Polymer Degradation and Stability 25 (1989) 143-160

Pyrolysis-GC and MS Applied to Study OligomerFormation in the Degradation of Polystyrene and Styrene

Copolymers

Louise Dean, Sally Groves, Robert Hancox, Gordon Lamb &Roy S. Lehrle*

Birmingham Polymer Group, Department of Chemistry, University of Birmingham,Birmingham 815 2TT, UK

(Received 7 September 1988; accepted 13 September 1988)

ABSTRACT

Polymer pyrolysis measurements can have quantitative significance only ifthe method of'pyrolysis and the method ofanalysis have been carefully chosento be appropriate for the problem being studied. Factors relevant to thesechoices are briefly discussed, with special reference to the technique ofpyrolysis-gas chromatography. General methods 0/obtaining mechanisms ofpyrolysis by this technique are reviewed.

Recent results on the application ofthe method to study oligomer/ormationin the degradation ofpolystyrene and some styrene copolymers are presentedin more detail. The overall conclusion from this work is that theintramolecular transfer mechanism, commonly assumed to be responsible/oroligomer formation, is not compatible with the results obtained frompolystyrene and its copolymers in certain situations. There is some evidence tosuggest that dimers and trimers are possibly formed by monomerrecombination, occurring both within the molten polymer and in the vapourphase above it.

1 INTRODUCTION

In Section 2 of this paper, the study of polymer pyrolysis by the technique ofpyrolysis-gas chromatography (Py-GC) is reviewed briefly with special

* To whom correspondence should be addressed.

143Polymer Degradation and Stability 0141-3910/89/$03'50 © 1989 Elsevier Science PublishersLtd, England. Printed in Great Britain

144 Louise Dean et al.

reference to the objective of obtaining results with quantitative significance.This is important when the target is to elucidate mechanisms of pyrolysis. Insuch work it may be required to examine the dependence of product yields,ratios, or rates of evolution, on a number of factors such as the molecularweight ofthe sample, or the temperature. If the experimental parameters arenot precisely controlled, not only will the results have little quantitativesignificance, but their reproducibility may be too poor to permit evenqualitative conclusions. Conversely, with well controlled experimentalparameters, Py-GC studies have allowed detailed mechanisms to beelucidated for a variety of polymer degradation systems.

Sections 3-9 provide an account of some recent work of the authors inwhich Py-GC studies, in conjunction with nuclear magnetic resonance(NMR) and mass spectrometry (MS), have been applied to examine thevalidity of the intramolecular transfer mechanism for oligomer formationduring thermal degradation. Results obtained from polystyrene alone haveprovided no reason to doubt the validity of that mechanism, butcomparisons of these results with those obtained from styrene copolymersystems are beginning to throw new light on this problem.

2 PYROLYSIS-GC USED QUANTITATIVELY TO OBTAINPYROLYSIS MECHANISMS

Any experimental study of polymer pyrolysis req uires that consideration begiven not only to choosing the most appropriate method of pyrolysis, butalso to selecting the technique for measuring what has occurred, and finallyto choosing the most fruitful way of dealing with the data. This generalbackground has been reviewed elsewhere.' In situations where the primaryinterest is in the volatile products evolved, it is highly desirable to analyseand measure the products 'in-line' after the pyrolysis, and one of the ways ofdoing this is to use the Py-GC technique. This method was first reportedin 1959,2 and recent reviewsv" have described its current breadth ofapplication.

In this and subsequent sections we focus our attention on therequirements for obtaining quantitative, meaningful results from thistechnique, and then to the questions of which data to obtain, and how toutilise them, to obtain mechanistic information.

