poly(vinyl chloride) stabilization with organo-tin compounds: part vi—the behavior of dibutyltin...

Polymer Degradation and Stability 9 (1984) 73-87

Poly(Vinyl Chloride) Stabilization with Organo-tin Compounds: Part VI--The Behavior of Dibutyltin

Dilaurate

T r a n Van H o a n g , A. Michel & A. G u y o t

CNRS, Laboratoire des Materiaux Organiques, BP 24, 69390 Lyon-Vernaison, France

(Received: 30 August, 1983)

ABSTRACT

The reaction between dibutyltin dilaurate (DBTL ) and chloro-4-hexene-2 (containing its isomer chloro-2-hexene-3) as a model compound for the allylic chlorine atom in poly(vinyl chloride) has been studied in dichlorobutane solution at 80°C. The reactions observed--elimination of HCl to give hexadiene and substitution of the laurate group to give hexenyl laurate--are believed to obey E2 and SN2 mechanisms through a common intermediate carbocation, with a rather low selectivity towards the substitution owing to the associated structure of the DBTL. The kinetics are complex because of the complex nature of the exchange reaction between DBTL and Bu2SnCl 2. A reverse reaction between the hexenyl laurate and the organo-tin chloride is proposed to explain the limitation of the substitution reaction.

I N T R O D U C T I O N

Although introduced quite early for the stabilization of poly(vinyl chloride) (PVC), dibutyltin dilaurate (DBTL) is now considered to be a poor stabilizer so that not much attention has been paid to it. However, it would be interesting to understand why it is so ineffective. Using chloro-4-pentene-2 as a model compound, Klemchuk I observed that the reaction takes place only on heating and then shows slow autocatalytic characteristics. He proposed a complex three-term kinetic equation and

73 Polymer Degradation and Stability 0141-3910/84/$03-00 © Elsevier Applied Science Publishers Ltd, England, 1984. Printed in Great Britain

74 Tran Van Hoang, A. Michel, A. Guyot

suggested that the actual reactant for the substitution reaction ofa laurate group at the allylic chlorine was the monochloroderivative, Bu2SnCIL. In an attempt to check this, Parker and Carman 2 carried out a 1H and 13C NMR study of the reaction between DBTL and Bu2SnC12. In contrast to the case of thioglycolate derivatives, where the monochloride is produced quantitatively, in the laurate case scrambling takes place, even at low temperature, and only one shifted signal is obtained: the pure monochloride cannot be obtained so that the equilibrium level remains unknown. More recently, Ayrey et al. 3 carried out a comparative study of the reaction of PVC with the dibutyltin derivatives of laurate, maleate and thioglycolate in trichlorobenzene solution at 180°C. They showed that the rate of laurate incorporation (using the 14C labelled compound) in the PVC was much lower than for the other compounds, and its ability to prevent the development of short, as well as long, sequences was also very low compared with the thioglycolate. In spite of this, the induction period before the evolution of HCI was twice as long in its presence: thus, a limited amount of substitution is effective in retarding the dehydro- chlorination reaction.

Our purpose in the present paper is to try to obtain further insight into the behavior of DBTL, using the mixture of isomers of chlorohexene (CH) (chloro-4-hexene-2 and chloro-2-hexene-3) as a model compound.

EXPERIMENTAL

Materials

The alkyltin chlorides were prepared and supplied by Ciba-Geigy, except for the methyl derivatives which were supplied by Rhone Poulenc. Diphenyl and triphenyltin chlorides are commercial products from Merck or Hoechst. Zinc chloride is a product of Merck. These compounds have not been further purified. The synthesis of chlorohexene has been previously described. 4 The solvents 1,4-dichlorobutane (1,4-DCB) and 1,2-dichloroethane ( 1,2-DCE) were purified by distillation at low pressure.

Apparatus

Gas chromatographic analyses of chlorohexene, hexadiene and organo- tin chlorides were carried out using an Intersmat IGC 12 apparatus as described previously, s.6

PVC stabilization with organo-tin compounds: Part VI 75

The IH NMR spectra were obtained using either an Hitachi Perkin- Elmer NMR Spectrometer R24 (60MHz) or a Brucker NMR Spec- trometer WP 80 (80 MHz). All chemical shifts were referred to TMS. The infra-red spectra were obtained using a Perkin-Elmer Spectrometer 297.

The liquid chromatographic experiments were carried out using an apparatus fitted with a chronatem 380 pump and a Waters refractive index detector. Two columns were used packed either with silica or with amine derivatives grafted onto silica. Pure iso-octane or iso-octane- dichloromethane mixtures were used as the carrier solvent.

