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Polymer Degradation and Stability 97 (2012) 893e904

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Polymer Degradation and Stability

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Novel solventless purification of poly(propylene carbonate):Tailoring the composition and thermal properties of PPC

Carlos Barreto a,b, Eddy Hansen a, Siw Fredriksen b,*

aDepartment of Chemistry, University of Oslo, Postboks 1033 Blindern, NO 0315 Oslo, NorwaybNorner AS, Department of Polymer Research, Asdalstrand 291, NO 3960 Stathelle, Norway

a r t i c l e i n f o

Article history:Received 1 November 2011Received in revised form10 March 2012Accepted 20 March 2012Available online 30 March 2012

Keywords:Poly(propylene carbonate)Maleic anhydrideThermal stabilityPoly(alkylene carbonate)PurificationMetal-ion coordination

Abbreviations: PAC, poly(alkylene carbonate); PPCCC, cyclic carbonates; PC, propylene carbonate;propylene oxide; W90, W90MA; WH1, See Section 3* Corresponding author. Tel.: þ47 35578098; fax: þ

E-mail address: siw.fredriksen@norner.no (S. Fred

0141-3910/$ e see front matter � 2012 Elsevier Ltd.doi:10.1016/j.polymdegradstab.2012.03.033

a b s t r a c t

Poly(propylene carbonate), PPC, is produced via a catalytic copolymerization of CO2 and propylene oxide.The common side product propylene carbonate and catalyst residues are detrimental to the thermal andmechanical properties of the resulting PPC. Thus, efficient purification procedures are needed. PPCproduced using zinc glutarate (ZnGA) catalyst was purified by a novel solideliquid extraction usingaqueous maleic acid. The resulting PPC exhibited a dramatically increased thermal stability as the onsetof the degradation was increased by 85 �C compared to that of a crude PPC reference sample. It issuggested that metal-ion coordination between some in situ produced zinc species and the carbonylmoieties in the PPC backbone may explain this. The stiffness of the PPC increased by 75% when plasticizerside products were removed by the solideliquid extraction. This novel purification method providesa sustainable alternative because only water and no organic solvent is used, and the method allows forthe tailoring of the metal residues from the catalyst in the final polymer. The novel solideliquidextraction procedure renders the PPC thermally stable at 200 �C for ca 60 min, thus expanding theprocessing window for PPC.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Current worldwide concerns regarding the greenhouse effectsfrom CO2 and the increased requirements for sustainable packagingmaterials have revitalized interest from industry and researchgroups around the world in poly(alkylene carbonates)(PACs) [1e4].These copolymers, made from epoxides and CO2, were first repor-ted by Inoue in the late 1960s [5]. Poly(propylene carbonate) (PPC)currently stands out as themost commercially interesting PAC [6,7],with a high potential for coating and packaging applications [3,4].

PPC is produced by the copolymerization of CO2 and propyleneoxide (PO) in the presence of a homogeneous or a heterogeneouscatalyst [2e5,8,9]. The catalyst and the polymerization conditionsused in this process determine the main features of the resultingpolymer, including the ratio of carbonate to polyether linkages, theformation of the cyclic side product propylene carbonate (PC)[2e4,8,9], the stereo- and regioregularity [2,3,8e10] and theinherent end groups [8,10].

, poly(propylene carbonate);MA, maleic anhydride; PO,Materials and methods.47 35578124.riksen).

All rights reserved.

PPC has shown limited applications as a stand-alone plasticmaterial due to its low glass transition temperature (Tg), which is inthe range of 25e46 �C [3,4]. The polymer has traditionally beenused as a sacrificial binder due to its low degradation temperatureand its favourable clean burn-off [3,4,11e15]. Alternative applica-tions for PPC are blends with other polymers or in hybrid multi-material compositions [3,4,15e22]. In these applications, themelting temperature of other polymers restricts their use incombination with PPC. PPC readily degrades at temperatures closeto 180 �C, while other engineering plastics like polyesters andpolyamides have higher melting temperatures and thus requireprocessing temperatures above 200 �C. Therefore, increasing thethermal stability of PPC to allow melt processing at highertemperatures is a major requirement to broaden its processing andapplication window. As a result, significant interest is beinggarnered in this particular area [12e14,23e42].

The commonly reported methods for purifying intermediateand high-molecular weight PACs (Mn > 30,000) are less advanta-geous viewed from economical, technical and sustainabilityperspectives. The use of large amounts of organic solvents [3,4] andthe removal of the solvent residues is relatively laborious.Furthermore, the subsequent separation of the solvents and themonomers to allow recirculation in an industrial process is veryenergy-demanding. Other concerns are related to toxicity and

C. Barreto et al. / Polymer Degradation and Stability 97 (2012) 893e904894

explosion hazards. Therefore, the development of simplified puri-fication processes that leave a low CO2 footprint and involve less- ornon-toxic solvents is of high interest.

Herein, we report a novel method for the purification of PPC,with the objective of removing catalyst residues and reaction sideproducts. The disclosed method uses solely water with the additionof an organic acid, and no organic solvents are employed. Theefficiency of the solideliquid extraction technique, as well as thechange in content of PC and the catalyst residues, was investigatedby FTIR, 1H NMR and AAS. The thermal stability was monitoredusing isothermal and dynamic TGA. Moreover, DMA was used toevaluate the thermomechanical properties of the polymer, and SECwas utilized to analyze any changes in the molecular weights of thesamples.

