Some Recent Chemistry Related to Applications of Organotin

12

Some Recent Chemistry Related to Applications of Organotin Compounds R. C. POLLER

Downloaded by UNIV OF CINCINNATI on May 26, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0157.ch012

Queen Elizabeth College, Campden Hill Rd., London W8 7AH, England

Studies of the interactions between PVC and Bu Sn( SBu) indicate that, although there is a small amount of exchange between SBu groups and reactive chlorine atoms in the polymer, the principal reaction is Bu Sn( SBu) + 2 H C l --> Bu SnCl + 2Bu SH. The thiol then adds to the polyene systems in the degrading polymer. The effect of changes in the organic groups bound to tin on the stabilizing efficiency is discussed as is the catalytic effect on dehydrochlorination of the various tin products. The nature of organotin biocides R SnX is considered and the preparation of compounds containing sucrose residues in the X group is described. These products have enhanced biocidal activity compared with compounds such as tributyltin oxide, even though they have only a fraction of the tin content. 2

35

2

35

35

2

2

2

2

35

3

T

he principal applications of organotin compounds are as thermal stabilizers for P V C and as biocidal agents. The technology was developed before there was any substantial understanding of the chemical and biological processes involved. Much has been learned of the chemistry of P V C stabilization since this topic was reviewed in 1974 (I ), and most of this paper is devoted to this topic. The structure of biocidal agents will be more briefly referred to at the end of this report. PVC Stabilizers Although other species are used, a typical organotin stabilizer is a dialkyltin compound. It has the general formula: R2SnX2, R = Me, Bu, C s H n , X = SR', O C O R " . A typical compound is Bu Sn(SCH COOC8Hi7-i)2. This structure was evolved empirically but we are now able to describe the function of these compounds in chemical terms. 2

2

177

Zuckerman; Organotin Compounds: New Chemistry and Applications Advances in Chemistry; American Chemical Society: Washington, DC, 1976.

178

ORGANOTIN

COMPOUNDS:

N E W

CHEMISTRY A N D

APPLICATIONS

The X Croups. Frye and his co-workers (2) studied reactions between P V C and organotin stabilizers carrying radioactive labels at various molecular positions. They concluded that exchange reactions occurred between chlorine atoms in the polymer and tin-bound X groups:

I 2

C


CI

I +

R2SnX — • 2 2

C

+ R,SnC\

2

X

Subsequent work showed (3, 4) that secondary chlorine atoms are inert to this reaction though allylic chlorines undergo exchange. When we examined uptake of radioactive sulfur by P V C treated with Bu2Sn( SBu)2 in vacuo, we obtained the results (5) shown in Figure 1. [These are quite different from the results obtained by Frye (2) who worked with thin films of polymer exposed to air.] There appears to be a slow uptake of radioactivity during an induction period. 35

40

R

Time in h Figure 1. Interaction between PVC and Bu Sn( SBu) at 180°Cfor different periods of time 2

Q5

2


12.

POLLER

Recent Chemistry and Applications

179


With the onset of discoloration the rate of absorption of radioactivity increases. During the induction period the stabilizer has a preventive role, and it is only at this stage that exchange reactions, probably accompanied by rearrangement (4) with the small number of allylic chlorine atoms present, are important. If this interpretation is accepted, then the radioactivity at the end of the induction period gives an approximate measure of the number of allylic chlorines present, and this is 3 per 100 polymer molecules. Subsequent experiments (6) which exploited differences in reactivity of secondary, tertiary, and allylic chlorine atoms toward labelled ethoxide ions confirmed this figure. The second and more important reaction of the X groups is with hydrogen chloride: Bu_,SnX

2

+

2HC1 — •

Bu SnCl 2

2

+

2HX

At least as important as hydrogen chloride absorption is the generation of H X , usually a thiol which has a healing function by adding to the polyene systems and thereby reducing the unsaturation and the color. This is the reason for the more rapid uptake of radioactivity which occurs after the induction period in Figure 1. We obtained clear evidence for this (5) by treating degraded P V C with B u ^ S H ; not only was the color discharged but the uptake of radioactivity was similar to that obtained by interaction between P V C and the labelled stabilizer. If this interpretation is correct, we would expect to see some correlation between the efficiency of a stabilizer R2Sn(SR02 and the ease of addition of the thiol R ' S H to alkenes. It is reasonable to assume that the latter process will be a radical one, and reactivity toward alkene addition is then given by the chain transfer constants (7,8). The values given in Table I indicate that, at least toward styrene addition, there is a correlation, and this may account for the fact that the organotin thioglycolates are the most effective stabilizers. In further studies (9) of the interactions between P V C samples and labelled stabilizers, we observed an inverse relationship between the amount of radioactivity taken up and the molecular weight of the polymer (Tables II, III). A l though we have not specifically investigated the mechanism of the degradation process, these results, and those reported by other workers (10,11,12,13,14,15, 16) are consistent with a predominantly radical process initiated at chain ends: CI- +

CH,CHC1CH CHC1 2

•

HC1 +

CHCHC1CH CHC1——•

CI- +

CH=CHCH CHC1

2

2

A high degree of chain transfer is postulated together with long kinetic chains leading to the observed, relatively short polyene sequences (17).