The associated structure of dibutyltin dilaurate was demonstrated by tonometry in 1,2-dichloroethane at 38.5 °C using an A.I.S. Tonometer.

RESULTS AND DISCUSSION

Neither reaction nor evidence for complex formation between the model compound, CH, and DBTL were observed in DCB at 80 °C. The reaction requires the presence of an organo-tin chloride as catalyst and shows autocatalytic character. This is true for both the elimination (dehydro- chlorination giving hexadiene) and the substitution leading to hexenyl laurate. The amount of the substitution reaction is obtained from the

al

b)

t-m-7

1 I I I

1600 7400 1200 7000

Fig. 1. Infra-red spectrum of the stoichiometric mixture of dibutyltin dilaurate and chlorohexene in DCB solution. (a) Initial spectrum. (b) After 55 rain at 80 °C. (c) After

200min at 80°C.


CI !

-CH = CH- CH-C

a)

b)

I I I I p.p.m.

7 6 5 4 Fig. 2. I H NMR spectrum (60 MHz in CDCI 3 as solvent) of the stoichiometric mixture of dibutyltin dilaurate and chlorohexene. (a) Initial spectrum. (b) After complete reaction

in the presence of dibutyltin dichloride.

difference between the consumption of chlorohexene and the formation of hexadiene, both being obtained by gas chromatographic analysis. But the formation of the hexenyl ester can be shown by infra-red or 1H NMR spectroscopy: in the infra-red spectra (Fig. 1), the ester band at 1720 cm - 1 is growing while the carboxylate band at 1570 1610cm -1 of the tin dilaurate is progressively vanishing. In the NMR spectrum (Fig. 2), the bands associated with the protons of the unsaturated structure (5.4-6.2ppm) and with the chlorinated carbon (4.1-4.8 ppm) both disappear, to be replaced by a new set in the range 5.1-6.2 ppm. As shown in Fig. 3 for the substitution reaction, the reaction rate is dependent on the Lewis acidity of the organo-tin chloride. The highest reaction rate is observed for the most acidic compound, BuSnCI 3 (Fig. 4). The results in


esCerified chlorohexene ( mmole . I - I ) A/

,oo /

I L L t

0 100 200 300 400

Fig. 3. Formation of hexenyl laurate from reaction of chlorohexene (500 + 50 mmoles/ litre) dibutyltin dilaurate (300 mmoles/litre) and dialkyltin dichloride (180 mmoles/litre) in DCB solution at 80°C; the alkyl groups are methyl (&), butyl (C)), iso-octyl ( , ) and

phenyl (11).

Fig. 4 show that when there is an excess of CH, enough tin catalyst and a rather high ratio ofCl/L, a maximum may be observed in the substitution reaction; the reason is that, when all the laurate groups have reacted, only the elimination reaction is possible, so that HC1 is produced and may react with the ester to give lauric acid and restore the CH which is readily degraded into hexadiene. As shown in Fig. 5, in conditions where the retrogradation reaction mentioned above is not observed, the competition between substitution and elimination is in favor of substitution by a factor of about 2.

In spite of their autocatalytic character, our data do not fit well the equation given by Klemchuk 1 and it is very difficult to propose a kinetic equation, even for the initial rate, which increases with both the concentration of the allylic chloride and the organo-tin chloride but decreases with the concentrat ion of the laurate. The precise values of the exponents of the kinetic relationships are dependent on the nature of the organo- tin chloride. The mechanism which is valid for the thioglycolate tin

78 Tran I/an Hoang, A. Michel, A. Guyot

;fied ch/orohexene (mmole . l - l )

700

- -A-- time (rain; - o | - - A - - - - ' - I I I I

0 50 700 150 200 250

Fig. 4. Hexenyl laurate (from difference between the consumption ofchlorohexene CH and the production of hexadiene) produced at 80 °C in DCB solution in the reaction with

D B T L and BuSnCI 3 in the following amounts (mmoles/litre). ( A ) CH = 365 D B T L = 120 BuSnCI 3 = 12 ((3) 369 60 12 ( O ) 180 120 48 ( , ) 370 60 24 (I--]) 380 120 48 (11) 562 120 48

derivatives 7 and also for the zinc or cadmium carboxylates 8 does not apply directly here; that is a two-step mechanism according to which the rate-determining step is the formation of an ion pair through reaction between the allylic carbocation and a metal chloride counter ion. The allylic carbocation may lead, in a second step, to the hexadiene through an elimination reaction or to an ester upon nucleophilic attack by the metal carboxylate. In the present case it is clear that the nucleophilic attack has a rather low efficiency, possibly due to the associated structure of DBTL. On the other hand, the exchange reaction may produce a variety of organo-tin chlorides leading to a complex first step of the reaction. These two factors will be examined more carefully.