Highly-developed GC apparatus is commercially available with excellentdetectors and data-handling systems, and provided that detector specificitiesare taken account of (if necessary by calibration), measurements of productyields are usually reliable. Undoubtedly, the weakest link in the quantitativeoperation of most Py-GC systems is the mounting of the sample for

Pyrolysis-G'C and MS applied to study oligomer formation 145

pyrolysis and the design of the pyrolysis unit itself. Such units can beclassified as (a) furnace types, (b)Curie-point filament types, and (c) resistivefilament types. The temperature of a resistive filament may be controlled bymeans of a feedback system involving the way in which the overall resistanceof the filament changes with temperature, but this is not a very good methodbecause of the temperature gradient which exists along the length of thefilament. An alternative method utilises a feedback signal from amicrothermocouple spotwelded on the sample-deposition region of thefilament." The present authors firmly recommend this arrangement inpreference to all other pyrolysis methods.

Planning which pyrolysis measurements to make, and how to interpretthem, requires some understanding of both the experimental problems andalso the underlying molecular behaviour involved in the thermaldegradation of polymers. A general order of attack might be:

(a) Attempt to deduce mechanism from product characterisation. It maybe possible to make preliminary suggestions about the mechanismfrom a consideration of the nature of the volatile products.

(b) Propose mechanisms on the basis ofkinetic measurements." The stagesin this approach are:(i) Verify that sample sizes are sufficiently small that the fractional

yield in a fixed pyrolysis time is independent of samplethickness.

(ii) Determine the pyrolysis time-limit below which first-orderevolution of volatiles is observed. Subsequent pyrolysis timesmust be restricted to within this limit.

(iii) For a chosen pyrolysis temperature, measure the observed rate­constant for product evolution from samples of different initialmolecular weight. Mechanisms of chain initiation and termin­ation may be inferred from the way in which the rate constantdepends on the initial molecular weight. The principles aresummarised in Ref. 3.

(c) Obtain information from temperature dependence and energetics.From an interpretation of the temperature dependence of the aboveresults, it may be possible to associate initiation processes with therupture of specific bonds."

(d) Measure trends in molecular weight with conversion. Since differentmechanisms give rise to different trends, these experiments may beperformed to confirm mechanisms proposed on the basis of thekinetic work. 7

146 Louise Dean et al.

Examples of the above approaches to obtain pyrolysis mechanisms will befound in Refs 5-10.

3 A PYROLYSIS~GC STUDY OF OLIGOMER FORMATIONFROM STYRENE POLYMERS AND COPOLYMERS

The formation of trimers during the thermal degradation of polystyrene isusually attributed to the occurrence of intramolecular transfer during unzip,followed by scission, as in channel (a) in Scheme I.

'-,'H 1---------- ----...'-.'" JI . J<

-CH-CH2-C-CH2-CH-CH2-CH

I I I IPh Ph Ph Ph

1

(a)

1(b)

1----CH+ CH 2=C-CH2-CH-CH2-CH2 --CH-CH2-C CH 2 +CH-CH2-CH2

I I I I I I [ [Ph Ph Ph Ph Ph Ph Ph Ph

TRIMERScheme I

In a similar way, intramolecular transfer involving neighbouring tertiaryhydrogen atoms in the chain would lead to the corresponding dimer orother oligomers.

In the subsequent sections of this paper we report measurements on thepyrolyses of several styrene polymer and copolymer systems, which wereperformed in order to assess whether this mechanism could be responsible

Pyrolysis-CC and MS applied to study oligomer formation 147

for oligomer formation, or whether alternative mechanisms must beproposed.

4 A COMPARISON OF THE PYROLYSIS BEHAVIOUR OF A'STYRENE/ETHYLENE' (STYRENE/HYDROGENATED

BUTADIENE) COPOLYMER WITH THAT OF POLYSTYRENE

Sincea full account of this section of the work has recently been presented,llonly the points relevant to the present study will be summarised here.

An NMR study of the copolymer showed that the number averagesequence length of butadiene units in the polymer is 2,0, and that for styreneunits is 1·3. This implies that most styrene units occur singly, though thereare occasional pairs, fewer triplets, and even fewer longer sequences.

