Makromol. Chem. 185,1277- 1284 (1984) 1277

Polyene sequence distribution in modified poly(viny1 chloride) after thermal degradation

Gerard0 Martinez, Carmen Mijangos, Jos&-Luis Millan

Instituto de Plfisticos y Caucho, C.S.I.C., Juan de la Cierva 3, Madrid-6, Spain

Donald L. Gerrard, William F. Maddams*

Research Development and Technical Services, The British Petroleum Company plc, Sunbury-on-Thames, Middlesex, England

(Date of receipt: October 17, 1983)

SUMMARY: The products resulting from the reaction of PVC with sodium benzenethiolate were degraded

to 0,3% at 180°C in the solid state and at 160°C in solution in trichlorobenzene. The polyene distribution of the polymers after degradation was studied by both UV-visible and resonance Raman spectroscopies, as a function of the degree of substitution. The results show that there are two types of behaviour: that of the PVC sample prior to the substitution reaction together with the samples modified up to a definite degree of substitution which depends on the starting isotactic content, and that of samples with higher degrees of substitution. The former group exhibits not only a steady improvement in thermal stability but also a preferential formation of polyenes of 7 - 9 double bonds whose concentration decreases with increasing degree of substitution. Conversely, for the second group of samples the thermal stability decreases with the degree of substitution and no specific absorption bands are observed. On the basis of earlier work on the selective substitution of the isotactic GTTG and heterotactic lTTG triads during the first stage of the reaction, the present results show that the bands at 393,416, and 437 nm are related to specific polyenes which result from initiation by the above quoted conformations in PVC, a conclusion for which confirmatory evidence was obtained by resonance Raman spectroscopic examination of the samples. There is, therefore, clear evidence for the occurrence of two distinct degradation mechanisms, one involving initiation by the unstable triad confor- mations and the other via random initiation at stable and normal structures. To this may be added the initiation by defect structures, which have been extensively documented in the literature.

Introduction

The type of polyene distribution in degraded PVC has recently been found to depend on the tacticity ' -3). The presence of some kind of isotactic and heterotactic triad conformations was shown to result in a decreased thermal stability that was concomitant with a narrow polyene distribution which mainly consists of polyenes of 7 - 9 double bonds2). Conversely, the presence of trans syndiotactic sequences had been previously shown to give rise to a broader polyene distribution containing sequences as long as 12 double bonds".

With reference to the specific polyenes of 7 - 9 double bonds, which absorb at 393, 41 6, and 437 nm, respectively, in the UV-visible spectrum, two different explanations have been tentatively given. They might be due either to specific propagations by both

Makromol. Chem. 185, No. 7, July 1984 0025-1 16X/84/$03 .OO

1278 G . Martinez et al.

sides of isolated cis double bonds resulting from initiations at some gauche conforma- tions, or to normal propagations, but restricted in length by the special geometry around the gauche conformations. Be that as it may, the specific relationship between the above quoted UV-visible bands and the content of some definite conformations in isotactic and heterotactic triads, has been established beyond all doubt3).

Since the above features relate to the PVC thermal degradation mechanisms it is of interest to extend the study, to examine the polyenes which result from the thermal degradation of a PVC modified to different extents by substitution reaction with sodium benzenethiolate. Actually, this reaction was previously shown to occur selectively by GTTG and TTTG triad conformation^^,^,^), and although the previous experimental conditions were different from those used now, the behaviour of modified samples in thermal degradation showed the bands at 393, 416, and 437 nm in the UV spectra to be due to polyenes resulting from initiations at GTTG and TTTG conformations. However, as indicated above, no definite conclusion could be drawn about the reason for such behaviour3).

The purpose of the present work is to confirm the previous results on the presence of some conformations as specific initiation and propagation determining structures, by comparing the evolution of the polyene sequence distribution with substitution extent as analysed by UV-visible spectroscopy and with that obtained by resonance Raman spectroscopy, which was shown to be an excellent tool to investigate the polyenes in degraded PVC7y8).

Experimental part

Polymer preparation and characterisation: The PVC sample used was prepared at 60 "C in bulk using 2,2'-azoisobutyronitrile (AIBN) as the initiator system. The full details have been published9). The number average molecular weight was determined by osmometric measure- ments at 34°C with solutions in cyclohexanone using a Knauer membrane osmometer (Tab. 1). The tacticity was measured by means of the I3C NMR spectra obtained at 20,l MHz and 100°C using a WP-80 Bruker spectrometer. The polymer was examined as a 10070 solution in a mixture of deuterated dimethylsulfoxide and o-dichlorobenzene (vol. ratio 1 : 4); a 10 mm sample tube was used. The calculation was carried out by measuring the areas with a compensating polar planimeter (Tab. 1).

