Polymer Degradation and Stability 9 (1984) 89 102

Poly(Vinyl Chloride) Stabilization with Organo-tin Compounds: Part Vll--Dibutyl Tin Maleate: Model

Compound Studies

Tran Van Hoang, A. Michel & A. Guyot CNRA, Laboratoire des Mat~riaux Organiques,

BP 24, 69390 Lyon-Vernaison, France

(Received: 30 August, 1983)

A B S T R A C T

The reaction between dibutyl tin maleate or dibutyl tin bisbutylmaleate and chlorohexene as a model compound for allylic chloride in PVC was studied at 80 °C in dichlorobutane solution. From kinetic studies by gas chromatographic analysis of the reaction medium, and also spectroscopic data (infra-red and 1H N M R ) it may be concluded that the main reaction is the substitution reaction which produces mono- and dihexenyl esters, as well as mixed diesters of maleic acid. The elimination reaction plays a minor rdle. Both activities are related to their Lewis acidities. The Diels-Alder condensation between maleates and hexadiene is not observed as a side reaction in these conditions," the main side reaction is the decomposition of monoesters of maleic acid (which is produced by the elimination reaction after one or two steps) into an alcohol and maleic anhydride--a reaction which can be catalysed by organo-tin chlorides. A Diels-Alder reaction between maleic anhydride and hexadiene takes place. The alcohol produced in the side reaction mentioned above may take part in a transesterification reaction with the organo-tin maleate and that reaction probably provides the best explanation of the earlier observation of Frye et al., 1 according to which there is at least temporary retention of reactivity of tin labelled organo-tin maleate by PVC.

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

In their study of the retention of radioactivity by poly(vinyl chloride) (PVC) thermally treated in the presence of labelled organo-tin com- pounds of the general formula Bu2SnY2, where Bu is an n-butyl group and Y is a more labile group such as a carboxyla~e group, Frye et al.1

89 Polymer Degradation and Stability 0141-3910/84/$03.00 © Elsevier Applied Science Publishers Ltd, England, 1984. Printed in Great Britain

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

observed rather intriguing differences between dibutyl tin bismethyl- maleate (DBTMM) and other organo-tin compounds, such as thiogly- colate, mercaptopropanoate or ethylhexanoate derivatives; first of all, when DBTMM was labelled (14C) on the maleate methyl group, practically no retention of radioactivity was observed; this is rather surprising because, in all other cases, the substitution of the Y group to the allylic chloride of PVC was well established and confirmed in many other studiesf1-6 Especially in the case of DBTMM, the authors suggested that, after reaction with HC1, cleavage of the ester group of the maleic acid hemiester may occur to give maleic anhydride and alcohol. Moreover, Frye et al. also observed definite radioactivity retention when the labelled group was the butyl group (14C) or the tin atom (113Sn); a part of that radioactivity was attributed to the formation of a co- ordination compound with the tin atom of donor atoms covalently bound to the polymer backbone (chlorine, oxygen. . . ) and that part may be eliminated under the action of solvents such as tetrahydrofuran, cyclo- hexanone or methanol and is readily eliminated by HCI; another part is more firmly attached and the authors thought that it was a consequence of a Diels-Alder reaction, but it can also be eliminated by HC1.

Using dibutyl tin maleate (DBTM) (a cyclic compound), Scott et al. 7

gave evidence for the formation of maleic anhydride through direct attack by HC1, as suggested by Mufti and Poller. 3 They also pointed out the possibility of Diels-Alder reactions involving both maleic anhydride (MA) and mono- or bis-reacted maleate stabilizer, because of the formation of gel during processing. More recently, in our laboratory, Michel and Owen 8 showed that the Diels-Alder process plays a minor role, although evidence for the accumulation of MA was given.

In order to clarify the various chemical processes encountered during PVC stabilization with tin maleate compounds, the present paper describes and discusses the model compound studies carried out in solution using chlorohexenes (CH) (both 4,2- and 2,3-isomers) as model compounds for the reactive allylic chlorines, which are present at the end of a growing polyene sequence in PVC.

EXPERIMENTAL DATA

Materials

Dibutyl tin maleate and methyl tin chlorides were supplied by Rhone Poulenc and dibutyl tin bisbutyl maleate and other alkyltin chlorides

PVC stabilization with organo-tin compounds." Part VII 91

were prepared and supplied by Ciba Geigy. Diphenyl and triphenyl tin chlorides were from Merck or Hoescht, maleic anhydride from Merck and maleic acid, 2,4-hexadiene, 2-hexene-4-ol, butanol and octanol from Fluka. /3-carotene was supplied by Eastman organic chemicals and deuterated chloroform by Spectrometrie Spinet Techniques. All these compounds were used without further purification.