The thermal degradation of the copolymer was studied at varioustemperatures in the range 52D-700°C by Py-GC. Very many pyrolysisproducts were obtained, and these were characterised by Py-GCMS. Fromthe sizes of the styrene monomer and trimer peaks, the molar ratio ofmonomer/trimer could be calculated, and this was plotted as a function ofthe pyrolysis temperature. The results are shown in Fig. 1, which alsoincludes corresponding results for a styrene homopolymer, studied underthe same conditions.

The fact that the monomer/trimer ratios for the copolymer and thehomopolymer are so similar (at least up to temperatures in the region of600°C) is very difficult to reconcile with the intramolecular transfer

L:()j

EL:::: 100

~co.5 50()jc::e'"ill 0 I-..__~__-:+::--_-::±::::--_--",.±-::,

500

Fig. 1. Styrene monomer/trimer molar ratio in the pyrolysis products from polystyrene (.)and styrene/hydrogenated butadiene copolymer (0), plotted as a function of pyrolysistemperature, I I Results obtained using a styrene/hydrogenated isoprene block copolymer (.)

have been superimposed on the original plot.

14& Louise Dean et al.

mechanism mentioned above. Bearing in mind the sequence lengths in thecopolymer, long styrene unzips with occasional intramolecular transfer arenot conceivable. Indeed, calculations indicate that approximately everytriplet sequence in the copolymer would have to yield a trimer, and could notitself unzip, in order to account for the observed yields of trimer. While sucha possibility cannot be excluded, it seems rather unlikely.

If the intramolecular transfer mechanism is not responsible for trimerformation in the pyrolysis of the copolymer, the most plausible mechanismwould seem to be that styrene monomer, perhaps while still energetic as aprimary pyrolysis product, is recombining to form trimers in such significantyield. And since polystyrene itself gives much the same monomer/trimerratio, does this not imply that trimer from the pyrolysis of polystyrene mayalso be formed by recombination?

In the above Py-GC work, the whole of the sample was completelypyrolysed in each case. (This had been verified by taking the filament up totemperature for a second time after each pyrolysis; the absence of furtherproducts shows that the reaction is effectively complete.) We have morerecently demonstrated that the situation is unaffected if partial pyrolyses areperformed. The same samples of polystyrene copolymer and homopolymeras used in the above work were now partially degraded at 480°C for 2·5 sineach case, and the ratio of the trimer to monomer peaks measured. Theaverage values obtained were approximately 15% for the copolymer, and20% for the homopolymer. Again, the similarity between these figuresargues against the intramolecular transfer mechanism for trimer formation.Furthermore, these partial pyrolysis results show that the conclusion doesnot depend upon the pyrolyses being complete.

5 A COMPARISON OF THE PYROLYSIS OF A STYRENE/HYDROGENATED ISOPRENE BLOCK COPOLYMER, A

STYRENE HOMOPOLYMER AND THE STYRENE!HYDROGENATED BUTADIENE COPOLYMER

A possible alternative explanation of the results in the previous section isthat some kind of compensation effectis occurring. Thus, it could be arguedthat intramolecular transfer is occurring in the pyrolysis of styrenehomopolymer and not in the copolymer, but the mere presence of productsfrom the hydrogenated diene has in some way favoured trimer formationfrom the copolymer. If this were the case, then the pyrolysis of acorresponding block copolymer should give much increased yields of trimer:the expected aliquot from intramolecular transfer in the styrene blocks, plusadditional trimer from the proposed favoured process.

Pyralysis-GC and US applied /0 study oligomer [ormation 149

Accordingly, a styrene/hydrogenated isoprene block copolymer withblock lengths of several hundred units was completely degraded attemperatures through the range 500-700°C in a thermocouple-feedback­filament Py-GC apparatus, the conditions being identical with those usedfor the original styrene/hydrogenated butadiene copolymer. The ratio of thearea of monomer to trimer peaks was again measured, and the results aresuperimposed on Fig. 1.

