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Distribution of active centres in Ziegler-Natta catalysts based on TiCl3 by their activity and stereospecificity
Et,ropcatl Polrm,,r Jot+mill V o l If*. pp 905 IO 970 Pergamon
Press L i d 1'4~¢1) P r i n l e d in G r e a t B r l l a m
DISTRIBUTION OF ACTIVE CENTRES IN ZIEGLER-NATTA CATALYSTS BASED ON TiCl3 BY THEIR ACTIVITY AND STEREOSPECIFICITY CATALYST POISONING L. A. RISHINAand E. I. VtZEN Institute of Chemical Physics Academy of Sciences of the USSR, Moscow B-334, U.S.S.R.
(Received 1 February 1980) Abstract--A fractional poisoning method has shown that all groups of Ziegler catalyst centres, both stereospecific and non-stereospecific, have a similar distribution of propagation rate constants. The most regular and highly molecular isotactic fraction is quite uniform by stereoregularity. Most of the poisonresistable part of active centres in ~STiCI3-AIEt3 systems have properties similar to those of initial system TiCI3-AIEt2CI,which may be caused by different adsorption abilities of co-catalysts AIEt3 and AIEt2CI at TiCI3 surface. Values np and/~p have been estimated for 6TiCI3(TACI-AIEh system.
Polypropylene (PP) made using solid complex Ziegler-Natta catalysts is non-uniform with respect to stereoregularity and molecular mass. The inhomogeneity is due to different types of active centres on the catalyst [1-6]. Conventional fractionation of isotactic polypropylene into three fractions (iso-, block-, atactic) gives semiquantitative data about stereospecific distribution of centres. According to the data on fractionation, all centres can be roughly put into three groups: highly isotactic centres (leading to a PP fraction insoluble in hot heptane), centres of intermediate stereospecificity (corresponding to the fraction soluble in hot heptane), and non-stereospecific centres producting atactic polymer. Previously we showed that active centres of Ziegler-Natta catalysts are characterized by quasicontinuous stereospecific distribution of centres [7]. Even a homogeneously stereoregular isotactic fraction is characterized by quite a broad molecular-mass distribution (MMD) [8, 9]. A mathematic model [9] was based on an exponential distribution of active centres with respect to propagation rate constants (kp). The model describes experimental observation of MMD width and shape of PP isotactic fraction changing with respect to polymerization time under the conditions of growing "living" chains. Further evidence for active centre distribution by various kinetic parameters can be obtained through fractional poisoning of polymerization centres. We have studied propylene polymerization using the catalytic system 6TiCI3(TAC-AIEt3(I), aged ~TiCI 3 (Stauffer)-AIEt3 (II) and 6TiCI3 (Stauffer)-AIEt2Cl (III) in the presence of carbon disulphide, which is an effective catalytic poison [10]. Propylene was polymerized in n-heptane at constant monomer pressure in a steel autoclave with a stirring rod according to a conventional procedure [7]. Total pressure of monomer and heptane vapours was 4.2-4.6 atm. Carbon disulphide was mixed with aluminium organic compounds. The mixture was t.Pa. i6/lO--D
placed in a reactor after storage for 15 min at 2 0 : then TiCla was introduced. i.r. Spectra of polymers were recorded with UR-20 spectrophotometer. Samples were pressed at 200 ° and then annealed in vacuum at 165° for 2 hr. Stereoregularity of polymers was determined as already described [11]. Viscosities of samples were measured in tetralin at 135°. Molecular mass was calculated from the relationship [12] [r/] = 1.91 x 10 -4 ~'i,r°'74. Gel-chromatograms of PP samples were taken on a Waters chromatograph at 135° in orthodichlorobenzene. The device was calibrated with PS-standards. The width of MMD was measured on the basis of universal calibration [13, 14].
