Study of the polymerization of acetylene with a TiCl4Al(isoC4H9)3 catalyst

STUDY OF THE POLYMERIZATION OF ACETYLENE WITH A TiCI~--AI(iso-C4Hg)3 CATALYST* N. F. NOSKOVA, I. 1~. ROSTOMYAI~, V. G. GRIGORYAI~ and G. A. CHUKHADZHYA~T Polymeric Products Planning Research Institute

(Received 13 July 1965) I ~ THE presence of typical Ziegler-Natta catalysts acetylene polymerizes to form a black, insoluble polymer [1-3] with semiconducting properties [4, 5]. Information in the literature on stereospecific polymerization of acetylene is scanty and unsystematic. I n view of the growing interest in polymers with a system of conjugated bonds, polyacetylene being the representative of these with the simplest structure, we felt t h a t it would be opportune to study the polymerization of acetylene in greater detail. The aim of the present work was to study the kinetics of polymerization of acetylene and the effect of various factors (the ratio of the catalyst components, catalyst concentration and electron-donating additives) on the yield of polymer, at temperatures from 60 ° to --60 ° . The experimental procedure and methods of purification of acetylene and the solvent (n-heptane) have been described in detail in a previous communication [3]. The components of the catalyst, tri-isobutylaluminium and TIC14, were used in the form of dilute solutions in n-heptane. The initial concentrations were Al(iso-C4Hg)3 0.38 g/ml and TiCI~ 0.08 g/ml, and the reaction time was 1 hr. I n experiments with electron-donating additives these were added after the Al(iso-C4Hg) 8 and TiC14 were mixed. The catalyst was prepared at the temperature of the experiment and it contained 0.16 g of TiC14 (12 mmole/1.). When the ratio of the components was varied the quantity of tri-isobutylaluminium was changed, without altering the quantity of TIC14. I t is well known that the ratio of the components is a decisive factor in the activity of the catalyst. This is primarily associated with change in the valence state of the titanium in the catalyst complex [6]. An excess of a trialkylaluminiu'm can function as an additive, modifying the catalyst [7]. I n the first series of experiments we studied the effect of the Al(iso-C~I-Ig)a: TiC14 ratio on the yield of polyacetylene at 60 °, 20 °, 0 °, --20 ° and --60 ° (Fig. 1). I t is seen from Fig. 1 that the rate of polymerization increases with decrease in * Vysokomol. soyed. 8: No. 9, 1524-1528, 1966. 1678

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temperature. For example, at an AI:Ti ratio of 6:1 the yield of polymer, calculated on 1 g of TiCI~, increases b y a factor of 5 with reduction in tempera, ture from -~60 ° to --60 °. Presumably the microstructure of the catalyst, which has a decisive effect on its activity, alters with change in temperature.

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FIG. 1. Dependence of the ~ield of polyacetylcnc on the ratio of the catalyst components" 1 -- -- 60°, 2 : -- 20°, 3-- 0°, 4-- 20°, 5-- 60°. Abscissa: moles Al(/so-C~Hg)a/mole of TiCl~ (A). Experiments aimed at finding the effect of temperature on catalyst formation showed that the activity of catalysts prepared at temperatures lower than the polymerization temperature is increased, b u t only slightly. The increase in the yield of polymer with decrease in temperature is probably not due to the increased concentration of acetylene in the solution, b u t primarily to decrease in the chemical adsorption of acetylene on the surface of the catalyst, because in our opinion strong adsorption of the acetylene is the rate-determining stage in this reaction [3]. The optimal ratio of A l : T i also increases with decrease in temperature. Whereas at 60 ° the highest yield of polyacetylene is obtained at an A1: Ti ratio of 2.3: 1, at --60 ° the ratio is 9:1 (Fig. 2). This is obviously explained b y the lower reducing activity of the trialkylaluminium at lower temperatures. When the concentration of the trialkylaluminium is increased above the optimal value the yield of polymer remains practically unchanged at 60 °, 20 ° and 0 °, b u t at --20 ° and --60 ° the yield falls. The dependence of the yield of polyacetylene on TiC14 concentration at the optimal AI:Ti ratio is shown in Fig. 3. I t is seen that at --60 ° the yield of polymer increases from 4 to 9.5 g/g of TiCla with increase in the concentration of TIC14 from 6 to 12 mmole/1. At a TiCla concentration of 24 mmole/1, the yield is 11.5 g/g of TIC14, i.e. the reaction rate has increased b u t less sharply. This is probably the result of impairment of the contact between the catalyst and the

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acetylene as a result of the increase in the quantity of solid polymer in the system. At a TiC14 concentration of 36 mmole/1, all the acetylene polymerizes (conversion 100%). Similar results were obtained at --20 °. At the other temperatures there is a linear relationship between the yield of polyacetylene and the q u a n t i t y of TiOl4.

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l~e. 2. Dependence of the optimal A1 : Ti ratio on temperature. Ordinate: moles A l ( i o o C,Hg)8/mole of TiCI~ (A) l ~ e . 3. V a r i a t i o n in y i e l d o f p o l y a c e t y l e n e w i t h TIC1, concentration: 1 - - - - 6 0 °, 2 - - - - 2 0 °, 3 - - 0 °, 4 -- 20 °, 5 -- 60 °.

