Reaction of organostannic derivatives of PVC with hydrogen chloride

1550

K . S . MrNsxER e~a/. REFERENCES

1. S. V. VINOGRADOVA, V. A. BASNEV and V. V. KORSHAK, Vysokomol. soyed. B9: 522, 1967 2. R. FOSTER, Chem. and Ind., 1942, 1959 3. P. W. MORGAN, J. Polymer Sci. A2: 437, 1964

REACTION

OF ORGANOSTANNIC DERIVATIVES HYDROGEN CHLORIDE*

O F PVC W I T H

K. S. I~I~INSKER,Yu. A. PURINSON, T. B. ZAVAROVA, N. A. PLATE, G. T. FEDOSErEVA and V. A. KARGIN (Received 6 July 1967)

0ROA~O-TIN compounds are widely used as thermostabilizers of PVC [1, 2]. Numerous reports [1-6] have shown the main role of these substances to be the binding of hydrogen chloride released during the thermal-oxidative decomposition of PVC. There is no clear idea at present about the mechanism of this process. Kenyon [3] for example, reports that the PVC stabilization with dibutyl diacetate gives rise to butyl radicals which recombine with the macroradicals and terminate the chain decomposition process. I t is not clear what happens to the hydrogen chloride in this case. Frye et al. [4] explain the stabilizing effect of dibutyl tin maleate b y an exchange reaction between the polymers and the stabilizer which causes the ester group to transfer to the polymer and replace the Cl-atoms, while the latter adds an atom of tin to give an organostannic compound. TABLE

Sample PVC-6 PVC-9 PVC-IO PVC-17 PVC-19 PVC-22 PVC-25

1. C O M P O S I T I O N OF T H E E X A M I N E D SAMI'LE

Organo-tin group

(C,H,), Sn Ditto (n-C4H,)sSn Ditto

Sn-content, %

Cl-content, %

16.85 3.68 13.64 9.88 12.10 5.50 4.20

26-26 49.38 29.82 40.68 36.94 47.50 45.66

* Vysokomol. soyed. AI0: No. 6, 1336-1342, 1968.

Reaction of organostannic derivatives of PVC with hydrogen chloride

1551

Frye's conculusions were based on the finding of the radioactivity after 4ecomposition in the presence of stabilizer, which increased only when the labelled C-atom was in the ester group. Where present in the butyl group, or if the Sn-atom was labelled, there was no radioactivity increase. There is, however, still another explanation for this experimental fact. The hydrogen chloride liberated will cleave off a maleate group from the stabilizer, which then adds on to the conjugated bonds of the polymer and produces its dehydrochlorination. This mechanism is specific for maleic acid derivatives, because they easily participate in the Diels-Alder reaction. This work aimed at studying the dehydrochlorination of PVC in which a portion of the Cl-atoms was replaced by triphenyl- or tributylstannyl groups. Such a stannized PVC (Sn-PVC) forms on reacting PVC with triphenyl- or tributylstannyl lithium [7] and has the following structure (1):

~ CHI-- CH--CH~--

CH ~ (1)

SnR3

The subjects of our study were triphenyl- and tributylstannyl derivatives of PVC with different Sn- and Cl-contents (Table 1). The control was the PVC which was converted to Sn-PVC. The dehydrochlorination of Sn-PVC-6 was studied b y the method described elsewhere [8] using a nitrogen stream at 175°C; it contained triphenylstannyl groups. The rate of dehydrochlorination was determined on the basis of Cl-ion concentration changes in the gaseous reaction products, which were absorbed in water (Fig. 1, curve 2) and from that bound b y the polymer (Fig. 1, curve 1). As the Figure shows, the bound Cl-time curve (Fig. 1, curve 1) passes through a peak, which also represents the start of Cl-ion appearance in the gaseous reaction products (Fig. 1, curve 2), i.e. the time of reaching the peak equals the induction period for free HC1 liberation. In addition to HC1, the gaseous products were also found to contain benzene and stannic chloride. The presence of SnC14 was proved in qualitative reactions with ammonia. Benzene was determined b y chromatography; its amount was found to be 3.5 times larger than that obtained when pure PVC was dehydrochlorinated under identical conditions. The above results can be explained b y assuming that the reaction between Sn-PVC and HC1 takes place as a consecutive phenyl group substitution on the Sn-atom b y C1, until the stage of trichlorostannyl group formation is reached, after which the Sn is completely liberated from the polymer in the form of SnCla: HCI ,,, U r l ~ - - c ; ~

~

HCI •

,-, c ; ~ 2 - - c ; H

In(O.Hs). -C.H.