RESULTS AND DISCUSSION The studied highly active catalytic systems including AIEt 3 (I, II) are very unstable. When using AIEt2CI (III), the decrease of velocity with time is much less marked (Fig. l). If CS2 is added to the TiCla-AIEt3 system, two effects are observed. The initial velocity of polymerization sharply drops but deactivation of the system also slows down. To determine the catalyst activity, we use the value of the initial effective constant of polymerization rate, k,ff, at 70°. Results are listed in Table I. By comparing i.r. spectra of AlEt3 and CS2 equimolar mixtures (0.1 tool/l) and the spectra of the separate substances, we found no evidence for chemical interaction between these substances at 20°. Thus, when TiCI3 is introduced in the system and active centres are produced, CS2 acts as a catalytic poison, not its complex with the aluminium-organic compound. Concentrations of CS2 were in the range 4 x 10 - 4 to 1 x 10 -2 mold, and the molar ratios CS2/TiCI3 were 0 to 1. Figure 2 shows the dependence of initial activity of system on CS,/'I'iCI3. Small CS2 additions
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the absolute amount of isotactic polymer produced within 20 min per g TiCI3 is reduced by a factor of about 8, that of atactic polymer by a factor of 1 I, and that of block-fraction by a factor of 15. Therefore, the weight percentage of iso-fraction in the polymer increases with poisoning of the catalyst from 60% at CS2/TiCI3 = 0 to 87% at Cs2/rl'iCl3 = 0.6 (Table 1). It is not possible to poison entirely non-stereospecific centres. Under the action of catalytic poison, the isotactic fractions of all three systems decreased in molecular weight (experiments 1 and 3, 8 and 11, 16--19, Table 1). The most drastic reduction of molecular weight is found for system III 6TiCI3-AIEt2CI. This is indicative of the fact that, in each system and in each group of centres, the centres show a range of activity; in the first place, those centres are poisoned which have the largest propagation rate constant. it should be noted that, no matter what amount of CS2 is introduced into the system, the quality of isotactic fraction of all studied samples does not change, i.e. stereospecific parameters of all centres remain unchanged (Table 1). Figure 5 shows gel-chromatograms of initial samples, for block- and atactic PP fractions obtained in standard catalytic systems II and III in the absence of catalytic poison. Whereas initial samples are characterized by a wide M M D (the width U = 11-13, Fig. 5a), the M M D width of block- and atactic parts
t, rain
Fig. 1. Kinetic curves for the systems: TiCI3(TACFAIEt3 (I); TiCl3lStauffer)-AIEt 3 (II); TiCI~ IStaufferFAIEt2Cl (1II). I 0 0 CS2friCl3 = 0.28; II and C 5 2 = 0 : /X A CS2/TiCl3 = 0.3; II1 H CS2 = 0, .... CS2/rI'iC13 = 1.09.
produce activation but systems I and show a sharp reduction of initial velocity in the presence of the poison; in system III, the initial activity is halved. The observed increase in the initial activity of systems including AIEt 3 causes a decreased proportion of isotactic fraction in the polymer, and an increase in the atactic and block fractions. It should be noted that, if CS2/'TiCI3 exceeds 0.3, the initial activity of systems II and III is almost unchanged; system I hardly changes its activity after CS2/TiCI3 reaches 0.1. For system III, the curve depicting change in the effective initial velocity constant is flatter. Thus, the curves of Fig. 2 show that systems I and II have two groups of centres with different activities. The first group includes centres of very high activity, easily poisoned. The second group includes centres of lower activity but greater stability. The same behaviour is displayed by the curves showing the change of effective initial propagation rate constants for the production of isotactic product iso block-polymer (k,,), ( kblock , t f ) , and atactic polymer ( ~ f ) with respect to CS2/TiCI3 (ki}~ = kc, x weight percentage of isofraction; k~, brook, k ,,, , were measured similarly) (Figs 3 and 4). Under the action of CS2 all types of catalyst centres, both stereospecific yielding an isotactic polymer and low-stereospecific, are poisoned. For low-stereospecific centres, this process is much more effective. For the system I at CS2/TiCI3 - 0.1,
250
20O
t--
(3
K30
(~5
1.0
(cs21 / (Ti%] Fig. 2. Initial activity of systems vs molar ratio CS2/"riCl3 I - O ; II--O; Ill - A .
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L. A. RISHINAand E. I, VIZEN 11 and 12, Table 1). For initial centres of systems I and II, this process is not observed due to a sharp deactivation of the catalyst restricting the chain growth. At the same time, molecular masses of polymers produced on poisoned systems are much higher than those of initial system II, despite a shorter polymerization time (experiments 6, 13, 19). One can therefore refer to a similarity between properties of unpoisoned centres of systems I and II and those of initial system II1, but not about their complete analogy. Apparently, part of the polymer in these cases is formed on unpoisoned stable centres produced from the interaction of TiCIa with AIEta. In accordance with Keii's assumptions [17], we suggest the existence of two types of active centres in c5 TiCI3-AIEI 3 systems. Centres of the first type are produced by AIEt3, and centres of the 2rid type are produced with both AIEta and AIEt2CI, which always appears in these systems as a result of alkylation. Initial activity is mostly provided by centres of the first type. These centres are not stable; they are highly active and, on average, are less stereospecific. So one can understand that the two types of centres should behave differently with respect to a catalytic poison.