2. Background

2.1. The copolymerization of propylene oxide and carbon dioxide

The copolymerization of PO and CO2 under moderate temper-ature and pressure conditions in the presence of homogeneous andheterogeneous organometallic catalysts yields a range of PPCs(Fig. 1) [2e5,8,9]. The type of catalyst and the polymerizationconditions (temperature, pressure, reaction times and solvents)that are applied in the process affect the polymer architecture(molecular weight, stereo- and regioregularity) and the variousside products, such as the undesirable cyclic carbonates and poly-mer chains that are rich in ether linkages [3,4]. The cyclic propylenecarbonates act as plasticizers in the final polymer and thereforedecrease the Tg significantly [3,4]. Moreover, the polymer chainswith high amounts of ether linkages decrease the efficiency of CO2fixation in the polymer and also affect the thermal stability[3,11,37,43,44].

Several review articles have described a range of advanced andsynthetically complex catalysts that are used to produce PPC[2e5,8,9]. Catalysts based on zinc carboxylates, in particular zincglutarate (ZnGA), have industrial relevance due to their ease ofsynthesis from non-toxic and inexpensive starting materials[2e5,8,9]. Heterogeneous ZnGA catalysts have the inherent capa-bility of producing high-molecular weights and broad molecularweight distributions, which are advantageous with respect toprocessing properties. These characteristics are especially impor-tant in moulding applications. However, they also inherently resultin the PPC containing some ether linkages, as well as in theformation of cyclic propylene carbonate. In contrast, homogeneouscatalyst systems produce narrow molecular weight distributions

Fig. 1. Catalytic copolymerization

with higher regioregularity and selectivity and a lower amount ofether linkages and cyclic carbonates [2e5,8,9].

With a few exceptions [45], the catalysts used to copolymerizePO and CO2 typically show low productivity (only a few hundredgrams of polymer per gram of catalyst) [2e4,8,9], which is far toolow to allow the catalyst residues to remain in the final polymerresin. The catalyst residues prove to be detrimental to the finalproperties of the polymer and are particularly unfavourable withrespect to the degradation properties [32,34,35,46]. Hence, there isa need for efficient and sustainable methods for handling thecatalyst residues and removing the undesired side products of PPC.

2.2. Purification of PPC

The polymer purification of PACs, and of PPC in particular, hasnot been the main focus of scientific investigations. To date, onlya few reports can be found in the literature [31,34,45,46]. Typically,the purification methods for PPCs use organic solvents. In suchtreatments, organic solvents such as chloroform, dichloromethane,benzene or acetone are used to dissolve the viscous reactor prod-ucts after the polymerization reaction. Subsequent deactivationand removal of the catalyst with aqueous inorganic acids (forexample, with hydrochloric acid or sulfuric acid) is performed andis often followed by an additional neutralization and water wash.The purified polymer is precipitated from an organic solution witha large excess of a non-solvent, typically alcohols such methanol orethanol, to remove the polyether-containing chains, cyclic prod-ucts, solvents and excess epoxides [3,13]. These procedures uselarge amounts of organic solvents and are surprisingly similar,irrespective of the nature of the catalyst and the specific type ofPAC.

Only a few reports discussing the selection of solvent/non-solvent pairs are found in the literature [47]. Recent studies havereviewed the influence of solvents on the thermal stability of PPC[13], pointing out the importance of solvent selection when tar-geting thermally stable materials. The solvents often modify theviscosity of the polymer solution, and the solvent/non-solvent pairgoverns not only the total amount of the solvents (depending onthe polymer solubility), the ease of removal of the solvent residuesand the extraction of low-molecular weight material but also themorphology of the precipitated polymer. From an industrial scale-up point of view, an efficient solvent system should allow for thedesired purification of the polymer in addition to fulfilling theadditional requirements regarding the use of low volumes, low CO2footprints and cost-efficient recovery and reuse, partnered togetherwith low hazard profiles.

of CO2 and propylene oxide.

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The removal of catalyst residues from a crude polymer afterpolymerization reactions has been reported using alternativemethods such as filtration, adsorption or ion exchange that aim atcatalyst recovery and reuse [10,45]. In these cases, additionalorganic solvents are required to reduce the viscosity of the polymersolution to allow its passage through porous media. Catalystdeactivation in hot water has been reported [31]. However, themethod of choice mainly employed today for the purification of PPCis based on using large amounts of organic solvents.

2.3. Degradation reactions in PPC

The thermal-degradation of PPC proceeds mainly through thenucleophilic attack of carbonyl groups in the polymer backbone[3,10,29,31,31,32,48]. An intramolecular reaction can be envisagedin which the nucleophile is the end group itself, and the PPC maydecompose into PC by a backbiting or chain-unzipping reaction(Fig. 2) [3,4,10,29,31,31,32,48]. Likewise, an intermolecular reactionmay take place, where the external nucleophile could be any endhydroxyl groups in any adjacent polymer chains [34]. In this case,the attack occurs at any carbonyl moiety along the polymer back-bone to form shorter polymer chains by random chain scission(Fig. 3) [3,4,10,29,31,31,32,48]. The resulting end groups will behydroxyl groups whenwater is the nucleophile [34] or unsaturatedend groups in the absence of water [3,4,29,31,48]. In the formercase, degradation has been reported to occur at low temperatures[34], while in the latter case, degradation has been reported tooccur at both high [3,4,32,48] and low temperatures [29,31,34].