Tab. 1. Characterization of polymer

Sample Xi,, .10-3 P(SS)a) P(S1 and IS)b) P(I1)")

0 38,9 0,311 0,495 0,194

a) P(SS): Fraction of syndiotactic triads. b, P(S1 and IS): Fraction of heterotactic triads.

P(I1): Fractions of isotactic triads.

Substitution reaction with sodium benzenethiolate: The substitution reaction was carried out at -15OC in purified cyclohexanone as solvent. The full experimental details have been published previously6).

Polyene sequence distribution in modified poly(viny1 chloride) . . . 1279

The samples, withdrawn at the indicated reaction times (Tab. 2), were characterised by UV spectroscopy in order to determine the degree of substitution, from the intensity of the benzene- thiolate 256 nm band. The correlation between the values so obtained (Tab. 2) with those from microanalysis was found to be very good.

Polymer degradation: The HCI evolved both in the solid state at 180°C and in solution at 160 "C in trichlorobenzene (twice distilled, analytical reagent grade) was estimated by conduc- tivity measurements, as described previouslyg).

Ultraraviolet risible spectroscopy: UV-visible absorption spectra of degraded samples were measured with solutions of 1,7 g/l in hexamethylphosphoric triamide. They were recorded on a Perkin Elmer 554 spectrometer in an inert atmosphere. 1,7 g/l solutions of the corresponding undegraded polymers were used as a reference.

Resonance Raman spectroscopy: The experimental method was as described p r e v i ~ u s l y ~ ~ ')

and the samples were examined as solutions in tetrahydrofuran. The concentrations were chosen to give solutions of similar absorbance in the visible region of the spectrum, to minimise any errors from partial absorption of the exciting radiation, although the concentrations used were sufficiently low to make such effects very small. All intensities were measured as ratios against that of the solvent peak at 915 cm-' and were corrected to a constant PVC concentra- tion of 30 mg/ml.

The Raman spectra were obtained with an Anaspec 33 spectrometer, using as exciting wavelengths the 457,9,475,6,488,0,501,7, and 514,5 nm lines from an argon ion laser and the 530,9 and 568,2 nm lines from a krypton ion laser. The 647,l nm line from this latter failed to excite resonance in any of the samples. The conjugated polyene sequence lengths ncorrespond- ing to the observed v, values were calculated from the recently establishedi0) relation between v,, the frequency of the C=C stretching mode and n, v, = 1461 + 151,24-0~07808",.

Results and discussion

The substitution data (Tab. 2) show two different behaviours with respect to reaction rate: that of samples 1 - 3 and that of samples 4 - 7, the rate being much higher in the case of the former samples.

This is well illustrated in Fig. 1 where sample 3 appears to be the transition between both groups of samples. As previously shown, this arises from the enhanced reactivity of a fraction of isotactic and heterotactic triad conformations6). Another feature of

Tab. 2. Substitution reaction dataa)

Sample No.

Time b, in h

Conversion in mol-Vo

0 1 4 8

12 21 48

103

0 0,57 0,91 1,17 1,39 1,61 2,16 2 7 6 4

a) [PVC] = 6,4. lo-, mol/l; [C,H,SNa] = 7,6. lo-, mol/l. b, The time after which the sample was withdrawn.

1280 G. Martinez et al.

s - C

Fig. 1 . Conversion as a function of time in the nucleophilic substitution of PVC with sodium benzen- ethiolate (cf. Tab. 2)

0 25 50 75 100 f / h

Tab. 3. Thermal degradation values

Sample (Degradation rate) Id a)

in min-'

solid state solution (at 180OC) (at 160°C)

0 4,42 0,50 1 3,73 0,48 2 3,85 - 3 4,61 0,65 4 10,oo - 5 11,79 0,93 6 12,31 - 7 15,38 1,60

a) ([HCl] / [HCl],) * t-'. [HCI], related to HCl available.

the substitution reaction is the increase in thermal stability of the polymer during the fast period of the reaction3s6), as demonstrated by the results in Tab. 3.

However, attention will be focussed mainly on the influence of substitution on the polyene distribution in the equally degraded polymers, The results are shown in Figs. 2 and 3 for degradations carried out in the solid state and in solution, respectively.

