Thermal degradation of poly(p-phenylene-graft-ɛ-caprolactone) copolymer

Polymer Degradation and Stability 92 (2007) 838e848www.elsevier.com/locate/polydegstab

Thermal degradation of poly( p-phenylene-graft-3-caprolactone)copolymer

Yusuf Nur a, Seda Yurteri b, Ioan Cianga b,c, Yusuf Yagci b, Jale Hacaloglu a,*

a Department of Chemistry, Middle East Technical University, 06531 Ankara, Turkeyb Istanbul Technical University, Department of Chemistry, Maslak, Istanbul 34469, Turkey

c ‘‘Petru Poni’’ Institute of Macromolecular Chemistry, Iasi, Romania

Received 28 November 2006; received in revised form 4 January 2007; accepted 21 January 2007

Available online 2 February 2007

Abstract

The thermal degradation of poly( p-phenylene-graft-3-caprolactone) (PPP), synthesized by Suzuki polycondensation of poly(3-caprolactone)(PCL) with a central 2,5-dibromo-1,4-benzene on the chain with 1,4-phenylene-diboronic acid, has been studied via direct pyrolysis mass spec-trometry. The thermal degradation occurred mainly in two steps. In the first step, decomposition of PCL chains occurred. A slight increase inthermal stability of PCL chains was noted. In the second stage of pyrolysis, the decomposition of the polyphenylene backbone takes place. Theevolution of CL monomer or small CL segments left on the phenyl ring continued also in the temperature region where degradation of PPPbackbone started.� 2007 Elsevier Ltd. All rights reserved.

Keywords: poly( p-Phenylene); poly(3-Caprolactone); Thermal degradation; Pyrolysis; Mass spectrometry

1. Introduction

Polyphenylene (PP) is a typical conjugated polymer withexcellent mechanical properties and thermal and thermo-oxi-dative stability. Other interesting and important propertiesthat PPs exhibit include liquid crystallinity [1] and photo-and electroluminescence [2]. Since the discovery of the elec-trical conductivity of polyphenylene (PP) when doped withoxidizing or reducing agents [3], PP has attracted significantinterest [4e6]. Yet, because of high crystallinity, insolubilityand high melting temperature the processability of PPs is lim-ited. Thus, attachment of side chains to obtain PP with im-proved solubility and processability has been the subject ofseveral studies. The effect of side chain chemistry and sidechain length on the planarity of PP backbones has been exten-sively investigated by various groups [7].

* Corresponding author.

E-mail address: [email protected] (J. Hacaloglu).

0141-3910/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.polymdegradstab.2007.01.023

Current methods for the direct synthesis of derivatized PPand other soluble polyarylenes are primarily based upon nickel-and palladium-mediated cross-coupling reactions due largely totheir preservation of regiochemistry and nearly quantitativeyields [8,9]. The cross-coupling of aryl halides and aryl boronicacids (Suzuki coupling) is one of the most common methods forthe synthesis of polyarylenes and has several advantages such assimplicity of the procedure and insensitivity to moisture[4,10,11]. We have previously reported the synthesis of PPs hav-ing various chemical structures and polymeric side groups [12e17]. PPs with polystyrene (PSt) side chains starting from atomtransfer radical polymerization (ATRP) initiators like 1,4-di-bromo-2,5-bis(bromomethyl)benzene [12] or benzene-2,5-di-bromomethyl-1,4-bis(boronic acid propanediol diester) [13]have been synthesized. The first initiator was also used for thesynthesis of polytetrahydrofuran (PTHF) based macromono-mers by cationic ring opening polymerization (CROP) andPPs with PTHF or with PTHF/PSt alternating side chains weresynthesized by Suzuki or Yamamoto methods [14].

