One Pot, One Feeding Step, Two-Stage Polymerization Synthesis andCharacterization of (PTT‑b‑PTMO‑b‑PTT)n Multiblock CopolymersQiaozhen 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 synthesisof poly(trimethylene terephthalate)-block-poly(tetramethyleneoxide) multiblock copolymers (PTT-b-PTMO-b-PTT)n bymelt polymerization of cycl ic oligo(tr imethyleneterephthalate)s (COTTs) with PTMO macroinitiator. Animproved quantitative 1H NMR characterization techniquewas developed and applied to investigate the multiblockcopolymers’ structure with different reaction time by the chainend and functional groups estimation. The polymerization kinetics was revealed, and the results indicated a two-stagepolymerization mechanism: melt ring-opening polymerization of cyclic oligo(trimethylene terephthalate)s (COTTs) by PTMOmacroinitiator to form triblock copolymers at first stage, followed by the in-situ condensation polymerization of blockcopolymers to produce multiblock copolymers at second stage. This was further confirmed by the gel permeationchromatography (GPC) and viscosity experiments. To our knowledge, this is the first reported poly(ether ester) multiblockcopolymers synthesized by one step polymerization process and with controlled structures. These multiblock copolymers showgood thermal stability and double crystalline properties.

■ INTRODUCTION

Poly(ether ester) copolymers are a class of important industrialthermal plastic polymers with polyethers as soft segments andcrystalline polyesters as hard segments, and have manyapplications in molded shoe soles, ski boots, automotiveparts, wires and cables, shock absorbers, and biomedicalapplications.1−5 The most frequently used soft segments arepoly(tetramethylene oxide) (PTMO, also known as poly-(tetrahydrofuran) or PTHF), while the hard segments arearomatic semicrystalline polyesters, due to their goodmechanical properties, toughness, chemical resistance, excellentsurface appearance, and stable electrical insulation proper-ties.2,6,7 Normally, these copolymers are synthesized bycondensation polymerization of low molecular weightsdihydroxyl-terminated polyethers with polyester monomers athigh temperatures.8−12 This process produces random multi-block copolymers with some drawbacks; e.g., the structure ofthe hard polyester segments is uncontrollable, the totalmolecular weights of the polymers are not too high, and itrequires 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 greatinterest recently due to its excellent elastic recovery andmoderate modulus properties, after the economically andecologically production of 1,3-propanediol monomer fromstarch by using a fermentation process.14 However, researchworks focused on poly(ether ester)s based on PTT are only a

few. Szymczyk et al. reported the synthesis of a series ofrandom segmented block copolymers of poly(trimethyleneterephthalate) and polyethers by two-step condensationpolymerization, with the rigid segments as well as the flexiblepoly(ethylene oxide) or poly(tetramethylene oxide) segmentsrandomly distributed along the chain.9,10,15,16 These types ofcopolymers have good thermoplastic elastomer properties, withthe phase structure as well as thermal and mechanicalproperties 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 touncontrolled polymerization process as stated above, and thereproducibility from two-step condensation polymerization isnot good. To resolve these problems, one needs to find otherpolymerization methods to produce poly(ether ester)s withwell-defined structure.Besides condensation polymerization, ring-opening polymer-

ization (ROP) of lactones and lactide can produce polyesterswith living polymerization characters.4,17−21 This method workswell with aliphatic polyesters but has problems on aromaticpolyesters due to the difficulty in synthesis of theircorresponding cyclic monomers, until Brunelle et al. reportedthe synthesis of corresponding cyclic oligomers.22−24 Com-pared to traditional condensation polymerization, ROP of cyclic