~

CI~n(CeHs), HCI

-~

-C.H.

nCl

,', u ; ~ 2 - - U H

CI,~nC.H,

--* ~CHI--CH~ --~ ~CHI--CH~ .~ %Sn C1i

I

SnC13

~

---~

-C,H. (2)

1552

K . S . MI~SXER et aJ.

This reaction progress fully agrees with t h e k n o w n d a t a on the reactivities o f various s u b s t i t u e n t s on the S n - a t o m in organo-Sn c o m p o u n d s . N u m e r o u s experiments established t h e following descending sequence o f the ease o f different, s u b s t i t u e n t liberation from the S n . a t o m b y HCI: o - t o l u y l > p - t o l u y l > p h c n y l ~

q

8

I 0

1

~ IU ~ J q Time ~h r

2

J

1 5

gJ

FIG. 1. The amount of HCI liberated as a function of thermal treatment time at 175°0. 1--0.05g Sn-PVC-6, bound HC1; 2--0.05g Sn-PVC-6, bound HC1; 3--0.05 g Sn PVC-6 and 1 g PVC, bound HC1; 4--0.05 g Sn-PVC-6 mixture with 1 g PVC, free HC1; 5--0.05 g Sn-PVC-6, total HC1; 6--0.05g Sn-PVC-6, free HC1; 7--0.05g Sn-PVC-6 mixture with l g PVC, total HC1 (experimental); 8--1 g PVC, free HC1; 9--0-05g Sn-PVC-6 mixture with 1 g PVC, total (calculated) HC1; •0--0.05 g Sn-PVC-22, bound HC1.

l

I

I

I

I

JO0

600

700

800

fO00

I

~200

I

NO0

I

I

I

t600 f800~800

I

,~000

~ C m "f

FzG. 2. Infrared spectra of Sn-PVC-9 before and after treatment with }tCI (spectrophotometer UR-10): /--original polymer; 2--after 15 rain treatment; 3--after 90 min treatment, with HC1.

Reaction of organostanni~ derivatives of PVC with hydrogen chloride

1553

~b~azyl ~vinyl ~ m e t h y l ~ e t h y l ~propyl ~isobutyl ~ b u t y l ~isoamyl >amyl~> ~ h e x y l ~ h e p t y l ~octyl [9]. It becomes clear from the sequence that the alkyl substituents are detached less easily than the aryl, and that this difficulty increases with increasing chain length. For example, the reaction of diethyldiphenyltin with HC1 [10] was found to yield 89.6% diethyltin dichloride. The probability of the phenyl groups on the Sn-atom being liberated will obviously be greater in the reaction of Sn-PVC with HC1 than from the polymer chain. To prove the above reaction mechanism, we examined the reaction of Sn-PVC with HC1 under conditions in which a dehydrochlorination of the actual polymer could not take place. This made it possible to eliminate all sorts of secondary reactions occurring during the thermal degradation of the polymer, and to observe the HC1 binding process in its pure form. Gaseous HC1 was bubbled through an Sn-PVC solution in tetrahydrofuran (THF) at a constant rate. There was an increase of the HCI content due to a replacement of the phenyl groups by Cl-atoms (Table 2). T A B L E 2. CHANGES OF C1 CONTENT I N THE S n - P V C - 9

SA~n'LE

D U R I N G ITS REACTION W I T H H C |

Time, rain Original polymer 15 30 45 60 75

C1 c o n t e n t , ~o test 1 51"6

53"3 54-8

54.8 56.3

test

2

51.6 53"8 55"0 55"0 55"4 55"6

Note: The feed rate was kept constant in each test, but differed from test to test.

