The stereospecific mechanism of olefin polymerization by heterogeneous complex catalysts—II. The model of the stereospecific polymerization site

European Polymer Journal, 1970, Vol. 6, pp. 525-535. Pergamon Press. Printed in England.

THE STEREOSPECIFIC MECHANISM OF OLEFIN POLYMERIZATION BY HETEROGENEOUS COMPLEX CATALYSTS--II T H E M O D E L OF T H E S T E R E O S P E C I F I C P O L Y M E R I Z A T I O N SITE Yu. V. KJSSIN and N. M. CmRKOV Polymer Department, Institute of Chemical Physics, Academy of Sciences U.S.S.R., Vorobyevskoy Shausee 2-b, Moscow, U.S.S.R.

(Received 12 April 1969) Abstract--There is further consideration of the model of the active site of polymerization on heterogeneous Ziegler-Natta catalysts. It is located on basal planes of lattice crystals of type a-TiCl3 or VCI3 and has two vacancies at the transition metal atom. One of these vacancies is blocked by the helical polymer chain and the other is available for olefin co-ordination. The study of the action of this model shows two types of stereospccific control over monomer co-ordination: by the growing helical polymer chain and by neighbouring transition metal atoms. Some kinetic aspects of olefin polymerization with preliminary co-ordination are discussed. INTRODUCTION QUESTIONS about the position and nature of active sites on the catalyst surface in heterogeneous systems of the type TiCI3-A1Et3 examined in the previous paper. Experimental data show that active sites cover practically all the surface of the heterogeneous part of the catalytic system and consequently are placed mostly on the basal faces of TIC13 crystals, because the area of these faces is a predominant part ( ~ 95 per cent) of the total catalyst area. The active sites are products of the reaction between TIC13 and aluminium-organic compounds. Accoridng to experimental studies of the reaction between transition metal halides and aluminium-organic compounds and to theoretical ideas about the olefin insertion in the transition metal-carbon bond, ~1~one can suppose that the active site for olefin polymerization is a complex compound of transition metal (usually Ti or V). This complex contains a metal-carbon bond, produced after the removal of chlorine atoms from basal faces of transition metal halides by means of an alkylation reaction. There is a strong bond between these complexes and the catalyst surface. Polymerization of olefins proceeds as an insertion reaction in a weak metal-carbon bond. According to experimental data, aluminium-organic compounds have a great influence on the activity of the site. In this paper, we give a model of the active site for olefin polymerization on heterogeneous Ziegler-Natta catalysts and describe two possible mechanisms for the stereospecific action of these sites. We examine also some kinetic aspects of the model. T H E S T R U C T U R E OF P O L Y M E R I Z A T I O N A C T I V E SITES First it is necessary to point out that practically all present ideas about active sites are speculative. Models must meet the following requirements: they must not contradict experimental data on polymerization with complex catalysts, they must explain adequately the stereospecific action of these centres. 525

