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Nature of active centers and stereospecificity of tetravalent titanium benzyl derivatives in polymerization of butadiene
European Polymer Journal. VoL I1, pp. 829 to 832. Pergamon Press 1975. Printed in Great Britain
NATURE OF ACTIVE CENTERS AND STEREOSPECIFICITY OF TETRAVALENT TITANIUM BENZYL DERIVATIVES IN POLYMERIZATION OF BUTADIENE B. A. DOLGOPLOSK, E. I. TINYAKOVA, O. K. SHARAEV, I. SH. GUZMAN and N. N. CHIGIR Institute of Petrochemical Synthesis, U.S.S.R. Academy of Sciences. Leninskii prospect 29, Moscow. U.S.S.R.
(Received 29 October 1974) Abstract--Stereospecificity of tetrabenzyltitanium and its halogeno-derivatives in the polymerization of butadiene has been investigated. The content of 1,2-units decreases while the content of 1,4-cis-units increases in the resulting polybutadiene for the series (C6H~CH2)4Ti, (C6H~CH2)3TiCt, (C6HsCH2)3TiBr, (C6HsCH2)3Til. Tribenzyltitanium iodide exhibits high stereospecificity for the formation of 1,4-cis-units and their content reaches 94-97°o. By determining the number of benzyl groups linked with titanium at different degrees of conversion, it has been shown that the active centre formed from tetrabenzyltitanium contains three benzyl groups and one polymer chain. Two benzyl groups. one iodine atom and one polymer chain are attached to a titanium atom in the active centre for the case of tribenzyltitanium iodide. Electron donors sharply change the stereospecificity of tribenzyltitanium iodide: the content of 1,2-units in the polymer rises to 68°°. and aliphatic solutions giving rise to high molecular Previous studies on the n-allyl complexes of nickel, products, Table I lists the data which characterize chromium and some other metals have demonstrated the stereospecificity of titanium benzyl derivatives in that the use of certain organometallic compounds as aromatic solutions. The content of 1,2-units decreases whereas the concatalyst for diene polymerization opens up wide postent of 1,4-cis-units increases in the series R4Ti, sibilities for investigating the nature of active centres R3TiCI, R3TiBr, R3RiI. In the case of R3TiI, the sysdiffering in their stereospecific action [I-4]. This paper deals with the polymerization of buta- tem becomes highly specific for the formation of 1,4-cisdiene under the action of benzyl derivatives of units and their content reaches 94-97~,. Dibenzyhitanium di-iodide is practically inactive in tetravalent titanium of general formula (C6HsCH2),TiX,,_ ., where n = 2,3,4 and X = CI. Br the polymerization. Polybutadiene formed in a small or I. Tetrabenzyltitanium was synthesized by reaction yield contains mainly 1,4-tra~s-units. The stereospeciof benzylmagnesium chloride with titanium tetra- ficity of RaTil decreases in aliphatic solvents: in polychloride. Tribenzyltianium chloride was prepared by butadiene the content of 1,4-cis-units decreases as a the reaction of tetrabenzyltitanium with equimolar result of increased content of 1,4-trans-units (Table amount of hydrogen chloride in toluene solution. Tri- 2). It cannot be excluded that higher stereospecificity benzyltitanium iodide, dibenzyhitanium di-iodide and in aromatic solvents may be due to the formation tribenzyltitanium bromide were synthesized by reac- of arene complexes of transition metals. Figure 1 shows typical kinetic curves for butadiene tion of tetrabenzyltitanium and appropriate amounts of iodine or bromine. All the products, excepting tri- polymerization in decane and o-xylene solutions with benzyltitanium chloride, were recrystallized from hex- tetrabenzyltitanium and tribenzyltitanium iodide. The ane and isolated as solids. Details regarding the syn- efficiency of tribenzyltitanium iodide at 25 ° is considerably higher than that of tetrabenzyltitanium at theses are given in the experimental section. 50 ° . The process is characterized by an induction period with duration increasing in aliphatic solvents. RESULTS AND DISCUSSION We studied the consumption of benzyl groups Tetrabenzyltitanium and its monohalogen deriva- linked with titanium as a function of the degree of tives initiate butadiene polymerization in aromatic conversion in order to find out the initiation rate and Table 1. Effect of substituent in R3TiX on stereospecificity the nature of active centres responsible for chain propagation. For this purpose, 20% aqueous solution of in butadiene polymerization in aromatic solutions [5] sulphuric acid free from oxygen was introduced into the reaction mixture at different stages of polymerizaMicrostructure of polybutadiene tion. The amount of toluene evolved during the hy(% of units) drolysis of the bonds ~ T i - - C H 2C6H s was determined Catalyst 1,4-cis1,4-trans- 1.2chromatographically (liquid phase Apiezon L, 10% on "'Spherochrom'; internal standard: iso-octane or n(C6H 5CH z)4Ti 26 15 59 deeane), (C6H sCH2hTiCI 35 19 46 The initial benzyl derivatives of titanium are hydro(C6H sCH 2)aTiBr 66 13 2I lysed with the evolution of quantitative amounts of (C6H~CH2hTil 96 2 "~ toluene: 4 and 3 moles of toluene/g atom of titanium 829 INTRODUCTION
830
B. A. DOLGOPLOSK,E. I. TINYAKOVA,O, K.
SHARAEV,
I. SH. GUZMANand N N. CHIGIR
Table 2. Effect of solvent on stereospecificity of titanium benzyl derivatives in butadiene polymerization
Catalyst
Solvent
Polymerization temp.
