Structural defects in poly(vinylchloride)—III. Degradation of vinyl chloride-phenylacetylene copolymers in solution

Eur. Polym. J. Vol. 20. No. 8, pp. 799 804, 1984 Printed in Great Britain. All rights reserved

0014-3057/84 S3.00+0.00 Copyright ,c 1984 Pergamon Press Ltd

STRUCTURAL DEFECTS IN POLY(VINYLCHLORIDE)--III DEGRADATION

OF VINYL CHLORIDE-PHENYLACETYLENE C O P O L Y M E R S IN S O L U T I O N *

D. BRAUNt, B. B()HRINGERt, F. T/JD6S*, T. KELENJ~ and T. T. NAGY+ tDeutsches Kunststoff-lnstitut, Schlossgartenstrasse 6R, 6100 Darmstadt, FRG and ++Central Research Institute of Chemistry of the Hungarian Academy of Scienccs, Budapest, Hungary (Receired 28 Not'ember 1983)

AIw,tract--Vinylchloride-phenylacetylene copolymers have been prepared and characterized. A known amount of defined defect sites, t,iz. double bonds, has been introduced into the main chain of the polymer. Thermal degradation behaviour of the copolymers has been studied in trichlorobenzene solution. A strong dependence of the kinetics of dehydrochlorination and polyene formation on the defect concentration has been found. Rate constants and activation parameters have been determined. The effects of different structures have been compared.

INTRODUCTION

As a result of many studies in the field of PVC degradation, it is now clear that the zip-like dehydrochlorination is initiated both at defect sites and randomly along the regular chain [e.g. 1-3]. The amount of irregularities is at most a few tenths of a percent in commercial PVCs, but the rate constant of initiation at defect sites is by orders of magnitude higher than that of random initiation, thus the rates themselves are comparable. In recent years great efforts have been made to reveal the chemical structure of the defect sites and to determine their quantity'. Even a IUPAC Working Party has been organized on the subject. Unsaturated chain-ends carrying primary allylic chlorines, saturated chain-ends with vicinal chlorines, internal double bonds, chloromethyl branches, other shortchain branches with tertiary chlorine atoms at the branch points and long chain branches have been identified by NMR spectroscopy, by GPC combined with viscometry and by chemical methods [3-6]. Attempts to correlate the rate of dehydrochlorination with the amount of certain defect structures have not given unambiguous results [2, 6-8]. For polymers prepared under the usual conditions, a strong intercorrelation exists among the amounts of various defect structures, the molecular weight and the polymerization temperature [4, 9], thus the individual effects cannot be separated by the investigation of industrial samples only. Dehydrochlorination studies on low molecular weight compounds carried out in the gaseous [10] or in the liquid [1 I] phase have contributed much to the basic understanding of the effect of labile structures. However, the reaction conditions used in these experiments are very different from those of PVC *Part II, D Braun and D. Sonderhof. Eur. Polvm. J. 18, 141 (1982). 799

degradation; thus one must be careful in drawing quantitative conclusions. The investigation of PVC samples containing known amounts of well defined defects gives valuable information about the problem. Thus, for example tertiary chlorine atoms can be incorporated into PVC by copolymerization with 2-chloropropene [12-15]. Generally, it was found that the rate of degradation linearly increases with the 2-chloropropene content of the copolymer. Copolymerization of vinyl chloride (VC) with acetylene did not lead to internal double bonds in PVC since acetylene, in consequence of degradative chain transfer, was incorporated at the chain-ends only [16]. Preliminary experiments to introduce allylic chlorine atoms into the chain by copolymerizing VC with l-chlorobutadiene also failed; presumably the latter acted as an inhibitor [17]. I-Chlorobutadiene could be homopolymerized but the polymer formed was found to be very unstable, turning dark even during the polymerization. PVCs containing internal allylic chlorine were prepared in a polymer-analogous reaction by low temperature potassium-t-butylate treatment of PVC solutions [18, 19]. The degradation of these polymers was investigated both in the bulk and in solution, and the rate constant of initiation at the allylic structures determined [20]. In a previous paper [21] the preparation and characterization of vinyl chloride--phenylacetylene copolymers (VC-PA) were described. It was shown that these copolymers contain phenyl substituted double bonds randomly in the main chain. Although such defects are absent from VC homopolymers, investigation of the degradation behaviour of these copolymers contributes to the structure-activity relationship of defects in PVC degradation. In the previous paper [21] degradation of these copolymers m the bulk was reported, Here we present results for degradation in solution, and compare initiation rates at different defect structures.