The examination of the infrared spectra of the polymer treated with HC1 (Fig. 2) shows the benzene ring absorption lines (730, 1478, 1574, 3050 and 3069 cm -1) to decrease in intensity with progressing phenyl group replacement by Cl-atoms, and then to disappear completely. The same happened in the case of the Sn--C bond absorption line (447 cm-1). The presence of an induction period to the start of free HC1 liberation from Sn-PVC can thus be explained by formation in the trichlorostannyl range of the polymer. The ascending part of the curve 1 (Fig. 1) here characterizes the increasing number of Cl-atoms replacing the phenyl groups. The descending part behind the peak represents the SnC14 formation, this being carried away by the nitrogen stream, so that the amount of bound C1 will decrease. The curves obtained during the thermal degradation of a 0.05 g Sn-PVC-6 mixture with 1 g pure PVC (Fig. 1, curves 3 and 4) are similar in shape. As the Sn-content in

K . S. I~IINSKER CA a~.

1554

the composition is much lower than in pure Sn-PVC, the induction period was much shorter in this case. Figure 1 shows that the dehydrochlorination of Sn-PVC (curves 1, 2 and 5) proceeds at a greater rate than that of the pure PVC (curve 6). Such an accelerated process was also observed with mixtures of 0.05 Sn-PVC-6 and 1 g PVC (curves 4, 7 and 8), the dehydrochlorination of the mixture (Fig. 1, curve 7) taking place at a greater rate than was the total of its components (Fig. 1, curve 9 produced by combining curves 8 and 8). The cases described compare the results of equal amounts by weight of Sn-PVC.

Bound1216 HC[,n!g

o

!

8

2

/4

A

•

3 I

2

Time,hf'

3

/4

5

6

F i e . 3. A m o u n t o f b o u n d H C I as a f u n c t i o n o f t h e r m a l t r e a t m e n t t i m e a t 175°0 w i t h lead

trisulpt~ate (0.15 g) of 0.3 g Sn-PVC. 1--PVC, 2--Sn-PVG-9, 3--Sn-PVC-10, ~--Sn-PVC-6. The acceleration of the dehydrochlorination could have been caused by the influence of the polymer structure (containing triphenylstannyl groups), or by the halogen derivatives of tin forming during the dehydrochlorination, especially SnC14. The cause of this acceleration was sought in kinetic studies of decomposition in the presence of lead trisulphate. The latter effectively binds the acid reaction products, HC1 and SnC14, giving rise to lead chlorides which do not affect the dehydrochlorination of PVC [11]. This method made it possible to eliminate SnC14 from the number of reaction products and thus clarify the influence of the triphenylstannyl groups. A linear relationship was found in this case between the amount of HC1 liberated during the dehydroehlorination of Sn-PVC and the thermal treatment time at 175°C (Fig. 3). The rate of Sn-PVC dehydrochlorination was here smaller than that of PVC alone and it decreased with increasing number of triphenylstannyl groups in the polymer (Fig. 4). The presence of the latter groups in the polymer thus retarded dehydrochlorination, so that the acceleration must have been due to the catalytic activity of SnC14, which agrees with the data given in the literature [12].

Reaction of organostannic derivatives of PVC with hydrogen chloride

1555

The acceleration of Sn-PVC dehydrochlorination was confirmed by kinetic studies in which another method was used. The polymer samples were held at 175°C in sealed ampoules filled with nitrogen. These were then cooled and opened, the polymer treated with a 20/o NH~OH solution to extract the HC1, which was k, tO 5,rain-1 25

~ , ( ?.

f5 5

r, I

n

¢

t

,~

~

I

i

ol i

f2

i

I

-e

f# Sn, 'J,

FIG. 4. Rate constants of Sn-PVC dehydrochlorination as a function of Sn-eontent: / - - t r i phenylstannyl derivative; 2--tributylstannyl derivative. then subjected to potentiometric titration. The amount of liberated HC] is shown as a function of time in Fig. 5. This shows, like Fig. 1, t h a t there is an induction period, and t h a t the latter increased with increasing Sn content of the polymer. The rate constants of the Sn-PVC-6 and Sn-PVC-10 dehydrochlorination, found Bound HCl,mg 18

