblock Copolymers at Different Time and in the Corresponding Structures Calculateda peak Integration (δ ppm) sample

reaction time (min)

d

30 60 90 120 150 180

4.00b 4.00 4.00 4.00 4.00 4.00 4.00

PTMO P1-1 P1-2 P1-3 P1-4 P1-5 P1-6

g

f

1.68 1.57 1.41 0.93 0.80 0.55

170.5 166.3 162.6 162.0 160.0 155.5 145.3

c

NTMO

NTT

STMO

STT

n

Mn (kg/mol)

28.1 27.4 27.3 27.4 26.8 25.3

43.6 101 106 118 176 199 272

16.7 17.5 19.4 29.5 33.5 46.0

42.6 41.7 41.7 41.0 39.9 37.3

7.03 6.8 6.8 6.8 6.7 6.3

2.38 2.55 2.84 4.30 5.00 7.27

3.1 10.7 11.2 12.5 18.7 21.2 29.1

a

NTMO and NTT are the corresponding average repeating units of TMO or TT in whole block copolymers, while STMO and STT are the corresponding polymer segments, respectively. n is the repeating units of (PTT-b-PTMO-b-PTT) segments in whole polymers, and Mn is the number-average molecular weight of polymer. Reaction conditions: mPTMO/mCOTT = 2/1 with 0.05 wt % Ti(n-C4H9O)4 at 240 °C. bCorresponding value for d* peak.

with increasing reaction time. Considering the fact that the total molecular weights of the polymer increased with reaction time, these results demonstrate that condensation polymerization of PTT-b-PTMO-b-PTT triblock copolymer was carried at these studied regions with the formation of corresponding multiblock copolymers as illustrated in Scheme 1. This is supported by the observation of small 1,3-propanediol droplets in glass tube near nitrogen outlet part. Since there is only PTT’s chain end groups observed while no PTMO’s observed from NMR, the structure of multiblock copolymer can be assigned as (PTT-b-PTMO-b-PTT)n (or PTT-b-(PTMO-b-PTT)n). The average repeating number of PTT-b-PTMO-b-PTT block segment, n, can be calculated by the following equation:

ratio of chain end groups to corresponding groups of repeating units by 1H NMR spectra. Here we choose the chain end group of g, aromatic hydrogen (peak c at 8.11 ppm), and methylene oxide hydrogen (peak f at 3.64 ppm and peak d at 4.43 ppm) for calculation using the following equations:

Ig Ic

=

4 4NTT

Ig Id + I f

=

4 4NTMO

(1)

(2)

where Ig, Ic, Id, and If are the integration values for the corresponding peak at g, c, d, and f, respectively. The total number-average molecular weights for the whole polymer (Mn) can thus be calculated by Mn/Da = 72.0 × NTMO + 206 × NTT, and the results are listed in Table 1. From Table 1, it clearly shows that the total molecular weights of the polymers increased with reaction time as the total repeating units of TT (NTT) and TMO (NTMO) increase. However, it also shows that the integration value of the aromatic hydrogen corresponding to COTTs at different reaction times is very small for each studied time period and decreases slowly with prolonged reaction time (see Table S2 in Supporting Information).41 The conversions of COTTs to PTT from NMR are all around 95%, similar to the reported results by Pang et al., where the ROP of cyclic dimer of PTT was studied.29 The results indicate that the ROP rate of COTTs by PTMO macroinitiators is rapid and finished within 30 min. On the other hand, the increment of molecular weights after 30 min is not from ROP, but from the condensation polymerization of block copolymers, as proposed in Scheme 1. To prove this, the average molecular weights of PTMO and PTT in each segment are calculated. As mentioned above, peak d in the NMR figure can be assigned to the methylene end group of PTMO linked with PTT by ester bond. Thus, the average repeating units of TMO (STMO) and TT (STT) in each segment can be calculated by the following equations: Id 4 = Ic 4STT

(3)