Received: May 9, 2013Revised: August 24, 2013Published: September 11, 2013

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oligo(ethylene terephthalate)s (COETs) or cyclic oligo-(butylene terephthalate)s (COBTs) produces their correspond-ing aromatic polyesters with rapid polymerization rate,25−28

much higher molecular weights,13,25 and little small molecularbyproduct. For example, high molecular weights PBT and itscopolymers with PET was prepared via ROP using severalcatalysts within 10−20 min, with the molecular weights about95 000−115 000 Da.23 However, the reported research workson ROP of cyclic oligo(trimethylene terephthalate)s (COTTs)are few. Kyeong Pang et al. reported the synthesis of PTT bythe ROP of cyclic trimethylene terephthalate dimer29 andfound that titanium(IV) butoxide was the most effectivecatalyst. Wan et al. studied the catalyst’s efficiency in ROP ofCOTTs to PTT, and Sb2O3 and titanium(IV) butoxide werefound to be effective.30

Since ring-opening polymerization of lactones with dihy-droxyl-terminated polyethers in solution at mild conditions canproduce polyester-block-polyether-block-polyester triblock co-polymers,17−21,31 we propose the melt ROP of cyclicoligo(aromatic ester)s by dihydroxyl-terminated polyetherswill produce triblock copolymers. Specially, PTT is chosen asthe polyester segment while PTMO as polyether segment inthis work. Since the polymerization must be carried above themelting temperature of the COTTs (210 °C), it is possible thatcondensation polymerization and transesterification reactionsoccur at this high temperature. As a result, multiblockcopolymers can be obtained by this process. With the callingof a good calibration method for the analysis of absolutemolecular weights and detailed structures of block copolymers,we developed an improved quantitative 1H NMR character-ization technique in this work to reveal the block copolymers’composition by chain end and functional groups analysis andthe polymerization kinetics. As a series of thermal plasticpolymers, block copolymers with different soft segment content

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

■ EXPERIMENTAL SECTIONMaterials. 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 afterstirring 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 thendistilled in the protection of nitrogen. Dihydroxyl-terminated PTMO(PTHF-2900, Sigma-Aldrich, numbers stand for average molecularweights) and titanium tetrabutoxide (Ti(n-C4H9O)4) (Alfa Aesar,98%) were used as received.

Synthesis of COTTs. COTTs were synthesized by using a similarprocedure as references by terephthaloyl chloride (TPC) and 1,3-PDOunder pseudo-high-dilution conditions in dichloromethane using 1,4-diazabicyclo[2.2.2]octane (DABCO) as catalyst.23,32 Yield: 36%. 1HNMR (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. Thesynthesis of the poly(ether ester) multiblock copolymers isrepresented in Scheme 1. In a typical polymerization procedure,predetermined amounts of COTTs and PTMO-2900 were charged ina three-necked flask with mechanical stirring and nitrogen inlet. Themixture was heated to 240 °C for 5 min under a nitrogen atmosphere,and then Ti(n-C4H9O)4 was added to start the polymerization. Forpolymerization kinetic studies, a small portion of mixture was takenout at different reaction time for 1H NMR, GPC, and viscositymeasurements.

Instrumentation. The 1H NMR spectrum was recorded on anINOVA 400 MHz nuclear magnetic resonance instrument usingCDCl3 or CF3COOD/CDCl3 (1:10 in volume, for all blockcopolymers) as the solvent and tetramethylsilane (TMS) as the

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

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internal standard, with the solution concentration of 0.02 g/mL. GPCexperiments were performed on a modular system comprising aWaters 1515 pump, a 717 plus autosampler, and a 2487 UV detectorwith three 300 mm (length) × 7.5 mm (inner diameter) columnsincluding particle size of 5 μm (PL gel Mixed-C, PolymerLaboratories). The polymers were dissolved (2.5 mg/mL) in amixture solvent (chloroform and 1,1,1,3,3,3-hexafluoro-2-propanol,95:5, v/v) and subjected for GPC characterization with the mobilephase flow rate of 0.60 mL/min at 35 °C. The molecular weights werecalculated using nine narrow distribution polystyrene standards from8 710 000 to 474 g/mol. The apparent viscosity test was taken onSNB-3 digital viscometer from Shanghai Nirun Intelligent TechnologyCo. 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 roomtemperature to 800 °C under a continuous nitrogen flow of 50 mLmin−1 with a TA Instruments SDT-2960TG/DTA. The temperatureof thermal degradation (Td) was measured at the point of 5% weightloss relative to the weight at room temperature. The differentialscanning calorimetry (DSC) was carried out on the TA Q100instrument under a nitrogen atmosphere in the temperature rangefrom −20 to 250 °C at a heating and cooling rate of 10 °C min−1. Thefirst cooling and second heating scans were used to determine themelting and crystallization peaks. For glass transition temperaturemeasurements, the temperature range is from −160 to 270 °C with aheating rate of 20 °C min−1 to enhance the sensitivity.