800

D. BRAUNet al. EXPERIMENTAL

VC-PA copolymers were prepared by suspension polymerization at 50 as described earlier [21, 22]. The samples were characterized by NMR and i.r. spectroscopy for comonomer content and by ozonolysis followed by GPC for internal double bonds [21, 22]. Thermal degradation tests were carried out in carefully purified 1,2,4-trichlorobenzene (TCB) solvent at l g/I concentration, in an argon stream, in the temperature range of 140-180'. The evolved HCI was measured by conductometry. U.V. and visible spectra were taken in separate experiments; for photometry, the degraded TCB solutions were diluted with tetrahydrofuran (THF) in l:l ratio [20]. RESULTS AND DISCUSSION

,.o 10

5 ~x~X~X_X_X_X_X_X.._x_x_x_x_x_x_x_x ~ '

Compared with industrial PVCs, the V C - P A copolymers are extremely unstable. The samples are white after preparation but, during storage at room temperature, they slowly turn pink (those with the highest PA content even brown) indicating dehydrochlorination. Of course, we have to note this fact, when evaluating the results. The investigated samples are listed in Table I. Samples 1,2 and 4 were stored for over a year before solution degradation experiments, while samples 3 and 5 were investigated within a few weeks of preparation. The dehydrochlorination kinetics of the V C - P A copolymers show three distinct phases (Fig. 1): a short induction period which corresponds to the time necessary to reach thermal equilibrium, thus having no kinetic significance. (It varies with the experimental conditions: it decreases if smaller apparatus and smaller sample size is applied; with smaller samples, however, we lose reproducibility. In the present experiments 20-30 ml solutions were used.) The induction period is followed by fast HCI elimination, and in the third phase a linear HCI elimination of relatively low rate is observed. In this stage, the degradation rate is nearly identical to that of commercial PVCs or of laboratory samples prepared under the usual conditions. The observed dehydrochlorination kinetics can be well interpreted in terms of a kinetic model considering slow random initiation, some orders of magnitude faster initiation at defects, very fast propagation (unzipping), and termination [l, 20, 23]. According to this model, the dehydrochlorination rate of usual PVCs is: d~ -ri[k,(1-~)+k2h dt

,4

15

exp(-k2t)]

(1)

where ~ is HCI loss conversion (mole HCI lost per mole VC units), t is time, ri is average polyene length (regarded initially to be identical with the kinetic chain length), k~ and k, denote rate constants of

[f x ~ ^ 0

commerclol PVC-*

, , t //~-/'-/77 I

5O

100 t ( mtn )

150

Fig. 1. HCI loss conversion curves of VC-PA copolymers at 180° in 0.1% TCB solution in argon stream. For the numbers on the curves, see Table I. initiation at random and at weak sites, respectively, and h is the initial fraction of the defects (h << 1). In the above approximation, the possible differences between the activities of the various defect structures were not considered. The zip-length was assumed to be independent of the mode of initiation. During the investigation of alkylaluminium treated PVCs, however, some doubts arose about this point: it was postulated that polyenes formed at defects are longer than those formed after random initiation [24]. If we consider different defect structures and assume a variation of the average kinetic chain length depending on the structure of the initiation sites, then the rate of dehydrochlorination can be given as follows: d~ = tit.k~ ( 1 - ~ ) + dt

~ri, k,hi (exp ( - k i t )

Sample

PA in the feed (tool ?,,)

PA in copolymer (h3) (mol ~o)

Chain scissions (h~) (tool ~°.o)

1

0.1

(l.094

0.102

2 3 4 S

0.2 0.2 0.3 0.4

0.445

0.32 0.36 0.60 0.73

0.634 0.8

46,400 29,800 26,900 24,100 19,400

(2)

i.2

where ri~ and ~i, are the average kinetic chain lengths for initiation at random and at the various defects, h. is the initial fraction of a certain defect, and k, is the rate constant of initiation at this defect. Comparison of the dehydrochlorination rates of commercial PVCs and of V C - P A copolymers (Fig. I) indicates that the incorporated phenylacetylene forms a very active defect site, i.e. the rate of initiation at these defects is relatively high. As an approximation, under the applied experimental conditions, the effect of defect sites other than PAs can be neglected, and the rate of HCI elimination can be regarded as

d~

d t = K + ri3 k3h3 • exp ( - k 3 t ) Table 1. The investigated VC-PA copolymers

200

(3)

where K is a constant which corresponds to the rate of HC1 elimination in the third phase of the curves, and the subscript 3 denotes quantities standing for the PAs (e.g. h 3 stands for the PA fraction of the copolymer). The rate of HCI elimination at early stages (v0) can be obtained from (3) with k~t <<1: v0 ~ K + ~i3k3h~

(4)

Structural defects in PVC--ilI

801 8

,o

/

_-

./Y"

c_

9

'I l I,oi 12

*

10

o x

5

08

•

06 /

04

0

t

1

05

10

_

h 3 (tool %)

Fig. 2. The rate of dehydrochlorination of VC-PA copolymers at 180 in 0.1";, TCB solution in argon stream as a function of PA content. (h 0 determined analytically or by chain ,~ission.