In our recent studies, we discussed the syntheses of poly( p-phenylene)s (PPPs) with alternating PSt and poly(3-caprolactone)

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839Y. Nur et al. / Polymer Degradation and Stability 92 (2007) 838e848

(PCL), and PCL and hexyl groups as lateral substituents eitherby Suzuki or Yamamoto methods [15e17]. Because of variousapplications, homo- and copolymers of polylactones, especiallypoly(3-caprolactone) (PCL), a non-polar aliphatic polyester,have gained significant interest [18e21]. When 2,5-dibromo-1,4-(dihydroxymethyl)benzene was used for ring opening poly-merization (ROP) of 3-caprolactone, well defined PCL basedmacromonomers were obtained as shown in Scheme 1. Thesemacromonomers were further used for the synthesis of PPs bySuzuki method in combination with 2,5-dihexylbenzene-1,4-diboronic acid [15].

In the present work, we report the synthesis of poly( p-phe-nylene)-graft-poly(3-caprolactone), PPP copolymer by usingthe Suzuki polycondensation method and its thermal charac-terization via pyrolysis mass spectrometry, to elucidate the ef-fect of modification on thermal characteristics.

Among the various thermal analysis techniques used in poly-mer characterization direct pyrolysis mass spectrometry pro-vides information not only on the thermal stability but also onthe primary thermal degradation products [22e24]. In this tech-nique, secondary and condensation reactions are almost totallyavoided as the high vacuum system not only favours vaporiza-tion but also causes removal of the degradation products fromthe heating zone.

2. Experimental

2.1. Materials

1,4-Phenylene-diboronic acid (Aldrich) was used asreceived. The synthesis of 2,5-dibromo-1,4-(dihydroxyme-thyl)benzene, the initiator for ROP of 3-caprolactone andthat of macromonomer by ring opening polymerization of(ROP) 3-caprolactone were described elsewhere as shown inScheme 1 [15]. Briefly, 51 mmol of monomer (CL),2.04 mmol of bifunctional initiator (2,5-dibromo-1,4-(dihy-droxymethyl)benzene) and a catalytic amount of Sn(Oct)2

([OH]/[Sn(Oct)2]¼ 200/1 molar ratio) were mixed under ni-trogen at 110 �C. After 24 h, the mixtures were diluted withCH2Cl2 and poured into 10-fold excess of cold methanol.The macromonomer was collected after filtration and driedat room temperature in vacuum for three days.

2.2. Synthesis of poly(p-phenylene) by Suzukipolycondensation

A 100-mL, three-neck, round bottom flask equipped witha condenser, a rubber septum, nitrogen inleteoutlet, and

magnetic stirrer, was charged with 20 mL 1 M NaHCO3 aque-ous solution and 30 mL THF. The solvents were previously de-gassed by bubbling nitrogen over a period of 30 min. Themixture was refluxed under nitrogen for 4 h, then allowed tocool to room temperature.

A 20-mL, three-neck, round bottom flask equipped inthe same way as the previous one was charged under inertatmosphere with 0.4896 g (0.15 mmol) of 1, 0.0248 g(0.15 mmol) of 1,4-phenylene-diboronic acid and 0.0026 g(0.00225 mmol) of Pd(PPh3)4. The mixture (1.2 mL) of sol-vents was introduced with a syringe through the septum.The reaction was maintained at reflux under vigorous stirringand with the exclusion of oxygen and light. After 24 h, a sup-plementary 1 mL of the mixture of solvents was added. After48 h, the polymer was obtained by precipitation in methanol.Reprecipitation from THF solution provided a white powder.

2.3. Structural and thermal characterization

1H NMR and 13C NMR spectra were recorded on a Bruker250 MHz spectrometer using CDCl3 as solvent and tetrame-thylsilane as the internal standard. GPC measurements wereperformed with an Agilent 1100 RI apparatus equipped withthree Waters Styragel columns HR series (4, 3, 2 narrowbore), at a flow rate of 0.3 mL/min and the temperature ofthe refractive index detector of 30 �C, and THF as eluent. Mo-lecular weights were calculated using polystyrene standards.

The direct insertion probe pyrolysis mass spectrometry(DP-MS) system used for the thermal analyses consists ofa 5973 HP quadruple mass spectrometer coupled to a JHPSIS direct insertion probe pyrolysis system. Samples(0.01 mg) were pyrolyzed in the flared glass sample vials.The temperature was increased at 10 �C/min and the scanrate was 2 scans/s. Pyrolysis experiments were repeated atleast twice to confirm reproducibility.