Content of units in polymer (%)
(C6HsCH2)4Ti
Toluene Pentane o-Xylene
50 50 50
24--26 20--27 20
(C6HsCH2)3TiI
Toluene o-Xylene Hexane Decane Pentane + o-Xylene (10:1 by vol.) Hexane + ooXylene (2:1 by vol.)
25 25 25 25
1,4-trans-
1,2-
15--26 16--19 19
56--59 57-63 61
97 96 81 80
0.5 I 15 15
2.5 3 4 5
25
91
4
5
25
94
2
4
1,4-cis-
approximately equal for R3TiI at 25 ° and R4Ti at 50 °. This suggests that the higher efficiency of the system with R3Til is a result of higher rate of chain propagation, Likewise it follows from Fig. 2 that, the higher the initial monomer to titanium ratio, the quicker is the
6C ,,: 5C
3O 2C
3"9
~0
3 36 35
Tlll~e, rnin
Fig. 1. Kinetic curves for butadiene polymerization under the action of tribenzyltitanium iodide (1-3) and tetraben° zyltitanium (4). I. [C4H~] = 3 mol/l. [Ti] -- 5 x 10 -3 mol/l, 25 °, o-xylene. 2. [C4H~] = 5 mol/I, [Ti] = 3 x 10 -3 mol/l, 25°, o-xylene. 3. [C,,H~] = 3 mol/l, [Ti] = 5 × 10 -3 tool/l, 25 °, n-decane. 4. [C4H6] = 3 mol/l, [I'i] = 7.5 x 10 -3 mol/l, 50°, o-xylene. are evolved in the case of tetrabenzyltitanium and tribenzyltitanium iodide, respectively. Polymerization of butadiene was carried out in o-xylene solution at different molar ratios of monomer to titanium. The experimental data listed in Fig. 2 clearly show that the toluene:titanium ratio gradually decreases as the process advances. The number of benzyl groups attached to the titanium atom decreases from 4 in the starting compound to 3 during the polymerization initiated by tetrabenzyltitanium (curves 1 and 2). As polymer chain propagates, the active centre retains all three benzyl groups. In the case of tribenzyltitanium iodide (curves 3 and 4), the number of benzyl groups attached to the titanium atom decreases from 3 to 2 during the initiation step and remains unchanged up to high conversions of the monomer. The induction period and an increase of the rate with time, as is evident from the data above, are due to the slow initiation reaction. The initiation rates determining the induction period of the process are
(o)
,~ 3.
~, ~
3.1
5
-
3
~ .... 7
28 ~
27
(b)
23 2"4
. ..0,.....~..,..,.,.~, ,.,0...
2J 20 19 1-8
3 4",~P -" ~~" -~ I IO
I 20
C 1 3O
x I 40
Conversion,
•
I 50
I 6O
I 70
.
%
Fig. 2. Consumption of benzyl groups linked with titanium during butadiene polymerization initiated by tetrabenzyltitanium (a) and tribenzyltitanium iodide (b). Solvent oxylene (I-6), n-decane (7). Temperature 50 (I, 2. 5) and 25 ° (3, 4, 6, 7). 1. f C , H 6 ] = 3 tool/l, [Ti] = 7.5 × 10 -3 mol/I. 2. [C4H6] = 3 mol/l. [Ti] = 5 x 10 -3 mol/l. 3. [C4H6] = 5 mol/I, [Ti] = 3 x 10 -3 mol/l. 4. [C4H6] = 5 mol/1. [Ti] = 1.5 x 10 -3 mol/I. 5. [C4H6] = 4 mol/l, [Ti] = 4 × 10-: mol/l. 6. 1 " C 4 H 6 ] = 3 moI/l. [Ti'l = 5 x 10 -3 mol/l. 7. [C,,H,] = 3 tool/1. [Til = 5 ×
10 - 3 m o t
I.