IOC
(.)
i= c
(
5O
(o)
0
05
[CS2]/[Tia3]
I0
Fig. 3. Changing of initial efficient velocity constants of isotactic production vs molar ratio of CS2/TiCI3: 1--O; I I - - 0 ; lll--A.
to
40
~6 B
of the product is much lower (U = 2.5-3, Fig. 5b). At 20 o the PP atactic fraction has a wide MMD (U = 7, Fig. 5c), which also indicates non-uniformity of all types of centres in the stereospecific polymerization of propylene. The observed narrowing of MMD of the atactic fraction with rising temperature is due to higher reactivity of iow-stereospecific centres for linear terminations of chain growth. Interesting results are found when comparing the data on poisoning of systems with AIEta and AIEt2Cl (in both cases 6TiCla (Stauffer) was used as the catalyst). As shown in Figs 1-3, unpoisoned systems (I and III) have very different stabilities of activity; poisonccl systems become very similar and are rather stable (Fig. 1), and they have the same effective propagation rate constant for producing isotactic polymers (Fig. 3). In the system lII, stable unpoisoned centres provide half of the initial catalyst activity; for the systems I and II, the contribution of such centres in the initial activity is only g-10~. Other data also show that properties of unpoisoned ¢¢ntres of systems I and II are similar to those of the centres of initial system IIl. Molecular mass of the polymer for both poisoned systems and the initial system III increases significantly with time up to considerable polymerization times (experiments 4 and 5,
,b [CS2]/[T~I~]
I-
4C
~
20
(b}
o'5 I [CS2] / [TiCI 3]
1'O
Fig. 4. Changing of initial effective velocity constants for production of block-polymer and atactic polymers vs molar ratio of CS2friCl3: I(a) and ll(b) O---atactic polymer; &--block-polymer.
Active centres in Ziegler-Natta catalysts
969
(b)
X
..1"
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/
6'o
5o
~o
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Fig. 5. (a) Cel-chromatograms of initial polypropylene samples produced from systems II ( ) and III (. . . . . ). (b) cel-chromatograms of block ( - - - ) and atactic ( ) polymers produced from system II. (c) Cel-chromatograms of atactic polymers produced from ctTiCl3-AIEt3system at 20° (. . . . . ) and at 70 ~ ( ).
Centres of the first type would be almost entirely poisoned when small amounts of CS2 are introduced. Centres of the second type are poisoned with greater difficulty, perhaps because of a higher adsorption ability of AIEt2CI in comparison with AIEt3 at the surface. Using the data obtained on poisoning of the system ~TiCI3 (TAC)--AIEt3 (Fig. 2, Table 1), we tried to estimate the number of initial active centres n r and the averaged (according to activity of centres) constant for chain propagation (/~p). As noted above, at small values of CS2/TiCI3, the system is activated• Since molecular masses of isotactic polymer change slightly when the system is activated (experiments 1, 2, Table 1), increase in the initial effective rate constant can be due to an increase in the
number of active centres. Such activation of the system can probably result from interaction of an electron donor CS2 with A1CI3 included in the titanium component of the catalyst. This can lead to removal of AICI3 from TiCI3 lattice, and the de-agglomeration of catalytic particles. The modification of catalytic systems at low concentrations of donor compounds was observed by other authors [15, 16]. After activation, a small increase in the amount of CS2 strongly inhibits polymerization (experiments 2 and 3). One can assume that CS2 is adsorbed at the TiCI3 surface almost irreversibly i.e. with a high equilibrium constant. The quantity of CS2 responsible for reduction of velocity in this part of the curve corresponds to the amount of active centres at the point of maximum
L. A. Rlsm~A and E. I. VlzFxq
970
activity of the system. If we make allowance for 20% of this value being due to the centres arising at activation, then the initial number of centres np would be about 0.03 tool per mol TiCts, and the propagation constant,/~p = 18 I/tool. sec. Such an estimate np and kp does not allow for centres of the 2nd type, which provide the final activity of the system. As mentioned above, the contribution of this part of the centres to the initial activity is small, corresponding to about 8%. Therefore, the estimated values of np and kp may serve as kinetic characteristics of the whole system. For the systems II and III, the curves of "poisoning" have a smooth slope indicating an equilibrium process at the surface of TiCla. Such an estimate of n~ and/~p, therefore, involves large errors in these cases.