The spectra in Fig. 2 clearly indicate that there are two different types of polyene distribution and that they correspond to the two above mentioned sample groups. The former group of spectra (samples 1 and 3) shows that absorpion bands at 393, 416, and 437 nm, which are the most pronounced for the starting material (sample 0), decrease progressively with the degree of substitution. After sample 3, there is a marked change in behaviour: all the observed absorptions are similar in relative intensity and they decrease with the degree of substitution.

The same effect is observed for degradation in solution (Fig. 3), where the bands at 393,416, and 437 nm for samples 0 and 1 are more pronounced than in the solid state.

Polyene sequence distribution in modified poly(viny1 chloride) . . . 1281

Fig. 2. UV-visible spectra of samples degraded to 0,3% at 180°C in solid state. (a): Sample 0; (b): sample 1; (c): sample 3; (d): sample 4; (e): sample 5; (9: sample 6; (g): sample 7

Fig. 3. UV-visible spectra of samples degraded to 0,3% at 160°C in solution in trichlorobenzene. (a): Sample 0; (b): sample 1; (c): sample 5 ; (d): sample 7

n

700 600 500 400 300 Unrn

Taking into account the fact that all the samples were degraded to the same extent (0,3%), the differences between the two groups of spectra in Figs. 2 and 3 would indicate either that polyenes with 7 - 9 double bonds are preferentially forming in samples 0, 1, and, to a lesser extent, 3, but not in the case of samples 4 -7, or that they disappear by secondary reactions. The former proposition is supported by the fact that the bands at 393, 416, and 437 nm are formed only when there are GTTG and TTTG conformations2), as is the case for samples 0 - 3 in Tab. 2, that is during the fast period of the substitution reactions". The latter proposition is more difficult to accept because secondary reactions of polyenes, particularly crosslinking, are known to occur with very long polyenes*).

Another conclusion from the results in Figs. 2 and 3 is that samples 4 - 7 after equal degradation must contain a higher concentration of very short polyenes which are not observable by the UV visible spectra (bands at A < 300 nm) of Figs. 2 and 3.

Resonance Raman Spectra

The results of the measurements are expressed in terms of the intensity ratio Iv2/19,5, the former being the intensity of the resonance Raman band and the latter is a

1282 G . Martinez et al.

band characteristic for the solvent. These intensity ratios are plotted as a function of the conjugated polyene sequence length n, thereby departing from previous practice l)

on two counts. The use of ZV2/Zgl5 rather than Zv,/Zg15 is a consequence of the fact that the former exploits more effectively the specificity of the spectra in terms of n. Furthermore, now that a reliable relation between v, and n has been established'O), it is more meaningful to plot ZV2/Zgl5 as a function of n rather than the exciting wave- length.

Although the results of the measurements on the samples degraded in solution and in the solid state are shown separately in Figs. 4 and 5 , respectively, for a given sample, they show very similar trends. This is very evident at the outset, with the material prior to reaction with sodium benzenethiolate. The spectra of this polymer degraded by the two routes show that the concentration of double bonds reaches a maximum for an n value of 10 - 11, and decreases steadily with increasing n, to the extent of about an order of magnitude for n = 18. As noted previously'), this behaviour is characteristic for the situation where the degradation has proceeded to the point where cross-linking reactions are beginning to occur.

Fig. 4. Resonance Raman spectra of samples degraded to 0,3% at 160 "C in solution in trichlorobenzene. (0): Sample 0; (0): sample 1; (x): sample 5 ; (A): sample 7

" 8 10 12 1L 16 18 n

There is a steady change in the appearance of the spectra with increasing degree of reaction with sodium benezenethiolate. Nevertheless, as with the results of the UV- visible absorption spectra, they fall into two groups: samples 1 - 3 and 4 - 7. There is an overall decrease in the level of the unsaturation in the range examined, n = 8 to 18, and a shift towards a greater relative concentration for small values of n. In that all the samples were degraded to a constant level of HC1 loss, the fall in the concentra- tion of the longer polyenes indicates that the concentration of shorter units must increase, as observed, and this is also in accord with the results of the UV-visible measurements. The overall trend of the resonance Raman results, together with the fact that the samples were readily soluble in tetrahydrofuran, rules out cross-linking as an explanation for the different type of distribution of sequence lengths which must, therefore, be characteristic for the primary degradation process, as influenced by the types of tactic placements present.