526

YU. V. KISSIN and N. M. CHIRKOV

The model presented below meets these requirements. As was mentioned above, the reaction between transition metal chloride and AIR3 results in partial or complete removal of chlorine atoms from the surface of the crystal. The alkyl groups which replace chlorine atoms also may leave the surface due to dealkylation reactions. When these rapid processes are over, there are two possible types of groups on the TiCI3 surface, which satisfy the conditions for a potentially active site, viz. the existence of a Ti-C bond and the existence of at least one vacancy at the Ti atom for olefin co-ordination (at this stage we do not take into account complexes with aluminiumorganic compounds. The role of such complexes will be discussed later). These two groups (potential polymerization active sites) are shown in Figs. la and lb. On these figures, as in others, chlorine atoms are represented by large balls, titanium atoms by cubes [corresponding to octahedral configuration of Ti(III) complexes], and carbon and hydrogen atoms by the standard Steward models. The site model in Fig. la is practically the same as the Arlman-Cossee active site. ('-~ Its structural formula is TiCI,Et and it has one vacancy at the Ti atom (shown by the arrow). The only difference between the site in Fig. la and the Arlman-Cossee site is that the former is placed on the basal plane of TiCl3 while the latter is on the lateral face; further one of the chlorine atoms situated "under" the Ti atom has, in the second case, a sterically different position from that of the two other chlorine atoms in respect of the catalyst surface (see Fig. 1 on page 101 of Ref. 2). The other site model, which can possibly exist on the TiCI3 basal surface after the removal of chlorine atoms, is presented in Fig. lb. Its structural formula is TiCI3Et and it has two vacancies at the Ti atom (shown by arrows). As was mentioned above, we suppose that complexes in Figs. la and lb can be potential active sites for olefin polymerization. Let us examine both these sites in the stationary state, i.e. at the time when they are linking with polymer chains of sufficient length. The following points must be considered: (I) In accordance with the co-ordination-anionic mechanism of catalysis, the metal atom must be bonded with the CH2-group of the first monomer unit in the polymer chain. (2) Most isotactic polyolefins (polypropylene, polystyrene, polybutene-1, poly-3methylbutene-1, etc.) and polyethylene under the conditions of polymerization (50-80 °, solvents--normal or branched hydrocarbons C3-Cto) do not dissolve in the polymerization media and form crystalline blocks. (3) The predominant configuration of isotactic macromolecules is a helix (for the first three polymers, helix 31 ; for poly-3-methylbutene-1, helix 4~) and the predominant conformation is T G T G . One can assume that the helix configuration is maintained in the vicinity of the active site. (4) The distance between the transition metal in the active site and the co-ordinated olefin must be not less than ~ 3 A. Figures 2a and 2b show the same active site models, as in Figs. la and lb, but titanium atoms are connected with polymer chains (polypropylene) but not with ethyl groups. The models in Figs. 2a and 2b are constructed in accord with the rules mentioned above; they are built up in such a manner that polymer chains are approximately perpendicular to the catalyst surface and the centres of helixes are situated almost above the transition metal atoms. Examination of the active site of Fig. 2a, similar to the Arlman-Cossee centre, c2)

i .._j,

(a)

....

,,J

(b)

F:G. 1. Models of the potential active sites of polymerization connected with ethyl groups. (a) Site with one ,,acancy, (b) Site with t~o ',acancies.

(a)

(b)

FIG. 2. Models of active sites of polymerization connected withisotacticpolypropylene chains. (a) Site with one vacancy, (b) Site with two vacancies.

[fx:-r.g p. 5261

(a)

{b}

FIG. 3. Scheme of propylene co-ordination on an active site with two vacancies. (a) Co-ordination leading to isotactic monomer addition, (b) Co-ordination leading to syndiotactic monomer addition.

;

U

iii,

--

FIG. 4. Molecular models used in measuring the distances between all ato~.s of the polymer chain and the approaching monomer.