Id 4 = Id + I f 4STMO

(4)

Id 4n = Ig 4

(5)

The calculated n values at different polymerization time are listed in Table 1. These results show the number of block segments increased with reaction time. When reacted for 30 min, a mixture of pentablock copolymers and heptablock copolymers was formed on average since the n value is 2.37, while pentadecablock copolymers and higher formed after reacted for 180 min. The influence of the catalyst amount on the polymerization time and copolymer structure is studied, and the results are presented in the Supporting Information (Figure S1 and Table S3 for 0.1 wt % catalyst, Figure S2 and Table S4 for 0.2 wt % catalyst). The average repeating units of TMO (STMO) and TT (STT) in each segments are almost the same during the studied reaction time and different catalyst amount, indicating they are not affected by the reaction time or catalyst amount. The total repeating units of TT (NTT) and TMO (NTMO) in copolymers, and the total molecular weights of the polymers, increased with reaction time, similar to the results observed in polymerization with 0.05 wt % catalyst, which reveals the formation of multiblock copolymers. However, with the same reaction time, NTT and NTMO of multiblock copolymers are higher than those with lower catalyst amount. Clearly, Ti(n-C4H9O)4 acts not only as a ring-opening polymerization catalyst but also as a condensation polymerization catalyst. This special property provides the facile one pot one step synthetic route to the multiblock copolymers. Figure 2 is the calculated number-average molecular weights (Mn) of the multiblock copolymers as a function of reaction time and catalyst amount. The molecular weights increased

The calculated values are listed in Table 1. The results show that the average repeating units of TMO and TT in corresponding PTMO or PTT segment are almost the same 7277

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Figure 2. Molecular weights calculated from 1H NMR spectra of (PTT-b-PTMO-b-PTT)n multiblock copolymers with different catalyst amounts and reaction times. Polymerization conditions of multiblock copolymers: mPTMO/mCOTT = 2/1 with Ti(n-C4H9O)4 as catalyst at 240 °C.

Figure 3. Degree of polymerization of multiblock copolymers at different reaction time with different amount of catalyst. Polymerization conditions: mPTMO/mCOTT = 2/1with Ti(n-C4H9O)4 as catalyst at 240 °C.

the reaction constant related linearly with the catalyst amount, supporting our deduction of eq 6. By using special eluent, the apparent molecular weights and polydispersity of polyesters can be estimated.42 To demonstrate the reliability of our results from NMR techniques, the multiblock copolymers in Table 1 were characterized by GPC, with chloroform and 1,1,1,3,3,3-hexafluoro-2-propanol (95:5, v/v) mixture solvent as eluent, and the results are presented in Figure 4. For COTTs, it clearly demonstrates the

with reaction time, in a somewhat linear trend, though the correlation fits only well for the data of 0.05 and 0.1 wt % catalyst amount, but not for 0.2 wt % catalyst amount. After reacted for 180 min with catalyst amount of 0.2 wt %, the molecular weights of the multiblock copolymers are higher than 90 000 g/mol. Such high molecular weights are very difficult to achieve by a traditional condensation polymerization and are the merit of current polymerization process. Since the ROP of COTTs by PTMO is finished before 30 min, and then condensation polymerization of PTT-b-PTMOb-PTT triblock copolymers occurs after that, the triblock copolymers can be looked as an AB monomer based on the reaction in Scheme 1. Then we have the following equation to describe the reaction kinetics if it follows:

X n = k′t + 1

(6)

where Xn is the average degree of polymerization for condensation polymerization of AB-type monomer, k′ is constant for a given condition which is related to the starting function groups concentration, the catalyst concentration, and the polymerization reaction constant, and t is the reaction time. Detailed deduction of eq 6 can be found in the Supporting Information. The degree of polymerization, Xn, can be calculated from NMR and is the same as repeating units of triblock copolymer (n) when the multiblock copolymer being represented as (PTT-b-PTMO-b-PTT)n. Figure 3 shows the degree of polymerization increases with reaction time. The lines are the linear fitting curves of the corresponding data. Clearly, the fitting lines for the reaction with 0.05 and 0.1 wt % catalyst amount fit well with the corresponding data, supporting the condensation polymerization mechanism as proposed in Scheme 1 at second stage. The exception in data with 0.2 wt % catalyst amount may come from their high molecular weights, since the errors of integration value of end groups are larger for polymers with higher molecular weights, or the reaction mechanism changed when the catalyst concentration is high. The slopes for the linear fitting curves are 0.031 min−1 for reaction with 0.05 wt % catalyst, 0.071 min−1 for reaction with 0.1 wt % catalyst, and 0.132 min−1 for reaction with 0.2 wt % catalyst. The linear increasing values of the slope with catalyst amount suggest that

Figure 4. GPC curves of (PTT-b-PTMO-b-PTT)n multiblock copolymers synthesized at different polymerization times. Polymerization conditions: mPTMO/mCOTT = 2/1 with 0.05 wt % Ti(n-C4H9O)4 at 240 °C with 30 (P1-1), 90 (P1-3), and 150 min (P1-5) reaction time, respectively. Eluent: chloroform and 1,1,1,3,3,3-hexafluoro-2propanol, 95:5, v/v.

cyclic oligomers with different size (p = 2−6), similar to other reports.23,32 For copolymers with increased polymerization time, the curves shifted to shorter retention time, indicating the increment of molecular weight, which agrees well with the NMR results. The polydispersity of the main peak is around 2.3 for all the copolymers (see Table S5), typical value for condensation polymerizations. The small peaks at longer retention time belong to the remaining COTTs (about 6%, see Table S5), with the cyclic dimer being the majority, and the calculated conversion agrees well with NMR results. Figure 5 represents the data from viscosity measurements of the multiblock copolymers at different reaction times. By fixing 7278

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most industrial polymers. It also has the advantage of versatile solvent selection over other methods and thus can be applied to determine the molecular weights of polymers that are hard to be dissolved in common GPC eluent. For example, the determination of molecular weights of aromatic polyesters is very important in industry, yet normally only apparent molecular weights (or viscosity) were reported. The NMR method we developed thus may have great applications in these fields. To illustrate if this one pot, one step polymerization technique can be applied in the synthesis of multiblock copolymers with different structure, a series of (PTT-b-PTMOb-PTT)n multiblock copolymers with different PTMO content were synthesized using similar polymerization conditions. The structures of these multiblock copolymers were characterized by 1H NMR, and the results are presented in Table 2. This polymerization technique works well for the synthesis of block copolymers with different PTMO weight content range from 40% to 75%, and the content of each block in multiblock copolymers is very close to the feeding ratio, indicating the easy control on the final copolymer composition by this synthetic method and the versatile application in multiblock copolymer synthesis. The thermal stability of these multiblock copolymers with different PTMO content was studied by TGA, and the results are listed in Table 2. Figure 6 shows the TGA curves of the

Figure 5. Apparent viscosity of (PTT-b-PTMO-b-PTT)n multiblock copolymers synthesized with different reaction time at the reaction condition of mPTMO/mCOTT = 2/1 and 0.2 wt % Ti(n-C4H9O)4 at 240 °C. Experimental conditions: phenol/tetrachloroethane (1:1, in mass) as solvent, concentration: 5 mg/mL, shear rate: 50 rpm, temperature: 20 °C.