The preference for a chain-unzipping reaction or a random chainscission reaction degradation pathway is determined by theirrespective activation energies. Thus, the prevalence of one mecha-nism or the other is temperature dependent [23,24,48]. The activa-tion energy is higher for the random chain scissionmechanism. Bothreaction types may occur simultaneously at low and mediumtemperatures, depending on the type and reactivity of the nucleo-philes present in the system. In PPC, with low amounts of nucleo-philes present, the degradation pathway occurs preferably ina backbitingmanner [24,48]. Small amounts of substances likewater,hydroxyl end groups [34], acids [11,13,14,26,48], bases [26], solvents[13] and metal-containing compounds [12,27,29,31,34,38,39,46,49]may modify the degradation of the PACs. Moreover, the presence ofrandom polyether inclusions [3,11,37,43,44], the availability and typeof end groups [32e34,36,37], the molecular weight and the regior-egularity and tacticity of PPC [3,50] affect the degradation process.

2.4. Thermal stability of PPC

Various approaches for enhancing the thermal stability of PPChave been reported. The approaches include the end-capping ofterminal hydroxyl groups, the introduction of metal-ion coordina-tion and the enhancement of intermolecular forces using

Fig. 2. Degradation via chain-unzipping in PPC: (

homogeneous and heterogeneous additives. The end-capping ofPPCs with isocyanates, acyl halides, carboxylic acid anhydrides, sili-cates, phosphorous containing compounds, among others, has beenstudied [23e25,38e40]. However, a thorough characterization of theresidual free hydroxyl groups after end-capping has not beendescribed in the literature. End-capping with maleic anhydride insolution [23,24,39] and in reactive-melt compounding [4,25,38,39]has been investigated. Hydrogen bonding has been shown tosignificantly improve the thermal stability of PPC in studiesemploying stearic acid [51], wood flour [52] and blends with otherpolymers [16e19,21,22]. The addition of nanoparticles [53e58], aswell as copolymerization with a second or third epoxide [35,59] ormaleic anhydride [60], have shown some improvements in thermalstability. Due to the wide range of testing conditions and the limitedinformation regarding the PPCs that have been used, reliablecomparisons are difficult to make for the thermal stabilities repor-ted. Furthermore, the actual ability of the aforementioned modifi-cations to enable PPC processing without degradation attemperatures higher than 180 �C has not been reported.

The presence of metal compounds has been shown to impartboth beneficial and detrimental deteriorating effects with respectto the thermal stability of polymers. This apparent contradictioncan be understood by the specific action of the metal used. Metalresidues in PACs exhibit catalytic effects for the nucleophilic attackof the carbonyl group in the polymer backbone [29,31,32,34,46,49].However, commercial polymer additive packages, typically con-taining zinc, calcium, tin, aluminium, magnesium, barium and thevery toxic but active cadmium and lead compounds, are used toenhance thermal stability. These additives allow for the facileprocessing of the polymers at elevated temperatures [61e63].

In the field of PACs, few examples in which organometalliccompounds have been utilized to improve thermal stability havebeen described. Very recently, the use of metal-ion coordination inPPCs by calcium [27,38] and copper compounds [12], resulting insignificant improvements in thermal stability, was reported. In thisinhibitory mechanism, certain metal-ions with activity moderatedby specific ligands coordinate to the carbonyl oxygen of PPC. Thus,the carbonyl moiety is stabilized against an attack by a nucleophile.The authors reported a decreased tendency for the degradation bythe chain-unzipping process [12,27].

2.5. Investigation of thermal decomposition reactions of PPC

Improvements in thermal stability are commonly quantifiedusing dynamic TGA in nitrogen or in air [3,4]. Low heating rates arepreferred to allow for efficient and reliable monitoring of thedecomposition behaviour [13]. However, different heating rates arereported (2e20 �C/min), and as a result, direct comparisons of thereported studies are difficult to make. Furthermore, the lack ofsufficient characterization of the starting polymers may lead toerroneous conclusions. Characterizations in terms of the molecular

a) carboxyl backbiting, (b) alkoxy backbiting.

Fig. 3. Degradation via random chain scission in PPC accompanied by the release of CO2: (a) leaving OH end groups, (b) leaving unsaturation.

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weight, the molecular weight distribution, the chain structure (e.g.,ether linkages), the nature and amount of the catalyst residues, thesolvent residues and any other components present are allimportant when evaluating the thermal properties ofpoly(propylene carbonate). Such parameters are often not fullydisclosed in reported results.

3. Material and methods

3.1. Preparation of materials

3.1.1. Poly(propylene carbonate) (PPC) by the copolymerization ofCO2 and propylene oxide

PPCwas prepared from the reaction of propylene oxide (99.999%purchased from SigmaeAldrich) and CO2 in the presence of a ZnGAcatalyst [64] that was prepared from zinc oxide (ZnO) and glutaricacid (GA), as described elsewhere [65]. The general reactionconditions for the polymerizationwere 60 �C, 40 h and 30 Bar. Afterthe desired polymerization time, the autoclave was vented toremove the excess monomer. The dry crude product was recoveredand analyzed. The zinc content was measured by atomic absorption(4870 wt ppm). The content of propylene carbonate (PC) wasestimated to be 4 wt % (by FTIR, which was calibrated by the resultsof 1H NMR analysis). The crude polymer product was worked-upaccording to the methods described below.