StereospecificMechanism of Olefin Polymerization by Heterogeneous Complex Catalysts--II 527 shows that the access to the only vacancy at the Ti atom (shown by the arrow) is largely blocked by CH3 and CH., groups of the second propylene unit in the polymer chain (counting from Ti atom). This effect is increased when the polypropylene molecule is replaced by molecules of polystyrene or polybutene-1. The models show also that this blocking cannot be avoided by rotation of T i - - C or C - - C bonds. This rotation of polymer chain is generally limited in such situations because both the ends of the chain are rigidly connected, one with the surface of catalyst and the other is included in the polymer crystallite close to the active siteJ ~. s) It is evident that the blocking of the only vacancy at the Ti atom decreases the opportunity for olefin to co-ordinate on this vacancy in the Arlman-Cossee model. It must be pointed out that previous authors (2) do not consider the possibility of the vacancy being blocked by a polymer molecule. In their model the hydrocarbon group at Ti atoms is represented by a ball of radius 2 A. This approximation is satisfactory in the case of the methyl group, but it is unrealistic for a polymer molecule of complex configuration. The active site model proposed by Rodriguez t6) also has one vacancy at the Ti atom. Examination of the model shows that in this case the vacancy also is blocked ; this blocking is increased by the aluminium compound being bonded to the Ti complex. Figure 2b shows the model of the second active site which can be proposed. This model is similar to the model of Fig. Ib, but here the Ti atom carries the polypropylene molecule instead of an ethyl group. This centre has two vacancies. One of them is strongly blocked by the CH2 group of the second monomer unit of macromolecule as for the Arlman-Cossee centre (Fig. 2a). (This vacancy is indicated by arrow 1.) The second vacancy is free and olefin can easily co-ordinate to it (arrow 2). Figures 2a and 2b demonstrate also that polymer molecules cannot be connected with neighbouring Ti atoms situated near the active site (this feature was pointed out in the previous paper) and only small molecules can be co-ordinated with these Ti atoms. We suppose that the most probable structure of the active site for olefin polymerization can be represented by the models shown in Figs. la and 2b, and the transition metal atom in this site has two vacancies. We shall show that such a site has two stereocontrol mechanisms in the course of olefin polymerization. T H E SCHEME OF T H E STEREOSPECIFIC G R O W T H OF A P O L Y M E R C H A I N AT T H E ACTIVE SITE W I T H TWO VACANCIES In accordance with theoretical proposals C~) and with experimental data, C7-~°) the insertion reaction for an olefin molecule at the metal--carbon bond of the active site has the following features: (1) The co-ordination of olefin with transition metal is a preliminary step of the insertion. The co-ordinated olefin is bonded to the metal atom by ~-electrons of the olefin molecule and d-orbitals of the metal. (2) In the co-ordinated state, the C=C---bond of the olefin must be parallel to the metal-carbon bond in the active site. (3) During the insertion, the C = C bond of the olefin is opened in a cis-position. Consider the addition of olefin to the polymer molecule, growing at the active site with two vacancies.

528

YU. V. KISSIN and N. M. CHIRKOV

If the oletin molecule is co-ordinated at the free vacancy in the active site on Fig. 2b (this vacancy is shown by arrow 2) in such a way that the C = C bond is parallel to the metal-carbon bond, four geometrically different positions of the co-ordinated monomer can be visualized. In two of them, the propylene molecule is placed with methyl group "downwards". In these cases, "head to head" addition must take place. It is evident that, in such an arrangement of the propylene molecule, strong repulsion must be observed between the atoms on the catalyst surface and the methyl group. For this reason "head to head" addition of monomer is hardly possible during heterogeneous catalysis. In two other arrangements of the propylene molecule on the vacancy, it can be sited with the methyl group "upwards" (relative to the catalyst surface); both are presented in Figs. 3a and 3b. Figure 3a demonstrates the type of olefin co-ordination which corresponds to isotatic addition of propylene to the chain. The insertion proceeds thus: after the breaking of the Ti--C bond and C---C bond of the olefin and the formation of a new Ti--C bond and a C--C bond between the olefin molecule and the chain, the polymer molecule appears to be connected with the transition metal atom at the site of the former free vacancy. During this process, the polymer chain moves away from the catalyst surface. When the insertion is complete, the methyl group of the added molecule has the same position, with respect to the polymer chain, as the methyl group marked 1 in Fig. 3a (these two methyl groups are separated by one turn of the helix). At this moment, the CH2 group of the second unit in the chain (marked 2 in Fig. 3a) as a result of the removal from the surface frees the previously blocked vacancy. Finally breaking of the bond between the Ti atom and the polymer molecule during the insertion results in the appearance of a new vacancy. This vacancy is however blocked by the CH~ group which was at the end of the polymer chain before the insertion. Thus, after the addition of monomer to the chain, the structure of the active site is completely regenerated: it has again one Ti--C bond and two vacancies, one of which is blocked and the other is free. The positions of the Ti--C bond and these vacancies are changed with respect to the catalyst surface: all of them turn through 120". During the insertion of the next propylene molecule, a further "turn" of the Ti--C bond and of the positions of vacancies takes place, etc. The active site of Fig. 3a therefore has three positions in contrast with the Cossee model which has only two positions (Fig. 2a). The structure of the centre proposed by us is the same for all positions. Figure 3b demonstrates the type of olefin co-ordination corresponding to the syndiotactic addition of monomer. It is evident from Fig. 3b that in this case CH3 and CH2 groups of the third unit in the polymer molecule prevent the approach of a propylene molecule to the active site, because they block the path for the methyl group of the propylene molecule. Moreover a repulsion occurs between methyl group of the approaching molecule and the CH and CH2 groups of the first unit and the CH2 group of the second unit; this repulsion begins when the distance between the Ti atom and olefin is approximately 3 A. This repulsion must result in addition to the syndio-position being less favoured than the iso-addition.