3. Results and discussion

The macromonomer retains the structural characteristics ofthe bifunctional initiator, 2,5-dibromo-1,4-(dihydroxymethyl)-benzene used in ROP of 3-caprolactone (Scheme 1) due to thepresence of bromine atoms in 2,5-positions [15e21] and givesthe possibility to use it in Suzuki or Yamamoto type polycon-densations for synthesis of polymeric substituted polypheny-lenes. The 1H NMR spectrum of the macromonomer showsnear the usual peaks of PCL also signals belonging to the restof the initiator (protons a and b in Fig. 1). By comparing theirintensities with those of CH2eO protons (g) or CH2eCOe ones

BrBr

CH2OH

HOH2C

OO +

Sn(Oct)2111ºC bulk

BrBr

CH2O--C-(CH2)5O--H

H--O(CH2)5CH2C--OCH2

n/2

n/2

O

On

Scheme 1. Synthesis of PCL macromonomer by ROP of 3-caprolactone.

840 Y. Nur et al. / Polymer Degradation and Stability 92 (2007) 838e848

Fig. 1. 1H NMR spectrum of PCL macromonomer (1) and the corresponding PPP with polymeric side chains (2) (CDCl3).

(c), a polymerization degree of 26 was calculated, correspond-ing to a molecular weight of Mn,HNMR¼ 3264 andMw/Mn¼ 1.27. This value was considered more reliable thanthe one found from GPC (calibrated with PSt standards) asMn,GPC¼ 4000.

The synthesis of PP with PCL side chains was performedby Suzuki polycondensation using as reaction partner for thearyldibromo-functionalized macromonomer, 1,4-phenylene-diboronic acid as shown in Scheme 2. A white polymer withMn,GPC¼ 32 300, Mw/Mn¼ 1.33 was obtained. The molecularweight measured by GPC based on PSt standards should betaken as the minimum estimate because of the highly branchedor comb-like structure of the obtained polyphenylene.

The proposed structure for PPP was confirmed by 1H NMRand 13C NMR spectra of the polymer. Fig. 1 shows that 1HNMR spectrum of the macromonomer has a single sharppeak at 7.54 ppm (a) due to the residue of initiator in the mid-dle of the PCL chain. The spectrum of PPP presents a verybroad peak in the range 7.62e7.29 ppm. Also the ratio ofthe integral of this peak compared to the other signal originat-ing from the residue of initiator (b) is lower than in the case ofthe spectrum of the macromonomer, due to the presence ofsupplementary aromatic protons (k) from the PP chain. Sup-plementary evidence of the proposed structure was obtainedfrom the 13C NMR spectrum (Fig. 2). The peak at 122 ppmassigned to CaromeBr (c) in the 13C NMR spectrum of the


BrBr

CH2O--C-(CH2)5O--H


n/2

n/2

O

OB(OH)2(HO)2B+

CH2O--C-(CH2)5O--H


n/2

n/2

O

O m

2

Pd(PPh3)4

THF, NaHCO3

Scheme 2. PPP with macromolecular side chains 2, by Suzuki

polycondensation.

macromonomer disappeared in that of PPP due to the lose ofBr atoms during the Suzuki polycondensation. Furthermore,two new peaks centered at 127.86 ppm and 130.38 ppm (oand p, respectively) are present in the spectrum of PPP corre-sponding to the carbons of phenylene rings originating fromthe diboronic acid.

3.1. Pyrolysis of PCL macromonomer

Thermal degradation of PCL has been studied in detail in theliterature. Though there were inconsistencies in the proposedmechanism, the formation of u-hydroxyl and ketene end groupshave been confirmed by several groups [25e29]. It has been de-termined that on isothermal heating, PCL degrades by pure un-zipping of the monomer from the hydroxyl end of the polymer,whereas a parallel mechanism including random chain scissionvia cis-elimination reaction and cyclic rupture via transesterifi-cation of PCL molecules was suggested [29].