REFERENCES
I. G. Natta, J. Polym. Sci. 34, 21 (1959). 2. A. Clark and G. C. Baily, J. Catalys. 2, 230 (1963). 3. C. Cosewith and G. Verstrate, Makromolecules 4, 482 (1971).
4. W. R. Shmeal and J. R. Street, J. Polym Sci. A-2 10, 2173 (1972). 5. L. L. B6hm, Polymer XIX, 562 (1978). 6. J. Boor Jr, J. Polym. Sci. Cl, 257 (1963); J. Boor Jr, J. Polym. Sci. A3, 995 (1965). 7. L. A. Rishina, E. I. Vizen and F. S. Dyachkovsky, Eur. Polym. J. 15, 93 (1979). 8. Y. Doi and T. Keii, Makromolek. Chem. 179, 2117 (1978). 9. E. I. Vizen and F. I. Yakobson, Highly Molec. Compounds, XX, 927 (1978) (in Russian). 10. K. Vesely, J. Ambroz, R. Vilim and O. Hamrik, International Symposium on Macromolecular Chemistry, Moscow, section II (1960). I I . . I . P . Luonga, J. appl. Polym. Sci. 3, 302 (1960). 12. G. Moraglio, Chim. Ind. 41, l0 (1959). 13. H. Benoit, Z. Crubisic and P. Rempp, J. Polym. Sci. BS, 753 (1967). 14. H. Coil and D. K. Gilding, J. Poly~ Sci. A-2 8, 89 (1970). 15. O. N. Pirogov and N. M. Chirkov, Highly Molec. Compounds 7, 491 (1965) (in Russian); O. N. Pirogov and N. M. Chirkov, ibid. 8, 1798 (1966). 16. T. Keii, Kinetics of Ziegler-Natta Polymerization. Kodansha, Tokyo (1972). 17. T. Keii, Coordination Polymerization. Academic Press, New York (1975).
Press L i d 1'4~¢1) P r i n l e d in G r e a t B r l l a m
DISTRIBUTION OF ACTIVE CENTRES IN ZIEGLER-NATTA CATALYSTS BASED ON TiCl3 BY THEIR ACTIVITY AND STEREOSPECIFICITY CATALYST POISONING L. A. RISHINAand E. I. VtZEN Institute of Chemical Physics Academy of Sciences of the USSR, Moscow B-334, U.S.S.R.
(Received 1 February 1980) Abstract--A fractional poisoning method has shown that all groups of Ziegler catalyst centres, both stereospecific and non-stereospecific, have a similar distribution of propagation rate constants. The most regular and highly molecular isotactic fraction is quite uniform by stereoregularity. Most of the poisonresistable part of active centres in ~STiCI3-AIEt3 systems have properties similar to those of initial system TiCI3-AIEt2CI,which may be caused by different adsorption abilities of co-catalysts AIEt3 and AIEt2CI at TiCI3 surface. Values np and/~p have been estimated for 6TiCI3(TACI-AIEh system.