StereospecificMechanism of Olefin Polymerization by Heterogeneous Complex Catalysts--II 529 This conclusion can be proved semiquantitatively by calculation of the steric interaction energy for approach of a propylene molecule to the polymer chain in iso- and syndio-positions. For this purpose, the method of atom-atom potentials developed by Kytaigorodski and Myrskaya c11~ can be used. This method allows an estimate of the interaction energy between nonbonded atoms in different structures. In accordance with the atom-atom potentials method, the interaction energy Ulj for i and j atoms is approximately Utj = - - A / r ~ -- Be -~" (r being the distance between the atoms). The first term of this expression corresponds to the attraction between the atoms, and the second member to the repulsion. The values of the constants A, B and a depend on the types of interacting atoms. We use the following values: for interaction of two C atoms

for interaction of two H atoms

A = B = c~ = A=

358 kcal. A6/mol 4.2 × 10" kcal/mol 3"58 A -1 57, B = 4.2 × 10*, a = 4.86

for interaction of C and H atoms A = 154, B = 4.2 × 10~, ct -----4.12 The distances between all the atoms of the helical polypropylene chain (4 units) and the propylene molecule, approaching the centre in iso- and syndio-positions, were measured with special models (Fig. 4) for various distances between the monomer molecule and the active site. Precision of the measurements is ~ 0.1 A. Figure 4 shows the models in the position where the propylene molecule is 3.5 A from the metal atom and in the iso-position in respect to the last unit of the chain. For the calculations, the C = C bond was taken as parallel to the metal-carbon bond at all distances. The steric interaction energies were calculated also for the polymer chain and the cis-"opened" propylene molecule in iso- and syndio-positions. In this case, all angles taken as tetrahedral and the distances between the metal atom and "opened" monomer molecule were taken as between 2.75 and 2.5 A. The results of the calculations are given in Fig. 5. These results were found to depend significantly on the rotation angle of the polymer molecule with respect to the metal-carbon bond. The data, presented in Fig. 5 were obtained for the minimum repulsion between the polymer and the olefin both in iso- and syndio-positions. It is evident from Fig. 5 that the approach o f a propylene molecule to the active site in the iso-position is more advantageous than in the syndio-position, especially at short distances between the monomer and the active site. Cis-opening of the olefin double bond and change of angles result in a decrease of the repulsion energy. But in this case also, the iso-position of monomer is more favourable than the syndio. It is supposed that, during the stereospecific polymerization of olefins with alkyl groups, any specifical electronic interactions between these groups and the polymer can be neglected, and that the preference for iso-addition can be explained in terms of steric factors only. The relation .Kiso/f~srndlo = e £xrR/r can be used for the evaluation of the ratio of the rate constants for isotactic and syndiotactic addition, where 2xE is the appropriate energy difference for the syndiotactic and isotactic placements. The calculations show (Fig. 5) that, at the reasonable distance of ~ 2-75 A between the "opened" monomer

530

YU. V. KISSIN and N. M. CHIRKOV

12o

/ /

IOO

I / !

u

80

/ /

5 /

-~ 60

/

/

/

40 / /

20 ...T.

4.0

- "~"

~oz

/

~x~/

.

3.5

3.0

2-5

FIo. 5. Dependence of the repulsion ener~,~¢ between the polymer chain and the monomer on the distance between the monomer and the active site (for iso- and syndio-approach of monomer). The right part of the figure refers to the monomer with "opened" C = C bond.

and the active site, ~ E is equal to ~ 7 kcal/mol. For this value, Ki~o/Ksynajo is ~2.10" at 70 °. Spectral data show (12) that, in most cases, Klso/K~ynaiodoes not exceed 102. But it must be noted that our calculations of AE were rather approximate and were based on some drastic limitations on the geometry of the active site (for example, C = C and M - - C bonds were assumed to be parallel during the approach of olefin). The previous results show that, in the steady state for the active site, i.e. when the polymer chain is bonded to the metal atom, the chain influences two aspects of the functioning of the site, viz. it governs the succession of monomer co-ordination on one of the two vacancies and it controls isotacticity of the addition.