W e . p l o t t e d rate curves for polymerization of acetylene at 20 °, 0 °, --20 ° land --60 ° at an AI:Ti ratio of 3.5:1 (Fig. 4). The ordinate represents the yield of polymer per minute per gramme of TiC14. I t is seen from Fig. 4 t h a t at all tern,p'pol.umer',/m~h 0-2 •

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:peratures the rate of polymerization is greates~ in the 10th minute of reaction. The rate then falls, and from the 20th to the 40th minute it remains constant

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(zero order). The stationary period begins earlier as the temperature is reduced. The catalyst prepared at --60 ° is somewhat more stable, it becomes deactivated after 70 min. Some authors have suggested t h a t the decrease in activity of the catalyst is the result o f deactivation of the active centre as the degree of conjugation of the chain increases [9-11]. The relatively s h o r t active life of the catalyst under these conditions is also possibly connected with the fact that the acetylene, acting as a strong electron acceptor, converts the titanium to the lower valence state, which is not active in this reaction [8]. The effect of electron-donating additives on the activity of Ziegler catalysts in the polymerization of propylene has been studied by Boor, Razuvayev, Vesel~ and their collaborators. Vesel~ [12] showed t h a t triethylamine acts as a poison for the AI(C~Hs)8-TiC13 system, even in small quantities. According to Boor [13] when triethylamine is added to a Zn(C~Hs)2-titanium chloride system the degree of conversion of propylene first falls, but with further addition of the amine the yield gradually increases. Razuvayev and his collaborators in the reports of their extensive work [7, 14, 15] have pointed out t h a t the effect of polar additives is complex, being dependent on the nature of the components of the catalyst, the additive and the monomer. The addition of pyridine, dioxan, diphenyl sulphide and thianthrene in certain proportions increases the molecular weight of polypropylene. I n the presence of pyridine and dioxan an increase in the rate of polymerization was noted. We have suggested t h a t in the polymerization of acetylene, which has strong aceeptor properties [16], donor additives should increase the reaction rate. I t is seen from Fig. 5 t h a t the addition of 0.05-0.1 mole of dioxan per mole of TiC14 does in fact increase the yield of polyaeetylene from 3.4 to 7.2 g/g of TIC14. Increase in the quantity of additive beyond this point causes a decrease in the yield of polymer, and at a concentration of 0.8 mole of dioxan per mole of TiC14 the yield of polymer is only 4.2 g/g of TiCI~ as compared with 3.4 g in the absence of the additive. Triethylamine in quantities from 0-1 to 0.3 mole also accelerates the polymerization of acetylene. I n a previous communication [3] we reported similar results from a study of the effect of various amines. Later experiments have shown however t h a t electron-donating additives have a positive effect only at AI:Ti ratios below the optimal. The above results were obtained with an A1: Ti ratio of 3.5: 1 at 0 °, whereas the optimal ratio at 0 ° is 6: 1. Figure 5 shows the effect of dioxan on the yield of polymer at the optimal AI:Ti ratios for 0 ° and --20 °. At 20 ° as little as 0.1 mole of dioxan per mole of TiC14 lowers the yield of polyaeetylene from 8.5 to 6.4 g/g of TiC14 (see also Table). The rate steadily falls with further addition of dioxan.

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FIG. 5. Dependence of the yield of polyacetylene on the quantity of electron-donating additive: 1--additive triethylamine, A1 : Ti=3"5 : 1, 0°; 2--additive dioxan, A1 : T i = 3"5 : 1, 0°; 3--the same, A I : T i = 6 : 1, 0°; d--the same, A I : T i = 7 : 1, --20% EFFECT

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* 0.1 mole of dioxan per mole of TICI~.

Modification of a Z i e g l e r - N a t t a c a t a l y s t b y a d d i t i v e s p r o b a b l y consists in bringing t h e electron d e n s i t y o f t h e c a t a l y s t to a certain o p t i m a l value, i.e. in t h e specific f u n c t i o n o f r e p l e n i s h m e n t of t h e electron deficit o f t h e c a t a l y t i c complex.

CONCLUSIONS (1) A study h a s b e e n m a d e of t h e p o l y m e r i z a t i o n of a c e t y l e n e w i t h t h e TiC14-AI(iso-CaH~)3 c a t a l y s t , w i t h a n d w i t h o u t e l e c t r o n - d o n a t i n g additives, w i t h v a r i o u s ratios of t h e c o m p o n e n t s o f t h e c a t a l y s t a t t e m p e r a t u r e s b e t w e e n 60 ° a n d - - 60 °. (2) T h e yield o f p o l y a c e t y l e n e a n d t h e a c t i v e life of t h e c a t a l y s t increase w i t h decrease in t e m p e r a t u r e a t a g i v e n A I : T i ratio. (3) U n d e r t h e g i v e n conditions e l e c t r o n - d o n a t i n g a d d i t i v e s h a v e a positive effect on t h e yield o f p o l y m e r o n l y w h e n t h e A I : T i r a t i o is below t h e o p t i m a l . At the optimal AI:Ti ratio the additives sharply retard polymerization.

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(4) I n t h e s t e a d y - s t a t e period o f p o l y m e r i z a t i o n the reaction is of t h e first order w i t h respect t o acetylene. (5) A t - - 2 0 ° a n d - - 6 0 ° a n d w i t h a TiCI~ c o n c e n t r a t i o n o f 36 mmole/1, the degree o f conversion o f acetylene is 100%. Trans/ated by E. O. PHILLIPS

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