Polypropylene (PP) made using solid complex Ziegler-Natta catalysts is non-uniform with respect to stereoregularity and molecular mass. The inhomogeneity is due to different types of active centres on the catalyst [1-6]. Conventional fractionation of isotactic polypropylene into three fractions (iso-, block-, atactic) gives semiquantitative data about stereospecific distribution of centres. According to the data on fractionation, all centres can be roughly put into three groups: highly isotactic centres (leading to a PP fraction insoluble in hot heptane), centres of intermediate stereospecificity (corresponding to the fraction soluble in hot heptane), and non-stereospecific centres producting atactic polymer. Previously we showed that active centres of Ziegler-Natta catalysts are characterized by quasicontinuous stereospecific distribution of centres [7]. Even a homogeneously stereoregular isotactic fraction is characterized by quite a broad molecular-mass distribution (MMD) [8, 9]. A mathematic model [9] was based on an exponential distribution of active centres with respect to propagation rate constants (kp). The model describes experimental observation of MMD width and shape of PP isotactic fraction changing with respect to polymerization time under the conditions of growing "living" chains. Further evidence for active centre distribution by various kinetic parameters can be obtained through fractional poisoning of polymerization centres. We have studied propylene polymerization using the catalytic system 6TiCI3(TAC-AIEt3(I), aged ~TiCI 3 (Stauffer)-AIEt3 (II) and 6TiCI3 (Stauffer)-AIEt2Cl (III) in the presence of carbon disulphide, which is an effective catalytic poison [10]. Propylene was polymerized in n-heptane at constant monomer pressure in a steel autoclave with a stirring rod according to a conventional procedure [7]. Total pressure of monomer and heptane vapours was 4.2-4.6 atm. Carbon disulphide was mixed with aluminium organic compounds. The mixture was t.Pa. i6/lO--D
placed in a reactor after storage for 15 min at 2 0 : then TiCla was introduced. i.r. Spectra of polymers were recorded with UR-20 spectrophotometer. Samples were pressed at 200 ° and then annealed in vacuum at 165° for 2 hr. Stereoregularity of polymers was determined as already described [11]. Viscosities of samples were measured in tetralin at 135°. Molecular mass was calculated from the relationship [12] [r/] = 1.91 x 10 -4 ~'i,r°'74. Gel-chromatograms of PP samples were taken on a Waters chromatograph at 135° in orthodichlorobenzene. The device was calibrated with PS-standards. The width of MMD was measured on the basis of universal calibration [13, 14].
RESULTS AND DISCUSSION The studied highly active catalytic systems including AIEt 3 (I, II) are very unstable. When using AIEt2CI (III), the decrease of velocity with time is much less marked (Fig. l). If CS2 is added to the TiCla-AIEt3 system, two effects are observed. The initial velocity of polymerization sharply drops but deactivation of the system also slows down. To determine the catalyst activity, we use the value of the initial effective constant of polymerization rate, k,ff, at 70°. Results are listed in Table I. By comparing i.r. spectra of AlEt3 and CS2 equimolar mixtures (0.1 tool/l) and the spectra of the separate substances, we found no evidence for chemical interaction between these substances at 20°. Thus, when TiCI3 is introduced in the system and active centres are produced, CS2 acts as a catalytic poison, not its complex with the aluminium-organic compound. Concentrations of CS2 were in the range 4 x 10 - 4 to 1 x 10 -2 mold, and the molar ratios CS2/TiCI3 were 0 to 1. Figure 2 shows the dependence of initial activity of system on CS,/'I'iCI3. Small CS2 additions
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40
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the absolute amount of isotactic polymer produced within 20 min per g TiCI3 is reduced by a factor of about 8, that of atactic polymer by a factor of 1 I, and that of block-fraction by a factor of 15. Therefore, the weight percentage of iso-fraction in the polymer increases with poisoning of the catalyst from 60% at CS2/TiCI3 = 0 to 87% at Cs2/rl'iCl3 = 0.6 (Table 1). It is not possible to poison entirely non-stereospecific centres. Under the action of catalytic poison, the isotactic fractions of all three systems decreased in molecular weight (experiments 1 and 3, 8 and 11, 16--19, Table 1). The most drastic reduction of molecular weight is found for system III 6TiCI3-AIEt2CI. This is indicative of the fact that, in each system and in each group of centres, the centres show a range of activity; in the first place, those centres are poisoned which have the largest propagation rate constant. it should be noted that, no matter what amount of CS2 is introduced into the system, the quality of isotactic fraction of all studied samples does not change, i.e. stereospecific parameters of all centres remain unchanged (Table 1). Figure 5 shows gel-chromatograms of initial samples, for block- and atactic PP fractions obtained in standard catalytic systems II and III in the absence of catalytic poison. Whereas initial samples are characterized by a wide M M D (the width U = 11-13, Fig. 5a), the M M D width of block- and atactic parts
t, rain
Fig. 1. Kinetic curves for the systems: TiCI3(TACFAIEt3 (I); TiCl3lStauffer)-AIEt 3 (II); TiCI~ IStaufferFAIEt2Cl (1II). I 0 0 CS2friCl3 = 0.28; II and C 5 2 = 0 : /X A CS2/TiCl3 = 0.3; II1 H CS2 = 0, .... CS2/rI'iC13 = 1.09.