Matrix action of the polymerization active site The stereocontrol of the polymer chain just considered is combined, in the case of heterogeneous catalysts, with another type of stereocontrol brought about by the catalyst surface near the active site. Figure 6 schematically shows part of the basal plane with an active centre having two vacancies and one alkyl group R. When the volume of R is small, both vacancies are free for olefin co-ordination (as in Fig. lb, when the Ti atom holds an ethyl group). One can see that this centre is asymmetric with respect to the direction of approach of the olefin. These directions are shown by arrows in Fig. 6 and correspond to orbital positions for octahedral symmetry. Transition metal atoms to the left of the active site are seen to be closer to the path and to prevent more efficiently the movement of the olefin than the metal atoms to the right. For this reason, when the olefin molecule approaches the vacancy in such a way that the C----C bond is nearly perpendicular to the surface (this position is preferred from the steric point of view), the most favourable situation is when the alkyl group of the olefin is opposite to the nearest metal atom on the path of the olefin (in Fig. 6, it will be at the right).

Stereospecific Mechanism of Olefin Polymerization by Heterogeneous Complex Catalysts--LI 531

This circumstance ensures the steric uniformity of olelin co-ordination on both vacancies, so leading to steric regularity in the growing polymer chain. The model study leads to the conclusion that, after the polymer helix formation, the stereoregulating actions of the catalyst matrix and of the polymer molecule work together to impose a tighter control on olefin co-ordination. A centre of this type is shown in Figs. 2-b, 3a and 3b. It is natural that the catalyst surface contains equal numbers of sites with opposite asymmetry. In accordance with this, equal quantities of enantiomorphous polymer chains will be produced by these sites. (It seems reasonable to neglect conformational transitions in polymer molecules which can occur in solution and in the melt and lead to change in the helix type, ~3) when considering the conditions for polymerization of the olefin since the polymer does not dissolve in the medium.) We want to point out that the matrix effect of the catalyst surface (considered above) differs from the matrix effects of the models described in (2) and (6). In these cases,

/

(

f

Fxo. 6. Scheme of the active site of polymerization with an alkyl group of small volume and with two vacancies. The arrows correspond to the directions of monomer approach.

orientational effect is provided by nonsymmetrically arranged halogen atoms (2) or by alkyl groups bonded with an aluminium atom; (6) in our case, however, the centre of asymmetry is due to nonsymmetric arrangement of transition metal atoms with respect to vacancies at the octahedrally co-ordinated central atom in the active site. This nonsymmetry is found in the geometry of most of the 4th and 5th period transition metal halides with lattice structure. We suppose that it is the reason for stereospeciflc action of catalysts based on TiCI3, VC13, CrC13, FeC13, ZrC13, etc. (14) At present it is difficult to assess the relative importances of the two proposed stereocontrol mechanisms (matrix effect and polymer effect) in the whole stereospecific action of the complex catalysts. Experimental data demonstrate that both effects are real. The role of interaction between the monomer and polymer molecules during polymerization is demonstrated by the fact that block-copolymers are produced in copolymerization of olelins with bulk side groups (16' 1~) while copolymerization of monomers with small or no substituents (for example ethylene-propylene pair) results in random copolymers.(~s. 19) E,P.J,