■ RESULTS AND DISCUSSIONThe chain-end estimation method by 1H NMR to determinethe number-average molecular weights of polymers is well-established.33−37 However, this method usually works well forpolymers with small molecular weights of several thousanddaltons. For polymers with molecular weights higher than 104

Da, the error becomes greater. This is due to the couplings of13C to 1H take at least 1% integrated peak area from thosemajor peaks of 1H connected with 12C.38,39 Another fact peopleoften overlooked is the different relaxation time of protons atthe chain end and on the backbone. If the relaxation delayprovided at the front of each acquisition scan was not longenough, NMR signals would be suppressed. Normally, thechain-end protons have longer relaxation time than those on

the backbone, which makes their peak integration value smallerdue to the shortage of NMR relaxation delay and causes biggersystem errors. For the better structural determination of ourmultiblock copolymers, we used an improved quantitative 1HNMR method where the decoupling of 13C to 1H was appliedonly during the acquisition, with the delay time (20 s) set as 10times of relaxation time (T1) and the experiments conducted at90° 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 differ-ent reaction time with the feeding ratio of 2/1 (mPTMO/mCOTT)and 0.05 wt % Ti(n-C4H9O)4. The corresponding assignmentof peaks to the polymer chemical structure is shown above thefigure. A detailed list of integration values for all peaks can befound in the Supporting Information (Table S1). It clearlyshows that after reacted for 30 min the peak at chemical shift of4.38 ppm corresponding to the functional end group (CH2OH)of PTMO disappeared, indicating all the end groups of PTMOwere reacted with COTTs. On the other hand, peaks atposition of 4.43, 3.97, and 2.14 ppm appeared. The peak at 4.43ppm corresponds to the PTMO’s methylene protons atchemical position d, which is bonded with PTT segments byester groups, while the other two correspond to the PTT’smethylene 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 methylenegroup at chain end (CH2OH, peak d*) to the total methyleneoxide groups (CH2O, peaks d* and f). The calculated value is43.6, which corresponds to the number-average molecularweights of 3100 g/mol. This value is very close to the molecularweights provided by the producer (2900 g/mol), indicating theaccuracy 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 groupsat chain end, clearly indicates the increment of polymermolecular weight. The total repeating units of TT (NTT) andTMO (NTMO) in copolymers can be calculated by integration

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.

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

=I

I N4

4g

c TT (1)

+=

I

I I N4

4g

d f TMO (2)

where Ig, Ic, Id, and If are the integration values for thecorresponding peak at g, c, d, and f, respectively. The totalnumber-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 thetotal repeating units of TT (NTT) and TMO (NTMO) increase.However, it also shows that the integration value of thearomatic hydrogen corresponding to COTTs at differentreaction times is very small for each studied time period anddecreases slowly with prolonged reaction time (see Table S2 inSupporting Information).41 The conversions of COTTs toPTT from NMR are all around 95%, similar to the reportedresults by Pang et al., where the ROP of cyclic dimer of PTTwas studied.29 The results indicate that the ROP rate ofCOTTs by PTMO macroinitiators is rapid and finished within30 min. On the other hand, the increment of molecular weightsafter 30 min is not from ROP, but from the condensationpolymerization 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, peakd in the NMR figure can be assigned to the methylene endgroup of PTMO linked with PTT by ester bond. Thus, theaverage repeating units of TMO (STMO) and TT (STT) in eachsegment can be calculated by the following equations:

=II S

44

d

c TT (3)

+=

II I S

44

d

d f TMO (4)

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

with increasing reaction time. Considering the fact that the totalmolecular weights of the polymer increased with reaction time,these results demonstrate that condensation polymerization ofPTT-b-PTMO-b-PTT triblock copolymer was carried at thesestudied regions with the formation of corresponding multiblockcopolymers as illustrated in Scheme 1. This is supported by theobservation of small 1,3-propanediol droplets in glass tube nearnitrogen outlet part.Since there is only PTT’s chain end groups observed while

no PTMO’s observed from NMR, the structure of multiblockcopolymer can be assigned as (PTT-b-PTMO-b-PTT)n (orPTT-b-(PTMO-b-PTT)n). The average repeating number ofPTT-b-PTMO-b-PTT block segment, n, can be calculated bythe following equation:

=II

n44

d

g (5)

The calculated n values at different polymerization time arelisted in Table 1. These results show the number of blocksegments increased with reaction time. When reacted for 30min, a mixture of pentablock copolymers and heptablockcopolymers was formed on average since the n value is 2.37,while pentadecablock copolymers and higher formed afterreacted for 180 min.The influence of the catalyst amount on the polymerization

time and copolymer structure is studied, and the results arepresented in the Supporting Information (Figure S1 and TableS3 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 studiedreaction time and different catalyst amount, indicating they arenot affected by the reaction time or catalyst amount. The totalrepeating units of TT (NTT) and TMO (NTMO) in copolymers,and the total molecular weights of the polymers, increased withreaction time, similar to the results observed in polymerizationwith 0.05 wt % catalyst, which reveals the formation ofmultiblock copolymers. However, with the same reaction time,NTT and NTMO of multiblock copolymers are higher than thosewith lower catalyst amount. Clearly, Ti(n-C4H9O)4 acts notonly as a ring-opening polymerization catalyst but also as acondensation polymerization catalyst. This special propertyprovides the facile one pot one step synthetic route to themultiblock copolymers.Figure 2 is the calculated number-average molecular weights

(Mn) of the multiblock copolymers as a function of reactiontime and catalyst amount. The molecular weights increased

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

peak Integration (δ ppm)

sample reaction time (min) d g f c NTMO NTT STMO STT n Mn (kg/mol)

PTMO 4.00b 170.5 43.6 3.1P1-1 30 4.00 1.68 166.3 28.1 101 16.7 42.6 7.03 2.38 10.7P1-2 60 4.00 1.57 162.6 27.4 106 17.5 41.7 6.8 2.55 11.2P1-3 90 4.00 1.41 162.0 27.3 118 19.4 41.7 6.8 2.84 12.5P1-4 120 4.00 0.93 160.0 27.4 176 29.5 41.0 6.8 4.30 18.7P1-5 150 4.00 0.80 155.5 26.8 199 33.5 39.9 6.7 5.00 21.2P1-6 180 4.00 0.55 145.3 25.3 272 46.0 37.3 6.3 7.27 29.1

aNTMO and NTT are the corresponding average repeating units of TMO or TT in whole block copolymers, while STMO and STT are the correspondingpolymer segments, respectively. n is the repeating units of (PTT-b-PTMO-b-PTT) segments in whole polymers, and Mn is the number-averagemolecular 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.