3.1.2. Standard solvent-based workup (WH1)The solid crude PPC (10 g) that was obtained from the poly-

merization reaction described above was dissolved in 100 ml ofdichloromethane (DCM) (99.5%) in a 250-ml reactor withmechanical stirring. HCl (0.5%, 100 ml) was added, and thesuspension was stirred for 10 min. The aqueous phase wasremoved, and the procedure was repeated using water. Methanol(MeOH) (99.8%, 160 ml) was added to the solution with vigorousstirring. The precipitated PPC was collected and dried for 72 h at35 �C under nitrogen flushing. The purified polymer was analyzed,and the following characteristics were recorded: Tg (midpoint):41.4 �C (DSC); Mn: 194.000, Mw: 537.000 (SEC in THF); randompolyether linkages: 10 mol % (1H NMR) [3].

3.1.3. Workup procedure in methanol (MeOH)The crude solid PPC (0.5e1 g) that was obtained from the

W90MA procedure was added to methanol (99.8%; 40 ml) in a 100-ml Erlenmeyer flask, which was held tightly capped during theprocess. The temperature was raised to 40 �C, and the suspensionwas heated at this temperature for the desired time (0, 2, 4, 20 h)with stirring (magnetic stirring bar, 450 rpm). The temperaturewas

maintained at �1 �C using a PEG bath. Agglomerates measuring2e3 mm were recovered and washed twice with deionised water(50 ml). The precipitate was collected and dried for 72 h at 35 �Cunder nitrogen flushing.

3.1.4. Workup procedure in aqueous maleic acid (W90MA)(solventless procedure)

The crude solid PPC that was obtained from the polymerizationreaction described above was quenched in liquid nitrogen and thenmilled to obtain particle sizes smaller than 1 mm. The PPC powder(2.0 g) was added to an aqueous solution (100 ml) containingaqueous maleic anhydride 99% (0, 0.1, 0.4, 1.0 and 3.0 wt/vol%) ina 150-ml Erlenmeyer flask. The flask was capped during thetemperature ramp from 60 �C (hold for 30 min) to 90 �C over a 2-hperiod with stirring (magnetic stirring bar, 450 rpm). The temper-ature was maintained at �1 �C using a PEG bath. Agglomeratesmeasuring 2e3 mm were recovered and washed twice withdeionised water (50 ml) to remove the excess unreacted maleicacid. The precipitate was collected and dried for 72 h at 35 �C undernitrogen flushing.

CO2 was kindly supplied by Yara Praxair, and all other reagentswere purchased from SigmaeAldrich.

3.2. Characterization methods

The thermal stability of PPC was investigated through the use ofthermogravimetric analyses (TGA) in N2. The thermograms wererecorded in a Q500 thermal analyzer from TA Instruments using 10-mg samples and a constant flow of N2. Dynamic TGA runs wererecorded over a temperature interval of 25e400 �C using a heatingrate of 5 �C/min. The isothermal TGA was run at a set-pointtemperature of 200 �C using an initial heating rate of 30 �C/min,and the tests were recorded over a period of 60e70 min. The TGAanalyses were performed according to the standard ISO 11358 aswell as ICTAC guidelines for data acquisition [66]. FTIR-ATR spectrawere recorded in a Perkin Elmer Spectrum GX in the range4000e650 cm�1(2-cm�1 resolution, 64 scans were signal-averaged), using test specimens cut from 0.5-mm-thick films hot-pressed at 150 �C. The molecular weight distribution was deter-mined by size-exclusion chromatography (SEC) in a Waters 150CVplus instrument equipped with a refractive index detector andcalibrated with narrow polystyrene standards. The determinationswere performed in THF as the eluent at 40 �C 1H NMR spectra wererecorded on a Bruker DPX300 instrument (15e25 mg in CDCl3).DMTA characterizations were performed in a DMA Q800 from TAInstruments using a single-cantilever fixture. The scanned intervalwas �5e70 �C using a dynamic strain of 0.01% and a frequency of

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1 Hz; the test specimens were 1 mm � 5 mm � 25 mm strips cutfrom hot-pressed films at 150 �C.

4. Results and discussion

4.1. Effect of solventless purification on PPC properties

Various methods were evaluated with the purpose of avoidingthe use of organic solvents and simplifying the polymer purificationoperations; an aqueous solideliquid extraction proved to give asa very interesting alternative. Preliminary screening tests wereconducted using a range of organic and inorganic acids. It wasfound that maleic anhydride, which hydrolyzes easily to maleicacid, successfully increased the thermal stability of PPC and enabledthe sufficient removal of the cyclic side product. The main effects ofusing a simple aqueous purification on the properties of PPC aredescribed below.

A series of PPC samples were prepared according to the W90MAprocedure, using various concentrations of maleic acid in thesolideliquid aqueous extraction step (0.1, 0.4, 1.0 and 3.0 wt % MA toproduce the samples PPCW90MA0.1, PPCW90MA0.4, PPCW90MA1.0and PPCW90MA3,0 respectively; abbreviated PPCW90MA0,1-3,0).The reference samples were the crude PPC product that wasobtained directly after the polymerization reaction, the PPCW90sample, which was purified using only water, and the PPCWH1sample, which was purified by solvent/non-solvent precipitation.