6/3--G

532

YU. v. KISSIN and N. M. CHIRKOV

On the other hand, examination of the propylene blocks in ethylene-propylene copolymers "9~ shows that propylene units are organized in isotactic sequences even in short blocks. This result demonstrates the importance of the matrix effect; further indications come from the results of polymerization on supported catalysts: ~z°~ semicrystalline polypropylene was obtained with a soluble catalytic system Ti(OR)4-AIEtzCI, supported by alumosilica or COC12, whereas unsupported catalyst produces irregular polymer. The whole of the preceding discussion ignores the role of aluminium--organic compounds. Experimental data (given partially in the previous paper) demonstrate that these substances are important in the catalysis. Their role is not limited to active site production on the surface of the transition metal halide but includes also a direct influence on the polymerization. (2 ~" 2:~ This effect is clear in examination of polymerization kinetics in the presence of various additives. It was found that compounds like AIEt2OEt and AIEt2OH are inhibitors of polymerization due to their adsorption on active sites, but they can be removed from the surface because of adsorption of AIEt3 simultaneously. ~2t~ There has been discussion of the role of aluminium-organic compounds in the catalysis. (6~ On the other hand, data on stereoregularity of polypropylene, obtained with different metal-organic compounds, c23~ indicate that the influence of these compounds on the polymer structure is relatively small; at least, it is not as significant as the role of transition metal compound3 ~2~ Consideration of the active site model with two vacancies at the moment when it is connected with polymer chain (Fig. 2b) indicates strong interference with complex formation between the Ti atom and the aluminium-organic compound especially when a vacancy is reserved for olefin co-ordination. At the same time, there is no obstruction to complex formation between aluminiumorganic compounds and transition metal atoms placed around the Ti atom with a polymer chain. In accordance with the previous paper, it is unlikely for steric reasons that these atoms can be active sites themselves. Apparently they are bonded with small ligands and among them there may be aluminium-organic compounds. (For the sake of simplicity these Ti atoms in Figs. 1, 2 and 3 do not have any ligands on the catalyst surface.) It can be supposed that, when co-ordinated at these points, metal-organic compounds can influence significantly the active site metal, with which they are connected through the "lower" layer of halogen atoms. These complexes can also increase the matrix effect of the active site for purely steric reasons. This scheme of active sites, occupying the basal plans of transition metal chlorides, is largely idealized. It does not take into account the chance of centre formation on some of the numerous defects that are always presented on the surface of real crystals, and also it ignores site formation on lateral less regular faces of crystals. ~2~ The interaction between transition metal halides and aluminium-organic compounds also leads to pronounced increase of surface irregularity. (25) All these factors lead to the formation of a large number of defect active sites, producing polymers of low stereoregularity.

Stereospecific Mechanism of Olefin Polymerizationby HeterogeneousComplex Catalysts--II 533

SOME KINETIC ASPECTS OF O L E F I N P O L Y M E R I Z A T I O N WITH P R E L I M I N A R Y CO-ORDINATION The specific feature of the proposed site model, as well as the models of Refs. I and 6, is the assumption that the first stage of the addition process is co-ordination of the monomer on the transition metal vacancy. The insertion reaction itself proceeds with participation of the co-ordinated olefin molecule. Some analogy can be found between the polymerization reaction and the insertion reaction of CO in M n - C bond in the process CH3Mn(CO)5 + CO ~ CH3COMn(CO)s. It was found with labelled CO that the insertion takes place with a co-ordinated CO molecule but not with the attacking molecule: CO CH3Mn(CO)5 ~-- CH3COMn(CO),

> CH3COMn(CO)5.

Kinetic data show that the nucleophile (attacking CO molecule) plays an important role in the reaction when the solvation ability of the solvent is low. c26) This same situation arises in polymerization. A similar kinetic scheme can be put forward for polymerization:

C--C--P

P

/

/

M

~

..C

C--C--P (+C----C)K2 /

M

"

Kt

" K_ 2

M

..C

"ll

r;

C

C

I

II

iII

Here KI = rate constant for olefin insertion, K2 ---- rate constant for olefin co-ordination, K_z = rate constant for removal of olefin from the complex. Under stationary conditions, when I and III cannot be distinguished kinetically (this assumption is commonly used in polymerization kinetics), every active site can be found either in state I, or in state II, and [I] + [II] = No (the number of active sites at the particular stage of growth). Accepting the stationary principle d[I]/dt = K2 [II] [M] -- K, [I] -- K_2 [I] = 0 one can obtain an expression for monomer consumption, i.e. for polymerization rate : -- d[M]/dt = N O