Tetravalent titanium benzyl derivatives completion of the initiation. At very high titanium concentrations (curves 5 and 6), the initiation reaction does not complete up to high monomer conversions as well as in the case of polymerization in aliphatic solvents (curve 7). The results obtained for the consumption of benzyl groups demonstrate that only one Ti-benzyl bond takes part in the polymerization reactions of butadiene with tetrabenzyititanium and tribenzyltitanium iodide. The absence of any ESR signal from the reaction mixture during the whole process of polymerization shows that titanium remains in the tetravalent state and the polymerization takes place by the insertion of monomer into the f-bond of carbon-titanium. Compound (I) is the active centre of the growing chain in the polymerization under the action of tetrabenzyltitanium: M (C6HsCH 2)3Ti
(C4H6).CH2C6H 5, (I)
where M indicates the point of monomer insertion. 1 The involvement of only one benzyl group in polymerization is probably due to the fact that Ti---C bonds in tetrabenzyltitanium are not equivalent. This assumption is corroborated by X-ray diffraction analysis data [6]. The authors have found that one of the benzyl groups in tetrabenzyltitanium is differently located. Compound (II) is the active centre of the growing chain in the polymerization under the action of tribenzyltitanium iodide: M (C6HsCH2)2--Ti
(C4H6),CH2C6H5.
J I
(ii) The presence of iodide ion in the reaction centre determines the cis-formation of butadiene unit inserted into the chain. Active centres (I) and (II) are
831
usual organometallic compounds with 6-bond of earbon-metal. In this respect, this process does not differ from polymerization under the action of organometallic compounds of alkali and alkali-earth metals. The active centres involving benzyl derivatives of titanium exhibit high stability. Our experiments have shown that, after the completion of polymerization under the action of tribenzyltitanium iodide, the organometallic polymer active centre retains its activity for 24hr at 25 ° towards a fresh portion of monomer. In this case, the molecular weight of the polymer increases from 175,000 to 275,000. The effective activation energy for polymerization of butadiene under the action of tribenzyltitanium iodide was determined with an initial monomer concentration of 5tool/1 and Ti = 3 x 10-3 tool/q, i.e. under the conditions where initiation is completed at about 15~ conversion. Polymerization was carried out up to ~ 40--80% conversion. The rates of the process were calculated from the steady portions of the kinetic curves. The effective activation energy was found to be 8 + 1 kcal/mol over the range from 10 to 30. The effect of electron-donor compounds on the stereospecificity of organometallic compounds of lithium and Ziegler-type catalysts in the polymerization of dienes is well known. From Table 3 it is clear that tetrahydrofuran and diethyl ether, when introduced in amounts up to 100mol/g atom of titanium, have no effect on the stereospecific action but slightly decrease the polymerization rate. For the case of tribenzyltitanium iodide, introduction of triphenylphosphine, pyridine and tetrahydrofuran in amounts of 1-10mol/g atom of titanium sharply decreases the content of 1.4-cis-units while the content of 1.2-units increases (up to 68~,i). The content of 1,2-units in the polymer practically remains constant and the content of IA-trans-units slightly increases in the presence of small amounts of diethyl ether (10mol/mol of RsTiI). Complete inversion of stereospecificity of R3TiI is observed only if diethyl ether is used as a solvent. Thus, in the presence of electron donor components the stereospecific action of RsTiI is similar to that of R+Ti.
Table 3. Effect of electron donor compounds on stereospecificity of titanium benzyl derivatives Polymerization temperature: 25-40 ° Catalyst (C~H 5CH ,)+Ti
(C6H sCH 2)sTiI
Donor (D) -Tetrahydrofu ran (C2Hs)20 Durene --
P(CeHs)s Pyridine Tetrahydrofuran Tetrahydrofuran (C2H s)20 (C2Hs)20 (C2Hs)20 * Polymerization temperature 0:.
D,rli (tool. ratio)
1,4-cis-
-100: 1 100:1 1:1
20-26
--
97
2:1 I:1 10:1 750: I t0:1 In ether In ether*
I0
20 17.5 21 24 27 17 79 21 18
Unit content (';o) 1,4-trans-
1,2-
15-26 22 18 195
56-63 68 62 63
0"5 15 15 14 15 17 20 I5
2-5 64 61 59 68 4 5q 67
832
B. A. I~3LGOPLOSK,E. I.
TINYAKOVA,O. K. SHaRAEV, I. SH. Gt;ZMAN and N. N. CmGm
Similar variations in the nature of stereoregulation under the influence of electron d o n o r c o m p o n e n t s is k n o w n for the polymerization of dienes with RLi in h y d r o c a r b o n media. This analogy is not yet sufficient to conclude that the mechanisms are identical because the disproportionation of R3TiI under the effect of electron d o n o r s still remains to be investigated for the system studied: Donor
2R3TiI
J RcTi + R2TiI,.