Fig. 2. 13C NMR spectrum of PCL macromonomer (1) and the corresponding PPP with polymeric side chains (2) (CDCl3).


Thermal degradation products of poly(3-caprolactone) PCLmacromonomer with a central 2,5-dibromo-1,4-benzene on thechain were recorded in a broad temperature range in accor-dance with TGA data [15]. The total ion current (the variation

of total ion yield as a function of temperature) TIC curve isshown in Fig. 3. Presence of more than one peak in the TICcurve indicates either a multi-component sample or a multi-stage thermal degradation mechanism. Pyrolysis of a highly

80 180 280 380Temperature °C

(a)

(b)

(c)

(d)

50 100 150 200 250 300 350 400 450

11555

84

21516928

296247 324276438

50 100 150 200 250 300 350 400 450

115

55

84

22928 169 324199 279 438

50 100 150 200 250 300 350 400 450 500 550 600 650 700 750

115

69

211169 324

438

400 450 500 550 600 650 700 750

438

397552457

415511379 666571480 529 780643

T=300°C

T=370°C

T= 430°C

m/z

TIC curve

Fig. 3. The TIC curve and the pyrolysis mass spectra recorded during the thermal degradation of the polymer.


ı.

C-(CH2)5-O

O

C-(CH2)nCH2CH2

O

(CH2)3-nOC-CH2CH2

O

C-(CH2)5-OO

C-(CH2)nCH2CH2

O

C-(CH2)5-O

O

C-(CH2)nCH=CH2

O

x

x+

x+

(CH2)3-nOC-CH2CH2

O

H(CH2)3-nOC-CH2CH2

O

ıı.

C-(CH2)5-O

O

C-(CH2)3CH-CH2-O

O

C-CH2CH2

O

C-(CH2)5-O

O

C-(CH2)3CH=CH2

O

x x+ O=C-CH2CH2

OHH

ııı.

C-(CH2)5-OO

C-CH2-CH2CH-CH2CH2O

O

C-(CH2)5-O

O

C=CH2

OH

x x+CH2=CHCH2

H

Scheme 3. Thermal degradation of PCL chains.

polydisperse polymer may also yield several broad peaks inthe TIC curve. The pyrolysis mass spectra recorded at themaxima present in the TIC curve were dominated by peaksthat can be associated mainly with the degradation productsof PCL side chains (Fig. 3). The base peak was at m/z¼115 Da due to the protonated caprolactone monomer through-out the pyrolysis. However, peaks that can directly be attrib-uted to 2,5-dibromo-1,4-(dihydroxymethyl)benzene were alsodetectable especially at early stages of pyrolysis. Amongthese, the isotopic molecular ion peaks at m/z¼ 294, 296and 298 Da with intensity ratio of 1:3:1 and isotopic[M� Br]þ peaks at m/z¼ 215 and 217 Da with an intensityratio of 1:1 were the most abundant. However, detection of2,5-dibromo-1,4-(dihydroxymethyl)benzene cannot be attrib-uted to presence of unreacted initiator, as the macromoleculewas precipitated in methanol in which the initiator is soluble.

Actually, the pyrolysis mass spectra of polymers are usuallyvery complex as thermal degradation products further dissoci-ate in the mass spectrometer during ionization. Furthermore,all the fragments with the same mass to charge ratio have con-tributions to the intensity of the same peak in the mass spec-trum. Thus, in pyrolysis MS analysis, not only the detectionof a peak but also the changes in its intensity (single ion

pyrograms, evolution profiles) as a function of temperaturehas significant importance.