produce activation but systems I and show a sharp reduction of initial velocity in the presence of the poison; in system III, the initial activity is halved. The observed increase in the initial activity of systems including AIEt 3 causes a decreased proportion of isotactic fraction in the polymer, and an increase in the atactic and block fractions. It should be noted that, if CS2/'TiCI3 exceeds 0.3, the initial activity of systems II and III is almost unchanged; system I hardly changes its activity after CS2/TiCI3 reaches 0.1. For system III, the curve depicting change in the effective initial velocity constant is flatter. Thus, the curves of Fig. 2 show that systems I and II have two groups of centres with different activities. The first group includes centres of very high activity, easily poisoned. The second group includes centres of lower activity but greater stability. The same behaviour is displayed by the curves showing the change of effective initial propagation rate constants for the production of isotactic product iso block-polymer (k,,), ( kblock , t f ) , and atactic polymer ( ~ f ) with respect to CS2/TiCI3 (ki}~ = kc, x weight percentage of isofraction; k~, brook, k ,,, , were measured similarly) (Figs 3 and 4). Under the action of CS2 all types of catalyst centres, both stereospecific yielding an isotactic polymer and low-stereospecific, are poisoned. For low-stereospecific centres, this process is much more effective. For the system I at CS2/TiCI3 - 0.1,
250
20O
t--
(3
K30
(~5
1.0
(cs21 / (Ti%] Fig. 2. Initial activity of systems vs molar ratio CS2/"riCl3 I - O ; II--O; Ill - A .
Active centres in Z i e g l e r - N a t t a c a t a l y s t s
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L. A. RISHINAand E. I, VIZEN 11 and 12, Table 1). For initial centres of systems I and II, this process is not observed due to a sharp deactivation of the catalyst restricting the chain growth. At the same time, molecular masses of polymers produced on poisoned systems are much higher than those of initial system II, despite a shorter polymerization time (experiments 6, 13, 19). One can therefore refer to a similarity between properties of unpoisoned centres of systems I and II and those of initial system II1, but not about their complete analogy. Apparently, part of the polymer in these cases is formed on unpoisoned stable centres produced from the interaction of TiCIa with AIEta. In accordance with Keii's assumptions [17], we suggest the existence of two types of active centres in c5 TiCI3-AIEI 3 systems. Centres of the first type are produced by AIEt3, and centres of the 2rid type are produced with both AIEta and AIEt2CI, which always appears in these systems as a result of alkylation. Initial activity is mostly provided by centres of the first type. These centres are not stable; they are highly active and, on average, are less stereospecific. So one can understand that the two types of centres should behave differently with respect to a catalytic poison.
IOC
(.)
i= c
(
5O
(o)
0
05
[CS2]/[Tia3]
I0
Fig. 3. Changing of initial efficient velocity constants of isotactic production vs molar ratio of CS2/TiCI3: 1--O; I I - - 0 ; lll--A.
to
40
~6 B
of the product is much lower (U = 2.5-3, Fig. 5b). At 20 o the PP atactic fraction has a wide MMD (U = 7, Fig. 5c), which also indicates non-uniformity of all types of centres in the stereospecific polymerization of propylene. The observed narrowing of MMD of the atactic fraction with rising temperature is due to higher reactivity of iow-stereospecific centres for linear terminations of chain growth. Interesting results are found when comparing the data on poisoning of systems with AIEta and AIEt2Cl (in both cases 6TiCla (Stauffer) was used as the catalyst). As shown in Figs 1-3, unpoisoned systems (I and III) have very different stabilities of activity; poisonccl systems become very similar and are rather stable (Fig. 1), and they have the same effective propagation rate constant for producing isotactic polymers (Fig. 3). In the system lII, stable unpoisoned centres provide half of the initial catalyst activity; for the systems I and II, the contribution of such centres in the initial activity is only g-10~. Other data also show that properties of unpoisoned ¢¢ntres of systems I and II are similar to those of the centres of initial system IIl. Molecular mass of the polymer for both poisoned systems and the initial system III increases significantly with time up to considerable polymerization times (experiments 4 and 5,
,b [CS2]/[T~I~]
I-
4C
~
20
(b}
o'5 I [CS2] / [TiCI 3]
1'O
Fig. 4. Changing of initial effective velocity constants for production of block-polymer and atactic polymers vs molar ratio of CS2friCl3: I(a) and ll(b) O---atactic polymer; &--block-polymer.