The similarity between the monomer/trimer ratios in the pyrolysisproducts from the block copolymer and the homopolymer demonstratesthat there is no additional trimer from any favoured process associated withthe presence of pyrolysis products from the hydrogenated diene. Again, thesimilarity of both of these results to those from the random copolymerargues against the intramolecular mechanism for trimer formation.

6 A PYROLYSIS-MS EXAMINATION OF THE 'STYRENE/ETHYLENE' (STYRENE/HYDROGENATED BUTADIENE)

COPOLYMER

If, during the pyrolysis of this polymer, the styrene trimer is formed byrecombination of monomer formed as a primary product, and if this processoccurs in the molten polymer, then a possible consequence is that there couldbe delayed formation of trimer until the monomer concentration in the melthas built up.

The technique of selective ion monitoring, in which the ion currentcorresponding to a particular mass is plotted against time, has been used toassess whether any such delay could be detected in Py-MS experiments. Thecopolymer was deposited on a temperature-programmable probe situatedadjacent to a port in the source of the mass spectrometer. The ion currentscorresponding to masses 104 and 312 provide (non-quantitative) measuresof the evolution ofmonomer and trimer respectively. Results were obtainedin two different mass spectrometers with different heated probe units.

Figure 2 shows results obtained with a Vacuum Generators TS250 doublefocusing mass spectrometer. The sample was contained in a small quartztube within the probe, which was programmed from 100 to 500a e in 2·5 min.A delay in the formation of trimer is clearly observed.

Figure 3 shows results obtained with a Kratos MS25 double focusingmass spectrometer, using a probe designed to give .faster temperatureresponse. The nominal temperature programme for the results in the figurewas 1a per second up to 8000 e,but the situation was the same when 0.10 persecond was used: no delay whatsoever in trimer formation can be detected.

Further work of this kind must be performed in order to reconcile the

100

80

60

40

20+' 0cQ)LL

'"UC0-100

80

60

40

20

020

004640

107

6368

601·28

80 1001·49 2'10

Time

1202·31

1202·31

134

1402·52

1402·52

160 scan3·13 Time

160 Scan313 Time

Fig. 2. Variation with time of the parent peak of styrene monomer ion (upper curve) and theparent peak of styrene trimer ion (lower curve) during a temperature-programmed pyrolysisof a styrene/hydrogenated butadiene copolymer. The figures above the curves refer to themass spectrometric scan number, which is a measure of time. Results were obtained with aprobe nominally programmed from 100 to 500De in 2'5 min. The mass spectrometer was a

Vacuum Generators double-focusing instrument, type TS250.

27087

8821

3380260

445711

O-bC""""'.,.,....".~""T~.......,....,....,.,.,..""'i'"~TTfTT~"'T""'~'T""""~T'""TT...",...,~~="""'T'="''mT'=ScanRl

100 TICTotal

50

100313.-650.

50-+'

0C<IIL

3100 312.-313.u

c.2

50

100 104.-105

50

m

"----iii' i fi

200 220 240 260 280 300 320 340 360 380 400 420 4405·16 5,48 6·20 6·52 7·23 7·55 8'27 8'59 931 10·02 1034 11-06 11·38

Time

Fig. 3. Variation with time of the parent peak of styrene monomer (m) and the parent peakof styrene trimer (t) during a temperature-programmed pyrolysis of a styrene/hydrogenatedbutadiene copolymer. The variation in total ion current (total) and summation ion current forall fragments between trimer and hexamer (0) are also shown for comparison. The nominaltemperature programme of the probe was 10 per second up to 800°e. The mass spectrometer

was a Kratos double-focusing instrument, type MS25.