~ ×~ .~×

3 ~2

I0

2

! 1

2 8 Time, hp

Fio. 5. Amount of bound HC1 as a function of thermal treatment time at 175°Cin ampoules of polymers: 1--Sn-PVC-9, 2--Sn-PVC-10, 3--Sn-PVC-6. on the basis of the experimental data in Fig. 5, are larger by one power of ten t h a n those calculated on the basis of experimental data contained in Fig. 3. The dehydrochlorination study of PVC containing tributylstannyl groups showed it to differ quite markedly from t h a t of a polymer with triphenylstannyl groups. The dehydrochlorination of such polymers (Sn-PVC-22) in a nitrogen stream had a very small induction period to appearance of free HCI and the amount of bound HC1, at less t h a n 30 min from the start of the test, reached a

1556

K.S.

MINSKER ¢~ a l .

constant value (Fig. 1). There was no acceleration of the dehydrochlorination after the induction period had terminated. The rate also remained unchanged, even after lead trisulphate had been added. The dehydrochlorination products did not contain stannic chloride, but they contained tributyltin chloride, which was identified in the shape of the fluoride (found: 36% Sn, 6.05% F; calculated: 38.41% Sn, 6.15~o F). These facts can be explained b y a scheme shown below, in which the tin is dissociated from the polymer chain during the reaction of the polymer with HC1, and is then present in the form of tributyltin chloride: ~ CH2--CH~ + HCI-~ ~ CH2--CH2 ~ + (C~Hg)aSnCl I

(3)

Sn(C~Hg)3

The earlier given descending series of the ease of detachment of different substituents from the Sn-atom in the presence of HC1 shows that the butyl groups will dissociate less easily than the phenyl groups. I t is very likely that the butyl groups will also dissociate less easily than the polymer chain. The dissociation of the tributylstannyl groups from the polymer chain in the form of the tributyltin chloride explains the small induction,period to free HC1 liberation. One Sn atom can bind one HC1 molecule in this case, while one phenylstannyl group is able to bind 3 molecules. There were no other reactions of the tributyltin chloride and the amount of bound C1 therefore remained constant. As no stannic chloride is formed in reaction (3), there will not be any acceleration of TABU 3. CH~OES OF C1 ~ D Sn co~vr~r rt~ THE Sn-PVC-22 SAMPLE DURING REACTION WITH HC1 Time, rain Original polymer 20 40 60 9O 120

C1 content, ~o Sn content, ~o 47.50 47.98 48.20 48.45 48.81 50.06

5"50 2"67 2"51 2"29 1.26 0

the dehydrocMorination after the induction period and the produced tributyltin chloride will not affect the dehydrochlorination kinetics [13]. The absence of SnC14 in the system also explains the constancy of the debydrochlorination rate after adding to the polymer the lead trisulphate. The study of the polymer reaction with HCI, as in the case of PVC containing triphenylstannyl groups, also confirmed the suggested mechanism. The dissociation of the tributylstannyl groups from the polymer results in an increase of the Cl-content and a reduction of that of tin (Table 3). Tributyltin chloride was not detected when the reaction was carried out in a model system. The monomeric reaction products, determined by gas-liquid

Reaction of organostannie derivatives of PVC with hydrogen chloride

1557

chromatography, contained butane instead and qualitative reactions showed that stannic chloride was present. The formation of these products was caused b y the very large HCI concentration in the model reaction system, which caused the tributyltin chloride to decompose into butyl groups and the Sn-atom: (C4H0)aSnC1+ 3HCI-* SnC1,+ 3C,H10.

C4)

The HC1 concentration during the actual dehydrochlorination of the polymer is much smaller than in the model system, so that reaction (4) cannot take place, and this makes it possible to detect the tributyltin chloride. The characteristics of the dehydrochlorination rate constant as a function of organo-tin group content of the polymer were the same for the tributyl derivatives of PVC as for the triphenylstannyl one. The decrease of the rate constant with increasing number of tributylstannyl groups in the polymer was very small indeed (Fig. 4). All the results of the physico-chemical studies of the Sn-PVC dehydrochlorination process show it to differ quite considerably from that of pure PVC. The established mechanism of the polymer reaction with HC1 permits the suggestion that the organo-tin groups chemically bound to the PVC have a stabilizing effect during its dehydrochlorination. CONCLUSIONS