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with reaction time, in a somewhat linear trend, though thecorrelation fits only well for the data of 0.05 and 0.1 wt %catalyst amount, but not for 0.2 wt % catalyst amount. Afterreacted for 180 min with catalyst amount of 0.2 wt %, themolecular weights of the multiblock copolymers are higher than90 000 g/mol. Such high molecular weights are very difficult toachieve by a traditional condensation polymerization and arethe merit of current polymerization process.Since the ROP of COTTs by PTMO is finished before 30

min, and then condensation polymerization of PTT-b-PTMO-b-PTT triblock copolymers occurs after that, the triblockcopolymers can be looked as an AB monomer based on thereaction in Scheme 1. Then we have the following equation todescribe the reaction kinetics if it follows:

= ′ +X k t 1n (6)

where Xn is the average degree of polymerization forcondensation polymerization of AB-type monomer, k′ isconstant for a given condition which is related to the startingfunction groups concentration, the catalyst concentration, andthe polymerization reaction constant, and t is the reaction time.Detailed deduction of eq 6 can be found in the SupportingInformation. The degree of polymerization, Xn, can becalculated from NMR and is the same as repeating units oftriblock copolymer (n) when the multiblock copolymer beingrepresented 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 thecorresponding data. Clearly, the fitting lines for the reactionwith 0.05 and 0.1 wt % catalyst amount fit well with thecorresponding data, supporting the condensation polymer-ization mechanism as proposed in Scheme 1 at second stage.The exception in data with 0.2 wt % catalyst amount may comefrom their high molecular weights, since the errors ofintegration value of end groups are larger for polymers withhigher molecular weights, or the reaction mechanism changedwhen the catalyst concentration is high. The slopes for thelinear 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, and0.132 min−1 for reaction with 0.2 wt % catalyst. The linearincreasing values of the slope with catalyst amount suggest that

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 demonstratethe reliability of our results from NMR techniques, themultiblock copolymers in Table 1 were characterized byGPC, with chloroform and 1,1,1,3,3,3-hexafluoro-2-propanol(95:5, v/v) mixture solvent as eluent, and the results arepresented in Figure 4. For COTTs, it clearly demonstrates the

cyclic oligomers with different size (p = 2−6), similar to otherreports.23,32 For copolymers with increased polymerizationtime, the curves shifted to shorter retention time, indicating theincrement of molecular weight, which agrees well with theNMR results. The polydispersity of the main peak is around 2.3for all the copolymers (see Table S5), typical value forcondensation polymerizations. The small peaks at longerretention time belong to the remaining COTTs (about 6%,see Table S5), with the cyclic dimer being the majority, and thecalculated conversion agrees well with NMR results.Figure 5 represents the data from viscosity measurements of

the multiblock copolymers at different reaction times. By fixing

Figure 2. Molecular weights calculated from 1H NMR spectra of(PTT-b-PTMO-b-PTT)n multiblock copolymers with different catalystamounts and reaction times. Polymerization conditions of multiblockcopolymers: mPTMO/mCOTT = 2/1 with Ti(n-C4H9O)4 as catalyst at240 °C.

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

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

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the same experimental conditions (concentration, temperature,solvent composition, and shear rate), a higher viscositymeasured indicates the higher molecular weights of themeasured polymer. It shows the apparent viscosity of thesolution increased with reaction time, coinciding well with theresults from NMR data. In the Mark−Houwink equation [η] =KMη

α, the K and α parameters are empirical parameters anddependent on the solvent−polymer interaction and temper-ature. For block copolymer systems, normally K and αparameters are different for different blocks with a givensolvent, so the equation can hardly be applied to estimate themolecular weights of block copolymers due to the difficulty infinding the universal parameters.43 This is more complicated inmultiblock copolymer systems, as the structures of themultiblock copolymers with different molecular weights arenot the same, and they may have lower dynamic viscositycompared to the similar homopolymer component.44 How canthis be applied in the multiblock copolymers is interesting, andwe are currently investigating that.Since the molecular weights measured from 1H NMR are the

absolute number-average molecular weights, the above resultsindicate the method we developed provides a good way for thecharacterization of polymer molecular weights and blockcopolymer structures. As listed in Table 1 as well as TablesS3 and S4, this method works well when the molecular weightsare below 105 Da, which covers the molecular weights range of