Pyrolysis-GC and MS applied to study oligomer formation 151

apparent discrepancy in the behaviour in the two mass spectrometers. In theopinion of the authors, the delay in trimer evolution observed in the TS250experiments is more likely to be due to a 'fractionation' effect arising fromsome difficulty in volatilising the trimer into the source region. (It seemslikely that this problem existed in the TS250 probe unit since monomer itselfis still evolving steadily even at high temperatures.) If this suggestion iscorrect, then there is no clear evidence for delayed trimer formation. Thismeans that the formation of trimer from monomer, ifit occurs, must eitherbe occurring rapidly in the melt, or be occurring in the vapour phase abovethe melt. Alternatively, ofcourse, it could be stated that this Py-MS evidencein itself gives us no reason to doubt the intramolecular transfer mechanismfor trimer formation.

7 THE PYROLYSIS OF STYRENE/METHYL METHACRYLATECOPOLYMERS, BLENDS, AND HOMOPOLYMERS

7.1 Using sample sizes in the milligram range*

A range of three styrene/MlvlA copolymers of different composition, andhomopolymers of styrene and MMA, were all prepared by a free radicalmechanism under identical conditions. Three blends of the homopolymers,having overall compositions over the same range as the copolymers, wereprepared by dissolving the appropriate amounts ofthe same homopolymers.Samples (c. 1 mg) of each of the above were totally pyrolysed at a nominaltemperature of 600°C on a simple resistive filament in a low-resolution Py­GC apparatus. The results were plotted in terms of a parameterfJ = A/(A + B), where A is the sum of the styrene and MMA monomer peaks,and B is the sum of the dimeric and trimeric peaks. Clearly f3 is defined insuch a way that the maximum value of unity corresponds to zero oligomeryield, whilst the value decreases towards zero as oligomer yields increase.

Results are shown in Fig. 4 for all of the samples. The measurements showlarge uncertainty because the apparatus used in this part of the work did notpermit good resolution. Nevertheless it can be seen that the incorporation ofMMA, either as a co-monomer or as a polymer blend, influences theoligomeric yield in a non-pro-rata way. Whilst we have no explanation ofthetrend in f3 with composition, the surprising thing is that the trend is verysimilar for both the copolymers and the blends. Since calculations (using thepublished monomer reactivity ratios for these monomers in free radical

* This part of the work was part of a project designed to test whether the addition of MMA,either as a co-monomer in a copolymer, or as a homopolymer in a blend, caused any morethan a pro-rata reduction in the yield of oligorners from polystyrene pyrolysis.

152 Louise Dean et al.

100

0·95

090

0·85

0·80

10050MMA (mol '/0)

0·75'--__--'- -'- -1-__-'

o

Fig. 4. Pyrolysis results for styrene/MMA copolymers (.) and homopolymer blends (.) asa function of composition. The parameter f3 is the ratio (total yield of'monomersl/Itotal yieldof monomers + oligomers). The diagonal dotted line represents the 'pro rata' effectdiscussed

in the text.

systems) show that the styrene/MlvlA copolymers have a largely alternatingstructure, again it seems unlikely that the oligomers from the copolymers areproduced by an intramolecular transfer mechanism, especially since there isno tertiary hydrogen atom on the alpha carbon atom in MMA. Hence onceagain we reach the conclusion that secondary reactions, possibly monomerrecombination, could be involved in the formation of oligomers from thesestyrene systems.

7.2 Using samples in the microgram range

Samples (6 J1.g) of the following polymers were completely pyrolysed at515°C in a thermocouple-feedback-filament higher resolution Py-GCapparatus: (1) styrene homopolymer; (II) a 3:1 molar copolymer ofstyrene:MMA; (III) a 1:2 molar copolymer of styrene:MMA; and (IV) a1: 3 molar copolymer of styrene: MMA. The chromatograms are shown inFig. 5, in which the peak assignments were made by Py-MS, and areconsistent with those published by Tsuge et al.12