4.1.1. Effects on the molecular weight distributionThe use of acidic aqueous conditions at elevated temperatures

for the purification of PPC renders hydrolytic degradationa possible reaction pathway, and previous reports of usinghomogeneous solutions over prolonged reaction times confirmthis [11,13,14,26,28,48]. Unexpectedly, the molecular weight of thePPC changed only insignificantly in the polymer workup reportedhere (Table 1) (PPC vs. (PPCW90 and PPCW90MA0,1-3,0)). Whencomparing the crude PPC material with the materials worked-upby an aqueous extraction (PPCW90 and PPCW90MA0,1-3,0) andthose in organic solvents (PPCWH1), only minor differences wereobserved. The latter purification method exhibited slightly higherMw and Mn values. This may be explained by the extraction oflow-molecular weight PPC from DCM/methanol mixtures, result-ing in a shift in the final molecular weight distribution to slightlyhigher values. In particular, the retention of the molecular weightin the case of the heterogeneous solideliquid extraction isnoticeable. This indicates that the solideliquid extraction rendersthe system less susceptible to severe degradation under theapplied conditions, i.e., temperature, time, and concentration andtype of acid.

4.1.2. Extraction of propylene carbonate (PC)The FTIR spectra shown in Fig. 4 clearly illustrate some of the

main results from the different purification procedures. The pres-ence of the PC side product is indicated by the carbonyl band at1800 cm�1 (C]O), which is clearly separated from the character-istic carbonyl band of PPC at 1740 cm�1. The peak intensity is in

Table 1Molecular weight distribution characteristics Mn and Mw for the crude and the worked-

PPC PPCWH1 PPCW90 PPCW

Mn (Kg/mol) 151 194 156 151Mn Std Dev (Kg/mol) 3 3 6 5Mw (Kg/mol) 487 537 520 500Mw Std Dev (Kg/mol) 4 4 29 46Mw/Mn 3,2 2,8 3,3 3,3

general significantly reduced but depends on the purificationprocedure. The lowest amounts of PC were found in the samplesthat were worked-up in aqueous solutions (PPCW90 andPPCW90MA0,1-3,0).Varying the MA (0e3% wt) had no observableeffect on the efficiency of the PC extraction. A slightly higheramount of PC was observed in the PPCWH1 sample. Repeatedextractions performed with methanol and/or by increasing themethanol:DCM ratio during solvent/anti-solvent purification maycompletely remove the PC.

PC is known to act as a plasticizer in PPC materials. Therefore,significant increases in the stiffness and in the glass transitiontemperature are obtained when the PC content is reduced. Anindication of the stiffness of the polymer is obtained from thestorage modulus, which is measured by DMA at a specifiedtemperature. From this same analysis, the glass transitiontemperature Tg can be obtained from the maximum value of thetangent delta curve. The dynamic DMA analysis results of selectedPPCmaterials are shown in Fig. 5. The storagemodulus at 20 �Cwasincreased from 1.95 GPa to values close to 3.4 GPawhen the PC wasremoved. The increase in the Tg followed the same trend and wasincreased by approximately five degrees for the sample PPCW90compared to the Tg of the crude PPC product.

4.1.3. Catalyst residuesThe presence of ZnGA (catalyst residues) was indicated by the

band at 1530 cm�1 (COO-) in the IR spectra (Fig. 4). The absorptionintensity of this peak was reduced after the purification of PPC inneat water (PPCW90) and was further reduced but not completelyremoved when the purification was performed in aqueous water(MA(aq); PPCW90MA0,1-3,0). The reference purification methodinvolving organic solvents (PPCWH1) in a precipitation procedureproved to be the most efficient in removing ZnGA residues, asevidenced by the virtual absence of the zinc glutarate signal in thiscase.

The significant decrease in the amount of ZnGA caused by thedifferent purification methods may be explained by hydrolysisreactions of the catalyst. Using organic solvents with aqueoushydrochloric acid (PPCWH1) may generate ZnGA species that aresoluble while the polymer precipitates from the solution. Usingonly hot water in the solideliquid extraction (PPCW90) may onlypartially hydrolyze the ZnGA, thus rendering the extraction ofcatalyst species inefficient. It is suggested that solideliquidextraction with hot water containing maleic acid may facilitatethe formation of extractable zinc glutarate species. However, even ifthe removal of zinc species is improved, it is not complete, andsome metal-containing compounds are still left in the purifiedsamples (PPCW90MA0,1-3,0). The presence of metal-containingcompounds in this series of samples was evident from thedynamic TGA analyses, which showed that residues were stillpresent at 400 �C (Table 3). The residues were confirmed to containzinc by AAS. The zinc content was quantified (Table 5) and showedthat 35% of the initial amount of zinc in the crude PPC was removedby extraction when 1% MA (PPCW90MA1,0) was used. Theremaining zinc content in the polymer was reduced to ca 20%(<1000 ppm) of its original value when increasing the amount of

up PPC as determined by SEC.

90MA0,1 PPCW90MA0,4 PPCW90MA1,0 PPCW90MA3,0

150 162 1508 8 10

500 530 51021 15 193,3 3,3 3,3

Fig. 4. FTIR-ATR spectra for the crude and the worked-up PPC samples. The arrows show the bands of interest (1800, 1675 and 1530 cm�1) when their intensities are maxima.