Chlorohexene synthesis, 1,4-dichlorobutane and 1,2-dichloroethane purifications were described in previous papers. 9"1°

Apparatus

Gas chromatographic analysis was described in a previous paper, lo but the temperature is programmed linearly from 65 °C to 175 °C at a rate of 8 °C/min. The integrator associated with gas chromatography is an ICAP 50 from Delsi. The substitution product is quantified by the difference between CH consumption and hexadiene formation. The tH NMR apparatus is a Perkin-Elmer Hitachi R 24. Infra-red studies were carried out using a Perkin-Elmer 297. The discs used were of KBr.

RESULTS AND DISCUSSION

Two maleate compounds have been used: the cyclic compound (DBTM) and the bis n,butyl maleate derivative (DBTBM).

0 ~ - - CH C 4 H g ~ s n / II

C4H9 / ~ O _ _ C ~ j / C H

O DBTM

0 0 II LI

C4H 9 . ~ . S n / O ~ - - C H = = C H - - C - - O ~ 4 H 9

C4H9 / "~O--C--C H~-----CH--C--O--C4 H 9 II II 0 0

DBTBM

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

Diester

J

Bu2SnCI 2

Mono / Ester

" 1

Hexadiene

Ol iqomer

j

6- DCB

/--CH V

Hexenol

Hexediene

Air 4--- V

Fig. 1. Programmed temperature (from 65 °C to 175 °C at 8 °C/min) gas chromatogram of a reaction mixture of 240 mmole/litre of DBTM and 675 mmole/litre of chlorohexene

(CH) in DCB at 80°C for 3h.

Both readily react with CH in DCB solution at 80°C. The main reaction is the substitution reaction. The product of the elimination reaction, hexadiene, is not observed initially in the case of DBTBM, and only in very small amounts in the case of DBTM, but in both cases its production rate increases during the process. From the initial reaction rate, the substitution reaction is first order with respect to each reactant,

PVC stabilization with organo-tin compounds: Part VII 93

CH and organo-tin compound, the rate constants at 80°C being, respectively, 37 x 10 -5 and 24 x 10-Smole- l l i t res -1 for DBTM and DBTBM. In the latter case (DBTBM) the same law is valid for experiments carried out at 60°C in dichloroethane, the rate constant being 12 × 10- 5 in the same units. Other products also appear, as shown by programmed temperature GC; first, the tin dichloride, Bu2SnCI2, in all cases. In the case of DBTM there were two heavy organic products which were identified (by identity of their retention time with synthetic com- pounds) as maleic acid hexenyl monoester (I) and malaic acid hexenyl diester (II). As shown in Fig. 1, monoester I appears on the GC chromatogram as a shoulder on the BuzSnCl 2 peak, and diester II is the major product. The production of a large amount of hexenol should be noted. Spectroscopic evidence (infra-red and 1H NMR) for the reaction includes the following: the disappearance of the infra-red bands of DBTM at 1580 and 1640cm-1 and the appearance of a very strong carboxylate band at 1725cm-1. the disappearance of the ~ H C I - - multiplet centred at 4.6ppm and modification of the pattern of the ethylenic protons in the 1H N MR spectrum, the most striking feature being the replacement of the singlet at 6.5 ppm of the DBTM ethylenic proton by a doublet (6.38 and 6.50) corresponding to two isomers of hexenenyl diester II. The infra-red spectrum also shows a new band at 1780 cm - 1 and a smaller one at 1850 cm- 1 which may be assigned to the formation of maleic anhydride (Fig. 2).

Similar observations are made in the case of DBTBM; in this case, maleic acid hexenyl butyl diester (VI) (Fig. 3) appears at the very beginning of the reaction, before Bu2SnC12 is formed. Some hexenyi diester II is also formed later. The infra-red spectra in Fig. 2 show the disappearance of the tin carboxylate band at 1600cm 1 and an enhancement of the ester band at 1730 cm- 1, the appearance of a very strong band at 1780cm -1, with a side band at 1850cm -1 (maleic anhydride) and, in addition, new OH bands at 3610 and 3680cm -1, which may come from acid groups and alcohol groups; from the GC chromatogram (Fig. 3) the formation of butanol is expected.