most industrial polymers. It also has the advantage of versatilesolvent selection over other methods and thus can be applied todetermine the molecular weights of polymers that are hard tobe dissolved in common GPC eluent. For example, thedetermination of molecular weights of aromatic polyesters isvery important in industry, yet normally only apparentmolecular weights (or viscosity) were reported. The NMRmethod we developed thus may have great applications in thesefields.To illustrate if this one pot, one step polymerization

technique can be applied in the synthesis of multiblockcopolymers with different structure, a series of (PTT-b-PTMO-b-PTT)n multiblock copolymers with different PTMO contentwere synthesized using similar polymerization conditions. Thestructures of these multiblock copolymers were characterizedby 1H NMR, and the results are presented in Table 2. Thispolymerization technique works well for the synthesis of blockcopolymers with different PTMO weight content range from40% to 75%, and the content of each block in multiblockcopolymers is very close to the feeding ratio, indicating the easycontrol on the final copolymer composition by this syntheticmethod and the versatile application in multiblock copolymersynthesis.The thermal stability of these multiblock copolymers with

different PTMO content was studied by TGA, and the resultsare listed in Table 2. Figure 6 shows the TGA curves of the

Figure 5. Apparent viscosity of (PTT-b-PTMO-b-PTT)n multiblockcopolymers synthesized with different reaction time at the reactioncondition 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.

Table 2. 1H NMR Results and Thermal Properties of (PTT-b-PTMO-b-PTT)n Multiblock Copolymers with Different PTMOContent 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 1:1.5 40.7 116 58.8 3.54 20.5 −77.9 51.7 19.4 225 352P3 1:1 51.2 356 119 9.30 50.1 −76.2 44.8 19.2 225 345P4 1.5:1 61.2 188 41.5 4.88 22.1 −98.0 53.8 18.2 224 344P5 2:1 68.1 453 74.1 11.8 47.9 −102.8 52.0 16.2 221 334P6 3:1 75.2 233 26.7 7.84 22.3 −119.2 15.5 186 333

aWPTMO is the weight fraction of PTMO in copolymers determined by NMR; Tg(PTMO) and Tg(PTT) are the glass transition temperatures forcorresponding PTMO and PTT blocks, determined by DSC measurements (scanning rate 20 °C/min); Tm(PTMO) and Tm(PTT) are the temperaturescorresponding 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 180min. bFeeding weight ratio of PTMO to COTTs.

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

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multiblock copolymers and the PTMO macroinitiator andCOTTs. The multiblock copolymers have similar thermalstability with 5% weight loss temperature around 340 °C,similar to the PTT homopolymers.45 It is interesting that themultiblock copolymers have much better thermal stability (∼90°C higher) than the PTMO macroinitiator. These resultsindicate that the decomposition of PTMO starts from the chainend. When the chain end is coupled with PTT in themultiblock copolymers, PTMO segments are stable anddecomposed when PTT decomposed. This is further supportedby the fact that only one decomposition stage is observed fromFigure 6, contrary to other block copolymer systems where twodecomposition stages were observed.46−49 These resultsindicate that the thermal stability of PTMO can be increasedif the end groups of PTMO is capped by functional groups withbetter thermal stability, which is useful for PTMO’sapplications.Figure 7 shows the DSC curves of (PTT-b-PTMO-b-PTT)n