C. Barreto et al. / Polymer Degradation and Stability 97 (2012) 893e904898

Fig. 5. Dynamic DMA plots for the crude and the worked-up PPC samples.

Table 4Thermal-degradation parameters from dynamic TGA in N2 at 5 �C/min forPPCW90MA3,0 and derived samples extracted with methanol after 2 and 4 h.

PPCW90MA3,0 PPCW90MA3,0MeOH(2h)

PPCW90MA3,0MeOH(4h)

aTjOnset 1 (�C) 117 � 5 e ebResiduej Onset

2 (% wt)96 � 1 99 � 1 100 � 1

cTjOnset 2 (�C) 260 � 3 252 � 3 244 � 3dTj 5% (�C) 250 � 3 249 � 3 243 � 3eResiduej 400 �C

(% wt)0,7 � 0.1 0,6 � 0.1 0,3 � 0.1

Note: a,c: Onset temperature for the main decomposition stage p. b: weight %residue at the onset of the second main decomposition stage. d: Temperature at 95%of weight residue. e: weight % residue at 400 �C.

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MA. This could be further reduced by optimizing the solideliquidexperimental parameters. It was concluded, by visual inspection,that simple IR analysis (Fig. 4) did not reveal the presence of zinccarboxylate species at concentrations �1000 Zn ppm due to theabsence of the characteristic band (1530 cm�1).

4.1.4. Thermal stabilityThe thermal stability quantified by TGA is a measure of the

ability of a polymer to withstand elevated temperatures duringlimited periods of time (typically minutes) without being subject toany force. This information can be used to determine appropriateprocessing conditions and applications. Both isothermal anddynamic TGA analyses revealed an exceptionally high increase inthe thermal stability of the PPC sample series that were purified bysolideliquid extraction in MA(aq) (PPCW90MA0,1-3,0).

Isothermal TGA (Fig. 6 and Table 2) showed that usingMA(aq) (PPCW90MA0,1-3,0) had a positive effect on the thermalstability when compared to the solideliquid extraction usingwater (PPCW90) and when compared with the conventionalpurification using organic solvents (PPCWH1). An increase in thethermal stability was observed for increasing concentrations ofMA up to a maximum concentration of 0.4%. A further increasein the MA concentration led to a decrease in the thermalstability. This may be due to the acid-catalyzed random chainscission that will come into play at higher acid concentrations

Table 2Thermal-degradation parameters from isothermal TGA in N2 at 200 �C for the crude and

PPCWH1 PPCW90 PPCW90MA0,1aWt%j60 min 40 � 2 8 � 4 93,0 � 1.5bWt%j20 min 85 � 1 29 � 2 96 � 1cDWt%j20e60 min �45 � 2 �20 � 4 �3 � 1.5

Note: a, b: Weight residue after x minutes of testing time. c: Loss of residue between 20

Table 3Thermal-degradation parameters from dynamic TGA in N2 at 5 �C/min for the crude and

PPC PPCWH1 PPCW90 PPaTjOnset 1 (�C) 72 � 5 e e 12bResiduej Onset 2 (% wt) 96 � 1 100 � 1 100 � 1 9cTj Onset 2 (�C) 172 � 3 209 � 3 168 � 3 25dTj 5% (�C) 158 � 3 212 � 3 175 � 3 25eResiduej 400 �C (% wt) 1,5 � 0.1 0,1 � 0.1 1,4 � 0.1 0

Note: a,c: Onset temperature for the main decomposition stage p. b: weight % residue atresidue. e: weight % residue at 400 �C.

(see previous discussion). The sample PPCW90 MA0,4 showedthe best thermal stability. In this case, the polymer remainedstable during a prolonged testing period of 60 min at 200 �C.This significant improvement in thermal stability may allow forthe processing of PPC at higher temperatures, where the meltviscosity is lower and the risk of shear degradation duringprocessing is reduced.

The dynamic TGA (Fig. 7 and Table 3) (heating rate 5 �C/min)showed that the samples worked-up with MA(aq)(PPCW90MA0,1-3,0) exhibit two main onsets of the thermal-degradation process. The first main decomposition stage, in termsof onset temperature and weight loss, was influenced by theconcentration of MA. The first onset was attributed to theremoval of volatile organic compounds such as propylene oxideand the desorption of bound water from the Zn species. Thisdecomposition behaviour is in line with reported DSC studies ofthe thermal stability of zinc maleate hydrates and other zinccarboxylates [67,68]. Such zinc-containing species are presum-ably formed by the hydrolysis of the zinc glutarate catalystduring polymer purification and are not completely removedduring the solideliquid extraction. This assumption fits well withthe TGA results shown in Fig. 10 and Table 4. The unsaturationascribed to the maleate species is evident in the FTIR spectrumnear the band at 1630 cm�1 (C]C) (Fig. 4) and in a broad andsmall peak in the 1H NMR spectra at 6.35 ppm (eCH]) (Fig. 8.).Quantification of the latter (by NMR) shows a good correlationwith the concentration of MA that was used in the purificationprocedure.

the worked-up PPC samples.