EXPERIMENTAL
Synthesis of organotitanium compounds Tetrabenzyltitanium was prepared by the reaction of benzylmagnesium chloride with titanium tetrachloride. The synthesis was carried out by the method described in [7] with the excess of C6HsCH2MgCI* (10-20% as compared to stoichiometry). The solution of TiCI4 in hexane was slowly added to the ether solution of benzylmagnesium chloride with efficient stirring. The temperature of the reaction mixture was maintained between - 2 0 and - 2 5 °. After the addition of the whole amount of TiCI4, the reaction mixture was agitated for 1 hr, and then the ether-hexane solution was filtered from the precipitate. The precipitate was extracted with several portions of ether which were added to the filtrate. The solvent was evaporated in. vacuum. The resulting Ti(CH2C6Hs) 4 was extracted with hexane and recrystallized. Dark-red crystals of Ti(CH2C6Hs)4 were obtained at about 65--75% of the theoretical yield. Tribenzyltitanium iodide, accordiz~ to [7], is obtained from tetrabenzyltitanium and equimolar amount of gaseous HI in toluene solution. The yield of final product is 10°/. of the theoretical value. We have developed a synthesis of tribenzyltitanium iodide by the reaction of toluene solution of (C6H~CH2hTi with benzene solution of iodine at - 2 0 °. Iodine solution in benzene, in a molar ratio of (C6HsCH2)4Ti:I2 = I:1, was slowly added to tetrabenzyltitanium solution cooled to - 2 0 °. The reaction mixture was agitated for 1 hr at this temperature, and then the solvent was evaporated in vacuum. Tribenzyltitanium iodide was recrystallized from hexane at - 7 8 ° (yield, 7075% of theory). (C6HsCH2)2TiI2 was prepared similarly using a molar ratio of (C6HsCH2)4Ti:I2 = 1:2. (CsHsCH2)3TiBr was prepared from Ti(CH2C6Hs), (in toluene) and bromine (in benzene) at - 2 0 °. All operations involved in the preparation and isolation of organotitanium compounds were carried out in argon atmosphere, as these products are very sensitive to oxygen and moisture. Analysis of titanium benzyl derivatives All organotitanium compounds were used in hydrocarbon solutions. Their concentrations in solutions were determined by the content of titanium and the number of Ti-----C bonds. In order to determine titanium content. a certain volume of the organotitanium compound solution was decomposed by aqueous alkali and then acidified with aqueous acid. Titanium was determined using Trilon B by back-titration with lead nitrate in the presence of hexamine and xylenol orange as indicator. Halide ions * In [7] the synthesis was carried out with 4:1 ratio of CbHsCH2MgX:TiCI 4. The yield of R4Ti was 400/0.
were determined by argentometric titration according to Volhard. The number of Ti---C bonds in organotitanium compounds were determined by two independent methods, viz. (I) chromatographically by the amount of toluene evolved during the hydrolysis of benzyl derivative by aqueous acid C6HsCH2--Ti~ + H2SO4 ~ CbHsCHa and (2) by the amount of iodine consumed. It was found that tetrabenzyltitanium and its halogen derivatives in aromatic hydrocarbons react with molecular iodine according to the equation: (C6HsCH,),TiX,_, + nl,--* nC6HsCH2I + TiI,X4_,, where n = 1,2,3,4. In other words, 2 g equiv, of iodine are consumed for every Ti---C bond. This reaction does not proceed in aliphatic hydrocarbons. Iodine dissolved in benzene was added to toluene solution of titanium benzyl derivatives in argon atmosphere. The mixture was maintained for 20 min at - 2 0 :, and then the unreacted iodine was titrated with sodium thiosulphate.
Polymeri'ation experiments Polymerization was carried out in ampoules or dilatometers connected to the vacuum line. The solvents and monomers were purified by procedures usually used for organometallic compounds. In investigating the consumption of benzyl groups linked with titanium during the polymerization, the reaction mixture was prepared in one "spider" type reactor and then it was poured into separate ampoules. The sealed ampoules were then placed in a thermostat and the temperature was maintained at the desired level. After cooling, the ampoules were opened and connected to the vacuum line. Non-polymerized monomer was condensed in vacuum, and then oxygen-free aqueous solution of sulphuric acid was introduced. The contents of the ampoules were agitated until the solutions were completely decolorized. The internal standard was introduced into the hydrocarbon solution and chromatographic analysis was carried out. The yield of polymer was determined after the completion of analysis. REFERENCES
I. B. A. Dolgoplosk, K. L. Makovetskii, E. I. Tinyakova and O. K. Sharaev, Polymerization of Dienes Under the Influence of n-AUyUic Complexes. Izd. Nauka, Moscow (1968). 2. B. A. Dolgoplosk and E. I. Tinyakova, I-.v. Akad. Nauk SSSR, set. Khim. No. 2, 344 (1970). 3. B. A. Dolgoplosk, E. I. Tinyakova, Int. Syrup. Macrotool. Chem., Budapest, p. 321, 1969. 4. E. I. Tinyakova, A. V. Alferov. T. G. Golenko, B. A. Dolgoplosk, I. A. Oreshkin, O. K. Sharaev, G. M. Chernenko and V. A. Yakovlev, J. Polym. Sci. C, 16, 2625 (1967). 5. I. Sh. Guzman, O. K. Sharaev. E. I. Tinyakova and B. A. Dolgoplosk, lzv. Akad. Nauk SSSR, ser. khhn. 3. 661 (19716; Dokl. Akad. Nauk SSSR 202. 1329 (1972); Dokl. Akad. Nauk ~SSR 208, 856 {1973). 6. G. R. Davies, J. A. J. Jarvis and B. T. Kilbourn, Chem. Commun. 23, 1511 (1971). 7. U. Zucchini, E. Albizzati and U. Giannini, J. Organomet. Chem. 26, 357 (1971).