Active centres in Ziegler-Natta catalysts
969
(b)
X
..1"
X',
/ ./"
/
6'o
5o
~o
log mm
Fig. 5. (a) Cel-chromatograms of initial polypropylene samples produced from systems II ( ) and III (. . . . . ). (b) cel-chromatograms of block ( - - - ) and atactic ( ) polymers produced from system II. (c) Cel-chromatograms of atactic polymers produced from ctTiCl3-AIEt3system at 20° (. . . . . ) and at 70 ~ ( ).
Centres of the first type would be almost entirely poisoned when small amounts of CS2 are introduced. Centres of the second type are poisoned with greater difficulty, perhaps because of a higher adsorption ability of AIEt2CI in comparison with AIEt3 at the surface. Using the data obtained on poisoning of the system ~TiCI3 (TAC)--AIEt3 (Fig. 2, Table 1), we tried to estimate the number of initial active centres n r and the averaged (according to activity of centres) constant for chain propagation (/~p). As noted above, at small values of CS2/TiCI3, the system is activated• Since molecular masses of isotactic polymer change slightly when the system is activated (experiments 1, 2, Table 1), increase in the initial effective rate constant can be due to an increase in the
number of active centres. Such activation of the system can probably result from interaction of an electron donor CS2 with A1CI3 included in the titanium component of the catalyst. This can lead to removal of AICI3 from TiCI3 lattice, and the de-agglomeration of catalytic particles. The modification of catalytic systems at low concentrations of donor compounds was observed by other authors [15, 16]. After activation, a small increase in the amount of CS2 strongly inhibits polymerization (experiments 2 and 3). One can assume that CS2 is adsorbed at the TiCI3 surface almost irreversibly i.e. with a high equilibrium constant. The quantity of CS2 responsible for reduction of velocity in this part of the curve corresponds to the amount of active centres at the point of maximum
L. A. Rlsm~A and E. I. VlzFxq
970
activity of the system. If we make allowance for 20% of this value being due to the centres arising at activation, then the initial number of centres np would be about 0.03 tool per mol TiCts, and the propagation constant,/~p = 18 I/tool. sec. Such an estimate np and kp does not allow for centres of the 2nd type, which provide the final activity of the system. As mentioned above, the contribution of this part of the centres to the initial activity is small, corresponding to about 8%. Therefore, the estimated values of np and kp may serve as kinetic characteristics of the whole system. For the systems II and III, the curves of "poisoning" have a smooth slope indicating an equilibrium process at the surface of TiCla. Such an estimate of n~ and/~p, therefore, involves large errors in these cases.
REFERENCES
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4. W. R. Shmeal and J. R. Street, J. Polym Sci. A-2 10, 2173 (1972). 5. L. L. B6hm, Polymer XIX, 562 (1978). 6. J. Boor Jr, J. Polym. Sci. Cl, 257 (1963); J. Boor Jr, J. Polym. Sci. A3, 995 (1965). 7. L. A. Rishina, E. I. Vizen and F. S. Dyachkovsky, Eur. Polym. J. 15, 93 (1979). 8. Y. Doi and T. Keii, Makromolek. Chem. 179, 2117 (1978). 9. E. I. Vizen and F. I. Yakobson, Highly Molec. Compounds, XX, 927 (1978) (in Russian). 10. K. Vesely, J. Ambroz, R. Vilim and O. Hamrik, International Symposium on Macromolecular Chemistry, Moscow, section II (1960). I I . . I . P . Luonga, J. appl. Polym. Sci. 3, 302 (1960). 12. G. Moraglio, Chim. Ind. 41, l0 (1959). 13. H. Benoit, Z. Crubisic and P. Rempp, J. Polym. Sci. BS, 753 (1967). 14. H. Coil and D. K. Gilding, J. Poly~ Sci. A-2 8, 89 (1970). 15. O. N. Pirogov and N. M. Chirkov, Highly Molec. Compounds 7, 491 (1965) (in Russian); O. N. Pirogov and N. M. Chirkov, ibid. 8, 1798 (1966). 16. T. Keii, Kinetics of Ziegler-Natta Polymerization. Kodansha, Tokyo (1972). 17. T. Keii, Coordination Polymerization. Academic Press, New York (1975).