multiblock copolymers with different PTMO content. All the

copolymers showed a crystal melting peak at around 10 °Cduring heating, corresponding to the PTMO segments’ meltingtemperature. The corresponding crystallization peak wasobserved during cooling process, at temperature range of −15to 0 °C. On the other hand, the PTT melting peak wasobserved for all samples, with temperature range from 200 to230 °C for copolymers P3 and P5. For copolymer P6, themelting peak of PTT is not too obvious and at much lowertemperature with the peak position at about 180 °C. Similarphenomena were observed from the cooling process, where thecrystallization temperature range of P6 is much lower than P3and P5. The main reason for these is due to the differentaverage length of PTT segments in multiblock copolymers,which can be deduced roughly by the total repeating units ofTT in multiblock copolymers over triblock copolymer segmentrepeating units (NTT/n). The calculated value is 12.8, 6.3, and3.4 for P3, P5, and P6, respectively. For multiblock copolymerP6, the average repeating length of PTT segments is too smallto form crystals. For multiblock copolymer P3 and P5, it isinteresting that P3 has a higher PTT crystallization temperaturethan P5 during cooling process, yet similar melting behaviorduring heating process. The small exothermic peak at around200 °C before PTT melting is the cold crystallization peak ofPTT. The above phenomena indicate these multiblock

copolymers are double crystalline polymers, and the behaviorcan 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 aheating rate of 20 °C/min to enhance the sensitivity, and theresults are presented in Table 2 and Figure S3. Interestingly,the glass transition temperature (Tg) of PTMO segments ismuch higher than the PTMO macroinitiator (−139 °C, FigureS3) and increased with PTT content until 50 wt %, while Tg ofPTT segments is similar to commercial PTT. Since thesecopolymers have the same PTMO segment length but differentcontent with similar multiblock structures, and the PTT andPTMO segments are phase separated and crystallized near Tgof PTMO, the increment of PTMO’s Tg should be related tothe copolymer’s hierarchical structures. The structure−propertyrelationship is under current investigation. With such low Tgsfrom PTMO segments, these multiblock copolymers havepotential applications as novel elastomers.

■ CONCLUSIONS

(PTT-b-PTMO-b-PTT)n multiblock copolymers were synthe-sized by a one pot, one step melting polymerization processusing COTTs as monomer, PTMO as macroinitiator, and Ti(n-C4H9O)4 as catalyst. The detailed structures of copolymerswere well characterized by an improved quantitative 1H NMRtechnique. The total block copolymer molecular weights,average total repeating units of TMO (NTMO) and TT (NTT)in polymer, and the degree of polymerization of triblockcopolymer segment (n) can be calculated by the integrationvalue of functional groups to the end groups. Our resultsrevealed that these values increased linearly with reaction timeat low catalyst amount (0.05 and 0.1 wt %). Investigation of thepolymerization kinetics revealed that the formation of multi-block copolymers consisted of two stages: at first, ring-openingpolymerization of COTTs by PTMO macroinitiator was carriedand PTT-b-PTMO-b-PTT triblock copolymers formed; thencondensation polymerization of the triblock copolymers wasstarted to form multiblock copolymers (PTT-b-PTMO-b-PTT)n at the second stage. By using this polymerizationmethod, (PTT-b-PTMO-b-PTT)n multiblock copolymers withdifferent PTMO content were synthesized and their thermalproperties were investigated. It was found that by coupling withPTT segments, the decomposition temperature of PTMOincreased to the same as PTT segments. DSC studies revealedthe double crystalline properties of the multiblock copolymers,indicating the potential application of these new poly(etherester)s as shape memory materials.

■ ASSOCIATED CONTENT

*S Supporting Information1H NMR figures of multiblock copolymers and summary ofintegration values of corresponding peaks, GPC and DSCresults, detailed deduction of eq 6. This material is available freeof charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*E-mail tuyingfeng@suda.edu.cn; Tel +86512 65882130; Fax+86512 65882130 (Y.T.).

NotesThe authors declare no competing financial interest.

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

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dx.doi.org/10.1021/ma400969a | Macromolecules 2013, 46, 7274−72817280

■ ACKNOWLEDGMENTSThe financial support from the National “973” Project (No.2011CB606004), the National Natural Science Foundation ofChina (Grant No. 21074079, 21274099), Specialized ResearchFund for the Doctoral Program of Higher Education of China(Grant No. 20103201120004), and a Project Funded by thePriority Academic Program Development of Jiangsu HigherEducation Institutions is gratefully acknowledged.

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