The main reactions to be expected are summarized in Scheme 1 (DBTM) and Scheme 2 (DBTBM). The following remarks can be made.

(a) There is no evidence for monochlorotin intermediates III, IV and V. This point will be discussed later.

(b) Initially, there is no evidence for the formation ofmaleic acid and its monohexenyl ester (I) which should result from product IV in the case of

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

i

B

i '7

B

A

>

e.

2

o

0 , .~

o ~

a

_=

e4

P V C s t a b i l i z a t i o n w i t h o r g a n o - t i n c o m p o u n d s : P a r t V I I 95

~a

~r

E

o ~

E

0

2~c

o 2~

&=

E

0

Y

- - -" .=-° - ' - : . " - -. .S -_- - " ,4 .

"h.

4 .

\ i l i I

J

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

0 H

O ~ C /

Bu2Sn II

~O___C/C

II O

+ C H 3--C H==C H---CH---CH 2---C H 3 I

CI (CH)

CH 3 /Cl I

Bu2__Sn. (III) CH2 I

\ O - - C O--CH==CH--O--- CH I

CH I

CH I

CH3

+(CH) / elimination/ / CH3

Bu2--SnCI 2 CH 2 I

+ H O - - C O - - C H = = C H ~ O ~ O - - C H I

CH I

CH I

CH3

(1)

+ Hexadiene

Secondary reactions BuzSnCI 2

I > Hexenol + Maleic anhydride

O II

o / C \

Bu2Sn. C " O - - C /

, II < ', i 0

CI H I I

C---C==C--C--CH - - C - - C substitution j / ~Elimination

O O II II

/O- -C- - -C==C--C- -OH Bu2--Sn (IV)

\ e l + Hexadiene

~ s u b s + (CH) titution % \ CH3

I CH 2 CH 2 I I

HC--O--CO~CH~-----CH--CO--O--CH I I

CH CH I I

CH (lI) CH I I

CH 3 CH 3

7 Elimination: Bu2SnCI 2 + Hexadiene + Maleic acid IV + C H "--x

Substitution: Bu2SnCI2==:CH 3 ~ H ~ C H - - C H ~ C H 2 ~ H 3 I

O I

(I) C O ~ C H = = C H ~ O O H Scheme I

PVC stabilization with organo-tin compounds." Part VlI 97

o o o o {l II II II Bu /O~H~---CH~ OBu

Bu2--Sn--OC~H~-----CH~--OBu 2 + CH ~ Sn. Bu// ( ) ~ C H = = C H ~ - - O B u

/ i II II CI H O O I I

C H 3 ~ C H ~ - - - - - - C H - - C H ~ H ~ H 3

CH2___CH 3 Elimination

(V) + B u O ~ O ~ C H = = C H ~ O - - O CH I O ~ C O-~H~--------~H-~ O - - O B u

CH / I Bu2--Sn (V) \

CH CI I

CH 3 + Hexadiene + H O C O ~ H = = C H - - C O O B u

(VII)

Then:

(V) + CH

~" Substitution: Bu/SnCI 2 + VI

Secondary reaction. Bu2SnCI2

H OCO--CH~-----CH~COOBu • (v11)

Scheme 2

• Elimination: Bu2SnCI 2 + Hexadiene + HOCO--4SH~--------CH~OOBu

BuOH + Maleic anhydride

DBTM. The fact that hexadiene is not formed initially, and the absence of derivatives from IV, leads one to conclude a quasi total selectivity for the substitution reaction as the first step in the reaction between DBTM and CH. Such selectivity is not valid to the same extent as in the case of DBTBM.

(c) The monoesters of maleic acid, as well as hexenyl ester I and butyl ester, a r e very sensitive to decomposition into maleic anhydride and alcohol. This fact seems to provide a good explanation for the apparent high selectivity for the substitution reaction in the second step of Scheme 1 from DBTM (where the amount of diester II is much larger than the amount of monoester I). It also accounts for the lower selectivity for substitution in the case of DBTBM.