NATURE OF ACTIVE CENTERS AND STEREOSPECIFICITY OF TETRAVALENT TITANIUM BENZYL DERIVATIVES IN POLYMERIZATION OF BUTADIENE B. A. DOLGOPLOSK, E. I. TINYAKOVA, O. K. SHARAEV, I. SH. GUZMAN and N. N. CHIGIR Institute of Petrochemical Synthesis, U.S.S.R. Academy of Sciences. Leninskii prospect 29, Moscow. U.S.S.R.
(Received 29 October 1974) Abstract--Stereospecificity of tetrabenzyltitanium and its halogeno-derivatives in the polymerization of butadiene has been investigated. The content of 1,2-units decreases while the content of 1,4-cis-units increases in the resulting polybutadiene for the series (C6H~CH2)4Ti, (C6H~CH2)3TiCt, (C6HsCH2)3TiBr, (C6HsCH2)3Til. Tribenzyltitanium iodide exhibits high stereospecificity for the formation of 1,4-cis-units and their content reaches 94-97°o. By determining the number of benzyl groups linked with titanium at different degrees of conversion, it has been shown that the active centre formed from tetrabenzyltitanium contains three benzyl groups and one polymer chain. Two benzyl groups. one iodine atom and one polymer chain are attached to a titanium atom in the active centre for the case of tribenzyltitanium iodide. Electron donors sharply change the stereospecificity of tribenzyltitanium iodide: the content of 1,2-units in the polymer rises to 68°°. and aliphatic solutions giving rise to high molecular Previous studies on the n-allyl complexes of nickel, products, Table I lists the data which characterize chromium and some other metals have demonstrated the stereospecificity of titanium benzyl derivatives in that the use of certain organometallic compounds as aromatic solutions. The content of 1,2-units decreases whereas the concatalyst for diene polymerization opens up wide postent of 1,4-cis-units increases in the series R4Ti, sibilities for investigating the nature of active centres R3TiCI, R3TiBr, R3RiI. In the case of R3TiI, the sysdiffering in their stereospecific action [I-4]. This paper deals with the polymerization of buta- tem becomes highly specific for the formation of 1,4-cisdiene under the action of benzyl derivatives of units and their content reaches 94-97~,. Dibenzyhitanium di-iodide is practically inactive in tetravalent titanium of general formula (C6HsCH2),TiX,,_ ., where n = 2,3,4 and X = CI. Br the polymerization. Polybutadiene formed in a small or I. Tetrabenzyltitanium was synthesized by reaction yield contains mainly 1,4-tra~s-units. The stereospeciof benzylmagnesium chloride with titanium tetra- ficity of RaTil decreases in aliphatic solvents: in polychloride. Tribenzyltianium chloride was prepared by butadiene the content of 1,4-cis-units decreases as a the reaction of tetrabenzyltitanium with equimolar result of increased content of 1,4-trans-units (Table amount of hydrogen chloride in toluene solution. Tri- 2). It cannot be excluded that higher stereospecificity benzyltitanium iodide, dibenzyhitanium di-iodide and in aromatic solvents may be due to the formation tribenzyltitanium bromide were synthesized by reac- of arene complexes of transition metals. Figure 1 shows typical kinetic curves for butadiene tion of tetrabenzyltitanium and appropriate amounts of iodine or bromine. All the products, excepting tri- polymerization in decane and o-xylene solutions with benzyltitanium chloride, were recrystallized from hex- tetrabenzyltitanium and tribenzyltitanium iodide. The ane and isolated as solids. Details regarding the syn- efficiency of tribenzyltitanium iodide at 25 ° is considerably higher than that of tetrabenzyltitanium at theses are given in the experimental section. 50 ° . The process is characterized by an induction period with duration increasing in aliphatic solvents. RESULTS AND DISCUSSION We studied the consumption of benzyl groups Tetrabenzyltitanium and its monohalogen deriva- linked with titanium as a function of the degree of tives initiate butadiene polymerization in aromatic conversion in order to find out the initiation rate and Table 1. Effect of substituent in R3TiX on stereospecificity the nature of active centres responsible for chain propagation. For this purpose, 20% aqueous solution of in butadiene polymerization in aromatic solutions [5] sulphuric acid free from oxygen was introduced into the reaction mixture at different stages of polymerizaMicrostructure of polybutadiene tion. The amount of toluene evolved during the hy(% of units) drolysis of the bonds ~ T i - - C H 2C6H s was determined Catalyst 1,4-cis1,4-trans- 1.2chromatographically (liquid phase Apiezon L, 10% on "'Spherochrom'; internal standard: iso-octane or n(C6H 5CH z)4Ti 26 15 59 deeane), (C6H sCH2hTiCI 35 19 46 The initial benzyl derivatives of titanium are hydro(C6H sCH 2)aTiBr 66 13 2I lysed with the evolution of quantitative amounts of (C6H~CH2hTil 96 2 "~ toluene: 4 and 3 moles of toluene/g atom of titanium 829 INTRODUCTION
830
B. A. DOLGOPLOSK,E. I. TINYAKOVA,O, K.
SHARAEV,
I. SH. GUZMANand N N. CHIGIR
Table 2. Effect of solvent on stereospecificity of titanium benzyl derivatives in butadiene polymerization
Catalyst
Solvent
Polymerization temp.