Kt K2[M] K2[M] + I(1 + K-2

In practice, in the case of olefin polymerization, the rate of monomer consumption is second order with respect to monomer at low concentrations and first order at high concentrations. (2~) The change of order is attributed to slow initiation steps. As can be seen from the preceding consideration, experimental data agree with the assumption of preliminary monomer co-ordination in the case where the metal-

534

YU. V. KISSIN and N. M. CHIRKOV

olefin complex is accepted to be weak, i.e. K - , is large and K-2 >> K_,[M] for all real monomer concentrations. In this case -- d[M]/dt ~ No

K~ K2 [M]. Kt + K_2

But in the situation where the condition K-2 >> K,[M] is not valid for all monomer concentrations, change of the order from 1 to 0 must occur. Polymerization first order with respect to monomer can be explained in principle also by another assumption, viz. the participation of a free monomer molecule in the insertion of a co-ordinated molecule in the metal-carbon bond (as in Ref. 26). Such participation might consist of attack on a free but "blocked" vacancy at the transition metal by the free monomer molecule (see Fig. 3), so assisting the insertion. In this case the expression for polymerization rate is -- d[M]/dt = No

2 K t / ( 2 [M] 2 (Kt + K2).[M] + K-2

and if the transition metal-olefin complex is stable (i.e. K_2 ,~ (K1 + K,)[M]), then 2 K1 K2 [M]. -- d[M]/dt ~ No K~ + / ( 2 Until now we have no experimental data suitable to discriminate between these possibilities. But the first of them (weak complexes between olefin and transition metal in the active site) appears to be more real; this is implied in Ref. 1. CONCLUSION The study of the reasons for the stereospecific action of heterogeneous complex catalysts in olefin polymerization is one o f the most interesting problems associated with polymerization processes. The experimental approaches to this problem are very complicated; most investigators choose the way involving construction of models, which are in agreement with the experimental results, c2' 6~ This report is a work of the same type. A model of the stereospecific active sites, situated on the basal faces of catalyst and having two vacancies for olefin co-ordination, is in good agreement with data on the character of polymerization with Ziegler-Natta catalysts; we believe that it is superior to a model in which a large part of the surface is filled with active sites and crystalline polymer blocks are formed in the vicinity of the catalyst surface. REFERENCES (I) P. Cossee, 3.. Catalysis 3, 80 (1964). (2) E. J. Arlman and P. Cossee~J. Catalysis 3, 99 (1964). (3) W. Klemm and E. Krose, Z. anorg, allg. Chem. 253, 209 (1947). (4) P. Blais and R. St. J. Mardey, J. Polym. Sci. A1, 6, 291 (1968). (5) Yu. V. Kissin, Vysokomolek. Soedin. All, 1569 (1969). (6) L. Rodriguez and H. M. van Loy, J. Polym. Sci. A1, 4, 1905 (1966). (7) T. Miyazawa, J. Polym. Sci. C7, 59 (1964). (8) G. Natta, A. Zambelli, A. L. Segre and M. Farina, Makromolek. Chem. 110, 1 (1967). (9) M. Tasumi and T. Shimanouchi, J. Polym. Sci. A2, 1607 (1964). (I0) M. Tasumi and G. Zerbi, J. Polym. Sci. B5, 985 (1967). (I1) L. I. Kytaigorodski and K. V. Myrskya, Soviet Phys. Crystallogr. 6, 507 (1961); 9, 174 (1964). (12) Yu. V. Kissin, V. I. Tsvetkova and N. M. Chirkov, Vysokomolek. Soeodin. Ag, 1104 (1967).