PPCW90MA0,4 PPCW90MA1,0 PPCW90MA3,0

98,7 � 1.5 97,9 � 1.5 95,2 � 1.599 � 1 98 � 1 96 � 10 � 1.5 0 � 1.5 0 � 1.5

and 60 min of testing time in wt%.

the worked-up PPC.

CW90MA0,1 PPCW90MA0,4 PPCW90MA1,0 PPCW90MA3,0

8 � 3 130 � 3 134 � 3 117 � 39 � 1 99 � 1 98 � 1 96 � 15 � 3 258 � 3 257 � 3 260 � 36 � 3 257 � 3 257 � 3 250 � 3,7 � 0.1 0,7 � 0.1 0,7 � 0.1 0,7 � 0.1

the onset of the second main decomposition stage. d: Temperature at 95% of weight

Table 5Characterization of a sequence of cascade workups W90MA1,0 and methanol extractions in crude PPC. Determination of zinc content by AAS and thermal decompositionparameters from dynamic TGA.

Technique Parameter PPC PPC W90MA1,0(B) PPC W90MA1,0(B)MeOH

PPC W90MA1,0(B)MeOH W90MA1,0

PPC W90MA1,0(B)MeOH W90MA1,0 MeOH

Dynamic TGA (N2-5 �C/min) aTjOnset 1 (�C) 72 � 5 118 � 5 e e ebResiduej Onset 2 (% wt) 96 � 1 98 � 1 100 � 1 100 � 1 100 � 1cTjOnset 2 (�C) 172 � 3 259 � 3 242 � 3 242 � 3 227 � 3dTj 5% (�C) 158 � 3 257 � 3 237 � 3 241 � 3 218 � 3eResiduej 400 �C (% wt) 1,5 � 0.1 0,9 � 0.1 0,4 � 0.1 0,3 � 0.1 0,2 � 0.1

Atomic Absorption Zn (wt ppm) 4870 � 30 3200 � 30 970 � 15 950 � 15 940 � 15

Note: a,c: Onset temperature for the main decomposition stage p. b: weight % residue at the onset of the second main decomposition stage. d: Temperature at 95% of weightresidue. e: weight % residue at 400 �C.

Fig. 6. Isothermal TGA curves in N2 at 200 �C for the crude and the worked-up PPCsamples.

C. Barreto et al. / Polymer Degradation and Stability 97 (2012) 893e904900

The first main decomposition onset in PPCW90MA3,0 wasabsent in the sample after an additional solideliquid extractionusingmethanol (Fig. 10 and Table 4). This was expected because thezinc species responsible for this were now efficiently removed videinfra (v. i.).

The second main decomposition stage in the sample seriesPPCW90MA0,1-3,0 represented the main thermal decompositionof the PPC itself. The onset of the thermal-degradation (Tjonset 2)was increased in all of the worked-up samples relative to theonset of degradation of the PPC crude products (Fig. 7 andTable 3). The sample worked-up in water (PPCW90) exhibiteda small improvement in thermal stability, ascribed to only partialdeactivation of the catalyst residues [31]. Using MA in thesolideliquid workup procedure gave the most striking results. For

Fig. 7. Dynamic TGA curves in N2 (5 �C/min) for

these samples (PPCW90MA0,1-3,0), the onset of the maindecomposition stage was increased by 85 �C. The increase in theonset of decomposition temperature proved to be almost insen-sitive to the concentration of MA(aq). The highest observedtemperature was Tj5% ¼ 260 �C for PPCW90MA1,0 and PPCW90MA0,4. This is higher than the results reported for end-cappedPPC (measured at 10 �C/min) [23], for PPC metal-ion coordina-tion with calcium ion coordination (measured at 10 �C/min) [27]and for copper ion coordination (measured at 2 �C/min) [12].However, higher values of Tj5% have been reported in samplesafter the melt end-capping with maleic anhydride measured athigher heating rates(20 �C/min) [39] and with hybrids of end-capping-metal-ion coordination prepared by compoundingcalcium stearate on melt end-capped PPC (20 �C/min) [38]. Highheating rates shift the measured onset of the degradation tohigher temperatures and vice versa [13,33]. The different TGAtesting conditions makes it difficult to compare the thermalstability of the various PPC materials.

The observed improvements in the thermal stability for samplesPPCW90MA0,1-3,0 correlate well with the results obtained fromthe isothermal analyses (Fig. 6 and Table 2).

4.2. Elucidation of the mechanism for the thermal stabilityenhancement

The unexpectedly high increase in the thermal stability of thePPC materials after the MA(aq) workup may be explained byconsidering the effects of residual zinc glutarate and maleatespecies on the PPC material. The polymer samples after workup inMA(aq) contained maleate species that were quantified by 1H NMR(6.35 ppm) (Fig. 8). Different explanations for the presence ofmaleate species and their effect on the thermal stability weresought.

the crude and the worked-up PPC samples.

Fig. 8. 1H NMR spectra for the crude and the worked-up PPC samples.

Fig. 9. 1H NMR spectra for PPCW90MA3,0 extracted with methanol after various contact times.

C. Barreto et al. / Polymer Degradation and Stability 97 (2012) 893e904 901

Fig. 11. TGA in N2 at 5 �C/min for a sequence of cascade workups for W90MA1,0 andmethanol extractions in crude PPC.

Fig. 10. TGA in N2 at 5 �C/min for PPCW90MA3,0 and the derived samples extractedwith methanol after 2 and 4 h.