Analysis of pyrolysis mass spectra and single ion pyrogramsof abundant and/or characteristic products indicated thatthermal degradation produced, besides 2,5-dibromo-1,4-(dihy-droxymethyl)benzene and the monomer with m/z¼ 114 Da,the protonated monomer and oligomers with m/z¼ 115, 229,343, 457, 571, 685 Da, and several fragments with various ter-minal groups through the mechanisms shown in Scheme 3. Asa result of these processes, chains bearing COOH (series offragments with m/z¼ 45, 159, 273, 387, 501, 615 .Da),O]C(CH2)nCH]CH2 (series of fragments with m/z¼ 55,169, 283, 397, 511 .; 69, 183, 297, 411, 525 .; 83, 197,311, 425, 539 .; and 97, 211, 325, 439, 553 .Da for n¼ 0to 3, respectively), and C(OH)]CH2 (series of fragmentswith 43, 157, 271, 385, 499, 613 .Da) were generated. Fur-thermore, peaks that may readily be assigned to fragmentsbearing different combinations of these groups at both endswere also detected. Among all these products relative intensityof protonated monomer and oligomers were the most abundant.The second most intense series of peaks were due to the prod-ucts bearing CO(CH3)CH]CH2 terminal groups due to therupture of labile CeO bonds.

Br

CH2O--C-(CH2)5O--H


n/2

n/2

O

OBrBr

CH2OH

HOH2C

+ C=CH(CH2)4O--C-(CH2)5O--H

O O

nBr

Scheme 4. Generation of 2,5-dibromo-1,4-(dihydroxymethyl)benzene.


Evolution of 2,5-dibromo-1,4-(dihydroxymethyl)benzenewas mainly detected in two regions: the initial stages andthe final stages of pyrolysis. It may be thought that the lowtemperature evolution was mainly due to dissociation of labileOeCO bonds of short PCL chains followed by H-abstractionreactions as shown in Scheme 4.

The pyrolysis mass spectra recorded above 350 �C involvedpeaks that can be attributed to products involving pheny-lene linkages indicating that coupling processes yieldingsubstituted phenylene oligomers have taken place as shownin Scheme 5.

Peaks due to dihydroxymethyl substituted phenylene oligo-mers ((CH2OH)2C6H2)x where x¼ 1, 2, 3, 4, and 5 at m/z¼136, 272, 408, 544, and 680 Da and unsubstituted phenyleneoligomers such as e(C6H4)xe where x¼ 1, 2, 3, 4, 5, 6 and7 at m/z¼ 76, 152, 228, 304, 380, 456 and 532 Da were

Phenylene oligomers based products

m/z=296 Da

m/z=115 Da

m/z=210 Da

m/z=324 Da

m/z=666 Da

m/z=55 Da

m/z=272 Da

m/z=408 Da

m/z=304 Da

m/z=532 Da

m/z=167 Da

PCL based products

•x1.6

•x11.9

•x277.6

•x14.9

•440°C

•380°C•305°C

•420°C

•x317.8

•x717.6

•x354.5

•x1934.9

•x41.7

•x14.3

•190 •290 •390Temperature °C

Fig. 4. Single ion pyrograms of some selected degradation products of the

polymer.

80 180 280 380

80 180 280 380

80 180 280 380

Temperature°C

(a)

(b)

(c)

Fig. 5. The TIC curves of (a) 0.010 mg, (b) 0.05 mg and (c) 0.025 mg of the

polymer.


(a) (b) (c)

x1.6

x11.9

x277.6

x14.9 x57.3

x51.5

x706.6

x1.5

x30.0

x24.0

x406.1

x1.1

440°C

380°C305°C

420°C

x317.8

x717.6

x354.5

x1934.9

x41.7

m/z=115 Da

m/z=210 Da

m/z=324 Da

m/z=666 Da

m/z=55 Da

m/z=272 Da

m/z=408 Da

m/z=304 Da

m/z=532 Da

m/z=167 Da

400°C320°C

300°C

180.7

x2719.3

x4933.5

x3139.5

400°C

350°C300°C

x914.7

x1229.6

x1022.5

x9882

x85.5


PCL based products

x14.3 x30.8

190 290 390

m/z=296 Da

190 290 390

x13.3

190 290 390

Temperature °C

Fig. 6. Single ion pyrograms of some selected degradation products of (a) 0.010 mg, (b) 0.05 mg and (c) 0.025 mg of the polymer.