It is evident that the yield of styrene trimer is decreasing in a more than

Pyrolysis-G'C and MS applied to study oligomer formation 153

IIIIrIIII

5S

t f.i.d.output

I

5

11

m

l 55

r~~II

I II II II I

,~, ? mms. msm

I \I \I I

J~Time ~

'1III

SSS

Fig. 5. Pyrolysis-GC results from 515"C pyrolyses of (1) styrene homopolymer, (II) a 3: 1molar copolymer ofstyrene:MMA, (III) a 1:2 molar copolymer of styrene: MMA, and (IV) a1:3 molar copolymer of styrene:MMA. The monomer, dimer, and trimer peaks are

characterised by use of M = MMA, S = styrene.

pro-rata way as the MMA content of the copolymer increases. Although thiscannot be quantified from the traces as presented here because the monomerpeak is off scale, integrator measurements of the peak areas gave the resultsshown in Table 1 for the ratios of the monomer to trimer peaks. It is evidentthat this ratio increases from c. 6 for polystyrene (sample I) to c. 500 for the1:3 styrene: MMA copolymer (sample IV). The table also includes resultsfor the homopolymer blends, V-VII, corresponding in composition to thecopolymers used, and these are identical, within experimental error, with theresult for polystyrene.

These results from microgram samples would not in themselves havegiven cause to doubt the intramolecular transfer mechanism; indeed, theyare consistent with it insofar as the styrene trimer yield decreasesdramatically as the styrene sequence length decreases. If we are to reconcilethese results with those above, we must question how the sample thickness

154 Louise Dean et al.

TABLE 1Styrene Monomer/Trimer Ratios Obtained from Complete Pyrolyses of Styrene/MMA

Systems at 515QC

Composition of system

Polystyrene ST:MMA = ST:MMA = ST:MMA =reference 3:1 1:2 1:3

CopolymersCopolymer sample numberMean sequence length of

styrene units in copolymerMonomer/trimer ratio

from copolymers

BlendsBlend sample numberMonomer/trimer ratio

[rom blends

(I)

(6'1)

(I)

(6'1)

II

13·3

v

6'0

III

1·2

70

VI

6·1

IV

500

VII

6·1

could be responsible for the difference in behaviour. Could it be thatmonomer recombination to give trimer requires the longer residence time ofthe primary monomer product which would be expected with a thicker layerofpolymer melt? Whilst this proposal does not entirely explain the differencein behaviour observed in this section, it was decided to investigate the effectof sample thickness on the degradation of polystyrene itself. These resultsare reported in the next section, together with other experiments designed toassess the possibility that recombination could also occur in the vapourphase above the melt.

8 PYROLYSIS EXPERIMENTS ON STYRENE HOMOPOLYMER

8.1 Dependence of product yields and ratios on sample size

Using a thermocouple-feedback-filament Py-GC technique, the dependenceof the ratio ofthe monomer/trimer peak areas and the monomer/dimer peakareas was measured as a function ofsample size. In each case two sample sizeranges were explored: 0·2 to 1·6 J1g (pyrolysis temperature 530°C), and up to25 J1g (pyrolysis temperature 480°C).

The results are plotted in Figs 6-9. It can be seen that there is somedependence of monomer/trimer ratio on sample size for small samples, butthe effect decreases with sample size. The dependence is very much larger formonomer/dimer ratios, however, and here the effect is detectable even into

Pyrotysis-GC and MS applied LO study oligomer formation

~07E-o .....c CllOLE"'6~l(l

QlEce 5e

155

4

0·6 0·8 1-0 1·2Sample size (I!g)

Fig. 6. Sample size dependence of the monomer/trimer ratio from polystyrene pyrolysed at530QC (sample size range 0,2-1,6 ,ug).

the larger size range. On the basis of the argument in the previous section,these results would be consistent with the proposal that oligomer formationfrom monomer is favoured if the latter has a greater residence time withinthe melt, i.e. the monomer/oligomer ratio decreases as the thickness of themelt increases. Thus the suggestion in earlier sections (i.e. that trimer may beformed by monomer recombination) must now be extended to dimerformation also. This possibility could not have been deduced from theexperiments involving the pyrolysis ofcopolymers because of the wide rangeof different dimeric products observed in those systems.