Fig. 12. Proposed mechanism for the enhancement in the thermal stability of PPC via metal-ion coordination after the solideliquid workup in aqueous maleic acid.

C. Barreto et al. / Polymer Degradation and Stability 97 (2012) 893e904902

C. Barreto et al. / Polymer Degradation and Stability 97 (2012) 893e904 903

Assuming that polymer-end-capping might take place, themaximum extent of end-capping for PPCW90MA3,0 was calculatedbased on the analyzed Mn of the PPC and assuming two availablehydroxyl end groups per polymer chain. The result showed thateven if the highest possible extent of end-capping had taken placewith maleic anhydride, excess (“free”) maleate species would stillbe present. In order to remove the assumed excess (“free”) maleatespecies, a solideliquid extraction in methanol was done onPPCW90MA3,0 for 2 and 4 h. The 1H NMR signal from the maleateat 6.35 ppm was significantly decreased after 2 h and completelyremoved after 4 h of extraction (Fig. 9). From this, it was concludedthat all maleate species were removed by extractionwith methanoland that no covalently bond maleate end groups had been formed.It should also be noted that it is not likely that an esterificationreactionwould take place under the aqueous conditions used in thefirst place. Hence, an explanation for the improved thermal stabilityhad to be sought elsewhere.

Alternatively, metal-ion coordination could take place if Znspecies are still present in the polymer, thus increasing the thermalstability [27]. TGA analyses of the methanol-extracted samples(PPCW90MA3,0MeOH-2 h and PPCW90MA3,0MeOH-4 h) (Fig. 10and Table 4) showed that the first main onset of degradation (at100e150 �C) had disappeared, indicating that compounds such zinchydrates were not present. Furthermore, the onset of the maindecomposition step was shifted to a lower temperature (Fig. 10 andTable 4), and the residues observed in the TGA curves at 400 �Ccorrelate well with the analyzed amount of metal residues revealedby AAS (Table 5). The decrease in the thermal stability is explainedby the removal of some zinc species by extraction, hence reducingany metal-ion coordination.

To further investigate the hypothesis of metal-ion coordination,an additional series of PPC purification experiments were con-ducted. Starting with the crude PPC, a workup was performed ina cascading fashion with alternating MA(aq) workup (W90MA1,0)followed by a MeOH extraction (W90MAMeOH). These stepswere subsequently repeated (W90MAMeOHW90MA and(W90MAMeOHW90MA-MeOH)). The material from each inter-mediate stage was characterized by dynamic TGA (Fig. 11 andTable 5). The Tonset values estimated from the TGA plots increasedafter the MA(aq) workup and decreased after the MeOH extrac-tions. However, the initial level of enhancement in the thermalstability was never achieved again after the repeated workupcycles. In general, a decline in the overall thermal stability wasobserved. Furthermore, the T5% values could not be correlated withthe zinc content alone (Table 5). It is hypothesized, therefore, thatzinc maleate-containing species are required to obtain an optimumincrease in thermal stability. The extraction with methanolremoves these species, hence the gradual reduction in the thermalstability.

It is suggested that the unexpectedly high increase in thethermal stability may be explained by the formation of activecoordination complexes comprising zinc ions and maleate speciesin addition to other species (Fig. 12). The zinc ions from the coor-dination complexes may be coordinated to the carbonyl groups inthe PPC backbone, in accordance with other reports [27]. Thecoordination of Zn species with PPC allows a stabilizing interactionand shifts the decomposition of PPC to higher temperatures.Complexes formed by the Zn maleate species and PPC are tenta-tively hypothesized (Fig. 12).

5. Conclusions

Poly(propylene carbonate) produced by a copolymerizationreaction catalyzed by zinc glutarate can be purified by a novelsolideliquid method using an aqueous organic acid.

The extraction of the common side product propylene carbonateis efficiently obtained and results in a desirable high glass transitiontemperature (Tg) and an increase in the stiffness of the polymer.

Signs of polymer degradation by an acid-catalyzed process areobserved only when high concentrations of maleic acid are used.

The aqueous solideliquid extraction may reduce but notcompletely remove catalyst residues from thepolymer. It is suggestedthat the resulting zinc species and the carbonyl groups in thepolymerenable metal-ion coordination and, consequently, a stabilization thatexplains the unexpectedly high thermal stability of the PPCmaterial.The novel solideliquid extraction procedure resulted in a single-stepdecompositionpath (TGA),which is in stark contrast to themulti-stepdecomposition path observed for the crude PPC product.

The simple solideliquid purification method allows for tailoredamounts of metal residues from the catalysts in the final productsand renders the PPC thermally stable at 200 �C for ca 60 min. Theprocessing at higher temperatures decreases the melt viscosity,reducing the occurrence of shear degradation, and allows forblending with polymers with higher melting temperatures.

These achievements may contribute to greener and more-simplified purification operations with lower carbon footprintsand broadened processing windows and may lead to new appli-cations for poly(propylene carbonate).

Acknowledgements

The support from the GASSMAKS program of the ResearchCouncil of Norway, Norner AS, Yara and RPC Superfos is gratefullyacknowledged.

The authors also acknowledge the collaboration with theScientific Laboratories at Norner AS and Sissel Jørgensen at UiO fortheir support in the characterization of samples.

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