Organo-tin chlorides act as catalysts for both substitution and

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

elimination reactions. For the latter, in the presence of organo-tin chlorides, the reaction is first order with respect to both CH and tin chloride, and is zero order with respect to DBTM or DBTBM. These kinetics indicate that the rate-determining step for the elimination reaction is the formation of an allylic carbocation from CH with an organo-tin chloride chlorinated counterion; such a mechanism has already been observed in many cases.5'l~ The rate constants at 80 °C in DCB solution are, respectively, 25 × 10- 5 and 30 x 10- 5 mole- 1 litre s- for dioctyltin dichloride (Oct2SnC12) and BUESnCI 2 in the case of DBTM and 8.6 x 10 -5 mole -2 litre 2 s -1 for Bu2SnC12 in the case of DBTBM, but, in that case, the reaction is also first order with respect to the tin maleate compound. The kinetic laws for the substitution reaction are more complex and tend to depend on the nature of tin chloride. With OctESnC12 and DBTM, the kinetic law observed in the absence of a catalyst is not changed but the rate constant is almost doubled (58 × 10- 5 instead of 37 × 10-5). With Bu2SnC12 and DBTM the law is the same as for the elimination reaction (first order with respect to both CH and Bu2SnCI 2 but zero order with respect to DBTM), but the rate constant (250 × 10- 5) is about ten times as great. For the same chloride, the kinetic law with DBTBM is more complex with fractional positive exponents for each reactant. Both elimination and substitution rates are very much accelerated if BuSnC13 is used as a catalyst. The usual methods cannot be applied here because the hexadiene produced by the elimination reaction takes part in side reactions (oligomerization and Diels-Alder conden- sation). CH consumption is actually first order with respect to CH and BuSnC13 but shows a negative order with respect to DBTM. It is clear that, as in many other cases, 1° the catalytic activity of organo-tin chlorides is related to their Lewis acidities.

The best explanation for the fractional or negative orders observed in the kinetic laws probably lies in the exchange reaction between the organo-tin maleates and the organo-tin chlorides, which obviously change the actual concentration of the catalysts and of the reactants. This exchange reaction can be studied independently in the absence of CH. A first approach is through GC analysis: using dimethyl tin dichloride (MeESnC12) or Oct2SnC12 or the trichloride derivatives, MeSnC13, BuSnC13 and OctSnCl3, mixed with either BDTM or DBTBM, the chromatogram of the solution kept after some time at 80°C shows Bu2SnC12 formation:

PVC stabilization with organo-tin compounds: Part VII 99

O--- / C I CI Bu2Sn / + R2SnC12 ~ Bu2Sn \ S n R 2

\O- - - \ O . . . . O / /O---

Bu2SnCI 2 + R2Sn" ~O--

A more powerful approach is by NMR spectroscopy. The details of that study will be published elsewhere, 12 but the conclusions are as follows: there is no evidence for the formation of a stable monochlorinated tin compound; this is at variance with the case of tin thioglycolate derivatives 5"12 which are of comparable (slightly better) stabilizing efficiency to maleate derivatives: the evidence is for scrambling of the resonances through a rapid equilibrium, as in the case of organo-tin carboxylates' a which are much less efficient as stabilizers. The scrambling is observed over a large range of temperatures ( - 60 °C, + 60 °C). When the ratio of Me2SnC12 to DBTBM is varied from 0.33 to 3, the chemical shift of the methyl protons (one singlet) is changed from 1.165 to 1.19ppm from tetramethylsilane, the value being 1.175 for pure Me2SnCl 2, while, for the olefinic protons of DBTBM, the shift varies from 6.285 to 6.33ppm (pure DBTBM: 6.25).

In addition to the decomposition of maleic acid monoesters into alcohol and maleic anhydride represented in Schemes 1 and 2, there are a few side reactions. First, when CH is in a large excess, after most of the tin maleate has been reacted, HCI is produced from the elimination reaction and is able to react with the various esters present; then butanol is produced in the case of DBTBM. A second possible side reaction is the Diels-Alder condensation of the maleic derivatives with the dienes. However, it was observed that in dichloroethane or in n-decane as solvent, there is no reaction at 60°C between DBTM or DBTBM and hexadiene or fl-carotene (a model compound for the polyenes in degraded PVC). The same observation is valid at 80°C in DCB; moreover, in these conditions, hexadiene does not react with maleic esters I or II. The only reaction observed with hexadiene was the formation of an adduct with maleic anhydride, the yield being about 20~o after 3h at 75°C in dichloroethane for an equimolar mixture. The reaction is not catalyzed by Bu2SnC12. These results tend to be at variance with the statement by Frye et al. 1 that the attachment of labelled tin or butyl groups to PVC might be caused by a Diels-Alder reaction between tin maleates and polyenes.

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

We suggest that a better explanation of the data reported by Frye et al. is a transesterification between the alkyl tin maleate ester and an allylic alcohol group in the polymer.