180

ORGANOTIN COMPOUNDS:

Table I.

NEW CHEMISTRY AND APPLICATIONS

Chain Transfer Constants for Thiols with Styrene at 60° (7,8)

Thiol

Constant

f-BuSH ΟΗ,ίΟΗΛ,,βΗ EtOCH CH CH SH BuSH EtOCOCH SH 2

2

3 19 21 22 58

2

2


Table II. Sample No.

Molecular Weights of PVC Samples M n

1 2 3 4 5 6

27500 31400 39800 45000 56000 79800

28000 38000 45500 55000 64000 82000

Determined in the ICI (Plastics Division) labs GPC. ^Determined in this laboratory by osmometry.

a

Retention of Radioactivity by P V C Samples after Interaction with Bu Sn( SBu) Retained Radioactivity x 10~ , (disintegration min~ g~ PVC)

Table III.

2

35

2

3 a

x

Sample No.

After 6 cycles

After 8 cycles

114 97.6 69.7 (73.3) 76.9 (78.7) 54.8 26.2

109 96.5 66.7 71.5 57.4 35.0

1 2 3 4 5 6 a

l

Figures in parenthesis are repeat experiments.

Table IV.

P V C Containing 2% of Stabilizer R S n ( S C H C O O C H , -z) Heated at 195° Approx. Time for Complete Discoloration, R min. 2

CH CH CH CH CH CH CH CH (CH ) C H CH BrCH C*H p-CH OC H 3

3

2

3

2

3

6

2

2

5

7

2

2

5

3

6

4

2

2

90 90 70 70 40 0 60 10


8

7


12.


POLLER

181

The R G r o u p s . There remains the possibility that organotin stabilizers may react directly with chain-carrying chlorine atoms. Experiments in progress (18) in which organotin stabilizers are exposed to chlorine atoms derived from the photolysis of carbon tetrachloride or from N-chlorosuccinimide indicate that such reactions are not important. This leaves the reaction between the stabilizer and hydrogen chloride as the dominant process. In an attempt to shed some light on this reaction, we examined (18) the effect on stabilizing efficiency of systematically varying the organic groups in organotin bis(isooctyl thioglycolates) (cf. Réf. 19). Some results are shown in Table IV. The stabilizers were milled into the P V C at 190°C and the resulting film heated at 195°C; samples were withdrawn at intervals and examined for color. Complete discoloration was taken as the point when the sample became uniformly black or very dark brown. Although the results must be regarded as approximate they show that the methyl- and ethyltin compounds are the most effective. Lengthening the alkyl chain causes a small reduction in stabilizing efficiency suggesting that electron release decelerates the stabilization reactions. It is unlikely that these differences are simply caused by lowering of the molar concentrations since the largest increase in molecular weight is between the dibutyl and dioctyl compounds which have equal stabilizing ability. Introduction of the electron withdrawing groups phenyl or bromine onto the tinbound methyl groups causes a much bigger reduction in stabilization. D i phenyltin bis(isooctylthioglycolate) is distinctly less efficient than the simple dialkyltin compounds but better than the dibenzyl analogue. Introducing the electron-donating p-methoxyl substituent onto the phenyl group reduces stabilization. The contradictions implied by these results can be resolved by recalling that two distinct reactions between stabilizer and hydrogen chloride may occur (19) . These are tin-sulfur bond cleavage which is a pro-stabilization reaction and tin-carbon cleavage which results in reduced stabilization since the final product will be stannic chloride, a powerful catalyst for dehydrochlorination (Figure 2). Simple dialkyltin stabilizers become converted in the polymer to dialkyltin chlorides but benzyl, bromomethyl, and phenyl groups are more readily

— C — S n — S —

I

I

- C — H

+

C l — S n — S —

destabilizing

Figure 2.

+

I

HC1

I

— C — S n — C l

+

— S H

stabilizing

Reaction of organotin dithioglycollates

with hydrogen chloride



182


cleaved from tin than alkyl, and the p-methoxyl substituent would be expected (20) to increase the rate of cleavage. Lewis Acidity Considerations. The dialkyltin dichloride formed from the stabilizer (either by absorption of hydrogen chloride or by exchange) is itself a Lewis acid catalyst for further dehydrochlorination, though much less active than stannic chloride. We set out to design a stabilizer in which the Lewis acidity in the ensuing organotin dichloride was suppressed. We prepared (21,22) di(4-ketopentyl)tin di(isooctylthioglycolate) in which it was expected that the keto groups would be free in the stabilizer but coordinated to tin in the corresponding dichloride. (CH COCHmCH,) Sn(SCH COOC H -0,


3

2

2

8

l 7

I HCl

J/ C H CH

2

Cl J

- C H .