8.2 Dependence of product yields on carrier gas flow-rate

Using the same technique as in Section 8.1, the effect of varying the carriergas flow-rate on the monomer/trimer ratio from polystyrene pyrolyses at480°C was investigated. The same sample size (10 J.l.g) was used in all cases.

10 12 14 16 18 20 22 24 26Samp le size (~g)

Fig. 7. Sample size dependence of the monomer/trimer ratio from polystyrene pyrolyscd at480QC (sample sizes up to 25,ug).

156 Louise Dean et al.

40

0s'-3581roE

~E~30~ •E0c0.s •ill 25 •ID • •'- • •~

lJ)

•20

Fig. 8. Sample size dependence of the monorner/dimer ratio from polystyrene pyrolysed at530aC (sample size range O·2-1·6,lg).

\\ ,,

,•-,-----. ----• -----1- -•

I018 -,~ .r; ..

::l •C1l ,

E 16 - "'L Iill ,E I

:0 \- I

e; '.~ 141-- I

oCo5

-

-

-

,25

I5

10L-_...L_-l-_-L__L-_~--l

oI I I

10 15 20Sample size (I-Ig)

Sample size dependence of the monorner/dimer ratio from polystyrene pyrolysed at480°C (sample sizes up to 25 flg).

Fig. 9.

Pyrolysis-GC and MS applied to study oligomer formation

I I ,wi

•525 I- -•

0

ftiL.

'" ~/~500 -E

,. -

L •ill •E-c

+-'

-;:: 475 I- -illE0c0 •.s~ 450 I- -illl- •~UJ

4-25 I I I0 10 20 30

Flow rate (cm 3 rmn")

157

Fig. 10. Effect of varying the carrier-gas flow rate on the monomer/trimer ratio frompolystyrene pyrolysed at 480°C.

The results are plotted in Fig. 10, from which it can be seen that themonomer/trimer yield increases with increasing flow-rate ofcarrier gas. Thistrend is consistent with the proposal that monomer may be recombining toform trimer in the vapour phase above the filament; increased flow of carriergas sweeps the monomer away more readily and trimer formation isreduced.

9 THE MECHANISM OF FORMATION OF OLIGOMERS IN THEPYROLYSIS OF POLYSTYRENE AND ITS COPOLYMERS:

DISCUSSION AND CONCLUSIONS

The experimental results described in Sections 3-8 throw considerabledoubt on whether the intramolecular transfer mechanism is exclusivelyresponsible for oligomer formation in the pyrolysis of polystyrene and itscopolymers. The principal elements of this pyrolysis evidence againstintramolecular transfer are as follows:

(a) The monomer/trimer ratio from a copolymer with exceedingly smallsequence lengths of styrene monomer (the styrene/hydrogenatedbutadiene copolymer) was found to be very close to that obtainedfrom styrene homopolymer over a considerable temperature range(Section 4).

158 Louise Dean et at.

(b) The monomer/trimer ratio from the above copolymer was againfound to be very close to that obtained from a styrene/hydrogenatedisoprene block copolymer with long sequence lengths of styrenemonomer (Section 5).

(c) For samples in the milligram range, monomer/(monomer +oligomer) ratios for a range of styrene/M'MA copolymers, whichhave structures with large degrees of alternation, were similar invalue to those measured for the corresponding blends of polystyreneand MMA (Section 7.1).

(d) Results from the pyrolysis of pure polystyrene display a dependenceon sample size which is consistent with monomer combination toform trimer, and especially dimer, within the melt (Section 8.1).

(e) Monomer/trimer yields from Py-GC pyrolyses of pure polystyreneshow a dependence on carrier gas flow-rate which is consistent withmonomer combination to form trimers in the vapour phase abovethe melt (Section 8.2).