Stereospecific Mechanism of Olefin Polymerization by Heterogeneous Complex Catalysts--II 535 (13) P. Corradini and G. AUegra, Atti Accad. naz. Lincei, classe di Sc. fis., mat. e nat. XXX, 516 (1961). (14) N. G. Gaylord and H. F. Mark, Linear and Stereoregular Addition Polymers, Lippincott, New York (1959). (15) F. X. Werber, C. J. Benning and G. E. Ashby, J. Polym. Sci. A1, 6, 755 (1968). (16) I. H. Anderson, G. M. Burnett and W. C. Geddes, Europ. Polym. J. 3, 161 (1967). (17) N. Ashikari, T. Kanernitsu, K. Yanagisawa, K. Nakagawa, H. Okamoto, S. Kobayashi and A. Nishioka, J. Polym. Sci. A2, 3009 (1964). (18) G. Bucci, T. Simonazzi, J. Polym. Sci. C7, 203 (1964). (19) Yu. V. Kissin, V. I. Tsvetkova and N. M. Chirkov, Vysokomolek. Soedin. A9, 1374 (1967). (20) G. Natta, J. Polym. Sci. 34, 21 (1959). (21) S. M. Mezhikovsky, Yu. V. Kissin and N. M. Chirkov, Vysokomolek. Soedin. A9, 2006 (1967); A10, 2231 (1968). (22) L. Kollar, A. Simon and J. Osvath, J. Polym. Sci. A1, 6, 927 (1968). (23) A. P. Firsov, B. G. Kashporov, Yu. V. Kissin and N. M. Chirkov, J. Polym. Sci. 62, 1104 (1962). (24) L. A. Novokshonova, G. P. Berseneva, V. I. Tsvetkova and N. M. Chirkov, Vysokomolek. Soedin. A_9,562 (1967). (25) E. Spousta, J. Dvorak and H. Nemynar, Chem. Prumysl. 17, 312 (1967). (26) R. J. Mawby, F. Basolo and R. G. Pearson, J. Am. chem. Soc. 86, 3994, 5043 (1964); G. E. Coates, Organometallic Compounds, 2nd edn. (1960). (27) A. P. Firsov, V. I. Tsvetkova and N. M. Chirkov, lzv. Akad. Nauk SSSR, Set. Chim. 1956 (1964) ; Dokl. Akad. Nauk SSSR 142, 149 (1962). R6sum~---On expose ici la suite des nos consid6rations sur le module du site actif de la polym6risation amorc6e par des catalyseurs h6t6rog6nes du type Ziegler-Natta. Ce site est localise~ sur les plans de base des r6seaux cristallins de type e du TiCI3 ou de VCI3 et poss~de deux lacunes sur l'atome du m6tal de transition. L'une de ces lacunes est occup6e par la chahae h61icoidale du polym6re et l'autre est utilis~e pour coordiner l'ol~.fine. L'6tude du mode d'action de ce module montre deux possibilit6s de contr61e st6r6osp6cifique sur la coordination du monom6re: au moyen de la chahae croissante h61icoidale du polym~re et au moyen des atomes voisins du m6tal de transition. On discute quelques aspects cin6tiques de la polym~risation des ol~fines avec une coordination pr61iminaire. Sommario--C'~ una ulteriore esaminazione del modello del luogo attivo di polimerizzazione sui catalizzatori eterogeni Ziegler-Natta. E locato sui piani basali di cristalli del reticolo del tipo a-TiCI~ o VCI3 e ha due vacanze all'atomo metallico di transizione. Una di queste vacanze ~ bloccata dalla catena elicoidale del polimero e l'altra ~ disponibile per la coordinazione delle olefine. Lo studio dell'azione di questo modello dimostra due tipi di controllo stereospecifico sulla coordinazione dei monomeri: quello secondo la catena e!icoidale crescente del polimero e quello secondo ~ atomi rnetaUici vicini di transizione. Alcuni aspetti cinetici della polimerizzazione delle olefine con coordinazione preliminare vengono discussi. Zusammenfassung--Das Modell der aktiven Polymerisationsstelle an heterogenen Ziegler-Natta Katalysatoren wird weiterhin untersucht. Es liegt auf Grundebenen der Gitter yon Kristallen des ~-TiC13 Typ oder VC13 und hat z~'ei Vakanzen am Metallatomtibergang. Eine dieser Vakanzen ist durch die spiralenf6rmige Polymerkette blockiert und die andere steht f'& Olefinkoordination verftigbar. Das Studium der Modellfunktion ergibt zwei Typen spezifischer Stereokontrolle iiber monomere Koordination: durch die wachsende, spiralenf~Srmige Polymerkette und durch benachbarte 0"bergangsmetallatome. Einige kinetische Gesichtspunkte der Olefin Polymerisation mit vorangehender Koordination werden besprochen.