The associated structure of tin carboxylates has been well documented in the literature. 9-1z Our measurements in DCE show a tonometric

P V C s tab i l i za t ion with organo- t in compounds." Par t V1 79

300 -

200 -

100 -

e s t e r i f i e d or degraded

c h l o r o h e x e n e

( mrnole . I - I )

/ o~°

J 0 t

~LJ j o

o ~ . ~ " I I I

0 I00 200 300 400

Fig. 5. Comparison between the substitution (C)} and the elimination {O} reactions in DCB solution at 80°C in the presence of: dibutyltin dichloride, 180mmoles/litre;

CH, 550mmoles/litre; DBTL, 300mmoles/litre.

molecular weight of 1005 (calculated for DBTL 632) and the GPC trace in THF clearly shows a bimodal chromatogram. So it may be a mixture of monomer and dimer. This structure is supported by the carbonyl region of the infra-red spectrum which shows two bands at 1570 (carboxylate anion) and 1610 cm- 1 (bridged carbonyl group). On dissolution in THF the relative intensity of these bands changes. The spectra (Fig. 6) show, in addition, two weak bands at 1710 or 1740 cm- 1 which may be caused by a small amount of lauric acid (as dimer 1710cm-1 or monomer 1740cm-113) as impurities (the dimer is dissociated in THF), which disappear if the material is dissolved in DCE. It may be supposed that, due to pentaco-ordination of the tin atom, the dimer is not able to react with the carbocation or to co-ordinate with an allylic chlorine atom:

L I O

C ~ Bu II Bu. 0 / "~0 I / O - - C - - L

Sn Sn L ~ - - O / [ ""O 0 / I

II Bu %C / Bu O I

L

80 Tran Van Hoang, A. Michel A. Guyot

Fig. 6.

2000 1800 1600 74,00 1200

InFra-red spectra of dibuty]tJn dilaurate. (a) Pure product. (b) In TI-IF solution (2.5 %).

The exchange reaction between DBTL and various organo-tin chlorides has been studied by liquid or gas chromatography, infra-red and 1H NMR spectroscopy. Because of the associated structure of both DBTL and the organo-tin chloride, the interpretation of the liquid chromatogram is quite difficult; however, the technique gives conclusive evidence for a reaction taking place in some cases with BuSnC13, Me2SnCI 2, t~2SnCl 2 and Me3SnC1 but not with Bu2SnC12, Oct2SnC12 or Bu3SnC1. The analysis of the reaction mixture by gas chromatography with programmed temperature shows a decrease in the concentration of Bu2SnCI 2 after heating the mixture of Bu2SnC12 and DBTL at 60°C in DCE. The decrease is about 10-15 ~o. When the mixture of BuSnCI 3 and DBTL is studied, the reaction is very rapid even at room temperature; Bu2SnCI2 is formed and BuSnCI 3 disappears totally if the ratio of DBTL/BuSnC13 is larger than 2. Conclusive evidence was obtained also for Me2SnC12, MeSnC13... and also by studying dimethyltin dilaurate.

Spectroscopic evidence for the exchange reaction is shown in Fig. 7 for the methyl tin chloride derivatives. Compared with the infra-red spectrum of DBTL, the main changes are as follows: the appearance of a strong band at 1710cm-1 and a relative increase of the 1570 band among the 1570-1610 bands with a shift towards lower wave numbers. In the case of


i

a

cm~l I 1 1

16 O0 14 O0 1200

Fig. 7. Infra-red spectra of mixtures ofdibutylt in di]aurate and (a) M%SnCI (ratio 1/1). (b) Me2SnC] 2 (2 DBTL/ I Mc2Snq]2). (c) M%SnCI 2 (ratio 1/1). (d) MeSnCl 3 (ratio 1/1).


the methyl derivatives, the changes are observed with all three chlorides and the importance of the effect is again related to the Lewis acidity of the organo-tin chloride, and also to the amount of the Lewis acid compound. Similar trends, but attenuated, are observed in the case of the butyl or the iso-octyl derivatives; in these cases the changes are hardly perceptible with the trialkyl derivatives. The change is also dependent on the bulkiness of the alkyl group; although the phenyl derivatives are more acidic, due to their bulkiness the changes are not very great (Fig. 8). It may be suggested here that the new band at 1718 should correspond to a carbonyl group co- ordinated to a tin atom bearing a chlorine atom. The associated structure might be:

O II

L - -CO . . . . CI I _ _ - C I \ /

R - - S n " - Sn--R

R \ O ~ c ~ O " C1 I L

The N MR data reported in Table 1 show that the chemical shifts caused by the exchange reaction are rather limited, and their extent follows the

TABLE 1 1H N M R Chemical Shift Variations (In ppm from T M S - 6 0 M H z ) for Equimolar

Mixtures of Dibutyltin Dilaurate and R ~SnCI~ (X + Y = 4)

R X CH 2 group in dibutyltin dilaurate Protons adjacent to tin Adjacent to tin Adjacent to CO0 in R~SnCI~.