the same experimental conditions (concentration, temperature, solvent composition, and shear rate), a higher viscosity measured indicates the higher molecular weights of the measured polymer. It shows the apparent viscosity of the solution increased with reaction time, coinciding well with the results from NMR data. In the Mark−Houwink equation [η] = KMηα, the K and α parameters are empirical parameters and dependent on the solvent−polymer interaction and temperature. For block copolymer systems, normally K and α parameters are different for different blocks with a given solvent, so the equation can hardly be applied to estimate the molecular weights of block copolymers due to the difficulty in finding the universal parameters.43 This is more complicated in multiblock copolymer systems, as the structures of the multiblock copolymers with different molecular weights are not the same, and they may have lower dynamic viscosity compared to the similar homopolymer component.44 How can this be applied in the multiblock copolymers is interesting, and we are currently investigating that. Since the molecular weights measured from 1H NMR are the absolute number-average molecular weights, the above results indicate the method we developed provides a good way for the characterization of polymer molecular weights and block copolymer structures. As listed in Table 1 as well as Tables S3 and S4, this method works well when the molecular weights are below 105 Da, which covers the molecular weights range of

Figure 6. TGA curves of (PTT-b-PTMO-b-PTT)n multiblock copolymers as well as PTMO and COTTs precursors under N2. Heating rate: 10 °C/min.

Table 2. 1H NMR Results and Thermal Properties of (PTT-b-PTMO-b-PTT)n Multiblock Copolymers with Different PTMO Content Synthesized by the One Pot, One Step Polymerization Techniquea Tg (°C)

Tm (°C)

sample

ratiob

WPTMO (wt %)

NTMO

NTT

n

Mn (kg/mol)

PTMO

PTT

PTMO

PTT

Td (°C)

P2 P3 P4 P5 P6

1:1.5 1:1 1.5:1 2:1 3:1

40.7 51.2 61.2 68.1 75.2

116 356 188 453 233

58.8 119 41.5 74.1 26.7

3.54 9.30 4.88 11.8 7.84

20.5 50.1 22.1 47.9 22.3

−77.9 −76.2 −98.0 −102.8 −119.2

51.7 44.8 53.8 52.0

19.4 19.2 18.2 16.2 15.5

225 225 224 221 186

352 345 344 334 333

a WPTMO is the weight fraction of PTMO in copolymers determined by NMR; Tg(PTMO) and Tg(PTT) are the glass transition temperatures for corresponding PTMO and PTT blocks, determined by DSC measurements (scanning rate 20 °C/min); Tm(PTMO) and Tm(PTT) are the temperatures corresponding to the melting of PTMO and PTT crystals, respectively, determined by DSC measurements (scanning rate 10 °C/min); Td: 5% weight loss temperature with a heating rate of 10 °C/min under nitrogen. Polymerization conditions: 0.25 wt % Ti(n-C4H9O)4 at 240 °C for 180 min. bFeeding weight ratio of PTMO to COTTs.

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copolymers are double crystalline polymers, and the behavior can be tuned by the block content in the copolymers. The glass transition temperatures for PTMO and PTT segments of P2−P7 copolymers were measured by DSC with a heating rate of 20 °C/min to enhance the sensitivity, and the results are presented in Table 2 and Figure S3. Interestingly, the glass transition temperature (Tg) of PTMO segments is much higher than the PTMO macroinitiator (−139 °C, Figure S3) and increased with PTT content until 50 wt %, while Tg of PTT segments is similar to commercial PTT. Since these copolymers have the same PTMO segment length but different content with similar multiblock structures, and the PTT and PTMO segments are phase separated and crystallized near Tg of PTMO, the increment of PTMO’s Tg should be related to the copolymer’s hierarchical structures. The structure−property relationship is under current investigation. With such low Tgs from PTMO segments, these multiblock copolymers have potential applications as novel elastomers.

multiblock copolymers and the PTMO macroinitiator and COTTs. The multiblock copolymers have similar thermal stability with 5% weight loss temperature around 340 °C, similar to the PTT homopolymers.45 It is interesting that the multiblock copolymers have much better thermal stability (∼90 °C higher) than the PTMO macroinitiator. These results indicate that the decomposition of PTMO starts from the chain end. When the chain end is coupled with PTT in the multiblock copolymers, PTMO segments are stable and decomposed when PTT decomposed. This is further supported by the fact that only one decomposition stage is observed from Figure 6, contrary to other block copolymer systems where two decomposition stages were observed.46−49 These results indicate that the thermal stability of PTMO can be increased if the end groups of PTMO is capped by functional groups with better thermal stability, which is useful for PTMO’s applications. Figure 7 shows the DSC curves of (PTT-b-PTMO-b-PTT)n multiblock copolymers with different PTMO content. All the