-----~H==CH---CH-- + Bu2SN (OCOCH==CHCOOCH3) 2 I

o . IT CH3OH + Bu2Sn/O--CO--CH~--CH--COOCH3

/ \ O - - C O--C H==C H---C OO---C H

I CH~-------CH~ .

The possibility of the formation of an allylic alcohol in the PVC chain is demonstrated by the presence of hexenol in the GC chromatogram of the reaction mixture of CH and DBTM (Fig. 1). Hexenol is also produced by reaction of CH and DBTBM (Fig. 3). Its production can be explained through sequences of transesterification and decomposition reactions according to Scheme 3.

DBTBM + CH ~ Diester Vl substitution

VI + BuOH • Hexenol + B u O C O - - C H = = C H - - C O O B u

(VIII)

VI + VII -~ Monoester 1 + VIII

Bu,SnCI 2 1 • Hexenol + Maleic anhydride

Scheme 3

Diester VI, produced through a substitution reaction, may react first with butanol (coming from the decomposition of monoester VII) to give butanol and maleic acid dibutyl ester (VIII); the same diester (VI) may lead to an exchange reaction with butyl monoester to give hexenyl monoester, which, through decomposition, will lead to hexenol and maleic anhydride.

The reaction of hexenol with DBTBM must then be studied inde- pendently. Spectroscopic and GC analyses showed evidence for both the consumption of hexenol and the production of butanol. A set of specific experiments showed the general character of the transesterification reaction between various maleic esters; for instance, between monohexenyl ester I and dibutyl ester, or between butyl hexenyl diester and monobutyl

PVC stabilization with organo-tin compounds': Part VII 101

esters. Other experiments showed the exchange between these esters and alcohols such as n-BuOH or iso-butanol.

These experiments confirm the explanation given by Frye et al. 1 for the disappearance of the labelling of PVC when its reaction product with methyl labelled dibutyl tin di-methyl maleate was treated with non- labelled methanol. They also explain the retention of radioactivity for tin of butyl group labelling. Indeed, the reaction product of hexenol and DBTBM keeps its stabilizer character; then, upon further reaction with CH, the butyl tin compound will finally give Bu2SnC12 and the tin atom will be detached from the hexenyl residue; on the other hand, the link between tin and that residue is broken upon reaction with HC1; both observations are in agreement with the data reported by Frye et al. 1

CONCLUSIONS

The mechanisms of the reaction between dialkyl-tin maleate PVC stabilizers and allylic chloride models of those in the polymer are rather complex: they show a very high selectivity towards substitution reactions, at least at the beginning of the reaction. However, that selectivity is perhaps more apparent than real because of side reactions of the inter- mediate products which cause the formation of alcohols and maleic anhydride. Various maleic acid mono- and diesters are produced, which are prone to transesterification reactions between themselves or with alcohols or organo-tin maleate; some of these transesterification reactions may explain the temporary attachment of tin-containing moieties to the PVC chain, as reported by Frye et al.1

REFERENCES

1. A. H. Frye, R. W. Horst and M. A. Paliobagis, J. Polymer Sci., A2, 1765, 1785, 1801 (1964).

2. P. P. Kelmchuck, A.C.S. Adv. in Chem. Ser., 85, 1 (1968). 3. A. S. Mufti and R. C. Poller, J. Chem. Soc. (C), 1362, 1767 (1967). 4. G. Ayrey, R. C. Poller and I. H. Siddiqui, J. Polymer Sci., B8, 1 (1970),

All0, 725 (1972). 5. A. Michel, A. Guyot and D. Nolle, Polym. Deg. and Stab., 2, 277 (1980). 6. G. Ayrey, S. Y. Hsu an R. C. Poller, A.C.S. Org. Coatings and Applied

Polym. Sci. Preprint, 46, 630 (1982).

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

7. G. Scott, M. Tahan and J. Vydova, Europ. Polym. J. 14, 913 (1972). 8. A. Michel and R. Owen (To be published.) 9. Tran Van Hoang, A. Michel, C. Pichot and A. Guyot, Europ. Polym. J. , l l ,

469 (1975). 10. Tran Van Hoang, A. Michel and A. Guyot, Polym. Deg. and Stab., 3,

137 (1981); 4, 213 (1982). II. A. Guyot and A. Michel, in Developments in polymer stabilization--2.

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

12. M. F. Llauro, C. Monnet and Tran Van Hoang (To be published.) 13. A. G. Parker and C. J. Carman, Adv. in Chem. Ser., 169, 363 (1978).