Me 3 0 0 - 0.08 2 +0.06 0 -0 .11 1 +0.07 - 0 . 0 3 - 0 . 0 2

Bu 3 0 0 0 2 + 0.06 + 0.02 - 0.05 1 +0.12 - 0 . 0 3 - 0 . 5 8

Oct 3 0 0 - 0.06 2 +0.06 0 -0 .11 1 +0.17 +0.12 - 0 . 6 5

~b 3 +0.07 +0.10 +0.05 2 +0.09 +0.12 0

+0.15 +0 '30

PVC stabilization with organo-tin compounds: Part V1 83

b

Fig. 8.

cm -7 I I I

1600 7~00 7200

Infra-red spectra of stoichiometric mixture of dibutyltin dilaurate and (a) ~b3SnC1; (b) ~b2SnC12; (c) Bu2SnC12.

order of Lewis acidity of the organo-tin chlorides. For the more acidic compound, butyltin trichloride, the data reported in Table 2 show that the extent of the exchange depends upon the concentration of the reactants. These data agree with the findings of Parker and Carman 2 according to which the exchange is a dynamic equilibrium. All our data are also consistent with the recent study of Burley and Hutton 14 concerning the thioglycolate derivatives: the exchange reaction tends to give the maximum scrambling of the various labile ligands of the tin (chlorine and carboxylate); however, in the case of the dilaurate, a quantitative description of the exchange is not possible with NMR spectroscopy.

An interesting additional point concerning the exchange reaction is


TABLE 2 1H NMR Data (100 MHz - TMS as Internal Reference - CCI3D as Solvent). Evolution of the Chemical Shift (in ppm) of Methylene Groups of Dibutyltin Dilaurate (Bu2SnL2) and Butyltin Trichloride (BuSnCI3) with Respect to their Molar Ratio R =

[BuSnCI3]/[BuzSnL2]

a* h* c* f** g** h** (ppm) (ppm) (ppm) (ppm) (ppm) (ppm)

1.68 2.38 1.32 1.3 1.6 1.72 2.2 2.38 Before reaction After reaction

R =0.5 1.72 2.38 1.31 1.31 R = 1 1.80 2.41 1.31 1.31 R = 2 - 7 1.82 2.50 1.31 1.31

1.31 1.73 1.31 1.80 1.31 1.82

O If

*(CH 3---C H 2 ~ C H u--CH 2 ) 2 S n ~ - ~ O ~ C H 2 (CH 2) 9C H 3) 2

b a c

** ~-CH 3~C H 2--CH 2--C H 2)-SnC13

f g h

demonstrated by the reaction between ZnC12 and DBTL. This reaction leads quantitatively to zinc dilaurate and Bu2SnC12, as shown by GC analysis of the latter and elemental analysis of the former. When a mixture of DBTL and ZnCI 2 is used in the presence of the model CH, the reaction takes place at a rate which increases with the amount of ZnC12 and also with an increasing selectivity compared with the substitution reaction (Fig. 9).

The above results concerning the exchange reaction make it possible to check the assumption of Klemchuk 1 that the true catalyst for the substitution reaction should be the monochloride. By reacting a stoichiometric mixture of DBTL and a trialkyl monochloride compound, it is possible to obtain some monochloride-laurate compound without the possibility of getting the tin dichloride. When reacted with CH, these mixtures give no reaction at all except in the case where the alkyl group is a methyl group; in that case a very small amount of hexadiene is produced but no evidence for the substitution reaction was obtained.

Careful examination of Fig. 4 shows that the maximum of the substitution reaction is observed well before all the ester groups have been engaged in the substitution reaction or have reacted with HCI to give


chtorohexene fransformafion (%)

, s - : /

100 200 Fig. 9. Reaction of DBTL (180 mmoles/litre) and chlorohexene (225 mmoles/litre) in dichlorethane solution at 60°C in the presence of ZnCI 2 (~): 5, , : 10 and & :

20 mmoles/litre). ( - - - ) Chlorohexene consumption. - Hexadiene formation.

lauric acid. The only possible explanation is to admit that the organo-tin chloride is able to react with the hexenyl laurate as follows:

CH3~CH~------CH---CH--CHz--CH 3 + RxSnC1 v • I

OCOC11H23 RxSnLCIv- 1 + CH

hexadiene + HCI

Because of the reaction of HC1 with the S n - - O C O L bond, the equilibrium should be shifted to the right as the reaction proceeds. The progressive disappearance of the hexenyl laurate is more rapid for the more acidic organo-tin chloride but it is also noticed that, in the case of Oct2SnC12 and Bu2SnCi z, the reaction needs several days to be completed at 80 °C.