Temperature-programmed pyrolysis-MS experiments, designed to assesswhether evidence for monomer recombination could be 0 btained in terms ofa delay in trimer formation, were also performed (Section 6). Althoughresults obtained with one mass spectrometer system seemed to suggest thatsuch a delay occurs, it was decided that this evidence must be discounted forthe present.

Only one system, styrene/MMA copolymers pyrolysed in the microgramrange (Section 7.2), has provided results where it has not been necessary todoubt the intramolecular transfer mechanism for oligomer formation.However, in view of the weight of the evidence above, we are cautious inciting these results as conclusive evidence for that mechanism.

There remains one other piece of evidence which on the face of it may beoffered in favour of the intramolecular transfer mechanism for oligomerformation from polystyrene: poly(alphamethyl styrene) yields onlymonomer on pyrolysis at moderate temperatures. Is not the reason for thisthat the methyl substitution ofthe tertiary hydrogen atom on the styrene hasprevented intramolecular transfer which can otherwise occur in polysty­rene? However, we might now rephrase this question: is it not that thealphamethyl group is discouraging monomer recombination and preventingoligomer formation in poly(alphamethyl styrene)? The steric hindrancepresented by the methyl group could be quite significant, especially if thepossibility of cyclic oligomers is considered.

In the case of the styrene/hydrogenated butadiene copolymers, it wasnoted in Section 4 that every styrene triplet sequence in the copolymer wouldhave to be evolved as trimer if the trimer yields are to be accounted for on the

Pyrolysis-GC and MS applied [0 study oligomer formation 159

basis of the intramolecular transfer mechanism. A consideration of thescission required confirms that this proposal is extremely unlikely. We mayrepresent the structure of the assumed product radical from intramoleculartransfer at a styrene triplet sequence as follows, where one hydrogenatedbutadiene unit is shown to the left of the styrene triplet:

I I

H H H HiH HiH H HI I I I! I . I i I I I

--C-C-C-C+C-C-C+C-C-C-HI I I I I I I I 1 \ I I

H H H H i H Ph H I Ph H PhI ,

x y

Subsequent scission would be required at x in order to form the trimer, butthis scission would give rise to an 'ethylenic' radical on the butadiene end ofthe chain. A much more probable scission would be at y; this would producea stabilised diphenyl propyl radical, but this would not lead to trimerformation. This certainly suggests that some other mechanism for trimerformation is required.

Further arguments against the intramolecular transfer mechanismbecome apparent when we realise that it would have consequential effects informing other products which should be detectable. Consider the alternativereaction channel (b) in Scheme 1. It seems reasonable to assume thatchannels (a) and (b) are equally probable, and since the product radical fromchannel (b) would be expected to unzip one monomer unit to give a tolylradical which would produce toluene after hydrogen abstraction, then onetoluene molecule should be produced for every trimer molecule inpolystyrene degradation. We have abundant evidence that this is by nomeans the case; toluene yields are more than an order ofmagnitude less thantrimer yields. Even if the product radical from channel (b) did not unzip amonomer unit, it would itself be expected to abstract hydrogen to form 1,3­diphenyl propane, but the amount of this detected is miniscule comparedwith trimer.

Our conclusion is, therefore, that the intramolecular transfer mechanismfor oligomer formation from the pyrolysis of polymers and copolymers ofstyrene must be very seriously questioned. The formation of oligomers byrecombination of monomer must now be regarded as an alternativemechanism.

ACKNOWLEDGEMENTS

The authors wish to thank Foseco International Ltd for the award of aResearch Scholarship to Robert Hancox, and Castrol Ltd for the award of a

160 Louise Dean et al.

Research Scholarship to Gordon Lamb. The styrenejMMA systems in themilligram range were studied by Louise Dean and Sally Groves as part oftheir undergraduate final year research work within the BirminghamPolymer Group.

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