The dependence of t',,, determined from the maximum slope of the HCI loss curves, on the PA content (h~) is shown in Fig. 2. From the integration of (3) we obtain: = Kt + ti~h~[1 - exp( - k~t)]

(5)

\

o

30

25

20 u'~10-3(cm

15 ~)

Fig. 4. The u.v. and visible spectra of a VC-PA copolymer (sample 2 in Table I) and of Ongrovi] S 470 PVC, both degraded for 50rain at 180". The numbers indicate the

polyene lengths. Note the difference in concentration and path length. Experimental conditions: VC-PA: degraded in 1g/l TCB solution, diluted to 0.5 g'l, path length 0.5 cm. PVC: degraded in 10 g/l TCB solution, diluted to 5 g,'l, path length I cm.

which gives for long degradation times (k~t >>1): = Kt + ti~h~

(6)

This means that the zero time intercept (~0) of the linear, low rate part of the kinetic curves [I, 23] is as follows: c~0= rich3 (7) The ~0 values obtained by extrapolation show a linear dependence on the PA content of the polymer (Fig. 3). From the slope of the line in Fig. 3, the average kinetic chain length for initiation at PA units, n~ --- 25 can be calculated. This value is very high if we consider that, under similar conditions, ~ usually does not exceed 10 12 [1,20]. Comparison of the u.v. spectra of a degraded VC-PA copolymer and of a degraded commercial PVC verifies this difference (Fig. 4).

./.

2o

/

0

I 05

I 10

h 3 (mol %)

Fig. 3. The dependence of the zero intercept (~) of the HCI loss conversion curves of VC-PA copolymers on PA content (h0` determined analytically or by chain scission. EPJ

20.8- D

It is known that the conjugation of a phenyl group with a polyene causes a shift of a polyene spectrum towards longer wave lengths (red shift). According to the data of Kuhn and Grundmann [30], one phenyl ring is equivalent to 3/2 double bonds in this respect for symmetrical structures. For asymmetric structures, the effect is even greater. The initial polyene length of a degraded VC-PA copolymer is however significantly greater than the initial polyene length of a degraded PVC, modified by the phenyl conjugation. We have to note that the polyene distribution of degraded VC-PA copolymers strongly deviates from the usually found geometric distribution. These results indicate that the incorporated PA influences the polyene length, meaning that it must have an effect on the rate of propagation and/or termination. The dependence of the polyene concentration of VC-PA copolymers (i.e. of the absorbance in the u.v. and visible spectrum) on the degradation time has also interesting features. At early stages, a rapid increase of the absorbance, especially for long polyenes, e.g. for those with 8-12 conjugated double bonds, can be observed. Later, however, the absorbance decreases (Figs 5 and 6). This is in good agreement with the expectations based on our earlier investigations on the secondary processes of PVC degradation [29]. It was shown that, during thermal degradation, polyenes are formed in primary reactions (which include initiation, polyene growth and termination) and are consumed in secondary reactions. Cyclization is the most important polyene consuming reaction during degradation in solution. Taking this into account, the observed decrease in the polyene concentration can be explained simply: the polyenes formed in high concentration in the early stages of degradation are

802

D. BRAUNet al. 14

14

12

12

10

10

08

08

06

06

04

04

/•8

",=- 5 0 m,n

12(

02

I

0 25

30

20 v

15

30

I 20

25 =,

x 10"S(cm-~)

15

xlO-3(cm

-~)

Fig. 5. The u.v. and visible spectra of degraded VC-PA (sample 2 in Table I), degradation at 180, I g/l TCB solution, spectra taken at 0.5 cm path length and 0.5 g/I concentration.