Content of units in polymer (%)
(C6HsCH2)4Ti
Toluene Pentane o-Xylene
50 50 50
24--26 20--27 20
(C6HsCH2)3TiI
Toluene o-Xylene Hexane Decane Pentane + o-Xylene (10:1 by vol.) Hexane + ooXylene (2:1 by vol.)
25 25 25 25
1,4-trans-
1,2-
15--26 16--19 19
56--59 57-63 61
97 96 81 80
0.5 I 15 15
2.5 3 4 5
25
91
4
5
25
94
2
4
1,4-cis-
approximately equal for R3TiI at 25 ° and R4Ti at 50 °. This suggests that the higher efficiency of the system with R3Til is a result of higher rate of chain propagation, Likewise it follows from Fig. 2 that, the higher the initial monomer to titanium ratio, the quicker is the
6C ,,: 5C
3O 2C
3"9
~0
3 36 35
Tlll~e, rnin
Fig. 1. Kinetic curves for butadiene polymerization under the action of tribenzyltitanium iodide (1-3) and tetraben° zyltitanium (4). I. [C4H~] = 3 mol/l. [Ti] -- 5 x 10 -3 mol/l, 25 °, o-xylene. 2. [C4H~] = 5 mol/I, [Ti] = 3 x 10 -3 mol/l, 25°, o-xylene. 3. [C,,H~] = 3 mol/l, [Ti] = 5 × 10 -3 tool/l, 25 °, n-decane. 4. [C4H6] = 3 mol/l, [I'i] = 7.5 x 10 -3 mol/l, 50°, o-xylene. are evolved in the case of tetrabenzyltitanium and tribenzyltitanium iodide, respectively. Polymerization of butadiene was carried out in o-xylene solution at different molar ratios of monomer to titanium. The experimental data listed in Fig. 2 clearly show that the toluene:titanium ratio gradually decreases as the process advances. The number of benzyl groups attached to the titanium atom decreases from 4 in the starting compound to 3 during the polymerization initiated by tetrabenzyltitanium (curves 1 and 2). As polymer chain propagates, the active centre retains all three benzyl groups. In the case of tribenzyltitanium iodide (curves 3 and 4), the number of benzyl groups attached to the titanium atom decreases from 3 to 2 during the initiation step and remains unchanged up to high conversions of the monomer. The induction period and an increase of the rate with time, as is evident from the data above, are due to the slow initiation reaction. The initiation rates determining the induction period of the process are
(o)
,~ 3.
~, ~
3.1
5
-
3
~ .... 7
28 ~
27
(b)
23 2"4
. ..0,.....~..,..,.,.~, ,.,0...
2J 20 19 1-8
3 4",~P -" ~~" -~ I IO
I 20
C 1 3O
x I 40
Conversion,
•
I 50
I 6O
I 70
.
%
Fig. 2. Consumption of benzyl groups linked with titanium during butadiene polymerization initiated by tetrabenzyltitanium (a) and tribenzyltitanium iodide (b). Solvent oxylene (I-6), n-decane (7). Temperature 50 (I, 2. 5) and 25 ° (3, 4, 6, 7). 1. f C , H 6 ] = 3 tool/l, [Ti] = 7.5 × 10 -3 mol/I. 2. [C4H6] = 3 mol/l. [Ti] = 5 x 10 -3 mol/l. 3. [C4H6] = 5 mol/I, [Ti] = 3 x 10 -3 mol/l. 4. [C4H6] = 5 mol/1. [Ti] = 1.5 x 10 -3 mol/I. 5. [C4H6] = 4 mol/l, [Ti] = 4 × 10-: mol/l. 6. 1 " C 4 H 6 ] = 3 moI/l. [Ti'l = 5 x 10 -3 mol/l. 7. [C,,H,] = 3 tool/1. [Til = 5 ×
10 - 3 m o t
I.
Tetravalent titanium benzyl derivatives completion of the initiation. At very high titanium concentrations (curves 5 and 6), the initiation reaction does not complete up to high monomer conversions as well as in the case of polymerization in aliphatic solvents (curve 7). The results obtained for the consumption of benzyl groups demonstrate that only one Ti-benzyl bond takes part in the polymerization reactions of butadiene with tetrabenzyititanium and tribenzyltitanium iodide. The absence of any ESR signal from the reaction mixture during the whole process of polymerization shows that titanium remains in the tetravalent state and the polymerization takes place by the insertion of monomer into the f-bond of carbon-titanium. Compound (I) is the active centre of the growing chain in the polymerization under the action of tetrabenzyltitanium: M (C6HsCH 2)3Ti
(C4H6).CH2C6H 5, (I)
where M indicates the point of monomer insertion. 1 The involvement of only one benzyl group in polymerization is probably due to the fact that Ti---C bonds in tetrabenzyltitanium are not equivalent. This assumption is corroborated by X-ray diffraction analysis data [6]. The authors have found that one of the benzyl groups in tetrabenzyltitanium is differently located. Compound (II) is the active centre of the growing chain in the polymerization under the action of tribenzyltitanium iodide: M (C6HsCH2)2--Ti
(C4H6),CH2C6H5.