T=275°C

T=335°C

T=380°C

80 180 280 380Temperature °C

TIC curve

100 200 300 400 500

55

11584

100 200 300 400

55

11584

100 200 300

55

11584

250 450 650

229

324

438552 666

m/z

(a)

(b)

(c)

(d)

Fig. 7. The TIC curve and the pyrolysis mass spectra recorded during the thermal degradation of PPP with alternating PCL side chains.

detected. No product involving PCL substituted phenyleneoligomers could be detected. Thus, it may be thought thatthey were either generated by coupling of 2,5-dibromo-1,4-(dihydroxymethyl)benzene generated readily at initial stagesof pyrolysis due to loss of short PCL chains or in the finalstages of pyrolysis after decomposition of long PCL chainshave nearly been completed. Actually, even if coupling ofthe macromolecules has occurred, the detection of PCLsubstituted phenylene oligomers should almost be impossibleas thermal stability of phenylene linkages should be noticeablyhigher than that of PCL side chains.

In Fig. 4, evolution profiles of some intense and/or charac-teristic products due to the 2,5-dibromo-1,4-(dihydroxyme-thyl)benzene (molecular ion at m/z¼ 296 Da), PCL basedproducts such as OC3H3 at m/z¼ 55 Da, protonated monomer,CL at m/z¼ 115 Da, CL oligomers stabilized by H-transfer

reactions (products with m/z¼ 210, 324 and 666 Da) andproducts due to phenylene oligomers C7H7$C6H4 at m/z¼167 Da, (C6H4)x with m/z¼ 304 and 532 Da where x¼ 4 or7, respectively, and ((CH2OH)2C6H2)x with m/z¼ 272 and408 Da where x¼ 2 or 3, respectively, are shown. Inspectionof evolution profiles indicated that PCL based productsoccurred in a broad temperature range (Fig. 4). The yield of2,5-dibromo-1,4-(dihydroxymethyl)benzene based productsdecreased significantly above 360 �C whereas those of PCLbased products increased especially above 380 �C. The maxi-mum 2,5-dibromo-1,4-(dihydroxymethyl)benzene yield wasdetected at 305 �C more than 100 �C lower than that ofPCL based products (420 �C). On the other hand, the peaksdue to the products that can be associated with phenyleneoligomers reached to maximum intensity values in the finalstages of pyrolysis, at 440 �C. Though, they were quite


weak and their presence pointed out existence of couplingprocesses.

No evidence for such a coupling reaction was noted inNMR spectra. Thus, it may be thought that these reactionshave taken place in the bulk although secondary reactionswere almost completely neglected under direct pyrolysis con-ditions and only 0.010 mg of polymer samples were used.The pyrolysis experiments were repeated reducing theamount of the sample by a factor of 1/2 and 1/4. It can benoted from Fig. 5 that as the amount of sample pyrolyzed de-creased, the high temperature evolutions were also decreasedsignificantly. A similar trend was detected when the singleion pyrograms were compared (Fig. 6). The peaks associatedwith products due to the coupling reactions diminished dras-tically and finally almost disappeared, while evolution of PCLbased products shifted slightly to lower temperatures as theamount of the sample pyrolyzed reduced. Thus, it may beconcluded that the radicals generated by the cleavage of la-bile phenyleBr bonds during the pyrolysis coupled generat-ing thermally more stable phenylene oligomers as shown inScheme 5.

Persenaire and his co-workers have determined that lowermolecular weight significantly decreases the thermal stabilityof PCL, more particularly the first degradation step [27].The degradation onsets were detected at 230 and 335 �C forPCLs of 1800 and 42 450 Mn. As PCL macromonomer was in-tended to be used in further polymerization reactions, effortshave been directed toward obtaining a low molecular weightcombined with low polydispersity [15]. Molecular weightand PDI of the macromonomer were determined to be3260 g mol�1, and 1.27, respectively. As there were twoPCL substituents per macromonomer, it can be thought thaton average each PCL chain had an Mn value of about1630 g mol�1. The degradation onset for the macromonomer(with minimum sample size) was around 240 �C and maxi-mum yield for PCL based products was detected at 320 �C.Thus, it may further be concluded that the presence of benzeneat the chain centre slightly increased the thermal stability ofPCL side chains.