(1) The thermal stability of PVC macromolecules can be improved b y the introduction of triphenylstannyl groups into its structure. (2) The triphenylstannyl groups are capable of binding the HC1 liberated during the dehydrochlorination of Sn-PVC, like any of the known stabilizers used for this purpose. (3) The sum of all the Rhysico-chemical studies made on the dehydrochlorination products of the triphenylstannyl derivative of PVC permit the suggestion that the HC1 binding consists of a consecutive substitution of the phenyl groups on the Sn-atom b y Cl-atoms. The final reaction stage is the dissociation of the Sn-atom from the polymer chain and the formation of stannic chloride, which then catalyses the dehydrochlorination of the polymer. Translated by K. A..ALLE!q

REFERENCES

1. G. Ya. GORDON, Stabilizatsiya sintetieheskikh polimerov (Stabilization of Synthetic Polymers). Gos. khim. izdat., 256, 1963 2. M. B. NEIMAN (Ed.), Starenie i stabflizatsiya polimerov (Ageing and Stabilization of Polymers). Izd. "Nauka", 174, 1964 3. A. S. KENYON, Nat. Bur. Standards Circ. 525: 81, 1953; Chem. Abs. 48: 7338, 1954 4. A. H. FRYE, R. W. HORST and M. A. PALIOBAGI, J. Polymer Sci. A2: 1765, 1964 5. S. R. ~TEPEK and ~. JIRKAL, Chem. Listy 59: 1201, 1965 6. E. SCALZO, Mat. Plast. 28: 682, 1962

1558

V. G. KA~ozov et, al.

7. Yu. A. PURINSON, N. A. PLATE, S. L. DAVYDOVA, Z. S. NURKEYEVA and V. A.

KARGIN, Vysokomol. soyed. BIO: 257, 1968 8. K. S. M1NSKER, T. B. ZAVAROVA, L. D. BUBIS, G. T. FEDOSEYEVA, R. I. BURLAKOVA and L K. PAKHOMOVA, Plast. massy, No. 9, 56, 1966 9. R. INGAM, S. ROZENBERG, G. GIL'MAN and F. RIKENS, Olovoorganicheskiye i germaniiorganicheskiye soyedineniya (Organo-tin and Organo-germanium Compounds). Izd. inostr, lit., 69, 1962 10. BULLARD and HOLDEN, J. Am. Chem. Soc. 53: 3150, 1931 11. K. S. MINSKER and L D. BUBIS, Vysokomol. soyed. Ag: 52, 1967 (Translated in Polymer Sei. U.S.S.R. 9A: 1, 57, 1967) 12. A. S. TROITSKAYA and B. B. TROITSKH~ Plast. massy, No. 7, 46, 1966 13. K. S. MINSKER, G. T. FEDOSEYEVA, T. B. ZAVAROVA and I. N. MALYSHEVA, Vysokomol. soyed. BIO: 454, 1968

KINETICS OF THE RADICAL POLYMERIZATION OF 2-METHYL5-VINYLPYRIDINE (MVP) AND OF ITS SALTS* V. G. KARKOZOV,R. K. GAVURINA, V. S. POLONSKII and A. I. SMm~OVA Lensoviet Institute of Technology, Leningrad (Received 8 August 1967)

THE polymerization of chemically activated monomers has been given more attention recently. The recently published papers by Kargin and Kabanova [1, 2] describe the analysis of existing experimental data and stress certain of the general principles. Suitable monomers for a study of the effect of chemical activation on their polymerization capacity are the vinylpyridines. The activator can come from a series of substances, particularly halogen alkyls and protie acids. The same authors [1, 2] studied the spontaneous polymerization of certain quaternary vinylpyridine salts which had a specific mechanism. An interesting s t u d y would be the tracing of the behaviour of activated monomers under radical polymerization conditions. A clarification of this problem produced several reports [3, 4], but the number of such publications is still small. The objects of our study were 2-methyl-5-vinylpyridine (MVP) and its hydrochloride (I) or its methyl iodide (II). The radical polymerization kinetics of MVP and its salts in aqueous ethanol were investigated under different conditions. * Vysokomol. soyed. A10: No. 6, 1343-1347, 1968.