J I
(ii) The presence of iodide ion in the reaction centre determines the cis-formation of butadiene unit inserted into the chain. Active centres (I) and (II) are
831
usual organometallic compounds with 6-bond of earbon-metal. In this respect, this process does not differ from polymerization under the action of organometallic compounds of alkali and alkali-earth metals. The active centres involving benzyl derivatives of titanium exhibit high stability. Our experiments have shown that, after the completion of polymerization under the action of tribenzyltitanium iodide, the organometallic polymer active centre retains its activity for 24hr at 25 ° towards a fresh portion of monomer. In this case, the molecular weight of the polymer increases from 175,000 to 275,000. The effective activation energy for polymerization of butadiene under the action of tribenzyltitanium iodide was determined with an initial monomer concentration of 5tool/1 and Ti = 3 x 10-3 tool/q, i.e. under the conditions where initiation is completed at about 15~ conversion. Polymerization was carried out up to ~ 40--80% conversion. The rates of the process were calculated from the steady portions of the kinetic curves. The effective activation energy was found to be 8 + 1 kcal/mol over the range from 10 to 30. The effect of electron-donor compounds on the stereospecificity of organometallic compounds of lithium and Ziegler-type catalysts in the polymerization of dienes is well known. From Table 3 it is clear that tetrahydrofuran and diethyl ether, when introduced in amounts up to 100mol/g atom of titanium, have no effect on the stereospecific action but slightly decrease the polymerization rate. For the case of tribenzyltitanium iodide, introduction of triphenylphosphine, pyridine and tetrahydrofuran in amounts of 1-10mol/g atom of titanium sharply decreases the content of 1.4-cis-units while the content of 1.2-units increases (up to 68~,i). The content of 1,2-units in the polymer practically remains constant and the content of IA-trans-units slightly increases in the presence of small amounts of diethyl ether (10mol/mol of RsTiI). Complete inversion of stereospecificity of R3TiI is observed only if diethyl ether is used as a solvent. Thus, in the presence of electron donor components the stereospecific action of RsTiI is similar to that of R+Ti.
Table 3. Effect of electron donor compounds on stereospecificity of titanium benzyl derivatives Polymerization temperature: 25-40 ° Catalyst (C~H 5CH ,)+Ti
(C6H sCH 2)sTiI
Donor (D) -Tetrahydrofu ran (C2Hs)20 Durene --
P(CeHs)s Pyridine Tetrahydrofuran Tetrahydrofuran (C2H s)20 (C2Hs)20 (C2Hs)20 * Polymerization temperature 0:.
D,rli (tool. ratio)
1,4-cis-
-100: 1 100:1 1:1
20-26
--
97
2:1 I:1 10:1 750: I t0:1 In ether In ether*
I0
20 17.5 21 24 27 17 79 21 18
Unit content (';o) 1,4-trans-
1,2-
15-26 22 18 195
56-63 68 62 63
0"5 15 15 14 15 17 20 I5
2-5 64 61 59 68 4 5q 67
832
B. A. I~3LGOPLOSK,E. I.
TINYAKOVA,O. K. SHaRAEV, I. SH. Gt;ZMAN and N. N. CmGm
Similar variations in the nature of stereoregulation under the influence of electron d o n o r c o m p o n e n t s is k n o w n for the polymerization of dienes with RLi in h y d r o c a r b o n media. This analogy is not yet sufficient to conclude that the mechanisms are identical because the disproportionation of R3TiI under the effect of electron d o n o r s still remains to be investigated for the system studied: Donor
2R3TiI
J RcTi + R2TiI,.