3.2. Pyrolysis of PPP with alternating PCL side chainsby Suzuki coupling

Both the trends observed in the TIC curve and the thermaldegradation products recorded during the pyrolysis of PPPwith alternating PCL side chains were independent of the sam-ple size for PPP sample unlike the macromonomer. Evolutionof thermal degradation products was detected above 250 �C.The TIC curve and the pyrolysis mass spectra recorded at275, 330, and 360 �C, are shown in Fig. 7. No peak in thelow temperature ranges was detected indicating that the poly-mer sample was not contaminated with any low molecularweight volatile reactants, and/or reagents adsorbed on thepolymer.

The shift of the TIC curve of PPP to higher temperatureranges pointed out an increase in thermal stability with respectto the macromonomer. The pyrolysis mass spectra were

dominated with characteristic peaks for PCL. Presence ofidentical PCL based products indicated that degradation ofPCL side chains followed similar reaction pathways with thosesummarized in Scheme 3. Yet, the intensity of low molecularweight product peaks enhanced while those of the oligomerpeaks reduced. The base peak was at 55 Da due toCOCH]CH2 which should also be generated during the ion-ization of CL chains bearing ketene end groups in the massspectrometry throughout the pyrolysis.

m/z=77 Da

m/z=91 Da

m/z=115 Da

m/z=324 Da

m/z=552 Da

m/z=55 Da

m/z=666 Da

150 250 350 450

107.2

X1.5

X502.1

x1316

x83.0

X87.7

X27.1

X1350

335°C

385°C 440°C

Temperature °C


PCL based products

Fig. 8. Single ion pyrograms of some selected degradation products of PPP

with alternating PCL side chains.


Br

CH2O

OCH2

nBr nBr nBr

CH2O

OCH2

+

CH2O

OCH2

CH2O

OCH2

n/2

Scheme 5. Formation of phenylene linkages.

In Fig. 8, evolution profiles of some of the intense and/orcharacteristic PCL and PPP based products, OC3H3 at m/z¼ 55 Da, protonated CL at m/z¼ 115 Da, CL oligomers stabi-lized by H-transfer reactions (products with m/z¼ 324, 532, and666 Da) and products due to degradation of PPP backbonenamely C6H5 and C7H7 are shown. In general, PCL based prod-ucts evolved slightly at higher temperatures than the degradationproducts of the macromonomer. It may be concluded that thethermal stability of PCL side chains increased upon copolymer-ization. Another point that should be noted was that unlike themacromonomer the evolution of protonated CL oligomers fol-lowed different paths. Single ion pyrogram of the protonatedmonomer showed two peak maxima at 340 and 390 �C. As thenumber of repeat units increased, the high temperature peakintensity diminished drastically and totally disappeared in thesingle ion pyrograms of CLn where n> 4.

Among the PPP based products only phenyl C6H5 and tro-pylium C7H7 ions had significant yield. In the evolution pro-files of these products again two maxima were detected. Thefirst at 390 �C was identical with the second maximum inthe evolution profiles of the protonated monomer and low mo-lecular weight CL fragments. The second peak had a maximumat 440 �C in the final stages of pyrolysis. At this temperature,an increase in the yield of phenyl, while a decrease in that oftropylium were detected.

In the light of pyrolysis data it can be concluded that ther-mal degradation of PPP with alternating PCL side chainsstarted with decomposition of PCL chains through the mech-anisms given in Scheme 5. The evolution of CL monomer orsmall CL segments left on the phenyl ring continued also inthe temperature region where degradation of PPP backbonestarted. In the final stage of pyrolysis, the decomposition ofthe polyphenylene generated as a result of loss of all PCLside chains occurred.

Acknowledgements

This work is partially supported by TUBITAK ResearchFunds TBAG-1691 and TBAG-1997.

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Thermal degradation of poly(p-phenylene-graft-ɛ-caprolactone) copolymer

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