Polyene sequence distribution in modified poly(viny1 chloride) . . . 1283

'" m

9 2 1.5 .-.

1.0 Fig. 5. Resonance Raman spectra of samples degraded to 0,3% at 180°C in solid state. (0): Sample 0; (0): sample 1; (x): sample 3; (A): sample 4; (0 ) : sample 5; (A): sample 6; (V):

0.5

sample 7 0

I I I

/'o\o,

n

All the above results may be accounted for by assuming that two different processes take place in the overall degradation of PVC: one involves initiation by the unstable triad conformations GTTG (isotactic) and TTTG (heterotactic), followed by a particular propagation which gives rise to an enhanced and specific UV absorbance at 393,416, and 437 nm. The second process involves random initiation at stable and normal structures followed by propagation to trans polyenes, the distribution of which depends on the distribution of trans syndiotactic sequences in the chain. The former mechanism is supported by the fact that specific polyenes of 7 -9 double bonds decrease or disappear either when selective substitution at the above mentioned conformations occurs (Tab. 2 and ref.2)) or when the content of these conformations is low, i.e. in the case of more overall syndiotactic polymers2). Moreover, a polymer similar to that used in this work (sample 0, Tab. 1) was recently extracted with acetone, which separates the more isotactic parts. Both the acetone soluble and insoluble fractions were degraded to the same extent and, surprisingly, the soluble fraction exhibited very strong bands at 393, 416, and 437 nm while the insoluble fraction gave a UV-visible spectrum very similar in shape to those of the samples 4 - 7 in Fig. 22). This confirms that bands at 393,416, and 437 nm are characteristic for the presence of some isotactic structures, i.e. the above quoted triad conformations.

As to the relationship between the presence of trans syndiotactic sequences in the polymer and the polyene distribution after degradation, this may be deduced on the basis of earlier result^^.^'). They show that the shapes of the UV-visible spectra of samples 4-7, that do not contain enhanced absorptions at 393, 416, and 437 nm, must be attributed to the relative increase of the content of trans syndiotactic parts as a result of the selective substitution process6).

The results of this work provide new approaches to the mechanisms of degradation of PVC. They are not at variance with the proposed influence of some defect structures on PVC stability, that has been widely studied by many authors12). It is, however, worthy of note that, to our knowledge, no influence of these defect structures on the propagation of the dehydrochlorination reaction has been found, in contradistinction to the isotactic conformations concerned in this work. Despite the fact that the concentration of defect structures in conventional PVC is very low and

1284 G. Martinez et al.

very difficult to determine with sufficient we believe them to be involved in the initiation process with PVC, but in our present study we are examining modified polymers in which the concentration of substituents, as the result of the reaction with sodium benzenethiolate (Tab. 2), far exceeds the concentration of defect structures in PVCI2). Hence, the relationship found between the presence of a definite type of isotactic conformations and both the PVC stability and the type of the polyenes resulting from the propagation process, is to be considered as a separate feature in the overall PVC degradation process. Moreover, it appears to be a very determining factor in this process.

1, G. Martinez, C. Mijangos, J. Milltin, D. L. Gerrard, W. F. Maddams, Makromol. Chem.

2, G. Martinez, C. Mijangos, J. Millitn, J. Appl. Polym. Sci. 28, 33 (1983) 3, G. Martinez, C. Mijangos, J. Milltin, J. Appl. Polym. Sci. in press 4, J. Millh, G. Martinez, C. Mijangos, J. Polym. Sci., Polym. Chem. Ed. 18, 505 (1980)

J. Milltin, G. Martinez, C. Mijangos, Polym. Bull. 5, 407 (1981) 6, G. Martinez, C. Mijangos, J. Milltin, J. Macromol. Sci., Chem. 17, 1129 (1982) ') D. L. Gerrard, W. F. Maddams, Macromolecules 8, 54 (1975) 8, D. L. Gerrard, W. F. Maddams, Macromolecules 10, 1221 (1977) 9, J. Millhn, M. Carranza, J. Guzmb, J. Polym. Sci., Polym. Symp. 42, 2422 (1973)

lo) A. Baruya, D. L. Gerrard, W. F. Maddams, Macromolecules 16, 578 (1983) ''1 W. H. Starnes, Jr., Dev. Polym. Degradation 3, 135 (1981) 12) A. Guyot, M. Bert, P. Burille, M. F. Llauro, A. Michel, Pure Appl. Chem. 53,401 (1981)

180, 2937 (1979)