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CONCLUSIONS (PTT-b-PTMO-b-PTT)n multiblock copolymers were synthesized by a one pot, one step melting polymerization process using COTTs as monomer, PTMO as macroinitiator, and Ti(nC4H9O)4 as catalyst. The detailed structures of copolymers were well characterized by an improved quantitative 1H NMR technique. The total block copolymer molecular weights, average total repeating units of TMO (NTMO) and TT (NTT) in polymer, and the degree of polymerization of triblock copolymer segment (n) can be calculated by the integration value of functional groups to the end groups. Our results revealed that these values increased linearly with reaction time at low catalyst amount (0.05 and 0.1 wt %). Investigation of the polymerization kinetics revealed that the formation of multiblock copolymers consisted of two stages: at first, ring-opening polymerization of COTTs by PTMO macroinitiator was carried and PTT-b-PTMO-b-PTT triblock copolymers formed; then condensation polymerization of the triblock copolymers was started to form multiblock copolymers (PTT-b-PTMO-bPTT)n at the second stage. By using this polymerization method, (PTT-b-PTMO-b-PTT)n multiblock copolymers with different PTMO content were synthesized and their thermal properties were investigated. It was found that by coupling with PTT segments, the decomposition temperature of PTMO increased to the same as PTT segments. DSC studies revealed the double crystalline properties of the multiblock copolymers, indicating the potential application of these new poly(ether ester)s as shape memory materials.

Figure 7. DSC curves of (PTT-b-PTMO-b-PTT)n multiblock copolymers. Scanning rate: 10 °C/min. Black line, P3; red line, P5; blue line, P6.

copolymers showed a crystal melting peak at around 10 °C during heating, corresponding to the PTMO segments’ melting temperature. The corresponding crystallization peak was observed during cooling process, at temperature range of −15 to 0 °C. On the other hand, the PTT melting peak was observed for all samples, with temperature range from 200 to 230 °C for copolymers P3 and P5. For copolymer P6, the melting peak of PTT is not too obvious and at much lower temperature with the peak position at about 180 °C. Similar phenomena were observed from the cooling process, where the crystallization temperature range of P6 is much lower than P3 and P5. The main reason for these is due to the different average length of PTT segments in multiblock copolymers, which can be deduced roughly by the total repeating units of TT in multiblock copolymers over triblock copolymer segment repeating units (NTT/n). The calculated value is 12.8, 6.3, and 3.4 for P3, P5, and P6, respectively. For multiblock copolymer P6, the average repeating length of PTT segments is too small to form crystals. For multiblock copolymer P3 and P5, it is interesting that P3 has a higher PTT crystallization temperature than P5 during cooling process, yet similar melting behavior during heating process. The small exothermic peak at around 200 °C before PTT melting is the cold crystallization peak of PTT. The above phenomena indicate these multiblock

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ASSOCIATED CONTENT

S Supporting Information *

H NMR figures of multiblock copolymers and summary of integration values of corresponding peaks, GPC and DSC results, detailed deduction of eq 6. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +86512 65882130; Fax +86512 65882130 (Y.T.). Notes

The authors declare no competing financial interest. 7280

dx.doi.org/10.1021/ma400969a | Macromolecules 2013, 46, 7274−7281

Macromolecules

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Article

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ACKNOWLEDGMENTS The financial support from the National “973” Project (No. 2011CB606004), the National Natural Science Foundation of China (Grant No. 21074079, 21274099), Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20103201120004), and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions is gratefully acknowledged.

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