/ 0 = C ^

^Sn

2

\ Q

H

> H

s\\

CH

C

» 2

/

CH2—CH2

Q

3

The IR spectra (Table V) showed that the C = 0 band in the dichloride at 1680 c m (i.e., a position characteristic of coordinated carbonyl) was displaced to the "free" position of 1705 c m " when the more powerful donor, 2,2'-bipyridyl was present. This band is masked in the isooctylthioglycolate, but the N M R spectra (22) (Table VI) demonstrate clearly that coordination of the keto groups - 1

1

Table V .

I R Spectra of 4 - K e t o p e n t y l C o m p o u n d s « v(C=0) (cm" ) 1708 1680 1705

Phase liq. film mull mull

1

RH R SnCl R SnCl -2,2'-bipyridyl a

2

2

2

2

R = CH COCH CH CH . 3

2

2

Table V I .

2

N M R Spectra of 4 - K e t o p e n t y l C o m p o u n d s a

b

R = CH COCH CH CH 3

2

RH R SnCl R SnCl -2,2'-bipyridyl R Sn(SCH COOC H -0 2

2

2

2

2

2

8

17

2

2

2

in CDC1

3

or C C 1

4

a

b

7.97 7.74 7.89 7.93

7.68 7.23 7.42 7.52


12.


POLLER

183

is absent in the stabilizer but present in the dichloride. Subsequent tests (22) showed that the ketopentyltin compounds are more than twice as effective in stabilization than the corresponding butyltin derivatives.


Conclusions Concerning the Structure of Stabilizers Although in the early stages of stabilization, exchange with the small number of allylic chlorine atoms present may be important, the dominant reaction is absorption of hydrogen chloride. The latter occurs by tin-sulfur bond cleavage, and structural changes which favor this but which do not promote tin-carbon bond cleavage will improve the stabilizer. The H X molecules liberated must be effective in radical additions to alkenes, i.e., the chain transfer constant should be high. Finally, reducing the Lewis acidity of the resulting organotin chloride will improve the efficiency of the stabilizer. Organotin Biocides The compounds of highest activity are those with three S n - C bonds; those in commercial use follow the formula: R SnX ( R = Bu, Ph, C Ô H H ) . Emphasis is usually placed on the nature of the R groups with the X groups considered to be of little importance. Once the ReSn group gets to the site of biochemical reaction i.e., at cell mitochondria, it may not matter what the X group is but the latter can be significantly involved in transporting the biocide to the reactive site. Although these compounds appear to pose little threat to the environment (23,24,25), their use in agriculture is limited by lack of specificity, and the host suffers as well as the predator. We considered that if an X group was designed so that the solubility of the resulting RsSnX compound was substantially increased this might have a considerable effect on biological activity and, hopefully, specificity. Since nature frequently uses glycoside formation to solubilize compounds and hence promote transport in living tissue, it was decided to incorporate the disaccharide sucrose into the X group. The most successful method used (26) was to treat the product obtained from the reaction between a cyclic anhydride and sucrose with an organotin hydroxide. 3

.COOsucrose sucroseOH

COOsucrose R SnOH 3

+ COOH

COOSnRj

(similarly for succinic and maleic anhydrides)

No attempt was made to purify the mixture of products obtained from the reaction between the anhydride and sucrose before the introduction of the organotin residue. We do not yet have information about the specificity of these products


184



Table V I I . C o m p o u n d s Tested against Enteromorpha M o d i f i e d w i t h A l g a l Nutrients

i n Sea Water

Concentration** lppm

0.1 ppm

+ + + +

+ + + +

Bu SnOCOC H COO-sucrose Ph SnOCOC H COO-sucrose (C H ) SnOCOC H COO-sucrose Bu SnOCOCH CH COO-sucrose Ph SnOCOCH CH COO-sucrose 3

6

4

3

6

4

6

+


fl

11

3

6

4

3

2

2

3

2

2

= effective; — = not effective.

Table V I I I .

A c t i o n o f Organotin C o m p o u n d s Against PaintDestroying F u n g i No. of Fungi Showing Inhibition of Spore Germination (max 16)

Bu SnOCOC H COO-sucrose Ph SnOCOC H COO-sucrose (C H ) SnOCOC H COO-sucrose Bu SnOCOCH CH COO-sucrose Ph SnOCOCH CH COO-sucrose (Bu Sn) 0 3

6

4

3

6

4

6

11

3

6

3

3

3

4

2

2

2

2

2

Table I X .