EXPERIMENTAL
Synthesis of organotitanium compounds Tetrabenzyltitanium was prepared by the reaction of benzylmagnesium chloride with titanium tetrachloride. The synthesis was carried out by the method described in [7] with the excess of C6HsCH2MgCI* (10-20% as compared to stoichiometry). The solution of TiCI4 in hexane was slowly added to the ether solution of benzylmagnesium chloride with efficient stirring. The temperature of the reaction mixture was maintained between - 2 0 and - 2 5 °. After the addition of the whole amount of TiCI4, the reaction mixture was agitated for 1 hr, and then the ether-hexane solution was filtered from the precipitate. The precipitate was extracted with several portions of ether which were added to the filtrate. The solvent was evaporated in. vacuum. The resulting Ti(CH2C6Hs) 4 was extracted with hexane and recrystallized. Dark-red crystals of Ti(CH2C6Hs)4 were obtained at about 65--75% of the theoretical yield. Tribenzyltitanium iodide, accordiz~ to [7], is obtained from tetrabenzyltitanium and equimolar amount of gaseous HI in toluene solution. The yield of final product is 10°/. of the theoretical value. We have developed a synthesis of tribenzyltitanium iodide by the reaction of toluene solution of (C6H~CH2hTi with benzene solution of iodine at - 2 0 °. Iodine solution in benzene, in a molar ratio of (C6HsCH2)4Ti:I2 = I:1, was slowly added to tetrabenzyltitanium solution cooled to - 2 0 °. The reaction mixture was agitated for 1 hr at this temperature, and then the solvent was evaporated in vacuum. Tribenzyltitanium iodide was recrystallized from hexane at - 7 8 ° (yield, 7075% of theory). (C6HsCH2)2TiI2 was prepared similarly using a molar ratio of (C6HsCH2)4Ti:I2 = 1:2. (CsHsCH2)3TiBr was prepared from Ti(CH2C6Hs), (in toluene) and bromine (in benzene) at - 2 0 °. All operations involved in the preparation and isolation of organotitanium compounds were carried out in argon atmosphere, as these products are very sensitive to oxygen and moisture. Analysis of titanium benzyl derivatives All organotitanium compounds were used in hydrocarbon solutions. Their concentrations in solutions were determined by the content of titanium and the number of Ti-----C bonds. In order to determine titanium content. a certain volume of the organotitanium compound solution was decomposed by aqueous alkali and then acidified with aqueous acid. Titanium was determined using Trilon B by back-titration with lead nitrate in the presence of hexamine and xylenol orange as indicator. Halide ions * In [7] the synthesis was carried out with 4:1 ratio of CbHsCH2MgX:TiCI 4. The yield of R4Ti was 400/0.
were determined by argentometric titration according to Volhard. The number of Ti---C bonds in organotitanium compounds were determined by two independent methods, viz. (I) chromatographically by the amount of toluene evolved during the hydrolysis of benzyl derivative by aqueous acid C6HsCH2--Ti~ + H2SO4 ~ CbHsCHa and (2) by the amount of iodine consumed. It was found that tetrabenzyltitanium and its halogen derivatives in aromatic hydrocarbons react with molecular iodine according to the equation: (C6HsCH,),TiX,_, + nl,--* nC6HsCH2I + TiI,X4_,, where n = 1,2,3,4. In other words, 2 g equiv, of iodine are consumed for every Ti---C bond. This reaction does not proceed in aliphatic hydrocarbons. Iodine dissolved in benzene was added to toluene solution of titanium benzyl derivatives in argon atmosphere. The mixture was maintained for 20 min at - 2 0 :, and then the unreacted iodine was titrated with sodium thiosulphate.
Polymeri'ation experiments Polymerization was carried out in ampoules or dilatometers connected to the vacuum line. The solvents and monomers were purified by procedures usually used for organometallic compounds. In investigating the consumption of benzyl groups linked with titanium during the polymerization, the reaction mixture was prepared in one "spider" type reactor and then it was poured into separate ampoules. The sealed ampoules were then placed in a thermostat and the temperature was maintained at the desired level. After cooling, the ampoules were opened and connected to the vacuum line. Non-polymerized monomer was condensed in vacuum, and then oxygen-free aqueous solution of sulphuric acid was introduced. The contents of the ampoules were agitated until the solutions were completely decolorized. The internal standard was introduced into the hydrocarbon solution and chromatographic analysis was carried out. The yield of polymer was determined after the completion of analysis. REFERENCES
I. B. A. Dolgoplosk, K. L. Makovetskii, E. I. Tinyakova and O. K. Sharaev, Polymerization of Dienes Under the Influence of n-AUyUic Complexes. Izd. Nauka, Moscow (1968). 2. B. A. Dolgoplosk and E. I. Tinyakova, I-.v. Akad. Nauk SSSR, set. Khim. No. 2, 344 (1970). 3. B. A. Dolgoplosk, E. I. Tinyakova, Int. Syrup. Macrotool. Chem., Budapest, p. 321, 1969. 4. E. I. Tinyakova, A. V. Alferov. T. G. Golenko, B. A. Dolgoplosk, I. A. Oreshkin, O. K. Sharaev, G. M. Chernenko and V. A. Yakovlev, J. Polym. Sci. C, 16, 2625 (1967). 5. I. Sh. Guzman, O. K. Sharaev. E. I. Tinyakova and B. A. Dolgoplosk, lzv. Akad. Nauk SSSR, ser. khhn. 3. 661 (19716; Dokl. Akad. Nauk SSSR 202. 1329 (1972); Dokl. Akad. Nauk ~SSR 208, 856 {1973). 6. G. R. Davies, J. A. J. Jarvis and B. T. Kilbourn, Chem. Commun. 23, 1511 (1971). 7. U. Zucchini, E. Albizzati and U. Giannini, J. Organomet. Chem. 26, 357 (1971).