CONCLUSIONS

The equilibrium nature of the exchange reaction between the organo- tin chlorides and the organo-tin carboxylates makes the kinetics of the


reaction of mixtures of these with the model allylic chloride very complex. The following reaction scheme is suggested:

. . . . , .CH=CH--CH~ + I

CI

\ S n / C I + \ S n / L / \ / \

L

*C1

CH---CH-=--CH \ S n i / \ C l f

CI, or L

I -C1 \ S n / \ , / Sn / \ / \ L

elimination • Isubs~utio n

----,-CH==CH--CH~-----CH~ + HCI--.-42H~----CH---CH-- +

C1 \ + Sn /

/ \

I , v • L, Y

e iminaio eror hane ..~.CH I _ ~ C H ~ H _ _ + r" ~" " .Sn/L

) / \

C1

\ /CI / S n \

and, in addition, the reaction of HC1 with the tin laurate compounds. Further, the nucleophilic attack of the carbocation by DBTL or the possible monolaurate compound is not very efficient because of the associated structure of the tin laurate compounds. Then the elimination reaction becomes more probable.

When comparing the low efficiency of DBTL with the high efficiency of the corresponding thioglycolate compounds, the main reasons for the difference are as follows:

(a) The lower association of the thioglycolate so that it is more efficient as a nucleophilic agent to react with the carbocation.


The weakening of the Sn- -S bond caused by internal co- ordination of the ester group to the tin makes the reaction with the allylic chloride easier.15

(c) The lauric acid produced by the acidolysis of DBTL by HC1 is not able to react with the allylic chlorine, while the thioglycolate ester R C O O - - C H 2 - - S H has been shown to be more reactive than the tin thioglycolate itself. 7

(d) The reverse reaction between the hexenyl thioglycolate and the organo-tin chloride was not observed, as it is with hexenyl laurate.

A C K N O W L E D G E M E N T

The authors are indebted to Dr A. Revillon for the GPC experiments.

R E F E R E N C E S

1. P. P. Klemchuk, Adv. in Chem. Ser., 85, 1 (1968). 2. R. G. Parker and C. J. Carman, Adv. in Chem. Ser., 169 (1978). 3. G. Ayrey, S. Y. Hsu and R. C. Poller, A.C.S. Org. Coatings and Applied

Poly. Sci. Preprints, 46, 630 (1982). 4. Tran Van Hoang, A. Michel, C. Pichot and A. Guyot, Europ. Polym. J~,

11,469 (1975). 5. Tran Van Hoang, A. Michel and A. Guyot, Polym. Deg. and Stab., 4,

427 39 (1982). 6. Tran Van Hoang, A. Michel and A. Guyot, Polym. Deg. and Stab., 3,

2 (1981). 7. A. Michel, A. Guyot and D. Nolle, Polym. Deg. and Stab., 2, 277 (1980). 8. A. Guyot and A. Michel, in Developments in polymer stabilization--2.

(G. Scott (Ed.)) (Chapter 3). Applied Science Publishers, London, 89 124 (1980).

9. M. Wada, M. Shindo and R. Okawara, J. Organomet. Chem., 1, 95 (1963). 10. R. Okamara and M. Ottara, J. Organomet. Chem., 1,360 (1964). 11. D. W. Allen and I. W. Nowell, J. Organomet. Chem., 24, 29 (1981). 12. V. I. Goldanski, V. V. Kharpov, O. Y. Okhlobystin and V. Y. Rocher, in,

Chemical applications of Mossbauer spectroscopy. (V. I. Goldanski and R. H. Hober (Ed.)). Academic Press, 336 76 (1980).

13. K. Volka, E. Szako and Z. Vymazal, Europ. Polym. J., 16, 149-50 (1980). 14. J. M. Burley and R. H. Hutton, Polym. Deg. and Stab., 3, 285 (1981). 15. M. Lequan, Y. Besace and R. C. Poller, Europ. Polym. J., 16, 1109 (1980).

poly(vinyl chloride) stabilization with organo-tin compounds: part vi—the behavior of dibutyltin...

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