slowly consumed by cyclization, and the polyene distribution approaches that of industrial PVCs (compare Figs 4 and 5). The rate constant of initiation at PA units was calculated by dividing the slope of the v0 vs h3 line in Fig. 2 by ri~ [see equation (4)]; k 3 = 4.9 × 10 -2 min -~ was obtained at 180L The value is considerably higher than the initiation rate constant at a "simple" internal allylic chlorine; it even exceeds the value determined for chemically dehydrochlorinated PVCs at 200 ° in TCB solution (I.06 x 10 2 m i n - ' ) . The k3 value found also exceeds the dehydrochlorination rate of some allylic model compounds at 180°: for 8-chloro-6-tridecene 8.5 × 1 0 - 3 m i n t and for 4chioro-2-dodecene 6.5 x 10 -3 min -~ were found [11]. The overall activation energy of dehydrochlorination of V C - P A copolymers (Table 2) in the early stages of degradation ranges between 19 and 24kcal/mol (80-100kJ/mol) which is, in line with expectations, lower than the activation energy for

15

10

O5

commercial PVCs degraded in the same solvent (26-29 kcal/mol [25]). On the other hand, it is much higher than the activation energy observed in "solid" phase degradation of VC-PA copolymers (6-10 kcal/ mol [21, 22]). This difference is even more surprising, since the activation energy of dehydrochlorination is generally lower in solution than in the bulk [26]. In order to compare the activities of different defect structures, we examined the relevant literature. The slopes of degradation rate vs defect concentration representations were regarded as a measure of destabilizing effect; the data are summarized in Table 3. The data for "'solid" samples at 180' suggest the following order of activity: PA >> internal allylic CI > tertiary CI. From the data of Table 3, the activity of internal double bonds seems to be lower in t-BuOK treated PVC [20, 27] than in "'usual" PVCs [211. This difference is probably due to the fact that, while in the case of the t-BuOK treatment only the a m o u n t of allylic chlorine atoms increases and the amount of other defects does not change, in the case of "usual" PVCs the increase of the a m o u n t of internal double bonds and the numbers of other weak sites also increase. In general, higher apparent activity and poor correlation can be expected if dehydrochlorination rates are correlated with the amount of a single type of defect. Thus Guyot e t al. found poor correlation

Table 2. The kinetic parameters o f dehydrochlorination solution

in l g/l

Arrhenius parameters o f % o o x I0 ~

o

I lOO

I 2o0 t ( mm

I 300

)

Fig. 6. The dependence of absorbance corresponding to 5 and 10 conjugated double bonds on degradation time of a VC-PA copolymer. For experimental conditions see Fig. 5.

(min-I)

~o

Sample

at 180

(5~,)

1 2 3 4 5

1.8 5.0 5.5 7.1 9.3

3.2 10.4 10.7 13.3 18.4

A (rain t) 4.2 7.1 4.3 8.7 1.4

x x x x x

I0 ~ 10: 107 106 10~

E (kcal/mol) 23.6 21.3 20.6 19.2 23.2

Structural defects in PVC- Ill

803

between dehydrochlorination rate in solution and total or internal unsaturation (correlation coefficient r = 0.78 and 0.63, respectively) [6]. Hjertberg and S6rvik [4, 28] tried to solve this problem by multiple regression analysis, in the form of V.ct = k-rCl-r + kACI~, + B

.E

,..,- xE

,--;o<-~o~o,~c~o:~,--~_

--

,'5 .o

.--a

o

oo

m

~

(8)

where Clr and CI,, are the amounts of tertiary and internal allylic chlorines, k+ and kA are the corresponding overall rate constants. CI~ was calculated as the sum of butyl and long branches. Unfortunately, a strong intercorrelation exists between their CI T and CI~ values; CIT = 5.10 CIA + 0.015, with r = 0.94, i.e. the a m o u n t of tertiary chlorine atoms was practically proportional to the a m o u n t of internal allylic atoms. Thus it is understandable that the effect of tertiary and allylic chlorines cannot be separated by multiple regression; also Hjertberg and S6rvik's conclusion that tertiary chlorine atoms are more active defect sites than internal allylic chlorines [4, 27] cannot be accepted without further experimental evidence. The two sets of data on the activity of tertiary chlorine differ by a factor of nearly 3. We have to note, however, the differences in VC-2-Cl-propene copolymer preparation: lvfin et al. [13, 27] prepared their samples at low polymerization conversions (up to 13!10), while Berens [15] used higher conversion (up to 92"i,). Acknowledgement--F'inancial support from the Deutsche

F-

Forschungsgemeinschaft and from the Hungarian Academy of Sciences is gratefully acknowledged. REFERENCES

N

.E E~ ,,..~ :~ ~

,

~


__~

E ~

- ca.a. "~F-

~,

_ ~ ci~,

~u~

~.~

,

v

<<-

~

o

~.=o.==~go =-

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