100 ppm

10 ppm

1 ppm

16 16 3 16 16 13

16 14 0 16 15 13

6 4 0 10 9 12

T i n C o n t e n t of B i o c i d a l Organotin C o m p o u n d s Compound

% Sn

Tributyltin oxide Tributyltin fluoride Bu SnOCOC H COO -sucrose Ph SnOCOC H COO-sucrose

39.8 38.4 15.2 14.1

3

3

6

4

6

4

but their biological activity is much higher than that of compounds in commercial use. As potential antifoulants, the compounds are at least three times more effective against the alga Enteromorpha than tributyltin oxide where a minimum concentration of 0.3 ppm for a complete kill is necessary under these conditions (Table VII). The results of screening against the paint-destroying fungi are shown in Table VIII. Although the tricyclohexyl derivative shows low activity, the other organotin-sucrose compounds inhibit spore germination in all 16 fungi at 100 ppm, and the two tributyltin compounds maintain this at 10 ppm whereas tributyltin oxide is effective against only 13 of the fungi at these concentrations. The high activity of these compounds is remarkable considering their very low tin content compared with commercial pesticides (Table IX). These results


12.

POLLER


185

show that the nature of the X group is important, and work is continuing on other organotin derivatives of carbohydrates. Acknowledgments I thank my co-workers, particularly S. Z. Abbas, F. Alavi-Moghadam, G. Ayrey, F. P. Man, and Ann Parkin. I am grateful to the Paint Research Associ ation (UK) for carrying out the biological tests.


Literature Cited 1. Ayrey, G., Head, B.C.,Poller, R.C.,Macromol. Rev. (1974) 8, 1. 2. Frye, A. H., Horst, R.W.,Paliobagis, Μ. Α., J. Polym. Sci. (1964) A2, 1765, 1785, 1801. 3. Klemchuk, P. P., Prog. Chem. (1968) 85, 1. 4. Ayrey,G.,Poller, R.C.,Siddiqui, I., J. Polym. Sci. A1 (1972) 10, 725. 5. Alavi-Moghadam, F., Ayrey,G.,and Poller, R.C.,Eur. Polym. J. (1975) 11, 649. 6. Alavi-Moghadam, F., Ayrey,G.,Poller, R. C., unpublished data. 7. Gregg, R. Α., Alderman, D. M., Mayo, F. R., J. Am. Chem. Soc. (1948) 70, 3740. 8. Walling,C.,J. Am. Chem. Soc. (1948) 70, 2561. 9. Alavi-Moghadam, F., Ayrey,G.,Poller, R.C.,Polymer (1975) 16, 833. 10. Bamford, C.H.,Fenton, D. F., Polymer (1969) 10, 63. 11. Yousufzai, Α. Η. K., Zafar, M. M., Shabik-ul-Hasan, Eur, Polym. J. (1972) 8, 1231. 12. Guyot, Α., Bert, M., Michel, Α., McNeill, I.C.,Eur. Polym. J. (1971) 7, 471. 13. Guyot, Α., Bert, M., Michel, Α., Spitz, R., J. Polym. Sci., A1 (1970) 8, 1596. 14. Michel, Α., Bert, M., Guyot, Α., J. Appl. Polym. Sci. (1969) 13, 929, 945. 15. McNeill, I.C.,Neil, D., Guyot,A.,Bert, M., Michel, Α., Eur. Polym. J. (1971) 7, 453. 16. McNeill, I.C.,Neil, D., Eur. Polym. J. (1970) 6, 143, 569. 17. Braun, D., Thallmaier, M., Makromol. Chem. (1966) 99, 59. 18. Ayrey,G.,Man, F. P., Poller, R.C.,unpublished data. 19. Rockett, B. W., Hadlington, M., Poyner, W. R., J. Appl. Polym. Sci. (1974) 18, 745. 20. Eaborn,C.,J. Organometal. Chem. (1975) 100, 43. 21. Abbas, S. Z., Poller, R.C.,J.Chem. Dalton . (1974) 1769. 22. Abbas, S. Z., Poller, R.C.,Polymer (1974) 15, 543. 23. Barnes, R. D., Bull, A. T., Poller, R.C.,Pest. Sci. (1973) 4, 305. 24. Bock, R., Freitag, K. D., Naturwissenschaften (1972) 59, 165. 25. Getzendaner, M. E., Corbin, H. B., J. Agr. Food Chem. (1972) 20, 881. 26. Parkin, Α., Poller, R.C.,British Provisional Patent 12168/76. RECEIVED May 3, 1976. Work is supported in part by the International Sugar Research Foundation.


Some Recent Chemistry Related to Applications of Organotin

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